Victor Kueh wrote:
>
> Dear all,
>
> I have a question regarding From & To header fields in SIP
> messages. After reading through the SIP-implementors mailing list archive,
> it makes me more confused.
>
> Basically, is it true to say that the 'To' field is never used
> in forwarding/sending a request, but the Request URI is used instead?
Yes. The To field, when the request originates, is usally the same as the
Request-URI (R-URI). Once set by the UAC, it will not change at all, except
for the addition of a tag when it reaches the destined UAS.
> How about when sending/forwarding a response? Is the 'From'
> field used? This is because in the SIP-implementors mailing list for
> December, Igor and Vijay answered 'No, never' when answering to this
> question 'Is From never considered while sending responses?'. However in
> the SIP FAQ, it is mentioned the 'From' field is used for subsequest
> requests from the callee to the caller if there is no Contact or Record
> Route header. If that applies for subsequent requests, how about for
> responses?
Responses *always* follow the Via list. What you mention above is the From
being used to initiate a subsequent request, and only in the absence of
(a) the Contact (the bis makes Contact mandatory, so all requests will have
a Contact, it was optional in the RFC, if I recall correctly) or, (b) a
Route list has not been created.
Maybe this will make it clear:
INVITE
UAC ---> P1 ---> P2 ---> UAS
The INVITE path is UAC->P1->P2->UAS. The response path will be
UAS->P2->P1->UAC. Let's assume that only P2 wants to be in subsequent
signaling, so it R-Rs. Subsequent requests (including the ACK) will go
through UAC->P2->UAS. The ACK engenders no response, but assume that the
UAS send a BYE. The BYE travels through the path UAS->P2->UAC and the
200 OK (BYE) travels through the path UAC->P2->UAS. Note that the responses
*always* follow the Via list, but subsequent requsts may not take the same
path as the original INVITE did.
- vijay
--
Vijay K. Gurbani vkg@{lucent.com,research.bell-labs.com,acm.org}
Internet Software and eServices Group
Lucent Technologies/Bell Labs Innovations 263 Shuman Blvd., Rm 1A-413
Naperville, Illinois 60566 Voice: +1 630 224 0216 Fax: +1 630 713 0184
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