> From: Sourav Dhar Chaudhuri <sourav_mi...@yahoo.co.in>
>
> Actually my scenario is attended call transfer using REFER
> method. So here I want to transfer an attended call using
> REFER method. But the User agent B whom I want to send REFER
> request with the existing dialog information does not having
> Support for replaces header. So in that case how does the User
> agent B will behave when it will receive the REFER request
> which is having existing Dialog information in that Refer to
> header with replaces parameter?
There is some subtlty to the correct answer:
"you" send a REFER request to User Agent B. The Refer-To value in the
REFER request contains a "replaces" "header parameter" (called
"header" in RFC 3261 section 25.1). The result of this header
parameter is that the INVITE that User Agent B generates will contain
a Replaces header field.
This behavior is *not* dependent on whether User Agent B implements
"replaces". The fact that UA B generates an INVITE is required by RFC
3515 section 2.4.3. The INVITE must be generated using the Refer-To
URI.
But the process of creating the INVITE from the URI is defined in RFC
3261 section 19.1.1 (among others) and it requires that *all* header
parameters in the URI are used to construct header fields in the
INVITE. (Excepting for certain header names that are known to be
"dangerous".) UA B must create the Replaces header field regardless
of whether it understands or implements Replaces headers itself.
Of course, for the attended call transfer to complete successfully,
the UA which *receives* the INVITE must implement Replaces.
One further consideration: If the other UA does not implement
Replaces, the best user experience is obtained if the other UA rejects
the INVITE -- otherwise, the user at that UA receives what appears to
be an additional incoming call. So it is desirable for the generated
INVITE to contain the header field "Require: replaces". This can be
accomplished by adjusting the Refer-To URI in the REFER request:
Refer-To: sip:user@host?Replaces=....&Require=replaces
Dale
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