7.6 Error Handling

29.2443GPPInterface between the Control Plane and the User Plane nodesRelease 17TS

7.6.1 Protocol Errors

A protocol error is defined as a message or an Information Element received from a peer entity with an unknown type, or if it is unexpected, or if it has an erroneous content.

The term silently discarded is used in the following clauses to mean that the receiving PFCP entity’s implementation shall discard such a message without further processing or that the receiving PFCP entity discards such an IE and continues processing the message. The conditions for the receiving PFCP entity to silently discard an IE are specified in the subsequent clauses.

The handling of unknown, unexpected or erroneous PFCP messages and IEs shall provide for the forward compatibility of PFCP. Therefore, the sending PFCP entity shall be able to safely include in a message a new conditional-optional or an optional IE. Such an IE may also have a new type value. Any legacy receiving PFCP entity shall, however, silently discard such an IE and continue processing the message.

If a protocol error is detected by the receiving PFCP entity, it should log the event including the erroneous message and may include the error in a statistical counter.

For Response messages containing a rejection Cause value, see clause 7.2.3.2.

The receiving PFCP entity shall apply the error handling specified in the subsequent clauses.

If the received erroneous message is a reply to an outstanding PFCP message, the PFCP transaction layer shall stop retransmissions and notify the PFCP application layer of the error even if the reply is silently discarded.

7.6.2 Different PFCP Versions

If a PFCP entity receives a message of an unsupported PFCP version, it shall return a PFCP Version Not Supported Response message and silently discard the received message.

7.6.3 PFCP Message of Invalid Length

If a PFCP entity receives a message, which is too short to contain the respective PFCP header, the PFCP-PDU shall be silently discarded.

If a PFCP entity receives a Request message within an IP/UDP packet of a length that is inconsistent with the value specified in the Length field of the PFCP header, then the receiving PFCP entity should log the error and shall send the Response message with Cause IE value set to "Invalid Length".

If a PFCP entity receives a Response message within an IP/UDP packet of a length that is inconsistent with the value specified in the Length field of the PFCP header, then the receiving PFCP entity should log the error and shall silently discard the message.

7.6.4 Unknown PFCP Message

If a PFCP entity receives a message with an unknown Message Type value, it shall silently discard the message.

7.6.5 Unexpected PFCP Message

If a PFCP entity receives an unexpected request message, for example a known message that is sent over an interface for which the message is not defined, or a message that is sent over an interface for which the message is defined, but the direction is incorrect, then the PFCP entity shall silently discard the message and shall log an error.

If a PFCP entity receives an unexpected response message which is not a request message, for example a message for which there is no corresponding outstanding request, it shall discard the message and may log an error.

7.6.6 Missing Information Elements

A PFCP entity shall check if all mandatory IEs are present in the received Request message. Apart from Heartbeat Request message, if one or more mandatory information elements are missing in the received Request message, the PFCP entity should log the error and shall send a Response message with Cause IE value set to "Mandatory IE missing" with the type of the missing mandatory IE.

If a PFCP entity receives a Response message with Cause IE value set to "Mandatory IE missing", it shall notify its upper layer.

A PFCP 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 PFCP entity shall notify the upper layer and should log the error.

A PFCP 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 PFCP entity should log the error and shall send a Response message with Cause IE value set to "Conditional IE missing" together with the type of the missing conditional IE.

A PFCP 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 PFCP entity shall notify the upper layer and should log the error.

Additional information elements may be included in Response messages containing a rejection Cause value, see clause 7.2.3.2.

Absence of an optional information element shall not trigger any error handling.

7.6.7 Invalid Length Information Element

An information element has an invalid length when the actual length of the IE is different from the value of the Length field in the IE header. Here, the actual length of the IE means the length of the content field of the received IE.

If a PFCP message contains more than one information elements and one or more of them have invalid length, the receiving PFCP entity can detect which of the IEs have invalid length only in the following cases:

– If the Length value in the IE header is greater than the overall length of the message;

– If the invalid length IE is the last one in the message.

Apart from Echo Request message, if a receiving PFCP entity detects information element with invalid length in a Request message, it shall send an appropriate error response with Cause IE value set to "Invalid length" together with the type of the offending IE.

7.6.8 Semantically incorrect Information Element

Apart from Echo Request message, the receiver of a PFCP signalling message Request including a mandatory or a verifiable conditional information element with a semantically invalid Value shall discard the request, should log the error, and shall send a response with Cause IE value set to "Mandatory IE incorrect" together with a type and instance of the offending IE.

The receiver of a PFCP signalling message Response including a mandatory or a verifiable conditional information element with a semantically invalid Value shall notify the upper layer that a message with this sequence number has been received and should log the error.

If a PFCP entity receives an information element with a value which is shown as reserved, it shall treat that information element as invalid and should log the error. If the invalid IE is received in a Request, and it is a mandatory IE or a verifiable conditional IE, the PFCP entity shall send a response with Cause set to "Mandatory IE incorrect" together with a type and instance of the offending IE.

The principle is: the use of reserved values invokes error handling; the use of spare values can be silently discarded and for IEs with spare values used, processing shall be continued ignoring the spare values.

The receiver of a PFCP signalling message including an optional information element with a Value that is not in the range defined for this information element value shall discard this IE, but shall treat the rest of the message as if this IE was absent and continue processing. The receiver shall not check the content of an information element field that is defined as "spare".

All semantically incorrect optional information elements in a PFCP signalling message shall be treated as not present in the message.

7.6.9 Unknown or unexpected Information Element

The receiver of a PFCP message including an unexpected information element with a known Type value that is not defined for this message shall discard the IE and log an error. The receiver shall process the message.

NOTE: An Information Element in an encoded PFCP message or grouped IE is identified by the IE Type.

7.6.10 Repeated Information Elements

An Information Element is repeated if there is more than one IE with the same IE Type in the scope of the PFCP message (or in the scope of the grouped IE). Such an IE is a member in a list.

If an information element is repeated in a PFCP signalling message in which repetition of the information element is not specified, only the contents of the information element appearing first shall be handled and all subsequent repetitions of the information element shall be ignored. When the number of repetitions of information elements is specified, only the contents of specified repeated information elements shall be handled and all subsequent repetitions of the information element shall be ignored.