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.

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.

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.

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).

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 or 6 (i.e. that the next octet is the length indicator or, for a type 6 IE, the next 2 octets are 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]".