Section: 6.1.1 Presence requirements of Information Elements
Content: There are four different presence requirements (Mandatory, Conditional, Optional, or Conditional-Optional) for an IE within a given GTP-PDU: - Mandatory means that the IE shall be included by the sending side, and that the receiver diagnoses a "Mandatory IE missing" error, when detecting that the IE is not present. A response including a "Mandatory IE missing" cause, shall include the type of the missing IE. - Conditional means: - that the IE shall be included by sending entity if the conditions specified in the relevant protocol specification are met; - the receiver shall check the conditions as specified in the corresponding message type description, based on the parameter combination in the message and/or on the state of the receiving node, to infer if a conditional IE shall be expected. Only if a receiver has sufficient information the following applies. A conditional IE, which is absolutely necessary for the receiving entity to complete the procedure, is missing, then the receiver shall abort the procedure. - Conditional-Optional means: - that the IE shall be included by the up-to-date sending entity, if the conditions specified in the relevant protocol specification are met. An entity, which is at an earlier version of the protocol and therefore is not up-to-date, obviously cannot send such new IE. - the receiver need not check the presence of the IE in the message. If the receiver checks the presence of the Conditional-Optional IE, then the IE's absence shall not trigger any of the error handling procedures. The handling of an absence or erroneous such IEs shall be treated as Optional IEs as specified in clause 7.7 "Error Handling". - Optional means: - that the IE shall be included as a service option. Therefore, the IE may be included or not in a message. The handling of an absent optional IE, or an erroneous optional IE is specified in clause 7.7 "Error Handling". For conditional IEs, the clause describing the GTP-PDU explicitly defines the conditions under which the inclusion of each IE becomes mandatory or optional for that particular GTP-PDU. These conditions shall be defined so that the presence of a conditional IE only becomes mandatory if it is critical for the receiving entity. The definition might reference other protocol specifications for final terms used as part of the condition. For grouped IEs, the presence requirement of the embedded IE shall follow the rules: - The grouped IE is Mandatory within a given message: the presence requirements of individual embedded IEs are as stated within the Mandatory grouped IE for the given message. - The grouped IE is Conditional within a given message: if the embedded IE in the grouped IE is Mandatory or Conditional, this embedded IE is viewed as Conditional IE by the receiver. If the embedded IE in the grouped IE is Conditional-Optional, this embedded IE is viewed as Optional IE by the receiver. If the embedded IE in the grouped IE is Optional, this embedded IE is viewed as Optional IE by the receiver. - The grouped IE is Conditional-Optional within a given message: if the embedded IE in the grouped IE is Mandatory or Conditional, this embedded IE is viewed as Conditional-Optional IE by the receiver. If the embedded IE in the grouped IE is Conditional-Optional, this embedded IE is viewed as Optional IE by the receiver. If the embedded IE in the grouped IE is Optional, this embedded IE is viewed as Optional IE by the receiver. - The grouped IE is Optional within a given message: all embedded IEs in the grouped IE are viewed as Optional IEs by the receiver. In all of the above cases, appropriate error handling as described in clause 7.7 shall be applied for protocol errors of the embedded IEs. Only the Cause information element at message level shall be included in the response if the Cause contains a value that indicates that the request is not accepted regardless of whether there are other mandatory or conditional information elements defined for a given response message. The following are exceptions: - Optionally, the Protocol Configuration Options, Recovery, User Location Information (ULI), Load Control Information, Overload Control Information, Bearer Context and Local Distinguished Name (LDN) information elements may be included. - For the rejection response of a Forward Relocation Request, the Forward Relocation Response message may also include an F-Cause IE as specified in clause 7.3.2. - For the rejection response of a SRVCC PS to CS Request, the SRVCC PS to CS Response message may also include an SRVCC Cause IE as specified in clause 5.2.3 in 3GPP TS 29.280[ Evolved Packet System (EPS); 3GPP Sv interface (MME to MSC, and SGSN to MSC) for SRVCC ]  [15]. - A Downlink Data Notification Acknowledge (with or) without an indication of success may also include a DL low priority traffic Throttling IE and the IMSI IE. - The PGW Back-Off Time IE may also be returned when rejecting a Create Session Request with the cause "APN Congestion". - Change Notification Response message may also include the IMSI and MEI information elements. - Failure Indication type messages do not have "Accept" types of Cause values, i.e. all used values indicate the rejection, therefore for Failure Indication type of triggered messages, other information elements, other than the Cause IE, shall also be included according to the conditions of presence specified in the respective message, if they are available. - The Context Response message (sent by an SGSN or MME) should also include the IMSI IE if the Cause IE contains the value "P-TMSI Signature mismatch", except if the UE is emergency or RLOS attached and the UE is UICCless. - The Create Bearer Response message, the Update Bearer Response message and the Delete Bearer Response message shall include the RAN/NAS Cause IE according to the conditions specified in clauses 7.2.4, 7.2.16 and 7.2.10.2. - The UE Registration Query Response message shall include IMSI to allow the SGSN to correlate the response message with the corresponding request. If the Cause information element at Grouped IE level contains a value that indicates that the Grouped IE is not handled correctly, e.g. "Context Not Found" at Bearer Context IE level, the other information elements in this Grouped IE, other than the Cause IE, may not be included.