Editing My 3GPP TS 24.007 Notes (section)

==== 11.2.3.1 Standard L3 message header ====

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For the 5GS protocols 5GMM and 5GSM, a standard L3 message can be <span style="color:red">either</span> <span style="color:red">a plain 5GS NAS message</span> or <span style="color:blue">a security protected 5GS NAS message</span>:
* <span style="color:red">The header of a plain 5GS NAS message</span> is composed of three octets for 5GMM NAS messages and composed of four octets for 5GSM NAS messages, and structured in four main parts, namely, the extended protocol discriminator (1 octet); an octet used as security header type (1/2 octet) plus a spare half octet in case of 5GMM NAS messages, and a PDU session identity of one octet in case of 5GSM NAS messages; an octet for procedure transaction identity (PTI) in case of 5GSM NAS messages; and one octet for message type. If the procedure transaction identity is present, it is preceding the message type octet.
* <span style="color:blue">The header of a security protected 5GS NAS message</span> is composed of seven octets, and structured in four main parts, the extended protocol discriminator (1 octet), an octet used as security header type (1/2 octet) plus a spare half octet, a message authentication code of four octets, and a sequence number of one octet. This header is followed by a complete plain 5GS NAS message (i.e. including the header of this plain 5GS NAS message).

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