Network Working Group Eiji Oki (Editor)
Internet Draft NTT
Category: Informational
Expires: April 2006
October 2005
PCC-PCE Communication Requirements for Inter-Layer Traffic
Engineering
draft-oki-pce-inter-layer-req-00.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
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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].
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.
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.
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 [PCE-ARCH] 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 technology (e.g., PSC, TDM VC4, TDM VC12, LSC) or a
distinction between client and server networking roles. 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
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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 VNT by
setting up and releasing LSPs in lower layers [MRN-REQ].
Inter-layer traffic engineering is included in the scope of the PCE
architecture [PCE-ARCH], and PCE can provide a suitable mechanism for
resolving inter-layer path computation issues.
The use of PCE for the control of the VNT is for further study.
This document presents a set of PCC-PCE communication protocol
requirements for inter-layer traffic engineering. It supplements the
generic requirements documented in [PCE-COM-REQ].
4. Inter-Layer Traffic Engineering
This section describes key topics of inter-layer traffic engineering
in MPLS and GMPLS networks.
4.1. Inter-Layer Path Computation
[LSP-HIER] defines a way to signal an upper-layer LSP, whose explicit
route includes lower-layer(s) LSP paths. The computation of end-to-
end paths across layers is called Inter-Layer Path Computation.
An LSR in the higher-layer may not have information on the lower-
layer topology, particularly in an overlay or augmented model, and
hence may 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 single PCE path computation where a single PCE have topology
information on multiple layers, and can compute an end-to-end path
considering all layers' topology, or on multiple PCEs computation
where a set of PCEs have information of a single layer topology and
collaborate together to build an end-to-end path.
A two-layer network is considered. The higher-layer network can be
considered as a packet-based IP/MPLS network or GMPLS network. The
lower-layer network is considered as a GMPLS optical network. The
bandwidth granularity of the lower layer is coarse. For example, the
bandwidth is equal to 2.5 Gbit/s or 10 Gbit/s. On the other hand, the
granularity of the higher layer is flexible and well engineered.
Consider the case where higher-layer LSPs are to be established end-
to-end across a lower-layer network. For example, packet LSPs carried
across an optical core. Connectivity across the lower-layer is
achieved by tunneling the higher-layer LSPs within lower-layer LSPs.
However, when the bandwidths of the higher-layer LSPs are much
smaller than the capacity of the lower-layer LSPs, the resources in
the lower layer are not fully utilized unless a mechanism is provided
to aggregate multiple higher-layer LSPs into a single lower-layer LSP.
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There are two main options for routing a higher-layer LSP over a
lower-layer network. A single hop route uses a single edge-to-edge
lower-layer LSP that is managed as a single hop within the higher
layer. A multiple hop route uses a series of lower-layer LSPs each of
which appears to the higher-layer LSP as a single hop.
Lower-layer LSPs form a Virtual Network Topology, which can be used
for routing higher-layer LSPs or to carry IP traffic. Inter-layer
path computation for end-to-end LSPs in the higher-layer network that
span the lower-layer network may utilize the VNT, and PCE is a
candidate for computing the paths of such higher-layer LSPs within
the higher-layer network. PCEs could:
- perform a single computation on behalf of the ingress LSR using
information gathered from more than one layer. This mode is referred
as to Single PCE Computation in [PCE-ARCH].
- perform a set of cooperated path computations on behalf of the
ingress LSR through cooperation between PCEs responsible for each
layer. This mode is referred as to Multiple PCE Computation with
inter-PCE communication in [PCE-ARCH].
- perform separate path computations on behalf of the TE LSP head-end
and each transit LSR that is the entry point to a new layer. This
mode is referred as to Multiple PCE Computation (without inter-PCE
communication) in [PCE-ARCH]. This option utilizes per-layer path
computation performed independently by successive PCEs. Since there
is no PCE-PCE communication, and since each PCE is responsible for a
single layer only, there are no requirements placed on the PCC-PCE
communications protocol above those already defined for single domain
operation described in [PCE-COM-REQ]. Therefore this option is not
discussed further in this document.
When PCE returns to PCC a computed explicit path that would be
acceptable for use for MPLS and GMPLS LSPs once converted to an
Explicit Route Object (ERO) for use in RSVP-TE signaling, two options
could be considered as:
-Option 1: Mono-layer path. There are two cases. The first case is
that the PCE computes a path that includes already established lower
layer-LSPs: that is the ERO includes sub-object(s) corresponding to
lower layer hierarchical LSPs. This does not trigger new lower layer-
LSP setup but the utilization of existing lower-layer LSPs. The other
is that the PCE computes a path that includes loose hop(s). The
higher layer would select which lower layers to use and would select
the entry and exit points from those layers, but would not select the
path across the layers. A transit LSR corresponding to the entry
point is expected to expand the loose hop. Path expansion process on
border LSR may result either in the selection of an existing lower
layer LSP, or in the computation and setup of a new lower-layer LSP.
-Option 2: Multi-layer path. The PCE computes a "multi-layer" path
that can include the complete path of one or more lower-layer LSPs
not yet established. In that case the ERO contains paths of lower-
layer LSPs to be established. The signaling of the higher-layer LSP
will trigger the establishment of the lower-layer LSPs (nested
signaling).
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4.2. VNT Management
As a result of inter-layer path computation, PCE may determine that
there are insufficient lower-layer LSPs to support this or future
higher-layer LSPs. New lower-layer LSPs are needed in order to
satisfy the high-layer LSP requests or to efficiently manage the
utilization of lower-layer network resources. In other words, the VNT
needs to be controlled or managed in cooperation with inter-layer
path computation.
While PCE is responsible for inter-layer path computation, VNT
management may be performed by other network elements. The
relationship between VNT management and PCE for inter-layer
computation is for further study.
5. Inter-layer Path Computation Models
The generic PCC-PCE communication protocol requirements [PCE-COM-REQ]
are limited to basic path computation scenarios and generic concerns.
They do not necessarily cover all the requirements for inter-layer
traffic engineering and further requirements are stated in section 6
of this document to address the specific problem statements set out
in this section.
As stated in Section 4.1, two PCE modes defined in the PCE
architecture can be used to perform inter-layer path computation.
They are discussed below.
5.1. Single PCE Inter-Layer Path Computation
In Figure 1, higher-layer LSRs (H1, H2, H3 and H4) are connected by
an end-to-end higher-layer LSP. This is supported by a lower-layer
LSP (H2-L1-L2-H3) that traverses the lower-layer LSRs (L1 and L2) but
is presented as a single hop (H2-H3) in the higher-layer. A single
PCE manages the entire network and has visibility into both layers.
-----
| PCE |
-----
----- ----- ----- -----
| LSR |--| LSR |................| LSR |--| LSR |
| H1 | | H2 | | H3 | | H4 |
----- -----\ /----- -----
\----- -----/
| LSR |--| LSR |
| L1 | | L2 |
----- -----
Figure 1 : Single PCE with Multi-Layer Visibility
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5.2. Multiple PCE Inter-Layer Path Computation
In Figure 2, there is one PCE in each layer. PCEs of each layer
collaborate together to compute an end-to-end path across layers. PCE
Hi is responsible for computations in the higher layer and may
consultEwith PCE Lo to compute paths across the lower layer. PCE Lo
is responsible for path computation in lower layer. A simple
cooperation could be: PCE Hi requests a path H2-H3 to PCE Lo. Of
course more complex cooperation may be required if an end-to-end
optimal path is desired.
-----
| PCE |
| Hi |
--+--
|
----- ----- | ----- -----
| LSR |--| LSR |............|...........| LSR |--| LSR |
| H1 | | H2 | | | H3 | | H4 |
----- -----\ --+-- /----- -----
\ | PCE | /
\ | Lo | /
\ ----- /
\ /
\----- -----/
| LSR |--| LSR |
| L1 | | L2 |
----- -----
Figure 2 : Cooperating Single-Layer PCEs
6. PCC-PCE Communication Requirements for Inter-Layer Traffic
Engineering
This section sets out additional requirements not covered in [PCE-
COM-REQ] specific to the problems of multi-layer TE.
6.1. 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.
6.1.1 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.
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Therefore, a request from a PCC to a PCE MUST support the inclusion
of such an indication.
6.1.2 Control of Type of Path to be Computed
A request from a PCC to a PCE MUST allow controlling the type path to
be computed: A mono-layer path that includes already established
lower layer-LSP, a mono-layer path that includes loose hop(s), or 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.
6.1.3 Communication of Inter-Layer Constraints
A request from a PCC to a PCE MUST support the inclusion of
constraints for multiple layers. This includes the switching type(s)
and encoding type(s) that can, must, or must not be used.
6.1.4 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 [PCE-
ARCH].
The PCC-PCE communication protocol MUST allow requests and replies
for cooperative inter-layer path computation.
6.1.5 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).
6.2. 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].
7. Manageability considerations
Manageability of inter-layer traffic engineering with PCE must
address the following consideration for section 6.1.
- need for a MIB module for control and monitoring
- need for built-in diagnostic tools
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- configuration implication for the protocol
8. 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.
9. Acknowledgment
We would like to thank Kohei Shiomoto and Ichiro Inoue for their
useful comments.
10. References
10.1 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.
10.2 Informative Reference
[PCE-ARCH] A. Farrel, JP. Vasseur and J. Ash, "Path Computation
Element (PCE) Architecture", draft-ietf-pce-architecture (work in
progress).
[PCE-COM-REQ] J. Ash, J.L Le Roux et al., "PCE Communication Protocol
Generic Requirements", draft-ietf-pce-comm-protocol-gen-reqs (work in
progress).
[PCE-DISC-REQ] JL Le Roux et al., "Requirements for Path Computation
Element (PCE) Discovery", draft-ietf-pce-discovery-reqs (work in
progress).
[MRN-REQ] K. Shiomoto et al., "Requirements for GMPLS-based multi-
region networks (MRN) ", draft-shiomoto-ccamp-gmpls-mrn-reqs (work in
progress).
11. AuthorsEAddresses
Eiji Oki
NTT
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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@francetelecom.com
Kenji Kumaki
KDDI Corporation
Garden Air Tower
Iidabashi, Chiyoda-ku,
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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