DOCBOOK: markup suggestions for protocol descriptions?

[eichin at thok dot org (Mark W. Eichin)]

I've documented a moderate sized software system, which includes
several components that send binary messages to each other.  I'm using
a variablelist for abstract descriptions of the messages; now I need to
describe the protocol at the byte-level.  Is there something more
abstract than a table that would be appropriate?  I think about the
packets as being a collection of fields, with type, size, name, "is
optional", perhaps range for some types.  I've done this in raw HTML
before with tables, each packet identified in a span and then the
fields each individual rows - which is tolerable for output, but I'd
hoped to do better for input...  suggestions?