



Internet Engineering Task Force (IETF)                      S. Sivabalan
Request for Comments: 9604                             Ciena Corporation
Category: Standards Track                                    C. Filsfils
ISSN: 2070-1721                                      Cisco Systems, Inc.
                                                             J. Tantsura
                                                                  Nvidia
                                                              S. Previdi
                                                              C. Li, Ed.
                                                     Huawei Technologies
                                                             August 2024


            Carrying Binding Label/SID in PCE-Based Networks

Abstract

   In order to provide greater scalability, network confidentiality, and
   service independence, Segment Routing (SR) utilizes a Binding Segment
   Identifier (BSID), as described in RFC 8402.  It is possible to
   associate a BSID to an RSVP-TE-signaled Traffic Engineering (TE)
   Label Switched Path (LSP) or an SR TE path.  The BSID can be used by
   an upstream node for steering traffic into the appropriate TE path to
   enforce SR policies.  This document specifies the concept of binding
   value, which can be either an MPLS label or a Segment Identifier
   (SID).  It further specifies an extension to Path Computation Element
   Communication Protocol (PCEP) for reporting the binding value by a
   Path Computation Client (PCC) to the Path Computation Element (PCE)
   to support PCE-based TE policies.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9604.

Copyright Notice

   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
     1.1.  Motivation and Example
     1.2.  Summary of the Extension
   2.  Requirements Language
   3.  Terminology
   4.  Path Binding TLV
     4.1.  SRv6 Endpoint Behavior and SID Structure
   5.  Operation
   6.  Binding SID in SR-ERO
   7.  Binding SID in SRv6-ERO
   8.  PCE Allocation of Binding Label/SID
   9.  Security Considerations
   10. Manageability Considerations
     10.1.  Control of Function and Policy
     10.2.  Information and Data Models
     10.3.  Liveness Detection and Monitoring
     10.4.  Verify Correct Operations
     10.5.  Requirements on Other Protocols
     10.6.  Impact on Network Operations
   11. IANA Considerations
     11.1.  PCEP TLV Type Indicators
       11.1.1.  TE-PATH-BINDING TLV
     11.2.  LSP Object
     11.3.  PCEP Error Type and Value
   12. References
     12.1.  Normative References
     12.2.  Informative References
   Acknowledgements
   Contributors
   Authors' Addresses

1.  Introduction

   A Path Computation Element (PCE) can compute Traffic Engineering (TE)
   paths through a network where those paths are subject to various
   constraints.  Currently, TE paths are set up using either the RSVP-TE
   signaling protocol or Segment Routing (SR).  We refer to such paths
   as "RSVP-TE paths" and "SR-TE paths", respectively, in this document.

   As per [RFC8402], SR allows a head-end node to steer a packet flow
   along a given path via an SR Policy.  As per [RFC9256], an SR Policy
   is a framework that enables the instantiation of an ordered list of
   segments on a node for implementing a source routing policy with a
   specific intent for traffic steering from that node.

   As described in [RFC8402], a Binding SID (BSID) is bound to an SR
   Policy, instantiation of which may involve a list of Segment
   Identifiers (SIDs).  Any packets received with an active segment
   equal to a BSID are steered onto the bound SR Policy.  A BSID may be
   either a local (SR Local Block (SRLB)) or a global (SR Global Block
   (SRGB)) SID.  As per Section 6.4 of [RFC9256], a BSID can also be
   associated with any type of interface or tunnel to enable the use of
   a non-SR interface or tunnel as a segment in a SID list.  In this
   document, the term "binding label/SID" is used to generalize the
   allocation of a binding value for both SR and non-SR paths.

   [RFC5440] describes the PCEP for communication between a Path
   Computation Client (PCC) and a PCE or between a pair of PCEs as per
   [RFC4655].  [RFC8231] specifies extensions to PCEP that allow a PCC
   to delegate its Label Switched Paths (LSPs) to a stateful PCE.  A
   stateful PCE can then update the state of LSPs delegated to it.
   [RFC8281] specifies a mechanism allowing a PCE to dynamically
   instantiate an LSP on a PCC by sending the path and characteristics.
   This document specifies an extension to PCEP to manage the binding of
   label/SID that can be applied to SR, RSVP-TE, and other path setup
   types.

   [RFC8664] provides a mechanism for a PCE (acting as a network
   controller) to instantiate SR-TE paths (candidate paths) for an SR
   Policy onto a head-end node (acting as a PCC) using PCEP.  For more
   information on the SR Policy Architecture, see [RFC9256].

1.1.  Motivation and Example

   A binding label/SID has local significance to the ingress node of the
   corresponding TE path.  When a stateful PCE is deployed for setting
   up TE paths, a binding label/SID reported from the PCC to the
   stateful PCE is useful for enforcing an end-to-end TE/SR policy.  A
   sample Data Center (DC) and IP/MPLS WAN use case is illustrated in
   Figure 1 with a multi-domain PCE.  In the IP/MPLS WAN, an SR-TE LSP
   is set up using the PCE.  The list of SIDs of the SR-TE LSP is {A, B,
   C, D}. The gateway Node-1 (which is the PCC) allocates a binding SID
   X and reports it to the PCE.  In the MPLS DC network, an end-to-end
   SR-TE LSP is established.  In order for the access node to steer the
   traffic towards Node-1 and over the SR-TE path in WAN, the PCE passes
   the SID stack {Y, X} where Y is the node SID of the gateway Node-1 to
   the access node and X is the BSID.  In the absence of the BSID X, the
   PCE would need to pass the SID stack {Y, A, B, C, D} to the access
   node.  This example also illustrates the additional benefit of using
   the binding label/SID to reduce the number of SIDs imposed by the
   access nodes with a limited forwarding capacity.

           SID stack
           {Y, X}              +--------------+
                               | Multi-domain |
    _ _ _ _ _ _ _ _ _ _ _ _ _ _|     PCE      |
   |                           +--------------+
   |                              ^
   |                              | Binding
   |           .-----.            | SID (X)     .-----.
   |          (       )           |            (       )
   V       .--(         )--.      |        .--(         )--.
+------+  (                 )  +-------+  (                 )  +-------+
|Access|_(  MPLS DC Network  )_|Gateway|_(    IP/MPLS WAN    )_|Gateway|
| Node | (  ==============>  ) |Node-1 | ( ================> ) |Node-2 |
+------+  (    SR-TE path   )  +-------+  (    SR-TE path   )  +-------+
           '--(         )--'    Node       '--(         )--'
               (       )        SID of         (       )
                '-----'         Node-1          '-----'
                                is Y            SIDs for SR-TE LSP:
                                                {A, B, C, D}

              Figure 1: A Sample Use Case of Binding SID

   Using the extension defined in this document, a PCC could report to
   the stateful PCE the binding label/SID it allocated via a Path
   Computation LSP State Report (PCRpt) message.  It is also possible
   for a stateful PCE to request a PCC to allocate a specific binding
   label/SID by sending a Path Computation LSP Update Request (PCUpd)
   message.  If the PCC can successfully allocate the specified binding
   value, it reports the binding value to the PCE.  Otherwise, the PCC
   sends an error message to the PCE indicating the cause of the
   failure.  A local policy or configuration at the PCC SHOULD dictate
   if the binding label/SID needs to be assigned.

1.2.  Summary of the Extension

   To implement the needed changes to PCEP, this document introduces a
   new OPTIONAL TLV that allows a PCC to report the binding label/SID
   associated with a TE LSP or a PCE to request a PCC to allocate any or
   a specific binding label/SID value.  This TLV is intended for TE LSPs
   established using RSVP-TE, SR-TE, or any other future method.  In the
   case of SR-TE LSPs, the TLV can carry a binding label (for SR-TE
   paths with the MPLS data plane) or a binding IPv6 SID (e.g., IPv6
   address for SR-TE paths with the IPv6 data plane).  Throughout this
   document, the term "binding value" means either an MPLS label or a
   SID.

   As another way to use the extension specified in this document, to
   support the PCE-based central controller [RFC8283] operation where
   the PCE would take responsibility for managing some part of the MPLS
   label space for each of the routers that it controls, the PCE could
   directly make the binding label/SID allocation and inform the PCC.
   See Section 8 for details.

   In addition to specifying a new TLV, this document specifies how and
   when a PCC and PCE can use this TLV, how they can allocate a binding
   label/SID, and the associated error handling.

2.  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 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Terminology

   The following terminologies are used in this document:

   BSID:  Binding SID

   binding label/SID:  a generic term used for the binding segment for
      both SR and non-SR paths

   binding value:  a generic term used for the binding segment as it can
      be encoded in various formats (as per the Binding Type (BT))

   LSP:  Label Switched Path

   PCC:  Path Computation Client

   PCEP:  Path Computation Element Communication Protocol

   RSVP-TE:  Resource Reservation Protocol - Traffic Engineering

   SID:  Segment Identifier

   SR:  Segment Routing

4.  Path Binding TLV

   The new optional TLV called "TE-PATH-BINDING TLV" (the format is
   shown in Figure 2) is defined to carry the binding label/SID for a TE
   path.  This TLV is associated with the LSP object specified in
   [RFC8231].  This TLV can also be carried in the PCEP-ERROR object
   [RFC5440] in case of error.  Multiple instances of TE-PATH-BINDING
   TLVs MAY be present in the LSP and PCEP-ERROR object.  The type of
   this TLV is 55.  The length is variable.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           Type = 55           |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      BT       |    Flags      |            Reserved           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ~            Binding Value (variable length)                    ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 2: TE-PATH-BINDING TLV

   The TE-PATH-BINDING TLV is a generic TLV such that it is able to
   carry binding label/SID (i.e., MPLS label or SRv6 SID).  It is
   formatted according to the rules specified in [RFC5440].  The value
   portion of the TLV comprises:

   *  Binding Type (BT): A one-octet field that identifies the type of
      binding included in the TLV.  This document specifies the
      following BT values:

      -  BT = 0: The binding value is a 20-bit MPLS label value.  The
         TLV is padded to 4-bytes alignment.  The Length MUST be set to
         7 (the padding is not included in the length, as per [RFC5440],
         Section 7.1), and the first 20 bits are used to encode the MPLS
         label value.

      -  BT = 1: The binding value is a 32-bit MPLS Label Stack Entry as
         per [RFC3032] with Label, Traffic Class (TC) [RFC5462], S, and
         TTL values encoded.  Note that the receiver MAY choose to
         override TC, S, and TTL values according to its local policy.
         The Length MUST be set to 8.

      -  BT = 2: The binding value is an SRv6 SID with the format of a
         16-octet IPv6 address, representing the binding SID for SRv6.
         The Length MUST be set to 20.

      -  BT = 3: The binding value is a 24-octet field, defined in
         Section 4.1, that contains the SRv6 SID as well as its Behavior
         and Structure.  The Length MUST be set to 28.

      Section 11.1.1 defines the IANA registry used to maintain these
      binding types as well as any future ones.  Note that multiple TE-
      PATH-BINDING TLVs with the same or different binding types MAY be
      present for the same LSP.  A PCEP speaker could allocate multiple
      TE-PATH-BINDING TLVs (of the same BT) and use different binding
      values in different domains or use cases based on a local policy.

   *  Flags: 1 octet of flags.  The following flag is defined in the new
      "TE-PATH-BINDING TLV Flag field" registry as described in
      Section 11.1.1:

       0 1 2 3 4 5 6 7
      +-+-+-+-+-+-+-+-+
      |R|             |
      +-+-+-+-+-+-+-+-+

                               Figure 3: Flags

      Where:

      -  R (Removal - 1 bit): When set, the requesting PCEP peer
         requires the removal of the binding value for the LSP.  When
         unset, the PCEP peer indicates that the binding value is added
         or retained for the LSP.  This flag is used in the PCRpt and
         PCUpd messages.  It is ignored in other PCEP messages.

      -  The unassigned flags MUST be set to 0 while sending and ignored
         on receipt.

   *  Reserved: MUST be set to 0 while sending and ignored on receipt.

   *  Binding Value: A variable-length field, padded with trailing zeros
      to a 4-octet boundary.  When the BT is 0, the 20 bits represent
      the MPLS label.  When the BT is 1, the 32 bits represent the MPLS
      label stack entry as per [RFC3032].  When the BT is 2, the 128
      bits represent the SRv6 SID.  When the BT is 3, the binding value
      also contains the SRv6 Endpoint Behavior and SID Structure,
      defined in Section 4.1.  In this document, the TE-PATH-BINDING TLV
      is considered to be empty if no binding value is present.  Note
      that the length of the TLV would be 4 in such a case.

4.1.  SRv6 Endpoint Behavior and SID Structure

   This section specifies the format of the binding value in the TE-
   PATH-BINDING TLV when the BT is set to 3 for the SRv6 Binding SIDs
   [RFC8986].  The format is shown in Figure 4.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                 SRv6 Binding SID (16 octets)                  |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         Reserved              |      Endpoint Behavior        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    LB Length  |    LN Length  | Fun. Length   |  Arg. Length  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure 4: SRv6 Endpoint Behavior and SID Structure

   The Binding Value consists of:

   *  SRv6 Binding SID: 16 octets.  The 128-bit IPv6 address,
      representing the binding SID for SRv6.

   *  Reserved: 2 octets.  It MUST be set to 0 on transmit and ignored
      on receipt.

   *  Endpoint Behavior: 2 octets.  The Endpoint Behavior code point for
      this SRv6 SID as defined by the "SRv6 Endpoint Behaviors" registry
      [RFC8986].  When the field is set with the value 0, the Endpoint
      Behavior is considered unknown.

   *  [RFC8986] defines an SRv6 SID as consisting of LOC:FUNCT:ARG,
      where a locator (LOC) is encoded in the L most significant bits of
      the SID, followed by F bits of function (FUNCT) and A bits of
      arguments (ARG).  A locator may be represented as B:N, where B is
      the SRv6 SID locator block (IPv6 prefix allocated for SRv6 SIDs by
      the operator) and N is the identifier of the parent node
      instantiating the SID, called "locator node".  The following
      fields are used to advertise the length of each individual part of
      the SRv6 SID:

      -  LB Length: 1 octet.  SRv6 SID Locator Block length in bits.

      -  LN Length: 1 octet.  SRv6 SID Locator Node length in bits.

      -  Function Length: 1 octet.  SRv6 SID Function length in bits.

      -  Arguments Length: 1 octet.  SRv6 SID Arguments length in bits.

   The total of the locator block, locator node, function, and arguments
   lengths MUST be less than or equal to 128 bits.  If this condition is
   not met, the corresponding TE-PATH-BINDING TLV is considered invalid.
   Also, if the Endpoint Behavior is found to be unknown or
   inconsistent, it is considered invalid.  A PCErr message with Error-
   Type = 10 ("Reception of an invalid object") and Error-value = 37
   ("Invalid SRv6 SID Structure") MUST be sent in such cases.

   The SRv6 SID Structure could be used by the PCE for ease of
   operations and monitoring.  For example, this information could be
   used for validation of SRv6 SIDs being instantiated in the network
   and checked for conformance to the SRv6 SID allocation scheme chosen
   by the operator as described in Section 3.2 of [RFC8986].  In the
   future, PCE could also be used for verification and for automatically
   securing the SRv6 domain by provisioning filtering rules at SR domain
   boundaries as described in Section 5 of [RFC8754].  The details of
   these potential applications are outside the scope of this document.

5.  Operation

   The binding value is usually allocated by the PCC and reported to a
   PCE via a PCRpt message (see Section 8 where PCE performs the
   allocation).  If a PCE does not recognize the TE-PATH-BINDING TLV, it
   would ignore the TLV in accordance with [RFC5440].  If a PCE
   recognizes the TLV but does not support the TLV, it MUST send a PCErr
   with Error-Type = 2 ("Capability not supported").

   Multiple TE-PATH-BINDING TLVs are allowed to be present in the same
   LSP object.  This signifies the presence of multiple binding SIDs for
   the given LSP.  In the case of multiple TE-PATH-BINDING TLVs, the
   existing instances of TE-PATH-BINDING TLVs MAY be included in the LSP
   object.  In case of an error condition, the whole message is
   rejected, and the resulting PCErr message MAY include the offending
   TE-PATH-BINDING TLV in the PCEP-ERROR object.

   If a PCE recognizes an invalid binding value (e.g., label value from
   the reserved MPLS label space), it MUST send a PCErr message with
   Error-Type = 10 ("Reception of an invalid object") and Error-value =
   2 ("Bad label value") as specified in [RFC8664].

   For SRv6 BSIDs, it is RECOMMENDED to always explicitly specify the
   SRv6 Endpoint Behavior and SID Structure in the TE-PATH-BINDING TLV
   by setting BT to 3.  This can enable the sender to have control of
   the SRv6 Endpoint Behavior and SID Structure.  A sender MAY choose to
   set the BT to 2, in which case the receiving implementation chooses
   how to interpret the SRv6 Endpoint Behavior and SID Structure
   according to local policy.

   If a PCC wishes to withdraw a previously reported binding value, it
   MUST send a PCRpt message with the specific TE-PATH-BINDING TLV with
   R flag set to 1.  If a PCC wishes to modify a previously reported
   binding, it MUST withdraw the former binding value (with R flag set
   in the former TE-PATH-BINDING TLV) and include a new TE-PATH-BINDING
   TLV containing the new binding value.  Note that other instances of
   TE-PATH-BINDING TLVs that are unchanged MAY also be included.  If the
   unchanged instances are not included, they will remain associated
   with the LSP.

   If a PCE requires a PCC to allocate one (or several) specific binding
   value(s), it may do so by sending a PCUpd or PCInitiate message
   containing one or more TE-PATH-BINDING TLVs.  If the values can be
   successfully allocated, the PCC reports the binding values to the
   PCE.  If the PCC considers the binding value specified by the PCE
   invalid, it MUST send a PCErr message with Error-Type = 32 ("Binding
   label/SID failure") and Error-value = 1 ("Invalid SID").  If the
   binding value is valid but the PCC is unable to allocate the binding
   value, it MUST send a PCErr message with Error-Type = 32 ("Binding
   label/SID failure") and Error-value = 2 ("Unable to allocate the
   specified binding value").  Note that, in case of an error, the PCC
   rejects the PCUpd or PCInitiate message in its entirety and can
   include the offending TE-PATH-BINDING TLV in the PCEP-ERROR object.

   If a PCE wishes to request the withdrawal of a previously reported
   binding value, it MUST send a PCUpd message with the specific TE-
   PATH-BINDING TLV with R flag set to 1.  If a PCE wishes to modify a
   previously requested binding value, it MUST request the withdrawal of
   the former binding value (with R flag set in the former TE-PATH-
   BINDING TLV) and include a new TE-PATH-BINDING TLV containing the new
   binding value.  If a PCC receives a PCUpd message with TE-PATH-
   BINDING TLV where the R flag is set to 1, but either the binding
   value is missing (empty TE-PATH-BINDING TLV) or the binding value is
   incorrect, it MUST send a PCErr message with Error-Type = 32
   ("Binding label/SID failure") and Error-value = 4 ("Unable to remove
   the binding value").

   In some cases, a stateful PCE may want to request that the PCC
   allocate a binding value of the PCC's own choosing.  It instructs the
   PCC by sending a PCUpd message containing an empty TE-PATH-BINDING
   TLV, i.e., no binding value is specified (bringing the Length field
   of the TLV to 4).  A PCE can also request that a PCC allocate a
   binding value at the time of initiation by sending a PCInitiate
   message with an empty TE-PATH-BINDING TLV.  Only one such instance of
   empty TE-PATH-BINDING TLV, per BT, SHOULD be included in the LSP
   object; others should be ignored on receipt.  If the PCC is unable to
   allocate a new binding value as per the specified BT, it MUST send a
   PCErr message with Error-Type = 32 ("Binding label/SID failure") and
   Error-value = 3 ("Unable to allocate a new binding label/SID").

   As previously noted, if a message contains an invalid TE-PATH-BINDING
   TLV that leads to an error condition, the whole message is rejected
   including any other valid instances of TE-PATH-BINDING TLVs, if any.
   The resulting error message MAY include the offending TE-PATH-BINDING
   TLV in the PCEP-ERROR object.

   If a PCC receives a TE-PATH-BINDING TLV in any message other than
   PCUpd or PCInitiate, it MUST close the corresponding PCEP session
   with the reason "Reception of a malformed PCEP message" (according to
   [RFC5440]).  Similarly, if a PCE receives a TE-PATH-BINDING TLV in
   any message other than a PCRpt or if the TE-PATH-BINDING TLV is
   associated with any object other than an LSP or PCEP-ERROR object,
   the PCE MUST close the corresponding PCEP session with the reason
   "Reception of a malformed PCEP message" (according to [RFC5440]).

   If a TE-PATH-BINDING TLV is absent in the PCRpt message and no
   binding values were previously reported, the PCE MUST assume that the
   corresponding LSP does not have any binding.  Similarly, if TE-PATH-
   BINDING TLV is absent in the PCUpd message and no binding values were
   previously reported, the PCC's local policy dictates how the binding
   allocations are made for a given LSP.

   Note that some binding types have similar information but different
   binding value formats.  For example, BT=(2 or 3) is used for the SRv6
   SID, and BT=(0 or 1) is used for the MPLS Label.  In case a PCEP
   speaker receives multiple TE-PATH-BINDING TLVs with the same SRv6 SID
   or MPLS Label but different BT values, it MUST send a PCErr message
   with Error-Type = 32 ("Binding label/SID failure") and Error-value =
   5 ("Inconsistent binding types").

6.  Binding SID in SR-ERO

   In PCEP messages, LSP route information is carried in the Explicit
   Route Object (ERO), which consists of a sequence of subobjects.
   [RFC8664] defines the "SR-ERO subobject" capable of carrying a SID as
   well as the identity of the Node or Adjacency Identifier (NAI)
   represented by the SID.  The NAI Type (NT) field indicates the type
   and format of the NAI contained in the SR-ERO.  In case of binding
   SID, the NAI MUST NOT be included and NT MUST be set to zero.
   Section 5.2.1 of [RFC8664] specifies bit settings and error handling
   in the case when NT=0.

7.  Binding SID in SRv6-ERO

   [RFC9603] defines the "SRv6-ERO subobject" for an SRv6 SID.
   Similarly to SR-ERO (Section 6), the NAI MUST NOT be included and the
   NT MUST be set to zero.  Section 5.2.1 of [RFC8664] specifies bit
   settings and error handling in the case when NT=0.

8.  PCE Allocation of Binding Label/SID

   Section 5 already includes the scenario where a PCE requires a PCC to
   allocate a specified binding value by sending a PCUpd or PCInitiate
   message containing a TE-PATH-BINDING TLV.  This section specifies an
   OPTIONAL feature for the PCE to allocate the binding label/SID of its
   own accord in the case where the PCE also controls the label space of
   the PCC and can make the label allocation on its own as described in
   [RFC8283].  Note that the act of requesting a specific binding value
   (Section 5) is different from the act of allocating a binding label/
   SID as described in this section.

   [RFC8283] introduces the architecture for PCE as a central controller
   as an extension of the architecture described in [RFC4655] and
   assumes the continued use of PCEP as the protocol used between PCE
   and PCC.  [RFC9050] specifies the procedures and PCEP extensions for
   using the PCE as the central controller.  It assumes that the
   exclusive label range to be used by a PCE is known and set on both
   PCEP peers.  A future extension could add the capability to advertise
   this range via a possible PCEP extension as well (see
   [PCE-ID-SPACE]).

   When PCE as a Central Controller (PCECC) operations are supported as
   per [RFC9050], the binding label/SID MAY also be allocated by the PCE
   itself.  Both peers need to exchange the PCECC capability as
   described in [RFC9050] before the PCE can allocate the binding label/
   SID on its own.

   A new P flag in the LSP object [RFC8231] is introduced to indicate
   that the allocation needs to be made by the PCE.  Note that the P
   flag could be used for other types of allocations (such as path
   segments [PCEP-SR]) in the future.

      P (PCE-allocation): If the bit is set to 1, it indicates that the
      PCC requests that the PCE make allocations for this LSP.  The TE-
      PATH-BINDING TLV in the LSP object identifies that the allocation
      is for a binding label/SID.  A PCC MUST set this bit to 1 and
      include a TE-PATH-BINDING TLV in the LSP object if it wishes to
      request an allocation for a binding label/SID by the PCE in the
      PCEP message.  A PCE MUST also set this bit to 1 and include a TE-
      PATH-BINDING TLV to indicate that the binding label/SID is
      allocated by PCE and encoded in the PCEP message towards the PCC.
      Further, if the binding label/SID is allocated by the PCC, the PCE
      MUST set this bit to 0 and follow the procedure described in
      Section 5.

   Note that:

   *  A PCE could allocate the binding label/SID of its own accord for a
      PCE-initiated or PCE-delegated LSP and inform the PCC in the
      PCInitiate message or PCUpd message by setting P=1 and including
      TE-PATH-BINDING TLV in the LSP object.

   *  To let the PCC allocate the binding label/SID, a PCE MUST set P=0
      and include an empty TE-PATH-BINDING TLV (i.e., no binding value
      is specified) in the LSP object in the PCInitiate/PCUpd message.

   *  To request that the PCE allocate the binding label/SID, a PCC MUST
      set P=1, D=1, and include an empty TE-PATH-BINDING TLV in the
      PCRpt message.  The PCE will attempt to allocate it and respond to
      the PCC with a PCUpd message that includes the allocated binding
      label/SID in the TE-PATH-BINDING TLV and P=1 and D=1 in the LSP
      object.  If the PCE is unable to allocate the binding label/SID,
      it MUST send a PCErr message with Error-Type = 32 ("Binding label/
      SID failure") and Error-value = 3 ("Unable to allocate a new
      binding label/SID").

   *  If one or both speakers (PCE and PCC) have not indicated support
      and willingness to use the PCEP extensions for the PCECC as per
      [RFC9050] and a PCEP peer receives P=1 in the LSP object, they
      MUST:

      -  send a PCErr message with Error-Type = 19 ("Invalid Operation")
         and Error-value = 16 ("Attempted PCECC operations when PCECC
         capability was not advertised") and

      -  terminate the PCEP session.

   *  A legacy PCEP speaker that does not recognize the P flag in the
      LSP object would ignore it in accordance with [RFC8231].

   It is assumed that the label range to be used by a PCE is known and
   set on both PCEP peers.  The exact mechanism is out of the scope of
   [RFC9050] and this document.  Note that the specific BSID could be
   from the PCE-controlled or the PCC-controlled label space.  The PCE
   can directly allocate the label from the PCE-controlled label space
   using P=1 as described above, whereas the PCE can request the
   allocation of a specific BSID from the PCC-controlled label space
   with P=0 as described in Section 5.

   Note that the P flag in the LSP object SHOULD NOT be set to 1 without
   the presence of TE-PATH-BINDING TLV or any other future TLV defined
   for PCE allocation.  On receipt of such an LSP object, the P flag is
   ignored.  The presence of TE-PATH-BINDING TLV with P=1 indicates the
   allocation is for the binding label/SID.  In the future, some other
   TLV (such as one defined in [PCEP-SR]) could also be used alongside
   P=1 to indicate allocation of a different attribute.  A future
   document should not attempt to assign semantics to P=1 without
   limiting the scope to one that both PCEP peers can agree on.

9.  Security Considerations

   The security considerations described in [RFC5440], [RFC8231],
   [RFC8281], [RFC8664], and [RFC9050] are applicable to this
   specification.  No additional security measure is required.

   As described in [RFC8402] and [RFC8664], SR intrinsically involves an
   entity (whether head-end or a central network controller) controlling
   and instantiating paths in the network without the involvement of
   (other) nodes along those paths.  Binding SIDs are in effect
   shorthand aliases for longer path representations, and the alias
   expansion is in principle known only by the node that acts on it.  In
   this document, the expansion of the alias is shared between PCC and
   PCE, and rogue actions by either PCC or PCE could result in shifting
   or misdirecting traffic in ways that are hard for other nodes to
   detect.  In particular, when a PCE propagates paths of the form {A,
   B, BSID} to other entities, the BSID values are opaque, and a rogue
   PCE can substitute a BSID from a different LSP in such paths to move
   traffic without the recipient of the path knowing the ultimate
   destination.

   The case of BT=3 provides additional opportunities for malfeasance,
   as it purports to convey information about internal SRv6 SID
   Structure.  There is no mechanism defined to validate this internal
   structure information, and mischaracterizing the division of bits
   into locator block, locator node, function, and argument can result
   in different interpretation of the bits by PCC and PCE.  Most
   notably, shifting bits into or out of the "argument" is a direct
   vector for affecting processing, but other attacks are also possible.

   Thus, as per [RFC8231], it is RECOMMENDED that these PCEP extensions
   only be activated on authenticated and encrypted sessions across PCEs
   and PCCs belonging to the same administrative authority, using
   Transport Layer Security (TLS) [RFC8253], as per the recommendations
   and best current practices in RFC 9325 [BCP195] (unless explicitly
   set aside in [RFC8253]).

10.  Manageability Considerations

   All manageability requirements and considerations listed in
   [RFC5440], [RFC8231], and [RFC8664] apply to PCEP protocol extensions
   defined in this document.  In addition, requirements and
   considerations listed in this section apply.

10.1.  Control of Function and Policy

   A PCC implementation SHOULD allow the operator to configure the
   policy the PCC needs to apply when allocating the binding label/SID.

   If BT is set to 2, the operator needs to have local policy set to
   decide the SID structure and the SRv6 Endpoint Behavior of the BSID.

10.2.  Information and Data Models

   The PCEP YANG module [PCEP-YANG] will be extended to include policy
   configuration for binding label/SID allocation.

10.3.  Liveness Detection and Monitoring

   The mechanisms defined in this document do not imply any new liveness
   detection and monitoring requirements in addition to those already
   listed in [RFC5440].

10.4.  Verify Correct Operations

   The mechanisms defined in this document do not imply any new
   operation verification requirements in addition to those already
   listed in [RFC5440], [RFC8231], and [RFC8664].

10.5.  Requirements on Other Protocols

   The mechanisms defined in this document do not imply any new
   requirements on other protocols.

10.6.  Impact on Network Operations

   The mechanisms defined in [RFC5440], [RFC8231], and [RFC8664] also
   apply to the PCEP extensions defined in this document.

11.  IANA Considerations

   IANA has allocated code points for the protocol elements described in
   this document in the "Path Computation Element Protocol (PCEP)
   Numbers" registry group.

11.1.  PCEP TLV Type Indicators

   This document defines a new PCEP TLV.  IANA has allocated the
   following in the "PCEP TLV Type Indicators" registry within the PCEP
   Numbers registry group:

                  +=======+=================+===========+
                  | Value | Description     | Reference |
                  +=======+=================+===========+
                  |   55  | TE-PATH-BINDING | RFC 9604  |
                  +-------+-----------------+-----------+

                                  Table 1

11.1.1.  TE-PATH-BINDING TLV

   IANA has created the "TE-PATH-BINDING TLV BT Field" registry to
   manage the values of the binding type field in the TE-PATH-BINDING
   TLV.  Initial values are shown below.  New values are assigned by
   Standards Action [RFC8126].

       +=======+======================================+===========+
       | Value | Description                          | Reference |
       +=======+======================================+===========+
       |   0   | MPLS Label                           | RFC 9604  |
       +-------+--------------------------------------+-----------+
       |   1   | MPLS Label Stack Entry               | RFC 9604  |
       +-------+--------------------------------------+-----------+
       |   2   | SRv6 SID                             | RFC 9604  |
       +-------+--------------------------------------+-----------+
       |   3   | SRv6 SID with Behavior and Structure | RFC 9604  |
       +-------+--------------------------------------+-----------+
       | 4-255 | Unassigned                           |           |
       +-------+--------------------------------------+-----------+

                                 Table 2

   IANA has created a new "TE-PATH-BINDING TLV Flag Field" registry to
   manage the Flag field in the TE-PATH-BINDING TLV.  New values are to
   be assigned by Standards Action [RFC8126].  Each bit should be
   tracked with the following qualities:

   *  Bit number (count from 0 as the most significant bit)
   *  Description
   *  Reference

                     +=====+=============+===========+
                     | Bit | Description | Reference |
                     +=====+=============+===========+
                     |  0  | R (Removal) | RFC 9604  |
                     +-----+-------------+-----------+
                     | 1-7 | Unassigned  |           |
                     +-----+-------------+-----------+

                                  Table 3

11.2.  LSP Object

   IANA has allocated a code point in the "LSP Object Flag Field"
   registry for the new P flag as follows:

                   +=====+================+===========+
                   | Bit | Description    | Reference |
                   +=====+================+===========+
                   |  0  | PCE-allocation | RFC 9604  |
                   +-----+----------------+-----------+

                                 Table 4

11.3.  PCEP Error Type and Value

   This document defines a new Error-Type and associated Error-values
   for the PCErr message.  IANA has allocated a new Error-Type and
   Error-values within the "PCEP-ERROR Object Error Types and Values"
   registry of the PCEP Numbers registry group, as follows:

        +============+================+===========================+
        | Error-Type | Meaning        | Error-value               |
        +============+================+===========================+
        | 32         | Binding label/ | 0: Unassigned             |
        |            | SID failure    +---------------------------+
        |            |                | 1: Invalid SID            |
        |            |                +---------------------------+
        |            |                | 2: Unable to allocate the |
        |            |                | specified binding value   |
        |            |                +---------------------------+
        |            |                | 3: Unable to allocate a   |
        |            |                | new binding label/SID     |
        |            |                +---------------------------+
        |            |                | 4: Unable to remove the   |
        |            |                | binding value             |
        |            |                +---------------------------+
        |            |                | 5: Inconsistent binding   |
        |            |                | types                     |
        +------------+----------------+---------------------------+

                                  Table 5

12.  References

12.1.  Normative References

   [BCP195]   Best Current Practice 195,
              <https://www.rfc-editor.org/info/bcp195>.
              At the time of writing, this BCP comprises the following:

              Moriarty, K. and S. Farrell, "Deprecating TLS 1.0 and TLS
              1.1", BCP 195, RFC 8996, DOI 10.17487/RFC8996, March 2021,
              <https://www.rfc-editor.org/info/rfc8996>.

              Sheffer, Y., Saint-Andre, P., and T. Fossati,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 9325, DOI 10.17487/RFC9325, November
              2022, <https://www.rfc-editor.org/info/rfc9325>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3032]  Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
              Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
              Encoding", RFC 3032, DOI 10.17487/RFC3032, January 2001,
              <https://www.rfc-editor.org/info/rfc3032>.

   [RFC5440]  Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation
              Element (PCE) Communication Protocol (PCEP)", RFC 5440,
              DOI 10.17487/RFC5440, March 2009,
              <https://www.rfc-editor.org/info/rfc5440>.

   [RFC5462]  Andersson, L. and R. Asati, "Multiprotocol Label Switching
              (MPLS) Label Stack Entry: "EXP" Field Renamed to "Traffic
              Class" Field", RFC 5462, DOI 10.17487/RFC5462, February
              2009, <https://www.rfc-editor.org/info/rfc5462>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8231]  Crabbe, E., Minei, I., Medved, J., and R. Varga, "Path
              Computation Element Communication Protocol (PCEP)
              Extensions for Stateful PCE", RFC 8231,
              DOI 10.17487/RFC8231, September 2017,
              <https://www.rfc-editor.org/info/rfc8231>.

   [RFC8253]  Lopez, D., Gonzalez de Dios, O., Wu, Q., and D. Dhody,
              "PCEPS: Usage of TLS to Provide a Secure Transport for the
              Path Computation Element Communication Protocol (PCEP)",
              RFC 8253, DOI 10.17487/RFC8253, October 2017,
              <https://www.rfc-editor.org/info/rfc8253>.

   [RFC8281]  Crabbe, E., Minei, I., Sivabalan, S., and R. Varga, "Path
              Computation Element Communication Protocol (PCEP)
              Extensions for PCE-Initiated LSP Setup in a Stateful PCE
              Model", RFC 8281, DOI 10.17487/RFC8281, December 2017,
              <https://www.rfc-editor.org/info/rfc8281>.

   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <https://www.rfc-editor.org/info/rfc8402>.

   [RFC8664]  Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W.,
              and J. Hardwick, "Path Computation Element Communication
              Protocol (PCEP) Extensions for Segment Routing", RFC 8664,
              DOI 10.17487/RFC8664, December 2019,
              <https://www.rfc-editor.org/info/rfc8664>.

   [RFC8986]  Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer,
              D., Matsushima, S., and Z. Li, "Segment Routing over IPv6
              (SRv6) Network Programming", RFC 8986,
              DOI 10.17487/RFC8986, February 2021,
              <https://www.rfc-editor.org/info/rfc8986>.

   [RFC9050]  Li, Z., Peng, S., Negi, M., Zhao, Q., and C. Zhou, "Path
              Computation Element Communication Protocol (PCEP)
              Procedures and Extensions for Using the PCE as a Central
              Controller (PCECC) of LSPs", RFC 9050,
              DOI 10.17487/RFC9050, July 2021,
              <https://www.rfc-editor.org/info/rfc9050>.

   [RFC9603]  Li, C., Ed., Kaladharan, P., Sivabalan, S., Koldychev, M.,
              and Y. Zhu, "Path Computation Element Communication
              Protocol (PCEP) Extensions for IPv6 Segment Routing",
              RFC 9603, DOI 10.17487/RFC9603, July 2024,
              <https://www.rfc-editor.org/info/rfc9603>.

12.2.  Informative References

   [PCE-ID-SPACE]
              Li, C., Shi, H., Ed., Wang, A., Cheng, W., and C. Zhou,
              "Path Computation Element Communication Protocol (PCEP)
              extension to advertise the PCE Controlled Identifier
              Space", Work in Progress, Internet-Draft, draft-ietf-pce-
              controlled-id-space-00, 4 June 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-pce-
              controlled-id-space-00>.

   [PCEP-SR]  Li, C., Chen, M., Cheng, W., Gandhi, R., and Q. Xiong,
              "Path Computation Element Communication Protocol (PCEP)
              Extension for Path Segment in Segment Routing (SR)", Work
              in Progress, Internet-Draft, draft-ietf-pce-sr-path-
              segment-09, 26 February 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-pce-sr-
              path-segment-09>.

   [PCEP-YANG]
              Dhody, D., Ed., Beeram, V., Hardwick, J., and J. Tantsura,
              "A YANG Data Model for Path Computation Element
              Communications Protocol (PCEP)", Work in Progress,
              Internet-Draft, draft-ietf-pce-pcep-yang-25, 21 May 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-pce-
              pcep-yang-25>.

   [RFC4655]  Farrel, A., Vasseur, J.-P., and J. Ash, "A Path
              Computation Element (PCE)-Based Architecture", RFC 4655,
              DOI 10.17487/RFC4655, August 2006,
              <https://www.rfc-editor.org/info/rfc4655>.

   [RFC8283]  Farrel, A., Ed., Zhao, Q., Ed., Li, Z., and C. Zhou, "An
              Architecture for Use of PCE and the PCE Communication
              Protocol (PCEP) in a Network with Central Control",
              RFC 8283, DOI 10.17487/RFC8283, December 2017,
              <https://www.rfc-editor.org/info/rfc8283>.

   [RFC8754]  Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J.,
              Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
              (SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,
              <https://www.rfc-editor.org/info/rfc8754>.

   [RFC9256]  Filsfils, C., Talaulikar, K., Ed., Voyer, D., Bogdanov,
              A., and P. Mattes, "Segment Routing Policy Architecture",
              RFC 9256, DOI 10.17487/RFC9256, July 2022,
              <https://www.rfc-editor.org/info/rfc9256>.

Acknowledgements

   We would like to thank Milos Fabian, Mrinmoy Das, Andrew Stone, Tom
   Petch, Aijun Wang, Olivier Dugeon, and Adrian Farrel for their
   valuable comments.

   Thanks to Julien Meuric for shepherding.  Thanks to John Scudder for
   the AD review.

   Thanks to Theresa Enghardt for the GENART review.

   Thanks to Martin Vigoureux, Benjamin Kaduk, Éric Vyncke, Lars Eggert,
   Murray Kucherawy, and Erik Kline for the IESG reviews.

Contributors

   Jonathan Hardwick
   Microsoft
   United Kingdom
   Email: jonhardwick@microsoft.com


   Dhruv Dhody
   Huawei Technologies
   Divyashree Techno Park, Whitefield
   Bangalore 560066
   Karnataka
   India
   Email: dhruv.ietf@gmail.com


   Mahendra Singh Negi
   RtBrick India
   N-17L, Floor-1, 18th Cross Rd, HSR Layout Sector-3
   Bangalore 560102
   Karnataka
   India
   Email: mahend.ietf@gmail.com


   Mike Koldychev
   Cisco Systems, Inc.
   2000 Innovation Drive
   Kanata Ontario K2K 3E8
   Canada
   Email: mkoldych@cisco.com


   Zafar Ali
   Cisco Systems, Inc.
   Email: zali@cisco.com


Authors' Addresses

   Siva Sivabalan
   Ciena Corporation
   Email: msiva282@gmail.com


   Clarence Filsfils
   Cisco Systems, Inc.
   Pegasus Parc
   De Kleetlaan 6a
   1831 Brabant
   Belgium
   Email: cfilsfil@cisco.com


   Jeff Tantsura
   Nvidia
   Email: jefftant.ietf@gmail.com


   Stefano Previdi
   Huawei Technologies
   Email: stefano@previdi.net


   Cheng Li (editor)
   Huawei Technologies
   Huawei Campus, No. 156 Beiqing Rd.
   Beijing
   100095
   China
   Email: c.l@huawei.com
