Correspondence from ITU-T SG 11 to SIP/SIPPING


APPROVAL:	APPROVED BY ITU-T Study Group 11 Rapporteurs meeting of
BICC/SIP interworking and AAL2/IP interworking (Newbury, June 2002)
CONTACT:	   Steve Norreys   
Tel: +44 20 7843 5189                       
Angel Centre                      
403 St Johns Street                      
London  UK                             
This message was agreed to at the ITU-T Study Group 11 Rapporteurs meeting on BICC/SIP interworking and AAL2/IP interworking (Newbury, June 2002).
As you know, ITU-T Study Group 11 has initiated a work program to define detailed signalling procedures for the interworking of SIP with BICC (Bearer Independent Call Control) and with ISUP (ISDN User Part of Signalling System 7).  The work done by the IETF on this topic has served as input to our own work.  We are basing our work on BICC CS2 (2001), ISUP (2000), and the recently approved version of SIP (2543bis-09).  The output documents consist of a SIP profile specification (draft Recommendation Q.sipprof) and the interworking specification itself (draft Recommendation Q.1912.sip).  The latest versions of these drafts are available on the informal SIPPING web page (or will be shortly).
The SG 11 BICC group invites the IETF SIP and SIPPING WGs to send comments on the interworking to the above contact in time for the next BICC Rapporteurs meeting in Ottawa (Canada), 9-13 September 2002. 
A strong consensus exists among the participants of the BICC Rapporteurs meeting that the first interworking recommendations on SIP- ISUP/BICC interworking will be finished in November 2002 for the approval at the SG11 meeting.  We have scheduled one more interim meeting to work toward this objective, in Ottawa, Canada, in September 2002.  Interested IETF participants are invited to contact Steve Norreys at the address given above for further information.
During the Rapporteurs' meeting in Newbury the following inconsistencies emerged between draft Q.1912.SIP and IETF documents.
1.	The use of ISUP Timer T10 for SIP procedures in
draft-ietf-sipping-overlap-00.txt is questionable, because this timer is part of the state machine on the ISUP side of the interworking point and using it might have unintended consequences.  It is recommended that a timer be defined specific to the interworking process instead. 
2.	The mapping of FCI and BCI within the IETF draft
draft-ietf-sipping-isup-02.txt is different from what the SG11 experts have seen proper to use within Q.1912.SIP.  The setting of the FCI in the interworking case (INVITE - IAM) is "no end-to-end method available, interworking encountered, ISDN not used all the way, originating access non-ISDN". The Setting of the BCI in the interworking Case (180-ACM) is "no end-to-end method available, interworking encountered, ISDN not used all the way, originating access non-ISDN". In the SIP-T case the FCI and BCI will be taken from the encapsulated ISUP.  It is not yet clear how to set the FCI and BCI if there is some updating of the received ISUP message using the contents of the SIP header mapping. 
3.	Cut through in the backward direction for tones and announcements
from the SIP network can be misused as there is a bearer path potentially available from the user equipment without charging.  (The called UA could send back a 180 Ringing, then send back its own media without bothering to send a 200 OK INVITE.)  We are currently taking a conservative approach and not cutting through until answer.  This is an issue of whether the SIP network can be trusted. 
4.	Regarding the SIP Status Code to ISDN Cause Code Mapping, some
changes to your proposed mappings will be integrated in the ITU Q.1912.SIP. The final mapping will be decided at the next meeting in Ottawa.  We have simplified the mappings a bit because the ISUP/BICC network won't be able to make good use of the information.  The general rule is that non-service-related responses will be mapped to "interworking, non-specific" unless the alternative is obvious.
Due to the lack of the meeting time, ITU wasn't able to make a complete review of the IETF interworking document. It was agreed that comments regarding this interworking document would be made directly by the individual experts and forward it to the IETF lists.
The SG 11 BICC group looks forward to further communications as this study progresses. 
Tom Taylor
Ph. +1 613 736 0961 (ESN 396 1490)