Internet Engineering Task Force (IETF)                          D. Dhody
Request for Comments: 9168                           Huawei Technologies
Category: Standards Track                                      A. Farrel
ISSN: 2070-1721                                       Old Dog Consulting
                                                                   Z. Li
                                                     Huawei Technologies
                                                            January 2022


  Path Computation Element Communication Protocol (PCEP) Extension for
                           Flow Specification

Abstract

   The Path Computation Element (PCE) is a functional component capable
   of selecting paths through a traffic engineering (TE) network.  These
   paths may be supplied in response to requests for computation or may
   be unsolicited requests issued by the PCE to network elements.  Both
   approaches use the PCE Communication Protocol (PCEP) to convey the
   details of the computed path.

   Traffic flows may be categorized and described using "Flow
   Specifications".  RFC 8955 defines the Flow Specification and
   describes how Flow Specification components are used to describe
   traffic flows.  RFC 8955 also defines how Flow Specifications may be
   distributed in BGP to allow specific traffic flows to be associated
   with routes.

   This document specifies a set of extensions to PCEP to support
   dissemination of Flow Specifications.  This allows a PCE to indicate
   what traffic should be placed on each path that it is aware of.

   The extensions defined in this document include the creation, update,
   and withdrawal of Flow Specifications via PCEP and can be applied to
   tunnels initiated by the PCE or to tunnels where control is delegated
   to the PCE by the Path Computation Client (PCC).  Furthermore, a PCC
   requesting a new path can include Flow Specifications in the request
   to indicate the purpose of the tunnel allowing the PCE to factor this
   into the path computation.

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/rfc9168.

Copyright Notice

   Copyright (c) 2022 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
   2.  Terminology
   3.  Procedures for PCE Use of Flow Specifications
     3.1.  Context for PCE Use of Flow Specifications
     3.2.  Elements of the Procedure
       3.2.1.  Capability Advertisement
         3.2.1.1.  PCEP Open Message
         3.2.1.2.  IGP PCE Capabilities Advertisement
       3.2.2.  Dissemination Procedures
       3.2.3.  Flow Specification Synchronization
   4.  PCE FlowSpec Capability TLV
   5.  PCEP FLOWSPEC Object
   6.  Flow Filter TLV
   7.  Flow Specification TLVs
   8.  Detailed Procedures
     8.1.  Default Behavior and Backward Compatibility
     8.2.  Composite Flow Specifications
     8.3.  Modifying Flow Specifications
     8.4.  Multiple Flow Specifications
     8.5.  Adding and Removing Flow Specifications
     8.6.  VPN Identifiers
     8.7.  Priorities and Overlapping Flow Specifications
   9.  PCEP Messages
   10. IANA Considerations
     10.1.  PCEP Objects
       10.1.1.  PCEP FLOWSPEC Object Flag Field
     10.2.  PCEP TLV Type Indicators
     10.3.  Flow Specification TLV Type Indicators
     10.4.  PCEP Error Codes
     10.5.  PCE Capability Flag
   11. Security Considerations
   12. Manageability Considerations
     12.1.  Management of Multiple Flow Specifications
     12.2.  Control of Function through Configuration and Policy
     12.3.  Information and Data Models
     12.4.  Liveness Detection and Monitoring
     12.5.  Verifying Correct Operation
     12.6.  Requirements for Other Protocols and Functional Components
     12.7.  Impact on Network Operation
   13. References
     13.1.  Normative References
     13.2.  Informative References
   Acknowledgements
   Contributors
   Authors' Addresses

1.  Introduction

   [RFC4655] defines the Path Computation Element (PCE), a functional
   component capable of computing paths for use in traffic engineering
   networks.  PCE was originally conceived for use in Multiprotocol
   Label Switching (MPLS) for traffic engineering (TE) networks to
   derive the routes of Label Switched Paths (LSPs).  However, the scope
   of PCE was quickly extended to make it applicable to networks
   controlled by Generalized MPLS (GMPLS), and more recent work has
   brought other traffic engineering technologies and planning
   applications into scope (for example, Segment Routing (SR)
   [RFC8664]).

   [RFC5440] describes the PCE Communication Protocol (PCEP).  PCEP
   defines the communication between a Path Computation Client (PCC) and
   a PCE, or between PCE and PCE, enabling computation of the path for
   MPLS-TE LSPs.

   Stateful PCE [RFC8231] specifies a set of extensions to PCEP to
   enable control of TE-LSPs by a PCE that retains state about the LSPs
   provisioned in the network (a stateful PCE).  [RFC8281] describes the
   setup, maintenance, and teardown of LSPs initiated by a stateful PCE
   without the need for local configuration on the PCC, thus allowing
   for a dynamic network that is centrally controlled.  [RFC8283]
   introduces the architecture for PCE as a central controller and
   describes how PCE can be viewed as a component that performs
   computation to place "flows" within the network and decide how these
   flows are routed.

   The description of traffic flows by the combination of multiple Flow
   Specification components and their dissemination as traffic flow
   specifications (Flow Specifications) is described for BGP in
   [RFC8955].  In BGP, a Flow Specification is comprised of traffic
   filtering rules and is associated with actions to perform on the
   packets that match the Flow Specification.  The BGP routers that
   receive a Flow Specification can classify received packets according
   to the traffic filtering rules and can direct packets based on the
   associated actions.

   When a PCE is used to initiate tunnels (such as TE-LSPs or SR paths)
   using PCEP, it is important that the head end of the tunnels
   understands what traffic to place on each tunnel.  The data flows
   intended for a tunnel can be described using Flow Specification
   components.  When PCEP is in use for tunnel initiation, it makes
   sense for that same protocol to be used to distribute the Flow
   Specification components that describe what data is to flow on those
   tunnels.

   This document specifies a set of extensions to PCEP to support
   dissemination of Flow Specification components.  We term the
   description of a traffic flow using Flow Specification components as
   a "Flow Specification".  This term is conceptually the same as the
   term used in [RFC8955]; however, no mechanism is provided to
   distribute an action associated with the Flow Specification because
   there is only one action that is applicable in the PCEP context (that
   is, directing the matching traffic to the identified LSP).

   The extensions defined in this document include the creation, update,
   and withdrawal of Flow Specifications via PCEP and can be applied to
   tunnels initiated by the PCE or to tunnels where control is delegated
   to the PCE by the PCC.  Furthermore, a PCC requesting a new path can
   include Flow Specifications in the request to indicate the purpose of
   the tunnel allowing the PCE to factor this into the path computation.

   Flow Specifications are carried in TLVs within a new object called
   the FLOWSPEC object defined in this document.  The flow filtering
   rules indicated by the Flow Specifications are mainly defined by BGP
   Flow Specifications.

   Note that PCEP-installed Flow Specifications are intended to be
   installed only at the head end of the LSP to which they direct
   traffic.  It is acceptable (and potentially desirable) that other
   routers in the network have Flow Specifications installed that match
   the same traffic but direct it onto different routes or to different
   LSPs.  Those other Flow Specifications may be installed using the
   PCEP extensions defined in this document, distributed using BGP per
   [RFC8955], or configured using manual operations.  Since this
   document is about PCEP-installed Flow Specifications, those other
   Flow Specifications at other routers are out of scope.  In this
   context, however, it is worth noting that changes to the wider
   routing system (such as the distribution and installation of BGP Flow
   Specifications, or fluctuations in the IGP link state database) might
   mean that traffic matching the PCEP Flow Specification never reaches
   the head end of the LSP at which the PCEP Flow Specification has been
   installed.  This may or may not be desirable according to the
   operator's traffic engineering and routing policies and is
   particularly applicable at LSPs that do not have their head ends at
   the ingress edge of the network, but it is not an effect that this
   document seeks to address.

2.  Terminology

   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.

   This document uses the following terms defined in [RFC5440]: PCC,
   PCE, and PCEP Peer.

   The following term from [RFC8955] is used frequently throughout this
   document:

   |  A Flow Specification is an n-tuple consisting of several matching
   |  criteria that can be applied to IP traffic.  A given IP packet is
   |  said to match the defined Flow Specification if it matches all the
   |  specified criteria.

   [RFC8955] also states that "[a] given Flow Specification may be
   associated with a set of attributes" and that "...attributes can be
   used to encode a set of predetermined actions."  However, in the
   context of this document, no action is explicitly specified as
   associated with the Flow Specification since the action of forwarding
   all matching traffic onto the associated path is implicit.

   How an implementation decides to filter traffic that matches a Flow
   Specification does not form part of this specification, but a flag is
   provided to indicate whether the sender of a PCEP message that
   includes a Flow Specification intends it to be installed as a Longest
   Prefix Match (LPM) route or as a Flow Specification policy.

   This document uses the terms "stateful PCE" and "active PCE" as
   advocated in [RFC7399].

3.  Procedures for PCE Use of Flow Specifications

3.1.  Context for PCE Use of Flow Specifications

   In the PCE architecture, there are five steps in the setup and use of
   LSPs:

   1.  Decide which LSPs to set up.  The decision may be made by a user,
       by a PCC, or by the PCE.  There can be a number of triggers for
       this, including user intervention and dynamic response to changes
       in traffic demands.

   2.  Decide what properties to assign to an LSP.  This can include
       bandwidth reservations, priorities, and the Differentiated
       Services Code Point (DSCP) (i.e., MPLS Traffic Class field).
       This function is also determined by user configuration or in
       response to predicted or observed traffic demands.

   3.  Decide what traffic to put on the LSP.  This is effectively
       determining which traffic flows to assign to which LSPs;
       practically, this is closely linked to the first two decisions
       listed above.

   4.  Cause the LSP to be set up and modified to have the right
       characteristics.  This will usually involve the PCE advising or
       instructing the PCC at the head end of the LSP, and the PCC will
       then signal the LSP across the network.

   5.  Tell the head end of the LSP what traffic to put on the LSP.
       This may happen after or at the same time as the LSP is set up.
       This step is the subject of this document.

3.2.  Elements of the Procedure

   There are three elements in the procedure:

   1.  A PCE and a PCC must be able to indicate whether or not they
       support the use of Flow Specifications.

   2.  A PCE or PCC must be able to include Flow Specifications in PCEP
       messages with a clear understanding of the applicability of those
       Flow Specifications in each case.  This includes whether the use
       of such information is mandatory, constrained, or optional and
       how overlapping Flow Specifications will be resolved.

   3.  Flow Specification information/state must be synchronized between
       PCEP peers so that, on recovery, the peers have the same
       understanding of which Flow Specifications apply just as is
       required in the case of stateful PCE and LSP delegation (see
       Section 5.6 of [RFC8231]).

   The following subsections describe these points.

3.2.1.  Capability Advertisement

   As with most PCEP capability advertisements, the ability to support
   Flow Specifications can be indicated in the PCEP Open message or in
   IGP PCE capability advertisements.

3.2.1.1.  PCEP Open Message

   During PCEP session establishment, a PCC or PCE that supports the
   procedures described in this document announces this fact by
   including the PCE FlowSpec Capability TLV (described in Section 4) in
   the OPEN object carried in the PCEP Open message.

   The presence of the PCE FlowSpec Capability TLV in the OPEN object in
   a PCE's Open message indicates that the PCE can distribute FlowSpecs
   to PCCs and can receive FlowSpecs in messages from PCCs.

   The presence of the PCE FlowSpec Capability TLV in the OPEN object in
   a PCC's Open message indicates that the PCC supports the FlowSpec
   functionality described in this document.

   If either one of a pair of PCEP peers does not include the PCE
   FlowSpec Capability TLV in the OPEN object in its Open message, then
   the other peer MUST NOT include a FLOWSPEC object in any PCEP message
   sent to the peer.  If a FLOWSPEC object is received when support has
   not been indicated, the receiver will respond with a PCErr message
   reporting the objects containing the FlowSpec as described in
   [RFC5440]: that is, it will use "Unknown Object" if it does not
   support this specification and "Not supported object" if it supports
   this specification but has not chosen to support FLOWSPEC objects on
   this PCEP session.

3.2.1.2.  IGP PCE Capabilities Advertisement

   The ability to advertise support for PCEP and PCE features in IGP
   advertisements is provided for OSPF in [RFC5088] and for IS-IS in
   [RFC5089].  The mechanism uses the PCE Discovery TLV, which has a
   PCE-CAP-FLAGS sub-TLV containing bit flags, each of which indicates
   support for a different feature.

   This document defines a new PCE-CAP-FLAGS sub-TLV bit, the FlowSpec
   Capable flag (bit number 16).  Setting the bit indicates that an
   advertising PCE supports the procedures defined in this document.

   Note that while PCE FlowSpec capability may be advertised during
   discovery, PCEP speakers that wish to use Flow Specification in PCEP
   MUST negotiate PCE FlowSpec capability during PCEP session setup, as
   specified in Section 3.2.1.1.  A PCC MAY initiate PCE FlowSpec
   capability negotiation at PCEP session setup even if it did not
   receive any IGP PCE capability advertisement, and a PCEP peer that
   advertised support for FlowSpec in the IGP is not obliged to support
   these procedures on any given PCEP session.

3.2.2.  Dissemination Procedures

   This section describes the procedures to support Flow Specifications
   in PCEP messages.

   The primary purpose of distributing Flow Specification information is
   to allow a PCE to indicate to a PCC what traffic it should place on a
   path (such as an LSP or an SR path).  This means that the Flow
   Specification may be included in:

   *  PCInitiate messages so that an active PCE can indicate the traffic
      to place on a path at the time that the PCE instantiates the path.

   *  PCUpd messages so that an active PCE can indicate or change the
      traffic to place on a path that has already been set up.

   *  PCRpt messages so that a PCC can report the traffic that the PCC
      will place on the path.

   *  PCReq messages so that a PCC can indicate what traffic it plans to
      place on a path when it requests that the PCE perform a
      computation in case that information aids the PCE in its work.

   *  PCRep messages so that a PCE that has been asked to compute a path
      can suggest which traffic could be placed on a path that a PCC may
      be about to set up.

   *  PCErr messages so that issues related to paths and the traffic
      they carry can be reported to the PCE by the PCC and problems with
      other PCEP messages that carry Flow Specifications can be
      reported.

   To carry Flow Specifications in PCEP messages, this document defines
   a new PCEP object called the "PCEP FLOWSPEC object".  The object is
   OPTIONAL in the messages described above and MAY appear more than
   once in each message.

   To describe a traffic flow, the PCEP FLOWSPEC object carries a Flow
   Filter TLV.

   The inclusion of multiple PCEP FLOWSPEC objects allows multiple
   traffic flows to be placed on a single path.

   Once a PCE and PCC have established that they can both support the
   use of Flow Specifications in PCEP messages, such information may be
   exchanged at any time for new or existing paths.

   The application and prioritization of Flow Specifications are
   described in Section 8.7.

   As per [RFC8231], any attributes of the path received from a PCE are
   subject to the PCC's local policy.  This holds true for the Flow
   Specifications as well.

3.2.3.  Flow Specification Synchronization

   The Flow Specifications are carried along with the LSP state
   information as per [RFC8231], making the Flow Specifications part of
   the LSP database (LSP-DB).  Thus, the synchronization of the Flow
   Specification information is done as part of LSP-DB synchronization.
   This may be achieved using normal state synchronization procedures as
   described in [RFC8231] or enhanced state synchronization procedures
   as defined in [RFC8232].

   The approach selected will be implementation and deployment specific
   and will depend on issues such as how the databases are constructed
   and what level of synchronization support is needed.

4.  PCE FlowSpec Capability TLV

   The PCE-FLOWSPEC-CAPABILITY TLV is an optional TLV that can be
   carried in the OPEN object [RFC5440] to exchange the PCE FlowSpec
   capabilities of the PCEP speakers.

   The format of the PCE-FLOWSPEC-CAPABILITY TLV follows the format of
   all PCEP TLVs as defined in [RFC5440] and is shown in Figure 1.


    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=51               |          Length=2             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Value=0             |          Padding              |
   +---------------------------------------------------------------+

                Figure 1: PCE-FLOWSPEC-CAPABILITY TLV Format

   The type of the PCE-FLOWSPEC-CAPABILITY TLV is 51, and it has a fixed
   length of 2 octets.  The Value field MUST be set to 0 and MUST be
   ignored on receipt.  The two bytes of padding MUST be set to zero and
   ignored on receipt.

   The inclusion of this TLV in an OPEN object indicates that the sender
   can perform FlowSpec handling as defined in this document.

5.  PCEP FLOWSPEC Object

   The PCEP FLOWSPEC object defined in this document is compliant with
   the PCEP object format defined in [RFC5440].  It is OPTIONAL in the
   PCReq, PCRep, PCErr, PCInitiate, PCRpt, and PCUpd messages and MAY be
   present zero, one, or more times.  Each instance of the object
   specifies a separate traffic flow.

   The PCEP FLOWSPEC object MAY carry a FlowSpec filter rule encoded in
   a Flow Filter TLV as defined in Section 6.

   The FLOWSPEC Object-Class is 43 (to be assigned by IANA).

   The FLOWSPEC Object-Type is 1.

   The format of the body of the PCEP FLOWSPEC object is shown in
   Figure 2.


    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            FS-ID                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         AFI                   |  Reserved     |   Flags   |L|R|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   //                             TLVs                            //
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 2: PCEP FLOWSPEC Object Body Format

   FS-ID (32 bits):  A PCEP-specific identifier for the FlowSpec
      information.  A PCE or PCC creates an FS-ID for each FlowSpec that
      it originates, and the value is unique within the scope of that
      PCE or PCC and is constant for the lifetime of a PCEP session.
      All subsequent PCEP messages can identify the FlowSpec using the
      FS-ID.  The values 0 and 0xFFFFFFFF are reserved and MUST NOT be
      used.  Note that [NUMERIC-IDS-SEC] gives advice on assigning
      transient numeric identifiers such as the FS-ID so as to minimize
      security risks.

   AFI (16 bits):  Address Family Identifier as used in BGP [RFC4760]
      (AFI=1 for IPv4 or VPNv4, AFI=2 for IPv6 and VPNv6 as per
      [RFC8956]).

   Reserved (8 bits):  MUST be set to zero on transmission and ignored
      on receipt.

   Flags (8 bits):  Two flags are currently assigned:

      R bit:  The Remove bit is set when a PCEP FLOWSPEC object is
         included in a PCEP message to indicate removal of the Flow
         Specification from the associated tunnel.  If the bit is clear,
         the Flow Specification is being added or modified.

      L bit:  The Longest Prefix Match (LPM) bit is set to indicate that
         the Flow Specification is to be installed as a route subject to
         LPM forwarding.  If the bit is clear, the Flow Specification
         described by the Flow Filter TLV (see Section 6) is to be
         installed as a Flow Specification.  If the bit is set, only
         Flow Specifications that describe IPv4 or IPv6 destinations are
         meaningful in the Flow Filter TLV, and others are ignored.  If
         the L is set and the receiver does not support the use of Flow
         Specifications that are present in the Flow Filter TLV for the
         installation of a route subject to LPM forwarding, then the
         PCEP peer MUST respond with a PCErr message with Error-Type 30
         (FlowSpec Error) and Error-value 5 (Unsupported LPM Route).

   Unassigned bits MUST be set to zero on transmission and ignored on
   receipt.

   If the PCEP speaker receives a message with the R bit set in the
   FLOWSPEC object and the Flow Specification identified with an FS-ID
   does not exist, it MUST generate a PCErr with Error-Type 30 (FlowSpec
   Error) and Error-value 4 (Unknown FlowSpec).

   If the PCEP speaker does not understand or support the AFI in the
   FLOWSPEC message, the PCEP peer MUST respond with a PCErr message
   with Error-Type 30 (FlowSpec Error) and Error-value 2 (Malformed
   FlowSpec).

   The following TLVs can be used in the FLOWSPEC object:

   Speaker Entity Identifier TLV:  As specified in [RFC8232], the
      SPEAKER-ENTITY-ID TLV encodes a unique identifier for the node
      that does not change during the lifetime of the PCEP speaker.
      This is used to uniquely identify the FlowSpec originator and thus
      is used in conjunction with the FS-ID to uniquely identify the
      FlowSpec information.  This TLV MUST be included.  If the TLV is
      missing, the PCEP peer MUST respond with a PCErr message with
      Error-Type 30 (FlowSpec Error) and Error-value 2 (Malformed
      FlowSpec).  If more than one instance of this TLV is present, the
      first MUST be processed, and subsequent instances MUST be ignored.

   Flow Filter TLV (variable):  One TLV MAY be included.  The Flow
      Filter TLV is OPTIONAL when the R bit is set.

   The Flow Filter TLV MUST be present when the R bit is clear.  If the
   TLV is missing when the R bit is clear, the PCEP peer MUST respond
   with a PCErr message with Error-Type 30 (FlowSpec Error) and Error-
   value 2 (Malformed FlowSpec).

   Filtering based on the L2 fields is out of scope of this document.

6.  Flow Filter TLV

   One new PCEP TLV is defined to convey Flow Specification filtering
   rules that specify what traffic is carried on a path.  The TLV
   follows the format of all PCEP TLVs as defined in [RFC5440].  The
   Type field values come from the code point space for PCEP TLVs and
   has the value 52 for Flow Filter TLV.

   The Value field of the TLV contains one or more sub-TLVs (the Flow
   Specification TLVs) as defined in Section 7, and they represent the
   complete definition of a Flow Specification for traffic to be placed
   on the tunnel.  This tunnel is indicated by the PCEP message in which
   the PCEP FLOWSPEC object is carried.  The set of Flow Specification
   TLVs in a single instance of a Flow Filter TLV is combined to
   indicate the specific Flow Specification.  Note that the PCEP
   FLOWSPEC object can include just one Flow Filter TLV.

   Further Flow Specifications can be included in a PCEP message by
   including additional FLOWSPEC objects.

   In the future, there may be a desire to add support for L2 Flow
   Specifications (such as described in [BGP-L2VPN]).

7.  Flow Specification TLVs

   The Flow Filter TLV carries one or more Flow Specification TLVs.  The
   Flow Specification TLV follows the format of all PCEP TLVs as defined
   in [RFC5440].  However, the Type values are selected from a separate
   IANA registry (see Section 10.3) rather than from the common PCEP TLV
   registry.

   Type values are chosen so that there can be commonality with Flow
   Specifications defined for use with BGP [RFC8955] [RFC8956].  This is
   possible because the BGP Flow Spec encoding uses a single octet to
   encode the type, whereas PCEP uses 2 octets.  Thus, the space of
   values for the Type field is partitioned as shown in Table 1.

           +===========+=======================================+
           | Range     | Description                           |
           +===========+=======================================+
           | 0-255     | Per BGP Flow Spec registry defined by |
           |           | [RFC8955] and [RFC8956].              |
           |           |                                       |
           |           | Not to be allocated in this registry. |
           +-----------+---------------------------------------+
           | 256-65535 | New PCEP Flow Specifications          |
           |           | allocated according to the registry   |
           |           | defined in this document.             |
           +-----------+---------------------------------------+

                Table 1: Flow Specification TLV Type Ranges

   [RFC8955] is the reference for the "Flow Spec Component Types"
   registry and defines the allocations it contains.  [RFC8956]
   requested the creation of the "Flow Spec IPv6 Component Types"
   registry, as well as its initial allocations.  If the AFI (in the
   FLOWSPEC object) is set to IPv4, the range 0..255 is as per "Flow
   Spec Component Types" [RFC8955]; if the AFI is set to IPv6, the range
   0..255 is as per "Flow Spec IPv6 Component Types" [RFC8956].

   The content of the Value field in each TLV is specific to the type/
   AFI and describes the parameters of the Flow Specification.  The
   definition of the format of many of these Value fields is inherited
   from BGP specifications.  Specifically, the inheritance is from
   [RFC8955] and [RFC8956], but it may also be inherited from future BGP
   specifications.

   When multiple Flow Specification TLVs are present in a single Flow
   Filter TLV, they are combined to produce a more detailed
   specification of a flow.  For examples and rules about how this is
   achieved, see [RFC8955].  As described in [RFC8955], where it says "A
   given component type MAY (exactly once) be present in the Flow
   Specification", a Flow Filter TLV MUST NOT contain more than one Flow
   Specification TLV of the same type: an implementation that receives a
   PCEP message with a Flow Filter TLV that contains more than one Flow
   Specification TLV of the same type MUST respond with a PCErr message
   with Error-Type 30 (FlowSpec Error) and Error-value 2 (Malformed
   FlowSpec) and MUST NOT install the Flow Specification.

   An implementation that receives a PCEP message carrying a Flow
   Specification TLV with a type value that it does not recognize or
   support MUST respond with a PCErr message with Error-Type 30
   (FlowSpec Error) and Error-value 1 (Unsupported FlowSpec) and MUST
   NOT install the Flow Specification.

   When used in other protocols (such as BGP), these Flow Specifications
   are also associated with actions to indicate how traffic matching the
   Flow Specification should be treated.  In PCEP, however, the only
   action is to associate the traffic with a tunnel and to forward
   matching traffic onto that path, so no encoding of an action is
   needed.

   Section 8.7 describes how overlapping Flow Specifications are
   prioritized and handled.

   All Flow Specification TLVs with Types in the range 0 to 255 have
   values defined for use in BGP (for example, in [RFC8955] and
   [RFC8956]) and are set using the BGP encoding but without the type
   octet (the relevant information is in the Type field of the TLV).
   The Value field is padded with trailing zeros to achieve 4-byte
   alignment.

   This document defines the following new types:

   +======+=====================+==================+
   | Type | Description         | Value Defined In |
   +======+=====================+==================+
   | 256  | Route Distinguisher | RFC 9168         |
   +------+---------------------+------------------+
   | 257  | IPv4 Multicast Flow | RFC 9168         |
   +------+---------------------+------------------+
   | 258  | IPv6 Multicast Flow | RFC 9168         |
   +------+---------------------+------------------+

     Table 2: Flow Specification TLV Types Defined
                    in this Document

   To allow identification of a VPN in PCEP via a Route Distinguisher
   (RD) [RFC4364], a new TLV, ROUTE-DISTINGUISHER TLV, is defined in
   this document.  A Flow Specification TLV with Type 256 (ROUTE-
   DISTINGUISHER TLV) carries an RD value, which is used to identify
   that other flow filter information (for example, an IPv4 destination
   prefix) is associated with a specific VPN identified by the RD.  See
   Section 8.6 for further discussion of VPN identification.


    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=256            |           Length=8            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Route Distinguisher                       |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 3: The Format of the ROUTE-DISTINGUISHER TLV

   The format of the RD is as per [RFC4364].

   Although it may be possible to describe a multicast Flow
   Specification from the combination of other Flow Specification TLVs
   with specific values, it is more convenient to use a dedicated Flow
   Specification TLV.  Flow Specification TLVs with Type values 257 and
   258 are used to identify a multicast flow for IPv4 and IPv6,
   respectively.  The Value field is encoded as 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Reserved           |S|G|  Src Mask Len | Grp Mask Len  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                        Source Address                         ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                   Group multicast Address                     ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 4: Multicast Flow Specification TLV Encoding

   The address fields and address mask lengths of the two Multicast Flow
   Specification TLVs contain source and group prefixes for matching
   against packet flows.  Note that the two address fields are 32 bits
   for an IPv4 Multicast Flow and 128 bits for an IPv6 Multicast Flow.

   The Reserved field MUST be set to zero and ignored on receipt.

   Two bit flags (S and G) are defined to describe the multicast
   wildcarding in use.  If the S bit is set, then source wildcarding is
   in use, and the values in the Source Mask Length and Source Address
   fields MUST be ignored.  If the G bit is set, then group wildcarding
   is in use, and the values in the Group Mask Length and Group
   multicast Address fields MUST be ignored.  The G bit MUST NOT be set
   unless the S bit is also set: if a Multicast Flow Specification TLV
   is received with S bit = 0 and G bit = 1, the receiver MUST respond
   with a PCErr with Error-Type 30 (FlowSpec Error) and Error-value 2
   (Malformed FlowSpec).

   The three multicast mappings may be achieved as follows:

      (S, G) - S bit = 0, G bit = 0, the Source Address and Group
      multicast Address prefixes are both used to define the multicast
      flow.

      (*, G) - S bit = 1, G bit = 0, the Group multicast Address prefix
      is used to define the multicast flow, but the Source Address
      prefix is ignored.

      (*, *) - S bit = 1, G bit = 1, the Source Address and Group
      multicast Address prefixes are both ignored.

8.  Detailed Procedures

   This section outlines some specific detailed procedures for using the
   protocol extensions defined in this document.

8.1.  Default Behavior and Backward Compatibility

   The default behavior is that no Flow Specification is applied to a
   tunnel.  That is, the default is that the FLOWSPEC object is not
   used, as is the case in all systems before the implementation of this
   specification.

   In this case, it is a local matter (such as through configuration)
   how tunnel head ends are instructed in terms of what traffic to place
   on a tunnel.

   [RFC5440] describes how receivers respond when they see unknown PCEP
   objects.

8.2.  Composite Flow Specifications

   Flow Specifications may be represented by a single Flow Specification
   TLV or may require a more complex description using multiple Flow
   Specification TLVs.  For example, a flow indicated by a source-
   destination pair of IPv6 addresses would be described by the
   combination of Destination IPv6 Prefix and Source IPv6 Prefix Flow
   Specification TLVs.

8.3.  Modifying Flow Specifications

   A PCE may want to modify a Flow Specification associated with a
   tunnel, or a PCC may want to report a change to the Flow
   Specification it is using with a tunnel.

   It is important to identify the specific Flow Specification so it is
   clear that this is a modification of an existing flow and not the
   addition of a new flow as described in Section 8.4.  The FS-ID field
   of the PCEP FLOWSPEC object is used to identify a specific Flow
   Specification in the context of the content of the Speaker Entity
   Identifier TLV.

   When modifying a Flow Specification, all Flow Specification TLVs for
   the intended specification of the flow MUST be included in the PCEP
   FLOWSPEC object.  The FS-ID MUST be retained from the previous
   description of the flow, and the same Speaker Entity Identifier TLV
   MUST be used.

8.4.  Multiple Flow Specifications

   It is possible that traffic from multiple flows will be placed on a
   single tunnel.  In some cases, it is possible to define these within
   a single PCEP FLOWSPEC object by widening the scope of a Flow
   Specification TLV: for example, traffic to two destination IPv4
   prefixes might be captured by a single Flow Specification TLV with
   type "Destination" with a suitably adjusted prefix.  However, this is
   unlikely to be possible in most scenarios, and it must be recalled
   that it is not permitted to include two Flow Specification TLVs of
   the same type within one Flow Filter TLV.

   The normal procedure, therefore, is to carry each Flow Specification
   in its own PCEP FLOWSPEC object.  Multiple objects may be present on
   a single PCEP message, or multiple PCEP messages may be used.

8.5.  Adding and Removing Flow Specifications

   The Remove bit in the PCEP FLOWSPEC object is left clear when a Flow
   Specification is being added or modified.

   To remove a Flow Specification, a PCEP FLOWSPEC object is included
   with the FS-ID matching the one being removed, and the R bit is set
   to indicate removal.  In this case, it is not necessary to include
   any Flow Specification TLVs.

   If the R bit is set and Flow Specification TLVs are present, an
   implementation MAY ignore them.  If the implementation checks the
   Flow Specification TLVs against those recorded for the FS-ID and
   Speaker Entity Identifier of the Flow Specification being removed and
   finds a mismatch, the Flow Specification matching the FS-ID MUST
   still be removed, and the implementation SHOULD record a local
   exception or log.

8.6.  VPN Identifiers

   VPN instances are identified in BGP using RDs [RFC4364].  These
   values are not normally considered to have any meaning outside of the
   network, and they are not encoded in data packets belonging to the
   VPNs.  However, RDs provide a useful way of identifying VPN instances
   and are often manually or automatically assigned to VPNs as they are
   provisioned.

   Thus, the RD provides a useful way to indicate that traffic for a
   particular VPN should be placed on a given tunnel.  The tunnel head
   end will need to interpret this Flow Specification not as a filter on
   the fields of data packets but rather using the other mechanisms that
   it already uses to identify VPN traffic.  These mechanisms could be
   based on the incoming port (for port-based VPNs) or may leverage
   knowledge of the VPN Routing and Forwarding (VRF) that is in use for
   the traffic.

8.7.  Priorities and Overlapping Flow Specifications

   Flow Specifications can overlap.  For example, two different Flow
   Specifications may be identical except for the length of the prefix
   in the destination address.  In these cases, the PCC must determine
   how to prioritize the Flow Specifications so as to know which path to
   assign packets that match both Flow Specifications.  That is, the PCC
   must assign a precedence to the Flow Specifications so that it checks
   each incoming packet for a match in a predictable order.

   The processing of BGP Flow Specifications is described in [RFC8955].
   Section 5.1 of that document explains the order of traffic filtering
   rules to be executed by an implementation of that specification.

   PCCs MUST apply the same ordering rules as defined in [RFC8955].

   Furthermore, it is possible that Flow Specifications will be
   distributed by BGP as well as by PCEP as described in this document.
   In such cases, implementations supporting both approaches MUST apply
   the prioritization and ordering rules as set out in [RFC8955]
   regardless of which protocol distributed the Flow Specifications.
   However, implementations MAY provide a configuration control to allow
   one protocol to take precedence over the other; this may be
   particularly useful if the Flow Specifications make identical matches
   on traffic but have different actions.  It is RECOMMENDED that a
   message be logged for the operator to understand the behavior when
   two Flow Specifications distributed by different protocols overlap,
   especially when one acts to replace another.

   Section 12.1 of this document covers manageability considerations
   relevant to the prioritized ordering of Flow Specifications.

   An implementation that receives a PCEP message carrying a Flow
   Specification that it cannot resolve against other Flow
   Specifications already installed (for example, because the new Flow
   Specification has irresolvable conflicts with other Flow
   Specifications that are already installed) MUST respond with a PCErr
   message with Error-Type 30 (FlowSpec Error) and Error-value 3
   (Unresolvable Conflict) and MUST NOT install the Flow Specification.

9.  PCEP Messages

   This section describes the format of messages that contain FLOWSPEC
   objects.  The only difference from previous message formats is the
   inclusion of that object.

   The figures in this section use the notation defined in [RFC5511].

   The FLOWSPEC object is OPTIONAL and MAY be carried in the PCEP
   messages.

   The PCInitiate message is defined in [RFC8281] and updated as below:

   <PCInitiate Message> ::= <Common Header>
                            <PCE-initiated-lsp-list>

   Where:
      <PCE-initiated-lsp-list> ::= <PCE-initiated-lsp-request>
                                   [<PCE-initiated-lsp-list>]

      <PCE-initiated-lsp-request> ::=
                                    ( <PCE-initiated-lsp-instantiation>|
                                      <PCE-initiated-lsp-deletion> )

      <PCE-initiated-lsp-instantiation> ::= <SRP>
                                            <LSP>
                                            [<END-POINTS>]
                                            <ERO>
                                            [<attribute-list>]
                                            [<flowspec-list>]

      Where:
         <flowspec-list> ::= <FLOWSPEC> [<flowspec-list>]

   The PCUpd message is defined in [RFC8231] and updated as below:

   <PCUpd Message> ::= <Common Header>
                       <update-request-list>

   Where:
      <update-request-list> ::= <update-request>
                                [<update-request-list>]

      <update-request> ::= <SRP>
                           <LSP>
                           <path>
                           [<flowspec-list>]

      Where:
         <path>::= <intended-path><intended-attribute-list>

         <flowspec-list> ::= <FLOWSPEC> [<flowspec-list>]

   The PCRpt message is defined in [RFC8231] and updated as below:

   <PCRpt Message> ::= <Common Header>
                       <state-report-list>

   Where:
      <state-report-list> ::= <state-report>[<state-report-list>]

      <state-report> ::= [<SRP>]
                         <LSP>
                         <path>
                         [<flowspec-list>]

       Where:
         <path>::= <intended-path>
                   [<actual-attribute-list><actual-path>]
                   <intended-attribute-list>

         <flowspec-list> ::= <FLOWSPEC> [<flowspec-list>]

   The PCReq message is defined in [RFC5440] and updated in [RFC8231];
   it is further updated below for a Flow Specification:

   <PCReq Message>::= <Common Header>
                      [<svec-list>]
                      <request-list>

   Where:
      <svec-list>::= <SVEC>[<svec-list>]

      <request-list>::= <request>[<request-list>]

      <request>::= <RP>
                   <END-POINTS>
                   [<LSP>]
                   [<LSPA>]
                   [<BANDWIDTH>]
                   [<metric-list>]
                   [<RRO>[<BANDWIDTH>]]
                   [<IRO>]
                   [<LOAD-BALANCING>]
                   [<flowspec-list>]

      Where:
         <flowspec-list> ::= <FLOWSPEC> [<flowspec-list>]

   The PCRep message is defined in [RFC5440] and updated in [RFC8231];
   it is further updated below for a Flow Specification:

   <PCRep Message> ::= <Common Header>
                       <response-list>

   Where:
      <response-list>::=<response>[<response-list>]

      <response>::=<RP>
                  [<LSP>]
                  [<NO-PATH>]
                  [<attribute-list>]
                  [<path-list>]
                  [<flowspec-list>]

      Where:
         <flowspec-list> ::= <FLOWSPEC> [<flowspec-list>]

10.  IANA Considerations

   This document requests that IANA allocate code points for the
   protocol elements defined in this document.

10.1.  PCEP Objects

   IANA maintains a subregistry called "PCEP Objects" within the "Path
   Computation Element Protocol (PCEP) Numbers" registry.  Each PCEP
   object has an Object-Class and an Object-Type, and IANA has allocated
   new code points in this subregistry as follows:

   +====================+==========+=======================+===========+
   | Object-Class Value | Name     | Object-Type           | Reference |
   +====================+==========+=======================+===========+
   | 43                 | FLOWSPEC | 0: Reserved           | RFC 9168  |
   |                    |          +-----------------------+-----------+
   |                    |          | 1: Flow               | RFC 9168  |
   |                    |          | Specification         |           |
   +--------------------+----------+-----------------------+-----------+

                Table 3: PCEP Objects Subregistry Additions

10.1.1.  PCEP FLOWSPEC Object Flag Field

   This document requests that a new subregistry, "FLOWSPEC Object Flag
   Field", be created within the "Path Computation Element Protocol
   (PCEP) Numbers" registry to manage the Flag field of the FLOWSPEC
   object.  New values are to be assigned by Standards Action [RFC8126].
   Each bit should be tracked with the following qualities:

   *  Bit number (counting from bit 0 as the most significant bit)

   *  Capability description

   *  Defining RFC

   The initial population of this registry is as follows:

   +=====+================+===========+
   | Bit | Description    | Reference |
   +=====+================+===========+
   | 0-5 | Unassigned     |           |
   +-----+----------------+-----------+
   | 6   | LPM (L bit)    | RFC 9168  |
   +-----+----------------+-----------+
   | 7   | Remove (R bit) | RFC 9168  |
   +-----+----------------+-----------+

     Table 4: Initial Contents of the
        FLOWSPEC Object Flag Field
                 Registry

10.2.  PCEP TLV Type Indicators

   IANA maintains a subregistry called "PCEP TLV Type Indicators" within
   the "Path Computation Element Protocol (PCEP) Numbers" registry.
   IANA has made the following allocations in this subregistry:

   +=======+=============================+===========+
   | Value | Description                 | Reference |
   +=======+=============================+===========+
   | 51    | PCE-FLOWSPEC-CAPABILITY TLV | RFC 9168  |
   +-------+-----------------------------+-----------+
   | 52    | FLOW FILTER TLV             | RFC 9168  |
   +-------+-----------------------------+-----------+

      Table 5: PCEP TLV Type Indicators Subregistry
                        Additions

10.3.  Flow Specification TLV Type Indicators

   IANA has created a new subregistry called "PCEP Flow Specification
   TLV Type Indicators" within the "Path Computation Element Protocol
   (PCEP) Numbers" registry.

   Allocations from this registry are to be made according to the
   following assignment policies [RFC8126]:

   +=============+===================================+
   | Range       | Registration Procedures           |
   +=============+===================================+
   | 0-255       | Reserved - must not be allocated. |
   |             |                                   |
   |             | Usage mirrors the BGP Flow Spec   |
   |             | registry [RFC8955] [RFC8956].     |
   +-------------+-----------------------------------+
   | 256-64506   | Specification Required            |
   +-------------+-----------------------------------+
   | 64507-65531 | First Come First Served           |
   +-------------+-----------------------------------+
   | 65532-65535 | Experimental Use                  |
   +-------------+-----------------------------------+

      Table 6: Registration Procedures for the PCEP
          Flow Specification TLV Type Indicators
                       Subregistry

   IANA has populated this registry with values defined in this document
   as follows, taking the new values from the range 256 to 64506:

   +=======+=====================+
   | Value | Meaning             |
   +=======+=====================+
   | 256   | Route Distinguisher |
   +-------+---------------------+
   | 257   | IPv4 Multicast      |
   +-------+---------------------+
   | 258   | IPv6 Multicast      |
   +-------+---------------------+

      Table 7: Initial Contents
           of the PCEP Flow
        Specification TLV Type
        Indicators Subregistry

10.4.  PCEP Error Codes

   IANA maintains a subregistry called "PCEP-ERROR Object Error Types
   and Values" within the "Path Computation Element Protocol (PCEP)
   Numbers" registry.  Entries in this subregistry are described by
   Error-Type and Error-value.  IANA has added the following assignment
   to this subregistry:

   +============+================+=========================+===========+
   | Error-Type | Meaning        | Error-value             | Reference |
   +============+================+=========================+===========+
   | 30         | FlowSpec error | 0: Unassigned           | RFC 9168  |
   |            |                +-------------------------+-----------+
   |            |                | 1: Unsupported          | RFC 9168  |
   |            |                | FlowSpec                |           |
   |            |                +-------------------------+-----------+
   |            |                | 2: Malformed            | RFC 9168  |
   |            |                | FlowSpec                |           |
   |            |                +-------------------------+-----------+
   |            |                | 3: Unresolvable         | RFC 9168  |
   |            |                | Conflict                |           |
   |            |                +-------------------------+-----------+
   |            |                | 4: Unknown              | RFC 9168  |
   |            |                | FlowSpec                |           |
   |            |                +-------------------------+-----------+
   |            |                | 5: Unsupported          | RFC 9168  |
   |            |                | LPM Route               |           |
   |            |                +-------------------------+-----------+
   |            |                | 6-255:                  | RFC 9168  |
   |            |                | Unassigned              |           |
   +------------+----------------+-------------------------+-----------+

       Table 8: PCEP-ERROR Object Error Types and Values Subregistry
                                 Additions

10.5.  PCE Capability Flag

   IANA has registered a new capability bit in the OSPF Parameters "Path
   Computation Element (PCE) Capability Flags" registry as follows:

   +=====+========================+===========+
   | Bit | Capability Description | Reference |
   +=====+========================+===========+
   | 16  | FlowSpec               | RFC 9168  |
   +-----+------------------------+-----------+

     Table 9: Path Computation Element (PCE)
       Capability Flags Registry Additions

11.  Security Considerations

   We may assume that a system that utilizes a remote PCE is subject to
   a number of vulnerabilities that could allow spurious LSPs or SR
   paths to be established or that could result in existing paths being
   modified or torn down.  Such systems, therefore, apply security
   considerations as described in [RFC5440], Section 2.5 of [RFC6952],
   [RFC8253], and [RFC8955].

   The description of Flow Specifications associated with paths set up
   or controlled by a PCE adds a further detail that could be attacked
   without tearing down LSPs or SR paths but causes traffic to be
   misrouted within the network.  Therefore, the use of the security
   mechanisms for PCEP referenced above is important.

   Visibility into the information carried in PCEP does not have direct
   privacy concerns for end users' data; however, knowledge of how data
   is routed in a network may make that data more vulnerable.  Of
   course, the ability to interfere with the way data is routed also
   makes the data more vulnerable.  Furthermore, knowledge of the
   connected endpoints (such as multicast receivers or VPN sites) is
   usually considered private customer information.  Therefore,
   implementations or deployments concerned with protecting privacy MUST
   apply the mechanisms described in the documents referenced above, in
   particular, to secure the PCEP session using IPsec per Sections 10.4
   to 10.6 of [RFC5440] or TLS per [RFC8253].  Note that TCP-MD5
   security as originally suggested in [RFC5440] does not provide
   sufficient security or privacy guarantees and SHOULD NOT be relied
   upon.

   Experience with Flow Specifications in BGP systems indicates that
   they can become complex and that the overlap of Flow Specifications
   installed in different orders can lead to unexpected results.
   Although this is not directly a security issue per se, the confusion
   and unexpected forwarding behavior may be engineered or exploited by
   an attacker.  Furthermore, this complexity might give rise to a
   situation where the forwarding behaviors might create gaps in the
   monitoring and inspection of particular traffic or provide a path
   that avoids expected mitigations.  Therefore, implementers and
   operators SHOULD pay careful attention to the manageability
   considerations described in Section 12 and familiarize themselves
   with the careful explanations in [RFC8955].

12.  Manageability Considerations

   The feature introduced by this document enables operational
   manageability of networks operated in conjunction with a PCE and
   using PCEP.  In the case of a stateful active PCE or with PCE-
   initiated services, in the absence of this feature, additional manual
   configuration is needed to tell the head ends what traffic to place
   on the network services (LSPs, SR paths, etc.).

   This section follows the advice and guidance of [RFC6123].

12.1.  Management of Multiple Flow Specifications

   Experience with Flow Specification in BGP suggests that there can be
   a lot of complexity when two or more Flow Specifications overlap.
   This can arise, for example, with addresses indicated using prefixes
   and could cause confusion about what traffic should be placed on
   which path.  Unlike the behavior in a distributed routing system, it
   is not important to the routing stability and consistency of the
   network that each head-end implementation applies the same rules to
   disambiguate overlapping Flow Specifications, but it is important
   that:

   *  a network operator can easily find out what traffic is being
      placed on which path and why.  This will facilitate analysis of
      the network and diagnosis of faults.

   *  a PCE be able to correctly predict the effect of instructions it
      gives to a PCC.  This will ensure that traffic is correctly placed
      on the network without causing congestion or other network
      inefficiencies and that traffic is correctly delivered.

   To that end, a PCC MUST enable an operator to view the Flow
   Specifications that it has installed, and these MUST be presented in
   order of precedence such that when two Flow Specifications overlap,
   the one that will be serviced with higher precedence is presented to
   the operator first.

   A discussion of precedence ordering for Flow Specifications is found
   in Section 8.7.

12.2.  Control of Function through Configuration and Policy

   Support for the function described in this document implies that a
   functional element that is capable of requesting that a PCE compute
   and control a path is also able to configure the specification of
   what traffic should be placed on that path.  Where there is a human
   involved in this action, configuration of the Flow Specification must
   be available through an interface (such as a graphical user interface
   or a Command Line Interface).  Where a distinct software component
   (i.e., one not co-implemented with the PCE) is used, a protocol
   mechanism will be required that could be PCEP itself or a data model,
   such as extensions to the YANG model for requesting path computation
   [TEAS-YANG-PATH].

   Implementations MAY be constructed with a configurable switch to
   indicate whether they support the functions defined in this document.
   Otherwise, such implementations MUST indicate that they support the
   function as described in Section 4.  If an implementation allows
   configurable support of this function, that support MAY be
   configurable per peer or once for the whole implementation.

   As mentioned in Section 12.1, a PCE implementation SHOULD provide a
   mechanism to configure variations in the precedence ordering of Flow
   Specifications per PCC.

12.3.  Information and Data Models

   The YANG model in [PCE-PCEP-YANG] can be used to model and monitor
   PCEP states and messages.  To make that YANG model useful for the
   extensions described in this document, it would need to be augmented
   to cover the new protocol elements.

   Similarly, as noted in Section 12.2, the YANG model defined in
   [TEAS-YANG-PATH] could be extended to allow the specification of Flow
   Specifications.

   Finally, as mentioned in Section 12.1, a PCC implementation SHOULD
   provide a mechanism to allow an operator to read the Flow
   Specifications from a PCC and to understand in what order they will
   be executed.  This could be achieved using a new YANG model.

12.4.  Liveness Detection and Monitoring

   The extensions defined in this document do not require any additional
   liveness detection and monitoring support.  See [RFC5440] and
   [RFC5886] for more information.

12.5.  Verifying Correct Operation

   The chief element of operation that needs to be verified (in addition
   to the operation of the protocol elements as described in [RFC5440])
   is the installation, precedence, and correct operation of the Flow
   Specifications at a PCC.

   In addition to the YANG model, for reading Flow Specifications
   described in Section 12.3, tools may be needed to inject Operations
   and Management (OAM) traffic at the PCC that matches specific
   criteria so that it can be monitored while traveling along the
   desired path.  Such tools are outside the scope of this document.

12.6.  Requirements for Other Protocols and Functional Components

   This document places no requirements on other protocols or
   components.

12.7.  Impact on Network Operation

   The use of the features described in this document clearly have an
   important impact on network traffic since they cause traffic to be
   routed on specific paths in the network.  However, in practice, these
   changes make no direct changes to the network operation because
   traffic is already placed on those paths using some pre-existing
   configuration mechanism.  Thus, the significant change is the
   reduction in mechanisms that have to be applied rather than a change
   to how the traffic is passed through the network.

13.  References

13.1.  Normative References

   [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>.

   [RFC4364]  Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
              Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February
              2006, <https://www.rfc-editor.org/info/rfc4364>.

   [RFC4760]  Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
              "Multiprotocol Extensions for BGP-4", RFC 4760,
              DOI 10.17487/RFC4760, January 2007,
              <https://www.rfc-editor.org/info/rfc4760>.

   [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>.

   [RFC5511]  Farrel, A., "Routing Backus-Naur Form (RBNF): A Syntax
              Used to Form Encoding Rules in Various Routing Protocol
              Specifications", RFC 5511, DOI 10.17487/RFC5511, April
              2009, <https://www.rfc-editor.org/info/rfc5511>.

   [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>.

   [RFC8232]  Crabbe, E., Minei, I., Medved, J., Varga, R., Zhang, X.,
              and D. Dhody, "Optimizations of Label Switched Path State
              Synchronization Procedures for a Stateful PCE", RFC 8232,
              DOI 10.17487/RFC8232, September 2017,
              <https://www.rfc-editor.org/info/rfc8232>.

   [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>.

   [RFC8955]  Loibl, C., Hares, S., Raszuk, R., McPherson, D., and M.
              Bacher, "Dissemination of Flow Specification Rules",
              RFC 8955, DOI 10.17487/RFC8955, December 2020,
              <https://www.rfc-editor.org/info/rfc8955>.

   [RFC8956]  Loibl, C., Ed., Raszuk, R., Ed., and S. Hares, Ed.,
              "Dissemination of Flow Specification Rules for IPv6",
              RFC 8956, DOI 10.17487/RFC8956, December 2020,
              <https://www.rfc-editor.org/info/rfc8956>.

13.2.  Informative References

   [BGP-L2VPN]
              Hao, W., Eastlake, D. E., Litkowski, S., and S. Zhuang,
              "BGP Dissemination of L2 Flow Specification Rules", Work
              in Progress, Internet-Draft, draft-ietf-idr-flowspec-
              l2vpn-18, 24 October 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-idr-
              flowspec-l2vpn-18>.

   [NUMERIC-IDS-SEC]
              Gont, F. and I. Arce, "Security Considerations for
              Transient Numeric Identifiers Employed in Network
              Protocols", Work in Progress, Internet-Draft, draft-gont-
              numeric-ids-sec-considerations-06, 5 December 2020,
              <https://datatracker.ietf.org/doc/html/draft-gont-numeric-
              ids-sec-considerations-06>.

   [PCE-PCEP-YANG]
              Dhody, D., Hardwick, J., Beeram, V. P., and J. Tantsura,
              "A YANG Data Model for Path Computation Element
              Communications Protocol (PCEP)", Work in Progress,
              Internet-Draft, draft-ietf-pce-pcep-yang-17, 23 October
              2021, <https://datatracker.ietf.org/doc/html/draft-ietf-
              pce-pcep-yang-17>.

   [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>.

   [RFC5088]  Le Roux, JL., Ed., Vasseur, JP., Ed., Ikejiri, Y., and R.
              Zhang, "OSPF Protocol Extensions for Path Computation
              Element (PCE) Discovery", RFC 5088, DOI 10.17487/RFC5088,
              January 2008, <https://www.rfc-editor.org/info/rfc5088>.

   [RFC5089]  Le Roux, JL., Ed., Vasseur, JP., Ed., Ikejiri, Y., and R.
              Zhang, "IS-IS Protocol Extensions for Path Computation
              Element (PCE) Discovery", RFC 5089, DOI 10.17487/RFC5089,
              January 2008, <https://www.rfc-editor.org/info/rfc5089>.

   [RFC5886]  Vasseur, JP., Ed., Le Roux, JL., and Y. Ikejiri, "A Set of
              Monitoring Tools for Path Computation Element (PCE)-Based
              Architecture", RFC 5886, DOI 10.17487/RFC5886, June 2010,
              <https://www.rfc-editor.org/info/rfc5886>.

   [RFC6123]  Farrel, A., "Inclusion of Manageability Sections in Path
              Computation Element (PCE) Working Group Drafts", RFC 6123,
              DOI 10.17487/RFC6123, February 2011,
              <https://www.rfc-editor.org/info/rfc6123>.

   [RFC6952]  Jethanandani, M., Patel, K., and L. Zheng, "Analysis of
              BGP, LDP, PCEP, and MSDP Issues According to the Keying
              and Authentication for Routing Protocols (KARP) Design
              Guide", RFC 6952, DOI 10.17487/RFC6952, May 2013,
              <https://www.rfc-editor.org/info/rfc6952>.

   [RFC7399]  Farrel, A. and D. King, "Unanswered Questions in the Path
              Computation Element Architecture", RFC 7399,
              DOI 10.17487/RFC7399, October 2014,
              <https://www.rfc-editor.org/info/rfc7399>.

   [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>.

   [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>.

   [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>.

   [TEAS-YANG-PATH]
              Busi, I., Belotti, S., Lopez, V., Sharma, A., and Y. Shi,
              "YANG Data Model for requesting Path Computation", Work in
              Progress, Internet-Draft, draft-ietf-teas-yang-path-
              computation-16, 6 September 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-teas-
              yang-path-computation-16>.

Acknowledgements

   Thanks to Julian Lucek, Sudhir Cheruathur, Olivier Dugeon, Jayant
   Agarwal, Jeffrey Zhang, Acee Lindem, Vishnu Pavan Beeram, Julien
   Meuric, Deborah Brungard, √Čric Vyncke, Erik Kline, Benjamin Kaduk,
   Martin Duke, Roman Danyliw, and Alvaro Retana for useful discussions
   and comments.

Contributors

   Shankara
   Huawei Technologies
   Divyashree Techno Park, Whitefield
   Bangalore 560066
   Karnataka
   India

   Email: shankara@huawei.com


   Qiandeng Liang
   Huawei Technologies
   Yuhuatai District
   101 Software Avenue,
   Nanjing, 210012
   China

   Email: liangqiandeng@huawei.com


   Cyril Margaria
   Juniper Networks
   200 Somerset Corporate Boulevard, Suite 4001
   Bridgewater, NJ 08807
   United States of America

   Email: cmargaria@juniper.net


   Colby Barth
   Juniper Networks
   200 Somerset Corporate Boulevard, Suite 4001
   Bridgewater, NJ 08807
   United States of America

   Email: cbarth@juniper.net


   Xia Chen
   Huawei Technologies
   Huawei Bld., No. 156 Beiqing Rd.
   Beijing, 100095
   China

   Email: jescia.chenxia@huawei.com


   Shunwan Zhuang
   Huawei Technologies
   Huawei Bld., No. 156 Beiqing Rd.
   Beijing, 100095
   China

   Email: zhuangshunwan@huawei.com


   Cheng Li
   Huawei Technologies
   Huawei Campus, No. 156 Beiqing Rd.
   Beijing, 100095
   China

   Email: c.l@huawei.com


Authors' Addresses

   Dhruv Dhody
   Huawei Technologies
   Divyashree Techno Park, Whitefield
   Bangalore 560066
   Karnataka
   India

   Email: dhruv.ietf@gmail.com


   Adrian Farrel
   Old Dog Consulting

   Email: adrian@olddog.co.uk


   Zhenbin Li
   Huawei Technologies
   Huawei Bldg., No. 156 Beiqing Rd.
   Beijing
   100095
   China

   Email: lizhenbin@huawei.com