Abstract:
In one embodiment, a protocol option is implemented in which a network packet stores a maximum transmission unit size to use for sending data on a path between a sender and a recipient. The stored value is replaced if a receiver of the network packet uses a maximum transmission unit (MTU) smaller than the stored value. The option may be used in an initiation packet sent during a communication initiation handshake to allow both establishment of a communication session as well as determination of an efficient transmission unit size. The initiation packet may be sent with a do-not-fragment option set so that if a receiver does not understand the option, and the packet is too large for the receiver, then receiver will reject the packet and place the sender on notice that a conventional trial-and-error approach is required to determine an allowable MTU.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention generally relates to discovering a network transmission unit size for use in transmitting data between a sender and recipient over a network in which intervening networked machines may have different maximum transmission unit sizes.  
         BACKGROUND  
         [0002]    A network, such as the Internet or other network, may have a variety of networked machines, e.g., routers, gateways, servers, clients, etc. Each networked machine incorporates a network interface card (NIC) (or other communication device) for communicating with other networked machines. Network communication essentially involves transmission of small bundles of data, commonly referred to as “datagrams” or “packets.” Unfortunately, there is no standard maximum datagram size, and therefore different machines on a route between communication endpoints (hereafter “path”) may utilize different communication mediums (e.g., Ethernet, 802.11) supporting different maximum datagram sizes.  
           [0003]    Consequently, communication protocols are designed to break a particular datagram into smaller chunks to accommodate the largest datagram size supported by machines on a path. For example, the Internet Protocol (IP) provides for “IP fragmentation,” which refers to the process of breaking IP packets into fragments to satisfy an underlying network interface&#39;s Maximum Transmission Unit (MTU). The MTU is the largest physical packet size, e.g., datagram size, supported by a network interface type. As data passes through machines on a network, any received packet larger than a particular machine&#39;s MTU is fragmented into smaller packets before being sent on.  
           [0004]    Thus, when a sending machine sends a packet to a receiving machine, ideally the MTU used by a sender is no larger than the largest MTU supported by all intervening machines on the path. If the packet is larger than one of the intervening MTUs, as discussed above, it will be fragmented. This fragmentation slows down transmission speed, and because of the best-effort nature of IP, increases the likelihood of the entire datagram being dropped. FIG. 1 illustrates packet fragmentation. A packet is sent  100  based on a sending machine&#39;s local MTU. The packet may be received  102  by one or more intervening machines  104  on the path, and the packet&#39;s associated length is compared against the intervening recipient&#39;s local MTU to determine if  106  the received packet&#39;s MTU is larger than the local MTU. If so, the packet is fragmented  108  to accommodate the local MTU. The packet fragments, or the received packet if fragmentation was unnecessary, are forwarded  110  to the recipient. The packet(s) are received  114 , and the originally sent packet is reconstructed  116  from the fragments if necessary.  
           [0005]    Such fragmentation can have a detrimental effect upon protocols, such as the Transmission Control Protocol (TCP), built on top of IP. A single TCP segment may require encoding as multiple IP packets (the number of packets depends on a sending machine&#39;s MTU). If the sending machine MTU is larger than the underlying network&#39;s MTU, as discussed above, the packets for the TCP segment are fragmented. A recipient must successfully receive all IP fragments in order to reconstruct the TCP segment. Even if only a single packet fragment is lost, the receiver is unable to reconstruct the datagram and the entire datagram is discarded, the TCP sender eventually times out waiting for an acknowledgment for the discarded TCP segment, resulting in retransmission of the entire TCP segment.  
           [0006]    To minimize retransmission risk, some TCP implementations attempt to identify the largest MTU supported along the path. Typically, a trial-and-error approach is taken. As illustrated in FIG. 2, a packet size is selected  200 ; initially the value is typically the sender&#39;s default/desired MTU. A packet is sent  202  with the IP packet “Don&#39;t Fragment” (DF) bit set. A router or other intermediary machine between the sender and recipient receives  204  the packet with the DF bit set. A comparison is made between the packet length and the receiver&#39;s local MTU. If  206  the local MTU is less than the packet length, the receiver cannot forward on the packet without fragmenting it.  
           [0007]    Since the DF bit is set, the receiver drops  208  the packet and sends the sender an Internet Control Message Protocol (ICMP) packet alerting the sender that the datagram was dropped. Sometimes the receiver, along with the ICMP, will report the MTU that should be used. Often, however, only the ICMP is sent, requiring the sender to guess what MTU to use. Thus, for each intermediary between a sender and recipient, processing loops  210  with the sender incrementally selecting  200  another MTU size. The sending host, using the new selected MTU, sends the TCP segment again, which of course may lead to a subsequent intermediary sending another ICMP message. Assuming the current receiver is not the intended recipient, if  206  the sender MTU was not larger than the local MTU, then the packet is forwarded  212  on towards the recipient, where it is received  204  and processed  206 - 210  as discussed above until the packet is ultimately received by the intended recipient.  
           [0008]    This incremental approach to discovering the largest path MTU may result in the sender sending the initial TCP segment numerous times before it finally reaches the intended destination. For short-lived connections, polling can add considerable overhead to the connection because of the retransmissions. This can be especially important for TCP-based applications that wish to be able to process short transactions at a very high rate. For further information, the reader is directed towards Internet Engineering Task Force (IETF) Request for Comments (RFC)  1191 . 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    The features and advantages of the present invention will become apparent from the following detailed description of the present invention in which:  
         [0010]    [0010]FIG. 1 illustrates packet fragmentation for data sent over a network.  
         [0011]    [0011]FIG. 2 illustrates a prior art trial-and-error approach for determining the smallest MTU along a network path.  
         [0012]    [0012]FIG. 3 illustrates according to one embodiment discovering the largest supported MTU for a path between communication endpoints.  
         [0013]    [0013]FIG. 4 illustrates an application of the FIG. 3 embodiment to the TCP/IP network communication protocol.  
         [0014]    [0014]FIG. 5 illustrates a suitable computing environment in which certain aspects of the invention may be implemented.  
     
    
     DETAILED DESCRIPTION  
       [0015]    [0015]FIG. 3 illustrates, according to one embodiment, discovering the largest supported MTU for a path between communication endpoints without relying on the incremental polling method currently used in the prior art. Unless indicated otherwise explicitly or implicitly by context, the terms “sender” and “recipient” will be used to refer to communication endpoints, and the term “receiver” will refer to an intermediary machine along the network path between the communication endpoints.  
         [0016]    The sender identifies  300  the largest sender packet (LSP) size supported by the sender&#39;s hardware and/or software. As will be discussed in FIG. 4, in a TCP/IP environment, the LSP may be the sender&#39;s MTU; however, it will be appreciated that the FIG. 3 embodiment is not limited to TCP/IP implementations. A packet associated with the sender&#39;s LSP is sent  302  to the recipient. In one embodiment, the LSP is stored within the packet in a header portion. In another embodiment, the LSP is stored in the packet as data payload. One or more intervening receivers along the network path receives the packet and compares  304  the packet&#39;s associate LSP with the receiver&#39;s largest local packet (LLP) size. As with the sender&#39;s LSP, in a TCP/IP implementation, the LLP may be the receiver&#39;s MTU.  
         [0017]    If  306  the LSP is larger than the LLP, then the receiver&#39;s LLP is associated  308  with the packet. For example, if the LSP is stored within the packet, then the appropriate portion of the packet is rewritten with the LLP data. If  306  the LSP is not larger than the LLP, or after associating  308  the receiver&#39;s LLP with the packet, the packet is updated  310  as appropriate, e.g., the packet&#39;s checksum may be recomputed to accommodate associating  308  the receiver&#39;s LLP, the Time To Live (TTL) for the packet may be revised, or some other routine update applied to the packet. After updating the packet, it is sent  312  to the recipient. Operations  304 - 312  are illustrated within box  314  to illustrate that these operations may loop  316  among multiple intervening receivers receipt  318  by the intended recipient.  
         [0018]    After receiving  318  the packet, which may now have an associated LSP size smaller than the sender&#39;s LSP originally associated with the packet, the currently associated LSP is compared  320  against the recipient&#39;s LSP. The recipient is to reply to the sender with a packet identifying the desired LSP to be used by the sender. If  322  the LSP associated with the packet is larger than the recipient&#39;s LLP, the recipient&#39;s LLP is the smallest LLP size of the machines on the path, and therefore the desired LSP is set  326  to be the recipient&#39;s LLP. A response packet is sent  328  to the sender identifying the recipients LLP as the LSP that should be used by the sender to communicate with the recipient. If  322  the LLP is larger than the LSP, the recipient&#39;s LLP is either larger than a previously encountered LLP, or larger than the sender&#39;s LSP. Therefore, the desired LSP is set  326  to be the currently associated LSP, and the response packet sent  328  accordingly.  
         [0019]    [0019]FIG. 4 illustrates an application of the FIG. 3 embodiment to the TCP/IP network communication protocol. In this embodiment, it is assumed communication is in accord with the TCP/IP communication protocol (or equivalent), and a new IP option (Record Maximum Transmission Unit (MTU)), and an existing TCP option (Maximum Segment Size (MSS)) are used to discover the MTU the sender should use without the sender having to engage in the incremental polling method currently used.  
         [0020]    As is known in the art, to initiate a TCP/IP connection, the communication endpoints engage in a “three-way handshake” to establish the connection. First, the sender sends a TCP/IP “SYN” packet, e.g., a packet with the SYN flag set, to the recipient endpoint. The recipient responds with a packet with a SYN/ACK packet, e.g., a packet with the SYN and Acknowledgement (ACK) flags set. The sender responds with an ACK packet. As discussed above, it is desirable to optimize the MTU used by the sender. To avoid the trial-and-error MTU discovery discussed above, the illustrated embodiment takes advantage of the SYN packet generally traveling along the same network path over which communication between the endpoints will occur.  
         [0021]    When the SYN packet is sent  400  to the recipient, in one embodiment, the packet is sent with a new IP header option, the “Record MTU” option, set to the sender&#39;s desired MTU, typically the largest MTU supported by the sender&#39;s outgoing network interface. In this embodiment, the Record MTU option is a field added to the IP options field, e.g., a new 6-byte type-length-value (TLV) field. Per current TCP/IP configurations, the type and length are one byte, and the MTU field is four bytes. It will be appreciated that the length field is unnecessary, but in order to conform to the IETF RFC  1812 , the length field must be present for routers that do not understand the new option (unknown options are ignored).  
         [0022]    As discussed above with respect to FIG. 3, the packet may be received by multiple intermediary receivers, collectively referenced by box  402  and the loop-back  404 . When the packet is received  406  by a particular intermediary, for example, a router between the sender and destination recipient, the Record MTU option is processed like the existing IP Record Route option (see, e.g., RFC  791 ). When a receiver that supports the Record MTU option detects the presence of the option, it compares the receiver&#39;s MTU, e.g., the MTU of the receiver&#39;s outgoing network interface, to that of the path MTU currently stored in the packet. If  408  the current path MTU is greater than the receiver&#39;s, the receiver replaces  410  the MTU field in the Record MTU option with the receiver&#39;s MTU. Thus, the smallest MTU supported by the path is kept. Per RFC  1812 , receivers that do not understand the Record MTU option will ignore it. If  408  the receiver&#39;s MTU exceeds the stored path MTU, or after replacing the stored path MTU, the packet is updated  412 , if necessary, e.g., the packet checksum is recomputed, the Time To Live (TTL) decremented, etc.  
         [0023]    The packet is then sent on towards the destination recipient; as noted above, processing may loop  404  through multiple intermediary receivers before the SYN packet is received  416  by the recipient. For sending the SYN/ACK to the sender, assuming the recipient understands the Record MTU option, the recipient compares  418  the path MTU with the recipient&#39;s MTU. If  420  the path MTU is less than the recipient&#39;s MTU, the recipient sets the Maximum Segment Size (MSS) option (see, e.g., RFC  793 ) for the SYN/ACK packet to the path MTU, e.g., the recipient reports the smallest MTU supported by the machines of the path between the communication endpoints. If  420  the recipient&#39;s MTU is less than the path MTU, the recipient sets the MSS option for the SYN/ACK packet to be the recipient&#39;s MTU. The SYN/ACK response packet is then sent  426  back to the sender.  
         [0024]    In conventional TCP implementations, if the SYN/ACK response does not contain the MSS option, then the sender must assume a default MSS size. However, if the MSS option is set, then the sender must not send a segment larger than the MSS. Thus, as described above, a host can determine an efficient path MTU for segments it will send to the recipient. It will be appreciated that the recipient may utilize the Record MTU option to determine the best MTU for communicating with the sender. Such checking by the recipient may be helpful since routes are not necessarily symmetric. It will also be appreciated that any packet may be sent with the new Record MTU option set. However, unless a long-loved connection is expected, the largest supported path MTU may be efficiently discovered during connection negotiation as discussed above.  
         [0025]    In FIG. 4, the intervening receivers and destination recipient are assumed to understand the new Record MTU option. However, this is not necessarily the case. In one embodiment, not illustrated, the sender sets the Don&#39;t Fragment (DF) flag for a sent TCP segment. Thus, if an intervening receiver, or the destination recipient does not understand the new Record MTU option, and the MTU for the segment is too large, then as discussed above with respect to FIG. 2, the segment is dropped and an ICMP error message sent to the sender. This would allow for transparent fallback to traditional polling or other path MTU discovery techniques.  
         [0026]    [0026]FIG. 5 and the following discussion are intended to provide a brief, general description of a suitable machine in which certain aspects of the illustrated invention may be implemented. As used herein, the term “machine” includes a single machine, such as a computer, handheld device, transportation device, etc., or a system of communicatively coupled machines or devices.  
         [0027]    Typically, a machine  500  includes a system bus  502  to which is attached processors  504 , a memory  506 , e.g., random access memory (RAM), read-only memory (ROM), or other state preserving medium, storage devices  508 , a video interface  510 , and input/output interface ports  512 . The machine may be controlled, at least in part, by input from conventional input devices, such as keyboards, mice, etc., as well as by directives received from another machine, interaction with a virtual reality (VR) environment, biometric feedback, or other input source or signal.  
         [0028]    The machine may include embedded controllers, such as programmable or non-programmable logic devices or arrays, Application Specific Integrated Circuits, embedded computers, smart cards, and the like. The machine may utilize connections to one or more remote machines  514 ,  516 , such as through a network interface  518 , modem  520 , or other communicative coupling. Machines may be interconnected by way of a physical and/or logical network  522 , such as an intranet, the Internet, local area networks, and wide area networks. One skilled in the art will appreciated that network  522  may utilize various wired and/or wireless short range or long range carriers and protocols, including radio frequency (RF), satellite, microwave, Institute of Electrical and Electronics Engineers (IEEE) 802.11, Bluetooth, optical, infrared, cable, laser, etc.  
         [0029]    The invention may be described by reference to or in conjunction with associated data including functions, procedures, data structures, application programs, etc. which when accessed by a machine results in the machine performing tasks or defining abstract data types or low-level hardware contexts. Associated data may be stored in, for example, volatile and/or non-volatile memory  506 , or in storage devices  508  and their associated storage media, including hard-drives, floppy-disks, optical storage, tapes, flash memory, memory sticks, digital video disks, biological storage, etc. Associated data may be delivered over transmission environments, including network  522 , in the form of packets, serial data, parallel data, propagated signals, etc., and may be used in a compressed or encrypted format. Associated data may be used in a distributed environment, and stored locally and/or remotely for access by single or multiprocessor machines, portable computers, handheld devices, e.g., Personal Digital Assistants (PDAs), cellular telephones, tablets, etc.  
         [0030]    Thus, for example, with respect to the illustrated embodiments, assuming machine  500  embodies a FIG. 4 sender machine establishing a communication session with a destination recipient, then remote machines  514 ,  516  may respectively be two intermediary receivers on a network path  524  between the communication endpoints. It will be appreciated that remote machines  514 ,  516  may be configured like machine  500 , and therefore include many or all of the elements discussed for machine.  
         [0031]    Having described and illustrated the principles of the invention with reference to illustrated embodiments, it will be recognized that the illustrated embodiments can be modified in arrangement and detail without departing from such principles. And, though the foregoing discussion has focused on particular embodiments, other configurations are contemplated. In particular, even though expressions such as “in one embodiment,” “in another embodiment,” or the like are used herein, these phrases are meant to generally reference embodiment possibilities, and are not intended to limit the invention to particular embodiment configurations. As used herein, these terms may reference the same or different embodiments that are combinable into other embodiments.  
         [0032]    Consequently, in view of the wide variety of permutations to the embodiments described herein, this detailed description is intended to be illustrative only, and should not be taken as limiting the scope of the invention. What is claimed as the invention, therefore, is all such modifications as may come within the scope and spirit of the following claims and equivalents thereto.