Packetizer

What is the format of the Cause IE? (Q114)


The information in this article applies to: H.323, H.225.0

The format of the Cause IE is not described in H.225.0 or H.323. H.225.0v1 through v3 says that the length is 1 octet for the ReleaseComplete message; however, there is no single-octet variant of this IE, so this is clearly a mistake. It was fixed in H.225.0v4.

The typical lengths are 2 and 3 octets, which would include octets 3 and 4 or 3, 3a, and 4, respectively (using Q.850/Q.931 octet numbering). The only field mentioned by H.323 is the Cause value in octet 4. Therefore, an entity should ignore all fields except this one.

Here are the suggested values for all fields in the 3-octet variant of the Cause IE:

For the 2-octet variant, set the length to "2", set the extension bit in octet 3 to "1", and omit octet 3a. We recommend that an EP transmit the 2-octet variant in order to minimize the risk of interoperability problems.

(Note that since H.323 does not specify this, other values may be present for the Coding standard, Location, and Recommendation fields. These make the most sense to us, though.)