Internet Engineering Task Force (IETF) S. Litkowski

Internet Engineering Task Force (IETF) S. Litkowski
Request for Comments: 8800 Cisco Systems, Inc.
Category: Standards Track S. Sivabalan
ISSN: 2070-1721 Ciena Corporation
C. Barth
Juniper Networks
M. Negi
RtBrick India
July 2020

Path Computation Element Communication Protocol (PCEP) Extension for
Label Switched Path (LSP) Diversity Constraint Signaling

Abstract

This document introduces a simple mechanism to associate a group of
Label Switched Paths (LSPs) via an extension to the Path Computation
Element Communication Protocol (PCEP) with the purpose of computing
diverse (disjointed) paths for those LSPs. The proposed extension
allows a Path Computation Client (PCC) to advertise to a Path
Computation Element (PCE) that a particular LSP belongs to a
particular Disjoint Association Group; thus, the PCE knows that the
LSPs in the same group need to be disjoint from each other.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8800.

Copyright Notice

Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.

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
1.1. Requirements Language
2. Terminology
3. Motivation
4. Applicability
5. Protocol Extension
5.1. Association Group
5.2. Disjoint TLVs
5.3. Disjointness Objective Functions
5.4. Relationship to SVEC
5.4.1. SVEC and OF
5.5. P Flag Considerations
5.6. Disjointness Computation Issues
6. Security Considerations
7. IANA Considerations
7.1. Association Type
7.2. PCEP TLVs
7.3. Objective Functions
7.4. NO-PATH-VECTOR Bit Flags
7.5. PCEP-ERROR Codes
8. Manageability Considerations
8.1. Control of Function and Policy
8.2. Information and Data Models
8.3. Liveness Detection and Monitoring
8.4. Verification of Correct Operations
8.5. Requirements on Other Protocols
8.6. Impact on Network Operations
9. References
9.1. Normative References
9.2. Informative References
Acknowledgments
Contributors
Authors' Addresses

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].

The 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.

[RFC8697] introduces a generic mechanism to create a grouping of LSPs
in the context of a PCE that 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
[RFC4655].

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. Sections 3 and 4 describe
the property and use of a Disjoint Association Group. A PCC can use
this extension to signal to a PCE that a particular LSP belongs to a
particular Disjoint Association Group. When a PCE receives LSP
states belonging to the same Disjoint Association 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

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

PLSP-ID: PCEP-specific identifier for the LSP

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:

* 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).

* Node disjointness: the paths of the associated LSPs should transit
different nodes (but may use different links that may have some
shared fate).

* SRLG disjointness: the paths of the associated LSPs should transit
different links that do not share fate (but may use common transit
nodes).

* 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 different head
end(s) and may terminate at the same or different tail end(s).

4. Applicability

_________________________________________
/ \
/ +------+ \
| | 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 a 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 that is
equally applicable to the active and passive modes of a stateful PCE
is described in [RFC8697].

Using the extension to PCEP defined in this document, the PCC uses
the extension defined in [RFC8697] to indicate that a group of LSPs
are required to be disjoint; such indication should include
disjointness parameters like the type of disjointness, the Disjoint
Association Group identifiers, and any customization parameters
according to the configured local policy.

The management of the Disjoint Association Group IDs will be a key
point for the operator as the Association ID field is limited to
65535. The local configuration of the IPv4/IPv6 Association Source,
or Global Association Source/Extended Association ID, can overcome
this limitation, as described in [RFC8697]. When a PCC or PCE
initiates all the LSPs in a particular Disjoint Association 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 Association
Group over PCEP Session

The Disjoint Association Group within a PCEP messages is used for:

* Configuration: Used to communicate the configured disjoint
requirement to a PCEP peer.

* Status: Used to communicate the status of the computed
disjointness.

5. Protocol Extension

5.1. Association Group

As per [RFC8697], LSPs are associated with other LSPs with which they
interact by adding them to a common association group. As described
in [RFC8697], the association group is uniquely identified by the
combination of the following 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, called "Disjoint
Association" (2), based on the generic ASSOCIATION object. This new
Association type is also called "DAT", for "Disjoint Association
Type".

[RFC8697] specifies the mechanism for the capability advertisement of
the Association types supported by a PCEP speaker by defining an
ASSOC-Type-List TLV to be carried within an OPEN object. This
capability exchange for the DAT (2) MUST be done before using the
disjoint 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 [RFC8697], the association group could
be manually created by the operator on the PCEP peers, and the LSPs
belonging to this association are conveyed via PCEP messages to the
PCEP peer; alternately, 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 are
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 [RFC8697].)

A Disjoint Association 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 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 the DISJOINTNESS-STATUS TLV (see Section 5.2).

There are different types of disjointness identified by the flags (T,
S, N, and L) in the DISJOINTNESS-CONFIGURATION TLV (see Section 5.2).
All LSPs in a particular Disjoint Association Group MUST use the same
combination of T, S, N, and L flags in the DISJOINTNESS-CONFIGURATION
TLV. If a PCEP peer receives a PCEP message for LSPs belonging to
the same Disjoint Association Group but having an inconsistent
combination of T, S, N, and L flags, the PCEP peer MUST NOT add the
LSPs to the Disjoint Association 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 multiple Disjoint Association
Groups, but in that case, the PCE SHOULD try to consider all the
Disjoint Association 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 MUST return a PCErr with Error-
Type 26 (Association Error) and Error-value 7 (Cannot join the
association group).

5.2. Disjoint TLVs

The Disjoint Association Group (ASSOCIATION object with Association
type = 2 for DAT) MUST carry the following TLV:

* DISJOINTNESS-CONFIGURATION TLV: Used to communicate some
disjointness configuration parameters. This is applicable for all
PCEP messages that include DAG.

In addition, the Disjoint Association Group (ASSOCIATION object with
Association type = 2 for DAT) MAY carry the following TLVs:

* 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 messages).

* VENDOR-INFORMATION-TLV: Used to communicate arbitrary vendor-
specific behavioral information, described in [RFC7470].

* OF-List TLV: Used to communicate the disjointness objective
function. See Section 5.3.

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 = 46 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags |T|P|S|N|L|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 3: DISJOINTNESS-CONFIGURATION TLV

Type: 46

Length: Fixed value of 4 bytes.

Flags:

L (Link Diverse) bit: When set, this indicates that the computed
paths within the Disjoint Association Group MUST NOT have any
link in common.

N (Node Diverse) bit: When set, this indicates that the computed
paths within the Disjoint Association Group MUST NOT have any
node in common.

S (SRLG Diverse) bit: When set, this indicates that the computed
paths within the Disjoint Association 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; 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, the PCE MUST return no path for LSPs that could not
be disjoint. When unset, the PCE is allowed to relax
disjointness by either applying a requested objective function
(cf. Section 5.3) 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).
See Section 5.6 for further details.

Unassigned bits: 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 Association Group (ASSOCIATION
object with Association type = 2 for DAT) without the DISJOINTNESS-
CONFIGURATION TLV, it SHOULD reply with a PCErr Error-Type 6
(Mandatory Object missing) and Error-value 15 (DISJOINTNESS-
CONFIGURATION TLV missing).

The DISJOINTNESS-STATUS TLV uses the same format as the DISJOINTNESS-
CONFIGURATION TLV with a different type 47 (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 the DISJOINTNESS-STATUS TLV MUST be specified in the document that
specifies the flag in the 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 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
defined:

MSL

Name: Minimize the number of Shared (common) Links.
Objective Function Code: 15
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: 16
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: 17
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.

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 32 (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, which specifies
the list of synchronized requests that can be either dependent or
independent. The SVEC object identifies the relationship between the
set of path computation requests, identified by 'Request-ID-number'
in the RP (Request Parameters) object. [RFC6007] further clarifies
the use of the SVEC list for synchronized path computations when
computing dependent requests and describes 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 Path Computation Request (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:

* 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 and LSP2.

* PCReq with SVEC object with link-diverse bit=1 (LSP1,LSP2) and DAG
with L=1 (LSP1,LSP3) - link-diverse paths between LSP1 and LSP2
and between LSP1 and 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 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 in 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:

* SVEC object with node-diverse bit=1 - ensure full node diversity.

* SVEC object with node-diverse bit=1 and OF=MSS - full node
diversity with as much SRLG diversity as possible.

* SVEC object with domain-diverse bit=1 [RFC8685]; node-diverse
bit=1, and OF=MSS - full domain and node diversity with as much
SRLG diversity as possible.

* 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 satisfy all constraints and objective
functions first without considering the diversity constraint.

This means that an LSP with the P flag set should be placed first, as
if the disjointness constraint has not been configured, while the
other LSPs in the association with the 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 on LSP 1 with P=0). Multiple LSPs in the
same Disjoint Association 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.

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 Association 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 /
\_________________________________________/

Figure 4: Example Topology with Six Internal Routers

Note: In Figure 4, the cost of all the links is 1, unless explicitly
marked otherwise.

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 the 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 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.

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 a 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 finding a disjoint path. In the
example above, if we consider that router R5 is down and 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 the
R1->R2 link leaving room for PE3->PE4 using the R3->R4 link. When
using the 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.

_________________________________________
/ \
/ +------+ \
| | PCE | |
| +------+ |
| |
| |
| +------+ 10 +------+ |
CE1 ****| PE 1 | ----- R1 ---- R2 ------- | PE 2 |**** CE2
| +------+ | \ | +------+ |
| | \2 | |
| | \ | |
| +------+ | \ | +------+ |
CE3 ****| PE 3 | ----- R3 ---- R4 ------- | PE 4 |**** CE4
| +------+ +------+ |
| |
\ /
\_________________________________________/

Figure 5: Example Topology with Four Internal Routers

Note: In Figure 5, the cost of all the links is 1, unless explicitly
marked otherwise.

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 the 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), if disjointness cannot
be ensured for one or more LSPs, the PCE MUST reply to a PCReq with a
PCRep message containing a NO-PATH object. In case of a 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
Explicit Route Object (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 Flags field of the NO-PATH-VECTOR
TLV:

* bit 11: When set, the PCE indicates that it could not find a
disjoint path for this LSP.

* bit 10: 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 out of scope of the document. See
[PCE-OPTIONAL] for a proposed mechanism.

6. Security Considerations

This document defines one new PCEP Association type, which by itself
does not add any new security concerns beyond those discussed in
[RFC5440], [RFC8231], [RFC7470], and [RFC8697]. But adding of a
spurious LSP into the Disjoint Association Group could lead to
recomputation and setup of all LSPs in the group, which 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. Also, certain combinations of
flags (notably, the 'T' bit) can result in conflicts that cannot be
resolved.

Also, as stated in [RFC8697], much of the information carried in the
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. This holds true for the DAT as well;
thus, this 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
[RFC8697]. IANA has assigned the following new value in the
"ASSOCIATION Type Field" subregistry [RFC8697] within the "Path
Computation Element Protocol (PCEP) Numbers" registry:

+======+======================+===========+
| Type | Name | Reference |
+======+======================+===========+
| 2 | Disjoint Association | RFC 8800 |
+------+----------------------+-----------+

Table 1: ASSOCIATION Type Field

7.2. PCEP TLVs

This document defines two new PCEP TLVs. IANA has assigned the
following values in the "PCEP TLV Type Indicators" subregistry within
the "Path Computation Element Protocol (PCEP) Numbers" registry:

+==========+============================+===========+
| TLV Type | TLV Name | Reference |
+==========+============================+===========+
| 46 | DISJOINTNESS-CONFIGURATION | RFC 8800 |
+----------+----------------------------+-----------+
| 47 | DISJOINTNESS-STATUS | RFC 8800 |
+----------+----------------------------+-----------+

Table 2: PCEP TLV Type Indicators

IANA has created a new subregistry, named "DISJOINTNESS-CONFIGURATION
TLV Flag Field", within the "Path Computation Element Protocol (PCEP)
Numbers" registry to manage the Flags 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:

* Bit number (count from 0 as the most significant bit)

* Flag Name

* Reference

The initial contents of this subregistry are shown below:

+======+=========================+===========+
| Bit | Name | Reference |
+======+=========================+===========+
| 31 | L - Link Diverse | RFC 8800 |
+------+-------------------------+-----------+
| 30 | N - Node Diverse | RFC 8800 |
+------+-------------------------+-----------+
| 29 | S - SRLG Diverse | RFC 8800 |
+------+-------------------------+-----------+
| 28 | P - Shortest Path | RFC 8800 |
+------+-------------------------+-----------+
| 27 | T - Strict Disjointness | RFC 8800 |
+------+-------------------------+-----------+
| 0-26 | Unassigned | |
+------+-------------------------+-----------+

Table 3: DISJOINTNESS-CONFIGURATION TLV
Flag Field

7.3. Objective Functions

This document defines three new objective functions. IANA has made
the following allocations in the "Objective Function" subregistry
within the "Path Computation Element Protocol (PCEP) Numbers"
registry:

+============+=======================+===========+
| Code Point | Name | Reference |
+============+=======================+===========+
| 15 | Minimize the number | RFC 8800 |
| | of Shared Links (MSL) | |
+------------+-----------------------+-----------+
| 16 | Minimize the number | RFC 8800 |
| | of Shared SRLGs (MSS) | |
+------------+-----------------------+-----------+
| 17 | Minimize the number | RFC 8800 |
| | of Shared Nodes (MSN) | |
+------------+-----------------------+-----------+

Table 4: Objective Function

7.4. NO-PATH-VECTOR Bit Flags

This document defines new bits for the NO-PATH-VECTOR TLV in the "NO-
PATH-VECTOR TLV Flag Field" subregistry of the "Path Computation
Element Protocol (PCEP) Numbers" registry. IANA has made the
following allocations:

+============+===========================+===========+
| Bit Number | Name | Reference |
+============+===========================+===========+
| 11 | Disjoint path not found | RFC 8800 |
+------------+---------------------------+-----------+
| 10 | Requested disjoint | RFC 8800 |
| | computation not supported | |
+------------+---------------------------+-----------+

Table 5: NO-PATH-VECTOR TLV Flag Field

7.5. PCEP-ERROR Codes

This document defines two new Error-values within existing Error-
Types related to disjoint association. IANA has allocated the
following new Error-values in the "PCEP-ERROR Object Error Types and
Values" subregistry within the "Path Computation Element Protocol
(PCEP) Numbers" registry:

+============+===========+============================+===========+
| Error-Type | Meaning | Error-value | Reference |
+============+===========+============================+===========+
| 6 | Mandatory | | [RFC5440] |
| | Object | | |
| | missing | | |
+------------+-----------+----------------------------+-----------+
| | | 15: DISJOINTNESS- | RFC 8800 |
| | | CONFIGURATION TLV missing | |
+------------+-----------+----------------------------+-----------+
| 10 | Reception | | [RFC5440] |
| | of an | | |
| | invalid | | |
| | object | | |
+------------+-----------+----------------------------+-----------+
| | | 32: Incompatible OF code | RFC 8800 |
+------------+-----------+----------------------------+-----------+

Table 6: PCEP-ERROR Object Error Types and Values

8. Manageability Considerations

8.1. Control of Function and Policy

An operator SHOULD be allowed to configure the Disjoint Association
Groups and disjoint parameters at the PCEP peers and associate them
with the LSPs. The operator MUST be allowed to set the Operator-
configured Association Range. The operator SHOULD be allowed to set
the local policies to define various disjoint computational behavior
at the PCE.

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 [PCEP-YANG] includes association group 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 Section 8.6 of [RFC5440] 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. References

9.1. 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>.

[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>.

[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>.

[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>.

[RFC8685] Zhang, F., Zhao, Q., Gonzalez de Dios, O., Casellas, R.,
and D. King, "Path Computation Element Communication
Protocol (PCEP) Extensions for the Hierarchical Path
Computation Element (H-PCE) Architecture", RFC 8685,
DOI 10.17487/RFC8685, December 2019,
<https://www.rfc-editor.org/info/rfc8685>.

[RFC8697] 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)", RFC 8697, DOI 10.17487/RFC8697, January 2020,
<https://www.rfc-editor.org/info/rfc8697>.

9.2. Informative References

[PCE-OPTIONAL]
Li, C., Zheng, H., and S. Litkowski, "Extension for
Stateful PCE to allow Optional Processing of PCEP
Objects", Work in Progress, Internet-Draft, draft-dhody-
pce-stateful-pce-optional-06, 9 July 2020,
<https://tools.ietf.org/html/draft-dhody-pce-stateful-pce-
optional-06>.

[PCEP-YANG]
Dhody, D., Hardwick, J., Beeram, V., and J. Tantsura, "A
YANG Data Model for Path Computation Element
Communications Protocol (PCEP)", Work in Progress,
Internet-Draft, draft-ietf-pce-pcep-yang-14, 7 July 2020,
<https://tools.ietf.org/html/draft-ietf-pce-pcep-yang-14>.

[RFC4655] Farrel, A., Vasseur, J.-P., 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>.

Acknowledgments

A special thanks to the authors of [RFC8697]; this document borrows
some text from it. The authors would also like to thank Adrian
Farrel and Julien Meuric for the valuable comments.

Thanks to Emmanuel Baccelli for the RTGDIR review.

Thanks to Dale Worley for a detailed GENART review.

Thanks to Alvaro Retana, Benjamin Kaduk, Suresh Krishnan, Roman
Danyliw, Alissa Cooper, and Éric Vyncke for the IESG review.

Contributors

Dhruv Dhody
Huawei Technologies
Divyashree Techno Park, Whitefiled
Bangalore 560066
Karnataka
India

Email: [email protected]

Authors' Addresses

Stephane Litkowski
Cisco Systems, Inc.

Email: [email protected]

Siva Sivabalan
Ciena Corporation

Email: [email protected]

Colby Barth
Juniper Networks

Email: [email protected]

Mahendra Singh Negi
RtBrick India
N-17L, Floor-1, 18th Cross Rd, HSR Layout Sector-3
Bangalore 560102
Karnataka
India

Email: [email protected]