8.2 Information Element Format
29.2743GPP3GPP Evolved Packet System (EPS)Evolved General Packet Radio Service (GPRS) Tunnelling Protocol for Control plane (GTPv2-C)Release 18Stage 3TS
8.2.1 General
Figure 8.2-1 depicts the format of an information element.
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Bits |
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Octets |
8 |
7 |
6 |
5 |
4 |
3 |
2 |
1 |
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1 |
Type = xxx (decimal) |
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2 to3 |
Length = n |
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4 |
Spare |
Instance |
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5 to (n+4) |
IE specific data or content of a grouped IE |
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Figure 8.2-1: Information Element Format
An IE has the following mandatory fields:
– Type field: This field indicates the type of Information Element. The valid values of the IE type are defined in clause 8.1.
– Length: This field contains the length of the information element excluding the first four octets, which are common for all information elements (Type, Length and the contents of octet 4) and is denoted "n" in Figure 8.2-1. For all the length fields, bit 8 of the lowest numbered octet is the most significant bit and bit 1 of the highest numbered octet is the least significant bit.
– Instance: This field shall be used to differentiate amongst different parameters in one specific message which use the same information element type (see also clause 6.1.3 "Information Element Instance").
An IE is said to be TLIV (Type, Length, Instance, Value) encoded.
8.2.1A Information Element with an IE Type Extension field
Figure 8.2.1A-1 depicts the format of an information element with an IE Type Extension field.
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Bits |
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Octets |
8 |
7 |
6 |
5 |
4 |
3 |
2 |
1 |
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1 |
Type = 254 (decimal) |
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2 to 3 |
Length = n |
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4 |
Spare |
Instance |
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5 to 6 7 to n+4 |
IE Type Extension |
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IE specific data or content of a group IE |
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Figure 8.2.1A-1: Information Element with an IE Type Extension field
The IE Type in octet 1 of an information element with an IE Type Extension field shall be set to 254. Such IE shall be further identified by the value encoded in the IE Type Extension field in octets 5 and 6.
The value of the IE Type Extension shall be encoded in full hexadecimal representation (binary, not ASCII encoding) from 256 up to 65535. This field indicates the type of the Information Element and the valid values of the IE type Extension field are defined in clause 8.1.
The semantics of the Length and Instance field remain the same as specified in clause 8.2.1.
8.2.2 Handling ASN.1/PER encoded parameters
During the TAU/RAU/HO procedures MME/S4-SGSN GTPv2 entities send some of the RANAP/S1AP/BSSGP parameters to a GTPv2 peer. Copying of the BSSGP parameters into GTPv2 IEs is straightforward. RANAP and S1AP, however, use ASN.1/PER encoding, which is different from GTPv2 specific TLV encoding.
Transparent copying of RANAP/S1AP parameters across GTPv2 interfaces:
- a GTPv2 entity shall transparently copy the respective information into one or more octets of the GTPv2 IE as specified in Annex B and clause 8.48. With this approach, GTPv2 will not be impacted if the contents of such RANAP/S1AP parameter changes over the time.
Non-transparent copying of RANAP/S1AP parameters across GTPv2 interfaces:
– GTPv2 entity decodes ASN.1/PER parameter and shall encode the value(s) into one or more octets of the GTPv2 IE according to what is specified in the present document.