Subject: [Sipping] Re: I-D ACTION:draft-camarillo-sipping-sbc-funcs-02.txt

From: Paul Kyzivat <[email-masked]>
Date: Mon, 17 Oct 2005 16:30:01 -0400

I can offer another SIP-Unfriendly aspect of NAT Traversal (3.4.2):

When the registration expiration interval has been decreased, and not 
all registrations are passed along into the network, then the SBC must 
make up a response to the registrations that are not forwarded.

If the registration *were* forwarded, then the response would include 
*all* the contacts that are registered at that time. But when faking a 
response, the SBC doesn't know all the contacts registered at that time. 
Whatever it does may be wrong part of the time. Some things it might try:

- Only return the contact registered.

   This of course is wrong if any other UA has registered.

- Cache the response from the most recent registration that was
   forwarded. If the SBC is clever, it will then remove any contacts
   from that response that should have expired.

   This will be wrong if any other UAs have updated the registration
   state since the last time the SBC forwarded a register for this UA.

- Cache response from the most recent registration to the same AOR,
   whether by the UA or another.

   Still wrong if another UA could register for the same AOR via a
   different SBC, or without an SBC.

In general, when performing this kind of trick, the SBC must depend on 
the registering UA behaving according to an undocumented subset of legal 
UA behavior, or else the UA could malfunction as a result.

	Paul

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From: "Jani Hautakorpi (JO/LMF)" <[email-masked]>
Date: Tue, 18 Oct 2005 13:24:03 +0300

Hi Paul,

Thank you for the feedback, I'll add your comments to the next version
of the draft.

All feedback to the draft is very welcome!

-- 
Jani H.