Message-Not-Understood Not Understood (Q13)

The information in this article applies to:

Prior to H.225.0v4, there was a mistake in H.225.0 regarding the Message Not Understood message, otherwise known as XRS and UnknownMessageResponse. The Recommendation said that if one cannot decode the requestSeqNum field from the message in question, zero shall be encoded in the Message Not Understood message’s requestSeqNum field. However, zero is not valid for this field! There is not a good solution to this problem, so my recommendation for pre-v4 entities is to at least encode something consistent, e.g., the value, one or 65,535. The receiver of MessageNotUnderstood will not know if this is a dummy or real value, but there is nothing more one can do.

This problem was cleared up in H.225.0v4 with the addition of the messageNotUnderstood field, which actually contains the incoming message that was not understood.