Network Working Group JL. Le Roux Request for Comments: 5541 France Telecom Category: Standards Track JP. Vasseur Cisco System Inc. Y. Lee Huawei June 2009
Encoding of Objective Functions in the Path Computation Element Communication Protocol (PCEP)
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This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.
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RFC 5541 Objective Functions in PCEP June 2009
Abstract
The computation of one or a set of Traffic Engineering Label Switched Paths (TE LSPs) in MultiProtocol Label Switching (MPLS) and Generalized MPLS (GMPLS) networks is subject to a set of one or more specific optimization criteria, referred to as objective functions (e.g., minimum cost path, widest path, etc.).
In the Path Computation Element (PCE) architecture, a Path Computation Client (PCC) may want a path to be computed for one or more TE LSPs according to a specific objective function. Thus, the PCC needs to instruct the PCE to use the correct objective function. Furthermore, it is possible that not all PCEs support the same set of objective functions; therefore, it is useful for the PCC to be able to automatically discover the set of objective functions supported by each PCE.
This document defines extensions to the PCE communication Protocol (PCEP) to allow a PCE to indicate the set of objective functions it supports. Extensions are also defined so that a PCC can indicate in a path computation request the required objective function, and a PCE can report in a path computation reply the objective function that was used for path computation.
This document defines objective function code types for six objective functions previously listed in the PCE requirements work, and provides the definition of four new metric types that apply to a set of synchronized requests.
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Table of Contents
1. Introduction ....................................................3 1.1. Conventions Used in This Document ..........................5 1.2. Terminology ................................................5 1.3. Message Formats ............................................6 2. Discovery of PCE Objective Functions ............................6 2.1. OF-List TLV ................................................6 2.2. Elements of Procedure ......................................7 3. Objective Function in PCEP Path Computation Request and Reply Messages ........................................................7 3.1. OF Object ..................................................7 3.1.1. Elements of Procedure ...............................8 3.2. Carrying The OF Object In a PCEP Message ...................9 3.3. New RP Object Flag ........................................12 3.3.1. Elements Of Procedure ..............................12 4. Objective Functions Definition .................................13 5. New Metric Types ...............................................14 6. IANA Considerations ............................................15 6.1. PCE Objective Function Sub-Registry .......................15 6.2. PCEP Code Points ..........................................16 6.2.1. OF Object ..........................................16 6.2.2. OF-List TLV ........................................16 6.2.3. PCEP Error Values ..................................16 6.2.4. RP Object Flag .....................................17 6.2.5. Metric Types .......................................17 7. Security Considerations ........................................17 8. Manageability Considerations ...................................18 8.1. Control of Function and Policy ............................18 8.2. Information and Data Models ...............................18 8.3. Liveness Detection and Monitoring .........................18 8.4. Verify Correct Operations .................................18 8.5. Requirements On Other Protocols ...........................19 8.6. Impact On Network Operations ..............................19 9. Acknowledgments ................................................19 10. References ....................................................19 10.1. Normative References .....................................19 10.2. Informative References ...................................19 Appendix A. RBNF Code Fragments ...................................21
The Path Computation Element-based network architecture [RFC4655] defines a Path Computation Element (PCE) as an entity capable of computing the paths of Traffic Engineered Label Switched Paths (TE LSPs) based on a network graph and of applying computational constraints. A PCE services path computation requests that are sent by Path Computation Clients (PCC).
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The PCE communication Protocol (PCEP), defined in [RFC5440], allows for communication between a PCC and a PCE or between two PCEs, in compliance with requirements and guidelines set forth in [RFC4657]. Such interactions include path computation requests and path computation replies.
The computation of one or a set of TE LSPs is subject to a set of one or more optimization criteria, called an objective function. An objective function is used by the PCE when it computes a path or a set of paths in order to select the "best" candidate paths. There is a variety of objective functions: an objective function could apply either to a set of non-synchronized path computation requests, or to a set of synchronized path computation requests. In the former case, the objective function refers to an individual path computation request (e.g., computation of the shortest constrained path where the metric is the IGP metric, computation of the least loaded constrained path, etc.). Conversely, in the latter case, the objective function refers to a set of path computation requests the computation of which is synchronized (e.g., minimize the aggregate bandwidth consumption of all LSPs, minimize the sum of the delays for two diverse paths or of the delta between those delays, etc.). Moreover, some objective functions relate to the optimization of a single metric and others to the optimization of a set of metrics (organized in a hierarchical manner, using a weighted function, etc.).
As spelled out in [RFC4674], it may be useful for a PCC to discover the set of objective functions supported by a PCE. Furthermore, [RFC4657] requires the ability for a PCC to indicate in a path computation request a required/desired objective function, as well as optional function parameters.
For these purposes, this document extends the PCE communication Protocol (PCEP). It defines PCEP extensions that allow a PCE to advertise a list of supported objective functions, as well as extensions to carry the objective function in PCEP request and reply messages. It complements the PCEP base specification [RFC5440].
Note that OSPF- and IS-IS-based PCE discovery mechanisms are defined in [RFC5088] and [RFC5089]. These mechanisms are dedicated to the discovery of a few generic parameters, while more detailed PCE parameters should be discovered using the PCE communication Protocol. Objective functions are in this second category; thus, the objective function discovery procedure is handled by PCEP.
A new PCEP TLV, named the OF-List TLV, is defined in Section 2. The OF-List TLV is carried in the PCEP OPEN object and allows a PCE to list, during PCEP session-setup phase, the objective functions that it supports.
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A new PCEP object, the OF object, is defined in Section 3. The OF object is carried within a PCReq (Path Computation Request) message to indicate the required/desired objective function to be applied by a PCE, or in a PCRep (Path Computation Reply) message to indicate the objective function that was used for path computation.
Six mandatory objective functions that must be supported by PCEP are listed in [RFC4657]. This document provides a definition of these six mandatory objective functions. Additional objective functions may be defined in other documents. Note that additional objective functions are defined for the PCE Global Concurrent Optimization (GCO) application, in [PCE-GCO].
This document also provides the definition of four new metric types that apply to a set of synchronized requests.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].
OF: Objective Function. A set of one or more optimization criteria used for the computation of a single path (e.g., path cost minimization) or for the synchronized computation of a set of paths (e.g., aggregate bandwidth consumption minimization, etc.).
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 of applying computational constraints.
PCEP: Path Computation Element communication Protocol.
This section defines PCEP extensions (see [RFC5440]) so as to support the advertisement of the objective functions supported by a PCE.
A new PCEP OF-List (Objective Function list) TLV is defined. The PCEP OF-List TLV is carried within an OPEN object. This way, during PCEP session-setup phase, a PCE can advertise to a PCEP peer the list of objective functions it supports.
The PCEP OF-List TLV is optional. It MAY be carried within an OPEN object sent by a PCE in an Open message to a PCEP peer so as to indicate the list of supported objective functions.
The OF-List TLV format is compliant with the PCEP TLV format defined in [RFC5440]. That is, the TLV is composed of 2 octets for the type, 2 octets specifying the TLV length, and a Value field. The Length field defines the length of the value portion in octets. The TLV is padded to 4-octet alignment, and padding is not included in the Length field (e.g., a 3-octet value would have a length of three, but the total size of the TLV would be eight octets).
The PCEP OF-List TLV has the following format:
TYPE: 4 LENGTH: N * 2 (where N is the number of objective functions) VALUE: list of 2-byte objective function code points, identifying the objective functions supported by the sender of the Open message.
A PCE MAY include an OF-List TLV within an OPEN object in an Open message sent to a PCEP peer in order to advertise a set of one or more objective functions. The OF-List TLV MUST NOT appear more than once in an OPEN object. If it appears more than once, the PCEP session MUST be rejected with error type 1 and error value 1 (PCEP session establishment failure / Reception of an invalid Open message). The absence of the OF-List TLV in an OPEN object MUST be interpreted as an absence of information on the list of supported objective functions by the PCE.
As specified in [RFC5440], a PCEP peer that does not recognize the OF-List TLV will silently ignore it.
3. Objective Function in PCEP Path Computation Request and Reply Messages
This section defines PCEP extensions [RFC5440] so as to support the communication of objective functions in PCEP path computation request and reply messages. A new PCEP OF (Objective Function) object is defined, to be carried within a PCReq message in order for the PCC to indicate the required/desired objective function.
The PCEP OF object may also be carried within a PCRep message in order for the PCE to indicate the objective function that was used by the PCE.
A new flag is defined in the RP (Request Parameters) object. The flag is used in a PCReq message to indicate that the PCE MUST include an OF object in the PCRep message to indicate the objective function that was used during path computation.
Also, new PCEP error types and values are defined.
The PCEP OF (Objective Function) object is optional. It MAY be carried within a PCReq message so as to indicate the desired/required objective function to be applied by the PCE during path computation or within a PCRep message so as to indicate the objective function that was used by the PCE during path computation.
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The OF object format is compliant with the PCEP object format defined in [RFC5440].
The OF Object-Class is 21. The OF Object-Type is 1.
To request the use of a specific objective function by the PCE, a PCC includes an OF object in the PCReq message.
[RFC5440] specifies a bit flag, referred to as the P bit, carried in the common PCEP object header. The P bit is set by a PCC to mandate that a PCE must take the information carried in the object into account during the path computation.
If the P bit is set in the OF object, the objective function is mandatory (required objective function) and the PCE MUST use the objective function during path computation. If the P bit is clear in the OF object, the objective function is optional (desired objective function) and the PCE SHOULD apply the function if it is supported but MAY choose to apply a different objective function, according to local capabilities and policies.
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On receipt of a PCReq message with an OF object, a PCE MUST proceed as follows:
- If the OF object is unknown/unsupported, the PCE MUST follow procedures defined in [RFC5440]. That is, if the P bit is set, the PCE sends a PCErr message with error type 3 or 4 (Unknown / Not supported object) and error value 1 or 2 (unknown / unsupported object class / object type), and the related path computation request MUST be discarded. If the P bit is cleared, the PCE is free to ignore the object.
- If the objective function is unknown/unsupported and the P bit is set, the PCE MUST send a PCErr message with error type 3 or 4 (Unknown / Not supported object) and error value 4 (Unrecognized/Unsupported parameter), and the related path computation request MUST be discarded.
- If the objective function is unknown/unsupported and the P bit is cleared, the PCE SHOULD apply another (default) objective function.
- If the objective function is supported but policy does not permit applying it and if the P bit is set, the PCE MUST send a PCErr message with the PCEP error type "policy-violation" (type 5) and a new error value, "objective function not allowed", which is defined in this document.
- If the objective function is supported but policy does not allow applying it and if the P bit is cleared, the PCE SHOULD apply another (default) objective function.
- If the objective function is supported and policy allows applying it and if the P bit is set, the PCE MUST apply the requested objective function. Otherwise, if the P bit is cleared, the PCE is free to apply any other objective function.
The default objective function may be locally configured.
The OF object MAY be carried within a PCReq message. If an objective function is to be applied to a set of synchronized path computation requests, the OF object MUST be carried just after the corresponding SVEC (Synchronization VECtor) object and MUST NOT be repeated for each elementary request.
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Similarly, if a metric is to be applied to a set of synchronized requests, the METRIC object MUST follow the SVEC object and MUST NOT be repeated for each elementary request. Note that metrics applied to a set of synchronized requests are defined in Section 5.
An OF object specifying an objective function that applies to an individual path computation request (non-synchronized case) MUST follow the RP object for which it applies.
The format of the PCReq message is updated as follows. Please see Appendix A for a full set of RBNF fragments defined in this document and the necessary code license.
The OF object MAY be carried within a PCRep message to indicate the objective function used by the PCE during path computation.
When the PCE wants to indicate to the PCC the objective function that was used for the synchronized computation of a set of paths, the PCRep message MUST include the corresponding SVEC object directly followed by the OF object, which MUST NOT be repeated for each elementary request. If a metric is applicable to the set of paths, the METRIC object MUST directly follow the SVEC object and MUST NOT be repeated for each elementary request.
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An OF object specifying an objective function used for an individual path computation (non-synchronized case) MUST follow the RP object for which it applies.
The format of the PCRep message is updated as follows. Please see Appendix A for a full set of RBNF fragments defined in this document and the necessary code license.
In some cases, where no objective function is specified in the request or an optional objective function is desired (P flag cleared in the OF object common header) but the PCE does not follow the request, the PCC may desire to know the objective function that was used by the PCE during path computation. To that end, a new flag is defined in the RP object, named the OF flag, allowing a PCC to request for the inclusion in the path computation reply of the objective function that was used by the PCE during path computation.
The following new bit flag of the RP object is defined:
The Supply OF on response flag (bit number 24). When set in a PCReq message, this indicates that the PCE MUST provide the applied objective function (should a path satisfying the constraints be found) in the PCRep message. When set in a PCRep message, this indicates that the objective function that was used during path computation is included.
If the PCC wants to know the objective function used by the PCE during path computation for a given request, it sets the OF flag in the RP object.
On receipt of a PCReq message with the OF flag in the RP object set, the PCE proceeds as follows:
- If policy permits, it MUST include in the PCRep message an OF object indicating the objective function it used during path computation.
- If policy does not permit, it MUST send a PCErr message with the PCEP error code "policy-violation" (type 5) and a new error value, "objective function indication not allowed", which is defined in this document.
Note that a legacy PCE might not recognize the OF flag in the RP object. According to the definition of the Flags field for the RP object (Section 7.4.1 of [RFC5440]), the legacy PCE will ignore the unknown flag, resulting in it sending a PCRep that does not contain an OF object. In this case, the PCC's behavior is an implementation choice. The PCC might:
- Discard the PCRep because it really wanted the OF object returned.
- Accept the PCRep without the knowledge of the OF that was applied.
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Note also that these procedures can give rise to the situation where a PCC receives a PCRep that contains an OF object with an objective function identifier that the PCC does not recognize. In this situation, the PCC behavior is dependent on implementation and configuration. The PCC could choose any of the following (or some other action):
- Ignore the OF object and use the computed path.
- Add the objective function to its view of the PCE's repertoire for inclusion in future computation requests.
- Discard the PCRep (i.e., the computed path) and send a PCReq to another PCE.
- Discard the PCRep (i.e., the computed path) and send another PCReq to the same PCE explicitly requiring the use of some other objective function (i.e., by setting the P bit in the OF object).
Six objective functions that must be supported by PCEP are listed in [RFC4657]. Objective function codes have been assigned by IANA and are described below.
Objective functions are formulated using the following terminology:
- A network comprises a set of N links {Li, (i=1...N)}.
- A path P is a list of K links {Lpi,(i=1...K)}.
- Metric of link L is denoted M(L). This can be the IGP metric, the TE metric, or any other metric.
- The cost of a path P is denoted C(P), where C(P) = sum {M(Lpi), (i=1...K)}.
- Residual bandwidth on link L is denoted r(L).
- Maximum reservable bandwidth on link L is denoted R(L).
There are three objective functions that apply to the computation of a single path:
Objective Function Code: 1 Name: Minimum Cost Path (MCP) Description: Find a path P such that C(P) is minimized.
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Objective Function Code: 2 Name: Minimum Load Path (MLP) Description: Find a path P such that ( Max {(R(Lpi) - r(Lpi)) / R(Lpi), i=1...K } ) is minimized.
Objective Function Code: 3 Name: Maximum residual Bandwidth Path (MBP) Description: Find a path P such that ( Min { r(Lpi), i=1...K } ) is maximized.
There are three objective functions that apply to a set of path computation requests the computation of which is synchronized:
Objective Function Code: 4 Name: Minimize aggregate Bandwidth Consumption (MBC) Description: Find a set of paths such that ( Sum {R(Li) - r(Li), i=1...N} ) is minimized.
Objective Function Code: 5 Name: Minimize the Load of the most loaded Link (MLL) Description: Find a set of paths such that ( Max { (R(Li) - r(Li)) / R(Li), i=1...N}) is minimized.
Objective Function Code: 6 Name: Minimize the Cumulative Cost of a set of paths (MCC) Description: Find a set of paths {P1...Pm} such that (Sum { C(Pi), i=1...m}) is minimized.
Other objective functions may be defined in separate documents.
Three metric types are defined in PCEP for the METRIC object: TE metric, IGP metric, and hop count. These metric types apply to an individual request. Here, we define four new metric types that apply to a set of synchronized requests:
Type 4: Aggregate bandwidth consumption.
Type 5: Load of the most loaded link.
Type 6: Cumulative IGP cost.
Type 7: Cumulative TE cost.
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These metrics may be used in a PCReq to indicate a bound (B bit set in the METRIC object) or to request the computation of a metric (C bit set in the METRIC object), or in a PCRep to indicate a computed metric.
A METRIC object with one of these four types follows the SVEC object for which it applies.
IANA manages the PCEP Objects code point registry (see [RFC5440]). This is maintained as the "PCEP Objects" sub-registry of the "Path Computation Element Protocol (PCEP) Numbers" registry.
This document defines a new PCEP object, the OF object, to be carried in PCReq and PCRep messages. IANA has made the following allocation:
Object Name Object Name Reference Class Type ------------------------------------------------------------ 21 OF 1 Objective Function RFC 5541
IANA manages the PCEP TLV code point registry (see [RFC5440]). This is maintained as the "PCEP TLV Type Indicators" sub-registry of the "Path Computation Element Protocol (PCEP) Numbers" registry.
This document defines a new PCEP TLV, the OF-List TLV, to be carried in the OPEN object. IANA has made the following allocation:
Type TLV name References ----------------------------------------------- 4 OF-List RFC 5541
IANA maintains a registry of Error-types and Error-values for use in PCEP messages. This is maintained as the "PCEP-ERROR Object Error Types and Values" sub-registry of the "Path Computation Element Protocol (PCEP) Numbers" registry.
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Two new Error-values are defined for the Error-type "policy violation" (type 5):
Error-type Meaning and error values Reference ------------------------------------------------------------------ 5 Policy violation
Error-value=3: objective function not RFC 5541 allowed (request rejected)
Error-value=4: OF bit of the RP object RFC 5541 set (request rejected)
A new flag of the RP object (specified in [RFC5440]) is defined in this document. IANA maintains a registry of RP object flags in the "RP Object Flag Field" sub-registry of the "Path Computation Element Protocol (PCEP) Numbers" registry.
IANA has made the following allocation:
Bit Description Reference ------------------------------------------- 24 Supply OF on response RFC 5541
Four new metric types are defined in this document for the METRIC object (specified in [RFC5440]). IANA maintains a registry of metric types in the "METRIC Object T Field" sub-registry of the "Path Computation Element Protocol (PCEP) Numbers" registry.
IANA has made the following allocations:
- Type 4: Aggregate bandwidth consumption - Type 5: Load of the most loaded link - Type 6: Cumulative IGP cost - Type 7: Cumulative TE cost
PCEP security mechanisms are described in [RFC5440] and are used to secure entire PCEP messages. Nothing in this document changes the message flows or introduces any new messages, so the security mechanisms set out in [RFC5440] continue to be applicable.
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This document introduces a single new object that may optionally be carried on PCEP messages and will be automatically secured using the mechanisms described in [RFC5440].
If a PCEP message is vulnerable to attack (for example, because the security mechanisms are not used), then the OF object could be used as part of an attack; however, it is likely that other objects will provide far more significant ways of attacking a PCE or PCC in this case.
It MUST be possible to configure the activation/deactivation of objective function discovery in PCEP.
In addition to the parameters already listed in Section 8.1 of [RFC5440], a PCEP implementation SHOULD allow configuring a list of authorized objective functions on a PCE. This may apply to any session the PCEP speaker participates in, to a specific session with a given PCEP peer, or to a specific group of sessions with a specific group of PCEP peers.
Note that it is not mandatory for an implementation to support all objective functions defined in Section 4.
It MUST be possible to configure a default objective function used for path computation when a path request is received that requests to use an optional objective function.
Mechanisms defined in this document do not imply any new liveness detection and monitoring requirements in addition to those already listed in [RFC5440].
[RFC4657] Ash, J., Ed., and J. Le Roux, Ed., "Path Computation Element (PCE) Communication Protocol Generic Requirements", RFC 4657, September 2006.
[RFC4674] Le Roux, J., Ed., "Requirements for Path Computation Element (PCE) Discovery", RFC 4674, October 2006.
[RFC5088] Le Roux, JL., Ed., Vasseur, JP., Ed., Ikejiri, Y., and R. Zhang, "OSPF Protocol Extensions for Path Computation Element (PCE) Discovery", RFC 5088, January 2008.
[RFC5089] Le Roux, JL., Ed., Vasseur, JP., Ed., Ikejiri, Y., and R. Zhang, "IS-IS Protocol Extensions for Path Computation Element (PCE) Discovery", RFC 5089, January 2008.
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[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008.
[PCE-GCO] Lee, Y., Le Roux, JL., King, D., and E. Oki, "Path Computation Element Communication Protocol (PCEP) Requirements and Protocol Extensions in Support of Global Concurrent Optimization", Work in Progress, March 2009.
[PCEP-MIB] Koushik, K., and E. Stephan, "PCE Communication Protocol (PCEP) Management Information Base", Work in Progress, January 2009.
[RFC5511] Farrel, A., "Routing Backus-Naur Form (RBNF) - A Syntax Used to Form Encoding Rules in Various Routing Protocol Specifications", RFC 5511, April 2009.
This appendix contains the full set of code fragments defined in this document.
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