Subject: [SIP] more tag headaches

From: Jonathan Rosenberg <[email-masked]>
Date: Sat, 17 Mar 2001 01:39:48 -0500

Don't you just love tags.

Here is the problem. Bis currently RECOMMENDS that tags be the same across a
calls for the same instance of the UA. The reasoning for this was recovery
from crashes.

However, there is great utility to having it be unique for each call. The
reason is that a call leg is identified by the Call-ID, To, and From,
including tags. Its helpful to have a call leg identifier that you can
guarantee to be unique in space and time (for billing, logging, customer
care, etc.), and even better, that uniqueness is not dependent on either the
caller or called party alone to do the right thing. As an example, one can
imagine all kinds of malicious attacks on gateways where a PC caller chooses
the same From tags and Call-ID for each call. But, if the gateway always
generates To tags that are globally unique, we have a way to identify the
call leg uniquely without cooperation of the caller. It works in the reverse
direction as well.

Crash and recovery does not require that the tags be the same across calls
for the same instance of the UA; it requires only that a UA be able to
identify a tag as its own. One can do this easily by generating the tag as a
concatenation of an instance ID and a globally unique number. This gives us
the best of both worlds.

So, does anyone object if I change the wording to discuss these issues, and
ultimately recomend tag selection as discussed in the previous paragraph?

-Jonathan R.

---
Jonathan D. Rosenberg                       72 Eagle Rock Ave.
Chief Scientist                             First Floor
dynamicsoft                                 East Hanover, NJ 07936
[email-masked]                     FAX:   (973) 952-5050
http://www.cs.columbia.edu/~jdrosen         PHONE: (973) 952-5000
http://www.dynamicsoft.com

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From: Henning Schulzrinne <[email-masked]>
Date: Sat, 17 Mar 2001 19:25:37 -0800

> call leg uniquely without cooperation of the caller. It works in the reverse

However, this again assumes that one or both of the end systems play
along.

> ultimately recomend tag selection as discussed in the previous paragraph?

I'm a bit concerned that this overlaps the identifiers proposed by the
DCS folks, inserted by proxies.

In general, while I don't mind stating that the tag should simply allow
the end system to identify itself, I'm not sure we need to get into the
reasoning behind this or suggest a particular algorithm or encoding.
This appears to be an implementation issue.