RFC 2191 (rfc2191) - Page 3 of 12

VENUS - Very Extensive Non-Unicast Service

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 RFC 2022 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