Path Computation Element Communication Protocol (PCEP) Procedures and Extensions for Using the PCE as a Central Controller (PCECC) of LSPs

Introduction The Path Computation Element (PCE) was developed to offload the path computation function from routers in an MPLS traffic-engineered (TE) network. It can compute optimal paths for traffic across a network and can also update the paths to reflect changes in the network or traffic demands. Since then, the role and function of the PCE have grown to cover a number of other uses (such as GMPLS ) and to allow delegated control and PCE-initiated use of network resources . According to , Software-Defined Networking (SDN) refers to a separation between the control elements and the forwarding components so that software running in a centralized system, called a controller, can act to program the devices in the network to behave in specific ways. A required element in an SDN architecture is a component that plans how the network resources will be used and how the devices will be programmed. It is possible to view this component as performing specific computations to place traffic flows within the network given knowledge of the availability of network resources, how other forwarding devices are programmed, and the way that other flows are routed. This is the function and purpose of a PCE, and the way that a PCE integrates into a wider network control system (including an SDN system) is presented in . In early PCE implementations, where the PCE was used to derive paths for MPLS Label Switched Paths (LSPs), paths were requested by network elements (known as Path Computation Clients (PCCs)), and the results of the path computations were supplied to network elements using the Path Computation Element Communication Protocol (PCEP) . This protocol was later extended to allow a PCE to send unsolicited requests to the network for LSP establishment . The PCE was developed to derive paths for MPLS LSPs, which are supplied to the head end of the LSP using the PCEP. But SDN has a broader applicability than signaled MPLS and GMPLS TE networks, and the PCE may be used to determine paths in a range of use cases. PCEP has been proposed as a control protocol for use in these environments to allow the PCE to be fully enabled as a central controller. introduces the architecture for the PCE as a central controller as an extension to the architecture described in and assumes the continued use of PCEP as the protocol used between the PCE and PCC. further examines the motivations and applicability for PCEP as a Southbound Interface (SBI) and introduces the implications for the protocol. describes the use cases for the PCECC architecture. A PCECC can simplify the processing of a distributed control plane by blending it with elements of SDN and without necessarily completely replacing it. Thus, the LSP can be calculated/set up/initiated and the label-forwarding entries can also be downloaded through a centralized PCE server to each network device along the path while leveraging the existing PCE technologies as much as possible. This document specifies the procedures and PCEP extensions for using the PCE as the central controller for static LSPs, where LSPs can be provisioned as explicit label instructions at each hop on the end-to-end path. Each router along the path must be told what label-forwarding instructions to program and what resources to reserve. The PCE-based controller keeps a view of the network and determines the paths of the end-to-end LSPs, and the controller uses PCEP to communicate with each router along the path of the end-to-end LSP. While this document is focused on the procedures for the static LSPs (referred to as the basic PCECC mode in ), the mechanisms and protocol encodings are specified in such a way that extensions for other use cases are easy to achieve. For example, the extensions for the PCECC for Segment Routing (SR) are specified in and .

Terminology The terminology used in this document is the same as that described in the .

Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

Basic PCECC Mode In this mode, LSPs are provisioned as explicit label instructions at each hop on the end-to-end path. Each router along the path must be told what label-forwarding instructions to program and what resources to reserve. The controller uses PCEP to communicate with each router along the path of the end-to-end LSP. examines the motivations and applicability for the PCECC and use of PCEP as an SBI. highlights the use of the PCECC for label allocation along the static LSPs, and it simplifies the processing of a distributed control plane by blending it with elements of SDN and without necessarily completely replacing it. This allows the operator to introduce the advantages of SDN (such as programmability) into the network. Further, describes some of the scenarios where the PCECC technique could be useful. also describes the implications on the protocol when used as an SDN SBI. The operator needs to evaluate the advantages offered by the PCECC against the operational and scalability needs of the PCECC. As per , the PCE-based controller will take responsibility for managing some part of the MPLS label space for each of the routers that it controls and may take wider responsibility for partitioning the label space for each router and allocating different parts for different uses. The PCC MUST NOT make allocations from the label space set aside for the PCE to avoid overlap and collisions of label allocations. It is RECOMMENDED that the PCE makes allocations (from the label space set aside for the PCE) for all nodes along the path. For the purpose of this document, it is assumed 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 ). The rest of the processing is similar to the existing stateful PCE mechanism. This document also allows a case where the label space is maintained by the PCC and the labels are allocated by it. In this case, the PCE should request the allocation from the PCC, as described in .

PCEP Requirements The following key requirements should be considered when designing the PCECC-based solution:

A PCEP speaker supporting this document needs to have the capability to advertise its PCECC capability to its peers.
A PCEP speaker needs means to identify PCECC-based LSPs in the PCEP messages.
PCEP procedures need to allow for PCC-based label allocations.
PCEP procedures need to provide a means to update (or clean up) label entries downloaded to the PCC.
PCEP procedures need to provide a means to synchronize the labels between the PCE and the PCC via PCEP messages.

Procedures for Using the PCE as a Central Controller (PCECC)

Stateful PCE Model Active stateful PCE is described in . A PCE as a Central Controller (PCECC) reuses the existing active stateful PCE mechanism as much as possible to control LSPs.

New LSP Functions Several new functions are required in PCEP to support the PCECC. This document extends the existing messages to support the new functions required by the PCECC:

PCInitiate:: A PCEP message described in . A PCInitiate message is used to set up a PCE-initiated LSP based on a PCECC mechanism. It is also extended for Central Controller Instructions (CCI) (download or clean up the label-forwarding instructions in the context of this document) on all nodes along the path, as described in .
PCRpt:: A PCEP message described in . A PCRpt message is used to send the PCECC LSP Reports. It is also extended to report the set of CCI (label-forwarding instructions in the context of this document) received from the PCE, as described in . describes the use of a PCRpt message during synchronization.
PCUpd:: A PCEP message described in . A PCUpd message is used to send the PCECC LSP Updates.

The new functions defined in this document are mapped onto the PCEP messages, as shown in . Functions Mapped to the PCEP Messages

Function	Message
PCECC Capability advertisement	Open
Label entry Add	PCInitiate
Label entry Clean up	PCInitiate
PCECC-Initiated LSP	PCInitiate
PCECC LSP Update	PCUpd
PCECC LSP State Report	PCRpt
PCECC LSP Delegation	PCRpt
PCECC Label Report	PCRpt

New PCEP Object This document defines a new PCEP object called CCI () to specify the Central Controller Instructions. In the scope of this document, this is limited to label-forwarding instructions. Future documents can create new CCI object-types for other types of Central Controller Instructions. The CC-ID is the unique identifier for the CCI in PCEP. The PCEP messages are extended in this document to handle the PCECC operations.

PCECC Capability Advertisement During the PCEP initialization phase, PCEP speakers (PCE or PCC) advertise their support of and willingness to use PCEP extensions for the PCECC using these elements in the OPEN message:

a new Path Setup Type (PST) () in the PATH-SETUP-TYPE-CAPABILITY TLV to indicate support for PCEP extensions for the PCECC - 2 (Traffic engineering path is set up using PCECC mode)
a new PCECC-CAPABILITY sub-TLV () with the L bit set to '1' inside the PATH-SETUP-TYPE-CAPABILITY TLV to indicate a willingness to use PCEP extensions for the PCECC-based Central Controller Instructions for label download
the STATEFUL-PCE-CAPABILITY TLV (with the I flag set )

The new PST is to be listed in the PATH-SETUP-TYPE-CAPABILITY TLV by all PCEP speakers that support the PCEP extensions for the PCECC in this document. The new PCECC-CAPABILITY sub-TLV is included in the PATH-SETUP-TYPE-CAPABILITY TLV in the OPEN object to indicate a willingness to use the PCEP extensions for the PCECC during the established PCEP session. Using the L bit in this TLV, the PCE shows the intention to function as a PCECC server, and the PCC shows a willingness to act as a PCECC client for label download instructions (see ). If the PCECC-CAPABILITY sub-TLV is advertised and the STATEFUL-PCE-CAPABILITY TLV is not advertised, or is advertised without the I flag set, in the OPEN object, the receiver MUST:

send a PCErr message with Error-Type=19 (Invalid Operation) and Error-value=17 (Stateful PCE capability was not advertised) and
terminate the session.

If a PCEP speaker receives the PATH-SETUP-TYPE-CAPABILITY TLV with the PCECC PST but without the PCECC-CAPABILITY sub-TLV, it MUST:

send a PCErr message with Error-Type=10 (Reception of an invalid object) and Error-value=33 (Missing PCECC Capability sub-TLV) and
terminate the PCEP session.

The PCECC-CAPABILITY sub-TLV MUST NOT be used without the corresponding PST being listed in the PATH-SETUP-TYPE-CAPABILITY TLV. If it is present without the corresponding PST listed in the PATH-SETUP-TYPE-CAPABILITY TLV, it MUST be ignored. If one or both speakers (PCE and PCC) have not indicated support and willingness to use the PCEP extensions for the PCECC, the PCEP extensions for the PCECC MUST NOT be used. If a PCECC operation is attempted when both speakers have not agreed in the OPEN messages, the receiver of the message 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 PCECC Capability sub-TLV) will ignore the sub-TLV in accordance with and . As per , the legacy PCEP speaker, on receipt of an unsupported PST in a Request Parameter (RP) / Stateful PCE Request Parameter (SRP) object, will:

send a PCErr message with Error-Type=21 (Invalid traffic engineering path setup type) and Error-value=1 (Unsupported path setup type) and
terminate the PCEP session.

LSP Operations The PCEP messages pertaining to a PCECC MUST include the PATH-SETUP-TYPE TLV in the SRP object with the PST set to '2' to clearly identify that the PCECC LSP is intended.

PCE-Initiated PCECC LSP The LSP instantiation operation is defined in . In order to set up a PCE-initiated LSP based on the PCECC mechanism, a PCE sends a PCInitiate message with the PST set to '2' for the PCECC (see ) to the ingress PCC. The label-forwarding instructions (see ) from the PCECC are sent after the initial PCInitiate and PCRpt message exchange with the ingress PCC, as per (see ). This is done so that the PCEP-specific identifier for the LSP (PLSP-ID) and other LSP identifiers can be obtained from the ingress and can be included in the label-forwarding instruction in the next set of PCInitiate messages along the path, as described below. An LSP-IDENTIFIERS TLV MUST be included for the PCECC LSPs; it uniquely identifies the LSP in the network. Note that the fields in the LSP-IDENTIFIERS TLV are described for the RSVP-signaled LSPs but are applicable to the PCECC LSP as well. The LSP object is included in the CCI (label download ) to identify the PCECC LSP for this instruction. The PLSP-ID is the original identifier used by the ingress PCC, so a transit/egress Label Switching Router (LSR) could have multiple Central Controller Instructions that have the same PLSP-ID. The PLSP-ID in combination with the source (in the LSP-IDENTIFIERS TLV) MUST be unique. The PLSP-ID is included for maintainability reasons to ease debugging. As per , the LSP object could also include the SPEAKER-ENTITY-ID TLV to identify the PCE that initiated these instructions. Also, the CC-ID is unique in each PCEP session, as described in . On receipt of a PCInitiate message for the PCECC LSP, the PCC responds with a PCRpt message with the status set to 'Going-up' and carrying the assigned PLSP-ID (see ). The ingress PCC also sets the D (Delegate) flag (see ) and C (Create) flag (see ) in the LSP object. When the PCE receives this PCRpt message with the PLSP-ID, it assigns labels along the path and sets up the path by sending a PCInitiate message to each node along the path of the LSP, as per the PCECC technique. The CC-ID uniquely identifies the Central Controller Instructions within a PCEP session. Each node along the path (PCC) responds with a PCRpt message to acknowledge the CCI with the PCRpt messages including the CCI and LSP objects. The ingress node would receive one CCI object with the O bit (out-label) set. The transit node(s) would receive two CCI objects with the in-label CCI without the O bit set and the out-label CCI with the O bit set. The egress node would receive one CCI object without the O bit set (see ). A node can determine its role based on the setting of the O bit in the CCI object(s) and the LSP-IDENTIFIERS TLV in the LSP object. The LSP deletion operation for the PCE-initiated PCECC LSP is the same as defined in . The PCE should further perform the label entry cleanup operation, as described in , for the corresponding LSP.

PCE-Initiated PCECC LSP | PCECC LSP +--------+ | | (GOING-UP) | | | | | |<-------PCInitiate,CC-ID=X,PLSP-ID=2----------------| Label | | | | download |--------PCRpt,CC-ID=X,PLSP-ID=2-------------------->| CCI | | | | | |<------PCInitiate,CC-ID=Y1,Y2,PLSP-ID=2-----| Label | | | | download | |-------PCRpt,CC-ID=Y1,Y2,PLSP-ID=2--------->| CCI | | | | | | |<----PCInitiate,CC-ID=Z,PLSP-ID=2-----| Label | | | | download | | |-----PCRpt,CC-ID=Z,PLSP-ID=2--------->| CCI | | | | | | |<---PCUpd,PLSP-ID=2,PST=2,D=1---------| PCECC LSP | | | (UP) | Update | | |----PCRpt,PLSP-ID=2,D=1,C=1---------->| | | | (UP) | ]]> Once the label operations are completed, the PCE MUST send a PCUpd message to the ingress PCC. As per , the PCUpd message is with the D flag set. The PCECC LSPs are considered to be 'up' by default (on receipt of a PCUpd message from the PCE). The ingress could further choose to deploy a data-plane check mechanism and report the status back to the PCE via a PCRpt message to make sure that the correct label instructions are made along the path of the PCECC LSP (and it is ready to carry traffic). The exact mechanism is out of scope of this document. In the case where the label allocations are made by the PCC itself (see ), the PCE could request an allocation to be made by the PCC; then, the PCC would send a PCRpt message with the allocated label encoded in the CC-ID object (as shown in ) in the configuration sequence from the egress towards the ingress along the path.

PCE-Initiated PCECC LSP (PCC Allocation) | PCECC LSP +--------+ | | (GOING-UP) | | | | | |<-------PCInitiate,CC-ID=X,PLSP-ID=2----------------| Label | | | C=1,O=0 | download |--------PCRpt,CC-ID=X,PLSP-ID=2-------------------->| CCI | | | Label=L1 | | |<------PCInitiate,PLSP-ID=2,----------------| Labels | | | CC-ID=Y1,C=1,O=0 | download | | | CC-ID=Y2,C=0,O=1,L1 | CCI | |-------PCRpt,PLSP-ID=2--------------------->| | | | CC-ID=Y1,O=0,Label=L2 | | | | CC-ID=Y2,O=1 | | | |<----PCInitiate,CC-ID=Z,PLSP-ID=2-----| Label | | | C=0,O=1,L2 | download | | |-----PCRpt,CC-ID=Z,PLSP-ID=2--------->| CCI | | | | | | |<---PCUpd,PLSP-ID=2,PST=2,D=1---------| PCECC LSP | | | (UP) | Update ]]> In this example, it should be noted that the request is made to the egress node with the C bit set in the CCI object to indicate that the label allocation needs to be done by the egress, and the egress responds with the allocated label to the PCE. The PCE further informs the transit PCC without setting the C bit to '1' in the CCI object for the out-label, but the C bit is set to '1' for the in-label, so the transit node makes the label allocation (for the in-label) and reports to the PCE. Similarly, the C bit is unset towards the ingress to complete all the label allocations for the PCECC LSP.

PCC-Initiated PCECC LSP In order to set up an LSP based on the PCECC mechanism where the LSP is configured at the PCC, a PCC MUST delegate the LSP by sending a PCRpt message with the PST set for the PCECC (see ) and D (Delegate) flag (see ) set in the LSP object (see ). When a PCE receives the initial PCRpt message with the D flag and PST set to '2', it SHOULD calculate the path and assign labels along the path in addition to setting up the path by sending a PCInitiate message to each node along the path of the LSP, as per the PCECC technique (see ). The CC-ID uniquely identifies the CCI within a PCEP session. Each PCC further responds with the PCRpt messages, including the CCI and LSP objects. Once the CCI (label operations) are completed, the PCE MUST send the PCUpd message to the ingress PCC. As per , this PCUpd message should include the path information calculated by the PCE. Note that the PCECC LSPs MUST be delegated to a PCE at all times. The LSP deletion operation for the PCECC LSPs is the same as defined in . If the PCE receives a PCRpt message for LSP deletion, then it does label the cleanup operation, as described in , for the corresponding LSP. The basic PCECC LSP setup sequence is as shown in .

PCC-Initiated PCECC LSP | PCECC LSP |PCC +--------+ | | |egress | | | | +--------+ | | | | | | | |<-------PCInitiate,CC-ID=X,PLSP-ID=1---------------| Label | | | L1,O=0 | download |--------PCRpt,CC-ID=X,PLSP-ID=1------------------->| CCI | | | | | |<------PCInitiate,PLSP-ID=1,---------------| Labels | | | CC-ID=Y1,O=0,L2 | download | | | CC-ID=Y2,O=1,L1 | CCI | |-------PCRpt,CC-ID=Y1,Y2,PLSP-ID=1-------->| | | | | | | |<----PCInitiate,CC-ID=Z,PLSP-ID=1----| Label | | | L2,O=1 | download | | |-----PCRpt,CC-ID=Z,PLSP-ID=1-------->| CCI | | | | | | |<---PCUpd,PLSP-ID=1,PST=2,D=1--------| PCECC LSP | | | | Update | | | | ]]> In the case where the label allocations are made by the PCC itself (see ), the PCE could request an allocation to be made by the PCC; then, the PCC would send a PCRpt message with the allocated label encoded in the CC-ID object, as shown in .

PCC-Initiated PCECC LSP (PCC Allocation) | PCECC LSP |PCC +--------+ | | |egress | | | | +--------+ | | | | | | | |<-------PCInitiate,CC-ID=X,PLSP-ID=1---------------| Label | | | C=1 | download |--------PCRpt,CC-ID=X,PLSP-ID=1------------------->| CCI | | | Label=L1 | | |<------PCInitiate,PLSP-ID=1,---------------| Labels | | | CC-ID=Y1,C=1 | download | | | CC-ID=Y2,C=0,L1 | CCI | |-------PCRpt,PLSP-ID=1-------------------->| | | | CC-ID=Y1,Label=L2 | | | | CC-ID=Y2 | | | |<----PCInitiate,CC-ID=Z,PLSP-ID=1----| Label | | | C=0,L2 | download | | |-----PCRpt,CC-ID=Z,PLSP-ID=1-------->| CCI | | | | | | |<---PCUpd,PLSP-ID=1,PST=2,D=1--------| PCECC LSP | | | | Update | | | | ]]> In the case where the label allocations are made by the PCC itself (see ), the procedure remains the same, with just an additional constraint on the configuration sequence. The rest of the PCC-initiated PCECC LSP setup operations are the same as those described in .

Central Controller Instructions The new CCI for the label operations in PCEP are done via the PCInitiate message () by defining a new PCEP object for CCI operations. The local label range of each PCC is assumed to be known by both the PCC and the PCE.

Label Download CCI In order to set up an LSP based on the PCECC, the PCE sends a PCInitiate message to each node along the path to download the label instructions, as described in Sections and . The CCI object MUST be included, along with the LSP object in the PCInitiate message. The LSP-IDENTIFIERS TLV MUST be included in the LSP object. The SPEAKER-ENTITY-ID TLV SHOULD be included in the LSP object. If a node (PCC) receives a PCInitiate message that includes a label to download (as part of CCI) that is out of the range set aside for the PCE, it MUST send a PCErr message with Error-Type=31 (PCECC failure) and Error-value=1 (Label out of range) and MUST include the SRP object to specify the error is for the corresponding label update via a PCInitiate message. If a PCC receives a PCInitiate message but fails to download the label entry, it MUST send a PCErr message with Error-Type=31 (PCECC failure) and Error-value=2 (Instruction failed) and MUST include the SRP object to specify the error is for the corresponding label update via a PCInitiate message. A new PCEP object for CCI is defined in .

Label Cleanup CCI In order to delete an LSP based on the PCECC, the PCE sends Central Controller Instructions via a PCInitiate message to each node along the path of the LSP to clean up the label-forwarding instruction. If the PCC receives a PCInitiate message but does not recognize the label in the CCI, the PCC MUST generate a PCErr message with Error-Type=19 (Invalid operation) and Error-value=18 (Unknown Label) and MUST include the SRP object to specify the error is for the corresponding label cleanup (via a PCInitiate message). The R flag in the SRP object defined in specifies the deletion of the label entry in the PCInitiate message.

Label Cleanup | CCI | | | R=1 | | |<------PCInitiate,CC-ID=Y1,Y2,PLSP-ID=2-----| Label | | | R=1 | cleanup | |-------PCRpt,CC-ID=Y1,Y2,PLSP-ID=2--------->| CCI | | | R=1 | | | |<----PCInitiate,CC-ID=Z,PLSP-ID=2-----| Label | | | R=1 | cleanup | | |-----PCRpt,CC-ID=Z,PLSP-ID=2--------->| CCI | | | R=1 | | | |<--PCInitiate,PLSP-ID=2,PST=2,R=1-----| PCECC LSP | | | | remove ]]> As per , following the removal of the label-forwarding instruction, the PCC MUST send a PCRpt message. The SRP object in the PCRpt message MUST include the SRP-ID-number from the PCInitiate message that triggered the removal. The R flag in the SRP object MUST be set. In the case where the label allocation is made by the PCC itself (see ), the removal procedure remains the same, adding the sequence constraint.

PCECC LSP Update The update is done as per the make-before-break procedures, i.e., the PCECC first updates new label instructions based on the updated path and then informs the ingress to switch traffic before cleaning up the former instructions. New CC-IDs are used to identify the updated instructions; the identifiers in the LSP object uniquely identify the existing LSP. Once new instructions are downloaded, the PCE further updates the new path at the ingress, which triggers the traffic switch on the updated path. The ingress PCC acknowledges with a PCRpt message, on receipt of the PCRpt message, the PCE does the cleanup operation for the former LSP, as described in .

PCECC LSP Update | new CCI | | | | | |<------PCInitiate,CC-ID=YY1,YY2,PLSP-ID=1--| Label | | | | download | |-------PCRpt,CC-ID=YY1,YY2,PLSP-ID=1------>| CCI | | | | | | |<----PCInitiate,CC-ID=ZZ,PLSP-ID=1---| Label | | | | download | | |-----PCRpt,CC-ID=ZZ,PLSP-ID=1------->| CCI | | | | | | |<---PCUpd,PLSP-ID=1,PST=2,D=1--------| PCECC | | | SRP=S | LSP Update | | | | | | |---PCRpt,PLSP-ID=1,PST=2,D=1-------->| Trigger | | | (SRP=S) | Delete | | | | former CCI | | | | |<-------PCInitiate,CC-ID=X,PLSP-ID=1---------------| Label | | | R=1 | cleanup |--------PCRpt,CC-ID=X,PLSP-ID=1------------------->| CCI | | | R=1 | | |<------PCInitiate,CC-ID=Y1,Y2,PLSP-ID=1----| Label | | | R=1 | cleanup | |-------PCRpt,CC-ID=Y1,Y2,PLSP-ID=1-------->| CCI | | | R=1 | | | |<----PCInitiate,CC-ID=Z,PLSP-ID=1----| Label | | | R=1 | cleanup | | |-----PCRpt,CC-ID=Z,PLSP-ID=1-------->| CCI | | | R=1 | ]]> The modified PCECC LSPs are considered to be 'up' by default. The ingress could further choose to deploy a data-plane check mechanism and report the status back to the PCE via a PCRpt message. The exact mechanism is out of scope of this document. In the case where the label allocations are made by the PCC itself (see ), the procedure remains the same.

Re-delegation and Cleanup As described in , a new PCE can gain control over an orphaned LSP. In the case of a PCECC LSP, the new PCE MUST also gain control over the CCI in the same way by sending a PCInitiate message that includes the SRP, LSP, and CCI objects and carries the CC-ID and PLSP-ID identifying the instructions that it wants to take control of. Further, as described in , the State Timeout Interval timer ensures that a PCE crash does not result in automatic and immediate disruption for the services using PCE-initiated LSPs. Similarly the Central Controller Instructions are not removed immediately upon PCE failure. Instead, they are cleaned up on the expiration of this timer. This allows for network cleanup without manual intervention. The PCC MUST support the removal of CCI as one of the behaviors applied on expiration of the State Timeout Interval timer. In the case of the PCC-initiated PCECC LSP, the control over the orphaned LSP at the ingress PCC is taken over by the mechanism specified in to request delegation. The control over the CCI is described above using .

Synchronization of Central Controller Instructions The purpose of CCI synchronization (labels in the context of this document) is to make sure that the PCE's view of CCI (labels) matches with the PCC's label allocation. This synchronization is performed as part of the LSP State Synchronization, as described in and . As per LSP State Synchronization , a PCC reports the state of its LSPs to the PCE using PCRpt messages and, as per , the PCE would initiate any missing LSPs and/or remove any LSPs that are not wanted. The same PCEP messages and procedures are also used for the CCI synchronization. The PCRpt message includes the CCI and the LSP object to report the label-forwarding instructions. The PCE would further remove any unwanted instructions or initiate any missing instructions.

PCECC LSP State Report As mentioned before, an ingress PCC MAY choose to apply any Operations, Administration, and Maintenance (OAM) mechanism to check the status of the LSP in the data plane and MAY further send its status in a PCRpt message to the PCE.

PCC-Based Allocations The PCE can request the PCC to allocate the label using the PCInitiate message. The C flag in the CCI object is set to '1' to indicate that the allocation needs to be made by the PCC. The PCC MUST try to allocate the label and MUST report to the PCE via a PCRpt or PCErr message. If the value of the label is 0 and the C flag is set to '1', it indicates that the PCE is requesting the allocation to be made by the PCC. If the label is 'n' and the C flag is set to '1' in the CCI object, it indicates that the PCE requests a specific value 'n' for the label. If the allocation is successful, the PCC MUST report via the PCRpt message with the CCI object. If the value of the label in the CCI object is invalid, it MUST send a PCErr message with Error-Type=31 (PCECC failure) and Error-value=3 (Invalid CCI). If it is valid but the PCC is unable to allocate it, it MUST send a PCErr message with Error-Type=31 (PCECC failure) and Error-value=4 (Unable to allocate the specified CCI). If the PCC wishes to withdraw or modify the previously assigned label, it MUST send a PCRpt message without any label or with the label containing the new value, respectively, in the CCI object. The PCE would further trigger the label cleanup of the older label, as per .

PCEP Messages As defined in , a PCEP message consists of a common header followed by a variable-length body made of a set of objects that can be either mandatory or optional. An object is said to be mandatory in a PCEP message when the object must be included for the message to be considered valid. For each PCEP message type, a set of rules is defined, which specifies the set of objects that the message can carry. An implementation MUST form the PCEP messages using the object ordering specified in this document. The LSP-IDENTIFIERS TLV MUST be included in the LSP object for the PCECC LSP. The message formats in this document are specified using Routing Backus-Naur Form (RBNF) encoding, as specified in .

The PCInitiate Message The PCInitiate message can be used to download or remove the labels; this document extends the message, as shown below. ::=

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Where:

<Common Header> is defined in .

::= [] ::= (| | ) ::=

::= [] ]]>

Where:

<PCE-initiated-lsp-instantiation> and <PCE-initiated-lsp-deletion> are as per .
The LSP and SRP object is defined in .

When a PCInitiate message is used for the CCI (labels), the SRP, LSP, and CCI objects MUST be present. The SRP object is defined in ; if the SRP object is missing, the receiving PCC MUST send a PCErr message with Error-Type=6 (Mandatory Object missing) and Error-value=10 (SRP object missing). The LSP object is defined in , and if the LSP object is missing, the receiving PCC MUST send a PCErr message with Error-Type=6 (Mandatory Object missing) and Error-value=8 (LSP object missing). The CCI object is defined in , and if the CCI object is missing, the receiving PCC MUST send a PCErr message with Error-Type=6 (Mandatory Object missing) and Error-value=17 (CCI object missing). More than one CCI object MAY be included in the PCInitiate message for a transit LSR. To clean up entries, the R (remove) bit MUST be set in the SRP object to be encoded along with the LSP and CCI objects. The CCI object received at the ingress node MUST have the O bit (out-label) set. The CCI object received at the egress MUST have the O bit unset. If this is not the case, the PCC MUST send a PCErr message with Error-Type=31 (PCECC failure) and Error-value=3 (Invalid CCI). Other instances of the CCI object, if present, MUST be ignored. For the point-to-point (P2P) LSP setup via the PCECC technique, at the transit LSR, two CCI objects are expected for incoming and outgoing labels associated with the LSP object. If any other CCI object is included in the PCInitiate message, it MUST be ignored. If the transit LSR did not receive two CCI objects, with one of them having the O bit set and another with the O bit unset, it MUST send a PCErr message with Error-Type=31 (PCECC failure) and Error-value=3 (Invalid CCI). Note that, on receipt of the PCInitiate message with CCI object, the ingress, egress, or transit role of the PCC is identified via the ingress and egress IP address encoded in the LSP-IDENTIFIERS TLV.

The PCRpt Message The PCRpt message can be used to report the labels that were allocated by the PCE to be used during the State Synchronization phase or as an acknowledgment to a PCInitiate message. ::=

]]>

Where: ::= [] ::= (| ) ::= []

::= []

::= [] ]]>

Where:

<path> is as per , and the LSP and SRP objects are also defined in .

When a PCRpt message is used to report the CCI (labels), the LSP and CCI objects MUST be present. The LSP object is defined in , and if the LSP object is missing, the receiving PCE MUST send a PCErr message with Error-Type=6 (Mandatory Object missing) and Error-value=8 (LSP object missing). The CCI object is defined in , and if the CCI object is missing, the receiving PCE MUST send a PCErr message with Error-Type=6 (Mandatory Object missing) and Error-value=17 (CCI object missing). Two CCI objects can be included in the PCRpt message for a transit LSR.

PCEP Objects The PCEP objects defined in this document are compliant with the PCEP object format defined in .

OPEN Object This document defines a new PST (2) to be included in the PATH-SETUP-TYPE-CAPABILITY TLV in the OPEN object. Further, a new sub-TLV for the PCECC capability exchange is also defined.

PCECC Capability Sub-TLV The PCECC-CAPABILITY sub-TLV is an optional TLV for use in the OPEN object in the PATH-SETUP-TYPE-CAPABILITY TLV when the Path Setup Type list includes the PCECC Path Setup Type 2. A PCECC-CAPABILITY sub-TLV MUST be ignored if the PST list does not contain PST=2. Its format is shown in .

PCECC Capability Sub-TLV The type of the TLV is 1, and it has a fixed length of 4 octets. The value comprises a single field: Flags (32 bits). Currently, the following flag bit is defined:

L bit (Label):: If set to '1' by a PCEP speaker, the L flag indicates that the PCEP speaker will support and is willing to handle the PCEC-based Central Controller Instructions for label download. The bit MUST be set to '1' by both a PCC and a PCE for the PCECC label download/report on a PCEP session.

Unassigned bits MUST be set to '0' on transmission and MUST be ignored on receipt.

PATH-SETUP-TYPE TLV The PATH-SETUP-TYPE TLV is defined in ; this document defines a new PST value:

PST=2:: Path is set up via the PCECC mode.

On a PCRpt/PCUpd/PCInitiate message, the PST=2 in the PATH-SETUP-TYPE TLV in the SRP object MUST be included for an LSP set up via the PCECC-based mechanism.

CCI Object The CCI object is used by the PCE to specify the forwarding instructions (label information in the context of this document) to the PCC and MAY be carried within a PCInitiate or PCRpt message for label download/report. CCI Object-Class is 44. CCI Object-Type is 1 for the MPLS label.

CCI Object The fields in the CCI object are as follows:

CC-ID:

A PCEP-specific identifier for the CCI information. A PCE creates a CC-ID for each instruction; the value is unique within the scope of the PCE and is constant for the lifetime of a PCEP session. The values 0 and 0xFFFFFFFF are reserved and MUST NOT be used. Note that gives advice on assigning transient numeric identifiers, such as the CC-ID, so as to minimize security risks.

Reserved1 (16 bit):

Set to 'zero' while sending; ignored on receipt.

Flags (16 bit):

A field used to carry any additional information pertaining to the CCI. Currently, the following flag bits are defined:

O bit (out-label) : If the bit is set to '1', it specifies the label is the out-label, and it is mandatory to encode the next-hop information (via Address TLVs () in the CCI object). If the bit is not set, it specifies the label is the in-label, and it is optional to encode the local interface information (via Address TLVs in the CCI object).
C Bit (PCC allocation): If the bit is set to '1', it indicates that the label allocation needs to be done by the PCC for the Central Controller Instruction. A PCE sets this bit to request the PCC to make an allocation from its label space. A PCC would set this bit to indicate that it has allocated the label and report it to the PCE.
All unassigned bits MUST be set to 'zero' at transmission and ignored at receipt.

Label (20-bit):

The label information.

Reserved2 (12 bit):

Set to 'zero' while sending; ignored on receive.

Address TLVs defines the IPV4-ADDRESS, IPV6-ADDRESS, and UNNUMBERED-ENDPOINT TLVs for the use of Generalized Endpoint. The same TLVs can also be used in the CCI object to associate the next-hop information in the case of an outgoing label and local interface information in the case of an incoming label. The next-hop information encoded in these TLVs needs to be a directly connected IP address/interface information. If the PCC is not able to resolve the next-hop information, it MUST reject the CCI and respond with a PCErr message with Error-Type=31 (PCECC failure) and Error-value=5 (Invalid next-hop information).

Security Considerations As per , the security considerations for a PCE-based controller are a little different from those for any other PCE system. That is, the operation relies heavily on the use and security of PCEP, so consideration should be given to the security features discussed in and the additional mechanisms described in . It further lists the vulnerability of a central controller architecture, such as a central point of failure, denial of service, and a focus for interception and modification of messages sent to individual Network Elements (NEs). In the PCECC operations, the PCEP sessions are also required to the internal routers, thus increasing the resources required for the session management at the PCE. The PCECC extension builds on the existing PCEP messages; thus, the security considerations described in , , and continue to apply. specifies the support of Transport Layer Security (TLS) in PCEP, as it provides support for peer authentication, message encryption, and integrity. It further provides mechanisms for associating peer identities with different levels of access and/or authoritativeness via an attribute in X.509 certificates or a local policy with a specific accept-list of X.509 certificates. This can be used to check the authority for the PCECC operations. Additional considerations are discussed in following sections.

Malicious PCE In this extension, the PCE has complete control over the PCC to download/remove the labels and can cause the LSPs to behave inappropriately and cause a major impact to the network. As a general precaution, it is RECOMMENDED that this PCEP extension be activated on mutually authenticated and encrypted sessions across PCEs and PCCs belonging to the same administrative authority, using TLS , as per the recommendations and best current practices in BCP 195 . Further, an attacker may flood the PCC with the PCECC-related messages at a rate that exceeds either the PCC's ability to process them or the network's ability to send them, by either spoofing messages or compromising the PCE itself. provides a mechanism to protect the PCC by imposing a limit. The same can be used for the PCECC operations as well. As specified in , a PCC needs to check if the label in the CCI object is in the range set aside for the PCE; otherwise, it MUST send a PCErr message with Error-Type=31 (PCECC failure) and Error-value=1 (Label out of range).

Malicious PCC The PCECC mechanism described in this document requires the PCE to keep labels (CCI) that it downloads and relies on the PCC responding (with either an acknowledgment or an error message) to request for LSP instantiation. This is an additional attack surface by placing a requirement for the PCE to keep a CCI/label replica for each PCC. It is RECOMMENDED that PCE implementations provide a limit on resources (in this case the CCI) a single PCC can occupy. provides a notification mechanism when such threshold is reached.

Manageability Considerations

Control of Function and Policy A PCE or PCC implementation SHOULD allow the PCECC capability to be enabled/disabled as part of the global configuration. list various controlling factors regarding the Path Setup Type. They are also applicable to the PCECC Path Setup Types. Further, describes the migration steps when the Path Setup Type of an existing LSP is changed.

Information and Data Models describes the PCEP MIB; this MIB can be extended to get the PCECC capability status. The PCEP YANG module could be extended to enable/disable the PCECC capability.

Liveness Detection and Monitoring Mechanisms defined in this document do not imply any new liveness detection and monitoring requirements in addition to those already listed in .

Verify Correct Operations The operator needs the following information to verify that PCEP is operating correctly with respect to the PCECC Path Setup Type.

An implementation SHOULD allow the operator to view whether the PCEP speaker sent the PCECC PST capability to its peer.
An implementation SHOULD allow the operator to view whether the peer sent the PCECC PST capability.
An implementation SHOULD allow the operator to view whether the PCECC PST is enabled on a PCEP session.
If one PCEP speaker advertises the PCECC PST capability, but the other does not, then the implementation SHOULD create a log to inform the operator of the capability mismatch.
If a PCEP speaker rejects a CCI, then it SHOULD create a log to inform the operator, giving the reason for the decision (local policy, label issues, etc.).

Requirements on Other Protocols PCEP extensions defined in this document do not put new requirements on other protocols.

Impact on Network Operations PCEP extensions defined in this document do not put new requirements on network operations.

IANA Considerations

PATH-SETUP-TYPE-CAPABILITY Sub-TLV Type Indicators detailed the creation of the "PATH-SETUP-TYPE-CAPABILITY Sub-TLV Type Indicators" subregistry. Further, IANA has allocated the following codepoint: PATH-SETUP-TYPE-CAPABILITY Sub-TLV Type Indicators Subregistry Addition

Value	Meaning	Reference
1	PCECC-CAPABILITY	RFC 9050

PCECC-CAPABILITY Sub-TLV's Flag Field This document defines the PCECC-CAPABILITY sub-TLV; IANA has created a new subregistry to manage the value of the PCECC-CAPABILITY sub-TLV's 32-bit Flag field. New values are to be assigned by Standards Action . Each bit should be tracked with the following qualities:

bit number (counting from bit 0 as the most significant bit)
capability description
defining RFC

Currently, there is one allocation in this registry. Initial Contents of the PCECC-CAPABILITY Sub-TLV Subregistry

Bit	Name	Reference
0-30	Unassigned	RFC 9050
31	Label	RFC 9050

PCEP Path Setup Type Registry created a subregistry within the "Path Computation Element Protocol (PCEP) Numbers" registry called "PCEP Path Setup Types". IANA has allocated a new codepoint within this registry, as follows: Path Setup Type Registry Codepoint Addition

Value	Description	Reference
2	Traffic engineering path is set up using PCECC mode	RFC 9050

PCEP Object IANA has allocated new codepoints in the "PCEP Objects" subregistry for the CCI object as follows: PCEP Objects Subregistry Additions

Object-Class Value	Name	Object-Type	Reference
44	CCI Object-Type	0: Reserved 1: MPLS Label 2-15: Unassigned	RFC 9050

CCI Object Flag Field IANA has created a new subregistry to manage the Flag field of the CCI object called "CCI Object Flag Field for MPLS Label". New values are to be assigned by Standards Action . Each bit should be tracked with the following qualities:

bit number (counting from bit 0 as the most significant bit)
capability description
defining RFC

Two bits are defined for the CCI Object flag field in this document as follows: CCI Object Flag Field for MPLS Label Initial Contents

Bit	Description	Reference
0-13	Unassigned
14	C Bit - PCC allocation	RFC 9050
15	O Bit - Specifies label is out-label	RFC 9050

PCEP-Error Object IANA has allocated new error types and error values within the "PCEP-ERROR Object Error Types and Values" subregistry of the "Path Computation Element Protocol (PCEP) Numbers" registry for the following errors: PCEP-ERROR Object Error Types and Values Additions

Error-Type	Meaning	Error-value	Reference
6	Mandatory Object missing	17: CCI object missing	RFC 9050
10	Reception of an invalid object	33: Missing PCECC Capability sub-TLV	RFC 9050
19	Invalid Operation	16: Attempted PCECC operations when PCECC capability was not advertised 17: Stateful PCE capability was not advertised 18: Unknown Label	RFC 9050
31	PCECC failure	1: Label out of range 2: Instruction failed 3: Invalid CCI 4: Unable to allocate the specified CCI 5: Invalid next-hop information	RFC 9050

References Normative References Informative References A Path Computation Element (PCE)-Based Architecture A YANG Data Model for Path Computation Element Communications Protocol (PCEP)

Acknowledgments We would like to thank , , , , and for their useful comments and suggestions. Thanks to for shepherding this document and providing valuable comments. Thanks to for being the responsible AD. Thanks to for a very detailed RTGDIR review. Thanks to for the SECDIR review. Thanks to for the Gen-ART review. Thanks to , , , , , , and for the IESG review.

Contributors Huawei Technologies

Divyashree Techno Park, Whitefield Bangalore Karnataka 560066 India dhruv.ietf@gmail.com

Huawei Technologies

Divyashree Techno Park, Whitefield Bangalore Karnataka 560066 India satishk@huawei.com

Old Dog Consulting

United Kingdom adrian@olddog.co.uk

Huawei Technologies

China gengxuesong@huawei.com

udayasreereddy@gmail.com

Futurewei Technologies

katherine.zhao@futurewei.com

Telus Ltd.

Toronto Canada boris.zhang@telus.com

Cisco Systems

Slovakia atokar@cisco.com