Movatterモバイル変換

Network Working Group                                      G. ArmitageRequest for Comments: 2191                         Lucent TechnologiesCategory: Informational                                 September 1997VENUS - Very Extensive Non-Unicast ServiceStatus 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.Abstract   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 scrutinizedArmitage                     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 insection 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 adoptionRFC2022 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 theArmitage                     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   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.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 participatingArmitage                     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   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   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 singleArmitage                     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 itsArmitage                     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   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?   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:      - 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      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).Armitage                     Informational                     [Page 10]

RFC 2191                         VENUS                    September 19976. 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.Acknowledgment   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 inRFC 2022.   It is an Informational memo, and does not mandate any additional   protocol behaviors beyond those described inRFC 2022.  As such, the   security implications are no greater or less than the implications   inherent inRFC 2022.  Should enhancements to security be required,   they would need to be added as an extension to the base architecture   inRFC 2022.Armitage                     Informational                     [Page 11]

RFC 2191                         VENUS                    September 1997Author's Address   Grenville Armitage   Bell Labs, Lucent Technologies.   101 Crawfords Corner Rd,   Holmdel, NJ, 07733   USA   EMail: gja@dnrc.bell-labs.comReferences   [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,RFC2121, 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]