5 X2 interface user plane protocol
36.4243GPPEvolved Universal Terrestrial Radio Access Network (E-UTRAN)Release 17TSX2 data transport
5.1 General
The transport layer for data streams over X2 is an IP based Transport. The following figure shows the transport protocol stacks over X2.
Figure 5.1: Transport network layer for data streams over X2
The GTP-U (TS 29.281 [2]) protocol over UDP over IP shall be supported as the transport for data streams on the X2 interface. The data link layer is as specified in clause 4.
There may be zero or one UL data stream and zero or one DL data stream per E-RAB at the X2 interface.
– The DL data stream is used for DL data forwarding from the source eNB to the target eNB.
– The UL data stream is used for UL data forwarding from the source eNB to the target eNB.
Each data stream is carried on a dedicated transport bearer.
The identity of a transport bearer signalled in the RNL control plane consists of the IP address and the TEID of the corresponding GTP tunnel, allocated by the target eNB (see TS 29.281 [2]).
5.2 GTP-U
The GTP-U (TS 29.281 [2]) protocol shall be used over the X2 interface between two eNBs.
5.3 UDP/IP
The path protocol used shall be UDP (IETF RFC 768 [3]).
The UDP port number for GTP-U shall be as defined in TS 29.281 [2].
The eNBs over the X2 interface shall support fragmentation and assembly of GTP packets at the IP layer.
The eNB shall support IPv6 (IETF RFC 2460 [5]) and/or IPv4 (IETF RFC 791 [6]).
There may be one or several IP addresses in the both eNBs. The packet processing function in the source eNB shall send downstream packets of a given E-RAB to the target eNB IP address (received in X2AP) associated to the DL transport bearer of that particular E-RAB. The packet processing function in the source eNB shall send upstream packets of a given E-RAB to the target eNB IP address (received in X2AP) associated to the UL transport bearer of that particular E-RAB.
The Transport Layer Address signalled in X2AP messages is a bit string of
a) 32 bits in case of IPv4 address according to IETF RFC 791 [6]; or
b) 128 bits in case of IPv6 address according to IETF RFC 2460 [5] ; or
c) 160 bits if both IPv4 and IPv6 addresses are signalled, in which case the IPv4 address is contained in the first 32 bits.
5.4 Diffserv code point marking
IP Differentiated Services code point marking (IETF RFC 2474 [4]) shall be supported. The mapping between traffic categories and Diffserv code points shall be configurable by O&M based on QoS Class Identifier (QCI)/ Label Characteristics and other E-UTRAN traffic parameters (e.g. ARP). Traffic categories are implementation-specific and may be determined from the application parameters.
5.5 Dual Connectivity
For the SCG bearer option, user data forwarding may be performed. The behaviour of the eNB from which user data is forwarded is the same as specified for the “source eNB”, the behaviour of the eNB to which user data is forwarded is the same as specified for the “target eNB”.
For the split bearer option:
– the GTP-U (TS 29.281 [2]) protocol over UDP over IP shall be supported as the transport for the data stream of PDCP PDUs on the X2 interface. The GTP-U PDU may include a RAN Container with flow control information as specified in TS 36.425 [9] which is carried in the GTP-U extension header. The transport bearer is identified by the GTP-U TEID (TS 29.281 [2]) and the IP address of the MeNB and SeNB respectively. There may be zero or one UL data stream and there is one DL data stream per E-RAB at the X2 interface;
– The DL data stream is used for DL data transmission from the MeNB to the SeNB;
– The UL data stream is used for UL data transmission from the SeNB to the MeNB;
– the packet processing function in the MeNB shall send downstream packets of a given E-RAB to the SeNB IP address (received in X2AP) associated to the DL transport bearer of that particular E-RAB. The packet processing function in the SeNB shall send upstream packets of a given E-RAB to the MeNB IP address (received in X2AP) associated to the UL transport bearer of that particular E-RAB;
– data forwarding may be performed by MeNB providing GTP-U TEID to receive the DL data forwarded by the SeNB.
5.6 E-UTRA-NR Dual Connectivity
User data forwarding may be performed for each E-RAB configured for EN-DC, towards or from the node hosting the PDCP entity. The behaviour of the E-UTRAN node from which user data is forwarded is the same as specified for the "source eNB", the behaviour of the E-UTRAN node to which user data is forwarded is the same as specified for the "target eNB".
If X2-U data bearer resources are allocated for EN-DC:
– the GTP-U (TS 29.281 [2]) protocol over UDP over IP shall be supported as the transport for the data stream of PDCP PDUs on the X2 interface. The GTP-U PDU may include an NR RAN Container with flow control information as specified in TS 38.425 [11] which is carried in the GTP-U extension header. The transport bearer is identified by the GTP-U TEID (TS 29.281 [2]) and the IP address of the E-UTRAN nodes involved in EN-DC;
– the packet processing function in the E-UTRAN node hosting the PDCP entity shall send downstream packets of a given E-RAB to the IP address indicated by the corresponding E-UTRAN node in X2AP associated to the DL transport bearer of that particular E-RAB. The packet processing function in the corresponding E-UTRAN node shall send upstream packets of a given E-RAB to the IP address indicated by the E-UTRAN node hosting the PDCP entity in X2AP associated to the UL transport bearer of that particular E-RAB;
Annex A (informative):
Change history
|
TSG # |
TSG Doc. |
CR |
Rev |
Subject/Comment |
New |
|
38 |
approved at TSG-RAN and placed under change control |
8.0.0 |
|||
|
39 |
RP-080078 |
0001 |
– |
Editorial correction on 36.424 |
8.1.0 |
|
39 |
RP-080078 |
0002 |
– |
Data link layer proposal |
8.1.0 |
|
40 |
RP-080302 |
0003 |
1 |
eGTP draft reference for X2 Data Transport |
8.2.0 |
|
40 |
RP-080302 |
0005 |
– |
Define format for TLA signalled in X2AP messages |
8.2.0 |
|
41 |
RP-080583 |
0006 |
1 |
X2 transport bearers |
8.3.0 |
|
42 |
RP-080845 |
0007 |
Correction of SAE Bearer |
8.4.0 |
|
|
43 |
RP-090083 |
0008 |
Correction on GTP-U version |
8.5.0 |
|
|
09/2009 |
– |
– |
– |
Creation of Rel-9 version based on v8.5.0 |
9.0.0 |
|
12/2010 |
Creation of Rel-10 version based on v. 9.0.0 |
10.0.0 |
|||
|
SP-49 |
SP-100629 |
Clarification on the use of References (TS 21.801 CR#0030) |
10.0.1 |
||
|
52 |
RP-110684 |
0009 |
Correction of references |
10.1.0 |
|
|
09/2012 |
Update to Rel-11 version (MCC) |
11.0.0 |
|||
|
63 |
RP-140297 |
0011 |
1 |
The content of Transport Layer Address |
12.0.0 |
|
66 |
RP-142089 |
0013 |
6 |
Data Forwarding and Data transmission |
12.1.0 |
|
67 |
RP-150351 |
0021 |
1 |
Correction on Data Transmission over X2 |
12.2.0 |
|
12/2015 |
Update to Rel-13 version (MCC) |
13.0.0 |
|||
|
71 |
RP-160449 |
0022 |
1 |
Rapporteur editorial corrections |
13.1.0 |
|
Change history |
|||||||
|
Date |
Meeting |
TDoc |
CR |
Rev |
Cat |
Subject/Comment |
New version |
|
2017-03 |
SA#75 |
Promotion to Release 14 without technical change |
14.0.0 |
||||
|
2017-06 |
RP#76 |
RP-171324 |
0024 |
2 |
F |
Derivation of Diffserv code point marking includes ARP |
14.1.0 |
|
2017-06 |
RP#76 |
RP-171324 |
0025 |
F |
Clarification of the use of the RAN Container |
14.1.0 |
|
|
2017-06 |
RP#76 |
RP-171324 |
0026 |
F |
Rapporteur editorial review |
14.1.0 |
|
|
2017-12 |
RP-78 |
RP-172672 |
0027 |
1 |
B |
Introduction of EN-DC |
15.0.0 |
|
2019-12 |
RP-86 |
RP-192915 |
0029 |
1 |
F |
Independent migration to IPv6 on S1-U for en-gNB’s |
15.1.0 |
|
2020-07 |
SA#88-e |
– |
– |
– |
– |
Update to Rel-16 version (MCC) |
16.0.0 |
|
2022-03 |
SA#95-e |
Promotion to Release 17 without technical change |
17.0.0 |
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