]> Huawei Technologies

No. 156 Beiqing Road Beijing China zhangli344@huawei.com

Huawei Technologies

No. 156 Beiqing Road Beijing China jie.dong@huawei.com

Arrcus, Inc.

United States of America keyur@arrcus.com

China Satellite Network Innovation Research Institute Co., Ltd.

China lijw3@chinasatnet.com.cn

China Mobile

China zhangxiaoqiu@chinamobile.com

Routing LSVR Internet-Draft For network scenarios such as Massively Scaled Data Centers (MSDCs), BGP is extended for Link-State (LS) distribution and the Shortest Path First (SPF) algorithm based calculation. BGP Link State Shortest Path First (BGP-LS-SPF) Routing leverages the mechanisms of both BGP protocol and BGP-LS protocol extensions. Segment Routing over IPv6 (SRv6) provides a source routing 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 SRv6 domain. In some networks, it may be useful to enable SRv6-based source routing mechanism together with BGP-LS-SPF. This document proposes to introduce the BGP Link-State (BGP-LS) extensions for SRv6 to the BGP-LS-SPF SAFI to enable SRv6 capabilities in BGP-LS-SPF. The usages and formats of these extensions are also provided in this document.

Introduction extends BGP for Link-State (LS) distribution and the Shortest Path First (SPF) algorithm based calculation. BGP Link State Shortest Path First (BGP-LS-SPF) Routing leverages the mechanisms of both BGP protocol and BGP-LS protocol extensions , with the extensions to BGP-LS attribute and new NLRI selection rules. Segment Routing over IPv6 (SRv6) allows for a flexible definition of end-to-end paths within various topologies by encoding paths as sequences of topological or functional sub-paths called "segments". SRv6 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 SRv6 domain. defines extensions to the BGP-LS to carry SR information. further defines extensions to BGP-LS to advertise SRv6 segments along with their behaviors and other attributes via BGP. In network scenarios such as Data Center networks, WAN networks or other networks where BGP-LS-SPF can be used as the underlay routing protocol, it may be useful to enable SRv6-based source routing mechanism for traffic engineering and optimization. This document proposes to introduce the BGP Link-State (BGP-LS) extensions for SRv6 to the BGP-LS-SPF SAFI to enable SRv6 capabilities in BGP-LS-SPF. The usages and formats of these extensions are also provided in this document.

Requirements Language 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.

SRv6 Node Attribute TLVs The following SRv6 Node Attributes TLVs described in are applicable to BGP-LS-SPF. Node Attribute TLVs for SRv6 with BGP-LS-SPF

Type	Description	Reference
1138	SRv6 Capabilities TLV	RFC 9514
266	Node MSD TLV	RFC 8814
1035	SR Algorithm TLV	RFC 9085

These SRv6 Node Attributes TLVs are advertised associated with the BGP-LS-SPF Node NLRI.

SRv6 Capabilities TLV The SRv6 Capabilities TLV defined in is used to announce the SRv6 capabilities of the node along with the BGP-LS Node NLRI and indicates the SRv6 support by the node. For BGP-LS-SPF, it SHOULD support this TLV for a BGP-LS-SPF node to advertise its support for the SRv6-related capabilities. This is an optional TLV of BGP-LS-SPF Node NLRI that MUST be advertised by an SRv6-capable node. This TLV MUST be advertised only once in the attributes of BGP-LS-SPF Node NLRI. When multiple SRv6 Capabilities TLVs are received from a given node, the receiver MUST use the first occurrence of the TLV in the attributes of BGP-LS-SPF Node NLRI. If no SRv6 Capabilities TLV is advertised in the BGP-LS-SPF Node NLRI, then it indicates that the originator of this NLRI does not support SRv6. The format and flags of SRv6 Capabilities TLV of BGP-LS-SPF is consistent with that in BGP-LS. The format of BGP-LS-SPF SRv6 Capability TLV is shown as follow:

SRv6 Capabilities TLV format

where: Type: 1038 Length: 4 Flags: 2-octet field. The following flags are defined: where: O-flag: If set, the node supports use of the O-bit in the SRH, as defined in . Other flags are not defined and are reserved for future use. They MUST be set to 0 on transmission and MUST be ignored on receipt. Reserved: 2-octet field that MUST be set to 0 when originated and ignored on receipt. This TLV is used by an SRv6‑capable node or controller to determine whether a node can participate in SRv6 TE path computation. If a node does not advertise its SRv6 Capabilities, then it MUST NOT be used for SRv6 TE path computation.

SRv6 Node MSD Types The Node MSD TLV defined in is used to advertise the limits and the Segment Routing Header (SRH) operations supported by the SRv6-capable node in BGP-LS. For BGP-LS-SPF, different SRv6-capable node may have different limits related to SRH processing, therefore, BGP-LS-SPF SHOULD support this TLV for nodes to advertise the limits and operations. The SRv6 Node MSD TLV is an optional TLV of BGP-LS-SPF Node Attribute that MAY be advertised by an SRv6-capable node. This TLV MUST be advertised only once in the attributes of BGP-LS-SPF Node NLRI. When multiple SRv6 Node MSD TLVs are received from a given node, the receiver MUST use the first occurrence of the TLV in the attributes of BGP-LS-SPF Node NLRI. The MSD types for SRv6 that are defined in for IS-IS are also used by BGP-LS-SPF. These MSD types are allocated in the "IGP MSD-Types" registry maintained by IANA and are shared by IS-IS, OSPF, and BGP-LS-SPF. They are described in the subsections below. The format of this TLV is the same as that in BGP-LS:

SRv6 Node MSD TLV format

where: Type: 266. Length: variable, represents the total length of the value field in octets. Value: consists of one or more pairs of a 1-octet MSD-Type and 1-octet MSD-Value. The detailed description of MSD-Type and MSD-Value is in . Node MSD is the smallest MSD supported by the node on the set of interfaces configured for use by the advertising BGP-LS-SPF instance. MSD values may be learned via a hardware API or may be provisioned. If there are multiple MSD-Types that have the same code point in a Node MSD TLV, then the Node MSD TLV MUST be ignored by the receiver. A SRv6‑capable node or controller SHOULD consider the Node MSD limits when computing SRv6 TE paths.

Maximum Segments Left MSD Type The Maximum Segments Left MSD Type signals the maximum value of the Segments Left field in the SRH of a received packet before applying the Endpoint behavior associated with a SID. If no value is advertised, the supported value is assumed to be 0.

Maximum End Pop MSD Type The Maximum End Pop MSD Type signals the maximum number of SIDs in the SRH to which the node can apply "Penultimate Segment Pop (PSP) of the SRH" or "Ultimate Segment Pop (USP) of the SRH" behaviors, which defined in . If the advertised value is zero or no value is advertised, then the node cannot apply the PSP or USP flavors.

Maximum H.Encaps MSD Type The Maximum H.Encaps MSD Type signals the maximum number of SIDs that can be added as part of the H.Encaps behavior as defined in . If the advertised value is zero or no value is advertised, then the headend can apply a SR Policy that only contains one segment without inserting any SRH. A non-zero SRH Max H.Encaps MSD indicates that the headend can insert a SRH with SIDs up to the advertised value.

Maximum End D MSD Type The Maximum End D MSD Type specifies the maximum number of SIDs present in a SRH when performing decapsulation. These include, but are not limited to, End.DX6, End.DT4, End.DT46, End with USD, and End.X with USD as defined in . If the advertised value is zero or no value is advertised, then the node cannot apply any behavior that results in decapsulation and forwarding of the inner packet when the outer IPv6 header contains a SRH.

SR-Algorithm TLV specifies that the algorithm support for SRv6 is advertised via the SR-Algorithm TLV specified in . The SR-Algorithm is used to advertise the SR algorithms supported by the node. This TLV is a basic TLV of SR and SHOULD be supported in BGP-LS-SPF. The SR-Algorithm TLV is an optional TLV of BGP-LS-SPF Node Attribute that MAY be advertised by an SRv6-capable node. This TLV MUST be advertised only once in the attributes of BGP-LS-SPF Node NLRI. If a node receiving multiple SR-Algorithm TLVs in the BGP-LS-SPF Node Attribute, the receiver MUST use the first occurrence of the TLV in the attributes of BGP-LS-SPF Node NLRI. If an SRv6-capable node does not advertise the SR-Algorithm TLV, it implies that algorithm 0 is the only algorithm supported by the node. If the originating node does advertise the SR-Algorithm sub-TLV, then algorithm 0 MUST be present while non-zero algorithms MAY be present. The format of Algorithm fields in this TLV is consistent with that in BGP-LS, as defined in .

SRv6 Algorithm TLV format

where: Type: 1035 Length: Variable Algorithm: 1 octet of algorithm. The algorithm values are allocated in the "IGP Algorithm Type" registry defined in , these values are shared by IS-IS, OSPF, and BGP-LS-SPF. This TLV MAY be omitted or advertised with only algorithm 0 when only the basic SPF algorithm is supported. Support for Flex-Algo is out of scope of this document.

SRv6 SIDs and Reachability An SRv6 SID is 128 bits and consists of locator, function, and argument parts as described in . An BGP-LS-SPF router is provisioned with algorithm-specific locators for each algorithm supported by that router. Each locator is a covering prefix for all SIDs provisioned on that router that have the matching algorithm. Locators MUST be advertised as BGP-LS-SPF Prefix NLRI objects along with the SRv6 Locator TLVs (see ) in its BGP-LS-SPF Attribute. Forwarding entries for the locators advertised in the BGP-LS-SPF Prefix NLRI MUST be installed in the forwarding plane of receiving SRv6-capable routers when the associated algorithm is supported by the receiving BGP-LS-SPF router. The processing of the prefix of the Locator, the calculation of its reachability, and the installation in the forwarding plane follows the process of BGP-LS-SPF . SRv6 SIDs are advertised as SRv6 SID Information TLVs (see ) in the SRv6 SID NLRI, except for SRv6 SIDs that are associated with a specific neighbor/link and are therefore advertised as SRv6 End.X SID TLV (see ). SRv6 SIDs received from other nodes are not directly routable and MUST NOT be installed in the forwarding plane. Reachability to SRv6 SIDs depends upon the existence of a covering locator. Adherence to the rules defined in this section will ensure that SRv6 SIDs associated with a supported algorithm will be forwarded correctly, while SRv6 SIDs associated with an unsupported algorithm will be dropped. NOTE: The drop behavior depends on the absence of a default/summary route covering a given locator.

SRv6 Link Attribute TLVs The following Link Attributes described in and are applicable to BGP-LS-SPF. Link Attribute TLVs for SRv6 with BGP-LS-SPF

Type	Description	Reference
1106	SRv6 End.X SID TLV	RFC 9514
1172	L2 Bundle Member Attributes TLV	RFC 9085
267	SRv6 Link MSD TLV	RFC 8814

These SRv6 Link Attribute TLVs are advertised associated with the BGP-LS-SPF Link NLRI.

SRv6 End.X SID TLV The SRv6 End.X SID TLV defined in is used to advertise the SRv6 SIDs associated with an IGP Adjacency SID behavior that correspond to a point-to-point or point-to-multipoint link or adjacency of the node running the IS-IS or OSPFv3 protocols. It is also used by BGP-LS to advertise the BGP EPE Peer Adjacency SID for SRv6 on the same lines as specified for SR-MPLS in . BGP-LS-SPF SHOULD support this TLV to advertise the SRv6 SIDs that correspond to a point-to-point link or adjacency of the node running BGP-LS-SPF. The SRv6 End.X SID TLV is an optional TLV of BGP-LS-SPF Link Attribute that MAY be advertised by an SRv6-capable node. More than one instance of this TLV (one for each SRv6 End.X SID) can be included in the BGP-LS-SPF Attribute. Multiple SRv6 End.X SID TLVs MAY be associated with the same adjacency. Every SRv6-enabled node SHOULD instantiate at least one unique SRv6 End.X SID corresponding to each of its neighbors, although it MAY omit doing so if features like TE or TI-LFA that require End.X SID are not in use. All End.X SIDs MUST be a subnet of a locator with matching algorithm that is advertised by the same BGP-LS-SPF node in an SRv6 Locator TLV. End.X SIDs that do not meet this requirement MUST be ignored. This ensures that the BGP-LS-SPF node advertising the End.X is also advertising its corresponding locator with the algorithm that will be used for computing paths destined to the SID. The format of this TLV is the same as in BGP-LS:

SRv6 End.X SID TLV format

where: Type: 1106 Length: variable Endpoint Behavior: 2-octet field. The Endpoint behavior code point for this SRv6 SID as defined in Section 10.2 of . Flags: 1 octet. The definition of flags are same as that in IS-IS SRv6 End.X SID sub-TLV () and the OSPFv3 SRv6 End.X SID sub-TLV (). Algorithm: 1-octet field. Algorithm associated with the SID. The algorithm associated with the SRv6 Locator from which the SID is allocated. Weight: 1-octet field. The value represents the weight of the SID for the purpose of load balancing. The use of the weight is defined in . Reserved: 1-octet field that MUST be set to 0 when originated and ignored on receipt. SID: 16-octet field. This field encodes the advertised SRv6 SID as a 128-bit value. Sub-TLVs: Used to advertise sub-TLVs that provide additional attributes for the specific SRv6 SID. The End.X SID advertised by this TLV is used for SRv6 TE path computation by an SRv6‑capable node or controller.

L2 Bundle Member Attributes TLV The L2 Bundle Member Attributes TLV, defined in , identifies an L2 Bundle Member link, which in turn is associated with a parent L3 link. This TLV is useful when entities external to BGP-LS-SPF wish to control traffic flows on the individual physical links that comprise the Layer 2 interface bundle. The network deployed BGP-LS-SPF may include trunk links. Therefore, BGP-LS-SPF MAY support this TLV to advertise L2 bundle member link attributes. The L2 Bundle Member Attributes TLV is an optional TLV of BGP-LS-SPF Link Attribute with BGP-LS-SPF Link NLRI that MAY be advertised by an SRv6-capable node. This TLV MAY include sub-TLVs that describe attributes associated with the bundle member. The identified bundle member represents a unidirectional path from the originating node to the neighbor specified in the parent L3 link. Multiple L2 Bundle Member Attributes TLVs MAY be associated with a BGP-LS-SPF Link NLRI. Advertisement of this TLV implies that the identified link is a member of the L2 Bundle associated with the Parent L3 Link, and the member link is operationally up. Therefore, advertisements MUST be withdrawn if the link becomes operationally down or it is no longer a member of the identified L2 Bundle. The format of this TLV is the same as that in BGP-LS:

L2 Bundle Member Attributes TLV format

Where: Type: 1172 Length: Variable. L2 Bundle Member Descriptor: 4-octet field that carries a link-local identifier as defined in . Link attribute Sub-TLVs: variable, the detailed description of these Sub-TLVs is specified in . In Trunk or LAG scenarios, SRv6‑capable nodes or controllers MAY consider the attributes of these Layer‑2 links in SRv6 TE path computation.

SRv6 Link MSD Types The Link MSD TLV defined in is used to advertise the limits and the SRH operations supported on the specific link by the SRv6-capable node. BGP-LS-SPF SHOULD support this TLV to advertise the limits and operations supported on the specific link to enable segment routing. The SRv6 MSD types specified in are also used with the BGP-LS-SPF Link MSD TLV, as these code points are shared between the IS-IS, OSPF, and BGP-LS-SPF. The SRv6 Link MSD TLV is an optional TLV of BGP-LS-SPF Link Attributes that MAY be advertised by an SRv6-capable node. This TLV MUST be advertised only once in the attributes of BGP-LS-SPF Link NLRI. When multiple SRv6 Link MSD TLVs are received from a given node, the receiver MUST use the first occurrence of the TLV in the attributes of BGP-LS-SPF Link NLRI. The format and different MSD Types of SRv6 Link MSD TLV is the same as , the Type of Link MSD TLV is 267. A SRv6‑capable node or controller SHOULD consider the link MSD limits when computing SRv6 TE paths. In addition to link MSD, the link MTU also SHOULD be considered during SRv6 TE path calculation and forwarding to avoid oversized packets.

SRv6 Prefix Attribute TLVs The following BGP-LS SRv6 Prefix Attributes introduced in are applicable to BGP-LS-SPF. Prefix Attribute TLVs for SRv6 with BGP-LS-SPF

Type	Description	Reference
1162	SRv6 Locator TLV	RFC 9514

These SRv6 Prefix Attribute TLVs are advertised associated with the BGP-LS-SPF Prefix NLRI.

SRv6 Locator TLV The SRv6 Locator TLV defined in is used to advertise the locators supported by each node. Locator is the key component of SRv6 SID, BGP-LS-SPF SHOULD support this TLV to enable segment routing. A node is provisioned with one or more locators supported by that node. Locators are covering prefixes for the set of SIDs provisioned on that node. If more than one locator is provisioned for a node, then each locator is advertised as an individual BGP-LS-SPF Prefix NLRI object along with the SRv6 Locator TLV in its BGP-LS-SPF Attribute. Only one SRv6 Locator TLVs SHOULD be advertised in the BGP-LS-SPF Attribute, associating with the BGP-LS-SPF prefix NLRI. This TLV MUST be advertised only once in the BGP-LS-SPF Attribute associated with the BGP-LS-SPF Prefix NLRI. When multiple SRv6 Locator TLVs are received from a given node in the BGP-LS-SPF Attribute, the receiver MUST use the first occurrence of the TLV. The format of the SRv6 Locator TLV is the same as BGP-LS ().

SRv6 Locator TLV format

where: Type: 1162 Length: variable Flags: 1 octet of flags. Currently, the flags field is not used and MUST be set to zero on transmission and MUST be ignored on receipt. Algorithm: 1-octet field. Algorithm associated with the SID, as defined in the "IGP Algorithm Types" registry . Reserved: 2-octet field. The value MUST be set to 0 when originated and ignored on receipt. Metric: 4-octet field. The value of the metric for the locator. Sub-TLVs: Used to advertise sub-TLVs that provide additional attributes for the given SRv6 Locator. Currently, none are defined. Since specifies that the Prefix Metric (TLV 1155) MUST be advertised associated with the prefix NLRI to be considered for route calculation, therefore the Metric field in this TLV is no longer usable. The Metric field in this TLV SHOULD be set to 0 and MUST be ignored on receipt. As specified in , this TLV is used by SRv6‑capable nodes to determine the reachability of SRv6 SIDs before installing them in the forwarding plane.

SRv6 SID NLRI The SRv6 SID NLRI defined in is used to carry the SRv6 SID information. When SRv6 SIDs need to be advertised in BGP-SPF, the following NLRI type and TLVs for SRv6 SID are applicable to BGP-LS-SPF SAFI: SRv6 SID NLRI with BGP-LS-SPF

Type	NLRI Type	Reference
6	SRv6 SID NLRI	RFC 9514
518	SRv6 SID Information TLV	RFC 9514

The format of SRv6 SID NLRI is the same as that in BGP-LS (). An SRv6-enabled node SHOULD advertise at least one SRv6 SID associated with an End behavior encapsulated in the SRv6 NLRI for itself as specified in . Per , the protocol-ID field in the SRv6 SID NLRI represents the origin of the advertised NLRI. For SRv6 SID NLRI, the specified Protocol-ID MUST be the direct protocol (4), as defiend in . The procedures for BGP Self-Originated NLRI specified in is also applicable to SRv6 SID NLRI. An SRv6-enabled node MAY advertise multiple instances of the SRv6 SID NLRI -- one for each of the SRv6 SIDs to be advertised.

SRv6 SID Information TLV The SRv6 SID Information TLV is used to carry the SRv6 SID that do not require a particular neighbor in an SRv6 SID NLRI. This TLV SHOULD be supported in BGP-LS-SPF to advertise SRv6 SIDs of each node. SRv6 SID Information TLV is a mandatory TLV in SRv6 NLRI. For each SRv6 SID NLRI, it MUST contain a single SRv6 SID Information TLV. When multiple SRv6 SID Information TLVs are received from a given node in a BGP-LS-SPF SRv6 SID NLRI for the same SID, the receiver MUST use the first occurrence of the TLV in the NLRI. The format of SRv6 SID Information TLV is the same as .

SRv6 SID Information TLV format

where: Type: 518 Length: 16 SID: 16-octet field. This field encodes the advertised SRv6 SID as a 128-bit value. The SRv6 SID MUST be allocated from its associated locator. SRv6 SIDs that are NOT allocated from the associated locator MUST be ignored. The SID information carried in this TLV is used by an SRv6‑capable node, controller or PCE to compute SRv6 TE paths.

SRv6 SID Attribute TLVs

SRv6 Endpoint Behavior TLV The Endpoint Behavior TLV defined in is used to advertise the behaviors associated with a SID. The BGP-LS-SPF SHOULD support this TLV to enable the advertisement of behaviors associated with an SRv6 SID. The SRv6 Endpoint Behavior TLV is a mandatory TLV that MUST be included once in the BGP-LS Attribute associated with the BGP-LS-SPF SRv6 SID NLRI. When multiple SRv6 Endpoint Behavior TLVs are received from a given node in the BGP-LS Attribute, the receiver MUST use the first occurrence of the TLV. The format of SRv6 Endpoint is the same as that in BGP-LS.

SRv6 Endpoint Behavior TLV format

Where: Type: 1250 Length: 4 Endpoint Behavior: 2-octet field. The Endpoint behavior code point for this SRv6 SID. Flags: 1 octet of flags. No flags are currently defined, and this field MUST be set to 0 on transmission and MUST be ignored on receipt. Algorithm: 1-octet field. Algorithm associated with the SID. Supported behavior values for this TLV are defined in of this document. Unsupported or unrecognized behavior values are ignored by the receiver. The endpoint behavior carried in this TLV is used together with the SRv6 SID information TLV for SRv6 TE path computation by an SRv6‑capable node or controller.

SRv6 SID Structure TLV The SRv6 SID Structure TLV defined in is used to advertise the length of each individual part of the SRv6 SID as defined in . BGP-LS-SPF MAY support this TLV to indicate the length of each individual part of the SRv6 SID of BGP-LS-SPF, which is useful in some scenarios using compressed SID. It is an optional TLV that MAY be used in the BGP-LS-SPF Attribute for SRv6 SID NLRI and as a sub-TLV of the SRv6 End.X SID TLV. The SRv6 SID Structure TLV MUST NOT appear more than once in the BGP-LS-SPF Attribute. If it appears more than once in the BGP-LS-SPF Attribute, then its parent SID TLV or NLRI MUST be ignored by the receiver. The format of SRv6 Structure TLV is the same as that in BGP-LS.

SRv6 SID Structure TLV format

where: Type: 1252 Length: 4 LB Length: 1-octet field. SRv6 SID Locator Block length in bits. LN Length: 1-octet field. SRv6 SID Locator Node length in bits. Fun. Length: 1-octet field. SRv6 SID Function length in bits. Arg. Length: 1-octet field. SRv6 SID Argument length in bits. The sum of all four sizes advertised in SRv6 SID Structure TLV MUST be less than or equal to 128 bits. If the sum of all four sizes advertised in the SRv6 SID Structure sub-TLV is larger than 128 bits, the parent TLV or NLRI MUST be ignored by the receiver. The SRv6 SID Structure sub-TLV is intended for informational use by the control and management planes, not for SRv6 TE path computation. The typical use cases for this information are described in the and .

SRv6 SID NLRI SPF Status TLV defines the SPF Status TLV for Node NLRI, Link NLRI, and Prefix NLRI. This TLV is defined to indicate the status of the node/link/prefix with respect to the BGP SPF calculation. It is used to expedite convergence for link/node/prefix failures or unreachability. A End SID is associated to a specific Node, when a node is unreachable, then the corresponding End SIDs are also unreachable. Therefore, the SPF Status TLV MAY be included in the SRv6 SID NLRI to indicate the unreachability of the NLRI. If the SPF Status TLV is not included with the SRv6 SID NLRI, the SID is considered to be reachable. The format of SRv6 SID NLRI SPF Status TLV is defined in , the values of it is shown in the following table. SRv6 SID NLRI Attribute Status Values

Value	Description
0	Reserved
1	SID unreachable with respect to BGP SPF
2-254	Unassigned
255	Reserved

The SPF Status TLV is used by an SRv6-capable node, controller, or PCE to exclude specific SRv6 SIDs from SRv6 TE path computation.

BGP-LS Attribute Sequence Number TLV Per the BGP-LS Attribute Sequence Number TLV of the BGP-LS-SPF NLRI types is defined to assure the most recent version of a given NLRI is used in the SPF computation. The Sequence Number TLV is mandatory for BGP-LS-SPF NLRI. The format of this TLV is introduced in . Since the SRv6 SID NLRI is a kind of the BGP-LS-SPF NLRI, the Sequence Number TLV is also mandatory for BGP-LS-SPF NLRI and MUST be included once. If the Sequence Number TLV is not received, then the corresponding NLRI is considered as malformed and MUST be handled as described in . An implementation SHOULD log an error for further analysis. The Sequence Number TLV is used by an SRv6-capable node, controller, or PCE to identify the most recent version of an SRv6 SID NLRI for SRv6 TE path computation.

Advertising Endpoint Behaviors Endpoint behaviors are defined in . The code points for the Endpoint behaviors are defined in the "SRv6 Endpoint Behaviors" registry of . This section lists the Endpoint behaviors and their code points, which MAY be advertised by BGP-LS-SPF and the TLVs in which each type MAY appear. SRv6 Endpoint Behaviors in BGP-LS-SPF

Endpoint Behavior	Endpoint Behavior Code Point	End SID	End.X SID
End (PSP, USP, USD)	1-4, 28-31	Y	N
End.X (PSP, USP, USD)	5-8, 32-35	N	Y

Security Considerations This document introduces no additional security vulnerabilities in addition to the ones as described in .

IANA Considerations This document has no IANA actions.

Acknowledgements This document refers extensively to the content of , , , and . The authors would like to thank the authors of these RFCs: James Uttaro, Hani Elmalky, Arjun Sreekantiah, Les Ginsberg, Shunwan Zhuang, Zhenbin Li, Zhibo Hu, Ketan Talaulikar, Peter Psenak, Clarence Filsfils, Ahmed Bashandy, Bruno Decraene, Stefano Previdi, Hannes Gredler, and Mach(Guoyi) Chen. The authors also would like to thank Acee Lindem for his valuable comments and suggestions.

References Normative References Many Massively Scaled Data Centers (MSDCs) have converged on simplified Layer 3 (L3) routing. Furthermore, requirements for operational simplicity have led many of these MSDCs to converge on BGP as their single routing protocol for both fabric routing and Data Center Interconnect (DCI) routing. This document describes extensions to BGP for use with BGP Link State (BGP-LS) distribution and the Shortest Path First (SPF) algorithm. In doing this, it allows BGP to be efficiently used as both the underlay protocol and the overlay protocol in MSDCs. This document discusses the Border Gateway Protocol (BGP), which is an inter-Autonomous System routing protocol. The primary function of a BGP speaking system is to exchange network reachability information with other BGP systems. This network reachability information includes information on the list of Autonomous Systems (ASes) that reachability information traverses. This information is sufficient for constructing a graph of AS connectivity for this reachability from which routing loops may be pruned, and, at the AS level, some policy decisions may be enforced. BGP-4 provides a set of mechanisms for supporting Classless Inter-Domain Routing (CIDR). These mechanisms include support for advertising a set of destinations as an IP prefix, and eliminating the concept of network "class" within BGP. BGP-4 also introduces mechanisms that allow aggregation of routes, including aggregation of AS paths. This document obsoletes RFC 1771. [STANDARDS-TRACK] 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. Segment Routing (SR) allows for a flexible definition of end-to-end paths by encoding paths as sequences of topological subpaths, called "segments". These segments are advertised by routing protocols, e.g., by the link-state routing protocols (IS-IS, OSPFv2, and OSPFv3) within IGP topologies. This document defines extensions to the Border Gateway Protocol - Link State (BGP-LS) address family in order to carry SR information via BGP. Segment Routing over IPv6 (SRv6) allows for a flexible definition of end-to-end paths within various topologies by encoding paths as sequences of topological or functional sub-paths called "segments". These segments are advertised by various protocols such as BGP, IS-IS, and OSPFv3. This document defines extensions to BGP - Link State (BGP-LS) to advertise SRv6 segments along with their behaviors and other attributes via BGP. The BGP-LS address-family solution for SRv6 described in this document is similar to BGP-LS for SR for the MPLS data plane, which is defined in RFC 9085. 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. This document defines a way for a Border Gateway Protocol - Link State (BGP-LS) speaker to advertise multiple types of supported Maximum SID Depths (MSDs) at node and/or link granularity. Such advertisements allow entities (e.g., centralized controllers) to determine whether a particular Segment Identifier (SID) stack can be supported in a given network. The Segment Routing (SR) architecture allows a flexible definition of the end-to-end path by encoding it as a sequence of topological elements called "segments". It can be implemented over the MPLS or the IPv6 data plane. This document describes the IS-IS extensions required to support SR over the IPv6 data plane. This document updates RFC 7370 by modifying an existing registry. The Segment Routing (SR) architecture allows a flexible definition of the end-to-end path by encoding it as a sequence of topological elements called segments. It can be implemented over an MPLS or IPv6 data plane. This document describes the OSPFv3 extensions required to support SR over the IPv6 data plane. Informative References This document describes how the existing IPv6 mechanisms for ping and traceroute can be used in a Segment Routing over IPv6 (SRv6) network. The document also specifies the OAM flag (O-flag) in the Segment Routing Header (SRH) for performing controllable and predictable flow sampling from segment endpoints. In addition, the document describes how a centralized monitoring system performs a path continuity check between any nodes within an SRv6 domain. The Segment Routing over IPv6 (SRv6) Network Programming framework enables a network operator or an application to specify a packet processing program by encoding a sequence of instructions in the IPv6 packet header. Each instruction is implemented on one or several nodes in the network and identified by an SRv6 Segment Identifier in the packet. This document defines the SRv6 Network Programming concept and specifies the base set of SRv6 behaviors that enables the creation of interoperable overlays with underlay optimization. Segment Routing (SR) allows a flexible definition of end-to-end paths within IGP topologies by encoding paths as sequences of topological subpaths called "segments". These segments are advertised by the link-state routing protocols (IS-IS and OSPF). This document describes the OSPFv2 extensions required for Segment Routing. A node steers a packet through a controlled set of instructions, called segments, by prepending the packet with a list of segment identifiers (SIDs). A segment can represent any instruction, topological or service based. SR segments allow steering a flow through any topological path and service chain while maintaining per-flow state only at the ingress node of the SR domain. This document describes an extension to Border Gateway Protocol - Link State (BGP-LS) for advertisement of BGP Peering Segments along with their BGP peering node information so that efficient BGP Egress Peer Engineering (EPE) policies and strategies can be computed based on Segment Routing. 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. This document specifies routing extensions in support of carrying link state information for Generalized Multi-Protocol Label Switching (GMPLS). This document enhances the routing extensions required to support MPLS Traffic Engineering (TE). [STANDARDS-TRACK]

Contributors Huawei Technologies

China wangyifan82@huawei.com

China Satellite Network Innovation Research Institute Co., Ltd.

China yangf4@chinasatnet.com.cn