The IP over ATM Mailing List Archive by date

[Rick Bubenik <rick@stl.nexen.com>]

[Jed writes:]
> >>  Also, only one
> >>such structure would exist in any switch for a multicast group
> >>(unlike with the mesh of VCs approach which has one point to
> >>multipoint VC for every leaf node transmitter in a multicast group).
> >>
[Grenville writes:]
> it is not entirely clear that you can easily construct a mpt-mpt
> VC service (to avoid the need for pt to mpt VC meshes).
> 
> The major difference between IP multicast routing and ATM multipoint
> VCs is that IDMR is routing self-contained entities with full
> source identification per packet. In ATM the unit being routed/switched
> is the cell, containing only path identification (VCI/VPI). This
> is a problem when using AAL5 - the reassembly engine at a/each receiver
> assumes a single segmentation engine produced a sequential flow
> of cells associated with one AAL_SDU. AAL5 could not operate over a mpt-mpt
> VC, as each receiver would be trying to reassemble AAL_SDUs from
> multiple sources whose cells were intermingled within the network.
> AAL3/4 incorporates a MID field that could be assigned uniquely
> to each sender on a mpt-mpt VC (up to 1023), but this AAL has been
> pushed aside by history (at least for the time being).
> 
> Given the desire to use AAL5, I think mpt-mpt will continue
> to be constructed from overlain pt-mpt VCs even after leaf
> initiated join signalling is deployed.

There is an alternative that allows AAL5 to be used for mpt-to-mpt
connections.  Namely, set up the connection as a switched VP and
either: 1) allocate a unique VCI to each transmitter or 2) set aside
a block of VCIs and have each transmitter randomly select a value
prior to sending a packet.  The first approach guarantees no
interleaving, but requires a VCI assignment protocol (relatively
simple, but not trivial).  The second approach allows occasional
interleaving, with an interleave probability that can be controlled
(based on the range of VCIs set aside and the number of expected
simultaneous transmitters).

The advantage of using a true mpt-to-mpt ATM level service is that
only one connection need be set up to each participant and there is
no single point of failure.  Also, this solution scales to very
large multicasts.

> Perhaps Joel would have some pointers to ATM Forum/PNNI related work
> on this topic?
> 
> cheers,
> gja
> 

	rick

  ++++++

	Rick Bubenik
	Ascom Nexion, Inc.
	1807 Park 270 Drive
	St. Louis, Mo. 63146
	(314) 579-6512 [office]
	(314) 542-0495 [fax]
	rick@nexen.com