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ECHO Message in GTP (an overview)

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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.
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- 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

ECHO Header

References

1) 3GPP TS 29.060
2) 3GPP TS 29.281
posted Feb 11, 2014 by Meenal Mishra

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GTP stands for the GPRS Tunneling Protocol and is used to encapsulate user data when passing through core network and also carries bearer specific signalling traffic between various network nodes.

Functionality Of GTP
- It provides mobility. When UE is mobile, the IP address remains same and packets are still forwarded since tunneling is provided between PGW and eNB via SGW
- Multiple tunnels (bearers) can be used by same UE to obtain different network QoS
- Main IP remains hidden so it provides security as well
- Creation, deletion and modification of tunnels in case of GTP-C

Types of GTP Protocol
Type of GTP

Various GTP Interfaces in LTE

GTP-C - Version 2
GTP-U - Version 1

In LTE network GTP-v2 is used on S5 and S11 interfaces and GTPv1 is used on S1-U, S5, X2-U interfaces (as shown below). In inter-RAT and inter PLMN connectivity, S3, S4, S8, S10, S12 and S16 interfaces also utilize GTP protocols
GTP Interfaces

GTP-U
GTP-U encapsulation of UE user plane traffic can be understood by following example. Lets see what happens when IP packet generated by UE reaches to eNodeB and is then forwarded to SGW.

Consider any application on UE creates an IP/TCP packet. This packet consist of actual data by application, TCP or UDP header and then IP field information which has source address of UE and destination address of application server (e.g. Twitter)
GTPU

When the eNodeB receives this packet over air interface, it will put the IP packet inside GTP header which has information related to tunnel IDs. Then further, it is encapsulated inside UDP and IP header and forwarded as ethernet frame towards SGW. Here the IP header contains eNodeB IP as a source address and SGW IP as a destination address

GTP-C
As GTP-Cv2 in LTE is used for tunnel management, some of the signalling messages cab be seen in the following figure -

GTP-C

GTP' (GTP prime)
GTP' uses the same message structure as GTP-C and GTP-U, but has an independent function. It can be used for carrying charging data from the charging data function (CDF) of the LTE network to the charging gateway function (CGF) over a Ga Interface.

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