Re: GTP sequence number equal to zero problem

[Pascal Quantin <pascal.quantin () gmail com>]

2015-11-03 16:13 GMT+01:00 POZUELO Gloria (BCS/PSD) <gloria.pozuelo () bics com
> :

> Hi all,
>
>
>
> I’m developing an extension for the GTPv1 dissector and while I was
> debugging I’ve encountered something that it seems to me a bit confusing.
> In the dissect_gtp_common function from the packet-gtp.c source, there is a
> section that makes the matching between requests and responses. The thing
> is that I have a pcap in which I have a create PDP context request with
> sequence number equal to 0, then when in the code asks if the message has
> sequence number, it never enters in that part because, I guess, that 0 is
> equal than NULL.
>
>
>
> If (seq_no) {
>
>                 /* matching is done */
>
> }
>
>
>
> I don’t know If this is a little bug or maybe the sequence number can’t be
> equal to 0. Can someone help me?
>
>
>
> Thanks!
>
>
>
> Regards.
Hi Gloria,

at first glance this seems to be a bug. Per 3GPP 29.060 chapter 9.3.1.1:
9.3.1.1            Usage of Sequence Number

The sending GGSN and SRNC shall use 0 for the value of the Sequence Number
of the first G-PDU in a tunnel, only during the PDP context activation, and
shall increment the Sequence Number for each following G-PDU. The value
shall wrap to zero after 65535.

The receiving GGSN and SRNC shall set the content of a counter to zero,
only during the PDP context activation. When the receiving GGSN and SRNC
receives a valid G-PDU, it shall increment this counter by one. This
counter shall wrap to zero after 65535. It defines the "Expected Sequence
Number".

Could you please fill a bug on https://bugs.wireshark.org and even better,
upload a fix on Gerrit (see
https://www.wireshark.org/docs/wsdg_html_chunked/ChSrcContribute.html for
details) ? Presumably we should have a booleab telling whether a sequence
number was retrieved or not and test it (instead of seq_no being different
from 0).

Regards,
Pascal.

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