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