[Sip] moving UDP support out of the core SIP document

[Rohan Mahy <rohan@cisco.com>]

Hello Everyone,

I wanted to share some thoughts that I had about removing UDP from the 
core SIP document when we go to draft standard. We are probably at 
least one year away from going to Draft Standard. I hope that during 
this time most implementations will support RFC3261, and therefore 
support TCP.  In addition, it is quite likely that when moving to Draft 
Standard that the IESG will REQUIRE TLS support on SIP User Agents. 
This would ensure a ubiquitous hop-by-hop channel security mechanism, 
but which requires a reliable transport (TCP or SCTP).

Obviously UDP is still very attractive in wireless networks (due to RTT 
issues) and in other typically private networks with well-understood 
delay and loss characteristics. To make most implementations simpler 
however, I am proposing that we make unreliable transports (UDP and 
DCCP) of SIP optional in User Agents, and that we put the UDP-specific 
mechanism (ex: much of the complication of the transaction layer for 
handling retransmissions) in a separate Draft Standard document.  It is 
very likely that if we did this there would also be a baseline security 
mechanism for UDP as well (probably some profile of IPsec).  The net 
effect is that SIP User Agents could also talk to each other using TCP 
or TLS+TCP, and that User Agents with a strong preference for UDP would 
have a consistent security mechanism.  Proxies would still support both 
transports, so you could easily communicate between a TCP-only User 
Agent, and as User Agent which prefers UDP without requiring the User 
Agent to switch over to TCP.

I welcome your discussion and comments on this idea.

thanks,
-rohan

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