[Sip] SIP Congestion Safety draft revised

["Dean Willis" <dwillis@dynamicsoft.com>]

Based on list discussion, I've made a few small revisions to:

http://www.softarmor.com/sipwg/drafts/draft-ietf-sip-congestsafe-01.html
http://www.softarmor.com/sipwg/drafts/draft-ietf-sip-congestsafe-01.txt
http://www.softarmor.com/sipwg/drafts/draft-ietf-sip-congestsafe-01.xml

The basic changes here are that we've added some discussion of dealing with
responses, and a new response code for a UAS to use if it needs to send a
big response and doesn't know that the path is congestion safe and therefore
can't send the big response, as well as some discussion of what we mean by
this.

The basic approach taken here is "If a UAS receives a request that is not
marked congestion-safe, then it can't send a response larger than the
request." We add a new response code (514) for the UAS to send if it can't
meet this constraint.

You might wonder "Gee, doesn't this mean that if some fool sends a HUGE
request through a bunch of unsafe proxies to a UAS, that the UAS can then
send a HUGE response?" The problem is that a complete solution requires
solving reverse path MTU discovery, and I just don't see that happening. So,
we have something of a compromise that makes a moderately reasonable guess
about non-deterministic things. Of course, life is much better if the
proxies in between implement congestion-safety, so that we don't run into
this problem at all.

--
Dean

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