SR Policies Extensions for Path Segment and Bidirectional Path in BGP-LS

Segment routing (SR) is a source routing paradigm that allows the ingress node steers packets into a specific path according to the Segment Routing Policy . However, the SR Policies defined in only supports unidirectional SR paths and there is no path ID in a Segment List to identify an SR path. For identifying an SR path and supporting bidirectional path , new policies carrying Path Segment and bidirectional path information are defined in , as well as the extensions to BGP to distribute new SR policies. The Path Segment can be a Path Segment in SR-MPLS and SRv6 , or other IDs that can identify a path. In many network scenarios, the configuration and state of each TE Policy is required by a controller which allows the network operator to optimize several functions and operations through the use of a controller aware of both topology and state information . To collect the TE Policy information that is locally available in a router, describes a new mechanism by using BGP-LS update messages. Based on the mechanism defined in , this document describes a mechanism to distribute configuration and states of the new SR policies defined in to external components using BGP-LS.

A mechanism to collect states of SR Policies via BGP-LS is proposed by . The characteristics of an SR policy can be described by a TE Policy State TLV, which is carried in the optional non-transitive BGP Attribute "LINK_STATE Attribute" defined in . The TE Policy State TLV contains several sub-TLVs such as SR TE Policy sub-TLVs. defines the BGP extensions for Path Segment. The encoding is shown below.

Attributes: Tunnel Encaps Attribute (23) Tunnel Type: SR Policy Binding SID Preference Priority Policy Name Explicit NULL Label Policy (ENLP) Segment List Weight Path Segment Segment Segment ... Segment List Weight Path Segment Segment Segment ... ... Figure 1. Path Segment in SR policy ]]> Also, defines SR policy extensions for bidirectional SR path, the encoding is shown below:

Attributes: Tunnel Encaps Attribute (23) Tunnel Type: SR Policy Binding SID Preference Priority Policy Name Explicit NULL Label Policy (ENLP) Segment List Weight Path Segment Segment Segment ... Reverse Segment List Weight Path Segment Segment Segment ... Figure 2. SR policy for Bidirectional path ]]> In order to collect configuration and states of unidirectional and bidirectional SR policies defined in , this document defines new sub-TLVs in SR TE Policy sub-TLVs.

This section defines the SR Path Segment sub-TLV to describe a Path Segment, and it can be included in the Segment List sub-TLV as defined in . An SR Path Segment sub-TLV can be associated with an SR path specified by a Segment List sub-TLV. Multiple Path Segment MAY be included in a Segment List for different use cases. When all the SID Lists within a candidate path share the same Path Segment ID, the Path Segment can be used to collect the aggregated information of the candidate path. The format of Path Segment TLV is shown below.

Where, Type: to be assigned by IANA. Length: the total length of the value field not including Type and Length fields. Flags: 2 octet field that indicates attribute and status of the Path Segment. The following bit positions are defined. Other bits SHOULD be cleared by originator and MUST be ignored by receiver.

Where: D-Flag : Indicates the dataplane for the BSIDs, it is set when Path Segment ID is a 16-octet SRv6 SID unset when the Path Segment ID is a 4-octet SR/MPLS label value. B-Flag: This flag, when set, indicates the presence of the SRv6 Endpoint Behavior and SID Structure encoding specified in . It MUST be ignored when D-flag is unset. They indicate the SRv6 Endpoint behavior and SID structure for the Path Segment ID value in the TLV. L-Flag: Local flag. Set when the Path Segment has local significance on an SR node. RESERVED: 2 octets. SHOULD be set to 0 by originator and MUST be ignored by receiver. Path Segment ID: It indicates the Path Segment ID value based on the status flags. The SRv6 Endpoint Behavior TLV (1250) and the SRv6 SID Structure TLV (1252) defined in are used as sub-TLVs of the SR Path Segment Sub-TLV to optionally indicate the SRv6 Endpoint behavior and SID structure for the Binding SID value in the TLV when the Path Segment is an SRv6 Path Segment.

In some scenarios like mobile backhaul transport network, there are requirements to support bidirectional path. In SR, a bidirectional path can be represented as a binding of two unidirectional SR paths . An SR policy carrying SR bidirectional path information is expressed in Figure 2. defines a new sub-TLV to describe a reversed SR path of an SID list. This section defines a Reverse Segment List sub-TLV to specify a reverse SR path associated with the path specified by the Segment List, and it reuses the format of SR Segment List TLV defined in :

All fields, except the type are defined in , and this TLV reuses it directly. The Type of this TLV is TBA. The SR Segment sub-TLV MUST be included as an ordered set of sub-TLVs within the SR Segment List TLV when the SID-List is not empty. A SID-List may be empty in certain cases (e.g. for a dynamic path) where the headend has not yet performed the computation and hence not derived the segments required for the path; in such cases, the SR Segment List TLV SHOULD NOT include any SR Segment sub-TLVs . Note: currently, only one reverse SID list is supported, so the weight field CAN be ignored when processing. However, multiple reverse SID list MAY be supported in the future, and the use case of supporting this still need to be discussed.