| PCE Working Group | D. Dhody |
| Internet-Draft | U. Palle |
| Intended status: Experimental | Huawei Technologies India Pvt Ltd |
| Expires: February 21, 2014 | August 20, 2013 |
Encoding of Data Structure (DS) in the Path Computation Element Communication Protocol (PCEP)
draft-dhody-pce-pcep-ds-04
The ability to compute shortest constrained Traffic Engineering Label Switched Paths (TE LSPs) in Multiprotocol Label Switching (MPLS) and Generalized MPLS (GMPLS) networks across multiple domains has been identified as a key requirement for point-to-point (P2P) and point-to-multipoint (P2MP) scenarios. Backward-Recursive Path Computation (BRPC) [RFC5441] defines Virtual Shortest Path Tree (VSPT) as a default de-facto data structure for path reply message in inter-domain scenarios.
As Path Computation Element (PCE) will get used in newer scenarios like inter-domain, protection, P2MP etc. As well as PCE is being explored to be used in Cross Stratrum Optimization (CSO) environment (see [CSO-PCE]) as well as in [ABNO]. Limiting PCE communication Protocol (PCEP) to just one data structure limits the usage of PCEP. Its important to keep PCEP generic enough to use differnt data structure and apply different algorithms.
This document defines extensions to the PCEP to allow multiple data structures. Extensions are defined for PCE to indicate the set of Data Structure (DS) it supports; also Path Computation Client (PCC) or PCE can indicate in a path computation request the required DS, and a PCE can report in a path computation reply the Data Structure that was used in the path reply message.
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The PCE architecture is defined in [RFC4655]. [RFC5441] describe a PCE-based path computation procedure to compute optimal inter-domain constrained (G)MPLS TE LSPs. It also defines Virtual Shortest Path Tree (VSPT) which is the only data structure and is used in all inter-domain scenarios.
This document describes the need for multiple data structure (DS). It may be useful for a PCC/PCE to discover the set of Data Structure (DS) supported by a PCE. Furthermore, PCC/PCE requires the ability to indicate in a path computation request a required/desired Data Structure, 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 Data Structure (DS), as well as extensions to carry Data Structure (DS) 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.
Data Structure (DS) are in this second category; thus, the Data Structure discovery procedure is handled by PCEP.
A new PCEP TLV, named the DS-List TLV, is defined in Section 4. The DS-List TLV is carried in the PCEP OPEN object and allows a PCE to list, during PCEP session-setup phase, the Data Structure (DS) that it supports.
A new PCEP object, the DS object, is defined in Section 5. The DS object is carried within a PCReq (Path Computation Request) message to indicate the required/desired data structure to be applied by a PCE, or in a PCRep (Path Computation Reply) message to indicate the data structure that was used for path computation and the reply message.
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].
The following terminology is used in this document.
[PCE-P2MP-PROCEDURES] describes the need for an extended VSPT for computation of the best core-tree. In case of Core tree based path computation, the PCE in a downstream domain does the pruning and returns the single optimal sub-path to its previous PCE, BRPC insures that the ingress PCE will get all the best optimal sub-paths for each LN (Leaf Border Nodes), but the combination of these single optimal sub-paths into a P2MP tree is not necessarily optimal even if each S2L (Source-to-Leaf) sub-path is optimal. Without trimming, the ingress PCE will get all the possible S2L sub-paths set for LN, and eventually by looking through all the combinations, and taking one sub-path from each set to built one P2MP tree it finds the optimal tree.
[RFC6007] describes the need for disjoint VSPT in case of Synchronized Dependent Path Computations.
The BRPC procedure constructs a VSPT to inform the enquiring PCE of potential paths to the destination node.
In the end-to-end diverse path computation, diversity (or disjointness) information among the potential paths must be preserved in the VSPT to ensure an end-to-end disjoint path. In order to preserve diversity (or disjointness) information, disjoint VSPTs are sent in the PCEP PCRep message. The PCReq containing a SVEC object with the appropriate diverse flag set would signal that the PCE should compute a disjoint VSPT.
A definition of the disjoint VSPT is a collection of VSPTs, in which each VSPT contains a potential set of primary and secondary paths.
In Hierarchical PCE model ([RFC6805]), Parent PCE MAY be used to return the domain-sequence which may be further applied by the ingress PCE to do the BRPC path computation; or Parent PCE MAY do the full end to end path computation.
In full end to end path computation model, Parent PCE MAY ask the child PCE to do the intra domain path computation between -
Here the results are a list of best paths between the nodes listed above, is is not a VSPT, which clearly defines it self as a P2MP Tree from entry boundary nodes to egress.
There exist clear instances like this where VSPT is not the only data structure in use.
VSPT does not work well with boundary constraints like HOP-LIMIT in inter-domain scenarios. Since there maybe a not-the-best-path in a domain which would have satisfied the end to end contraint, but was prunded.
Since PCEP allow multiple Objective Function (OF) [RFC5541]; it is natural to extend PCEP to support multiple Data Structure based on path computation scenarios.
[CSO-PCE] describes the use of PCE in Cross Stratrum Optimization (CSO) environment. A request can be made to the PCE with different sets of computation mode that are not currently supported by PCE. For instance, NCG may request PCE a multi-destination and multi-source path computation request. This scenario arises when there are many possible Data Center choices for a given application request and there could be multiple sources for this request. Multi-destination with a single source (aka., anycast) is a default case for multi-destination and multi-source path computation.
In addition, with this architecture, NCG may have different sets of objectives and constraints than typical path computation requests. For instance, multi-criteria objective functions that combine the bandwidth requirement and latency may be very useful for some applications.
[ABNO] describes Application-Based Network Operations using PCE.
Its important to keep PCEP generic to support new requirements in the future.
This is being achieved in some extent by an RP object bit.
For example, if P2MP bit is set and Objective function (OF) is Minimum Cost Tree (MCT), the extended VSPT should be used. We beleive this not a sustainable mechanism.
Making PCEP generic allowing use of multiple DS will make PCEP protocol behave in a better way.
This section defines PCEP extensions (see [RFC5440]) so as to support the advertisement of the Data Structure (DS) supported by a PCE.
A new PCEP DS-List (Data Structure list) TLV is defined. The PCEP DS-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 data structure it supports.
The PCEP DS-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 data structures.
The DS-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 DS-List TLV has the following format:
TYPE: 4
LENGTH: N * 2 (where N is the number of Data Structures)
VALUE: list of 2-byte data structure code points, identifying
the data structures supported by the sender of the Open
message.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DS Code #1 | DS Code #2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DS Code #N | padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
DS Code (2 bytes): Data Structure code point identifier. IANA
manages the "PCE Data Structure" code point registry
(see Section 6).
A PCE MAY include a DS-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 supported Data Structures. The DS-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 DS-List TLV in an OPEN object MUST be interpreted as an absence of information on the list of supported data structures by the PCE, default data stricture VSPT is always supported.
As specified in [RFC5440], a PCEP peer that does not recognize the DS-List TLV will silently ignore it.
This section defines PCEP extensions [RFC5440] so as to support the communication of Data Structure (DS) in PCEP path computation request and reply messages. A new PCEP DS (Data Structure) object is defined, to be carried within a PCReq message in order for the PCC/PCE to indicate the required/desired data structure.
The PCEP DS object may also be carried within a PCRep message in order for the PCE to indicate the data structure that was used by the PCE and used in the reply message.
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 a DS object in the PCRep message to indicate the data structure that was used during path computation and encoded in the reply message.
Also, new PCEP error types and values are defined.
The PCEP DS (Data Structure) object is optional. It MAY be carried within a PCReq message so as to indicate the desired/required data structure to be applied by the PCE during path computation or within a PCRep message so as to indicate the data structure that was used by the PCE during path computation and in the reply message.
The DS object format is compliant with the PCEP object format defined in [RFC5440].
The DS Object-Class is <TBA by IANA>.
The DS Object-Type is 1.
The format of the DS object body is:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DS Code | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
// Optional TLV(s) //
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
DS Code (2 bytes): The identifier of the Data Structure. IANA
manages the "PCE Data Structure" code point registry
Reserved (2 bytes): This field MUST be set to zero on transmission
and MUST be ignored on receipt.
Optional TLVs may be defined in the future.
To request the use of a specific data structure by the PCE, a PCC/PCE includes a DS 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/PCE 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 DS object, the data structure is mandatory (required data structure) and the PCE MUST use the data structure during path computation. If the P bit is clear in the DS object, the data structure is optional (desired data structure) and the PCE SHOULD apply the data structure if it is supported but MAY choose to apply a different data structure, according to local capabilities and policies.
On receipt of a PCReq message with a DS object, a PCE MUST proceed as follows:
The default data structure is VSPT or may be locally configured.
The DS object MAY be carried within a PCReq message. If a data structure is to be applied to a set of synchronized path computation requests, the DS object MUST be carried just after the corresponding SVEC (Synchronization VECtor) object and MUST NOT be repeated for each elementary request.
A DS object specifying a data structure 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.
<PCReq Message> ::= <Common Header>
[<svec-list>]
<request-list>
where:
<svec-list> ::= <SVEC>
[<OF>]
[<DS>]
[<metric-list>]
[<svec-list>]
<request-list> ::= <request> [<request-list>]
<request> ::= <RP>
<END-POINTS>
[<LSPA>]
[<BANDWIDTH>]
[<metric-list>]
[<OF>]
[<DS>]
[<RRO>[<BANDWIDTH>]]
[<IRO>]
[<LOAD-BALANCING>]
and where:
<metric-list> ::= <METRIC>[<metric-list>]
The DS object MAY be carried within a PCRep message to indicate the data structure used by the PCE during path computation and in the reply message.
When the PCE wants to indicate to the PCC/PCE the data structure 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 DS object, which MUST NOT be repeated for each elementary request.
A DS object specifying a data structure 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.
<PCRep Message> ::= <Common Header>
[<svec-list>]
<response-list>
where:
<svec-list> ::= <SVEC>
[<OF>]
[<DS>]
[<metric-list>]
[<svec-list>]
<response-list> ::= <response> [<response-list>]
<response> ::= <RP>
[<NO-PATH>]
[<attribute-list>]
[<path-list>]
<path-list> ::= <path> [<path-list>]
<path> ::= <ERO>
<attribute-list>
and where:
<attribute-list> ::= [<OF>]
[<DS>]
[<LSPA>]
[<BANDWIDTH>]
[<metric-list>]
[<IRO>]
<metric-list> ::= <METRIC> [<metric-list>]
Note: The DS object MAY be associated to a negative reply, i.e., a reply with a NO-PATH object.
In some cases, where no data structure is specified in the request or an optional data structure is desired (P flag cleared in the DS object common header) but the PCE does not follow the request, the PCC/PCE may desire to know the data structure that was used by the PCE during path computation. To that end, a new flag is defined in the RP object, named the DS flag, allowing a PCC/PCE to request for the inclusion in the path computation reply of the data structure that was used by the PCE during path computation.
The following new bit flag of the RP object is defined: The Supply DS on response flag (bit number <TBA>). When set in a PCReq message, this indicates that the PCE MUST provide the applied data structure in the PCRep message. When set in a PCRep message, this indicates that the data structure that was used during path computation is included.
If the PCC/PCE wants to know the data structure used by the PCE during path computation for a given request, it sets the DS flag in the RP object.
On receipt of a PCReq message with the DS flag in the RP object set, the PCE proceeds as follows:
Note that a legacy PCE might not recognize the DS 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 a DS object. In this case, the PCC/PCE’s behavior is an implementation choice. It might:
Note also that these procedures can give rise to the situation where a PCC/PCE receives a PCRep that contains a DS object with a data structure identifier that the PCC/PCE does not recognize. In this situation, the PCC/PCE behavior is dependent on implementation and configuration. The PCC/PCE could choose any of the following (or some other action):
This document defines a 16-bit PCE data structure identifier to be carried within the PCEP DS object, and also defines the PCEP DS-List TLV. IANA should create and manages the 16-bit "PCE Data Structure" code point registry. Values are TBD.
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 DS object, to be carried in PCReq and PCRep messages.
IANA should make the following allocation:
Object Name Object Name Reference
Class Type
------------------------------------------------------------
TBA DS 1 Data Structure This ID
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 DS-List TLV, to be carried in the OPEN object.
IANA should make the following allocation:
Type TLV name References
-----------------------------------------------
TBA DS-List This ID
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.
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=TBA: data This ID
structure not
allowed (request rejected)
Error-value=TBA: DS bit This ID
of the RP object 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 should make the following allocation:
Bit Description Reference
----------------------------------------------
TBA Supply DS on response This ID
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.
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 DS 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 data structure 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 data structure 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 data structure defined. It MUST be possible to configure a default data structure used for path computation when a path request is received that requests to use an optional data structure.
The PCEP MIB Module defined in [PCEP-MIB] could be extended to include data structure.
Mechanisms defined in this document do not imply any new liveness detection and monitoring requirements in addition to those already listed in [RFC5440].
Mechanisms defined in this document do not imply any new operation verification requirements in addition to those already listed in [RFC5440].
Mechanisms defined in this document do not imply any requirements on other protocols in addition to those already listed in [RFC5440].
Mechanisms defined in this document do not have any impact on network operations in addition to those already listed in [RFC5440].
We would like to thank Pradeep Shastry, Suresh babu, Quintin Zhao and Chen Huaimo for their useful comments and suggestions.
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. |