Subject: [Sipping] Comment on draft-ietf-sipping-nat-scenarios-03.txt

From: "Ar c'hasour" <[email-masked]>
Date: Sat, 21 Jan 2006 04:54:57 -0800 (PST)

Hi,

When a SIP "outbound" connection is used between a UE1
(SIP ip1-port1) and a UE2 (SIP ip2-port2), can the
media (eg RTP packets) also use ip1-port1 and
ip2-port2 in their IP src and dst addresses? Or, in
other words, is SIP/RTP interleaving supported?

Whatever the answer is, it would helpful to add a
comment on this point

Thanks
P. Kasour

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---

From: Jonathan Rosenberg <[email-masked]>
Date: Sun, 22 Jan 2006 19:34:12 -0500

This is a good question, actually.

I am aware of several implementations that do this. The practice is 
definitely not documented anywhere. I believe it will 'just work' in 
theory, in that, you can choose to do this on your UA, and the peer UA 
or proxies don't need to be the wiser. Demuxing SIP and RTP packets is 
relatively easy. However, I'm not sure its a good practice. A few 
reasons for that:

* May interwork poorly with whatever tricks a downstream SBC is doing; 
generally hard to say what that might be

* Separating signaling and media allows for installation of QoS 
mechanisms in the network just for voice, which is more readily 
characterized. This is more likely to be a big issue if there is a lot 
of signaling, for example, SIP MESSAGE requests.

* Same issue for any kind of middlebox accounting systems which count 
RTP packets in the network based on the SDP. They'll be fooled and 
confused by the signaling packets.

Note that none of the above are problems in theory if the servers in the 
nework didn't involve themselves in the contents of the SDP and 
processing of RTP packets. But, in practice all of the above three are 
likely to be real issues.

As such, I don't think the nat scenarios document should recommend this 
practice. I do think it would be valuable to have a sectoin discussing 
it though, primarily to enumerate the issues above and then discourage it.

Thanks,
Jonathan R.

-- 
Jonathan D. Rosenberg, Ph.D.                   600 Lanidex Plaza
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