Internet DRAFT - draft-ietf-pce-association-diversity
draft-ietf-pce-association-diversity
PCE Working Group S. Litkowski
Internet-Draft S. Sivabalan
Intended status: Standards Track Cisco Systems, Inc.
Expires: December 23, 2020 C. Barth
Juniper Networks
M. Negi
RtBrick India
June 21, 2020
Path Computation Element Communication Protocol (PCEP) Extension for LSP
Diversity Constraint Signaling
draft-ietf-pce-association-diversity-15
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 (disjointed) 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
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This Internet-Draft will expire on December 23, 2020.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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 . . . . . . . . . . . . . . . . . . 12
5.4.1. SVEC and OF . . . . . . . . . . . . . . . . . . . . . 13
5.5. P Flag Considerations . . . . . . . . . . . . . . . . . . 13
5.6. Disjointness Computation Issues . . . . . . . . . . . . . 16
6. Security Considerations . . . . . . . . . . . . . . . . . . . 17
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
7.1. Association Type . . . . . . . . . . . . . . . . . . . . 18
7.2. PCEP TLVs . . . . . . . . . . . . . . . . . . . . . . . . 18
7.3. Objective Functions . . . . . . . . . . . . . . . . . . . 19
7.4. NO-PATH-VECTOR Bit Flags . . . . . . . . . . . . . . . . 19
7.5. PCEP-ERROR Codes . . . . . . . . . . . . . . . . . . . . 20
8. Manageability Considerations . . . . . . . . . . . . . . . . 20
8.1. Control of Function and Policy . . . . . . . . . . . . . 20
8.2. Information and Data Models . . . . . . . . . . . . . . . 21
8.3. Liveness Detection and Monitoring . . . . . . . . . . . . 21
8.4. Verification of Correct Operations . . . . . . . . . . . 21
8.5. Requirements on Other Protocols . . . . . . . . . . . . . 21
8.6. Impact on Network Operations . . . . . . . . . . . . . . 21
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 21
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 22
10.1. Normative References . . . . . . . . . . . . . . . . . . 22
10.2. Informative References . . . . . . . . . . . . . . . . . 23
Appendix A. Contributor Addresses . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24
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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 (disjointed) paths,
including the requested type of diversity. Section 3 and Section 4
describe the property and use of a disjoint-group. 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 {DAG Y}
+-----+ ----------------> +-----+
_ _ _ _ _ _| PCE | | | PCE |
| +-----+ | ----------> +-----+
| PCInitiate | | PCReq/PCRpt
|{DAG X} | | {DAG Y}
| | |
| .-----. | | .-----.
| ( ) | +-----+ ( )
| .--( )--. | |PCC 2|--.--( )--.
V ( ) | +-----+ ( )
+---+ ( ) | ( )
|PCC|----( (G)MPLS network ) +-----+ ( (G)MPLS network )
+---+ ( ) |PCC 1|-----( )
{DAG 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 group is uniquely identified by the combination of
these fields in the ASSOCIATION object: 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 DAT (TBD1) MUST be
done before using the disjointness association. Thus the PCEP
speaker MUST include the DAT in the ASSOC-Type-List TLV and MUST
receive the same from the PCEP peer before using the Disjoint
Association Group (DAG) in PCEP messages.
This association type is considered to be both dynamic and operator-
configured in nature. As per [I-D.ietf-pce-association-group], 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 of the computed path is informed to the PCC via
DISJOINTNESS-STATUS-TLV (see Section 5.2).
There are differet types of disjointness identified by the flags (T,
S, N, L) in the DISJOINTNESS-CONFIGURATION-TLV (see Section 5.2).
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 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 MUST NOT add the LSPs to the disjoint group and MUST reply
with a PCErr with Error-type 26 (Association Error) and Error-Value 6
(Association information mismatch).
A particular LSP MAY be associated to the multiple disjoint groups,
but in that case, the PCE SHOULD try to consider all the disjoint
groups during path computation if possible. Otherwise a local policy
MAY define the computational behavior. If a PCE does not support
such a path computation it MUST NOT add the LSP into the association
group and return a PCErr with Error-type 26 (Association Error) and
Error-Value 7 (Cannot join the association group).
5.2. Disjoint TLVs
The disjoint group (ASSOCIATION object with Association type = TBD1
for DAT) MUST carry the following TLV:
o DISJOINTNESS-CONFIGURATION-TLV: Used to communicate some
disjointness configuration parameters. This is applicable for all
PCEP message that includes DAG.
In addition, the disjoint group (ASSOCIATION object with Association
type = TBD1 for DAT) MAY carry the following TLVs:
o DISJOINTNESS-STATUS-TLV: Used to communicate the status of the
computed disjointness. This is applicable for messages from a PCE
to a PCC only (i.e. 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.
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The DISJOINTNESS-CONFIGURATION-TLV is shown in the following figure:
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 as if the disjointness
constraint has not been configured, and then with those LSPs
fixed, the other LSPs with P flag unset in the association
group are computed by taking into account the disjointness
constraint. The role of P flag is further described with
examples in Section 5.5.
* T (Strict disjointness) bit: when set, if disjoint paths cannot
be found, PCE MUST return no path for LSPs that could not be be
disjoint. When unset, the PCE is allowed to relax disjointness
by Section 5.5 either 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.
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* Unassigned bits are considered reserved. They MUST be set to 0
on transmission and MUST be ignored on receipt.
If a PCEP speaker receives a disjoint-group (ASSOCIATION object with
Association type = TBD1 for DAT) without DISJOINTNESS-CONFIGURATION-
TLV, it SHOULD reply with a PCErr Error-type=6 (Mandatory Object
missing) and Error-value=TBD10 (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 indicates that
the computed path is the shortest path (computed first without taking
disjointness constraints into consideration, but considering other
constraints).
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 with the same name and in the
same location in DISJOINTNESS-STATUS-TLV; the semantics of the flag
in 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
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* Description: Find a set of paths such that it passes through the
least number of shared (common) links.
* A network comprises a set of N links {Li, (i=1...N)}.
* A path P passes through K links {Lpi,(i=1...K)}.
* A set of paths {P1...Pm} have L links that are common to more
than one path {Lci,(i=1...L)}.
* Find a set of paths such that the value of L is minimized.
MSS
* Name: Minimize the number of shared (common) SRLGs.
* Objective Function Code: TBD5
* Description: Find a set of paths such that it passes through the
least number of shared (common) SRLGs.
* A network comprises a set of N links {Li, (i=1...N)}.
* A path P passes through K links {Lpi,(i=1...K)} belonging to
unique M SRLGs {Spi,(i=1..M)}.
* A set of paths {P1...Pm} have L SRLGs that are common to more
than one path {Sci,(i=1...L)}.
* Find a set of paths such that the value of L is minimized.
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.
* A network comprises a set of N nodes {Ni, (i=1...N)}.
* A path P passes through K nodes {Npi,(i=1...K)}.
* A set of paths {P1...Pm} have L nodes that are common to more
than one path {Nci,(i=1...L)}.
* Find a set of paths such that the value of L is minimized.
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If the OF-list TLV is included in the Association Object, the first
OF-code inside the OF Object MUST be 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 identifies 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 requests 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 DAG. 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 (including the
flags set inside 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]. It should be noted that the LSPs in the association group
can be fully same or partially overlapping with the LSPs grouped by
the SVEC object in PCReq message.
Some examples of usage are listed below:
o PCReq with SVEC object with node-diverse bit=1 (LSP1,LSP2) and DAG
with S=1 (LSP1,LSP2) - both node and SRLG diverse path between
LSP1, LSP2.
o PCReq with SVEC object with link-diverse bit=1 (LSP1,LSP2) and DAG
with L=1 (LSP1,LSP3) - link diverse paths between LSP1 & LSP2, and
LSP1 & LSP3. If the DAG is part of the stateful database, any
future change in LSP3 will have an impact on LSP1. But any future
change in LSP2 will have no impact on LSP1, as LSP2 is part of
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SVEC object (which is considered once on receipt of the PCReq
message only).
5.4.1. SVEC and OF
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 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 first 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 the P flag changes
the relationship between LSPs to a one-sided relationship (LSP 1 with
P=0 depends on 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 other 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 link 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
link 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 link 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
link 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 link 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 link 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 a 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. See
[I-D.dhody-pce-stateful-pce-optional] for a proposed mechanism.
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], [RFC7470] 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
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legitimate LSPs to fail with an error. Also, certain combinations of
flags (notably, the 'T' bit) can result in conflicts that cannot be
resolved.
Also, as stated in [I-D.ietf-pce-association-group], much of the
information carried in the Disjointness Association object 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 and understand the network topology.
Thus securing the PCEP session using Transport Layer Security (TLS)
[RFC8253], as per the recommendations and best current practices in
BCP 195 [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 | Disjointness Association Type | [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
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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:
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 | Name | Reference |
| Number | | |
+-----------+-----------------------------------------+-------------+
| 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=TBD10: | [This.I-D] |
| | DISJOINTNESS- | |
| | CONFIGURATION TLV | |
| | missing | |
| 10 | Reception of an | [RFC5440] |
| | invalid 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. The 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. Furthermore,
implementations 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
Apart from the operation verification requirements already listed in
[RFC5440], a PCEP implementation SHOULD provide parameters related to
disjoint path computation, such as number of DAG, number of disjoint
path computation failures etc. A PCEP implementation SHOULD log
failure events (e.g., incompatible Flags).
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 review.
Thanks to Dale Worley for a detailed GENART review.
Thanks to Alvaro Retana, Benjamin Kaduk, Suresh Krishnan, Roman
Danyliw, Alissa Cooper and Eric Vyncke for IESG review.
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10. References
10.1. Normative References
[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>.
[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>.
[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>.
[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>.
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[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>.
[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>.
[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>.
[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-13 (work in progress), October 2019.
[I-D.dhody-pce-stateful-pce-optional]
Li, C., Zheng, H., and S. Litkowski, "Extension for
Stateful PCE to allow Optional Processing of PCEP
Objects", draft-dhody-pce-stateful-pce-optional-05 (work
in progress), January 2020.
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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
Cisco Systems, Inc.
EMail: slitkows.ietf@gmail.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
RtBrick India
N-17L, Floor-1, 18th Cross Rd, HSR Layout Sector-3
Bangalore, Karnataka 560102
India
EMail: mahend.ietf@gmail.com
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