Subject: RE: [SIP] Proxies modifying SDP

From: "Neil Deason" <[email-masked]>
Date: Fri, 15 Dec 2000 18:29:55 -0000

>Bernie Hoeneisen wrote:
> > I have question concerning the case, when SIP proxies modify SDP,
> > which is AFAIK against SIP principles, isn't it?
>I do not think so.
>Why is that *against* SIP principles?  What principles?'

The fundamental concept that payloads are opaque to SIP Proxy
Servers. To do this sort of thing you probably want to use a
back to back UA.

Cheers,
Neil.
--
Ubiquity Software, UK                    www.ubiquity.net

> > In the concrete case, proxies would be able to remove certain codecs
> > from the INVITE messages, depending on the current policy in the
> > network. (The UAS gets a subset of the codecs, which were originally
> > sent by the UAC.)
> > What impact does such a proxy behavior have to SIP? I can think of
> > problems with authentication and message integrity checking (UAC signs
> > message with its private key).
>It certainly depends on the security mechanism used.  For e.g., with the
>simple basic md5, it does not cause any issue.
>
>Jean-Francois

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From: "Jean-Francois Mule" <[email-masked]>
Date: Fri, 15 Dec 2000 11:35:11 -0800

Independently of the Record-Route discussions, the 2543bis-02 (dated
11-24-00) has a small issue:

In the same section 6.35.1 of 2543bis-02, it is stated:
"a request route, once established, persists until the end of the call leg,
*regardless of whether the Record-Route header is present in subsequent
requests.*"
...
"Note that proxy servers have to add Record-Route headers to each request
*as long as* they want to be "visited" by the next request for the call
leg."
The persistence of the record-route is confusing.  The suggestion is to just
remove the note.

Comments?

Jean-Francois.

---

From: Billy Biggs <[email-masked]>
Date: Fri, 15 Dec 2000 13:52:14 -0600

Jean-Francois Mule ([email-masked]):

> "a request route, once established, persists until the end of the call
> leg, *regardless of whether the Record-Route header is present in
> subsequent requests.*"
> ...
> "Note that proxy servers have to add Record-Route headers to each
> request *as long as* they want to be "visited" by the next request for
> the call leg." The persistence of the record-route is confusing.  The
> suggestion is to just remove the note.

  The statement is correct: The Route persists until the end of the call
leg regardless.  Proxies may not ever leave the Route for a call-leg.
The only thing that can be updated is the Contact address at the far
end.

  That said, proxies must still add a Record-Route header, just to make
the protocol more explicit.

  Is there some other statement in the bis which makes this confusing?
Hopefully when this is re-written it will all become clearer.

-- 
Billy Biggs, 3Com               Email: [email-masked]
http://www.div8.net/billy/      Phone: [email-masked]

---

From: Vijay Gurbani <[email-masked]>
Date: Fri, 15 Dec 2000 14:56:56 -0600

> leg, *regardless of whether the Record-Route header is present in 

Right.

> The persistence of the record-route is confusing.  The suggestion is to 
> just remove the note.
> 
> Comments?

If I recall correctly, it was done to make the protocol resiliant to UA 
failures; doing so would insulate a proxy against a UA that may crash and
reboot between different requests in the same call.  If each proxy adds a
R-R header to all requests, the rebooted UA can resurrect the Route list 
from the R-R's in the request it got after it rebooted.  Assuming none of 
the UA's crash, the route will be established after a successful INVITE 
transaction; R-R in subsequent requests will be a no-op.

- vijay
-- 
Vijay K. Gurbani  vkg@{lucent.com,research.bell-labs.com,acm.org}
Internet Software and eServices Group 
Lucent Technologies/Bell Labs Innovations 263 Shuman Blvd., Rm 1A-413
Naperville, Illinois 60566     Voice: +1 630 224 0216   Fax: +1 630 713 0184