the GTP-C header (the first 4 octets). The TEID (if present) and the Sequence Number shall be included in the length count. Connectivity mechanisms Apart from the common message structure, there is also a common mechanism for verifying connectivity from one GSN to another GSN. This uses two messages. echo request echo response As often as every 60 seconds, a GSN can send an echo request to every other GSN with which it has an active connection. If the other end does not respond it can be treated as down and the active connections to it will be deleted. Apart from the two messages previously mentioned, there are no other messages common across all GTP variants meaning that, for the most part, they effectively form three completely separate protocols. GTP-C - GTP control The GTP-C protocol is the control section of the GTP standard. When a subscriber requests a PDP context, the SGSN will send a create PDP context request GTP-C message to the GGSN giving details of the subscriber's request. The GGSN will then respond with a create PDP context response GTP-C message which will either give details of the PDP context actually activated or will indicate a failure and give a reason for that failure. This is a UDP message on port 2123. The eGTP-C (or, GTPv2-C) protocol is responsible for creating, maintaining and deleting tunnels on multiple Sx interfaces. It is used for the control plane path management, tunnel management and mobility management. It also controls forwarding relocation messages; SRNS context and creating forward tunnels during inter LTE handovers. GTP-U - GTP user data tunneling GTP-U is, in effect a relatively simple IP based tunnelling protocol which permits many tunnels between each set of end points. When used in the UMTS, each subscriber will have one or more tunnel, one for each PDP context that they have active, as well as possibly having separate tunnels for specific connections with different quality of service requirements. The separate tunnels are identified by a TEID (Tunnel Endpoint Identifier) in the GTP-U messages, which should be a dynamically allocated random number. If this random number is of cryptographic quality, then it will provide a measure of security against certain attacks. Even so, the requirement of the 3GPP standard is that all GTP traffic, including user data should be sent within secure private networks, not directly connected to the Internet. This happens