New H3C Technologies

linchangwang.04414@h3c.com

China Mobile

chengweiqiang@chinamobile.com

Cisco Systems, Inc

zali@cisco.com

Cisco Systems, Inc

aramahen@cisco.com

General Network Working Group Segment Routing SR Policy BGP-LS Segment Routing is a source routing paradigm that explicitly indicates the forwarding path for packets at the ingress node. An SR Policy is associated with one or more candidate paths, and each candidate path is either dynamic, explicit or composite. This document specifies the extensions to BGP Link State (BGP-LS) to carry composite candidate path information in the advertisement of an SR policy.

As described in , BGP Link State (BGP-LS) provides a mechanism by which link-state and TE information can be collected from networks and shared with external components using the BGP routing protocol. Segment routing (SR) is a source routing paradigm that explicitly indicates the forwarding path for packets at the ingress node. The ingress node steers packets into a specific path according to the Segment Routing Policy (SR Policy) as defined in . An SR Policy is associated with one or more candidate paths. A composite candidate path acts as a container for grouping of SR Policies. As described in section 2.2 in , the composite candidate path construct enables combination of SR Policies, each with explicit candidate paths and/or dynamic candidate paths with potentially different optimization objectives and constraints, for a load-balanced steering of packet flows over its constituent SR Policies. defines extensions for BGP to distribute SR policies carrying composite candidate path information. While as defined in Section 3.6 of , PCEP signals the Composite Candidate Path. describes a mechanism to collect the SR policy information that is locally available in a node and advertise it into BGP-LS updates. This document extends it to provide some extra information to carry composite candidate path information in the BGP-LS advertisement.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

defines the BGP-LS NLRI that can be a Node NLRI, a Link NLRI or a Prefix NLRI. The corresponding BGP-LS attribute is a Node Attribute, a Link Attribute or a Prefix Attribute. describes a mechanism to collect the SR Policy information that is locally available in a node and advertise it into BGP Link State (BGP-LS) updates. This section defines a new sub-TLV which is carried in the optional non-transitive BGP Attribute "LINK_STATE Attribute" defined in .

Segment Routing Policy (SR Policy) architecture is specified in . A SR Policy can comprise of one or more candidate paths, and each candidate path is either dynamic, explicit or composite. A composite candidate path can comprise of one or more constituent SR policies. The endpoints of the constituent SR Policies and the parent SR Policy MUST be identical, and the colors of each of the constituent SR Policies and the parent SR Policy MUST be different. The Composite Candidate Path TLV is used to report the constituent SR policy(s) of a composite candidate path. Only a single instance of this TLV is advertised for a given candidate path. If multiple instances are present, then the first valid (i.e., not determined to be malformed as per section 8.2.2 of [RFC9552]) one is used and the rest are ignored.The TLV has 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Color | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Weight | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Sub-TLVs (variable) // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

where: Type: to be assigned by IANA. Length: the total length of the value field not including Type and Length fields. Reserved: 32 bits reserved and MUST be set to 0 on transmission and MUST be ignored on receipt. Color: 4 octets that indicates the color of the constituent SR Policy. Weight: 4 octet field that indicates the weight associated with the SID-List for weighted load-balancing. Refer Section 2.2 and 2.11 of . Sub-TLVs: variable and contains any other optional attributes associated with the Composite Candidate Path. Currently, the sub-TLV only defines the Per-Flow Forwarding Class TLV.

Per-Flow Candidate Path builds on top of the concept of the Composite Candidate Path. Each Path in a Per-Flow Candidate Path is assigned a 3-bit forward class value, which allows Quality of Service (QoS) classified traffic to be steered depending on the forward class. The Per-Flow Forwarding Class TLV is an optional sub-TLV of the Composite Candidate Path TLV.Only a single instance of this sub-TLV is advertised for a given candidate path. If multiple instances are present, then the first valid (i.e., not determined to be malformed as per section 8.2.2 of [RFC9552]) one is used and the rest are ignored.

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 | FC | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

where: Type (16 bits): TBD1 for "PER-FLOW-FORWARD-CLASS" TLV. Length (16 bits): 4. Reserved: This field MUST be set to zero on transmission and MUST be ignored on receipt. FC (3 bits): Forward class value that is given by the QoS classifier to traffic entering the given Candidate Path. Different classes of traffic that enter the given Candidate Path can be differentially steered into different Colors.

The document does not bring new operation beyond the description of operations defined in and . The existing operations defined in and can apply to this document directly. Typically but not limit to, the BGP-LS messages carrying composite candidate path information along with the SR policy are distributed to a controller. After configuration, the composite candidate path information will be advertised by BGP update messages. The operation of advertisement is the same as defined in and , as well as the reception.

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

This document defines a new TLV in the BGP-LS Link Descriptor and Attribute TLVs: Value Description Reference TBA Composite Candidate Path TLV This document TBA Per-Flow Forwarding Class TLV This document

Huawei Technologies Cisco Systems Huawei Technologies RtBrick Inc. Microsoft This document describes a mechanism to collect the Segment Routing Policy information that is locally available in a node and advertise it into BGP Link-State (BGP-LS) updates. Such information can be used by external components for path computation, re-optimization, service placement, network visualization, etc. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Segment Routing (SR) leverages the source routing paradigm. A node steers a packet through an ordered list of instructions, called "segments". A segment can represent any instruction, topological or service based. A segment can have a semantic local to an SR node or global within an SR domain. SR provides a mechanism that allows a flow to be restricted to a specific topological path, while maintaining per-flow state only at the ingress node(s) to the SR domain. SR can be directly applied to the MPLS architecture with no change to the forwarding plane. A segment is encoded as an MPLS label. An ordered list of segments is encoded as a stack of labels. The segment to process is on the top of the stack. Upon completion of a segment, the related label is popped from the stack. SR can be applied to the IPv6 architecture, with a new type of routing header. A segment is encoded as an IPv6 address. An ordered list of segments is encoded as an ordered list of IPv6 addresses in the routing header. The active segment is indicated by the Destination Address (DA) of the packet. The next active segment is indicated by a pointer in the new routing header. In many environments, a component external to a network is called upon to perform computations based on the network topology and the current state of the connections within the network, including Traffic Engineering (TE) information. This is information typically distributed by IGP routing protocols within the network. This document describes a mechanism by which link-state and TE information can be collected from networks and shared with external components using the BGP routing protocol. This is achieved using a BGP Network Layer Reachability Information (NLRI) encoding format. The mechanism applies to physical and virtual (e.g., tunnel) IGP links. The mechanism described is subject to policy control. Applications of this technique include Application-Layer Traffic Optimization (ALTO) servers and Path Computation Elements (PCEs). This document obsoletes RFC 7752 by completely replacing that document. It makes some small changes and clarifications to the previous specification. This document also obsoletes RFC 9029 by incorporating the updates that it made to RFC 7752.