Relationship of H.323 and H.245 versions

Paul Long Plong at SMITHMICRO.COM
Wed Jan 20 23:15:03 EST 1999

Dave, Pete, et al.,

We need to be very clear about how products of different versions can be
interoperable.  Terms such as "compatible," "backward compatible," and
"supports" are too ambiguous to be useful in normative text.  Here's my
attempt at splitting hairs.  The first paragraph is taken straight from Dave's

Products claiming compliance with Version 2 of H.323 shall comply with all of
the mandatory requirements of this document, H.323 (1998), which references
H.225.0 (1998) and H.245 (1998).  The protocol identifiers used in messages
defined in H.225.0 (1998) and H.245 (1998) are specified in those

Interoperability between products of possibly different versions is assured by
the following.  Note that what is commonly referred to as a "message" is an
ASN.1 component of a choice type of typically the first or second level of an
ASN.1 syntax tree. Therefore, in the following paragraph, a "message" that is
not a component of the extension root is treated the same as any other choice
extension addition.

Assume that an H.323vM product and an H.323vN product are in communication
with each other, where M and N are version numbers and M <= N.  The H.323
version number shall be taken to be the same as the version number indicated
in the H.225.0 protocolIdentifier field.  Each product shall encode and decode
the H.245 and H.225.0 ASN.1 syntax trees defined in their respective version
of H.323.  The H.323vM product shall behave according to H.323vM and shall
assume that the H.323vN product also behaves according to H.323vM, with these
two exceptions:
*       the H.323vN product encodes values for the protocolIdentifier fields
for H.323vN, not H.323vM, and
*       the H.323vN product encodes ASN.1 sequence and choice extension
additions defined in H.323vN that may not be defined in H.323vM.
Although the H.323vM product shall parse extension additions defined in later
versions of H.323 (as prescribed by ASN.1), it shall not recognize or
otherwise act upon them.  Therefore, the H.323vN product shall not rely on
behavior in the H.323vM product that is based on the value or presence of
extension additions defined in H.323vN but not H.323vM.  This in effect forces
the H.323vN product to behave like an H.323vM product.  In addition, if a
product is relaying an ASN.1 value, it shall encode the same value that it
decoded, regardless of its own version or the version of the value's sender.

Paul Long
Smith Micro Software, Inc.

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