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IETF RFC 8232
Last modified on Friday, September 22nd, 2017
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Internet Engineering Task Force (IETF) E. Crabbe
Request for Comments: 8232 Oracle
Category: Standards Track I. Minei
ISSN: 2070-1721 Google, Inc.
J. Medved
Cisco Systems, Inc.
R. Varga
Pantheon Technologies SRO
X. Zhang
D. Dhody
Huawei Technologies
September 2017
Optimizations of Label Switched Path State Synchronization
Procedures for a Stateful PCE
 Abstract
A stateful Path Computation Element (PCE) has access to not only the
information disseminated by the network's Interior Gateway Protocol
(IGP) but also the set of active paths and their reserved resources
for its computation. The additional Label Switched Path (LSP) state
information allows the PCE to compute constrained paths while
considering individual LSPs and their interactions. This requires a
State Synchronization mechanism between the PCE and the network, the
PCE and Path Computation Clients (PCCs), and cooperating PCEs. The
basic mechanism for State Synchronization is part of the stateful PCE
specification. This document presents motivations for optimizations
to the base State Synchronization procedure and specifies the
required Path Computation Element Communication Protocol (PCEP)
extensions.
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/RFC 8232.
Crabbe, et al. Standards Track PAGE 1 
RFC 8232 Optimizations of State Synchronization September 2017
Copyright Notice
Copyright (c) 2017 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.
Crabbe, et al. Standards Track PAGE 2
RFC 8232 Optimizations of State Synchronization September 2017
Table of Contents
1. Introduction ....................................................4
1.1. Requirements Language ......................................4
2. Terminology .....................................................5
3. State Synchronization Avoidance .................................5
3.1. Motivation .................................................5
3.2. State Synchronization Avoidance Procedure ..................5
3.2.1. IP Address Change during Session Re-establishment ..10
3.3. PCEP Extensions ...........................................11
3.3.1. LSP-DB Version Number TLV ..........................11
3.3.2. Speaker Entity Identifier TLV ......................12
4. Incremental State Synchronization ..............................13
4.1. Motivation ................................................13
4.2. Incremental Synchronization Procedure .....................14
5. PCE-Triggered Initial Synchronization ..........................17
5.1. Motivation ................................................17
5.2. PCE-Triggered Initial State Synchronization Procedure .....18
6. PCE-Triggered Resynchronization ................................19
6.1. Motivation ................................................19
6.2. PCE-Triggered State Resynchronization Procedure ...........19
7. Advertising Support of Synchronization Optimizations ...........20
8. IANA Considerations ............................................21
8.1. PCEP-Error Object .........................................21
8.2. PCEP TLV Type Indicators ..................................22
8.3. STATEFUL-PCE-CAPABILITY TLV ...............................22
9. Manageability Considerations ...................................22
9.1. Control of Function and Policy ............................22
9.2. Information and Data Models ...............................22
9.3. Liveness Detection and Monitoring .........................23
9.4. Verify Correct Operations .................................23
9.5. Requirements on Other Protocols ...........................23
9.6. Impact on Network Operations ..............................23
10. Security Considerations .......................................23
11. References ....................................................24
11.1. Normative References .....................................24
11.2. Informative References ...................................24
Acknowledgments ...................................................25
Contributors ......................................................25
Authors' Addresses ................................................26
Crabbe, et al. Standards Track PAGE 3
RFC 8232 Optimizations of State Synchronization September 2017
1. Introduction
The Path Computation Element Communication Protocol (PCEP) provides
mechanisms for Path Computation Elements (PCEs) to perform path
computations in response to Path Computation Client (PCC) requests.
[RFC 8231] describes a set of extensions to PCEP to provide stateful
control. A stateful PCE has access to not only the information
carried by the network's Interior Gateway Protocol (IGP) but also the
set of active paths and their reserved resources for its
computations. The additional state allows the PCE to compute
constrained paths while considering individual LSPs and their
interactions. This requires a State Synchronization mechanism
between the PCE and the network, the PCE and the PCC, and cooperating
PCEs. [RFC 8231] describes the basic mechanism for State
Synchronization. This document specifies following optimizations for
State Synchronization and the corresponding PCEP procedures and
extensions:
o State Synchronization Avoidance: To skip State Synchronization if
the state has survived and not changed during session restart.
(See Section 3.)
o Incremental State Synchronization: To do incremental (delta) State
Synchronization when possible. (See Section 4.)
o PCE-Triggered Initial Synchronization: To let PCE control the
timing of the initial State Synchronization. (See Section 5.)
o PCE-Triggered Resynchronization: To let PCE resynchronize the
state for sanity check. (See Section 6.)
Support for each of the synchronization optimization capabilities is
advertised during the PCEP initialization phase. See Section 7 for
the new flags defined in this document. The handling of each flag is
described in the relevant section.
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 [RFC 2119] [RFC 8174] when, and only when, they appear in all
capitals, as shown here.
Crabbe, et al. Standards Track PAGE 4
RFC 8232 Optimizations of State Synchronization September 2017
2. Terminology
This document uses the following terms defined in [RFC 5440]: PCC,
PCE, and PCEP Peer.
This document uses the following terms defined in [RFC 8051]: Stateful
PCE, Delegation, and LSP State Database (LSP-DB).
This document uses the following terms defined in [RFC 8231]:
Redelegation Timeout Interval, LSP State Report, and LSP Update
Request.
Within this document, when describing PCE-PCE communications, the
requesting PCE fills the role of a PCC as usual.
3. State Synchronization Avoidance
3.1. Motivation
The purpose of State Synchronization is to provide a
checkpoint-in-time state replica of a PCC's LSP state in a stateful
PCE. State Synchronization is performed immediately after the
initialization phase [RFC 5440]. [RFC 8231] describes the basic
mechanism for State Synchronization.
State Synchronization is not always necessary following a PCEP
session restart. If the state of both PCEP peers did not change, the
synchronization phase may be skipped. This can result in significant
savings in both control-plane data exchanges and the time it takes
for the stateful PCE to become fully operational.
3.2. State Synchronization Avoidance Procedure
State Synchronization MAY be skipped following a PCEP session restart
if the state of both PCEP peers did not change during the period
prior to session re-initialization. To be able to make this
determination, state must be exchanged and maintained by both PCE and
PCC during normal operation. This is accomplished by keeping track
of the changes to the LSP-DB, using a version tracking field called
the LSP-DB Version Number.
The INCLUDE-DB-VERSION (S) bit in the STATEFUL-PCE-CAPABILITY TLV
(Section 7) is advertised on a PCEP session during session startup to
indicate that the LSP-DB Version Number is to be included when the
LSPs are reported to the PCE. The LSP-DB Version Number, carried in
LSP-DB-VERSION TLV (see Section 3.3.1), is owned by a PCC, and it
MUST be incremented by 1 for each successive change in the PCC's LSP-
DB. The LSP-DB Version Number MUST start at 1 and may wrap around.
Crabbe, et al. Standards Track PAGE 5
RFC 8232 Optimizations of State Synchronization September 2017
Values 0 and 0xFFFFFFFFFFFFFFFF are reserved. If either of the two
values are used during LSP State (re)Synchronization, the PCE speaker
receiving this value MUST send back a PCEP Error (PCErr) with Error-
type=20 and Error-value=6 'Received an invalid LSP-DB Version
Number', and close the PCEP session. Operations that trigger a
change to the local LSP-DB include a change in the LSP operational
state, delegation of an LSP, removal or setup of an LSP, or change in
any of the LSP attributes that would trigger a report to the PCE.
If the include LSP-DB version capability is enabled, a PCC MUST
increment its LSP-DB Version Number when the 'Redelegation Timeout
Interval' timer expires (see [RFC 8231] for the use of the
Redelegation Timeout Interval).
If both PCEP speakers set the S flag in the OPEN object's
STATEFUL-PCE-CAPABILITY TLV to 1, the PCC MUST include the LSP-DB-
VERSION TLV in each LSP object of the Path Computation LSP State
Report (PCRpt) message. If the LSP-DB-VERSION TLV is missing in a
PCRpt message, the PCE will generate an error with Error-type=6
(Mandatory Object missing) and Error-value=12 'LSP-DB-VERSION TLV
missing', and close the session. If the include LSP-DB version
capability has not been enabled on a PCEP session, the PCC SHOULD NOT
include the LSP-DB-VERSION TLV in the LSP Object, and the PCE MUST
ignore it, were it to receive one.
If a PCE's LSP-DB survived the restart of a PCEP session, the PCE
will include the LSP-DB-VERSION TLV in its OPEN object, and the TLV
will contain the last LSP-DB Version Number received on an LSP State
Report from the PCC in the previous PCEP session. If a PCC's LSP-DB
survived the restart of a PCEP session, the PCC will include the LSP-
DB-VERSION TLV in its OPEN object, and the TLV will contain the
latest LSP-DB Version Number. If a PCEP speaker's LSP-DB did not
survive the restart of a PCEP session or at startup when the database
is empty, the PCEP speaker MUST NOT include the LSP-DB-VERSION TLV in
the OPEN object.
If both PCEP speakers include the LSP-DB-VERSION TLV in the OPEN
object and the TLV values match, the PCC MAY skip State
Synchronization, and the PCE does not wait for the end-of-
synchronization marker [RFC 8231]. Otherwise, the PCC MUST perform
full State Synchronization (see [RFC 8231]) or incremental State
Synchronization (see Section 4 if this capability is advertised) to
the stateful PCE. In other words, if the incremental State
Synchronization capability is not advertised by the peers, based on
the LSP-DB Version Number match, either the State Synchronization is
skipped or a full State Synchronization is performed. If the PCC
attempts to skip State Synchronization, by setting the SYNC flag to 0
and PLSP-ID to a non-zero value on the first LSP State Report from
Crabbe, et al. Standards Track PAGE 6
RFC 8232 Optimizations of State Synchronization September 2017
the PCC as per [RFC 8231], the PCE MUST send back a PCErr with Error-
type=20 and Error-value=2 'LSP-DB version mismatch', and close the
PCEP session.
If State Synchronization is required, then prior to completing the
initialization phase, the PCE MUST mark any LSPs in the LSP-DB that
were previously reported by the PCC as stale. When the PCC reports
an LSP during State Synchronization, if the LSP already exists in the
LSP-DB, the PCE MUST update the LSP-DB and clear the stale marker
from the LSP. When it has finished State Synchronization, the PCC
MUST immediately send an end-of-synchronization marker. The end-of-
synchronization marker is a PCRpt message with an LSP object
containing a PLSP-ID of 0 and with the SYNC flag set to 0 [RFC 8231].
The LSP-DB-VERSION TLV MUST be included in this PCRpt message. On
receiving this state report, the PCE MUST purge any LSPs from the
LSP-DB that are still marked as stale.
Note that a PCE/PCC MAY force State Synchronization by not including
the LSP-DB-VERSION TLV in its OPEN object.
Since a PCE does not make changes to the LSP-DB Version Number, a PCC
should never encounter this TLV in a message from the PCE (other than
the OPEN message). A PCC SHOULD ignore the LSP-DB-VERSION TLV, were
it to receive one from a PCE.
Figure 1 shows an example sequence where the State Synchronization is
skipped.
Crabbe, et al. Standards Track PAGE 7
RFC 8232 Optimizations of State Synchronization September 2017
+-+-+ +-+-+
|PCC| |PCE|
+-+-+ +-+-+
| |
|--Open--, |
| DBv=42 \ ,---Open--|
| S=1 \ / DBv=42 |
| \/ S=1 |
| /\ |
| / `-------->| (OK to skip sync)
(Skip sync) |<--------` |
| . |
| . |
| . |
| |
|--PCRpt,DBv=43,SYNC=0-->| (Regular
| | LSP State Report)
|--PCRpt,DBv=44,SYNC=0-->| (Regular
| | LSP State Report)
|--PCRpt,DBv=45,SYNC=0-->|
| |
Figure 1: State Synchronization Skipped
Figure 2 shows an example sequence where the State Synchronization is
performed due to LSP-DB version mismatch during the PCEP session
setup. Note that the same State Synchronization sequence would
happen if either the PCC or the PCE would not include the LSP-DB-
VERSION TLV in their respective Open messages.
Crabbe, et al. Standards Track PAGE 8
RFC 8232 Optimizations of State Synchronization September 2017
+-+-+ +-+-+
|PCC| |PCE|
+-+-+ +-+-+
| |
|--Open--, |
| DBv=46 \ ,---Open--|
| S=1 \ / DBv=42 |
| \/ S=1 |
| /\ |
| / `-------->| (Expect sync)
(Do sync) |<--------` |
| |
|--PCRpt,DBv=46,SYNC=1-->| (Sync start)
| . |
| . |
| . |
|--PCRpt,DBv=46,SYNC=0-->| (Sync done)
| . | (Purge LSP state
| . | if applicable)
| . |
|--PCRpt,DBv=47,SYNC=0-->| (Regular
| | LSP State Report)
|--PCRpt,DBv=48,SYNC=0-->| (Regular
| | LSP State Report)
|--PCRpt,DBv=49,SYNC=0-->|
| |
Figure 2: State Synchronization Performed
Figure 3 shows an example sequence where the State Synchronization is
skipped, but because one or both PCEP speakers set the S flag to 0,
the PCC does not send LSP-DB-VERSION TLVs in subsequent PCRpt
messages to the PCE. If the current PCEP session restarts, the PCEP
speakers will have to perform State Synchronization, since the PCE
does not know the PCC's latest LSP-DB Version Number information.
Crabbe, et al. Standards Track PAGE 9
RFC 8232 Optimizations of State Synchronization September 2017
+-+-+ +-+-+
|PCC| |PCE|
+-+-+ +-+-+
| |
|--Open--, |
| DBv=42 \ ,---Open--|
| S=0 \ / DBv=42 |
| \/ S=0 |
| /\ |
| / `-------->| (OK to skip sync)
(Skip sync) |<--------` |
| . |
| . |
| . |
|------PCRpt,SYNC=0----->| (Regular
| | LSP State Report)
|------PCRpt,SYNC=0----->| (Regular
| | LSP State Report)
|------PCRpt,SYNC=0----->|
| |
Figure 3: State Synchronization Skipped;
No LSP-DB-VERSION TLVs Sent from the PCC
3.2.1. IP Address Change during Session Re-establishment
There could be a case during PCEP session re-establishment when the
PCC's or PCE's IP address can change. This includes, but is not
limited to, the following cases:
o A PCC could use a physical interface IP address to connect to the
PCE. In this case, if the line card that the PCC connects from
changes, then the PCEP session goes down and comes back up again,
with a different IP address associated with a new line card.
o The PCC or PCE may move in the network, either physically or
logically, which may cause its IP address to change. For example,
the PCE may be deployed as a virtual network function (VNF), and
another virtualized instance of the PCE may be populated with the
original PCE instance's state, but it may be given a different IP
address.
To ensure that a PCEP peer can recognize a previously connected peer,
each PCEP peer includes the SPEAKER-ENTITY-ID TLV described in
Section 3.3.2 in the OPEN message.
Crabbe, et al. Standards Track PAGE 10
RFC 8232 Optimizations of State Synchronization September 2017
This TLV is used during the State Synchronization procedure to
identify the PCEP session as a re-establishment of a previous session
that went down. Then State Synchronization optimizations such as
state sync avoidance can be applied to this session. Note that this
usage is only applicable within the State Timeout Interval [RFC 8231].
After the State Timeout Interval expires, all state associated with
the PCEP session is removed, which includes the SPEAKER-ENTITY-ID
received. Note that the PCEP session initialization [RFC 5440]
procedure remains unchanged.
3.3. PCEP Extensions
A new INCLUDE-DB-VERSION (S) bit is added in the stateful
capabilities TLV (see Section 7 for details).
3.3.1. LSP-DB Version Number TLV
The LSP-DB Version Number (LSP-DB-VERSION) TLV is an optional TLV
that MAY be included in the OPEN object and the LSP object.
This TLV is included in the LSP object in the PCRpt message to
indicate the LSP-DB version at the PCC. This TLV SHOULD NOT be
included in other PCEP messages (Path Computation Update Request
(PCUpd), Path Computation Request (PCReq), and Path Computation Reply
(PCRep)) and MUST be ignored if received.
The format of the LSP-DB-VERSION 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=23 | Length=8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LSP-DB Version Number |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: LSP-DB-VERSION TLV Format
The type of the TLV is 23, and it has a fixed length of 8 octets.
The value contains a 64-bit unsigned integer, carried in network byte
order, representing the LSP-DB Version Number.
Crabbe, et al. Standards Track PAGE 11
RFC 8232 Optimizations of State Synchronization September 2017
3.3.2. Speaker Entity Identifier TLV
The Speaker Entity Identifier TLV (SPEAKER-ENTITY-ID) is an optional
TLV that MAY be included in the OPEN object when a PCEP speaker
wishes to determine if State Synchronization can be skipped when a
PCEP session is restarted. It contains a unique identifier for the
node that does not change during the lifetime of the PCEP speaker.
It identifies the PCEP speaker to its peers even if the speaker's IP
address is changed.
In case of a remote peer IP address change, a PCEP speaker would
learn the Speaker Entity Identifier on receiving the open message,
but it MAY have already sent its open message without realizing that
it is a known PCEP peer. In such a case, either a full
synchronization is done or the PCEP session is terminated. This may
be a local policy decision. The new IP address is associated with
the Speaker Entity Identifier for the future either way. In the
latter case when the PCEP session is re-established, it would be
correctly associated with the Speaker Entity Identifier and not be
considered as an unknown peer.
The format of the SPEAKER-ENTITY-ID 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=24 | Length (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
// Speaker Entity Identifier //
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: SPEAKER-ENTITY-ID TLV Format
The type of the TLV is 24, and it has a variable length, which MUST
be greater than 0. The value is padded to a 4-octet alignment. The
padding is not included in the Length field. The value contains the
Speaker Entity Identifier (an identifier of the PCEP speaker
transmitting this TLV). This identifier is required to be unique
within its scope of visibility, which is usually limited to a single
domain. It MAY be configured by the operator. Alternatively, it can
be derived automatically from a suitably stable unique identifier,
Crabbe, et al. Standards Track PAGE 12
RFC 8232 Optimizations of State Synchronization September 2017
such as a Media Access Control (MAC) address, serial number, Traffic
Engineering Router ID, or similar. In the case of inter-domain
connections, the speaker SHOULD prefix its usual identifier with the
domain identifier of its residence, such as an Autonomous System
number, an IGP area identifier, or similar to make sure it remains
unique.
The relationship between this identifier and entities in the Traffic
Engineering database is intentionally left undefined.
From a manageability point of view, a PCE or PCC implementation
SHOULD allow the operator to configure this Speaker Entity
Identifier.
If a PCEP speaker receives the SPEAKER-ENTITY-ID on a new PCEP
session, that matches with an existing alive PCEP session, the PCEP
speaker MUST send a PCErr with Error-type=20 and Error-value=7
'Received an invalid Speaker Entity Identifier', and close the PCEP
session.
4. Incremental State Synchronization
[RFC 8231] describes the LSP State Synchronization mechanism between
PCCs and stateful PCEs. During the State Synchronization, a PCC
sends the information of all its LSPs (i.e., the full LSP-DB) to the
stateful PCE. In order to reduce the State Synchronization overhead
when there is a small number of LSP state changes in the network
between the PCEP session restart, this section defines a mechanism
for incremental (Delta) LSP-DB synchronization.
4.1. Motivation
According to [RFC 8231], if a PCE restarts and its LSP-DB survived,
PCCs with a mismatched LSP-DB Version Number will send all their LSPs
information (full LSP-DB) to the stateful PCE, even if only a small
number of LSPs underwent state change. It can take a long time and
consume large communication channel bandwidth.
Figure 6 shows an example of LSP State Synchronization.
Crabbe, et al. Standards Track PAGE 13
RFC 8232 Optimizations of State Synchronization September 2017
+-----+
| PCE |
+-----+
/
/
/
/
+------+ +------+
| PCC1 |------------| PCC2 |
+------+ +------+
| |
| |
+------+ +------+
| PCC3 |------------| PCC4 |
+------+ +------+
Figure 6: Topology Example
Assume that there are 320 LSPs in the network, with each PCC having
80 LSPs. During the time when the PCEP session is down, 20 LSPs of
each PCC (i.e., 80 LSPs in total), are changed. Hence, when the PCEP
session restarts, the stateful PCE needs to synchronize 320 LSPs with
all PCCs. But actually, 240 LSPs stay the same. If performing full
LSP State Synchronization, it can take a long time to carry out the
synchronization of all LSPs. It is especially true when only a low
bandwidth communication channel is available (e.g., in-band control
channel for optical transport networks), and there is a substantial
number of LSPs in the network. Another disadvantage of full LSP
synchronization is that it is a waste of communication bandwidth to
perform full LSP synchronization given the fact that the number of
LSP changes can be small during the time when the PCEP session is
down.
An incremental (Delta) LSP-DB State Synchronization is described in
this section, where only the LSPs that underwent state change are
synchronized between the session restart. This may include
new/modified/deleted LSPs.
4.2. Incremental Synchronization Procedure
[RFC 8231] describes State Synchronization and Section 3 of this
document describes State Synchronization avoidance by using
LSP-DB-VERSION TLV in its OPEN object. This section extends this
idea to only synchronize the delta (changes) in case of version
mismatch.
Crabbe, et al. Standards Track PAGE 14
RFC 8232 Optimizations of State Synchronization September 2017
If both PCEP speakers include the LSP-DB-VERSION TLV in the OPEN
object and the LSP-DB-VERSION TLV values match, the PCC MAY skip
State Synchronization. Otherwise, the PCC MUST perform State
Synchronization. Incremental State Synchronization capability is
advertised on a PCEP session during session startup using the
DELTA-LSP-SYNC-CAPABILITY (D) bit in the capabilities TLV (see
Section 7). Instead of dumping full LSP-DB to the stateful PCE
again, the PCC synchronizes the delta (changes) as described in
Figure 7 when the D and S flags are set to 1 by both the PCC and PCE.
Other combinations of D and S flags set by the PCC and PCE result in
full LSP-DB synchronization procedures as described in [RFC 8231]. By
setting the D flag to zero in the OPEN message, a PCEP speaker can
skip the incremental synchronization optimization, resulting in a
full LSP-DB synchronization.
Crabbe, et al. Standards Track PAGE 15
RFC 8232 Optimizations of State Synchronization September 2017
+-+-+ +-+-+
|PCC| |PCE|
+-+-+ +-+-+
| |
|--Open--, |
| DBv=46 \ ,---Open--|
| S=1 \ / DBv=42 |
| D=1 \/ S=1 |
| /\ D=1 |
| / \ |
| / `-------->| (Expect delta sync)
(Do sync)|<--------` | (DO NOT purge LSP
(Delta) | | state)
| |
(Delta sync starts) |--PCRpt,DBv=46,SYNC=1-->|
| . |
| . |
| . |
| . |
|--PCRpt,DBv=46,SYNC=0-->| (Sync done,
| | PLSP-ID=0)
| |
|--PCRpt,DBv=47,SYNC=0-->| (Regular
| | LSP State Report)
|--PCRpt,DBv=48,SYNC=0-->| (Regular
| | LSP State Report)
|--PCRpt,DBv=49,SYNC=0-->|
| |
Figure 7: Incremental Synchronization Procedure
As per Section 3, the LSP-DB Version Number is incremented each time
a change is made to the PCC's local LSP-DB. Each LSP is associated
with the DB version at the time of its state change. This is needed
to determine which LSP and what information needs to be synchronized
in incremental State Synchronization. The incremental state sync is
done from the last LSP-DB version received by the PCE to the latest
DB version at the PCC. Note that the LSP-DB Version Number can wrap
around, in which case the incremental state sync would also wrap till
the latest LSP-DB Version Number at the PCC.
In order to carry out incremental State Synchronization, it is not
necessary for a PCC to store a complete history of LSP-DB change for
all time, but remember the LSP state changes (including LSP
modification, setup, and deletion) that the PCE did not get to
process during the session down. Note that, a PCC would be unaware
that a particular LSP report has been processed by the PCE before the
session to the PCE went down. So a PCC implementation MAY choose to
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RFC 8232 Optimizations of State Synchronization September 2017
store the LSP-DB Version Number with each LSP at the time its status
changed, so that when a session is re-established, an incremental
synchronization can be attempted based on the PCE's last LSP-DB
Version Number. For an LSP that is deleted at the PCC, the PCC
implementation would need to remember the deleted LSP in some way to
make sure this could be reported as part of incremental
synchronization later. The PCC would discard this information based
on a local policy or when it determines that this information is no
longer needed with sufficient confidence. In the example shown in
Figure 7, the PCC needs to store the LSP state changes that happened
between DB Versions 43 to 46 and synchronize these changes, when
performing incremental LSP state update.
If a PCC finds out it does not have sufficient information to
complete incremental synchronization after advertising incremental
LSP State Synchronization capability, it MUST send a PCErr with
Error-type=20 and Error-value=5 'A PCC indicates to a PCE that it can
not complete the State Synchronization' (defined in [RFC 8231]), and
terminate the session. The PCC SHOULD re-establish the session with
the D bit set to 0 in the OPEN message.
The other procedures and error checks remain unchanged from the full
State Synchronization [RFC 8231].
5. PCE-Triggered Initial Synchronization
5.1. Motivation
In networks such as optical transport networks, the control channel
between network nodes can be realized through in-band overhead, thus
it has limited bandwidth. With a stateful PCE connected to the
network via one network node, it is desirable to control the timing
of PCC State Synchronization so as not to overload the low
communication channel available in the network during the initial
synchronization (be it incremental or full) when the session
restarts, when there is a comparatively large amount of control
information needing to be synchronized between the stateful PCE and
the network. The method proposed, i.e., allowing PCE to trigger the
State Synchronization, is similar to the function proposed in
Section 6 but is used in different scenarios and for different
purposes.
Crabbe, et al. Standards Track PAGE 17
RFC 8232 Optimizations of State Synchronization September 2017
5.2. PCE-Triggered Initial State Synchronization Procedure
Support of PCE-triggered initial State Synchronization is advertised
during session startup using the TRIGGERED-INITIAL-SYNC (F) bit in
the STATEFUL-PCE-CAPABILITY TLV (see Section 7).
In order to allow a stateful PCE to control the LSP-DB
synchronization after establishing a PCEP session, both PCEP speakers
MUST set the F bit to 1 in the OPEN message. If the LSP-DB-VERSION
TLV is included by both PCEP speakers and the TLV value matches, the
State Synchronization can be skipped as described in Section 3.2. If
the TLV is not included or the LSP-DB Version is mismatched, the PCE
can trigger the State Synchronization process by sending a PCUpd
message with PLSP-ID = 0 and SYNC = 1. The PCUpd message SHOULD
include an empty Explicit Route Object (ERO) (with no ERO sub-object
and object length of 4) as its intended path and SHOULD NOT include
the optional objects for its attributes for any parameter update.
The PCC MUST ignore such an update when the SYNC flag is set. If the
TRIGGERED-INITIAL-SYNC capability is not advertised by a PCE and the
PCC receives a PCUpd with the SYNC flag set to 1, the PCC MUST send a
PCErr with the SRP-ID-number of the PCUpd, Error-type=20, and
Error-value=4 'Attempt to trigger a synchronization when the PCE
triggered synchronization capability has not been advertised' (see
Section 8.1). If the TRIGGERED-INITIAL-SYNC capability is advertised
by a PCE and the PCC, the PCC MUST NOT trigger State Synchronization
on its own. If the PCE receives a PCRpt message before the PCE has
triggered the State Synchronization, the PCE MUST send a PCErr with
Error-type=20 and Error-value=3 'Attempt to trigger synchronization
before PCE trigger' (see Section 8.1).
In this way, the PCE can control the sequence of LSP synchronization
among all the PCCs that are re-establishing PCEP sessions with it.
When the capability of PCE control is enabled, only after a PCC
receives this message, it will start sending information to the PCE.
This PCE-triggering capability can be applied to both full and
incremental State Synchronization. If applied to the latter, the
PCCs only send information that PCE does not possess, which is
inferred from the LSP-DB version information exchanged in the OPEN
message (see Section 4.2 for a detailed procedure).
Once the initial State Synchronization is triggered by the PCE, the
procedures and error checks remain unchanged [RFC 8231].
If a PCC implementation that does not implement this extension should
not receive a PCUpd message to trigger State Synchronization as per
the capability advertisement, but if it were to receive it, it will
behave as per [RFC 8231].
Crabbe, et al. Standards Track PAGE 18
RFC 8232 Optimizations of State Synchronization September 2017
6. PCE-Triggered Resynchronization
6.1. Motivation
The accuracy of the computations performed by the PCE is tied to the
accuracy of the view the PCE has on the state of the LSPs.
Therefore, it can be beneficial to be able to resynchronize this
state even after the session has been established. The PCE may use
this approach to continuously sanity check its state against the
network or to recover from error conditions without having to tear
down sessions.
6.2. PCE-Triggered State Resynchronization Procedure
Support of PCE-triggered state resynchronization is advertised by
both PCEP speakers during session startup using the TRIGGERED-RESYNC
(T) bit in the STATEFUL-PCE-CAPABILITY TLV (see Section 7). The PCE
can choose to resynchronize its entire LSP-DB or a single LSP.
To trigger resynchronization for an LSP, the PCE sends a Path
Computation State Update (PCUpd) for the LSP, with the SYNC flag in
the LSP object set to 1. The PCE SHOULD NOT include any parameter
updates for the LSP, and the PCC MUST ignore such an update when the
SYNC flag is set. The PCC MUST respond with a PCRpt message with the
LSP state, SYNC flag set to 0 and MUST include the SRP-ID-number of
the PCUpd message that triggered the resynchronization. If the PCC
cannot find the LSP in its database, PCC MUST also set the R (remove)
flag [RFC 8231] in the LSP object in the PCRpt message.
The PCE can also trigger resynchronization of the entire LSP-DB. The
PCE MUST first mark all LSPs in the LSP-DB that were previously
reported by the PCC as stale, and then send a PCUpd with an LSP
object containing a PLSP-ID of 0 and with the SYNC flag set to 1.
The PCUpd message MUST include an empty ERO (with no ERO sub-object
and object length of 4) as its intended path and SHOULD NOT include
the optional objects for its attributes for any parameter update.
The PCC MUST ignore such update if the SYNC flag is set. This PCUpd
message is the trigger for the PCC to enter the synchronization phase
as described in [RFC 8231] and start sending PCRpt messages. After
the receipt of the end-of-synchronization marker, the PCE will purge
LSPs that were not refreshed. The SRP-ID-number of the PCUpd that
triggered the resynchronization SHOULD be included in each of the
PCRpt messages. If the PCC cannot resynchronize the entire LSP-DB,
the PCC MUST respond with a PCErr message with Error-type=20 and
Error-value=5 'cannot complete the State Synchronization' [RFC 8231],
and it MAY terminate the session. The PCE MUST remove the stale mark
for the LSPs that were previously reported by the PCC. Based on the
local policy, the PCE MAY reattempt synchronization at a later time.
Crabbe, et al. Standards Track PAGE 19
RFC 8232 Optimizations of State Synchronization September 2017
If the TRIGGERED-RESYNC capability is not advertised by a PCE and the
PCC receives a PCUpd with the SYNC flag set to 1, it MUST send a
PCErr with the SRP-ID-number of the PCUpd, Error-type=20, and
Error-value=4 'Attempt to trigger a synchronization when the PCE
triggered synchronization capability has not been advertised' (see
Section 8.1).
Once the state resynchronization is triggered by the PCE, the
procedures and error checks remain unchanged from the full state
synchronization [RFC 8231]. This would also include the PCE
triggering multiple state resynchronization requests while
synchronization is in progress.
If a PCC implementation that does not implement this extension should
not receive a PCUpd message to trigger resynchronization as per the
capability advertisement, but if it were to receive it, it will
behave as per [RFC 8231].
7. Advertising Support of Synchronization Optimizations
Support for each of the optimizations described in this document
requires advertising the corresponding capabilities during session
establishment time.
The STATEFUL-PCE-CAPABILITY TLV is defined in [RFC 8231]. This
document defines the following new flags in the
STATEFUL-PCE-CAPABILITY TLV:
Bit Description
------------------------- ---------------------------------
30 S bit (INCLUDE-DB-VERSION)
27 D bit (DELTA-LSP-SYNC-CAPABILITY)
26 F bit (TRIGGERED-INITIAL-SYNC)
28 T bit (TRIGGERED-RESYNC)
If the S bit (INCLUDE-DB-VERSION) is set to 1 by both PCEP speakers,
the PCC will include the LSP-DB-VERSION TLV in each LSP object. See
Section 3.2 for details.
If the D bit (DELTA-LSP-SYNC-CAPABILITY) is set to 1 by a PCEP
speaker, it indicates that the PCEP speaker allows incremental
(delta) State Synchronization. See Section 4.2 for details.
If the F bit (TRIGGERED-INITIAL-SYNC) is set to 1 by both PCEP
speakers, the PCE SHOULD trigger initial (first) State
Synchronization. See Section 5.2 for details.
Crabbe, et al. Standards Track PAGE 20
RFC 8232 Optimizations of State Synchronization September 2017
If the T bit (TRIGGERED-RESYNC) is set to 1 by both PCEP speakers,
the PCE can trigger resynchronization of LSPs at any point in the
life of the session. See Section 6.2 for details.
See Section 8.3 for IANA allocations.
8. IANA Considerations
IANA has allocated code points for the protocol elements defined in
this document.
8.1. PCEP-Error Object
IANA has allocated the following values in the "PCEP-ERROR Object
Error Types and Values" registry.
Error-Type Meaning Reference
------------------------------------------------------------
6 Mandatory Object missing [RFC 5440]
Error-value
12: LSP-DB-VERSION TLV missing This document
20 LSP State Synchronization Error [RFC 8231]
Error-value
2: LSP-DB version mismatch. This document
3: Attempt to trigger This document
synchronization before PCE
trigger.
4: Attempt to trigger a This document
synchronization when the
PCE triggered synchronization
capability has not been
advertised.
6: Received an invalid This document
LSP-DB Version Number.
7: Received an invalid This document
Speaker Entity Identifier.
Crabbe, et al. Standards Track PAGE 21
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8.2. PCEP TLV Type Indicators
IANA has allocated the following values in the "PCEP TLV Type
Indicators" registry.
Value Meaning Reference
------------------------- ----------------- -------------
23 LSP-DB-VERSION This document
24 SPEAKER-ENTITY-ID This document
8.3. STATEFUL-PCE-CAPABILITY TLV
The STATEFUL-PCE-CAPABILITY TLV is defined in [RFC 8231]. The
"STATEFUL-PCE-CAPABILITY TLV Flag Field" registry has been created to
manage the flags in the TLV. IANA has allocated the following values
in this registry.
Bit Description Reference
-------------------------- -------------------------- -------------
26 TRIGGERED-INITIAL-SYNC This document
27 DELTA-LSP-SYNC-CAPABILITY This document
28 TRIGGERED-RESYNC This document
30 INCLUDE-DB-VERSION This document
9. Manageability Considerations
All manageability requirements and considerations listed in [RFC 5440]
and [RFC 8231] apply to PCEP protocol extensions defined in this
document. In addition, requirements and considerations listed in
this section apply.
9.1. Control of Function and Policy
A PCE or PCC implementation MUST allow configuring the State
Synchronization optimization capabilities as described in this
document. The implementation SHOULD also allow the operator to
configure the Speaker Entity Identifier (Section 3.3.2). Further,
the operator SHOULD be to be allowed to trigger the resynchronization
procedures as per Section 6.2.
9.2. Information and Data Models
An implementation SHOULD allow the operator to view the stateful
capabilities advertised by each peer and the current synchronization
status with each peer. To serve this purpose, the PCEP YANG module
[PCEP-YANG] can be extended to include advertised stateful
capabilities and synchronization status.
Crabbe, et al. Standards Track PAGE 22
RFC 8232 Optimizations of State Synchronization September 2017
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 [RFC 5440].
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
[RFC 5440] and [RFC 8231].
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 [RFC 5440] and [RFC 8231] also apply to PCEP
extensions defined in this document.
The State Synchronization optimizations described in this document
can result in a reduction of the amount of data exchanged and the
time taken for a stateful PCE to be fully operational when a PCEP
session is re-established. The ability to trigger resynchronization
by the PCE can be utilized by the operator to sanity check its state
and recover from any mismatch in state without tearing down the
session.
10. Security Considerations
The security considerations listed in [RFC 8231] apply to this
document as well. However, this document also introduces some new
attack vectors. An attacker could spoof the SPEAKER-ENTITY-ID and
pretend to be another PCEP speaker. An attacker may flood the PCC
with triggered resynchronization requests at a rate that exceeds the
PCC's ability to process them by either spoofing messages or
compromising the PCE itself. The PCC can respond with a PCErr
message as described in Section 6.2 and terminate the session. Thus,
securing the PCEP session using Transport Layer Security (TLS)
[PCEPS], as per the recommendations and best current practices in
[RFC 7525], is RECOMMENDED. An administrator could also expose the
Speaker Entity Identifier as part of the certificate, for the peer
identity verification.
Crabbe, et al. Standards Track PAGE 23
RFC 8232 Optimizations of State Synchronization September 2017
11. References
11.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<https://www.rfc-editor.org/info/RFC 2119>.
[RFC 5440] Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation
Element (PCE) Communication Protocol (PCEP)", RFC 5440,
DOI 10.17487/RFC 5440, March 2009,
<https://www.rfc-editor.org/info/RFC 5440>.
[RFC 8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC 8174,
May 2017, <https://www.rfc-editor.org/info/RFC 8174>.
[RFC 8231] Crabbe, E., Minei, I., Medved, J., and R. Varga, "Path
Computation Element Communication Protocol (PCEP)
Extensions for Stateful PCE", RFC 8231,
DOI 10.17487/RFC 8231, September 2017,
<http://www.rfc-editor.org/info/RFC 8231>.
11.2. Informative References
[PCEP-YANG]
Dhody, D., Hardwick, J., Beeram, V., and j.
jefftant@gmail.com, "A YANG Data Model for Path
Computation Element Communications Protocol (PCEP)", Work
in Progress, draft-ietf-pce-pcep-yang-05, July 2017.
[PCEPS] Lopez, D., Dios, O., Wu, Q., and D. Dhody, "Secure
Transport for PCEP", Work in Progress,
draft-ietf-pce-pceps-18, September 2017.
[RFC 7525] 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/RFC 7525, May
2015, <https://www.rfc-editor.org/info/RFC 7525>.
[RFC 8051] Zhang, X., Ed. and I. Minei, Ed., "Applicability of a
Stateful Path Computation Element (PCE)", RFC 8051,
DOI 10.17487/RFC 8051, January 2017,
<https://www.rfc-editor.org/info/RFC 8051>.
Crabbe, et al. Standards Track PAGE 24
RFC 8232 Optimizations of State Synchronization September 2017
Acknowledgments
We would like to thank Young Lee, Sergio Belotti, and Cyril Margaria
for their comments and discussions.
Thanks to Jonathan Hardwick for being the document shepherd and
providing comments and guidance.
Thanks to Tomonori Takeda for the Routing Area Directorate review.
Thanks to Adrian Farrel for the TSVART review and providing detailed
comments and suggestions.
Thanks to Daniel Franke for the SECDIR review.
Thanks to Alvaro Retana, Kathleen Moriarty, and Stephen Farrell for
comments during the IESG evaluation.
Thanks to Deborah Brungard for being the responsible AD and guiding
the authors as needed.
Contributors
Gang Xie
Huawei Technologies
F3-5-B R&D Center, Huawei Industrial Base, Bantian, Longgang District
Shenzhen, Guangdong, 518129
China
Email: xiegang09@huawei.com
Crabbe, et al. Standards Track PAGE 25
RFC 8232 Optimizations of State Synchronization September 2017
Authors' Addresses
Edward Crabbe
Oracle
Email: edward.crabbe@gmail.com
Ina Minei
Google, Inc.
1600 Amphitheatre Parkway
Mountain View, CA 94043
United States of America
Email: inaminei@google.com
Jan Medved
Cisco Systems, Inc.
170 West Tasman Dr.
San Jose, CA 95134
United States of America
Email: jmedved@cisco.com
Robert Varga
Pantheon Technologies SRO
Mlynske Nivy 56
Bratislava 821 05
Slovakia
Email: robert.varga@pantheon.tech
Xian Zhang
Huawei Technologies
F3-5-B R&D Center, Huawei Industrial Base, Bantian, Longgang District
Shenzhen, Guangdong 518129
China
Email: zhang.xian@huawei.com
Dhruv Dhody
Huawei Technologies
Divyashree Techno Park, Whitefield
Bangalore, Karnataka 560066
India
Email: dhruv.ietf@gmail.com
Crabbe, et al. Standards Track PAGE 26
RFC TOTAL SIZE: 56167 bytes
PUBLICATION DATE: Friday, September 22nd, 2017
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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