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Network Working Group R. Ramanathan Request for Comments: 2102 BBN Systems and Technologies Category: Informational February 1997

Multicast Support for Nimrod :  Requirements and Solution Approaches

Status of this Memo

 This memo provides information for the Internet community.  This memo
 does not specify an Internet standard of any kind.  Distribution of
 this memo is unlimited.


 Nimrod does not specify a particular solution for multicasting.
 Rather, Nimrod may use any of a number of emerging multicast
 techniques.  We identify the requirements that Nimrod has of a
 solution for multicast support.  We compare existing approaches for
 multicasting within an internetwork and discuss their advantages and
 disadvantages.  Finally, as an example, we outline the mechanisms to
 support multicast in Nimrod using the scheme currently being
 developed within the IETF - namely, the Protocol Indpendent Multicast
 (PIM) protocol.

Table of Contents

 1  Introduction.................................................  2
 2  Multicast vs Unicast.........................................  3
 3  Goals and Requirements.......................................  4
 4  Approaches...................................................  6
 5  A Multicasting Scheme based on PIM........................... 10
    5.1 Overview ................................................ 10
    5.2 Joining and Leaving a Tree .............................. 12
        5.2.1 An Example ........................................ 15
    5.3 Establishing a Shared Tree .............................. 16
    5.4 Switching to a Source-Rooted Shortest Path Tree.......... 18
    5.5 Miscellaneous Issues..................................... 20
 6  Security Considerations...................................... 21
 7  Summary...................................................... 21
 8  References................................................... 22
 9  Acknowledgements............................................. 23
 10 Author's Address............................................. 23

Ramanathan Informational [Page 1] RFC 2102 Nimrod Multicast Support February 1997

1 Introduction

 The nature of emerging applications such as videoconferencing, remote
 classroom, etc.  makes the support for multicasting essential for any
 future routing architecture.  Multicasting is performed by using a
 multicast delivery tree whose leaves are the multicast destinations.
 Nimrod does not propose a solution for the multicasting problem.
 There are two chief reasons for this.  First, multicasting is a non-
 trivial problem whose requirements are still not well understood.
 Second, a number of groups (for instance the IDMR working group of
 the IETF) are studying the problem by itself and it is not our
 intention to duplicate those efforts.
 This attitude towards multicasting is consistent with Nimrod's
 general philosophy of flexibility, adaptability and incremental
 While a multicasting solution per se is not part of the "core" Nimrod
 architecture, Nimrod does require that the solution have certain
 characteristics.  It is the purpose of this document to discuss some
 of these requirements and evaluate approaches towards meeting them.
 This document is organized as follows.  In section 2 we discuss why
 multicasting is treated a little differently than unicast despite the
 fact that the former is essentially a generalization of the latter.
 Following that, in section 4 we discuss current approaches toward
 multicasting .  In section 5, we give an example of how Nimrod
 multicasting may be done using PIM [DEF+94a].  For readers who do not
 have the time to go through the entire document, a summary is given
 at the end.
 This document uses many terms and concepts from the Nimrod
 Architecture document [CCS96] and some terms and concepts (in section
 5) from the Nimrod Functionality document [RS96].  Much of the
 discussion assumes that you have read at least the Nimrod
 Architecture document [CCS96].

Ramanathan Informational [Page 2] RFC 2102 Nimrod Multicast Support February 1997

2 Multicast vs Unicast

 We begin by looking at the similarities and differences between
 unicast routing and multicast routing.  Both unicast and multicast
 routing require two phases - route generation and packet forwarding.
 In the case of unicast routing, Nimrod specifies modes of packet
 forwarding; route generation itself is not specified but left to the
 particular routing agent.  For multicasting, Nimrod leaves both route
 generation and packet forwarding mechanisms unspecified.  To explain
 why, we first point out three aspects that make multicasting quite
 different from unicasting :

o Groups and group dynamism. In multicasting, the destinations are part

of a group, whose membership is dynamic.  This brings up the following
issues :
  1. An association between the multicast group and the EIDs and

locators of the members comprising that group. This is especially

   relevant in the case of sender initiated multicasting and policy
  1. A mechanism to accommodate new group members in the delivery in

response to addition of members, and a mechanism to "prune" the

   delivery in response to departures.

o State creation. Most solutions to multicasting can essentially be

viewed as creating state in routers for multicast packet forwarding.
Based on who creates the state, multicasting solutions differ.  In
multicasting, we have several options for this - e.g., the sender, the
receivers or the intermediate routers.

o Route generation. Even more so than in unicast routing, one can choose

from a rich spectrum of heuristics with different tradeoffs between a
number of parameters (such as cost and delay, algorithmic time
complexity and optimality etc.).  For instance, some heuristics produce
a low-cost tree with high end-to-end delay and some produce trees that
give the shortest path to each destination but with a higher cost.
Heuristics for multicasting are a significant research area today, and
we expect advances to result in sophisticated heuristics in the near
 Noting that there are various possible combinations of route
 generation, group dynamism handling and state creation for a solution
 and that each solution conceivably has applications for which it is
 the most suitable, we do not specify one particular approach to
 multicasting in Nimrod.  Every implementation of Nimrod is free to
 use its own multicasting technique, as long as it meets the goals and
 requirements of Nimrod.  However, for interoperability, it is

Ramanathan Informational [Page 3] RFC 2102 Nimrod Multicast Support February 1997

 necessary that certain things are agreed upon - for instance, the
 structure of the forwarding information database that they create (we
 discuss this in more detail in section 4).
 Thus, we do not discuss the details of any multicast solution here,
 only its requirements in the context of Nimrod.  Specifically, we
 structure the discussion in the remainder of this document on the
 following two themes :
o What are the goals that we want to meet in providing multicasting in
  Nimrod, and what specific requirements do these goals imply for the
  multicast solution?
o What are some of the approaches to multicasting being discussed
  currently, and how relevant are each of these approaches to Nimrod?

3 Goals and Requirements

 The chief goals of Nimrod multicasting and their implications on
 solution requirements are as follows:

1. Scalability. Nimrod multicasting must scale in terms of the size of

 the internetwork, the number of groups supported and the number of
 members per group.  It must also support group dynamism efficiently.
 This has the following implications for the solution:
 o Routers not on the direct path to the multicast destinations should
   not be involved in state management.  In a network with a large
   number of routers, a solution that does involve such routers is
   unlikely to scale.
 o It is likely that there will be a number of applications that have
   a few members per group (e.g., medical imaging) and a number of
   applications that have a large number of members per group (e.g.,
   news distribution).  Nimrod multicasting should scale for both
   these situations.  If no single mechanism adequately scales for
   both sparse and dense group memberships simultaneously, a
   combination of mechanisms should be considered.
 o In the face of group membership change, there must be a facility
   for incremental addition or deletion of "branches" in the
   multicast tree.  Reconstructing the tree from scratch is not likely
   to scale.

Ramanathan Informational [Page 4] RFC 2102 Nimrod Multicast Support February 1997

 o It is likely that we will have some well-known groups (i.e., groups
   which are more or less permanent in existence) and some ephemeral
   groups.  The dynamics of group membership are likely to be
   different for each class of groups, and the solution should take
   that into account as appropriate.

2. Policy support. This includes both quality of service (QOS) as

 well as access restrictions, although currently, demand is probably
 higher for QOS. In particular, every path from a source to each
 destination in the multicast group should satisfy the requested
 quality of service and conform to the access restrictions.  The
 implications for the multicasting solution are :
o It is likely that many multicasting applications will be cost
  conscious in addition to having strict quality of service bounds
  (such as delay and jitter).  Balancing these will necessitate
  dealing with some new parameters - e.g., the tree cost (sum of the
  "cost" of each link), the tree delay (maximum, mean and variance
  in end-to-end delay) etc.
o In order to support policy-based routing, we need to know where the
  destinations are (so that we can decide what route we can take to
  them).  In such a case, a mechanism that provides an association
  between a group id and a set of destination locators is probably
o Some policy constraints are likely to be destination specific.  For
  instance, a domain might refuse transit service to traffic going to
  certain destination domains.  This presents certain unique problems
  - in particular, for a single group, multiple trees may need to be
  built, each tree "servicing" disjoint partitions of the multicast

3. Resource sharing. Multicasting typically goes hand in hand with large

 traffic volume or applications with a high demand for resources.
 These, in turn, imply efficient resource management and sharing if
 possible.  Therefore, it is important that we place an emphasis on
 interaction with resource reservation.  For instance, Nimrod must be
 able to provide information on which tree resources are shareable and
 which are not so that resource reservation may use it while allocating
 resources to flows.

4. Interoperability. There are two issues in this context. First, the

 solution must be independent of mechanisms that provide the solution
 with information it needs.  For instance, many multicast solutions
 (e.g., PIM) make use of information supplied by unicast routing
 protocols.  The multicast solution must not be dependent on which
 unicast protocol is used.

Ramanathan Informational [Page 5] RFC 2102 Nimrod Multicast Support February 1997

 Second, a multicast solution must interoperate with other multicast
 solutions in the construction of a delivery tree.  This implies some
 kind of "agreement" at some "level".  For instance, the agreement
 could be that everybody use the same structure for storing forwarding
 information in the routers.  Since the delivery tree is defined by the
 nature of forwarding information in the routers and not by the
 particular mechanism used to create that information, multiple
 implementations can coexist.

4 Approaches

 The approaches to multicasting currently in operation and those being
 considered by the IETF include the following :

1. Distance vector multicast routing protocol (DVMRP)[DC90]. This

 approach is based upon distance-vector routing information distribution
 and hop-by-hop forwarding.  It uses Reverse Path Forwarding (RPF)[DM78]
 - a distributed algorithm for constructing an internetwork broadcast
 tree.  DVMRP uses a modified RPF algorithm, essentially a truncated
 broadcast tree, to build a reverse shortest path sender-based multicast
 delivery tree.  A reverse shortest path from s to d is a path that uses
 the same intermediate nodes as those in the shortest path from d to
 s (If the paths are symmetric (i.e., cost the same) in either
 direction, the reverse shortest path is same as the shortest path.)
 An implementation of RPF exists in the current Internet in what
 is commonly referred to as the MBONE. An improvement to this is in the
 process of being deployed.  It incorporates "prune" messages to
 truncate further the routers not on the path to the destinations and
 "graft" messages to undo this truncation, if later necessary.
 The main advantage of this scheme is that it is simple.  The major
 handicap is scalability.  Two issues have been raised in this
 context[BFC93].  First, if S is the number of active sources and G
 the number of groups, then the state overhead is O(GS) and might be
 unacceptable when resources are limited.  Second, routers not on a
 multicast tree are involved (in terms of sending/tracking prune and
 graft messages) even though they might not be interested in the
 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 any

Ramanathan 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 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
 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
 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
 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

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) :
 1. S-EID : The EID of the initiator of the flow.
 2. T-EID : The EID of the target of the flow.
 3. Flow-id :  A label denoting the flow.
 4. Direction :  The direction of the flow - whether from the initiator
    to the target (FORW) or from the target to the initiator (REVERSE)
    or both (BOTH).
 5. Code :  Denotes whether the packet is for joining a flow
    (NMFReq-Join) for leaving a flow (NMFReq-Prune).
 6. Source Route :  A sequence of node locators through which the packet
    must travel.

Ramanathan Informational [Page 12] RFC 2102 Nimrod Multicast Support February 1997

 The processing of the NMFReq by a forwarding agent at node N is
 similar to that of the unicast flow request (see [CCS96]), except for
 the fact that now we provide the ability for the new flow to "splice"
 onto an existing delivery tree or "un-splice" from an existing
 delivery tree.  Specifically,
 o If the Code is NMFReq-Join then the algorithm executed by the
   forwarding agent for node N is shown in Figure 1.
 o If the Code is NMFReq-Prune then the algorithm is executed by the
   forwarding agent at node N is shown in Figure 2.
 The NMFRep packet is used to accept or reject an NMFReq-Join or
 NMFReq-Prune.  The packet format is the same as that for unicast flow
 request.  However, an NMFRep packet is generated now by the first
 node N that grafts the new flow to the existing tree.  This may be
 different from the target of the NMFReq.
 It is required that a leaf router keep track of all hosts currently
 joined to the group and send a prune message only if there is no host
 in the local network for the group.
 The NMFReq - NMFRep exchanges constitute a procedure for joining a
 multicast delivery tree (when the Code is Join) and for leaving a
 multicast delivery tree (when the Code is Prune).  We term these
 procedures Tree-Join and Tree-Leave respectively; we shall be using
 these procedures as "building-blocks" in the construction of shared
 trees (section 5.3) and of source-based trees (section 5.4).

Ramanathan Informational [Page 13] RFC 2102 Nimrod Multicast Support February 1997

begin if the flow-id F in NMFReq-Join is in flow-list then

 if T-EID in NMFReq-Join = target in flow state for F then
    if Direction in NMFReq-Join is REVERSE or BOTH then
       Add the node preceding N in source route to child list for F
       discard packet
    discard packet


   install state for F in N, i.e.,
      assign parent(F) = node succeeding N in source route
      assign child(F)  = node preceeding N in source route
      assign target(F) = T-EID in NMFReq-Join
   forward NMFReq-Join to parent(F)


Figure 1: Algorithm executed by a forwarding agent for node N when when it receives an NMFReq-Join.


if the flow-id F in NMFReq-Prune is in flow-list
then begin
     delete previous hop in source route from child list for F, if exists
     if child list for F is empty
     then begin
           delete the flow-id and state associated with it
           forward to next hop in source route
     else discard packet
else forward to next hop in source-route


Figure 2: Algorithm executed by a forwarding agent for node N when it receives an NMFReq-Prune.

Ramanathan Informational [Page 14] RFC 2102 Nimrod Multicast Support February 1997

5.2.1 An Example

 An example of how a tree is joined is given here with the help of
 Figure 3.  In the figure, bold lines indicate an existing tree.
 Representative R on behalf of host H joins the tree by sending an
 NMFJoin-Req towards a target T. When used in the shared tree mode,
 the target is the RP and when used in the source tree mode, it is the
 source (root) of the multicast tree.  Suppose that a host H wants to
 join the multicast tree.  The following steps are executed :

Step 1. A representative R of H queries the route agent for a route

  from T to R. It obtains the route T - C- B - A - R. It builds a
  NMFJoin-Req packet with source route as R, A, B, C, T and flow
  as F forwards it to A.

Step 2. A looks for flow F in its installed flow database and

  doesn't find it.  It installs state for F (makes R a child and
  B a parent in the multicast tree) and sends the NMFJoin-Req packet
  to B.

Step 3. B looks for flow F in its installed flow database and finds it.

  It adds B to its child list and constructs an NMFJoin-Rep packet and
  sends it to A.

Step 4. A forwards the packet to R and the tree joining is complete.

  Branch B-A-R is now added to the tree.

Ramanathan Informational [Page 15] RFC 2102 Nimrod Multicast Support February 1997

5.3 Establishing a Shared Tree

 There are two parts to establishing a shared tree - the receiver-to-
 RP communication wherein the receiver joins the delivery tree rooted
 at RP and the sender-to-RP communication wherein the RP joins the
 delivery tree rooted at the sender.
                                  |   |\
                                  +---+  \
                                    /      \
                                   /         \
                                C /            \ X
                              +---+           +---+
                              |   |           |   |
                              +---+           +---+
    R    join-req           join-req     \  B
    +---+ - - - - ->  +---+ - - - - -> +---+
    |   |<------------|   |<-----------|   |
    +---+   join-rep  +---+   join-rep +---+
      |                 A                 \
      |                                     \
      |                                       \     Y
     ( )                                        +---+
       H                                        |   |

Figure 3: Illustration for the example describing joining an existing multicast tree.

 Receiver-RP Communications:  When an endpoint wishes to join a
 multicast group G, the endpoint representative obtains the Rendezvous
 Point EID for G.  We assume that the association database contains
 such a mapping.  For details on how the association database query is
 implemented, please refer [CCS96].
 The representative also obtains the flow-id to be used for the flow.
 The flow-id is constructed as the tuple (RP-EID, G) or an equivalent
 thereof.  Note that the flow-id must be unique to the particular
 multicast flow.  This is not the only method or perhaps even the best
 method for obtaining a flow id.  Alternate methods for obtaining the
 flow-id are discussed in section 5.5.

Ramanathan Informational [Page 16] RFC 2102 Nimrod Multicast Support February 1997

 The representative then initiates a Tree-Join procedure.
 The NMFReq packet fields are as follows:
   o S-EID : The EID of the endpoint wishing to join.
   o T-EID : The RP EID (obtained from the Association Database).
   o Flow-id : The flow-id for this group (obtained as mentioned
   o Direction : REVERSE (from the RP to the receiving endpoint).
   o Code : Join.
   o Source Route : Reverse of the route obtained from the map agent
     for a query "from RP-EID to Receiver-EID".
 At the first node already containing this Flow-id or the RP, an
 NMFRep packet is generated.  The S-EID, T-EID, Direction and Flow-id
 fields are copied from the NMFReq packet and the Code is set to
 Join-Accept or Join-Refuse as the case may be.  The source route is
 reversed from the NMFReq packet.
 Sender-RP Communications: When an endpoint wishes to send to a
 multicast group G, the endpoint representative obtains the Rendezvous
 Point EID for G.  We assume that the association database contains
 such a mapping.  For details on how the association database query is
 implemented, please refer [CCS96].
 The representative also obtains the flow-id to be used for the flow.
 The flow-id is constructed as the tuple (Sender-EID, G) or an
 equivalent thereof.  Note that the flow-id must be unique to the
 particular multicast flow.  This is not the only method or perhaps
 even the best method for obtaining a flow id.  Alternate methods for
 obtaining the flow-id are discussed in section 5.5.
 The representative then sends a RP-Register Message to the RP. This
 register message is equivalent to the PIM-Register described in
 [DEF+94b].  The RP-Register message contains the group G and the
 flow-id (obtained as discussed above) and the sender EID.
 The RP then initiates a Tree-Join with the Sender EID as the target.
 The NMFReq fields are as follows :
   o S-EID : RP-EID.
   o T-EID : Sender EID (copied from RP-Register Message).

Ramanathan Informational [Page 17] RFC 2102 Nimrod Multicast Support February 1997

   o Flow-id :  The flow-id field from RP-Register Message.
   o Code :  Join.
   o Direction :  REVERSE.
   o Source Route :  Reverse of the route obtained from map agent
     query "from Sender-EID to RP-EID".
 The NMFRep fields are obvious.
 Shared Tree Data Forwarding: Packets sent from the source for group G
 contain the Flow-id used by the sender(s) and receiver(s) for setting
 up the delivery tree.  The packets from the sender are sent to the RP
 where they are multicast, using the state created for the flow, into
 the delivery tree rooted at the RP to all of the receivers that did a

5.4 Switching to a Source-Rooted Shortest Path Tree

 There are two parts involved in switching to a Source-Rooted Shortest
 Path Tree - the receiver-source communications wherein the receiver
 joins a multicast delivery tree rooted at the source and the
 receiver-RP communications wherein the receiver leaves the shared
 Receiver-Source Communications:  An endpoint E that is receiving
 packets through the shared tree from source S has the option of
 switching to a delivery tree rooted at the source such that packets
 from S to E traverse the shortest path (using whatever metric).
 The endpoint representative of E obtains the flow-id to be used for
 the flow.  The flow-id is constructed equivalently to the tuple
 (Source-EID, G).  Note that the flow-id must be unique to the
 particular multicast flow.  This is not the only method or perhaps
 even the best method for obtaining a flow id.  Alternate methods for
 obtaining the flow-id are discussed in section 5.5.
 The representative for E initiates a Tree-Join toward S with NMFReq
 fields as follows:
 o S-EID : EID of the Endpoint E.
 o T-EID : EID of the source.
 o Flow-id :  Flow id for the multicast (obtained as mentioned above).
 o Code :  Join.

Ramanathan Informational [Page 18] RFC 2102 Nimrod Multicast Support February 1997

 o Direction :  REVERSE.
 o Source Route : To obtain the route, the route agent is queried for
   a shortest path route (based on the chosen metric, typically, the
   delay) from the source to the endpoint.  We note that the quality
   of the route is constrained by the amount of routing information
   available, directly or indirectly, to the route agent.  The Source
   Route is the reverse of the route thus obtained.
 A comment on the difference between the shortest-path trees obtained
 using the RPF tree as in [DEF+94b, DC90] and the trees that are be
 obtained here.  When using the RPF scheme, the packets from the
 source S to the endpoint E follow a path that is the shortest path
 from E to S. This is the desired path if and only if the path is
 symmetric in either direction.  However, in the mechanism described
 here for Nimrod, the packets do follow the "actual" shortest path
 from S to E whether or not the path is symmetric.
 The NMFRep fields are obvious.
 Receiver-RP Communications: After the receiver has joined the
 source-rooted tree, it can optionally disassociate itself from the
 shared tree.  This is done by initiating a Tree-Leave procedure.
 The representative sends a NMFReq packet toward the RP with the
 fields as follows.
 o S-EID : The EID of the endpoint wishing to leave the shared tree.
 o T-EID : The RP-EID.
 o Flow-id :  The flow-id it used to join the shared tree.
 o Code :  Prune.
 o Direction :  REVERSE.
 o Source Route :  Obtained as for the Tree-Join.
 The prune packet is processed by the intermediate forwarding agents
 as mentioned in section 5.2.  When the receiver gets back the NMFRep
 packet, the receiver has left the shared tree.
 Source Tree Data Forwarding: Packets from the source contain the
 flow-id that was used to join the source tree for a given multicast
 group.  Forwarding agents simply use the state created by the Tree-
 Join procedure in order to duplicate and forward packets toward the

Ramanathan Informational [Page 19] RFC 2102 Nimrod Multicast Support February 1997

5.5 Miscellaneous Issues

 Obtaining the Flow-Id: In the above scheme the flow-id for a
 particular multicast group G was obtained by combining the RP-EID and
 the group set-id (G-SID) (in case of shared tree) or by combining the
 Source-EID and the G-SID (in case of source-based tree).  A
 disadvantage of this approach is that the bit-length of EID/SID is
 potentially high (more than 64 bits) and thus the flow-id could be
 very long.  While there do exist bit-based data structures and search
 algorithms (such as Patricia Trees) that may be used for an efficient
 implementation, it is worth considering some other methods in lieu of
 using the EID/SID combination.  We describe some methods below :

1. For shared trees, the flow-id for a particular group G may be stored

 and updated in the association database.  Since we have to use the
 association database anyway to obtain the RP-EID, these does not cause
 much additional burden.
 However, this cannot be used efficiently for source-based trees because
 we need a flow-id for each combination of Source and Group.

2. The flow-id for shared trees could be done as above. When the sender

 does an RP-Register, it could send the RP the flow-id that it wishes to
 be used by receivers when they switch to a source-based tree.  This
 could be included in the RP-Register message.  The RP could then
 multicast that flow-id to all receivers in a special packet.  When the
 receivers wish to switch, they use that flow-id.
 This needs the definition of the "special" packet.

3. The flow-id is handed out only by the source (for source-based trees)

 or the RP (for shared trees).  The receivers use a "dummy" flow-id in
 the NMFReq when doing a Tree-Join.  The correct flow-id to be used is
 returned in the NMFRep message generated by the forwarding agent where
 the new branch meets the existing tree.  Forwarding agents in the path
 of the NMFRep packet update the state information by rewriting the
 dummy flow-id by the correct flow-id contained in the NMFRep packet.
 This requires the re-definition of the NMFRep packet.  Note that now
 there must be space for two flow-ids in the NMFRep packet - one for the
 "dummy" flow-id and the other for the "correct" flow-id that must
 replace the dummy flow-id.
 We claim that each of the above schemes achieves synchronization in
 the flow-id in various parts of the internetwork and that each flow-
 id is unique to the multicast delivery tree.  A formal proof of these
 claims is beyond the scope of this document.

Ramanathan Informational [Page 20] RFC 2102 Nimrod Multicast Support February 1997

 Dense Mode Multicast:  The PIM architecture [DEF+94a] includes a
 multicast protocol when the group membership is densely distributed
 within the internetwork.  In this mode, no Rendezvous Points are used
 and a source rooted tree is formed based on Reverse Path Forwarding
 in a manner similar to that of DVMRP [DC90].
 We do not give details of how Nimrod can implement Dense Mode
 Multicast here.
 Multiple RPs:  Our discussion above has been based on the assumption
 that there is one RP per group.  PIM allows more than one RP per
 group.  We do not discuss multiple-RP PIM here.

6 Security Considerations

 Security issues are not discussed in this memo.

7 Summary

o Nimrod does not specify a particular multicast route generation

algorithm or state creation procedure.  Nimrod can accommodate diverse
multicast techniques and leaves the choice of the technique to the
particular instantiation of Nimrod.

o A solution for multicasting within Nimrod should be capable of:

  1. Scaling to large networks, large numbers of multicast groups and

large multicast groups.

  1. Supporting policy, including quality of service and access


  1. Resource sharing.
  1. Interoperability with other solutions.

o Multicasting typically requires the setting up of state in intermediate

routers for packet forwarding.  The state setup may be initiated by the
sender (e.g., IDPR), by the receiver (e.g., CBT), by both (e.g., PIM)
or by neither.  The architecture of Nimrod provides sufficient
flexibility to accommodate any of these approaches.

o A receiver-initiated multicast protocol, PIM, is being designed by the

IDMR working group of the IETF. The facilities provided by Nimrod make
the use of PIM as a multicast protocol quite straightforward.

Ramanathan Informational [Page 21] RFC 2102 Nimrod Multicast Support February 1997

8 References

[BFC93] A. J. Ballardie, P. F. Francis, and J. Crowcroft. Core based

        trees. In Proceedings of ACM SIGCOMM, 1993.

[CCS96] Castineyra, I., Chiappa, N., and M. Steenstrup, "The Nimrod

        Routing Architecture", RFC 1992, August 1996.

[DC90] S. Deering and D. Cheriton. Multicast routing in datagram

        internetworks and extended lans. ACM Transactions on Computer
        Systems, pages 85--111, May 1990.

[DEF+94a] Deering, S., Estrin, D., Farinacci, D., Jacobson, V., Liu,

        C., and L. Wei, "Protocol Independent Multicast (PIM) :
        Motivation and Architecture, Work in Progress.

[DEF+94b] Deering, S., Estrin, D., Farinacci, D., Jacobson, V., Liu,

        C., and L. Wei, "Protocol Independent Multicast (PIM) :
        Sparse Mode Protocol Specification, Work in Progress.

[DEFJ93] Deering, S., Estrin, D., Farinacci, D., and V. Jacobson,

        "IGMP router extensions for routing to sparse multicast
        groups, Work in Progress.

[DM78] Y. K. Dalal and R. M. Metcalfe. Reverse path forwarding of

        broadcast packets. Communications of the ACM, 21(12), pages
        1040--1048, 1978.

[Moy92] Moy, J., "Multicast Extensions to OSPF, RFC 1584, March 1994.

[RS96] Ramanathan, S., and M. Steenstrup, "Nimrod functional and

        protocol specifications, Work in Progress.

[Ste] Steenstrup, M., "Inter-domain policy routing protocol

        specification:  Version 2", Work in Progress.

Ramanathan Informational [Page 22] RFC 2102 Nimrod Multicast Support February 1997

9 Acknowledgements

 We thank Isidro Castineyra (BBN), Charles Lynn (BBN), Martha
 Steenstrup (BBN) and other members of the Nimrod Working Group for
 their comments and suggestions on this memo.

10 Author's Address

 Ram Ramanathan
 BBN Systems and Technologies
 10 Moulton Street
 Cambridge, MA 02138
 Phone:  (617) 873-2736

Ramanathan Informational [Page 23]

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