GPRS Tunnelling Protocol - General Features - Header

GTP Version 1

GTPv1 headers contain the following fields:

Version: It is a 3-bit field. For GTPv1, this has a value of 1.
Protocol Type (PT): a 1-bit value that differentiates GTP (value 1) from GTP' (value 0).
Reserved: a 1-bit reserved field (must be 0).
Extension header flag(E): a 1-bit value that states whether there is an extension header optional field.
Sequence number flag(S): a 1-bit value that states whether there is a Sequence Number optional field.
N-PDU number flag(PN): a 1-bit value that states whether there is a N-PDU number optional field.
Message Type: an 8-bit field that indicates the type of GTP message. Different types of messages are defined in 3GPP TS 29.060 section 7.1
Length: a 16-bit field that indicates the length of the payload in bytes (rest of the packet following the mandatory 8-byte GTP header). Includes the optional fields.
Tunnel endpoint identifier (TEID): A 32-bit(4-octet) field used to multiplex different connections in the same GTP tunnel.
Sequence number: an (optional) 16-bit field. This field exists if any of the E, S, or PN bits are on. The field must be interpreted only if the S bit is on.
N-PDU number: an (optional) 8-bit field. This field exists if any of the E, S, or PN bits are on. The field must be interpreted only if the PN bit is on.
Next extension header type: an (optional) 8-bit field. This field exists if any of the E, S, or PN bits are on. The field must be interpreted only if the E bit is on.

Next Extension Headers are as follows:

Length: an 8-bit field. This field states the length of this extension header, including the length, the contents, and the next extension header field, in 4-octet units. The length must be a multiple of 4.
Contents: extension header contents.
Next extension header: an 8-bit field. It states the type of the next extension, or 0 if no next extension exists. This permits chaining several next extension headers.