Subject: [SIP] On SIP-H323 Interworking

From: "rafi" <[email-masked]>
Date: Thu, 13 Jul 2000 17:19:05 +0530

Hi,

Following are my understanding when a call is established 
from H.323 terminal to a SIP terminal ( draft-singh-sip-h323-01.txt).
Following could be  the call flow diagram  for such a call setup.
(Assuming no  faststart in the SetUp message)

SIP          IWF            GK               H323Terminal
Terminal
                                   GRQ
                               <---------------
                                   GCF
                               ---------------->

                                   RRQ
                             <----------------
                                   RCF
                              ----------------->

                                   ARQ
                             <----------------
                                   ACF
                             ----------------->
      
                        SETUP
                 <------------------------------

                      ARQ
                   ----------->
                      ACF
                   <-----------

   INVITE(SDP=C)

 <----------------

   200 OK (SDP=C1)
------------------->

   ACK
<------------------
                              TCS ( Cap=C1)
                    -------------------------------->
                              TCS ACK
                    <-------------------------------

                              TCS (Cap = C2)
                    <-------------------------------
                              TCS ACK
                     ------------------------------->

                     /* MSD is not shown */

                               OLC
                     -------------------------------->
                               OLC ACK
                     <--------------------------------

                               OLC
                      <--------------------------------
                               OLC ACK
                      --------------------------------->

   INVITE ( Cap = C3)
<---------------------

      183
---------------------->
                               Alerting
                       ---------------------------------->

    200 OK
 --------------------->        CONNECT
                         -------------------------------->

    ACK
<-----------------------

   Following are the difficulties with the above type of call set up.

  (i)  IWF must know default capability set of H323 terminal. There 
       is no way to  know actual capabilities in H323 without establishing 
       the call.

  (ii)  The capability final agreed upon by the Terminals (C3) is always
        a  subset of C ( default capability), thus  making selection of C  
        a critical one.

  (iiI)   When  a call is established initially at SIP Terminal ( first
        ACK), H323 terminal MAY  miss the media info.

   In RFC2543bis sending empty SDP is allowed  in INVITE message.
   Thus instead of above call scenario can we have call flow something 
   like  this

SIP          IWF            GK               H323Terminal

                                   GRQ
                               <--------------------
                                   GCF
                               ------------------------>

                                   RRQ
                               <-----------------------
                                   RCF
                               ------------------------>

                                   ARQ
                               <------------------------
                                   ACF
                               ------------------------>
     
                        SETUP
                    <-----------------------------------

                      ARQ
                  ----------->
                      ACF
                  <-----------

   INVITE(SDP=0)

 <----------------

   200 OK (SDP=C)
------------------->

                              TCS ( Cap=C)
                      ------------------------------->
                              TCS ACK
                     <-------------------------------

                              TCS (Cap = C1)
                      <-------------------------------
                              TCS ACK
                       ------------------------------->

                       /* MSD is not shown */

                               OLC
                       -------------------------------->
                               OLC ACK
                       <--------------------------------

                               OLC
                       <--------------------------------
                               OLC ACK
                       --------------------------------->

                             CONNECT
                         -------------------------------->

    ACK ( SDP = C2)   C2 is the maximal subset of C and C1
<-----------------------

  Any comments?

Regards
Rafi Assadi H.M.