Request for Comments: 3717 Consultant
Category: Informational J. Luciani
Marconi Communications
D. Awduche
MCI
March 2004
IP over Optical Networks: A Framework
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
The Internet transport infrastructure is moving towards a model of
high-speed routers interconnected by optical core networks. The
architectural choices for the interaction between IP and optical
network layers, specifically, the routing and signaling aspects, are
maturing. At the same time, a consensus has emerged in the industry
on utilizing IP-based protocols for the optical control plane. This
document defines a framework for IP over Optical networks,
considering both the IP-based control plane for optical networks as
well as IP-optical network interactions (together referred to as "IP
over optical networks").
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology and Concepts . . . . . . . . . . . . . . . . . . . 4
3. The Network Model. . . . . . . . . . . . . . . . . . . . . . . 8
3.1. Network Interconnection. . . . . . . . . . . . . . . . . 8
3.2. Control Structure. . . . . . . . . . . . . . . . . . . . 11
4. IP over Optical Service Models and Requirements. . . . . . . . 13
4.1. Domain Services Model. . . . . . . . . . . . . . . . . . 13
4.2. Unified Service Model. . . . . . . . . . . . . . . . . . 14
4.3. Which Service Model? . . . . . . . . . . . . . . . . . . 15
4.4. What are the Possible Services?. . . . . . . . . . . . . 16
5. IP transport over Optical Networks . . . . . . . . . . . . . . 16
5.1. Interconnection Models . . . . . . . . . . . . . . . . . 17
5.2. Routing Approaches . . . . . . . . . . . . . . . . . . . 18
5.3. Signaling-Related. . . . . . . . . . . . . . . . . . . . 21
5.4. End-to-End Protection Models . . . . . . . . . . . . . . 23
6. IP-based Optical Control Plane Issues. . . . . . . . . . . . . 25
6.1. Addressing . . . . . . . . . . . . . . . . . . . . . . . 25
6.2. Neighbor Discovery . . . . . . . . . . . . . . . . . . . 27
6.3. Topology Discovery . . . . . . . . . . . . . . . . . . . 28
6.4. Protection and Restoration Models. . . . . . . . . . . . 29
6.5. Route Computation. . . . . . . . . . . . . . . . . . . . 30
6.6. Signaling Issues . . . . . . . . . . . . . . . . . . . . 32
6.7. Optical Internetworking. . . . . . . . . . . . . . . . . 34
7. Other Issues . . . . . . . . . . . . . . . . . . . . . . . . . 35
7.1. WDM and TDM in the Same Network. . . . . . . . . . . . . 35
7.2. Wavelength Conversion. . . . . . . . . . . . . . . . . . 36
7.3. Service Provider Peering Points. . . . . . . . . . . . . 36
7.4. Rate of Lightpath Set-Up . . . . . . . . . . . . . . . . 36
7.5. Distributed vs. Centralized Provisioning . . . . . . . . 37
7.6. Optical Networks with Additional Configurable
Components . . . . . . . . . . . . . . . . . . . . . . . 38
7.7. Optical Networks with Limited Wavelength Conversion
Capability . . . . . . . . . . . . . . . . . . . . . . . 38
8. Evolution Path for IP over Optical Architecture. . . . . . . . 39
9. Security Considerations. . . . . . . . . . . . . . . . . . . . 41
9.1. General Security Aspects . . . . . . . . . . . . . . . . 42
9.2. Security Considerations for Protocol Mechanisms. . . . . 43
10. Summary and Conclusions. . . . . . . . . . . . . . . . . . . . 44
11. Informative References . . . . . . . . . . . . . . . . . . . . 44
12. Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . 45
13. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 46
14. Authors’ Addresses . . . . . . . . . . . . . . . . . . . . . . 47
15. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 48
1. Introduction
Optical network technologies are evolving rapidly in terms of
functions and capabilities. The increasing importance of optical
networks is evidenced by the copious amount of attention focused on
IP over optical networks and related photonic and electronic
interworking issues by all major network service providers,
telecommunications equipment vendors, and standards organizations. In
this regard, the term "optical network" is used generically in
practice to refer to both SONET/SDH-based transport networks, as well
as switched optical networks (including all-optical networks).
It has been realized that optical networks must be survivable,
flexible, and controllable. There is, therefore, an ongoing trend to
introduce intelligence in the control plane of optical networks to
make them more versatile [1]. An essential attribute of intelligent
optical networks is the capability to instantiate and route optical
layer connections in real-time or near real-time, and to provide
capabilities that enhance network survivability. Furthermore, there
is a need for multi-vendor optical network interoperability, when an
optical network may consist of interconnected vendor-specific optical
sub-networks.
The optical network must also be versatile because some service
providers may offer generic optical layer services that may not be
client-specific. It would therefore be necessary to have an optical
network control plane that can handle such generic optical services.
There is general consensus in the industry that the optical network
control plane should utilize IP-based protocols for dynamic
provisioning and restoration of optical channels within and across
optical sub-networks. This is based on the practical view that
signaling and routing mechanisms developed for IP traffic engineering
applications could be re-used in optical networks. Nevertheless, the
issues and requirements that are specific to optical networking must
be understood to suitably adopt and adapt the IP-based protocols.
This is especially the case for restoration, and for routing and
signaling in all-optical networks. Also, there are different views
on the model for interaction between the optical network and client
networks, such as IP networks. Reasonable architectural alternatives
in this regard must be supported, with an understanding of their
relative merits.
Thus, there are two fundamental issues related to IP over optical
networks. The first is the adaptation and reuse of IP control plane
protocols within the optical network control plane, irrespective of
the types of digital clients that utilize the optical network. The
second is the transport of IP traffic through an optical network
together with the control and coordination issues that arise
therefrom.
This document defines a framework for IP over optical networks
covering the requirements and mechanisms for establishing an IP-
centric optical control plane, and the architectural aspects of IP
transport over optical networks. In this regard, it is recognized
that the specific capabilities required for IP over optical networks
would depend on the services expected at the IP-optical interface as
well as the optical sub-network interfaces. Depending on the
specific operational requirements, a progression of capabilities is
possible, reflecting increasingly sophisticated interactions at these
interfaces. This document therefore advocates the definition of
"capability sets" that define the evolution of functionality at the
interfaces as more sophisticated operational requirements arise.
This document is organized as follows. In the next section,
terminology covering some basic concepts related to this framework
are described. The definitions are specific to this framework and
may have other connotations elsewhere. In Section 3, the network
model pertinent to this framework is described. The service model
and requirements for IP-optical, and multi-vendor optical
internetworking are described in Section 4. This section also
considers some general requirements. Section 5 considers the
architectural models for IP-optical interworking, describing the
relative merits of each model. It should be noted that it is not the
intent of this document to promote any particular model over the
others. However, particular aspects of the models that may make one
approach more appropriate than another in certain circumstances are
described. Section 6 describes IP-centric control plane mechanisms
for optical networks, covering signaling and routing issues in
support of provisioning and restoration. The approaches described in
Section 5 and 6 range from the relatively simple to the
sophisticated. Section 7 describes a number of specialized issues in
relation to IP over optical networks. Section 8 describes a possible
evolution path for IP over optical networking capabilities in terms
of increasingly sophisticated functionality that may be supported as
the need arises. Section 9 considers security issues pertinent to
this framework. Finally, the summary and conclusion are presented in
Section 10.
2. Terminology and Concepts
This section introduces terminology pertinent to this framework and
some related concepts. The definitions are specific to this
framework and may have other interpretations elsewhere.
WDM
Wavelength Division Multiplexing (WDM) is a technology that allows
multiple optical signals operating at different wavelengths to be
multiplexed onto a single optical fiber and transported in parallel
through the fiber. In general, each optical wavelength may carry
digital client payloads at a different data rate (e.g., OC-3c, OC-
12c, OC- 48c, OC-192c, etc.) and in a different format (SONET,
Ethernet, ATM, etc.). For example, there are many commercial WDM
networks in existence today that support a mix of SONET signals
operating at OC-48c (approximately 2.5 Gbps) and OC-192
(approximately 10 Gbps) over a single optical fiber. An optical
system with WDM capability can achieve parallel transmission of
multiple wavelengths gracefully while maintaining high system
performance and reliability. In the near future, commercial dense
WDM systems are expected to concurrently carry more than 160
wavelengths at data rates of OC-192c and above, for a total of 1.6
Tbps or more. The term WDM will be used in this document to refer to
both WDM and DWDM (Dense WDM).
In general, it is worth noting that WDM links are affected by the
following factors, which may introduce impairments into the optical
signal path:
1. The number of wavelengths on a single fiber.
2. The serial bit rate per wavelength.
3. The type of fiber.
4. The amplification mechanism.
5. The number and type of nodes through which the signals pass before
reaching the egress node or before regeneration.
All these factors (and others not mentioned here) constitute domain
specific features of optical transport networks. As noted in [1],
these features should be taken into account in developing standards
based solutions for IP over optical networks.
Optical cross-connect (OXC)
An OXC is a space-division switch that can switch an optical data
stream from an input port to a output port. Such a switch may
utilize optical-electrical conversion at the input port and
electrical-optical conversion at the output port, or it may be all-
optical. An OXC is assumed to have a control-plane processor that
implements the signaling and routing protocols necessary for
computing and instantiating optical channel connectivity in the
optical domain.
Optical channel trail or Lightpath
An optical channel trail is a point-to-point optical layer connection
between two access points in an optical network. In this document,
the term "lightpath" is used interchangeably with optical channel
trail.
Optical mesh sub-network
An optical sub-network, as used in this framework, is a network of
OXCs that supports end-to-end networking of optical channel trails
providing functionality like routing, monitoring, grooming, and
protection and restoration of optical channels. The interconnection
of OXCs in this network can be based on a general mesh topology. The
following sub-layers may be associated with this network:
(a) An optical multiplex section (OMS) layer network: The optical
multiplex section layer provides transport for the optical
channels. The information contained in this layer is a data
stream comprising a set of optical channels, which may have a
defined aggregate bandwidth.
(b) An optical transmission section (OTS) layer network: This layer
provides functionality for transmission of optical signals
through different types of optical media.
This framework does not address the interaction between the optical
sub-network and the OMS, or between the OMS and OTS layer networks.
Mesh optical network (or simply, "optical network")
A mesh optical network, as used in document, is a topologically
connected collection of optical sub-networks whose node degree may
exceed 2. Such an optical network is assumed to be under the purview
of a single administrative entity. It is also possible to conceive
of a large scale global mesh optical network consisting of the
voluntary interconnection of autonomous optical networks, each of
which is owned and administered by an independent entity. In such an
environment, abstraction can be used to hide the internal details of
each autonomous optical cloud from external clouds.
Optical internetwork
An optical internetwork is a mesh-connected collection of optical
networks. Each of these networks may be under a different
administration.
Wavelength continuity property
A lightpath is said to satisfy the wavelength continuity property if
it is transported over the same wavelength end-to-end. Wavelength
continuity is required in optical networks with no wavelength
conversion feature.
Wavelength path
A lightpath that satisfies the wavelength continuity property is
called a wavelength path.
Opaque vs. transparent optical networks
A transparent optical network is an optical network in which optical
signals are transported from transmitter to receiver entirely in the
optical domain without OEO conversion. Generally, intermediate
switching nodes in a transparent optical network do not have access
to the payload carried by the optical signals.
Note that amplification of signals at transit nodes is permitted in
transparent optical networks (e.g., using Erbium Doped Fiber
Amplifiers << EDFAs).
On the other hand, in opaque optical networks, transit nodes may
manipulate optical signals traversing through them. An example of
such manipulation would be OEO conversion which may involve 3R
operations (reshaping, retiming, regeneration, and perhaps
amplification).
Trust domain
A trust domain is a network under a single technical administration
in which adequate security measures are established to prevent
unauthorized intrusion from outside the domain. Hence, it may be
assumed that most nodes in the domain are deemed to be secure or
trusted in some fashion. Generally, the rule for "single"
administrative control over a trust domain may be relaxed in practice
if a set of administrative entities agree to trust one another to
form an enlarged heterogeneous trust domain. However, to simplify
the discussions in this document, it will be assumed, without loss of
generality, that the term trust domain applies to a single
administrative entity with appropriate security policies. It should
be noted that within a trust domain, any subverted node can send
control messages which can compromise the entire network.
Flow
In this document, the term flow will be used to signify the smallest
non-separable stream of data, from the point of view of an endpoint
or termination point (source or destination node). The reader should
note that the term flow is heavily overloaded in contemporary
networking literature. In this document, we will consider a
wavelength to be a flow, under certain circumstances. However, if
there is a method to partition the bandwidth of the wavelength, then
each partition may be considered a flow, for example using time
division multiplexing (TDM), it may be feasible to consider each
quanta of time within a given wavelength as a flow.
Traffic Trunk
A traffic trunk is an abstraction of traffic flow traversing the same
path between two access points which allows some characteristics and
attributes of the traffic to be parameterized.
3. The Network Model
3.1. Network Interconnection
The network model considered in this memo consists of IP routers
attached to an optical core internetwork, and connected to their
peers over dynamically established switched optical channels. The
optical core itself is assumed to be incapable of processing
individual IP packets in the data plane.
The optical internetwork is assumed to consist of multiple optical
networks, each of which may be administered by a different entity.
Each optical network consists of sub-networks interconnected by
optical fiber links in a general topology (referred to as an optical
mesh network). This network may contain re-configurable optical
equipment from a single vendor or from multiple vendors. In the near
term, it may be expected that each sub-network will consist of
switches from a single vendor. In the future, as standardization
efforts mature, each optical sub-network may in fact contain optical
switches from different vendors. In any case, each sub-network
itself is assumed to be mesh-connected internally. In general, it
can be expected that topologically adjacent OXCs in an optical mesh
network will be connected via multiple, parallel (bi-directional)
optical links. This network model is shown in Figure 1.
In this environment, an optical sub-network may consist entirely of
all-optical OXCs or OXCs with optical-electrical-optical (OEO)
conversion. Interconnection between sub-networks is assumed to be
implemented through compatible physical interfaces, with suitable
optical-electrical conversions where necessary. The routers that
have direct physical connectivity with the optical network are
referred to as "edge routers" with respect to the optical network. As
shown in Figure 1, other client networks (e.g., ATM) may also connect
to the optical network.
The switching function in an OXC is controlled by appropriately
configuring the cross-connect fabric. Conceptually, this may be
viewed as setting up a cross-connect table whose entries are of the
form <input port i, output port j>, indicating that the data stream
entering input port i will be switched to output port j. In the
context of a wavelength selective cross-connect (generally referred
to as a WXC), the cross-connect tables may also indicate the input
and output wavelengths along with the input and output ports. A
lightpath from an ingress port in an OXC to an egress port in a
remote OXC is established by setting up suitable cross-connects in
the ingress, the egress and a set of intermediate OXCs such that a
continuous physical path exists from the ingress to the egress port.
Optical paths tend to be bi-directional, i.e., the return path from
the egress port to the ingress port is typically routed along the
same set of intermediate interface cards as the forward path, but
this may not be the case under all circumstances.
Optical Network
+---------------------------------------+
| |
| Optical Subnetwork |
+---------+ | +-----------------------------------+ |
| | | | +-----+ +-----+ +-----+ | |
| IP | | | | | | | | | | |
| Network +-UNI --+-+ OXC +------+ OXC +------+ OXC + | |
| | | | | | | | | | | |
+---------+ | | +--+--+ +--+--+ +--+--+ | |
| +----|------------|------------|----+ |
| | | | |
| INNI INNI INNI |
+---------+ | | | | |
| | | +----+------+ | +-------+----+ |
| IP + UNI- | | +-----+ | | |
| Network | | | Optical | | Optical | |
| | | |Subnetwork +---INNI---+ Subnetwork | |
+---------+ | | | | | |
| +-----+-----+ +------+-----+ |
| | | |
+-------+-----------------------+-------+
| |
ENNI ENNI