📄 rfc2102.txt
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particular source-group pair. The performance of this scheme is expected to be relatively poor for large networks with sparsely distributed group membership. Furthermore, no support for policies or QOS is provided.2. Core Based Trees (CBT)[BFC93]. This scheme uses a single tree shared by all sources per group. This tree has a single router as the core (with additional routers for robustness) from which branches emanate. The chief distinguishing characteristic of CBT is that it is receiver initiated, i.e., receivers wishing to join a multicast group find the tree (or its core) and attach themselves to it, without anyRamanathan Informational [Page 6] RFC 2102 Nimrod Multicast Support February 1997 participation from the sources. The chief motivation behind this scheme is the reduction of the state overhead, to O(G), in comparison to DVMRP and PIM(described below). Also, only routers in the path between the core and the potential members are involved in the process. Core-based tree formation and packet flow are decoupled from underlying unicast routing. The main disadvantage is that packets no longer traverse the shortest path from the source to their destinations. The performance in general depends on judicious placement of cores and coordination between them. Traffic concentration on links incident to the core is another problem. There is also a dependence on network entities (in other administrative domains, for instance) for resource reservation and policy routing.3. Protocol Independent Multicasting (PIM)[DEFJ93]. Yet another approach based on the receiver initiated philosophy, this is designed to reap the advantages of DVMRP and CBT. Using a "rendezvous point", a concept similar to the core discussed above, it allows for the simultaneous existence of shared and source-specific multicast trees. In the steady state, data can be delivered over the reverse shortest path from the sender to the receiver (for better end-to-end delay) or over the shared tree. Using two modes of operation, sparse and dense, this provides improved performance, both when the group membership in an internetwork is sparse and when it is dense. It is however, a complex protocol. A limitation of PIM is that the shortest paths are based on the reverse metrics and therefore truly "shortest" only when the links are symmetric.4. Multicast Open Shortest Path First (MOSPF)[Moy92]. Unlike the abovementioned approaches, this is based on link-state routing information distribution. The packet forwarding mechanism is hop-by-hop. Since every router has complete topology information, every router computes the shortest path multicast tree from any source to any group using Dijkstra's algorithm. If the router doing the computation falls within the tree computed, it can determine which links it must forward copies onto.Ramanathan Informational [Page 7] RFC 2102 Nimrod Multicast Support February 1997 MOSPF inherits advantages of OSPF and link-state distribution, namely localized route computation (and easy verification of loop-freedom), fast convergence to link-state changes etc. However, group membership information is sent throughout the network, including links that are not in the direct path to the multicast destinations. Thus, like DVMRP, this is most suitable for small internetworks, that is, as an intra-domain routing mechanism.5. Inter-Domain Policy Routing (IDPR)[Ste]. This approach uses link-state routing information distribution like MOSPF, but uses source-specified packet forwarding. Using the link-state database, the source generates a policy multicast route to the destinations. Using this, the IDPR path-setup procedure sets up state in intermediate entities for packet duplication and forwarding. The state contains information about the next-hop entities for the multicast flow. When a data packet arrives, it is forwarded to each next hop entity obtained from the state. Among the advantages of this approach are its ability to support policy based multicast routing with ease and independence (flexibility) in the choice of multicasting algorithm used at the source. IDPR also allows resource sharing over multiple multicast trees. The major disadvantage is that it makes it relatively more difficult to handle group membership changes (additions and deletions) since such changes must be first communicated to the source of the tree which will then add branches appropriately. We now discuss the applicability of these approaches to Nimrod. Common to all of the approaches described is the fact that we need to set up state in the intermediate routers for multicast packet forwarding. The approaches differ mainly on who initiates the state creation - the sender (e.g., IDPR, PIM), the receiver (e.g., CBT, PIM) or the routers themselves create state without intitiation by the sender or receivers (e.g., DVMRP, MOSPF). Nimrod should be able to accommodate both sender initiated as well as receiver initiated state creation for multicasting. In the remainder of this section, we discuss the pros and cons of these approaches for Nimrod. Nimrod uses link-state routing information distribution (topology maps) and has four modes of packet forwarding - flow mode, Connectivity Specification Chain (CSC) mode, Connectivity Specification Sequence (CSS) mode and datagram mode [CCS96].Ramanathan Informational [Page 8] RFC 2102 Nimrod Multicast Support February 1997 An approach similar to that used in IDPR is viable for multicasting using the flow mode. The source can set up state in intermediate routers which can then appropriately duplicate packets. For the CSC, BTES and datagram modes, an approach similar to the one used in MOSPF is applicable. In these situations, the advantages and disadvantages of these approaches in the context of Nimrod is similar to the advantages and disadvantages of IDPR and MOSPF respectively. Sender based trees can be set up using an approach similar to IDPR and generalizing it to an "n" level hierarchy. A significant advantage of this approach is policy-based routing. The source knows about the policies of nodes that care to advertise them and can choose a route the way it wants (i.e., not depend upon other entities to choose the route, as in some schemes mentioned above). Another advantage is that each source can use the multicast route generation algorithm and packet forwarding scheme that best suits it, instead of being forced to use whatever is implemented elsewhere in the network. Further, this approach allows for incrementally deploying new multicast tree generation algorithms as research in that area progresses. CBT-like methods may be used to set up receiver initiated trees. Nimrod provides link-state maps for generating routes and a CBT-like method is compatible with this. For instance, a receiver wishing to join a group may generate a (policy) route to the core for that group using its link-state map and attach itself to the tree. A disadvantage of sender based methods in general seems to be the support of group dynamism. Specifically, if there is a change in the membership of the group, the particular database which contains the group-destination mapping must be updated. In comparison, receiver oriented approaches seem to be able to accommodate group dynamism more naturally. Nimrod does not preclude the simultaneous existence of multiple approaches to multicasting and the possibility of switching from one to the other depending on the dynamics of group distributions. Interoperability is an issue - that is, the question of whether or not different implementations of Nimrod can participate in the same tree. However, as long as there is agreement in the structure of the state created (i.e., the states can be interpreted uniformly for packet forwarding), this should not be a problem. For instance, a receiver wishing to join a sender created tree might set up state on a path between itself and a router on the tree with the sender itself being unaware of it. Packets entering the router would now be additionally forwarded along this new "branch" to the new receiver.Ramanathan Informational [Page 9] RFC 2102 Nimrod Multicast Support February 1997 In conclusion, the architecture of Nimrod can accommodate diverse approaches to multicasting. Each approach has its disadvantages with respect to the requirements mentioned in the previous section. The architecture does not demand that one particular solution be used, and indeed, we expect that a combination of approaches will be employed and engineered in a manner most appropriate to the requirements of the particular application or subscriber.5 A Multicasting Scheme based on PIM The Inter-Domain Multicast Routing (IDMR) working group of the IETF has developed a specification for a new multicast scheme, namely, Protocol Independent Multicasting (PIM) for use in the Internet [DEF+94a, DEF+94b]. In this section, we decribe how the schemes mentioned therein may be implemented using the facilities provided by Nimrod. We note that the path setup facility provided in Nimrod makes it very conducive to PIM-style multicasting; despite the length of the description given here, we assure the reader that it is quite simple to implement PIM style multicasting in Nimrod. Before reading this section, we recommend that the reader acquire some familiarity with PIM (see [DEF+94a, DEF+94b]).5.1 Overview The PIM architecture maintains the traditional IP multicast service model of receiver-initiated membership and is independent of any specific unicast routing protocol (hence the name). A significant aspect of PIM is that it provides mechanisms for establishing two kinds of trees - a shared tree, which is intended for low "cost" multicasting and a source-based tree, intended for low delay multicasting. A shared tree is rooted at a rendezvous point (RP), which is typically a prespecified router for the multicast group in question. In order to establish a shared tree, a designated router (DR) for a host wishing to join a group G initiates a flow setup from the RP for G to the DR. A source S wishing to send to a group G initiates a flow setup between S and the RP for group G. At the conclusion of these flow setups, packets can be forwarded from S to H through the RP. For details on the protocol used to implement this flow setup please refer to [DEF+94b].Ramanathan Informational [Page 10] RFC 2102 Nimrod Multicast Support February 1997 After the shared tree has been setup, a recipient for group G has the option of switching to a source-based shortest path tree. In such a tree, packets are delivered from the source to each recipient along the shortest path. To establish a source-based shortest path tree, the DR for H looks at the source S of the packets it is receiving via the shared tree and establishes a flow between S and the DR. The flow is established along the shortest path from the DR to S (Thus, strictly speaking, it is the reverse shortest path that is being used.) Subsequently, packets can be forwarded from S to H using this shortest path and thereby bypassing the RP. For details on the protocol used to implement source-based trees in PIM please refer to [DEF+94b]. When a host wishes to leave a multicast group, its designated router sends a prune message towards the source (for source-based trees) or towards the RP (for shared trees). For details on this and other features of PIM please refer to [DEF+94b]. In Nimrod, PIM is implemented as follows (we refer to PIM based multicast as Nimpim). In order to join a shared tree, an endpoint (or an agent acting on behalf of the endpoint) wishing to join a group G queries the association database for the EID and locator of the RP for G (for well-known groups the association may be configured). It is required that such an association be maintained for every multicast group G. The endpoint gets a route for the RP and initiates a multicast flow setup to the RP (a multicast flow setup is similar to an unicast flow setup described in [CCS96] except for one feature - when a multicast flow setup request reaches a node that already has that flow present, the request is not forwarded further. The new flow gets "spliced" in as a new branch of the existing multicast tree). Similarly, the source establishes a flow to the RP. The RP creates state to associate these two flows and now packets can be forwarded to the endpoints from the source. Note that each flow setup may be "hierarchical" and involve many subflows. All this, however, is transparent to Nimpim. For details on management of hierarchical flows please refer to [CCS96]. To create the source-based tree, the representative for a recipient node N obtains the EID or locator of the source from the data packets and initiates a multicast flow setup to the source. The route agent for the node N uses its map in order to calculate the shortest path from the source to N. The flow request is sent along the reverse of this path. We note that the "shortness" of the path is constrained by the amount of routing information available locally. However, since the map is available locally, one can find the actual shortest path from the source to N and not use the shortest path from N to S. Thus, with Nimrod one can actually surmount a shortcoming of PIM with relative ease.Ramanathan Informational [Page 11] RFC 2102 Nimrod Multicast Support February 1997 We now discuss some more details of Nimpim. We start with a description of multicast flow setup. This is the "basic" functionality required to implement multicasting. Having this "building-block" spelt out, we use this to specify the establishment of the shared tree (in section 5.3) and the establishment of a source-based tree (in section 5.4). We only discuss sparse-mode multicasting, as described in [DEF+94a] here. Further, to simplify the discussion, we assume a single Rendezvous Point per group. Finally, we "address" all entities in terms of their EIDs alone for reasons of conciseness - the locators could be used in conjuction to reduce the overhead of database lookups.5.2 Joining and Leaving a Tree Nimpim uses two control packets in order to setup a flow - the Nimrod Multicast Flow-Request packet (NMFReq) and the Nimrod Multicast Flow-Reply packet (NMFRep). The NMFReq packet is a control packet identified by a prespecified "payload type". The protocol-specific part of this packet includes the following fields (except for the Code field, these fields are present in the Unicast Flow-Request packet too) :⌨️ 快捷键说明
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