Path Computation Element Communication Protocol (PCEP) Extension for LSP Diversity Constraint Signaling

describes the Path Computation Element communication Protocol (PCEP) which enables the communication between a Path Computation Client (PCC) and a Path Control Element (PCE), or between two PCEs based on the PCE architecture . PCEP Extensions for Stateful PCE Model describes a set of extensions to PCEP to enable active control of MPLS-TE and GMPLS tunnels. describes the setup and teardown of PCE-initiated LSPs under the active stateful PCE model, without the need for local configuration on the PCC, thus allowing for a dynamic network. introduces a generic mechanism to create a grouping of LSPs in the context of a PCE which can then be used to define associations between a set of LSPs and a set of attributes (such as configuration parameters or behaviors) and is equally applicable to the active and passive modes of a stateful PCE or a stateless PCE . This document specifies a PCEP extension to signal that a set of LSPs in a particular group should use diverse paths, including the requested type of diversity. A PCC can use this extension to signal to a PCE that a particular LSP belongs to a particular disjoint-group. When a PCE receives LSP states belonging to the same disjoint-group from some PCCs, the PCE should ensure that the LSPs within the group are disjoint from each other.

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.

The following terminology is used in this document. Disjoint Association Type. Disjoint Association Group. Multiprotocol Label Switching. Objective Function. Path Computation Client. Any client application requesting a path computation to be performed by a Path Computation Element. Path Computation Element. An entity (component, application, or network node) that is capable of computing a network path or route based on a network graph and applying computational constraints. Path Computation Element communication Protocol. Shared Risk Link Group.

Path diversity is a very common use case in today's IP/MPLS networks especially for layer 2 transport over MPLS. A customer may request that the operator provide two end-to-end disjoint paths across the operator's IP/MPLS core. The customer may use these paths as primary/backup or active/active configuration. Different levels of disjointness may be offered: Link disjointness: the paths of the associated LSPs should transit different links (but may use common nodes or different links that may have some shared fate). Node disjointness: the paths of the associated LSPs should transit different nodes (but may use different links that may have some shared fate). SRLG disjointness: the paths of the associated LSPs should transit different links that do not share fate (but may use common transit nodes). Node+SRLG disjointness: the paths of the associated LSPs should transit different links that do not have any common shared fate and should transit different nodes. The associated LSPs may originate from the same or from different head-end(s) and may terminate at the same or different tail-end(s).

_________________________________________ / \ / +------+ \ | | PCE | | | +------+ | | | | ***********************> | | +------+ 10 +------+ | CE1 ****| PE 1 | ----- R1 ---- R2 ------- | PE 2 |**** CE2 | +------+ | | +------+ | | | | | | | | | | +------+ | | +------+ | CE3 ****| PE 3 | ----- R3 ---- R4 ------- | PE 4 |**** CE4 | +------+ ***********************> +------+ | | | \ / \_________________________________________/ In the figure above, let us consider that the customer wants to have two disjoint paths, one between CE1 and CE2 and one between CE3 and CE4. From an IP/MPLS network point view, in this example, the CEs are connected to different PEs to maximize their disjointness. When LSPs originate from different head-ends, distributed computation of diverse paths can be difficult, whereas, computation via a centralized PCE ensures path disjointness, correctness and simplicity. describes the relationship between the Disjoint Association Group (DAG) and Synchronization VECtor (SVEC) object. The PCEP extension for stateful PCE defined new PCEP messages - Path Computation Report (PCRpt), Path Computation Update (PCUpd) and Path Computation Initiate (PCInitiate) . These messages use PLSP-ID in the LSP object for identification. Moreover to allow diversity between LSPs originating from different PCCs, the generic mechanism to create a grouping of LSPs is described in (that is equally applicable to the active and passive modes of a stateful PCE). Using the extension to PCEP defined in this document, the PCC uses the extension to indicate that a group of LSPs are required to be disjoint; such indication should include disjointness parameters such as the type of disjointness, the disjoint group identifiers, and any customization parameters according to the configured local policy. The management of the disjoint group IDs will be a key point for the operator as the Association ID field is limited to 65535. The local configuration of IPv4/IPv6 association source, or Global Association Source/Extended Association ID allows to overcome this limitation as described in . When a PCC or PCE initiates all the LSPs in a particular disjoint-group, it can set the IPv4/IPv6 association source as one of its own IP address. When disjoint LSPs are initiated from different head-ends, the association source could be the PCE address or any other unique value to identify the DAG.

+-----+ _ _ _ _ _ _| PCE | | | PCE | | +-----+ | ----------> +-----+ | PCInitiate | | PCReq/PCRpt |{Disjoint-group X} | | {Disjoint-group Y} | | | | .-----. | | .-----. | ( ) | +-----+ ( ) | .--( )--. | |PCC 2|--.--( )--. V ( ) | +-----+ ( ) +---+ ( ) | ( ) |PCC|----( (G)MPLS network ) +-----+ ( (G)MPLS network ) +---+ ( ) |PCC 1|-----( ) Disjoint-group X ) +-----+ ( ) '--( )--' '--( )--' ( ) ( ) '-----' '-----' Case 1: Disjointness initiated by Case 2: Disjointness initiated by PCE and enforced by PCC PCC and enforced by PCE ]]> Using the disjoint-group within a PCEP messages is used for: Configuration: Used to communicate the configured disjoint requirement to a PCEP peer. Status: Used to communicate the status of the computed disjointness.

As per , LSPs are associated with other LSPs with which they interact by adding them to a common association group. As described in the association group is uniquely identified by the combination of these fields in the ASSOCIATION object: Association Type, Association ID, Association Source, and (if present) Global Association Source or Extended Association ID. This document defines a new Association type, based on the generic Association object: Association type = TBD1 Disjoint Association Type (DAT). specifies the mechanism for the capability advertisement of the association types supported by a PCEP speaker by defining a ASSOC-Type-List TLV to be carried within an OPEN object. This capability exchange for the Disjointness Association Type (TBD1) MUST be done before using the disjointness association. Thus the PCEP speaker MUST include the Disjointness Association Type in the ASSOC-Type-List TLV before using the Disjoint Association Group (DAG) in PCEP messages. This association type is considered to be both dynamic and operator-configured in nature. The association group could be created by the operator manually on the PCEP peers and the LSPs belonging to this associations is conveyed via PCEP messages to the PCEP peer; or the association group could be created dynamically by the PCEP speaker and both the association group information and the LSPs belonging to the association group is conveyed to the PCEP peer. The Operator-configured Association Range MUST be set for this association-type to mark a range of association identifiers that are used for operator-configured associations to avoid any association identifier clash within the scope of the association source. (Refer to .) A disjoint group can have two or more LSPs, but a PCE may be limited in the number of LSPs it can take into account when computing disjointness. If a PCE receives more LSPs in the group than it can handle in its computation algorithm, it SHOULD apply disjointness computation to only a subset of LSPs in the group. The subset of disjoint LSPs will be decided by PCE as a local policy. Local polices MAY define the computational behavior for the other LSPs in the group. For example, the PCE may provide no path, a shortest path, or a constrained path based on relaxing disjointness, etc. The disjoint status is informed to the PCC. All LSPs in a particular disjoint group MUST use the same combination of T, S, N, L flags in the DISJOINTNESS-CONFIGURATION-TLV. If a PCEP peer receives a PCEP messages for LSPs belonging to the same disjoint group but having an inconsistent combination of T, S, N, L flags, the PCEP peer SHOULD NOT try to add the LSPs in disjoint group and SHOULD reply with a PCErr with Error-type 26 (Association Error) and Error- Value 6 (Association information mismatch). Associating a particular LSP to multiple disjoint groups is authorized from a protocol perspective, however there is no assurance that the PCE will be able to compute properly the multi-disjointness constraint.

The disjoint group MUST carry the following TLV: DISJOINTNESS-CONFIGURATION-TLV: Used to communicate some disjointness configuration parameters. In addition, the disjoint group MAY carry the following TLV: DISJOINTNESS-STATUS-TLV: Used to communicate the status of the computed disjointness. This is applicable for messages from PCE to PCC (PCUpd, PCInitiate or PCRep message). VENDOR-INFORMATION-TLV: Used to communicate arbitrary vendor-specific behavioral information, described in . OF-List TLV: Used to communicate the disjointness objective function. See . The DISJOINTNESS-CONFIGURATION-TLV is shown in the following figure:

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 = TBD2 | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Flags |T|P|S|N|L| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type: TBD2. Length: Fixed value of 4 bytes. Flags: L (Link diverse) bit: when set, this indicates that the computed paths within the disjoint group MUST NOT have any link in common. N (Node diverse) bit: when set, this indicates that the computed paths within the disjoint group MUST NOT have any node in common. S (SRLG diverse) bit: when set, this indicates that the computed paths within the disjoint group MUST NOT share any SRLG (Shared Risk Link Group). P (Shortest path) bit: when set, this indicates that the computed path of the LSP SHOULD satisfy all the constraints and objective functions first without considering the diversity constraint, this means that all of the LSPs with P flag set in the association group are computed first, and then with those LSPs fixed, the other LSPs with P flag unset in the association group are computed. The role of P flag is further described with example in T (Strict disjointness) bit: when set, if disjoint paths cannot be found, PCE SHOULD return no path for LSPs that could not be be disjoint. When unset, the PCE is allowed to relax disjointness by either applying a requested objective function (cf. below) or using the local policy if no objective function is requested (e.g. using a lower disjoint type (link instead of node) or fully relaxing disjointness constraint). Further see for details. Unassigned bits are considered reserved. They MUST be set to 0 on transmission and MUST be ignored on receipt. If a PCEP speaker receives a disjoint-group without DISJOINTNESS-CONFIGURATION-TLV, it SHOULD reply with a PCErr Error-type=6 (Mandatory Object missing) and Error-value=TBD8 (DISJOINTNESS-CONFIGURATION-TLV missing). The DISJOINTNESS-STATUS-TLV uses the same format as the DISJOINTNESS-CONFIGURATION-TLV with a different type TBD3 (in the TLV). The L, N, and S flags are set if the respective disjointness criterion was requested and the computed paths meet it. The P flag is set to indicate that the computed path is the shortest. The T flag has no meaning in the DISJOINTNESS-STATUS-TLV and MUST NOT be set while sending and MUST be ignored on receipt. Any document defining a new flag for the DISJOINTNESS-CONFIGURATION-TLV automatically defines a new flag with the same name and in the same location in DISJOINTNESS-STATUS-TLV; the semantics of the flag in DISJOINTNESS-STATUS-TLV MUST be specified in the document that specifies the flag in DISJOINTNESS-CONFIGURATION-TLV.

An objective function (OF) MAY be applied to the disjointness computation to drive the PCE computation behavior. In this case, the OF-List TLV (defined in () is used as an optional TLV in the Association Group Object. Whereas the PCEP OF-List TLV allows multiple OF-codes inside the TLV, a sender SHOULD include a single OF-code in the OF-List TLV when included in the Association Group, and the receiver MUST consider the first OF-code only and ignore others if included. To minimize the common shared resources (Node, Link or SRLG) between a set of paths during path computation three new OF-codes are proposed: MSL Minimize the number of shared (common) Links. TBD4 Find a set of paths such that it passes through the least number of shared (common) links. MSS Minimize the number of shared (common) SRLGs. TBD5 Find a set of paths such that it passes through the least number of shared (common) SRLGs. MSN Minimize the number of shared (common) Nodes. TBD6 Find a set of paths such that they pass through the least number of shared (common) nodes. If the OF-list TLV is included in the Association Object, the OF-code inside the OF Object MUST include one of the disjoint OFs defined in this document. If this condition is not met, the PCEP speaker MUST respond with a PCErr message with Error-Type=10 (Reception of an invalid object) and Error-Value=TBD9 (Incompatible OF code).

defines a mechanism for the synchronization of a set of path computation requests by using the SVEC object, that specifies the list of synchronized requests that can either be dependent or independent. The SVEC object identify the relationship between the set of path computation requests, identified by 'Request-ID-number' in RP (Request Parameters) object. further clarified the use of the SVEC list for synchronized path computations when computing dependent requests as well as described a number of usage scenarios for SVEC lists within single-domain and multi-domain environments. The SVEC object includes a Flags field that indicates the potential dependency between the set of path computation request in a similar way as the Flags field in the TLVs defined in this document. The path computation request in the PCReq message MAY use both the SVEC and ASSOCIATION objects to identify the related path computation request as well as the diversity association group. The PCE MUST try to find a path that meets both the constraints. It is possible that the diversity requirement in the association group is different from the one in the SVEC object. The PCE MUST consider both the objects as per the processing rules and aim to find a path that meets both of these constraints. In case no such path is possible, the PCE MUST send a path computation reply (PCRep) with a NO-PATH object indicating path computation failure as per . uses SVEC diversity flag for node, link or SRLG to describe the potential disjointness between the set of path computation requests used in PCEP protocol. This document defines three new OF-codes to maximize diversity as much as possible, in other words, new OF-codes allow specification of minimization of common shared resources (Node, Link or SRLG) among a set of paths during path computation. It may be interesting to note that the diversity flags in the SVEC object and OF for diversity can be used together. Some examples of usage are listed below: SVEC object with node-diverse bit=1 - ensure full node-diversity. SVEC object with node-diverse bit=1 and OF=MSS - full node diverse with as much as SRLG-diversity as possible. SVEC object with domain-diverse bit=1;link diverse bit=1 and OF=MSS - full domain and node diverse path with as much as SRLG-diversity as possible. SVEC object with node-diverse bit=1 and OF=MSN - ensure full node-diversity. In the last example above, it is interesting to note that "OF" becomes redundant as "SVEC object" ensures full node-diversity, however this specification does not prohibit redundant constraints while using "SVEC object" and "OF" together for diversity.

As mentioned in , the P flag (when set) indicates that the computed path of the LSP SHOULD satisfies all constraints and objective functions first without considering the diversity constraint. This means that an LSP with P flag set should be placed as if the disjointness constraint has not been configured, while the other LSPs in the association with P flag unset should be placed by taking into account the disjointness constraint. Setting P flag changes the relationship between LSPs to a one-sided relationship (LSP 1 with P=0 depends of LSP 2 with P=1, but LSP 2 with P=1 does not depend of LSP 1 with P=0). Multiple LSPs in the same disjoint group may have the P flag set. In such a case, those LSPs may not be disjoint from each other but will be disjoint from others LSPs in the group that have the P flag unset. This could be required in some primary/backup scenarios where the primary path should use the more optimal path available (taking into account the other constraints). When disjointness is computed, it is important for the algorithm to know that it should try to optimize the path of one or more LSPs in the disjoint group (for instance the primary path) while other paths are allowed to be costlier (compared to a similar path without the disjointness constraint). Without such a hint, the disjointness algorithm may set a path for all LSPs that may not completely fulfill the customer's requirement.

_________________________________________ / \ / +------+ \ | | PCE | | | +------+ | | | | | | +------+ 10 +------+ | CE1 ****| PE 1 | ----- R1 ---- R2 ------- | PE 2 |**** CE2 | +------+ | | +------+ | | | | | | | | | | +------+ | | +------+ | CE3 ****| PE 3 | ----- R3 ---- R4 ------- | PE 4 |**** CE4 | +------+ \ | / +------+ | | \ | 10 / | \ +-- R5 --------- R6 / \_________________________________________/ Cost of all the links is 1, unless explicitly marked otherwise. In the figure above, a customer has two dual homed sites (CE1/CE3 and CE2/CE4). Let us consider that this customer wants two disjoint paths between the two sites. Due to physical meshing, the customer wants to use CE1 and CE2 as primary (and CE3 and CE4 are hosted in a remote site for redundancy purpose). Without any hint (constraint) provided, the PCE may compute the two disjoint LSPs together, leading to PE1->PE2 using a path PE1->R1->R2->PE2 and PE3->PE4 using PE3->R3->R4->PE4. In this case, even if the disjointness constraint is fulfilled, the path from PE1 to PE2 does not use the best optimal path available in the network (path delay may be higher): the customer requirement is thus not completely fulfilled. The usage of the P flag allows the PCE to know that a particular LSP should be tied to the best path as if the disjointness constraint was not requested. In our example, if the P flag is set to the LSP PE1->PE2, the PCE should use the path PE1->R1->R3->R4->R2->PE2 for this LSP, while the other LSP should be disjoint from this path. The second LSP will be placed on PE3->R5->R6->PE4 as it is allowed to be costlier. Driving the PCE disjointness computation may be done in other ways, for instance setting a metric boundary reflecting an path delay boundary. Other constraints may also be used. The P flag allows to simply express that the disjointness constraint should not make the LSP worst. Any constraint added to a path disjointness computation may reduce the chance to find suitable paths. The usage of the P flag, as any other constraint, may prevent to find a disjoint path. In the example above, if we consider that the router R5 is down, if PE1->PE2 has the P flag set, there is no room available to place PE3->PE4 (the disjointness constraint cannot be fulfilled). If PE1->PE2 has the P flag unset, the algorithm may be able to place PE1->PE2 on R1->R2 link leaving a room for PE3->PE4 using the R3->R4 link. When using P flag or any additional constraint on top of the disjointness constraint, the user should be aware that there is less chance to fulfill the disjointness constraint.

_________________________________________ / \ / +------+ \ | | PCE | | | +------+ | | | | | | +------+ 10 +------+ | CE1 ****| PE 1 | ----- R1 ---- R2 ------- | PE 2 |**** CE2 | +------+ | \ | +------+ | | | \2 | | | | \ | | | +------+ | \ | +------+ | CE3 ****| PE 3 | ----- R3 ---- R4 ------- | PE 4 |**** CE4 | +------+ +------+ | | | \ / \_________________________________________/ Cost of all the links is 1, unless explicitly marked otherwise. In the figure above, we still consider the same previous requirements, so PE1->PE2 LSP should be optimized (P flag set) while PE3->PE4 should be disjoint and may use a costlier path. Regarding PE1->PE2, there are two paths that are satisfying the constraints (ECMP): PE1->R1->R4->R2->PE2 (path 1) and PE1->R1->R3->R4->R2->PE2 (path 2). An implementation may choose one of the paths. If the implementation elects only one path, there is a chance that picking up one path may prevent disjointness. In our example, if path 2 is used for PE1->PE2, there is no room left for PE3->PE4 while if path 1 is used, PE3->PE4 can be placed on R3->R4 link. When P flag is set for an LSP and when ECMPs are available, an implementation should aim to select a path that allows disjointness.

There may be some cases where the PCE is not able to provide a set of disjoint paths for one or more LSPs in the association. When the T flag is set (Strict disjointness requested), if disjointness cannot be ensured for one or more LSPs, the PCE MUST reply to a Path Computation Request (PCReq) with a Path Computation Reply (PCRep) message containing a NO-PATH object. In case of PCRpt message, the PCE MUST return a PCErr message with Error-Type 26 "Association Error" and Error-Value 7 "Cannot join the association group". In case of network event leading to an impossible strict disjointness, the PCE MUST send a PCUpd message containing an empty ERO to the corresponding PCCs. In addition to the empty ERO Object, the PCE MAY add the NO-PATH-VECTOR TLV () in the LSP Object. This document adds new bits in the NO-PATH-VECTOR TLV: bit "TBD7": when set, the PCE indicates that it could not find a disjoint path for this LSP. bit "TBD8": when set, the PCE indicates that it does not support the requested disjointness computation. When the T flag is unset, the PCE is allowed to relax disjointness by applying a requested objective function () if specified. Otherwise, if no objective function is specified, the PCE is allowed to reduce the required level of disjointness as it deems fit. The actual level of disjointness of the paths computed by the PCE can be reported through the DISJOINTNESS-STATUS-TLV by setting the appropriate flags in the TLV. While the DISJOINTNESS-CONFIGURATION-TLV defines the desired level of disjointness required by configuration, the DISJOINTNESS-STATUS-TLV defines the achieved level of disjointness computed. There are some cases where the PCE may need to completely relax the disjointness constraint in order to provide a path to all the LSPs that are part of the association. A mechanism that allows the PCE to fully relax a constraint is considered by the authors as more global to PCEP rather than linked to the disjointness use case. As a consequence, it is considered as out of scope of the document.

This document defines one new PCEP association type, which on itself does not add any new security concerns beyond those discussed in , and . But, adding of a spurious LSP into the disjointness association group could lead to re-computation and set-up of all LSPs in the group, that could be used to overwhelm the PCE and the network. A spurious LSP can have flags that are inconsistent with those of the legitimate LSPs of the group and thus cause LSP allocation for the legitimate LSPs to fail with an error. Also, as stated in , much of the information carried in the Disjointness Association object, as per this document is not extra sensitive. It often reflects information that can also be derived from the LSP Database, but association provides a much easier grouping of related LSPs and messages. The disjointness association could provide an adversary with the opportunity to eavesdrop on the relationship between the LSPs. Thus securing the PCEP session using Transport Layer Security (TLS) , as per the recommendations and best current practices in , is RECOMMENDED.

This document defines a new Association type, originally described in . IANA is requested to make the assignment of a new value for the sub-registry "ASSOCIATION Type Field" (request to be created in ), as follows: Association typeAssociation NameReference TBD1 Disjoint-group Association [This.I-D]

This document defines the following new PCEP TLVs and the IANA is requested to make the assignment of new values for the existing "PCEP TLV Type Indicators" registry as follows: TLV TypeTLV NameReference TBD2 Disjointness Configuration TLV [This.I-D] TBD3 Disjointness Status TLV [This.I-D] This document requests that a new sub-registry, named "Disjointness Configuration TLV Flag Field", is created within the "Path Computation Element Protocol (PCEP) Numbers" registry to manage the Flag field in the Disjointness Configuration TLV. New values are to be assigned by Standards Action . Each bit should be tracked with the following qualities: Bit number (count from 0 as the most significant bit) Flag Name Reference Bit Number Name Reference 31 L - Link Diverse [This.I-D] 30 N - Node Diverse [This.I-D] 29 S - SRLG Diverse [This.I-D] 28 P - Shortest Path [This.I-D] 27 T - Strict Disjointness [This.I-D]

Three new Objective Functions have been defined in this document. IANA is requested to make the following allocations from the PCEP "Objective Function" sub-registry: Code PointName Reference TBD4 Minimize the number of shared Links (MSL) [This.I-D] TBD5 Minimize the number of shared SRLGs (MSS) [This.I-D] TBD6 Minimize the number of shared Nodes (MSN) [This.I-D]

This documents defines new bits for the NO-PATH-VECTOR TLV in the "NO-PATH-VECTOR TLV Flag Field" sub-registry of the "Path Computation Element Protocol (PCEP) Numbers" registry. IANA is requested to make the following allocation: Bit Number Name Reference TBD7 Disjoint path not found [This.I-D] TBD8 Requested disjoint computation not supported [This.I-D]

This document defines new Error-Value within existing Error-Type related to path protection association. IANA is requested to allocate new error values within the "PCEP-ERROR Object Error Types and Values" sub-registry of the PCEP Numbers registry, as follows: Error-Type Meaning Reference 6 Mandatory Object missing Error-value=TBD8: DISJOINTNESS-CONFIGURATION TLV missing [This.I-D] 10 Reception of an invalid object Error-value=TBD9: Incompatible OF code [This.I-D]

An operator SHOULD be allowed to configure the disjointness association groups and disjoint parameters at the PCEP peers and associate it with the LSPs. The Operator-configured Association Range MUST be allowed to be set by the operator. Operator SHOULD be allowed to set the local policies to define various disjoint computational behavior at the PCE.

An implementation SHOULD allow the operator to view the disjoint associations configured or created dynamically. Further implementation SHOULD allow to view disjoint associations reported by each peer, and the current set of LSPs in this association. The PCEP YANG module includes association groups information.

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

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

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

Mechanisms defined in , Section 8.6 also apply to PCEP extensions defined in this document. Additionally, a PCEP implementation SHOULD allow a limit to be placed on the number of LSPs that can belong to a DAG.

A special thanks to authors of , this document borrow some of the text from it. Authors would also like to thank Adrian Farrel and Julien Meuric for the valuable comments. Thanks to Emmanuel Baccelli for RTGDIR reviews.