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 is used as local address by UAS and remote address by the UAC for
e-2-e call leg setup. However, requests are forwarded based on request URI,
which may be changed by intermediate proxies while To remains unchanged.
The request URI may be set to the last Record-Route entry, the Contact header
field or the To itself in that order depending on what was rx in the previous
response.
>
>
> 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?
The To/From are swapped for all subsequent requests made by callee and request
URI set appropriately as the case may be. So if RR & Contact are absent this
works. However replies are expected to take the same path as the requests and
therefore use the Via header field.
-Gaurav
>
>
> Thanks.
>
> Regards,
> Victor
>
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