[Wireshark-bugs] [Bug 14105] gsmtap: SACCH/SDCCH dissection has changed between 2.2.7 and 2.4.1

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https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=14105

Pascal Quantin  changed:

   What|Removed |Added

 Resolution|--- |FIXED
 Status|CONFIRMED   |RESOLVED

--- Comment #19 from Pascal Quantin  ---
The sample pcap is now decoded properly, so I will mark the bug as fixed.
It can be reopened if needs be.

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[Wireshark-bugs] [Bug 14105] gsmtap: SACCH/SDCCH dissection has changed between 2.2.7 and 2.4.1

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https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=14105

--- Comment #18 from Harald Welte  ---
I think from my point of view it can be closed, but it might make sense to get
some feedback from the original reporter (or somebody else independent of me)
on whether his problem is really gone?

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[Wireshark-bugs] [Bug 14105] gsmtap: SACCH/SDCCH dissection has changed between 2.2.7 and 2.4.1

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https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=14105

--- Comment #17 from Pascal Quantin  ---
Harald,

can we close the bug despite the Ping-Bug tag in Git commit message instead of
Bug? Or do you want to keep it open for a long term solution?

Thanks,
Pascal.

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[Wireshark-bugs] [Bug 14105] gsmtap: SACCH/SDCCH dissection has changed between 2.2.7 and 2.4.1

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https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=14105

--- Comment #16 from Gerrit Code Review  ---
Change 26984 had a related patch set uploaded by Pascal Quantin:
lapdm: Hand B4 frames into a dissector supporting L2 pseudo-length

https://code.wireshark.org/review/26984

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[Wireshark-bugs] [Bug 14105] gsmtap: SACCH/SDCCH dissection has changed between 2.2.7 and 2.4.1

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https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=14105

--- Comment #15 from Gerrit Code Review  ---
Change 26983 had a related patch set uploaded by Pascal Quantin:
lapdm: Hand B4 frames into a dissector supporting L2 pseudo-length

https://code.wireshark.org/review/26983

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[Wireshark-bugs] [Bug 14105] gsmtap: SACCH/SDCCH dissection has changed between 2.2.7 and 2.4.1

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https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=14105

--- Comment #14 from Gerrit Code Review  ---
Change 26982 had a related patch set uploaded by Pascal Quantin:
rsl: Fix treatment of SACCH FILL / SACCH INFO MODIFY

https://code.wireshark.org/review/26982

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[Wireshark-bugs] [Bug 14105] gsmtap: SACCH/SDCCH dissection has changed between 2.2.7 and 2.4.1

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https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=14105

--- Comment #13 from Gerrit Code Review  ---
Change 26981 had a related patch set uploaded by Pascal Quantin:
rsl: Fix treatment of SACCH FILL / SACCH INFO MODIFY

https://code.wireshark.org/review/26981

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[Wireshark-bugs] [Bug 14105] gsmtap: SACCH/SDCCH dissection has changed between 2.2.7 and 2.4.1

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https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=14105

--- Comment #12 from Harald Welte  ---
I presume everyone just has way too much work on their plate already and hence
patches haven't received any review so far.

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[Wireshark-bugs] [Bug 14105] gsmtap: SACCH/SDCCH dissection has changed between 2.2.7 and 2.4.1

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https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=14105

Guy Harris  changed:

   What|Removed |Added

 OS|Linux   |All

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[Wireshark-bugs] [Bug 14105] gsmtap: SACCH/SDCCH dissection has changed between 2.2.7 and 2.4.1

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https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=14105

--- Comment #11 from Harald Welte  ---
the LAPDm side patch has been submitted as
https://code.wireshark.org/review/26798

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[Wireshark-bugs] [Bug 14105] gsmtap: SACCH/SDCCH dissection has changed between 2.2.7 and 2.4.1

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https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=14105

--- Comment #10 from Harald Welte  ---
Created attachment 16252
  --> https://bugs.wireshark.org/bugzilla/attachment.cgi?id=16252=edit
patch as submitted to gerrit for LAPDm side

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[Wireshark-bugs] [Bug 14105] gsmtap: SACCH/SDCCH dissection has changed between 2.2.7 and 2.4.1

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https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=14105

--- Comment #8 from Harald Welte  ---
I've pushed https://code.wireshark.org/review/26797 as a possible fix.  It's
fairly trivial, but it feels quite wrong to call a "gsm_a_ccch" dissector with
something that clearly is transportetd over the downlink SACCH and not over the
CCCH.

As dissect_ccch() comments state: "The name CCCH might not be correct!"

So in reality, what dissect_ccch() is, is a RR dissector for the "information
field of LAPDm B4 and Bbis frames".  Maybe it should be renamed? But then, we'd
change the name for register_dissector and would have to rename this in various
places.

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[Wireshark-bugs] [Bug 14105] gsmtap: SACCH/SDCCH dissection has changed between 2.2.7 and 2.4.1

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https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=14105

--- Comment #9 from Harald Welte  ---
Created attachment 16251
  --> https://bugs.wireshark.org/bugzilla/attachment.cgi?id=16251=edit
patch as submitted to gerrit

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[Wireshark-bugs] [Bug 14105] gsmtap: SACCH/SDCCH dissection has changed between 2.2.7 and 2.4.1

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https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=14105

--- Comment #7 from Harald Welte  ---
Yes, I will post patches.  But it may take a fed days or even a week due to
travel plans and the need to get things right this time, testing with handsets,
...

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[Wireshark-bugs] [Bug 14105] gsmtap: SACCH/SDCCH dissection has changed between 2.2.7 and 2.4.1

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https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=14105

--- Comment #6 from Pascal Quantin  ---
Soudns a good plan. Will you follow-up with patches?

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[Wireshark-bugs] [Bug 14105] gsmtap: SACCH/SDCCH dissection has changed between 2.2.7 and 2.4.1

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https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=14105

Harald Welte  changed:

   What|Removed |Added

 Status|UNCONFIRMED |CONFIRMED
 Ever confirmed|0   |1

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[Wireshark-bugs] [Bug 14105] gsmtap: SACCH/SDCCH dissection has changed between 2.2.7 and 2.4.1

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https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=14105

--- Comment #5 from Harald Welte  ---
Sorry for the late response here.  The issue is rather complicated, but has now
shown up several other places like https://osmocom.org/issues/3059 and
https://lists.osmocom.org/pipermail/openbsc/2017-December/011545.html

In short:
* The wireshark RSL dissector has been wrong regarding the decoding of the "L3
Info" Information Element of SACCH filling on Abis.  It expects that L3 Info
payload to be _without_ the L2 pseudo-length field at the beginning
* OpenBSC / OsmoBSC introduced a regression in April 2017 which removed the L2
pseudo-length from the RSL side.  It probably was never detected as in
wireshark, it just looked great
* As wireshark decoded RSL correctly, but not the GSMTAP / Um / air interface
traces, the LAPDm dissector was changed in
https://code.wireshark.org/review/21767 to also remove that one byte

The last change is not without merit, as 3GPP TS 44.006 specifies explicitly
that the B4 frame format is used on downlink SACCH, and that frame format has
no length octet. Rather, the length octet magically appeared in the sytem
information messages themselves.

So from a spec point of view, it seemed that initially, a length octet was part
of the L2 header, but then has subsequently removed from the spec, while that
very same length octet of the same position of the message was later added to
the beginning of the layer 3 information.  That change likely happened around
the GSM phase1 -> phase2 change in the 1990ies and likely predates any
wireshark code.

So my line of thinking is:
* fix OsmoBSC to encode the messages properly again (in progress)
* fix wireshark dissector for RSL
* fix wireshark dissector for LAPDm/RR stacking on SACCH (i.e. keep the 2-byte
B4 frame format introduced in https://code.wireshark.org/review/21767 as it is
correct since the late 1990ies but adjust the dissector stacked on top of it

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[Wireshark-bugs] [Bug 14105] gsmtap: SACCH/SDCCH dissection has changed between 2.2.7 and 2.4.1

[bugzilla-daemon]

https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=14105

Pascal Quantin  changed:

   What|Removed |Added

 CC||lafo...@gnumonks.org

--- Comment #4 from Pascal Quantin  ---
So this is a consequence of https://code.wireshark.org/review/21767 that
purposely removed 1 extra byte to the LAPDm header, leading to the behavior
seen.
GSM is way too old for me, so I'm CCing Harald (the patch author) to know
whether there is a subtlety that was missed in the initial patch, or whether
this is due to the use of a newer Wireshark with an older gsmtap.

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[Wireshark-bugs] [Bug 14105] gsmtap: SACCH/SDCCH dissection has changed between 2.2.7 and 2.4.1

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https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=14105

--- Comment #3 from Pascal Quantin  ---
1O years is a long time, I did not remember some SI could be transmitted over
dedicated channels and not only broadcast ones...
Having a look right now.

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[Wireshark-bugs] [Bug 14105] gsmtap: SACCH/SDCCH dissection has changed between 2.2.7 and 2.4.1

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https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=14105

--- Comment #2 from Peter Wu  ---
Anonymous user said:
"SI1/2/2ter/2quater/3/4 are usually transmitted on CCCH/BCCH,
SI5/5ter/6 are not, these are usually transmitted on SACCH/SDCCH"

No idea what that means though.

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[Wireshark-bugs] [Bug 14105] gsmtap: SACCH/SDCCH dissection has changed between 2.2.7 and 2.4.1

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https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=14105

--- Comment #1 from Pascal Quantin  ---
It has been 10 years or so that I have not done 2G, but how could a System
Information be transmitted over SACCH instead of CCCH?

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