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RFC 9433 - Segment Routing over IPv6 for the Mobile User Plane

Field Value
Title Segment Routing over IPv6 for the Mobile User Plane
Authors S. Matsushima, C. Filsfils, M. Kohno, P. Camarillo, D. Voyer, C. Perkins
Date July 2023
Status Proposed Standard
Link datatracker.ietf.org/doc/rfc9433

Summary

RFC 9433 defines how SRv6 can be used as the mobile user plane protocol in 3GPP networks, replacing GTP-U tunneling between mobile network functions. By encoding mobile session state directly into SRv6 SIDs, the architecture eliminates the need for GTP-U encapsulation between the UPF (User Plane Function) nodes, reducing per-packet overhead and simplifying the data plane.

The document specifies several new SRv6 endpoint behaviors purpose-built for mobile user plane interworking. These behaviors handle the translation between GTP-U and SRv6 at network boundaries and the pure SRv6 forwarding within the mobile transport. The SID structure carries mobile-specific arguments such as QFI (QoS Flow Identifier) and TEID (Tunnel Endpoint Identifier) information, enabling stateless translation at domain boundaries.

This approach allows operators to converge their mobile backhaul and transport networks onto a single SRv6 infrastructure, reducing operational complexity while enabling advanced capabilities like network slicing using SRv6 and Flex-Algo.

Key Sections

Section Topic Description
3 Reference Architecture 5G user plane architecture with SRv6 replacing GTP-U
4 SRv6 SID for Mobile User Plane SID structure carrying mobile session context
5 SRv6 Endpoint Behaviors New behaviors for GTP-U interworking
6 Uplink and Downlink Scenarios Packet flow for both directions
7 IPv4 User Plane Support Handling IPv4 UE traffic over SRv6

MUP Endpoint Behaviors

Behavior Direction Function
End.M.GTP6.D Downlink Decapsulates SRv6 and encapsulates into GTP-U toward the gNB (IPv6 payload)
End.M.GTP6.Di Downlink Same as above but for IPv6 traffic with inner header insertion
End.M.GTP6.E Uplink Decapsulates GTP-U from gNB and encapsulates into SRv6
End.M.GTP4.E Uplink Decapsulates GTP-U (IPv4 outer) and encapsulates into SRv6
H.M.GTP4.D Downlink Headend behavior: encodes GTP-U parameters into SRv6 SID args for IPv4 N3

MUP SID Structure

The MUP SID encodes mobile session information directly in the SRv6 SID:

|<-------- Locator -------->|<-- Function -->|<--- Argument --->|
|   Block     |    Node     |    Behavior    |  TEID + QFI etc  |
Field Purpose
Locator Identifies the UPF or mobile anchor node
Function Identifies the MUP behavior (e.g., End.M.GTP6.D)
Argument Carries TEID, QFI, and other GTP-U session parameters

GTP-U to SRv6 Interworking

Scenario Source Translation Point Destination
Uplink (N3) gNB sends GTP-U Ingress UPF: GTP-U to SRv6 Anchor UPF (SRv6 endpoint)
Downlink (N3) Anchor UPF sends SRv6 Egress UPF: SRv6 to GTP-U gNB receives GTP-U
N9 (inter-UPF) UPF sends SRv6 No translation needed UPF receives SRv6

Common Pitfalls

  • The MUP behaviors require the argument portion of the SID to be large enough to encode the TEID (32 bits) and QFI (6 bits). An undersized Locator:Function leaves insufficient room for arguments.
  • The interworking functions at domain boundaries must maintain awareness of the GTP-U tunnel state for proper TEID mapping, even though the SRv6 core is stateless.
  • IPv4 user plane traffic requires special handling (End.M.GTP4.E / H.M.GTP4.D) because of the different N3 encapsulation header format.
  • Deploying MUP without Flex-Algo for network slicing loses one of the key benefits -- the ability to map 5G network slices to distinct SRv6 topologies.
  • Not all SRv6 implementations support MUP behaviors. Verify platform support before planning a GTP-U replacement deployment.

References

  1. RFC 9433 - Segment Routing over IPv6 for the Mobile User Plane
  2. RFC 8986 - SRv6 Network Programming
  3. 3GPP TS 29.281 - GPRS Tunnelling Protocol User Plane (GTPv1-U)
  4. 3GPP TS 23.501 - 5G System Architecture