Cellular Network Infrastructure » Core Network (CN) » OsmoSGSN

I purchased a KRZR 3 for the sysmocom lab. Not sure when somebody will be able to try to reproduce, but at least we should be able to do now.

I've been running SGSN now for some weeks with this patch in place to allow the KRZR to attach and I have not seen any related problems. (with any other phone)

TS 04.06 [EDIT correction TS 04.64 6.4.1.6 ] is kind of confusing here.

On the one had it says:
As an optimisation, parameters confirming the requested values may be omitted from the XID response.

also:

The responding side may respond with parameters that were not included in the XID command. A parameter that was
not included in the XID command shall in this case be treated as if the current value of the parameter was included in
the XID command. The responding side shall include such a parameter in every XID response until the parameter has
been explicitly negotiated, either by responding to an XID command that included the parameter, or by explicitly
including the parameter the next time an XID command is transmitted.

also:

Negotiated XID parameters shall apply to the LLE identified by the DLCI of the XID frames used, except Version,
Reset, and IOV-UI that applies to an LLME (i.e., a TLLI), and except Layer-3 Parameters that apply to the layer 3
above the LLE.

So from that I understand that while we may not actually be applying this L3 param to the LLE identified by the DLCI (to be honest, I haven't even looked up what the dlci is), we still MUST include it in the response.

Maybe this is a mis understanding that is the origin of this

if (xid_field->type != GPRS_LLC_XID_T_L3_PAR)

in the code?

Ref: https://www.etsi.org/deliver/etsi_ts/101300_101399/101351/08.07.00_60/ts_101351v080700p.pdf#page=29

note to self: attach patch and .pcap

The point is that the L3 parameters are handled in SNDCP, and not in LLC. So the LLC code is correct to only handle the non-L3. The behavioral difference (and possible bug) hence is in the SNDCP XID handling.

What we need is a pcap file showing request and response for both the modified and the unmodified code.

The interesting function might be the sndcp_sn_xid_ind() in the sgsn.

Posted https://gerrit.osmocom.org/#/c/osmo-sgsn/+/13811/ for review.

I'm writing a testcase for this. It should be reasonably simple to send a MO XID with empty L3, and check the response.

note: We don't really have reasonable XID related tests yet.

Hi laforge Was that test case ever merged? Maybe we can reference it and then close this.

keith wrote:

Hi laforge Was that test case ever merged? Maybe we can reference it and then close this.

commit 2aaac1b1d1655732055e6ad5da5965fe94af33e5
Author: Harald Welte <laforge@gnumonks.org>
Date:   Thu May 2 10:02:53 2019 +0200

    sgsn: Add initial XID handshake related tests

    Change-Id: I5d4b3cba2fe05dffe10c843f16cfec343bc67b5f

That commit includes a case with empty L3: TC_xid_empty_l3