Subject: [SIP] Computing SIP-URL from TEL-URL

From: Bryan Byerly <[email-masked]>
Date: Tue, 11 Jul 2000 17:44:53 -0400

Hi guys,

From a SIP proxy's perspective,

Can anyone think of a reason that it would not be safe to assume that a
TEL-URL:
tel:5551234
is equivalent to the SIP-URL:
sip:5551234@this_proxy;user=phone

where "this_proxy" is the IP address of the proxy processing the
message.

Is this assumption safe for all known headers ("To:", "From:",
"Contact:", "Route:", "Remote-Party-ID", ...)?

Thanks!

Bryan

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From: Igor Slepchin <[email-masked]>
Date: Tue, 11 Jul 2000 19:13:12 -0400

> sip:5551234@this_proxy;user=phone

Well, they are two different URLs and may or may not refer to the same
resource. Proxies may (or may not) treat them differently. E.g., a proxy
may consider sip:5551234@this_proxy;user=phone to be a valid username in
its address space (user=phone notwithstanding), look for registrations
for that user and forward the request to the registered hosts. 

> "Contact:", "Route:", "Remote-Party-ID", ...)?

Note that only SIP URLs are allowed in Contact and Route headers. Both
Contact and Route are used for forwarding SIP messages and tel URLs
don't make sense in that context.

Thank you,
Igor Slepchin

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From: David Williams <[email-masked]>
Date: Tue, 11 Jul 2000 21:27:08 -0400 (EDT)

Overall, it appears that Redirect servers should behave statefully and
retransmit their final response.  But is it also legal for them to
behave statelessly and only send a single final response?  In a manner
similiar to that of a stateless proxy?

Some relevant sections of the 2453bis:

  Section 12.1 (Behavior of Redirect Servers) says: "The Redirect
server maintains transaction state for the whole SIP transaction."
  Section 10.1.2 (Responses) says: "Responses with status 300 and
higher are retransmitted by each stateful proxy or UAS until the next
upstream proxy or UAC sends an ACK or CANCEL."  But no meantion of a
redirect server.
  Section 10.5.1 (UDP) indicates that servers should reliably
retransmit responses.  Without meantioning that stateless proxies
cannot.

thanks,
-david

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From: Jonathan Rosenberg <[email-masked]>
Date: Fri, 14 Jul 2000 03:08:12 -0400

This is actually possible so long as (1) your redirect server never
sends a provisional response, (2) your location service logic yields the
same exact result when invoked with the same request. Request
retransmissions will arrive at the server, and appear as new requests.
These cause the location service lookup to be redone, and a new redirect
response to be sent. This redirect response will be the same as previous
ones from the same invite, thus resulting in triggered response
retransmissions, and thus reliability.

Sending a provisional response screws this up, since you will no longer
get request retransmissions (for INVITE, at least).

-Jonathan R.

-- 
Jonathan D. Rosenberg                       72 Eagle Rock Ave.
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