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Issue #6: Media failure in case of codec reordering

Category

Incorrect implementation of the specification

Description

While it should be possible for a UAC and a UAS to use different audio codecs in the same call, we have seen that some UAs are unable to handle this situation.

Implications

Such a behavior by UAs could result in no audio being heard by one or both end users, despite successful signaling.

Instance #1

The packet trace below shows an SDP offer and answer exchange between a UAC (Cisco 7960) and a UAS (Voicemail server of Sipphone), where a codec preference specified in the offer is changed during the answer by codec reordering. Once this codec negotiation is complete, the UAC sends its audio packets in the PCMU format while the UAS in G.729. But no audio could be heard at the UAC.

Packet traces / Call-flows

[Offer] v=0 o=UA1 aa bb IN IP4 x.x.x.x s= SIP Call t=0 0 m=audio 22238 RTP/AVP 0 8 18 101 c=IN IP4 xx.xx.xx.xx a=rtpmap:0 PCMU/8000 a=rtpmap:8 PCMA/8000 a=rtpmap:18 G729/0 a=fmtp:18 annexb=no a=rtpmap:101 telephone-event /8000 a=fmtp:101 0-15 a=sendrecv

[Answer] v=0 o=UA2 cc dd IN IP4 y.y.y.y s= session c=IN IP4 y.y.y.y t=0 0 m=audio17088 RTP/AVP 18 0 8 101 a=rtpmap:18 G729/8000 a=fmtp:18 annexb=no a=rtpmap:0 PCMU/8000 a=rtpmap:8 PCMA/8000 a=rtpmap:101 telephone-event /8000 a=fmtp:101 0-16 a=silenceSupp:off - - -

Fixes / Workarounds

None.