The IP over ATM Mailing List Archive by date

[Keith McCloghrie <kzm@cisco.com>]

Grenville,

> >>Another reason (applicable to IP multicast) is the use of the "expanding
> >>ring search" technique to progressively search a greater and greater
> >>set of receivers of a particular multicast group.  LIS's with a large
> >>number of hosts defeat this technique, particularly with respect to
> >>finding the "closest" receiver.
> 
> This is a bit off tangent, but could you elaborate on the
> actual use of this technique?
 

                                          By expanding ring search is
            meant the following. Hosts can restrict the frowarding
            extent of the IP multicast datagrams that they send by
            appropriate setting of the TTL value in the datagram's IP
            header.  Then, for example, to search for the nearest server
            the host can send multicasts first with TTL set to 1, then
            2, etc.


      o MOSPF provides an optimization for IP multicast's "expanding
        ring search" (sometimes called "TTL scoping") procedure. In
        an expanding ring search, an application finds the nearest
        server by sending out successive multicasts, each with a
        larger TTL. The first responding server will then be the
        closest (in terms of hops, but not necessarily in terms of
        the OSPF metric).

It is also referenced in the BOOTP spec (RFC 1497), and Host Requirements 
(RFC 1122).  Router Requirements (RFC 1716) says:

         (6a) Based on the IP source and destination addresses found in
              the datagram header, the router determines the datagram's
              outgoing interfaces. In order to implement IP multicast's
              expanding ring search (see [INTERNET:4]) a minimum TTL
              value is specified for each outgoing interface. A copy of
              the multicast datagram is forwarded out each outgoing
              interface whose minimum TTL value is less than or equal to
              the TTL value in the datagram header, by separately
              applying the remaining steps on each such interface.

Keith.