Subject: [Sip] Open Issue #146: reuse of disabled media streams

From: Jonathan Rosenberg <[email-masked]>
Date: Wed, 5 Dec 2001 16:05:54 -0500

Issue:

Bis currently specifies that if a client disables a media stream (by setting
its port to zero), it cannot remove that stream from the SDP. As such, a UA
which constantly adds/removes media streams will cause the SDP to
continually grow, and never shrink. You definitely cannot remove them (since
the ordering is needed to match up streams between the offer/answer), but it
can work for a client to re-use a disabled stream. That is, when it wants to
add a new stream, it uses the "placeholder" held by the disabled stream. So,
instead of adding a new m line, the disabled m line is replaced with the m
line of the stream you want to add.

This works because there is never any confusion about which streams
correspond to which. Think of the sequence of m lines in an SDP as a TDM
system, with each stream a "channel". Streams in the offer and answer are
correlated when they sit in the same channel. Once a stream is in a channel,
it can't move to another one, since then you lose correlation. When a stream
is disabled, its channel becomes empty. You can then insert a new stream
into that channel later on, and its clear that this is a new one (because it
replaces a disabled stream).

So, the question is this: do we allow reuse of disabled streams in this
fashion?

Points of discussion:

Why do this at all? The only real reason is savings of bandwidth, since you
can reduce the size of the SDP. This is not a very common case for general
usage. However, the 3gpp guys are thinking about having the proxy disable
streams (egads!) based on some network policy about what codecs to use. They
don't want to be carrying around these zeroed out media streams. However,
our recommended approach with issues like this has been compression (ala the
signaling compression work in rohc). These streams will likely be compressed
very well, so the overhead may be moot.

Gonzalo pointed out that an alternative is to use FID, since that could
provide a way to correlate the streams. In that case, you can remove
disabled streams altogether if they can be correlated. However, it wasn't
clear that FID actually allows for correlation of streams between offer and
answer. Rather, its meant for association of streams within the same SDP.

Subsequent discussion on the list, in a long twisted and tedious thread
resulted in the conclusion that fid doesn't solve this problem.

There was one comment advocating reuse if it doesn't break any
implementations, and another advocating it generally. I personally am on the
fence. We need to make a decision. So, I propose:

Proposal:
---------

Allow reuse.

Thanks,
Jonathan R.
---
Jonathan D. Rosenberg, Ph.D.                72 Eagle Rock Ave.
Chief Scientist                             First Floor
dynamicsoft                                 East Hanover, NJ 07936
[email-masked]                     FAX:   (973) 952-5050
http://www.jdrosen.net                      PHONE: (973) 952-5000
http://www.dynamicsoft.com

_______________________________________________
Sip mailing list  http://www1.ietf.org/mailman/listinfo/sip
This list is for NEW development of the core SIP Protocol
Use [email-masked] for questions on current sip
Use [email-masked] for new developments on the application of sip

---

From: "Bert Culpepper" <[email-masked]>
Date: Thu, 6 Dec 2001 13:42:20 -0500

I agree with the proposal to allow reuse.

Bert

> -----Original Message-----
> From: [email-masked] [mailto:[email-masked]]On Behalf Of
> Jonathan Rosenberg
> Sent: Wednesday, December 05, 2001 4:06 PM
> To: [email-masked]'
> Subject: [Sip] Open Issue #146: reuse of disabled media streams
> 
> 
> Issue:
>