Subject: [Sipping] PRACK: Does non-200 response cease re-transmission of reliable 18x?

From: "Christer Holmberg" <[email-masked]>
Date: Mon, 30 Mar 2009 08:38:55 +0200

Hi,

One of the PRACK related issues presented in SFO is whether a PRACK
non-200 response would cease re-transmission of the assoicated reliable
18x response.

Based on the mail discussions we've had I proposed that the non-200
response DOES NOT ceasee the re-transmission. However, some people
seemed to have an issue with that, so I was asked to "take it to the
list".

The main justification why a non-200 response would not cease the
re-transmission is because an intermediate may reject the PRACK, in
which case the UAS will never receive it (read: the UAS will continue to
re-transmit the reliable 18x). So, when the UAC receives the PRACK
non-200 response it should send a new PRACK, which hopefully will reach
the UAS and trigger a 200 response.

So, anyone who does not agree with the justification aboive, and thinks
that a non-200 response DOES cease the re-transmission of the associated
reliable 18x response, please indicate so.

Regards,

Christer

---

From: Paul Kyzivat <[email-masked]>
Date: Tue, 31 Mar 2009 14:39:40 -0400

Christer Holmberg wrote:
> seemed to have an issue with that, so I was asked to "take it to the list".
> 

I agree with Christer here. Another scenario where this would be 
important is:

-> INVITE
<- 180rel
-> PRACK
<- 401

It this case it is essential to retry the PRACK with updated 
credentials. Certainly the UAS doesn't want to consider the 180 to have 
been acknowledged in this case, since it might have been from a MiTM.

This could come about in the case of a reINVITE, where the nonce used in 
the original INVITE finally becomes stale by the time the PRACK is received.

	Thanks,
	Paul