METHOD AND APPARATUS FOR EARLY RANDOM DISCARD OF PACKETS

Abstract: A system and method are provided for randomly discarding a packet received at a high performance network interface if the rate of packet transfers cannot keep pace with the rate of packet arrivals. A selected packet may be dropped as it arrives at a packet queue, or a packet already in the queue may be discarded. The queue has multiple defined regions, any of which may overlap or share a common boundary. A probability indicator is associated with a region to specify the probability of a packet being discarded when the level of traffic in the queue is within the region. Probability indicators may differ from region to region so that the probability of discarding a packet fluctuates as the level of traffic stored in the queue changes. Information gleaned from a packet may be applied to prevent certain types of packets from being dropped.

Description: METHOD AND APPARATUS FOR EARLY RANDOM DISCARD OF PACKETS BACKGROUND This invention relates to the fields of computer systems and computer networks. In particular, the present invention relates to a Network Interface Circuit (NIC) for processing communication packets exchanged between a computer network and a host computer system. The interface between a computer and a network is often a bottleneck for communications passing between the computer and the network. While computer performance (e.g., processor speed) has increased exponentially over the years and computer network transmission speeds have undergone similar increases, inefficiencies in the way network interface circuits handle communications have become more and more evident. With each incremental increase in computer or network speed, it becomes ever more apparent that the interface between the computer and the network cannot keep pace. These inefficiencies involve several basic problems in the way communications between a network and a computer are handled. Today's most popular forms of networks tend to be packet-based. These types of networks, including the Internet and many local area networks, transmit information in the form of packets. Each packet is separately created and transmitted by an originating endstation and is separately received and processed by a destination endstation. In addition, each packet may, in a bus topology network for example, be received and processed by numerous stations located between the originating and destination endstations. One basic problem with packet networks is that each packet must be processed through multiple protocols or protocol levels (known collectively as a "protocol stack") on both the origination and destination endstations. When data transmitted between stations is longer than a certain minimal length, the data is divided into multiple portions, and each portion is carried by a separate packet. The amount of data that a packet can carry is generally limited by the network that conveys the packet and is often expressed as a maximum transfer unit (MTU). The original aggregation of data is sometimes known as a "datagram," and each packet carrying part of a single datagram is processed very similarly to the other packets of the datagram. Communication packets are generally processed as follows. In the origination endstation, each separate data portion of a datagram is processed through a protocol stack. During this processing multiple protocol headers (e.g., TCP, IP, Ethernet) are added to the data portion to form a packet that can be transmitted across the network. The packet is received by a network interface circuit, which transfers the packet to the destination endstation or a host computer that serves the destination endstation. In the destination endstation, the packet is processed through the protocol stack in the opposite direction as in the origination endstation. During this processing the protocol headers are removed in the opposite order in which they were applied. The data portion is thus recovered and can be made available to a user, an application program, etc. Several related packets (e.g., packets carrying data from one datagram) thus undergo substantially the same process in a serial manner (i.e., one packet at a time). The more data that must be transmitted, the more packets must be sent, with each one being separately handled and processed through the protocol stack in each direction. Naturally, the more packets that must be processed, the greater the demand placed upon an endstation' s processor. The number of packets that must be processed is affected by factors other than just the amount of data being sent in a datagram. For example, as the amount of data that can be encapsulated in a packet increases, fewer packets need to be sent. As stated above, however, a packet may have a maximum allowable size, depending on the type of network in use (e.g., the maximum transfer unit for standard Ethernet traffic is approximately 1,500 bytes). The speed of the network also affects the number of packets that a NIC may handle in a given period of time. For example, a gigabit Ethernet network operating at peak capacity may require a NIC to receive approximately 1.48 million packets per second. Thus, the number of packets to be processed through a protocol stack may place a significant burden upon a computer's processor. The situation is exacerbated by the need to process each packet separately even though each one will be processed in a substantially similar manner. A related problem to the disjoint processing of packets is the manner in which data is moved between "user space" (e.g., an application program's data storage) and "system space" (e.g., system memory) during data transmission and receipt. Presently, data is simply copied from one area of memory assigned to a user or application program into another area of memory dedicated to the processor's use. Because each portion of a datagram that is transmitted in a packet may be copied separately (e.g., one byte at a time), there is a nontrivial amount of processor time required and frequent transfers can consume a large amount of the memory bus' bandwidth. Illustratively, each byte of data in a packet received from the network may be read from the system space and written to the user space in a separate copy operation, and vice versa for data transmitted over the network. Although system space generally provides a protected memory area (e.g., protected from manipulation by user programs), the copy operation does nothing of value when seen from the point of view of a network interface circuit. Instead, it risks over-burdening the host processor and retarding its ability to rapidly accept additional network traffic from the NIC. Copying each packet's data separately can therefore be very inefficient, particularly in a high-speed network environment. In addition to the inefficient transfer of data (e.g., one packet's data at a time), the processing of headers from packets received from a network is also inefficient. Each packet carrying part of a single datagram generally has the same protocol headers (e.g., Ethernet, IP and TCP), although there may be some variation in the values within the packets' headers for a particular protocol. Each packet, however, is individually processed through the same protocol stack, thus requiring multiple repetitions of identical operations for related packets. Successively processing unrelated packets through different protocol stacks will likely be much less efficient than progressively processing a number of related packets through one protocol stack at a time. Another basic problem concerning the interaction between present network interface circuits and host computer systems is that the combination often fails to capitalize on the increased processor resources that are available in multi-processor computer systems. In other words, present attempts to distribute the processing of network packets (e.g., through a protocol stack) among a number of protocols in an efficient manner are generally ineffective. In particular, the performance of present NICs does not come close to the expected or desired linear performance gains one may expect to realize from the availability of multiple processors. In some multi-processor systems, little improvement in the processing of network traffic is realized from the use of more than 4-6 processors, for example. In addition, the rate at which packets are transferred from a network interface circuit to a host computer or other communication device may fail to keep pace with the rate of packet arrival at the network interface. One element or another of the host computer (e.g., a memory bus, a processor) may be over-burdened or otherwise unable to accept packets with sufficient alacrity. In this event one or more packets may be dropped or discarded. Dropping packets may cause a network entity to re-transmit some traffic and, if too many packets are dropped, a network connection may require re-initialization. Further, dropping one packet or type of packet instead of another may make a significant difference in overall network traffic. If, for example, a control packet is dropped, the corresponding network connection may be severely affected and may do little to alleviate the packet saturation of the network interface circuit because of the typically small size of a control packet. Therefore, unless the dropping of packets is performed in a manner that distributes the effect among many network connections or that makes allowance for certain types of packets, network traffic may be degraded more than necessary. Thus, present NICs fail to provide adequate performance to interconnect today's high-end computer systems and high-speed networks. In addition, a network interface circuit that cannot make allowance for an over-burdened host computer may degrade the computer's performance. SUMMARY In one embodiment of the invention packets are received from a network and stored in a packet queue prior to being transferred to a host computer. If the rate of packet transfers to the host computer cannot keep pace with the rate of packet arrivals at the queue, one or more packets may be dropped. Therefore, a system and method of discarding packets in a random manner is provided, such that the effect of lost packets is fairly distributed among network communicants. In one embodiment of the invention a packet queue that is used to store packets received from a network is divided into multiple regions. Each region is distinct yet shares a boundary with an adjacent region. In an alternative embodiment regions may overlap. A fullness gauge or indicator is employed to indicate how full the packet queue is. In particular, read and write pointers that are used to update the packet queue can also be used to determine how full the queue is. This fullness indicator thus fluctuates as the level of network traffic stored in the packet queue ebbs and flows. For one or more of the multiple packet queue regions, a programmable probability indicator is assigned. Each probability indicator indicates the probability of dropping a packet when the fullness indicator indicates that the level of traffic stored in the queue is within the probability indicator's associated region. Probability indicators may be programmed and re-programmed as the level of traffic in the packet queue changes. The probability indicator may take the form of a percentage or ratio that is configured to randomly select packets to be discarded. In one particular embodiment of the invention a probability indicator takes the form of a bit or flag mask. Each bit or flag may take one of two possible values (e.g., zero and one). In this embodiment, a counter tracks the number of packets received at the packet queue by repeatedly counting through a limited range of numbers, such as zero through N. The bit or flag mask correspondingly contains N+l bits or flags. Thus, for each counter value the corresponding bit or flag in the mask indicates whether the packet received during that counter value is dropped. In an alternative embodiment, a random number is generated when a packet is received. The random number may be compared to a threshold to determine whether the received packet is dropped. Each region may have a separate threshold for determining whether a packet is dropped. In yet another embodiment of the invention, a packet may be immunized or exempted from being discarded because it exhibits a particular characteristic or status. For example, a control packet may be one type of packet that is not dropped. In this embodiment, the counter is not incremented when a non-discardable packet is received. Other packets that may be exempt from discarding may be packets within a particular network connection or flow, packets associated with a particular application, packets formatted according to a particular protocol, etc. A relevant characteristic or detail of a packet may be extracted during a process in which one or more of the packet's headers are parsed. In one embodiment of the invention, when a probability indicator indicates that a packet should be dropped the packet that is dropped may be one just received at the packet queue. In another embodiment, however, a packet already stored in the packet queue may be dropped. BRIEF DESCRIPTION OF THE FIGURES FIG. 1 A is a block diagram depicting a network interface circuit (NIC) for receiving a packet from a network in accordance with an embodiment of the present invention. FIG. IB is a flow chart demonstrating one method of operating the NIC of FIG. 1 A to transfer a packet received from a network to a host computer in accordance with an embodiment of the invention. FIG. 2 is a diagram of a packet transmitted over a network and received at a network interface circuit in one embodiment of the invention. FIG. 3 is a block diagram depicting a header parser of a network interface circuit for parsing a packet in accordance with an embodiment of the invention. FIGs. 4A-4B comprise a flow chart demonstrating one method of parsing a packet received from a network at a network interface circuit in accordance with an embodiment of the present invention. FIG. 5 is a block diagram depicting a network interface circuit flow database in accordance with an embodiment of the invention. FIGs. 6A-6E comprise a flowchart illustrating one method of managing a network interface circuit flow database in accordance with an embodiment of the invention. FIG. 7 is a flow chart demonstrating one method of distributing the processing of network packets among multiple processors on a host computer in accordance with an embodiment of the invention. FIG. 8 is a diagram of a packet queue for a network interface circuit in accordance with an embodiment of the invention. FIG. 9 is a diagram of a control queue for a network interface circuit in accordance with an embodiment of the invention. FIG. 10 depicts a system for randomly discarding a packet from a network interface in accordance with an embodiment of the invention. FIGs. 11 A-l IB comprise a flow chart demonstrating one method of discarding a packet from a network interface in accordance with an embodiment of the invention. FIG. 12 depicts one set of dynamic instructions for parsing a packet in accordance with an embodiment of the invention. DETAILED DESCRIPTION The following description is presented to enable any person skilled in the art to make and use the invention, and is provided in the context of particular applications of the invention and their requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the present invention. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. In particular, embodiments of the invention are described below in the form of a network interface circuit (NIC) receiving communication packets formatted in accordance with certain communication protocols compatible with the Internet. One skilled in the art will recognize, however, that the present invention is not limited to communication protocols compatible with the Internet and may be readily adapted for use with other protocols and in communication devices other than a NIC. The program environment in which a present embodiment of the invention is executed illustratively incorporates a general-purpose computer or a special purpose device such a hand-held computer. Details of such devices (e.g., processor, memory, data storage, input/output ports and display) are well known and are omitted for the sake of clarity. It should also be understood that the techniques of the present invention might be implemented using a variety of technologies. For example, the methods described herein may be implemented in software running on a programmable microprocessor, or implemented in hardware utilizing either a combination of microprocessors or other specially designed application specific integrated circuits, programmable logic devices, or various combinations thereof. In particular, the methods described herein may be implemented by a series of computer-executable instructions residing on a storage medium such as a carrier wave, disk drive, or other computer-readable medium. Introduction In one embodiment of the present invention, a network interface circuit (NIC) is configured to receive and process communication packets exchanged between a host computer system and a network such as the Internet. In particular, the NIC is configured to receive and manipulate packets formatted in accordance with a protocol stack (e.g., a combination of communication protocols) supported by a network coupled to the NIC. A protocol stack may be described with reference to the seven-layer ISO-OSI (International Standards Organization - Open Systems Interconnection) model framework. Thus, one illustrative protocol stack includes the Transport Control Protocol (TCP) at layer four, Internet Protocol (IP) at layer three and Ethernet at layer two. For purposes of discussion, the term "Ethernet" may be used herein to refer collectively to the standardized IEEE (Institute of Electrical and Electronics Engineers) 802.3 specification as well as version two of the non-standardized form of the protocol. Where different forms of the protocol need to be distinguished, the standard form may be identified by including the "802.3" designation. Other embodiments of the invention are configured to work with communications adhering to other protocols, both known (e.g., AppleTalk, IPX (Internetwork Packet Exchange), etc.) and unknown at the present time. The methods provided by this invention are easily adaptable for new communication protocols. In addition, the processing of packets described below may be performed on communication devices other than a NIC. For example, a modem, switch, router or other communication port or device (e.g., serial, parallel, USB, SCSI) may be similarly configured and operated. In embodiments of the invention described below, a NIC receives a packet from a network on behalf of a host computer system or other communication device. The NIC analyzes the packet (e.g., by retrieving certain fields from one or more of its protocol headers) and takes action to increase the efficiency with which the packet is transferred or provided to its destination entity. Equipment and methods discussed below for increasing the efficiency of processing or transferring packets received from a network may also be used for packets moving in the reverse direction (i.e., from the NIC to the network). One technique that may be applied to incoming network traffic involves examining or parsing one or more headers of an incoming packet (e.g., headers for the layer two, three and four protocols) in order to identify the packet's source and destination entities and possibly retrieve certain other information. Using identifiers of the communicating entities as a key, data from multiple packets may be aggregated or re-assembled. Typically, a datagram sent to one destination entity from one source entity is transmitted via multiple packets. Aggregating data from multiple related packets (e.g., packets carrying data from the same datagram) thus allows a datagram to be re-assembled and collectively transferred to a host computer. The datagram may then be provided to the destination entity in a highly efficient manner. For example, rather than providing data from one packet at a time (and one byte at a time) in separate "copy" operations, a "page-flip" operation may be performed. In a page-flip, an entire memory page of data may be provided to the destination entity, possibly in exchange for an empty or unused page. In another technique, packets received from a network are placed in a queue to await transfer to a host computer. While awaiting transfer, multiple related packets may be identified to the host computer. After being transferred, they may be processed as a group by a host processor rather than being processed serially (e.g., one at a time). Yet another technique involves submitting a number of related packets to a single processor of a multi-processor host computer system. By distributing packets conveyed between different pairs of source and destination entities among different processors, the processing of packets through their respective protocol stacks can be distributed while still maintaining packets in their correct order. The techniques discussed above for increasing the efficiency with which packets are processed may involve a combination of hardware and software modules located on a network interface and/or a host computer system. In one particular embodiment, a parsing module on a host computer's NIC parses header portions of packets. Illustratively, the parsing module comprises a microsequencer operating according to a set of replaceable instructions stored as micro-code. Using information extracted from the packets, multiple packets from one source entity to one destination entity may be identified. A hardware reassembly module on the NIC may then gather the data from the multiple packets. Another hardware module on the NIC is configured to recognize related packets awaiting transfer to the host computer so that they may be processed through an appropriate protocol stack collectively, rather than serially. The re-assembled data and the packet's headers may then be provided to the host computer so that appropriate software (e.g., a device driver for the NIC) may process the headers and deliver the data to the destination entity. Where the host computer includes multiple processors, a load distributor (which may also be implemented in hardware on the NIC) may select a processor to process the headers of the multiple packets through a protocol stack. In another embodiment of the invention, a system is provided for randomly discarding a packet from a NIC when the NIC is saturated or nearly saturated with packets awaiting transfer to a host computer. One Embodiment of a High Performance Network Interface Circuit FIG. 1A depicts NIC 100 configured in accordance with an illustrative embodiment of the invention. A brief description of the operation and interaction of the various modules of NIC 100 in this embodiment follows. A communication packet may be received at NIC 100 from network 102 by a medium access control (MAC) module (not shown in FIG. 1 A). The MAC module performs low-level processing of the packet such as reading the packet from the network, performing some error checking, detecting packet fragments, detecting over-sized packets, removing the layer one preamble, etc. Input Port Processing (IPP) module 104 then receives the packet. The IPP module stores the entire packet in packet queue 116, as received from the MAC module or network, and a portion of the packet is copied into header parser 106. In one embodiment of the invention IPP module 104 may act as a coordinator of sorts to prepare the packet for transfer to a host computer system. In such a role, IPP module 104 may receive information concerning a packet from various modules of NIC 100 and dispatch such information to other modules. Header parser 106 parses a header portion of the packet to retrieve various pieces of information that will be used to identify related packets (e.g., multiple packets from one same source entity for one destination entity) and that will affect subsequent processing of the packets. In the illustrated embodiment, header parser 106 communicates with flow database manager (FDBM) 108, which manages flow database (FDB) 110. In particular, header parser 106 submits a query to FDBM 108 to determine whether a valid communication flow (described below) exists between the source entity that sent a packet and the destination entity. The destination entity may comprise an application program, a communication module, or some other element of a host computer system that is to receive the packet. In the illustrated embodiment of the invention, a communication flow comprises one or more datagram packets from one source entity to one destination entity. A flow may be identified by a flow key assembled from source and destination identifiers retrieved from the packet by header parser 106. In one embodiment of the invention a flow key comprises address and/or port information for the source and destination entities from the packet's layer three (e.g., IP) and/or layer four (e.g., TCP) protocol headers. For purposes of the illustrated embodiment of the invention, a communication flow is similar to a TCP end-to-end connection but is generally shorter in duration. In particular, in this embodiment the duration of a flow may be limited to the time needed to receive all of the packets associated with a single datagram passed from the source entity to the destination entity. Thus, for purposes of flow management, header parser 106 passes the packet's flow key to flow database manager 108. The header parser may also provide the flow database manager with other information concerning the packet that was retrieved from the packet (e.g., length of the packet). Flow database manager 108 searches FDB 110 in response to a query received from header parser 106. Illustratively, flow database 110 stores information concerning each valid communication flow involving a destination entity served by NIC 100. Thus, FDBM 108 updates FDB 110 as necessary, depending upon the information received from header parser 106. In addition, in this embodiment of the invention FDBM 108 associates an operation or action code with the received packet. An operation code may be used to identify whether a packet is part of a new or existing flow, whether the packet includes data or just control information, the amount of data within the packet, whether the packet data can be re-assembled with related data (e.g., other data in a datagram sent from the source entity to the destination entity), etc. FDBM 108 may use information retrieved from the packet and provided by header parser 106 to select an appropriate operation code. The packet's operation code is then passed back to the header parser, along with an index of the packet's flow within FDB 110. In one embodiment of the invention the combination of header parser 106, FDBM 108 and FDB 110, or a subset of these modules, may be known as a traffic classifier due to their role in classifying or identifying network traffic received at NIC 100. In the illustrated embodiment, header parser 106 also passes the packet's flow key to load distributor 112. In a host computer system having multiple processors, load distributor 112 may determine which processor an incoming packet is to be routed to for processing through the appropriate protocol stack. Load distributor 112 may, for example, ensure that related packets are routed to a single processor. By sending all packets in one communication flow or end-to-end connection to a single processor, the correct ordering of packets can be enforced. Load distributor 112 may be omitted in an alternative embodiment of the invention. In an alternative embodiment, header parser 106 may also communicate directly with other modules of NIC 100 besides the load distributor and flow database manager. Thus, after header parser 106 parses a packet FDBM 108 alters or updates FDB 110 and load distributor 112 identifies a processor in the host computer system to process the packet. After these actions, the header parser passes various information back to IPP module 104. Illustratively, this information may include the packet's flow key, an index of the packet's flow within flow database 110, an identifier of a processor in the host computer system, and various other data concerning the packet (e.g., its length, a length of a packet header). Now the packet may be stored in packet queue 116, which holds packets for manipulation by DMA (Direct Memory Access) engine 120 and transfer to a host computer. In addition to storing the packet in a packet queue, a corresponding entry for the packet is made in control queue 118 and information concerning the packet's flow may also be passed to dynamic packet batching module 122. Control queue 118 contains related control information for each packet in packet queue 116. Packet batching module 122 draws upon information concerning packets in packet queue 116 to enable the batch (i.e., collective) processing of headers from multiple related packets. In one embodiment of the invention packet batching module 122 alerts the host computer to the availability of headers from related packets so that they may be processed together. Although the processing of a packet's protocol headers is performed by a processor on a host computer system in one embodiment of the invention, in another embodiment the protocol headers may be processed by a processor located on NIC 100. In the former embodiment, software on the host computer (e.g., a device driver for NIC 100) can reap the advantages of additional memory and a replaceable or upgradeable processor (e.g., the memory may be supplemented and the processor may be replaced by a faster model). During the storage of a packet in packet queue 116 checksum generator 114 may perform a checksum operation. The checksum may be added to the packet queue as a trailer to the packet. Illustratively, checksum generator 114 generates a checksum from a portion of the packet received from network 102. In one embodiment of the invention a checksum is generated from the TCP portion of a packet (e.g., the TCP header and data). If a packet is not formatted according to the TCP protocol a checksum may be generated on another portion of the packet and the result may be adjusted in later processing as necessary. For example, if the checksum calculated by checksum generator 114 was not calculated on the correct portion of the packet, the checksum may be adjusted to capture the correct portion. This adjustment may be made by software operating on a host computer system (e.g., a device driver). Checksum generator 114 may be omitted or merged into another module of NIC 100 in an alternative embodiment of the invention. From the information obtained by header parser 106 and the flow information managed by flow database manager 108, the host computer system served by NIC 100 in the illustrated embodiment is able to process network traffic very efficiently. For example, data portions of related packets may be re-assembled by DMA engine 120 to form aggregations that can be more efficiently manipulated. And, by assembling the data into buffers the size of a memory page, the data can be more efficiently transferred to a destination entity through "page-flipping," in which an entire memory page filled by DMA engine 120 is provided at once. One page-flip can thus take the place of multiple copy operations. Meanwhile, the header portions of the re-assembled packets may similarly be processed as a group through their appropriate protocol stack. As already described, in another embodiment of the invention the processing of network traffic through appropriate protocol stacks may be efficiently distributed in a multi-processor host computer system. In this embodiment, load distributor 112 assigns or distributes related packets (e.g., packets in the same communication flow) to the same processor. In particular, packets having the same source and destination addresses in their layer three protocol (e.g., IP) headers and/or the same source and destination ports in their layer four protocol (e.g., TCP) headers may be sent to a single processor. In the NIC illustrated in FIG. 1 A, the processing enhancements discussed above (e.g., re-assembling data, batch processing packet headers, distributing protocol stack processing) are possible for packets received from network 102 that are formatted according to one or more pre-selected protocol stacks. In this embodiment of the invention network 102 is the Internet and NIC 100 is therefore configured to process packets using one of several protocol stacks compatible with the Internet. Packets not configured according to the pre-selected protocols are also processed, but may not receive the benefits of the full suite of processing efficiencies provided to packets meeting the pre-selected protocols. For example, packets not matching one of the pre-selected protocol stacks may be distributed for processing in a multi-processor system on the basis of the packets' layer two (e.g., medium access control) source and destination addresses rather than their layer three or layer four addresses. Using layer two identifiers provides less granularity to the load distribution procedure, thus possibly distributing the processing of packets less evenly than if layer three/four identifiers were used. FIG. IB depicts one method of using NIC 100 of FIG. 1 A to receive one packet from network 102 and transfer it to a host computer. State 130 is a start state, possibly characterized by the initialization or resetting of NIC 100. In state 132, a packet is received by NIC 100 from network 102. As already described, the packet may be formatted according to a variety of communication protocols. The packet may be received and initially manipulated by a MAC module before being passed to an IPP module. In state 134, a portion of the packet is copied and passed to header parser 106. Header parser 106 then parses the packet to extract values from one or more of its headers and/or its data. A flow key is generated from some of the retrieved information to identify the communication flow that includes the packet. The degree or extent to which the packet is parsed may depend upon its protocols, in that the header parser may be configured to parse headers of different protocols to different depths. In particular, header parser 106 may be optimized (e.g., its operating instructions configured) for a specific set of protocols or protocol stacks. If the packet conforms to one or more of the specified protocols it may be parsed more fully than a packet that does not adhere to any of the protocols. In state 136, information extracted from the packet's headers is forwarded to flow database manager 108 and/or load distributor 112. The FDBM uses the information to set up a flow in flow database 110 if one does not already exist for this communication flow. If an entry already exists for the packet's flow, it may be updated to reflect the receipt of a new flow packet. Further, FDBM 108 generates an operation code to summarize one or more characteristics or conditions of the packet. The operation code may be used by other modules of NIC 100 to handle the packet in an appropriate manner, as described in subsequent sections. The operation code is returned to the header parser, along with an index (e.g., a flow number) of the packet's flow in the flow database. In state 138, load distributor 112 assigns a processor number to the packet, if the host computer includes multiple processors, and returns the processor number to the header processor. Illustratively, the processor number identifies which processor is to conduct the packet through its protocol stack on the host computer. State 138 may be omitted in an alternative embodiment of the invention, particularly if the host computer consists of only a single processor. In state 140, the packet is stored in packet queue 116. As the contents of the packet are placed into the packet queue, checksum generator 114 may compute a checksum. The checksum generator may be informed by IPP module 104 as to which portion of the packet to compute the checksum on. The computed checksum is added to the packet queue as a trailer to the packet. In one embodiment of the invention, the packet is stored in the packet queue at substantially the same time that a copy of a header portion of the packet is provided to header parser 106. Also in state 140, control information for the packet is stored in control queue 118 and information concerning the packet's flow (e.g., flow number, flow key) may be provided to dynamic packet batching module 122. In state 142, NIC 100 determines whether the packet is ready to be transferred to host computer memory. Until it is ready to be transferred, the illustrated procedure waits. When the packet is ready to be transferred (e.g., the packet is at the head of the packet queue or the host computer receives the packet ahead of this packet in the packet queue), in state 144 dynamic packet batching module 122 determines whether a related packet will soon be transferred. If so, then when the present packet is transferred to host memory the host computer is alerted that a related packet will soon follow. The host computer may then process the packets (e.g., through their protocol stack) as a group. In state 146, the packet is transferred (e.g., via a direct memory access operation) to host computer memory. And, in state 148, the host computer is notified that the packet was transferred. The illustrated procedure then ends at state 150. The procedure described above is just one method of employing the modules of NIC 100 to receive a single packet from a network and transfer it to a host computer system. Other suitable methods are also contemplated within the scope of the invention. An Illustrative Packet FIG. 2 is a diagram of an illustrative packet received by NIC 100 from network 102. Packet 200 comprises data portion 202 and header portion 204, and may also contain trailer portion 206. Depending upon the network environment traversed by packet 200, its maximum size (e.g., its maximum transfer unit or MTU) may be limited. In the illustrated embodiment, data portion 202 comprises data being provided to a destination or receiving entity within a computer system (e.g., user, application program, operating system) or a communication subsystem of the computer. Header portion 204 comprises one or more headers prefixed to the data portion by the source or originating entity or a computer system comprising the source entity. Each header normally corresponds to a different communication protocol. In a typical network environment, such as the Internet, individual headers within header portion 204 are attached (e.g., prepended) as the packet is processed through different layers of a protocol stack (e.g., a set of protocols for communicating between entities) on the transmitting computer system. For example, FIG. 2 depicts protocol headers 210, 212, 214 and 216, corresponding to layers one through four, respectively, of a suitable protocol stack. Each protocol header contains information to be used by the receiving computer system as the packet is received and processed through the protocol stack. Ultimately, each protocol header is removed and data portion 202 is retrieved. In one embodiment of the invention a system and method are provided for parsing packet 200 to retrieve various bits of information. In this embodiment, packet 200 is parsed in order to identify the beginning of data portion 202 and to retrieve one or more values for fields within header portion 204. Illustratively, however, layer one protocol header or preamble 210 corresponds to a hardware-level specification related to the coding of individual bits. Layer one protocols are generally only needed for the physical process of sending or receiving the packet across a conductor. Thus, in this embodiment of the invention layer one preamble 210 is stripped from packet 200 shortly after being received by NIC 100 and is therefore not parsed. The extent to which header portion 204 is parsed may depend upon how many, if any, of the protocols represented in the header portion match a set of pre-selected protocols. For example, the parsing procedure may be abbreviated or aborted once it is determined that one of the packet's headers corresponds to an unsupported protocol. In particular, in one embodiment of the invention NIC 100 is configured primarily for Internet traffic. Thus, in this embodiment packet 200 is extensively parsed only when the layer two protocol is Ethernet (either traditional Ethernet or 802.3 Ethernet, with or without tagging for Virtual Local Area Networks), the layer three protocol is IP (Internet Protocol) and the layer four protocol is TCP (Transport Control Protocol). Packets adhering to other protocols may be parsed to some (e.g., lesser) extent. NIC 100 may, however, be configured to support and parse virtually any communication protocol's header. Illustratively, the protocol headers that are parsed, and the extent to which they are parsed, are determined by the configuration of a set of instructions for operating header parser 106. As described above, the protocols corresponding to headers 212, 214 and 216 depend upon the network environment in which a packet is sent. The protocols also depend upon the communicating entities. For example, a packet received by a network interface may be a control packet exchanged between the medium access controllers for the source and destination computer systems. In this case, the packet would be likely to include minimal or no data, and may not include layer three protocol header 214 or layer four protocol header 216. Control packets are typically used for various purposes related to the management of individual connections. Another communication flow or connection could involve two application programs. In this case, a packet may include headers 212, 214 and 216, as shown in FIG. 2, and may also include additional headers related to higher layers of a protocol stack (e.g., session, presentation and application layers in the ISO-OSI model). In addition, some applications may include headers or header-like information within data portion 202. For example, for a Network File System (NFS) application, data portion 202 may include NFS headers related to individual NFS datagrams. A datagram may be defined as a collection of data sent from one entity to another, and may comprise data transmitted in multiple packets. In other words, the amount of data constituting a datagram may be greater than the amount of data that can be included in one packet. One Embodiment of a Header Parser FIG. 3 depicts header parser 106 of FIG. 1A in accordance with a present embodiment of the invention. Illustratively, header parser 106 comprises header memory 302 and parser 304, and parser 304 comprises instruction memory 306. Although depicted as distinct modules in FIG. 3, in an alternative embodiment of the invention header memory 302 and instruction memory 306 are contiguous. Advantageously, methods for parsing a packet that are described herein are readily adaptable for packets formatted in accordance with virtually any communication protocol. In the illustrated embodiment, parser 304 parses a header stored in header memory 302 according to instructions stored in instruction memory 306. The instructions are designed for the parsing of particular protocols or a particular protocol stack, as discussed above. In one embodiment of the invention, instruction memory 306 is modifiable (e.g., the memory is implemented as RAM, EPROM, EEPROM or the like), so that new or modified parsing instructions may be downloaded or otherwise installed. Instructions for parsing a packet are further discussed in the following section. In FIG. 3, a header portion of a packet stored in IPP module 104 (shown in FIG. 1 A) is copied into header memory 302. Illustratively, a specific number of bytes (e.g., 114) at the beginning of the packet are copied. In an alternative embodiment of the invention, the portion of a packet that is copied may be of a different size. The particular amount of a packet copied into header memory 302 should be enough to capture one or more protocol headers, or at least enough information (e.g., whether included in a header or data portion of the packet) to retrieve the information described below. The header portion stored in header memory 302 may not include the layer one header, which may be removed prior to or in conjunction with the packet being processed by IPP module 104. After a header portion of the packet is stored in header memory 302, parser 304 parses the header portion according to the instructions stored in instruction memory 306. Instructions for operating parser 304 in the presently described embodiment apply the formats of selected protocols to step through the contents of header memory 302 and retrieve specific information. In particular, specifications of communication protocols are well known and widely available. Thus, a protocol header may be traversed byte by byte or some other fashion by referring to the protocol specifications. Thus, in a present embodiment of the invention the parsing algorithm is dynamic, with information retrieved from one field of a header often altering the manner in which another part is parsed. For example, it is known that the Type field of a packet adhering to the traditional, form of Ethernet (e.g., version two) begins at the thirteenth byte of the (layer two) header. By comparison, the Type field of a packet following the IEEE 802.3 version of Ethernet begins at the twenty-first byte of the header. The Type field is in yet other locations if the packet forms part of a Virtual Local Area Network (VLAN) communication (which illustratively involves tagging or encapsulating an Ethernet header). Thus, in a present embodiment of the invention, the values in certain fields are retrieved and tested in order to ensure that the information needed from a header is drawn from the correct portion of the header. Details concerning the form of a VLAN packet may be found in specifications for the IEEE 802.3p and IEEE 802.3q forms of the Ethernet protocol. The operation of header parser 106 also depends upon other differences between protocols, such as whether the packet uses version four or version six of the Internet Protocol, etc. Specifications for versions four and six of IP may be located in IETF (Internet Engineering Task Force) RFCs (Request for Comment) 791 and 2460, respectively. The more protocols that are "known" by parser 304, the more protocols a packet may be tested for, and the more complicated the parsing of a packet's header portion may become. The protocols that may be parsed by parser 304 are limited only by the instructions according to which it operates. Thus, by augmenting or replacing the parsing instructions stored in instruction memory 306, virtually all known protocols may be handled by header parser 106 and virtually any information may be retrieved from a packet's headers. If, of course, a packet header does not conform to an expected or suspected protocol, the parsing operation may be terminated. In this case, the packet may not be suitable for one more of the efficiency enhancements offered by NIC 100 (e.g., data reassembly, packet batching, load distribution). Illustratively, the information retrieved from a packet's headers is used by other portions of NIC 100 when processing that packet. For example, as a result of the packet parsing performed by parser 304 a flow key is generated to identify the communication flow or communication connection that comprises the packet. Illustratively, the flow key is assembled by concatenating one or more addresses corresponding to one or more of the communicating entities. In a present embodiment, a flow key is formed from a combination of the source and destination addresses drawn from the IP header and the source and destination ports taken from the TCP header. Other indicia of the communicating entities may be used, such as the Ethernet source and destination addresses (drawn from the layer two header), NFS file handles or source and destination identifiers for other application datagrams drawn from the data portion of the packet. The communicating entities may be identified with greater resolution by using indicia drawn from the higher layers of the protocol stack associated with a packet. Thus, a combination of IP and TCP indicia may identify the entities with greater particularity than layer two information. Besides a flow key, parser 304 also generates a control or status indicator to summarize additional information concerning the packet. In one embodiment of the invention a control indicator includes a sequence number (e.g., TCP sequence number drawn from a TCP header) to ensure the correct ordering of packets when re-assembling their data. The control indicator may also reveal whether certain flags in the packet's headers are set or cleared, whether the packet contains any data, and, if the packet contains data, whether the data exceeds a certain size. Other data are also suitable for inclusion in the control indicator, limited only by the information that is available in the portion of the packet parsed by parser 304. In one embodiment of the invention, header parser 106 provides the flow key and all or a portion of the control indicator to flow database manager 108. As discussed in a following section, FDBM 108 manages a database or other data structure containing information relevant to communication flows passing through NIC 100. In other embodiments of the invention, parser 304 produces additional information derived from the header of a packet for use by other modules of NIC 100. For example, header parser 106 may report the offset, from the beginning of the packet or from some other point, of the data or payload portion of a packet received from a network. As described above, the data portion of a packet typically follows the header portion and may be followed by a trailer portion. Other data that header parser 106 may report include the location in the packet at which a checksum operation should begin, the location in the packet at which the layer three and/or layer four headers begin, diagnostic data, payload information, etc. The term "payload" is often used to refer to the data portion of a packet. In particular, in one embodiment of the invention header parser 106 provides a payload offset and payload size to control queue 118. In appropriate circumstances, header parser 106 may also report (e.g., to IPP module 104 and/or control queue 118) that the packet is not formatted in accordance with the protocols that parser 304 is configured to manipulate. This report may take the form of a signal (e.g., the No_Assist signal described below), alert, flag or other indicator. The signal may be raised or issued whenever the packet is found to reflect a protocol other than the pre-selected protocols that are compatible with the processing enhancements described above (e.g., data re-assembly, batch processing of packet headers, load distribution). For example, in one embodiment of the invention parser 304 may be configured to parse and efficiently process packets using TCP at layer four, IP at layer three and Ethernet at layer two. In this embodiment, an IPX (Internetwork Packet Exchange) packet would not be considered compatible and IPX packets therefore would not be gathered for data reassembly and batch processing. At the conclusion of parsing in one embodiment of the invention, the various pieces of information described above are disseminated to appropriate modules of NIC 100. After this (and as described in a following section), flow database manager 108 determines whether an active flow is associated with the flow key derived from the packet and sets an operation code to be used in subsequent processing. In addition, IPP module 104 transmits the packet to packet queue 116. IPP module 104 may also receive some of the information extracted by header parser 106, and pass it to another module of NIC 100. In the embodiment of the invention depicted in FIG. 3, an entire header portion of a received packet to be parsed is copied and then parsed in one evolution, after which the header parser turns its attention to another packet. However, in an alternative embodiment multiple copy and/or parsing operations may be performed on a single packet. In particular, an initial header portion of the packet may be copied into and parsed by header parser 106 in a first evolution, after which another header portion may be copied into header parser 106 and parsed in a second evolution. A header portion in one evolution may partially or completely overlap the header portion of another evolution. In this manner, extensive headers may be parsed even if header memory 302 is of limited size. Similarly, it may require more than one operation to load a full set of instructions for parsing a packet into instruction memory 306. Illustratively, a first portion of the instructions may be loaded and executed, after which other instructions are loaded. With reference now to FIGs. 4A-4B, a flow chart is presented to illustrate one method by which a header parser may parse a header portion of a packet received at a network interface circuit from a network. In this implementation, the header parser is configured, or optimized, for parsing packets conforming to a set of pre-selected protocols (or protocol stacks). For packets meeting these criteria, various information is retrieved from the header portion to assist in the re-assembly of the data portions of related packets (e.g., packets comprising data from a single datagram). Other enhanced features of the network interface circuit may also be enabled. The information generated by the header parser includes, in particular, a flow key with which to identify the communication flow or communication connection that comprises the received packet. In one embodiment of the invention, data from packets having the same flow key may be identified and re-assembled to form a datagram. In addition, headers of packets having the same flow key may be processed collectively through their protocol stack (e.g., rather than serially). In another embodiment of the invention, information retrieved by the header parser is also used to distribute the processing of network traffic received from a network. For example, multiple packets having the same flow key may be submitted to a single processor of a multi-processor host computer system. In the method illustrated in FIGs. 4A-4B, the set of pre-selected protocols corresponds to communication protocols frequently transmitted via the Internet. In particular, the set of protocols that may be extensively parsed in this method include the following. At layer two: Ethernet (traditional version), 802.3 Ethernet, Ethernet VLAN (Virtual Local Area Network) and 802.3 Ethernet VLAN. At layer three: IPv4 (with no options) and IPv6 (with no options). Finally, at layer four, only TCP protocol headers (with or without options) are parsed in the illustrated method. Header parsers in alternative embodiments of the invention parse packets formatted through other protocol stacks. In particular, a NIC may be configured in accordance with the most common protocol stacks in use on a given network, which may or may not include the protocols compatible with the header parser method illustrated in FIGs. 4A-4B. As described below, a received packet that does not correspond to the protocols parsed by a given method may be flagged and the parsing algorithm terminated for that packet. Because the protocols under which a packet has been formatted can only be determined, in the present method, by examining certain header field values, the determination that a packet does not conform to the selected set of protocols may be made at virtually any time during the procedure. Thus, the illustrated parsing method has as one goal the identification of packets not meeting the formatting criteria for re-assembly of data. Various protocol header fields appearing in headers for the selected protocols are discussed below. Communication protocols that may be compatible with an embodiment of the present invention (e.g., protocols that may be parsed by a header parser) are described with great particularity in a number of references. They therefore need not be visited in minute detail herein. In addition, the illustrated method of parsing a header portion of a packet for the selected protocols is merely one method of gathering the information described below. Other parsing procedures capable of doing so are equally suitable. In a present embodiment of the invention, the illustrated procedure is implemented as a combination of hardware and software. For example, updateable micro-code instructions for performing the procedure may be executed by a microsequencer. Alternatively, such instructions may be fixed (e.g., stored in read-only memory) or may be executed by a processor or microprocessor. In FIGs. 4A-4B, state 400 is a start state during which a packet is received by NIC 100 (shown in FIG. 1 A) and initial processing is performed. NIC 100 is coupled to the Internet for purposes of this procedure. Initial processing may include basic error checking and the removal of the layer one preamble. After initial processing, the packet is held by IPP module 104 (also shown in FIG. 1 A). In one embodiment of the invention, state 400 comprises a logical loop in which the header parser remains in an idle or wait state until a packet is received. In state 402, a header portion of the packet is copied into memory (e.g., header memory 302 of FIG. 3). In a present embodiment of the invention a predetermined number of bytes at the beginning (e.g., 114 bytes) of the packet are copied. Packet portions of different sizes are copied in alternative embodiments of the invention, the sizes of which are guided by the goal of copying enough of the packet to capture and/or identify the necessary header information. Illustratively, the full packet is retained by IPP module 104 while the following parsing operations are performed, although the packet may, alternatively, be stored in packet queue 116 prior to the completion of parsing. Also in state 402, a pointer to be used in parsing the packet may be initialized. Because the layer one preamble was removed, the header portion copied to memory should begin with the layer two protocol header. Illustratively, therefore, the pointer is initially set to point to the twelfth byte of the layer two protocol header and the two-byte value at the pointer position is read. These two bytes may be part of a number of different fields, depending upon which protocol constitutes layer two of the packet's protocol stack. For example, these two bytes may comprise the Type field of a traditional Ethernet header, the Length field of an 802.3 Ethernet header or the TPID (Tag Protocol IDentifier) field of a VLAN-tagged header. In state 404, a first examination is made of the layer two header to determine if it comprises a VLAN-tagged layer two protocol header. Illustratively, this determination depends upon whether the two bytes at the pointer position store the hexadecimal value 8100. If so, the pointer is probably located at the TPID field of a VLAN-tagged header. If not a VLAN header, the procedure proceeds to state 408. If, however, the layer two header is a VLAN-tagged header, in state 406 the CFI (Canonical Format Indicator) bit is examined. If the CFI bit is set (e.g., equal to one), the illustrated procedure jumps to state 430, after which it exits. In this embodiment of the invention the CFI bit, when set, indicates that the format of the packet is not compatible with (i.e., does not comply with) the pre-selected protocols (e.g., the layer two protocol is not Ethernet or 802.3 Ethernet). If the CFI bit is clear (e.g., equal to zero), the pointer is incremented (e.g., by four bytes) to position it at the next field that must be examined. In state 408, the layer two header is further tested. Although it is now known whether this is or is not a VLAN-tagged header, depending upon whether state 408 was reached through state 406 or directly from state 404, respectively, the header may reflect either the traditional Ethernet format or the 802.3 Ethernet format. At the beginning of state 408, the pointer is either at the twelfth or sixteenth byte of the header, either of which may correspond to a Length field or a Type field. In particular, if the two-byte value at the position identified by the pointer is less than 0600 (hexadecimal), then the packet corresponds to 802.3 Ethernet and the pointer is understood to identify a Length field. Otherwise, the packet is a traditional (e.g., version two) Ethernet packet and the pointer identifies a Type field. If the layer two protocol is 802.3 Ethernet, the procedure continues at state 410. If the layer two protocol is traditional Ethernet, the Type field is tested for the hexadecimal values of 0800 and 08DD. If the tested field has one of these values, then it has also been determined that the packet's layer three protocol is the Internet Protocol. In this case the illustrated procedure continues at state 412. Lastly, if the field is a Type field having a value other than 0800 or 86DD (hexadecimal), then the packet's layer three protocol does not match the pre-selected protocols according to which the header parser was configured. Therefore, the procedure continues at state 430 and then ends. In one embodiment of the invention the packet is examined in state 408 to determine if it is a jumbo Ethernet frame. This determination would likely be made prior to deciding whether the layer two header conforms to Ethernet or 802.3 Ethernet. Illustratively, the jumbo frame determination may be made based on the size of the packet, which may be reported by IPP module 104 or a MAC module. If the packet is a jumbo frame, the procedure may continue at state 410; otherwise, it may resume at state 412. In state 410, the procedure verifies that the layer two protocol is 802.3 Ethernet with LLC SNAP encapsulation. In particular, the pointer is advanced (e.g., by two bytes) and the six-byte value following the Length field in the layer two header is retrieved and examined. If the header is an 802.3 Ethernet header, the field is the LLC_SNAP field and should have a value of AAAA03000000 (hexadecimal). The original specification for an LLC SNAP header may be found in the specification for IEEE 802.2. If the value in the packet's LLC_SNAP field matches the expected value the pointer is incremented another six bytes, the two-byte 802.3 Ethernet Type field is read and the procedure continues at state 412. If the values do not match, then the packet does not conform to the specified protocols and the procedure enters state 430 and then ends. In state 412, the pointer is advanced (e.g., another two bytes) to locate the beginning of the layer three protocol header. This pointer position may be saved for later use in quickly identifying the beginning of this header. The packet is now known to conform to an accepted layer two protocol (e.g., traditional Ethernet, Ethernet with VLAN tagging, or 802.3 Ethernet with LLC SNAP) and is now checked to ensure that the packet's layer three protocol is IP. As discussed above, in the illustrated embodiment only packets conforming to the IP protocol are extensively processed by the header parser. Illustratively, if the value of the Type field in the layer two header (retrieved in state 402 or state 410) is 0800 (hexadecimal), the layer three protocol is expected to be IP, version four. If the value is 86DD (hexadecimal), the layer three protocol is expected to be IP, version six. Thus, the Type field is tested in state 412 and the procedure continues at state 414 or state 418, depending upon whether the hexadecimal value is 0800 or 86DD, respectively. In state 414, the layer three header's conformity with version four of IP is verified. In one embodiment of the invention the Version field of the layer three header is tested to ensure that it contains the hexadecimal value 4, corresponding to version four of IP. If in state 414 the layer three header is confirmed to be IP version four, the procedure continues at state 416; otherwise, the procedure proceeds to state 430 and then ends at state 432. In state 416, various pieces of information from the IP header are saved. This information may include the IHL (IP Header Length), Total Length, Protocol and/or Fragment Offset fields. The IP source address and the IP destination addresses may also be stored. The source and destination address values are each four bytes long in version four of IP. These addresses are used, as described above, to generate a flow key that identifies the communication flow in which this packet was sent. The Total Length field stores the size of the IP segment of this packet, which illustratively comprises the IP header, the TCP header and the packet's data portion. The TCP segment size of the packet (e.g., the size of the TCP header plus the size of the data portion of the packet) may be calculated by subtracting twenty bytes (the size of the IP version four header) from the Total Length value. After state 416, the illustrated procedure advances to state 422. In state 418, the layer three header's conformity with version six of IP is verified by testing the Version field for the hexadecimal value 6. If the Version field does not contain this value, the illustrated procedure proceeds to state 430. In state 420, the values of the Payload Length (e.g., the size of the TCP segment) and Next Header field are saved, plus the IP source and destination addresses. Source and destination addresses are each sixteen bytes long in version six of IP. In state 422 of the illustrated procedure, it is determined whether the IP header (either version four or version six) indicates that the layer four header is TCP. Illustratively, the Protocol field of a version four IP header is tested while the Next Header field of a version six header is tested. In either case, the value should be 6 (hexadecimal). The pointer is then incremented as necessary (e.g., twenty bytes for IP version four, forty bytes for IP version six) to reach the beginning of the TCP header. If it is determined in state 422 that the layer four header is not TCP, the procedure advances to state 430 and ends at end state 432. In one embodiment of the invention, other fields of a version four IP header may be tested in state 422 to ensure that the packet meets the criteria for enhanced processing by NIC 100. For example, an IHL field value other than 5 (hexadecimal) indicates that IP options are set for this packet, in which case the parsing operation is aborted. A fragmentation field value other than zero indicates that the IP segment of the packet is a fragment, in which case parsing is also aborted. In either case, the procedure jumps to state 430 and then ends at end state 432. In state 424, the packet's TCP header is parsed and various data are collected from it. In particular, the TCP source port and destination port values are saved. The TCP sequence number, which is used to ensure the correct re-assembly of data from multiple packets, is also saved. Further, the values of several components of the Flags field - illustratively, the URG (urgent), PSH (push), RST (reset), SYN (synch) and FIN (finish) bits - are saved. In one embodiment of the invention these flags signal various actions to be performed or statuses to be considered in the handling of the packet. Other signals or statuses may be generated in state 424 to reflect information retrieved from the TCP header. For example, the point from which a checksum operation is to begin may be saved (illustratively, the beginning of the TCP header); the ending point of a checksum operation may also be saved (illustratively, the end of the data portion of the packet). An offset to the data portion of the packet may be identified by multiplying the value of the Header Length field of the TCP header by four. The size of the data portion may then be calculated by subtracting the offset to the data portion from the size of the entire TCP segment. In state 426, a flow key is assembled by concatenating the IP source and destination addresses and the TCP source and destination ports. As already described, the flow key may be used to identify a communication flow or communication connection, and may be used by other modules of NIC 100 to process network traffic more efficiently. Although the sizes of the source and destination addresses differ between IP versions four and six (e.g., four bytes each versus sixteen bytes each, respectively), in the presently described embodiment of the invention all flow keys are of uniform size. In particular, in this embodiment they are thirty-six bytes long, including the two-byte TCP source port and two-byte TCP destination port. Flow keys generated from IP, version four, packet headers are padded as necessary (e.g., with twenty-four clear bytes) to fill the flow key's allocated space. In state 428, a control or status indicator is assembled to provide various information to one or more modules of NIC 100. In one embodiment of the invention a control indicator includes the packet's TCP sequence number, a flag or identifier (e.g., one or more bits) indicating whether the packet contains data (e.g., whether the TCP payload size is greater than zero), a flag indicating whether the data portion of the packet exceeds a pre-determined size, and a flag indicating whether certain entries in the TCP Flags field are equivalent to pre-determined values. The latter flag may, for example, be used to inform another module of NIC 100 that components of the Flags field do or do not have a particular configuration. After state 428, the illustrated procedure ends with state 432. State 430 may be entered at several different points of the illustrated procedure. This state is entered, for example, when it is determined that a header portion that is being parsed by a header parser does not conform to the pre-selected protocol stacks identified above. As a result, much of the information described above is not retrieved. A practical consequence of the inability to retrieve this information is that it then cannot be provided to other modules of NIC 100 and the enhanced processing described herein may not be performed for this packet. In particular, and as discussed previously, in a present embodiment of the invention one or more enhanced operations may be performed on parsed packets to increase the efficiency with which they are processed. Illustrative operations that may be applied include the re-assembly of data from related packets (e.g., packets containing data from a single datagram), batch processing of packet headers through a protocol stack, load distribution or load sharing of protocol stack processing, efficient transfer of packet data to a destination entity, etc. In the illustrated procedure, in state 430 a flag or signal (illustratively termed No_Assist) is set or cleared to indicate that the packet presently held by IPP module 104 (e.g., which was just processed by the header parser) does not conform to any of the preselected protocol stacks. This flag or signal may be relied upon by another module of NIC 100 when deciding whether to perform one of the enhanced operations. Another flag or signal may be set or cleared in state 430 to initialize a checksum parameter indicating that a checksum operation, if performed, should start at the beginning of the packet (e.g., with no offset into the packet). Illustratively, incompatible packets cannot be parsed to determine a more appropriate point from which to begin the checksum operation. After state 430, the procedure ends with end state 432. After parsing a packet, the header parser may distribute information generated from the packet to one or more modules of NIC 100. For example, in one embodiment of the invention the flow key is provided to flow database manager 108, load distributor 112 and one or both of control queue 118 and packet queue 116. Illustratively, the control indicator is provided to flow database manager 108. This and other control information, such as TCP payload size, TCP payload offset and the No_Assist signal may be returned to IPP module 104 and provided to control queue 118. Yet additional control and/or diagnostic information, such as offsets to the layer three and/or layer four headers, may be provided to IPP module 104, packet queue 116 and/or control queue 118. Checksum information (e.g., a starting point and either an ending point or other means of identifying a portion of the packet from which to compute a checksum) may be provided to checksum generator 114. After a received packet is parsed on NIC 100 (e.g., by header parser 106), the packets are still processed (e.g., through their respective protocol stacks) on the host computer system in the illustrated embodiment of the invention. However, after parsing a packet in an alternative embodiment of the invention, NIC 100 also performs one or more subsequent processing steps. For example, NIC 100 may include one or more protocol processors for processing one or more of the packet's protocol headers. Dynamic Header Parsing Instructions in One Embodiment of the Invention In one embodiment of the present invention, header parser 106 parses a packet received from a network according to a dynamic sequence of instructions. The instructions may be stored in the header parser's instruction memory (e.g., RAM, SRAM, DRAM, flash) that is re-programmable or that can otherwise be updated with new or additional instructions. In one embodiment of the invention software operating on a host computer (e.g., a device driver) may download a set of parsing instructions for storage in the header parser memory. The number and format of instructions stored in a header parser's instruction memory may be tailored to one or more specific protocols or protocol stacks. An instruction set configured for one collection of protocols, or a program constructed from that instruction set, may therefore be updated or replaced by a different instruction set or program. For packets received at the network interface that are formatted in accordance with the selected protocols (e.g., "compatible" packets), as determined by analyzing or parsing the packets, various enhancements in the handling of network traffic become possible as described herein. In particular, packets from one datagram that are configured according to a selected protocol may be re-assembled for efficient transfer in a host computer. In addition, header portions of such packets may be processed collectively rather than serially. And, the processing of packets from different datagrams by a multi-processor host computer may be shared or distributed among the processors. Therefore, one objective of a dynamic header parsing operation is to identify a protocol according to which a received packet has been formatted or determine whether a packet header conforms to a particular protocol. FIG. 12, discussed in detail shortly, presents an illustrative series of instructions for parsing the layer two, three and four headers of a packet to determine if they are Ethernet, IP and TCP, respectively. The illustrated instructions comprise one possible program or microcode for performing a parsing operation. After a particular set of parsing instructions is loaded into a parser memory, a number of different programs may be assembled. FIG. 12 thus presents merely one of a number of programs that may be generated from the stored instructions. The instructions presented in FIG. 12 may be performed or executed by a microsequencer, a processor, a microprocessor or other similar module located within a network interface circuit. In particular, other instruction sets and other programs may be derived for different communication protocols, and may be expanded to other layers of a protocol stack. For example, a set of instructions could be generated for parsing NFS (Network File System) packets. Illustratively, these instructions would be configured to parse layer five and six headers to determine if they are Remote Procedure Call (RPC) and External Data Representation (XDR), respectively. Other instructions could be configured to parse a portion of the packet's data (which may be considered layer seven). An NFS header may be considered a part of a packet's layer six protocol header or part of the packet's data. One type of instruction executed by a microsequencer may be designed to locate a particular field of a packet (e.g., at a specific offset within the packet) and compare the value stored at that offset to a value associated with that field in a particular communication protocol. For example, one instruction may require the microsequencer to examine a value in a packet header at an offset that would correspond to a Type field of an Ethernet header. By comparing the value actually stored in the packet with the value expected for the protocol, the microsequencer can determine if the packet appears to conform to the Ethernet protocol. Illustratively, the next instruction applied in the parsing program depends upon whether the previous comparison was successful. Thus, the particular instructions applied by the microsequencer, and the sequence in which applied, depend upon which protocols are represented by the packet's headers. The microsequencer may test one or more field values within each header included in a packet. The more fields that are tested and that are found to comport with the format of a known protocol, the greater the certainty that the packet conforms to that protocol. One communication protocol may be quite different than another protocol, thus requiring examination of different parts of packet headers for different protocols. Illustratively, the parsing of one packet may end in the event of an error or because it was determined that the packet being parsed does or does not conform to the protocol(s) the instructions are designed for. Each instruction in FIG. 12 may be identified by a number and/or a name. A particular instruction may perform a variety of tasks other than comparing a header field to an expected value. An instruction may, for example, call another instruction to examine another portion of a packet header, initialized, load or configure a register or other data structure, prepare for the arrival and parsing of another packet, etc. In particular, a register or other storage structure may be configured in anticipation of an operation that is performed in the network interface after the packet is parsed. For example, a program instruction in FIG. 12 may identify an output operation that may or may not be performed, depending upon the success or failure of the comparison of a value extracted from a packet with an expected value. An output operation may store a value in a register, configure a register (e.g., load an argument or operator) for a post-parsing operation, clear a register to await a new packet, etc. A pointer may be employed to identify an offset into a packet being parsed. In one embodiment, such a pointer is initially located at the beginning of the layer two protocol header. In another embodiment, however, the pointer is situated at a specific location within a particular header (e.g., immediately following the layer two destination and/or source addresses) when parsing commences. Illustratively, the pointer is incremented through the packet as the parsing procedure executes. In one alternative embodiment, however, offsets to areas of interest in the packet may be computed from one or more known or computed locations. In the parsing program depicted in FIG. 12, a header is navigated (e.g., the pointer is advanced) in increments of two bytes (e.g., sixteen-bit words). In addition, where a particular field of a header is compared to a known or expected value, up to two bytes are extracted at a time from the field. Further, when a value or header field is copied for storage in a register or other data structure, the amount of data that may be copied in one operation may be expressed in multiples of two-byte units or in other units altogether (e.g., individual bytes). This unit of measurement (e.g., two bytes) may be increased or decreased in an alternative embodiment of the invention. Altering the unit of measurement may alter the precision with which a header can be parsed or a header value can be extracted. In the embodiment of the invention illustrated in FIG. 12, a set of instructions loaded into the header parser's instruction memory comprises a number of possible operations to be performed while testing a packet for compatibility with selected protocols. Program 1200 is generated from the instruction set. Program 1200 is thus merely one possible program, microcode or sequence of instructions that can be formed from the available instruction set. In this embodiment, the loaded instruction set enables the following sixteen operations that may be performed on a packet that is being parsed. Specific implementations of these operations in program 1200 are discussed in additional detail below. These instructions will be understood to be illustrative in nature and do not limit the composition of instruction sets in other embodiments of the invention. In addition, any subset of these operations may be employed in a particular parsing program or microcode. Further, multiple instructions may employ the same operation and have different effects. A CLR_REG operation allows the selective initialization of registers or other data structures used in program 1200 and, possibly, data structures used in functions performed after a packet is parsed. Initialization may comprise storing the value zero. A number of illustrative registers that may be initialized by a CLR_REG operation are identified in the remaining operations. A LD FID operation copies a variable amount of data from a particular offset within the packet into a register configured to store a packet's flow key or other flow identifier. This register may be termed a FLOWID register. The effect of an LD_FID operation is cumulative. In other words, each time it is invoked for one packet the generated data is appended to the flow key data stored previously. A LD_SEQ operation copies a variable amount of data from a particular offset within the packet into a register configured to store a packet's sequence number (e.g., a TCP sequence number). This register may be assigned the label SEQNO. This operation is also cumulative - the second and subsequent invocations of this operation for the packet cause the identified data to be appended to data stored previously. A LD_CTL operation loads a value from a specified offset in the packet into a CONTROL register. The CONTROL register may comprise a control indicator for identifying whether a packet is suitable for data re-assembly, packet batching, load distribution or other enhanced functions of NIC 100. In particular, a control indicator may indicate whether a No_Assist flag should be raised for the packet, whether the packet includes any data, whether the amount of packet data is larger than a predetermined threshold, etc. Thus, the value loaded into a CONTROL register in a LD_CTL operation may affect the post-parsing handling of the packet. A LD_SAP operation loads a value into the CONTROL register from a variable offset within the packet. The loaded value may comprise the packet's ethertype. In one option that may be associated with a LD_SAP operation, the offset of the packet's layer three header may also be stored in the CONTROL register or elsewhere. A packet's layer three header may immediately follow its layer two ethertype field if the packet conforms to the Ethernet and IP protocols. A LD_R1 operation may be used to load a value into a temporary register (e.g., named RI) from a variable offset within the packet. A temporary register may be used for a variety of tasks, such as accumulating values to determine the length of a header or other portion of the packet. A LD Rl operation may also cause a value from another variable offset to be stored in a second temporary register (e.g., named R2). The values stored in the RI and/or R2 registers during the parsing of a packet may or may not be cumulative. A LD L3 operation may load a value from the packet into a register configured to store the location of the packet's layer three header. This register may be named L3OFFSET. In one optional method of invoking this operation, it may be used to load a fixed value into the L3OFFSET register. As another option, the LDJ 3 operation may add a value stored in a temporary register (e.g., RI) to the value being stored in the L3OFFSET register. A LD_SUM operation stores the starting point within the packet from which a checksum should be calculated. The register in which this value is stored may be named a CSUMSTART register. In one alternative invocation of this operation, a fixed or predetermined value is stored in the register. As another option, the LD_SUM operation may add a value stored in a temporary register (e.g., RI) to the value being stored in the CSUMSTART register. A LD HDR operation loads a value into a register configured to store the location within the packet at which the header portion may be split. The value that is stored may, for example, be used during the transfer of the packet to the host computer to store a data portion of the packet in a separate location than the header portion. The loaded value may thus identify the beginning of the packet data or the beginning of a particular header. In one invocation of a LD_HDR operation, the stored value may be computed from a present position of a parsing pointer described above. In another invocation, a fixed or predetermined value may be store. As yet another alternative, a value stored in a temporary register (e.g., RI) and/or a constant may be added to the loaded value. A LD LEN operation stores the length of the packet's payload into a register (e.g., a PAYLOADLEN register). An IM_FID operation appends or adds a fixed or predetermined value to the existing contents of the FLOWID register described above. An IM_SEQ operation appends or adds a fixed or predetermined value to the contents of the SEQNO register described above. An IM_S AP operation loads or stores a fixed or predetermined value in the CSUMSTART register described above. An IM R1 operation may add or load a predetermined value in one or more temporary registers (e.g., RI, R2). An IM_CTL operation loads or stores a fixed or predetermined value in the CONTROL register described above. A ST_FLAG operation loads a value from a specified offset in the packet into a FLAGS register. The loaded value may comprise one or more fields or flags from a packet header. The labels assigned to the operations and registers described above and elsewhere in this section are merely illustrative in nature and in no way limit the operations and parsing instructions that may be employed in other embodiments of the invention. Instructions in program 1200 comprise instruction number field 1202, which contains a number of an instruction within the program, and instruction name field 1204, which contains a name of an instruction. In an alternative embodiment of the invention instruction number and instruction name fields may be merged or one of them may be omitted. Instruction content field 1206 includes multiple portions for executing an instruction. An "extraction mask" portion of an instruction is a two-byte mask in hexadecimal notation. An extraction mask identifies a portion of a packet header to be copied or extracted, starting from the current packet offset (e.g., the current position of the parsing pointer). Illustratively, each bit in the packet's header that corresponds to a one in the hexadecimal value is copied for comparison to a comparison or test value. For example, a value of OxFFOO in the extraction mask portion of an instruction signifies that the entire first byte at the current packet offset is to be copied and that the contents of the second byte are irrelevant. Similarly, an extraction mask of 0x3 FFF signifies that all but the two most significant bits of the first byte are to be copied. A two-byte value is constructed from the extracted contents, using whatever was copied from the packet. Illustratively, the remainder of the value is padded with zeros. The format of an extraction mask (or an output mask, described below) may be adjusted as necessary to reflect little endian or big endian representation. One or more instructions in a parsing program may not require any data extracted from the packet at the pointer location to be able to perform its output operation. These instructions may have an extraction mask value of 0x0000 to indicate that although a two- byte value is still retrieved from the pointer position, every bit of the value is masked off. Such an extraction mask thus yields a definite value of zero. This type of instruction may be used when, for example, an output operation needs to be performed before another substantive portion of header data is extracted with an extraction mask other than 0x0000. A "compare value" portion of an instruction is a two-byte hexadecimal value with which the extracted packet contents are to be compared. The compare value may be a value known to be stored in a particular field of a specific protocol header. The compare value may comprise a value that the extracted portion of the header should match or have a specified relationship to in order for the packet to be considered compatible with the preselected protocols. An "operator" portion of an instruction identifies an operator signifying how the extracted and compare values are to be compared. Illustratively, EQ signifies that they are tested for equality, NE signifies that they are tested for inequality, LT signifies that the extracted value must be less than the compare value for the comparison to succeed, GE signifies that the extracted value must be greater than or equal to the compare value, etc. An instruction that awaits arrival of a new packet to be parsed may employ an operation of NP. Other operators for other functions may be added and the existing operators may be assigned other monikers. A "success offset" portion of an instruction indicates the number of two-byte units that the pointer is to advance if the comparison between the extracted and test values succeeds. A "success instruction" portion of an instruction identifies the next instruction in program 1200 to execute if the comparison is successful. Similarly, "failure offset" and "failure instruction" portions indicate the number of two-byte units to advance the pointer and the next instruction to execute, respectively, if the comparison fails. Although offsets are expressed in units of two bytes (e.g., sixteen-bit words) in this embodiment of the invention, in an alternative embodiment of the invention they may be smaller or larger units. Further, as mentioned above an instruction may be identified by number or name. Not all of the instructions in a program are necessarily used for each packet that is parsed. For example, a program may include instructions to test for more than one type or version of a protocol at a particular layer. In particular, program 1200 tests for either version four or six of the IP protocol at layer three. The instructions that are actually executed for a given packet will thus depend upon the format of the packet. Once a packet has been parsed as much as possible with a given program or it has been determined that the packet does or does not conform to a selected protocol, the parsing may cease or an instruction for halting the parsing procedure may be executed. Illustratively, a next instruction portion of an instruction (e.g., "success instruction" or "failure instruction") with the value "DONE" indicates the completion of parsing of a packet. A DONE, or similar, instruction may be a dummy instruction. In other words, "DONE" may simply signify that parsing to be terminated for the present packet. Or, like instruction eighteen of program 1200, a DONE instruction may take some action to await a new packet (e.g., by initializing a register). The remaining portions of instruction content field 1206 are used to specify and complete an output or other data storage operation. In particular, in this embodiment an "output operation" portion of an instruction corresponds to the operations included in the loaded instruction set. Thus, for program 1200, the output operation portion of an instruction identifies one of the sixteen operations described above. The output operations employed in program 1200 are further described below in conjunction with individual instructions. An "operation argument" portion of an instruction comprises one or more arguments or fields to be stored, loaded or otherwise used in conjunction with the instruction's output operation. Illustratively, the operation argument portion takes the form of a multi-bit hexadecimal value. For program 1200, operation arguments are eleven bits in size. An argument or portion of an argument may have various meanings, depending upon the output operation. For example, an operation argument may comprise one or more numerical values to be stored in a register or to be used to locate or delimit a portion of a header. Or, an argument bit may comprise a flag to signal an action or status. In particular, one argument bit may specify that a particular register is to be reset; a set of argument bits may comprise an offset into a packet header to a value to be stored in a register, etc. Illustratively, the offset specified by an operation argument is applied to the location of the parsing pointer position before the pointer is advanced as specified by the applicable success offset or failure offset. The operation arguments used in program 1200 are explained in further detail below. An "operation enabler" portion of an instruction content field specifies whether or when an instruction's output operation is to be performed. In particular, in the illustrated embodiment of the invention an instruction's output operation may or may not be performed, depending on the result of the comparison between a value extracted from a header and the compare value. For example, an output enabler may be set to a first value (e.g., zero) if the output operation is never to be performed. It may take different values if it is to be performed only when the comparison does or does not satisfy the operator (e.g., one or two, respectively). An operation enabler may take yet another value (e.g., three) if it is always to be performed. A "shift" portion of an instruction comprises a value indicating how an output value is to be shifted. A shift may be necessary because different protocols sometime require values to be formatted differently. In addition, a value indicating a length or location of a header or header field may require shifting in order to reflect the appropriate magnitude represented by the value. For example, because program 1200 is designed to use two-byte units, a value may need to be shifted if it is to reflect other units (e.g., bytes). A shift value in a present embodiment indicates the number of positions (e.g., bits) to right-shift an output value. In another embodiment of the invention a shift value may represent a different shift type or direction. Finally, an "output mask" specifies how a value being stored in a register or other data structure is to be formatted. As stated above, an output operation may require an extracted, computed or assembled value to be stored. Similar to the extraction mask, the output mask is a two-byte hexadecimal value. For every position in the output mask that contains a one, in this embodiment of the invention the corresponding bit in the two-byte value identified by the output operation and/or operation argument is to be stored. For example, a value of OxFFFF indicates that the specified two-byte value is to be stored as is. Illustratively, for every position in the output mask that contains a zero, a zero is stored. Thus, a value of OxFOOO indicates that the most significant four bits of the first byte are to be stored, but the rest of the stored value is irrelevant, and may be padded with zeros. An output operation of "NONE" may be used to indicate that there is no output operation to be performed or stored, in which case other instruction portions pertaining to output may be ignored or may comprise specified values (e.g., all zeros). In the program depicted in FIG. 12, however, a CLR REG output operation, which allows the selective reinitialization of registers, may be used with an operation argument of zero to effectively perform no output. In particular, an operation argument of zero for the CLR_REG operation indicates that no registers are to be reset. In an alternative embodiment of the invention the operation enabler portion of an instruction could be set to a value (e.g., zero) indicating that the output operation is never to be performed. The format and sequence of instructions in FIG. 12 will be understood to represent just one method of parsing a packet to determine whether it conforms to a particular communication protocol. In particular, the instructions are designed to examine one or more portions of one or more packet headers for comparison to known or expected values and to configure or load a register or other storage location as necessary. Instructions for parsing a packet may take any of a number of forms and be performed in a variety of sequences without exceeding the scope of the invention. With reference now to FIG. 12, instructions in program 1200 may be described in detail. Prior to execution of the program depicted in FIG. 12, a parsing pointer is situated at the beginning of a packet's layer two header. The position of the parsing pointer may be stored in a register for easy reference and update during the parsing procedure. In particular, the position of the parsing pointer as an offset (e.g., from the beginning of the layer two header) may be used in computing the position of a particular position within a header. Program 1200 begins with a WAIT instruction (e.g., instruction zero) that waits for a new packet (e.g., indicated by operator NP) and, when one is received, sets a parsing pointer to the twelfth byte of the layer two header. This offset to the twelfth byte is indicated by the success offset portion of the instruction. Until a packet is received, the WAIT instruction loops on itself. In addition, a CLR REG operation is conducted, but the operation enabler setting indicates that it is only conducted when the comparison succeeds (e.g., when a new packet is received). The specified CLR_REG operation operates according to the WAIT instruction's operation argument (i.e., 0x3FF). In this embodiment, each bit of the argument corresponds to a register or other data structure. The registers initialized in this operation may include the following: ADDR (e.g., to store the parsing pointer's address or location), FLOWID (e.g., to store the packet's flow key), SEQNO (e.g., to store a TCP sequence number), SAP (e.g., the packet's ethertype) and PAYLOADLEN (e.g., payload length). The following registers configured to store certain offsets may also be reset: FLOWOFF (e.g., offset within FLOWID register), SEQOFF (e.g., offset within SEQNO register), L3OFFSET (e.g., offset of the packet's layer three header), HDRSPLIT (e.g., location to split packet) and CSUMSTART (e.g., starting location for computing a checksum). Also, one or more status or control indicators (e.g., CONTROL or FLAGS register) for reporting the status of one or more flags of a packet header may be reset. In addition, one or more temporary registers (e.g., RI, R2) or other data structures may also be initialized. These registers are merely illustrative of the data structures that may be employed in one embodiment of the invention. Other data structures may be employed in other embodiments for the same or different output operations. Temporary registers such as RI and/or R2 may be used in program 1200 to track various headers and header fields. The number of possible combinations of communication protocols and the effect of those various combinations on the structure and format of a packet's headers. More information may need to be examined or gathered from a packet conforming to one protocol or set of protocols than from a packet conforming to another protocol or set of protocols. For example, if extension headers are used with an Internet Protocol header, values from those extension headers and/or their lengths may need to be stored, which values are not needed if extension headers are not used. When calculating a particular offset, such as an offset to the beginning of a packet's data portion for example, multiple registers may need to be maintained and their values combined or added. In this example, one register or temporary register may track the size or format of an extension header, while another register tracks the base IP header. Instruction VLAN (e.g., instruction one) examines the two-byte field at the parsing pointer position (possibly a Type, Length or TPID field) for a value indicating a VLAN- tagged header (e.g., 8100 in hexadecimal). If the header is VLAN-tagged, the pointer is incremented a couple of bytes (e.g., one two-byte unit) and execution continues with instruction CFI; otherwise, execution continues with instruction 802.3. In either event, the instruction's operation enabler indicates that an IM_CTL operation is always to be performed. As described above, an IM CTL operation causes a control register or other data structure to be populated with one or more flags to report the status or condition of a packet. A control indicator may indicate whether a packet is suitable for enhanced processing (e.g., whether a No_Assist signal should be generated for the packet), whether a packet includes any data and, if so, whether the size of the data portion exceeds a specified threshold. The operation argument OxOOA for instruction VLAN comprises the value to be stored in the control register, with individual bits of the argument corresponding to particular flags. Illustratively, flags associated with the conditions just described may be set to one, or true, in this IM_CTL operation. Instruction CFI (e.g., instruction two) examines the CFI bit or flag in a layer two header. If the CFI bit is set, then the packet is not suitable for the processing enhancements of a present embodiment of the invention, and the parsing procedure ends by calling instruction DONE (e.g., instruction eighteen). If the CFI bit is not set, then the pointer is incremented another couple of bytes and execution continues with instruction 802.3. As explained above, a null output operation (e.g., "NONE") indicates that no output operation is performed. In addition, the output enabler value (e.g., zero) further ensures that no output operation is performed. In instruction 802.3 (e.g., instruction three), a Type or Length field (depending on the location of the pointer and format of the packet) is examined to determine if the packet's layer two format is traditional Ethernet or 802.3 Ethernet. If the value in the header field appears to indicate 802.3 Ethernet (e.g., contains a hexadecimal value less than 0600), the pointer is incremented two bytes (to what should be an LLC SNAP field) and execution continues with instruction LLC 1. Otherwise, the layer two protocol may be considered traditional Ethernet and execution continues with instruction IPV4_1. Instruction 802.3 in this embodiment of the invention does not include an output operation. In instructions LLC 1 and LLC_2 (e.g., instructions four and five), a suspected layer two LLC SNAP field is examined to ensure that the packet conforms to the 802.3 Ethernet protocol. In instruction LLC 1, a first part of the field is tested and, if successful, the pointer is incremented two bytes and a second part is tested in instruction LLC_2. If instruction LLC_2 succeeds, the parsing pointer is advanced four bytes to reach what should be a Type field and execution continues with instruction IPV4_1. If either test fails, however, the parsing procedure exits. In the illustrated embodiment of the invention, no output operation is performed while testing the LLC SNAP field. In instruction IPV4_1 (e.g., instruction six), the parsing pointer should be at an Ethernet Type field. This field is examined to determine if the layer three protocol appears to correspond to version four of the Internet Protocol. If this test is successful (e.g., the Type field contains a hexadecimal value of 0800), the pointer is advanced two bytes to the beginning of the layer three header and execution of program 1200 continues with instruction IPV4_2. If the test is unsuccessful, then execution continues with instruction IPV6 1. Regardless of the test results, the operation enabler value (e.g., three) indicates that the specified LD_SAP output operation is always performed. As described previously, in a LD SAP operation a packet's ethertype (or Service Access Point) is stored in a register. Part of the operation argument of 0x100, in particular the right-most six bits (e.g., zero) constitute an offset to a two-byte value comprising the ethertype. The offset in this example is zero because, in the present context, the parsing pointer is already at the Type field that contains the ethertype. In the presently described embodiment, the remainder of the operation argument constitutes a flag specifying that the starting position of the layer three header (e.g., an offset from the beginning of the packet) is also to be saved (e.g., in the L3OFFSET register). In particular, the beginning of the layer three header is known to be located immediately after the two-byte Type field. Instruction IPV4 2 (e.g., instruction seven) tests a suspected layer three version field to ensure that the layer three protocol is version four of IP. In particular, a specification for version four of IP specifies that the first four bits of the layer three header contain a value of 0x4. If the test fails, the parsing procedure ends with instruction DONE. If the test succeeds, the pointer advances six bytes and instruction IPV4_3 is called. The specified LD_SUM operation, which is only performed if the comparison in instruction IPV4_2 succeeds, indicates that an offset to the beginning of a point from which a checksum may be calculated should be stored. In particular, in the presently described embodiment of the invention a checksum should be calculated from the beginning of the TCP header (assuming that the layer four header is TCP). The value of the operation argument (e.g., OxOOA) indicates that the checksum is located twenty bytes (e.g., ten two- byte increments) from the current pointer. Thus, a value of twenty bytes is added to the parsing pointer position and the result is stored in a register or other data structure (e.g., the CSUMSTART register). Instruction IPV4_3 (e.g., instruction eight) is designed to determine whether the packet's IP header indicates IP fragmentation. If the value extracted from the header in accordance with the extraction mask does not equal the comparison value, then the packet indicates fragmentation. If fragmentation is detected, the packet is considered unsuitable for the processing enhancements discussed herein and the procedure exits (e.g., through instruction DONE). Otherwise, the pointer is incremented two bytes and instruction IPV4_4 is called after performing a LD LEN operation. In accordance with the LD LEN operation, the length of the IP segment is saved. The illustrated operation argument (e.g., 0x03E) comprises an offset to the Total Length field where this value is located. In particular, the least-significant six bits constitute the offset. Because the pointer has already been advanced past this field, the operation argument comprises a negative value. This binary value (e.g., 111110) may be used to represent the decimal value of negative two. Thus, the present offset of the pointer, minus four bytes (e.g., two two-byte units), is saved in a register or other data structure (e.g., the PAYLOADLEN register). Any other suitable method of representing a negative offset may be used. Or, the IP segment length may be saved while the pointer is at a location preceding the Total Length field (e.g., during a previous instruction). In instruction IPV4_4 (e.g., instruction nine), a one-byte Protocol field is examined to determine whether the layer four protocol appears to be TCP. If so, the pointer is advanced fourteen bytes and execution continues with instruction TCP_1; otherwise the procedure ends. The specified LD_FID operation, which is only performed when the comparison in instruction IPV4_4 succeeds, involves retrieving the packet's flow key and storing it in a register or other location (e.g., the FLOWID register). In order for the comparison in instruction IPV4_4 to be successful, the packet's layer three and four headers must conform to IP (version four) and TCP, respectively. If so, then the entire flow key (e.g., IP source and destination addresses plus TCP source and destination port numbers) is stored contiguously in the packet's header portion. In particular, the flow key comprises the last portion of the IP header and the initial portion of the TCP header and may be extracted in one operation. The operation argument (e.g., 0x182) thus comprises two values needed to locate and delimit the flow key. Illustratively, the right-most six bits of the argument (e.g., 0x02) identify an offset from the pointer position, in two-byte units, to the beginning of the flow key. The other five bits of the argument (e.g., 0x06) identify the size of the flow key, in two-byte units, to be stored. In instruction IPV6_1 (e.g., instruction ten), which follows the failure of the comparison performed by instruction IPV4 1, the parsing pointer should be at a layer two Type field. If this test is successful (e.g., the Type field holds a hexadecimal value of 86DD), instruction IPV6_2 is executed after a LD_SUM operation is performed and the pointer is incremented two bytes to the beginning of the layer three protocol. If the test is unsuccessful, the procedure exits. The indicated LD_SUM operation in instruction IPV6_1 is similar to the operation conducted in instruction IPV4 2 but utilizes a different argument. Again, the checksum is to be calculated from the beginning of the TCP header (assuming the layer four header is TCP). The specified operation argument (e.g., 0x015) thus comprises an offset to the beginning of the TCP header - twenty-one two-byte steps ahead. The indicated offset is added to the present pointer position and saved in a register or other data structure (e.g., the CSUMSTART register). Instruction IPV6_2 (e.g., instruction eleven) tests a suspected layer three version field to further ensure that the layer three protocol is version six of IP. If the comparison fails, the parsing procedure ends with the invocation of instruction DONE. If it succeeds, instruction IPV6_3 is called. Operation IM R1, which is performed only when the comparison succeeds in this embodiment, saves the length of the IP header from a Payload Length field. The Total Length field (e.g., IP segment size) of an IP, version four, header includes the size of the version four header. However, the Payload Length field (e.g., IP segment size) of an IP, version six, header does not include the size of the version six header. Thus, the size of the version six header, which is identified by the right-most eight bits of the output argument (e.g., 0x14, indicating twenty two-byte units) is saved. Illustratively, the remainder of the argument identifies the data structure in which to store the header length (e.g., temporary register RI). Because of the variation in size of layer three headers between protocols, in one embodiment of the invention the header size is indicated in different units to allow greater precision. In particular, in one embodiment of the invention the size of the header is specified in bytes in instruction IPV6_2, in which case the output argument could be 0x128. Instruction IPV6_3 (e.g., instruction twelve) in this embodiment does not examine a header value. In this embodiment, the combination of an extraction mask of 0x0000 with a comparison value of 0x0000 indicates that an output operation is desired before the next examination of a portion of a header. After the LD FID operation is performed, the parsing pointer is advanced six bytes to a Next Header field of the version six IP header. Because the extraction mask and comparison values are both 0x0000, the comparison should never fail and the failure branch of instruction should never be invoked. As described previously, a LD_FID operation stores a flow key in an appropriate register or other data structure (e.g., the FLOWID register). Illustratively, the operation argument of 0x484 comprises two values for identifying and delimiting the flow key. In particular, the right-most six bits (e.g., 0x04) indicates that the flow key portion is located at an offset of eight bytes (e.g., four two-byte increments) from the current pointer position. The remainder of the operation argument (e.g., 0x12) indicates that thirty-six bytes (e.g., the decimal equivalent of 0x12 two-byte units) are to be copied from the computed offset. In the illustrated embodiment of the invention the entire flow key is copied intact, including the layer three source and destination addresses and layer four source and destination ports. In instruction IPV6_4 (e.g., instruction thirteen), a suspected Next Header field is examined to determine whether the layer four protocol of the packet's protocol stack appears to be TCP. If so, the procedure advances thirty-six bytes (e.g., eighteen two-byte units) and instruction TCP_1 is called; otherwise the procedure exits (e.g., through instruction DONE). Operation LD LEN is performed if the value in the Next Header field is 0x06. As described above, this operation stores the IP segment size. Once again the argument (e.g., 0x03F) comprises a negative offset, in this case negative one. This offset indicates that the desired Payload Length field is located two bytes before the pointer's present position. Thus, the negative offset is added to the present pointer offset and the result saved in an appropriate register or other data structure (e.g., the PAYLOADLEN register). In instructions TCP_1, TCP_2, TCP_3 and TCP_4 (e.g., instructions fourteen through seventeen), no header values - other than certain flags specified in the instruction's output operations - are examined, but various data from the packet's TCP header are saved. In the illustrated embodiment, the data that is saved includes a TCP sequence number, a TCP header length and one or more flags. For each instruction, the specified operation is performed and the next instruction is called. As described above, a comparison between the comparison value of 0x0000 and a null extraction value, as used in each of these instructions, will never fail. After instruction TCP_4, the parsing procedure returns to instruction WAIT to await a new packet. For operation LD_SEQ in instruction TCP_1, the operation argument (e.g., 0x081) comprises two values to identify and extract a TCP sequence number. The right-most six bits (e.g., 0x01) indicate that the sequence number is located two bytes from the pointer's current position. The rest of the argument (e.g., 0x2) indicates the number of two-byte units that must be copied from that position in order to capture the sequence number. Illustratively, the sequence number is stored in the SEQNO register. For operation ST_FLAG in instruction TCP 2, the operation argument (e.g., 0x145) is used to configure a register (e.g., the FLAGS register) with flags to be used in a post- parsing task. The right-most six bits (e.g., 0x05) constitute an offset, in two-byte units, to a two-byte portion of the TCP header that contains flags that may affect whether the packet is suitable for post-parsing processing enhancements of an embodiment of the present invention. For example, URG, PSH, RST, SYN and FIN flags may be located at the offset position and be used to configure the register. The output mask (e.g., 0x002F) indicates that only particular portions (e.g., bits) of the TCP header's Flags field are stored. Operation LD_R1 of instruction TCP 3 is similar to the operation conducted in instruction IPV6_2. Here, an operation argument of 0x205 includes a value (e.g., the least- significant six bits) identifying an offset of five two-byte units from the current pointer position. That location should include a Header Length field to be stored in a data structure identified by the remainder of the argument (e.g., temporary register RI). The output mask (e.g., OxFOOO) indicates that only the first four bits are saved (e.g., the Header Length field is only four bits in size). The value extracted from the Header Length field may need to be adjusted in order to reflect the use of two-byte units (e.g., sixteen bit words) in the illustrated embodiment. Therefore, in accordance with the shift portion of instruction TCP_3, the value extracted from the field and configured by the output mask (e.g., OxFOOO) is shifted to the right eleven positions when stored in order to simplify calculations. Operation LD_HDR of instruction TCP 4 causes the loading of an offset to the first byte of packet data following the TCP header. Illustratively, packets that are compatible with a pre-selected protocol stack may be separated at some point into header and data portions. Saving an offset to the data portion now makes it easier to split the packet later. Illustratively, the right-most seven bits of the OxOFF operation argument comprise a first element of the offset to the data. One skilled in the art will recognize the bit pattern (e.g., 1111111) as equating to negative one. Thus, an offset value equal to the current parsing pointer (e.g., the value in the ADDR register) minus two bytes - which locates the beginning of the TCP header - is saved. The remainder of the argument signifies that the value of a temporary data structure (e.g., temporary register RI ) is to be added to this offset. In this particular context, the value saved in the previous instruction (e.g., the length of the TCP header) is added. These two values combine to form an offset to the beginning of the packet data, which is stored in an appropriate register or other data structure (e.g., the HDRSPLIT register). Finally, and as mentioned above, instruction DONE (e.g., instruction eighteen) indicates the end of parsing of a packet when it is determined that the packet does not conform to one or more of the protocols associated with the illustrated instructions. This may be considered a "clean-up" instruction. In particular, output operation LD_CTL, with an operation argument of 0x001 indicates that a No Assist flag is to be set (e.g., to one) in the control register described above in conjunction with instruction VLAN. The No_Assist flag, as described elsewhere, may be used to inform other modules of the network interface that the present packet, is unsuitable for one or more processing enhancements described elsewhere. The illustrated program or microcode demonstrates just one method of parsing a packet. Other programs, comprising the same instructions in a different sequence or different instructions altogether, with similar or dissimilar formats, may be employed to examine and store portions of headers and to configure registers and other data structures. The efficiency gains to be realized from the application of the enhanced processing of a present embodiment of the invention more than offset the time required to parse a packet with the illustrated program. Further, even though a header parser parses a packet on a NIC in a current embodiment, the packet may still need to be processed through its protocol stack (e.g., to remove the protocol headers) by a processor on a host computer. Doing so avoids burdening the communication device (e.g., network interface) with such a task. One Embodiment of a Flow Database FIG. 5 depicts flow database (FDB) 110 according to one embodiment of the invention. Illustratively FDB 110 is implemented as a CAM (Content Addressable Memory) using a re-writeable memory component (e.g., RAM, SRAM, DRAM). In this embodiment, FDB 110 comprises associative portion 502 and associated portion 504, and may be indexed by flow number 506. The scope of the invention does not limit the form or structure of flow database 110. In alternative embodiments of the invention virtually any form of data structure may be employed (e.g., database, table, queue, list, array), either monolithic or segmented, and may be implemented in hardware or software. The illustrated form of FDB 110 is merely one manner of maintaining useful information concerning communication flows through NIC 100. Advantageously, the structure of a CAM allows highly efficient and fast associative searching. In the illustrated embodiment of the invention, the information stored in FDB 110 and the operation of flow database manager (FDBM) 108 (described below) permit functions such as data re-assembly, batch processing of packet headers, and other enhancements. These functions may be briefly described as follows. One form of data re-assembly involves the re-assembly or combination of data from multiple related packets (e.g., packets from a single communication flow or a single datagram). One method for the batch processing of packet headers entails processing protocol headers from multiple related packets through a protocol stack collectively rather than one packet at a time. Another illustrative function of NIC 100 involves the distribution or sharing of such protocol stack processing (and/or other functions) among processors in a multi-processor host computer system. Yet another possible function of NIC 100 is to enable the transfer of re-assembled data to a destination entity (e.g., an application program) in an efficient aggregation (e.g., a memory page), thereby avoiding piecemeal and highly inefficient transfers of one packet's data at a time. Thus, in this embodiment of the invention, one purpose of FDB 110 and FDBM 108 is to generate information for the use of NIC 100 and/or a host computer system in enabling, disabling or performing one or more of these functions. Associative portion 502 of FDB 110 in FIG. 5 stores the flow key of each valid flow destined for an entity served by NIC 100. Thus, in one embodiment of the invention associative portion 502 includes IP source address 510, IP destination address 512, TCP source port 514 and TCP destination port 516. As described in a previous section these fields may be extracted from a packet and provided to FDBM 108 by header parser 106. Although each destination entity served by NIC 100 may participate in multiple communication flows or end-to-end TCP connections, only one flow at a time will exist between a particular source entity and a particular destination entity. Therefore, each flow key in associative portion 502 that corresponds to a valid flow should be unique from all other valid flows. In alternative embodiments of the invention, associative portion 502 is composed of different fields, reflecting alternative flow key forms, which may be determined by the protocols parsed by the header parser and the information used to identify communication flows. Associated portion 504 in the illustrated embodiment comprises flow validity indicator 520, flow sequence number 522 and flow activity indicator 524. These fields provide information concerning the flow identified by the flow key stored in the corresponding entry in associative portion 502. The fields of associated portion 504 may be retrieved and/or updated by FDBM 108 as described in the following section. Flow validity indicator 520 in this embodiment indicates whether the associated flow is valid or invalid. Illustratively, the flow validity indicator is set to indicate a valid flow when the first packet of data in a flow is received, and may be reset to reassert a flow's validity every time a portion of a flow's datagram (e.g., a packet) is correctly received. Flow validity indicator 520 may be marked invalid after the last packet of data in a flow is received. The flow validity indicator may also be set to indicate an invalid flow whenever a flow is to be torn down (e.g., terminated or aborted) for some reason other than the receipt of a final data packet. For example, a packet may be received out of order from other packets of a datagram, a control packet indicating that a data transfer or flow is being aborted may be received, an attempt may be made to re-establish or re-synchronize a flow (in which case the original flow is terminated), etc. In one embodiment of the invention flow validity indicator 520 is a single bit, flag or value. Flow sequence number 522 in the illustrated embodiment comprises a sequence number of the next portion of data that is expected in the associated flow. Because the datagram being sent in a flow is typically received via multiple packets, the flow sequence number provides a mechanism to ensure that the packets are received in the correct order. For example, in one embodiment of the invention NIC 100 re-assembles data from multiple packets of a datagram. To perform this re-assembly in the most efficient manner, the packets need to be received in order. Thus, flow sequence number 522 stores an identifier to identify the next packet or portion of data that should be received. In one embodiment of the invention, flow sequence number 522 corresponds to the TCP sequence number field found in TCP protocol headers. A packet's TCP sequence number identifies the position of the packet's data relative to other data being sent in a datagram. For packets and flows involving protocols other than TCP, an alternative method of verifying or ensuring the receipt of data in the correct order may be employed. Flow activity indicator 524 in the illustrated embodiment reflects the recency of activity of a flow or, in other words, the age of a flow. In this embodiment of the invention flow activity indicator 524 is associated with a counter, such as a flow activity counter (not depicted in FIG. 5). The flow activity counter is updated (e.g., incremented) each time a packet is received as part of a flow that is already stored in flow database 110. The updated counter value is then stored in the flow activity indicator field of the packet's flow. The flow activity counter may also be incremented each time a first packet of a new flow that is being added to the database is received. In an alternative embodiment, a flow activity counter is only updated for packets containing data (e.g., it is not updated for control packets). In yet another alternative embodiment, multiple counters are used for updating flow activity indicators of different flows. Because it can not always be determined when a communication flow has ended (e.g., the final packet may have been lost), the flow activity indicator may be used to identify flows that are obsolete or that should be torn down for some other reason. For example, if flow database 110 appears to be fully populated (e.g., flow validity indicator 520 is set for each flow number) when the first packet of a new flow is received, the flow having the lowest flow activity indicator may be replaced by the new flow. In the illustrated embodiment of the invention, the size of fields in FDB 110 may differ from one entry to another. For example, IP source and destination addresses are four bytes large in version four of the protocol, but are sixteen bytes large in version six. In one alternative embodiment of the invention, entries for a particular field may be uniform in size, with smaller entries being padded as necessary. In another alternative embodiment of the invention, fields within FDB 110 may be merged. In particular, a flow's flow key may be stored as a single entity or field instead of being stored as a number of separate fields as shown in FIG. 5. Similarly, flow validity indicator 520, flow sequence number 522 and flow activity indicator 524 are depicted as separate entries in FIG. 5. However, in an alternative embodiment of the invention one or more of these entries may be combined. In particular, in one alternative embodiment flow validity indicator 520 and flow activity indicator 524 comprise a single entry having a first value (e.g., zero) when the entry's associated flow is invalid. As long as the flow is valid, however, the combined entry is incremented as packets are received, and is reset to the first value upon termination of the flow. In one embodiment of the invention FDB 110 contains a maximum of sixty-four entries, indexed by flow number 506, thus allowing the database to track sixty-four valid flows at a time. In alternative embodiments of the invention, more or fewer entries may be permitted, depending upon the size of memory allocated for flow database 110. In addition to flow number 506, a flow may be identifiable by its flow key (stored in associative portion 502). In the illustrated embodiment of the invention, flow database 110 is empty (e.g., all fields are filled with zeros) when NIC 100 is initialized. When the first packet of a flow is received header parser 106 parses a header portion of the packet. As described in a previous section, the header parser assembles a flow key to identify the flow and extracts other information concerning the packet and/or the flow. The flow key, and other information, is passed to flow database manager 108. FDBM 108 then searches FDB 110 for an active flow associated with the flow key. Because the database is empty, there is no match. In this example, the flow key is therefore stored (e.g., as flow number zero) by copying the IP source address, IP destination address, TCP source port and TCP destination port into the corresponding fields. Flow validity indicator 520 is then set to indicate a valid flow, flow sequence number 522 is derived from the TCP sequence number (illustratively provided by the header parser), and flow activity indicator 524 is set to an initial value (e.g., one), which may be derived from a counter. One method of generating an appropriate flow sequence number, which may be used to verify that the next portion of data received for the flow is received in order, is to add the TCP sequence number and the size of the packet's data. Depending upon the configuration of the packet (e.g., whether the SYN bit in a Flags field of the packet's TCP header is set), however, the sum may need to be adjusted (e.g., by adding one) to correctly identify the next expected portion of data. As described above, one method of generating an appropriate initial value for a flow activity indicator is to copy a counter value that is incremented for each packet received as part of a flow. For example, for the first packet received after NIC 100 is initialized, a flow activity counter may be incremented to the value of one. This value may then be stored in flow activity indicator 524 for the associated flow. The next packet received as part of the same (or a new) flow causes the counter to be incremented to two, which value is stored in the flow activity indicator for the associated flow. In this example, no two flows should have the same flow activity indicator except at initialization, when they may all equal zero or some other predetermined value. Upon receipt and parsing of a later packet received at NIC 100, the flow database is searched for a valid flow matching that packet's flow key. Illustratively, only the flow keys of active flows (e.g., those flows for which flow validity indicator 520 is set) are searched. Alternatively, all flow keys (e.g., all entries in associative portion 502) may be searched but a match is only reported if its flow validity indicator indicates a valid flow. With a CAM such as FDB 110 in FIG. 5, flow keys and flow validity indicators may be searched in parallel. If a later packet contains the next portion of data for a previous flow (e.g., flow number zero), that flow is updated appropriately. In one embodiment of the invention this entails updating flow sequence number 522 and incrementing flow activity indicator 524 to reflect its recent activity. Flow validity indicator 520 may also be set to indicate the validity of the flow, although it should already indicate that the flow is valid. As new flows are identified, they are added to FDB 110 in a similar manner to the first flow. When a flow is terminated or torn down, the associated entry in FDB 110 is invalidated. In one embodiment of the invention, flow validity indicator 520 is merely cleared (e.g., set to zero) for the terminated flow. In another embodiment, one or more fields of a terminated flow are cleared or set to an arbitrary or predetermined value. Because of the bursty nature of network packet traffic, all or most of the data from a datagram is generally received in a short amount of time. Thus, each valid flow in FDB 110 normally only needs to be maintained for a short period of time, and its entry can then be used to store a different flow. Due to the limited amount of memory available for flow database 110 in one embodiment of the invention, the size of each field may be limited. In this embodiment, sixteen bytes are allocated for IP source address 510 and sixteen bytes are allocated for IP destination address 512. For IP addresses shorter than sixteen bytes in length, the extra space may be padded with zeros. Further, TCP source port 514 and TCP destination port 516 are each allocated two bytes. Also in this embodiment, flow validity indicator 520 comprises one bit, flow sequence number 522 is allocated four bytes and flow activity indicator 524 is also allocated four bytes. As described above, a flow is similar, but not identical, to an end-to-end TCP connection. A TCP connection may exist for a relatively extended period of time, sufficient to transfer multiple datagrams from a source entity to a destination entity. A flow, however, may exist only for one datagram. Thus, during one end-to-end TCP connection, multiple flows may be set up and torn down (e.g., once for each datagram). As described above, a flow may be set up (e.g., added to FDB 110 and marked valid) when NIC 100 detects the first portion of data in a datagram and may be torn down (e.g., marked invalid in FDB 110) when the last portion of data is received. Illustratively, each flow set up during a single end-to-end TCP connection will have the same flow key because the layer three and layer four address and port identifiers used to form the flow key will remain the same. In the illustrated embodiment, the size of flow database 110 (e.g., the number of flow entries) determines the maximum number of flows that may be interleaved (e.g., simultaneously active) at one time while enabling the functions of data re-assembly and batch processing of protocol headers. In other words, in the embodiment depicted in FIG. 5, NIC 100 can set up sixty-four flows and receive packets from up to sixty-four different datagrams (i.e., sixty-four flows may be active) without tearing down a flow. If a maximum number of flows through NIC 100 were known, flow database 110 could be limited to the corresponding number of entries. The flow database may be kept small because a flow only lasts for one datagram in the presently described embodiment and, because of the bursty nature of packet traffic, a datagram's packets are generally received in a short period of time. The short duration of a flow compensates for a limited number of entries in the flow database. In one embodiment of the invention, if FDB 110 is filled with active flows and a new flow is commenced (i.e., a first portion of data in a new datagram), the oldest (e.g., the least recently active) flow is replaced by the new one. In an alternative embodiment of the invention, flows may be kept active for any number of datagrams (or other measure of network traffic) or for a specified length or range of time. For example, when one datagram ends its flow in FDB 110 may be kept "open" (i.e., not torn down) if the database is not full (e.g., the flow's entry is not needed for a different flow). This scheme may further enhance the efficient operation of NIC 100 if another datagram having the same flow key is received. In particular, the overhead involved in setting up another flow is avoided and more data re-assembly and packet batching (as described below) may be performed. Advantageously, a flow may be kept open in flow database 110 until the end-to-end TCP connection that encompasses the flow ends. One Embodiment of a Flow Database Manager FIGs. 6A-6E depict one method of operating a flow database manager (FDBM), such as flow database manager 108 of FIG. 1 A, for managing flow database (FDB) 110. Illustratively, FDBM 108 stores and updates flow information stored in flow database 110 and generates an operation code for a packet received by NIC 100. FDBM 108 also tears down a flow (e.g., replaces, removes or otherwise invalidates an entry in FDB 110) when the flow is terminated or aborted. In one embodiment of the invention a packet's operation code reflects the packet's compatibility with pre-determined criteria for performing one or more functions of NIC 100 (e.g., data re-assembly, batch processing of packet headers, load distribution). In other words, depending upon a packet's operation code, other modules of NIC 100 may or may not perform one of these functions. In another embodiment of the invention, an operation code indicates a packet status. For example, an operation code may indicate that a packet: contains no data, is a control packet, contains more than a specified amount of data, is the first packet of a new flow, is the last packet of an existing flow, is out of order, contains a certain flag (e.g., in a protocol header) that does not have an expected value (thus possibly indicating an exceptional circumstance), etc. The operation of flow database manager 108 depends upon packet information provided by header parser 106 and data drawn from flow database 110. After FDBM 108 processes the packet information and/or data, control information (e.g., the packet's operation code) is stored in control queue 118 and FDB 110 may be altered (e.g., a new flow may be entered or an existing one updated or torn down). With reference now to FIGs. 6A-6E, state 600 is a start state in which FDBM 108 awaits information drawn from a packet received by NIC 100 from network 102. In state 602, header parser 106 or another module of NIC 100 notifies FDBM 108 of a new packet by providing the packet's flow key and some control information. Receipt of this data may be interpreted as a request to search FDB 110 to determine whether a flow having this flow key already exists. In one embodiment of the invention the control information passed to FDBM 108 includes a sequence number (e.g., a TCP sequence number) drawn from a packet header. The control information may also indicate the status of certain flags in the packet's headers, whether the packet includes data and, if so, whether the amount of data exceeds a certain size. In this embodiment, FDBM 108 also receives a No_Assist signal for a packet if the header parser determines that the packet is not formatted according to one of the preselected protocol stacks (i.e., the packet is not "compatible"), as discussed in a previous section. Illustratively, the No_Assist signal indicates that one or more functions of NIC 100 (e.g., data re-assembly, batch processing, load-balancing) may not be provided for the packet. In state 604, FDBM 108 determines whether a No_Assist signal was asserted for the packet. If so, the procedure proceeds to state 668 (FIG. 6E). Otherwise, FDBM 108 searches FDB 110 for the packet's flow key in state 606. In one embodiment of the invention only valid flow entries in the flow database are searched. As discussed above, a flow's validity may be reflected by a validity indicator such as flow validity indicator 520 (shown in FIG. 5). If, in state 608, it is determined that the packet's flow key was not found in the database, or that a match was found but the associated flow is not valid, the procedure advances to state 646 (FIG. 6D). If a valid match is found in the flow database, in state 610 the flow number (e.g., the flow database index for the matching entry) of the matching flow is noted and flow information stored in FDB 110 is read. Illustratively, this information includes flow validity indicator 520, flow sequence number 522 and flow activity indicator 524 (shown in FIG. 5). In state 612, FDBM 108 determines from information received from header parser 106 whether the packet contains TCP payload data. If not, the illustrated procedure proceeds to state 638 (FIG. 6C); otherwise the procedure continues to state 614. In state 614, the flow database manager determines whether the packet constitutes an attempt to reset a communication connection or flow. Illustratively, this may be determined by examining the state of a SYN bit in one of the packet's protocol headers (e.g., a TCP header). In one embodiment of the invention the value of one or more control or flag bits (such as the SYN bit) are provided to the FDBM by the header parser. One TCP entity may attempt to reset a communication flow or connection with another entity (e.g., because of a problem on one of the entity's host computers) and send a first portion of data along with the re-connection request. This is the situation the flow database manager attempts to discern in state 614. If the packet is part of an attempt to re-connect or reset a flow or connection, the procedure continues at state 630 (FIG. 6C). In state 616, flow database manager 108 compares a sequence number (e.g., a TCP sequence number) extracted from a packet header with a sequence number (e.g., flow sequence number 522 of FIG. 5) of the next expected portion of data for this flow. As discussed in a previous section, these sequence numbers should correlate if the packet contains the flow' s next portion of data. If the sequence numbers do not match, the procedure continues at state 628. In state 618, FDBM 108 determines whether certain flags extracted from one or more of the packet's protocol headers match expected values. For example, in one embodiment of the invention the URG, PSH, RST and FIN flags from the packet's TCP header are expected to be clear (i.e., equal to zero). If any of these flags are set (e.g., equal to one) an exceptional condition may exist, thus making it possible that one or more of the functions (e.g., data re-assembly, batch processing, load distribution) offered by NIC 100 should not be performed for this packet. As long as the flags are clear, the procedure continues at state 620; otherwise the procedure continues at state 626. In state 620, the flow database manager determines whether more data is expected during this flow. As discussed above, a flow may be limited in duration to a single datagram. Therefore, in state 620 the FDBM determines if this packet appears to be the final portion of data for this flow's datagram. Illustratively, this determination is made on the basis of the amount of data included with the present packet. A datagram comprising more data than can be carried in one packet is sent via multiple packets. The typical manner of disseminating a datagram among multiple packets is to put as much data as possible into each packet. Thus, each packet except the last is usually equal or nearly equal in size to the maximum transfer unit (MTU) allowed for the network over which the packets are sent. The last packet will hold the remainder, usually causing it to be smaller than the MTU. Therefore, one manner of identifying the final portion of data in a flow's datagram is to examine the size of each packet and compare it to a figure (e.g., MTU) that a packet is expected to exceed except when carrying the last data portion. It was described above that control information is received by FDBM 108 from header parser 106. An indication of the size of the data carried by a packet may be included in this information. In particular, header parser 106 in one embodiment of the invention is configured to compare the size of each packet's data portion to a pre-selected value. In one embodiment of the invention this value is programmable. This value is set, in the illustrated embodiment of the invention, to the maximum amount of data a packet can carry without exceeding MTU. In one alternative embodiment, the value is set to an amount somewhat less than the maximum amount of data that can be carried. Thus, in state 620, flow database manager 108 determines whether the received packet appears to carry the final portion of data for the flow's datagram. If not, the procedure continues to state 626. In state 622, it has been ascertained that the packet is compatible with pre-selected protocols and is suitable for one or more functions offered by NIC 100. In particular, the packet has been formatted appropriately for one or more of the functions discussed above. FDBM 108 has determined that the received packet is part of an existing flow, is compatible with the pre-selected protocols and contains the next portion of data for the flow (but not the final portion). Further, the packet is not part of an attempt to re-set a flow/connection, and important flags have their expected values. Thus, flow database 110 can be updated as follows. The activity indicator (e.g., flow activity indicator 524 of FIG. 5) for this flow is modified to reflect the recent flow activity. In one embodiment of the invention flow activity indicator 524 is implemented as a counter, or is associated with a counter, that is incremented each time data is received for a flow. In another embodiment of the invention, an activity indicator or counter is updated every time a packet having a flow key matching a valid flow (e.g., whether or not the packet includes data) is received. In the illustrated embodiment, after a flow activity indicator or counter is incremented it is examined to determine if it "rolled over" to zero (i.e., whether it was incremented past its maximum value). If so, the counter and/or the flow activity indicators for each entry in flow database 110 are set to zero and the current flow's activity indicator is once again incremented. Thus, in one embodiment of the invention the rolling over of a flow activity counter or indicator causes the re-initialization of the flow activity mechanism for flow database 110. Thereafter, the counter is incremented and the flow activity indicators are again updated as described previously. There are many other suitable methods that may be applied in an embodiment of the present invention to indicate that one flow was active more recently than another was. Also in state 622, flow sequence number 522 is updated. Illustratively, the new flow sequence number is determined by adding the size of the newly received data to the existing flow sequence number. Depending upon the configuration of the packet (e.g., values in its headers), this sum may need to be adjusted. For example, this sum may indicate simply the total amount of data received thus far for the flow's datagram. Therefore, a value may need to be added (e.g., one byte) in order to indicate a sequence number of the next byte of data for the datagram. Other suitable methods of ensuring that data is received in order may be used in place of the scheme described here. Finally, in state 622 in one embodiment of the invention, flow validity indicator 520 is set or reset to indicate the flow's validity. Then, in state 624, an operation code is associated with the packet. In the illustrated embodiment of the invention, operation codes comprise codes generated by flow database manager 108 and stored in control queue 118. In this embodiment, an operation code is three bits in size, thus allowing for eight operation codes. Operation codes may have a variety of other forms and ranges in alternative embodiments. For the illustrated embodiment of the invention, TABLE 1 describes each operation code in terms of the criteria that lead to each code's selection and the ramifications of that selection. For purposes of TABLE 1, setting up a flow comprises inserting a flow into flow database 110. Tearing down a flow comprises removing or invalidating a flow in flow database 110. Reassembly of data may be performed by DMA engine 120. In the illustrated embodiment of the invention, operation code 4 is selected in state 624 for packets in the present context of the procedure (e.g., compatible packets carrying the next, but not last, data portion of a flow). Thus, the existing flow is not torn down and there is no need to set up a new flow. As described above, a compatible packet in this embodiment is a packet conforming to one or more of the pre-selected protocols. By changing or augmenting the pre-selected protocols, virtually any packet may be compatible in an alternative embodiment of the invention. Returning now to FIGs. 6A-6E, after state 624 the illustrated procedure ends at state 670. In state 626 (reached from state 618 or state 620), operation code 3 is selected for the packet. Illustratively, operation code 3 indicates that the packet is compatible and matches a valid flow (e.g., the packet's flow key matches the flow key of a valid flow in FDB 110). Operation code 3 may also signify that the packet contains data, does not constitute an attempt to re-synchronize or reset a communication flow/connection and the packet's sequence number matches the expected sequence number (from flow database 110). But, either an important flag (e.g., one of the TCP flags URG, PSH, RST or FIN) is set (determined in state 618) or the packet's data is less than the threshold value described above (in state 620), thus indicating that no more data is likely to follow this packet in this flow. Therefore, the existing flow is torn down but no new flow is created. Illustratively, the flow may be torn down by clearing the flow's validity indicator (e.g., setting it to zero). After state 626, the illustrated procedure ends at state 670. In state 628 (reached from state 616), operation code 2 is selected for the packet. In the present context, operation code 2 may indicate that the packet is compatible, matches a valid flow (e.g., the packet's flow key matches the flow key of a valid flow in FDB 110), contains data and does not constitute an attempt to re-synchronize or reset a communication flow/connection. However, the sequence number extracted from the packet (in state 616) does not match the expected sequence number from flow database 110. This may occur, for example, when a packet is received out of order. Thus, the existing flow is torn down but no new flow is established. Illustratively, the flow may be torn down by clearing the flow's validity indicator (e.g., setting it to zero). After state 628, the illustrated procedure ends at state 670. State 630 is entered from state 614 when it is determined that the received packet constitutes an attempt to reset a communication flow or connection (e.g., the TCP SYN bit is set). In state 630, flow database manager 108 determines whether more data is expected to follow. As explained in conjunction with state 620, this determination may be made on the basis of control information received by the flow database manager from the header parser. If more data is expected (e.g., the amount of data in the packet equals or exceeds a threshold value), the procedure continues at state 634. In state 632, operation code 2 is selected for the packet. Operation code 2 was also selected in state 628 in a different context. In the present context, operation code 2 may indicate that the packet is compatible, matches a valid flow and contains data. Operation code 2 may also signify in this context that the packet constitutes an attempt to re- synchronize or reset a communication flow or connection, but that no more data is expected once the flow/connection is reset. Therefore, the existing flow is torn down and no new flow is established. Illustratively, the flow may be torn down by clearing the flow's validity indicator (e.g., setting it to zero). After state 632, the illustrated procedure ends at state 670. In state 634, flow database manager 108 responds to an attempt to reset or re- synchronize a communication flow/connection whereby additional data is expected. Thus, the existing flow is torn down and replaced as follows. The existing flow may be identified by the flow number retrieved in state 610 or by the packet's flow key. The flow's sequence number (e.g., flow sequence number 522 in FIG. 5) is set to the next expected value. Illustratively, this value depends upon the sequence number (e.g., TCP sequence number) retrieved from the packet (e.g., by header parser 106) and the amount of data included in the packet. In one embodiment of the invention these two values are added to determine a new flow sequence number. As discussed previously, this sum may need to be adjusted (e.g., by adding one). Also in state 634, the flow activity indicator is updated (e.g., incremented). As explained in conjunction with state 622, if the flow activity indicator rolls over, the activity indicators for all flows in the database are set to zero and the present flow is again incremented. Finally, the flow validity indicator is set to indicate that the flow is valid. In state 636, operation code 7 is selected for the packet. In the present context, operation code 7 indicates that the packet is compatible, matches a valid flow and contains data. Operation code 7 may further signify, in this context, that the packet constitutes an attempt to re-synchronize or reset a communication flow/connection and that additional data is expected once the flow/connection is reset. In effect, therefore, the existing flow is torn down and a new one (with the same flow key) is stored in its place. After state 636, the illustrated procedure ends at end state 670. State 638 is entered after state 612 when it is determined that the received packet contains no data. This often indicates that the packet is a control packet. In state 638, flow database manager 108 determines whether one or more flags extracted from the packet by the header parser match expected or desired values. For example, in one embodiment of the invention the TCP flags URG, PSH, RST and FIN must be clear in order for DMA engine 120 to re-assemble data from multiple related packets (e.g., packets having an identical flow key). As discussed above, the TCP SYN bit may also be examined. In the present context (e.g., a packet with no data), the SYN bit is also expected to be clear (e.g., to store a value of zero). If the flags (and SYN bit) have their expected values the procedure continues at state 642. If, however, any of these flags are set, an exceptional condition may exist, thus making it possible that one or more functions offered by NIC 100 (e.g., data re-assembly, batch processing, load distribution) are unsuitable for this packet, in which case the procedure proceeds to state 640. In state 640, operation code 1 is selected for the packet. Illustratively, operation code 1 indicates that the packet is compatible and matches a valid flow, but does not contain any data and one or more important flags or bits in the packet's header(s) are set. Thus, the existing flow is torn down and no new flow is established. Illustratively, the flow may be torn down by clearing the flow's validity indicator (e.g., setting it to zero). After state 640, the illustrated procedure ends at end state 670. In state 642, the flow's activity indicator is updated (e.g., incremented) even though the packet contains no data. As described above in conjunction with state 622, if the activity indicator rolls over, in a present embodiment of the invention all flow activity indicators in the database are set to zero and the current flow is again incremented. The flow's validity indicator may also be reset, as well as the flow's sequence number. In state 644, operation code 0 is selected for the packet. Illustratively, operation code 0 indicates that the packet is compatible, matches a valid flow, and that the packet does not contain any data. The packet may, for example, be a control packet. Operation code 0 further indicates that none of the flags checked by header parser 106 and described above (e.g., URG, PSH, RST and FIN) are set. Thus, the existing flow is not torn down and no new flow is established. After state 644, the illustrated procedure ends at end state 670. State 646 is entered from state 608 if the packet's flow key does not match any of the flow keys of valid flows in the flow database. In state 646, FDBM 108 determines whether flow database 110 is full and may save some indication of whether the database is full. In one embodiment of the invention the flow database is considered full when the validity indicator (e.g., flow validity indicator 520 of FIG. 5) is set for every flow number (e.g., for every flow in the database). If the database is full, the procedure continues at state 650, otherwise it continues at state 648. In state 648, the lowest flow number of an invalid flow (e.g., a flow for which the associated flow validity indicator is equal to zero) is determined. Illustratively, this flow number is where a new flow will be stored if the received packet warrants the creation of a new flow. After state 648, the procedure continues at state 652. In state 650, the flow number of the least recently active flow is determined. As discussed above, in the illustrated embodiment of the invention a flow's activity indicator (e.g., flow activity indicator 524 of FIG. 5) is updated (e.g., incremented) each time data is received for a flow. Therefore, in this embodiment the least recently active flow can be identified as the flow having the least recently updated (e.g., lowest) flow activity indicator. Illustratively, if multiple flows have flow activity indicators set to a common value (e.g., zero), one flow number may be chosen from them at random or by some other criteria. After state 650, the procedure continues at state 652. In state 652, flow database manager 108 determines whether the packet contains data. Illustratively, the control information provided to FDBM 108 by the header parser indicates whether the packet has data. If the packet does not include data (e.g., the packet is a control packet), the illustrated procedure continues at state 668. In state 654, flow database manager 108 determines whether the data received with the present packet appears to contain the final portion of data for the associated datagram/flow. As described in conjunction with state 620, this determination may be made on the basis of the amount of data included with the packet. If the amount of data is less than a threshold value (a programmable value in the illustrated embodiment), then no more data is expected and this is likely to be the only data for this flow. In this case the procedure continues at state 668. If, however, the data meets or exceeds the threshold value, in which case more data may be expected, the procedure proceeds to state 656. In state 656, the values of certain flags are examined. These flags may include, for example, the URG, PSH, RST, FIN bits of a TCP header. If any of the examined flags do not have their expected or desired values (e.g., if any of the flags are set), an exceptional condition may exist making one or more of the functions of NIC 100 (e.g., data reassembly, batch processing, load distribution) unsuitable for this packet. In this case the procedure continues at state 668; otherwise the procedure proceeds to state 658. In state 658, the flow database manager retrieves the information stored in state 646 concerning whether flow database 110 is full. If the database is full, the procedure continues at state 664; otherwise the procedure continues at state 660. In state 660, a new flow is added to flow database 110 for the present packet. Illustratively, the new flow is stored at the flow number identified or retrieved in state 648. The addition of a new flow may involve setting a sequence number (e.g., flow sequence number 522 from FIG. 5). Flow sequence number 522 may be generated by adding a sequence number (e.g., TCP sequence number) retrieved from the packet and the amount of data included in the packet. As discussed above, this sum may need to be adjusted (e.g., by adding one). Storing a new flow may also include initializing an activity indicator (e.g., flow activity indicator 524 of FIG. 5). In one embodiment of the invention this initialization involves storing a value retrieved from a counter that is incremented each time data is received for a flow. Illustratively, if the counter or a flow activity indicator is incremented past its maximum storable value, the counter and all flow activity indicators are cleared or reset. Also in state 660, a validity indicator (e.g., flow validity indicator 520 of FIG. 5) is set to indicate that the flow is valid. Finally, the packet's flow key is also stored in the flow database, in the entry corresponding to the assigned flow number. In state 662, operation code 6 is selected for the packet. Illustratively, operation code 6 indicates that the packet is compatible, did not match any valid flows and contains the first portion of data for a new flow. Further, the packet's flags have their expected or necessary values, additional data is expected in the flow and the flow database is not full. Thus, operation code 6 indicates that there is no existing flow to tear down and that a new flow has been stored in the flow database. After state 662, the illustrated procedure ends at state 670. In state 664, an existing entry in the flow database is replaced so that a new flow, initiated by the present packet, can be stored. Therefore, the flow number of the least recently active flow, identified in state 650, is retrieved. This flow may be replaced as follows. The sequence number of the existing flow (e.g., flow sequence number 522 of FIG. 5) is replaced with a value derived by combining a sequence number extracted from the packet (e.g., TCP sequence number) with the size of the data portion of the packet. This sum may need to be adjusted (e.g., by adding one). Then the existing flow's activity indicator (e.g., flow activity indicator 524) is replaced. For example, the value of a flow activity counter may be copied into the flow activity indicator, as discussed above. The flow's validity indicator (e.g., flow validity indicator 520 of FIG. 5) is then set to indicate that the flow is valid. Finally, the flow key of the new flow is stored. In state 666, operation code 7 is selected for the packet. Operation code 7 was also selected in state 636. In the present context, operation code 7 may indicate that the packet is compatible, did not match the flow key of any valid flows and contains the first portion of data for a new flow. Further, the packet's flags have compatible values and additional data is expected in the flow. Lastly, however, in this context operation code 7 indicates that the flow database is full, so an existing entry was torn down and the new one stored in its place. After state 666, the illustrated procedure ends at end state 670. In state 668, operation code 5 is selected for the packet. State 668 is entered from various states and operation code 5 thus represents a variety of possible conditions or situations. For example, operation code 5 may be selected when a No_Assist signal is detected (in state 604) for a packet. As discussed above, the No_Assist signal may indicate that the corresponding packet is not compatible with a set of pre-selected protocols. In this embodiment of the invention, incompatible packets are ineligible for one or more of the various functions of NIC 100 (e.g., data re-assembly, batch processing, load distribution). State 668 may also be entered, and operation code 5 selected, from state 652, in which case the code may indicate that the received packet does not match any valid flow keys and, further, contains no data (e.g., it may be a control packet). State 668 may also be entered from state 654. In this context operation code 5 may indicate that the packet does not match any valid flow keys. It may further indicate that the packet contains data, but that the size of the data portion is less than the threshold discussed in conjunction with state 654. In this context, it appears that the packet's data is complete (e.g., comprises all of the data for a datagram), meaning that there is no other data to reassemble with this packet's data and therefore there is no reason to make a new entry in the database for this one-packet flow. Finally, state 668 may also be entered from state 656. In this context, operation code 5 may indicate that the packet does not match any valid flow keys, contains data, and more data is expected, but at least one flag in one or more of the packet's protocol headers does not have its expected value. For example, in one embodiment of the invention the TCP flags URG, PSH, RST and FIN are expected to be clear. If any of these flags are set an exceptional condition may exist, thus making it possible that one of the functions offered by NIC 100 is unsuitable for this packet. As TABLE 1 reflects, there is no flow to tear down and no new flow is established when operation code 5 is selected. Following state 668, the illustrated procedure ends at state 670. The procedure illustrated in FIGs. 6A-6E and discussed above is but one suitable procedure for maintaining and updating a flow database and for determining a packet's suitability for certain processing functions. In particular, different operation codes may be utilized or may be implemented in a different manner, a goal being to produce information for later processing of the packet through NIC 100. Although operation codes are assigned for all packets by a flow database manager in the illustrated procedure, in an alternative procedure an operation code assigned by the FDBM may be replaced or changed by another module of NIC 100. This may be done to ensure a particular method of treating certain types of packets. For example, in one embodiment of the invention IPP module 104 assigns a predetermined operation code (e.g., operation code 2 of TABLE 1) to jumbo packets (e.g., packets greater in size than MTU) so that DMA engine 120 will not re-assemble them. In particular, the IPP module may independently determine that the packet is a jumbo packet (e.g., from information provided by a MAC module) and therefore assign the predetermined code. Illustratively, header parser 106 and FDBM 108 perform their normal functions for a jumbo packet and IPP module 104 receives a first operation code assigned by the FDBM. However, the IPP module replaces that code before storing the jumbo packet and information concerning the packet. In one alternative embodiment header parser 106 and/or flow database manager 108 may be configured to recognize a particular type of packet (e.g., jumbo) and assign a predetermined operation code. The operation codes applied in the embodiment of the invention illustrated in FIGs. 6A-6E are presented and explained in the following TABLE 1. TABLE 1 includes illustrative criteria used to select each operation code and illustrative results or effects of each code. TABLE 1 One Embodiment of a Load Distributor In one embodiment of the invention, load distributor 112 enables the processing of packets through their protocol stacks to be distributed among a number of processors. Illustratively, load distributor 112 generates an identifier (e.g., a processor number) of a processor to which a packet is to be submitted. The multiple processors may be located within a host computer system that is served by NIC 100. In one alternative embodiment, one or more processors for manipulating packets through a protocol stack are located on NIC 100. Without an effective method of sharing or distributing the processing burden, one processor could become overloaded if it were required to process all or most network traffic received at NIC 100, particularly in a high-speed network environment. The resulting delay in processing network traffic could deteriorate operations on the host computer system as well as other computer systems communicating with the host system via the network. Simply distributing packets among processors in a set of processors (e.g., such as in a round-robin scheme) may not be an efficient plan. Such a plan could easily result in packets being processed out of order. For example, if two packets from one communication flow or connection that are received at a network interface in the correct order were submitted to two different processors, the second packet may be processed before the first. This could occur, for example, if the processor that received the first packet could not immediately process the packet because it was busy with another task. When packets are processed out of order a recovery scheme must generally be initiated, thus introducing even more inefficiency and more delay. Therefore, in a present embodiment of the invention packets are distributed among multiple processors based upon their flow identities. As described above, a header parser may generate a flow key from layer three (e.g., IP) and layer four (e.g., TCP) source and destination identifiers retrieved from a packet's headers. The flow key may be used to identify the communication flow to which the packet belongs. Thus, in this embodiment of the invention all packets having an identical flow key are submitted to a single processor. As long as the packets are received in order by NIC 100, they should be provided to the host computer and processed in order by their assigned processor. Illustratively, multiple packets sent from one source entity to one destination entity will have the same flow key even if the packets are part of separate datagrams, as long as their layer three and layer four identifiers remain the same. As discussed above, separate flows are set up and torn down for each datagram within one TCP end-to-end connection. Therefore, just as all packets within one flow are sent to one processor, all packets within a TCP end-to-end connection will also be sent to the same processor. This helps ensure the correct ordering of packets for the entire connection, even between datagrams. Depending upon the network environment in which NIC 100 operates (e.g., the protocols supported by network 102), the flow key may be too large to use as an identifier of a processor. In one embodiment of the invention described above, for example, a flow key measures 288 bits. Meanwhile, the number of processors participating in the load- balancing scheme may be much smaller. For example, in the embodiment of the invention described below in conjunction with FIG. 7, a maximum of sixty-four processors is supported. Thus, in this embodiment only a six-bit number is needed to identify the selected processor. The larger flow key may therefore be mapped or hashed into a smaller range of values. FIG. 7 depicts one method of generating an identifier (e.g., a processor number) to specify a processor to process a packet received by NIC 100, based on the packet's flow key. In this embodiment of the invention, network 102 is the Internet and a received packet is formatted according to a compatible protocol stack (e.g., Ethernet at layer two, IP at layer three and TCP at layer four). State 700 is a start state. In state 702 a packet is received by NIC 100 and a header portion of the packet is parsed by header parser 106 (a method of parsing a packet is described in a previous section). In state 704, load distributor 112 receives the packet's flow key that was generated by header parser 106. Because a packet's flow key is 288 bits wide in this embodiment, in state 706 a hashing function is performed to generate a value that is smaller in magnitude. The hash operation may, for example, comprise a thirty-two bit CRC (cyclic redundancy check) function such as ATM (Asynchronous Transfer Mode) Adaptation Layer 5 (AAL5). AAL5 generates thirty-two bit numbers that are fairly evenly distributed among the 2 32 possible values. Another suitable method of hashing is the standard Ethernet CRC-32 function. Other hash functions that are capable of generating relatively small numbers from relatively large flow keys, where the numbers generated are well distributed among a range of values, are also suitable. With the resulting hash value, in state 708 a modulus operation is performed over the number of processors available for distributing or sharing the processing. Illustratively, software executing on the host computer (e.g., a device driver for NIC 100) programs or stores the number of processors such that it may be read or retrieved by load distributor 112 (e.g., in a register). The number of processors available for load balancing may be all or a subset of the number of processors installed on the host computer system. In the illustrated embodiment, the number of processors available in a host computer system is programmable, with a maximum value of sixty-four. The result of the modulus operation in this embodiment, therefore, is the number of the processor (e.g., from zero to sixty-three) to which the packet is to be submitted for processing. In this embodiment of the invention, load distributor 112 is implemented in hardware, thus allowing rapid execution of the hashing and modulus functions. In an alternative embodiment of the invention, virtually any number of processors may be accommodated. In state 710, the number of the processor that will process the packet through its protocol stack is stored in the host computer's memory. Illustratively, state 710 is performed in parallel with the storage of the packet in a host memory buffer. In one embodiment of the invention a descriptor ring in the host computer's memory is constructed to hold the processor number and possibly other information concerning the packet (e.g., a pointer to the packet, its size, its TCP checksum). A descriptor ring in this embodiment is a data structure comprising a number of entries, or "descriptors," for storing information to be used by a network interface circuit's host computer system. In the illustrated embodiment, a descriptor temporarily stores packet information after the packet has been received by NIC 100, but before the packet is processed by the host computer system. The information stored in a descriptor may be used, for example, by the device driver for NIC 100 or for processing the packet through its protocol stack. In state 712, an interrupt or other alert is issued to the host computer to inform it that a new packet has been delivered from NIC 100. In an embodiment of the invention in which NIC 100 is coupled to the host computer by a PCI (Peripheral Component Interconnect) bus, the INTA signal may be asserted across the bus. A PCI controller in the host receives the signal and the host operating system is alerted (e.g., via an interrupt). In state 714, software operating on the host computer (e.g., a device driver for NIC 100) is invoked (e.g., by the host computer's operating system interrupt handler) to act upon a newly received packet. The software gathers information from one or more descriptors in the descriptor ring and places information needed to complete the processing of each new packet into a queue for the specified processor (i.e., according to the processor number stored in the packet's descriptor). Illustratively, each descriptor corresponds to a separate packet. The information stored in the processor queue for each packet may include a pointer to a buffer containing the packet, the packet's TCP checksum, offsets of one or more protocol headers, etc. In addition, each processor participating in the load distribution scheme may have an associated queue for processing network packets. In an alternative embodiment of the invention, multiple queues may be used (e.g., for multiple priority levels or for different protocol stacks). Illustratively, one processor on the host computer system is configured to receive all alerts and/or interrupts associated with the receipt of network packets from NIC 100 and to alert the appropriate software routine or device driver. This initial processing may, alternatively, be distributed among multiple processors. In addition, in one embodiment of the invention a portion of the retrieval and manipulation of descriptor contents is performed as part of the handling of the interrupt that is generated when a new packet is stored in the descriptor ring. The processor selected to process the packet will perform the remainder of the retrieval/manipulation procedure. In state 716, the processor designated to process a new packet is alerted or woken. In an embodiment of the invention operating on a Solaris™ workstation, individual processes executed by the processor are configured as "threads." A thread is a process running in a normal mode (e.g., not at an interrupt level) so as to have minimal impact on other processes executing on the workstation. A normal mode process may, however, execute at a high priority. Alternatively, a thread may run at a relatively low interrupt level. A thread responsible for processing an incoming packet may block itself when it has no packets to process, and awaken when it has work to do. A "condition variable" may be used to indicate whether the thread has a packet to process. Illustratively, the condition variable is set to a first value when the thread is to process a packet (e.g., when a packet is received for processing by the processor) and is set to a second value when there are no more packets to process. In the illustrated embodiment of the invention, one condition variable may be associated with each processor's queue. In an alternative embodiment, the indicated processor is alerted in state 716 by a "cross-processor call." A cross-processor call is one way of communicating among processors whereby one processor is interrupted remotely by another processor. Other methods by which one processor alerts, or dispatches a process to, another processor may be used in place of threads and cross-processor calls. In state 718, a thread or other process on the selected processor begins processing the packet that was stored in the processor's queue. Methods of processing a packet through its protocol stack need not be described here. The illustrated procedure then ends with end state 720. In one alternative embodiment of the invention, a high-speed network interface is configured to receive and process ATM (Asynchronous Transfer Mode) traffic. In this embodiment, a load distributor is implemented as a set of instructions (e.g., as software) rather than as a hardware module. ATM traffic is connection-oriented and may be identified by a virtual connection identifier (VCI), which corresponds to a virtual circuit established between the packet's source and destination entities. Each packet that is part of a virtual circuit includes the VCI in its header. Advantageously, a VCI is relatively small in size (e.g., sixteen bits). In this alternative embodiment, therefore, a packet's VCI may be used in place of a flow key for the purpose of distributing or sharing the burden of processing packets through their protocol stacks. Illustratively, traffic from different VCIs is sent to different processors, but, to ensure correct ordering of packets, all packets having the same VCI are sent to the same processor. When an ATM packet is received at a network interface, the VCI is retrieved from its header and provided to the load distributor. The modulus of the VCI over the number of processors that are available for load distribution is then computed. Similar to the illustrated embodiment, the packet and its associated processor number are then provided to the host computer. As described above, load distribution in a present embodiment of the invention is performed on the basis of a packet's layer three and/or layer four source and destination entity identifiers. In an alternative embodiment of the invention, however, load distribution may be performed on the basis of layer two addresses. In this alternative embodiment, packets having the same Ethernet source and destination addresses, for example, are sent to a single processor. However, this may result in a processor receiving many more packets than it would if layer three and/or layer four identifiers were used. For example, if a large amount of traffic is received through a router situated near (in a logical sense) to the host computer, the source Ethernet address for all of the traffic may be the router's address even though the traffic is from a multitude of different end users and/or computers. In contrast, if the host computer is on the same Ethernet segment as all of the end users/computers, the layer two source addresses will show greater variety and allow more effective load sharing. Other methods of distributing the processing of packets received from a network may differ from the embodiment illustrated in FIG. 7 without exceeding the scope of the invention. In particular, many alternative procedures for assigning a flow's packets to a processor and delivering those packets to the processor may be employed. One Embodiment of a Packet Queue As described above, packet queue 116 stores packets received from IPP module 104 prior to their re-assembly by DMA engine 120 and their transfer to the host computer system. FIG. 8 depicts packet queue 116 according to one embodiment of the invention. In the illustrated embodiment, packet queue 116 is implemented as a FIFO (First-In First-Out) queue containing up to 256 entries. Each packet queue entry in this embodiment stores one packet plus various information concerning the packet. For example, entry 800 includes packet portion 802 plus a packet status portion. Because packets of various sizes are stored in packet queue 116, packet portion 802 may include filler 802a to supplement the packet so that the packet portion ends at an appropriate boundary (e.g., byte, word, double word). Filler 802a may comprise random data or data having a specified pattern. Filler 802a may be distinguished from the stored packet by the pattern of the filler data or by a tag field. Illustratively, packet status information includes TCP checksum value 804 and packet length 806 (e.g., length of the packet stored in packet portion 802). Storing the packet length may allow the packet to be easily identified and retrieved from packet portion 802. Packet status information may also include diagnostic/status information 808. Diagnostic/status information 808 may include a flag indicating that the packet is bad (e.g., incomplete, received with an error), an indicator that a checksum was or was not computed for the packet, an indicator that the checksum has a certain value, an offset to the portion of the packet on which the checksum was computed, etc. Other flags or indicators may also be included for diagnostics, filtering, or other purposes. In one embodiment of the invention, the packet's flow key (described above and used to identify the flow comprising the packet) and/or flow number (e.g., the corresponding index of the packet's flow in flow database 110) are included in diagnostic/status information 808. In another embodiment, a tag field to identify or delimit filler 802a is included in diagnostic/status information 808. In one alternative embodiment of the invention, any or all of the packet status information described above is stored in control queue 118 rather than packet queue 116. In the illustrated embodiment of the invention packet queue 116 is implemented in hardware (e.g., as random access memory). In this embodiment, checksum value 804 is sixteen bits in size and may be stored by checksum generator 114. Packet length 806 is fourteen bits large and may be stored by header parser 106. Finally, portions of diagnostic/status information 808 may be stored by one or more of IPP module 104, header parser 106, flow database manager 108, load distributor 112 and checksum generator 114. Packet queue 116 in FIG. 8 is indexed with two pointers. Read pointer 810 identifies the next entry to be read from the queue, while write pointer 812 identifies the entry in which the next received packet and related information is to be stored. In a present embodiment of the invention, the packet stored in packet portion 802 of an entry is extracted from packet queue 116 when its data is to be-reassembled by DMA engine 120 and/or transferred to the host computer system. One Embodiment of a Control Queue In one embodiment of the invention control queue 118 stores control and status information concerning a packet received by NIC 100. In this embodiment, control queue 118 retains information used to enable the batch processing of protocol headers and/or the re-assembly of data from multiple related packets. Control queue may also store information to be used by the host computer or a series of instructions operating on a host computer (e.g., a device driver for NIC 100). The information stored in control queue 118 may supplement or duplicate information stored in packet queue 116. FIG. 9 depicts control queue 118 in one embodiment of the invention. The illustrated control queue contains one entry for each packet stored in packet queue 116 (e.g., up to 256 entries). In one embodiment of the invention each entry in control queue 118 corresponds to the entry (e.g., packet) in packet queue 116 having the same number. FIG. 9 depicts entry 900 having various fields, such as CPU number 902, No_Assist signal 904, operation code 906, payload offset 908, payload size 910 and other status information 912. An entry may also include other status or control information (not shown in FIG. 9). Entries in control queue 118 in alternative embodiments of the invention may comprise different information. CPU (or processor) number 902, discussed in a previous section, indicates which one of multiple processors on the host computer system should process the packet's protocol headers. Illustratively, CPU number 902 is six bits in size. No_Assist signal 904 indicates whether the packet is compatible with (e.g., is formatted according to) any of a set of pre-selected protocols that may be parsed by header parser 106. No_Assist signal 904 may comprise a single flag (e.g. one bit). In one embodiment of the invention the state or value of No_Assist signal 904 may be used by flow database manager 108 to determine whether a packet's data is re-assembleable and/or whether its headers may be processed with those of related packets. In particular, the FDBM may use the No_Assist signal in determining which operation code to assign to the packet. Operation code 906 provides information to DMA engine 120 to assist in the re- assembly of the packet's data. As described in a previous section, an operation code may indicate whether a packet includes data or whether a packet's data is suitable for reassembly. Illustratively, operation code 906 is three bits in size. Payload offset 908 and payload size 910 correspond to the offset and size of the packet's TCP payload (e.g., TCP data), respectively. These fields may be seven and fourteen bits large, respectively. In the illustrated embodiment, other status information 912 includes diagnostic and/or status information concerning the packet. Status information 912 may include a starting position for a checksum calculation (which may be seven bits in size), an offset of the layer three (e.g., IP) protocol header (which may also be seven bits in size), etc. Status information 912 may also include an indicator as to whether the size of the packet exceeds a first threshold (e.g., whether the packet is greater than 1522 bytes) or falls under a second threshold (e.g., whether the packet is 256 bytes or less). This information may be useful in re-assembling packet data. Illustratively, these indicators comprise single-bit flags. In one alternative embodiment of the invention, status information 912 includes a packet's flow key and/or flow number (e.g., the index of the packet's flow in flow database 110). The flow key or flow number may, for example, be used for debugging or other diagnostic purposes. In one embodiment of the invention, the packet's flow number may be stored in status information 912 so that multiple packets in a single flow may be identified. Such related packet may then be collectively transferred to and/or processed by a host computer. FIG. 9 depicts a read pointer and a write pointer for indexing control queue 118. Read pointer 914 indicates an entry to be read by DMA engine 120. Write pointer 916 indicates the entry in which to store information concerning the next packet stored in packet queue 116. In an alternative embodiment of the invention, a second read pointer (not shown in FIG. 9) may be used for indexing control queue 118. Illustratively, when a packet is to be transferred to the host computer, information drawn from entries in the control queue is searched to determine whether a related packet (e.g., a packet in the same flow as the packet to be transferred) is also going to be transferred. This may be done by dynamic packet batching module 122 in the embodiment of the invention depicted in FIG. 1. If a related packet is found, the host computer is alerted so that protocol headers from the related packets may be processed collectively. In this alternative embodiment of the invention related packets are identified by matching their flow numbers (or flow keys) in status information 912. The second read pointer may be used to look ahead in the control queue for packets with matching flow numbers. In one embodiment of the invention CPU number 902 may be stored in the control queue by load distributor 112 and No Assist signal 904 may be stored by header parser 106. Operation code 906 may be stored by flow database manager 108, and payload offset 908 and payload size 910 may be stored by header parser 106. Portions of other status information may be written by the preceding modules and/or others, such as IPP module 104 and checksum generator 114. In one particular embodiment of the invention, however, many of these items of information are stored by IPP module 104 or some other module acting in somewhat of a coordinator role. Early Random Packet Discard in One Embodiment of the Invention Packets may arrive at a network interface from a network at a rate faster than they can be transferred to a host computer. When such a situation exists, the network interface must often drop, or discard, one or more packets. Therefore, in one embodiment of the present invention a system and method for randomly discarding a packet are provided. Systems and methods discussed in this section may be applicable to other communication devices as well, such as gateways, routers, bridges, modems, etc. One reason that a packet may be dropped is that a network interface is already storing the maximum number of packets that it can store for transfer to a host computer. In particular, a queue that holds packets to be transferred to a host computer, such as packet queue 116 (shown in FIG. 1 A), may be fully populated when another packet is received from a network. Either the new packet or a packet already stored in the queue may be dropped. Partly because of the bursty nature of much network traffic, multiple packets may often be dropped when a network interface is congested. And, in some network interfaces, if successive packets are dropped one particular network connection or flow (e.g., a connection or flow that includes all of the dropped packets) may be penalized even if it is not responsible for the high rate of packet arrival. If a network connection or flow is penalized too heavily, the network entity generating the traffic in that connection or flow may tear it down in the belief that a "broken pipe" has been encountered. A broken pipe occurs when a network entity interprets a communication problem as indicating that a connection has been severed. For certain network traffic (e.g., TCP traffic), the dropping of a packet may initiate a method of flow control in which a network entity's window (e.g., number of packets it transmits before waiting for an acknowledgement) shrinks or is reset to a very low number. Thus, every time a packet from a TCP communicant is dropped by a network interface at a receiving entity, the communicant must re-synchronize its connection with the receiving entity. If one or a subset of communicants are responsible for a large percentage of network traffic received at the entity, then it seems fair that those communicants should be penalized in proportion to the amount of traffic for which it is responsible. In addition, it may be wise to prevent certain packets or types of packets from being discarded. For example, discarding a small control packet may do very little to alleviate congestion in a network interface and yet have a drastic or negative effect upon a network connection or flow. Further, if a network interface is optimized for packets adhering to a particular protocol, it may be more efficient to avoid dropping such packets. Even further, particular connections, flows or applications may be prioritized, in which case higher priority traffic should not be dropped. Thus, in one embodiment of a network interface according to the present invention, a method is provided for randomly discarding a packet when a communication device's packet queue is full or is filled to some threshold level. Intelligence may be added to such a method by selecting certain types of packets for discard (e.g., packets from a particular flow, connection or application) or excepting certain types of packets from being discarded (e.g., control packets, packets conforming to a particular protocol or set of protocols). A provided method is random in that discarded packets are selected randomly from those packets that are considered discardable. Applying a random discard policy may be sufficient to avoid broken pipes by distributing the impact of dropped packets among multiple connections or flows. In addition, if a small number of transmitting entities are responsible for a majority of the traffic received at a network interface, dropping packets randomly may ensure that the offending entities are penalized proportionately. Different embodiments of the invention that are discussed below provide various combinations of randomness and intelligence, and one of these attributes may be omitted in one or more embodiments. FIG. 10 depicts a system and method for randomly discarding packets in a present embodiment of the invention. In this embodiment, packet queue 1000 is a hardware FIFO (e.g., first-in first-out) queue that is 16 ICB in size. In other embodiments of the invention the packet queue may be smaller or larger or may comprise another type of data structure (e.g., list, array, table, heap) implemented in hardware or software. Similar to packet queue 116 discussed in a previous section, packet queue 1000 receives packets from a network and holds them for transfer to a host computer. Packets arriving from a network may arrive from the network at a high rate and may be processed or examined by one or more modules (e.g., header parser 106, flow database manager 108) prior to being stored in packet queue 1000. For example, where the network is capable of transmitting one gigabit of traffic per second, packets conforming to one set of protocols (e.g., Ethernet, IP and TCP) may be received at a rate of approximately 1.48 million packets per second. After being stored in packet queue 1000, packets are transferred to a host computer at a rate partially dependent upon events and conditions internal to the host computer. Thus, the network interface may not be able to control the rate of packet transmittal to the host computer. In the illustrated embodiment, packet queue 1000 is divided into a plurality of zones or regions, any of which may overlap or share a common boundary. Packet queue 1000 may be divided into any number of regions, and the invention is not limited to the three regions depicted in FIG. 10. Illustratively, region zero (represented by the numeral 1002) encompasses the portion of packet queue 1000 from 0 KB (e.g., no packets are stored in the queue) to 8 KB (e.g., half full). Region one (represented by the numeral 1004) encompasses the portion of the packet queue from 8 KB to 12 KB. Region two (represented by the numeral 1006) encompasses the remaining portion of the packet queue, from 12 ICB to 16 KB. In an alternative embodiment, regions may only be defined for a portion of packet queue 1000. For example, only the upper half (e.g., above 8 ICB) may be divided into one or more regions. The number and size of the different regions and the location of boundaries between the regions may vary according to several factors. Among the factors are the type of packets received at the network interface (e.g., the protocols according to which the packets are configured), the size of the packets, the rate of packet arrival (e.g., expected rate, average rate, peak rate), the rate of packet transfer to the host computer, the size of the packet queue, etc. For example, in another embodiment of the invention, packet queue 1000 is divided into five regions. A first region extends from 0 KB to 8 KB; a second region ranges from 8 KB to 10 KB; a third from 10 KB to 12 KB; a fourth from 12 KB to 14 KB; and a final region extends from 14 KB to 16 KB. During operation of a network interface according to a present embodiment, traffic indicator 1008 indicates how full packet queue 1000 is. Traffic indicator 1008, in one embodiment of the invention, comprises read pointer 810 and/or write pointer 812 (shown in FIG. 8). In the presently discussed embodiment in which packet queue 1000 is fully partitioned, traffic indicator 1008 will generally be located in one of the regions into which the packet queue was divided or at a dividing boundary. Thus, during operation of a network interface appropriate action may be taken, as described below, depending upon how full the packet queue is (e.g., depending upon which region is identified by traffic indicator 1008). In FIG. 10, counter 1010 is incremented as packets arrive at packet queue 1000. In the illustrated embodiment, counter 1010 continuously cycles through a limited range of values, such as zero through seven. In one embodiment of the invention, each time a new packet is received the counter is incremented by one. In an alternative embodiment, counter 1010 may not be incremented when certain "non-discardable" packets are received. Various illustrative criteria for identifying non-discardable packets are presented below. For one or more regions of packet queue 1000, an associated programmable probability indicator indicates the probability that a packet will be dropped when traffic indicator 1008 indicates that the level of traffic in the packet queue has reached the associated region. Therefore, in the illustrated embodiment probability indicator 1012 indicates the probability that a packet will be dropped while the packet queue is less than half full (e.g., when traffic indicator 1008 is located in region zero). Similarly, probability indicators 1014 and 1016 specify the probability that a new packet will be dropped when traffic indicator 1008 identifies regions one and two, respectively. In the illustrated embodiment, probability indicators 1012, 1014 and 1016 each comprise a set, or mask, of sub-indicators such as bits or flags. Illustratively, the number of sub-indicators in a probability indicator matches the range of counter values - in this case, eight. In one embodiment of the invention, each sub-indicator may have one of two values (e.g., zero or one) indicating whether a packet is dropped. Thus, the sub-elements of a probability indicator may be numbered from zero to seven (illustratively, from right to left) to correspond to the eight possible values of counter 1010. For each position in a probability indicator that stores a first value (e.g., one), when the value of counter 1010 matches the number of that bit, the next discardable packet received for packet queue 1000 will be dropped. As discussed above, certain types of packets (e.g., control packets) may not be dropped. Illustratively, counter 1010 is only incremented for discardable packets. In FIG. 10, probability indicator 1012 (e.g., 00000000) indicates that no packets are to be dropped as long as the packet queue is less than half full (e.g., as long as traffic indicator 1008 is in region zero). Probability indicator 1014 (e.g., 00000001) indicates that every eighth packet is to be dropped when there is at least 8 KB stored in the packet queue. In other words, when traffic indicator 1008 is located in region one, there is a 12.5% probability that a discardable packet will be dropped. In particular, when counter 1010 equals zero the next discardable packet, or a packet already stored in the packet queue, is discarded. Probability indicator 1016 (e.g., 01010101) specifies that every other discardable packet is to be dropped. There is thus a 50% probability that a discardable packet will be dropped when the queue is more than three-quarters full. Illustratively, when a packet is dropped, counter 1010 is still incremented. As another example, in the alternative embodiment described above in which the packet queue is divided into five regions, suitable probability indicators may include the following. For regions zero and one, 00000000; for region two, 00000001 ; for region three, 00000101 ; and for region four, 01111111. Thus, in this alternative embodiment, region one is treated as an extension to region zero. Further, the probability of dropping a packet has a wider range, from 0% to 87.5%. In one alternative embodiment described above, only a portion of a packet queue is partitioned into regions. In this alternative embodiment, a default probability or null probability (e.g., 00000000) of dropping a packet may be associated with the un-partitioned portion. Illustratively, this ensures that no packets are dropped before the level of traffic stored in the queue reaches a first threshold. Even in an embodiment where the entire queue is partitioned, a default or null probability may be associated with a region that encompasses or borders a 0 KB threshold. Just as a packet queue may be divided into any number of regions for purposes of the present invention, probability indicators may comprise bit masks of any size or magnitude, and need not be of equal size or magnitude. Further, probability indicators are programmable in a present embodiment, thus allowing them to be altered even during the operation of a network interface. Discarding packets on the basis of a probability indicator injects randomness into the discard process. A random early discard policy may be sufficient to avoid the problem of broken pipes discussed above. In particular, in one embodiment of the invention, all packets are considered discardable, such that all packets are counted by counter 1010 and all are candidates for being dropped. As already discussed, however, in another embodiment of the invention intelligence is added in the process of excluding certain types of packets from being discarded. It will be understood that probability indicators and a counter simply constitute one system for enabling the random discard of packets in a network interface. Other mechanisms are also suitable. In one alternative embodiment, a random number generator may be employed in place of a counter and/or probability indicators to enable a random discard policy. For example, when a random number is generated, such as M, the Mth packet (or every Mth packet) after the number is generated may be dropped. Or, the random number may specify a probability of dropping a packet. The random number may thus be limited to (e.g., hashed into) a certain range of values or probabilities. As another alternative, a random number generator may be used in tandem with multiple regions or thresholds within a packet queue. In this alternative embodiment a programmable value, represented here as N, may be associated with a region or queue threshold. Then, when a traffic indicator reaches that threshold or region, the Nth packet (or every Nth packet) may be dropped until another threshold or boundary is reached. In yet another alternative embodiment of the invention, the probability of dropping a packet is expressed as a binary fraction. A binary fraction consists of a series of bits in which each bit represents one half of the magnitude of its more significant neighbor. For example, a binary fraction may use four digits in one embodiment of the invention. From left to right, the bits may represent 0.5, 0.25, 0.125 and 0.0625, respectively. Thus, a binary fraction of 1010 would be interpreted as indicating a 62.5% probability of dropping a packet (e.g., 50% plus 12.5%). The more positions (e.g., bits) used in a binary fraction, the greater precision that may be attained. In one implementation of this alternative embodiment a separate packet counter is associated with each digit. The counter for the leftmost bit increments at twice the rate of the next counter, which increments twice as fast as the next counter, etc. In other words, when the counter for the most significant (e.g., left) bit increments from 0 to 1 the other counters do not change. When the most significant counter increments again, from 1 back to 0, then the next counter increments from 0 to 1. Likewise, the counter for the third bit does not increment from 0 to 1 until the second counter returns to 0. In summary, the counter for the most significant bit changes (i.e., increments) each time a packet is received. The counter for the next most significant bit maintains each value (i.e., 0 or 1) for two packets before incrementing. Similarly, the counter for the third most significant bit maintains each counter value for four packets before incrementing and the counter for the least significant bit maintains its values for eight packets before incrementing. Each time a packet is received or a counter is incremented the counters are compared to the probability indicator (e.g., the specified binary fraction). In one embodiment the determination of whether a packet is dropped depends upon which of the fraction's bits are equal to one. Illustratively, for each fraction bit equal to one a random packet is dropped if the corresponding counter is equal to one and the counters for any bits of higher significance are equal to zero. Thus for the example fraction 1010, whenever the most significant bit's counter is equal to one a random packet is dropped. In addition, a random packet is also dropped whenever the counter for the third bit is equal to one and the counters for the first two bits are equal to zero. Other suitable mechanisms for specifying and enforcing a probability of dropping a packet received at a network interface may be derived without exceeding the scope of the present invention. As already mentioned, intelligence may be imparted to a random discard policy in order to avoid discarding certain types of packets. In a previous section, methods of parsing a packet received from a network were described. In particular, in a present embodiment of the invention a packet received from a network is parsed before it is placed into a packet queue such as packet queue 1000. During the parsing procedure various information concerning the packet may be gleaned. This information may be used to inject intelligence into a random discard policy. In particular, one or more fields of a packet header may be copied, an originating or destination entity of the packet may be identified, a protocol may be identified, etc. Thus, in various embodiments of the invention certain packets or types of packets may be immune from being discarded. In the embodiment illustrated in FIG. 10, for example, control packets are immune. Control packets often contain information essential to the establishment, re-establishment or maintenance of a communication connection. Dropping a control packet may thus have a more serious and damaging effect than dropping a packet that is not a control packet. In addition, because control packets generally do not contain data, dropping a control packet may save very little space in the packet queue. Many other criteria for immunizing packets are possible. For example, when a packet is parsed according to a procedure described in a previous section, a No_Assist flag or signal may be associated with the packet to indicate whether the packet is compatible with a set of pre-selected communication protocols. Illustratively, if the flag is set to a first value (e.g., one) or the signal is raised, the packet is considered incompatible and is therefore ineligible for certain processing enhancements (e.g., re-assembly of packet data, batch processing of packet headers, load-balancing). Because a packet for which a No_Assist flag is set to the first value may be a packet conforming to an unexpected protocol or unique format, it may be better not to drop such packets. For example, a network manager may want to ensure receipt of all such packets in order to determine whether a parsing procedure should be augmented with the ability to parse additional protocols. Another reason for immunizing a No Assist packet (e.g., packets that are incompatible with a set of selected protocols) from being discarded concerns the reaction to dropping the packet. Because the packet's protocols were not identified, it may not be known how the packet's protocols respond to the loss of a packet. In particular, if the sender of the packet does not lower its transmission rate in response to the dropped packet (e.g., as a form of congestion control), then there is no benefit to dropping it. A packet's flow number may be used to immunize certain packets in another alternative embodiment of the invention. As discussed in a previous section, a network interface may include a flow database and flow database manager to maintain a record of multiple communication flows received by the network interface. It may be efficacious to prevent packets from one or more certain flows from being discarded. Immunized flows may include a flow involving a high-priority network entity, a flow involving a particular application, etc. For example, it may be considered relatively less damaging to discard packets from an animated or streaming graphics application in which a packet, or a few packets, may be lost without seriously affecting the destination entity and the packets may not even need to be retransmitted. In contrast, the consequences may be more severe if a few packets are dropped from a file transfer connection. The packets will likely need to be retransmitted, and the transmitting entity's window may be shrunk as a result - thus decreasing the rate of file transfer. In yet another alternative embodiment of the invention, a probability indicator may comprise a bit mask in which each bit corresponds to a separate, specific flow through the network interface. In particular, the bits may correspond to the flows maintained in the flow database described in a previous section. Although embodiments of the invention discussed thus far in this section involve discarding packets as they arrive at a packet queue, in an alternative embodiment packets may be discarded from within the packet queue. In particular, as the packet queue is filled (e.g., as a traffic indicator reaches pre-defined regions or thresholds), packets already stored in the queue may be discarded at random according to one or more probability indicators. In the embodiment illustrated in FIG. 10, for example, when traffic indicator 1008 reaches a certain threshold, such as the boundary between regions one and two or the end of the queue, packets may be deleted in one or more regions according to related probability indicators. Such probability indicators would likely have different values than those indicated in FIG. 10. In a present embodiment of the invention, probability indicators and/or the specifications (e.g., boundaries) into which a packet queue is partitioned are programmable and may be adjusted by software operating on a host computer (e.g., a device driver). Criteria for immunizing packets may also be programmable. Methods of discarding packets in a network interface or other communication device may thus be altered in accordance with the embodiments described in this section, even during continued operation of such a device. Various other embodiments and criteria for randomly discarding packets and/or applying criteria for the intelligent discard of packets may be derived. FIGS. 11A-1 IB comprise a flow chart demonstrating one method of implementing a policy for randomly discarding packets in a network interface according to the embodiment of the invention substantially similar to the embodiment illustrated in FIG. 10. In this embodiment, a packet is received while packet queue 1000 is not yet full. As one skilled in the will appreciate, this embodiment provides a method of determimng whether to discard the packet. Once packet queue 1000 is full, when another packet is received the network interface generally must drop a packet - either the one just received or one already stored in the queue - in which case the only decision is which packet to drop. In FIG. 11 A, state 1100 is a start state. State 1100 may reflect the initialization of the network interface (and packet queue 1000) or may reflect a point in the operation of the network interface at which one or more parameters or aspects concerning the packet queue and the random discard policy are to be modified. In state 1102, one or more regions are identified in packet queue 1000, perhaps by specifying boundaries such as the 8 KB and 12 KB boundaries depicted in FIG. 10. Although the regions depicted in FIG. 10 fully encompass packet queue 1000 when viewed in unison, regions in an alternative embodiment of the invention may encompass less than the entire queue. In state 1104, one or more probability indicators are assigned and configured. In the illustrated embodiment, one probability indicator is associated with each region. Alternatively, multiple regions may be associated with one probability indicator. Even further, one or more regions may not be explicitly associated with a probability indicator, in which case a default or null probability indicator may be assumed. As described above, a probability indicator may take the form of a multi-bit mask, whereby the number of bits in the mask reflect the range of possible values maintained by a packet counter. In another embodiment of the invention, a probability indicator may take the form of a random number or a threshold value against which a randomly generated number is compared when a decision must be whether to discard a packet. In state 1106, if certain types of packets are to be prevented from being discarded, criteria are expressed to identify the exempt packets. Some packets that may be exempted are control packets, packets conforming to unknown or certain known protocols, packets belonging to a particular network connection or flow, etc. In one embodiment of the invention, no packets are exempt from being discarded. In state 1108, a packet or traffic counter is initialized. As described above, the counter may be incremented, possibly through a limited range of values, when a discardable packet is received for storage in packet queue 1000. The limited range of counter values may correspond to the number of bits in a mask form of a probability indicator. Alternatively, the counter may be configured to increment through a greater range, in which case a counter value may be filtered through a modulus or hash function prior to being compared to a probability indicator as described below. In state 1110, a packet is received from a network and may be processed through one or more modules (e.g., a header parser, an IPP module) prior to its arrival at packet queue 1000. Thus, in state 11 10 the packet is ready to be stored in the packet queue. One or more packets may already be stored in the packet queue and a traffic indicator (e.g., a pointer or index) identifies the level of traffic stored in the queue (e.g., by a storage location and/or region in the queue). In state 1112, it may be determined whether the received packet is discardable. For example, if the random discard policy that is in effect allows for the exemption of some packets from being discarded, in state 1112 it is determined whether the received packet meets any of the exemption criteria. If so, the illustrated procedure continues at state 1122. Otherwise, the procedure continues at state 1114. In state 1114, an active region of packet queue 1000 is identified. In particular, the region of the packet queue to which the queue is presently populated with traffic is determined. The level of traffic stored in the queue depends upon the number and size of packets that have been stored in the queue to await transfer to a host computer. The slower the transfer process, the higher the level of traffic may reach in the queue. Although the level of traffic stored in the queue rises and falls as packets are stored and transferred, the level may be identified at a given time by examining the traffic indicator. The traffic indicator may comprise a pointer identifying the position of the last or next packet to be stored in the queue. Such a pointer may be compared to another pointer that identifies the next packet to be transferred to the host computer in order to reveal how much traffic is stored in the queue. In state 1116, the counter value (e.g., a value between zero and seven in the embodiment of FIG. 10) is compared to the probability indicator associated with the active region. As previously described, the counter is incremented as discardable packets are received at the queue. This comparison is conducted so as to determine whether the received packet should be discarded. As explained above, in the embodiment of FIG. 10 the setting of the probability indicator bit corresponding to the counter value is examined. For example, if the counter has a value of N, then bit number N of the probability indicator mask is examined. If the bit is set to a first state (e.g., one) the packet is to be discarded; otherwise it is not to be discarded. In state 1118, the counter is incremented to reflect the receipt of a discardable packet, whether or not the packet is to be discarded. In the presently discussed embodiment of the invention, if the counter contains its maximum value (e.g., seven) prior to being incremented, incrementing it entails resetting it to its minimum value (e.g., zero). In state 1120, if the packet is to be discarded the illustrated procedure continues at state 1124. Otherwise, the procedure continues at state 1122. In state 1122, the packet is stored in packet queue 1000 and the illustrated procedure ends with end state 1126. In state 1124, the packet is discarded and the illustrated procedure ends with end state 1126. U.S. Patent Application Serial No. 09/259,765, entitled "A High Performance Network Interface" and filed on March 1 , 1999, provides additional details of a network interface in which a present embodiment of the invention may be practiced and is hereby incorporated by reference. Sun, Sun Microsystems, SPARC and Solaris are trademarks or registered trademarks of Sun Microsystems, Incorporated in the United States and other countries. The foregoing descriptions of embodiments of the invention have been presented for purposes of illustration and description only. They are not intended to be exhaustive or to limit the invention to the forms disclosed. Accordingly, the above disclosure is not intended to limit the invention; the scope of the invention is defined by the appended claims.

Claims: What Is Claimed Is: 1. A method of randomly discarding a packet received from a network, comprising: identifying multiple regions in a packet storage device configured to store packets received from a network; maintaining a probability of discarding a packet received from said network; receiving a first packet from a network; and applying said probability to determine whether to discard a packet. 2. The method of claim 1 , further comprising maintaining a packet count, wherein said packet count is incremented when a discardable packet is received from said network. 3. The method of claim 2, wherein a discardable packet is a packet other than a control packet. 4. The method of claim 2, wherein all packets received for storage in said packet storage device are discardable. 5. The method of claim 2, wherein said packet count is repeatedly incrementable through N values. 6. The method of claim 5, wherein: said maintaining a probability comprises maintaining a probability indicator comprising N sub-indicators; and said applying comprises examining an Mth sub-indicator of said probability indicator to determine whether to discard said first packet when said packet count is M, wherein M < N. 7. The method of claim 5, wherein: said maintaining a probability comprises maintaining a probability indicator comprising N sub-indicators; and said applying comprises examining an Mth sub-indicator of said probability indicator to determine whether a packet stored in said packet storage device prior to said receiving is to be discarded when said packet count is M, wherein M < N. 8. The method of claim 6, wherein said Mth sub-indicator comprises a first value if said first packet is to be discarded, and said Mth indicator comprises a second value if said first packet is not to be discarded. 9. The method of claim 1 , wherein said probability comprises a randomly generated value. 10. The method of claim 1 , wherein said probability comprises a first value for comparison to a second value randomly generated when said packet is received for storage in said packet storage device. 11. The method of claim 1 , further comprising maintaining a traffic indicator configured to indicate how full said packet storage device is. 12. The method of claim 11 , wherein said traffic indicator is configured to examine one or more pointers associated with said packet storage device. 13. The method of claim 11 , wherein said traffic indicator identifies one of said multiple regions. 14. The method of claim 1 , wherein said identifying multiple regions comprises identifying a packet storage device threshold separating a first region of said packet storage device from a second region of said packet storage device. 15. The method of claim 14, wherein said maintaining a probability comprises: assigning a first probability to said first region; and assigning a second probability to said second region. 16. The method of claim 1 , wherein said receiving a first packet from a network comprises examining one or more headers of a first packet received from a network. 17. The method of claim 16, wherein said examining comprises determining a characteristic of said packet by extracting a field in one of said headers. 18. The method of claim 1 , wherein said applying said probability comprises comparing said probability to a packet count. 19. The method of claim 18, wherein said probability comprises a set of indicators, and wherein said applying further comprises: discarding said packet if a first indicator of said set of indicators is in a first state, wherein said first indicator is associated with said packet count; and storing said packet in said packet storage device if said first indicator is in a second state. 20. The method of claim 19, wherein said first indicator stores a first value in said first state and said first indicator stores a second value in said second state. 21. A method of randomly discarding a packet received at a communication device from a network, comprising: identifying multiple regions in a packet queue, wherein said packet queue is configured to store packets received from a network; associating a probability indicator with one or more of said multiple regions; maintaining a counter configured to store a packet count, wherein said packet count is repeatedly incrementable through a pre-determined range of packet counts, and wherein said packet count is incremented when a discardable packet is received for storage in said packet queue; maintaining a traffic indicator to determine a level of traffic stored in said packet queue; receiving a packet from a network for storage in said packet queue; determining whether said packet is discardable; identifying a first region of said multiple regions, wherein said first region includes said level of traffic stored in said packet queue; examining said probability indicator to determine whether to discard said packet; and discarding said packet if said probability indicator comprises a first value. 22. The method of claim 21 , wherein said communication device is a network interface. 23. The method of claim 21 , wherein said identifying multiple regions comprises selecting one or more boundaries, wherein each of said one or more boundaries separates one of said multiple regions from another of said multiple regions. 24. The method of claim 21 , wherein said associating a probability indicator comprises associating separate probability indicators with each of said multiple regions. 25. The method of claim 21 , wherein said associating a probability indicator comprises associating separate probability indicators with each of said multiple regions other than an initial region. 26. The method of claim 21 , wherein said determining whether said packet is discardable comprises determining whether said packet satisfies any of a set of criteria for exempting a packet from being discarded. 27. The method of claim 21 , wherein said examining said probability indicator comprises: retrieving said packet count from said count, wherein said packet count is N; and examining a portion of said probability indicator corresponding to said packet count. 28. The method of claim 27, wherein said portion comprises an Nth sub- indicator in said probability indicator, and wherein said packet is to be discarded if said Nth indicator is in a first state. 29. The method of claim 21 , further comprising storing said packet in said packet queue if said probability indicator comprises a second value. 30. A communication device for randomly discarding a packet received from a network, comprising: a packet storage device configured to store packets received from a network, wherein said packet storage device includes multiple regions; a traffic indicator configured to identify a level of packets stored in said packet storage device; and a first probability indicator configured to indicate a probability of randomly dropping a packet when said level of packets is included in a first region of said multiple regions. 31. The communication device of claim 30, wherein said communication device is a network interface. 32. The communication device of claim 30, further comprising a counter configured to store a packet count, wherein said packet count is configured to increment after a packet is received from said network. 33. The communication device of claim 30, further comprising a second probability indicator configured to indicate a probability of randomly dropping a packet when said level of packets is included in a second region of said multiple regions. 34. The communication device of claim 30, wherein said packet storage device is further configured to store packets for transfer to a host computer. 35. The communication device of claim 30, wherein said first probability indicator comprises a randomly generated number for determining whether to drop a packet. 36. The communication device of claim 30, wherein said first probability indicator comprises a first value for comparison to a second value randomly generated when a packet is received from said network. 37. The communication device of claim 30, wherein said first probability indicator comprises a set of sub-indicators, and wherein a first sub-indicator comprises either a first value indicating a first packet received from said network is to be dropped or a second value indicating said first packet is not to be dropped. 38. The communication device of claim 37, further comprising: a counter configured to store a packet count, wherein said packet count is incremented when a packet is received from said network; wherein a packet count stored in said counter when said first packet is received identifies said first sub-indicator. 39. A computer readable storage medium storing instructions that, when executed by a computer, cause the computer to perform a method for randomly discarding a packet from a network, the method comprising: identifying multiple regions in a packet storage device configured to store packets received from a network; maintaining a probability of discarding a packet received from said network; receiving a first packet from a network; and applying said probability to determine whether to discard a packet.

Inventors: CHENG LINDA, GENTRY DENTON, MULLER SHIMON, CHENG, LINDA, GENTRY, DENTON, MULLER, SHIMON
Applicants: SUN MICROSYSTEMS INC, SUN MICROSYSTEMS, INC.
Application year: 2000
Main classification: H04L12
IPC codes: H04L12