Internet Engineering Task Force (IETF) A. Raghuram
Request for Comments: 8741 A. Goddard
Category: Standards Track AT&T
ISSN: 2070-1721 J. Karthik
S. Sivabalan
Cisco Systems, Inc.
M. Negi
Huawei Technologies
March 2020
Ability for a Stateful Path Computation Element (PCE) to Request and
Obtain Control of a Label Switched Path (LSP)
Abstract
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 that it is aware of but 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.
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/rfc8741.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction
2. Terminology
2.1. Requirements Language
3. LSP Control Request Flag
4. Operation
5. Security Considerations
6. IANA Considerations
7. Manageability Considerations
7.1. Control of Function and Policy
7.2. Information and Data Models
7.3. Liveness Detection and Monitoring
7.4. Verify Correct Operations
7.5. Requirements on Other Protocols
7.6. Impact on Network Operations
8. References
8.1. Normative References
8.2. Informative References
Acknowledgements
Contributors
Authors' Addresses
1. Introduction
"Path Computation Element Communication Protocol (PCEP) Extensions
for Stateful PCE" [RFC8231] specifies a set of extensions to PCEP
[RFC5440] to enable stateful control of Traffic Engineering Label
Switched Paths (TE LSPs) between and across PCEP sessions in
compliance with [RFC4657]. It includes mechanisms to synchronize LSP
state between Path Computation Clients (PCCs) and PCEs, delegate
control of LSPs to PCEs, and allow PCEs to control the timing and
sequence of path computations within and across PCEP sessions. The
stateful PCEP defines the following two useful network operations:
Delegation: As per [RFC8051], 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 [RFC8231], 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 [RFC8231]), during a
PCE failure, one of the redundant PCEs 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 can 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 [RFC8231] 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 with [RFC8231]. Ultimately, it is the PCC
that decides which PCE to delegate the orphaned LSP to.
This specification provides a simple extension that allows a PCE to
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 [RFC8231] unless
explicitly set aside in this document.
2. Terminology
This document uses the following terms defined in [RFC5440]:
PCC: Path Computation Client
PCE: Path Computation Element
PCEP: Path Computation Element communication Protocol
This document uses the following terms defined in [RFC8231]:
PCRpt: Path Computation State Report message
PCUpd: Path Computation Update Request message
PLSP-ID: A PCEP-specific identifier for the LSP
SRP: Stateful PCE Request Parameters
Readers of this document are expected to have some familiarity with
[RFC8231].
2.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. LSP Control Request Flag
The Stateful PCE Request Parameters (SRP) object is defined in
Section 7.2 of [RFC8231] and includes a Flags field.
A new "LSP Control Request" flag (30), also called the C flag, is
introduced in the SRP object. In 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 value other than 0 and 0xFFFFF is used to
identify the LSP for which the PCE requests control. A 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.
The C flag is ignored in case the R flag [RFC8281] in the SRP object
is set.
4. Operation
During normal operation, a PCC that wishes to delegate the control of
an LSP sets the Delegate (D) flag (Section 7.3 of [RFC8231]) to 1 in
all PCRpt messages pertaining to the LSP. The PCE confirms the
delegation by setting the 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 the D flag to 0 in PCRpt messages pertaining to
the LSP. If the PCE wishes to relinquish the control of the LSP, it
sets the 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 the C flag set to 1 in the SRP object. The LSP for which the
PCE requests control is identified by the PLSP-ID in the associated
LSP object. A PLSP-ID value 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 per [RFC8231]).
The D flag and C flag are mutually exclusive in a PCUpd message. The
PCE MUST NOT send a control request for the LSP that 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 the 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 the D
flag set to 1 and 0, respectively, in accordance with stateful PCEP
[RFC8231]. 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 an exponentially increasing timer. Note that, if
the PCUpd message with the 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 [RFC8231] (a
PCErr with Error-type=19 (Invalid Operation) and error-value 1
(Attempted LSP Update Request for a non-delegated LSP)).
As per [RFC8231], 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 may receive an LSP control request: a PCUpd
message with the C flag set and the D flag unset. The legacy
implementation would ignore the C flag and trigger the error
condition for the D flag, as specified in [RFC8231] (i.e., 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 a
PLSP-ID value of 0, the error condition, as specified in [RFC8231],
(i.e., a PCErr with Error-type=19 (Invalid Operation) and error-value
3 (Attempted LSP Update Request for an LSP identified by an unknown
PLSP-ID)) would be triggered.
[RFC8281] 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 [RFC8281].
5. Security Considerations
The security considerations listed in [RFC8231] and [RFC8281] apply
to this document as well. However, this document also introduces a
new attack vector. An attacker may flood the PCC with requests to
delegate all of its LSPs at a rate that 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 [RFC8231], a
local policy at the 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 that would
in turn respond with an error, as described in Section 4. So, a PCE
would learn this fact only when it wants to take control over an LSP.
A PCE might also be susceptible to downgrade attacks by falsifying
the error condition.
As per [RFC8231], 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) [RFC8253], as per the recommendations and best
current practices in BCP 195 [RFC7525] (unless explicitly excluded in
[RFC8253]).
6. IANA Considerations
IANA has allocated the following code point in the "SRP Object Flag
Field" subregistry in the "Path Computation Element Protocol (PCEP)
Numbers" registry.
+-----+---------------------+-----------+
| Bit | Description | Reference |
+=====+=====================+===========+
| 30 | LSP Control Request | RFC 8741 |
+-----+---------------------+-----------+
Table 1
7. Manageability Considerations
All manageability requirements and considerations listed in [RFC5440]
and [RFC8231] apply to PCEP extensions defined in this document. In
addition, requirements and considerations listed in this section
apply.
7.1. Control of Function and Policy
A PCC implementation SHOULD allow the operator to configure the
policy rules that specify the conditions under 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 for the delegation
requests received 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.
7.2. Information and Data Models
The PCEP YANG module [PCEP-YANG] could be extended to include a
mechanism to trigger the LSP control request.
7.3. Liveness Detection and Monitoring
Mechanisms defined in this document do not imply any new liveness
detection and monitoring requirements in addition to those already
listed in [RFC5440].
7.4. Verify Correct Operations
Mechanisms defined in this document do not imply any new operation
verification requirements in addition to those already listed in
[RFC5440] and [RFC8231].
7.5. Requirements on Other Protocols
Mechanisms defined in this document do not imply any new requirements
on other protocols.
7.6. Impact on Network Operations
Mechanisms defined in [RFC5440] and [RFC8231] 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.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC5440] Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation
Element (PCE) Communication Protocol (PCEP)", RFC 5440,
DOI 10.17487/RFC5440, March 2009,
<https://www.rfc-editor.org/info/rfc5440>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8231] Crabbe, E., Minei, I., Medved, J., and R. Varga, "Path
Computation Element Communication Protocol (PCEP)
Extensions for Stateful PCE", RFC 8231,
DOI 10.17487/RFC8231, September 2017,
<https://www.rfc-editor.org/info/rfc8231>.
[RFC8281] Crabbe, E., Minei, I., Sivabalan, S., and R. Varga, "Path
Computation Element Communication Protocol (PCEP)
Extensions for PCE-Initiated LSP Setup in a Stateful PCE
Model", RFC 8281, DOI 10.17487/RFC8281, December 2017,
<https://www.rfc-editor.org/info/rfc8281>.
8.2. Informative References
[PCEP-YANG]
Dhody, D., Hardwick, J., Beeram, V., and J. Tantsura, "A
YANG Data Model for Path Computation Element
Communications Protocol (PCEP)", Work in Progress,
Internet-Draft, draft-ietf-pce-pcep-yang-13, 31 October
2019,
<https://tools.ietf.org/html/draft-ietf-pce-pcep-yang-13>.
[RFC4657] Ash, J., Ed. and J.L. Le Roux, Ed., "Path Computation
Element (PCE) Communication Protocol Generic
Requirements", RFC 4657, DOI 10.17487/RFC4657, September
2006, <https://www.rfc-editor.org/info/rfc4657>.
[RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
2015, <https://www.rfc-editor.org/info/rfc7525>.
[RFC8051] Zhang, X., Ed. and I. Minei, Ed., "Applicability of a
Stateful Path Computation Element (PCE)", RFC 8051,
DOI 10.17487/RFC8051, January 2017,
<https://www.rfc-editor.org/info/rfc8051>.
[RFC8253] Lopez, D., Gonzalez de Dios, O., Wu, Q., and D. Dhody,
"PCEPS: Usage of TLS to Provide a Secure Transport for the
Path Computation Element Communication Protocol (PCEP)",
RFC 8253, DOI 10.17487/RFC8253, October 2017,
<https://www.rfc-editor.org/info/rfc8253>.
Acknowledgements
Thanks to Jonathan Hardwick for reminding 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 his Security Directorate review.
Thanks to Francesca Palombini for GENART review.
Thanks to Benjamin Kaduk, Martin Vigoureux, Alvaro Retana, and Barry
Leiba for IESG reviews.
Contributors
The following people contributed substantially to the content of this
document and should be considered coauthors:
Dhruv Dhody
Huawei Technologies
Divyashree Techno Park, Whitefield
Bangalore 560066
Karnataka
India
Email: dhruv.ietf@gmail.com
Jon Parker
Cisco Systems, Inc.
2000 Innovation Drive
Kanata Ontario K2K 3E8
Canada
Email: jdparker@cisco.com
Chaitanya Yadlapalli
AT&T
200 S Laurel Avenue
Middletown, NJ 07748
United States of America
Email: cy098@att.com
Authors' Addresses
Aswatnarayan Raghuram
AT&T
200 S Laurel Avenue
Middletown, NJ 07748
United States of America
Email: ar2521@att.com
Al Goddard
AT&T
200 S Laurel Avenue
Middletown, NJ 07748
United States of America
Email: ag6941@att.com
Jay Karthik
Cisco Systems, Inc.
125 High Street
Boston, Massachusetts 02110
United States of America
Email: jakarthi@cisco.com
Siva Sivabalan
Cisco Systems, Inc.
2000 Innovation Drive
Kanata Ontario K2K 3E8
Canada
Email: msiva@cisco.com
Mahendra Singh Negi
Huawei Technologies
Divyashree Techno Park, Whitefield
Bangalore 560066
Karnataka
India
Email: mahend.ietf@gmail.com
