ETSI
ETSI TS 129 060 V11.7.0 (2013
3GPP TS 29.060 version 11.7.0 Release 11
6 GTP Header
The GTP header is a variable length header used for both the GTP-C and the GTP-U protocols. The minimum length of
the GTP header is 8 bytes. There are three flags that are used to signal the presence of additional optional fields: the PN
flag, the S flag and the E flag. The PN flag is used to signal the presence of N-PDU Numbers. The S flag is used to
signal the presence of the GTP Sequence Number field. The E flag is used to signal the presence of the Extension
Header field, used to enable future extensions of the GTP header defined in this document, without the need to use
another version number. If and only if one or more of these three flags are set, the fields Sequence Number, N-PDU and
Extension Header shall be present. The sender shall set all the bits of the unused fields to zero. The receiver shall not
evaluate the unused fields.
The GTP-C and the GTP-U use some of the fields in the GTP header differently. The detailed use of such fields is
described in the sections related to GTP-C and to GTP-U.
Always present fields:
- Version field: This field is used to determine the version of the GTP protocol. For the treatment of other
versions, see clause 11.1.1, "Different GTP versions". The version number shall be set to "1".
- Protocol Type (PT): This bit is used as a protocol discriminator between GTP (when PT is "1") and GTP' (when
PT is "0"). GTP is described in this document and the GTP' protocol in 3GPP TS 32.295 [33]. Note that the
interpretation of the header fields may be different in GTP' than in GTP.
- Extension Header flag (E): This flag indicates the presence of a meaningful value of the Next Extension Header
field. When it is set to "0", the Next Extension Header field either is not present or, if present, shall not be
interpreted. When it is set to "1", the Next Extension Header field is present, and shall be interpreted, as
described below in this section.
- Sequence number flag (S): This flag indicates the presence of a meaningful value of the Sequence Number field.
When it is set to "0", the Sequence Number field either is not present or, if present, shall not be interpreted.
When it is set to "1", the Sequence Number field is present, and shall be interpreted, as described below in this
section.
- N-PDU Number flag (PN): This flag indicates the presence of a meaningful value of the N-PDU Number field.
When it is set to "0", the N-PDU Number field either is not present, or, if present, shall not be interpreted. When
it is set to "1", the N-PDU Number field is present, and shall be interpreted, as described below in this section.
- Message Type: This field indicates the type of GTP message. The valid values of the message type are defined in
clause 7.1 for both GTP-C and GTP-U.
- Length: This field indicates the length in octets of the payload, i.e. the rest of the packet following the mandatory
part of the GTP header (that is the first 8 octets). The Sequence Number, the N-PDU Number or any Extension
headers shall be considered to be part of the payload, i.e. included in the length count.
- Tunnel Endpoint Identifier (TEID): This field unambiguously identifies a tunnel endpoint in the receiving
GTP-U or GTP-C protocol entity. The receiving end side of a GTP tunnel locally assigns the TEID value the
transmitting side has to use. The TEID values are exchanged between tunnel endpoints using GTP-C
(or RANAP, over the Iu) messages.
Optional fields:
- Sequence Number: This field is an optional field in G -PDUs. It is used as a transaction identity for signalling
messages having a response message defined for a request message, that is the Sequence Number value is
copied from the request to the response message header. In the user plane, an increasing sequence number for T-
PDUs is transmitted via GTP-U tunnels, when transmission order must be preserved.
- N-PDU Number: This field is used at the Inter SGSN Routeing Area Update procedure and some inter-system
handover procedures (e.g. between 2G and 3G radio access networks). This field is used to co-ordinate the data
transmission for acknowledged mode of communication between the MS and the SGSN. The exact meaning of
this field depends upon the scenario. (For example, for GSM/GPRS to GSM/GPRS, the SNDCP N-PDU number
is present in this field).
-
Next Extension Header Type: This field defines the type of Extension Header that follows this field in the
GTP-PDU.