PCEP path setup type: Conveying Path Setup Type in PCE Communication Protocol (PCEP) Messages
How PCEP peers announce and select the method used to set up a traffic engineering path, beyond RSVP-TE.
Sivabalan, S., Tantsura, J., Minei, I., Varga, R., and J. Hardwick, "Conveying Path Setup Type in PCE Communication Protocol (PCEP) Messages", RFC 8408, DOI 10.17487/RFC8408, July 2018, <https://www.rfc-editor.org/info/rfc8408>.
@misc{rfc8408,
series = {Request for Comments},
number = 8408,
howpublished = {RFC 8408},
publisher = {RFC Editor},
doi = {10.17487/RFC8408},
url = {https://www.rfc-editor.org/info/rfc8408},
author = {S. Sivabalan and J. Tantsura and I. Minei and R. Varga and J. Hardwick},
title = {{Conveying Path Setup Type in PCE Communication Protocol (PCEP) Messages}},
pagetotal = 12,
year = 2018,
month = jul,
} RFC text
Contents
Internet Engineering Task Force (IETF) S. Sivabalan
Request for Comments: 8408 Cisco Systems, Inc.
Category: Standards Track J. Tantsura
ISSN: 2070-1721 Nuage Networks
I. Minei
Google, Inc.
R. Varga
Pantheon Technologies SRO
J. Hardwick
Metaswitch Networks
July 2018
Conveying Path Setup Type
in PCE Communication Protocol (PCEP) Messages
Abstract
A Path Computation Element (PCE) can compute Traffic Engineering (TE)
paths through a network; these paths are subject to various
constraints. Currently, TE paths are Label Switched Paths (LSPs)
that are set up using the RSVP-TE signaling protocol. However, other
TE path setup methods are possible within the PCE architecture. This
document proposes an extension to the PCE Communication Protocol
(PCEP) to allow support for different path setup methods over a given
PCEP session.
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/rfc8408.
Copyright Notice
Copyright (c) 2018 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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Path Setup Type Capability TLV . . . . . . . . . . . . . . . 4
4. Path Setup Type TLV . . . . . . . . . . . . . . . . . . . . . 6
5. Operation . . . . . . . . . . . . . . . . . . . . . . . . . . 7
6. Manageability Considerations . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
8.1. Additions to PCEP TLV Type Indicators Registry . . . . . 9
8.2. New PCEP Path Setup Types Registry . . . . . . . . . . . 9
8.3. Additions to PCEP-ERROR Object Error Types and Values
Registry . . . . . . . . . . . . . . . . . . . . . . . . 10
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
9.1. Normative References . . . . . . . . . . . . . . . . . . 10
9.2. Informative References . . . . . . . . . . . . . . . . . 11
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 11
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
[RFC5440] describes the PCE Communication Protocol (PCEP) for
communication between a Path Computation Client (PCC) and a Path
Computation Element (PCE) or between a PCE and a PCE. A PCC
requests, from a PCE, a path subject to various constraints and
optimization criteria. The PCE responds to the PCC with a hop-by-hop
path in an Explicit Route Object (ERO). The PCC uses the ERO to set
up the path in the network.
[RFC8231] specifies extensions to PCEP that allow a PCC to delegate
its LSPs to a PCE. The PCE can then update the state of LSPs
delegated to it. In particular, the PCE may modify the path of an
LSP by sending a new ERO. The PCC uses this ERO to reroute the LSP
in a make-before-break fashion. [RFC8281] specifies a mechanism that
allows a PCE to dynamically instantiate an LSP on a PCC by sending
the ERO and the characteristics of the LSP. The PCC creates the LSP
using the ERO and other attributes sent by the PCE.
So far, PCEP and its extensions have assumed that the TE paths are
label switched and are established via the RSVP-TE signaling
protocol. However, other methods of LSP setup are possible in the
PCE architecture (see [RFC4655] and [RFC4657]). This document
generalizes PCEP to allow other LSP setup methods to be used. It
defines two new TLVs and specifies the base procedures to facilitate
this:
o The PATH-SETUP-TYPE-CAPABILITY TLV allows a PCEP speaker to
announce which LSP setup methods it supports when the PCEP session
is established.
o The PATH-SETUP-TYPE TLV allows a PCEP speaker to specify which
setup method should be used for a given LSP. When multiple path
setup types are deployed in a network, a given PCEP session may
have to simultaneously support more than one path setup type. A
PCEP speaker uses the PATH-SETUP-TYPE TLV to explicitly indicate
the intended path setup type in the appropriate PCEP messages,
unless the path setup type is RSVP-TE (which is assumed to be the
path setup type if no other setup type is indicated). This is so
that both the PCC and the PCE can take the necessary steps to set
up the path.
This document defines a path setup type code for RSVP-TE. When a new
path setup type (other than RSVP-TE) is introduced for setting up a
path, a path setup type code and, optionally, a sub-TLV pertaining to
the new path setup type will be defined by the document that
specifies the new path setup type.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Terminology
The following terminology is used in this document:
ERO: Explicit Route Object
PCC: Path Computation Client
PCE: Path Computation Element
PCEP: PCE Communication Protocol
PST: Path Setup Type
TLV: Type, Length, and Value
3. Path Setup Type Capability TLV
A PCEP speaker indicates which PSTs it supports during the PCEP
initialization phase using the following process. When the PCEP
session is created, it sends an Open message with an OPEN object
containing the PATH-SETUP-TYPE-CAPABILITY TLV. The format of this
TLV is as follows.
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 (34) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Num of PSTs |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PST#1 | ... | PST#N | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
// Optional sub-TLVs (variable) //
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: PATH-SETUP-TYPE-CAPABILITY TLV
The TLV Type is 34. Its Reserved field MUST be set to zero by the
sender and MUST be ignored by the receiver. The other fields in the
TLV are as follows.
Length: The total length in bytes of the remainder of the TLV, that
is, excluding the Type and Length fields.
Num of PSTs: The number of PSTs in the following list, excluding
padding.
List of PSTs: A list of the PSTs that the PCEP speaker supports.
Each PST is a single byte in length. Duplicate entries in this
list MUST be ignored. The PCEP speaker MUST pad the list with
zeros so that it is a multiple of four bytes in length. This
document defines the following PST value:
* PST = 0: Path is set up using the RSVP-TE signaling protocol
Optional sub-TLVs: A list of sub-TLVs associated with the supported
PSTs. Each PST has zero or one sub-TLVs associated with it, and
each sub-TLV is associated with exactly one PST. Each sub-TLV
MUST obey the rules for TLV formatting defined in [RFC5440]. That
is, each sub-TLV is padded to a four-byte alignment, and the
Length field of each sub-TLV does not include the padding bytes.
This document does not define any sub-TLVs; an example sub-TLV can
be found in [PCEP-EXTENSIONS].
A PCEP speaker MUST check that this TLV is correctly formatted, as
follows.
o If there are no sub-TLVs, then the TLV Length field MUST be equal
to four bytes plus the size of the PST list, excluding any padding
bytes.
o If there are sub-TLVs, then the TLV Length field MUST be equal to
four bytes plus the size of the PST list (rounded up to the
nearest multiple of four) plus the size of the appended sub-TLVs,
excluding any padding bytes in the final sub-TLV.
o The Num of PSTs field MUST be greater than zero.
If a PCEP speaker receives a PATH-SETUP-TYPE-CAPABILITY TLV that
violates these rules, then the PCEP speaker MUST send a PCErr message
with Error-Type = 10 (Reception of an invalid object) and Error-value
= 11 (Malformed object) and MUST close the PCEP session. The PCEP
speaker MAY include the malformed OPEN object in the PCErr message as
well.
If a PCEP speaker receives an OPEN object with more than one PATH-
SETUP-TYPE-CAPABILITY TLV, then it MUST ignore all but the first
instance of this TLV.
The absence of the PATH-SETUP-TYPE-CAPABILITY TLV from the OPEN
object is equivalent to a PATH-SETUP-TYPE-CAPABILITY TLV containing a
single PST value of 0 (Path is set up using the RSVP-TE signaling
protocol) and no sub-TLVs. A PCEP speaker MAY omit the PATH-SETUP-
TYPE-CAPABILITY TLV if the only PST it supports is RSVP-TE. If a
PCEP speaker supports other PSTs besides RSVP-TE, then it SHOULD
include the PATH-SETUP-TYPE-CAPABILITY TLV in its OPEN object.
If a PCEP speaker does not recognize the PATH-SETUP-TYPE-CAPABILITY
TLV, it will ignore the TLV in accordance with [RFC5440].
4. Path Setup Type TLV
When a PCEP session is used to set up TE paths using different
methods, the corresponding PCE and PCC must be aware of the path
setup method used. This means that a PCE must be able to specify
paths in the correct format, and a PCC must be able to take control-
plane and forwarding-plane actions appropriate to the PST.
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 (28) | Length (4) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | PST |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: PATH-SETUP-TYPE TLV
The PATH-SETUP-TYPE TLV is an optional TLV associated with the
Request Parameters (RP) [RFC5440] and the Stateful PCE Request
Parameters (SRP) [RFC8231] objects. Its format is shown in Figure 2.
The TLV type is 28. Its Reserved field MUST be set to zero. The
one-byte PST field contains the PST as defined for the PATH-SETUP-
TYPE-CAPABILITY TLV.
The absence of the PATH-SETUP-TYPE TLV is equivalent to a PATH-SETUP-
TYPE TLV with a PST value of 0 (Path is set up using the RSVP-TE
signaling protocol). A PCEP speaker MAY omit the TLV if the PST is
RSVP-TE. If the RP or SRP object contains more than one PATH-SETUP-
TYPE TLV, only the first TLV MUST be processed, and the rest MUST be
ignored.
If a PCEP speaker does not recognize the PATH-SETUP-TYPE TLV, it will
ignore the TLV in accordance with [RFC5440] and use RSVP-TE to set up
the path.
5. Operation
During the PCEP initialization phase, if a PCEP speaker receives a
PATH-SETUP-TYPE-CAPABILITY TLV from its peer, it MUST assume that the
peer supports only the PSTs listed in the TLV. If the PCEP speaker
and its peer have no PSTs in common, then the PCEP speaker MUST send
a PCErr message with Error-Type = 21 (Invalid traffic engineering
path setup type) and Error-value = 2 (Mismatched path setup type) and
close the PCEP session.
If the peer has sent no PATH-SETUP-TYPE-CAPABILITY TLV, then the PCEP
speaker MUST infer that the peer supports path setup using at least
RSVP-TE. The PCEP speaker MAY also infer that the peer supports
other path setup types, but the means of inference are outside the
scope of this document.
When a PCC sends a PCReq message to a PCE [RFC5440], it MUST include
the PATH-SETUP-TYPE TLV in the RP object, unless the intended PST is
RSVP-TE (in which case it MAY omit the PATH-SETUP-TYPE TLV). If the
PCE is capable of expressing the path in a format appropriate to the
intended PST, it MUST use the appropriate ERO format in the PCRep
message.
When a PCE sends a PCRep message to a PCC [RFC5440], it MUST include
the PATH-SETUP-TYPE TLV in the RP object, unless the PST is RSVP-TE
(in which case it MAY omit the PATH-SETUP-TYPE TLV). If the PCE does
not support the intended PST, it MUST send a PCErr message with
Error-Type = 21 (Invalid traffic engineering path setup type) and
Error-value = 1 (Unsupported path setup type) and close the PCEP
session. If the PSTs corresponding to the PCReq and PCRep messages
do not match, the PCC MUST send a PCErr message with Error-Type = 21
(Invalid traffic engineering path setup type) and Error-value = 2
(Mismatched path setup type) and close the PCEP session.
When a stateful PCE sends a PCUpd message [RFC8231] or a PCInitiate
message [RFC8281] to a PCC, it MUST include the PATH-SETUP-TYPE TLV
in the SRP object, unless the intended PST is RSVP-TE (in which case
it MAY omit the PATH-SETUP-TYPE TLV). If the PCC does not support
the PST associated with the PCUpd or PCInitiate message, it MUST send
a PCErr message with Error-Type = 21 (Invalid traffic engineering
path setup type) and Error-value = 1 (Unsupported path setup type)
and close the PCEP session.
When a PCC sends a PCRpt message to a stateful PCE [RFC8231], it MUST
include the PATH-SETUP-TYPE TLV in the SRP object, unless the PST is
RSVP-TE (in which case it MAY omit the PATH-SETUP-TYPE TLV). The PCC
MUST include the SRP object in the PCRpt message if the PST is not
RSVP-TE, even when the SRP-ID-number is the reserved value of
0x00000000. If the PCRpt message is triggered by a PCUpd or
PCInitiate message, then the PST that the PCC indicates in the PCRpt
message MUST match the PST that the stateful PCE intended in the
PCUpd or PCInitiate message. If it does not match, then the PCE MUST
send a PCErr message with Error-Type = 21 (Invalid traffic
engineering path setup type) and Error-value = 2 (Mismatched path
setup type) and close the PCEP session.
6. Manageability Considerations
This document generalizes PCEP to allow path setup methods other than
RSVP-TE to be used by the network (but does not define any new path
setup types besides RSVP-TE). It is possible that, in a given
network, multiple path setup methods will be used. It is also
possible that not all devices will support the same set of path setup
methods. Managing networks that combine multiple path setup methods
may therefore raise some challenges from a configuration and
observability point of view.
Each document that defines a new path setup type in the "PCEP Path
Setup Types" registry (Section 8.2) must include a Manageability
Considerations section. The Manageability Considerations section
must explain how operators can manage PCEP with the new path setup
type. It must address the following questions, which are generally
applicable when working with multiple path setup types in PCEP.
o What are the criteria for when devices will use the new path setup
type in PCEP, and how can the operator control this?
o How can the network be migrated to the new path setup type, and
are there any backwards-compatibility issues that operators need
to be aware of?
o Are paths set up using the new path setup type intended to coexist
with other paths over the long term, and if so, how is this
situation managed with PCEP?
o How can operators verify the correct operation of PCEP in the
network with respect to the new path setup type? Which fault
conditions must be reported to the operators?
o Are there any existing management interfaces (such as YANG models)
that must be extended to model the operation of PCEP in the
network with respect to the new path setup type?
See [RFC5706] for further guidance on how to write Manageability
Considerations sections in Standards Track documents.
7. Security Considerations
The security considerations described in [RFC5440] and [RFC8281] are
applicable to this specification. No additional security measure is
required.
Note that if the security mechanisms of [RFC5440] and [RFC8281] are
not used, then the protocol described in this document could be
attacked in the following new way. An attacker, using a TCP man-in-
the-middle attack, could inject error messages into the PCEP session
when a particular PST is (or is not) used. Doing this could
potentially force the use of a specific PST, which may allow the
attacker to subsequently attack a weakness in that PST.
8. IANA Considerations
8.1. Additions to PCEP TLV Type Indicators Registry
IANA has allocated the following code points in the "PCEP TLV Type
Indicators" registry.
Value Description Reference
----- -------------------------- ---------
28 PATH-SETUP-TYPE RFC 8408
34 PATH-SETUP-TYPE-CAPABILITY RFC 8408
8.2. New PCEP Path Setup Types Registry
IANA has created a new sub-registry within the "Path Computation
Element Protocol (PCEP) Numbers" registry called "PCEP Path Setup
Types". The allocation policy for this new registry is IETF Review
[RFC8126]. This new registry contains the following value:
Value Description Reference
----- -------------------------- ---------
0 Path is set up using the RFC 8408
RSVP-TE signaling protocol
8.3. Additions to PCEP-ERROR Object Error Types and Values Registry
IANA has allocated the following code points in the "PCEP-ERROR
Object Error Types and Values" registry.
Error-Type Meaning Reference
---------- ------------------------------------------- ---------
10 Reception of an invalid object RFC 5440
Error-value = 11: Malformed object RFC 8408
21 Invalid traffic engineering path setup type RFC 8408
Error-value = 0: Unassigned RFC 8408
Error-value = 1: Unsupported path setup type RFC 8408
Error-value = 2: Mismatched path setup type RFC 8408
9. References
9.1. 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>.
9.2. Informative References
[PCEP-EXTENSIONS]
Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W.,
and J. Hardwick, "PCEP Extensions for Segment Routing",
Work in Progress, draft-ietf-pce-segment-routing-12, June
2018.
[RFC4655] Farrel, A., Vasseur, J., and J. Ash, "A Path Computation
Element (PCE)-Based Architecture", RFC 4655,
DOI 10.17487/RFC4655, August 2006,
<https://www.rfc-editor.org/info/rfc4655>.
[RFC4657] Ash, J., Ed. and J. 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>.
[RFC5706] Harrington, D., "Guidelines for Considering Operations and
Management of New Protocols and Protocol Extensions",
RFC 5706, DOI 10.17487/RFC5706, November 2009,
<https://www.rfc-editor.org/info/rfc5706>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
Acknowledgements
We would like to thank Marek Zavodsky for valuable comments.
Contributors
The following people contributed to this document:
- Jan Medved
- Edward Crabbe
Authors' Addresses
Siva Sivabalan
Cisco Systems, Inc.
2000 Innovation Drive
Kanata, Ontario K2K 3E8
Canada
Email: msiva@cisco.com
Jeff Tantsura
Nuage Networks
755 Ravendale Drive
Mountain View, CA 94043
United States of America
Email: jefftant.ietf@gmail.com
Ina Minei
Google, Inc.
1600 Amphitheatre Parkway
Mountain View, CA 94043
United States of America
Email: inaminei@google.com
Robert Varga
Pantheon Technologies SRO
Mlynske Nivy 56
Bratislava, 821 05
Slovakia
Email: nite@hq.sk
Jon Hardwick
Metaswitch Networks
100 Church Street
Enfield, Middlesex
United Kingdom
Email: jonathan.hardwick@metaswitch.com
ISO Plain Language
On this page
§1Why PCEP Needs a Path Setup Type
RFC 5440 describes the PCE Communication Protocol (PCEP) for communication between a Path Computation Client (PCC) and a Path Computation Element (PCE), or between one PCE and another. A PCC requests, from a PCE, a path subject to various constraints and optimization criteria. The PCE responds to the PCC with a hop-by-hop path in an Explicit Route Object (ERO), and the PCC uses the ERO to set up the path in the network.
RFC 8231 specifies extensions that allow a PCC to delegate its LSPs to a PCE, so that the PCE can update their state, for instance by sending a new ERO that the PCC uses to reroute the LSP in a make-before-break fashion. RFC 8281 specifies a mechanism that allows a PCE to dynamically instantiate an LSP on a PCC.
So far, PCEP and its extensions have assumed that the traffic engineering paths are label switched and are established with the RSVP-TE signalling protocol. However, other methods of LSP setup are possible in the PCE architecture. This document generalizes PCEP to allow other LSP setup methods to be used, and defines two new TLVs to facilitate it.
The PATH-SETUP-TYPE-CAPABILITY TLV allows a PCEP speaker to announce which LSP setup methods it supports when the PCEP session is established. The PATH-SETUP-TYPE TLV allows a PCEP speaker to specify which setup method should be used for a given LSP. When multiple path setup types are deployed in a network, a given PCEP session may have to support more than one at the same time, so a PCEP speaker uses the PATH-SETUP-TYPE TLV to explicitly indicate the intended path setup type in the appropriate PCEP messages, unless the path setup type is RSVP-TE, which is assumed when no other setup type is indicated. That is so that both the PCC and the PCE can take the necessary steps to set up the path.
This document defines a path setup type code for RSVP-TE. When a new path setup type other than RSVP-TE is introduced, the document that specifies it will define a path setup type code and, optionally, a sub-TLV for it.
PCEP worked out routes for tunnels that were always built with RSVP-TE. This RFC lets the same protocol carry a tunnel built a different way, by agreeing on the method first.
§2Terms
The document uses a short set of terms. An ERO is an Explicit Route Object, the hop-by-hop path a PCE hands back. A PCC is a Path Computation Client, the element that asks for a path. A PCE is a Path Computation Element, the element that computes one. PCEP is the PCE Communication Protocol itself. A PST is a Path Setup Type, the method used to set the path up in the network. A TLV is a Type, Length, and Value.
The two new objects are named after those terms: the PATH-SETUP-TYPE-CAPABILITY TLV carries a list of the PSTs a speaker supports, and the PATH-SETUP-TYPE TLV carries the single PST that applies to one LSP.
The only new term is "path setup type": the name for the method used to build a tunnel, such as RSVP-TE.
§3The Path Setup Type Capability TLV
A PCEP speaker indicates which path setup types it supports during the PCEP initialization phase. When the session is created, it sends an Open message with an OPEN object containing the PATH-SETUP-TYPE-CAPABILITY TLV, whose type is 34. Its Reserved field MUST be set to zero by the sender and MUST be ignored by the receiver.
The rest of the TLV holds a count and a list. The Num of PSTs field is the number of path setup types in the following list, excluding padding. The list itself holds the supported types, each a single byte, and duplicate entries in the list MUST be ignored. The speaker MUST pad the list with zeros so that it is a multiple of four bytes in length. This document defines one value: PST 0 means the path is set up using the RSVP-TE signalling protocol. Optional sub-TLVs may follow, each associated with exactly one path setup type and each obeying the TLV formatting rules of RFC 5440.
The speaker has to check that the TLV is correctly formatted. With no sub-TLVs, the Length field must equal four bytes plus the size of the path setup type list, excluding padding. With sub-TLVs, it must equal four bytes plus the size of the list rounded up to the nearest multiple of four, plus the size of the appended sub-TLVs, excluding any padding in the final sub-TLV. The Num of PSTs field must be greater than zero. A speaker that receives a TLV violating these rules MUST send a PCErr message with Error-Type 10, reception of an invalid object, and Error-value 11, malformed object, and MUST close the PCEP session. If an OPEN object carries more than one of these TLVs, all but the first instance MUST be ignored.
The absence of the TLV is equivalent to one holding a single path setup type value of 0 and no sub-TLVs. A speaker MAY omit it if the only type it supports is RSVP-TE, and SHOULD include it if it supports others.
Each end of the session lists the methods it can build tunnels with, as a short list of numbers. If a reader does not recognise the list at all, it ignores it and assumes the older method.
| Object | Type | Where it appears | What it does |
|---|---|---|---|
| PATH-SETUP-TYPE-CAPABILITY | 34 | The OPEN object | Lists the setup methods the speaker supports |
| PATH-SETUP-TYPE THIS RFC | 28 | The RP and SRP objects | Names the setup method for one LSP |
§4The Path Setup Type TLV
When a PCEP session is used to set up traffic engineering paths using different methods, the PCE and the PCC must be aware of the method used. The PCE must be able to specify paths in the correct format, and the PCC must be able to take the control-plane and forwarding-plane actions appropriate to the path setup type.
The PATH-SETUP-TYPE TLV is an optional TLV associated with the Request Parameters (RP) object of RFC 5440 and the Stateful PCE Request Parameters (SRP) object of RFC 8231. Its type is 28, and its Reserved field MUST be set to zero. The one-byte PST field holds the path setup type, defined exactly as for the capability TLV.
The absence of the TLV is equivalent to one with a PST value of 0, meaning the path is set up using RSVP-TE. A speaker MAY omit it if the type is RSVP-TE. If the RP or SRP object contains more than one PATH-SETUP-TYPE TLV, only the first MUST be processed and the rest MUST be ignored. A speaker that does not recognise the TLV ignores it in accordance with RFC 5440 and uses RSVP-TE to set up the path.
The second object is smaller: one number saying which method to use for one particular tunnel, carried inside the request for that tunnel.
§5How the Two Ends Agree
During the initialization phase, a PCEP speaker that receives a PATH-SETUP-TYPE-CAPABILITY TLV from its peer MUST assume that the peer supports only the types listed in it. If the speaker and its peer have no path setup type in common, the speaker MUST send a PCErr message with Error-Type 21, invalid traffic engineering path setup type, and Error-value 2, mismatched path setup type, and close the PCEP session. If the peer sent no capability TLV, the speaker MUST infer that the peer supports at least RSVP-TE, and MAY infer that it supports others, though the means of inference are outside the scope of this document.
When a PCC sends a PCReq message, it MUST include the PATH-SETUP-TYPE TLV in the RP object, unless the intended type is RSVP-TE, in which case it MAY omit it. If the PCE is capable of expressing the path in a format appropriate to the intended type, it MUST use the appropriate ERO format in the PCRep message. When a PCE sends a PCRep, it MUST include the TLV in the RP object, unless the type is RSVP-TE. If the PCE does not support the intended type, it MUST send a PCErr message with Error-Type 21 and Error-value 1, unsupported path setup type, and close the session. If the types in the request and the reply do not match, the PCC MUST send a PCErr message with Error-Type 21 and Error-value 2 and close the session.
The same pattern applies to the stateful messages. When a stateful PCE sends a PCUpd or a PCInitiate message, it MUST include the TLV in the SRP object unless the type is RSVP-TE, and a PCC that does not support that type MUST send a PCErr message with Error-Type 21 and Error-value 1 and close the session. When a PCC sends a PCRpt message, it MUST include the TLV in the SRP object unless the type is RSVP-TE, and it MUST include the SRP object even when the SRP-ID-number is the reserved value of 0x00000000. If the report was triggered by a PCUpd or PCInitiate message, the type the PCC indicates MUST match the one the stateful PCE intended; if it does not, the PCE MUST send a PCErr message with Error-Type 21 and Error-value 2 and close the session.
If the two ends do not share a method, the session is closed with an error. If they do, each message names the method it is about, so neither end builds the tunnel the wrong way.
- 10 / 11Reception of an invalid object — Malformed object, sent when the capability TLV is malformed.
- 21 / 1Invalid path setup type — Unsupported path setup type.
- 21 / 2Invalid path setup type — Mismatched path setup type.
§6Manageability Considerations
This document generalizes PCEP to allow path setup methods other than RSVP-TE to be used by the network, although it defines no new path setup types beyond RSVP-TE itself. It is possible that a given network will use several methods at once, and that not all devices will support the same set of methods. Managing networks that combine multiple path setup methods may therefore raise challenges from a configuration and observability point of view.
Each document that defines a new path setup type in the "PCEP Path Setup Types" registry must include a Manageability Considerations section, and that section must explain how operators can manage PCEP with the new type. It must address five questions. What are the criteria for when devices will use the new path setup type in PCEP, and how can the operator control this? How can the network be migrated to the new path setup type, and are there backwards-compatibility issues that operators need to be aware of? Are paths set up using the new type intended to coexist with other paths over the long term, and if so, how is that situation managed with PCEP? How can operators verify the correct operation of PCEP in the network with respect to the new type, and which fault conditions must be reported to the operators? Are there any existing management interfaces, such as YANG models, that must be extended to model the operation of PCEP in the network with respect to the new type?
Each new setup method has to come with a note on how operators run it: when it is used, how to migrate to it, how it coexists with the old one, how to check it works, and what has to be added to the management models.
§7Security Considerations
The security considerations described in RFC 5440 and RFC 8281 are applicable to this specification. No additional security measure is required.
The RFC then adds one note. If the security mechanisms of RFC 5440 and RFC 8281 are not used, the protocol described in this document could be attacked in a new way. An attacker using a TCP man-in-the-middle attack could inject error messages into the PCEP session when a particular path setup type is, or is not, used. Doing this could potentially force the use of a specific path setup type, which may allow the attacker to subsequently attack a weakness in that type.
Nothing new is required, but there is a new way to attack an unprotected session: an attacker could push both ends towards one setup method and then exploit a weakness in it.
§8IANA Considerations
Two code points are allocated in the "PCEP TLV Type Indicators" registry: value 28 for PATH-SETUP-TYPE and value 34 for PATH-SETUP-TYPE-CAPABILITY.
A new sub-registry is created within the "Path Computation Element Protocol (PCEP) Numbers" registry, called "PCEP Path Setup Types", with IETF Review as the allocation policy. Its first value is 0, meaning that the path is set up using the RSVP-TE signalling protocol.
IANA keeps the lists of object numbers and of setup methods. This RFC adds two object numbers and the first method, RSVP-TE, numbered zero.