[magma] MLDv2 router in the presence of MLDv1 Multicast Address Listeners

"K.Kawaguchi" <kawaguti@ysknet.co.jp> Tue, 28 September 2010 08:28 UTC

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Subject: [magma] MLDv2 router in the presence of MLDv1 Multicast Address Listeners
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Hi all,

I have another question.

About a size limit of a Multicast Address Specific Query in MLDv2.

How is a query transmitted when the number of multicast source
addresses of a query message is limited by the MTU?

Is a Query split like a Report type IS_IN?
(It isn't a Query like Report type IS_EX, is it?)

5.  Message Formats

5.1.  Multicast Listener Query Message

5.1.10.  Number of Sources (N)

   The Number of Sources (N) field specifies how many source addresses
   are present in the Query.  This number is zero in a General Query or
   a Multicast Address Specific Query, and non-zero in a Multicast
   Address and Source Specific Query.  This number is limited by the MTU
   of the link over which the Query is transmitted.  For example, on an
   Ethernet link with an MTU of 1500 octets, the IPv6 header (40 octets)
   together with the Hop-By-Hop Extension Header (8 octets) that
   includes the Router Alert option consume 48 octets; the MLD fields up
   to the Number of Sources (N) field consume 28 octets; thus, there are
   1424 octets left for source addresses, which limits the number of
   source addresses to 89 (1424/16).

5.2.  Version 2 Multicast Listener Report Message

5.2.15.  Multicast Listener Report Size

   If the set of Multicast Address Records required in a Report does not
   fit within the size limit of a single Report message (as determined
   by the MTU of the link on which it will be sent), the Multicast
   Address Records are sent in as many Report messages as needed to
   report the entire set.

   If a single Multicast Address Record contains so many source
   addresses that it does not fit within the size limit of a single
   Report message, then:

   o  if its Type is not IS_EX or TO_EX, it is split into multiple
      Multicast Address Records; each such record contains a different
      subset of the source addresses, and is sent in a separate Report.

   o  if its Type is IS_EX or TO_EX, a single Multicast Address Record
      is sent, with as many source addresses as can fit; the remaining
      source addresses are not reported.  Although the choice of which
      sources to report is arbitrary, it is preferable to report the
      same set of sources in each subsequent report, rather than
      reporting different sources each time.

Best Regards
Kiyoaki Kawaguchi