Carrying SR-Algorithm in Path Computation Element Communication Protocol (PCEP)

Carrying SR-Algorithm in Path Computation Element Communication Protocol (PCEP) Cisco Systems, Inc.

Eurovea Central 3. Pribinova 10 Bratislava 811 09 Slovakia ssidor@cisco.com

Cisco Systems, Inc.

2300 East President George Richardson TX 75082 United States of America atokar@cisco.com

ZTE Corporation

No.50 Software Avenue Nanjing Jiangsu 210012 China peng.shaofu@zte.com.cn

Huawei Technologies

Huawei Campus, No. 156 Beiqing Rd. Beijing 100095 China pengshuping@huawei.com

Nokia

andrew.stone@nokia.com

RTG pce Prefix-SID Algorithm Flexible Algorithm IGP Algorithm Types This document specifies extensions to the Path Computation Element Communication Protocol (PCEP) to enhance support for Segment Routing (SR) with a focus on the use of Segment Identifiers (SIDs) and SR-Algorithms in Traffic Engineering (TE). The SR-Algorithm associated with a SID defines the path computation algorithm used by Interior Gateway Protocols (IGPs). It introduces mechanisms for PCEP peers to signal the SR-Algorithm associated with SIDs by encoding this information in Explicit Route Object (ERO) and Record Route Object (RRO) subobjects, enables SR-Algorithm constraints for path computation, and defines new metric types for the METRIC object. This document updates RFC 8664 and RFC 9603 to allow such extensions.

Introduction describes the Path Computation Element Communication Protocol (PCEP) for communication between a Path Computation Client (PCC) and a Path Computation Element (PCE) or between a pair of PCEs. and specify PCEP extensions to support Segment Routing (SR) over MPLS and IPv6 data planes, respectively. This document specifies extensions to PCEP to enhance support for SR Traffic Engineering (TE). Specifically, it focuses on the use of Segment Identifiers (SIDs) and SR-Algorithms. An SR-Algorithm associated with a SID defines the path computation algorithm used by IGPs. The PCEP extensions specified in this document are as follows:

Signaling SR-Algorithm in ERO and RRO:: Mechanisms are introduced for PCEP peers to exchange information about the SR-Algorithm associated with each SID. This includes extending SR-ERO, SR-RRO, SRv6-ERO, and SRv6-RRO subobjects to carry an Algorithm field. This document updates and to enable such encoding.
SR-Algorithm Constraint for Path Computation:: Mechanisms are defined for signaling a specific SR-Algorithm as a constraint to the PCE for path computation. This includes a new SR-Algorithm TLV carried in the Label Switched Path Attributes (LSPA) object.
Extensions to METRIC object:: Several new metric types are introduced for the METRIC object to support optimization metrics derived from Flexible Algorithm Definitions (FADs) during Flexible Algorithm path computation; their application is not restricted to Flexible Algorithms, and they may be used with Label Switched Paths (LSPs) set up using different Path Setup Types (PSTs).

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.

Terminology This document uses the following terms defined in : Explicit Route Object (ERO), Label Switched Path Attributes (LSPA), Path Computation Client (PCC), Path Computation Element (PCE), Path Computation Element Communication Protocol (PCEP), PCEP peer, PCEP speaker, Record Route Object (RRO), and Traffic Engineering Database (TED). This document uses the following term defined in : Label Switched Path (LSP). This document uses the following term defined in and : Application-Specific Link Attributes (ASLA). This document uses the following terms defined in : Node or Adjacency Identifier (NAI) and Segment Routing Database (SR-DB). This document uses the following terms defined in : Flexible Algorithm Definition (FAD) and winning FAD. Note that the base PCEP specification originally defined the use of the PCE architecture for MPLS and GMPLS networks with LSPs instantiated using the RSVP-TE signaling protocol. Over time, support for additional PSTs, such as SRv6, has been introduced . The term "LSP" is used extensively in PCEP specifications and, in the context of this document, refers to a Candidate Path within an SR Policy, which may be an SRv6 path (still represented using the LSP object as specified in ). The term "extension block" is used in this document to identify the additional bytes appended to a PCEP object, which may exist depending on the inclusion of a flag in that object The following terminologies are used in this document:

P2MP:: Point-to-Multipoint
Subobject Extension Block:: Optional, variable-length extension block for SR-ERO and SR-RRO subobjects defined in of this document.
Subobject Extension Block Flag (SEBF):: Any flag in the Flags field of SR-ERO or SR-RRO subobjects that is used to signal that the corresponding field is encoded in the Subobject Extension Block.

Motivation Existing PCEP specifications lack mechanisms to explicitly signal and negotiate SR-Algorithm capabilities and constraints. This limits the ability of PCEs to make informed path computation decisions based on the specific SR-Algorithms supported and desired within the network. The absence of an explicit SR-Algorithm specification in PCEP messages implied no specific constraint on the SR-Algorithm to be used for path computation, effectively allowing the use of SIDs with any SR-Algorithm. A primary motivation for these extensions is to enable the PCE to leverage the path computation logic and topological information derived from IGPs, including Flexible Algorithms. Aligning PCE path computation with these IGP algorithms enables network operators to obtain paths that are congruent with the underlying routing behavior, which can result in segment lists with a reduced number of SIDs. The support for SR-Algorithm constraints in PCE path computation simplifies the deployment and management of Flexible Algorithm paths in multi-domain network scenarios. The PCE and the PCC may independently compute SR-TE paths with different SR-Algorithms. This information needs to be exchanged between PCEP peers for purposes such as network monitoring and troubleshooting. In scenarios involving multiple PCEs, when a PCC receives a path from the primary PCE, it needs to be able to report the complete path information, including the SR-Algorithm, to a backup PCE. This is essential for high availability (HA) scenarios, ensuring that the backup PCE can correctly verify Prefix SIDs. The introduction of an SR-Algorithm TLV within the LSPA object allows operators to specify SR-Algorithm constraints directly, thereby refining path computations to meet specific needs, such as low-latency paths. The ability to specify an SR-Algorithm per SID in ERO and RRO is crucial for multiple reasons, for example:

SID types without algorithm specified - Certain SID types, such as Binding SIDs (BSIDs) , may not have an SR-Algorithm specified. It may be inaccurate to state that an entire end-to-end path adheres to a specific algorithm if it includes a BSID from another policy.

Topologies with two IGP domains, each using the same FAD but with differing algorithm numbers.

Object Formats

OPEN Object

SR PCE Capability Sub-TLV The SR-PCE-CAPABILITY sub-TLV is defined in to be included in the PATH-SETUP-TYPE-CAPABILITY TLV. This document defines the following flag in the SR-PCE-CAPABILITY Sub-TLV Flags field:

S (SR-Algorithm Capability) - bit 5:: If the S flag is set to 1, a PCEP speaker indicates support for the Algorithm field and the Subobject Extension Block in the SR-ERO subobject described in and the SR-Algorithm TLV described in for LSPs set up using PST 1 (Segment Routing) . It does not indicate support for these extensions for other PSTs. If the S flag is set to 0, behavior reverts to the procedures defined in existing specifications prior to the introduction of this extension.

SRv6 PCE Capability Sub-TLV The SRv6-PCE-CAPABILITY sub-TLV is defined in to be included in the PATH-SETUP-TYPE-CAPABILITY TLV. This document defines the following flag in the SRv6-PCE-CAPABILITY Sub-TLV Flags field:

SR-Algorithm Capability (S) - bit 13:: If the S flag is set to 1, a PCEP speaker indicates support for the Algorithm field in the SRv6-ERO subobject described in and the SR-Algorithm TLV described in for LSPs set up using PST 3 (SRv6) . It does not indicate support for these extensions for other PSTs. If the S flag is set to 0, behavior reverts to the procedures defined in existing specifications prior to the introduction of this extension.

SR-ERO Subobject This document updates the SR-ERO subobject format defined in with a new optional, variable-length Subobject Extension Block field. The block is used to convey additional information, such as the Algorithm field, and is designed to allow future extensibility. Further, a new A flag in the Flags field is introduced as shown in .

SR-ERO Subobject Format A new flag in the Flags field:

A (SR-Algorithm Flag):

If set by a PCEP speaker, the Subobject Extension Block MUST be included in the SR-ERO subobject, as shown in , along with the specified algorithm. The length of this block is variable and determined by subtracting the size of the fixed fields and any optional SID or NAI fields from the total subobject Length. The length of the Subobject Extension Block MUST always be a multiple of 4 bytes. If this flag is set to 0, then either:

the Subobject Extension Block is not included and processing described in applies or
the Subobject Extension Block is included (due to an SEBF in a future specifications) and the Algorithm field MUST be ignored.

This document updates the SR-ERO subobject validation defined in by extending existing validation to include the Subobject Extension Block and the A flag, as follows. On receiving an SR-ERO subobject, a PCC MUST validate that the Length field, S bit, F bit, A bit, NT field, and any present SEBFs are consistent, as follows:

If the Subobject Extension Block is included (i.e., if any SEBF, such as A or a future flag, is set to 1), the length of the subobject MUST include the size of the entire Subobject Extension Block as determined by the set of SEBFs.
- The minimum size of the Subobject Extension Block is 4 bytes when only a single SEBF (such as A) is set and may be longer (in multiples of 4 bytes) if additional SEBFs are set and require more space.
- The total subobject Length is the sum of the sizes of the fixed and optional fields (SID, NAI, etc.) and the total size of the Subobject Extension Block required by the set of SEBFs.
- The exact calculation of Length for each NT, S, F, and set of SEBFs is as follows:
  - If NT=0, the F bit MUST be 1, the S bit MUST be 0, and the Length MUST be 8 + the size of the Subobject Extension Block.
  - If NT=1, the F bit MUST be 0.
    - If the S bit is 1, the Length MUST be 8 + the size of the Subobject Extension Block.
    - If the S bit is 0, the Length MUST be 12 + the size of the Subobject Extension Block.
  - If NT=2, the F bit MUST be 0.
    - If the S bit is 1, the Length MUST be 20 + the size of the Subobject Extension Block.
    - If the S bit is 0, the Length MUST be 24 + the size of the Subobject Extension Block.
  - If NT=3, the F bit MUST be 0.
    - If the S bit is 1, the Length MUST be 12 + the size of the Subobject Extension Block.
    - If the S bit is 0, the Length MUST be 16 + the size of the Subobject Extension Block.
  - If NT=4, the F bit MUST be 0.
    - If the S bit is 1, the Length MUST be 36 + the size of the Subobject Extension Block.
    - If the S bit is 0, the Length MUST be 40 + the size of the Subobject Extension Block.
  - If NT=5, the F bit MUST be 0.
    - If the S bit is 1, the Length MUST be 20 + the size of the Subobject Extension Block.
    - If the S bit is 0, the Length MUST be 24 + the size of the Subobject Extension Block.
  - If NT=6, the F bit MUST be 0.
    - If the S bit is 1, the Length MUST be 44 + the size of the Subobject Extension Block.
    - If the S bit is 0, the Length MUST be 48 + the size of the Subobject Extension Block.
If no SEBF (including the A flag defined in this document) is set, the Length value MUST follow the requirements defined in .

Subobject Extension Block The Subobject Extension Block is an optional, extensible field in the SR-ERO subobject. Its presence is indicated by the setting of any SEBF in the subobject's Flags field (e.g., the A flag defined in this document or flags defined by future specifications).

Block length and presence:

If the A flag is set, and no other SEBF is set, the block length MUST be 4.
The block length is at least 4 bytes when present.
The block length MUST always be a multiple of 4 bytes.
The block MUST be included if any SEBF is set in the Flags field.
Future extensions may define additional SEBFs and corresponding fields, allowing the block to be increased in size beyond the initial 4 bytes as needed.

The first 4 bytes of the Subobject Extension Block are described in .

Subobject Extension Block Format

Unassigned (24 bits):: This field is reserved for future use and MUST be set to zero when sending and ignored when receiving.
Algorithm (8 bits):: The SR-Algorithm value from the "IGP Algorithm Types" registry of the "Interior Gateway Protocol (IGP) Parameters" registry group (see ).

Future extensions SHOULD first use the Unassigned portion of the initial 4 bytes to carry new information. If additional space is needed, the Subobject Extension Block may be extended in 4-byte increments. Each such extension must be indicated by a dedicated SEBF in the Flags field (similar to the A flag) and must be accompanied by capability signaling in an appropriate capability sub-TLV. The specific sub-TLV to be used is not restricted by this specification and may include, for example, the SR-PCE-CAPABILITY sub-TLV, the SRv6-PCE-CAPABILITY sub-TLV, or other capability TLVs, depending on the context of the extension. Interoperability procedures and the precise signaling mechanisms for each new SEBF and its associated capability will be defined by future specifications or procedures describing those extensions. When receiving a Subobject Extension Block longer than 4 bytes, receivers that do not recognize or have not negotiated support for additional flags MUST ignore the unknown additional bytes beyond those defined in this document.

Guidance for Future Extensions Future enhancements extending the Subobject Extension Block must:

Define a new SEBF in the Flags field to indicate the presence of a new extension and specify the corresponding capability signaling for that extension.
Specify which parts of the reserved/extension block are used and how the block length is calculated when their extension is present.
Ensure the reserved bits in the initial 4 bytes are used when possible and the block is extended only when additional space is necessary.
Have future extensions define additional SEBFs and corresponding fields, allowing the block to be increased in size beyond the initial 4 bytes as needed.

Example: Future extension introducing a Z flag and a new Z field (8 bits):

If the A flag and/or the Z flag are set, the Subobject Extension Block is included. The Z field may use 8 bits of the reserved portion. A field is only considered valid if its corresponding flag is set. For example, if the Z flag is set to 1 but the A flag is set to 0, the Z field is valid but the Algorithm field is ignored.
If space beyond the initial 4 bytes is needed, the extension document specifies the new block layout and total length. To simplify parsing, if a flag for such an extension is set, the full extended block is encoded, including the initial 4 bytes, even if the A flag is set to 0.

SRv6-ERO Subobject This document updates the SRv6-ERO subobject format defined in with the Algorithm field carved out of the Reserved field. Further, a new A flag is defined in the existing Flags field as shown in .

SRv6-ERO Subobject Format

Flags field:: A (SR-Algorithm Flag): If set by a PCEP speaker, the Algorithm field is included in the SRv6-ERO subobject as specified in . If this flag is set to 0, then the Algorithm field is absent and processing described in applies.
Reserved (8 bits):: It MUST be set to 0 while sending and ignored on receipt.
Algorithm (8 bits):: The SR-Algorithm value from the "IGP Algorithm Types" registry of the "Interior Gateway Protocol (IGP) Parameters" registry group.

SR-Algorithm TLV A new TLV for the LSPA object is introduced to carry the SR-Algorithm constraint (). This TLV MUST only be used when PST = 1 or 3 for SR-MPLS and SRv6, respectively. Only the first instance of this TLV MUST be processed; subsequent instances MUST be ignored. The format of the SR-Algorithm TLV is as follows:

SR-Algorithm TLV Format

Type (16 bits):: 66
Length (16 bits):: 4

The 32-bit value is formatted as follows.

Reserved (16 bits):

MUST be set to 0 by the sender and MUST be ignored by the receiver.

Flags (8 bits):

This document defines the following flag. The other flags MUST be set to 0 by the sender and MUST be ignored by the receiver.

S (Strict):: If set, the path computation at the PCE MUST fail if the specified SR-Algorithm constraint cannot be satisfied. If the S (Strict) bit is unset and the PCE is unable to compute a path that satisfies the specified SR-Algorithm constraint, the PCE MUST attempt to compute a path as if no SR-Algorithm constraint had been requested. This means the PCE may use any available SR-Algorithm for the computation, consistent with the default behavior in the absence of SR-Algorithm constraint.

Algorithm (8 bits):

The SR-Algorithm to be used during path computation (see ).

Extensions to METRIC Object The METRIC object is defined in . This document specifies additional types for the METRIC object to enable the encoding of optimization metric types derived from the FAD during Flexible Algorithm path computation (see ). While these new metric types are defined to support this specific use case, their use is not restricted to Flexible Algorithm path computation or to any specific PST.

T=22: Path Min Delay metric ()
T=23: P2MP Path Min Delay metric ()
T=24: Path Bandwidth metric ()
T=25: P2MP Path Bandwidth metric ()
T=128-255: User-defined metric ()

The following terminology is used and expanded along the way.

A network comprises a set of N links {Li, (i=1...N)}.
A path P of a point-to-point (P2P) LSP is a list of K links {Lpi,(i=1...K)}.
A P2MP tree T comprises a set of M destinations {Dest_j,(j=1...M)}.

Path Min Delay Metric and define the "Min/Max Unidirectional Link Delay" sub-TLV to advertise the link minimum and maximum delay in microseconds in a 24-bit field. defines the METRIC object with a 32-bit metric value encoded in IEEE floating point format (see ). The encoding for the Path Min Delay metric value is quantified in units of microseconds and encoded in IEEE floating point format. For use in the PCEP METRIC object, the 24-bit unsigned integer delay value is converted to a 32-bit IEEE floating point value. This conversion follows the procedure specified in .

P2P Path Min Delay Metric The minimum Link Delay metric is defined in and as "Min Unidirectional Link Delay". The Path Min Link Delay metric represents the measured minimum link delay value over a configurable interval. The Path Min Delay metric type of the METRIC object in PCEP represents the sum of the Min Link Delay metric of all links along a P2P path.

A Min Link Delay metric of link L is denoted by D(L).
A Path Min Delay metric for the P2P path P = Sum {D(Lpi), (i=1...K)}.

P2MP Path Min Delay Metric The P2MP Path Min Delay metric type of the METRIC object in PCEP encodes the Path Min Delay metric for the destination that observes the worst (i.e., highest value) delay metric among all destinations of the P2MP tree.

The P2P Path Min Delay metric of the path to destination Dest_j is denoted by PMDM(Dest_j).
The P2MP Path Min Delay metric for the P2MP tree T = Maximum{PMDM(Dest_j), (j=1...M)}.

Path Bandwidth Metric defines a new metric type, "Bandwidth metric", which may be advertised in their link metric advertisements. When performing Flexible Algorithm path computation as described in , procedures described in Sections and from MUST be followed with automatic metric calculation. For path computations in contexts other than Flexible Algorithm (including PSTs other than 1 or 3 for SR-MPLS and SRv6, respectively), if the Generic Metric sub-TLV with the Bandwidth metric type is not advertised for a link, the PCE implementation MAY apply a local policy to derive a metric value (similar to the procedures in Sections and of ) or the link MAY be treated as if the metric value is unavailable (e.g., by using a default value). If the Bandwidth metric value is advertised for a link, the PCE MUST use the advertised value to compute the path metric in accordance with Sections and . The Path Bandwidth metric value is encoded in IEEE floating point format (see ). For use in the PCEP METRIC object, the 24-bit unsigned integer delay value is converted to a 32-bit IEEE floating point value. This conversion follows the procedure specified in .

P2P Path Bandwidth Metric The Path Bandwidth metric type of the METRIC object in PCEP represents the sum of the Bandwidth metric of all links along a P2P path.

A Bandwidth metric of link L is denoted by B(L).
A Path Bandwidth metric for the P2P path P = Sum {B(Lpi), (i=1...K)}.

P2MP Path Bandwidth Metric The Bandwidth metric type of the METRIC object in PCEP encodes the Path Bandwidth metric for the destination that observes the worst Bandwidth metric among all destinations of the P2MP tree.

The P2P Bandwidth metric of the path to destination Dest_j is denoted by BM(Dest_j).
The P2MP Path Bandwidth metric for the P2MP tree T = Maximum{BM(Dest_j), (j=1...M)}.

User-Defined Metric defined a new metric type range for "User-defined metric", which may be advertised in their link metric advertisements. These are user defined and can be assigned by an operator for local use. User-defined metric values are encoded using the IEEE floating point format (see ). For use in the PCEP METRIC object, the 24-bit unsigned integer delay value is converted to a 32-bit IEEE floating point value. This conversion follows the procedure specified in . The metric type range was chosen to allow mapping with values assigned in the "IGP Metric-Type" registry. For example, the User-defined metric type 130 of the METRIC object in PCEP can represent the sum of the User-defined metric 130 of all links along a P2P path. User-defined metrics are equally applicable to P2P and P2MP paths.

Operation The PCEP extensions defined in Sections and of this document MUST NOT be used unless both PCEP speakers have indicated support by setting the S flag in the Path Setup Type sub-TLV corresponding to the PST of the LSP. If this condition is not met, the receiving PCEP speaker MUST respond with a PCErr message with Error-Type 19 (Invalid Operation) and Error-value 33 (Attempted use of SR-Algorithm without advertised capability). The SR-Algorithm used in this document refers to a complete range of SR-Algorithm values (0-255) if a specific section does not specify otherwise. Valid SR-Algorithm values are defined in the "IGP Algorithm Types" registry of the "Interior Gateway Protocol (IGP) Parameters" registry group. Refer to and for the definition of SR-Algorithm in SR. and describe the use of the SR-Algorithm in IGP. Note that some RFCs refer to SR-Algorithm with different names, for example, "Prefix-SID Algorithm" and "SR Algorithm".

ERO and RRO Subobjects If a PCC receives the Algorithm field in the ERO subobject within PCInitiate, PCUpd, or PCRep messages and the path received from those messages is being included in the ERO of PCRpt message, then the PCC MUST include the Algorithm field in the encoded subobjects with the received SR-Algorithm value. As per , the format of the SR-RRO subobject is the same as that of the SR-ERO subobject but without the L flag; therefore, the SR-RRO subobject may also carry the A flag and Algorithm field in the Subobject Extension Block. Similarly, as per , the format of the SRv6-RRO subobject is the same as that of the SRv6-ERO subobject but without the L flag; therefore, the SRv6-RRO subobject may also carry the A flag and Algorithm field.

SR-ERO A PCEP speaker MAY set the A flag and include the Algorithm field as part of the Subobject Extension Block in an SR-ERO subobject if the S flag has been advertised in the SR-PCE-CAPABILITY sub-TLV by both PCEP speakers. If the PCEP peer receives an SR-ERO subobject with the A flag set but the S flag was not advertised in SR-PCE-CAPABILITY sub-TLV, then it MUST consider the entire ERO as invalid, as described in . The Subobject Extension Block field in the SR-ERO subobject MUST be included after the optional SID, NAI, or SID structure, and the length of the SR-ERO subobject MUST be increased by the size of the Subobject Extension Block, as determined by the set of SEBFs. If the length and the A flag are not consistent, as specified in , the PCEP peer MUST consider the entire ERO invalid and MUST send a PCErr message with Error-Type = 10 ("Reception of an invalid object") and Error-value = 11 ("Malformed object"). If the SID value is absent (S flag is set to 1), the NAI value is present (F flag is set to 0), and the Algorithm field is set (the A flag is set to 1), the PCC is responsible for choosing the SRv6-SID value based on values specified in the NAI and Algorithm fields. If the PCC cannot find a SID index in the SR-DB, it MUST send a PCErr message with Error-Type = 10 ("Reception of an invalid object") and Error-value = 14 ("Unknown SID").

SRv6-ERO A PCEP speaker MAY set the A flag and include the Algorithm field in an SRv6-ERO subobject if the S flag has been advertised in SRv6-PCE-CAPABILITY sub-TLV by both PCEP speakers. If the PCEP peer receives an SRv6-ERO subobject with the A flag set or with the SR-Algorithm included, but the S flag was not advertised in SRv6-PCE-CAPABILITY sub-TLV, then it MUST consider the entire ERO as invalid, as described in . The Algorithm field in the SRv6-ERO subobject MUST be included in the position specified in ; the length of the SRv6-ERO subobject is not impacted by the inclusion of the Algorithm field. If the SRv6-SID value is absent (S flag is set to 1), the NAI value is present (F flag is n), and the Algorithm field is set (the A flag is set to 1), the PCC is responsible for choosing the SRv6-SID value based on values specified in the NAI and Algorithm fields. If the PCC cannot find a SID index in the SR-DB, it MUST send a PCErr message with Error-Type = 10 ("Reception of an invalid object") and Error-value = 14 ("Unknown SID").

SR-Algorithm Constraint To signal a specific SR-Algorithm constraint to the PCE, the headend MUST encode the SR-Algorithm TLV inside the LSPA object. If a PCC receives an LSPA object with the SR-Algorithm TLV as part of PCInitiate or PCUpd messages, then it MUST include an LSPA object with the SR-Algorithm TLV in a PCRpt message as part of intended-attribute-list. If a PCE receives an LSPA object with the SR-Algorithm TLV in PCRpt or PCReq, then it MUST include the LSPA object with the SR-Algorithm TLV in a PCUpd message, or a PCRep message in case of an unsuccessful path computation based on rules described in . A PCEP peer that did not advertise the S flag in the Path Setup Type sub-TLV corresponding to the LSP's PST MUST ignore the SR-Algorithm TLV on receipt. The PCE MUST NOT use Prefix SIDs associated with an SR-Algorithm other than the one specified in the SR-Algorithm constraint. If a protected Adjacency SID is used without an associated SR-Algorithm, there is a risk that the backup path may fail to forward traffic over parts of the topology that are not included in the specified SR-Algorithm. Consequently, it is NOT RECOMMENDED to use protected Adjacency SIDs without an explicitly specified SR-Algorithm. If an Adjacency SID has an associated SR-Algorithm, the PCE MUST ensure that the SR-Algorithm matches the one specified in the SR-Algorithm constraint. Other SID types, such as BSIDs, are allowed. Furthermore, the inclusion of a path BSID from another policy is permitted only if the path associated with that policy fully satisfies all the constraints of the current path computation. The specified SR-Algorithm constraint is applied to the end-to-end SR Policy path. Using different SR-Algorithm constraints or using winning FAD with different optimization metrics or constraints for the same SR-Algorithm in each domain or part of the topology in single path computation is out of the scope of this document. If the PCE is unable to find a path with the given SR-Algorithm constraint, it does not support a combination of specified constraints, or if the FAD contains constraints, optimization metrics, or other attributes, which the PCE does not support or recognize, it MUST use an empty ERO in PCInitiate for LSP instantiation, a PCUpd message if an update is required, or a NO-PATH object in PCRep to indicate that it was not able to find the valid path. If the Algorithm field value is in the range 128-255, the PCE MUST perform path computation according to the Flexible Algorithm procedures outlined in . Otherwise, the PCE MUST adhere to the path computation procedures with SID filtering as defined in . If the NO-PATH object is included in PCRep, then the PCE MAY include the SR-Algorithm TLV to indicate constraint, which cannot be satisfied as described in . SR-Algorithm does not replace the objective function defined in .

Path Computation for SR-Algorithms 0-127 The SR-Algorithm constraint acts as a filter, restricting which SIDs may be used as a result of the path computation function. Path computation is done based on optimization metric type and constraints specified in the PCEP message received from the PCC. The mechanism described in this section is applicable only to SR-Algorithm values in the range 0-127. It is not applicable to Flexible Algorithms (range 128-255), which are handled as described in . Within the 0-127 range, currently defined algorithms are 0 (Shortest Path First (SPF)) and 1 (Strict-SPF), as introduced in . Future algorithms defined within this range that do not require explicit PCEP extensions beyond the SR-Algorithm TLV may also utilize this SID filtering approach. If a PCE implementation receives a request with an SR-Algorithm value in the 0-127 range that it does not support for path computation, it MUST reject the PCEP message and send a PCErr message with Error-Type 19 (Invalid Operation) and Error-value 34 (Unsupported combination of constraints).

Path Computation for Flexible Algorithms This section is applicable only to the Flexible Algorithms range of SR-Algorithm values. The PCE performs Flexible Algorithm path computation based on topology information stored in its TED . The TED is expected to be populated with necessary information, including Flexible Algorithm Definitions (FADs), node participation, and ASLA-specific link attributes, through standard mechanisms, such as IGPs with Traffic Engineering extensions or BGP - Link State (BGP-LS) . The PCE must follow the IGP Flexible Algorithm path computation logic as described in . This includes performing the FAD selection as described in and other sections, determining the topology associated with specific a Flexible Algorithm based on the FAD, the node participation (), using ASLA-specific link attributes (), and applying other rules for Flexible Algorithm path calculation (). While defines the base procedures for IGP Flexible Algorithms, these procedures are further extended by other documents, such as ; a PCE implementation may need to support these IGP extensions to allow use of specific constraints in FAD. created an IANA registry called "IGP Flex-Algorithm Path Computation Rules" within the "Interior Gateway Protocol (IGP) Parameters" registry group with the ordered set of rules that MUST be used to prune links from the topology during the Flexible Algorithm path computation. The PCE MUST optimize the computed path based on the metric type specified in the FAD. The optimization metric type included in PCEP messages from the PCC MUST be ignored. The PCE MUST use the metric type from the FAD in messages sent to the PCC unless that metric type is not defined in PCEP or not supported by the PCEP peer. It is allowed to use SID types other than Prefix SID (e.g., Adjacency or BSID) but only from nodes participating in the specified SR-Algorithm. There are corresponding metric types in PCEP for IGP and TE metrics from FAD introduced in , but there were no corresponding metric types defined for "Min Unidirectional Link Delay" from and "Bandwidth metric" and "User-defined metric" from . of this document introduces them. Note that the defined "Path Bandwidth metric" is accumulative and is different from the BANDWIDTH object defined in . The PCE MUST use the constraints specified in the FAD and also constraints (except optimization metric type) directly included in PCEP messages from the PCC. The PCE implementation MAY decide to ignore specific constraints received from the PCC based on existing processing rules for PCEP objects and TLVs, e.g., the P flag described in and processing rules described in . If the PCE does not support a specified combination of constraints, it MUST fail path computation and respond with a PCEP message with a PCInitiate or PCUpd message with an empty ERO or PCRep with NO-PATH object. The PCC MUST NOT include constraints from the FAD in the PCEP message sent to the PCE, as it can result in undesired behavior in various cases. The PCE SHOULD NOT include constraints from the FAD in PCEP messages sent to the PCC. The combinations of the constraints specified in the FAD and constraints directly included in PCEP messages from the PCC may decrease the chance that Flexible-Algorithm-specific Prefix SIDs represent an optimal path while satisfying all specified constraints; as a result, a longer SID list may be required for the computed path. Adding more constraints on top of the FAD requires complex path computation and may reduce the benefit of this scheme.

Metric Types All the rules of processing the METRIC object as explained in and are applicable to the metric types defined in this document.

Manageability Considerations All manageability requirements and considerations listed in , , , and apply to the PCEP extensions defined in this document. In addition, the requirements and considerations listed in this section apply.

Control of Function and Policy A PCE or PCC implementation MAY allow the capability of supporting the PCEP extensions introduced in this document to be enabled or disabled as part of the global configuration. By default, this capability SHOULD be enabled.

Information and Data Models An implementation SHOULD allow the operator to view the capability defined in this document. Sections and of should be extended to include the capabilities introduced in Sections and for the PCEP peer.

Liveness Detection and Monitoring This document does not define any new mechanism that impacts the liveness detection and monitoring of PCEP.

Verify Correct Operations An implementation SHOULD also allow the operator to view FADs, which may be used in Flexible Algorithm path computation as defined in . An implementation SHOULD allow the operator to view nodes participating in the specified SR-Algorithm.

Requirements on Other Protocols and Functional Components This document does not put new requirements but relies on the necessary IGP extensions.

Impact on Network Operations This document inherits considerations from documents describing IGP Flexible Algorithm -- for example, and .

Operational Considerations This document inherits operational considerations from documents describing IGP Flexible Algorithm -- for example, and .

Security Considerations The security considerations described in , , , , , , and apply to the extensions described in this document as well. Note that this specification introduces the possibility of computing paths by the PCE based on Flexible-Algorithm-related topology attributes and based on the metric type and constraints from the FAD. This creates additional vulnerabilities, which are already described for the path computation done by IGP, like those described in the Security Considerations section of but which are also applicable to path computation done by the PCE. Hence, securing the PCEP session using Transport Layer Security (TLS) is RECOMMENDED as per the recommendations and best current practices described in .

IANA Considerations

SR Capability Flag IANA maintains a registry named "SR Capability Flag Field" within the "Path Computation Element Protocol (PCEP) Numbers" registry group to manage the Flags field of the SR-PCE-CAPABILITY sub-TLV. IANA has registered the following:

Bit	Description	Reference
5	SR-Algorithm Capability	RFC 9933

SRv6 PCE Capability Flag IANA maintains a registry named "SRv6 Capability Flag Field" within the "Path Computation Element Protocol (PCEP) Numbers" registry group to manage the Flags field of SRv6-PCE-CAPABILITY sub-TLV. IANA has registered the following:

Bit	Description	Reference
13	SR-Algorithm Capability	RFC 9933

SR-ERO Flag IANA maintains a registry named "SR-ERO Flag Field" within the "Path Computation Element Protocol (PCEP) Numbers" registry group to manage the Flags field of the SR-ERO Subobject. IANA has registered the following:

Bit	Description	Reference
7	SR-Algorithm Flag (A)	RFC 9933

SRv6-ERO Flag IANA maintains a registry named "SRv6-ERO Flag Field" within the "Path Computation Element Protocol (PCEP) Numbers" registry group to manage the Flags field of the SRv6-ERO subobject. IANA has registered the following:

Bit	Description	Reference
7	SR-Algorithm Flag (A)	RFC 9933

PCEP TLV Types IANA maintains a registry named "PCEP TLV Type Indicators" within the "Path Computation Element Protocol (PCEP) Numbers" registry group. IANA has registered the following TLV type for the new LSPA TLV specified in this document.

Value	Description	Reference
66	SR-Algorithm	RFC 9933

Metric Types IANA maintains a registry named "METRIC Object T Field" within the "Path Computation Element Protocol (PCEP) Numbers" registry group. IANA has registered these codepoints as follows:

Value	Description	Reference
22	Path Min Delay metric	RFC 9933
23	P2MP Path Min Delay metric	RFC 9933
24	Path Bandwidth metric	RFC 9933
25	P2MP Path Bandwidth metric	RFC 9933
128-255	User-defined metric	RFC 9933

PCEP-Error Object IANA has registered the following Error-Types and Error-values within the "PCEP-ERROR Object Error Types and Values" registry of the "Path Computation Element Protocol (PCEP) Numbers" registry group.

Error-Type	Meaning	Error-value	Reference
19	Invalid Operation	33: Attempted use of SR-Algorithm without advertised capability	RFC 9933
		34: Unsupported combination of constraints	RFC 9933

References Normative References Updates for PCEPS: TLS Connection Establishment Restrictions Huawei sn3rd Vigil Security, LLC Informative References IEEE Standard for Floating-Point Arithmetic IEEE IGP Algorithm Types IANA

Acknowledgements Thanks to for shepherding the document and for their contributions and suggestions. The authors would like to thank , , , , , , , , , , , , , , , , and for review and suggestions.

Contributors Ciena Corporation

mkoldych@proton.me

Cisco Systems, Inc.

zali@cisco.com

Cisco Systems, Inc.

slitkows.ietf@gmail.com

Ciena

msiva282@gmail.com

Cisco Systems, Inc.

tsaad.net@gmail.com

RtBrick Inc

mahend.ietf@gmail.com

ietfc@btconnect.com