Wireshark-bugs: [Wireshark-bugs] [Bug 5055] wrogn decoding of GTP Prime (GTP') Packets
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From: bugzilla-daemon@xxxxxxxxxxxxx
Date: Tue, 27 Jul 2010 01:43:07 -0700 (PDT)
https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=5055

--- Comment #2 from ernst.vogel@xxxxxxxxxxxx 2010-07-27 01:42:59 PDT ---
(In reply to comment #1)
> The problem is the version beeing set to 2 making Wireshark beleve this is a
> GTPv2 message. Should the version realy be 2?
> Regards
> Anders

The version-field depends on the protocol-type-flag: when the flag is set to 0,
its a gtp_prime packet and the version field indicates the
gtp_prime(GTP')-version, not the GTP-version.
I have pasted a short extract from the Specs (3GPP TS 32.295 V9.0.0 (2009-12)):

--------------------------
6    Data description for the transfer
6.1    The GTP' charging protocol
This clause describes the features of GTP'. The message types described in
subclause 6.2.2 ("Reused GTP message types") are also described in the related
clauses of TS 29.060 [200]. 
6.1.1    Usage of GTP Header in charging
In GTP' messaging only the signalling plane of GTP is partly reused. The GTP'
header is shown in Figure 6.1.
==> Bit 5 of octet 1 of the GTP header is the Protocol Type (PT) flag:  it is
'0' if the message is GTP'.
==> The Version bits indicate the GTP' protocol version when the Protocol Type
flag is '0'.
Bit 1 of octet 1 is not used in GTP' (except in v0), and it is marked '0' in
the GTP' header. It is in use in GTP' v0 and distinguishes the used
header-length. In the case of GTP' v0, this bit being marked one (1) indicates
the usage of the 6 octets header. If the bit is set to '0' (usually the case)
the 20-octet header is used. For all other versions of GTP', this bit is not
used and is set to '0'. However, this does not suggest the use of the 20-octet
header, rather a shorter 6-octet header.
The Length indicates the length of payload (number of octets after the GTP'
header). The Sequence Number of the packet is part of the GTP' header.
--------------------------

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