Subject: [SPIRITS] Redirection in SPIRITS

From: "Vijay K. Gurbani" <[email-masked]>
Date: Wed, 19 Mar 2003 16:33:45 -0600

Folks:

One issue that has come up in conjunction to the services
enabled by SPIRITS is how to best do redirection.  For
instance, an application server may subscribe to the equivalent
of a Terminating Attempt Authorized trigger (or Terminating
Busy trigger) to perform redirection services.  How do we
realize such services in SPIRITS?

So far, using ICW as a canonical example, the PSTN sends an
INVITE and expect a 3xx response redirecting it to alternative
destinations.  However, given the advent of events in the SIP
WG, there is a better way to do these types of services which
does not require the heavy-weight processing of the INVITE
transaction.  And that is the use of "message/sipfrag" MIME
type (http://www.ietf.org/rfc/rfc3420.txt).

Using this, the 200 OK (NOTIFY) from the application server (which
is a SPIRITS server as well), instead of containing an empty
body, will instead contain a body of MIME type "message/sipfrag".
The body will include fragments of a 3xx final response,
including the Contacts to redirect the call to.

Here is a sample call flow:

PSTN               App Server
   |<-------------------+ SUBS (body: application/spirits-event)
   +------------------->| 200 OK
  ...                  ...
   +------------------->| NOT (body: application/spirits-event)
   |<-------------------+ 200 OK (body: message/sipfrag)

Of course, this does not deprecate or obsolete the use of an
INVITE request; but rather it is yet another (light-weight) way
to do redirection services in SIP.

Comments/questions/opinions welcome.

Regards,

- vijay
-- 
Vijay K. Gurbani  vkg@{lucent.com,research.bell-labs.com,acm.org}
Wireless Networks Group/Internet Software and Services
Lucent Technologies/Bell Labs Innovations, 2000 Lucent Lane, Rm 6G-440
Naperville, Illinois 60566     Voice: +1 630 224 0216

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