Section: 7.7.6 Missing Information Elements
Content: A GTP entity shall check if all mandatory IEs are present in the received Request message. Apart from Echo Request message, if one or more mandatory information elements are missing in the received Request message, the GTP entity should log the error and shall send a Response message with Cause IE value set to "Mandatory IE missing" together with the type and instance of the missing mandatory IE. If a GTP entity receives a Response message with Cause IE value set to "Mandatory IE missing", it shall notify its upper layer. A GTP entity shall check if all mandatory IEs are present in the received Response message without a rejection Cause value. If one or more mandatory information elements are missing, the GTP entity shall notify the upper layer and should log the error. If a mandatory IE is missing in a Response message, which the SGW shall forward over another interface (e.g. when SGW forwards a message from PGW to MME), the SGW shall include the rejection Cause "Invalid Reply from remote peer" (see clause 8.4) in the forwarded Response message. A GTP entity shall check if conditional information elements are present in the received Request message, if possible (i.e. if the receiving entity has sufficient information available to check if the respective conditions were met). If one or more conditional information elements are missing, a GTP entity should log the error and shall send a Response message with Cause IE value set to "Conditional IE missing" together with the type and instance of the missing conditional IE. A GTP entity shall check if conditional information elements are present in the received Response message without a rejection Cause value, if possible (i.e. if the receiving entity has sufficient information available to check if the respective conditions were met). If one or more conditional information elements are missing, a GTP entity shall notify the upper layer and should log the error. For Response messages containing a rejection Cause value, see clause 6.1.1. If the Indication IE is applicable for the message as a conditional IE and if it is not present, the GTP entity shall not reject the message unless there are other reasons to reject the message. If the Indication IE is applicable for the message as conditional IE and if it is present with the value of all the applicable flags set to "0", the GTP entity shall not reject the message unless there are other reasons to reject the message. Absence of an optional information element shall not trigger any of the error handling processes.