Subject: RE: [Sip] another modest proposal: change call-leg ID

From: "Fairlie-Cuninghame, Robert" <[email-masked]>
Date: Wed, 11 Jul 2001 14:55:26 -0700

I think people can always build rogue endpoints or proxies but as far as I
can see this proposal doesn't change the "security" situation. If the rogue
entity can change/replicate the tag value then it could just as easily
change/replicate the whole URL (and vice versa). Additionally if
authentication is employed then the tag is just as protected as the whole
URL (since it is a subset).
 
It does however raise the point that if a message is signed then an
intermediate proxy may not modify the rest of the URL (while maintaining the
same tag value) but I think this is a desirable rather than undesirable
property. [Just as a proxy may not currently change the display name on a
signed message even though this does not affect the call-leg id.]
 
Cheers,

Robert.

-----Original Message-----
From: Jay Batson [mailto:[email-masked]]
Sent: Wednesday, July 11, 2001 10:28 PM
To: Zmolek, Andrew (Andrew); Jonathan Rosenberg
Cc: Grothe, Lew E (Lew); [email-masked]
Subject: RE: [Sip] another modest proposal: change call-leg ID

/me resists saying something like "Rogue endpoints?  Nah -- nobody would
ever *think* of building something uncontrolled, would they?"  ;-)

Just kidding.
-jb

At 12:32 PM 7/11/01, Zmolek, Andrew (Andrew) wrote:

Jonathan- 

Would your proposal in any way enable a rogue endpoint to alter or take over
a session by replicating a valid set of Call-ID and tags? 

--Andy Zmolek 
    Technology & Standards Engineer 
      CTO Standards 
        Avaya Inc. 

            [email-masked] 
              +1 720 444 4001 

-----Original Message----- 
From: Jonathan Rosenberg [ mailto:[email-masked]
<mailto:[email-masked]> ] 
Sent: Wednesday, July 11, 2001 12:30 AM 
To: [email-masked]' 
Subject: [Sip] another modest proposal: change call-leg ID 

Folks, 

While we're on the subject of modest proposals, here is another that I've 
been toying with: 

  Change the definition of call-leg ID. Instead of To, To-tag, From, 
  From-tag, and Call-ID, its *only* the tags and the Call-ID. That is, 
  the ID no longer contains the URLs. 

There are quite a few benefits from this: 

1. A calls B, and its forwarded to C. C accepts, and later re-INVITEs. The 
From field in this re-INVITE is NOT actually the identity of C, but rather, 
B. This screws up the idempotency of SIP INVITE. The >From field has one 
meaning in INVITE, and another in re-INVITE. This makes defining policies 
based on the From field more complex, since they no longer work properly for

re-INVITE. 

2. call-leg ID lookups in gateways are much faster now, since the tags and 
call-ids are just strings. Matching is now case-sensitive string compares. 
Much easier than URL comparison. 

3. Overlap dialing is easier. The reason we've had to send each digit as a 
new call leg, is that we needed to change the To field and the request URI. 
By definition, this meant a new call leg. Now, we can send all of that 
within a single call leg. Much cleaner. 

4. The Replaces draft benefits. We've already identified a need for the 
Replaces header to identify the entire call leg thats being replaced. This 
gets big when it carries two URLs, two tags, and a call-id. Now, we can 
throw away the URLs. This shortens the header and makes matching of the 
call-leg IDs easier. 

5. It works with existing proxies. 

THe main drawback, of course, is that this won't interoperate with rfc2543 
UAs. Thats fixable with Supported; the UAC indicates Supported: 
new-call-leg-id in the INVITE if it understands this. In that case, any 
responses or re-invites that change the URLs include a Require: 
new-call-leg-id. 

I fully admit that this may not be worth the pain. However, I'd welcome 
comments from others on whether its useful to consider further. 

-Jonathan R. 

--- 
Jonathan D. Rosenberg, Ph.D.                72 Eagle Rock Ave. 
Chief Scientist                             First Floor 
dynamicsoft                                 East Hanover, NJ 07936 
[email-masked]                     FAX:   (973) 952-5050 
http://www.jdrosen.net <http://www.jdrosen.net>                       PHONE:
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---

From: James Undery <[email-masked]>
Date: Thu, 12 Jul 2001 09:20:02 +0100

> URL (since it is a subset).

Just a small point, the tags are in headers which authentication leaves
unprotected, I submitted a draft yesterday attempting to fix this issue.

James Undery