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Content for  TS 24.501  Word version:  17.5.0

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9  General message format and information elements codingWord‑p. 645

9.1  OverviewWord‑p. 645

9.1.1  NAS message formatWord‑p. 645

Within the protocols defined in the present document, every 5GS NAS message is a standard L3 message as defined in TS 24.007. This means that the message consists of the following parts:
  1. if the message is a plain 5GS NAS message:
    1. extended protocol discriminator;
    2. security header type associated with a half spare octet or PDU session identity;
    3. procedure transaction identity;
    4. message type;
    5. other information elements, as required.
  2. if the message is a security protected 5GS NAS message:
    1. extended protocol discriminator;
    2. security header type associated with a half spare octet;
    3. message authentication code;
    4. sequence number;
    5. plain 5GS NAS message, as defined in item 1
The organization of a plain 5GS NAS message is illustrated in the example shown in Figure 9.1.1.1.
Reproduction of 3GPP TS 24.501, Figure 9.1.1.1: General message organization example for a plain 5GS NAS message
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The PDU session identity and the procedure transaction identity are only used in messages with extended protocol discriminator 5GS session management. Octet 2a with the procedure transaction identity shall only be included in these messages.
The organization of a security protected 5GS NAS message is illustrated in the example shown in Figure 9.1.1.2.
Reproduction of 3GPP TS 24.501, Figure 9.1.1.2: General message organization example for a security protected 5GS NAS message
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Unless specified otherwise in the message descriptions of clause 8 and Annex D, a particular information element shall not be present more than once in a given message.

9.1.2  Field format and mappingWord‑p. 646

When a field is contained within a single octet, the lowest numbered bit of the field represents the least significant bit.
When a field extends over more than one octet, the order of bit values progressively decreases as the octet number increases. In that part of the field contained in a given octet, the lowest numbered bit represents the least significant bit. The most significant bit of the field is represented by the highest numbered bit of the lowest numbered octet of the field. The least significant bit of the field is represented by the lowest numbered bit of the highest numbered octet of the field.
For example, a bit number can be identified as a couple (o, b) where o is the octet number and b is the relative bit number within the octet. Figure 9.1.2.1 illustrates a field that spans from bit (1, 3) to bit (2, 7). The most significant bit of the field is mapped on bit (1, 3) and the least significant bit is mapped on bit (2, 7).
Reproduction of 3GPP TS 24.501, Figure 9.1.2.1: Field mapping convention
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9.2  Extended protocol discriminatorWord‑p. 646

Bits 1 to 8 of the first octet of every 5GS NAS message contain the Extended protocol discriminator (EPD) IE. The EPD and its use are defined in TS 24.007. The extended protocol discriminator in the header (see TS 24.007) of a security protected 5GS NAS message is encoded as "5GS mobility management messages".

9.3  Security header typeWord‑p. 647

Bits 1 to 4 of the second octet of every 5GMM message contain the Security header type IE. This IE includes control information related to the security protection of a 5GMM message. The total size of the Security header type IE is 4 bits.
The Security header type IE can take the values shown in Table 9.3.1.
Security header type (octet 1)
Bits
4 3 2 1
0 0 0 0Plain 5GS NAS message, not security protected
Security protected 5GS NAS message:
0 0 0 1Integrity protected
0 0 1 0Integrity protected and ciphered
0 0 1 1Integrity protected with new 5G NAS security context (NOTE 1)
0 1 0 0Integrity protected and ciphered with new 5G NAS security context (NOTE 2)
All other values are reserved.
NOTE 1:
This codepoint may be used only for a SECURITY MODE COMMAND message.
NOTE 2:
This codepoint may be used only for a SECURITY MODE COMPLETE message.
A 5GMM message received with the security header type encoded as 0000 shall be treated as not security protected, plain 5GS NAS message. A protocol entity sending a not security protected 5GMM message shall send the message as plain 5GS NAS message and encode the security header type as 0000.
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9.4  PDU session identityWord‑p. 647

Bits 1 to 8 of the second octet of every 5GSM message contain the PDU session identity IE. The PDU session identity and its use to identify a message flow are defined in TS 24.007.

9.5  Spare half octetWord‑p. 647

This element is used in the description of 5GMM and 5GSM messages when an odd number of half octet type 1 information elements are used. This element is filled with spare bits set to zero and is placed in bits 5 to 8 of the octet unless otherwise specified.

9.6  Procedure transaction identityWord‑p. 647

Bits 1 to 8 of the third octet of every 5GSM message contain the procedure transaction identity. Bits 1 to 8 of the first octet of every UE policy delivery message contain the procedure transaction identity. The procedure transaction identity and its use are defined in TS 24.007.

9.7  Message typeWord‑p. 647

The Message type IE and its use are defined in TS 24.007. Table 9.7.1 and Table 9.7.2 define the value part of the message type IE used in the 5GS mobility management protocol and 5GS session management protocol.
Bits
8 7 6 5 4 3 2 1
0 1 - - - - - -5GS mobility management messages
0 1 0 0 0 0 0 1Registration request
0 1 0 0 0 0 1 0Registration accept
0 1 0 0 0 0 1 1Registration complete
0 1 0 0 0 1 0 0Registration reject
0 1 0 0 0 1 0 1Deregistration request (UE originating)
0 1 0 0 0 1 1 0Deregistration accept (UE originating)
0 1 0 0 0 1 1 1Deregistration request (UE terminated)
0 1 0 0 1 0 0 0Deregistration accept (UE terminated)
0 1 0 0 1 1 0 0Service request
0 1 0 0 1 1 0 1Service reject
0 1 0 0 1 1 1 0Service accept
0 1 0 0 1 1 1 1Control plane service request
0 1 0 1 0 0 0 0Network slice-specific authentication command
0 1 0 1 0 0 0 1Network slice-specific authentication complete
0 1 0 1 0 0 1 0Network slice-specific authentication result
0 1 0 1 0 1 0 0Configuration update command
0 1 0 1 0 1 0 1Configuration update complete
0 1 0 1 0 1 1 0Authentication request
0 1 0 1 0 1 1 1Authentication response
0 1 0 1 1 0 0 0Authentication reject
0 1 0 1 1 0 0 1Authentication failure
0 1 0 1 1 0 1 0Authentication result
0 1 0 1 1 0 1 1Identity request
0 1 0 1 1 1 0 0Identity response
0 1 0 1 1 1 0 1Security mode command
0 1 0 1 1 1 1 0Security mode complete
0 1 0 1 1 1 1 1Security mode reject
0 1 1 0 0 1 0 05GMM status
0 1 1 0 0 1 0 1Notification
0 1 1 0 0 1 1 0Notification response
0 1 1 0 0 1 1 1UL NAS transport
0 1 1 0 1 0 0 0DL NAS transport
Bits
8 7 6 5 4 3 2 1
1 1 - - - - - -5GS session management messages
1 1 0 0 0 0 0 1PDU session establishment request
1 1 0 0 0 0 1 0PDU session establishment accept
1 1 0 0 0 0 1 1PDU session establishment reject
1 1 0 0 0 1 0 1PDU session authentication command
1 1 0 0 0 1 1 0PDU session authentication complete
1 1 0 0 0 1 1 1PDU session authentication result
1 1 0 0 1 0 0 1PDU session modification request
1 1 0 0 1 0 1 0PDU session modification reject
1 1 0 0 1 0 1 1PDU session modification command
1 1 0 0 1 1 0 0PDU session modification complete
1 1 0 0 1 1 0 1PDU session modification command reject
1 1 0 1 0 0 0 1PDU session release request
1 1 0 1 0 0 1 0PDU session release reject
1 1 0 1 0 0 1 1PDU session release command
1 1 0 1 0 1 0 0PDU session release complete
1 1 0 1 0 1 1 05GSM status
1 1 0 1 1 0 0 0Service-level authentication command
1 1 0 1 1 0 0 1Service-level authentication complete
1 1 0 1 0 1 1 1Remote UE report
1 1 0 1 1 0 0 0Remote UE report response
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9.8  Message authentication codeWord‑p. 649

The message authentication code (MAC) information element contains the integrity protection information for the message. The MAC IE shall be included in the SECURITY PROTECTED 5GS NAS MESSAGE message if a valid 5G NAS security context exists and security functions are started.
The message authentication code (MAC) is also included in the Intra N1 mode NAS transparent container IE and in the S1 mode to N1 mode NAS transparent container IE.
The usage of MAC is specified in subclause 4.4.3.3.
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9.9  Plain 5GS NAS messageWord‑p. 649

This IE includes a complete plain 5GS NAS message as specified in subclauses 8.2 and 8.3. The SECURITY PROTECTED 5GS NAS MESSAGE (see subclause 8.2.28) includes a complete plain 5GS NAS message as specified in subclauses 8.2. The SECURITY PROTECTED 5GS NAS MESSAGE message (see subclause 8.2.28) is not plain 5GS NAS messages and shall not be included in this IE.
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9.10  Sequence numberWord‑p. 649

This IE includes the NAS message sequence number (SN) which consists of the eight least significant bits of the NAS COUNT for a SECURITY PROTECTED 5GS NAS MESSAGE message.
The NAS message sequence number (SN) with the eight least significant bits of the NAS COUNT is also included in the Intra N1 mode NAS transparent container IE and in the N1 mode to S1 mode NAS transparent container IE.
The usage of SN is specified in subclause 4.4.3.
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9.11  Other information elementsWord‑p. 650

9.11.1  GeneralWord‑p. 650

The different formats (V, LV, T, TV, TLV, LV-E, TLV-E) and the five categories of information elements (type 1, 2, 3, 4 and 6) are defined in TS 24.007.
The first octet of an information element in the non-imperative part contains the IEI of the information element. If this octet does not correspond to an IEI known in the message, the receiver shall determine whether this IE is of type 1 or 2 (i.e. it is an information element of one octet length) or an IE of type 4 (i.e. that the next octet is the length indicator indicating the length of the remaining of the information element) (see TS 24.007).
This allows the receiver to jump over unknown information elements and to analyse any following information elements of a particular message.
The definitions of information elements which are:
  1. common for the 5GMM and 5GSM protocols;
  2. used by access stratum protocols; or
  3. sent to upper layers
are described in subclause 9.11.2.
The information elements of the 5GMM or 5GSM protocols can be defined by reference to an appropriate specification which provides the definition of the information element, e.g., "see subclause 10.5.6.3A in TS 24.008 [12]".
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