Re-invite rules

Hi all ,

I looking for the rules of a re-invite.
I find in few site this rules :
-csqeq must be increased by one
-to tag and from tag must be the same of the rest of sip message.

Can you help me to find the rfc that explain this

Thx a lot


This is core SIP stuff, so RFC 3261.

cseq is numbered independently in each direction and incremented for each new request in the dialog, not just for INVITEs. From and to tags should be fixed within a dialog, with the exception that the initial INVITE has no To tag. The Call ID must also be constant within the dialog.

Thx David ,

As far as I understand, If my device sends an INVITE with “cseq 20”, the callee sends us a 200 OK with “cseq 20”, the callee can send re-INVITE with “cseq 1”. is it ok ? because it doesn’t fit what I read on other sites, and RFC3261 is a bit confusing to me.
But the tags should not change during the dialog.

see the pictures attached

Thanks for your help


The Call-ID of the request MUST be set to the Call-ID of the dialog.
Requests within a dialog MUST contain strictly monotonically
increasing and contiguous CSeq sequence numbers (increasing-by-one)
in each direction (excepting ACK and CANCEL of course, whose numbers
equal the requests being acknowledged or cancelled). Therefore, if
the local sequence number is not empty, the value of the local
sequence number MUST be incremented by one, and this value MUST be
placed into the CSeq header field. If the local sequence number is
empty, an initial value MUST be chosen using the guidelines of
Section The method field in the CSeq header field value
MUST match the method of the request.

Fairly clearly says there independent sequences in the two directions. In any case both sides could choose to send at once and both requests would be valid.

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