Procedures for Communication between Stateful Path Computation Elements
draft-ietf-pce-state-sync-10
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| Authors | Haomian Zheng , Stephane Litkowski , Siva Sivabalan , Cheng Li | ||
| Last updated | 2024-11-29 (Latest revision 2024-10-21) | ||
| Replaces | draft-litkowski-pce-state-sync | ||
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draft-ietf-pce-state-sync-10
PCE Working Group H. Zheng, Ed.
Internet-Draft Huawei Technologies
Intended status: Standards Track S. Litkowski
Expires: 2 June 2025 Cisco
S. Sivabalan
Ciena Corporation
C. Li
Huawei Technologies
29 November 2024
Procedures for Communication between Stateful Path Computation Elements
draft-ietf-pce-state-sync-10
Abstract
The Path Computation Element (PCE) Communication Protocol (PCEP)
provides mechanisms for PCEs to perform path computation in response
to a Path Computation Client (PCC) request. The Stateful PCE
extensions allow stateful control of Multi-Protocol Label Switching
(MPLS) Traffic Engineering (TE) Label Switched Paths (LSPs) using
PCEP.
A Path Computation Client (PCC) can synchronize LSP state information
to a Stateful Path Computation Element (PCE). A PCC can have
multiple PCEP sessions towards multiple PCEs. There are some use
cases, where an inter-PCE stateful communication can bring additional
resiliency in the design, for instance when some PCC-PCE session
fails.
This document describes the procedures to allow stateful
communication between PCEs for various use-cases and also the
procedures to prevent computations loops.
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."
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This Internet-Draft will expire on 2 June 2025.
Copyright Notice
Copyright (c) 2024 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 Revised BSD License text as
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction and Problem Statement . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
1.2. Reporting LSP Changes . . . . . . . . . . . . . . . . . . 5
1.3. Split-Brain . . . . . . . . . . . . . . . . . . . . . . . 5
1.4. Applicability to H-PCE . . . . . . . . . . . . . . . . . 8
2. Solution . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.1. State-sync Session . . . . . . . . . . . . . . . . . . . 8
2.2. Primary/Secondary Relationship between PCE . . . . . . . 10
3. Procedures and Protocol Extensions . . . . . . . . . . . . . 10
3.1. Opening a state-sync session . . . . . . . . . . . . . . 10
3.1.1. Capability Advertisement . . . . . . . . . . . . . . 11
3.2. State Synchronization . . . . . . . . . . . . . . . . . . 11
3.3. Incremental Updates and Report Forwarding Rules . . . . . 12
3.4. Maintaining LSP States from Different Sources . . . . . . 13
3.5. Computation Priority between PCEs and Sub-delegation . . 14
3.5.1. Association Group . . . . . . . . . . . . . . . . . . 16
3.6. Passive Stateful Procedures . . . . . . . . . . . . . . . 16
3.7. PCE Initiation Procedures . . . . . . . . . . . . . . . . 16
4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 16
4.1. Example 1 - Successful disjoint paths (requiring
reroute) . . . . . . . . . . . . . . . . . . . . . . . . 16
4.2. Example 2 - Successful disjoint paths (simultaneous
turnup) . . . . . . . . . . . . . . . . . . . . . . . . . 18
4.3. Example 3 - Unfeasible disjoint paths (insufficient
state-sync sessions) . . . . . . . . . . . . . . . . . . 20
5. Using Primary/Secondary Computation and State-sync Sessions to
Increase Scaling . . . . . . . . . . . . . . . . . . . . 21
6. PCEP-PATH-VECTOR TLV . . . . . . . . . . . . . . . . . . . . 23
7. Security Considerations . . . . . . . . . . . . . . . . . . . 24
8. Implementation Status . . . . . . . . . . . . . . . . . . . . 25
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9. Manageability Considerations . . . . . . . . . . . . . . . . 25
9.1. Control of Function and Policy . . . . . . . . . . . . . 25
9.2. Information and Data Models . . . . . . . . . . . . . . . 26
9.3. Liveness Detection and Monitoring . . . . . . . . . . . . 26
9.4. Verify Correct Operations . . . . . . . . . . . . . . . . 26
9.5. Requirements On Other Protocols . . . . . . . . . . . . . 26
9.6. Impact On Network Operations . . . . . . . . . . . . . . 26
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 26
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26
11.1. PCEP-Error Object . . . . . . . . . . . . . . . . . . . 26
11.2. PCEP TLV Type Indicators . . . . . . . . . . . . . . . . 27
11.3. STATEFUL-PCE-CAPABILITY TLV . . . . . . . . . . . . . . 27
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 27
12.1. Normative References . . . . . . . . . . . . . . . . . . 27
12.2. Informative References . . . . . . . . . . . . . . . . . 28
Appendix A. Contributors . . . . . . . . . . . . . . . . . . . . 30
Appendix B. Scenarios . . . . . . . . . . . . . . . . . . . . . 30
B.1. Scenario 1 . . . . . . . . . . . . . . . . . . . . . . . 30
B.2. Scenario 2 . . . . . . . . . . . . . . . . . . . . . . . 30
B.3. Scenario 3 . . . . . . . . . . . . . . . . . . . . . . . 31
B.4. Scenario 4 . . . . . . . . . . . . . . . . . . . . . . . 32
B.5. Scenario 5 . . . . . . . . . . . . . . . . . . . . . . . 33
B.6. Scenario 6 . . . . . . . . . . . . . . . . . . . . . . . 34
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 35
1. Introduction and Problem Statement
The Path Computation Element communication Protocol (PCEP) [RFC5440]
provides mechanisms for Path Computation Elements (PCEs) to perform
path computations in response to Path Computation Clients' (PCCs)
requests.
A stateful PCE [RFC8231] is capable of considering, for the purposes
of path computation, not only the network state in terms of links and
nodes (referred to as the Traffic Engineering Database or TED) but
also the status of active services (previously computed paths), and
currently reserved resources, stored in the Label Switched Paths
Database (LSP-DB).
[RFC8051] describes general considerations for a stateful PCE
deployment and examines its applicability and benefits, as well as
its challenges and limitations through a number of use cases.
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A PCC can synchronize LSP state information to a Stateful PCE. The
stateful PCE extension allows a redundancy scenario where a PCC can
have redundant PCEP sessions towards multiple PCEs. In such a case,
a PCC gives control of an LSP to a single PCE, and only one PCE is
responsible for path computation for this delegated LSP. The
document does not state the procedures related to an inter-PCE
stateful communication.
There are some use cases, where an inter-PCE stateful communication
can bring additional resiliency in the design, for instance when some
PCC-PCE session fails. The inter-PCE stateful communication may also
provide a faster update of the LSP states when such an event occurs.
Finally, when, in a redundant PCE scenario, there is a need to
compute a set of paths that are part of a group (so there is a
dependency between the paths), there may be some cases where the
computation of all paths in the group is not handled by the same PCE:
this situation is called a split-brain. This split-brain scenario
may lead to computation loops between PCEs or suboptimal path
computation.
In the scope of this document, the term 'computation loop' is used to
describe a behaviour of PCEP message exchange looping between PCC and
PCE or between PCEs, resulting in frequent path calculations, path
reporting and path updates to the network resulting in constant load
on the PCE and oscillation of data plane traffic after each
subsequent path update.
This document describes the procedures to allow a stateful
communication between PCEs for various use-cases and also the
procedures to prevent computations loops. Hierarchical PCE use case
is out of scope of this document.
This section contains illustrative examples to showcase the need for
inter-PCE stateful PCEP sessions.
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.
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1.2. Reporting LSP Changes
When using a stateful PCE ([RFC8231]), a PCC can synchronize LSP
state information to the stateful PCE. If the PCC grants the control
of the LSP to the PCE (called delegation [RFC8231]), the PCE can
update the LSP parameters at any time.
In a multi PCE deployment (redundancy, loadbalancing...), with the
specification defined in [RFC8231], when a PCE makes an update, the
PCC is responsible for reporting the LSP parameter updates all PCEs.
This delay may affect the reaction time of the other PCEs if they
need to take action after being notified of the LSP parameter change.
Apart from the synchronization from the PCC, it is also useful if
there is a synchronization mechanism between the stateful PCEs. As a
stateful PCE makes changes to its delegated LSPs, these changes
(pending LSPs and the sticky resources [RFC7399]) can be synchronized
to the other PCEs.
+----------+
| PCC | LSP1
+----------+
/ \
/ \
+---------+ +---------+
| PCE1 |----| PCE2 |
+---------+ +---------+
Figure 1: Active and Standby PCEs
In Figure 1, we consider PCE1 is responsible for computing paths for
PCC1, and PCE2 is standing by. When there is a change in LSP1, the
PCC should report to PCE1. From PCE2's perspective, PCC1 reporting
the update of LSP1 to PCE2 is slower than sync it from PCE1 to PCE2.
1.3. Split-Brain
In a resiliency case, a PCC has redundant PCEP sessions towards
multiple PCEs. In such a case, a PCC gives control on an LSP to a
single PCE only, and only this PCE is responsible for the path
computation for the delegated LSP: the PCC achieves this by setting
the D flag only towards the active PCE [RFC8231] selected for
delegation. The election of the active PCE to delegate an LSP is
controlled by each PCC. The PCC usually elects the active PCE by a
local configured policy (by setting a priority). Upon PCEP session
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failure, or active PCE failure, the PCC may decide to elect a new
active PCE by sending a new PCRpt message with D flag set to this new
active PCE. When the failed PCE or PCEP session comes back online,
it will be up to the implementation whether to revert back to the
original primary PCE. Reverting may lead to some disruption on the
existing path if computation results from both PCEs are not exactly
the same. By considering a network with multiple PCCs and
implementing multiple stateful PCEs for redundancy purpose, it is not
likely that all PCCs delegate their LSPs to the same PCE.
+----------+
| PCC1 | LSP1
+----------+
/ \
/ \
+---------+ +---------+
| PCE1 | | PCE2 |
+---------+ +---------+
\ /
*fail* \ /
+----------+
| PCC2 | LSP2
+----------+
Figure 2: Two PCEs with Shared Responsibility
In the example in Figure 2, we consider that by configuration, both
PCCs will firstly delegate their LSPs to PCE1. So, PCE1 is
responsible for computing a path for both LSP1 and LSP2. If the PCEP
session between PCC2 and PCE1 fails, PCC2 will delegate LSP2 to PCE2.
So PCE1 becomes responsible only for LSP1 path computation while PCE2
is responsible for the path computation of LSP2. When the PCC2-PCE1
session is back online, PCC2 will keep using PCE2 as active PCE
(consider no preemption in this example). So the result is a
permanent situation where each PCE is responsible for a subset of
path computation.
This situation is called a split-brain scenario, as there are
multiple computation brains running at the same time while a central
computation unit was required in some deployments/use cases.
Further, there are use cases where a particular LSP path computation
is linked to another LSP path computation: the most common use case
is path disjointness (see [RFC8800]) and Bidirectional LSPs (see
[RFC9059]). The set of LSPs that are dependent to each other may
start from different head-ends.
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_________________________________________
/ \
/ +------+ +------+ \
| | PCE1 | | PCE2 | |
| +------+ +------+ |
| |
| +------+ +------+ |
| | PCC1 | ----------------------> | PCC2 | |
| | | <---------------------- | | |
| +------+ +------+ |
| |
| |
| +------+ +------+ |
| | PCC3 | ----------------------> | PCC4 | |
| | | <---------------------- | | |
| +------+ +------+ |
| |
\ /
\_________________________________________/
_________________________________________
/ \
/ +------+ +------+ \
| | PCE1 | | PCE2 | |
| +------+ +------+ |
| |
| +------+ 10 +------+ |
| | PCC1 | ----- R1 ---- R2 ------- | PCC2 | |
| +------+ | | +------+ |
| | | |
| | | |
| +------+ | | +------+ |
| | PCC3 | ----- R3 ---- R4 ------- | PCC4 | |
| +------+ +------+ |
| |
\ /
\_________________________________________/
Figure 3: Managing Link-Disjoint LSPs
In Figure 3, the requirement is to create two link-disjoint LSPs:
PCC1->PCC2 and PCC3->PCC4. In the topology, all link cost metrics
are set to 1 except for the link 'R1-R2' which has a metric of 10.
The PCEs are responsible for the path computation and PCE1 is the
active primary PCE for all PCCs in the nominal case.
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Appendix B provides several scenarios for illustrative purposes.
There are many other cases where the solution defined in this
document are also applicable.
1.4. Applicability to H-PCE
[RFC8751] describes general considerations and use cases for the
deployment of Stateful PCE(s) using the Hierarchical PCE [RFC6805]
architecture. In this architecture, there is a clear need to
communicate between a child stateful PCE and a parent stateful PCE.
The procedures and extensions as described in Section 3 are equally
applicable to the H-PCE scenario.
2. Solution
The solution specified in this document is based on:
* The creation of the inter-PCE stateful PCEP session with specific
procedures.
* A Primary/Secondary relationship between stateful PCEs.
The solution builds upon the protocol extensions for stateful PCE in
[RFC8231], synchronization optimizations in [RFC8232], and PCE-
initiation in [RFC8281].
2.1. State-sync Session
This document specify a mechanism to set-up a PCEP session between
the stateful PCEs. Creating a PCEP session between PCEs is already
enabled for multiple scenarios like the one described in [RFC4655]
(multiple PCEs that are handling part of a path computation) and
[RFC6805] (hierarchical PCE). But that earlier work focused only on
the sessions between stateless PCEs.
Stateful PCE brings additional features (LSP state synchronization,
path update, delegation, ...). Thus some new behaviors need to be
defined on the inter-PCE PCEP session.
This inter-PCE PCEP session will allow the exchange of LSP states
between PCEs that can help in some scenarios where PCEP sessions are
lost between PCCs and PCEs. This inter-PCE PCEP session is called a
"state-sync session" in this document.
For example, in the scenario in Figure 4, there is no possibility to
compute disjointness as there is no PCE that is aware of both LSPs.
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+----------+
| PCC1 | LSP: PCC1->PCC2
+----------+
/
D=1 /
+---------+ +---------+
| PCE1 | | PCE2 |
+---------+ +---------+
/ D=1
/
+----------+
| PCC3 | LSP: PCC3->PCC4
+----------+
Figure 4: Partitioned Visibility Amongst PCEs
If we add a state-sync session as shown in Figure 5, PCE1 will be
able to do state synchronization via PCRpt messages for its LSP to
PCE2 and PCE2 will do the same. All the PCEs will be aware of all
LSPs even if a PCC->PCE session is down. PCEs will then be able to
compute disjoint paths.
+----------+
| PCC1 | LSP : PCC1->PCC2
+----------+
/
D=1 /
+---------+ PCEP +---------+
| PCE1 | ----- | PCE2 |
+---------+ +---------+
/ D=1
/
+----------+
| PCC3 | LSP : PCC3->PCC4
+----------+
Figure 5: Partitioned Visibility With State Synchronization
The procedures associated with this state-sync session are defined in
Section 3.
By just adding this state-sync session, it does not ensure that a
path with LSP association based constraints can always be computed
and does not prevent the computation loop, but it increases
resiliency and ensures that PCEs will have the state information for
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all LSPs. Also, this session will allow a PCE to update the other
PCEs providing a faster synchronization mechanism than relying on
PCCs only.
2.2. Primary/Secondary Relationship between PCE
As seen in Section 1, performing a path computation in a split-brain
scenario (multiple PCEs responsible for computation) may provide a
non-optimal LSP placement, no path, or computation loops. To achieve
better efficiency, an LSP association constraint-based computation
may require that a single PCE performs the path computation for all
LSPs in the association group. Note that, it could be all LSPs
belonging to a particular association group, or all LSPs from a
particular PCC, or all LSPs in the network that need to be delegated
to a single PCE based on the deployment scenarios.
This document specifies a mechanism to add a priority mechanism
between PCEs to elect a single computing 'primary' PCE. Using this
priority mechanism, PCEs can agree on the PCE that will be
responsible for the computation for a particular association group,
or set of LSPs. The priority could be set per association, per PCC,
or for all PCEs. The rest of the text considers the association
group as an example.
When a single PCE is performing the computation for a particular
association group, no computation loop can happen and an optimal
placement will be provided. The other PCEs will only act as state
collectors and forwarders.
In the scenario described in Section 2.1, PCE1 and PCE2 will decide
that PCE1 will be responsible for the path computation of both LSPs.
If we first configure PCC1->PCC2, PCE1 computes the shortest path as
it is the only LSP in the disjoint-group that it is aware of:
R1->R3->R4->R2->PCC2 (shortest path). When PCC3->PCC4 is configured,
PCE2 will not perform computation even if it has delegation but
forwards the delegation via PCRpt message to PCE1 through the state-
sync session. PCE1 will then perform disjointness computation and
will move PCC1->PCC2 onto R1->R2->PCC2 and provides an ERO to PCE2
for PCC3->PCC4: R3->R4->PCC4. The PCE2 will further update the PCC3
with the new path.
3. Procedures and Protocol Extensions
3.1. Opening a state-sync session
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3.1.1. Capability Advertisement
A PCE indicates its support of state-sync procedures during the PCEP
Initialization phase [RFC5440]. The OPEN object in the Open message
MUST contains the "Stateful PCE Capability" TLV defined in [RFC8231].
A new P (INTER-PCE-CAPABILITY) flag is introduced to indicate the
support of state-sync.
This document adds a new bit in the Flags field with :
* P (INTER-PCE-CAPABILITY - 1 bit - TBD4): If set to 1 by a PCEP
Speaker, the PCEP speaker indicates that the session MUST follow
the state-sync procedures as described in this document. If the P
bit is set by both speakers, the procedures MUST be used. If a
PCEP speaker receives a STATEFUL-PCE-CAPABILITY TLV with P=0 while
it advertised P=1 or if both set P flag to 0, the session SHOULD
be set-up but the state-sync procedures MUST NOT be applied on
this session. A PCE MAY decide to close a session if the received
setting of the P flag is not acceptable.
The U flag [RFC8231] MUST be set when sending the STATEFUL-PCE-
CAPABILITY TLV with the P flag set. In case the U flag is not set
along with the P flag, the state sync capability is not enabled and
it is considered as if the P flag is not set. The S flag MAY be set
if optimized synchronization is required as per [RFC8232].
3.2. State Synchronization
When the state sync capability has been negotiated between stateful
PCEs, each PCEP speaker will behave as a PCE and as a PCC at the same
time regarding the state synchronization as defined in [RFC8231].
This means that each PCEP Speaker:
* MUST send a PCRpt message towards its neighbor with S flag set for
each LSP in its LSP database learned from a PCC. (PCC role)
* MUST send the End Of Synchronization Marker towards its neighbor
when all LSPs have been reported. (PCC role)
* MUST wait for the LSP synchronization from its neighbor to end
(receiving an End Of Synchronization Marker). (PCE role)
The process of synchronization runs in parallel on each PCE (with no
defined order).
The optimized state synchronization procedures MAY be used, as
defined in [RFC8232].
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When a PCEP Speaker sends a PCRpt on a state-sync session, it MUST
add the SPEAKER-ENTITY-ID TLV (defined in [RFC8232]) in the LSP
Object, the value used will refer to the 'owner' PCC of the LSP. If
a PCEP Speaker receives a PCRpt on a state-sync session without this
TLV, it MUST discard the PCRpt message and it MUST reply with a PCErr
message using error-type=6 (Mandatory Object missing) and error-
value=TBD1 (SPEAKER-ENTITY-ID TLV missing).
3.3. Incremental Updates and Report Forwarding Rules
During the life of an LSP, its state may change (path, constraints,
operational state...) and a PCC will advertise a new PCRpt to the PCE
for each such change.
When propagating LSP state changes from a PCE to other PCEs, it MUST
ensure that a PCE always uses the freshest state coming from the PCC.
When a PCE receives a new PCRpt from a PCC with the LSP-DB-VERSION,
the PCE MUST forward the PCRpt to all its state-sync sessions and
MUST add the appropriate SPEAKER-ENTITY-ID TLV in the PCRpt. In
addition, it MUST add a new ORIGINAL-LSP-DB-VERSION TLV (described
below). The ORIGINAL-LSP-DB-VERSION contains the LSP-DB-VERSION
coming from the PCC.
When a PCE receives a new PCRpt from a PCC without the LSP-DB-
VERSION, it SHOULD NOT forward the PCRpt on any state-sync sessions
and SHOULD log such an event on the first occurrence.
When a PCE receives a new PCRpt from a PCC with the R flag (Remove)
set and an LSP-DB-VERSION TLV, the PCE MUST forward the PCRpt to all
its state-sync sessions keeping the R flag set (Remove) and MUST add
the appropriate SPEAKER-ENTITY-ID TLV and ORIGINAL-LSP-DB-VERSION TLV
in the PCRpt message.
When a PCE receives a PCRpt from a state-sync session, it MUST NOT
forward the PCRpt to other state-sync sessions. This helps to
prevent message loops between PCEs. As a consequence, a full mesh of
PCEP sessions between PCEs are REQUIRED.
When a PCRpt is forwarded, all the original objects and values are
kept. As an example, the PLSP-ID used in the forwarded PCRpt will be
the same as the original one used by the PCC. Thus an implementation
supporting this document MUST consider SPEAKER-ENTITY-ID TLV and
PLSP-ID together to uniquely identify an LSP on the state-sync
session.
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The ORIGINAL-LSP-DB-VERSION TLV is encoded as shown in Figure 6 and
MUST always contain the LSP-DB-VERSION received from the owner PCC of
the LSP.
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=8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LSP State DB Version Number |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: The ORIGINAL-LSP-DB-VERSION TLV
Using the ORIGINAL-LSP-DB-VERSION TLV allows a PCE to keep using
optimized synchronization ([RFC8232]) with another PCE. In such a
case, the PCE will send a PCRpt to another PCE with both ORIGINAL-
LSP-DB-VERSION TLV and LSP-DB-VERSION TLV. The ORIGINAL-LSP-DB-
VERSION TLV will contain the version number as allocated by the PCC
while the LSP-DB-VERSION will contain the version number allocated by
the local PCE.
3.4. Maintaining LSP States from Different Sources
When a PCE receives a PCRpt on a state-sync session, it stores the
LSP information into the original PCC address context (as the LSP
belongs to the PCC). A PCE SHOULD maintain a single state for a
particular LSP and SHOULD maintain the list of sources it learned a
particular state from.
A PCEP speaker may receive state information for a particular LSP
from different sources: the PCC that owns the LSP (through a regular
PCEP session) and some PCEs (through PCEP state-sync sessions). A
PCEP speaker MUST always keep the freshest state in its LSP database,
overriding the previously received information.
A PCE, receiving a PCRpt from a PCC, updates the state of the LSP in
its LSP-DB with the newly received information. When receiving a
PCRpt from another PCE, a PCE SHOULD update the LSP state only if the
ORIGINAL-LSP-DB-VERSION present in the PCRpt indicates it is newer
than the current ORIGINAL-LSP-DB-VERSION of the stored LSP state
taking wrap around into account. This ensures that a PCE never tries
to update its stored LSP state with an old information. Each time a
PCE updates an LSP state in its LSP-DB, it SHOULD reset the source
list associated with the LSP state and SHOULD add the source speaker
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address in the source list. When a PCE receives a PCRpt which has an
ORIGINAL-LSP-DB-VERSION (if coming from a PCE) or an LSP-DB-VERSION
(if coming from the PCC) equals to the current ORIGINAL-LSP-DB-
VERSION of the stored LSP state, it SHOULD add the source speaker
address in the source list.
When a PCE receives a PCRpt requesting an LSP deletion from a
particular source, it SHOULD remove this particular source from the
list of sources associated with this LSP.
When the list of sources becomes empty for a particular LSP, the LSP
state MUST be removed. This means that all the sources must send a
PCRpt with R=1 for an LSP to make the PCE remove the LSP state.
Note that a PCC uses the Open message exchange during PCEP session
establishment to inform the PCE about its capabilities and
parameters. Currently, there is no mechanism to pass that
information to other PCEs via the state-sync session.
3.5. Computation Priority between PCEs and Sub-delegation
A computation priority is necessary to ensure that a single PCE will
perform the computation for all the LSPs in an association group:
this will allow for a more optimized LSP placement and will prevent
computation loops.
All PCEs in the network that are handling LSPs in a common LSP
association group SHOULD be aware of each other including the
computation priority of each PCE. Note that there is no need for PCC
to be aware of this. The computation priority is a number and the
PCE having the highest priority MUST be responsible for the
computation. If several PCEs have the same priority value, their IP
address MUST be used as a tie-breaker to provide a rank: the highest
IP address has more priority.
The computation priorities could be set through local configurations.
The priority for local and remote PCEs could be set at global level
so the highest priority PCE will handle all path computations or more
granular, so a PCE may have the highest priority for only a subset of
LSPs or association-groups. See Section 9.1 for more details. In
future, PCEs could also advertise and discover these parameters via
PCEP, those details are out of the scope of this document and left
for future specification.
A PCEP Speaker receiving a PCRpt from a PCC with the D flag set that
does not have the highest computation priority, SHOULD forward the
PCRpt on all state-sync sessions (as per Section 3.3) and SHOULD set
D flag on the state-sync session towards the highest priority PCE, D
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flag will be unset to all other state-sync sessions. This behavior
is similar to the delegation behavior handled at the PCC side and is
called a sub-delegation (the PCE sub-delegates the control of the LSP
to another PCE). When a PCEP Speaker sub-delegates an LSP to another
PCE, it loses control of the LSP and cannot update it anymore by its
own decision. When a PCE receives a PCRpt with D flag set on a
state-sync session, as a regular PCE, it is granted control over the
LSP.
If the highest priority PCE is failing or if the state-sync session
between the local PCE and the highest priority PCE failed, the
operator MAY decide to instruct a switch-over to delegate the LSP to
the next highest priority PCE or to take back control of the LSP. It
is a local policy decision.
When a PCE has the delegation for an LSP and needs to update this
LSP, it MUST send a PCUpd message to all state-sync sessions and to
the PCC session on which it received the delegation. The D-Flag
would be unset in the PCUpd for state-sync sessions whereas the
D-Flag would be set for the PCC. In the case of sub-delegation, the
computing PCE will send the PCUpd only to all state-sync sessions (as
it has no direct delegation from a PCC). The D-Flag would be set for
the state-sync session to the PCE that sub-delegated this LSP and the
D-Flag would be unset for other state-sync sessions.
The PCUpd sent over a state-sync session MUST contain the SPEAKER-
ENTITY-ID TLV in the LSP Object (the value used must identify the
target PCC). The PLSP-ID used is the original PLSP-ID generated by
the PCC and learned from the forwarded PCRpt. If a PCE receives a
PCUpd on a state-sync session without the SPEAKER-ENTITY-ID TLV, it
MUST discard the PCUpd and MUST reply with a PCErr message using
error-type=6 (Mandatory Object missing) and error-value=TBD1
(SPEAKER-ENTITY-ID TLV missing).
When a PCE receives a valid PCUpd on a state-sync session, it SHOULD
forward the PCUpd to the appropriate PCC (identified based on the
SPEAKER-ENTITY-ID TLV value) that delegated the LSP originally and
SHOULD remove the SPEAKER-ENTITY-ID TLV from the LSP Object. The
acknowledgment of the PCUpd is done through a cascaded mechanism, and
the PCC is the only responsible for triggering the acknowledgment:
when the PCC receives the PCUpd from the local PCE, it acknowledges
it with a PCRpt as per [RFC8231]. When receiving the new PCRpt from
the PCC, the local PCE uses the defined forwarding rules on the
state-sync session so the acknowledgment is relayed to the computing
PCE.
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3.5.1. Association Group
All LSPs belonging to the same association group SHOULD have the same
computation priorities for the PCEs. A PCE SHOULD only compute a
path using an association-group constraint if it has delegation for
all of LSPs in the association-group. In this case, an
implementation MAY use a local policy on PCE to decide if PCE does
not compute path at all for this set of LSP or if it can compute a
path by relaxing the association-group constraint.
3.6. Passive Stateful Procedures
In the passive stateful PCE architecture, the PCC is responsible for
triggering a path computation request using a PCReq message to its
PCE. Similarly to PCRpt Message, which remains unchanged for passive
mode, if a PCE receives a PCReq for an LSP and if this PCE finds that
it does not have the highest computation priority of this LSP, or
groups, it MUST forward the PCReq message to the highest priority PCE
over the state-sync session. When the highest priority PCE receives
the PCReq, it computes the path and generates a PCRep message towards
the PCE that made the request. This PCE will then forward the PCRep
to the requesting PCC. The handling of LSP object and the SPEAKER-
ENTITY-ID TLV in PCReq and PCRep is similar to PCRpt/PCUpd messages.
3.7. PCE Initiation Procedures
It is possible that a PCE does not have a PCEP session with the
headend to initiate an LSP as per [RFC8281]. A PCE could send the
PCInitiate message on the state-sync sessions to another PCE to
request it to create a PCE-Initiated LSP on its behalf. If the PCE
is able to initiate the LSP it would report it on the state-sync
session via PCRpt message. If the PCE does not have a session to the
headend, it MUST send a PCErr message with Error-type=24 (PCE
instantiation error) and Error-value=TBD5 (No PCEP session with the
headend). PCE could try to initiate via another state-sync PCE if
available.
4. Examples
The examples in this section are for illustrative purpose only, to
show how the behavior of the state sync inter-PCE session works.
4.1. Example 1 - Successful disjoint paths (requiring reroute)
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_________________________________________
/ \
/ +------+ +------+ \
| | PCE1 | | PCE2 | |
| +------+ +------+ |
| |
| +------+ 10 +------+ |
| | PCC1 | ----- R1 ---- R2 ------- | PCC2 | |
| +------+ | | +------+ |
| | | |
| | | |
| +------+ | | +------+ |
| | PCC3 | ----- R3 ---- R4 ------- | PCC4 | |
| +------+ +------+ |
| |
\ /
\_________________________________________/
+----------+
| PCC1 | LSP : PCC1->PCC2
+----------+
/
D=1 /
+---------+ +---------+
| PCE1 |----| PCE2 |
+---------+ +---------+
/ D=1
/
+----------+
| PCC3 | LSP : PCC3->PCC4
+----------+
PCE1 computation priority 100
PCE2 computation priority 200
Figure 7: Disjoint Paths Requiring Reroute
Consider the PCEP sessions in Figure 7, where computation priority is
global for all the LSPs and a link disjoint path between LSPs
PCC1->PCC2 and PCC3->PCC4 is required.
Consider the PCC1->PCC2 is configured first and PCC1 delegates the
LSP to PCE1, but as PCE1 does not have the highest computation
priority, it sub-delegates the LSP to PCE2 by sending a PCRpt with
D=1 and including the SPEAKER-ENTITY-ID TLV over the state-sync
session. PCE2 receives the PCRpt and as it has delegation for this
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LSP, it computes the shortest path: R1->R3->R4->R2->PCC2. It then
sends a PCUpd to PCE1 (including the SPEAKER-ENTITY-ID TLV) with the
computed ERO. PCE1 forwards the PCUpd to PCC1 (removing the SPEAKER-
ENTITY-ID TLV). PCC1 acknowledges the PCUpd by a PCRpt to PCE1.
PCE1 forwards the PCRpt to PCE2.
When PCC3->PCC4 is configured, PCC3 delegates the LSP to PCE2, PCE2
can compute a disjoint path as it has knowledge of both LSPs and has
delegation also for both. The only solution found is to move
PCC1->PCC2 LSP on another path, PCE2 can move PCC1->PCC2 as it has
sub-delegation for it. It creates a new PCUpd with a new ERO:
R1->R2-PCC2 towards PCE1 which forwards to PCC1. PCE2 sends a PCUpd
to PCC3 with the path: R3->R4->PCC4.
In this set-up, PCEs are able to find a disjoint path while without
state-sync and computation priority they could not.
4.2. Example 2 - Successful disjoint paths (simultaneous turnup)
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_____________________________________
/ \
/ +------+ +------+ \
| | PCE1 | | PCE2 | |
| +------+ +------+ |
| |
| +------+ 100 +------+ |
| | | -------------------- | | |
| | PCC1 | ----- R1 ----------- | PCC2 | |
| +------+ | +------+ |
| | | | |
| 6 | | 2 | 2 |
| | | | |
| +------+ | +------+ |
| | PCC3 | ----- R3 ----------- | PCC4 | |
| +------+ 10 +------+ |
| |
\ /
\_____________________________________/
+----------+
| PCC1 | LSP : PCC1->PCC2
+----------+
/ \
D=1 / \ D=0
+---------+ +---------+
| PCE1 |----| PCE2 |
+---------+ +---------+
D=0 \ / D=1
\ /
+----------+
| PCC3 | LSP : PCC3->PCC4
+----------+
PCE1 computation priority 200
PCE2 computation priority 100
Figure 8: Disjoint Paths with Simultaneous Turnup
In this example (see Figure 8), suppose both LSPs are configured
almost at the same time. PCE1 sub-delegates PCC1->PCC2 to PCE2 while
PCE2 keeps delegation for PCC3->PCC4, PCE2 computes a path for
PCC1->PCC2 and PCC3->PCC4 and can achieve disjointness computation
easily. No computation loop happens in this case.
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4.3. Example 3 - Unfeasible disjoint paths (insufficient state-sync
sessions)
_________________________________________
/ \
/ +------+ +------+ \
| | PCE1 | | PCE2 | |
| +------+ +------+ |
| |
| +------+ 10 +------+ |
| | PCC1 | ----- R1 ---- R2 ------- | PCC2 | |
| +------+ | | +------+ |
| | | |
| | | |
| +------+ | | +------+ |
| | PCC3 | ----- R3 ---- R4 ------- | PCC4 | |
| +------+ +------+ |
| |
\ /
\_________________________________________/
+----------+
| PCC1 | LSP : PCC1->PCC2
+----------+
/
D=1 /
+---------+ +---------+ +---------+
| PCE1 |----| PCE2 |----| PCE3 |
+---------+ +---------+ +---------+
/ D=1
/
+----------+
| PCC3 | LSP : PCC3->PCC4
+----------+
PCE1 computation priority 100
PCE2 computation priority 200
PCE3 computation priority 300
Figure 9: Unfeasible Disjoint Paths
With the PCEP sessions as in Figure 9, consider the need to have link
disjoint LSPs PCC1->PCC2 and PCC3->PCC4.
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Suppose PCC1->PCC2 is configured first, PCC1 delegates the LSP to
PCE1, but as PCE1 does not have the highest computation priority, it
will sub-delegate the LSP to PCE2 (as it not aware of PCE3 and has no
way to reach it). PCE2 cannot compute a path for PCC1->PCC2 as it
does not have the highest priority and is not allowed to sub-delegate
the LSP again towards PCE3 as per Section 3.
When PCC3->PCC4 is configured, PCC3 delegates the LSP to PCE2 that
performs sub-delegation to PCE3. As PCE3 will have knowledge of only
one LSP in the group, it cannot compute disjointness and can decide
to fall-back to a less constrained computation to provide a path for
PCC3->PCC4. In this case, it will send a PCUpd to PCE2 that will be
forwarded to PCC3.
Disjointness cannot be achieved in this scenario because of lack of
state-sync session between PCE1 and PCE3, but no computation loop
happens. Thus it is required for all PCEs that support state-sync to
have a full mesh sessions between each other.
5. Using Primary/Secondary Computation and State-sync Sessions to
Increase Scaling
The Primary/Secondary computation and state-sync sessions
architecture can be used to increase the scaling of the PCE
architecture. If the number of PCCs is really high, it may be too
resource consuming for a single PCE instance to maintain all the PCEP
sessions while at the same time performing all path computations.
Using primary/secondary computation and state-sync sessions may allow
to create groups of PCEs that manage a subset of the PCCs and perform
some or no path computations. Decoupling PCEP session maintenance
and computation will allow increasing scaling of the PCE
architecture.
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+----------+
| PCC500 |
+----------+-+
| PCC1 |
+----------+
/ \
/ \
+---------+ +---------+
| PCE1 |---| PCE2 |
+---------+ +---------+
| \ / |
| \/ |
| /\ |
| / \ |
+---------+ +---------+
| PCE3 |---| PCE4 |
+---------+ +---------+
\ /
\ /
+----------+
| PCC501 |
+----------+-+
| PCC1000 |
+----------+
Figure 10: Improved Scalability
In Figure 10, two groups of PCEs are created: PCE1/2 maintain PCEP
sessions with PCC1 up to PCC500, while PCE3/4 maintain PCEP sessions
with PCC501 up to PCC1000. A granular primary/secondary policy is
set-up as follows to load-share computation between PCEs:
* PCE1 has priority 200 for association ID 1 up to 300, association
source 0.0.0.0. All other PCEs have a decreasing priority for
those associations.
* PCE3 has priority 200 for association ID 301 up to 500,
association source 0.0.0.0. All other PCEs have a decreasing
priority for those associations.
If some PCCs delegate LSPs with association ID 1 up to 300 and
association source 0.0.0.0, the receiving PCE (if not PCE1) will sub-
delegate the LSPs to PCE1. PCE1 becomes responsible for the
computation of these LSP associations while PCE3 is responsible for
the computation of another set of associations.
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The procedures described in this document could help greatly in load-
sharing between a group of stateful PCEs.
6. PCEP-PATH-VECTOR TLV
This specification allows PCEP messages to be propagated among PCEP
speakers. It may be useful to track information about the
propagation of the messages. One of the use cases is a message loop
detection mechanism, but other use cases like hop by hop information
recording may also be implemented in future.
This document introduces the PCEP-PATH-VECTOR TLV (type TBD3) to be
encoded in the LSP Object with the format shown in Figure 11.
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=TBD3 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PCEP-SPEAKER-INFORMATION#1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PCEP-SPEAKER-INFORMATION#n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 11: The PCEP-PATH-VECTOR TLV
The TLV format and padding rules are as per [RFC5440].
The PCEP-SPEAKER-INFORMATION field has the format shown in Figure 12.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | ID Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// Speaker Entity identity (variable) //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// Sub-TLVs (optional) //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Figure 12: The PCEP-SPEAKER-INFORMATION TLV
* Length: defines the total length of the PCEP-SPEAKER-INFORMATION
field.
* ID Length: defines the length of the Speaker Entity identity field
not counting any padding.
* Speaker Entity identity: same possible values as the SPEAKER-
IDENTIFIER-TLV. Padded with trailing zeros to a 4-byte boundary.
* The PCEP-SPEAKER-INFORMATION may also carry some optional sub-TLVs
so each PCEP speaker can add local information that could be
recorded. This document does not define any sub-TLV.
The PCEP-PATH-VECTOR TLV MAY be carried in the LSP Object. Its usage
is purely optional.
If a PCEP speaker receives a message with PCEP-PATH-VECTOR TLV and
finds its speaker information already present in the PCEP-PATH-VECTOR
TLV, it MUST ignore the PCEP message and SHOULD log it as an error
because this represents a message loop.
The list of speakers within the PCEP-PATH-VECTOR TLV MUST be ordered.
When sending a PCEP message (PCRpt, PCUpd, or PCInitiate), a PCEP
Speaker MAY add the PCEP-PATH-VECTOR TLV with a PCEP-SPEAKER-
INFORMATION containing its own information. If the PCEP message sent
is the result of a previously received PCEP message, and if the PCEP-
PATH-VECTOR TLV was already present in the initial message, the PCEP
speaker MAY append a new PCEP-SPEAKER-INFORMATION containing its own
information at the end of the TLV.
7. Security Considerations
The security considerations described in [RFC8231] and [RFC5440]
apply to the extensions described in this document as well.
Additional considerations related to state synchronization and sub-
delegation between stateful PCEs are introduced, as it could be
spoofed and could be used as an attack vector. An attacker could
attempt to create too much state in an attempt to load the PCEP peer.
The PCEP peer could respond with a PCErr message as described in
[RFC8231]. An attacker could impact LSP operations by creating bogus
state. Further, state synchronization between stateful PCEs could
provide an adversary with the opportunity to eavesdrop on the
network. Thus, securing the PCEP session using Transport Layer
Security (TLS) [RFC8253], as per the recommendations and best current
practices in [RFC9325], is RECOMMENDED.
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8. Implementation Status
[Note to the RFC Editor - remove this section before publication, as
well as remove the reference to RFC 7942.]
This section records the status of known implementations of the
protocol defined by this specification at the time of posting of this
Internet-Draft, and is based on a proposal described in [RFC7942].
The description of implementations in this section is intended to
assist the IETF in its decision processes in progressing drafts to
RFCs. Please note that the listing of any individual implementation
here does not imply endorsement by the IETF. Furthermore, no effort
has been spent to verify the information presented here that was
supplied by IETF contributors. This is not intended as, and must not
be construed to be, a catalog of available implementations or their
features. Readers are advised to note that other implementations may
exist.
According to [RFC7942], "this will allow reviewers and working groups
to assign due consideration to documents that have the benefit of
running code, which may serve as evidence of valuable experimentation
and feedback that have made the implemented protocols more mature.
It is up to the individual working groups to use this information as
they see fit".
At the time of posting the -06 version of this document, there are no
known implementations of this mechanism. It is believed that some
vendors are considering implementations, but these plans are too
vague to make any further assertions.
9. Manageability Considerations
9.1. Control of Function and Policy
An operator MUST be allowed to configure the capability to support
state-sync procedures for an inter-PCE session. They MUST be allowed
to configure a computation priority of the local and remote PCEs at
the global level. They MAY also be allowed to configure computation
priority of the local and remote PCEs per association (or a range of
them). Further, they MAY also be allowed to configure computation
priority per PCC (or range of them). An implementation MAY support
other such configuration levels for computation priority of the local
and remote PCEs.
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9.2. Information and Data Models
An implementation SHOULD allow the operator to view the capability
defined in this document. To serve this purpose, the PCEP YANG
module [I-D.ietf-pce-pcep-yang] could be extended in the future.
9.3. 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].
9.4. Verify Correct Operations
Mechanisms defined in this document do not imply any new operation
verification requirements in addition to those already listed in
[RFC5440].
9.5. Requirements On Other Protocols
Mechanisms defined in this document do not imply any new requirements
on other protocols.
9.6. Impact On Network Operations
Mechanisms defined in this document improves the network operations
by alleviating the problems described in Section 1.
10. Acknowledgements
Thanks to [I-D.knodel-terminology] urging for better use of terms.
11. IANA Considerations
This document requests IANA actions to allocate code points for the
protocol elements defined in this document.
11.1. PCEP-Error Object
IANA is requested to allocate a new Error Value for the Error Type 6
and 24.
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+============+============================+===========+
| Error-Type | Meaning | Reference |
+============+============================+===========+
| 6 | Mandatory Object Missing | [RFC5440] |
+------------+----------------------------+-----------+
| | Error-value=TBD1: SPEAKER- | This |
| | ENTITY-ID TLV missing | document |
+------------+----------------------------+-----------+
| 24 | LSP instantiation error | [RFC8281] |
+------------+----------------------------+-----------+
| | Error-value=TBD5: No PCEP | This |
| | session with the headend | document |
+------------+----------------------------+-----------+
Table 1
11.2. PCEP TLV Type Indicators
IANA is requested to allocate new TLV Type Indicator values within
the "PCEP TLV Type Indicators" sub-registry of the PCEP Numbers
registry, as follows:
+=======+=============================+===============+
| Value | Meaning | Reference |
+=======+=============================+===============+
| TBD2 | ORIGINAL-LSP-DB-VERSION TLV | This document |
+-------+-----------------------------+---------------+
| TBD3 | PCEP-PATH-VECTOR TLV | This document |
+-------+-----------------------------+---------------+
Table 2
11.3. STATEFUL-PCE-CAPABILITY TLV
IANA is requested to allocate a new bit value in the STATEFUL-PCE-
CAPABILITY TLV Flag Field sub-registry.
+======+======================+===============+
| Bit | Description | Reference |
+======+======================+===============+
| TBD4 | INTER-PCE-CAPABILITY | This document |
+------+----------------------+---------------+
Table 3
12. References
12.1. 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>.
[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>.
[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>.
[RFC8232] Crabbe, E., Minei, I., Medved, J., Varga, R., Zhang, X.,
and D. Dhody, "Optimizations of Label Switched Path State
Synchronization Procedures for a Stateful PCE", RFC 8232,
DOI 10.17487/RFC8232, September 2017,
<https://www.rfc-editor.org/info/rfc8232>.
[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>.
12.2. Informative References
[I-D.ietf-pce-pcep-yang]
Dhody, D., Beeram, V. P., Hardwick, J., and J. Tantsura,
"A YANG Data Model for Path Computation Element
Communications Protocol (PCEP)", Work in Progress,
Internet-Draft, draft-ietf-pce-pcep-yang-26, 19 October
2024, <https://datatracker.ietf.org/doc/html/draft-ietf-
pce-pcep-yang-26>.
[I-D.knodel-terminology]
Knodel, M. and N. ten Oever, "Terminology, Power, and
Inclusive Language in Internet-Drafts and RFCs", Work in
Progress, Internet-Draft, draft-knodel-terminology-14, 24
August 2023, <https://datatracker.ietf.org/doc/html/draft-
knodel-terminology-14>.
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[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>.
[RFC6805] King, D., Ed. and A. Farrel, Ed., "The Application of the
Path Computation Element Architecture to the Determination
of a Sequence of Domains in MPLS and GMPLS", RFC 6805,
DOI 10.17487/RFC6805, November 2012,
<https://www.rfc-editor.org/info/rfc6805>.
[RFC7399] Farrel, A. and D. King, "Unanswered Questions in the Path
Computation Element Architecture", RFC 7399,
DOI 10.17487/RFC7399, October 2014,
<https://www.rfc-editor.org/info/rfc7399>.
[RFC7942] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", BCP 205,
RFC 7942, DOI 10.17487/RFC7942, July 2016,
<https://www.rfc-editor.org/info/rfc7942>.
[RFC8051] Zhang, X., Ed. and I. Minei, Ed., "Applicability of a
Stateful Path Computation Element (PCE)", RFC 8051,
DOI 10.17487/RFC8051, January 2017,
<https://www.rfc-editor.org/info/rfc8051>.
[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>.
[RFC8751] Dhody, D., Lee, Y., Ceccarelli, D., Shin, J., and D. King,
"Hierarchical Stateful Path Computation Element (PCE)",
RFC 8751, DOI 10.17487/RFC8751, March 2020,
<https://www.rfc-editor.org/info/rfc8751>.
[RFC8800] Litkowski, S., Sivabalan, S., Barth, C., and M. Negi,
"Path Computation Element Communication Protocol (PCEP)
Extension for Label Switched Path (LSP) Diversity
Constraint Signaling", RFC 8800, DOI 10.17487/RFC8800,
July 2020, <https://www.rfc-editor.org/info/rfc8800>.
[RFC9059] Gandhi, R., Ed., Barth, C., and B. Wen, "Path Computation
Element Communication Protocol (PCEP) Extensions for
Associated Bidirectional Label Switched Paths (LSPs)",
RFC 9059, DOI 10.17487/RFC9059, June 2021,
<https://www.rfc-editor.org/info/rfc9059>.
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[RFC9325] Sheffer, Y., Saint-Andre, P., and T. Fossati,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 9325, DOI 10.17487/RFC9325, November
2022, <https://www.rfc-editor.org/info/rfc9325>.
[RFC9552] Talaulikar, K., Ed., "Distribution of Link-State and
Traffic Engineering Information Using BGP", RFC 9552,
DOI 10.17487/RFC9552, December 2023,
<https://www.rfc-editor.org/info/rfc9552>.
Appendix A. Contributors
Dhruv Dhody
Huawei
India
Email: dhruv.ietf@gmail.com
Appendix B. Scenarios
This appendix provides several scenarios for illustrative purposes.
There are many other cases where the solution defined in this
document are also applicable.
B.1. Scenario 1
In the normal case (PCE1 as active primary PCE), consider that
PCC1->PCC2 LSP is configured first with the link disjointness
constraint, PCE1 sends a PCUpd message to PCC1 with the Explicit
Routing Object (ERO): R1->R3->R4->R2->PCC2 (shortest path). PCC1
signals and installs the path. When PCC3->PCC4 is configured, the
PCEs already knows the path of PCC1->PCC2 and can compute a link-
disjoint path: the solution requires to move PCC1->PCC2 onto a new
path to let room for the new LSP. PCE1 sends a PCUpd message to PCC1
with the new ERO: R1->R2->PCC2 and a PCUpd to PCC3 with the following
ERO: R3->R4->PCC4. In the normal case, there is no issue for PCE1 to
compute a link-disjoint path.
B.2. Scenario 2
Consider that PCC1 lost its PCEP session with PCE1 (all other PCEP
sessions are UP) as shown in Figure 13. PCC1 delegates its LSP to
PCE2.
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+----------+
| PCC1 | LSP: PCC1->PCC2
+----------+
\
\ D=1
+---------+ +---------+
| PCE1 | | PCE2 |
+---------+ +---------+
D=1 \ / D=0
\ /
+----------+
| PCC3 | LSP: PCC3->PCC4
+----------+
Figure 13: Scenario 2
Consider that the PCC1->PCC2 LSP is configured first with the link
disjointness constraint, PCE2 (which is the new active primary PCE
for PCC1) sends a PCUpd message to PCC1 with the ERO:
R1->R3->R4->R2->PCC2 (shortest path). When PCC3->PCC4 is configured,
PCE1 is not aware of LSPs from PCC1 any more, so it cannot compute a
disjoint path for PCC3->PCC4 and will send a PCUpd message to PCC3
with the shortest path ERO: R3->R4->PCC4. When PCC3->PCC4 LSP will
be reported to PCE2 by PCC3, PCE2 will ensure disjointness
computation and will correctly move PCC1->PCC2 (as it owns delegation
for this LSP) on the following path: R1->R2->PCC2. With this
sequence of events and these PCEP sessions, disjointness is ensured.
B.3. Scenario 3
+----------+
| PCC1 | LSP: PCC1->PCC2
+----------+
/ \
D=1 / \ D=0
+---------+ +---------+
| PCE1 | | PCE2 |
+---------+ +---------+
/ D=1
/
+----------+
| PCC3 | LSP: PCC3->PCC4
+----------+
Figure 14: Scenario 3
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Consider the PCEP sessions in Figure 14, and the PCC1->PCC2 LSP is
configured first with the link disjointness constraint, PCE1 computes
the shortest path as it is the only LSP in the disjoint association
group that it is aware of: R1->R3->R4->R2->PCC2 (shortest path).
When PCC3->PCC4 is configured, PCE2 must compute a disjoint path for
this LSP. The only solution found is to move PCC1->PCC2 LSP on
another path, but PCE2 cannot do it as it does not have delegation
for this LSP. In this set-up, PCEs are not able to find a disjoint
path.
B.4. Scenario 4
+----------+
| PCC1 | LSP: PCC1->PCC2
+----------+
/ \
D=1 / \ D=0
+---------+ +---------+
| PCE1 | | PCE2 |
+---------+ +---------+
D=0 \ / D=1
\ /
+----------+
| PCC3 | LSP: PCC3->PCC4
+----------+
Figure 15: Scenario 4
Consider the PCEP sessions in Figure 15. and that PCEs are configured
to fall-back to the shortest path if disjointness cannot be found as
described in [RFC8800]. The PCC1->PCC2 LSP is configured first, PCE1
computes the shortest path as it is the only LSP in the disjoint
association group that it is aware of: R1->R3->R4->R2->PCC2 (shortest
path). When PCC3->PCC4 is configured, PCE2 must compute a disjoint
path for this LSP. The only solution found is to move PCC1->PCC2 LSP
on another path, but PCE2 cannot do it as it does not have delegation
for this LSP. PCE2 then provides the shortest path for PCC3->PCC4:
R3->R4->PCC4. When PCC3 receives the ERO, it reports it back to both
PCEs. When PCE1 becomes aware of the PCC3->PCC4 path, it recomputes
the constrained shortest path first (CSPF) algorithm and provides a
new path for PCC1->PCC2: R1->R2->PCC2. The new path is reported back
to all PCEs by PCC1. PCE2 recomputes also CSPF to take into account
the new reported path. The new computation does not lead to any path
update.
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B.5. Scenario 5
_____________________________________
/ \
/ +------+ +------+ \
| | PCE1 | | PCE2 | |
| +------+ +------+ |
| |
| +------+ 100 +------+ |
| | | -------------------- | | |
| | PCC1 | ----- R1 ----------- | PCC2 | |
| +------+ | +------+ |
| | | | |
| 6 | | 2 | 2 |
| | | | |
| +------+ | +------+ |
| | PCC3 | ----- R3 ----------- | PCC4 | |
| +------+ 10 +------+ |
| |
\ /
\_____________________________________/
Figure 16: Scenario 5
Now, consider a new network topology in Figure 16 with the same PCEP
sessions as the previous example. Suppose that both LSPs are
configured almost at the same time. PCE1 will compute a path for
PCC1->PCC2 while PCE2 will compute a path for PCC3->PCC4. As each
PCE is not aware of the path of the second LSP in the association
group (not reported yet), each PCE is computing the shortest path for
the LSP. PCE1 computes ERO: R1->PCC2 for PCC1->PCC2 and PCE2
computes ERO: R3->R1->PCC2->PCC4 for PCC3->PCC4. When these shortest
paths will be reported to each PCE. Each PCE will recompute
disjointness. PCE1 will provide a new path for PCC1->PCC2 with ERO:
PCC1->PCC2. PCE2 will provide also a new path for PCC3->PCC4 with
ERO: R3->PCC4. When those new paths will be reported to both PCEs,
this will trigger CSPF again. PCE1 will provide a new more optimal
path for PCC1->PCC2 with ERO: R1->PCC2 and PCE2 will also provide a
more optimal path for PCC3->PCC4 with ERO: R3->R1->PCC2->PCC4. So we
come back to the initial state. When those paths will be reported to
both PCEs, this will trigger CSPF again. An infinite loop of CSPF
computation is then happening with a permanent flap of paths because
of the split-brain situation.
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Another common example to note would be two LSPs with link-diverse
paths that share a common node in its path but delegated to different
PCEs. In case of the common node failure, both PCEs would detect the
same and each could independently compute a new path that might both
choose the same new link.
This permanent computation loop comes from the inconsistency between
the state of the LSPs as seen by each PCE due to the split-brain:
each PCE is trying to modify at the same time its delegated path
based on the last received path information which de facto
invalidates this received path information.
B.6. Scenario 6
Domain/Area 1 Domain/Area 2
________________ ________________
/ \ / \
/ +------+ | | +------+ \
| | PCE1 | | | | PCE3 | |
| +------+ | | +------+ |
| | | |
| +------+ | | +------+ |
| | PCE2 | | | | PCE4 | |
| +------+ | | +------+ |
| | | |
| +------+ | | +------+ |
| | PCC1 | | | | PCC2 | |
| +------+ | | +------+ |
| | | |
| | | |
| +------+ | | +------+ |
| | PCC3 | | | | PCC4 | |
| +------+ | | +------+ |
\ | | |
\_______________/ \________________/
Figure 17: Scenario 6
In the example in Figure 17, suppose that the disjoint LSPs from PCC1
to PCC2 and from PCC4 to PCC3 are created. All the PCEs have the
knowledge of both domain topologies (e.g. using BGP-LS [RFC9552]).
For operation/management reasons, each domain uses its own group of
redundant PCEs. PCE1/PCE2 in domain 1 have PCEP sessions with PCC1
and PCC3 while PCE3/PCE4 in domain 2 have PCEP sessions with PCC2 and
PCC4. As PCE1/2 does not know about LSPs from PCC2/4 and PCE3/4 do
not know about LSPs from PCC1/3, there is no possibility to compute
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the disjointness constraint. This scenario can also be seen as a
split-brain scenario. This multi-domain architecture (with multiple
groups of PCEs) can also be used in a single domain, where an
operator wants to limit the failure domain by creating multiple
groups of PCEs maintaining a subset of PCCs. As for the multi-domain
example, there will be no possibility to compute the disjoint path
starting from head-ends managed by different PCE groups.
In this document, we specify a solution that addresses the
possibility to compute LSP association based constraints (like
disjointness) in split-brain scenarios while preventing computation
loops.
Authors' Addresses
Haomian Zheng (editor)
Huawei Technologies
H1, Huawei Xiliu Beipo Village, Songshan Lake
Dongguan
Guangdong, 523808
China
Email: zhenghaomian@huawei.com
Stephane Litkowski
Cisco
Email: slitkows.ietf@gmail.com
Siva Sivabalan
Ciena Corporation
Email: msiva282@gmail.com
Cheng Li
Huawei Technologies
Huawei Campus, No. 156 Beiqing Rd.
Beijing
100095
China
Email: c.l@huawei.com
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