| Internet-Draft | Segment List Protection | June 2023 |
| Liu, et al. | Expires 30 December 2023 | [Page] |
This document proposes extensions of BGP in order to provide protection information for segment lists when delivering SR policy via BGP.¶
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Segment Routing [RFC8402] allows a headend node to steer a packet flow along any path. [RFC9256] 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 [I-D.ietf-pce-multipath].¶
[I-D.ietf-idr-segment-routing-te-policy] specifies BGP extensions for the advertisement of SR Policy. [I-D.lin-idr-sr-policy-seglist-id] 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.¶
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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
Segment List sub-TLV is introduced in [I-D.ietf-idr-segment-routing-te-policy] and it includes the elements of the paths (i.e., segments).¶
This document introduces a one-bit flag in the RESERVED field, where,¶
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 |B| RESERVED |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// sub-TLVs //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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.¶
This document introduces a new sub-sub-tlv of Segment List sub-TLV, where,¶
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 | RESERVED |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Backup List ID 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Backup List ID N |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
As defined in [I-D.ietf-idr-segment-routing-te-policy], the SR Policy encoding structure is as follows:¶
SR Policy SAFI NLRI: <Distinguisher, Policy-Color, Endpoint>
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:¶
SR Policy SAFI NLRI: <Distinguisher, Policy-Color, Endpoint>
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
...
...
¶
This document introduces a one-bit flag field in the Segment List sub-TLV [I-D.ietf-idr-segment-routing-te-policy] for the Backup Flag (B-Flag).¶
This document defines a new sub-TLV in the registry "SR Policy List Sub-TLVs" [I-D.ietf-idr-segment-routing-te-policy] to be assigned by IANA:¶
Codepoint Description Reference
-------------------------------------------------------------
TBD List Protection Sub-TLV This document
¶
Procedures and protocol extensions defined in this document do not affect the security considerations discussed in [I-D.ietf-idr-segment-routing-te-policy].¶