[Devel] SIP To: Header Tags issue

[Ovidiu Sas]

The RFC is not vague at all (see 9.1 Client Behavior):

    The following procedures are used to construct a CANCEL request.  The
    Request-URI, Call-ID, To, the numeric part of CSeq, and From header
    fields in the CANCEL request MUST be identical to those in the
    request being cancelled, including tags.

The INVITE has no To tag and therefor CANCEL shall have no To tag.


-ovi

alex at pilosoft.com wrote:
> Hi,
> 
> Having somewhat of an interesting issue that appears to result from 
> vagueness of SIP RFC.
> 
> Issue is that if requester sent a SIP message and there is no ;tag on the 
> To: field, the UAS is supposed to add the tag and send it back, and 
> requester should use that value in all future references to this call.
> 
> Now, this causes problems in the following sequence of events, assuming 
> SER is  doing stateful forwarding.
> 
> 
> Requester         SER           UAS
> INVITE->
>          <- 100 trying
>                       INVITE->
>                       <- 100 trying (with the new tag)
>         
> Now, note that at this point, the message with the new tag is 'consumed'
> by ser, and requester remains oblivious to the new tag. (Yes, once 183
> Ringing or OK comes through - requester will get the tag, but, if
> requester needs to CANCEL the call, UAS requires the tag to be present - 
> and the CANCEL will fail).
> 
> So, what is the more appropriate thing to do? 
> 
> Options as I see them:
> 
> a) I should make SER add a tag to the To: field if it is missing.  
> RFC is silent whether a proxy is allowed/required to add a tag to the To
> field.
> 
> b) I should make SER do a stateless-forwarding of "100 Trying" messages. 
> This will result in client getting two acks (one stateful from SER, one 
> stateless from UAS), which I'm not sure is a correct thing.
> 
> Which one is the "right thing"?
> 
> 
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