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Re: [Sip] B2BUA & Max-Forwards

inline.

Adam Roach wrote:

-----Original Message-----
From: Phil Reynolds [mailto:preynolds@ridgewaysystems.com]



Now your back to restricting what I can and can't do in an
intermediary. It can't be a B2BUA 'cos it doesn't need to
honour require headers (the UA at the other end does that,
if they were meant for my intermediary then they would be
proxy-require?), and it can't be a proxy 'cos of the rules
about not changing the body so what is it?


Broken.

Adam is correct.


The restrictions placed on nodes by 3261 (which I'm merely
reiterating, not imposing) aren't an authoritarian attempt
at unnecessary proscription. They serve a purpose.

I'll describe one of the (several) purposes that applies
to the situation you propose.

One of the things that UAs are required to be able to do is
sign and/or encrypt the bodies and/or some subset of the
headers to ensure confidentiality and/or integrity. The
protocol places restrictions on various nodes to ensure
that this can be done. Your box is broken precisely because
it interferes with these abilities (or, more precisely,
these abilities interfere with it).
Let me add some more here.

Anything that deviates at all from proxy rules will need to be a b2bua and follow b2bua rules, including processing of Require. There is no black-or-white here. Let me give some examples.

Lets say my b2bua is a proxy in all ways, BUT it happens to muck with SDP to enable firewall traversal. Surely then it can ignore Require, right? WRONG. Lets say I define extension foo, which means "the content of this SDP is not what it seems - look at this header to instead find the real port/address for media". This extension needs a Require (because only a UA ever looks at bodies), but not a Proxy-Require, because proxies don't. Thus, your almost-a-proxy will fail in odd ways because it has ignored Require when it shouldn't have.

Indeed, for any operation prohibited to a proxy, but allowed for a UA, I can come up with an extension, indicated in a Require header, that would need to be understood by a b2bua which breaks the proxy rules and performs this operation. Indeed, the rules on what is a UA, and what is a proxy, are meant precisely to help define what goes in Require, and what goes in Proxy-Require.

Whether or not Require gets passed on by such a b2bua depends entirely on the application logic.

-Jonathan R.

--
Jonathan D. Rosenberg, Ph.D. 72 Eagle Rock Ave.
Chief Scientist First Floor
dynamicsoft East Hanover, NJ 07936
jdrosen@dynamicsoft.com FAX: (973) 952-5050
http://www.jdrosen.net PHONE: (973) 952-5000
http://www.dynamicsoft.com

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