Ability for a Stateful Path Computation Element (PCE) to request and obtain control of a Label Switched Path (LSP)
AT&T
200 S Laurel Aevenue
Middletown
NJ
07748
USA
ar2521@att.com
AT&T
200 S Laurel Aevenue
Middletown
NJ
07748
USA
ag6941@att.com
Cisco Systems, Inc.
125 High Street
Boston
Massachusetts
02110
USA
jakarthi@cisco.com
Cisco Systems, Inc.
2000 Innovation Drive
Kanata
Ontario
K2K 3E8
Canada
msiva@cisco.com
Huawei Technologies
Divyashree Techno Park, Whitefield
Bangalore
Karnataka
560066
India
mahend.ietf@gmail.com
PCE Working Group
A Stateful Path Computation Element (PCE) retains information about the placement of Multiprotocol Label Switching (MPLS) Traffic Engineering Label Switched Paths (TE LSPs). When a PCE has stateful control over LSPs it may send indications to LSP head-ends to modify the attributes (especially the paths) of the LSPs. A Path Computation Client (PCC) that has set up LSPs under local configuration may delegate control of those LSPs to a stateful PCE.
There are use-cases in which a stateful PCE may wish to obtain control of locally configured LSPs of which it is aware but that have not been delegated to the PCE.
This document describes an extension to the Path Computation Element
communication Protocol (PCEP) to enable a PCE to make requests for
such control.
Stateful Path Computation Element (PCE) communication Protocol (PCEP) extensions specifies a set of extensions to PCEP to enable stateful control of Traffic Engineering Label Switched Paths (TE LSPs) between and across PCEP sessions in compliance with . It includes mechanisms to synchronize LSP state between Path Computation Clients (PCCs) and PCEs, delegate control of LSPs to PCE, and PCE-control of timing and sequence of path computations within and across PCEP sessions. The stateful PCEP defines the following two useful network operations:
Delegation: As per , an operation to grant a PCE temporary rights to modify a
subset of LSP parameters on one or more LSPs of a PCC. LSPs are
delegated from a PCC to a PCE and are referred to as "delegated"
LSPs.
Revocation: As per , an operation performed by a PCC on a previously
delegated LSP. Revocation revokes the rights granted to the PCE
in the delegation operation.
For Redundant Stateful PCEs (section 5.7.4. of ), during a PCE failure, one of the redundant PCE might want to request to take control over an LSP. The redundant PCEs may use a local policy or a proprietary election mechanism to decide which PCE would take control. In this case, a mechanism is needed for a stateful PCE to request control of one or more LSPs from a PCC, so that a newly elected primary PCE can request to take over control.
In case of virtualized PCEs (vPCEs) running in virtual network function (VNF) mode, as the computation load in the network increases, a new instance of vPCE could be instantiated to balance the current load. The PCEs could use a proprietary algorithm to decide which LSPs to be assigned to the new vPCE. Thus, having a mechanism for the PCE to request control of some LSPs is needed.
In some deployments, the operator would like to use stateful PCE for global optimization algorithms but would still like to keep the control of the LSP at the PCC. In such cases, a stateful PCE could request to take control during the global optimization and return the delegation once done.
Note that specifies a mechanism for a PCC to delegate an orphaned LSP to another PCE. The mechanism defined in this document can be used in conjunction to . Ultimately, it is the PCC that decides which PCE to delegate the orphaned LSP to.
This specification provides a simple extension: by using it a PCE can request control of one or more LSPs from any PCC over the stateful PCEP session. The procedures for granting and relinquishing control of the LSPs are specified in accordance with the specification unless explicitly set aside in this document.
This document uses the following terms defined in :
Path Computation Client.
Path Computation Element.
Path Computation Element communication Protocol.
This document uses the following terms defined in :
Path Computation State Report message.
Path Computation Update Request message.
A PCEP-specific identifier for the LSP.
Stateful PCE Request Parameters.
Readers of this document are expected to have some familiarity with .
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 when, and only when, they
appear in all capitals, as shown here.
The Stateful PCE Request Parameters (SRP) object is defined in Section 7.2 of and it includes a Flags field.
A new flag, the "LSP-Control Request Flag" (C) - TBD, is introduced in the SRP object. On a PCUpd message, a PCE sets the C Flag to 1 to indicate that it wishes to gain control of LSPs. The LSPs are identified by the PLSP-ID in the LSP object following the SRP object. A PLSP-ID of value other than 0 and 0xFFFFF is used to identify the LSP for which the PCE requests control. The PLSP-ID value of 0 indicates that the PCE is requesting control of all LSPs originating from the PCC that it wishes to delegate. The C Flag has no meaning in other PCEP messages that carry SRP objects and for which the C flag MUST be set to 0 on transmission and MUST be ignored on receipt.
During normal operation, a PCC that wishes to delegate the control of an LSP sets the D Flag (delegate, Section 7.3 of ) to 1 in all PCRpt messages pertaining to the LSP. The PCE confirms the delegation by setting D Flag to 1 in all PCUpd messages pertaining to the LSP. The PCC revokes the control of the LSP from the PCE by setting D Flag to 0 in PCRpt messages pertaining to the LSP. If the PCE wishes to relinquish the control of the LSP, it sets D Flag to 0 in all PCUpd messages pertaining to the LSP.
If a PCE wishes to gain control over an LSP, it sends a PCUpd message with C Flag set to 1 in SRP object. The LSP for which the PCE requests control is identified by the PLSP-ID in the associated LSP object. The PLSP-ID of 0 indicates that the PCE wants control over all LSPs originating from the PCC. An implementation of this feature needs to make sure to check for the LSP control feature (C flag set to 1) before any check for PLSP-ID (as prescribed in ). The D Flag and C Flag are mutually exclusive in a PCUpd message. The PCE MUST NOT send a control request for the LSP which is already delegated to the PCE, i.e. if the D Flag is set in the PCUpd message, then the C Flag MUST NOT be set. If a PCC receives a PCUpd message with D Flag set in the LSP object (i.e. LSP is already delegated) and
the C Flag is also set (i.e. PCE is making a control request), the PCC MUST ignore the C Flag. A PCC can decide to delegate the control of the LSP at its own discretion. If the PCC grants or denies the control, it sends a PCRpt message with D Flag set to 1 and 0 respectively in accordance with stateful PCEP . If the PCC does not grant the control, it MAY choose to not respond, and the PCE MAY choose to retry requesting the control preferably using exponentially increasing timer. Note that, if the PCUpd message with C Flag set is received for a currently non-delegated LSP (for which the PCE is requesting delegation), this MUST NOT trigger the error handling as specified in (a PCErr with Error-type=19 (Invalid Operation)
and error-value 1 (Attempted LSP Update Request for a non-delegated
LSP)).
As per , a PCC cannot delegate an LSP to more than one PCE at any time. If a PCE requests control of an LSP that has already been delegated by the PCC to another PCE, the PCC MAY ignore the request, or MAY revoke
the delegation to the first PCE before delegating it to the second. This choice is a matter of local policy.
It should be noted that a legacy implementation of PCC that does not
support this extension would receive an LSP control request: PCUpd message with C flag set and D flag (delegate) unset, it would ignore the C flag and trigger the error condition for the D flag as specified in (a PCErr with Error-type=19 (Invalid Operation) and
error-value 1 (Attempted LSP Update Request for a non-delegated LSP)). Further, in case of PLSP-ID of 0, the error condition as specified
in (a PCErr with Error-type=19 (Invalid Operation) and
error-value 3 (Attempted LSP Update Request for an LSP identified by an unknown PSP-ID)) would be triggered.
describes the setup,
maintenance and teardown of PCE-initiated LSPs under the stateful PCE
model. It also specifies how a PCE may obtain control over an orphaned LSP
that was PCE-initiated. A PCE implementation can apply the mechanism described
in this document in conjunction with those in .
[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
. 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 , "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".
The PCE function was developed in the ONOS open source platform. This extension was implemented on a private version as a proof of concept to enable multi-instance support.
Organization: Huawei
Implementation: Huawei's PoC based on ONOS
Description: PCEP as a southbound plugin was added to ONOS. To support multi-instance ONOS deployment in a cluster, this extension in PCEP is used. Refer https://wiki.onosproject.org/display/ONOS/PCEP+Protocol
Maturity Level: Prototype
Coverage: Full
Contact: satishk@huawei.com
The security considerations listed in and
apply to this document as well. However, this document also
introduces a new attack vector. An attacker may flood the PCC with request to delegate all of its LSPs
at a rate which exceeds the PCC's ability to process them, either by spoofing messages or by compromising the PCE itself.
The PCC SHOULD be configured with a threshold rate for the delegation requests received from the PCE. If the threshold is reached, it is RECOMMENDED to log the issue.
A PCC is the ultimate arbiter of delegation. As per , a local policy at PCC is used to influence the delegation. A PCC can also revoke the delegation at any time. A PCC need not blindly trust the control requests and SHOULD take local policy and other factors into consideration before honoring the request.
Note that, a PCE may not be sure if a PCC supports this feature. A PCE would try sending a control request to a 'legacy' PCC, which would in turn respond with an error as described in . So a PCE would learn this fact only when it wants to take control over an LSP. A PCE might also be susceptible to a downgrade attacks by falsifying the error condition.
As per , it is RECOMMENDED
that these PCEP extensions only be activated on authenticated and
encrypted sessions across PCEs and PCCs belonging to the same
administrative authority, using Transport Layer Security (TLS)
, as per the recommendations and best current practices in
BCP 195 (unless explicitly excluded in ).
IANA maintains a registry called the "Path Computation Element Protocol (PCEP) Numbers" registry. It contains a subregistry called
the "SRP Object Flag Field" registry. This document requests IANA to allocate following code point in the "SRP Object Flag Field" subregistry.
Bit
Description
Reference
TBD
LSP-Control Request Flag
This document
All manageability requirements and considerations listed in
and
apply to PCEP protocol extensions defined in this document. In addition, requirements and considerations listed in this section apply.
A PCC implementation SHOULD allow the operator to configure the policy based on which it honors the request to control the LSPs. This includes the handling of the case where an LSP
control request is received for an LSP that is currently delegated to some other PCE. A PCC implementation SHOULD also allow the operator to configure the threshold rate based on which it accepts the delegation requests from the PCE.
Further, the operator MAY be allowed to trigger the LSP control request for a particular LSP at the PCE.
A PCE implementation SHOULD also allow the operator to configure an exponentially increasing timer to retry the control requests for which the PCE did not get a response.
The PCEP YANG module could be extended to include mechanism to trigger the LSP control request.
Mechanisms defined in this document do not imply any new liveness detection and monitoring requirements in addition to those already listed in .
Mechanisms defined in this document do not imply any new operation verification requirements in addition to those already listed in
and .
Mechanisms defined in this document do not imply any new requirements on other protocols.
Mechanisms defined in
and
also apply to PCEP extensions defined in this document.
Further, the mechanism described in this document can help the operator to request control of the LSPs at a particular PCE.
Thanks to Jonathan Hardwick to remind the authors to not use suggested values in IANA section.
Thanks to Adrian Farrel, Haomian Zheng and Tomonori Takeda for their valuable comments.
Thanks to Shawn M. Emery for security directorate's review.
Thanks to Francesca Palombini for GENART review.
Thanks to Benjamin Kaduk, Martin Vigoureux, Alvaro Retana, and Barry Leiba for IESG reviews.