Purpose of ECHO message in GTP
- If path management is enabled, the broadband gateway sends periodic echo requests to all peers identified in the peer information table.
- When an echo-request message is received from a peer, the broadband gateway sends an echo-response message.
- If a peer does not respond after a specified number of echo requests, the peer is declared down and all subscriber sessions with the peer are brought down.
- Echo messages are sent to the peer using the GTP version that the peer supports. A broadband gateway configured as a GGSN, P-GW, or GGSN/P-GW supports sending echo replies to GTPv0, GTPv1, and GTPv2 echo requests from a peer SGSN or S-GW.
Format/Usage of ECHO message
ECHO message is same as other GTP-C/U message header format but only one difference is the 4 bytes TEID filed is missing in the header format.
The ECHO message is also called as path management message in the GTP interface because this message is used for to ping an GTP endpoint in the S11, S5, S8 interfaces. As well this message is used for restart procedure.
A GTP-U/C peer may send an Echo Request on a path to the other GTP-U/C peer to find out if it is alive (see section Path Failure). Echo Request messages may be sent for each path in use. A path is considered to be in use if at least one PDP context/Session, EPS Bearer, MBMS UE context, or MBMS bearer context uses the path to the other GTP-U/C peer. When and how often an Echo Request message may be sent is implementation specific but an Echo Request shall not be sent more often than every 60 s on each path. This doesn’t prevent resending an Echo Request with the same sequence number according to the T3-RESPONSE timer.
A GTP-U peer shall be prepared to receive an Echo Request at any time and it shall reply with an Echo Response. The optional Private Extension contains vendor or operator specific information