Having finished reading the rtp-usage draft, I am impressed. This is good work, and will help a lot going forward. One thing I want to check, because it seems that the only place that says "Using ... is REQUIRED" rather than "Support ... is REQUIRED": 5.3. Symmetric RTP/RTCP RTP entities choose the RTP and RTCP transport addresses, i.e., IP addresses and port numbers, to receive packets on and bind their respective sockets to those. When sending RTP packets, however, they may use a different IP address or port number for RTP, RTCP, or both; e.g., when using a different socket instance for sending and for receiving. Symmetric RTP/RTCP requires that the IP address and port number for sending and receiving RTP/RTCP packets are identical. Using Symmetric RTP and RTCP [RFC4961] is REQURIED. In the STUN-based firewall traversal scenario, STUN will discover a <sender address/port, recipient address/port> at the sender that will cause delivery of packets with a corresponding (not necessarily identical) <sender address/port, recipient address/port> at the recipient, and that the recipient can swap those addresses around and have the packets delivered to the sender (the STUN connectivity check is bidirectional). No guarantees are made for any other <sender address/port, recipient address/port> pair, so non-symmetric RTP/RTCP seems likely to fail. Is this the (only) reasoning that led to this requirement? If so, should it be inserted into the document so that people can reproduce the thinking? (and if there are other reasons, which I haven't thought of, it might be good to document those too). Harald