| PCE Working Group | S. Litkowski |
| Internet-Draft | Orange |
| Intended status: Standards Track | S. Sivabalan |
| Expires: December 22, 2018 | Cisco Systems, Inc. |
| C. Barth | |
| Juniper Networks | |
| D. Dhody | |
| Huawei | |
| June 20, 2018 |
Path Computation Element communication Protocol extension for signaling LSP diversity constraint
draft-ietf-pce-association-diversity-04
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 paths for those LSPs. The proposed extension allows a PCC to advertise to a PCE the belonging of a particular LSP to a disjoint-group, thus the PCE knows that LSPs in the same group must be disjoint from each other.
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This Internet-Draft will expire on December 22, 2018.
Copyright (c) 2018 IETF Trust and the persons identified as the document authors. All rights reserved.
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[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].
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.
[I-D.ietf-pce-association-group] introduces a generic mechanism to create a grouping of LSPs which can then be used to define associations between a set of LSPs and a set of attributes (such as configuration parameters or behaviors) and is equally applicable to the active and passive modes of a stateful PCE [RFC8231] or a stateless PCE [RFC5440].
This document specifies a PCEP extension to signal that a particular group of LSPs should use diverse paths including the requested type of diversity. A PCC can use this extension to signal to a PCE the belonging of a particular LSP to a disjoint-group. When a PCE receives LSP states belonging to the same disjoint-group from some PCCs, the PCE should ensure that the LSPs within the group are disjoint from each other.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.
The following terminology is used in this document.
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 IP/MPLS core. The customer may use those paths as primary/backup or active/active.
Different level of disjointness may be offered:
The associated LSPs may originate from the same or from different head-end(s) and may terminate at the same or different tail-end(s).
_________________________________________
/ \
/ +------+ \
| | PCE | |
| +------+ |
| |
| ***********************> |
| +------+ 10 +------+ |
CE1 ****| PE 1 | ----- R1 ---- R2 ------- | PE 2 |**** CE2
| +------+ | | +------+ |
| | | |
| | | |
| +------+ | | +------+ |
CE3 ****| PE 3 | ----- R3 ---- R4 ------- | PE 4 |**** CE4
| +------+ ***********************> +------+ |
| |
\ /
\_________________________________________/
Figure 1 - Disjoint paths with different head-ends and tail-ends
In the figure above, consider that the customer wants to have two disjoint paths between CE1/CE2 and CE3/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.
[RFC5440] defines a mechanism for the synchronization of a set of path computation requests by using the SVEC (Synchronization VECtor) object, that specifies the list of synchronized requests that can either be dependent or independent. The SVEC object identify the relationship between the set of path computation requests, identified by 'Request-ID-number' in RP (Request Parameters) object. [RFC6007] further clarified the use of the SVEC list for synchronized path computations when computing dependent requests as well as described a number of usage scenarios for SVEC lists within single-domain and multi-domain environments.
The SVEC object includes a Flags field that indicates the potential dependency between the set of path computation request in a similar way as the Flags field in the TLVs defined in this document. The path computation request in the PCReq message MAY use both SVEC object to identify the related path computation request as well as to identify the diversity association group. The PCE MUST try to find a path that meets both the constraints. It is possible that the diversity set in the association group is different from the one in SVEC object, this might be true for the same LSP as well. The PCE would consider both the objects as per the processing rules and aim to find a path that meets both these constraints. In case no such path is possible (or the constraints are incompatible), the PCE MUST send a path computation reply (PCRep) with NO-PATH object indicating path computation failure as per [RFC5440].
The PCEP extension for stateful PCE [RFC8231] defined new PCEP messages - PCRpt, PCUpd and PCInitiate [RFC8281]. These messages uses PLSP-ID in the LSP object for identification. Moreover to allow diversity between LSPs originating from different PCCs, the generic mechanism to create a grouping of LSPs is described in [I-D.ietf-pce-association-group] (that is equally applicable to the active and passive modes of a stateful PCE).
Using PCEP, the PCC could indicate that the disjoint path computation is required, such indication should include disjointness parameters such as the type of disjointness, the disjoint group identifiers, and any customization parameters according to the configured local policy. As mentioned previously, the extension described in [I-D.ietf-pce-association-group] is well suited to associate a set of LSPs with a particular disjoint-group.
The management of the disjoint group-ids will be a key point for the operator as the Association ID field is limited to 65535. The local configuration of IPv4/IPv6 association source, or Global Association Source/Extended Association ID should allow to overcome this limitation as described in [I-D.ietf-pce-association-group]. When a PCC or PCE initiates all the LSPs in a particular disjoint-group, it can set the IPv4/IPv6 association source as one of its own IP address. When disjoint LSPs are initiated from different head-ends, association source could be the PCE address or any other unique value to identify the disjoint association group.
Initiate Disjoint LSPs
|
| PCReq/PCRpt
V {Disjoint-group Y}
+-----+ ----------------> +-----+
_ _ _ _ _ _| PCE | | | PCE |
| +-----+ | ----------> +-----+
| PCInitiate | | PCReq/PCRpt
|{Disjoint-group X} | | {Disjoint-group Y}
| | |
| .-----. | | .-----.
| ( ) | +----+ ( )
| .--( )--. | |PE 1|--.--( )--.
V ( ) | +----+ ( )
+---+ ( ) | ( )
|PCC|----( (G)MPLS network ) +----+ ( (G)MPLS network )
+---+ ( ) |PE 3|-----( )
Disjoint-group 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-group over PCEP session
Using the disjoint-group within a PCEP messages may have two purpose:
As per [I-D.ietf-pce-association-group], LSPs are associated with other LSPs with which they interact by adding them to a common association group. The Association parameters, as described in [I-D.ietf-pce-association-group] as the combination of the mandatory fields Association type, Association ID and Association Source in the ASSOCIATION object, that uniquely identify the association group, uniquely identify the disjoint group. If the optional TLVs - Global Association Source or Extended Association ID are included, then they are included in combination with mandatory fields to uniquely identifying the association group. This document defines a new Association type, based on the generic Association object -
[I-D.ietf-pce-association-group] specify the mechanism for the capability advertisement of the association types supported by a PCEP speaker by defining a ASSOC-Type-List TLV to be carried within an OPEN object. This capability exchange for the association type described in this document (i.e. Disjointness Association Type) MUST be done before using the disjointness association. Thus the PCEP speaker MUST include the Disjointness Association Type (TBD1) in the ASSOC-Type-List TLV before using the disjoint association group in the PCEP messages.
This association type is considered to be both dynamic and operator-configured in nature. The association group could be created by the operator manually on the PCEP peers and the LSPs belonging to this associations is conveyed via PCEP messages to the PCEP peer; or the association group could be created dynamically by the PCEP speaker and both the association group information and the LSPs belonging to the association group is conveyed to the PCEP peer. The Operator-configured Association Range MUST be set for this association-type to mark a range of association identifiers that are used for operator-configured associations to avoid any association identifier clash within the scope of the association source. (Refer [I-D.ietf-pce-association-group].)
A disjoint group can have two or more LSPs. But a PCE may be limited in how many 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 matter.
Local polices on the PCC or PCE 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.
Associating a particular LSP to multiple disjoint groups is authorized from a protocol perspective, however there is no insurance that the PCE will be able to compute properly the multi-disjointness constraint.
The disjoint group MUST carry the following TLV:
In addition, the disjoint group MAY carry the following TLV:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = [TBD2] | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags |T|P|S|N|L|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The DISJOINTNESS-CONFIGURATION-TLV is shown in the following figure:
Type: TBD2.
Length: Fixed value of 4 bytes.
If a PCEP speaker receives a disjoint-group without DISJOINTNESS-CONFIGURATION-TLV, it SHOULD reply with a PCErr Error-type=6 (Mandatory Object missing) and Error-value=TBD7 (DISJOINTNESS-CONFIGURATION-TLV missing).
The DISJOINTNESS-STATUS-TLV uses the same format as the DISJOINTNESS-CONFIGURATION-TLV with a different type TBD3 (in TLV):
Any new flag defined for the DISJOINTNESS-CONFIGURATION-TLV is be automatically applicable to the DISJOINTNESS-STATUS-TLV.
An objective function (OF) MAY be applied to the disjointness computation to drive the PCE computation behavior. In this case, the OF-List TLV (defined in ([RFC5541]) is used as an optional TLV in the Association Group Object. The PCEP OF-List TLV allow multiple OF-Codes inside the TLV, a sender SHOULD include a single OF-Code in the OF-List TLV when included in the Association Group, and the receiver MUST consider the first OF-code only and ignore others if included.
To minimize the common shared resources (Node, Link or SRLG) between a set of paths during path computation three new OF codes are proposed:
MSL
MSS
MSN
[RFC5440] uses SVEC diversity flag for node, link or SRLG to describe the potential disjointness between the set of path computation requests used in PCEP protocol.
This document defines three new OF codes to maximize diversity as much as possible, in other words, minimize the common shared resources (Node,Link or SRLG) between a set of paths.
It may be interesting to note that the diversity flags in the SVEC object and OF for diversity can be used together. Some example of usage are listed below -
As mentioned in Section 4.2, the P-flag (when set) indicates that the computed path of the LSP SHOULD satisfies all constraints and objective functions first without considering the diversity constraint. This could be required in some primary/backup scenarios where the primary path should use the more optimal path available (taking into account the other constraints). When disjointness is computed, it is important for the algorithm to know that it should try to optimize the path of one or more LSPs in the disjoint group (for instance the primary path) while other paths are allowed to be longer (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 requirement.
_________________________________________
/ \
/ +------+ \
| | PCE | |
| +------+ |
| |
| |
| +------+ 10 +------+ |
CE1 ****| PE 1 | ----- R1 ---- R2 ------- | PE 2 |**** CE2
| +------+ | | +------+ |
| | | |
| | | |
| +------+ | | +------+ |
CE3 ****| PE 3 | ----- R3 ---- R4 ------- | PE 4 |**** CE4
| +------+ \ | / +------+ |
| \ | 10 / |
\ +-- R5 --------- R6 /
\_________________________________________/
Figure 3
In the figure above, a customer has two dual homed sites (CE1/CE3 and CE2/CE4). Consider, this customer wants two disjoint paths between the two sites. Due to physical meshing, the customer wants to use CE1 and CE2 as primary ( and CE3 and CE4 are hosted in a remote site for redundancy purpose).
Without any hint (constraint) provided, the PCE may compute the two disjoint LSPs together, leading to PE1->PE2 using a path PE1->R1->R2->PE2 and PE3->PE4 using PE3->R3->R4->PE4. In this case, even if the disjointness constraint is fulfilled, the path from PE1 to PE2 does not use the best optimal path available in the network (RTD may be higher): the customer requirement is thus not completely fulfilled.
The usage of the P-Flag allows the PCE to know that a particular LSP should be tied to the best path as if the disjointness constraint was not requested.
In our example, if the P-Flag is set to the LSP PE1->PE2, the PCE should use the path PE1->R1->R3->R4->R2->PE2 for this LSP, while the other LSP should be disjoint from this path. The second LSP will be placed on PE3->R5->R6->PE4 as it is allowed to be longer.
Driving the PCE disjointness computation may be done in other ways by for instance setting a metric boundary reflecting an RTD boundary. Other constraints may also be used.
The P-Flag allows a simple expression that the disjointness constraint should not make the LSP worst.
Any constraint added to a path disjointness computation may reduce the chance to find suitable paths. The usage of the P-flag, as any other constraint, may prevent to find a disjoint path. In the example above, if we consider that the router R5 is down, if PE1->PE2 has the P-flag set, there is no room available to place PE3->PE4 (the disjointness constraint cannot be fulfilled). If PE->PE2 has the P-flag unset, the algorithm may be able to place PE1->PE2 on R1->R2 link leaving a room for PE3->PE4 using the R3->R4 link. When using P-flag or any additional constraint on top of the disjointness constraint, the user should be aware that there is less chance to fulfill the disjointness constraint.
Multiple LSPs in the same disjoint group may have the P-flag set. In such a case, those LSPs may not be disjoint from each other but will be disjoint from others LSPs in the group that have the P-flag unset.
_________________________________________
/ \
/ +------+ \
| | PCE | |
| +------+ |
| |
| |
| +------+ 10 +------+ |
CE1 ****| PE 1 | ----- R1 ---- R2 ------- | PE 2 |**** CE2
| +------+ | \ | +------+ |
| | \2 | |
| | \ | |
| +------+ | \ | +------+ |
CE3 ****| PE 3 | ----- R3 ---- R4 ------- | PE 4 |**** CE4
| +------+ +------+ |
| |
\ /
\_________________________________________/
Figure 4
In the figure above, we still consider the same previous requirements, so PE1->PE2 LSP should be optimized (P-flag set) while PE3->PE4 should be disjoint and may use a longer 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 or even use both (using both may happen in case Segment Routing TE is used, allowing ECMP).
If the implementation elects only one path, there is a chance that picking up one path may prevent disjointness. In our example, if path 2 is used for PE1->PE2, there is no room left for PE3->PE4 while if path 1 is used, PE3->PE4 can be placed on R3->R4 link.
When P-flag is set for an LSP and when ECMPs are available, an implementation MAY select a path that allows disjointness.
There may be some cases where the PCE is not able to provide a set of disjoint paths for one or more LSPs in the association.
When the T-bit is set (Strict disjointness requested), if disjointness cannot be ensured for one or more LSPs, the PCE SHOULD reply with a PCUpd message containing an empty ERO. 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 NO-PATH-VECTOR TLV:
When the T-bit is unset, the PCE is allowed to reduce the required level of disjointness. The actual level of disjointness 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 expected level of disjointness required by configuration, the DISJOINTNESS-STATUS-TLV defines the actual level of disjointness computed.
There are some cases where the PCE may need to completely relax the disjointness constraint in order to provide a path to all the LSPs that are part of the association. A mechanism that allows the PCE to fully relax a constraint is considered by the authors as more global to PCEP rather than linked to the disjointness use case. As a consequence, it is considered as out of scope of the document.
All LSPs in a particular disjoint group MUST use the same combination of T,S,N,L flags in the DISJOINTNESS-CONFIGURATION-TLV. If a PCE receives PCRpt messages for LSPs belonging to the same disjoint group but having an inconsistent combination of T,S,N,L flags, the PCE SHOULD NOT try to compute disjointness path and SHOULD reply a PCErr with Error-type 26 (Association Error) and Error-Value 6 (Association information mismatch) to all PCCs involved in the disjoint group.
This document defines one new type for association, which do not add any new security concerns beyond those discussed in [RFC5440], [RFC8231] and [I-D.ietf-pce-association-group] in itself.
This document defines the following new association type originally defined in [I-D.ietf-pce-association-group].
Value Name Reference
TBD1 Disjoint-group
Association Type [This I.D.]
This document defines the following new PCEP TLVs:
Value Name Reference TBD2 DISJOINTNESS-CONFIGURATION-TLV [This I.D.] TBD3 DISJOINTNESS-STATUS-TLV [This I.D.]
IANA is requested to manage the space of flags carried in the DISJOINTNESS-CONFIGURATION-TLV defined in this document, numbering them from 0 as the least significant bit.
New bit numbers may be allocated in future.
IANA is requested to allocate the following bit numbers in the DISJOINTNESS-CONFIGURATION-TLV flag space:
Bit Number Name Reference
0 Link disjointness [This I.D.]
1 Node disjointness [This I.D.]
2 SRLG disjointness [This I.D.]
3 Shortest-path [This I.D.]
4 Strict disjointness [This I.D.]
three new Objective Functions have been defined. IANA has made the following allocations from the PCEP "Objective Function" sub-registry:
Value Description Reference TBD4 MSL [This I.D.] TBD5 MSN [This I.D.] TBD6 MSS [This I.D.]
This documents defines new bits for the NO-PATH-VECTOR TLV in the "NO-PATH-VECTOR TLV Flag Field" sub-registry of the "Path Computation Element Protocol (PCEP) Numbers" registry:
Bit Number Name Reference
TBD7 Disjoint path not found [This I.D.]
TBD8 Requested disjointness [This I.D.]
computation not supported
Error-Type Meaning
6 Mandatory Object missing
Error-value=TBD7: DISJOINTNESS-CONFIGURATION
TLV missing
IANA is requested to allocate new Error Types and Error Values within the " PCEP-ERROR Object Error Types and Values" sub-registry of the PCEP Numbers registry, as follows:
An operator MUST be allowed to configure the disjointness associations and parameters at PCEP peers and associate it with the LSPs.
[RFC7420] describes the PCEP MIB, there are no new MIB Objects for this document.
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 new requirements on other protocols.
Mechanisms defined in this document do not have any impact on network operations in addition to those already listed in [RFC5440].
A special thanks to author of [I-D.ietf-pce-association-group], this document borrow some of the text from it. Authors would also like to thank Adrian Farrel for his useful comments.
| [RFC4655] | Farrel, A., Vasseur, J. and J. Ash, "A Path Computation Element (PCE)-Based Architecture", RFC 4655, DOI 10.17487/RFC4655, August 2006. |
| [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. |
| [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. |
| [RFC7420] | Koushik, A., Stephan, E., Zhao, Q., King, D. and J. Hardwick, "Path Computation Element Communication Protocol (PCEP) Management Information Base (MIB) Module", RFC 7420, DOI 10.17487/RFC7420, December 2014. |
| [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. |