SR Policies for Path Segment and Bi-directional Path in BGP-LS
draft-li-idr-bgp-ls-sr-policy-path-segment-00

Abstract

This document specifies the way of collecting configuration and states of SR policies carrying path ID and bi-directional path information by using BPG-LS. Such information can be used by external conponents for many use cases such as performance measurement, path re-optimization and end-to-end protection.

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 RFC 2119.

Status of This Memo

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Copyright Notice

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Table of Contents

• 1. Introduction
• 2. Terminology
• 3. Carrying SR Path Sub-TLVs in BGP-LS
• 3.1. SR Path ID Sub-TLV
• 3.2. Sub-TLVs for Bi-directional Path
• 3.2.1. SR Bi-directional Path Sub-TLV
• 3.2.2. SR Reverse Path Segment List Sub-TLV
• 4. Operations
• 5. IANA Considerations
• 6. Security Considerations
• 7. Acknowledgements
• 8. Normative References
• Authors' Addresses

1. Introduction

Segment routing (SR) [I-D.ietf-spring-segment-routing] is a source routing paradigm that allows the ingress node steers packets into a specific path according to the Segment Routing Policy [I-D.filsfils-spring-segment-routing-policy].

However, the SR Policies defined in [I-D.filsfils-spring-segment-routing-policy] only supports uni-directional SR paths and there is no path ID in a Segment List to identify an SR path. For identifying an SR path and supporting bi-directional path [I-D.cheng-spring-mpls-path-segment], new policies carrying Path ID and bi-directional path information are defined in [I-D.li-idr-sr-policy-path-segment-distribution], as well as the extensions to BGP to distribute new SR policies. The path ID can be a path segment in SR-MPLS [I-D.cheng-spring-mpls-path-segment] , or a path ID in SRv6 [I-D.li-spring-passive-pm-for-srv6-np], 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 [I-D.ietf-idr-te-lsp-distribution].

To collect the TE Policy information that is locally available in a router, [I-D.ietf-idr-te-lsp-distribution] describes a new mechanism by using BGP-LS update messages.

Based on the mechanism defined in [I-D.ietf-idr-te-lsp-distribution], this document describes a mechanism to distribute configuration and states of the new SR policies defined in [I-D.li-idr-sr-policy-path-segment-distribution] to external components using BGP-LS.

2. Terminology

This memo makes use of the terms defined in [I-D.ietf-spring-segment-routing] and [I-D.ietf-idr-te-lsp-distribution].

3. Carrying SR Path Sub-TLVs in BGP-LS

A mechanism to collect states of SR Policies via BGP-LS is proposed by [I-D.ietf-idr-te-lsp-distribution]. 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 [RFC7752]. The TE Policy State TLV contains several sub-TLVs such as SR TE Policy sub-TLVs. Rather than replicating SR TE Policy sub-TLVs, [I-D.ietf-idr-te-lsp-distribution] reuses the equivalent sub-TLVs as defined in [I-D.ietf-idr-segment-routing-te-policy].

As defined in [I-D.li-idr-sr-policy-path-segment-distribution], the new SR policies for bi-directional path has the following format:

    SR Policy SAFI NLRI: <Distinguisher, Policy-Color, Endpoint>
        Attributes: Tunnel Encaps Attribute (23)
        Tunnel Type: SR Policy
            Binding SID
            Preference
            Bi-directioanl Path
                Segment List 
                    Weight
                    Path ID
                    Segment
                    Segment
                    ...
                Reverse Segment List
                    Weight
                    Path ID
                    Segment
                    Segment
                    ...

          Figure 1. SR policy for Bi-directional path

For collecting configuration and states of uni-directional and bi-directional SR policies defined in [I-D.li-idr-sr-policy-path-segment-distribution], new sub-TLVs in SR TE Policy sub-TLVs should be defined. Likewise, rather than replicating SR Policy sub-TLVs, this document can reuse the equivalent sub-TLVs as defined in [I-D.li-idr-sr-policy-path-segment-distribution].

3.1. SR Path ID Sub-TLV

This section defines an SR Path ID sub-TLV to specify an SR path, and it is included in the Segment List sub-TLV as defined in [I-D.ietf-idr-te-lsp-distribution].

An SR Path ID sub-TLV is associated with an SR path specified by a Segment List sub-TLV, and it MUST appear only once within a Segment List sub-TLV. It 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     |    Flag       |      PIT      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                       Path ID (Variable)                      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     Figure 2. Path ID sub-TLV

All fields, including type and length, are defined in [I-D.li-idr-sr-policy-path-segment-distribution].

3.2. Sub-TLVs for Bi-directional Path

In some scenarios like mobile backhaul transport network, there are requirements to support bi-directional path. In SR, a bi-directional path can be represented as a binding of two uni-directional SR paths [I-D.cheng-spring-mpls-path-segment]. [I-D.li-idr-sr-policy-path-segment-distribution] defines new sub-TLVs to describe an SR bi-directional path. An SR policy carrying SR bi-directional path information is expressed in Figure 1.

3.2.1. SR Bi-directional Path Sub-TLV

This section defines an SR bi-directional path sub-TLV to specify a bi-directional path, which contains a Segment List sub-TLV [I-D.ietf-idr-segment-routing-te-policy] and an associated Reverse Path Segment List as defined in [I-D.li-idr-sr-policy-path-segment-distribution]. The SR bi-directional path 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            |   RESERVED    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        Sub-TLVs (Variable)                    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              Figure 3. SR Bi-directional path sub-TLV

All fields, including type and length, are defined in [I-D.li-idr-sr-policy-path-segment-distribution].

3.2.2. SR Reverse Path Segment List Sub-TLV

This section defines an SR Reverse Path Segment List sub-TLV to specify an SR reverse path associated with the path specified by the Segment List in the same SR Bi-directional Path Sub-TLV, and it 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            |   RESERVED    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        Sub-TLVs (Variable)                    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           Figure 4. SR Reverse Path Segment List Sub-TLV

[I-D.li-idr-sr-policy-path-segment-distribution].

4. Operations

No new operation procedures are defined in this document, the operations procedures of [RFC7752] can apply to this document.

Typically but not limited to, the uni/bi-directional SR policies carrying path identification information can be distributed by the ingress node.

Generally, BGP-LS is used for collecting link states and synchronizing with the external component. The consumer of the uni/bi-directional SR policies carrying path identification information is not BGP LS process by itself, and it can be any applications such as performance measurement and path re-coputation or re-optimization, etc. The operation of sending information to other precesses is out of scope of this document.

5. IANA Considerations

TBA

6. Security Considerations

TBA

7. Acknowledgements

TBA

8. Normative References

Authors' Addresses