Editing My 3GPP TS 24.007 Notes (section)

=== 11.2.1	Components of a standard L3 message ===

A standard L3 message consists of an imperative part, itself composed of a header and the rest of imperative part, followed by a non-imperative part. Both the non-header part of the imperative part and the non-imperative part are composed of successive parts referred as standard information elements.

==== 11.2.1.1 Format of standard information elements ====

A standard IE may have the following parts, in that order:
* <span style="color:blue">an information element identifier (IEI)</span>;
* <span style="color:blue">a length indicator (LI)</span>;
* <span style="color:blue">a value (V) part</span>.

A standard IE has one of the formats shown in table 11.1:

{| class="wikitable"
|+ Table 11.1: Formats of information elements
|-
! Format !! Meaning !! IEI present !! LI present !! Value part present
|-
| T || Type only || yes || no || no
|-
| V || Value only || no || no || yes
|-
| TV || Type and Value || yes || no || yes
|-
| LV || Length and Value || no || yes || yes
|-
| TLV || Type, Length, and Value || yes || yes || yes
|-
| LV-E || Length and Value || no || yes || yes
|-
| TLV-E || Type, Length and Value || yes || yes || yes
|}

Some IEs may appear in the structure, but not in all instances of messages. An IE is then said to be present or not present in the message instance. If an IE is not present in a message instance, none of the three parts is present. Otherwise, parts must be present according to the IE format.

In the message structure, an IE that is allowed not to be present in all message instances is said not to be mandatory. Other IEs are said to be mandatory.

LV-E and TLV-E are used for 5GS Mobility Management (5GMM), 5GS Session Management (5GSM), EPS Mobility Management (EMM), EPS Session Management (ESM), GPRS Mobility Management (GMM) and GPRS Session Management (SM) only. In GPRS GMM and GPRS SM messages, IEs of format LV-E and TLV-E may be used only after MS and network have successfully negotiated support of such IEs.

===== 11.2.1.1.1 Information element type and value part =====
Every standard IE has an information element type which determines the values possible for the value part of the IE, and the basic meaning of the information. The information element type describes only the value part. Standard IEs of the same information element type may appear with different formats. The format used for a given standard IE in a given message is specified within the description of the message.

The value part of a standard IE either consists of a half octet or one or more octets; the value part of a standard IE with format LV or TLV consists of an integral number of octets, between 0 and 255 inclusive; it then may be empty, i.e., consist of zero octets; if it consists of a half octet and has format TV, its IEI consists of a half octet, too. For LV-E and TLV-E, the value part of a standard IE consists of an integral number of octets, between 0 and 65535 inclusive. The value part of a standard IE may be further structured into parts, called fields.

===== 11.2.1.1.2 Length indicator =====
For LV or TLV, the length indicator (LI) of a standard IE consists of one octet. For LV-E and TLV-E, the LI of a standard IE consists of two octets where bit 8 of octet n contains the most significant bit and bit 1 of octet n+1 contains the least significant bit (refer to figure 11.9 in clause 11.2.1.1.4 for the relative ordering of the 2 octets). The LI contains the binary encoding of the number of octets of the IE value part. The LI of a standard IE with empty value part indicates 0 octets. Standard IE of an information element type such that the possible values may have different values must be formatted with a length field, i.e., LV, TLV, LV-E or TLV-E.

===== 11.2.1.1.3 Information element identifier =====
When present, the IEI of a standard IE consists of a half octet or one octet. A standard IE with IEI consisting of a half octet has format TV, and its value part consists of a half octet. The value of the IEI depends on the standard IE, not on its information element type. The IEI, if any, of a given standard IE in a given message is specified within the description of the message. In some protocol specifications, default IEI values can be indicated. They are to be used if not indicated in the message specification. Non mandatory standard IE in a given message, i.e., IE which may be not be present (formally, for which the null string is acceptable in the message), must be formatted with an IEI, i.e., with format T, TV, TLV or TLV-E.

===== 11.2.1.1.4 Categories of IEs; order of occurrence of IEI, LI, and value part =====
Totally five categories of standard information elements are defined:
;type 1:information elements of format V or TV with value part consisting of 1/2 octet
;type 2:information elements of format T with value part consisting of 0 octets
;type 3:information elements of format V or TV with value part that has fixed length of at least one octet
;type 4:information elements of format LV or TLV with value part consisting of zero, one or more octets and a maximum of 255 octets
;type 6:information elements of format LV-E or TLV-E with value part consisting of zero, one or more octets and a maximum of 65535 octets. This category is used in 5GS, EPS and GPRS only.

Type 1 standard information elements of format V provide the value in bit positions 8, 7, 6, 5 of an octet (see figure 11.1) or bits 4, 3, 2, 1 of an octet (see figure 11.2).

[[File:Type1 IE of format V.png|center|Figure 11.1 and 11.2 of Type 1 IE of format V]]

See spec for details...

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

...

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).

...

<center>[[Telecommunications info|To Telecommunications info]]</center>