RE: [Sip] Interesting problem with PRACK and resent INVITEs

["Christer Holmberg" <[e-mail]>]

Hi, 

>Interesting. But I'm not certain I buy it. CSeqs only make 
>sense within a dialog. F13 doesn't contain the To-tag abc, so 
>it ought not be expected to use a valid CSeq for that dialog.

Correct.

And, the INVITEs shall contain different From-tag values (and Call-ID
values), so UAS 2 will be able to differentiate the dialogs based on
those parameters.

Regards,

Christer




> 
>A different issue here is that that F13 ought not go to UAS1 
>at all. But we treat it as a new transaction, and don't 
>expect the proxy to maintain any history across transactions, 
>so we don't have a way to prevent that.
> 
>There ought to be something in F11 that causes the request to 
>go only to (in this case) UA2. (In general to whichever node generated 
>the challenge, which might have been a proxy rather than a UA.)
> 
> 	Paul
> 
> [e-mail] wrote:
> > I was looking at a problem were were having with a phone and 
> > discovered the following interesting situation with PRACK 
> and INVITEs 
> > that have to be resent with authentication.
> > 
> > Consider an INVITE that gets serially forked to two 
> destinations, the 
> > first one doesn't answer and the second one demands authentication:
> > 
> >     UAC             Proxy           UAS 1           UAS 2
> > 
> >      |                |               |               |
> > F1   | INVITE CSeq 1  |               |               |
> >      |--------------->|               |               |
> > F2   |                | INVITE CSeq 1 |               |
> >      |                |-------------->|               |
> > F3   |                | 180 CSeq 1    |               |
> >      |                | to-tag abc    |               |
> >      |                |<--------------|               |
> > F4   | 180 CSeq 1     |               |               |
> >      | to-tag abc     |               |               |
> >      |<---------------|               |               |
> > F5   | PRACK CSeq 2   |               |               |
> >      | to-tag abc     |               |               |
> >      |------------------------------->|               |
> > F6   |                | 200 CSeq 2    |               |
> >      |                | to-tag abc    |               |
> >      |<-------------------------------|               |
> >      |                |               |               |
> >     ---------------------- delay ------------------------
> >      |                |               |               |
> > F7   |                | CANCEL CSq 1  |               |
> >      |                |-------------->|               |
> > F8   |                | 200 CANCEL    |               |
> >      |                |<--------------|               |
> > F9   |                | 487 CSeq 1    |               |
> >      |                | to-tag abc    |               |
> >      |                |<--------------|               |
> > F10  |                | INVITE CSeq 1 |               |
> >      |                |------------------------------>|
> > F11  |                |               | 407 CSeq 1    |
> >      |                |               | to-tag def    |
> >      |                |<------------------------------|
> > F12  | 407 CSeq 1     |               |               |
> >      | to-tag def     |               |               |
> >      |<---------------|               |               |
> > F13  | INVITE CSeq 2  |               |               |
> >      |--------------->|               |               |
> > F14  |                | INVITE CSeq 2 |               |
> >      |                |-------------->|               |
> > F15  |                | 500 CSeq 2    |               |
> >      |                | to-tag pqr    |               |
> >      |                |<--------------|               |
> > F16  | 500 CSeq 2     |               |               |
> >      | to-tag pqr     |               |               |
> >      |<---------------|               |               |
> >      |                |               |               |
> > 
> > Since the PRACK F5 is sent within a forked dialog, we don't 
> think of 
> > it as using up CSeq number space in the space of out-of-dialog 
> > requests.  So when it comes time to re-send the INVITE F13, 
> it's easy 
> > to think that you can just use a CSeq one higher than the original 
> > INVITE F1.  But transaction processing at UAS 1 is likely to notice 
> > that it's already seen CSeq 2 with that Call-Id.  It looks like one 
> > must make sure that the re-sent INVITE has a CSeq higher 
> than has been 
> > used in any derived dialog of the original INVITE.
> > 
> > Dale
> > 
> > 
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> 
> 
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