Abstract:
This document describes, among other things, a router providing service-enabled multicast forwarding for a flow-based architecture. Services are enabled using inbound/outbound policies from Access Control Lists (ACLs). In a first mode, per-flow services are not enabled, and flows are aggregated for efficiency. In a second mode, per-flow services are enabled, and control information for an outbound interface (OIF) is shared across all multicast flows that do not have per-flow service enabled. A flow management scheme for these modes allows dynamic multicast membership updates while reducing disruption of active flows. This supports a large multicast OIF list and a high rate of multicast “Joins” and “Leaves”. This provides scalability and performance, such as desired for deploying multicast for broadband subscriber applications.

Description:
TECHNICAL FIELD  
       [0001]     This patent document pertains generally to communications, and more particularly, but not by way of limitation, to network switches and systems and methods for multicast internet protocol (IP) forwarding.  
       BACKGROUND  
       [0002]     The use of computer or communications networks, including Local Area Networks (LANs), Wide-Area Networks (WANs), and the Internet continues to grow at ever increasing rates. Each day, more and more computer systems or communications devices are becoming interconnected in such wired or wireless networks, which typically communicate data in packets. This has created a need for high performance network switches, such as for use by network service providers. Many such switches comprise multiple modules, with many data flows between the modules themselves and between the interfaces to external networks. A data flow is sometimes called an “IP flow,” which refers to a stream of packets that enter and exit the same set of interfaces. The packets of a particular IP flow have the same values in the IP packet header for the following six attributes of the IP packet header: (1) Source IP Address, (2) Source L4 Port, (3) Type of Service (TOS), (4) Destination IP Address, (5) Destination L4 Port, and (6) Protocol.  
         [0003]     In some cases, the network switch modules, including the processors residing in the modules, can be partitioned into virtual routers (VRs), that is, software running on the processors that emulates the functioning of an individual physical hardware router. As a result of the combination of hundreds of thousands of data flows for the virtual routers in these network switches, there is a need for efficiently processing packet data flows, and for controlling the resources consumed within the network switch.  
         [0004]     As broadband network access becomes more available, individual subscribers of network service providers have more available options for different services and service levels. Even the same subscriber may have different service needs at different times. As an illustrative example, a first subscriber may desire high definition television (HDTV) service over a network. A second subscriber may desire mobile telephone service over the network. The first subscriber may occasionally desire video-on-demand (VOD). The second subscriber may need to switch between voice communication and high-speed digital data communication.  
         [0005]     A “unicast” communication typically refers to a communication from a single source device to a single destination device over a network. By contrast, a “multicast” communication typically refers to a communication to a group of destination devices from one or more source devices. Multicast packet forwarding raises additional complexity because of the many destination devices. Many existing router devices will be unable to provide the desired scalability to accommodate such additional destination devices. This is particularly true when each individual data flow may require “per-flow” services for the multicast traffic. Allocating resources efficiently for a large number of multicast data flows is a challenging problem. Moreover, multicast broadcasting of content presents additional complexity because individual users may join or leave a particular multicast group at will and often. Such “channel surfing” creates an additional burden for keeping track of the participants of a multicast group so that the content can be routed appropriately.  
       SUMMARY  
       [0006]     The present inventors have recognized a need in the art for multicast packet forwarding that is scalable, for example, in terms of the number of subscribers or in terms of the number of flows. Moreover, the present inventors have also recognized a need in the art for multicast packet forwarding that can efficiently accommodate users joining or leaving a multicast group without imposing substantial increased overhead that might otherwise bog down the rate at which content can be multicast. Still further, the present inventors have recognized a need for accommodating the same or different subscriber requiring different services or service levels in a scalable and efficient manner. The present inventors have also recognized a need for improved or optimized use of resources.  
         [0007]     This document discusses, among other things, a router providing multicast packet forwarding. Each multicast session includes an identifier, such as a {Source, Group} pair defining one or more flows of the multicast session, and the participants in the multicast group. Each multicast session also includes a first pointer to a chain of one or more blocks of second pointers. Each second pointer corresponds to an outbound interface (OIF) participating in the multicast session. Each second pointer points to a transmit control block (TCB) that includes information about which OIF from which a replicated packet should be transmitted and what processing associated with that replicated packet. Each TCB can be shared by multiple second pointers, for example, from different multicast sessions. Multicast flows not requiring specific services can share a default TCB. The blocks of second pointers are independently updateable (such as when a participant joins or leaves a multicast session) to permit other blocks to be used concurrent to such updating. Advantages include one or more of scalability, extensibility, dynamic allocation, and dynamic updating that reduces the likelihood of interruptions to multicast transmission.  
         [0008]     In one example, the present systems and methods includes the following machine-assisted method of multicast forwarding. A first pointer is assigned to each of one of one or more multicast sessions. The first pointer points to a chain of one or more blocks of second pointers. Each second pointer corresponds to an outbound interface (OIF) participating in the multicast session defined by the first pointer. One of one or more transmit control blocks (TCBs) is assigned to each second pointer. Each TCB identifies the OIF out which packets of the multicast session are transmitted. Packets of a first multicast session are received. The packet of the first multicast session is replicated for each OIF participating in the multicast session. The replicated packet of the first multicast session is transmitted out from the OIF participating in the multicast session. This typically includes using the TCB, such as to determine the OIF participating in the multicast session.  
         [0009]     In another example, the present systems and methods includes a system comprising at least one router. The router comprises at least one virtual router. The at least one virtual router comprises virtual interfaces (VIs) and a packet forwarding engine (PFE). The packet forwarding engine includes the following: an identifier for each one of one or more multicast sessions being handled by the router; a first pointer for each multicast session; at least one chain of one or more blocks of second pointers, in which each first pointer points to one chain of one or more blocks of second pointers, and in which each second pointer corresponds to an outbound interface (OIF) participating in the multicast session defined by the first pointer; one or more transmit control blocks (TCBs), in which each second pointer points to one of the TCBs, in which each TCB the OIF out which packets of the multicast session are transmitted from the router; and a direct memory access (DMA) engine that replicates a packet of a multicast session for each OIF participating in the multicast session, and in which the replicated packet is forwarded to the OIF according to the TCB defined by the second pointer.  
         [0010]     In another example, the present systems and methods include a system comprising at least one router. The router comprises at least one subrouter. The at least one subrouter comprises input/output (I/O) interfaces and a packet forwarding engine (PFE). The packet forwarding engine includes: an identifier for each one of one or more multicast sessions being handled by the router; a first pointer for each multicast session; at least one chain of one or more blocks of second pointers, in which each first pointer points to one chain of one or more blocks of second pointers, and in which each second pointer corresponds to an outbound interface (OIF) participating in the multicast session defined by the first pointer; one or more transmit control blocks (TCBs), in which each second pointer points to one of the TCBs, in which each TCB identifies the OIF out which packets of the multicast session are transmitted from the router; and a direct memory access (DMA) engine that replicates a packet of a multicast session for each OIF participating in the multicast session, and in which the replicated packet is forwarded to the OIF according to the TCB defined by the second pointer.  
         [0011]     In another example, the present systems and methods include a machine-assisted method of managing multicast IP flows. In this example, an IP flow is identified, such as by using information from a packet header. For any newly identified IP flow, if flow-specific (also referred to as “per-flow”) services are required, then at least one new first transmit/control block (TCB) is created for that newly identified IP flow. Otherwise a default second TCB is used for the newly identified flow. The first TCB includes one or more attributes relating to flow-specific services required by the newly identified flow. The second TCB excludes any attributes relating to flow-specific services required by the newly identified flow. The second TCB includes one or more attributes related to a virtual interface (VI) serving as an outbound interface (OIF) for the newly identified IP flow.  
         [0012]     In another example, the present systems and methods include a machine-assisted multimode multicast forwarding protocol. The protocol includes a first mode, in which each flow of a multicast session uses a default transmit/control block (TCB) that excludes any attributes relating to flow-specific services. The protocol also includes a second mode. In the second mode, each flow of a multicast session uses only one of (1) a TCB that includes at least one attribute relating to a flow-specific service; and (2) a TCB that excludes any attributes relating to flow-specific services, and wherein the TCB that excludes any attributes relating to flow-specific services is shared across all flows without flow-specific services enabled that are present in the same or a different multicast session.  
         [0013]     The following detailed description, which includes the accompanying drawings, describes, among other things, systems, devices, and methods addressing one or more of such needs, or other needs in the art, or providing other advantages that will be better understood in the context of this entire document, including the appended claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     In the drawings, which are not necessarily drawn to scale, like numerals describe substantially similar components throughout the several views. Like numerals having different letter suffixes represent different instances of substantially similar components. The drawings illustrate generally, by way of example, but not by way of limitation, various embodiments discussed in the present document.  
         [0015]      FIG. 1  is a block diagram of one example of an operating environment for the present system and methods.  
         [0016]      FIG. 2  is a block diagram of one example of a Virtual Router (VR).  
         [0017]      FIG. 3  is a block diagram of one example of a Packet Forwarding Engine (PFE) and a main memory of a VR.  
         [0018]      FIG. 4  is a flow chart of one example of a multicast internet protocol (IP) packet forwarding method such as can be performed, for example, by using the PFE and the main memory of  FIG. 3 .  
         [0019]      FIG. 5  is a block diagram of one example of an OIF module and multicast TCB module for a set of multicast sessions.  
         [0020]      FIG. 6  is a flow chart of one example of a process that is invoked if multicast packet forwarding is invoked in  FIG. 4 .  
         [0021]      FIG. 7  is a flow chart of one example of packet retrieval and replication.  
         [0022]      FIG. 8  is a flow chart of one example of processing by an egress module.  
         [0023]      FIG. 9  is a flow chart of one example of more detail of acts included in the multicast forwarding. 
     
    
     DETAILED DESCRIPTION  
       [0024]     The following detailed description includes references to the accompanying drawings, which form a part of the detailed description. The drawings show, by way of illustration, specific embodiments in which the invention may be practiced. These embodiments, which are also referred to herein as “examples,” are described in enough detail to enable those skilled in the art to practice the invention. The embodiments may be combined, other embodiments may be utilized, or structural, logical and electrical changes may be made without departing from the scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims and their equivalents.  
         [0025]     In this document, the terms “a” or “an” are used, as is common in patent documents, to include one or more than one. In this document, the term “or” is used to refer to a nonexclusive or, unless otherwise indicated. Furthermore, all publications, patents, and patent documents referred to in this document are incorporated by reference herein in their entirety, as though individually incorporated by reference. In the event of inconsistent usages between this document and those documents so incorporated by reference, the usage in the incorporated reference(s) should be considered supplementary to that of this document; for irreconcilable inconsistencies, the usage in this document controls.  
         [0026]     Some portions of the following detailed description are presented in terms of algorithms and symbolic representations of operations on data bits within a computer memory. These algorithmic descriptions and representations are the ways used by those skilled in the data processing arts to most effectively convey the substance of their work to others skilled in the art. An algorithm includes a self-consistent sequence of steps leading to a desired result. The steps are those requiring physical manipulations of physical quantities. Usually, though not necessarily, these quantities take the form of electrical or magnetic signals capable of being stored, transferred, combined, compared, and otherwise manipulated. It has proven convenient at times, principally for reasons of common usage, to refer to these signals as bits, values, elements, symbols, characters, terms, numbers, or the like. It should be borne in mind, however, that all of these and similar terms are to be associated with the appropriate physical quantities and are merely convenient labels applied to these quantities. Unless specifically stated otherwise as apparent from the following discussions, terms such as “processing” or “computing” or “calculating” or “determining” or “displaying” or the like, refer to the action and processes of a computer system, or similar computing device, that manipulates and transforms data represented as physical (e.g., electronic) quantities within the computer system&#39;s registers and memories into other data similarly represented as physical quantities within the computer system memories or registers or other such information storage, transmission or display devices.  
         [0027]      FIG. 1  is a block diagram of one example of an operating environment for the present system and methods. In the example of  FIG. 1 , a system  100  typically includes personal computers (PCs)  102  that are respectively connected to modems  106 . The modems  106  are typically respectively connected to a digital subscriber line access module (DSLAM)  116 . The DSLAM  116  multiplexes signals from the modems  106  onto the Internet Protocol (IP) network  118 . The IP network  118  is typically connected to a router box  114  that includes virtual routers (VRs)  128 . The router box  114  is typically connected to the Internet  112 . The router box  114  is also typically connected to a dynamic host configuration protocol (DHCP) server  120 , a web portal  122 , a RADIUS server  124 , and a control server  126 .  
         [0028]     Although, in this example, the router  114  includes three VRs  128 , other examples call for any number of VRs  128 . In one example, one or more of the VRs  128  can establish subscriber connections, such as to users of the PCs  102 . When establishing such connections, the VRs  128  can use the DHCP server  120  for assigning IP network addresses to the PCs  102 . The VRs  128  can use the RADIUS server  124  to authenticate subscribers. After authenticating subscribers, the VRs  128  can configure subscriber connections according to service profiles, which refer to subscriber-specific services that individual subscribers receive during connections. In one example, the VRs  128  can receive service profiles information from the control server  126  or the RADIUS server  224 .  
         [0029]     After the VRs  128  establish subscriber connections, they typically provide access to the web portal  122 , where users can select new services. Additionally, after establishing subscriber connections, the VRs  128  typically process and forward packets over the IP network  118  and the Internet  112 . Although  FIG. 1  illustrates an example in which the users accessing the Internet  112  via PCs, this is merely an illustrative example. In other examples, the individual users may access the Internet  112  or other computer or communications network wirelessly, such as by using a 3rd Generation (3G) or other mobile phone or other handheld or portable device, or by using a laptop or other portable computing device with Wireless Fidelity (WiFi) (e.g., using IEEE 802.11b wireless networking) capability or the like. In still other examples, the individual users may access the Internet  112  or other communications or computer network using an Ethernet connection, a Very High bit rate Digital Subscriber Line (VHDSL) connection, a Fiber To The Premises (FTTP) connection, or a cable TV line or like connection, or the like. Thus, the present systems and methods are not limited to any particular devices or techniques for accessing the Internet  112 , such as through the router box  114 . The exemplary system  100  typically provides network services to thousands of subscribers. Each subscriber can receive a particular set of services upon establishing a connection with the system  100 .  
         [0030]      FIG. 2  is a block diagram of one example of a VR  128 . In this example, the VR  128  includes a packet forwarding engine (PFE)  206 , and one or more virtual interfaces (VIs)  208  from a source or to a destination. A VI over which multicast packets are forwarded is sometimes referred to as an Outbound Interface (OIF). Different services can be applied to multicast packet forwarding traffic as well as unicast packet forwarding traffic. Which services are applied to a particular packet are determined, in one example, by an inbound policy or an outbound policy associated with a particular VI  208 . In one example, the packet header (e.g., one or more of the above-described six packet header attributes defining an IP flow) is examined (e.g., such as by comparing such attribute(s) to one or more matching criteria in an access control list (ACL)) to determine whether any services should be applied to the packet and, if so, which services should be applied.  
         [0031]      FIG. 3  is a block diagram of one example of a Packet Forwarding Engine (PFE)  300  and a main memory  302  of a VR  128 . In this example, PFE  300  includes an ingress module  304 , an egress module  306 , a PFE memory  308 , a Direct Memory Access (DMA) engine  310 , a packet input interface  312 , a packet output interface  314 , and a main memory interface  316 . In this example, the ingress module  304  includes an ingress rate limit module  318 , an ingress statistics module  320 , a flow classification module  322 , and a multicast forwarding module  324 . In this example, the PFE memory  308  includes a Flow Control Block (FCB)  326 , a multicast block  328 , an Outbound InterFace (OIF) module  330 , a default Transmit Control Block (TCB)  332 , a multicast TCB module  334 , a metering block  336 , and a statistics block  338 . In this example, the egress module  306  includes a TCB processing module  340 , a header transform module  342 , an egress rate limit module  344 , and an egress statistics module  346 .  
         [0032]      FIG. 4  is a flow chart of one example of a multicast internet protocol (IP) packet forwarding method such as can be performed, for example, by using the PFE  300  and main memory  302  of  FIG. 3 . At  400 , a packet is received at the packet input interface  312  on a particular virtual interface (VI) of a particular VR in the router box  114 . When a packet is received at  400 , it is not known whether the packet is part of a multicast data flow or a unicast data flow. At  402 , in one example, the ingress rate limit module  318  performs a rate limiting function to control a packet ingress data rate through the ingress module  304 . At  404 , in one example, the ingress statistics module  320  computes packet ingress statistics, such as packet count, byte count, etc. Such ingress statistics may be important for managing subscriber service levels, among other things.  
         [0033]     At  406 , in one example, the flow classification module  322  is used to classify the data flow, for example, as a unicast flow or a multicast flow. The flow classification module  322  typically uses a predefined portion of the packet header to classify the data flow, and to identify the particular FCB associated with the flow. For example, the “destination address” portion of the packet header is used to identify the packet as a multicast packet. In one example, in a first mode (sometimes referred to as a “strict-optimized mode”), the data flow classification uses the source IP address and the destination IP address portions of the packet header to classify the data flow. In a second mode (sometimes referred to as a “adaptive-optimized mode”), in which subscriber-specific services are needed, additional portions of the packet header are used to further classify the data flow in accordance with the appropriate services.  
         [0034]     In one example, the flow classification at  406  uses the information extracted from the packet header to look up a corresponding FCB entry in FCB  326 . If the data flow is a multicast data flow then, in one example, the corresponding FCB entry will have a “multicast” flag set, and a “forwarding action” field of the FCB entry will indicate that hardware forwarding of packets is to be used for the multicast data flow. At  408 , if the classification indicates a multicast data flow, then, at  410 , multicast packet forwarding is invoked. Otherwise, at  412 , unicast packet forwarding is invoked.  
         [0035]     Each FCB entry in FCB  326  includes information identifying a particular multicast session. Each multicast session is defined by a {Source, Group} pair, which is sometimes referred to as an {S, G} pair. The Source field of the {S, G} pair defines the source of the multicast transmission. In one example, this is a single multicast transmission source. In another example, there are multiple (e.g., redundant) transmission sources for the same multicast transmission. The Group field of the {S, G} pair defines a group corresponding to the multicast session. In one example, the group can be conceptualized as a “channel” of content. There may be one recipient or a very large number of recipients of the content. Such recipients of the multicast content can join or leave the Group at will, such as by issuing the appropriate Internet Group Management Protocol (IGMP) request or using one or more other protocols. Thus, scalability and the ability to easily update the Group are desirable qualities of the present multicast forwarding systems and methods.  
         [0036]     Since each multicast session can have multiple IP flows associated with that particular multicast session, there can be multiple FCBs associated with the same {S, G}, where each FCB corresponds to one of these IP flows, and the {S, G} defines the particular multicast session. This may be true, for example, in the adaptive-optimized mode case, where because of the different services levels needed, there are different IP flows associated with the same multicast session.  
         [0037]      FIG. 5  is a block diagram of one example of an OIF module  330  and multicast TCB module  334  for a set of multicast sessions defined by respective {S, G} pairs {S, G} 1  through {S, G} n . The {S, G} pair  500  of a particular multicast session includes a first pointer  501  that points to a dynamically allocated set of OIF Blocks  502 . The particular number of OIF Blocks  502  depends on how many OIFs are then participating in that multicast session. For a particular multicast session, each OIF block  502  points to a subsequent OIF block  502  (with the exception of the last OIF block  502  in this conceptual “chain” of OIF blocks).  
         [0038]     Each OIF block includes a reasonably small number of slots  503  for storing corresponding second pointers  504  to a TCB  506  for a particular OIF. The example of  FIG. 5  illustrates eight slots  503  per OIF block  502 , each slot for storing a corresponding second pointer  504  to a TCB  506 . Another example includes six second pointer slots  503  per OIF block  502 . Each second pointer  504  points to a particular TCB  506  for a particular OIF, which may service one or more users participating in the corresponding multicast session. Each OIF block  502  typically has the same number of second pointer slots  503  as every other OIF block  502 , however, the number of OIF blocks  502  can vary between different {S, G} pairs, or even for the same {S, G} pair, such as at different points in time when different numbers of OIFs are part of that particular multicast session. More particularly, as OIFs are added or removed from a multicast session (such as may happen when users join or leave the multicast session) corresponding second pointers  504  are added or removed, respectively. If needed, additional OIF blocks  502  are added or removed, such as to accommodate the addition or removal of the second pointers  504 . Using the present systems and methods, dynamically adding or removing such OIF blocks  502  as needed is easy because, among other things, each multicast session includes OIF blocks  502  that are chained together by third pointers  505  from one OIF block  502  to another OIF block  502  (except for the last OIF block  502  in the chain). When a user joins or leaves a multicast session under circumstances that require adding or removing an OIF to that multicast session, the OIF list can be updated by simply updating a single OIF block  502 , during which time the other OIF blocks  502  in that chain are still available and usable for performing multicast forwarding. Although  FIG. 5  illustrates a typical example in which each multicast session (defined by a particular {S, G} pair) points to its own chain of OIF blocks  502 , it is possible that, in one example implementation, different multicast sessions point to the same (shared) chain of OIF blocks  502 . This will likely be the less typical case, for example, in which these two or more different multicast sessions each have the same OIFs participating in that multicast session. This can be conceptualized as two or more different channels that are being “watched” by the same OIFs. When this occurs, the pointers from each such multicast session can point to the same (shared) chain of OIF blocks  502 , if desired. Alternatively, separate chains of OIF blocks  502  can be maintained for each multicast session, for example, if such a simplified implementation is desired.  
         [0039]     Each second pointer  504  points to a particular TCB  506 , which typically includes information relevant to processing or routing packets to the particular OIF that is associated with that second pointer  504 , or to services associated with the particular OIF that is associated with that second pointer  504 . For example, if the packet header matches particular services in the ACL, attributes in the TCB are adjusted accordingly to obtain such services. Each second pointer  504  corresponds to a particular outbound interface (OIF) out which multicast packets are being forwarded, such as from the packet output interface  314  of the VR out over the network.  
         [0040]     Because more than one multicast session can use the same OIF of the VR, second pointers  504  from different multicast sessions can point to the same (shared) TCB  506  for that OIF. In the illustrative example of  FIG. 5 , the second pointer PTR 2 ( 2 , 4 ) from the second multicast session points to the shared TCB( 1 ) as the second pointer PTR 2 (n, 5 ) from the nth multicast session. Thus, second pointers  504  from different multicast sessions may share the same TCB  506 .  
         [0041]     Similarly, because multiple IP flows can use the same OIF, there can be multiple TCBs  506  for the same OIF, such as for multiple IP flows on the same OIF, where such multiple flows use different services and, therefore, have different corresponding TCBs  506 .  
         [0042]     In  FIG. 5 , for example, a particular TCB  506  typically includes, among other things, OIF information  508 , header transformation information  510 , metering information  512 , and statistics information  514 . The OIF information  508  includes, for example, information identifying which OIF will be used by the packet output interface  314  to output the packets from the VR. The header transformation information  510  includes, for example, Media Access Control (MAC) address generation information and protocol independent multicast (PIM) encapsulation information for that particular OIF. The metering information  512  includes, for example, egress rate limiting or other like information for that particular OIF. The statistics information  514  includes egress statistics collection information for that particular OIF.  
         [0043]     The schema depicted in  FIG. 5  provides numerous advantages. As discussed above, scalability from one to very many users is a desirable property. The ability to update the multicast forwarding schema as many users join or leave different multicast sessions (which sometimes results in adding or removing OIFs) is another desirable property. For example, when a user joins or leaves a multicast session under circumstances that require adding or removing an OIF to that multicast session, the OIF list can be updated by simply updating a single OIF block  502 , during which time the other OIF blocks  502  in that chain are still available and usable for performing multicast forwarding.  
         [0044]     The schema depicted in  FIG. 5  allows many users to be managed very efficiently because of, among other things, its use of first pointers  501  from {S, G} pairs to shared or independent chains of OIF blocks  502 , and per-OIF second pointers  504  to shared or independent TCBs  506 . Moreover, each OIF block  502  is typically apportioned into a small number of second pointer slots  503 . Each OIF block  502  is typically independently addressable and updatable when updating that OIF block to add or remove a particular OIF&#39;s second pointer  504 . As an illustrative example, if the OIF corresponding to second pointer PTR 2 ( 1 , 3 ) in OIF Block ( 1 , 1 ) was removed from the multicast session of {S, G} 1  (for example, because all of the one or more users of that OIF left that multicast session), then the second pointer PTR 2 ( 1 ,  3 ) in that OIF Block ( 1 ,  1 ) is removed, opening one second pointer slot  503  in OIF Block ( 1 ,  1 ) that could later be filled by another second pointer for another OIF being added (e.g., to service one or more users joining that multicast session).  
         [0045]     While such updating of a particular OIF block  502  is occurring, other OIF blocks  502  in the same or a different chain of OIF blocks  502  are still usable to carry out multicast forwarding to the users represented by the second pointers  504  in those other OIF blocks  502 . This improves the ability to multicast content, without interruption, to a large number of recipient users on different OIFs of a particular multicast session, even as other second pointers  504  are added or removed, such as to accommodate other recipient users of that multicast session that are joining or leaving that multicast session. In one example, both OIF blocks  502  and TCBs  506  are capable of being dynamically allocated as needed. Together with the sharing of TCBs  506  or even of OIF chains, as discussed above, the schema illustrated in FIG. typically offers one or more of the advantages of scalability, updatability, efficiency in memory usage, and high throughput performance with reduced interruptions.  
         [0046]      FIG. 6  is a flow chart of one example of a process that is invoked if multicast packet forwarding is invoked at  410  of  FIG. 4 . At  600 , a multicast forwarding operation is initiated, such as by calling executable or interpretable instructions of the multicast forwarding module  324 . At  602 , information in the packet header is mapped to an FCB entry in FCB module  326 . Each FCB entry in the FCB module  326  identifies an IP flow or a group of IP flows for an {S, G} pair  500  corresponding to a particular multicast session. At  604 , from that FCB entry, a first pointer  501  is extracted to the first OIF block  502  in the chain of one or more OIF blocks  502  corresponding to that multicast session.  
         [0047]     At  606  the next second pointer  504  in the current OIF block  502  is retrieved. At  606 , the retrieved second pointer  504  to a TCB  506  is used to build a portion of a control block that will be sent to the DMA engine  310 . At  606 , if other second pointers  504  exist in the current OIF block  502 , then process flow returns to  606 . Otherwise, process flow proceeds to  606  and the control block that was constructed for the completed OIF block  502  is sent to the DMA engine  310 . In this manner, one control block corresponding to each OIF block  502  is sent to the DMA engine  310  after that control block is constructed from the corresponding OIF block  502 . At  610 , if other OIF blocks  502  exist in that chain, then the next OIF block  502  is retrieved and made the current OIF block, a new control block is initiated, and process flow returns to  606 . Otherwise, at  610 , if no other OIF blocks  501  exist in the chain, then process flow proceeds to  614  to process (or wait for) the next received packet (e.g., at  400  of  FIG. 4 ).  
         [0048]      FIG. 7  is a flow chart of one example of packet retrieval and replication. At  700 , the DMA engine  310  receives a control block (such as from  608  in  FIG. 6 ). At  702 , the next entry in the received control block is retrieved. At  704 , the stored packet is retrieved from a packet buffer in the main memory  302  by DMA engine  310 . At  706 , the retrieved packet is sent to the egress module  306  for egress transmission, along with the corresponding control block entry, which provides information to the egress module  306  about how that particular packet is to be processed for the particular recipient user corresponding to the control block entry, which, in turn, corresponded to a particular second pointer  504 , as discussed above. At  708 , if there are more entries in the control block, then process flow returns to  702  to retrieve the next entry in the control block. Otherwise, process flow returns to  700  to receive (or wait for) another control block. In the manner illustrated in  FIG. 7 , a packet is held in the packet buffer in the main memory  302  so that it can be replicated. The replicated packets are sent to the egress module  306  for further processing (particular to the user that is to receive that replicated packet) and transmission that OIF.  
         [0049]      FIG. 8  is a flow chart of one example of processing by the egress module  306 . At  800 , a replicated packet is received from the DMA engine  310 . At  802 , the replicated packet is processed according to the TCB  506  corresponding to the particular OIF&#39;s second pointer  504 . As discussed above, such information is encapsulated into the control block that was submitted to the DMA engine  310 , and communicated to the egress module  306  along with the replicated packet. At  804 , header transformation occurs. In one example, this includes MAC address generation or encapsulation appropriate for the designated OIF over which the replicated packet will be transmitted. At  806 , egress rate limiting, if any, is applied. At  808 , egress statistics, if any, are computed. At  810 , the replicated packet is transmitted out over the computer network on the designated OIF.  
         [0050]      FIG. 9  is a flow chart of one example of more detail of acts included in the multicast forwarding, such as at  602  of  FIG. 6  or elsewhere, as appropriate. Among other things, the flow chart of  FIG. 9  illustrates one example of how TCBs  506  are created and, where possible, shared.  
         [0051]     At  900 , the system determines whether a received packet represents a new IP flow. This can be determined by looking at the above-described attributes in the packet header that identify a particular IP flow. If the packet corresponds to a previously identified multicast IP flow, then process flow proceeds to  606 , and a previously defined FCB entry and a previously defined TCB  506  are used for further multicast forwarding processing. If a new flow is detected at  900 , there will be no matching FCB entry in FCB  326 . Therefore, for a new flow detected at  900 , a new FCB entry will be created in FCB  326 , as discussed below.  
         [0052]     If a new flow is detected at  900 , then, at  902 , is its determined whether the new flow is a strict optimized mode or, instead, is in an adaptive optimized mode that provides one or more services for that particular flow. This determination is typically made using a configurable attribute.  
         [0053]     At  902 , if in the strict optimized mode, then, at  904 , an OIF list (e.g., a chain of OIF blocks, as illustrated in  FIG. 5 ) is built for the {S, G} entry corresponding to the newly identified multicast flow. Because no flow-specific services are required, this OIF list includes second pointers  504  to a default TCB corresponding to each OIF in that multicast session. This default TCB does not include any per-flow of ACL-based service-specific attributes. Instead this default TCB typically depends only on attributes and services that are applicable to all flows associated with the particular VI serving as the OIF. Each OIF participating in the multicast session will have a corresponding second pointer  504  to its corresponding default TCB. Then, at  906 , the OIF module  330  of the PFE  300  is updated with each OIF block  502  in the chain of OIF blocks that make up the OIF list of that particular multicast flow. The OIF module  330  of the PFE  300  is typically updated with such OIF blocks  502  on a block-by-block basis. Then, at  908 , the FCB  326  of the PFE  300  is updated to include, for example, a pointer to the OIF list for the newly identified multicast flow. Then, process flow proceeds to  606  of  FIG. 6  (or elsewhere, if appropriate).  
         [0054]     At  902 , if in the adaptive optimized mode instead of the strict optimized mode, then, at  910  it is determined whether any ingress services are needed. In one example, this includes checking for such ingress services on the VI  208  at which the packet is received. At  910 , if one or more such ingress services are needed, then, at  912 , a TCB  506  is created to control the providing of any such ingress services, (otherwise process flow proceeds to  916 ). Then, at  914 , a second pointer  504  is created to point to this newly created TCB  506 . This newly created TCB  506  for the ingress services includes an OIF field  508  that specifies a null OIF (the PFE  300  does not actually forward any packets out any such null OIF).  
         [0055]     At  916 , it is determined whether there is a next OIF entry (that is, a second pointer  504 ) in the OIF list for the new multicast flow. If there is no such next OIF entry (e.g., upon specification of an invalid {S, G} entry or a null OIF), then process flow proceeds to  906 . Otherwise, at  918 , it is determined whether any outbound services are needed on the next OIF entry in the OIF module  330 . If so, then, at  920 , a new TCB  506  is created for that OIF entry to control the providing of any such outbound services, otherwise, at  922 , the VI default TCB  332  is used for that OIF entry. Then, at  924 , a second pointer  504  is created to point to the new TCB  506  or the default TCB  332 , as appropriate, and the OIF list for that multicast session is updated accordingly. Then, at  926 , it is determined if there is a next OIF entry in the OIF list for the multicast session. If so, process flow returns to  918 , otherwise process flow proceeds to  906 .  
         [0056]     Using the above process described with respect to  FIG. 9 , some TCBs  506  may be shared by multiple second pointers  504 . For example, the default TCB  332  is likely shared by multiple second pointers  504 . Other TCBs  506  may correspond to individual second pointers  504 .  
         [0057]     Although the above examples have been discussed with respect to a router box providing virtual routers (e.g., VRs  128 ), the present systems and methods are not so limited. For example, certain aspects of the present systems and methods are also applicable to alternative systems using hardware routers instead of the virtual routers.  
         [0058]     The Abstract of the Disclosure is provided to comply with 37 C.F.R. § 1.72(b), requiring an abstract that will allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. In addition, in the foregoing Detailed Description, it can be seen that various features may be grouped together in a single embodiment for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claimed embodiments require more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive subject matter lies in less than all features of a single disclosed embodiment. Thus the following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separate embodiment.  
         [0059]     It is to be understood that the above description is intended to be illustrative, and not restrictive. For example, the above-described embodiments (and/or aspects thereof) may be used in combination with each other. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. In the appended claims, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein.” Also, in the following claims, the terms “including” and “comprising” are open-ended, that is, a system, device, article, or process that includes elements in addition to those listed after such a term in a claim are still deemed to fall within the scope of that claim. Moreover, in the following claims, the terms “first,” “second,” and “third,” etc. are used merely as labels, and are not intended to impose numerical requirements on their objects.