18A GTP-U based restart procedure

23.0073GPPRelease 18Restoration proceduresTS

The support of GTP-U based restart procedure is optional for a GTP-U entity.

When the feature is supported, across GTP-U based interfaces, i.e. the S1-U, S11-U, S2a, S2b, X2, S4, S5, S8, S12, M1 and Sn interfaces of the Evolved Packet System in EPS, and the F1-U, Xn, N3, N9, N19, N3mb and N19mb interfaces of the 5G System in 5GS, a GTP-U entity shall utilize GTP-U Echo Request, Echo Response messages and GTP-U Error Indication message containing the Recovery Time Stamp Information Element to detect and handle a restart.

A GTP-U entity shall be prepared to receive an Echo Request message at any time (even from unknown peers), and it shall reply with an Echo Response message.

A GTP-U entity shall maintain two Recovery Time Stamps:

– in volatile memory a remote Recovery Time Stamp of a peer GTP-U entity with which the entity is in contact;

– in non-volatile memory own, or local Recovery Time Stamp that was sent to a peer GTP-U entity.

After a GTP-U entity has (re)started and all its GTP-U contexts have been lost, and if the GTP-U entity knows these GTP-U contexts are not to be restored, e.g., by its Control Plane function, it shall immediately update all local Recovery Time Stamps and shall clear all remote Recovery Time Stamps. When peer GTP-U entities information is available, e.g. when the first GTP-U tunnel towards the peer GTP-U entity is established, the (re)started GTP-U entity may send its (updated) Recovery Time Stamps in an Echo Request message to the peer GTP-U entity before sending GTP-U packets.

NOTE: The GTP-U entity needs to be configured to know if the GTP-U contexts can be restored or not. It is implemention specific how to handle the scenario that the GTP-U entity interworks with some control plane nodes supporting restoration of GTP-U contexts while others not supporting, for example, it can use different IP addresses.

A GTP-U entity may have a common local Recovery Time Stamp for all peer GTP-U entities, or it may have a separate local Recovery Time Stamp for each peer GTP-U entity.

A GTP-U entity may probe the liveliness of each peer GTP-U entity with which it is in contact by sending an Echo Request message.

The Recovery Time Stamp signalled in the GTP-U Echo Request and Response messages is associated with the GTP-U entity identified by the source IP address of the message.

The Recovery Time Stamp signalled in the GTP-U Error Indication is associated with the source IP address of the GTP-U Error Indication or associated with a list of IP address(es) which are sharing the same Recovery Time Stamp if those IP address(es) are explicitly included in the GTP-U Error Indication message.

The GTP-U entity that receives a Recovery Time Stamp Information Element from a peer GTP-U entity shall compare the received remote Recovery Time Stamp value with the previous Recovery Time Stamp value stored for that peer GTP-U entity.

– If no previous value was stored, the Recovery Time Stamp value received in the Echo Request or Response messages or the GTP Error Indication message shall be stored for the peer GTP-U entity.

– If the value of a Recovery Time Stamp previously stored for a peer GTP-U entity is smaller than the Recovery Time Stamp received in the Echo Request or Response messages or the GTP-U Error Indication messages, this indicates that the entity that sent the Echo Request or Response messages or the GTP-U Error Indication messages has restarted. The received, new Recovery Time Stamp value shall be stored by the receiving entity, replacing the value previously stored for the peer GTP-U entity.

– If the value of a Recovery Time Stamp previously stored for a peer GTP-U entity is larger than the Recovery Time Stamp value received in the Echo Request or Response messages or the GTP-U Error Indication messages, this indicates a possible race condition (newer message arriving before the older one). The received new Recovery Time Stamp value shall be discarded and an error may be logged.

Based on operator’s policy, when a Recovery Time Stamp IE is received in an Echo Request from a peer GTP-U entity, with a Recovery Time Stamp larger than the value of the Recovery Time Stamp previously stored for the peer GTP-U entity, the GTP-U entity may verify whether the peer GTP-U entity has really restarted by:

– sending one or more Echo Request message(s) towards the peer GTP-U entity; and

– determining that the peer GTP-U entity has restarted if the Recovery Time Stamp in the Echo Response message is larger than the value of the Recovery Time Stamp previously stored for the peer GTP-U entity.