Hi Krishna,

See my answers below...

Hi ..!! I have a few queries regarding H.225.0 RAS/AnnexG:

1. AnnexGCommonInfo does not contain Protocol Identifier. Why is it not

required for AnnexG messages? Keeping in view, the possible future versions of AnnexG; how shall a Border Element, on receiving a PDU from some other Border Element know

the

version of protocol being used by the sender of the PDU? Also, how does

the

sender ascertain which version of protocol the receiver understands? e.g.

if

an element is added in version 3 of the protocol, and the recepient can handle upto version 7, how can it be ascertained that the peer entity

would

understand the information sent/received?

Actually, Annex G does has a Protocol Identifier - it is the "version" field that is part of the common message info and is included in every message:

AnnexGCommonInfo ::= SEQUENCE { sequenceNumber INTEGER (0..65535), version AnnexGVersion, hopCount INTEGER (1..255), replyAddress SEQUENCE OF TransportAddress OPTIONAL, -- Must be present in request integrityCheckValue ICV OPTIONAL, tokens SEQUENCE OF ClearToken OPTIONAL, cryptoTokens SEQUENCE OF CryptoH323Token OPTIONAL, nonStandard SEQUENCE OF NonStandardParameter OPTIONAL, ..., serviceID ServiceID OPTIONAL }

In the first version of Annex G, it is defined as:

AnnexGVersion ::= OBJECT IDENTIFIER -- shall be set to -- {itu-t (0) recommendation (0) h(8) 2250 annex (1) g (7) version (0) 1}

2. Why is Sequence Number from 0 to 65535 for AnnexG, while for RAS its

from

1 to 65535 (ASN.1 Message Syntax)? Also, in section 7.17 (Message Not Understood) of H.225.0 it is mentioned that RequestSeqNum shall be zero if the message cannot be

decoded;

PDU encoding shall fail in such a case.

You're right. I am guessing that nobody ever noticed the discrepancy between H.225.0 (RAS) and Annex G. From the section you quote (H.225.0, section 7.17)), specifying the sequence number to zero seems to be a bug in the standard, since it defines the sequence number as:

RequestSeqNum ::= INTEGER (1..65535)

Regards Krishna

Michael Fortinsky ----------------------------------------------------------------- Senior Program Manager, IP Telephony Group, VocalTec Communications Ltd. Email: mike@vocaltec.com Tel: 972 9 9707768 Fax: 972 9 9561867