One document matched: draft-armitage-ion-venus-00.txt

Internet-Draft                                      Grenville Armitage
                                                              Bellcore
                                                       July 22nd, 1996


               VENUS - Very Extensive Non-Unicast Service
                   <draft-armitage-ion-venus-00.txt>


Status of this Memo

   This document was submitted to the IETF Internetworking over NBMA
   (ION) WG.  Publication of this document does not imply acceptance by
   the ION WG of any ideas expressed within.  Comments should be
   submitted to the ion@nexen.com mailing list.

   Distribution of this memo is unlimited.

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Abstract

   The MARS model is our current solution to IP multicasting over ATM,
   establishing and managing the use of ATM pt-mpt SVCs for intra-LIS 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 `cut-through' mechanisms (e.g.
   NHRP) has led some people to request the development of a Multicast
   equivalent. This document describes a hypothetical solution, dubbed
   `Very Extensive NonUnicast Service' (VENUS), shows how complex such a
   service would be, and asserts that analogies with unicast cut-through



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   mechanisms are not entirely appropriate. It is also noted that VENUS
   ultimately has the look and feel of a single, large cluster using a
   distributed MARS - a problem space we are already tackling for other
   reasons. This document is being issued to help focus ION efforts, and
   optionally challenge people to prove its thesis wrong.


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], to allow unicast forwarding paths to
   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 cannot be simplistically re-applied. 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 our hypothetical multicast
   cut-through 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
   `cut-through 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 group members at any given time. However, once you
   have sources within a VENUS Domain establishing cut-through
   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



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   similar to a single, large distributed MARS Cluster.

   A second problem is the impact that cut-through 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 cut-through 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 networks with sparsely distributed multicast group
   membership. VENUS introduces tougher requirements by assuming that
   multicast group membership may be dense across the region of
   interest.)

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


2. What does it mean to `cut-through' ?

   Before going further it is worth considering both the definition of
   the Cluster, and two possible definitions of `cut-through'.

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



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   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 `cut-through'

   The currently popular definition of `cut-through' 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 "cutting through" the logical IP
   topology (and very real routers) along the unicast path from source
   to destination.

   For simplicity of early adoption it was recommended that a Cluster's
   scope be made equivalent to that of a LIS. Under these circumstances
   the `Classical IP' routing model would place Mrouters an the
   LIS/Cluster boundaries, and multicast cut-through involves bypassing
   the same physical routing entities as unicast routing. 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 `cut-through'

   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 `cut
   through' 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,



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   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 cut-through 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.)

   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 we are currently developing 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 MARSs 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 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



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   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).

3.1 SVC establishment - answering a MARS_REQUEST.

   The SVC establishment phase contains a number of inter-related
   problems.

   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.

   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 degree of
   `cut through' 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



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   address of the Mrouter into that cluster. The nett effect would be
   that the MARS Client establishes a cut-through 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 (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 prematurely end 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 cut-through to different
   (sub)sets of the group members scattered across the participating
   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 `cut through' 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' of participating clusters 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 you don't
   simply solve your multiple-LIS multicasting by creating a single big
   MARS Cluster.)

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



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   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 cut-through 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
   cut-through 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
   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 cut-through 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



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   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
   cut-through SVC directly into a remote cluster while a second client
   - in the same source cluster - may have been forced to terminate its
   cut-through 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 imagination.

   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?

   A good question, since the whole aim of developing a cut-through
   connection mechanism is predicated on the assumption that bypassing
   IP level entities is always a `win'. However, this is arguably not
   true for multicast.



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   The most important observation that should be made about cut-through
   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. And
   this immediately begs the question of why you'd bypass an Mrouter. As
   noted in [4] they 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 cut- through connections.

   (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 minor benefits in removing the additional re-
   assembly/segmentation latencies of having packets pass through an
   Mrouter. However, if you've already ascertained that your VENUS
   Domain is small enough to avoid the scaling limits in [4], you might
   as well simply construct a single large MARS Cluster. A large cluster
   avoids a topological mismatch between IP Mrouters and ATM switches
   too.)


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:

      - The Cluster is partitioned into sub-clusters.

      - 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.

      - The MARS_REQUEST from any sub-cluster member must return
      information from all the sub-clusters, so as to ensure that all a



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      group's members across the cluster are identified.

      - 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
   VENUS:

      - There are no Mrouters (IDMR nodes) within the scope of the
      distributed Cluster.

      - 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).

6. Conclusion.

   This document has described a hypothetical multicast cut-through
   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 you
   expand the potential scope of a pt-mpt SVC at establishment phase,
   you must similarly expand the scope of the SVC management mechanism.
   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 cut-through 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, and it is the author's contention
   that developing a general VENUS mechanism is not a valuable use of
   the ION working group's time. Developing solutions for large clusters
   using a distributed MARS, and single clusters spanning multiple
   Logical IP Subnets, is a much better focus for ION's work.



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Security Considerations

   Security considerations are not addressed in this document.


Author's Address

   Grenville Armitage
   Bellcore, 445 South Street
   Morristown, NJ, 07960
   USA

   Email: gja@thumper.bellcore.com
   Ph. +1 201 829 2635



References

   [1] Laubach, M., "Classical IP and ARP over ATM", RFC1577, Hewlett-
   Packard Laboratories, December 1993.

   [2] G. Armitage, "Support for Multicast over UNI 3.0/3.1 based ATM
   Networks.", Bellcore, INTERNET DRAFT, draft-ietf-ipatm-ipmc-12.txt,
   February 1996.

   [3] J. Luciani, et al, "NBMA Next Hop Resolution Protocol (NHRP)",
   INTERNET DRAFT, draft-ietf-rolc-nhrp-09.txt, July 1996.

   [4] G. Armitage, "Issues affecting MARS Cluster Size", Bellcore,
   INTERNET DRAFT, draft-armitage-ion-cluster-size-00.txt, July 1996.

   [5] G. Armitage, "Redundant MARS architectures and SCSP", Bellcore,
   INTERNET DRAFT, draft-armitage-ion-mars-scsp-00.txt, June 1996.

   [6] J. Luciani, G. Armitage, J. Jalpern, "Server Cache
   Synchronization Protocol (SCSP) - NBMA", INTERNET DRAFT, draft-
   luciani-rolc-scsp-03.txt, June 1996.

   [7] Y. Rekhter, D. Farinacci, " Support for Sparse Mode PIM over
   ATM", Cisco Systems, INTERNET DRAFT, draft-ietf-rolc-pim-atm-00.txt,
   April 1996.









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