6A S121 Message Header
29.2763GPP3GPP Evolved Packet System (EPS)Optimized handover procedures and protocols between E-UTRAN access and cdma2000 HRPD AccessRelease 17Stage 3TS
6A.1 Introduction
The S121 Message Header is conformant to the GTPv2-C Message Header, see 3GPP TS 29.274 [6] section 5. All S121 messages shall have a header that includes specific parameters. The following header parameters are defined for the S121 interface:
– Version
– Flags (T = TEID Included)
– Message Type
– Length
6A.2 S121 Message Header
The S121 header is a variable length header. The minimum length of the S121 header is eight octets. Space has been reserved for four flags that may be used in the future to signal the presence of additional optional header fields or utilities.
– Bit 4 (the T bit) indicates whether a TEID is present in the header, as per 3GPP TS 29.274 [6]. The T bit shall be set to zero to indicate that the TEID field shall not be present in any message sent on the S121 interface.
If the header fields do not occupy a full eight octets, then spare octets shall be added after the last valid field in the S121 header to complete eight octets. Spare octets and bits shall be set to zero.
Always present fields:
– Version field: This field is used to determine the version of the S121 protocol. The version number shall be set to ‘010’.
– Message Type: This field indicates the type of S121 message. The valid values of the message type are defined in clause 7A.1. Note that values chosen for Message Type shall be coordinated with and shall not overlap the Message Type values chosen for GTPv2-C in 3GPP TS 29.274 [6].
– Length: This field indicates the length in octets of the payload, i.e. the rest of the packet following the mandatory part of the S121 header (that is the first 4 octets).
– Sequence Number: This field enables the target system to identify any missing receipt of messages and is used also for acknowledgement of messages.
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Bits |
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Octets |
8 |
7 |
6 |
5 |
4 |
3 |
2 |
1 |
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|
1 |
Version=010 |
(*) |
T=0 |
(*) |
(*) |
(*) |
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|
2 |
Message Type |
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|
3 |
Length (1st Octet) |
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|
4 |
Length (2nd Octet) |
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|
5 |
Sequence Number (1st Octet) |
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6 |
Sequence Number (2nd Octet) |
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7 |
Sequence Number (3rd Octet) |
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|
8 |
Spare |
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NOTE 0: (*) This bit is a spare bit. It shall be sent as ‘0’. The receiver shall not evaluate this bit.
Figure 6A.2-1: Layout of the S121 Message Header