3GPP 24.501 v20.0.0 — the document's own text

9.1.1 NAS message format

Taught in 2. Stage 3 — the difference between a plan and a protocol (CT1 — the group that writes what your phone says, in depth), 5. Finding your way around TS 24.501 (CT1 — the group that writes what your phone says, in depth), 7. What a NAS message looks like on the wire (CT1 — the group that writes what your phone says, in depth), 4. Inside TS 24.501, the document at the centre (CT1 — the group that writes what your phone says, overview), 2. What the group produces, and what one of its documents looks like (CT1 — the group that writes what your phone says, quick start).

Within the protocols defined in the present document, every 5GS NAS message is a standard L3 message as defined in 3GPP TS 24.007 [11]. This means that the message consists of the following parts:

if the message is a plain 5GS NAS message:

extended protocol discriminator;
security header type associated with a half spare octet or PDU session identity;
procedure transaction identity;
message type;
other information elements, as required.

if the message is a security protected 5GS NAS message:

extended protocol discriminator;
security header type associated with a half spare octet;
message authentication code;
sequence number;
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.

8	7	6	5	4	3	2	1
Extended protocol discriminator	octet 1
Security header type associated with a spare half octet; orPDU session identity	octet 2
Procedure transaction identity	octet 2a*
Message type	octet 3
	octet 4
Other information elements as required
	octet n

Figure 9.1.1.1: General message organization example for a plain 5GS NAS message

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.

8	7	6	5	4	3	2	1
Extended protocol discriminator	octet 1
Security header type associated with a spare half octet	octet 2
	octet 3
Message authentication code

	octet 6
Sequence number	octet 7
	octet 8
Plain 5GS NAS message
	octet n

Figure 9.1.1.2: General message organization example for a security protected 5GS NAS message

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.