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author | Thomas Voss <mail@thomasvoss.com> | 2024-11-27 20:54:24 +0100 |
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committer | Thomas Voss <mail@thomasvoss.com> | 2024-11-27 20:54:24 +0100 |
commit | 4bfd864f10b68b71482b35c818559068ef8d5797 (patch) | |
tree | e3989f47a7994642eb325063d46e8f08ffa681dc /doc/rfc/rfc8800.txt | |
parent | ea76e11061bda059ae9f9ad130a9895cc85607db (diff) |
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diff --git a/doc/rfc/rfc8800.txt b/doc/rfc/rfc8800.txt new file mode 100644 index 0000000..30e229c --- /dev/null +++ b/doc/rfc/rfc8800.txt @@ -0,0 +1,1123 @@ + + + + +Internet Engineering Task Force (IETF) S. Litkowski +Request for Comments: 8800 Cisco Systems, Inc. +Category: Standards Track S. Sivabalan +ISSN: 2070-1721 Ciena Corporation + C. Barth + Juniper Networks + M. Negi + RtBrick India + July 2020 + + + Path Computation Element Communication Protocol (PCEP) Extension for + Label Switched Path (LSP) Diversity Constraint Signaling + +Abstract + + This document introduces a simple mechanism to associate a group of + Label Switched Paths (LSPs) via an extension to the Path Computation + Element Communication Protocol (PCEP) with the purpose of computing + diverse (disjointed) paths for those LSPs. The proposed extension + allows a Path Computation Client (PCC) to advertise to a Path + Computation Element (PCE) that a particular LSP belongs to a + particular Disjoint Association Group; thus, the PCE knows that the + LSPs in the same group need to be disjoint from each other. + +Status of This Memo + + This is an Internet Standards Track document. + + This document is a product of the Internet Engineering Task Force + (IETF). It represents the consensus of the IETF community. It has + received public review and has been approved for publication by the + Internet Engineering Steering Group (IESG). Further information on + Internet Standards is available in Section 2 of RFC 7841. + + Information about the current status of this document, any errata, + and how to provide feedback on it may be obtained at + https://www.rfc-editor.org/info/rfc8800. + +Copyright Notice + + Copyright (c) 2020 IETF Trust and the persons identified as the + document authors. All rights reserved. + + This document is subject to BCP 78 and the IETF Trust's Legal + Provisions Relating to IETF Documents + (https://trustee.ietf.org/license-info) in effect on the date of + publication of this document. Please review these documents + carefully, as they describe your rights and restrictions with respect + to this document. Code Components extracted from this document must + include Simplified BSD License text as described in Section 4.e of + the Trust Legal Provisions and are provided without warranty as + described in the Simplified BSD License. + +Table of Contents + + 1. Introduction + 1.1. Requirements Language + 2. Terminology + 3. Motivation + 4. Applicability + 5. Protocol Extension + 5.1. Association Group + 5.2. Disjoint TLVs + 5.3. Disjointness Objective Functions + 5.4. Relationship to SVEC + 5.4.1. SVEC and OF + 5.5. P Flag Considerations + 5.6. Disjointness Computation Issues + 6. Security Considerations + 7. IANA Considerations + 7.1. Association Type + 7.2. PCEP TLVs + 7.3. Objective Functions + 7.4. NO-PATH-VECTOR Bit Flags + 7.5. PCEP-ERROR Codes + 8. Manageability Considerations + 8.1. Control of Function and Policy + 8.2. Information and Data Models + 8.3. Liveness Detection and Monitoring + 8.4. Verification of Correct Operations + 8.5. Requirements on Other Protocols + 8.6. Impact on Network Operations + 9. References + 9.1. Normative References + 9.2. Informative References + Acknowledgments + Contributors + Authors' Addresses + +1. Introduction + + [RFC5440] 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 [RFC4655]. + + The PCEP Extensions for Stateful PCE Model [RFC8231] describes a set + of extensions to PCEP to enable active control of MPLS-TE and GMPLS + tunnels. [RFC8281] 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. + + [RFC8697] introduces a generic mechanism to create a grouping of LSPs + in the context of a PCE that 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 [RFC8231] or a stateless PCE + [RFC4655]. + + This document specifies a PCEP extension to signal that a set of LSPs + in a particular group should use diverse (disjointed) paths, + including the requested type of diversity. Sections 3 and 4 describe + the property and use of a Disjoint Association Group. A PCC can use + this extension to signal to a PCE that a particular LSP belongs to a + particular Disjoint Association Group. When a PCE receives LSP + states belonging to the same Disjoint Association Group from some + PCCs, the PCE should ensure that the LSPs within the group are + disjoint from each other. + +1.1. Requirements Language + + The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", + "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and + "OPTIONAL" in this document are to be interpreted as described in + BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all + capitals, as shown here. + +2. Terminology + + The following terminology is used in this document. + + DAT: Disjoint Association Type + + DAG: Disjoint Association Group + + MPLS: Multiprotocol Label Switching + + OF: Objective Function + + PCC: Path Computation Client. Any client application requesting + a path computation to be performed by a Path Computation + Element. + + PCE: 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. + + PCEP: Path Computation Element Communication Protocol + + PLSP-ID: PCEP-specific identifier for the LSP + + SRLG: Shared Risk Link Group + +3. Motivation + + 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 different head + end(s) and may terminate at the same or different tail end(s). + +4. Applicability + + _________________________________________ + / \ + / +------+ \ + | | PCE | | + | +------+ | + | | + | ***********************> | + | +------+ 10 +------+ | + CE1 ****| PE 1 | ----- R1 ---- R2 ------- | PE 2 |**** CE2 + | +------+ | | +------+ | + | | | | + | | | | + | +------+ | | +------+ | + CE3 ****| PE 3 | ----- R3 ---- R4 ------- | PE 4 |**** CE4 + | +------+ ***********************> +------+ | + | | + \ / + \_________________________________________/ + + Figure 1: Disjoint Paths with Different Head Ends and Tail Ends + + 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. + + Section 5.4 describes the relationship between the Disjoint + Association Group (DAG) and Synchronization VECtor (SVEC) object. + + The PCEP extension for stateful PCE [RFC8231] defined new PCEP + messages -- Path Computation Report (PCRpt), Path Computation Update + (PCUpd), and Path Computation Initiate (PCInitiate) [RFC8281]. These + messages use a 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 that is + equally applicable to the active and passive modes of a stateful PCE + is described in [RFC8697]. + + Using the extension to PCEP defined in this document, the PCC uses + the extension defined in [RFC8697] to indicate that a group of LSPs + are required to be disjoint; such indication should include + disjointness parameters like the type of disjointness, the Disjoint + Association Group identifiers, and any customization parameters + according to the configured local policy. + + The management of the Disjoint Association Group IDs will be a key + point for the operator as the Association ID field is limited to + 65535. The local configuration of the IPv4/IPv6 Association Source, + or Global Association Source/Extended Association ID, can overcome + this limitation, as described in [RFC8697]. When a PCC or PCE + initiates all the LSPs in a particular Disjoint Association 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. + + + Initiate Disjoint LSPs + | + | PCReq/PCRpt + V {DAG Y} + +-----+ ----------------> +-----+ + _ _ _ _ _ _| PCE | | | PCE | + | +-----+ | ----------> +-----+ + | PCInitiate | | PCReq/PCRpt + |{DAG X} | | {DAG Y} + | | | + | .-----. | | .-----. + | ( ) | +-----+ ( ) + | .--( )--. | |PCC 2|--.--( )--. + V ( ) | +-----+ ( ) + +---+ ( ) | ( ) + |PCC|----( (G)MPLS network ) +-----+ ( (G)MPLS network ) + +---+ ( ) |PCC 1|-----( ) + {DAG X} ( ) +-----+ ( ) + '--( )--' ( )--' + ( ) ( ) + '-----' '-----' + + Case 1: Disjointness initiated by Case 2: Disjointness initiated by + PCE and enforced by PCC PCC and enforced by PCE + + Figure 2: Sample Use Cases for Carrying Disjoint Association + Group over PCEP Session + + The Disjoint Association 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. + +5. Protocol Extension + +5.1. Association Group + + As per [RFC8697], LSPs are associated with other LSPs with which they + interact by adding them to a common association group. As described + in [RFC8697], the association group is uniquely identified by the + combination of the following 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, called "Disjoint + Association" (2), based on the generic ASSOCIATION object. This new + Association type is also called "DAT", for "Disjoint Association + Type". + + [RFC8697] specifies the mechanism for the capability advertisement of + the Association types supported by a PCEP speaker by defining an + ASSOC-Type-List TLV to be carried within an OPEN object. This + capability exchange for the DAT (2) MUST be done before using the + disjoint association. Thus, the PCEP speaker MUST include the DAT in + the ASSOC-Type-List TLV and MUST receive the same from the PCEP peer + before using the Disjoint Association Group (DAG) in PCEP messages. + + This Association type is considered to be both dynamic and operator- + configured in nature. As per [RFC8697], the association group could + be manually created by the operator on the PCEP peers, and the LSPs + belonging to this association are conveyed via PCEP messages to the + PCEP peer; alternately, 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 are + 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 [RFC8697].) + + A Disjoint Association 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 of the computed path is informed to the PCC + via the DISJOINTNESS-STATUS TLV (see Section 5.2). + + There are different types of disjointness identified by the flags (T, + S, N, and L) in the DISJOINTNESS-CONFIGURATION TLV (see Section 5.2). + All LSPs in a particular Disjoint Association Group MUST use the same + combination of T, S, N, and L flags in the DISJOINTNESS-CONFIGURATION + TLV. If a PCEP peer receives a PCEP message for LSPs belonging to + the same Disjoint Association Group but having an inconsistent + combination of T, S, N, and L flags, the PCEP peer MUST NOT add the + LSPs to the Disjoint Association Group and MUST reply with a PCErr + with Error-Type 26 (Association Error) and Error-value 6 (Association + information mismatch). + + A particular LSP MAY be associated to multiple Disjoint Association + Groups, but in that case, the PCE SHOULD try to consider all the + Disjoint Association Groups during path computation, if possible. + Otherwise, a local policy MAY define the computational behavior. If + a PCE does not support such a path computation, it MUST NOT add the + LSP into the association group and MUST return a PCErr with Error- + Type 26 (Association Error) and Error-value 7 (Cannot join the + association group). + +5.2. Disjoint TLVs + + The Disjoint Association Group (ASSOCIATION object with Association + type = 2 for DAT) MUST carry the following TLV: + + * DISJOINTNESS-CONFIGURATION TLV: Used to communicate some + disjointness configuration parameters. This is applicable for all + PCEP messages that include DAG. + + In addition, the Disjoint Association Group (ASSOCIATION object with + Association type = 2 for DAT) MAY carry the following TLVs: + + * DISJOINTNESS-STATUS TLV: Used to communicate the status of the + computed disjointness. This is applicable for messages from a PCE + to a PCC only (i.e., PCUpd, PCInitiate, or PCRep messages). + + * VENDOR-INFORMATION-TLV: Used to communicate arbitrary vendor- + specific behavioral information, described in [RFC7470]. + + * OF-List TLV: Used to communicate the disjointness objective + function. See Section 5.3. + + 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 = 46 | Length | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | Flags |T|P|S|N|L| + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + + Figure 3: DISJOINTNESS-CONFIGURATION TLV + + Type: 46 + + Length: Fixed value of 4 bytes. + + Flags: + + L (Link Diverse) bit: When set, this indicates that the computed + paths within the Disjoint Association Group MUST NOT have any + link in common. + + N (Node Diverse) bit: When set, this indicates that the computed + paths within the Disjoint Association Group MUST NOT have any + node in common. + + S (SRLG Diverse) bit: When set, this indicates that the computed + paths within the Disjoint Association 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, as if the + disjointness constraint has not been configured; then, with + those LSPs fixed, the other LSPs with P flag unset in the + association group are computed by taking into account the + disjointness constraint. The role of P flag is further + described with examples in Section 5.5. + + T (Strict Disjointness) bit: When set, if disjoint paths cannot + be found, the PCE MUST return no path for LSPs that could not + be disjoint. When unset, the PCE is allowed to relax + disjointness by either applying a requested objective function + (cf. Section 5.3) 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). + See Section 5.6 for further details. + + Unassigned bits: 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 Association Group (ASSOCIATION + object with Association type = 2 for DAT) without the DISJOINTNESS- + CONFIGURATION TLV, it SHOULD reply with a PCErr Error-Type 6 + (Mandatory Object missing) and Error-value 15 (DISJOINTNESS- + CONFIGURATION TLV missing). + + The DISJOINTNESS-STATUS TLV uses the same format as the DISJOINTNESS- + CONFIGURATION TLV with a different type 47 (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 indicates that + the computed path is the shortest path (computed first without taking + disjointness constraints into consideration but considering other + constraints). + + 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 the DISJOINTNESS-STATUS TLV MUST be specified in the document that + specifies the flag in the DISJOINTNESS-CONFIGURATION TLV. + +5.3. Disjointness Objective Functions + + 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 [RFC5541]) is used as an optional TLV in the + ASSOCIATION 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 + defined: + + MSL + + Name: Minimize the number of Shared (common) Links. + Objective Function Code: 15 + Description: Find a set of paths such that it passes through the + least number of shared (common) links. + - A network comprises a set of N links {Li, (i=1...N)}. + - A path P passes through K links {Lpi,(i=1...K)}. + - A set of paths {P1...Pm} have L links that are common to + more than one path {Lci,(i=1...L)}. + - Find a set of paths such that the value of L is minimized. + + MSS + + Name: Minimize the number of Shared (common) SRLGs. + Objective Function Code: 16 + Description: Find a set of paths such that it passes through the + least number of shared (common) SRLGs. + - A network comprises a set of N links {Li, (i=1...N)}. + - A path P passes through K links {Lpi,(i=1...K)} belonging to + unique M SRLGs {Spi,(i=1..M)}. + - A set of paths {P1...Pm} have L SRLGs that are common to + more than one path {Sci,(i=1...L)}. + - Find a set of paths such that the value of L is minimized. + + MSN + + Name: Minimize the number of Shared (common) Nodes. + Objective Function Code: 17 + Description: Find a set of paths such that they pass through the + least number of shared (common) nodes. + - A network comprises a set of N nodes {Ni, (i=1...N)}. + - A path P passes through K nodes {Npi,(i=1...K)}. + - A set of paths {P1...Pm} have L nodes that are common to + more than one path {Nci,(i=1...L)}. + - Find a set of paths such that the value of L is minimized. + + If the OF-List TLV is included in the ASSOCIATION object, the first + OF-code inside the OF object MUST be 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 32 (Incompatible OF code). + +5.4. Relationship to SVEC + + [RFC5440] defines a mechanism for the synchronization of a set of + path computation requests by using the SVEC object, which specifies + the list of synchronized requests that can be either dependent or + independent. The SVEC object identifies the relationship between the + set of path computation requests, identified by 'Request-ID-number' + in the RP (Request Parameters) object. [RFC6007] further clarifies + the use of the SVEC list for synchronized path computations when + computing dependent requests and describes 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 requests in a similar + way as the Flags field in the TLVs defined in this document. The + path computation request in the Path Computation Request (PCReq) + message MAY use both the SVEC and ASSOCIATION objects to identify the + related path computation request, as well as the DAG. 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 (including the flags set inside 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 [RFC5440]. It should be noted that the + LSPs in the association group can be fully same or partially + overlapping with the LSPs grouped by the SVEC object in PCReq + message. + + Some examples of usage are listed below: + + * PCReq with SVEC object with node-diverse bit=1 (LSP1,LSP2) and DAG + with S=1 (LSP1,LSP2) - both node- and SRLG-diverse path between + LSP1 and LSP2. + + * PCReq with SVEC object with link-diverse bit=1 (LSP1,LSP2) and DAG + with L=1 (LSP1,LSP3) - link-diverse paths between LSP1 and LSP2 + and between LSP1 and LSP3. If the DAG is part of the stateful + database, any future change in LSP3 will have an impact on LSP1. + But any future change in LSP2 will have no impact on LSP1, as LSP2 + is part of SVEC object (which is considered once on receipt of the + PCReq message only). + +5.4.1. SVEC and OF + + This document defines three new OF-codes in Section 5.3 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 + diversity with as much SRLG diversity as possible. + + * SVEC object with domain-diverse bit=1 [RFC8685]; node-diverse + bit=1, and OF=MSS - full domain and node diversity with as much + 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. + +5.5. P Flag Considerations + + As mentioned in Section 5.2, the P flag (when set) indicates that the + computed path of the LSP SHOULD satisfy all constraints and objective + functions first without considering the diversity constraint. + + This means that an LSP with the P flag set should be placed first, as + if the disjointness constraint has not been configured, while the + other LSPs in the association with the P flag unset should be placed + by taking into account the disjointness constraint. Setting the P + flag changes the relationship between LSPs to a one-sided + relationship (LSP 1 with P=0 depends on LSP 2 with P=1, but LSP 2 + with P=1 does not depend on LSP 1 with P=0). Multiple LSPs in the + same Disjoint Association 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 other 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 Association 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 / + \_________________________________________/ + + Figure 4: Example Topology with Six Internal Routers + + Note: In Figure 4, the 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 link disjoint + paths between the two sites. Due to physical meshing, the customer + wants to use CE1 and CE2 as the 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 + link disjoint LSPs together, leading to PE1->PE2 using 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 link 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 a 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 finding a disjoint path. In the + example above, if we consider that router R5 is down and if PE1->PE2 + has the P flag set, there is no room available to place PE3->PE4 (the + link disjointness constraint cannot be fulfilled). If PE1->PE2 has + the P flag unset, the algorithm may be able to place PE1->PE2 on the + R1->R2 link leaving room for PE3->PE4 using the R3->R4 link. When + using the 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 + | +------+ +------+ | + | | + \ / + \_________________________________________/ + + Figure 5: Example Topology with Four Internal Routers + + Note: In Figure 5, the 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 link 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 link 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 the P flag is set for an LSP and when ECMPs are available, an + implementation should aim to select a path that allows disjointness. + +5.6. Disjointness Computation Issues + + 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), if disjointness cannot + be ensured for one or more LSPs, the PCE MUST reply to a PCReq with a + PCRep message containing a NO-PATH object. In case of a 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 a network event leading to an impossible strict + disjointness, the PCE MUST send a PCUpd message containing an empty + Explicit Route Object (ERO) to the corresponding PCCs. In addition + to the empty ERO object, the PCE MAY add the NO-PATH-VECTOR TLV + [RFC5440] in the LSP object. + + This document adds new bits in the Flags field of the NO-PATH-VECTOR + TLV: + + * bit 11: When set, the PCE indicates that it could not find a + disjoint path for this LSP. + + * bit 10: 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 (Section 5.3) 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 out of scope of the document. See + [PCE-OPTIONAL] for a proposed mechanism. + +6. Security Considerations + + This document defines one new PCEP Association type, which by itself + does not add any new security concerns beyond those discussed in + [RFC5440], [RFC8231], [RFC7470], and [RFC8697]. But adding of a + spurious LSP into the Disjoint Association Group could lead to + recomputation and setup of all LSPs in the group, which 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, certain combinations of + flags (notably, the 'T' bit) can result in conflicts that cannot be + resolved. + + Also, as stated in [RFC8697], much of the information carried in the + ASSOCIATION object reflects information that can also be derived from + the LSP database, but association provides a much easier grouping of + related LSPs and messages. This holds true for the DAT as well; + thus, this could provide an adversary with the opportunity to + eavesdrop on the relationship between the LSPs and understand the + network topology. + + Thus, securing the PCEP session using Transport Layer Security (TLS) + [RFC8253], as per the recommendations and best current practices in + BCP 195 [RFC7525], is RECOMMENDED. + +7. IANA Considerations + +7.1. Association Type + + This document defines a new Association type, originally described in + [RFC8697]. IANA has assigned the following new value in the + "ASSOCIATION Type Field" subregistry [RFC8697] within the "Path + Computation Element Protocol (PCEP) Numbers" registry: + + +======+======================+===========+ + | Type | Name | Reference | + +======+======================+===========+ + | 2 | Disjoint Association | RFC 8800 | + +------+----------------------+-----------+ + + Table 1: ASSOCIATION Type Field + +7.2. PCEP TLVs + + This document defines two new PCEP TLVs. IANA has assigned the + following values in the "PCEP TLV Type Indicators" subregistry within + the "Path Computation Element Protocol (PCEP) Numbers" registry: + + +==========+============================+===========+ + | TLV Type | TLV Name | Reference | + +==========+============================+===========+ + | 46 | DISJOINTNESS-CONFIGURATION | RFC 8800 | + +----------+----------------------------+-----------+ + | 47 | DISJOINTNESS-STATUS | RFC 8800 | + +----------+----------------------------+-----------+ + + Table 2: PCEP TLV Type Indicators + + IANA has created a new subregistry, named "DISJOINTNESS-CONFIGURATION + TLV Flag Field", within the "Path Computation Element Protocol (PCEP) + Numbers" registry to manage the Flags field in the DISJOINTNESS- + CONFIGURATION TLV. New values are to be assigned by Standards Action + [RFC8126]. Each bit should be tracked with the following qualities: + + * Bit number (count from 0 as the most significant bit) + + * Flag Name + + * Reference + + The initial contents of this subregistry are shown below: + + +======+=========================+===========+ + | Bit | Name | Reference | + +======+=========================+===========+ + | 31 | L - Link Diverse | RFC 8800 | + +------+-------------------------+-----------+ + | 30 | N - Node Diverse | RFC 8800 | + +------+-------------------------+-----------+ + | 29 | S - SRLG Diverse | RFC 8800 | + +------+-------------------------+-----------+ + | 28 | P - Shortest Path | RFC 8800 | + +------+-------------------------+-----------+ + | 27 | T - Strict Disjointness | RFC 8800 | + +------+-------------------------+-----------+ + | 0-26 | Unassigned | | + +------+-------------------------+-----------+ + + Table 3: DISJOINTNESS-CONFIGURATION TLV + Flag Field + +7.3. Objective Functions + + This document defines three new objective functions. IANA has made + the following allocations in the "Objective Function" subregistry + within the "Path Computation Element Protocol (PCEP) Numbers" + registry: + + +============+=======================+===========+ + | Code Point | Name | Reference | + +============+=======================+===========+ + | 15 | Minimize the number | RFC 8800 | + | | of Shared Links (MSL) | | + +------------+-----------------------+-----------+ + | 16 | Minimize the number | RFC 8800 | + | | of Shared SRLGs (MSS) | | + +------------+-----------------------+-----------+ + | 17 | Minimize the number | RFC 8800 | + | | of Shared Nodes (MSN) | | + +------------+-----------------------+-----------+ + + Table 4: Objective Function + +7.4. NO-PATH-VECTOR Bit Flags + + This document defines new bits for the NO-PATH-VECTOR TLV in the "NO- + PATH-VECTOR TLV Flag Field" subregistry of the "Path Computation + Element Protocol (PCEP) Numbers" registry. IANA has made the + following allocations: + + +============+===========================+===========+ + | Bit Number | Name | Reference | + +============+===========================+===========+ + | 11 | Disjoint path not found | RFC 8800 | + +------------+---------------------------+-----------+ + | 10 | Requested disjoint | RFC 8800 | + | | computation not supported | | + +------------+---------------------------+-----------+ + + Table 5: NO-PATH-VECTOR TLV Flag Field + +7.5. PCEP-ERROR Codes + + This document defines two new Error-values within existing Error- + Types related to disjoint association. IANA has allocated the + following new Error-values in the "PCEP-ERROR Object Error Types and + Values" subregistry within the "Path Computation Element Protocol + (PCEP) Numbers" registry: + + +============+===========+============================+===========+ + | Error-Type | Meaning | Error-value | Reference | + +============+===========+============================+===========+ + | 6 | Mandatory | | [RFC5440] | + | | Object | | | + | | missing | | | + +------------+-----------+----------------------------+-----------+ + | | | 15: DISJOINTNESS- | RFC 8800 | + | | | CONFIGURATION TLV missing | | + +------------+-----------+----------------------------+-----------+ + | 10 | Reception | | [RFC5440] | + | | of an | | | + | | invalid | | | + | | object | | | + +------------+-----------+----------------------------+-----------+ + | | | 32: Incompatible OF code | RFC 8800 | + +------------+-----------+----------------------------+-----------+ + + Table 6: PCEP-ERROR Object Error Types and Values + +8. Manageability Considerations + +8.1. Control of Function and Policy + + An operator SHOULD be allowed to configure the Disjoint Association + Groups and disjoint parameters at the PCEP peers and associate them + with the LSPs. The operator MUST be allowed to set the Operator- + configured Association Range. The operator SHOULD be allowed to set + the local policies to define various disjoint computational behavior + at the PCE. + +8.2. Information and Data Models + + An implementation SHOULD allow the operator to view the disjoint + associations configured or created dynamically. Furthermore, + implementations SHOULD allow to view disjoint associations reported + by each peer and the current set of LSPs in this association. The + PCEP YANG module [PCEP-YANG] includes association group information. + +8.3. 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 [RFC5440]. + +8.4. Verification of Correct Operations + + Apart from the operation verification requirements already listed in + [RFC5440], a PCEP implementation SHOULD provide parameters related to + disjoint path computation, such as number of DAG, number of disjoint + path computation failures, etc. A PCEP implementation SHOULD log + failure events (e.g., incompatible Flags). + +8.5. Requirements on Other Protocols + + Mechanisms defined in this document do not imply any new requirements + on other protocols. + +8.6. Impact on Network Operations + + Mechanisms defined in Section 8.6 of [RFC5440] 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. + +9. References + +9.1. Normative References + + [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate + Requirement Levels", BCP 14, RFC 2119, + DOI 10.17487/RFC2119, March 1997, + <https://www.rfc-editor.org/info/rfc2119>. + + [RFC5440] Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation + Element (PCE) Communication Protocol (PCEP)", RFC 5440, + DOI 10.17487/RFC5440, March 2009, + <https://www.rfc-editor.org/info/rfc5440>. + + [RFC5541] Le Roux, JL., Vasseur, JP., and Y. Lee, "Encoding of + Objective Functions in the Path Computation Element + Communication Protocol (PCEP)", RFC 5541, + DOI 10.17487/RFC5541, June 2009, + <https://www.rfc-editor.org/info/rfc5541>. + + [RFC7470] Zhang, F. and A. Farrel, "Conveying Vendor-Specific + Constraints in the Path Computation Element Communication + Protocol", RFC 7470, DOI 10.17487/RFC7470, March 2015, + <https://www.rfc-editor.org/info/rfc7470>. + + [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for + Writing an IANA Considerations Section in RFCs", BCP 26, + RFC 8126, DOI 10.17487/RFC8126, June 2017, + <https://www.rfc-editor.org/info/rfc8126>. + + [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC + 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, + May 2017, <https://www.rfc-editor.org/info/rfc8174>. + + [RFC8231] Crabbe, E., Minei, I., Medved, J., and R. Varga, "Path + Computation Element Communication Protocol (PCEP) + Extensions for Stateful PCE", RFC 8231, + DOI 10.17487/RFC8231, September 2017, + <https://www.rfc-editor.org/info/rfc8231>. + + [RFC8253] Lopez, D., Gonzalez de Dios, O., Wu, Q., and D. Dhody, + "PCEPS: Usage of TLS to Provide a Secure Transport for the + Path Computation Element Communication Protocol (PCEP)", + RFC 8253, DOI 10.17487/RFC8253, October 2017, + <https://www.rfc-editor.org/info/rfc8253>. + + [RFC8685] Zhang, F., Zhao, Q., Gonzalez de Dios, O., Casellas, R., + and D. King, "Path Computation Element Communication + Protocol (PCEP) Extensions for the Hierarchical Path + Computation Element (H-PCE) Architecture", RFC 8685, + DOI 10.17487/RFC8685, December 2019, + <https://www.rfc-editor.org/info/rfc8685>. + + [RFC8697] Minei, I., Crabbe, E., Sivabalan, S., Ananthakrishnan, H., + Dhody, D., and Y. Tanaka, "Path Computation Element + Communication Protocol (PCEP) Extensions for Establishing + Relationships between Sets of Label Switched Paths + (LSPs)", RFC 8697, DOI 10.17487/RFC8697, January 2020, + <https://www.rfc-editor.org/info/rfc8697>. + +9.2. Informative References + + [PCE-OPTIONAL] + Li, C., Zheng, H., and S. Litkowski, "Extension for + Stateful PCE to allow Optional Processing of PCEP + Objects", Work in Progress, Internet-Draft, draft-dhody- + pce-stateful-pce-optional-06, 9 July 2020, + <https://tools.ietf.org/html/draft-dhody-pce-stateful-pce- + optional-06>. + + [PCEP-YANG] + Dhody, D., Hardwick, J., Beeram, V., and J. Tantsura, "A + YANG Data Model for Path Computation Element + Communications Protocol (PCEP)", Work in Progress, + Internet-Draft, draft-ietf-pce-pcep-yang-14, 7 July 2020, + <https://tools.ietf.org/html/draft-ietf-pce-pcep-yang-14>. + + [RFC4655] Farrel, A., Vasseur, J.-P., and J. Ash, "A Path + Computation Element (PCE)-Based Architecture", RFC 4655, + DOI 10.17487/RFC4655, August 2006, + <https://www.rfc-editor.org/info/rfc4655>. + + [RFC6007] Nishioka, I. and D. King, "Use of the Synchronization + VECtor (SVEC) List for Synchronized Dependent Path + Computations", RFC 6007, DOI 10.17487/RFC6007, September + 2010, <https://www.rfc-editor.org/info/rfc6007>. + + [RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre, + "Recommendations for Secure Use of Transport Layer + Security (TLS) and Datagram Transport Layer Security + (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May + 2015, <https://www.rfc-editor.org/info/rfc7525>. + + [RFC8281] Crabbe, E., Minei, I., Sivabalan, S., and R. Varga, "Path + Computation Element Communication Protocol (PCEP) + Extensions for PCE-Initiated LSP Setup in a Stateful PCE + Model", RFC 8281, DOI 10.17487/RFC8281, December 2017, + <https://www.rfc-editor.org/info/rfc8281>. + +Acknowledgments + + A special thanks to the authors of [RFC8697]; this document borrows + some text from it. The authors would also like to thank Adrian + Farrel and Julien Meuric for the valuable comments. + + Thanks to Emmanuel Baccelli for the RTGDIR review. + + Thanks to Dale Worley for a detailed GENART review. + + Thanks to Alvaro Retana, Benjamin Kaduk, Suresh Krishnan, Roman + Danyliw, Alissa Cooper, and Éric Vyncke for the IESG review. + +Contributors + + Dhruv Dhody + Huawei Technologies + Divyashree Techno Park, Whitefiled + Bangalore 560066 + Karnataka + India + + Email: dhruv.ietf@gmail.com + + +Authors' Addresses + + Stephane Litkowski + Cisco Systems, Inc. + + Email: slitkows.ietf@gmail.com + + + Siva Sivabalan + Ciena Corporation + + Email: msiva282@gmail.com + + + Colby Barth + Juniper Networks + + Email: cbarth@juniper.net + + + Mahendra Singh Negi + RtBrick India + N-17L, Floor-1, 18th Cross Rd, HSR Layout Sector-3 + Bangalore 560102 + Karnataka + India + + Email: mahend.ietf@gmail.com |