Advertisement of Segment Routing Policies using BGP Link-State

This section defines the various TLVs which enable the headend to report the state at the SR Policy candidate path level. These TLVs (and their sub-TLVs) are carried in the optional non-transitive BGP-LS Attribute defined in associated with the SR Policy Candidate Path NLRI type. The detailed procedures for the advertisement are described in .

The SR Binding SID (BSID) is an optional TLV that is used to report the BSID and its attributes for the SR Policy candidate path. The TLV MAY also optionally contain the Specified BSID value for reporting as described in section 6.2.3 of . Only a single instance of this TLV is advertised for a given candidate path. If multiple instances are present, then the first one is considered valid and the rest are ignored. The TLV has the following format:

Type: 1201 Length: variable (valid values are 12 or 36 octets) BSID Flags: 2-octet field that indicates attribute and status of the Binding SID (BSID) associated with this candidate path. The following bit positions are defined and the semantics are described in detail in section 6.2 of . Other bits MUST be cleared by the originator and MUST be ignored by a receiver.

D-Flag: Indicates the dataplane for the BSIDs and if they are 16 octet SRv6 SID (when set) or are 4 octet SR/MPLS label value (when clear). B-Flag: Indicates the allocation of the value in the BSID field when set and indicates that BSID is not allocated when clear. U-Flag: Indicates the specified BSID value is unavailable when set. When clear it indicates that this candidate path is using the specified BSID. This flag is ignored when there is no specified BSID. L-Flag: Indicates the BSID value is from the Segment Routing Local Block (SRLB) of the headend node when set and is from the local dynamic label pool when clear. F-Flag: Indicates the BSID value is one allocated from dynamic label pool due to fallback (e.g. when specified BSID is unavailable) when set and indicates that there has been no fallback for BSID allocation when clear. RESERVED: 2 octets. MUST be set to 0 by the originator and MUST be ignored by a receiver. Binding SID: It indicates the operational or allocated BSID value based on the status flags. Specified BSID: It is used to report the explicitly specified BSID value regardless of whether it is successfully allocated or not. The field is set to value 0 when BSID has not been specified. The BSID fields above depend on the dataplane (SRv6 or MPLS) indicated by the D-Flag. If D-Flag set (SRv6 dataplane), then the length of the BSID fields is 16 octets. If the D-Flag is clear (MPLS dataplane), then the length of the BSID fields is 4 octets. When carrying the MPLS Label, as shown in the figure below, the TC, S, and TTL (total of 12 bits) are RESERVED and MUST be set to 0 by the originator and MUST be ignored by a receiver.

In the case of an SRv6, the SR Binding SID sub-TLV does not have the ability to signal the SRv6 Endpoint Behavior or the structure of the SID. Therefore, the SR Binding SID sub-TLV SHOULD NOT be used for the advertisement of an SRv6 Binding SID. Instead, the SRv6 Binding SID TLV defined in SHOULD be used for signaling of an SRv6 Binding SID. The use of the SR Binding SID sub-TLV for advertisement of the SRv6 Binding SID has been deprecated, and is documented here only for backward compatibility with implementations that followed early versions of this specification.

The SRv6 Binding SID (BSID) is an optional TLV that is used to report the SRv6 BSID and its attributes for the SR Policy candidate path. The TLV MAY also optionally contain the Specified SRv6 BSID value for reporting as described in section 6.2.3 of . Multiple instances of this TLV may be used to report each of the SRv6 BSIDs associated with the candidate path. The TLV has the following format:

Type: 1212 Length: variable BSID Flags: 2-octet field that indicates attribute and status of the Binding SID (BSID) associated with this candidate path. The following bit positions are defined and the semantics are described in detail in section 6.2 of . Other bits MUST be cleared by the originator and MUST be ignored by a receiver.

B-Flag: Indicates the allocation of the value in the BSID field when set and indicates that BSID is not allocated when clear. U-Flag: Indicates the specified BSID value is unavailable when set. When clear it indicates that this candidate path is using the specified BSID. This flag is ignored when there is no specified BSID. F-Flag: Indicates the BSID value is one allocated dynamically due to fallback (e.g. when specified BSID is unavailable) when set and indicates that there has been no fallback for BSID allocation when clear. RESERVED: 2 octets. MUST be set to 0 by the originator and MUST be ignored by a receiver. Binding SID: It indicates the operational or allocated BSID value based on the status flags. Specified BSID: It is used to report the explicitly specified BSID value regardless of whether it is successfully allocated or not. The field is set to value 0 when BSID has not been specified. Sub-TLVs: variable and contains any other optional attributes associated with the SRv6 BSID. The SRv6 Endpoint Behavior TLV (1250) and the SRv6 SID Structure TLV (1252) MAY optionally be used as sub-TLVs of the SRv6 Binding SID TLV to indicate the SRv6 Endpoint behavior and SID structure for the Binding SID value in the TLV. defines SRv6 Endpoint Behavior TLV And SRv6 SID Structure TLV.

The SR Candidate Path State TLV provides the operational status and attributes of the SR Policy at the candidate path level. Only a single instance of this TLV is advertised for a given candidate path. If multiple instances are present, then the first one is considered valid and the rest are ignored. The TLV has the following format:

Type: 1202 Length: 8 octets Priority: 1-octet value which indicates the priority of the candidate path. Refer Section 2.12 of . RESERVED: 1 octet. MUST be set to 0 by the originator and MUST be ignored by a receiver. Flags: 2-octet field that indicates attribute and status of the candidate path. The following bit positions are defined and the semantics are described in section 5 of unless stated otherwise for individual flags. Other bits MUST be cleared by the originator and MUST be ignored by a receiver.

S-Flag: Indicates the candidate path is in an administrative shut state when set and not in administrative shut state when clear. A-Flag: Indicates the candidate path is the active path (i.e. one provisioned in the forwarding plane as specified in section 2.9 of ) for the SR Policy when set and not the active path when clear. B-Flag: Indicates the candidate path is the backup path (i.e. one identified for path protection of the active path as specified in section 9.3 of ) for the SR Policy when set and not the backup path when clear. E-Flag: Indicates that the candidate path has been evaluated for validity (e.g. headend may evaluate candidate paths based on their preferences) when set and has not been evaluated for validity when clear. V-Flag: Indicates the candidate path has at least one valid SID-List when set and indicates no valid SID-List is available or evaluated when clear. When the E-Flag is clear (i.e. the candidate path has not been evaluated), then this flag MUST be set to 0 by the originator and ignored by the receiver. O-Flag: Indicates the candidate path was instantiated by the headend due to an on-demand nexthop trigger based on a local template when set and that the candidate path has not been instantiated due to on-demand nexthop trigger when clear. Refer to section 8.5 of for details. D-Flag: Indicates the candidate path was delegated for computation to a PCE/controller when set and indicates that the candidate path has not been delegated for computation when clear. C-Flag: Indicates the candidate path was provisioned by a PCE/controller when set and indicates that the candidate path was not provisioned by a PCE/controller when clear. I-Flag: Indicates the candidate path is to perform the "drop upon invalid" behavior when no other valid candidate path is available for this SR Policy when the flag is set. Refer to section 8.2 of for details. When clear, it indicates that the candidate path is not enabled for the "drop upon invalid" behavior. T-Flag: Indicates the candidate path has been marked as eligible for use as a transit policy on the headend when set and not eligible for use as a transit policy when clear. Transit policy is a policy whose BSID can be used in the segment list of another SR Policy. Refer to section 8.3 of for steering into a transit policy using its BSID. U-Flag: Indicates that this candidate path is reported as active and is dropping traffic as a result of the "drop upon invalid" behavior being activated for the SR Policy when set. When clear, it indicates that the candidate path is not dropping traffic as a result of the "drop upon invalid" behavior. Refer to section 8.2 of for details. Preference: 4-octet value which indicates the preference of the candidate path. Refer to section 2.7 of for details.

The SR Policy Name TLV is an optional TLV that is used to carry the symbolic name associated with the SR Policy. Only a single instance of this TLV is advertised for a given candidate path. If multiple instances are present, then the first one is considered valid and the rest are ignored. The TLV has the following format:

Type: 1213 Length: variable SR Policy Name: Symbolic name for the SR Policy without a NULL terminator as specified in section 2.1 of . It is RECOMMENDED that the size of the symbolic name be limited to 255 bytes. Implementations MAY choose to truncate long names to 255 bytes when signaling via BGP-LS.

The SR Candidate Path Name TLV is an optional TLV that is used to carry the symbolic name associated with the candidate path. Only a single instance of this TLV is advertised for a given candidate path. If multiple instances are present, then the first one is considered valid and the rest are ignored. The TLV has the following format:

Type: 1203 Length: variable Candidate Path Name: Symbolic name for the SR Policy candidate path without a NULL terminator as specified in section 2.6 of . It is RECOMMENDED that the size of the symbolic name be limited to 255 bytes. Implementations MAY choose to truncate long names to 255 bytes when signaling via BGP-LS.

The SR Candidate Path Constraints TLV is an optional TLV that is used to report the constraints associated with the candidate path. The constraints are generally applied to a dynamic candidate path which is computed either by the headend or may be delegated to a controller. The constraints may also be applied to an explicit path where the computation entity is expected to validate that the path satisfies the specified constraints and if not the path is to be invalidated (e.g., due to topology changes). Only a single instance of this TLV is advertised for a given candidate path. If multiple instances are present, then the first one is considered valid and the rest are ignored. The TLV has the following format:

Type: 1204 Length: variable Flags: 2-octet field that indicates the constraints that are being applied to the candidate path. The following bit positions are defined and the other bits MUST be cleared by the originator and MUST be ignored by a receiver.

D-Flag: Indicates that the candidate path uses SRv6 dataplane when set and SR/MPLS dataplane when clear P-Flag: Indicates that the candidate path prefers the use of only protected SIDs when set and indicates that the candidate path does not prefer the use of only protected SIDs when clear. This flag is mutually exclusive with the U-Flag (i.e., both these flags cannot be set at the same time). U-Flag: Indicates that the candidate path prefers the use of only unprotected SIDs when set and indicates that the candidate path does not prefer the use of only unprotected SIDs when clear. This flag is mutually exclusive with the P-Flag (i.e., both these flags cannot be set at the same time). A-Flag: Indicates that the candidate path uses only the SIDs belonging to the specified SR Algorithm when set and indicates that the candidate path does not use only the SIDs belonging to the specified SR Algorithm when clear. T-Flag: Indicates that the candidate path uses only the SIDs belonging to the specified topology when set and indicates that the candidate path does not use only the SIDs belonging to the specified topology when clear. S-Flag: Indicates that the use of protected (P-Flag) or unprotected (U-Flag) SIDs becomes a strict constraint instead of a preference when set and indicates that there is no strict constraint (and only a preference) when clear. F-Flag: Indicates that the candidate path is fixed once computed and not modified except on operator intervention and indicates that the candidate path may be modified as part of recomputation when clear. H-Flag: Indicates that the candidate path uses only adjacency SIDs and traverses hop-by-hop over the links corresponding to those adjacency SIDs when set and indicates that the candidate path is not restricted to using only hop-by-hop adjacency SIDs when clear. RESERVED1: 2 octets. MUST be set to 0 by the originator and MUST be ignored by a receiver. MTID: Indicates the multi-topology identifier of the IGP topology that is preferred to be used when the path is set up. When the T-flag is set then the path is strictly using the specified topology SIDs only. Algorithm: Indicates the algorithm that is preferred to be used when the path is set up. When the A-flag is set then the path is strictly using the specified algorithm SIDs only. The algorithm values are from IGP Algorithm Types registry under the IANA Interior Gateway Protocol (IGP) Parameters. RESERVED2: 1 octet. MUST be set to 0 by the originator and MUST be ignored by a receiver. sub-TLVs: one or more optional sub-TLVs MAY be included in this TLV to describe other constraints. These sub-TLVs are: SR Affinity Constraint, SR SRLG Constraint, SR Bandwidth Constraint, SR Disjoint Group Constraint, SR Bidirectional Group Constraint, and SR Metric Constraint. These constraint sub-TLVs are defined below.

The SR Affinity Constraint sub-TLV is an optional sub-TLV of the SR Candidate Path Constraints TLV that is used to carry the affinity constraints associated with the candidate path. The affinity is expressed in terms of Extended Admin Group (EAG) as defined in . Only a single instance of this sub-TLV is advertised for a given candidate path. If multiple instances are present, then the first one is considered valid and the rest are ignored. The sub-TLV has the following format:

Type: 1208 Length: variable, dependent on the size of the Extended Admin Group. MUST be a non-zero multiple of 4 octets. Exclude-Any-Size: one octet to indicate the size of Exclude-Any EAG bitmask size in multiples of 4 octets. (e.g. value 0 indicates the Exclude-Any EAG field is skipped, value 1 indicates that 4 octets of Exclude-Any EAG is included) Include-Any-Size: one octet to indicate the size of Include-Any EAG bitmask size in multiples of 4 octets. (e.g. value 0 indicates the Include-Any EAG field is skipped, value 1 indicates that 4 octets of Include-Any EAG is included) Include-All-Size: one octet to indicate the size of Include-All EAG bitmask size in multiples of 4 octets. (e.g. value 0 indicates the Include-All EAG field is skipped, value 1 indicates that 4 octets of Include-All EAG is included) RESERVED: 1 octet. MUST be set to 0 by the originator and MUST be ignored by a receiver. Exclude-Any EAG: the bitmask used to represent the affinities that have been excluded from the path. Include-Any EAG: the bitmask used to represent the affinities that have been included in the path. Include-All EAG: the bitmask used to represent all the affinities that have been included in the path.

The SR SRLG Constraint sub-TLV is an optional sub-TLV of the SR Candidate Path Constraints TLV that is used to carry the Shared Risk Link Group (SRLG) values that have been excluded from the candidate path. Only a single instance of this sub-TLV is advertised for a given candidate path. If multiple instances are present, then the first one is considered valid and the rest are ignored. The sub-TLV has the following format:

Type: 1209 Length: variable, dependent on the number of SRLGs encoded. MUST be a non-zero multiple of 4 octets. SRLG Values: One or more SRLG values. Each SRLG value is of 4 octets.

The SR Bandwidth Constraint sub-TLV is an optional sub-TLV of the SR Candidate Path Constraints TLV that is used to indicate the bandwidth that has been requested for the candidate path. Only a single instance of this sub-TLV is advertised for a given candidate path. If multiple instances are present, then the first one is considered valid and the rest are ignored. The sub-TLV has the following format:

Type: 1210 Length: 4 octets Bandwidth: 4 octets which specify the desired bandwidth in unit of bytes per second in IEEE floating point format .

The SR Disjoint Group Constraint sub-TLV is an optional sub-TLV of the SR Candidate Path Constraints TLV that is used to carry the disjointness constraint associated with the candidate path. The disjointness between two SR Policy Candidate Paths is expressed by associating them with the same disjoint group identifier and then specifying the type of disjointness required between their paths. The types disjointness are described in section 3 of where the level of disjointness increases in the order: link, node, SRLG, Node + SRLG. The computation is expected to achieve the highest level of disjointness requested and when that is not possible then fallback to a lesser level progressively based on the levels indicated. Only a single instance of this sub-TLV is advertised for a given candidate path. If multiple instances are present, then the first one is considered valid and the rest are ignored. The sub-TLV has the following format:

Type: 1211 Length: Variable. Minimum of 8 octets. Request Flags: one octet to indicate the level of disjointness requested as specified in the form of flags. The following flags are defined and the other bits MUST be cleared by the originator and MUST be ignored by a receiver.

S-Flag: Indicates that SRLG disjointness is requested when set and indicates that SRLG disjointness is not requested when clear. N-Flag: Indicates that node disjointness is requested when set and indicates that node disjointness is not requested when clear. L-Flag: Indicates that link disjointness is requested when set and indicates that the link disjointness is not requested when clear. F-Flag: Indicates that the computation may fallback to a lower level of disjointness amongst the ones requested when all cannot be achieved when set and indicates that fallback to a lower level of disjointness is not allowed when clear. I-Flag: Indicates that the computation may fallback to the default best path (e.g. IGP path) in case of none of the desired disjointness can be achieved when set and indicates that fallback to the default best path is not allowed when clear. Status Flags: one octet to indicate the level of disjointness that has been achieved by the computation as specified in the form of flags. The following flags are defined and the other bits MUST be cleared by the originator and MUST be ignored by a receiver.

S-Flag: Indicates that SRLG disjointness is achieved when set and indicates that SRLG disjointness is not achieved when clear. N-Flag: Indicates that node disjointness is achieved when set and indicates that node disjointness was not achieved when clear. L-Flag: Indicates that link disjointness is achieved when set and indicates that link disjointness was not achieved when clear. F-Flag: Indicates that the computation has fallen back to a lower level of disjointness than requested when set and indicates that there has been no fallback to a lower level of disjointness when clear. I-Flag: Indicates that the computation has fallen back to the best path (e.g. IGP path) and disjointness has not been achieved when set and indicates that there has been no fallback to best path when clear. X-Flag : Indicates that the disjointness constraint could not be achieved and hence path has been invalidated when set and indicates that the path has not been invalidated due to unmet disjointness constraints when clear. RESERVED: 2 octets. MUST be set to 0 by the originator and MUST be ignored by a receiver. Disjoint Group Identifier: 4-octet value that is the group identifier for a set of disjoint paths. Alternatively, this field MAY contain the entire PCEP Association Object as specified in section 6.1 of (including its optional TLVs) when PCEP is used for the signaling the SR Policy candidate path and where the BGP-LS Producer is unable to determine the group identifier that can be accommodated in a 4-octet value (since PCEP supports multiple methods of encoding an association identifier). Note that the parsing of the PCEP object is expected to be performed only by the BGP-LS Consumer (hence, outside the scope of this document) and not by any BGP Speaker as specified in . If the PCEP object size is such that the update for a single SR Policy Candidate Path NLRI would exceed the supported BGP message size by the implementation, then the PCEP Association Object MUST NOT be encoded and this sub-TLV skipped along with an error log. Refer section 5.3 of for discussion on implications of encoding large sets of information into BGP-LS.

The SR Bidirectional Group Constraint sub-TLV is an optional sub-TLV of the SR Candidate Path Constraints TLV that is used to carry the bidirectional constraint associated with the candidate path. The bidirectional relationship between two SR Policy Candidate Paths is expressed by associating them with the same bidirectional group identifier and then specifying the type of bidirectional routing required between their paths. Only a single instance of this sub-TLV is advertised for a given candidate path. If multiple instances are present, then the first one is considered valid and the rest are ignored. The sub-TLV has the following format:

Type: 1214 Length: Variable. Minimum of 8 octets. Flags: two octets to indicate the bidirectional path setup information as specified in the form of flags. The following flags are defined and the other bits MUST be cleared by the originator and MUST be ignored by a receiver.

R-Flag: Indicates that this candidate path of the SR Policy forms the reverse path when the R-Flag is set. If the R-Flag is clear, this candidate path forms the forward path. C-Flag: Indicates that the bidirectional path is co-routed when set and indicates that the bidirectional path is not co-routed when clear. RESERVED: 2 octets. MUST be set to 0 by the originator and MUST be ignored by a receiver. Bidirectional Group Identifier: 4-octet value that is the group identifier for a set of bidirectional paths. Alternatively, this field MAY contain the entire PCEP Association Object as specified in section 6.1 of (including its optional TLVs) when PCEP is used for the signaling the SR Policy candidate path and where the BGP-LS Producer is unable to determine the group identifier that can be accommodated in a 4-octet value (since PCEP supports multiple methods of encoding an association identifier). Note that the parsing of the PCEP object is expected to be performed only by the BGP-LS Consumer (hence, outside the scope of this document) and not by any BGP Speaker as specified in . If the PCEP object size is such that the update for a single SR Policy Candidate Path NLRI would exceed the supported BGP message size by the implementation, then the PCEP Association Object MUST NOT be encoded and this sub-TLV skipped along with an error log. Refer section 5.3 of for discussion on implications of encoding large sets of information into BGP-LS.

The SR Metric Constraint sub-TLV is an optional sub-TLV of the SR Candidate Path Constraints TLV that is used to report the optimization metric of the candidate path. For a dynamic path computation, it is used to report the optimization metric used along with its parameters. For an explicit path, this sub-TLV MAY be used to report the metric margin or bound to be used for validation (i.e., the path is invalidated if the metric is beyond specified values). Multiple instances of this sub-TLV may be used to report different metric type uses. The sub-TLV has the following format:

Type: 1215 Length: 12 octets Metric Type: 1-octet field which identifies the type of the metric being used. The Table 1 below lists the metric types introduced by this document along with reference for each. Where the references are for IS-IS and OSPF specifications, those metric types are defined for a link while in the SR Policy context those relate to the candidate path or the segment list. The metric type code points that may be used in this sub-TLV are also listed in of this document. Note that the metric type in this field is not taken from the "IGP Metric Type" registry from IANA "IGP Parameters" and is a separate registry that includes IGP Metric Types as well as metric types specific to SR Policy path computation. Additional metric types may be introduced by future documents. This document does not make any assumption of a smaller metric value being better than a higher metric value; that is something dependent on the semantics of the specific metric type. The document uses the words "best" and "worst" to abstract this aspect when referring to metric margins and bounds. Type 0: IGP: In IS-IS, this is known as the default metric and specified in section 3 of . This is known as metric in both OSPFv2 and OSPFv3 . Type 1: Min Unidirectional Delay: This is specified in section 4.2 of for IS-IS and in section 4.2 of for OSPFv2/OSPFv3. Type 2: TE: This is specified in section 3.7 of as the TE default metric for IS-IS, in section 2.5.5 of for OSPFv2, and in section 4 of for OSPFv3. Type 3: Hop Count: This is specified in section 7.8 of . Type 4: SID List Length: This is specified in section 4.5 of . Type 5: Bandwidth: This is specified in section 4 of . Type 6: Average Unidirectional Delay: This is specified in section 4.1 of for IS-IS and in section 4.1 of for OSPFv2/OSPFv3. Type 7: Unidirectional Delay Variation: This is specified in section 4.3 of for IS-IS and in section 4.3 of for OSPFv2/OSPFv3. Type 8: Loss: This is specified in section 4.4 of for IS-IS and in section 4.4 of for OSPFv2/OSPFv3. Types 128 to 255 (both inclusive): User Defined: This is specified for IS-IS and OSPF in section 2 of . Flags: 1-octet field that indicates the validity of the metric fields and their semantics. The following bit positions are defined and the other bits MUST be cleared by the originator and MUST be ignored by a receiver.

O-Flag: Indicates that this is the optimization metric being reported for a dynamic candidate path when set and indicates that the metric is not the optimization metric when clear. This bit MUST NOT be set in more than one instance of this TLV for a given candidate path advertisement. M-Flag: Indicates that the metric margin allowed is specified when set and indicates that metric margin allowed is not specified when clear. A-Flag: Indicates that the metric margin is specified as an absolute value when set and is expressed as a percentage of the minimum metric when clear. B-Flag: Indicates that the metric bound allowed for the path is specified when set and indicates that metric bound is not specified when clear. RESERVED: 2 octets. MUST be set to 0 by the originator and MUST be ignored by a receiver. Metric Margin: 4-octet value which indicates the metric margin when the M-flag is set. The metric margin is specified, depending on the A-flag, as either an absolute value or as a percentage of the best computed path metric based on the specified constraints for path calculation. The metric margin allows for the metric value of the computed path to vary (depending on the semantics of the specific metric type) from the best metric value possible to optimize for other factors (that are not specified as constraints) such as bandwidth availability, minimal SID stack depth, and maximizing of ECMP for the SR path computed. Metric Bound: 4-octet value which indicates the worst metric value (depending on the semantics of the specific metric type) that is allowed when the B-flag is set. If the computed path metric crosses the specified bound value then the path is considered invalid. The absolute metric margin and the metric bound values are encoded as specified for each metric type. For metric types that are smaller than 4 octets in size, the most significant bits are filled with zeros. The percentage metric margin is encoded as an unsigned integer percentage value.

The SR Segment List TLV is used to report a single SID-List of a candidate path. Multiple instances of this TLV may be used to report multiple SID-Lists of a candidate path. The TLV has the following format:

Type: 1205 Length: variable Flags: 2-octet field that indicates attribute and status of the SID-List.The following bit positions are defined and the semantics are described in detail in . Other bits MUST be cleared by the originator and MUST be ignored by a receiver.

D-Flag: Indicates the SID-List is comprised of SRv6 SIDs when set and indicates it is comprised of SR/MPLS labels when clear. E-Flag: Indicates that SID-List is associated with an explicit candidate path when set and with a dynamic candidate path when clear. All segment lists of a given candidate path MUST be either explicit or dynamic and in case of inconsistency, the receiver MAY consider them all to be dynamic. C-Flag: Indicates that SID-List has been computed for a dynamic path when set. It is always reported as set for explicit paths. When clear, it indicates that the SID-List has not been computed for a dynamic path. V-Flag: Indicates the SID-List has passed verification or its verification was not required when set and failed verification when clear. R-Flag: Indicates that the first Segment has been resolved when set and failed resolution when clear. F-Flag: Indicates that the computation for the dynamic path failed when set and succeeded (or not required in case of explicit path) when clear. A-Flag: Indicates that all the SIDs in the SID-List belong to the specified algorithm when set and indicates that not all the SIDs belong to the specified algorithm when clear. T-Flag: Indicates that all the SIDs in the SID-List belong to the specified topology (identified by the multi-topology ID) when set and indicates that not all the SIDs belong to the specified topology when clear. M-Flag: Indicates that the SID-list has been removed from the forwarding plane due to fault detection by a monitoring mechanism (e.g. BFD) when set and indicates no fault detected or monitoring is not being done when clear. RESERVED: 2 octets. MUST be set to 0 by the originator and MUST be ignored by a receiver. MTID: 2 octets that indicates the multi-topology identifier of the IGP topology that is to be used when the T-flag is set. Algorithm: 1 octet that indicates the algorithm of the SIDs used in the SID-List when the A-flag is set. The algorithm values are from IGP Algorithm Types registry under the IANA Interior Gateway Protocol (IGP) Parameters. RESERVED: 1 octet. MUST be set to 0 by the originator and MUST be ignored by a receiver. Weight: 4-octet field that indicates the weight associated with the SID-List for weighted load-balancing. Refer to section 2.2 and 2.11 of . Sub-TLVs: variable and contains the ordered set of Segments and any other optional attributes associated with the specific SID-List. The SR Segment sub-TLV (defined in ) 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 situations (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 where the SID-LIST is empty, the SR Segment List TLV SHOULD NOT include any SR Segment sub-TLVs.

The SR Segment sub-TLV describes a single segment in a SID-List. One or more instances of this sub-TLV in an ordered manner constitute a SID-List for an SR Policy candidate path. It is a sub-TLV of the SR Segment List TLV and it has the following format:

Type: 1206 Length: variable Segment Type: 1 octet which indicates the type of segment. Initial values are specified by this document (see for details). Additional segment types are possible, but out of scope for this document. RESERVED: 1 octet. MUST be set to 0 by the originator and MUST be ignored by a receiver. Flags: 2-octet field that indicates attribute and status of the Segment and its SID. The following bit positions are defined and the semantics are described in section 5 of . Other bits MUST be cleared by the originator and MUST be ignored by a receiver.

S-Flag: Indicates the presence of SID value in the SID field when set and that no value is indicated when clear. E-Flag: Indicates the SID value is explicitly provisioned value (locally on headend or via controller/PCE) when set and is a dynamically resolved value by headend when clear V-Flag: Indicates the SID has passed verification or did not require verification when set. When V-Flag is clear, it indicates the SID has failed verification. R-Flag: Indicates the SID has been resolved or did not require resolution (e.g. because it is not the first SID) when set. When R-Flag is clear, it indicates the SID has failed resolution. A-Flag: Indicates that the Algorithm indicated in the Segment descriptor is valid when set. When clear, it indicates that the headend is unable to determine the algorithm of the SID. SID: 4 octets carrying the MPLS Label or 16 octets carrying the SRv6 SID based on the Segment Type. When carrying the MPLS Label, as shown in the figure below, the TC, S, and TTL (total of 12 bits) are RESERVED and MUST be set to 0 by the originator and MUST be ignored by a receiver.

Segment Descriptor: variable size Segment descriptor based on the type of segment (refer to for details) Sub-Sub-TLVs: variable and contains any other optional attributes associated with the specific segment. The SRv6 Endpoint Behavior TLV (1250) and the SRv6 SID Structure TLV (1252) defined in are used as sub-sub-TLVs of the SR Segment sub-TLV. These two sub-sub-TLVs are used to optionally indicate the SRv6 Endpoint behavior and SID structure when advertising the SRv6 specific segment types.

Section 4 of defines multiple types of segments and their description. This section defines the encoding of the Segment Descriptors for each of those Segment types to be used in the Segment sub-TLV described previously in . The following types are currently defined and their mapping to the respective segment types defined in :

The Segment is SR-MPLS type and is specified simply as the label. The format of its Segment Descriptor is as follows:

Algorithm: 1-octet value that indicates the algorithm used for picking the SID. This is valid only when the A-flag has been set in the Segment TLV. The algorithm values are from IGP Algorithm Types registry under the IANA Interior Gateway Protocol (IGP) Parameters.

The Segment is SRv6 type and is specified simply as the SRv6 SID address. The format of its Segment Descriptor is as follows:

The Segment is SR-MPLS Prefix SID type and is specified as an IPv4 node address. The format of its Segment Descriptor is as follows:

Algorithm: 1-octet value that indicates the algorithm used for picking the SID. The algorithm values are from IGP Algorithm Types registry under the IANA Interior Gateway Protocol (IGP) Parameters. IPv4 Node Address: 4-octet value which carries the IPv4 address associated with the node

The Segment is SR-MPLS Prefix SID type and is specified as an IPv6 global address. The format of its Segment Descriptor is as follows:

Algorithm: 1-octet value that indicates the algorithm used for picking the SID. The algorithm values are from IGP Algorithm Types registry under the IANA Interior Gateway Protocol (IGP) Parameters. IPv6 Node Global Address: 16-octet value which carries the IPv6 global address associated with the node

The Segment is SR-MPLS Adjacency SID type and is specified as an IPv4 node address along with the local interface ID on that node. The format of its Segment Descriptor is as follows:

IPv4 Node Address: 4-octet value which carries the IPv4 address associated with the node Local Interface ID: 4-octet value which carries the local interface ID of the node identified by the Node Address

The Segment is SR-MPLS Adjacency SID type and is specified as a pair of IPv4 local and remote addresses. The format of its Segment Descriptor is as follows:

IPv4 Local Address: 4-octet value which carries the local IPv4 address associated with the node's interface IPv4 Remote Address: 4-octet value which carries the remote IPv4 address associated with interface on the node's neighbor. This is optional and MAY be set to 0 when not used (e.g. when identifying point-to-point links).

The Segment is SR-MPLS Adjacency SID type and is specified as a pair of IPv6 global address and interface ID for local and remote nodes. The format of its Segment Descriptor is as follows:

IPv6 Local Node Global Address: 16-octet value which carries the IPv6 global address associated with the local node Local Node Interface ID : 4-octet value which carries the interface ID of the local node identified by the Local Node Address IPv6 Remote Node Global Address: 16-octet value which carries the IPv6 global address associated with the remote node. This is optional and MAY be set to 0 when not used (e.g. when identifying point-to-point links). Remote Node Interface ID: 4-octet value which carries the interface ID of the remote node identified by the Remote Node Address. This is optional and MAY be set to 0 when not used (e.g. when identifying point-to-point links).

The Segment is SR-MPLS Adjacency SID type and is specified as a pair of IPv6 Global addresses for local and remote interface addresses. The format of its Segment Descriptor is as follows:

IPv6 Local Address: 16-octet value which carries the local IPv6 address associated with the node's interface IPv6 Remote Address: 16-octet value which carries the remote IPv6 address associated with the interface on the node's neighbor

The Segment is SRv6 END SID type and is specified as an IPv6 global address. The format of its Segment Descriptor is as follows:

Algorithm: 1-octet value that indicates the algorithm used for picking the SID. The algorithm values are from IGP Algorithm Types registry under the IANA Interior Gateway Protocol (IGP) Parameters. IPv6 Node Global Address: 16-octet value which carries the IPv6 global address associated with the node

The Segment is SRv6 END.X SID type and is specified as a pair of IPv6 global address and interface ID for local and remote nodes. The format of its Segment Descriptor is as follows:

IPv6 Local Node Global Address: 16-octet value which carries the IPv6 global address associated with the local node Local Node Interface ID: 4-octet value which carries the interface ID of the local node identified by the Local Node Address IPv6 Remote Node Global Address: 16-octet value which carries the IPv6 global address associated with the remote node. This is optional and MAY be set to 0 when not used (e.g. when identifying point-to-point links). Remote Node Interface ID: 4-octet value which carries the interface ID of the remote node identified by the Remote Node Address. This is optional and MAY be set to 0 when not used (e.g. when identifying point-to-point links).

The Segment is SRv6 END.X SID type and is specified as a pair of IPv6 Global addresses for local and remote interface addresses. The format of its Segment Descriptor is as follows:

The SR Segment List Metric sub-TLV reports the computed metric of the specific SID-List. It is used to report the type of metric and its computed value by the computation entity (i.e., either the headend or the controller when the path is delegated) when available. More than one instance of this sub-TLV may be present in SR Segment List to report metric values of different metric types. The metric margin and bound may be optionally reported using this sub-TLV when this information is not being reported using the SR Metric Constraint sub-TLV (refer to ) at the SR candidate path level. It is a sub-TLV of the SR Segment List TLV and has the following format:

Type: 1207 Length: 16 octets Metric Type: 1-octet field which identifies the type of metric. The semantics are the same as the Metric Type field in the SR Metric Constraints sub-TLV in of this document. Flags: 1-octet field that indicates the validity of the metric fields and their semantics. The following bit positions are defined and the other bits MUST be cleared by the originator and MUST be ignored by a receiver.

M-Flag: Indicates that the metric margin allowed for this path computation is specified when set and indicates that metric margin allowed is not specified when clear. A-Flag: Indicates that the metric margin is specified as an absolute value when set and is expressed as a percentage of the minimum metric when clear. B-Flag: Indicates that the metric bound allowed for the path is specified when set and indicates that metric bound is not specified when clear. V-Flag: Indicates that the metric value computed is being reported when set and indicates that the computed metric value is not being reported when clear. RESERVED: 2 octets. MUST be set to 0 by the originator and MUST be ignored by a receiver. Metric Margin: 4-octet value which indicates the metric margin value when the M-flag is set. The metric margin is specified, depending on the A-flag, as either an absolute value or as a percentage of the best computed path metric based on the specified constraints for path calculation. The metric margin allows for the metric value of the computed path to vary (depending on the semantics of the specific metric type) from the best metric value possible to optimize for other factors (that are not specified as constraints) such as bandwidth availability, minimal SID stack depth, and maximizing of ECMP for the SR path computed. Metric Bound: 4-octet value which indicates the worst metric value (depending on the semantics of the specific metric type) that is allowed when the B-flag is set. If the computed path metric crosses the specified bound value then the path is considered invalid. Metric Value: 4-octet value which indicates the metric of the computed path when the V-flag is set. This value is available and reported when the computation is successful and a valid path is available. The absolute metric margin, metric bound, and metric values are encoded as specified for each metric type. For metric types that are smaller than 4 octets in size, the most significant bits are filled with zeros. The percentage metric margin is encoded as an unsigned integer percentage value.

The SR Segment List Bandwidth sub-TLV is an optional sub-TLV used to report the bandwidth allocated to the specific SID-List by the path computation entity. Only a single instance of this sub-TLV is advertised for a given Segment List. If multiple instances are present, then the first one is considered valid and the rest are ignored. It is a sub-TLV of the SR Segment List TLV and has the following format:

Type: 1216 Length: 4 octets Bandwidth: 4 octets which specify the allocated bandwidth in unit of bytes per second in IEEE floating point format .

The SR Segment List Identifier sub-TLV is an optional sub-TLV used to report an identifier associated with the specific SID-List. Only a single instance of this sub-TLV is advertised for a given Segment List. If multiple instances are present, then the first one is considered valid and the rest are ignored. It is a sub-TLV of the SR Segment List TLV and has the following format:

Type: 1217 Length: 4 octets Segment List Identifier: 4 octets which carry a 32-bit unsigned non-zero integer that serves as the identifier associated with the segment list. A value of 0 indicates that there is no identifier associated with the Segment List. The scope of this identifier is the SR Policy Candidate path.