Network Working Group Eiji Oki (Editor)
Internet Draft NTT
Category: Informational
Expires: September 2007
March 2007
PCC-PCE Communication Requirements for Inter-Layer Traffic
Engineering
draft-ietf-pce-inter-layer-req-04.txt
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Abstract
The Path Computation Element (PCE) provides functions of path
computation in support of traffic engineering in Multi-Protocol
Label Switching (MPLS) and Generalized MPLS (GMPLS) networks.
MPLS and GMPLS networks may be constructed from layered service
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 "PCE
Communication Protocol Generic Requirements". This document
complements the generic requirements and presents a detailed set of
PCC-PCE communication protocol requirements for inter-layer traffic
engineering.
Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in RFC 2119
[RFC2119].
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Table of Contents
1. Contributors...................................................2
2. Terminology....................................................2
3. Introduction...................................................3
4. Motivation for PCE-Based Inter-Layer Path Computation..........3
5. PCC-PCE Communication Requirements for Inter-Layer Traffic
Engineering.......................................................4
5.1. PCC-PCE Communication.......................................4
5.1.1. Control of Inter-Layer Path Computation...................4
5.1.2. Control of The Type of Path to be Computed................4
5.1.3. Communication of Inter-Layer Constraints..................5
5.1.4. Adaptation Capability.....................................5
5.1.5. Cooperation Between PCEs..................................5
5.1.6. Inter-Layer Diverse paths.................................6
5.2. Supportive Network Models...................................6
6. Manageability considerations...................................6
6.1. Control of Function and Policy...............................6
6.2. Information and Data Models..................................6
6.3. Liveness Detection and Monitoring............................6
6.4. Verifying Correct Operation..................................7
6.5. Requirements on Other Protocols and Functional Components....7
6.6. Impact on Network Operation..................................7
7. Security Considerations........................................7
8. Acknowledgments................................................8
9. References.....................................................8
9.1. Normative Reference.........................................8
9.2. Informative Reference.......................................8
10. Authors' Addresses...........................................8
11. Intellectual Property Statement..............................9
1. Contributors
The following are the authors that contributed to the present
document:
Eiji Oki (NTT)
Jean-Louis Le Roux (France Telecom)
Kenji Kumaki (KDDI)
Adrian Farrel (Old Dog Consulting)
2. Terminology
LSP: Label Switched Path.
LSR: Label Switching Router.
PCC: Path Computation Client: any client application requesting a
path computation to be performed by a Path Computation Element.
PCE: Path Computation Element: an entity (component, application or
network node) that is capable of computing a network path or route
based on a network graph and applying computational constraints.
PCECP: PCE Communication Protocol, a protocol for communication
between PCCs and PCEs.
TED: Traffic Engineering Database which contains the topology and
resource information of the domain. The TED may be fed by IGP
extensions or potentially by other means.
TE LSP: Traffic Engineering Label Switched Path.
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TE LSP head-end: head/source/ingress of the TE LSP.
TE LSP tail-end: tail/destination/egress of the TE LSP.
3. Introduction
The Path Computation Element (PCE) defined in [RFC4655] 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 of multiple layers. These layers may
represent separations of technologies (e.g., packet switch capable
(PSC), time division multiplex (TDM), lambda switch capable (LSC))
[RFC3945], separation of data plane switching granularity levels
(e.g., PSC-1 and PSC-2, or VC4 and VC12) [MRN-REQ], 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, LSP 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) [MRN-REQ].
It is important to optimize network resource utilization globally,
i.e. taking into account all layers, rather than optimizing resource
utilization at each layer independently. This allows achieving
better network efficiency. This is what we call Inter-layer traffic
engineering. This includes mechanisms allowing to compute end-to-end
paths across layers, as 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 [MRN-REQ].
Inter-layer traffic engineering is included in the scope of the PCE
architecture [RFC4655], 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 [PCE-INTER-LAYER-FRWK].
This document presents a set of PCC-PCE communication protocol
(PCECP) requirements for inter-layer traffic engineering. It
supplements the generic requirements documented in [RFC4657].
4. Motivation for PCE-Based Inter-Layer Path Computation
[RFC4206] defines a way to signal a higher-layer LSP, whose explicit
route includes hops traversed by LSPs in lower layers. 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 lower-
layer topology, particularly in an overlay or augmented model, and
hence 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
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of PCEs has information about the topology of one or more layers,
but not all layers, and 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 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 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 setup between border LSRs, considering both
layers' TE topologies.
Further discussion of the application of PCE to inter-layer path
computation can be found in [PCE-INTER-LAYER-FRWK].
5. PCC-PCE Communication Requirements for Inter-Layer Traffic
Engineering
This section sets out additional requirements not covered in
[RFC4657] specific to the problems of multi-layer TE.
5.1. PCC-PCE Communication
The PCC-PCE communication protocol MUST allow requests and replies
for inter-layer path computation.
This requires no additional messages, but implies the following
additional constraints to be added to the PCC-PCE communication
protocol.
5.1.1. Control of Inter-Layer Path Computation
A request from a PCC to a PCE SHOULD indicate 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.
Therefore, a request from a PCC to a PCE MUST support the inclusion
of such an indication.
5.1.2. 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-TE signaling. There are
two options [PCE-INTER-LAYER-FRWK].
- 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 [RFC4206].
A request from a PCC to a PCE MUST allow control of the type of the
path to be computed by selection from the following list:
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- 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 complete path of one or
more lower-layer LSPs not yet established.
When multi-layer path computation is requested, a response from a
PCE to a PCC MUST support the inclusion, as part of end-to-end path,
of the path of the lower-layer LSPs to be established.
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), or
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 (nested signaling). The nested
signaling may increase the higher-layer connection setup latency. An
ingress LSR for the higher-layer LSP, or a PCC, needs to know
whether nested signaling is required or not.
A request from a PCC to a PCE MUST allow indicating whether nested
signaling is acceptable or not.
A response from a PCE to a PCC MUST allow indicating whether the
computed path triggers nested signaling or not.
5.1.3. Communication of Inter-Layer Constraints
A request from a PCC to a PCE MUST support the inclusion of
constraints for 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.
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 under-lying TDM network might be optimized for fewest hops.
5.1.4. Adaptation Capability
It MUST be possible for the path computation request to indicate the
desired adaptation function at the egress of the LSP that is being
computed. This will be particularly important where the egress LSR
participates in more than one layer network but may not be capable
of all associated adaptations.
5.1.5. Cooperation Between PCEs
When each layer is controlled by a 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
[RFC4655].
The PCC-PCE communication protocol MUST allow requests and replies
for multiple PCE inter-layer path computation.
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5.1.6. Inter-Layer Diverse paths
The PCE communication protocol 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 request, with the desired
level of diversity at each layer (link, node, SRLG).
5.2. Supportive Network Models
The PCC-PCE communication protocol SHOULD allow several
architectural alternatives for interworking between MPLS and GMPLS
networks: overlay, integrated and augmented models [RFC3945].
6. Manageability considerations
6.1. 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 PCECP 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.
6.2. Information and Data Models
PCECP protocol 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
PCECP control and monitoring or MAY be provided in a new MIB module.
The MIB objects MUST provide the ability to control and monitor all
aspects of PCECP relevant to inter-layer path computation.
6.3. Liveness Detection and Monitoring
No changes are necessary to the liveness detection and monitoring
requirements as already embodied in [RFC4657]. It should be noted,
however, that inter-layer path computations might require extended
cooperation between PCEs (as is also the case for inter-AS and
inter-area computations) and so the liveness detection and
monitoring SHOULD be applied to each PCECP communication and
aggregated to report the behavior of an individual PCECP request to
the originating PCC.
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In particular, where a request is forwarded between PCEs multiple
times neither the PCC not the first PCE can monitor the liveness of
inter PCE-PCE connections or of the PCEs themselves. In this case,
suitable performance of the original PCECP request relies on each
PCE operating correct monitoring procedures and correlating any
failures back to the PCECP requests that are outstanding. These
requirements are no different from those for any cooperative PCE
usage, and are expected to be already covered by general and by
inter-AS and inter-area implementations.
6.4. Verifying Correct Operation
There are no additional requirements beyond those expressed in
[RFC4657] for verifying the correct operation of the PCECP. 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.
6.5. 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 is used to collect the information to build the
topology graph MUST include the requisite information. If the TE
extensions to the routing protocol are used, these SHOULD satisfy
the requirements as described in [MRN-REQ].
6.6. Impact on Network Operation
The use of a PCE to compute inter-layer paths is not expected to
have significant impact on network operations. But it should 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.
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.
7. Security Considerations
Inter-layer traffic engineering with PCE may raise new security
issues when PCE-PCE communication is done 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.
It is expected that solutions for inter-layer protocol extensions
will address these issues in detail using security techniques such
as authentication.
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8. Acknowledgments
We would like to thank Kohei Shiomoto, Ichiro Inoue, and Dean Cheng
for their useful comments.
9. References
9.1. Normative Reference
[RFC2119] Bradner, S., "Key words for use in RFCs to indicate
requirements levels", RFC 2119, March 1997.
[RFC3945] Mannie, E., "Generalized Multi-Protocol Label Switching
Architecture", RFC 3945, October 2004.
[RFC4206] Kompella, K., and Rekhter, Y., "Label Switched Paths (LSP)
Hierarchy with Generalized Multi-Protocol Label Switching (GMPLS)
Traffic Engineering (TE)", RFC 4206, October 2005.
9.2. Informative Reference
[RFC4655] A. Farrel, JP. Vasseur and J. Ash, "A Path Computation
Element (PCE)-Based Architecture", RFC 4655, September 2006.
[RFC4657] J. Ash, J.L Le Roux et al., " Path Computation Element
(PCE) Communication Protocol Generic Requirements", RFC 4657,
September 2006.
[RFC4674] JL Le Roux et al., "Requirements for Path Computation
Element (PCE) Discovery", RFC 4674, September 2006.
[MRN-REQ] K. Shiomoto et al., "Requirements for GMPLS-based multi-
region and multi-layer networks (MRN/MLN)", draft-ietf-ccamp-gmpls-
mln-reqs (work in progress).
[PCE-INTER-LAYER-FRWK] E. Oki et al., "Framework for PCE-Based
Inter-Layer MPLS and GMPLS Traffic Engineering", draft-oki-pce-
inter-layer-frwk (work in progress)
10. Authors' Addresses
Eiji Oki
NTT
3-9-11 Midori-cho,
Musashino-shi, Tokyo 180-8585, Japan
Email: oki.eiji@lab.ntt.co.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,
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Tokyo 102-8460, JAPAN
Phone: +81-3-6678-3103
Email: ke-kumaki@kddi.com
Adrian Farrel
Old Dog Consulting
Email: adrian@olddog.co.uk
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Copyright (C) The IETF Trust (2007).
This document is subject to the rights, licenses and restrictions
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