13 Detection and handling of late arriving requests

29.2743GPP3GPP Evolved Packet System (EPS)Evolved General Packet Radio Service (GPRS) Tunnelling Protocol for Control plane (GTPv2-C)Release 18Stage 3TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

13.1 General

The procedures specified in this clause aim at handling more efficiently requests which may arrive late at upstreams entities, e.g. in networks experiencing processing or transport delays.

These procedures are optional to support. When supported, the use of these procedures is dependent on operator policy.

The procedure specified in clause 13.2 may be used with or without the procedure specified in clause 13.3. The procedure specified in clause 13.3 shall only be used in conjunction with the procedure specified in clause 13.2.

13.2 Detection and handling of requests which collide with an existing session context

13.2.1 General

This procedure enables an entity, which receives a request colliding with an existing session context, to know the time at which the new request and the existing session were originated, and to accept the new request only if it is more recent than the existing session.

The originating entities within the PLMN (i.e. MME, SGSN, TWAN and ePDG) shall be NTP synchronized.

13.2.2 Principles

The following principles shall apply if this procedure is supported and enabled by operator policy.

The SGW shall forward this parameter over the S5/S8 interface, if it is received from the MME/SGSN.

Upon receipt of a session establishment request which collides with an existing session context, the PGW shall accept the new session establishment request only if it contains a more recent time stamp than the time stamp stored for the existing session. An incoming session request shall be considered as more recent than an existing session and be accepted if no Origination Time Stamp information was provided for at least one of the two sessions. The PGW shall reject an incoming request whose time stamp is less recent than the time stamp of the existing session with the cause ‘Late Overlapping Request’.

3GPP TS 29.212 [29] and 3GPP TS 29.273 [68] further specify:

– the PGW requirements regarding the forwarding of the Origination Time Stamp parameter over the Gx and/or S6b interfaces, when received from the SGW or TWAN/ePDG;

– the handling of the Origination Time Stamp parameter by the PCRF and 3GPP AAA Server for an incoming request colliding with an existing session context.

An originating entity which detects a NTP failure shall not include the Origination Time Stamp towards other entities.

13.3 Detection and handling of requests which have timed out at the originating entity

13.3.1 General

This procedure enables an entity which receives a request to know when the request times out at the originating entity, and to stop further processing, at the receiver and further upstream entities, a request which has timed out at the originating entity.

The originating entities (i.e. MME, SGSN, TWAN and ePDG) and the receiving entities (i.e. SGW, PGW, PCRF, 3GPP AAA Server) shall be NTP synchronized. This procedure may be used between entities pertaining to the same PLMN, and if allowed by operator policy, between entities pertaining to different PLMNs.

This procedure shall not affect the setting of the retransmission timers by intermediate entities. E.g. the SGW shall set its T3 and N3 retransmission parameters as specified in this specification, irrespective of the time at which the request times out at the originating entity.

13.3.2 Principles

The following principles shall apply if this procedure is supported, enabled by operator policy and if the PGW pertains to the same PLMN as the originating entity or if the PGW pertains to a different PLMN and operator policy in the originating entity allows to use this procedure towards this PLMN.

A GTP-C entity originating a Create Session Request (i.e. MME, SGSN, TWAN or ePDG) shall include in the message the Origination Time Stamp indicating the absolute time at which the request is initiated, as specified in clause 13.2.2, and the Maximum Wait Time indicating the maximum time period to complete the processing of the request. The Maximum Wait Time, together with the Origination Time Stamp, indicates the absolute time at which the request times out at the originating entity.

The MME/SGSN shall set the Maximum Wait Time to a value smaller or equal to (N3+1) x T3 set in the SGW, to avoid upstream entities continuing to process requests which would have ceased to be processed by the SGW, which could result in hanging contexts in upstream entities.

NOTE 1: If the Maximum Wait Time is set to a value smaller than N3 x T3 set in the SGW, the SGW actually stops retransmitting a given GTP-C Create Session Request as soon as it receives a rejection response from the PGW due to the expiry of the Maximum Wait Time. I.e. the Maximum Wait Time actually leads to shorten the duration during which the SGW retransmits the GTP-C request.

The SGW shall forward the Origination Time Stamp over the S5/S8 interface as specified in clause 13.2.2. The SGW shall also forward the Maximum Wait Time over the S5/S8 interface, if received from the MME/SGSN.

Upon receipt of a request which contains the Origination Time Stamp and the Maximum Wait Time parameters, the receiving entity should check that the request has not already timed out at the originating entity. The receiving entity may perform additional similar checks during the processing of the request, e.g. upon receipt of a response from the next upstream entity.

The receiving entity should reject a request that is known to have timed out with the cause ‘Timed Out Request’; it may alternatively drop the request. Besides, the receiving entity should initiate the release of any session it may have successfully created towards an upstream entity, to avoid hanging sessions in the network.

NOTE 2: Sending a rejection response over the last hop towards the originating node, i.e. over S11/S4 or S2a/S2b, is not useful as the request has timed out at the originated entity.

In the context of this specification, the receiving entity refers to an SGW or PGW.

3GPP TS 29.212 [29] and 3GPP TS 29.273 [68] further specify:

– the PGW requirements regarding the forwarding of the Maximum Wait Time parameter over the Gx and/or S6b interfaces, when received from the SGW or TWAN/ePDG;

– the handling of the Maximum Wait Time parameter by the PCRF and 3GPP AAA Server.

An originating entity which detects a NTP failure shall not insert the Origination Time Stamp and the Maximum Wait Time parameters towards other entities. A receiving entity which detects a NTP failure shall ignore the the Maximum Wait Time parameter.