BGP Extensions of SR Policy for Segment List Protection

Introduction Segment Routing allows a headend node to steer a packet flow along any path. details the concept of SR Policy and steering into an SR Policy. An SR Policy is a set of candidate paths, each consisting of one or more segment lists. The headend of an SR Policy may learn multiple candidate paths for an SR Policy. Candidate path can be used for path protection, that is, the lower preference candidate path may be designated as the backup for a specific or all (active) candidate path(s). Backup candidate path provide protection only when all the segment lists in the active CP are invalid.If a candidate path is associated with a set of Segment-Lists, each Segment-List is associated with weight for weighted load balancing. The protection mechanism for SR Policy is not flexible enough. For example, there're two active segment lists(SL1, SL2) in the primary candidate path CP1, SL1 and SL2 can together carry 80 Gbps. If SL1 fails, CP1 are still the primary path, but the bandwith of CP1 is probably not enough. If there's a backup segment list for SL1, e.g, SL3, in CP1, traffic will be load-balanced between SL3 and SL2 after SL1 fails. The pcep extensions for segment list identification and protection relationship among segment lists specification are proposed in . specifies BGP extensions for the advertisement of SR Policy. defines extensions to BGP SR Policy to specify the identifier of segment list. This document proposes extensions of BGP in order to provide the protection information of segment lists when delivering SR policy via BGP.

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

BGP Extensions for Advertising Segment List

Extensions of Segment List sub-TLV Segment List sub-TLV is introduced in and it includes the elements of the paths (i.e., segments). This document introduces a one-bit flag in the RESERVED field, where,

B-Flag in Segment List sub-TLV

B-Flag(Backup Flag): One bit. If set, indicates a pure backup path. This is a segment list that only carries rerouted traffic after the protected segment list fails. If this flag is not set, it indicates that the segment list acts as the active member in the candidate path that carries normal traffic.

Using segment lists for path protection can be compatible with using candidate paths. When a path fails, the backup segment list within the same candidate path is used preferentially for path protection. If the backup list is also invalid, then other candidate path can be enabled for protection.

List Protection Sub-TLV This document introduces a new sub-sub-tlv of Segment List sub-TLV, where,

List Protection Sub-TLV

Type: 1 octet. TBD.
Length: 1 octet, specifies the length of the value field not including Type and Length fields.
RESERVED: 2 octet of reserved bits. SHOULD be unset on transmission and MUST be ignored on receipt.
Backup List ID: 4 octet of ID for the back up segment list, the segment list id is delivered in Segment List ID Sub-TLV as define in . If there're multiple backup paths, the list ID of each path should be included in the TLV.

As defined in , the SR Policy encoding structure is as follows: Attributes: Tunnel Encaps Attribute (23) Tunnel Type: SR Policy Binding SID Preference Priority Policy Name Explicit NULL Label Policy (ENLP) Segment List Weight Segment Segment ... Segment List ... ... ]]> The new SR Policy encoding structure with List Protection sub-TLV is shown as below: Attributes: Tunnel Encaps Attribute (23) Tunnel Type: SR Policy Binding SID SRv6 Binding SID Preference Priority Policy Name Policy Candidate Path Name Explicit NULL Label Policy (ENLP) Segment List List Protection Weight Segment Segment ... Segment List ... ... ]]>

IANA Considerations

New Registry: Flag Field of Segment List sub-TLV This document introduces a one-bit flag field in the Segment List sub-TLV for the Backup Flag (B-Flag).

Existing Registry: BGP Tunnel Encapsulation Attribute sub-TLVs This document defines a new sub-TLV in the registry "SR Policy List Sub-TLVs" to be assigned by IANA:

Security Considerations Procedures and protocol extensions defined in this document do not affect the security considerations discussed in .