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Network Working Group G. Armitage Request for Comments: 2191 Lucent Technologies Category: Informational September 1997

             VENUS - Very Extensive Non-Unicast Service

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.


 The MARS model (RFC2022) provides a solution to intra-LIS IP
 multicasting over ATM, establishing and managing the use of ATM pt-
 mpt SVCs for IP multicast packet forwarding. Inter-LIS multicast
 forwarding is achieved using Mrouters, in a similar manner to which
 the "Classical IP over ATM" model uses Routers to inter-connect LISes
 for unicast traffic. The development of unicast IP shortcut
 mechanisms (e.g.  NHRP) has led some people to request the
 development of a Multicast equivalent. There are a number of
 different approaches. This document focuses exclusively on the
 problems associated with extending the MARS model to cover multiple
 clusters or clusters spanning more than one subnet. It describes a
 hypothetical solution, dubbed "Very Extensive NonUnicast Service"
 (VENUS), and shows how complex such a service would be. It is also
 noted that VENUS ultimately has the look and feel of a single, large
 cluster using a distributed MARS.  This document is being issued to
 help focus ION efforts towards alternative solutions for establishing
 ATM level multicast connections between LISes.

1. Introduction

 The classical model of the Internet running over an ATM cloud
 consists of multiple Logical IP Subnets (LISs) interconnected by IP
 Routers [1].  The evolving IP Multicast over ATM solution (the "MARS
 model" [2]) retains the classical model. The LIS becomes a "MARS
 Cluster", and Clusters are interconnected by conventional IP
 Multicast routers (Mrouters).
 The development of NHRP [3], a protocol for discovering and managing
 unicast forwarding paths that bypass IP routers, has led to some
 calls for an IP multicast equivalent.  Unfortunately, the IP
 multicast service is a rather different beast to the IP unicast
 service. This document aims to explain how much of what has been
 learned during the development of NHRP must be carefully scrutinized

Armitage Informational [Page 1] RFC 2191 VENUS September 1997

 before being re-applied to the multicast scenario. Indeed, the
 service provided by the MARS and MARS Clients in [2] are almost
 orthogonal to the IP unicast service over ATM.
 For the sake of discussion, let's call this hypothetical multicast
 shortcut discovery protocol the "Very Extensive Non-Unicast Service"
 (VENUS). A "VENUS Domain" is defined as the set of hosts from two or
 more participating Logical IP Subnets (LISs). A multicast shortcut
 connection is a point to multipoint SVC whose leaf nodes are
 scattered around the VENUS Domain. (It will be noted in section 2
 that a VENUS Domain might consist of a single MARS Cluster spanning
 multiple LISs, or multiple MARS Clusters.)
 VENUS faces a number of fundamental problems. The first is exploding
 the scope over which individual IP/ATM interfaces must track and
 react to IP multicast group membership changes. Under the classical
 IP routing model Mrouters act as aggregation points for multicast
 traffic flows in and out of Clusters [4]. They also act as
 aggregators of group membership change information - only the IP/ATM
 interfaces within each Cluster need to know the specific identities
 of their local (intra-cluster) group members at any given time.
 However, once you have sources within a VENUS Domain establishing
 shortcut connections the data and signaling plane aggregation of
 Mrouters is lost. In order for all possible sources throughout a
 VENUS Domain to manage their outgoing pt-mpt SVCs they must be kept
 aware of MARS_JOINs and MARS_LEAVEs occuring in every MARS Cluster
 that makes up a VENUS Domain. The nett effect is that a VENUS domain
 looks very similar to a single, large distributed MARS Cluster.
 A second problem is the impact that shortcut connections will have on
 IP level Inter Domain Multicast Routing (IDMR) protocols. Multicast
 groups have many sources and many destinations scattered amongst the
 participating Clusters. IDMR protocols assume that they can calculate
 efficient inter-Cluster multicast trees by aggregating individual
 sources or group members in any given Cluster (subnet) behind the
 Mrouter serving that Cluster. If sources are able to simply bypass an
 Mrouter we introduce a requirement that the existence of each and
 every shortcut connection be propagated into the IDMR decision making
 processes. The IDMR protocols may need to adapt when a source's
 traffic bypasses its local Mrouter(s) and is injected into Mrouters
 at more distant points on the IP-level multicast distribution tree.
 (This issue has been looked at in [7], focussing on building
 forwarding trees within networks where the termination points are
 small in number and sparsely distributed. VENUS introduces tougher
 requirements by assuming that multicast group membership may be dense
 across the region of interest.)

Armitage Informational [Page 2] RFC 2191 VENUS September 1997

 This document will focus primarily on the internal problems of a
 VENUS Domain, and leave the IDMR interactions for future analysis.

2. What does it mean to "shortcut" ?

 Before going further it is worth considering both the definition of
 the Cluster, and two possible definitions of "shortcut".

2.1 What is a Cluster?

 In [2] a MARS Cluster is defined as the set of IP/ATM interfaces that
 are willing to engage in direct, ATM level pt-mpt SVCs to perform IP
 multicast packet forwarding. Each IP/ATM interface (a MARS Client)
 must keep state information regarding the ATM addresses of each leaf
 node (recipient) of each pt-mpt SVC it has open. In addition, each
 MARS Client receives MARS_JOIN and MARS_LEAVE messages from the MARS
 whenever there is a requirement that Clients around the Cluster need
 to update their pt-mpt SVCs for a given IP multicast group.
 It is worth noting that no MARS Client has any concept of how big its
 local cluster is - this knowledge is kept only by the MARS that a
 given Client is registered with.
 Fundamentally the Cluster (and the MARS model as a whole) is a
 response to the requirement that any multicast IP/ATM interface using
 pt-mpt SVCs must, as group membership changes, add and drop leaf
 nodes itself. This means that some mechanism, spanning all possible
 group members within the scopes of these pt-mpt SVCs, is required to
 collect group membership information and distribute it in a timely
 fashion to those interfaces.  This is the MARS Cluster, with certain
 scaling limits described in [4].

2.2 LIS/Cluster boundary "shortcut"

 The currently popular definition of "shortcut" is based on the
 existence of unicast LIS boundaries. It is tied to the notion that
 LIS boundaries have physical routers, and cutting through a LIS
 boundary means bypassing a router. Intelligently bypassing routers
 that sit at the edges of LISs has been the goal of NHRP. Discovering
 the ATM level identity of an IP endpoint in a different LIS allows a
 direct SVC to be established, thus shortcutting the logical IP
 topology (and very real routers) along the unicast path from source
 to destination.
 For simplicity of early adoption RFC2022 recommends that a Cluster's
 scope be made equivalent to that of a LIS. Under these circumstances
 the "Classical IP" routing model places Mrouters at LIS/Cluster
 boundaries, and multicast shortcutting must involve bypassing the

Armitage Informational [Page 3] RFC 2191 VENUS September 1997

 same physical routing entities as unicast shortcutting. Each MARS
 Cluster would be independent and contain only those IP/ATM interfaces
 that had been assigned to the same LIS.
 As a consequence, a VENUS Domain covering the hosts in a number of
 LIS/Clusters would have to co-ordinate each individual MARS from each
 LIS/Cluster (to ensure group membership updates from around the VENUS
 Domain were propagated correctly).

2.3 Big Cluster, LIS boundary "shortcut"

 The MARS model's fundamental definition of a Cluster was deliberately
 created to be independent of unicast terminology. Although not
 currently well understood, it is possible to build a single MARS
 Cluster that encompasses the members of multiple LISs. As expected,
 inter-LIS unicast traffic would pass through (or bypass, if using
 NHRP) routers on the LIS boundaries. Also as expected, each IP/ATM
 interface, acting as a MARS Client, would forward their IP multicast
 packets directly to intra-cluster group members. However, because the
 direct intra-cluster SVCs would exist between hosts from the
 different LISs making up the cluster, this could be considered a
 "shortcut" of the unicast LIS boundaries.
 This approach immediately brings up the problem of how the IDMR
 protocols will react. Mrouters only need to exist at the edges of
 Clusters. In the case of a single Cluster spanning multiple LISs,
 each LIS becomes hidden behind the Mrouter at the Cluster's edge.
 This is arguably not a big problem if the Cluster is a stub on an
 IDMR protocol's multicast distribution tree, and if there is only a
 single Mrouter in or out of the Cluster. Problems arise when two or
 more Mrouters are attached to the edges of the Cluster, and the
 Cluster is used for transit multicast traffic. Each Mrouter's
 interface is assigned a unicast identity (e.g. that of the unicast
 router containing the Mrouter). IDMR protocols that filter packets
 based on the correctness of the upstream source may be confused at
 receiving IP multicast packets directly from another Mrouter in the
 same cluster but notionally "belonging" to an LIS multiple unicast IP
 hops away.
 Adjusting the packet filtering algorithms of Mrouters is something
 that needs to be addressed by any multicast shortcut scheme. It has
 been noted before and a solution proposed in [7]. For the sake of
 argument this document will assume the problem solvable. (However, it
 is important that any solution scales well under general topologies
 and group membership densities.)

Armitage Informational [Page 4] RFC 2191 VENUS September 1997

 A multi-LIS MARS Cluster can be considered a simple VENUS Domain.
 Since it is a single Cluster it can be scaled using the distributed
 MARS solutions currently being developed within the IETF [5,6].

3. So what must VENUS look like?

 A number of functions that occur in the MARS model are fundamental to
 the problem of managing root controlled, pt-mpt SVCs. The initial
 setup of the forwarding SVC by any one MARS Client requires a
 query/response exchange with the Client's local MARS, establishing
 who the current group members are (i.e. what leaf nodes should be on
 the SVC). Following SVC establishment comes the management phase -
 MARS Clients need to be kept informed of group membership changes
 within the scopes of their SVCs, so that leaf nodes may be added or
 dropped as appropriate.
 For intra-cluster multicasting the current MARS approach is our
 solution for these two phases.
 For the rest of this document we will focus on what VENUS would look
 like when a VENUS Domain spans multiple MARS Clusters. Under such
 circumstances VENUS is a mechanism co-ordinating the MARS entities of
 each participating cluster. Each MARS is kept up to date with
 sufficient domain-wide information to support both phases of client
 operation (SVC establishment and SVC management) when the SVC's
 endpoints are outside the immediate scope of a client's local MARS.
 Inside a VENUS Domain a MARS Client is supplied information on group
 members from all participating clusters.
 The following subsections look at the problems associated with both
 of these phases independently. To a first approximation the problems
 identified are independent of the possible inter-MARS mechanisms. The
 reader may assume the MARS in any cluster has some undefined
 mechanism for communicating with the MARSs of clusters immediately
 adjacent to its own cluster (i.e. connected by a single Mrouter hop).

3.1 SVC establishment - answering a MARS_REQUEST.

 The SVC establishment phase contains a number of inter-related
 First, the target of a MARS_REQUEST (an IP multicast group) is an
 abstract entity. Let us assume that VENUS does not require every MARS
 to know the entire list of group members across the participating
 clusters.  In this case each time a MARS_REQUEST is received by a
 MARS from a local client, the MARS must construct a sequence of
 MARS_MULTIs based on locally held information (on intra-cluster
 members) and remotely solicited information.

Armitage Informational [Page 5] RFC 2191 VENUS September 1997

 So how does it solicit this information? Unlike the unicast
 situation, there is no definite, single direction to route a
 MARS_REQUEST across the participating clusters. The only "right"
 approach is to send the MARS_REQUEST to all clusters, since group
 members may exist anywhere and everywhere. Let us allow one obvious
 optimization - the MARS_REQUEST is propagated along the IP multicast
 forwarding tree that has been established for the target group by
 whatever IDMR protocol is running at the time.
 As noted in [4] there are various reasons why a Cluster's scope be
 kept limited. Some of these (MARS Client or ATM NIC limitations)
 imply that the VENUS discovery process not return more group members
 in the MARS_MULTIs that the requesting MARS Client can handle. This
 provides VENUS with an interesting problem of propagating out the
 original MARS_REQUEST, but curtailing the MARS_REQUESTs propagation
 when a sufficient number of group members have been identified.
 Viewed from a different perspective, this means that the scope of
 shortcut achievable by any given MARS Client may depend greatly on
 the shape of the IP forwarding tree away from its location (and the
 density of group members within clusters along the tree) at the time
 the request was issued.
 How might we limit the number of group members returned to a given
 MARS Client? Adding a limit TLV to the MARS_REQUEST itself is
 trivial. At first glance it might appear that when the limit is being
 reached we could summarize the next cluster along the tree by the ATM
 address of the Mrouter into that cluster. The nett effect would be
 that the MARS Client establishes a shortcut to many hosts that are
 inside closer clusters, and passes its traffic to more distant
 clusters through the distant Mrouter. However, this approach only
 works passably well for a very simplistic multicast topology (e.g. a
 linear concatenation of clusters).
 In a more general topology the IP multicast forwarding tree away from
 the requesting MARS Client will branch a number of times, requiring
 the MARS_REQUEST to be replicated along each branch. Ensuring that
 the total number of returned group members does not exceed the
 client's limit becomes rather more difficult to do efficiently.
 (VENUS could simply halve the limit value each time it split a
 MARS_REQUEST, but this might cause group member discovery on one
 branch to end prematurely while all the group members along another
 branch are discovered without reaching the subdivided limit.)
 Now consider this decision making process scattered across all the
 clients in all participating clusters. Clients may have different
 limits on how many group members they can handle - leading to
 situations where different sources can shortcut to different
 (sub)sets of the group members scattered across the participating

Armitage Informational [Page 6] RFC 2191 VENUS September 1997

 clusters (because the IP multicast forwarding trees from senders in
 different clusters may result in different discovery paths being
 taken by their MARS_REQUESTs.)
 Finally, when the MARS_REQUEST passes a cluster where the target
 group is MCS supported, VENUS must ensure the ATM address of the MCS
 is collected rather than the addresses of the actual group members.
 (To do otherwise would violate the remote cluster's intra-cluster
 decision to use an MCS. The shortcut in this case must be content to
 directly reach the remote cluster's MCS.)
 (A solution to part of this problem would be to ensure that a VENUS
 Domain never has more MARS Clients throughout than the clients are
 capable of adding as leaf nodes. This may or may not appeal to
 people's desire for generality of a VENUS solution. It also would
 appear to beg the question of why the problem of multiple-LIS
 multicasting isn't solved simply by creating a single big MARS

3.2 SVC management - tracking group membership changes.

 Once a client's pt-mpt SVC is established, it must be kept up to
 date.  The consequence of this is simple, and potentially
 devastating: The MARS_JOINs and MARS_LEAVEs from every MARS Client in
 every participating cluster must be propagated to every possible
 sender in every participating cluster (this applies to groups that
 are VC Mesh supported - groups that are MCS supported in some or all
 participating clusters introduce complications described below).
 Unfortunately, the consequential signaling load (as all the
 participating MARSs start broadcasting their MARS_JOIN/LEAVE
 activity) is not localized to clusters containing MARS Clients who
 have established shortcut SVCs.  Since the IP multicast model is Any
 to Multipoint, and you can never know where there may be source MARS
 Clients, the JOINs and LEAVEs must be propagated everywhere, always,
 just in case. (This is simply a larger scale version of sending JOINs
 and LEAVEs to every cluster member over ClusterControlVC, and for
 exactly the same reason.)
 The use of MCSs in some clusters instead of VC Meshes significantly
 complicates the situation, as does the initial scoping of a client's
 shortcut during the SVC establishment phase (described in the
 preceding section).
 In Clusters where MCSs are supporting certain groups, MARS_JOINs or
 MARS_LEAVEs are only propagated to MARS Clients when an MCS comes or
 goes. However, it is not clear how to effectively accommodate the
 current MARS_MIGRATE functionality (that allows a previously VC Mesh
 based group to be shifted to an MCS within the scope of a single

Armitage Informational [Page 7] RFC 2191 VENUS September 1997

 cluster). If an MCS starts up within a single Cluster, it is possible
 to shift all the intra-cluster senders to the MCS using MARS_MIGRATE
 as currently described in the MARS model. However, MARS Clients in
 remote clusters that have shortcut SVCs into the local cluster also
 need some signal to shift (otherwise they will continue to send their
 packets directly to the group members in the local cluster).
 This is a non-trivial requirement, since we only want to force the
 remote MARS Clients to drop some of their leaf nodes (the ones to
 clients within the Cluster that now has an MCS), add the new MCS as a
 leaf node, and leave all their other leaf nodes untouched (the cut-
 through connections to other clusters). Simply broadcasting the
 MARS_MIGRATE around all participating clusters would certainly not
 work.  VENUS needs a new control message with semantics of "replaced
 leaf nodes {x, y, z} with leaf node {a}, and leave the rest alone".
 Such a message is easy to define, but harder to use.
 Another issue for SVC management is that the scope over which a MARS
 Client needs to receive JOINs and LEAVEs needs to respect the
 Client's limited capacity for handling leaf nodes on its SVC. If the
 MARS Client initially issued a MARS_REQUEST and indicated it could
 handle 1000 leaf nodes, it is not clear how to ensure that subsequent
 joins of new members wont exceed that limit. Furthermore, if the SVC
 establishment phase decided that the SVC would stop at a particular
 Mrouter (due to leaf node limits being reached), the Client probably
 should not be receiving direct MARS_JOIN or MARS_LEAVE messages
 pertaining to activity in the cluster "behind" this Mrouter. (To do
 otherwise could lead to multiple copies of the source client's
 packets reaching group members inside the remote cluster - one
 version through the Mrouter, and another on the direct SVC connection
 that the source client would establish after receiving a subsequent,
 global MARS_JOIN regarding a host inside the remote cluster.)
 Another scenario involves the density of group members along the IDMR
 multicast tree increasing with time after the initial MARS_REQUEST is
 answered. Subsequent JOINs from Cluster members may dictate that a
 "closer" Mrouter be used to aggregate the source's outbound traffic
 (so as not to exceed the source's leaf node limitations). How to
 dynamically shift between terminating on hosts within a Cluster, and
 terminating on a cluster's edge Mrouter, is an open question.
 To complicate matters further, this scoping of the VENUS domain-wide
 propagation of MARS_JOINs and MARS_LEAVEs needs to be on a per-
 source- cluster basis, at least. If MARS Clients within the same
 cluster have different leaf node limits, the problem worsens. Under
 such circumstances, one client may have been able to establish a
 shortcut SVC directly into a remote cluster while a second client -
 in the same source cluster - may have been forced to terminate its

Armitage Informational [Page 8] RFC 2191 VENUS September 1997

 shortcut on the remote cluster's Mrouter. The first client obviously
 needs to know about group membership changes in the remote cluster,
 whilst the second client does not. Propagating these JOIN/LEAVE
 messages on ClusterControlVC in the source cluster will not work -
 the MARS for the source cluster will need to explicitly send copies
 of the JOIN/LEAVE messages only to those MARS Clients whose prior SVC
 establishment phase indicates they need them. Propagation of messages
 to indicate a VC Mesh to MCS transition within clusters may also need
 to take account of the leaf node limitations of MARS Clients. The
 scaling characteristics of this problem are left to the readers
 It was noted in the previous section that a VENUS domain could be
 limited to ensure there are never more MARS Clients than any one
 client's leaf node limit. This would certainly avoid the need to for
 complicated MARS_JOIN/LEAVE propagation mechanisms. However, it begs
 the question of how different the VENUS domain then becomes from a
 single, large MARS Cluster.

4. What is the value in bypassing Mrouters?

 This is a good question, since the whole aim of developing a shortcut
 connection mechanism is predicated on the assumption that bypassing
 IP level entities is always a "win". However, this is arguably not
 true for multicast.
 The most important observation that should be made about shortcut
 connection scenarios is that they increase the exposure of any given
 IP/ATM interface to externally generated SVCs. If there are a
 potential 1000 senders in a VENUS Domain, then you (as a group
 member) open yourself up to a potential demand for 1000 instances of
 your re-assembly engine (and 1000 distinct incoming SVCs, when you
 get added as a leaf node to each sender's pt-mpt SVC, which your
 local switch port must be able to support).
 It should be no surprise that the ATM level scaling limits applicable
 to a single MARS Cluster [4] will also apply to a VENUS Domain. Again
 we're up against the question of why you'd bypass an Mrouter. As
 noted in [4] Mrouters perform a useful function of data path
 aggregation - 100 senders in one cluster become 1 pt-mpt SVC out of
 the Mrouter into the next cluster along the tree. They also hide MARS
 signaling activity - individual group membership changes in one
 cluster are hidden from IP/ATM interfaces in surrounding clusters.
 The loss of these benefits must be factored into any network designed
 to utilize multicast shortcut connections.

Armitage Informational [Page 9] RFC 2191 VENUS September 1997

 (For the sake of completeness, it must be noted that extremely poor
 mismatches of IP and ATM topologies may make Mrouter bypass
 attractive if it improves the use of the underlying ATM cloud. There
 may also be benefits in removing the additional re-
 assembly/segmentation latencies of having packets pass through an
 Mrouter. However, a VENUS Domain ascertained to be small enough to
 avoid the scaling limits in [4] might just as well be constructed as
 a single large MARS Cluster. A large cluster also avoids a
 topological mismatch between IP Mrouters and ATM switches.)

5. Relationship to Distributed MARS protocols.

 The ION working group is looking closely at the development of
 distributed MARS architectures. An outline of some issues is provided
 in [5,6]. As noted earlier in this document the problem space looks
 very similar that faced by our hypothetical VENUS Domain. For
 example, in the load-sharing distributed MARS model:
  1. The Cluster is partitioned into sub-clusters.
  1. Each Active MARS is assigned a particular sub-cluster, and uses

its own sub-ClusterControlVC to propagate JOIN/LEAVE messages to

    members of its sub-cluster.
  1. The MARS_REQUEST from any sub-cluster member must return

information from all the sub-clusters, so as to ensure that all a

    group's members across the cluster are identified.
  1. Group membership changes in any one sub-cluster must be

immediately propagated to all the other sub-clusters.

 There is a clear analogy to be made between a distributed MARS
 Cluster, and a VENUS Domain made up of multiple single-MARS Clusters.
 The information that must be shared between sub-clusters in a
 distributed MARS scenario is similar to the information that must be
 shared between Clusters in a VENUS Domain.
 The distributed MARS problem is slightly simpler than that faced by
  1. There are no Mrouters (IDMR nodes) within the scope of the

distributed Cluster.

  1. In a distributed MARS Cluster an MCS supported group uses the

same MCS across all the sub-clusters (unlike the VENUS Domain,

    where complete generality makes it necessary to cope with mixtures
    of MCS and VC Mesh based Clusters).

Armitage Informational [Page 10] RFC 2191 VENUS September 1997

6. Conclusion.

 This document has described a hypothetical multicast shortcut
 connection scheme, dubbed "Very Extensive NonUnicast Service"
 (VENUS).  The two phases of multicast support - SVC establishment,
 and SVC management - are shown to be essential whether the scope is a
 Cluster or a wider VENUS Domain. It has been shown that once the
 potential scope of a pt-mpt SVC at establishment phase has been
 expanded, the scope of the SVC management mechanism must similarly be
 expanded. This means timely tracking and propagation of group
 membership changes across the entire scope of a VENUS Domain.
 It has also been noted that there is little difference in result
 between a VENUS Domain and a large MARS Cluster. Both suffer from the
 same fundamental scaling limitations, and both can be arranged to
 provide shortcut of unicast routing boundaries. However, a completely
 general multi-cluster VENUS solution ends up being more complex. It
 needs to deal with bypassed Mrouter boundaries, and dynamically
 changing group membership densities along multicast distribution
 trees established by the IDMR protocols in use.
 No solutions have been presented. This document's role is to provide
 context for future developments.


 This document was prepared while the author was with the
 Internetworking Research group at Bellcore.

Security Considerations

 This memo addresses specific scaling issues associated with the
 extension of the MARS architecture beyond that described in RFC 2022.
 It is an Informational memo, and does not mandate any additional
 protocol behaviors beyond those described in RFC 2022.  As such, the
 security implications are no greater or less than the implications
 inherent in RFC 2022.  Should enhancements to security be required,
 they would need to be added as an extension to the base architecture
 in RFC 2022.

Armitage Informational [Page 11] RFC 2191 VENUS September 1997

Author's Address

 Grenville Armitage
 Bell Labs, Lucent Technologies.
 101 Crawfords Corner Rd,
 Holmdel, NJ, 07733


 [1] Laubach, M., "Classical IP and ARP over ATM", RFC 1577, Hewlett-
 Packard Laboratories, December 1993.
 [2] Armitage, G., "Support for Multicast over UNI 3.0/3.1 based ATM
 Networks.", Bellcore, RFC 2022, November 1996.
 [3] Luciani, J., et al, "NBMA Next Hop Resolution Protocol (NHRP)",
 Work in Progress, February 1997.
 [4] Armitage, G., "Issues affecting MARS Cluster Size", Bellcore, RFC
 2121, March 1997.
 [5] Armitage, G., "Redundant MARS architectures and SCSP", Bellcore,
 Work in Progress, November 1996.
 [6] Luciani, J., G. Armitage, J. Jalpern, "Server Cache
 Synchronization Protocol (SCSP) - NBMA", Work in Progress, March 1997.
 [7] Rekhter, Y., D. Farinacci, " Support for Sparse Mode PIM over
 ATM", Cisco Systems, Work in Progress, April 1996.

Armitage Informational [Page 12]

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