Columbia VoIP Testbed
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Issue #2: Lack of support for non-symmetric signaling Category Incorrect implementation of the specification Description The Contact header field provides a SIP URI that can be used to contact the UA for subsequent requests. Specifically, a UAS, in its responses, adds a Contact header field that indicates the address where it would like to be contacted for subsequent requests in the dialog (which includes the ACK for a 2XX response in case of an INVITE). We have observed that a few proxy servers send ACKs back to the source port from where the “180 Ringing” and “200 OK” responses were received, rather than to the port in the contact header of UAS. Implications This results in incorrect/incomplete signaling – leaving the UAs in undesired states, usually resulting in multiple retransmissions and failed transaction.
Instance #1 The sipphone proxy exhibits this behavior, while proxying the INVITEs to a registered SIP end-client that does non-symmetric signaling. Packet traces / Call-flows The call flow and the packet traces shows this behavior when a Cisco 7960 recieves an incoming call from Sipphone proxy. Fixes / Workarounds None.