[Asterisk-Dev] Yet more non-RFC compliance with SIP

William Waites asterisk at lists.styx.org
Thu Feb 26 11:31:30 MST 2004

This time with INFO -- and likely other request methods.

The scenario is Asterisk talking to eXosip and configured
to use INFO for DTMF. The first INFO message goes through
fine, subsequent ones are treated as retransmissions by
eXosip. Why does this happen? Because the branch tag in
the VIA header constructed by Asterisk is always the same.

>From RFC3621 Via


   The branch parameter value MUST be unique across space and time for
   all requests sent by the UA.  The exceptions to this rule are CANCEL
   and ACK for non-2xx responses.


In order to make your implementation compliant to this part
of the specification, you must either:

	- not send branch parameters that start with the magic
	  cookie "z9hG4bK" that cause other SIP implementations
	  to mistakenly believe that Asterisk actually complies
	  with RFC3621
	- arrange it so that the branch parameter is unique for
	  each request as required by RFC3621 setion

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