1. ROHC for GTP-U draft-lei-tsvwg-gtpu-compression-00
IETF 79th meeting
Nov 2010
Lei Zhu

2. Scenarios to compress GTP-U header
LTE-A (to addressing IMT-Advanced requirements) developed Relay architectures by 3GPP RAN1/2/3 in Release 10.
Relay architecture is to address the needs to optimize coverage and cell-edge user throughput
Scenario
Mobility
Hops
Purposes
Urban Hot Spot
Fixed, Nomadic
Two hops
Coverage and Throughput
Dead spot
Fixed
Two hops or Multi-hops
Coverage
Indoor Hot Spot
Throughput
Rural Area
Emergency or Temporary Coverage
Nomadic
Wireless Backhaul Only
Coverage or Throughput

3. Needs to compress GTP-U header
The layer 3 Relay architecture was selected together with protocol stacks through L1/2 and L3 protocols. Shown below.
For some reasons, protocol stack of the added interface (Un interface) cross IP/UDP/GTP-U at user plane and IP/SCTP at control plane.
The traffic model at Un interface is not 100% sure, but VoIP applications are assumed by service providers when specify it.
The mechanisms of basic ROHC profile are very good to be reused. But the alternative proposals are also possible to create since the signaling to exchange status and information are not owned by IETF.

4. Classification of GTP-U header fields
STATIC-DEF: Version, Protocol type, Extension Header Flag, Sequence Number Flag, N-PDU Number Flag, Message Type and TEID.
PATTERN: Sequence number
Result:
IANA reserves a new compression profile identifier for IP/UDP/GTP-U header chain.
The compression profile for GTP-U headers would be developed with minor changes to existing compression profile for IP/UDP/RTP. GTP-U header compression profile should take care the reuse the different sequence number(s) of GTP-U header, ROHC header and further RTP header and perform same behavior as profiles before, even it seems SN of GTP-U could not exist in real traffic.

5. Changes to ROHC IR packet
| Add-CID octet | if for small CIDs and (CID != 0)
| | IR type octet
: :
/ 0-2 octets of CID / 1-2 octets if for large CIDs
: :
| Profile = 0xXXXX | 1 octet
| CRC | 1 octet
| |
/ Static chain / variable length
| |
| |
+ Dynamic chain variable length
/ Payload / variable length

6. Extension to profile identifiers
The proposed compression profile need a reserved identifier which is to be carried in ROHC IR and CO packet.
Existing compression profiles are historically referenced by 3GPP PDCP layer spec ( and ). The referenced RFCs are shown in the table.
Profile Identifier
Usage:
Reference
0x0000
No compression
RFC 4995
0x0001
RTP/UDP/IP
RFC 3095, RFC 4815
0x0002
UDP/IP
0x0003
ESP/IP
0x0004 IP
RFC 3843, RFC 4815
0x0006
TCP/IP
RFC 4996
0x0101
RFC 5225
0x0102
0x0103
0x0104
Ox010n
IP/IDP/GTP-U
n/a

7. Conclusions
The compression profile for GTP-U header is highly possible to IETF turn to develop a new profile in somewhere of tsv area.
A new work item is expected to be approved ASAP.
A working group draft addressing the needs to a compression profile for GTP-U are done before the end of next year and acts as fundamental reference.