9.1 Overview

24.5013GPPNon-Access-Stratum (NAS) protocol for 5G System (5GS)Release 18Stage 3TS

9.1.1 NAS message format

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:

1) if the message is a plain 5GS NAS message:

a) extended protocol discriminator;

b) security header type associated with a half spare octet or PDU session identity;

c) procedure transaction identity;

d) message type;

e) other information elements, as required.

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

a) extended protocol discriminator;

b) security header type associated with a half spare octet;

c) message authentication code;

d) sequence number;

e) 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; or PDU 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.

9.1.2 Field format and mapping

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).

8	7	6	5	4	3	2	1
					24	23	22	1st octet of field
21	20							2nd octet of field

Figure 9.1.2.1: Field mapping convention