RFC 2365 (rfc2365) - Page 2 of 8


Administratively Scoped IP Multicast



Alternative Format: Original Text Document



RFC 2365          Administratively Scoped IP Multicast         July 1998


   TTL scoping has been used to control the distribution of multicast
   traffic with the objective of easing stress on scarce resources
   (e.g., bandwidth), or to achieve some kind of improved privacy or
   scaling properties. In addition, the TTL is also used in its
   traditional role to limit datagram lifetime. Given these often
   conflicting roles, TTL scoping has proven difficult to implement
   reliably, and the resulting schemes have often been complex and
   difficult to understand.

   A more serious architectural problem concerns the interaction of TTL
   scoping with broadcast and prune protocols (e.g., DVMRP [DVMRP]). The
   particular problem is that in many common cases, TTL scoping can
   prevent pruning from being effective. Consider the case in which a
   packet has either had its TTL expire or failed a TTL threshold. The
   router which discards the packet will not be capable of pruning any
   upstream sources, and thus will sink all multicast traffic (whether
   or not there are downstream receivers). Note that while it might seem
   possible to send prunes upstream from the point at which a packet is
   discarded, this strategy can result in legitimate traffic being
   discarded, since subsequent packets could take a different path and
   arrive at the same point with a larger TTL.

   On the other hand, administratively scoped IP multicast can provide
   clear and simple semantics for scoped IP multicast. The key
   properties of administratively scoped IP multicast are that (i).
   packets addressed to administratively scoped multicast addresses do
   not cross configured administrative boundaries, and (ii).
   administratively scoped multicast addresses are locally assigned, and
   hence are not required to be unique across administrative boundaries.

4. Definition of the Administratively Scoped IPv4 Multicast Space

   The administratively scoped IPv4 multicast address space is defined
   to be the range 239.0.0.0 to 239.255.255.255.

5. Discussion

   In order to support administratively scoped IP multicast, a router
   should support the configuration of per-interface scoped IP multicast
   boundaries. Such a router, called a boundary router, does not forward
   packets matching an interface's boundary definition in either
   direction (the bi-directional check prevents problems with multi-
   access networks). In addition, a boundary router always prunes the
   boundary for dense-mode groups [PIMDM], and doesn't accept joins for
   sparse-mode groups [PIMSM] in the administratively scoped range.






Meyer                    Best Current Practice