SRH Reduction for SRv6 End.M.GTP6.E Behavior

Introduction Segment Routing over IPv6 for the Mobile User Plane defines interworking between SRv6 networks and GTP-U networks including required behaviors such as End.M.GTP6.E. This End.M.GTP6.E behavior converts SRv6 packets to GTP-U packets for downlink(DL) traffic at an egress MUP-PE when a gNB is using IPv6/GTP. In End.M.GTP6.E behavior, an ingress MUP-PE needs two SIDs in an SRH with the remote endpoint information (IP address and TEID) in different places in the SRH and an egress MUP-PE also needs to fetch the last SID next to the active SID before outer IPv6 and SRH decapsulation to restore the IPv6/GTP-U header from those SIDs, in which current hardware pipelines may be unfamiliar or insufficient to implement. This document specifies a new behavior named End.M.GTP6.E.Red which makes End.M.GTP6.E behavior more hardware-friendly by indicating the behavior with one SID. This behavior utilizes an Interwork Segment Discovery (ISD) Route and Type 1 Session Transformed (ST) Route of MUP SAFI , specified in to restore the gNB address from the reduced SRH .

Conventions and Definitions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

Terminology Terminology used in this document is given by and .

End.M.GTP6.E.Red Behavior End.M.GTP6.E.Red (Endpoint Behavior with encapsulation for IPv6/GTP-U tunnel with reduced SRH) is used in the interworking scenarios described in for the downlink toward the legacy gNB using IPv6/GTP. depicts a topology used for the example. This topology is the same as Figure 4 described in Section 5.3 of but terminology is replaced by one used in .

Example Topology for Interworking In this topology, we assume the addressing as below:

The prefix length of the Interwork Segment, that is, actual RAN IP Prefix is 'a'.
The length of the LOC+FUNCT field of the SID for End.M.GTP6.E.Red behavior on the Egress PE is 'b'.

shows the relationship between RAN IP Prefix, gNB address and End.M.GTP6.E.Red SID.

Relationship between RAN IP Prefix and gNB address and SID In one of deployment scenarios, the length of actual RAN IP Prefix can be 64 bits (a=64) or shorter (a<64) and the length of SRGW-IPv6-LOC-FUNC can be 48 bits (b=48) in both cases of full SID and uSID. These are given by the addressing design of the RAN and the SRv6 domain. In this case, the stuff field is 24 bits (or more) and then, the prefix length of advertised RAN IP Prefix (the NLRI in in the ISD Route) is 88 bits.

Control Plane Specification

Egress PE If the actual RAN IP Prefix is shorter than b+40 bit-length, then the Egress PE makes up the missing 40-a+b bits(stuff field) from the gNB address so that the Egress PE can generate a prefix of b+40 bit length (advertised RAN IP Prefix). The Egress PE generates SID prefixes of End.M.GTP6.E.Red behavior ('b' bits of SRGW-IPv6-LOC-FUNC field) for each advertised RAN IP Prefixes and holds the mapping. The Egress PE MUST advertise an Interwork Segment Discovery (ISD) Route which NLRI contains the advertised RAN IP Prefix with the corresponding SID information.

Ingress PE The Ingress PE receives a Type 1 Session Transformed (ST) Route for the UE from the MUP Controller and an ISD Route for the gNB from the Egress PE. When the Type 1 ST Route can be resolved with the RAN IP Prefix in the NLRI field of the ISD Route, the Ingress PE MUST generate a complete SID value by merging b+40 bit-length SID value stored in the ISD Route and the last 128-40-b bits of the Endpoint Address (the IPv6 address of the gNB) then store the complete SID as H.Encaps(.Red) behavior for the host route of the UE in the FIB.

Data Plane Specification

Ingress PE When the Ingress PE receives a packet toward the UE and finds the corresponding local SID in the FIB, then just perform H.Encaps(.Red) behavior.

Egress PE When Egress PE node N receives a packet destined to D, and D is a local End.M.GTP6.E.Red SID, N does the following: Notes:

The source address S SHOULD be an End.M.GTP6.D SID instantiated at N or IPv6 address of the UPF.
The higher b+40 bits IPv6 DA can be restored from the advertised RAN IP Prefix corresponding to the SID in the mapping, and lower 128-40-b bits can be restored from lower 128-40-b bits of the End.M.GTP6.E.Red SID (remainder bits field in ).
GTP-U TEID is restored from the Args.Mob.Session field in the SID as defined in .

Security Considerations The security considerations for Segment Routing are discussed in . More specifically, for SRv6, the security considerations and the mechanisms for securing an SR domain are discussed in . Together, they describe the required security mechanisms that allow establishment of an SR domain of trust to operate SRv6-based services for internal traffic while preventing any external traffic from accessing or exploiting the SRv6-based services. The technology described in this document is applied to a mobile network that is within the SR domain. It's important to note the resemblance between the SR domain and the 3GPP Packet Core Domain. This document introduces new SRv6 Endpoint Behaviors. Those behaviors operate on control plane information, including information within the received SRH payload on which the behaviors operate. Altering the behaviors requires that an attacker alter the SR domain as defined in . Those behaviors do not need any special security consideration given that they are deployed within that SR domain.

IANA Considerations The following values have been allocated in the "SRv6 Endpoint Behaviors" subregistry within the top-level "Segment Routing Parameters" registry: New SRv6 Mobile User-plane Endpoint Behavior Types

Value	Hex	Endpoint behavior	Reference
168	0x00a8	End.M.GTP6.E.Red	This.ID