BGP SR Policy Extensions for Network Resource Partition

Abstract

Segment Routing (SR) Policy is a set of candidate paths, each consisting of one or more segment lists and the associated information. The header of a packet steered in an SR Policy is augmented with an ordered list of segments associated with that SR Policy. A Network Resource Partition (NRP) is a subset of network resources allocated in the underlay network which can be used to support one or a group of RFC 9543 network slice services.¶

In networks where there are multiple NRPs, an SR Policy may be associated with a particular NRP. The association between SR Policy and NRP needs to be specified, so that for service traffic which is steered into the SR Policy, the header of the packets can be augmented with the information associated with the NRP. An SR Policy candidate path can be distributed using BGP SR Policy. This document defines the extensions to BGP SR policy to specify the NRP which the SR Policy candidate path is associated with.¶

2. NRP Identifier of SR Policy

In order to specify the NRP the candidate path of SR policy is associated with, a new sub-TLV called "NRP sub-TLV" is defined in the BGP Tunnel Encapsulation Attribute [RFC9012]. The NRP sub-TLV can be carried in the BGP Tunnel Encapsulation Attribute with the tunnel type set to SR Policy.¶

The NRP sub-TLV is optional and MUST NOT appear more than once for one SR Policy candidate path. If the NRP sub-TLV appears more than once, the associated BGP SR Policy NLRI is considered malformed and the "treat-as-withdraw" strategy of [RFC7606] is applied.¶

The NRP sub-TLV has the following format:¶

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |   Length      |     Flags     |   Reserved    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         NRP ID (4 octets)                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                      Figure 1. NRP Sub-TLV

where:¶

• Type: 123¶

• Length: 6¶

• Flags: 1-octet flag field. None is defined at this stage. The flags SHOULD be set to zero on transmission and MUST be ignored on receipt.¶

• RESERVED: 1 octet of reserved bits. It SHOULD be set to zero on transmission and MUST be ignored on receipt.¶

• NRP ID: A 32-bit domain significant identifier which is used to identify an NRP. Value 0 and 0xFFFFFFFF are reserved.¶

The encoding structure of BGP SR Policy with the NRP sub-TLV is expressed as below:¶

         SR Policy SAFI NLRI: <Distinguisher, Policy-Color, Endpoint>
         Attributes:
            Tunnel Encaps Attribute (23)
               Tunnel Type: SR Policy (15)
                   Binding SID
                   SRv6 Binding SID
                   Preference
                   Priority
                   Policy Name
                   Policy Candidate Path Name
                   Explicit NULL Label Policy (ENLP)
                   NRP
                   Segment List
                       Weight
                       Segment
                       Segment
                       ...
                   ...
             Figure 2. SR Policy Encoding with NRP sub-TLV

3. Procedures

When a candidate path of SR Policy is instantiated within an NRP, and a network-wide data plane NRP ID is used for identifying the resources of the NRP, the originating node of SR Policy SHOULD include the NRP sub-TLV in the BGP Tunnel Encapsulation Attribute of the BGP SR Policy. The setting of other fields and attributes in BGP SR Policy SHOULD follow the mechanism as defined in [I-D.ietf-idr-sr-policy-safi].¶

On reception of an SR Policy NLRI, a BGP speaker determines if it is acceptable and usable according to the rules defined in Section 4.2 of [I-D.ietf-idr-sr-policy-safi]. If the SR Policy candidate path selected as the best candidate path is associated with an NRP, the headend node of the SR Policy SHOULD encapsulate the NRP ID and the segment list of the selected candidate path in the header of packets which are steered to the SR Policy. For SR Policy with IPv6 data plane, the approach to encapsulate the NRP ID in IPv6 Hop-by-Hop Options header is defined in [I-D.ietf-6man-enhanced-vpn-vtn-id]. For SR Policy with MPLS data plane, one approach to encapsulate the NRP ID to the packet is defined in [I-D.li-mpls-enhanced-vpn-vtn-id].¶

Although the proposed mechanism allows that different candidate paths in one SR policy be associated with different NRPs, in normal network scenarios it is considered that the association between an SR Policy and NRP is consistent, in such case all candidate paths of one SR policy SHOULD be associated with the same NRP.¶

4. Scalability Considerations

The mechanism specified in this document adds additional information to the SR Policy candidate paths. In order to steer traffic into different NRPs using SR Policy, the SR Policies used for different NRPs need to be different. As the number of NRP increases, the number of SR Policies would also increase accordingly. When BGP is used for distributing SR Policy candidate paths, the amount of control plane information exchanged between the network controller and the headend nodes would also increase. However, since the SR Policies candidate paths distributed in BGP are only installed by the corresponding headend nodes, the impacts to the BGP control plane are considered acceptable.¶

8. References