[Guy Harris <gharris@xxxxxxxxxxxx>]

> However, if the URL I gave is right, v120 uses basic mode only.

Well, the V.120 spec says "The data link control sublayer provides the
procedures and formats of fields for data link layer peer-to-peer
communication. ... For formats and the elements of procedures, see
Recommendation Q.922", and Q.922 seems to at least allow for modulo 128
operation; it also speaks of SABME, as does V.120, and I *think* the
"extended" in "Set asynchronous balanced mode extended" means "modulo
128".

The V.120 trace Bert sent out a while ago has what appears to be 16-bit
control fields - and, in fact, the first V.120 frame in that trace is an
SABME.  That frame is, of course, a U frame, which means the control
field is only one octet - and would be treated as only one octet by
"dissect_xdlc_control()" regardless of whether "is_extended" is true or
not, so perhaps the V.120 dissector needn't check whether "v120len" is
3, and can just pass TRUE as the "is_extended" argument to
"dissect_xdlc_control()".