Section: 4.2 Protocol stack 4.2.0 General
Content: The protocol stack for GTPv2 shall be as depicted in Figure 4.2.0-1. Figure 4.2.0-1: GTPv2 stack The GTPv2 headers are specified in the respective clauses of this specification. The source and destination IP addresses and UDP ports used for each GTP-C message depend on the role that the message plays in a message exchange. A message can be an Initial message, or a Triggered message, or a Triggered Reply message to Triggered message. An Initial message is sent to a peer GTP entity with a sequence number chosen by the sending entity (see clause 7.6). A Triggered message is sent in response to an Initial message. Triggered Reply message may be sent in response to a Triggered message. See clause 7.6 for the sequence number usage. Typically, a Request message is an Initial message, but a Request message may be a Triggered messages in certain procedures where they are triggered by an Initial Command message. See clause 4.2.5 for classification of the Initial messages and their possible Triggered messages, as well as cases where there are Triggered Reply messages to the Triggered messages. Piggybacking is an optional feature, which is described in Annex F of 3GPP TS 23.401[ General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access ]  [3]. If the feature is supported, then the piggybacking of the initial messages on triggered response messages for EUTRAN Initial Attach, a Handover from Trusted or Untrusted Non-3GPP IP Access to E-UTRAN (see clauses 8.6 and 16.11 of 3GPP TS 23.402[ Architecture enhancements for non-3GPP accesses ]  [45]) and UE-requested PDN Connectivity procedures shall be implemented as per requirements in clauses 4.2.0 and 5.5.1 of this specification .When piggybacking is used, a common IP header and a common UDP header shall be used for the triggered response message and the piggybacked initial message as depicted in Figure 4.2.0-2. Immediately following the triggered response message is the piggybacked initial message, following which no additional information shall be present. The clause 5.5 specifies the usage of piggybacking-specific fields in the GTP-C header. Figure 4.2.0-2: Packet Format for the Piggybacking of messages