rsl: Fix treatment of SACCH FILL / SACCH INFO MODIFY

The RSL SACCH INFO MODIFY / RSL SACCH FILL messages contain a
SI5 / SI6 / SI5bis / SI5ter message.  Those SI are (like the SI on CCCH)
sent as UI frame in downlink direction.  Since Phase 2, the "L2
pseudo-length field is part of the L3 message, and not stripped /
interpreted at L2.

3GPP TS 44.006 states that a special B4 frame format is used on the
SACCH downlink for UI frames, which is basically a normal B frame, but
with no length field at L2, shifting the length field into L3 where it
becomes the L2 pseudo-length.

From RSL, we need to call a variant of the RR dissector that is able to
decode a L3 message that includes a L2 pseudo-length.  This is,
paradoxically, not the "gsm_a_sacch" dissector, as that one is only used
for B-frames, i.e. actual LAPDm ABM frames.  We must use the "gsm_a_ccch"
dissector.

See also the discussion in
https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=14105
as well as https://lists.osmocom.org/pipermail/openbsc/2017-December/011545.html
and https://osmocom.org/issues/3059 for further background information.

Ping-Bug: 14105
Change-Id: Icdad2b7698d5d7d613cacceec6a3d848b946306c
Reviewed-on: https://code.wireshark.org/review/26797
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>
Petri-Dish: Pascal Quantin <pascal.quantin@gmail.com>
Tested-by: Petri Dish Buildbot
Reviewed-by: Anders Broman <a.broman58@gmail.com>

0 files changed, 0 insertions, 0 deletions