PCE Communication Protocol (PCEP) Extensions for Using the PCE as a Central Controller (PCECC) of point-to-multipoint (P2MP) LSPs

Introduction The 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 the 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 the network elements (known as Path Computation Clients (PCCs)), and the results of the path computations were supplied to network elements using the PCE Communication Protocol (PCEP) . This protocol was later extended to allow a PCE to send unsolicited requests to the network for LSP establishment . introduces the architecture for PCE as a central controller as an extension of the architecture described in and assumes the continued use of PCEP as the protocol used between PCE and PCC. further examines the motivations and applicability for PCEP as a Southbound Interface (SBI), and introduces the implications for the protocol. 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. specify the procedures and PCEP extensions for using the PCE as the central controller for static P2P 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. describes how to set up point-to-multipoint (P2MP) Traffic Engineering Label Switched Paths (TE LSPs) for use in Multiprotocol Label Switching (MPLS) and Generalized MPLS (GMPLS) networks. The PCE has been identified as a suitable application for the computation of paths for P2MP TE LSPs (). The extensions of PCEP to request path computation for P2MP TE LSPs are described in . Further specify the extensions that are necessary in order for the deployment of stateful PCEs to support P2MP TE LSPs as well as the setup, maintenance and teardown of PCE-initiated P2MP LSPs under the stateful PCE model. This document extends to specify the procedures and PCEP extensions for using the PCE as the central controller for static P2MP LSPs, where LSPs can be provisioned as explicit label instructions at each hop on the end-to-end path with the added functionality of a P2MP branch node. As per , a branch node is an LSR that replicates the incoming data on to one or more outgoing interfaces. describes the use cases for P2MP in PCECC architecture.

Terminology Terminologies used in this document are the same as described in the draft .

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 Section 3 of describe the PCECC model of operation. This document extends the functionality to include support for central control instruction for replication at the branch nodes for the P2MP LSP. The rest of the processing at the root node is similar to the existing stateful PCE mechanism for P2MP .

Procedures for Using the PCE as a Central Controller (PCECC) for P2MP

Stateful PCE Model Active stateful PCE is described in and extended for P2MP . A PCE as a Central Controller (PCECC) reuses the existing active stateful PCE mechanism as much as possible to control the LSPs. extends PCEP messages - PCInitiate, PCRpt, and PCUpd message for the Central Controller's Instructions (CCI) (label-forwarding instructions in the context of this document). This document specifies the procedure for additional instruction for the branch node needed for P2MP.

PCECC Capability Advertisement As per Section 5.4 of , during the PCEP initialization phase, PCEP Speakers (PCE or PCC) advertise their support of and willingness to use PCEP extension for the PCECC using a new Path Setup Type (PST) in PATH-SETUP-TYPE-CAPABILITY TLV and a new PCECC-CAPABILITY sub-TLV. A new M bit is added in the PCECC-CAPABILITY sub-TLV to indicate support for PCECC-P2MP. A PCC MUST set the M bit in the PCECC-CAPABILITY sub-TLV and include STATEFUL-PCE-CAPABILITY TLV with the P2MP bits set (as per ) in the OPEN object to support the PCECC P2MP extensions defined in this document. If the M bit is set in PCECC-CAPABILITY sub-TLV and the STATEFUL-PCE-CAPABILITY TLV is not advertised or is advertised without the N bit set, in the OPEN object, the receiver MUST:

send a PCErr message with Error-Type=19 (Invalid Operation) and Error-value=TBD2 (P2MP capability was not advertised) and
terminate the session.

The rest of the processing is as per .

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

PCE-Initiated PCECC LSP The LSP instantiation operation is the same as defined in and . In order to set up a PCE-Initiated P2MP LSP based on the PCECC mechanism, a PCE sends a PCInitiate message with the PST set to '2' for the PCECC () to the ingress PCC (root node). As described in , the label-forwarding instructions from PCECC are sent after the initial PCInitiate and PCRpt exchange. 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. An P2MP-LSP-IDENTIFIER TLV MUST be included for the PCECC P2MP LSPs, it uniquely identifies the P2MP LSP in the network. As per , the LSP object is included in the central controller's instructions (label download) to identify the PCECC P2MP LSP for this instruction. The handling of PLSP-ID is as per . The ingress PCC (root) also sets the D (Delegate) flag (see ) and C (Create) flag (see ) in the LSP object of the PCRpt message. As per , 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 P2MP Tree as per the PCECC technique. The CC-ID uniquely identifies the central controller instruction within a PCEP session. Each node along the path (PCC) responds with the PCRpt messages to acknowledge the CCI with the PCRpt messages, including the CCI and the LSP objects. The only new extension required is the instructions on the branch nodes for replications to more than one outgoing interface with the respective label. The rest of the operations remains the same as and .

PCC-Initiated PCECC LSP In order to set up a P2MP LSP based on the PCECC mechanism where the LSP is configured at the PCC, a PCC MUST delegate the P2MP LSP by sending a PCRpt message with the PST set for the PCECC and D (Delegate) flag (see ) set in the LSP object. When a PCE receives the initial PCRpt message with the D flags and PST Type set to '2', it SHOULD calculate the P2MP tree and assign labels along the P2MP tree in addition to setting up the P2MP LSP by sending a PCInitiate message to each node along the path of the P2MP LSP as per . The only new extension required is the instructions on the branch nodes for replications to more than one outgoing interface with the respective label. The rest of the operations remains the same as and .

Central Control Instructions The CCI for the label operations in PCEP is done via the PCInitiate message as described in , by defining a PCEP Objects 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 and . The CCI object MUST be included, along with the LSP object in the PCInitiate message. As per , there are 2 instances of CCI object in the PCInitiate message in a transit node for the P2P LSP. For PCECC-P2MP operations, multiple instances of CCI objects for out-labels are allowed at the branch node. Similarly, to acknowledge the central controller instructions, the PCRpt message allows multiple instances of CCI object for PCECC-P2MP operations. The P2MP-LSP-IDENTIFIERS TLV MUST be included in the LSP object for the PCECC-based P2MP LSP. The SPEAKER-ENTITY-ID TLV SHOULD be included in LSP object. As described in , 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 sends a PCErr message with Error-type=3 (PCECC failure) and Error-value=1 (Label out of range) (). If a PCC receives a PCInitiate message but fails to download the label entry, it sends a PCErr message with Error-type=3 (PCECC failure) and Error-value=2 (Instruction failed) (). Consider the example in the Section 3.6.1 of - PCECC would provision each node along the path and assign incoming and outgoing labels from R1 to {R6, R8} with the path as "R1-L0-R2-L2-R5-L4-R6" and "R1-L0-R2-L1-R4-L3-R3-L5-R8":

R1: Outgoing label 9000 on link L0
R2: Incoming label 9000 on link L0
R2: Outgoing label 9001 on link L1 (*)
R2: Outgoing label 9002 on link L2 (*)
R5: Incoming label 9002 on link L2
R5: Outgoing label 9004 on link L4
R6: Incoming label 9004 on link L4
R4: Incoming label 9001 on link L1
R4: Outgoing label 9003 on link L3
R3: Incoming label 9003 on link L3
R3: Outgoing label 9005 on link L5
R8: Incoming label 9005 on link L5

This can also be represented as : {R1, 6000}, {6000, R2, {9001,9002}}, {9001, R4, 9003}, {9002, R5, 9004} {9003, R3, 9005}, {9004, R6}, {9005, R8}. The main difference (*) is in the branch node instruction at R2 where two copies of packet are sent towards R4 and R5 with 9001 and 9002 labels respectively. The operations on all nodes except R2 are same as . The branch node (R2) needs to be instructed to replicate two copies of the incoming packet, and send it towards R4 and R5 with 9001 and 9002 labels respectively). This is done via including 3 instances of CCI objects in the PCEP messages, one for each label in the example, 9000 for incoming and 9001/9002 for outgoing (along with remote nexthop). The message and procedure remain exactly as with only the distinction that more than one outgoing CCI MAY be present for the P2MP LSP.

Label Cleanup CCI In order to delete a P2MP LSP based on the PCECC, the PCE sends a Central Controller Instructions via a PCInitiate message to each node along the path of the P2MP tree to clean up the label-forwarding instruction as per . In the case of branch nodes, all instances of CCIs needs to be present in the PCEP message.

PCECC LSP Update In case of a modification of PCECC P2MP LSP with a new path, the procedure, and instructions as described in apply.

Re-delegation and Cleanup In case of a re-delegation and clean up of PCECC P2MP LSP, the procedure, and instructions as described in apply.

Synchronization of Central Controllers Instructions The procedure and instructions are as per .

PCECC LSP State Report 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 the PCRpt message (as per ) to the PCE.

PCC-Based Allocations The PCE can request the PCC to allocate the label using the PCInitiate message. The procedure and instructions are as per Section 5.5.8 of .

PCEP Messages specify the extension to PCInitiate and PCRpt message for PCECC. For P2P LSP, only two instances of CCI objects can be included. In the case of the P2MP LSP, multiple CCI objects are allowed. The message format and other procedures continue to apply.

PCEP Objects

OPEN Object

PCECC Capability sub-TLV The PCECC-CAPABILITY sub-TLV is an optional TLV for use in the OPEN Object for PCECC capability advertisement in PATH-SETUP-TYPE-CAPABILITY TLV as specified in . This document adds a new flag (M Bit) in the PCECC-CAPABILITY sub-TLV to indicate the support for P2MP in PCECC. M (PCECC-P2MP-CAPABILITY - 1 bit - TBD1): If set to 1 by a PCEP speaker, it indicates that the PCEP speaker is capable of PCECC-P2MP capability. A PCC MUST set the M Bit in the PCECC-CAPABILITY sub-TLV and set the N (P2MP-CAPABILITY), the M (P2MP-LSP-UPDATE-CAPABILITY), and the P (P2MP-LSP-INSTANTIATION-CAPABILITY) bits (as per ) in the STATEFUL-PCE-CAPABILITY TLV to support the PCECC-P2MP extensions defined in this document. If the M Bit is set in the PCECC-CAPABILITY sub-TLV, and the P2MP bits (in the STATEFUL-PCE-CAPABILITY TLV) are not set in the OPEN Object, a PCEP speaker SHOULD send a PCErr message with Error-Type=19 (Invalid Operation) and Error-value=TBD2 (P2MP capability was not advertised) and terminate the session.

PATH-SETUP-TYPE TLV The PATH-SETUP-TYPE TLV is defined in ; defines a PST value for PCECC as '2', which is applicable for P2MP LSP as well.

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 optionally carried within PCInitiate or PCRpt message for label download/report. The CCI Object Type 1 for MPLS Label is defined in , which is used for the P2MP LSPs as well. The address TLVs are defined in , they associate the next-hop information in case of an outgoing label. If a node (PCC) receives a PCInitiate message with more than one CCI with O-bit set for the outgoing label and the node does not support the P2MP branch/replication capability, it MUST respond with PCErr message with Error-Type=2 (Capability not supported) (defined in ). The rest of the processing is same as .

Implementation Status [Note to the RFC Editor - remove this section before publication, as well as remove the reference to RFC 7942.] This section records the status of known implementations of the protocol defined by this specification at the time of posting of this Internet-Draft, and is based on a proposal described in . The description of implementations in this section is intended to assist the IETF in its decision processes in progressing drafts to RFCs. Please note that the listing of any individual implementation here does not imply endorsement by the IETF. Furthermore, no effort has been spent to verify the information presented here that was supplied by IETF contributors. This is not intended as, and must not be construed to be, a catalog of available implementations or their features. Readers are advised to note that other implementations may exist. According to , "this will allow reviewers and working groups to assign due consideration to documents that have the benefit of running code, which may serve as evidence of valuable experimentation and feedback that have made the implemented protocols more mature. It is up to the individual working groups to use this information as they see fit". Currently, there are no known implementations of the extensions as specified.

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). The security considerations described in , , , and apply to the extensions described in this document. As per , it is RECOMMENDED that these PCEP extensions only be activated on authenticated and encrypted sessions across PCEs and PCCs belonging to the same administrative authority, using Transport Layer Security (TLS) as per the recommendations and best current practices in (unless explicitly set aside in ).

Manageability Considerations

Control of Function and Policy A PCE or PCC implementation SHOULD allow to configure to enable/disable PCECC-P2MP capability as a global configuration.

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 PCECC-P2MP 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 Mechanisms defined in this document do not imply any new operation verification requirements in addition to those already listed in and .

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

PCECC-CAPABILITY sub-TLV defines the PCECC-CAPABILITY sub-TLV and requests that IANA create a registry to manage the value of the PCECC-CAPABILITY sub-TLV's Flag field. IANA is requested to allocate a new bit in the PCECC-CAPABILITY sub-TLV Flag Field registry, as follows: PCECC-CAPABILITY sub-TLV's Flag field

Bit	Description	Reference
TBD1	P2MP	This document

PCEP-Error Object IANA is requested to allocate a new error value within the "PCEP-ERROR Object Error Types and Values" sub-registry of the PCEP Numbers registry for the following errors: PCEP-ERROR Object Error Types and Values

Error-Type	Meaning	Reference
19	Invalid operation
	Error-value = TBD2: P2MP capability was not advertised	This document

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