Re: 4941bis and multicast & QOS

Gyan Mishra <> Fri, 14 February 2020 00:52 UTC

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From: Gyan Mishra <>
Date: Thu, 13 Feb 2020 19:52:28 -0500
Message-ID: <>
Subject: Re: 4941bis and multicast & QOS
To: "Manfredi (US), Albert E" <>
Cc: 6MAN <>
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Sorry Bert I missed your reply - responding now

On Sun, Feb 2, 2020 at 3:36 PM Manfredi (US), Albert E <> wrote:

> From: ipv6 <> On Behalf Of Gyan Mishra
> > How would multicast ASM or SSM work with the privacy extension temporary
> address enabled.  With the privacy temporary address enabled the IGMP Join
> would go out the temporary addresses, just as would normal unicast traffic.
> That's a good question, although only SSM would be a problem. In ASM, the
> IGMP/MLD join reports always go to the group multicast address anyway, and
> presumably, that won’t change. The unique source address of the client
> joining, or of the multicast source, should not matter. When responding to
> a new IGMP query, a new source address could be used in the report, still
> indicating to the router that membership exists. I would think, no problem.

   You make a good point as to where the problem may fall.  So with ASM
model the receiver is not source aware and so sends a IGMP join via
temporary address RPF interface for outgoing “client like” unicast
connections.  The LHR DR on the subnet creates MRIB state and forwards *,G
shared tree join along RPF path to the Anycast RP /MSDP - which has active
SA cache entry for the Source within the multicast routing domain.  The LHR
now learns the SA for the group and initiates SPT switchover to the SPT
tree and builds a tree directly to the source.

With SSM the receiver is source aware via MLDv2, which now has source field
in the packet for the SSM channel - along with well know default SSM group
232/8 - receiver is able to send via temporary address S,G join and to its
LHR DR router on the subnet - which now builds SPT tree directly to the

So this all works well in a client / server multicast model with pim
sparse-mode where the clients are receiver only and the server is source

In a peer2peer multicast model where the source acts as both receiver and
source and the receiver acts as both receiver and source is the “bi-dir
PIM” model -  this is the MPLS MVPN world translates into MP2MP LMDT
instantiated via mLDP.

Multicast is predominately used in enterprises.  The internet MBONE never
really took off due to multicast use of UDP RTP  transport for streaming
and thus a requirement for end to end QOS for reliability.

So in the enterprise bi-dir pim model application requiring peer2peer
multicast is the only scenario where you would run into issues with the
privacy temporary address changing and becoming invalid resulting in
session reset.  The app based source discovery would have to trigger
changing of SSM channel when the source becomes invalid so the session
would not reset.

I think for this peer2peer model to work you really need a stable address
on the host.

This could be one use case where RFC 4941bis would fail - with removing the
stable address as it’s currently written.

> In SSM, the IP destination address of the reports is a well-known
> multicast address, to routers, but of course, if you want to include only a
> certain set of sources, those source IP address(es) in the IGMP/MLD report
> either have to be valid, or you get nothing.
> > The temporary address would be the  RPF path shared and SPT tree.
> Isn’t this just related to multicast routing, between routers? Their
> addresses would not change at a dizzying rate?
> Bert
> --

Gyan  Mishra

Network Engineering & Technology


Silver Spring, MD 20904

Phone: 301 502-1347