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IETF RFC 6457
Last modified on Saturday, December 3rd, 2011
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Internet Engineering Task Force (IETF) T. Takeda, Ed.
Request for Comments: 6457 NTT
Category: Informational A. Farrel
ISSN: 2070-1721 Old Dog Consulting
December 2011
PCC-PCE Communication and PCE Discovery Requirements for
Inter-Layer Traffic Engineering
 Abstract
The Path Computation Element (PCE) provides functions of path
computation in support of traffic engineering in networks controlled
by Multi-Protocol Label Switching (MPLS) and Generalized MPLS
(GMPLS).
MPLS and GMPLS networks may be constructed from layered client/server
networks. It is advantageous for overall network efficiency to
provide end-to-end traffic engineering across multiple network
layers. PCE is a candidate solution for such requirements.
Generic requirements for a communication protocol between Path
Computation Clients (PCCs) and PCEs are presented in RFC 4657, "Path
Computation Element (PCE) Communication Protocol Generic
Requirements". Generic requirements for a PCE discovery protocol are
presented in RFC 4674, "Requirements for Path Computation Element
(PCE) Discovery".
This document complements the generic requirements and presents
detailed sets of PCC-PCE communication protocol requirements and PCE
discovery protocol requirements for inter-layer traffic engineering.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
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). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/RFC 6457.
Takeda, et al. Informational PAGE 1 
RFC 6457 PCE Inter-Layer Requirements December 2011
Copyright Notice
Copyright (c) 2011 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.
Table of Contents
1. Introduction ....................................................2
1.1. Terminology ................................................3
2. Motivation for PCE-Based Inter-Layer Path Computation ...........4
3. PCC-PCE Communication and Discovery Requirements for
Inter-Layer .....................................................4
3.1. PCC-PCE Communication ......................................5
3.1.1. Control of Inter-Layer Path Computation .............5
3.1.2. Control of the Type of Path to Be Computed ..........5
3.1.3. Communication of Inter-Layer Constraints ............6
3.1.4. Adaptation Capability ...............................7
3.1.5. Cooperation between PCEs ............................7
3.1.6. Inter-Layer Diverse Paths ...........................7
3.2. Capabilities Advertisements for PCE Discovery ..............7
3.3. Supported Network Models ...................................8
4. Manageability Considerations ....................................8
4.1. Control of Function and Policy .............................8
4.2. Information and Data Models ................................8
4.3. Liveness Detection and Monitoring ..........................8
4.4. Verifying Correct Operation ................................9
4.5. Requirements on Other Protocols and Functional Components ..9
4.6. Impact on Network Operation ................................9
5. Security Considerations ........................................10
6. Acknowledgments ................................................10
7. References .....................................................10
7.1. Normative References ......................................10
7.2. Informative References ....................................10
1. Introduction
The Path Computation Element (PCE) defined in [RFC 4655] is an entity
that is capable of computing a network path or route based on a
network graph and applying computational constraints.
A network may comprise multiple layers. These layers may represent
the separation of technologies (e.g., Packet Switch Capable (PSC),
Time Division Multiplex (TDM), lambda switch capable (LSC)) into
GMPLS regions [RFC 3945], the separation of data plane switching
Takeda, et al. Informational PAGE 2
RFC 6457 PCE Inter-Layer Requirements December 2011
granularity levels (e.g., PSC-1 and PSC-2 or Virtual Circuit 4 (VC4)
and VC12) into GMPLS layers [RFC 5212], or a distinction between
client and server networking roles (e.g., commercial or
administrative separation of client and server networks). In this
multi-layer network, Label Switched Paths (LSPs) in lower layers are
used to carry upper-layer LSPs. The network topology formed by
lower-layer LSPs and advertised to the higher layer is called a
"Virtual Network Topology (VNT)" [RFC 5212].
In layered networks under the operation of Multiprotocol Label
Switching Traffic Engineering (MPLS-TE) and Generalized MPLS (GMPLS)
protocols, it is important to provide mechanisms to allow global
optimization of network resources. That is, to take into account all
layers, rather than optimizing resource utilization at each layer
independently. This allows better network efficiency to be achieved.
This is what we call "inter-layer traffic engineering". This
includes mechanisms allowing computation of end-to-end paths across
layers (known as "inter-layer path computation") and mechanisms for
control and management of the VNT by setting up and releasing LSPs in
the lower layers [RFC 5212].
Inter-layer traffic engineering is included in the scope of the PCE
architecture [RFC 4655], and PCE can provide a suitable mechanism for
resolving inter-layer path computation issues. The applicability of
the PCE-based path computation architecture to inter-layer traffic
engineering is described in [RFC 5623].
This document presents sets of requirements for communication between
Path Computation Clients (PCCs) and PCEs using the PCE Communication
Protocol (PCEP) and for PCE discovery for inter-layer traffic
engineering. It supplements the generic requirements documented in
[RFC 4657], [RFC 4674], and the framework provided in [RFC 5623].
1.1. Terminology
LSP: Label Switched Path.
LSR: Label Switching Router.
PCC: A Path Computation Client is any client entity (component,
application or network node) requesting a path computation to
be performed by a Path Computation Element.
PCE: A Path Computation Element is an entity that is capable of
computing a network path or route based on a network graph and
applying computational constraints.
PCEP: A PCE Communication Protocol is a protocol for communication
between PCCs and PCEs.
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RFC 6457 PCE Inter-Layer Requirements December 2011
Although this requirements document is informational and not a
protocol specification, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in RFC 2119
[RFC 2119] for clarity of requirement specification.
2. Motivation for PCE-Based Inter-Layer Path Computation
[RFC 4206] defines a way to signal an MPLS or a GMPLS LSP with an
explicit route in a higher layer of a network that includes hops
traversed by LSPs in lower layers of the network. The computation of
end-to-end paths across layers is called "inter-layer path
computation".
An LSR in the higher layer might not have information on the topology
of lower layers, particularly in an overlay or augmented model;
hence, it might not be able to compute an end-to-end path across
layers.
PCE-based inter-layer path computation consists of relying on one or
more PCEs to compute an end-to-end path across layers. This could
rely on a single PCE path computation where the PCE has topology
information about multiple layers and can directly compute an end-to-
end path across layers considering the topology of all of the layers.
Alternatively, the inter-layer path computation could be performed as
a multiple PCE computation, where each member of a set of PCEs has
information about the topology of one or more layers, but not all
layers, and they collaborate to compute an end-to-end path.
Consider a two-layer network where the higher-layer network is a
packet-based IP/MPLS or GMPLS network and the lower-layer network is
a GMPLS-controlled optical network. An ingress LSR in the higher-
layer network tries to set up an LSP to an egress LSR also in the
higher-layer network across the lower-layer network, and it needs a
path in the higher-layer network. However, suppose that there is no
TE link between border LSRs, which are located on the boundary
between the higher-layer and lower-layer networks, and that the
ingress LSR does not have topology visibility in the lower layer. If
a single-layer path computation is applied for the higher layer, the
path computation fails. On the other hand, inter-layer path
computation is able to provide a route in the higher layer and a
suggestion that a lower-layer LSP be set up between border LSRs,
considering both layers as TE topologies.
Further discussion of the application of PCE to inter-layer path
computation can be found in [RFC 5623].
3. PCC-PCE Communication and Discovery Requirements for Inter-Layer
Traffic Engineering
This section provides additional requirements specific to the
problems of multi-layer TE that are not covered in [RFC 4657] or
[RFC 4674].
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RFC 6457 PCE Inter-Layer Requirements December 2011
3.1. PCC-PCE Communication
PCEP MUST allow requests and replies for inter-layer path
computation.
This requires no additional messages, but it implies the following
additional constraints to be added to PCEP.
3.1.1. Control of Inter-Layer Path Computation
A request from a PCC to a PCE MUST support the inclusion of an
optional indication of whether inter-layer path computation is
allowed. In the absence of such an indication, the default is that
inter-layer path computation is not allowed.
3.1.2. Control of the Type of Path to Be Computed
The PCE computes and returns a path to the PCC that the PCC can use
to build a higher-layer or lower-layer LSP once converted to an
Explicit Route Object (ERO) for use in RSVP - Traffic Engineering
(RSVP-TE) signaling. There are two options [RFC 5623].
- Option 1: Mono-Layer Path. The PCE computes a "mono-layer" path,
i.e., a path that includes only TE links from the same layer.
- Option 2: Multi-Layer Path. The PCE computes a "multi-layer"
path, i.e., a path that includes TE links from distinct layers
[RFC 4206].
It may be necessary or desirable for a PCC to control the type of
path that is produced by a PCE. For example, a PCC may know that it
is not possible, for technological or policy reasons, to signal a
multi-layer path and that a mono-layer path is required, or the PCC
may know that it does not wish the layer border node to have control
of path computation. In order to make this level of control
possible, PCEP MUST allow the PCC to select the path types to be
computed, and that may be returned, by choosing one or more from the
following list:
- A mono-layer path that is specified by strict hop(s). The path
may include virtual TE link(s).
- A mono-layer path that includes loose hop(s).
- A multi-layer path that can include the path (as strict or loose
hops) of one or more lower-layer LSPs not yet established.
The path computation response from a PCE to a PCC MUST report the
type of path computed, and where a multi-layer path is returned, PCEP
MUST support the inclusion, as part of end-to-end path, of the path
of the lower-layer LSPs to be established.
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RFC 6457 PCE Inter-Layer Requirements December 2011
If a response message from a PCE to PCC carries a mono-layer path
that is specified by strict hops but includes virtual TE link(s),
includes loose hop(s), or carries a multi-layer path that can include
the complete path of one or more lower-layer LSPs not yet
established, the signaling of the higher-layer LSP may trigger the
establishment of the lower-layer LSPs (triggered signaling). The
triggered signaling may increase the higher-layer connection setup
latency. An ingress LSR for the higher-layer LSP, or a PCC, needs to
know whether or not triggered signaling is required.
A request from a PCC to a PCE MUST allow indicating whether or not
triggered signaling is acceptable.
A response from a PCE to a PCC MUST allow indicating whether or not
the computed path requires triggered signaling.
Note that a PCE may not be able to distinguish virtual TE links from
regular TE links. In such cases, even if a request from a PCC to a
PCE indicates that triggered signaling is not acceptable, a PCE may
choose virtual TE links in path computation. Therefore, when a
network uses virtual TE links and a PCE is not able to distinguish
virtual TE links from regular TE links, a PCE MAY choose virtual TE
links even if a request from a PCC to a PCE indicates triggered
signaling is not acceptable.
Also, note that an ingress LSR of a higher-layer or lower-layer LSP
may be present in multiple layers. Thus, even when a mono-layer path
is requested or supplied, PCEP MUST be able to indicate the
required/provided path layer.
3.1.3. Communication of Inter-Layer Constraints
A request from a PCC to a PCE MUST support the inclusion of
constraints for a multi-layer path. This includes control over which
network layers may, must, or must not be included in the computed
path. Such control may be expressed in terms of the switching types
of the layer networks.
Furthermore, it may be desirable to constrain the number of layer
boundaries crossed (i.e., the number of adaptations in the sense used
in [RFC 5212] performed on the end-to-end path), so PCEP SHOULD
include a constraint or objective function to minimize or cap the
number of adaptations on a path and a mechanism to report that number
when a path is supplied.
The path computation request MUST also allow for different objective
functions to be applied within different network layers. For
example, the path in a packet-network may need to be optimized for
least delay using the IGP metric as a measure of delay, while the
path in an underlying TDM network might be optimized for fewest hops.
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RFC 6457 PCE Inter-Layer Requirements December 2011
3.1.4. Adaptation Capability
The concept of adaptation is used here as introduced in [RFC 5212].
It MUST be possible for the path computation request to indicate the
desired adaptation function at the end points of the lower-layer LSP
that is being computed. This will be particularly important where
the ingress and egress LSR participate in more than one layer network
but may not be capable of all associated adaptations.
3.1.5. Cooperation between PCEs
When each layer is in scope of a different PCE, which only has access
to the topology information of its layer, the PCEs of each layer need
to cooperate to perform inter-layer path computation. In this case,
communication between PCEs is required for inter-layer path
computation. A PCE that behaves as a client is defined as a PCC
[RFC 4655].
PCEP MUST allow requests and replies for multiple PCE inter-layer
path computation.
3.1.6. Inter-Layer Diverse Paths
PCEP MUST allow for the computation of diverse inter-layer paths. A
request from a PCC to a PCE MUST support the inclusion of multiple
path requests, with the desired level of diversity at each layer
(link, node, Shared Risk Link Group (SRLG)).
3.2. Capabilities Advertisements for PCE Discovery
In the case where there are several PCEs with distinct capabilities
available, a PCC has to select one or more appropriate PCEs. For
that purpose, the PCE discovery mechanism MAY support the disclosure
of some detailed PCE capabilities. A PCE MAY (to be consistent with
the above text and RFC 4674) be able to advise the following PCE
capabilities related to inter-layer path computation:
- Support for inter-layer path computation
- Support for mono-layer/multi-layer paths
- Support for inter-layer constraints
- Support for adaptation capability
- Support for inter-PCE communication
- Support for inter-layer diverse path computation
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RFC 6457 PCE Inter-Layer Requirements December 2011
3.3. Supported Network Models
PCEP SHOULD allow several architectural alternatives for interworking
between MPLS- and GMPLS-controlled networks: overlay, integrated, and
augmented models [RFC 3945] [RFC 5145] [RFC 5146].
4. Manageability Considerations
4.1. Control of Function and Policy
An individual PCE MAY elect to support inter-layer computations and
advertise its capabilities as described in the previous sections.
PCE implementations MAY provide a configuration switch to allow
support of inter-layer path computations to be enabled or disabled.
When the level of support is changed, this SHOULD be re-advertised.
However, a PCE MAY also elect to support inter-layer computations,
but not to advertise the fact, so that only those PCCs configured to
know of the PCE and its capabilities can use it.
Support for, and advertisement of support for, inter-layer path
computation MAY be subject to policy and a PCE MAY hide its inter-
layer capabilities from certain PCCs by not advertising them through
the discovery protocol and not reporting them to the specific PCCs in
any PCEP capabilities exchange. Further, a PCE MAY be directed by
policy to refuse an inter-layer path computation request for any
reason including, but not limited to, the identity of the PCC that
makes the request.
A further discussion of policy-enabled path computation can be found
in [RFC 5394].
4.2. Information and Data Models
PCEP extensions to support inter-layer computations MUST be
accompanied by MIB objects for the control and monitoring of the
protocol and of the PCE that performs the computations. The MIB
objects MAY be provided in the same MIB module as used for general
PCEP control and monitoring [PCEP-MIB] or MAY be provided in a new
MIB module.
The MIB objects MUST provide the ability to control and monitor all
aspects of PCEP relevant to inter-layer path computation.
4.3. Liveness Detection and Monitoring
No changes are necessary to the liveness detection and monitoring
requirements as already embodied in [RFC 4657]. It should be noted,
however, that inter-layer path computations might require extended
cooperation between PCEs (as is also the case for inter-AS
(Autonomous System) and inter-area computations), and so the liveness
detection and monitoring SHOULD be applied to each PCEP communication
and aggregated to report the behavior of an individual PCEP request
to the originating PCC.
Takeda, et al. Informational PAGE 8
RFC 6457 PCE Inter-Layer Requirements December 2011
In particular, where a request is forwarded between multiple PCEs,
neither the PCC nor the first PCE can monitor the liveness of all
PCE-PCE connections or of the PCEs themselves. In this case,
suitable performance of the original PCEP request relies on each PCE
operating correct monitoring procedures and correlating any failures
back to the PCEP requests that are outstanding. These requirements
are no different from those for any cooperative PCE usage, and they
are expected already to be covered by general, and by inter-AS and
inter-area, implementations. Such a procedure is specified in
[RFC 5441]. In addition, [RFC 5886] specifies mechanisms to gather
various state metrics along the path computation chain.
4.4. Verifying Correct Operation
There are no additional requirements beyond those expressed in
[RFC 4657] for verifying the correct operation of the PCEP. Note that
verification of the correct operation of the PCE and its algorithms
is out of scope for the protocol requirements, but a PCC MAY send the
same request to more than one PCE and compare the results.
4.5. Requirements on Other Protocols and Functional Components
A PCE operates on a topology graph that may be built using
information distributed by TE extensions to the routing protocol
operating within the network. In order that the PCE can select a
suitable path for the signaling protocol to use to install the inter-
layer LSP, the topology graph must include information about the
inter-layer signaling and forwarding (i.e., adaptation) capabilities
of each LSR in the network.
Whatever means are used to collect the information to build the
topology graph, the graph MUST include the requisite information. If
the TE extensions to the routing protocol are used, these SHOULD
satisfy the requirements as described in [RFC 5212].
4.6. Impact on Network Operation
This section examines the impact on network operations of the use of
a PCE for inter-layer traffic engineering. It does not present any
further requirements on the PCE or PCC, for the PCEP or for
deployment.
The use of a PCE to compute inter-layer paths is not expected to have
significant impact on network operations if the upper-layer traffic
engineering practices are aware of the frequent changes that might
occur in the VNT. It should also be noted that the introduction of
inter-layer support to a PCE that already provides mono-layer path
computation might change the loading of the PCE and that might have
an impact on the network behavior especially during recovery periods
immediately after a network failure.
Takeda, et al. Informational PAGE 9
RFC 6457 PCE Inter-Layer Requirements December 2011
On the other hand, it is envisioned that the use of inter-layer path
computation will have significant benefits to the operation of a
multi-layer network including improving the network resource usage
and enabling a greater number of higher-layer LSPs to be supported.
5. Security Considerations
Inter-layer traffic engineering with PCE may raise new security
issues when PCE-PCE communication is used between different layer
networks for inter-layer path computation. Security issues may also
exist when a single PCE is granted full visibility of TE information
that applies to multiple layers.
The formal introduction of a VNT Manager component, as described in
[RFC 5623], provides the basis for the application of inter-layer
security and policy.
It is expected that solutions for inter-layer protocol extensions
will address these issues in detail.
6. Acknowledgments
We would like to thank Kohei Shiomoto, Ichiro Inoue, Dean Cheng,
Meral Shirazipour, Julien Meuric, and Stewart Bryant for their useful
comments. Thanks to members of ITU-T Study Group 15, Question 14 for
their constructive comments during the liaison process.
7. References
7.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 3945] Mannie, E., Ed., "Generalized Multi-Protocol Label
Switching (GMPLS) Architecture", RFC 3945, October 2004.
[RFC 4206] Kompella, K. and Y. Rekhter, "Label Switched Paths (LSP)
Hierarchy with Generalized Multi-Protocol Label Switching
(GMPLS) Traffic Engineering (TE)", RFC 4206, October
2005.
7.2. Informative References
[RFC 4655] Farrel, A., Vasseur, J.-P., and J. Ash, "A Path
Computation Element (PCE)-Based Architecture", RFC 4655,
August 2006.
[RFC 4657] Ash, J., Ed., and J. Le Roux, Ed., "Path Computation
Element (PCE) Communication Protocol Generic
Requirements", RFC 4657, September 2006.
[RFC 4674] Le Roux, J., Ed., "Requirements for Path Computation
Element (PCE) Discovery", RFC 4674, October 2006.
Takeda, et al. Informational PAGE 10
RFC 6457 PCE Inter-Layer Requirements December 2011
[RFC 5145] Shiomoto, K., Ed., "Framework for MPLS-TE to GMPLS
Migration", RFC 5145, March 2008.
[RFC 5146] Kumaki, K., Ed., "Interworking Requirements to Support
Operation of MPLS-TE over GMPLS Networks", RFC 5146,
March 2008.
[RFC 5212] Shiomoto, K., Papadimitriou, D., Le Roux, JL., Vigoureux,
M., and D. Brungard, "Requirements for GMPLS-Based Multi-
Region and Multi-Layer Networks (MRN/MLN)", RFC 5212,
July 2008.
[RFC 5394] Bryskin, I., Papadimitriou, D., Berger, L., and J. Ash,
"Policy-Enabled Path Computation Framework", RFC 5394,
December 2008.
[RFC 5623] Oki, E., Takeda, T., Le Roux, JL., and A. Farrel,
"Framework for PCE-Based Inter-Layer MPLS and GMPLS
Traffic Engineering", RFC 5623, September 2009.
[PCEP-MIB] A. Koushik, and E. Stephan, "PCE communication protocol
(PCEP) Management Information Base", Work in Progress,
July 2010.
[RFC 5441] Vasseur, JP., Ed., Zhang, R., Bitar, N., and JL. Le Roux,
"A Backward-Recursive PCE-Based Computation (BRPC)
Procedure to Compute Shortest Constrained Inter-Domain
Traffic Engineering Label Switched Paths", RFC 5441,
April 2009.
[RFC 5886] Vasseur, JP., Ed., Le Roux, JL., and Y. Ikejiri, "A Set
of Monitoring Tools for Path Computation Element
(PCE)-Based Architecture", RFC 5886, June 2010.
Takeda, et al. Informational PAGE 11
RFC 6457 PCE Inter-Layer Requirements December 2011
Contributing Authors
Eiji Oki
University of Electro-Communications
Tokyo, Japan
EMail: oki@ice.uec.ac.jp
Jean-Louis Le Roux
France Telecom R&D,
Av Pierre Marzin,
22300 Lannion, France
EMail: jeanlouis.leroux@orange-ftgroup.com
Kenji Kumaki
KDDI Corporation
Garden Air Tower
Iidabashi, Chiyoda-ku,
Tokyo 102-8460, JAPAN
EMail: ke-kumaki@kddi.com
Authors' Addresses
Tomonori Takeda (editor)
NTT
3-9-11 Midori-cho,
Musashino-shi, Tokyo 180-8585, Japan
EMail: takeda.tomonori@lab.ntt.co.jp
Adrian Farrel
Old Dog Consulting
EMail: adrian@olddog.co.uk
Takeda, et al. Informational PAGE 12
RFC TOTAL SIZE: 29107 bytes
PUBLICATION DATE: Saturday, December 3rd, 2011
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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