Path Computation Element Communication Protocol (PCEP) Extension for LSP Diversity Constraint Signaling
draft-ietf-pce-association-diversity-11
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 8800.
|
|
|---|---|---|---|
| Authors | Stephane Litkowski , Siva Sivabalan , Colby Barth , Mahendra Singh Negi | ||
| Last updated | 2019-10-28 (Latest revision 2019-08-31) | ||
| Replaces | draft-litkowski-pce-association-diversity, draft-dhody-pce-of-diverse | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews |
GENART Last Call review
(of
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by Dale Worley
Ready w/issues
|
||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | Submitted to IESG for Publication | |
| Document shepherd | Julien Meuric | ||
| Shepherd write-up | Show Last changed 2019-07-05 | ||
| IESG | IESG state | Became RFC 8800 (Proposed Standard) | |
| Consensus boilerplate | Yes | ||
| Telechat date |
(None)
Needs a YES. Needs 7 more YES or NO OBJECTION positions to pass. |
||
| Responsible AD | Deborah Brungard | ||
| Send notices to | Julien Meuric <julien.meuric@orange.com> | ||
| IANA | IANA review state | Version Changed - Review Needed |
draft-ietf-pce-association-diversity-11
PCE Working Group S. Litkowski
Internet-Draft Orange
Intended status: Standards Track S. Sivabalan
Expires: April 30, 2020 Cisco Systems, Inc.
C. Barth
Juniper Networks
M. Negi
Huawei Technologies
October 28, 2019
Path Computation Element Communication Protocol (PCEP) Extension for LSP
Diversity Constraint Signaling
draft-ietf-pce-association-diversity-11
Abstract
This document introduces a simple mechanism to associate a group of
Label Switched Paths (LSPs) via an extension to the Path Computation
Element (PCE) communication Protocol (PCEP) with the purpose of
computing diverse paths for those LSPs. The proposed extension
allows a Path Computation Client (PCC) to advertise to a PCE that a
particular LSP belongs to a particular disjoint-group, thus the PCE
knows that the LSPs in the same group need to be disjoint from each
other.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 30, 2020.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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 . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Protocol Extension . . . . . . . . . . . . . . . . . . . . . 7
5.1. Association Group . . . . . . . . . . . . . . . . . . . . 7
5.2. Disjoint TLVs . . . . . . . . . . . . . . . . . . . . . . 8
5.3. Disjointness Objective Functions . . . . . . . . . . . . 10
5.4. Relationship to SVEC . . . . . . . . . . . . . . . . . . 11
5.5. P Flag Considerations . . . . . . . . . . . . . . . . . . 12
5.6. Disjointness Computation Issues . . . . . . . . . . . . . 15
6. Security Considerations . . . . . . . . . . . . . . . . . . . 16
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
7.1. Association Type . . . . . . . . . . . . . . . . . . . . 17
7.2. PCEP TLVs . . . . . . . . . . . . . . . . . . . . . . . . 17
7.3. Objective Functions . . . . . . . . . . . . . . . . . . . 18
7.4. NO-PATH-VECTOR Bit Flags . . . . . . . . . . . . . . . . 18
7.5. PCEP-ERROR Codes . . . . . . . . . . . . . . . . . . . . 19
8. Manageability Considerations . . . . . . . . . . . . . . . . 19
8.1. Control of Function and Policy . . . . . . . . . . . . . 19
8.2. Information and Data Models . . . . . . . . . . . . . . . 20
8.3. Liveness Detection and Monitoring . . . . . . . . . . . . 20
8.4. Verification of Correct Operations . . . . . . . . . . . 20
8.5. Requirements on Other Protocols . . . . . . . . . . . . . 20
8.6. Impact on Network Operations . . . . . . . . . . . . . . 20
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 20
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
10.1. Normative References . . . . . . . . . . . . . . . . . . 20
10.2. Informative References . . . . . . . . . . . . . . . . . 21
Appendix A. Contributor Addresses . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
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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].
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 in the context of a PCE 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 set of LSPs
in a particular group should use diverse paths, including the
requested type of diversity. A PCC can use this extension to signal
to a PCE that a particular LSP belongs to a particular 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.
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.
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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.
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:
o 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).
o Node disjointness: the paths of the associated LSPs should transit
different nodes (but may use different links that may have some
shared fate).
o SRLG disjointness: the paths of the associated LSPs should transit
different links that do not share fate (but may use common transit
nodes).
o 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 from different
head-end(s) and may terminate at the same or different tail-end(s).
4. Applicability
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_________________________________________
/ \
/ +------+ \
| | 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 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 the extension to PCEP defined in this document, the PCC uses
the [I-D.ietf-pce-association-group] extension to indicate that a
group of LSPs are required to be disjoint; such indication should
include disjointness parameters such as the type of disjointness, the
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disjoint group identifiers, and any customization parameters
according to the configured local policy.
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 allows 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, the association
source could be the PCE address or any other unique value to identify
the DAG.
Initiate Disjoint LSPs
|
| PCReq/PCRpt
V {Disjoint-group Y}
+-----+ ----------------> +-----+
_ _ _ _ _ _| PCE | | | PCE |
| +-----+ | ----------> +-----+
| PCInitiate | | PCReq/PCRpt
|{Disjoint-group X} | | {Disjoint-group Y}
| | |
| .-----. | | .-----.
| ( ) | +-----+ ( )
| .--( )--. | |PCC 2|--.--( )--.
V ( ) | +-----+ ( )
+---+ ( ) | ( )
|PCC|----( (G)MPLS network ) +-----+ ( (G)MPLS network )
+---+ ( ) |PCC 1|-----( )
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 is used for:
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o Configuration: Used to communicate the configured disjoint
requirement to a PCEP peer.
o Status: Used to communicate the status of the computed
disjointness.
5. Protocol Extension
5.1. Association Group
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. As described in [I-D.ietf-pce-association-group]
the association A A group is uniquely identified by the combination
of these fields in the ASSOCIATION object:A Association Type,
Association ID, Association Source, and (if present) Global
Association Source or Extended Association ID.
This document defines a new Association type, based on the generic
Association object:
o Association type = TBD1 Disjoint Association Type (DAT).
[I-D.ietf-pce-association-group] specifies 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 Disjointness
Association Type (TBD1) MUST be done before using the disjointness
association. Thus the PCEP speaker MUST include the Disjointness
Association Type in the ASSOC-Type-List TLV before using the Disjoint
Association Group (DAG) in 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 to
[I-D.ietf-pce-association-group].)
A disjoint 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
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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 is informed to the PCC.
All LSPs in a particular disjoint group MUST use the same combination
of T, S, N, L flags in the DISJOINTNESS-CONFIGURATION-TLVSection 5.2.
If a PCEP peer receives a PCEP messages for LSPs belonging to the
same disjoint group but having an inconsistent combination of T, S,
N, L flags, the PCEP peer SHOULD NOT try to add the LSPs in disjoint
group and SHOULD reply with a PCErr with Error-type 26 (Association
Error) and Error- Value 6 (Association information mismatch).
Associating a particular LSP to multiple disjoint groups is
authorized from a protocol perspective, however there is no assurance
that the PCE will be able to compute properly the multi-disjointness
constraint.
5.2. Disjoint TLVs
The disjoint group MUST carry the following TLV:
o DISJOINTNESS-CONFIGURATION-TLV: Used to communicate some
disjointness configuration parameters.
In addition, the disjoint group MAY carry the following TLV:
o DISJOINTNESS-STATUS-TLV: Used to communicate the status of the
computed disjointness. This is applicable for messages from PCE
to PCC (PCUpd, PCInitiate or PCRep message).
o VENDOR-INFORMATION-TLV: Used to communicate arbitrary vendor-
specific behavioral information, described in [RFC7470].
o OF-List TLV: Used to communicate the disjointness objective
function. See Section 5.3.
The DISJOINTNESS-CONFIGURATION-TLV is shown in the following figure:
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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|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: TBD2.
Length: Fixed value of 4 bytes.
Flags:
* L (Link diverse) bit: when set, this indicates that the
computed paths within the disjoint group MUST NOT have any link
in common.
* N (Node diverse) bit: when set, this indicates that the
computed paths within the disjoint group MUST NOT have any node
in common.
* S (SRLG diverse) bit: when set, this indicates that the
computed paths within the disjoint 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, and then with those
LSPs fixed, the other LSPs with P flag unset in the association
group are computed. The role of P flag is further described
with example inA Section 5.5
* T (Strict disjointness) bit: when set, if disjoint paths cannot
be found, PCE SHOULD return no path for LSPs that could not be
be disjoint. When unset, the PCE is allowed to relax
disjointness by Section 5.5either applying a requested
objective function (cf. Section 5.3 below) 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). Further see Section 5.6 for details.
* Unassigned bits are considered reserved. They MUST be set to 0
on transmission and MUST be ignored on receipt.
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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=TBD8 (DISJOINTNESS-
CONFIGURATION-TLV missing).
The DISJOINTNESS-STATUS-TLV uses the same format as the DISJOINTNESS-
CONFIGURATION-TLV with a different type TBD3 (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 is set to
indicate that the computed path is the shortest.
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 A A with the same name and in
the same location in DISJOINTNESS-STATUS-TLV; the semantics of the
flag in A A DISJOINTNESS-STATUS-TLV MUST be specified in the
document that specifies the flag in 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 Group 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
proposed:
MSL
* Name: Minimize the number of shared (common) Links.
* Objective Function Code: TBD4
* Description: Find a set of paths such that it passes through the
least number of shared (common) links.
MSS
* Name: Minimize the number of shared (common) SRLGs.
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* Objective Function Code: TBD5
* Description: Find a set of paths such that it passes through the
least number of shared (common) SRLGs.
MSN
* Name: Minimize the number of shared (common) Nodes.
* Objective Function Code: TBD6
* Description: Find a set of paths such that they pass through the
least number of shared (common) nodes.
If the OF-list TLV is included in the Association Object, the OF-code
inside the OF Object MUST include 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=TBD9 (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, 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 the SVEC
and ASSOCIATION objects to identify the related path computation
request as well as the diversity association group. 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
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].
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[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 Section 5.3 to maximize
diversity as much as possible, in other words, new OF-codes allow
specification of minimization of common A A 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:
o SVEC object with node-diverse bit=1 - ensure full node-diversity.
o SVEC object with node-diverse bit=1 and OF=MSS - full node diverse
with as much as SRLG-diversity as possible.
o SVEC object with domain-diverse bit=1;link diverse bit=1 and
OF=MSS - full domain and node diverse path with as much as SRLG-
diversity as possible.
o 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 satisfies all constraints and
objective functions first without considering the diversity
constraint.
This means that an LSP with P flag set should be placed as if the
disjointness constraint has not been configured, while the other LSPs
in the association with P flag unset should be placed by taking into
account the disjointness constraint. Setting P flag changes the
relationship between LSPs to a one-sided relationship (LSP 1 with P=0
depends of LSP 2 with P=1, but LSP 2 with P=1 does not depend of LSP
1 with P=0). 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.
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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 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 /
\_________________________________________/
Cost of all the links is 1, unless explicitly marked otherwise.
Figure 3
In the figure above, a customer has two dual homed sites (CE1/CE3 and
CE2/CE4). Let us consider that 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
(path delay may be higher): the customer requirement is thus not
completely fulfilled.
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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 costlier.
Driving the PCE disjointness computation may be done in other ways,
for instance setting a metric boundary reflecting an 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 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 PE1->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.
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_________________________________________
/ \
/ +------+ \
| | PCE | |
| +------+ |
| |
| |
| +------+ 10 +------+ |
CE1 ****| PE 1 | ----- R1 ---- R2 ------- | PE 2 |**** CE2
| +------+ | \ | +------+ |
| | \2 | |
| | \ | |
| +------+ | \ | +------+ |
CE3 ****| PE 3 | ----- R3 ---- R4 ------- | PE 4 |**** CE4
| +------+ +------+ |
| |
\ /
\_________________________________________/
Cost of all the links is 1, unless explicitly marked otherwise.
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 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 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 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 requested), if
disjointness cannot be ensured for one or more LSPs, the PCE MUST
reply to a Path Computation Request (PCReq) with a Path Computation
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Reply (PCRep) message containing a NO-PATH object. In case of 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 network event leading to an impossible strict
disjointness, the PCE MUST send a PCUpd message containing an empty
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 NO-PATH-VECTOR TLV:
bit "TBD7": when set, the PCE indicates that it could not find a
disjoint path for this LSP.
bit "TBD8": 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 as out of scope of the document.
6. Security Considerations
This document defines one new PCEP association type, which on itself
does not add any new security concerns beyond those discussed in
[RFC5440], [RFC8231] and [I-D.ietf-pce-association-group]. But,
adding of a spurious LSP into the disjointness association group
could lead to re-computation and set-up of all LSPs in the group,
that 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.
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Also, as stated in [I-D.ietf-pce-association-group], much of the
information carried in the Disjointness Association object, as per
this document is not extra sensitive. It often reflects information
that can also be derived from the LSP Database, but association
provides a much easier grouping of related LSPs and messages. The
disjointness association could provide an adversary with the
opportunity to eavesdrop on the relationship between the LSPs.
Thus securing the PCEP session using Transport Layer Security (TLS)
[RFC8253], as per the recommendations and best current practices in
[RFC7525], is RECOMMENDED.
7. IANA Considerations
7.1. Association Type
This document defines a new Association type, originally described in
[I-D.ietf-pce-association-group]. IANA is requested to make the
assignment of a new value for the sub-registry "ASSOCIATION Type
Field" (request to be created in [I-D.ietf-pce-association-group]),
as follows:
+------------------+-----------------------------+-------------+
| Association type | Association Name | Reference |
+------------------+-----------------------------+-------------+
| TBD1 | Disjoint-group Association | [This.I-D] |
+------------------+-----------------------------+-------------+
7.2. PCEP TLVs
This document defines the following new PCEP TLVs and the IANA is
requested to make the assignment of new values for the existing "PCEP
TLV Type Indicators" registry as follows:
+----------+---------------------------------+-------------+
| TLV Type | TLV Name | Reference |
+----------+---------------------------------+-------------+
| TBD2 | Disjointness Configuration TLV | [This.I-D] |
| TBD3 | Disjointness Status TLV | [This.I-D] |
+----------+---------------------------------+-------------+
This document requests that a new sub-registry, named "Disjointness
Configuration TLV Flag Field", is created within the "Path
Computation Element Protocol (PCEP) Numbers" registry to manage the
Flag 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:
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o Bit number (count from 0 as the most significant bit)
o Flag Name
o Reference
+------------+-------------------------+-------------+
| Bit Number | Name | Reference |
+------------+-------------------------+-------------+
| 31 | L - Link Diverse | [This.I-D] |
| 30 | N - Node Diverse | [This.I-D] |
| 29 | S - SRLG Diverse | [This.I-D] |
| 28 | P - Shortest Path | [This.I-D] |
| 27 | T - Strict Disjointness | [This.I-D] |
+------------+-------------------------+-------------+
Table 1: Disjointness Configuration TLV
7.3. Objective Functions
Three new Objective Functions have been defined in this document.
IANA is requested to make the following allocations from the PCEP
"Objective Function" sub-registry:
+------------+----------------------------------------+-------------+
| Code Point | Name | Reference |
+------------+----------------------------------------+-------------+
| TBD4 | Minimize the number of shared Links | [This.I-D] |
| | (MSL) | |
| TBD5 | Minimize the number of shared SRLGs | [This.I-D] |
| | (MSS) | |
| TBD6 | Minimize the number of shared Nodes | [This.I-D] |
| | (MSN) | |
+------------+----------------------------------------+-------------+
7.4. NO-PATH-VECTOR Bit Flags
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. IANA is requested to make
the following allocation:
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+------------+-----------------------------------------+------------+
| Bit Number | Name | Reference |
+------------+-----------------------------------------+------------+
| TBD7 | Disjoint path not found | [This.I-D] |
| TBD8 | Requested disjoint computation not | [This.I-D] |
| | supported | |
+------------+-----------------------------------------+------------+
Table 2: NO-PATH-VECTOR TLV
7.5. PCEP-ERROR Codes
This document defines new Error-Value within existing Error-Type
related to path protection association. IANA is requested to
allocate new error values within the "PCEP-ERROR Object Error Types
and Values" sub-registry of the PCEP Numbers registry, as follows:
+----------+-------------------------+------------------------------+
| Error- | Meaning | Reference |
| Type | | |
+----------+-------------------------+------------------------------+
| 6 | Mandatory Object | [I-D.ietf-pce-association-gr |
| | missing | oup] |
| | Error-value=TBD8: | [This.I-D] |
| | DISJOINTNESS- | |
| | CONFIGURATION TLV | |
| | missing | |
| 10 | Reception of an invalid | [RFC5440] |
| | object | |
| | Error-value=TBD9: | [This.I-D] |
| | Incompatible OF code | |
+----------+-------------------------+------------------------------+
8. Manageability Considerations
8.1. Control of Function and Policy
An operator SHOULD be allowed to configure the disjointness
association groups and disjoint parameters at the PCEP peers and
associate it with the LSPs. The Operator-configured Association
Range MUST be allowed to be set by the operator. Operator SHOULD be
allowed to set the local policies to define various disjoint
computational behavior at the PCE.
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8.2. Information and Data Models
An implementation SHOULD allow the operator to view the disjoint
associations configured or created dynamically. Further
implementation SHOULD allow to view disjoint associations reported by
each peer, and the current set of LSPs in this association. The PCEP
YANG module [I-D.ietf-pce-pcep-yang] includes association groups
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
Mechanisms defined in this document do not imply any new operation
verification requirements in addition to those already listed in
[RFC5440].
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 [RFC5440], Section 8.6 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. Acknowledgments
A special thanks to authors of [I-D.ietf-pce-association-group], this
document borrow some of the text from it. Authors would also like to
thank Adrian Farrel and Julien Meuric for the valuable comments.
Thanks to Emmanuel Baccelli for RTGDIR reviews.
10. References
10.1. Normative References
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[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>.
[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>.
[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>.
[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>.
[I-D.ietf-pce-association-group]
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)", draft-ietf-pce-association-group-10 (work in
progress), August 2019.
10.2. Informative References
[RFC4655] Farrel, A., Vasseur, J., 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>.
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[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>.
[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>.
[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>.
[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>.
[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>.
[I-D.ietf-pce-pcep-yang]
Dhody, D., Hardwick, J., Beeram, V., and J. Tantsura, "A
YANG Data Model for Path Computation Element
Communications Protocol (PCEP)", draft-ietf-pce-pcep-
yang-12 (work in progress), July 2019.
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Appendix A. Contributor Addresses
Dhruv Dhody
Huawei Technologies
Divyashree Techno Park, Whitefield
Bangalore, Karnataka 560066
India
EMail: dhruv.ietf@gmail.com
Authors' Addresses
Stephane Litkowski
Orange
EMail: stephane.litkowski@orange.com
Siva Sivabalan
Cisco Systems, Inc.
2000 Innovation Drive
Kanata, Ontario K2K 3E8
Canada
EMail: msiva@cisco.com
Colby Barth
Juniper Networks
EMail: cbarth@juniper.net
Mahendra Singh Negi
Huawei Technologies
Divyashree Techno Park, Whitefield
Bangalore, Karnataka 560066
India
EMail: mahend.ietf@gmail.com
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