A case when 491 is send is :
UA1(UAC) sends a Invite to UA2(UAS) and establishes a call.(Invite/200/ACK)
If the UA2 wants to Keep UA1 on hold it will sent a re-Invite on the same
Dialog.
Before this is completed, UA1 may send a re-invite to UA2 to change session
parameters (or)keep on hold...
during these situations to resolve the glare condition when both sides send
re-Invite 491 is used.
ua1 ps ua2
Invite
-------> Invite
------>
200
200 <------
<--------
Ack
-------->
Ack
------->
=====RTP==
Invite (Invite on same Dialog)
Invite<-------
<--------
Invite
--------->
Invite
------->
491
<-------
491
<--------
HTH,
Regards,
Sreeram
________________________________
From: [EMAIL PROTECTED] on behalf of Carlos Lasa
Sent: Wed 7/20/2005 2:02 PM
To: [email protected]
Subject: [Sip-implementors] 491/500 responses to Re-INVITEs
Someone can help me in understanding section 14.2:
a) "A UAS that receives a second INVITE before it
sends the final response to a first INVITE with a
lower CSeq sequence number on the same dialog MUST
return a 500 (Server Internal Error) response to the
second INVITE and MUST include a Retry-After header
field with a randomly chosen value of between 0 and 10
seconds."
Q1: I understood for this that a UAS MUST send a 500
for any Re-INVITE that arrives while is processing a
previous (minor CSeq) Re-INVITE in the same dialog. Ok
?
b) "A UAS that receives an INVITE on a dialog while an
INVITE it had sent on that dialog is in progress MUST
return a 491 (Request Pending) response to the
received INVITE."
Q2: This 2 paragraph is what I did not understand.
It's suppossed that a UAS will receive Requests and
will send responses, why the RFC says that the UAS
"had sent an INVITE" ? Could you explain me, in others
words, in what case I should respond 491 ?
Regards,
Carlos.-
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