[Sip] Outbound-10 comments

  Hi Cullen and Rohan,

  In the draft, it requires the UA configure the next hop route header with "keep-stun", "keep-crlf", or "timed-keepalive" tags. This would cause some problems.

  1. As an example, the route-set contains this route header as indicated in section 9 example: 
  Route: <sip:pri.example.com;lr;keep-stun>
  If the DNS NAPTR resolution for pri.example.com is TCP, SCTP or TLS, the keep-stun will be useless. Vice versa, if the pri.example.com is resolved as UDP, and if "keep-crlf" was manually configured, it is not working either.

  2. Before sending the REGISTER request, the admin/or user does not know what keep-alive mechanism the proxy (or edge proxy) supports. Blindly configure the keep-stun, or keep-crlf would cause the problem that the draft indicated itself in section 8: "the node could be blacklisted for UDP traffic".
  The better approach is to let the UA and the proxy to negotiate, not manually configure from UA side. 
  a. UA sends REGISTER to the proxy with a "outbound" tag in the Supported header.
  b. Proxy insert the "outbound" tag in the 200 OK, if the UA indicated that it supports the outbound.
  c. If the "outbound" tag is present in the 200 OK, and if the transport is UDP, using the STUN keep alive, other connection based transport using crlf keep alive.
  There would be no way to mass up by configurations with this approach. Let me know if I missed something.

  Regards,
  Jerry Yin

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