Subject: evolving behavior of proxy servers

From: Bill Newman <[email-masked]>
Date: Thu, 28 Oct 1999 16:04:09 -0700 (PDT)

I'm trying to update a presentation summarizing how SIP works.

In reading RFC2543 and looking at some older slides on SIP, it
looks as though the original default behavior of SIP was to 
have ACKs and BYEs travel back through proxy servers, through
the same paths as INVITEs and OKs.

>From a number of more recent sources, notably the "Wandering ACKs"
entry in the SIP bugs page
  <http://www.cs.columbia.edu/~hgs/sip/notes.html>
it appears that that behavior is no longer favored, and the 
future standard will send ACKs and BYEs directly from user agent
to user agent, bypassing any proxy servers.

Is that correct?

Furthermore, is it correct that the issue here is largely
a tradeoff of two desirable features
  (1) In the old way, proxy servers had the opportunity to 
      (crudely, modulo network errors and ill-behaved clients
      and other real-world stuff) keep track of what calls
      were in progress.
  (2) In the new way, proxy servers not only don't need to retain
      call state, but don't need to have any sort of constant
      state, so all sorts of hairy time-dependent or random
      behavior (e.g. ACDs) can be implemented without messing up
      calls.
and that the consensus is now that (2) is more important than (1)?

  Bill Newman
  Nortel Networks
  [email-masked]

---

From: Jonathan Rosenberg <[email-masked]>
Date: Fri, 29 Oct 1999 10:08:35 -0400

> the same paths as INVITEs and OKs.

They do if the proxies add the Record-Route header. This allows each
proxy to individually decide whether it will see the ACK and requests
that follow, or whether they bypass that particular proxy.

> and that the consensus is now that (2) is more important than (1)?

The spec does not mandate either one; one is not fundamentally better
than the other. It depends on your application. The protocol explicitly
has a Record-Route feature to allow each administrator to decide which
operating point is preferred, and this decision can be different for
each proxy from call to call.

-Jonathan R.
-- 
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