Subject: RE: [SIP] Max-Forwards + Proxies

From: "Fairlie-Cuninghame, Robert" <[email-masked]>
Date: Mon, 26 Jun 2000 20:52:32 -0700

It is my understanding that the IP TTL operates in the way that Johnathan
described. That is, a router should not forward packets with a TTL of 0 or 1
(returning an ICMP in both cases). A Host however is free to process a
received packet with a TTL of 0 or 1 (although it should not normally
receive a packet with a TTL of 0). Traceroute knows that it reached the
destination because the endpoint returns an ICMP Echo reply (rather than
ICMp Time Exceeded).

This is not necessarily the behavior we want in this case but I thought I
would straighten out the analogy.

Cheers,

Robert.

-----Original Message-----
From: Henning Schulzrinne [mailto:[email-masked]]
Sent: Tuesday, June 27, 2000 4:34 AM
To: Jonathan Rosenberg
Cc: Shail Bhatnagar; Neil Deason; Sean Olson; [email-masked];
[email-masked]
Subject: Re: [SIP] Max-Forwards + Proxies

Jonathan Rosenberg wrote:
> 
> Shail Bhatnagar wrote:
> >
> > > > I thought behavior of registrars/redirect servers closely matches
> > > > user agents, in terms of request handling. Of course
> > > > there are differences. Per this proposal, a user agent will process
a
> > > > request with Max-Forwards of 0 but registrars/redirect servers and
> > > > proxies will not. Is my understanding correct ??
> > >
> > > No. Any server (UA, proxy, redirect, registrar) receiving a request
with
> > > Max-Forwards zero responds with a 483 and does not process.
> > >
> > > -Jonathan R.
> > >
> >
> > Okay, so any proxy should not forward a request with incoming
> > Max-Forwards 1, since if it forwards it, Max-Forwards will become
> > 0 and the downstream guy will return 483.
> >
> > Right ??
> 
> I suppose so, yes.

But that would defeat the whole point of being able to figure out the
downstream capabilities, returned in the 483. Thus, a proxy should *not*
try to be too smart. After all, if IP routers did this, traceroute
wouldn't work...

-- 
Henning Schulzrinne   http://www.cs.columbia.edu/~hgs

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---

From: Henning Schulzrinne <[email-masked]>
Date: Tue, 27 Jun 2000 08:23:03 -0400

Not to belabor the point, but please see RFC 1009 (Requirements for
Internet Gateways), Section 4.8. Your statement about TTL appears to be
at variance with the facts. (Not forwarding a packet with TTL 1 also
wouldn't work since receiving a packet with TTL 0 is perfectly valid.)
-- 
Henning Schulzrinne   http://www.cs.columbia.edu/~hgs