| PCE Working Group | E. Crabbe |
| Internet-Draft | Oracle |
| Intended status: Standards Track | I. Minei |
| Expires: September 28, 2017 | Google, Inc. |
| J. Medved | |
| Cisco Systems, Inc. | |
| R. Varga | |
| Pantheon Technologies SRO | |
| X. Zhang | |
| D. Dhody | |
| Huawei Technologies | |
| March 27, 2017 |
Optimizations of Label Switched Path State Synchronization Procedures for a Stateful PCE
draft-ietf-pce-stateful-sync-optimizations-10
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, PCE and path computation clients (PCCs), and between 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.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].
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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 http://datatracker.ietf.org/drafts/current/.
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This Internet-Draft will expire on September 28, 2017.
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 (http://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.
The Path Computation Element Communication Protocol (PCEP) provides mechanisms for Path Computation Elements (PCEs) to perform path computations in response to Path Computation Clients (PCCs) requests.
[I-D.ietf-pce-stateful-pce] 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, PCE and PCC, and between cooperating PCEs. [I-D.ietf-pce-stateful-pce] describes the basic mechanism for state synchronization. This document specifies following optimizations for state synchronization and the corresponding PCEP procedures and extensions:
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.
This document uses the following terms defined in [RFC5440]: PCC, PCE, PCEP Peer.
This document uses the following terms defined in [RFC8051]: Stateful PCE, Delegation, LSP State Database.
This document uses the following terms defined in [I-D.ietf-pce-stateful-pce]: Redelegation Timeout Interval, LSP State Report, LSP Update Request.
Within this document, when describing PCE-PCE communications, one of the PCEs fills the role of a PCC. This provides a saving in documentation without loss of function.
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 ([RFC5440]). [I-D.ietf-pce-stateful-pce] 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.
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 state database, using a version tracking field called the LSP State Database 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 State Database Version Number is to be included when the LSPs are reported to the PCE. The LSP State Database 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 state database. The LSP State Database Version Number MUST start at 1 and may wrap around. 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 PCErr with Error-type 20 Error-value TBD6 (suggested value - 6) 'Received an invalid LSP DB Version Number', and close the PCEP session. Operations that trigger a change to the local LSP state database 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 State Database Version Number when the 'Redelegation Timeout Interval' timer expires (see [I-D.ietf-pce-stateful-pce] 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 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 TBD1 (suggested 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 state database 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 State Database Version Number received on an LSP State Report from the PCC in the previous PCEP session. If a PCC's LSP State Database 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 State Database Version Number. If a PCEP speaker's LSP state database 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 [I-D.ietf-pce-stateful-pce]. Otherwise, the PCC MUST perform full state synchronization (see [I-D.ietf-pce-stateful-pce]) 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 database 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 the PCC as per [I-D.ietf-pce-stateful-pce], the PCE MUST send back a PCErr with Error-Type 20 Error-Value TBD2 (suggested value - 2) 'LSP Database 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 database 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 database, the PCE MUST update the LSP database 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 Path Computation State Report (PCRpt) message with an LSP object containing a PLSP-ID of 0 and with the SYNC flag set to 0 ([I-D.ietf-pce-stateful-pce]). 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 database 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 State Database 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.
+-+-+ +-+-+
|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 state database 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.
+-+-+ +-+-+
|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 State Database Version Number information.
+-+-+ +-+-+
|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 PCC
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:
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.
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 [I-D.ietf-pce-stateful-pce]. 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 [RFC5440] procedure remains unchanged.
A new INCLUDE-DB-VERSION (S) bit is added in the stateful capabilities TLV (see Section 7 for details).
The LSP State Database 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 (PCUpd, PcReq, 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=TBD5 | Length=8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LSP State DB Version Number |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: LSP-DB-VERSION TLV format
The type of the TLV is TBD5 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 State DB Version Number.
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 PCEP session is terminated. This may be a local policy decision. The new IP address is associated with the speaker entity identifier for future either way. In the latter case when PCEP session is re-established, it would be correctly associated with 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=TBD13 | Length (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
// Speaker Entity Identifier //
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: SPEAKER-ENTITY-ID TLV format
The type of the TLV is TBD13 and it has a variable length, which MUST be greater than 0. The Value is padded to 4-octet alignment. The padding is not included in the Length field. The value contains the entity identifier of the 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, such as a 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 Autonomous System number, 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 Error-value TBD7 (suggested value - 7) 'Received an invalid Speaker Entity Identifier', and close the PCEP session.
[I-D.ietf-pce-stateful-pce] 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 change in the network between PCEP session restart, this section defines a mechanism for incremental (Delta) LSP Database (LSP-DB) synchronization.
According to [I-D.ietf-pce-stateful-pce], if a PCE restarts and its LSP-DB survived, PCCs with mismatched LSP State Database 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.
+-----+
| PCE |
+-----+
/
/
/
/
+------+ +------+
| PCC1 |------------| PCC2 |
+------+ +------+
| |
| |
+------+ +------+
| PCC3 |------------| PCC4 |
+------+ +------+
Figure 6: Topology Example
Assuming 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 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 PCEP session is down.
An incremental (Delta) LSP Database (LSP-DB) state synchronization is described in this section, where only the LSPs underwent state change are synchronized between the session restart. This may include new/modified/deleted LSPs.
[I-D.ietf-pce-stateful-pce] 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.
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 D flag and S flag is set to 1 by both PCC and PCE. Other combinations of D and S flags setting by PCC and PCE result in full LSP-DB synchronization procedure as described in [I-D.ietf-pce-stateful-pce]. 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.
+-+-+ +-+-+
|PCC| |PCE|
+-+-+ +-+-+
| |
|--Open--, |
| DBv=46 \ ,---Open--|
| S=1 \ / DBv=42 |
| D=1 \/ S=1 |
| /\ D=1 |
| / \ |
| / `-------->| (Expect Delta sync)
(Do sync)|<--------` | (DONOT 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 State Database Version Number is incremented each time a change is made to the PCC's local LSP State Database. 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 State Database Version Number can wrap around, and in which case the incremental state sync would also wrap till the latest 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 Database 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 PCE went down. So a PCC implementation MAY choose to store the LSP State Database 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 State Database 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 Version 43 to 46 and synchronizes 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 [I-D.ietf-pce-stateful-pce]) 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 ([I-D.ietf-pce-stateful-pce]).
In networks such as optical transport networks, the control channel between network nodes can be realized through in-band overhead thus 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 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.
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 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 mis-matched, 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 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 TBD4 (suggested value - 4) 'Attempt to trigger synchronization when the TRIGGERED-SYNC 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 TBD3 (suggested 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 detailed procedure).
Once the initial state synchronization is triggered by the PCE, the procedures and error checks remain unchanged ([I-D.ietf-pce-stateful-pce]).
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 [I-D.ietf-pce-stateful-pce].
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 re-synchronize 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.
Support of PCE-triggered state re-synchronization 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 re-synchronize its entire LSP database or a single LSP.
To trigger re-synchronization 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 [I-D.ietf-pce-stateful-pce] in the LSP object in the PCRpt message.
The PCE can also trigger re-synchronization of the entire LSP database. The PCE MUST first mark all LSPs in the LSP database 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 [I-D.ietf-pce-stateful-pce] and start sending PCRpt messages. After the receipt of the end-of-synchronization marker, the PCE will purge LSPs which were not refreshed. The SRP-ID-number of the PCUpd that triggered the re-synchronization SHOULD be included in each of the PCRpt messages. If the PCC cannot re-synchronize the entire LSP database, the PCC MUST respond with PCErr message with Error-type 20 Error-value 5 'cannot complete the state synchronization' [I-D.ietf-pce-stateful-pce], and MAY terminate the session. The PCE MUST remove the stale mark for the LSP that were previously reported by the PCC. Based on the local policy, the PCE MAY reattempt synchronization at a later time.
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 TBD4 (suggested value - 4) 'Attempt to trigger synchronization when the TRIGGERED-SYNC capability has not been advertised' (see Section 8.1).
Once the state re-synchronization is triggered by the PCE, the procedures and error checks remain unchanged from the full state synchronization ([I-D.ietf-pce-stateful-pce]). This would also include PCE triggering multiple state re-synchronization requests while synchronization is in progress.
If a PCC implementation that does not implement this extension should not receive a PCUpd message to trigger re-synchronization as per the capability advertisement, but if it were to receive it, it will behave as per [I-D.ietf-pce-stateful-pce].
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 [I-D.ietf-pce-stateful-pce]. This document defines following new flags in the STATEFUL-PCE-CAPABILITY TLV:
| Bit | Description |
|---|---|
| TBD9 (suggested value 30) | S bit (INCLUDE-DB-VERSION) |
| TBD10 (suggested value 27) | D bit (DELTA-LSP-SYNC-CAPABILITY) |
| TBD11 (suggested value 26) | F bit (TRIGGERED-INITIAL-SYNC) |
| TBD12 (suggested value 28) | T bit (TRIGGERED-RESYNC) |
If the S (INCLUDE-DB-VERSION) bit 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 (DELTA-LSP-SYNC-CAPABILITY) bit 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 (TRIGGERED-INITIAL-SYNC) bit is set to 1 by both PCEP Speakers, the PCE SHOULD trigger initial (first) state synchronization. See Section 5.2 for details.
If the T (TRIGGERED-RESYNC) bit is set to 1 by both PCEP Speakers, the PCE can trigger re-synchronization of LSPs at any point in the life of the session. See Section 6.2 for details.
See Section 8.3 for IANA allocations.
This document requests IANA actions to allocate code points for the protocol elements defined in this document.
IANA is requested to make the following allocation in the "PCEP-ERROR Object Error Types and Values" registry.
Error-Type Meaning Reference
6 Mandatory Object missing [RFC5440]
Error-Value= TBD1(suggested This document
value 12): LSP-DB-VERSION TLV
missing
20 LSP State synchronization [I-D.ietf-pce-stateful-pce]
error
Error-Value= TBD2(suggested This document
value 2): LSP Database version
mismatch.
Error-Value=TBD3(suggested This document
value 3): Attempt to trigger
synchronization before PCE
trigger.
Error-Value=TBD4(suggested This document
value 4): Attempt to trigger a
synchronization when the
PCE triggered synchronization
capability has not been
advertised.
Error-Value=TBD6(suggested This document
value 6): Received an invalid
LSP DB Version Number.
Error-Value=TBD7(suggested This document
value 7): Received an invalid
Speaker Entity Identifier.
IANA is requested to make the following allocation in the "PCEP TLV Type Indicators" registry.
| Value | Meaning | Reference |
|---|---|---|
| TBD5(suggested value 23) | LSP-DB-VERSION | This document |
| TBD13(suggested value 24) | SPEAKER-ENTITY-ID | This document |
The STATEFUL-PCE-CAPABILITY TLV is defined in [I-D.ietf-pce-stateful-pce] and a registry is requested to be created to manage the flags in the TLV. IANA is requested to make the following allocation in the aforementioned registry.
| Bit | Description | Reference |
|---|---|---|
| TBD11 (suggested value 26) | TRIGGERED-INITIAL-SYNC | This document |
| TBD10 (suggested value 27) | DELTA-LSP-SYNC-CAPABILITY | This document |
| TBD12 (suggested value 28) | TRIGGERED-RESYNC | This document |
| TBD9 (suggested value 30) | INCLUDE-DB-VERSION | This document |
All manageability requirements and considerations listed in [RFC5440] and [I-D.ietf-pce-stateful-pce] apply to PCEP protocol extensions defined in this document. In addition, requirements and considerations listed in this section apply.
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 re-synchronization procedures as per Section 6.2.
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 [I-D.ietf-pce-pcep-yang] can be extended to include advertised stateful capabilities, and synchronization status.
Mechanisms defined in this document do not imply any new liveness detection and monitoring requirements in addition to those already listed in [RFC5440].
Mechanisms defined in this document do not imply any new operation verification requirements in addition to those already listed in [RFC5440] and [I-D.ietf-pce-stateful-pce].
Mechanisms defined in this document do not imply any new requirements on other protocols.
Mechanisms defined in [RFC5440] and [I-D.ietf-pce-stateful-pce] 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 re-synchronization 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.
The security considerations listed in [I-D.ietf-pce-stateful-pce] 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 re-synchronization request at a rate which exceeds the PCC's ability to process them, either by spoofing messages or by compromising the PCE itself. The PCC can respond with PCErr message as described in Section 6.2 and terminate the session. Thus securing the PCEP session using Transport Layer Security (TLS) [I-D.ietf-pce-pceps], as per the recommendations and best current practices in [RFC7525], is RECOMMENDED. An administrator could also expose the speaker entity id as part of the certificate, for the peer identity verification.
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 provide comments and guidance.
Thanks to Tomonori Takeda for Routing Area Directorate review.
Thanks to Adrian Farrel for TSVART review and providing detailed comments and suggestions.
Thanks to Daniel Franke for 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.
Gang Xie
Huawei Technologies
F3-5-B R&D Center, Huawei Industrial Base, Bantian, Longgang District
Shenzhen, Guangdong, 518129
P.R. China
Email: xiegang09@huawei.com
| [I-D.ietf-pce-stateful-pce] | Crabbe, E., Minei, I., Medved, J. and R. Varga, "PCEP Extensions for Stateful PCE", Internet-Draft draft-ietf-pce-stateful-pce-18, December 2016. |
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |
| [RFC5440] | Vasseur, JP. and JL. Le Roux, "Path Computation Element (PCE) Communication Protocol (PCEP)", RFC 5440, DOI 10.17487/RFC5440, March 2009. |
| [RFC7525] | Sheffer, Y., Holz, R. and P. Saint-Andre, "Recommendations for Secure Use of Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May 2015. |
| [RFC8051] | Zhang, X. and I. Minei, "Applicability of a Stateful Path Computation Element (PCE)", RFC 8051, DOI 10.17487/RFC8051, January 2017. |
| [I-D.ietf-pce-pcep-yang] | Dhody, D., Hardwick, J., Beeram, V. and j. jefftant@gmail.com, "A YANG Data Model for Path Computation Element Communications Protocol (PCEP)", Internet-Draft draft-ietf-pce-pcep-yang-02, March 2017. |
| [I-D.ietf-pce-pceps] | Lopez, D., Dios, O., Wu, W. and D. Dhody, "Secure Transport for PCEP", Internet-Draft draft-ietf-pce-pceps-11, January 2017. |