Subject: same multicast address for different RTP sessions???

From: Sylvie Brunelle <[email-masked]>
Date: Thu, 08 Jun 2000 11:48:24 +0100

Hi everybody,

I was wondering whether it was possible to have two different RTP
sessions with the same multicast address but different UDP ports. If I
understand well the passage below (paragraph 2.2 from
draft-ietf-avt-rtp-new-07.txt), it seems to be possible.

" If both audio and video media are used in a conference, they are
   transmitted as separate RTP sessions RTCP packets are transmitted for

   each medium using two different UDP port pairs and/or multicast
   addresses. There is no direct coupling at the RTP level between the
   audio and video sessions, except that a user participating in both
   sessions should use the same distinguished (canonical) name in the
   RTCP packets for both so that the sessions can be associated."

But in that case that would mean that, if RTP session 1 and RTP session
2 do use the same multicast address, one participant from RTP session 1
will take from the network the traffic from RTP session 1 and RTP
session2 and will only differentiate the packets coming from these two
different sessions when he will process the UDP header!

Furthermore,  in case there is congestion and the participant of session
1 and 2 (which are using the same multicast address) decides to
unsubscribe from the RTP session 2 (in order to reduce congestion), he
will not solve congestion at all, as the router will continue forwarding
him all the packets corresponding to the multicast address of RTP
session 1 (and thus RTP session2).

Cheers,
Sylvie

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From: Jon Crowcroft <[email-masked]>
Date: Thu, 08 Jun 2000 12:27:42 +0100

>>draft-ietf-avt-rtp-new-07.txt), it seems to be possible.

well, under most circumstances, one would expect traffic sources from
the same place with receviers at the same locations to go over the
same tree roughly (ok, so there are  equal cost multipath OSPFs out
there, but ....) so whether you use the same multicast address or
different ones, they are sharing the same bottlenecks unless there's
very explicit multicast policies setup (could use different GLOP
addresses etc etc) - but yes, yo uare right, receiver based congestion
control, leaving one group and not the other, wouldnt work if both
flows are distinguished only by UDP ports - nor would (i think) source
specific (IGMP v3) if they are sources from same IP address......

of course
if you want explicit differentiation, its done either by intserv (which
includes ports) or in diffserv by edge marking of DSCP fields in IP
Header, which can be different for things with same src+dst IP addresses and
ports

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From: Colin Perkins <[email-masked]>
Date: Thu, 08 Jun 2000 13:04:22 +0100

Sylvie,

>draft-ietf-avt-rtp-new-07.txt), it seems to be possible.

Yes, it is possible.

>different sessions when he will process the UDP header!

Correct.

>session 1 (and thus RTP session2).

Correct.

These are the reasons why it is best to use different multicast groups for
different sessions.

Colin

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From: "kjposter2" <[email-masked]>
Date: Thu, 8 Jun 2000 07:32:11 -0500

I would really really like to unsubscribe from this

the email address I would like to unsubscribe

[email-masked]

any help would be appreciated

I requested this but still receive emails