Subject: Re: [Sip] Contact header in 1xx responses/updates remote target ornot?

From: [email-masked]
Date: Sun, 3 Jun 2007 19:03:50 -0400

From: "Jeroen van Bemmel" <[email-masked]>

   Not sure if I read your comment right, but RFC3261 12.2.1.2 says

   When a UAC receives a 2xx response to a target refresh request, it
      MUST replace the dialog's remote target URI with the URI from the
      Contact header field in that response, if present.

   It does not say anything about 1xx responses here though...

Ack, you're right, I'm wrong.  I tried to check but didn't see
12.2.1.2, so I assumed that responses aren't target-refreshing.

However, there is a problem that was discussed long ago, which is that
a response (except to INVITE) isn't acknowledged, the UAS doesn't know
that the UAC has received it, and so doesn't know whether it can
abandon listening to its previous target address.  So a UA shouldn't
use responses to non-INVITE requests to update its target.

In regard to 1xx, it seems like a US should use the same contact in
1xx as it does in the 2xx.  If the 1xx is not reliable, the UA can't
depend on it for target refresh, so the same target refresh must be
done in the 2xx.  If the 1xx is reliable, the UA can depend on it, but
it seems unsafe to then provide a different target address in the 2xx
-- which one should be the resulting target?  (But upon receiving the
PRACK, the UAS does know that it can abandon the previous target.)

(Since provisionals and PRACKs are only used in INVITE transactions,
final responses are acknowledged, so my first rule of thumb is
satisfied.)

Dale

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