Internet Draft
Internet Engineering Task Force
INTERNET-DRAFT
Daniel O. Awduche
Expiration Date: January 2001 UUNET (Worldcom)
Yakov Rekhter
Cisco Systems
John Drake
Calient Networks
Rob Coltun
Redback Networks
July 2000
Multi-Protocol Lambda Switching:
Combining MPLS Traffic Engineering Control With Optical Crossconnects
draft-awduche-mpls-te-optical-02 .txt
Status of this Memo
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Abstract
This document describes an approach to the design of control planes
for optical crossconnects (OXCs), which leverages existing control
plane techniques developed for MPLS Traffic Engineering. The
proposed approach combines recent advances in MPLS traffic
engineering control plane constructs with OXC technology to: (1)
provide a framework for real-time provisioning of optical channels in
automatically switched optical networks, (2) foster the expedited
development and deployment of a new class of versatile OXCs, and (3)
allow the use of uniform semantics for network management and
operations control in hybrid networks consisting of OXCs and label
switching routers (LSRs). The proposed approach is particularly
advantageous for OXCs intended for data-centric optical
internetworking systems. In such environments, it will help to
simplify network administration. This approach also paves the way
for the eventual incorporation of DWDM multiplexing capabilities in
IP routers.
1. Introduction
This document describes an approach to the design of control planes
for optical crossconnects (OXCs), which is based on the Multiprotocol
Label Switching (MPLS) traffic engineering control plane model. In
this approach, the main idea it to leverage recent advances in
control plane technology developed for MPLS traffic engineering (see
[1,2,3,4,8,9,10]). This approach is driven by pragmatic
considerations, as it exploits an existing technology base to foster
rapid development and deployment of a new class of versatile OXCs
that address the optical transport needs of the rapidly evolving
Internet. This approach will assist in optical channel layer
bandwidth management, dynamic provisioning of optical channels, and
network survivability through enhanced protection and restoration
capabilities in the optical domain.
As used in this document, an OXC is a path switching element in an
optical transport network that establishes routed paths for optical
channels by locally connecting an optical channel from an input port
(fiber) to an output port (fiber) on the switch element. Additional
characteristics of OXCs, as used in this document, are discussed in
Section 4.1.
The proposed OXC control plane uses the IGP extensions for MPLS
traffic engineering (with additional enhancements) to distribute
relevant optical transport network state information, including
topology state information. This state information is subsequently
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used by a constraint-based routing system to compute paths for
point-to-point optical channels through the optical transport
network. The proposed OXC control plane also uses an MPLS signaling
protocol (see [3,4]) to instantiate point-to-point optical channels
between access points in the optical transport network.
This document does not specify the details of the extensions and
domain specific adaptations required to map the MPLS traffic
engineering control plane model onto the optical domain. These
aspects will be covered in a number of supplementary documents that
will follow. However, in Section 7 of this memo, we provide a high
level overview of the architectural issues involved in making such
adaptations.
2. Advantages
The advantages of the proposed approach are numerous.
- It offers a framework for optical bandwidth management
and the real-time provisioning of optical channels in
automatically switched optical networks.
- It exploits recent advances in MPLS control plane technology
and also leverages accumulated operational experience with IP
distributed routing control.
- It obviates the need to reinvent a new class of control
protocols for optical transport networks and allows reuse
of software artifacts originally developed for the MPLS
traffic engineering application. Consequently, it fosters
the rapid development and deployment of a new class of
versatile OXCs.
- It facilitates the introduction of control coordination concepts
between data network elements and optical network elements.
- It simplifies network administration in facilities based service
provider networks by providing uniform semantics for network
management and control in both the data and optical domains.
- It paves the way for the eventual introduction of DWDM
multiplexing capabilities on IP routers
- Lastly, it establishes a preliminary framework for the notion
of an optical Internet.
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3. Background
The growth, performance, and survivability requirements of the
Internet (which is also becoming very mission critical) are mandating
IP-centric networks to be cost effective, survivable, scalable, and
to provide control capabilities that facilitate network performance
optimization. Some of these requirements are being addressed by the
Multiprotocol Label Switching (MPLS) traffic engineering capabilities
under development by the IETF [1,2,3,4].
The underlying optical transport network also needs to be versatile,
reconfigurable, cost effective, and to support a variety of
protection and restoration capabilities due to the rapidly changing
requirements of the Internet.
A result of these trends, therefore, is the evolution of optical
transport networks from simple linear and ring topologies to mesh
topologies. By a mesh (not necessarily fully meshed) topology, we
mean a connected (not necessarily fully connected) network of
arbitrary topology in which the node degree is typically more than
two. In mesh optical networks, optical crossconnects engender
versatility and reconfigurability by performing switching and
rearrangement functions.
Underscoring the importance of versatile networking capabilities in
the optical domain, a number of standardization organizations and
interoperability forums have initiated work items to study the
requirements and architectures for reconfigurable optical networks.
Refer, for example, to ITU-T recommendation G.872 [5]. This document
defines a functional architecture for an optical transport network
(OTN) that supports the transport of digital client signals. ITU-T
G.872 speaks of an OTN as "a transport network bounded by optical
channel access points"[5]. The ITU-T G.872 OTN architecture is based
on a layered structure, which includes:
(a) an optical channel (OCh) layer network,
(b) an optical multiplex section (OMS) layer network, and
(c) an optical transmission section (OTS) layer network.
The optical channel layer is the most relevant to the discussions in
this document. The optical channel layer network supports end-to-end
networking of optical channel trails between access points. The OCh
layer network provides the following functions: routing, monitoring,
grooming, and protection and restoration of optical channels. In
this situation, programmable Optical crossconnects, with
rearrangeable switch fabrics and relatively smart control planes,
will be critical to the realization of the OCh layer functions,
especially in mesh optical networks. In the data network domain,
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routing, monitoring, and survivability are crucial functions
performed by the MPLS traffic engineering control plane (see
[1,2,3,4,8,9,10]).
Note: Although we mention the ITU-T recommendation G.872, the OXC
control plane design approach described here is not restricted to
G.872 compliant networks. Instead, it can be applied to any mesh
optical network that uses programmable and reconfigurable OXCs.
Other standards organizations and interoperability forums that are
actively pursuing projects related to dynamically reconfigurable
optical networks include the ANSI T1X1.5 committee, the Optical
Internetworking Forum (OIF), and now by virtue of this memo the IETF.
Recent contributions to ANSI T1X1.5 emphasize the need for automation
of the OCh layer network by using appropriate signaling protocols to
establish optical channels in real time (see [12] and [13]).
The Optical Internetworking Forum (http://www.oiforum.com), an
international organization engaged in the development and promotion
of interoperability agreements for optical internetworking systems,
is also evaluating architectural and signaling options related to the
internetworking of data network elements with reconfigurable optical
networks -- to facilitate rapid provisioning, efficient
protection/restoration, and other services in optical internetworking
systems.
In all these cases, the successful realization of the vision of
versatile optical networking depends very much on the synthesis of
appropriate control plane technologies with programmable and
reconfigurable OXCs.
4. OXCs, LSRs, Optical Trails, and Explicit LSPs
Consider a hybrid, IP-centric optical internetworking environment
consisting of both label switching routers (LSRs) and OXCs, where the
OXCs are programmable and support wavelength conversion/translation.
At a level of abstraction, an LSR and an OXC exhibit a number of
isomorphic relations. It is important to enumerate these relations
because they help to expose the reusable software artifacts from the
MPLS traffic engineering control plane model. Architecturally, both
LSRs and OXCs emphasize problem decomposition by decoupling the
control plane from the data plane.
The data plane of an LSR uses the label swapping paradigm to transfer
a labeled packet from an input port to an output port (see e.g.,
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[6,7]). The data plane of an OXC uses a switching matrix to connect
an optical channel trail from an input port to an output port.
An LSR performs label switching by first establishing a relation
between an tuple. Likewise, an OXC provisions optical channel
trails by first establishing a relation between an
tuple. These relations are determined by the control plane of the
respective network elements, and are locally instantiated on the
device through a switch controller. In the LSR, the next hop label
forwarding entry (NHLFE) maintains the input-output relations. In the
OXC, the switch controller reconfigures the internal interconnection
fabric to establish the relations. These relations cannot be altered
by the payload of the data plane.
The functions of the control plane (for both LSRs and OXCs) include
resource discovery, distributed routing control, and connection
management. In particular, the control plane of the LSR is used to
discover, distribute, and maintain relevant state information
associated with the MPLS network, and to instantiate and maintain
label switched paths (LSPs) under various MPLS traffic engineering
rules and policies. An LSP is the path through one or more LSRs
followed by a specific forwarding equivalence class (FEC) (see [7]).
An explicit LSP is one whose route is defined at its origination
node.
The control plane of the OXC, on the other hand, is used to discover,
distribute, and maintain relevant state information associated with
the OTN, and to establish and maintain optical channel trails under
various optical internetworking traffic engineering rules and
policies. An optical channel trail provides a point-to-point optical
connection between two access points. An optical channel trail may
consist of just one wavelength or a concatenation of multiple
wavelengths. If an optical trail consists of just one wavelength,
then it is said to satisfy the "wavelength continuity property." At
each intermediate OXC along the route of an optical channel trail,
OXC switch fabric connects the trail from an input port to an output
port.
A distinction between the current generation of OXCs and LSRs is that
the former do not perform packet level processing in the data plane,
while the later are datagram devices which may perform certain packet
level operations in the data plane. A significant conceptual
difference is that with LSRs the forwarding information is carried
explicitly as part of the labels appended to data packets, while with
OXCs the switching information is implied from the wavelength or
optical channel.
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4.1 Review of Relevant OXC Characteristics
The following section contains a brief overview of relevant OXC
characteristics, focusing on the switching functions and underlying
technologies.
As used in this document, the switching function of an OXC may be
electrical or optical. If the switching fabric is purely electrical,
then the crossconnect is typically referred to as a digital
crossconnect (DXC), or a broadband digital cross-connect (BBDXC) --
if the capacity and port density are sufficiently high. Optical-
Electrical-Optical (OEO) conversion is required in BBDXCs. A BBDXC
may or may not have WDM multiplexing capabilities. If a BBDXC has WDM
multiplexing capabilities, then it may be connected directly to other
compatible WDM devices through optical fiber links that carry
multiple wavelengths per fiber. If a BBDXC does not have WDM
multiplexing capabilities, then it may be connected to an external
DWDM multiplexer through a set of discrete fibers, where each fiber
carries only one wavelength. A BBDXC may also perform regeneration,
reshaping, and re-timing functions.
If the switching fabric of an OXC is completely photonic, then we
refer to the cross-connect as a pure OXC. If the granularity of
channel switching is the wavelength, then the OXC is called a
wavelength routing switch (WRS), or simply a wavelength router. The
problem of establishing optical channel trails using WRS is called
the "Routing and Wavelength Assignment problem" (RWA) [11].
An OXC may also be equipped with both electrical and optical
switching capabilities. In this situation, some channels may be
switched in the electrical domain and others in the optical domain.
Other terms commonly used within the context of optical transport
network switching elements include: wavelength selective
crossconnects (WSXC) and wavelength interchanging crossconnects
(WIXC).
In this document, we use the generic term OXC to refer to all
categories of programmable and reconfigurable crossconnects for
optical transport networks, irrespective of the technologies that
underlie them.
The OXC control plane design approach described in this document is
independent of the underlying OXC switch technologies. It is also
independent of specific OXC implementation details. Local adaptation
mechanisms can be used to tailor the control plane onto various OXC
implementations with different hardware capabilities. As an example,
a local adaption function can map a channel/port input/output
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relation into specialized low level instructions to actuate a
rearrangement of the crossconnect switch fabric such that the
required input/output relation is realized.
A number of forthcoming supplementary documents will describe in some
detail the extensions needed to adapt the control plane approach
described in this memo to various OXC technologies and optical
transport network contexts.
4.2 Explicit LSPs and Optical Channel Trails
At a conceptual level, explicit LSPs and optical channel trails
exhibit certain commonalities. Essentially, they are both
fundamentally unidirectional, point-to-point virtual path connection
abstractions. An explicit LSP provides a parameterized packet
forwarding path (traffic-trunk) between an ingress LSR and an egress
LSR. Correspondingly, an optical channel trail provides a (possibly
parameterized) optical channel between two endpoints for the
transport of client digital signals.
The payload carried by both LSPs and optical trails are transparent
to intermediate nodes along their respective paths. Both LSPs and
optical trails can be parameterized to stipulate their performance,
behavioral, and survivability requirements from the network. A set of
LSPs induce a virtual graph on a data network topology, while a set
of optical trails induce a virtual graph on the topology of a fiber
plant.
A constraint-based routing scheme can be used to select appropriate
paths for both LSPs and optical trails. Generally such paths may
satisfy some demands and policy requirements subject to some
constraints imposed by the operational environment.
There are also commonalities in the allocation of labels to LSPs and
in the allocation of wavelengths to optical trails. Two different
LSPs that traverse through a given LSR port or interface cannot be
allocated the same label. The exception is for LSP aggregation using
label merge or label stacking. Similarly, two different optical
trails that traverse through a given OXC port cannot be allocated the
same wavelength.
5. Generic Requirements for the OXC Control Plane
The following section contains the requirements for the OXC control
plane, with emphasis on the routing components of these requirements.
There are three key aspects to these requirements:
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(1) the capability to establish optical channel trails
expeditiously, (in seconds or even milliseconds rather
than days or months).
(2) the capability to support traffic engineering functions,
(see note below)
(3) the capability to support various protection and restoration
schemes.
Note: the introduction of DWDM and automatically switched optical
networks is unlikely to eliminate the need for traffic engineering.
Instead, it will simply mandate OXCs to also support some traffic
engineering capabilities.
Historically, the "control plane" of optical transport networks has
been implemented via network management. This approach has the
following drawbacks:
(1) It leads to relatively slow convergence following failure
events (typical restoration times are measured in minutes,
or even days and weeks especially in systems that require
explicit manual intervention). The only way to expedite
service recovery in such environments is to pre-provision
dedicated protection channels.
(2) It complicates the task of interworking equipment from
different manufacturers, especially at the management level
(generally, a custom "umbrella network management system
-NMS- or operations support system -OSS-" is required to
integrate otherwise incompatible Element Management Systems
from different vendors)
(3) It precludes the use of distributed dynamic routing control
capabilities in such environments.
(4) It complicates the task of inter-network provisioning (due
to the lack of EDI between operator NMSs).
Thus, another important motivation for the approach described in this
document is to improve the responsiveness of the optical transport
network, and to increase the level of interoperability within and
between service provider networks.
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6. MPLS Traffic Engineering as a Generic Control Plane for OXCs
The requirements for the OXC control plane described in the previous
section have already been addressed by the MPLS Traffic Engineering
control plane, under development by the IETF (see e.g.,
[1,2,3,4,8,9]).
6.1 Overview of the MPLS Traffic Engineering Control Plane
Let us now discuss the components of the MPLS traffic engineering
control plane model. The MPLS traffic engineering control plane is a
synthesis of new concepts in IP traffic engineering (enabled by MPLS)
and the conventional IP network layer control plane. The high level
requirements for traffic engineering over MPLS were articulated in
RFC-2702 [1 ]. It is the combination of the notions defined in RFC-
2702 (including relevant extensions) along with the conventional IP
control plane constructs that effectively establishes a framework for
the MPLS traffic engineering control plane model [1] (see also [2]).
The components of the MPLS traffic engineering control plane model
include the following modules:
- Resource discovery
- State information dissemination, which is used to distribute
relevant information concerning the state of the network,
including topology and resource availability information.
In the MPLS context, this is accomplished by extending
conventional IP link state interior gateway protocols to carry
additional information in their link state advertisements
(see [8,9]).
- Path selection, which is used to select an appropriate route
through the MPLS network for explicit routing. It is implemented
by introducing the concept of constraint-based routing which is
used to compute paths that satisfy certain specifications subject
to certain constraints, including constraints imposed by the
operational environment (see [1]).
- Path management, which includes label distribution, path
placement, path maintenance, and path revocation. These are
used to establish, maintain, and tear down LSPs in the MPLS
context. The label distribution, path placement, and path
revocation functions are implemented through a signaling
protocol, such as the RSVP extensions [3] or through CR-LDP [4].
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These components of the MPLS traffic engineering control plane are
separable, and independent of each other. This is a very attractive
feature because it allows an MPLS control plane to be implemented
using a composition or synthesis of best of breed modules.
In RFC-2702 [1 ], several new MPLS control plane capabilities were
proposed that allow various traffic engineering policies to be
actualized in MPLS networks. Many of these capabilities are also
relevant and applicable to automatically switched optical transport
networks with reconfigurable OXCs.
We summarize some of these capabilities below, focusing on the set of
attributes that can be associated with traffic-trunks. A traffic-
trunk is an aggregation of traffic belonging to the same class which
are forwarded through a common path. In general, the traffic-trunk
concept is a technology independent abstraction. In [1], it was used
within the context of MPLS and allowed certain attributes of the
traffic transported through LSPs to be parameterized. The traffic-
trunk concept can also be extended, in an obvious manner, to the
optical transport network.
As stipulated in RFC-2702 [1 ], the attributes that can can be
associated with traffic-trunks include: (1) traffic parameters which
indicate the bandwidth requirements of the traffic-trunk, (2)
adaptivity attributes which specify the sensitivity of the traffic-
trunk to changes in the state of the network and in particular
indicates whether the traffic-trunk can be re-routed when "better"
paths become available, (3) priority attributes which impose a
partial order on the set of traffic-trunks and allow path selection
and path placement operations to be prioritized, (4) preemption
attributes which indicate whether a traffic-trunk can preempt an
existing traffic-trunk from its path, (5) resilience attributes which
stipulate the survivability requirements of the traffic-trunk and in
particular the response of the system to faults that impact the path
of the traffic-trunk, and (6) resource class affinity attributes
which further restrict route selection to specific subsets of
resources and in particular allow generalized inclusion and exclusion
policies to be implemented. Other policy attributes and options are
also defined by Awduche et al in RFC-2702 [1 ] for traffic-trunks,
including policing attributes [1] (policing is irrelevant in the OXC
context). Concepts of subscription (booking) factors are also
supported to either bound the utilization of network resources
through under-subscription or to exploit statistical multiplexing
gain through over-subscription (this aspect is also not very relevant
in the OXC context).
It should be clear that a subset of these capabilities can be mapped
onto an optical transport network by substituting the term "traffic-
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trunk" with the term "optical channel trail."
The MPLS control plane also supports the notion of abstract nodes.
An abstract node is essentially a set of nodes (e.g., a subnet, an
autonomous system, etc) whose internal topology is opaque to the
origination node of an explicit LSP. So, in the most general manner,
the route of an explicit LSP (or traffic-trunk) can be specified as a
sequence of single hops and/or as a sequence of abstract nodes.
The MPLS control plane is very general and is also oblivious of the
specifics of the data plane technology. In this regard, the MPLS
control plane can be used in conjunction with a data plane that (a)
does not necessarily process IP packet headers and (b) does not know
about IP packet boundaries. For an existence proof, note that the
MPLS control plane has been implemented on IP-LSRs, ATM-LSRs, and
Frame Relay-LSRs.
The MPLS control plane may also be implemented on OXCs as discussed
in this document.
6.2 Synthesizing the MPLS Traffic Engineering Control Plane with OXCs
Given that that both OXCs and LSRs require control planes, one option
would be to have two separate, independent, and incompatible control
planes - one for OXCs, and another for LSRs. To understand the
drawbacks of this approach, especially in IP-centric optical
internetworking systems, one need to look no further than the
experience with IP over ATM, where IP has its own control plane (BGP,
IS-IS, OSPF), and ATM its own control plane (PNNI) [12]. For some of
the drawbacks see [1,2].
Given that the control planes for both OXCs and LSRs have relatively
similar requirements, an alternative approach is to develop a
coherent control plane technology that can be used for LSRs and for
OXCs. Such a uniform control plane will eliminate the administrative
complexity of managing hybrid optical internetworking systems with
separate, dissimilar control and operational semantics.
Specializations may be introduced in the control plane, as necessary,
to account for inherent peculiarities of the underlying technologies
and networking contexts.
All of the above observations suggest, therefore, that the MPLS
Traffic Engineering control plane (with some minor extensions) would
be very suitable as the control plane for OXCs. An OXC that uses the
MPLS traffic engineering control plane would effectively become an IP
addressable device. Thus, this proposition also solves the problem of
addressing for OXCs. The distribution of topology state information,
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establishment of optical channel trails, OTN traffic engineering
functions, and protection and restoration capabilities would be
faciliated by the MPLS Traffic Engineering control plane.
An out-of-band IP communications system can be used to carry and
distribute control traffic between the control planes of OXCs,
perhaps through dedicated supervisory channels (using e.g., dedicated
wavelengths or channels, or an independent out-of-band IP network).
In this environment, SNMP, or some other network management
technology, could be used for element management. From the
perspective of control semantics, an OXC with an MPLS Traffic
Engineering control plane would resemble a Label Switching Router.
If the OXC is a wavelength routing switch, then the physical fiber
between a pair of OXCs would represent a single link in the OTN
network topology. Individual wavelengths or channels would be
analogous to labels. If there are multiple fibers between a pair of
OXCs, then as an option, these multiple fibers could be logically
grouped together through a process called bundling and represented as
a single link in the OTN network topology.
If a fiber terminates on a device that functions as both an OXC and
an IP router, then the following situation may be possible:
- A subset of optical channels within the fiber may be uncommitted.
That is, they are not currently in use and hence are available for
allocation.
- A second subset of channels may already be committed for transit
purposes. That is, they are already cross-connected by the OXC
element to other out-bound optical channels and thus are not
immediately available for allocation.
- Another subset of optical channels (within the same fiber) could
be
in use as terminal channels. That is, they are already allocated
but terminate on the local OXC/router device, for example, as
SONET
interfaces.
In the above scenario one way to represent the fiber in the OTN
network topology is to depict it is as several links, where one of
these links would represent the set of uncommitted channels which
constitute the residual capacity of the fiber; while each terminal
channel that terminates on the OXC/router could be represented as an
individual link.
In the control plane model described here, IS-IS or OSPF, with
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extensions for traffic engineering ([8] or [9]) and possibly
additional optical network specific extensions would be used to
distribute information about the optical transport network topology
and information about available bandwidth and available channels per
fiber, as well as other OTN network topology state information. This
information will then be used to compute explicit routes for optical
channel trails. An MPLS signaling protocol, such as RSVP extensions
(see [RSVP]), will be used to instantiate the optical channel trails.
Using the RSVP extensions, for example, the wavelength information or
optical channel information (as the case may be) will be carried in
the LABEL object, which will be used to control and reconfigure the
OXCs.
The use of a uniform control plane technology for both LSRs and OXCs
introduces a number of interesting (and potentially advantageous)
architectural possibilities. One such possibility is that a single
control plane (MPLS Traffic Engineering) may be able to span both
routers and OXCs. In such an environment a Label Switched Path could
traverse an intermix of routers and OXCs, or could span just routers,
or just OXCs. This offers the potential for real bandwidth-on-demand
networking, in which an IP router may dynamically request bandwidth
services from the optical transport network. Another possibility is
that OXCs and LSRs may run different instances of the control plane
which are decoupled with little or no interaction between the control
plane instances.
To bootstrap the system, OXCs must be able to exchange control
information. One way to support this is to pre-configure a dedicated
control wavelength between each pair of adjacent OXCs, or between an
OXC and a router, and to use this wavelength as a supervisory channel
for exchange of control traffic. Another possibility, which has
already been mentioned, is to construct a dedicated out of band IP
network for the distribution of control traffic.
Even though an OXC equipped with an MPLS traffic engineering control
plane would (from a control perspective) resemble a Label Switching
Router, there are some important distinctions and limitations. One
distinction concerns the fact that there are no analogs of label
merging in the optical domain. This implies that an OXC cannot merge
several wavelengths into one wavelength. Another distinction is that
an OXC cannot perform the equivalent of label push and pop operations
in the optical domain. This is because the analog of a label in the
OXC is a wavelength or an optical channel, and the concept of pushing
and popping wavelengths is infeasible with contemporary commercial
optical technologies.
In the proposed control plane approach, an OXC will maintain a WFIB
(Wavelength Forwarding Information Base) per interface (or per
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fiber). This is because lambdas and/or channels (labels) are specific
to a particular interface (fiber), and the same lambda and/or channel
(label) could be used concurrently on multiple interfaces (fibers).
The MPLS traffic engineering control plane is already being
implemented on data plane technologies that exhibit some of the
aforementioned distinctions. For example, an ATM-LSR supports only a
subset of the MPLS functionality. In particular, most ATM-LSRs are
incapable of merging Label Switching Paths, and may not be able to
perform label push/pop operations as well. Also, similar to the
approach proposed here for OXCs, ATM-LSRs have per interface LFIB
(Label Forwarding Information Base).
Yet another important distinction concerns the granularity of
resource allocation. An MPLS Label Switching Router which operates in
the electrical domain can potentially support an arbitrary number of
LSPs with arbitrary bandwidth reservation granularities (bounded by
the maximum reservable bandwidth per interface, the label space, and
the amount of required control overhead). In sharp contrast, an OXC
can only support a relatively small number of optical channel trails
(this may change as the technology evolves), each of which will have
coarse discrete bandwidth granularities (e.g.,OC-12, OC-48, OC-192,
and OC-768). A special degenerate case occurs when the control plane
is used to establish optical channel trails which all have a fixed
bandwidth (e.g., OC-48).
If the bandwidth associated with an LSP is small relative to the
capacity of an optical channel trail, then very inefficient
utilization of network resources could result if only one LSP is
mapped onto a given optical channel trail. To improve utilization of
resources, therefore, it is necessary to be able to map several low
bandwidth LSPs onto a relatively high capacity optical channel trail.
For this purpose, a generalized notion of "nested LSPs" may be used.
Note that since an OXC cannot perform label push/pop operations, the
start/end of a nested LSP has to be on a router (as nesting requires
label push/pop). Also note that in this nesting situation, it is the
wavelength of the "container" optical channel trail itself that
effectively constitutes the outermost label.
The transparency and multiprotocol properties of the MPLS Control
Plane approach would allow an OXC to route optical channel trails
carrying various types of digital payloads (including IP, ATM, Sonet,
etc) in a coherent and uniform way.
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7. Control Adaptation
This section provides a high level overview of the architectural
considerations involved in tailoring the MPLS traffic engineering
control plane model to the optical domain. More detailed discussions
of these issues will be provided in a number of supplementary
documents to follow.
In adapting the MPLS traffic engineering control plane model to OXCs,
a number of critical issues should be considered. One critical issue
concerns the development of OTN specific domain models which
abstracts and captures relevant characteristics of the OTN. The
domain models help to delineate the design space for the control
plane problem in OXCs, and to construct domain specific software
reference architectures.
A domain model includes functional and structural aspects. For the
purpose of the present discussions, however, we have grouped the
considerations pertaining to OTN domain models into two broad
categories: (1) a horizontal dimension and (2) a vertical dimension.
The horizontal dimension pertains to the specialized networking
requirements of the OTN environment. It indicates the enhancements
needed to the MPLS TE control plane model to address the peculiar OTN
networking requirements. The vertical dimension pertains to
localized hardware and software characteristics of the OXCs, which
helps to determine the device specific adaptations and support
mechanisms needed to port and reuse the MPLS TE control plane
software artifacts on an OXC.
Horizontal dimension considerations include the following aspects:
- What type of OTN state information should be discovered and
disseminated to support path selection for optical channel
trails? Such state information may include domain specific
characteristics of the OTN (encoded as metrics), such as
attenuation, dispersion (chromatic, PMD), etc. This aspect will
determine the type of additional extensions that are required
to IGP link state advertisements to allow distribute such
information.
- What infrastructure will be used to propagate the control
information?
- How are constrained paths computed for optical channel trails
which fulfill a set of performance and policy requirements
subject to a set of system constraints?
- What are the domain specific requirements for setting up optical
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channel trails and what are the enhancements needed to existing
MPLS signaling protocols to address these requirements?
Vertical dimension considerations include the aspects required to
practically port MPLS control plane software onto an OXC. In terms
of vertical dimensions, a candidate system architecture for an OXC
equipped with an MPLS control plane model is shown in Figure 1 below.
--------------------------------
| OXC WITH MPLS CONTROL PLANE |
| |
| ------------------- |
| | | |
| | MPLS Control Plane| |
| | | |
| ------------------- |
| | |
| ------------------- |
| | | |
| |Control Adaptation | |
| ------------------- |
| | OXC Switch | |
| | Controller | |
| ------------------- |
| | |
| ------------------- |
| | | |
| | OXC Switch Fabric | |
| | OXC Data Plane | |
| ------------------- |
| |
--------------------------------
Figure 1: Candidate OXC systems architecture
8. Architectural Considerations for Deployment in Operational Networks
This section provides a high level overview of the architectural
considerations for deployment of the proposed control plane in
operational networks consisting of LSRs and OXCs. These architectural
issues have implications on the degree of control isolation, control
coupling, and control cohesion between LSRs and OXCs.
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Essentially, there are two extremal architectural options for
deployment of the proposed control plane in an operational context
consisting of LSRs and OXCs.
- Overlay Option: One option is to use different instances of the
control plane in the OTN (OXC) and IP (LSR) domains. In this
situation, each instance of the control plane will operate
independent of the other. Interworking (including control
coordination) between the two domains can be established through
static configuration or through some other procedures that are
outside the scope of this document. This partitioned and
explicitely decoupled deployment option allows maximal
control isolation between the OTN and IP domains. This scheme is
conceptually similar to the model in use today, whereby the OTN
simply provides point-to-point channels between IP network
elements with very minimal control interaction between the two
domains.
- Peer Option: Another option is to use a single instance of the
control plane that subsumes and spans LSRs and OXCs.
Other architectural options are also possible which allow various
degrees of control isolation and control integration between the OXCs
and LSRs.
To improve scalability the control plane may use routing hierarchy
(e.g., routing areas). Hierarchy may be applied in either of the
situations mentioned above. Furthermore, in the overlay option with
different control plane instances for OXCs and LSRs, hierarchy could
be enabled for each control plane instance independent of the other.
In the deployment option with a single instance of the control plane,
each routing area may maintain a link state database that contains:
(1) physical LSPs (fiber links), (2) optical LSPs (optical channel
trails), and (3) logical LSPs (conventional label switched paths). As
a general rule, all of these path-oriented connection entities could
simply be considered as LSPs with different characteristics. The
origination LSR (the head-end) of each LSP entity may locally decide
whether to advertise the LSP (with appropriate attributes), so that
other LSRs could use it as a link for subsequent path computations.
There are significant tradeoffs to the above deployment options,
including aspects related to scalability and fault isolation.
Additional documents to follow may elaborate on some of these
aspects.
One of the advantages of the control plane design approach described
in this memo is that it potentially allows network administrators the
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leeway to make these deployment architectural decisions based on
their specific objectives, network contexts, and service models.
9. Summary
This document outlined how the MPLS Traffic Engineering control plane
could be adapted and reused as the control plane for optical
crossconnects. Such a control plane would be used to distribute
optical transport network topology state information and to setup
optical channel trails. Such a control plane would support various
traffic engineering functions in the optical domain, and enable a
variety of protection and restoration capabilities. Furthermore,
such a control plane technology would expedite the development and
deployment of a new class of versatile data-centric OXCs.
Additionally, the proposed control plane approach would simplify
integration of OXCs and label switching routers. Finally, the
proposed control plane approach would provide coherent semantics for
network management and operations control in hybrid optical
internetworking systems consisting of LSRs and OXCs.
References:
[1] D. Awduche, J. Malcolm, J. Agogbua, M. O'Dell, and J.
McManus,"Requirements for Traffic Engineering Over MPLS," RFC-
2702 ,September 1999.
[2] D. Awduche, "MPLS and Traffic Engineering in IP Networks," IEEE
Communications Magazine, December, 1999.
[3] D. Awduche, L. Berger, D. Gan, T. Li, G. Swallow, and V.
Srinivasan, "Extensions to RSVP for LSP Tunnels," Internet Draft,Work
in Progress, 1999
[4] B. Jamoussi et al, "Constraint-Based LSP Setup using
LDP,"Internet Draft, Work in Progress, 1999
[5] ITU-T G.872, "Architecture for Optical Transport Networks," 1999.
[6] R. Callon, P. Doolan, N. Feldman, A. Fredette, G. Swallow,A.
Viswanathan, "A Framework for Multiprotocol Label Switching,"Internet
Draft, Work in Progress, 1999
[7] E. Rosen, A. Viswanathan, R. Callon, "Multiprotocol Label
Awduche/Rekhter, et al [Page 19]
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Switching Architecture," Internet Draft, Work in Progress, 1999
[8] H. Smit and T. Li, "IS-IS extensions for Traffic
Engineering,"Internet Draft, Work in Progress, 1999
[9] D. Katz, D. Yeung, "Traffic Engineering Extensions to
OSPF,"Internet Draft, Work in Progress, 1999
[10] Y. Rekhter and T. Li, "A Border Gateway Protocol 4 (BGP-
4),"RFC-1171 , March 1995
[11] B. Mukherjee, "Optical Communications Networks," McGraw-Hill,
1997.[14]
[12] G. Newsome and P. Bonenfant, "The Automatic Switched Optical
Network," Contribution to T1 STANDARDS PROJECT - T1X1.5, 1999.
[13] P. Bonenfant and and X. Liu, "A Proposal for Automatically
Switched Optical Channel Networks (ASON)," Contribution to T1
STANDARDS PROJECT - T1X1.5, 1999.
[14] "T. Worster, "General Switch Management Protocol," Internet
Draft, Work in Progress, 1999.
[15] E. Crawley, R. Nair, B. Rajagopalan, and H. Sandick, "A
Framework for QoS-Based Routing in the Internet," RFC-2386 , August,
1998.
Security Considerations
It is imperative to guarantee the integrity and confidentiality of
control information used by the proposed OXC control plane. This can
be accomplished by using existing security mechanisms for the various
components of the MPLS traffic engineering control plane model.
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Author's Addresses
Daniel O. Awduche
UUNET (Worldcom)
22001 Loudoun County Parkway
Ashburn, VA 20147
Phone: 703-886-5277
Email: awduche@uu.net
Yakov Rehkter
Cisco Systems
170 W. Tasman Dr.
San Jose, CA 95134
Email: yakov@cisco.com
John Drake
Calient Networks
5853 Rue Ferrari
San Jose, CA 95138
Phone: (408) 972-3720
Email: jdrake@calient.net
Rob Coltun
Redback Networks
300 Ferguson Drive
Mountain View, CA 94043
Phone: (650) 390-9030
Email: redback.com
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