Subject: [SIP] Forking proxy and early call leg (also : Early media draft)

From: Tan Ya-Ching ICM N MC MI E73 <[email-masked]>
Date: Tue, 14 Aug 2001 18:38:13 +0200

Figure 3 and Figure 5 of draft-rosenberg-sip-early-media-00 do not show the
correct handling of the 489 response (10) by the forking proxy. Figure 3
shows that the 489 response is forwarded immediately upstream by the forking
proxy. According to the bis draft, for 4xx/5xx response received by the
forking proxy, "the proxy MUST send an ACK. It remembers the response if it
has a lower status code class than any previous 4xx and 5xx response. On
completion, a response with the lowerst response class is returned if there
were no 2xx, 3xx or 6xx responses". A SIP bis-compliant forking proxy would
therefore silently discards this 489 response, and forwards only the 200 OK
(19) upstream to the UAC.

This brings us to a question concerning "early call leg". J. Rosenberg
proposed in an earlier SIP mail that "a 1xx with tags and record-routes
establish an early call leg. Within that early call leg, you can send
non-INVITE requests, including PRACK, INFO, BYE, etc., as you would for an
answered call leg". But how would the client be aware that the forking proxy
has received a non-2xx final response and as a result, that particular early
call leg has been terminated ?

We therefore propose the following slight modification of forking proxy
behavior to section 17.4 (Forking Proxy) of the bis draft :

Once the forking proxy has received and forwarded a provisional response
with a To tag upstream, it MUST forward upstream any final non-2xx responses
received on that branch, regardless of the responses received on the other
branches. On branches where no provisional responses with To tag were
received, the forking proxy behaves as usual, by selecting the "best
available final status" for those branches.

- Ya-Ching Tan & Giuseppe Del Castillo

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