ECHO message handling in GTP-C

Introduction: 
ECHO message is a primary message communication in GTP interface. This message is used for to ping an end point as well used for restoration procedure for an end point. We will go detail in the following description.

Format of ECHO message:

		Bits
Octets		8	7	6	5	4	3	2	1
1		Version			P	T=0	Spare	Spare	Spare
2		Message Type
3		Message Length (1st Octet)
4		Message Length (2nd Octet)
5		Sequence Number (1st Octet)
6		Sequence Number (2nd Octet)
7		Sequence Number (3rd Octet)
8		Spare

This message is same as other GTP-C 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.

The format of GTP-C ECHO message is like this:

ECHO message is a combination of ECHO GTP-C header and some Informational Element.
The Informational elements are

Information elements	P	Condition / Comment	IE Type	Ins.
Recovery	M		Recovery	0
Sending Node Features	CO	This IE shall be sent towards a peer node on any GTPv2 interface if the sending node supports at least one feature on this interface or if the sending node supports at least one feature and does not know the interface type towards the peer node. This IE may be present otherwise.	Node Features	0
Private Extension	O		Private Extension	VS

This message contains one IE called Recovery IE to set the restart counter value for an GTP endpoint.

Suppose one GTP endpoint restarted then that end point will send an ECHO request to the other GTP end point  with the recovery IE.

Restart procedure Handling at SGW

During or immediately after an SGW Restart the SGW shall place local SGW restart counter value in all GTPv2 Echo requests/responses messages.

The SGW will receive the MME restart counter in GTPv2 Echo requests and Echo response messages that the SGW receives from the MME.

The SGW will receive the PGW restart counter in GTPv2 Echo requests/ responses receives from the PGW.

When an SGW detects that a peer MME has restarted it shall delete all PDN connection table data associated with the peer node that fails as well as freeing any internal SGW resources associated with those PDN connections.

When an SGW detects that a peer PGW has restarted it shall delete all PDN connection table data associated with the peer node that fails as well as freeing any internal SGW resources associated with those PDN connections. In addition, if the optional feature PGW Restart Notification is supported by the SGW  the SGW shall initiate the cleanup of the hanging PDN connections associated with the SGW and the restarted PGW at the corresponding MMEs by sending GTPv2 message(s) PGW Restart Notification, with the control plane IP address of the restarted PGW and the control plane IP address of the SGW on the S11 interface included. 

When the MME receives this message, according to the control plane IP address of the restarted PGW and the control plane IP address of the SGW on the S11 interface included in the message, MME should delete all PDN connection table data associated with the SGW and the restarted PGW as well as freeing any internal MME resources associated with those PDN connections. The MME may optionally perform other implementation specific actions such as to clear external resources (e.g. S1-MME messages to clear eNodeB resources) or more advanced forms of restoration. 

Restart procedure Handling at PGW

After a PGW restart, the PGW shall delete all MM Bearer contexts affected by the restart that it may have stored.

During or immediately after a PGW Restart, the PGW shall place this PGW restart counter value in all GTPv2 echo requests/responses the PGW sends.

The PGW will receive the SGW  restart counters in GTPv2 echo requests/responses. When a PGW detects that a peer SGW has restarted it shall delete all PDN connection table data associated with the peer node that fails as well as freeing any internal PGW resources associated with those PDN connections.