| PCE Working Group | D. Dhody |
| Internet-Draft | Q. Wu |
| Intended status: Experimental | U. Palle |
| Expires: May 1, 2017 | X. Zhang |
| Huawei Technologies | |
| October 28, 2016 |
PCE support for Domain Diversity
draft-dwpz-pce-domain-diverse-06
The Path Computation Element (PCE) may be used for computing path for services that traverse multi-area and multi-AS Multiprotocol Label Switching (MPLS) and Generalized MPLS (GMPLS) Traffic Engineered (TE) networks.
Path computation should facilitate the selection of paths with domain diversity. This document examines the existing mechanisms to do so and further propose some extensions to Path Computation Element Protocol (PCEP).
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This Internet-Draft will expire on May 1, 2017.
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The ability to compute shortest constrained TE LSPs in Multiprotocol Label Switching (MPLS) and Generalized MPLS (GMPLS) networks across multiple domains has been identified as a key requirement. In this context, a domain is a collection of network elements within a common sphere of address management or path computational responsibility such as an Interior Gateway Protocol (IGP) area or an Autonomous Systems (AS).
In a multi-domain environment, Domain Diversity is defined in [RFC6805]. A pair of paths are domain-diverse if they do not traverse any of the same transit domains. Domain diversity may be maximized for a pair of paths by selecting paths that have the smallest number of shared domains. Path computation should facilitate the selection of domain diverse paths as a way to reduce the risk of shared failure and automatically helps to ensure path diversity for most of the route of a pair of LSPs.
The main motivation behind domain diversity is to avoid fate sharing, but it can also be because of some geo-political reasons and commercial relationships that would require domain diversity. for example, a pair of paths should choose different transit Autonomous System (AS) because of some policy considerations.
In case when full domain diversity could not be achieved, it is helpful to minimize the common shared domains. Also it is interesting to note that other scope of diversity (node, link, SRLG etc) can still be applied inside the common shared domains.
This document examine a way to achieve domain diversity with existing inter-domain path computation mechanism like per-domain path computation technique [RFC5152], Backward Recursive Path Computation (BRPC) mechanism [RFC5441] and Hierarchical PCE [RFC6805]. This document also considers synchronized as well as non-synchronized dependent path computations. Since independent and synchronized path computation cannot be used to apply diversity, it is not discussed in this document.
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 terminology is as per [RFC5440].
As described in [RFC6805], a set of paths are considered to be domain diverse if they do not share any transit domains, apart from ingress and egress domains.
Some additional parameters to consider would be -
The per domain path computation technique [RFC5152] defines a method where the path is computed during the signaling process (on a per-domain basis). The entry Boundary Node (BN) of each domain is responsible for performing the path computation for the section of the LSP that crosses the domain, or for requesting that a PCE for that domain computes that piece of the path.
The BRPC [RFC5441] technique involves cooperation and communication between PCEs in order to compute an optimal end-to-end path across multiple domains. The sequence of domains to be traversed maybe known before the path computation, but it can also be used when the domain path is unknown and determined during path computation.
In H-PCE [RFC6805] architecture, the parent PCE is used to compute a multi-domain path based on the domain connectivity information. The parent PCE may be requested to provide a end to end path or only the sequence of domains.
[Editor's Note: It has been requested to move this section to the HPCE-Extension document - draft-ietf-pce-hierarchy-extensions. This section would be removed from this document once that is done.]
[RFC5440] defines SVEC object which includes flags for the potential dependency between the set of path computation requests (Link, Node and SRLG diverse). This document proposes a new flag O for domain diversity.
The format of the SVEC object body is as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Flags |O|P|D|S|N|L|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Request-ID-number #1 |
// //
| Request-ID-number #M |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: SVEC Body Object Format
Following new bit is added in the Flags field:
The Domain Diverse O-bit can be used in Hierarchical PCE path computation to compute synchronized domain diverse end to end path or diverse domain sequences as described in Section 3.3.
When domain diverse O bit is set, it is applied to the transit domains. The other bit in SVEC object (N, L, S etc) is set, should still be applied in the ingress and egress domain.
In case when full domain diversity could not be achieved, it maybe helpful to minimize the common shared domains. It's interesting to note that diversity (node, link etc) can still be applied inside the common shared transit domains as well as for ingress and egress domain via the bits in SVEC object (N, L, S etc).
A new Objective function (OF) [RFC5541] code for synchronized path computation requests is proposed:
MCTD
The MCTD OF can be used in Hierarchical PCE path computation to request synchronized domain diverse end to end paths or diverse domain sequences as described in Section 3.3.
[Editor's Note: A new document is created for the OF for minimizing shared node, links, SRLGs inside the domain - [PCE-OF-DIVERSE].]
For non synchronized diverse domain path computation the X bit in XRO or EXRS [RFC5521] sub-objects can be used, where X bit set as 1 indicates that the domain specified SHOULD be excluded from the path computed by the PCE, but MAY be included subject to PCE policy and the absence of a viable path that meets the other constraints and excludes the domain.
[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 further extends by adding domain-diverse O-bit in SVEC object and a new OF Code for minimizing the number of shared transit domain.
Further [PCE-OF-DIVERSE] 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 -
In case of non-synchronized path computation, Ingress node (i.e. a PCC) should be aware of transit domain identifiers of the primary path. So during the path computation or signaling of the primary path, the transit domain should be identified.
A possible solution for path computation could be a flag in RP object requesting domain identifier to be returned in the PCEP path reply message.
[Editor's Note: There should be a mechanism in signaling and path computation to obtain the domain information. Further details - TBD]
In case of non-synchronized path computation, PCE may be requested to provide an optimal primary path first and then PCC requests for a backup path with exclusion. Note that this approach does not guarantee diversity compared to disjoint path computations for primary and backup path in a synchronized manner.
A synchronized path computation with diversity flags and/or objective function is used to make sure that both the primary path and the backup path can be computed simultaneously with full diversity or optimized to be as diverse as possible. In the latter case we may sacrifice optimal path for diversity, thus there is a trade-off between the two.
An implementation may further choose to analyze the trade-off i.e. it may send multiple request to PCE asking to optimize based on diversity as well as say, cost and make an intelligent choice between them.
This document add a new bit to SVEC object and define a new object function. The security of the procedures described in this document depends on PCEP [RFC5440]. [RFC6007] describe security considerations when SVEC are supported.
In addition to [RFC5440], the PCC should construct the SVECs to identify and associate domain diverse SVEC relationships. Considerations for use of objective functions are mentioned in [RFC5541].
The PCEP MIB Module defined in [RFC7420], there are no additional parameters identified in this document.
The domain-diverse path computation in this document allows PCEs to compute optimal sets of diverse paths. This type of path computation and cooperation may require more time to obtain its results. Therefore, it is recommended for PCEP to support the PCE monitoring mechanism specified in [RFC5886].
[RFC5440] provides a sufficient description for this document. There are no additional considerations.
There should be a mechanism in signaling and path computation to obtain the domain information during primary path computation. Details to be added in future revision.
Mechanisms defined in this document do not have any impact on network operations in addition to those already listed in [RFC5440] and [RFC6007].
TBD.
We would like to thank Qilei Wang for starting this discussion in the mailing list.
Ramon Casellas
CTTC - Centre Tecnologic de Telecomunicacions de Catalunya
Av. Carl Friedrich Gauss n7
Castelldefels, Barcelona 08860
Spain
EMail: ramon.casellas@cttc.es
Avantika
Huawei Technologies
Divyashree Techno Park, Whitefield
Bangalore, Karnataka 560066
India
EMail: avantika.sushilkumar@huawei.com