RFC 2121 (rfc2121) - Page 2 of 12

Issues affecting MARS Cluster Size

RFC 2121           Issues affecting MARS Cluster Size         March 1997


   The geographic distribution of clients affects the latency between a
   client issuing a MARS_JOIN, and it finally being added onto the pt-
   mpt VCs of the other MARS Clients transmitting to the specified
   multicast group. (This latency is made up of both the time to
   propagate the MARS_JOIN, and the delay in the underlying ATM cloud's
   reaction to the subsequent ADD_PARTY messages.)

   When architecting an IP/ATM network it is important to understand the
   worst case scaling limits applicable to your Clusters. This document
   provides a primarily qualitative look at the design choices that
   impose the most dramatic constraints on Cluster size. Since the focus
   is on worst-case scenarios, most of the analysis will assume
   multicast groups that are VC Mesh based and have all cluster members
   as sources and receivers. Engineering using the worst-case boundary
   conditions, then applying optimisations such as Multicast Servers
   (MCS), provides the Cluster with a margin of safety.  It is hoped
   that more detailed quantitative analysis of Cluster sizing limits
   will be prompted by this document.

   Section 2 comments on the VC state requirements of the MARS model,
   while Sections 3 and 4 identify the group change processing load and
   latency characteristics of a cluster as a function of its size.
   Section 5 looks at how Multicast Routers (both conventional and
   combination router/switch architectures) increase the scale of a
   multicast capable IP/ATM network. Finally, Section 6 discusses how
   the use of Multicast Servers (MCS) might impact on the worst case
   Cluster size limits.


2. VC state limitations.

   Two characteristics of ATM NICs and switches will limit the number of
   members a Cluster may contain. They are:

      The maximum number of VCs that can be originated from, or
      terminate on, a port (VCmax).

      The maximum number of leaf nodes supportable by a root node
      (LEAFmax).

   We'll assume that the MARS node has similar VCmax and LEAFmax values
   as Cluster members.  VCmax affects the Cluster size because of the
   following:

      The MARS terminates a pt-pt control VC from each cluster member,
      and originates a VC for ClusterControlVC and ServerControlVC.





Armitage                     Informational