6 Handling of unknown, unforeseen, and erroneous protocol data

24.2443GPPRelease 17Stage 3TSWireless LAN control plane protocol for trusted WLAN access to EPC

6.1 General

The procedures specified in the present document apply to those messages which pass the checks described in this subclause.

This clause also specifies procedures for the handling of unknown, unforeseen, and erroneous protocol data by the receiving entity. These procedures are called "error handling procedures", but in addition to providing recovery mechanisms for error situations they define a compatibility mechanism for future extensions of the protocols.

Subclauses 6.1 to 6.8 shall be applied in order of precedence, starting with subclause 6.1.

Most error handling procedures are mandatory for the UE.

Detailed error handling procedures in the TWAG are implementation dependent and may vary from PLMN to PLMN. However, when extensions of this protocol are developed, TWAG will be assumed to have the error handling that is indicated in this subclause as mandatory ("shall") and that is indicated as strongly recommended ("should").

Also, the error handling of the TWAG is only considered as mandatory or strongly recommended when certain thresholds for errors are not reached during a dedicated connection.

6.2 Message too short

When the UE receives a WLCP message which is too short to contain a complete message type information element, the UE shall discard the message.

The TWAG shall take the same approach.

6.3 Unknown or unforeseen procedure transaction identity or PDN connection ID

6.3.1 Procedure transaction identity

The following TWAG procedures shall apply for handling an unknown, erroneous, or unforeseen PTI received in a WLCP message:

a) If the TWAG receives a PDN CONNECTIVITY REQUEST message with a reserved PTI value, the TWAG shall respond with a PDN CONNECTIVITY REJECT message including ESM cause #81 "invalid PTI value";

b) If the TWAG receives a PDN DISCONNECT REQUEST message with a reserved PTI value, the TWAG shall respond with a PDN DISCONNECT REJECT message including ESM cause #81 "invalid PTI value"; and

c) If the TWAG receives a WLCP message other than those listed in items a through b above with a reserved PTI value, the TWAG shall ignore the message.

The following UE procedures shall apply for handling an unknown, erroneous, or unforeseen PTI received in a WLCP message:

a) If the UE receives a PDN CONNECTIVITY REJECT message in which the PTI value is an unassigned or reserved value, or an assigned value that does not match any PTI in use, the UE shall ignore the message;

b) If the UE receives a PDN DISCONNECT REJECT message in which the PTI value is an unassigned or reserved value, or an assigned value that does not match any PTI in use, the UE shall ignore the message; and

c) If the UE receives a WLCP message other than those listed in items a through b with a reserved PTI value or an assigned value that does not match any PTI in use, the UE shall ignore the message.

6.3.2 PDN connection ID

The following TWAG procedures shall apply for handling an unknown, erroneous, or unforeseen PDN connection ID received in the header of a WLCP message:

a) If the TWAG receives a PDN CONNECTIVITY REQUEST message which includes an assigned or reserved PDN connection ID value, the TWAG shall respond with a PDN CONNECTIVITY REJECT message including ESM cause #43 "invalid EPS bearer identity";

b) If the TWAG receives a PDN DISCONNECT REQUEST message which includes an unassigned or reserved PDN connection ID value, the TWAG shall respond with a PDN DISCONNECT REJECT message including ESM cause #43 "invalid EPS bearer identity"; and

c) If the TWAG receives a WLCP message other than those listed in items a through b above in which the message includes a reserved PDN connection ID value or an assigned value that does not match an existing PDN connection ID, the TWAG shall ignore the message.

The following UE procedures shall apply for handling an unknown, erroneous, or unforeseen PDN connection ID received in the header of a WLCP message:

a) If the UE receives a PDN CONNECTIVITY REJECT message which includes an assigned or reserved PDN connection ID value, the UE shall ignore the message;

b) If the UE receives a PDN DISCONNECT REJECT message which includes an unassigned or reserved PDN connection ID value or an assigned PDN connection ID value which does not match existing PDN connection, the UE shall ignore the message;

c) If the UE receives a PDN DISCONNECT REQUEST message which includes an unassigned or reserved PDN connection ID value or an assigned PDN connection ID value which does not match existing PDN connection, the UE shall ignore the message; and

d) If the UE receives a WLCP message other than those listed in items a through c in which the message includes an unassigned or reserved PDN connection ID value or a value that does not match an existing PDN connection ID, the UE shall ignore the message.

6.4 Unknown or unforeseen message type

If UE receives a WLCP message with message type not defined or not implemented, the UE shall return a status message with cause #97 "message type non-existent or not implemented".

If the TWAG receives a WLCP message with message type not defined or not implemented, the TWAG shall ignore the message except that the TWAG should return a status message with cause #97 "message type non-existent or not implemented".

6.5 Non-semantical mandatory information element errors

6.5.1 Common procedures

When on receipt of a message,

– an "imperative message part" error; or

– a "missing mandatory IE" error

is diagnosed or when a message containing:

– a syntactically incorrect mandatory IE;

– an IE unknown in the message, but encoded as "comprehension required" (see 3GPP TS 24.007 [7]); or

– an out of sequence IE encoded as "comprehension required" (see 3GPP TS 24.007 [7]) is received,

the UE shall proceed as follows:

The UE shall return a status message with cause #96 "invalid mandatory information"; and

the TWAG shall proceed as follows:

The TWAG shall either:

– try to treat the message (the exact further actions are implementation dependent); or

– ignore the message except that the TWAG should return a status message with cause #96 "invalid mandatory information".

6.5.2 PDN connection management

The following UE procedures shall apply for handling an error encountered with a mandatory information element in a WLCP message:

a) If the message is a PDN CONNECTIVITY REQUEST, a PDN CONNECTIVITY REJECT message with ESM cause #96 "invalid mandatory information", shall be returned.

b) If the message is a PDN DISCONNECT REQUEST, a PDN DISCONNECT ACCEPT message shall be returned. All resources associated with that PDN connection shall be released.

The following TWAG procedures shall apply for handling an error encountered with a mandatory information element in a WLCP message:

a) If the message is a PDN CONNECTIVITY REQUEST, a PDN CONNECTIVITY REJECT message with ESM cause #96 "invalid mandatory information", shall be returned.

b) If the message is a PDN DISCONNECT REQUEST, a PDN DISCONNECT REJECT message with ESM cause #96 "invalid mandatory information", shall be returned.

6.6 Unknown and unforeseen IEs in the non-imperative message part

6.6.1 IEIs unknown in the message

The UE shall ignore all IEs unknown in a message which are not encoded as "comprehension required" (see 3GPP TS 24.301 [5]).

The TWAG shall take the same approach.

6.6.2 Out of sequence IEs

The UE shall ignore all out of sequence IEs in a message which are not encoded as "comprehension required" (see 3GPP TS 24.301 [5]).

The TWAG shall take the same approach.

6.6.3 Repeated IEs

If an information element with format V, TV, or TLV is repeated in a message in which repetition of the information element is not specified in clause 7 of the present document, the UE shall handle only the contents of the information element appearing first and shall ignore all subsequent repetitions of the information element. When repetition of information elements is specified, the UE shall handle only the contents of specified repeated information elements. If the limit on repetition of information elements is exceeded, the UE shall handle the contents of information elements appearing first up to the limit of repetitions and shall ignore all subsequent repetitions of the information element.

The TWAG shall follow the same procedures.

6.7 Non-imperative message part errors

6.7.1 General

This category includes:

– syntactically incorrect optional IEs; and

– conditional IE errors.

6.7.2 Syntactically incorrect optional IEs

The UE shall treat all optional IEs that are syntactically incorrect in a message as not present in the message.

The TWAG shall take the same approach.

6.7.3 Conditional IE errors

When upon receipt of a WLCP message the UE diagnoses a "missing conditional IE" error or an "unexpected conditional IE" error, or when the UE receives a WLCP message containing at least one syntactically incorrect conditional IE, the UE shall ignore the message and shall return a status message with cause #100 "conditional IE error".

When the TWAG receives a message and diagnoses a "missing conditional IE" error or an "unexpected conditional IE" error or when the TWAG receives a message containing at least one syntactically incorrect conditional IE, the TWAG shall either:

– try to treat the message (the exact further actions are implementation dependent); or

– ignore the message except that the TWAG should return a status message with cause #100 "conditional IE error".

6.8 Messages with semantically incorrect contents

When a message with semantically incorrect contents is received, the UE shall perform the foreseen reactions of the procedural part of the present document (i.e. of clauses 5). If however no such reactions are specified, the UE shall ignore the message except that the UE shall return a status message with cause #95 "semantically incorrect message".

The TWAG should follow the same procedure except that a status message is not normally transmitted.