One document matched: draft-many-ip-optical-framework-01.txt
Differences from draft-many-ip-optical-framework-00.txt
Bala Rajagopalan
Internet Draft Tellium, Inc.
draft-many-ip-optical-framework-01.txt James Luciani
Expires on: 1/14/2001 Tollbridge Technologies
Daniel Awduche
UUNET (MCI Worldcom)
Brad Cain, Bilel Jamoussi
Nortel Networks
IP over Optical Networks: A Framework
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026 except that the right to
produce derivative works is not granted.
Internet Drafts are working documents of the Internet Engineering
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progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
1. Abstract
The Internet transport infrastructure is moving towards a model of
high-speed routers interconnected by optical core networks. A
consensus is emerging in the industry on utilizing IP-centric
protocols for the optical control plane. At the same time, there is
ongoing activity in defining models for IP transport over optical
networks, specifically, the routing and signaling aspects. This
draft defines a framework for IP over Optical networks, considering
both the IP control plane for optical networks as well as IP
transport over optical networks together referred to as "IP over
optical networks"). This document is complementary to the
framework of Multi Protocol Lambda Switching proposed in [1].
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2. 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 [1].
3. Introduction
Optical network technologies have evolved rapidly in terms of
functions, capabilities, and densities. The increasing importance of
optical transport networks is evidenced by the copious amount of
attention focused on IP over optical networks and related photonic
and electronic interworking issues by all the major network service
providers, telecommunications equipment vendors, and standards
organizations.
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 transport
systems to make them more versatile [1]. An essential attribute of
intelligent optical transport networks is the capability to
instantiate and route lightpaths 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 transport network may consist of
interconnected vendor-specific optical sub-networks [2].
The optical network must also be versatile because some service
providers and network contexts may provide generic optical layer
services that may not be specific to any digital clients above the
optical transport network. In the context of an automatically
switched optical network, it would be necessary to have a control
layer that can handle such generic optical services.
There is wide consensus in the industry that the optical control
plane should be IP-centric, utilizing IP-based protocols for
dynamic provisioning and perhaps restoration of lightpaths 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 the IP-based
protocols. This is especially the case for restoration. 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 pros and cons.
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
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second is the transport of IP traffic through an optical network
together with the control and coordination issues that arise
therefrom.
This draft defines a framework for IP over optical network 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 are
possible, reflecting increasingly sophisticated interactions at
these interfaces. This draft therefore advocates the definition of
"capability sets" that define the evolution of functionality at the
interfaces as more sophisticated operational requirements arise.
This draft is organized as follows. In the next section, terminology
covering certain concepts related to this framework are described.
In Section 5, 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 6.
This section presently considers certain general requirements.
Specific operational requirements may be accommodated in this
section as they arise. Section 7 considers the architectural models
for IP-optical interworking, describing the pros and cons of each
model. It should be noted that it is not the intent of this draft 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 8
describes IP-centric control plane mechanisms for optical networks,
covering signaling and routing issues in support of provisioning and
restoration. Section 9 describes certain specialized issues in
relation to IP over optical networks. The approaches described in
Section 7 and 8 range from the relatively simple to the
sophisticated. Section 10 describes a possible evolution path for IP
over optical networking capabilities in terms of increasingly
sophisticated functionality supported. Section 11 considers security
aspects. Finally, summary and conclusion are presented in Section
12.
4. Terminology and Concepts
This section introduces some terminology for describing common
concepts in IP over optical networks pertinent to this framework.
WDM
---
Wavelength Division Multiplexing (WDM) is a technology that allows
multiple optical signals operating at different wavelengths to be
multiplexed onto a single fiber so that they can be transported in
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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 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 and 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 of nodes through which the signal passes before
it reaches 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 channel trail or Lightpath
----------------------------------
An optical channel trail is a point-to-point optical connection
between two access points in an optical network. In this draft, the
term "lightpath" is used interchangeably with optical channel trail.
Optical Cross-Connect (OXC)
---------------------------
An OXC is a space-division switch that can switch an optical data
stream on an input port to a output port. Such a switch may have
optical-electrical conversion at the input port and electrical-
optical conversion at the output port, or it can be all-optical. An
OXC is assumed to have a control-plane processor that implements
signaling and routing protocols necessary for realizing an optical
network.
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Optical mesh sub-network
------------------------
An optical sub-network, as considered 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
topology (also called "mesh" topology) Underlying this network could
be the following:
(a) An optical multiplex section (OMS) layer network : The optical
multiplex section layer provides the transport of the optical
channels. The information contained in this layer is a data
stream comprising a set of n optical channels, which have a
defined aggregate bandwidth.
(b) An optical transmission section (OTS) layer network : This
provides functionality for transmission of the optical signal on
optical media of different types.
This framework does not address the interaction between the optical
sub-network and the OMS, or between the OMS and OTS layer networks.
Optical mesh network
--------------------
An optical mesh network, as considered in draft, is a mesh-connected
collection of optical sub-networks.
Wavelength continuity property
------------------------------
A lightpath is said to satisfy the wavelength continuity property if
it consists of just one 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 each
transit node along a path passes optical transmission without
transducing the optical signal into an electrical signal and back
to an optical signal. More generally, all non-terminus nodes in a
transparent optical network will pass optical signals without
performing retiming and reshaping and thus such nodes are unaware of
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many characteristics of the carried signals. One could, for
example, carry analog signals side by side with digital signals
(potentially of varying bit rate) on different wavelengths over such
a network.
Note that re-powering of signals at transit nodes is potentially
permitted in transparent optical networks. This is a result of the
availability of all optical amplification devices such as Erbium
Doped Fiber Amplifiers (EDFAs).
In opaque optical networks, by comparison, transit nodes will
perform manipulation of the optical signals which they are carrying.
An example of such manipulation would be 3R (reshaping, retiming,
regeneration/amplification).
IP Routed hop
-------------
Within the context of this draft, an IP routed hop is any device
that is IP aware and is able to forward IP packets from an input
port to an output after possibly performing some operations on the
packets. An IP routed hop device will participate in IP routing
protocols such as OSPF, or IS-IS, or derivatives thereof. An IP
routed hop device is thus distinguished from a pure switch which
does not participate in an IP routing protocol. Switch routers,
routing switches, and label switched routers are all potentially
capable of acting as both pure switches and IP routed hop elements.
Trust domain
------------
A trust domain is a network under a single technical administration
in which most nodes in the network are considered to be secure or
trusted in some fashion. An example of a trust domain is a campus
or corporate network in which all routing protocol packets are
considered to be authentic, without the need for additional security
schemes to prevent unauthorized access to the network
infrastructure. Generally, the "single" administrative control rule
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 draft, it will assumed,
without loss of generality, that the term trust domain applies to a
single administrative entity.
Shortcut/cut-through path
-------------------------
A shortcut (used synonymously in this document with the term cut-
through) is defined as an LP or optical channel trail which causes
packets between its endpoints to bypass a number of normally routed
hops within the trust domain. To be more precise, a shortcut makes
its end-points appear to be adjacent to each other with respect to
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routed hops, even though the short-cut LP itself may traverse a
number of intermediate nodes.
Flow
----
For the purpose of this document, the term flow will be used to
represent the smallest separable stream of data, as seen by an
endpoint (source or destination node). It is to be noted that the
term flow is heavily overloaded in the networking literature. Within
the context of this document, it may be convenient to consider a
wavelength as 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 by quantizing time
into some nicely manageable intervals, it may be feasible to
consider each quanta of time within a given wavelength as a flow.
Traffic Trunk
-------------
A abstraction of traffic flow that follows the same path between two
access points which allows some characteristics and attributes of
the traffic to be parameterized.
5. The Network Model
5.1 Network Interconnection
The network model considered in this draft consists of of IP routers
attached to an optical core network, and connected to their peers
over dynamically established switched lightpaths. The optical core
itself is assumed to be incapable of processing individual IP
packets.
The optical network is assumed to consist of multiple optical
sub-networks interconnected by optical links in a general topology
(referred to as an optical mesh network). This network may be multi-
vendor. In the near term, it may be expected that each sub-network
will consist of a single vendor switches. 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. 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.
Here, an optical sub-network may consist entirely of all-optical
OXCs or OXCs with optical-electrical-optical conversion.
Interconnection between sub-networks are assumed to be 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
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routers". As shown in the figure, other client networks (e.g.,
ATM) may connect to the optical network.
The switching function in an OXC is controlled by appropriately
configuring a cross-connect table. Conceptually, the cross-connect
table consists of entries of the form <input port i, output port j>,
indicating that data stream entering input port i will be switched
to output port j. 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 are assumed to be bi-directional,
i.e., the return path from the egress port to the ingress port
follows the same path as the forward path.
Optical Network
+---------------------------------------+
| |
+--------------+ | |
| | | +------------+ +------------+ |
| IP | | | | | | |
| Network +--IF1--+ Optical +---IF2--+ Optical | |
| | | Subnetwork | | Subnetwork | |
+--------------+ | | | +-----+ | |
| +------+-----+ | +------+-----+ |
| | | | |
| IF2 IF2 IF2 |
+--------------+ | | | | |
| | | +------+-----+ | +------+-----+ |
| IP +--IF1--| +--+ | | |
| Network | | | Optical | | Optical | |
| | | | Subnetwork +---IF2--+ Subnetwork | |
+--------------+ | | | | | |
| +------+-----+ +------+-----+ |
| | | |
+-------IF1-------------------IF1-------+
| |
| |
+------+-------+ +------------+
| | | |
| Other Client | |Other Client|
| Network | | Network |
| (e.g., ATM) | | |
+--------------+ +------------+
Figure 1: Optical Network Model
Multiple data streams output from an OXC may be multiplexed onto an
optical link using WDM technology. The WDM functionality may exist
outside of the OXC, and be transparent to the OXC. Or, this function
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may be built into the OXC. In the latter case, the cross-connect
table (conceptually) consists of pairs of the form, <{input port
i, Lambda(j)}, {output port k, Lambda(l)}>. This indicates that the
data stream received on wavelength Lambda(j) over input port i is
switched to output port k on Lambda(l). Automated establishment of
lightpaths involves setting up the cross-connect table entries in
the appropriate OXCs in a coordinated manner such that the desired
physical path is realized.
Under this network model, a switched lightpath must be established
between a pair of IP routers before they can communicate. This
lightpath might traverse multiple optical sub-networks and be
subject to different provisioning and restoration procedures in each
sub-network. The IP-based control plane issue is that of designing
standard signaling and routing protocols for coherent end-to-end
provisioning and restoration of lightpaths across multiple optical
sub-networks. Similarly, IP transport over such an optical network
involves determining IP reachability and seamless establishment of
paths from one IP endpoint to another over an optical core.
5.2 Control Structure
There are two logical control interfaces identified in Figure 1.
These are the client-optical network interface (IF1), and the
optical sub-network interface (IF2). These interfaces are also
referred to as the User-Network Interface (UNI) and the Network-
Network Interface(NNI). The services defined at these interfaces to
a large degree determine the type and amount of control flow across
them. It is possible to have a unified service definition across
both these interfaces such that there is virtually no difference in
the control flow across them. In this case, there will be no
distinction between IF1 and IF2 as far as control flow is concerned.
On the other hand, it may be required to minimize control flow
information, especially routing-related information, over IF1. In
this case, IF1 and IF2 may look different in some respects. In this
draft, these interfaces are treated as distinct.
Each of these interfaces can be categorized as public or private
depending upon context and service models. If IF1 (or IF2) is
private, then routing information (ie, topology state information)
can be exchanged across it. If IF1 (or IF2) is public, then routing
information is not exchanged across it, or such information may be
exchanged across it with very explicit restrictions (including for
example abstraction, filtration, etc). Thus, different relationships
(e.g., peer or over-lay, Section 7) may occur across private and
public logical interfaces.
The physical control structure used to realize these logical
interfaces may vary. For instance, for the client-optical interface,
some of the possibilities are:
1.
Direct interface: An in-band or out-of-band IP control channel
(IPCC) may be implemented between an edge router and each OXC
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that it connects to. This control channel is used for exchanging
signaling and routing messages between the router and the OXC.
With a direct interface, the edge router and the OXC it connects
to are peers in the control plane. This is shown in Figure 2.
The type of routing and signaling information exchange across the
direct interface would vary depending on the service definition.
This issue is dealt with in the next section. Some choices for
the routing protocol are OSPF/ISIS (with traffic engineering
extensions) or BGP. Other directory-based routing information
exchanges are also possible. Some of the signaling protoco choices
are adaptations of RSVP-TE or CR-LDP. The details of how the IP
control channel is realized is outside the scope of this draft.
2.
Indirect interface: An out-of-band IP control channel may be
implemented between the client and a device in the optical
network to signal service requests and responses. For instance, a
management system or a server in the optical network may receive
service requests from clients. Similarly, out-of-band signaling
may be used between a device in the client network (e.g., a
management system) and the OXC, or between devices in client and
optical networks to signal service requests. In these cases, there
is no direct control interaction between clients and respective
OXCs. One reason to have an indirect interface would be that the
OXCs and/or clients do not support a direct interface.
3. Provisioned interface: In this case, the optical network services
are manually provisioned and there is no control interactions
between edge router and the optical network.
+-----------------------------+ +-----------------------------+
| | | |
| +---------+ +---------+ | | +---------+ +---------+ |
| | | | | | | | | | | |
| | Routing | |Signaling| | | | Routing | |Signaling| |
| | Protocol| |Protocol | | | | Protocol| |Protocol | |
| | | | | | | | | | | |
| +-----+---+ +---+-----+ | | +-----+---+ +---+-----+ |
| | | | | | | |
| | | | | | | |
| +--+-----------+---+ | | +--+-----------+---+ |
| | | | | | | |
| | IP Layer +......IPCC.......+ IP Layer | |
| | | | | | | |
| +------------------+ | | +------------------+ |
| | | |
| Edge Router | | OXC |
+-----------------------------+ +-----------------------------+
Figure 2: Direct Interface
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Although different control structures are possible, further
descriptions in this framework assume direct interfaces for IP-
optical and optical sub-network control interactions.
6. IP over Optical Service Models and Requirements
In this section, the service models and requirements at the IP-
optical interface (IF1 in Figure 1) and the optical sub-network
interface (IF2) are considered. Presently, there are not many
clearly stated operational requirements at these interfaces.
However, two general models have emerged for the services at the IP-
optical interface (which can also be applied at the optical sub-
network interface). These models are as follows.
6.1 Domain Services Model
Under this model, the optical network primarily offers high
bandwidth connectivity in the form of lightpaths. This is the model
being considered in the ODSI and OIF [3]. Standardized signaling
across the interface IF1 (Figure 1) is used to invoke the following
services:
1.
Lightpath creation: This service allows a lightpath with the
specified attributes to be created between a pair of termination
points in the optical network. Lightpath creation may be subject
to network-defined policies (e.g., connectivity restrictions) and
security procedures.
2.
Lightpath deletion: This service allows an existing lightpath to
be deleted.
3.
Lightpath modification: This service allows certain parameters of
the lightpath to be modified.
4.
Lightpath status enquiry: This service allows the status of
certain parameters of the lightpath (referenced by its ID) to be
queried by the router that created the lightpath.
Additionally, services related to end-system discovery may be
offered. For example,
1.
Address registration/de-registration: This service allows a router
to register/de-register its address(es) and user group
identifier(s) (e.g., VPN id) with the optical network.
2.
Remote end-system discovery: This service allows a router to
obtain the addresses of remote clients that belong to the same
user group. These services may also be implemented as part of a
routing protocol across IF1, for instance, BGP.
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Because a few, well-defined services are offered across IF1, the
requirements on the signaling protocol are minimal. Specifically,
the signaling protocol is required to convey a few messages with
certain attributes point-to-point between the router and the optical
network. Such a protocol may be based on RSVP or LDP, or even a
messaging application over a TCP connection.
The optical domain services model does not specify the type and
nature of routing protocols within the optical network. Furthermore,
the integration of multiple, optical sub-networks across IF2 will
require the specification of a standard routing protocol, say, BGP.
The optical domain services model would result in the establishment
of a lightpath topology between routers at the edge of the optical
network. The resulting overlay model for IP over optical networks
is discussed in Section 7.
6.2 Unified Service Model
Under this model, the IP and optical networks are treated together
as a single integrated network that is managed and traffic
engineered in a unified manner. In this regard, the OXCs are treated
just like any other router as far as the control plane is
considered. Thus, from a routing and signaling point of view, there
is no distinction between IF1, IF2 and any other router-to-router
interface. It is assumed that this control plane is MPLS-based, as
described in [1].
Under the unified service model, there are no specific services
defined at IF1, but the services are implied by the semantics of
MPLS signaling protocols. Specifically, an edge router can create
a lightpath with specified attributes, or delete and modify
lightpaths as it creates label-switched paths (LSPs). In this
regard, the services obtained from the optical network are similar
to the domain services model. These services, however, may be
invoked in a more seamless manner as compared to the domain services
model. For instance, in principle, a remote router can compute an
end-to-end path across the optical network utilizing, say, OSPF with
traffic engineering extensions [4]. It can then establish an LSP
across the optical network. But the edge routers must still
recognize that an LSP across the optical network is a lightpath, or
a conduit for multiple LSPs. The concept of "forwarding adjacency"
can be used to specify virtual links across optical networks in
routing protocols such as OSPF [4]. In essence, once a lightpath is
established across an optical network between two edge routers, it
can be advertised as a forwarding adjacency (a virtual link) between
these routers. Thus, from a data plane point of view, the
lightpaths result in a overlay between edge routers. The decisions
as to when to create such lightpaths, and the bandwidth management
for these lightpaths is identical in both the domain services model
and the unified service model. The routing model for unified
services is described in Section 7.
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6.3 Which Service Model?
The pros and cons of the above service models can be debated at
length, but the approach recommended in this framework is to define
routing and signaling mechanisms in support of both. As pointed out
above, signaling for service request can be unified to cover both
models. The significant difference, however, is in routing
protocols, as described in Section 7.
6.4 What are the possible services?
Specialized services may be built atop the point-to-point
connectivity service offered by the optical network. For example,
6.4.1 Virtual Private Optical Networks (VPON)
Given that the data plane between IP routers over an optical network
is an overlay, it is easy to imagine a virtual private network of
lightpaths that interconnect routers (or any other set of clients).
Indeed, in the case where the optical network provides connectivity
for multiple sets of external client networks, there has to be a
way to enforce routing policies that ensure connectivity only
between eligible sets of clients. Thus, the VPON service must be
provided by an optical network.
6.4.2 Traffic engineering for fault tolerance and load balancing
In the mesh optically switched configurations, multiple feasible
paths may exist between ingress and egress nodes at the boundaries
of the optical cloud. In this case, for the purposes of traffic
engineering, it might be valuable to have capability to setup
lightpaths that satisfy certain requirements subject to certain
constraints. There is nothing new in this idea, except that the
connection oriented infrastructure is built from optical rather than
traditional L2 devices. Nonetheless, this potential should be noted
and should be considered when defining routing mechanisms.
6.4 IP-over-Optical Network Requirements
TBD.
7. IP transport over Optical Networks
To examine the architectural alternatives for IP over optical
networks, it is important to distinguish between the data and
control planes over the router-optical-net interface, IF1. As
described in Section 6, the optical network provides a service to
external entities in the form of fixed bandwidth transport pipes
(optical paths). IP routers at the edge of the optical networks must
necessarily establish such paths before communication at the IP
layer can begin. Thus, the IP data plane over optical networks is
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realized over an overlay network of optical paths. On the other
hand, IP routers and OXCs can have a peer relation on the control
plane, especially for the implementation of a routing protocol that
allows dynamic discovery of IP endpoints attached to the optical
network. The IP over optical network architecture is defined
essentially by the organization of the control plane. The assumption
in this framework is that an MPLS-based control plane [1] is used.
Depending on the service model(Section 6), however, the control
planes in the IP and optical networks can be loosely or tightly
coupled. This coupling determines
o the details of the topology and routing information advertised by
the optical network across IF1;
o Level of control that IP routers can exercise in selecting
specific paths for connections across the optical network; and
o Policies regarding the dynamic provisioning of optical paths
between routers. This includes access control, accounting and
security issues.
The following interconnection models are then possible:
7.1 Overview of Interconnection Models
7.1.1 The Peer Model
Under the peer model, the IP/MPLS layers act as peers of the optical
transport network, such that a single routing protocol instance runs
over both the IP/MPLS and optical domains. Presumably a common IGP
such as OSPF or IS-IS, with appropriate extensions, will be used to
distribute topology information [4]. In the case of OSPF, opaque
LSAs will be used to advertise topology state information. In the
case of IS-IS, extended TLVs will have to be defined to propagate
topology state information. One tacit assumption here is that a
common addressing scheme will also be used for the optical and IP
networks. A common address space can be trivially realized by using
IP addresses in both IP and optical domains. Thus, the optical
network elements become IP addressable entities as noted in [1].
7.1.2 The Overlay Model
Under the overlay model, the IP/MPLS routing, topology distribution,
and signaling protocols are independent of the routing, topology
distribution, and signaling protocols at the optical layer. This
model is conceptually similar to the classical IP over ATM or MPOA
models, but applied to a optical sub-network directly. MPLS may
nonetheless provide a mechanism to cut through or bypass routed
hops. In the overlay model, lambda routing, topology
distribution, and signaling protocols would have to be defined for
the optical domain. In certain circumstances, it may also be
feasible to statically configure the optical channels that provide
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connectivity in the overlay model through network management. Static
configuration is, however, unlikely to scale in very large networks.
7.1.3 The Augmented Model
Under the augmented model, there are actually separate routing
instances in the IP and optical domains, but information from one
routing instance is passed through the other routing instance. For
example, external IP addresses could be carried within the optical
routing protocols to allow reachability information to be passed to
IP clients.
The overlay, augmented, and peer models can also be described using
the terminology of "termination capable" (TC) and "terminology
incapable" (TI) interfaces, in conjunction with the generalized
notion of LSP nesting described in [5]. The routing approaches to
corresponding to these interconnection models are described below.
7.2 Routing Approaches
7.2.1 Integrated Routing
This routing approach supports the peer model described above. Under
this approach, the IP and optical networks are assumed to run the
same instance of an IP routing protocol, e.g., OSPF with suitable
"optical" extensions. These extensions must capture optical link
parameters, and any constraints that are specific to optical
networks. The topology and link state information maintained by all
nodes (OXCs and routers) is identical. This permits a router to
compute an end-to-end path to another router across the optical
network. Suppose the path computation is triggered by the need to
route a label switched path (LSP). Such an LSP can be established
using MPLS signaling, e.g., RSVP-TE or CR-LDP. When the LSP is
routed over the optical network, a lightpath must be established
between two edge routers. This lightpath is in essence a tunnel
across the optical network, and may have capacity much larger than
that required to route the first LSP. Thus, it is essential that
other routers in the network realize the availability of resources
in the lightpath for other LSPs to be routed over it. The
lightpath must therefore be advertised as a virtual link in the
topology.
The notion of "forwarding adjacency" (FA) described in [4] is
essential in propagating lightpath information to routers. An FA is
essentially a virtual link advertised into a link state routing
protocol. Thus, an FA could be described by the same parameters that
define resources in any regular link. While it is necessary to
specify the mechanism for creating an FA, it is not necessary to
specify how an FA is used by the routing scheme. Once an FA is
advertised in a link state protocol, its usage for routing LSPs is
defined by the route computation and traffic engineering algorithms
implemented.
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It should be noted that at the IP-optical interface, the
connectivity and resource availability is defined by the physical
ports that connect routers to OXCs. Suppose a router R1 is connected
to OXC O1 over two ports, P1 and P2. Under integrated routing, the
connectivity between R1 and O1 over the two ports would have been
captured in the link state representation of the network. Now,
suppose an FA is created from R1 to another router R2 over port P1.
While this FA is advertised as a virtual link between R1 and R2, it
is also necessary to remove the link R1-O1 (over P1) from the link
state representation since that port is no longer available for
creating a lightpath. Thus, as FAs are created, an overlaid set of
virtual links are introduced into the link state representation,
replacing the links previously advertised at the IP-Optical
interface. Finally, the details of the optical network captured in
the link state representation is replaced by a network of virtual
links that represent lightpaths. In this regard, there is a great
deal of similarity between integrated routing and domain-specific
routing (described next). Both ultimately deal with the creation of
the overlaid lightpath topology to meet the traffic engineering
objectives.
7.2.2 Domain-Specific Routing
This routing approach supports the augmented interconnection model.
Under this approach, routing within the optical and IP domains are
separated, with a standard routing protocol running between domains.
This is similar to the IP inter-domain routing model. Two choices
for this are considered.
7.2.2.1 Domain-Specific Routing using BGP
The inter-domain IP routing protocol, BGP [6], may be adapted for
exchanging routing information between IP and optical domains. This
would allow the routers to advertise IP address prefixes within
their network to the optical network and to receive external IP
address prefixes from the optical network. The optical network
transports the reachability information from one IP network to
others. For instance, edge routers and OXCs can run exterior BGP
(EBGP). Within the optical network, interior BGP (IBGP) is used
between border OXCs within the same sub-network, and EBGP is used
between sub-networks (over IF2,
Figure 1).
Under this scheme, it is necessary to identify the egress points in
the optical network corresponding to externally reachable IP
addresses. This is due to the following. Suppose an edge router
desires to establish an LSP to a destination reachable across the
optical network. It could directly request a lightpath to that
destination, without explicitly specifying the egress optical port
for the lightpath as the optical network has knowledge of
externally reachable IP addresses. However, if the same edge router
has to establish another LSP to a different external destination, it
must first determine whether there is a lightpath already available
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(with sufficient residual capacity) that leads to that destination.
To identify this, it is necessary for edge routers to keep track of
which egress ports in the optical network lead to which external
destinations. Thus, a border OXC receiving external IP prefixes from
an edge router through EBGP must include its own IP address as the
egress point before propagating these prefixes to other border OXCs
or edge routers. An edge router receiving this information need
not propagate the egress address further, but it must keep the
association between external IP addresses and egress OXC
addresses. Specific BGP mechanisms for propagating egress OXC
addresses are to be determined, considering prior examples
described in [7,8]. When Virtual Private Optical Networks (VPONs)
are implemented, the address prefixes advertised by the border OXCs
must be accompanied by some VPON identification (for example, VPN
IPv4 addresses, as defined in [8], may be used).
7.2.2.2 Domain Specific Routing using OSPF/ISIS
The routing information exchanged across the IP-optical UNI could be
summarized using a hierarchical routing protocol such as OSPF/ISIS.
The following description is based on OSPF.
OSPF supports a two-level hierarchical routing scheme through the
use of OSPF areas. Routing within each area is flat, while detailed
knowledge of an areaÆs topology is hidden from all other areas.
Routers attached to two or more areas are called Area Border Routers
(ABRs). ABRs propagate IP addressing information from one area to
another using summary LSAs. Within an OSPF routing domain, all areas
are attached directly to a special area called the OSPF backbone
area. The exchange of information between areas can be controlled
to implement domain specific routing in each area. For instance, the
optical network can be a collection of one or more areas in which
certain link parameters and information specific to optical networks
is incorporated into a version of OSPF. The client network (e.g.,
IP) could be separate OSPF area(s), running OSPF with TE extensions.
The summary LSAs exchanged between the optical and client areas can
be designed such that optical domain specific information is hidden
from client networks while providing adequate routing information
for end-to-end routing of lightpaths.
While the use of BGP or OSPF/ISIS allows edge routers to learn about
reachability of destinations across the optical network, the
determination of how many lightpaths to establish and to what egress
points are traffic engineering decisions.
7.2.3 Overlay Routing
This routing approach supports the overlay interconnection model.
Under this approach, overlay mechanism that allows edge routers to
register and query for external addresses is implemented. This is
similar to address resolution for IP over ATM. Under this approach,
the optical network could implement a registry that allows edge
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routers to register IP addresses and VPON identifiers. An edge
router may be allowed to query for external addresses belonging
to the same set of VPONs it belongs to. A successful query would
return the address of the egress optical port through which the
external destination can be reached.
Because IP-optical interface connectivity is limited, the
determination of how many lightpaths must be established and to what
endpoints are traffic engineering decisions. Furthermore, after an
initial set of such lightpaths are established, these may be used as
adjacencies within VPONs for a VPON-wide routing scheme, for
example, OSPF. With this approach, an edge router could first
determine other edge routers of interest by querying the registry.
After it obtains the appropriate addresses, an initial overlay
lightpath topology may be formed. Routing adjacencies may then be
established across the lightpaths and further routing information
may be exchanged to establish VPON-wide routing.
7.3 Signaling-related
7.3.1 The Role of MPLS
It is possible to model wavelengths, and potentially TDM channels
within a wavelength as "labels" (see [1] for an enumeration of
relevant commonalties, and [21] for encodings). Furthermore, the
wavelength `labels' need not change at every hop. For instance, an
ADM can serve as an LSR just as well as an optical switch. An IP
router with WDM capability can serve as an LER. WDM LSPs can be
established solely using RSVP or LDP, or in conjunction with some
other signaling protocol. If separate signaling protocols are used
to establish parts of the LSP, then some extensions to LDP may be
needed. If RSVP or LDP is used solely for label provisioning, then
IP router functionality must be associated with each potential label
switched hop. Also, each physical interface involved in an LSP must
also be associated with a IP addressable interface. The use of WDM
wavelengths and channels is not unlike the use of labels in
conventional label switched technologies as noted in [1].
For both label switching and WDM switches (OXCs), once the label
has been provisioned by the protocol, the IP router no longer
participates in forwarding of the traffic in the FEC. Instead, the
native capabilities of the device are used to switch traffic to the
eventual egress LER. WDM label switching is compatible with label
merging. Label merging is a technique that can be used to merge
paths from several related sources into a common path.
7.3.2 Role of NHRP and MPOA
NHRP [20] may be applied as a signaling mechanism for what is
referred to as optical flow switching. To request a shortcut, the
existing packet format for the NHRP Resolution Request would be used
with a new extension in the form of a modified Forward Transit NHS
Extension is included. The extension would include enough
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information at each hop (including the source and destination) to
uniquely identify which wavelength (and potentially a `time
interval' quanta within some cyclic measure of time, such as an epoc
in sonet) to use when bypassing each routed/forwarded hop and which
port that the request was received on. When the egress NHS receives
the Resolution request, and assuming there are sufficient resources
at each bypassed hop (resources include both the willingness to
forward another WL as well as the existence of an unused
wavelength), the egress NHS will send a resolution reply back to the
sender. For simplicity, it will be assumed that a given port is bi-
directional. The architecture does not require this per se, but can
work with unidirectional links (only less elegantly). When the
egress NHS sends the reply, it sends the reply back toward the
receiver through the port on which the NHS received the resolution
request (as stated previously, this is mostly a logical construct
and does not preclude the existence of unidirectional links).
However before the egress NHS does this, it programs its forwarding
engine to drop the data which it receives on the appropriate
wavelength (and potentially in a more granular the quanta) from the
upstream NHS. Upon receiving an NHRP reply from a downstream
neighbor, the upstream NHS programs its forwarding engine to send
data for the shortcut on the wavelength it dedicated during the
resolution request process. It also programs its forwarding engine
to receive the data from its upstream neighbor on the wavelength
which the upstream neighbor said it would use and then the current
NHS propagates the NHRP resolution reply back upstream on the port
from which it received the resolution request. This process
continues until the source of the resolution request is reached. In
this way the shortcut is setup from ingress to egress.
If wavelength conversion is not done on a hop by hop basis than the
problem is difficult to do in a fully distributed manner. There is
still merit in using signaling however. In this case, the ingress
device would need to query some server which is administering the
wavelength allocation process to ask permission and to ask for a
wavelength to be allocated from ingress to egress. If the request
is granted then the same process would be carried out as
previously described except that a given wavelength would be
required to be allocated at each hop. Other ways of doing this are
possible but this is by far the most simple.
Note that an option here would be (whether or not wavelength
conversion exists) to allow a transit NHS to terminate the shortcut
if the downstream transit NHS has insufficient resources to sync the
bypass. Note in this case, no bandwidth is gained by using
shortcuts since all data would then need to go over the default link
between the current NHS and the downstream NHS, however, it is
possible that some delay and jitter performance might be gained in
this context.
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7.3.3 RSVP and CR-LDP
References [4, 9] and [21] discuss how RSVP can be extended to serve
as a signaling protocol for the establishment of optical channels
and lightpaths. Furthermore, in the previous sections, if the terms
NHS is substituted with`RSVP capable router', `NHRP Resolution
Request' with `Path message', and `NHRP Resolution Reply' with `ReSV
message,' then RSVP can be used to address the problem described in
the NHRP example above with more richly defined semantics for QoS.
7.4 End-to-end restoration models
TBD.
8. IP-based Optical Control Plane Issues
Provisioning and restoring lightpaths end-to-end between IP networks
requires protocol and signaling support within optical sub-networks
and across the interface IF2. In this regard, a distinction is made
between control procedures within an optical sub-network (Figure 1)
and those between sub-networks. The general guideline followed in
this framework is to separate these two cases, and allow the
possibility that different control procedures are followed inside
different sub-networks, while a common set of procedures are
followed across sub-networks (over interface IF2). Clearly, it is
possible to follow the same control procedures inside a sub-network
as defined for control across sub-networks. But this is left as a
choice as per this framework, rather than a mandate. In the
following, signaling and routing within and across sub-networks are
considered.
8.1 Addressing
For interoperability across optical sub-networks using an IP-centric
control plane, the fundamental issue is that of addressing. What
entities should be identifiable from a signaling and routing point
of view? How should they be addressed? This section presents some
guidelines on this.
Identifiable entities in optical networks includes OXCs, optical
links, optical channels and sub-channels, Shared Risk Link Groups
(SRLGs), etc. An issue here is how granular the identification
should be as far as the establishment of optical trails are
concerned. The scheme for identification must accommodate the
specification of the termination points in the optical network with
adequate granularity when establishing optical trails. For instance,
an OXC could have many ports, each of which may in turn terminate
many optical channels, each of which contain many subchannels etc.
It is perhaps not reasonable to assume that every sub-channel or
channel termination, or even OXC ports could be assigned a unique IP
address. Also, the routing of an optical trail within the network
does not depend on the precise termination point information, but
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rather only on the terminating OXC. Thus, finer granularity
identification of termination points is of relevance only to the
terminating OXC and not to intermediate OXCs (of course, resource
allocation at each intermediate point would depend on the
granularity of resources requested). This suggests an identification
scheme whereby OXCs are identified by a unique IP address and a
"selector" identifies further fine-grain information of relevance at
an OXC. This, of course, does not preclude the identification of
these termination points directly with IP addresses(with a null
selector). The selector can be formatted to have adequate number of
bits and a structure that expresses port, channel, sub-channel, etc,
identification.
Within the optical network, the establishment of trail segments
between adjacent OXCs require the identification of specific port,
channel, sub-channel, etc. With an MPLS-based control plane, a label
serves this function. The structure of the "optical label" must be
such that it can encode the required information.
Optical links between adjacent OXCs may be bundled for advertisement
into a link state protocol [10, 11]. A bundled interface may be
numbered or unnumbered. In either case, the component links within
the bundle must be identifiable. In concert with SRLG
identification, this information is necessary for correct path
computation [11].
Finally, another entity that must be identified is the SRLG [12]. An
SRLG is an identifier assigned to a group of optical links that
share a physical resource. For instance, all optical channels routed
over the same fiber could belong to the same SRLG. Similarly, all
fibers routed over a conduit could belong to the same SRLG. The
notable characteristic of SRLGs is that a given link could belong to
more than one SRLG, and two links belonging to a given SRLG may
individually belong to two other SRLGs. This is illustrated in
Figure 3. Here, the links 1,2,3 and 4 may belong to SRLG 1, links
1,2 and 3 could belong to SRLG 2 and link 4 could belong to SRLG 3.
Similarly, links 5 and 6 could belong to SRLG 1, and links 7 and 8
could belong to SRLG 4. (In this example, the same SRLG, i.e., 1,
contains links from two different adjacencies).
While the classification of physical resources into SRLGs is a
manual operation, the assignment of unique identifiers to these
SRLGs within an optical network is essential to ensure correct
SRLG-disjoint path computation for protection. SRLGs could be
identified with a flat identifier (e.g., 32 bit integer).
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+--------------+ +------------+ +------------+
| +-1:OC48---+ +-5:OC192-+ |
| +-2:OC48---+ +-6:OC192-+ |
| OXC1 +-3:OC48---+ OXC2 +-7:OC48--+ OXC3 |
| +-4:OC192--+ +-8:OC48--+ |
| | | | +------+ |
+--------------+ +----+-+-----+ | +----+------+-----+
| | | | |
| | | | |
+--------------+ | | | | |
| | +----+-+-----+ | | +------+-----+
| +----------+ +--+ | | |
| OXC4 +----------+ +----+ | |
| +----------+ OXC5 +--------+ OXC6 |
| | | +--------+ |
+--------------+ | | | |
+------+-----+ +------+-----+
Figure 3: Mesh Optical Network with SRLGs
8.2 Neighbor Discovery
Routing within the optical network relies on knowledge of network
topology and resource availability. This information may be gathered
and used by a centralized system, or by a distributed link state
routing protocol. In either case, the first step towards network-
wide link state determination is the discovery of the status of
local links to all neighbors by each OXC. Specifically, each OXC
must determine the up/down status of each optical link, the
bandwidth and other parameters of the link, and the identity of the
remote end of the link (e.g., remote port number). The last piece of
information is used to specify an appropriate label when signaling
for lightpath provisioning. The determination of these parameters
could be based on a combination of manual configuration and an
automated protocol running between adjacent OXCs. The
characteristics of such a protocolwould depend on the type of OXCs
that are adjacent (e.g., transparent or opaque). Generically, the
protocol may be refered to as the "Neighbor Discovery Protocol
(NDP)" although other functions such as link management and fault
isolation may be performed as part of the protocol (e.g., LMP [13]).
NDP would typically require in-band communication on the bearer
channels to determine local connectivity and link status. In the
case of opaque OXCs with SONET termination, one instance of NDP
would run on each OXC port, communicating with the corresponding NDP
instance at the neighboring OXC. The protocol would utilize the
SONET overhead bytes to transmit the (configured) local attributes
periodically to the neighbor. Thus, two neighboring switches can
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automatically determine the identities of each other and the local
connectivity,and also keep track of the up/down status of local
links. Neighbor discovery with transparent OXCs is described in
[13].
8.3 Topology Discovery
Topology discovery is the procedure by which the topology and
resource state of all the links in a sub-network are determined.
Topology discovery may be done using a link state routing protocol
(e.g., OSPF, ISIS), or it can be through a management protocol (in
the case of centralized path computation). The focus in this
framework is on fully distributed route computation using an IP link
state protocol.
In general, most of the link state routing functionality is
maintained when applied to optical networks. However, the
representation of optical links, as well as some link parameters,
are changed in this setting. Specifically,
o The link state information may consist of link bundles [10,11].
Each link bundle is represented as an abstract link in the network
topology. Different bundling representations are possible. For
instance, the parameters of the abstract link may include the
number, bandwidth and the type of optical links contained in the
underlying link bundle [11]. Also, the SRLGs corresponding to each
optical link in the bundle may be included as a parameter.
o The link state information should capture restoration-related
parameters for optical links. Specifically, with shared protection
(Section 8.5), the link state updates must have information that
allows the computation of shared protection paths.
o A single routing adjacency could be maintained between neighbors
which may have multiple optical links (or even multiple link
bundles) between them. This reduces the protocol messaging
overhead.
o Since link availability information changes dynamically,a flexible
policy for triggering link state updates based on availability
thresholds may be implemented. For instance, changes in
availability of links of a given bandwidth (e.g., OC-48) may
trigger updates only after the availability figure changes by a
certain percentage.
These concepts are relatively well-understood. On the other hand,
the resource representation models and the topology discovery
process for hierarchical routing (e.g., OSPF with multiple areas)
are areas that need further work.
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8.4 Restoration Models
Automatic restoration of lightpaths is a service offered by optical
networks. There could be local and end-to-end mechanisms for
restoration of lightpaths within a sub-network. Local mechanisms are
used to select an alternate link between two adjacent OXCs when a
failure affects the primary link over which the (protected)
lightpath is being routed. Local restoration does not affect the
end-to-end route of the lightpath. When local restoration is not
possible (e.g., no alternate link is available between the adjacent
OXCs in question), end-to-end restoration may be performed. With
this, the affected lightpath may be rerouted over an alternate path
that completely avoids the OXCs or the link segment where the
failure occurred. For end-to-end restoration, alternate paths are
typically pre-computed. Such back-up paths may have to be physically
diverse from the corresponding primary paths.
End-to-end restoration may be based on two types of protection
schemes; "1 + 1" protection or shared protection. Under 1 + 1
protection, a back-up path is established for the protected primary
path along a physically diverse route. Both paths are active and the
failure along the primary path results in an immediate switch-over
to the back-up path. Under shared protection, back-up paths
corresponding to physically diverse primary paths may share the same
network resources. When a failure affects a primary path, it is
assumed that the same failure will not affect the other primary
paths whose back-ups share resources.
8.5 Route Computation
The computation of a primary route for a lightpath within an optical
sub-network is essentially a constraint-based routing problem. The
constraint is typically the bandwidth required for the lightpath,
perhaps along with administrative and policy constraints. The
objective of path computation could be to minimize the total
capacity required for routing lightpaths [14].
Route computation with constraints may be accomplished using a
number of algorithms [15]. When 1+1 protection is used, a back-up
path that does not traverse on any link which is part of the same
SRLG as links in the primary path must be computed. Thus, it is
essential that the SRLGs in the primary path be known during
alternate path computation, along with the availability of
resources in links that belong to other SRLGs. This requirement has
certain implications on optical link bundling. Specifically, a
bundled LSA must include adequate information such that a remote OXC
can determine the resource availability under each SRLG that the
bundled link refers to, and the relationship between links
belonging to different SRLGs in the bundle. For example,
considering Figure 3, if links 1,2,3 and 4 are bundled
together in an LSA, the bundled LSA must indicate that there are
three SRLGs which are part of the bundle (i.e., 1, 2 and 3), and
that links in SRLGs 2 and 3 are also part of SRLG 1.
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It is somewhat complex to encode the SRLG relationships in a link
bundle LSA. This information, however, is naturally captured if the
link bundle is encoded as a set of "link groups", each specifying
the links that belong to exactly the same set of SRLGs. Within the
link group, it is possible to specify the number of links of a
particular type, for example, OC-48. With reference to Figure 3,
for example, a bundle LSA can be advertised for the entire set of
links between OXC1 and OXC2, with the following information:
Link Group ID SRLGs Link Type Number Other Info
------------- ----- --------- ------ ----------
1 1,2 OC-48 3 ---
2 1,3 OC-192 1 ---
Assuming that the above information is available for each bundle at
every node, there are several approaches possible for path
computation. For instance,
1. The primary path can be computed first, and the (exclusive
or shared) back-up is computed next based on the SRLGs chosen
for the primary path. In this regard,
o The primary path itself can be computed by taking into account
specific link groups in a bundle. That is, the primary path
computation procedure can output a series of link groups the
path is routed over. Since a link group is uniquely identified
with a set of SRLGs, the alternate path can be computed right
away based on this knowledge. In this case, if the primary path
set up does not succeed for lack of resources in a chosen link
group, the primary and backup paths muse be recomputed.
o It might be desirable to compute primary paths using bundle-
level information (i.e., resource availability in all link
groups in a bundle) rather than specific link group level
information. In this case, the primary path computation
procedure would output a series of bundles the path
traverses. Each OXC in the path would have the freedom to
choose the particular link group to route that segment of the
primary path. This procedure would increase the chances of
successfully setting up the primary path when link state
information is not up to date everywhere. But the specific link
group chosen, and hence the SRLGs in the primary path, must be
captured during primary path set-up, for example, using the
RSVP-TE Route Record Object [16]. This SRLG information is
then used for computing the back-up path. The back-up path may
also be established specifying only which SRLGs to AVOID in a
given segment, rather than which link groups to use. This would
maximize the chances of establishing the back-up.
2. The primary path and the back-up path are comptuted together in
one step, for example, using Suurbaale's algorithm [17]. In this
case, the paths must be computed using specific link group
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information.
To summarize, it is essential to capture sufficient information in
link bundle LSAs to accommodate different path computation
procedures and to maximize the chances of successful path
establishment. Depending on the path computation procedure used,
the type of support needed during path establishment (e.g., the
recording of link group or SRLG information during path
establishment) may differ.
When shared protection is used, the route computation algorithm must
take into account the possibility of sharing links among multiple
back-up paths. Under shared protection, the back-up paths
corresponding to SRLG-disjoint primary paths can be assigned the
same links. The assumption here is that since the primary paths
are not routed over links that have the same SRLG, a given failure
will affect only one of them. Furthermore, it is assumed that
multiple failure events affecting links belonging to more than one
SRLG will not occur concurrently. Unlike the case of 1+1
protection, the back-up paths are not established apriori. Rather, a
failure event triggers the establishment of a single back-up path
corresponding to the affected primary path.
The distributed implementation of route computation for shared back-
up paths require knowledge about the routing of all primary and
back-up paths at every node. This raises scalability concerns. For
this reason, it may be practical to consider the centralization of
the route computation algorithm in a route server that has complete
knowledge of the link state and path routes. Heuristics for fully
distributed route computation without complete knowledge of path
routes are to be determined. Path computation for restoration is
further described in [18].
8.6 Signaling Issues
Signaling within an optical sub-network for lightpath provisioning
is a relatively simple operation. After a route is determined for a
lightpath, each OXC in the path must establish appropriate cross-
connects in a coordinated fashion. This coordination is akin to
selecting incoming and outgoing labels in a label-switched
environment. Thus, protocols like RSVP-TE [16] and CR-LDP [19] can
be used for this. A few new concerns, however, must be addressed.
8.6.1 Bidirectional Lightpath Establishment
Lightpaths are typically bi-directional. That is, the output port
selected at an OXC for the forward direction is also the input port
for the reverse direction of the path. Since signaling for optical
paths may be autonomously initiated by different nodes, it is
possible that two path set-up attempts are in progress at the same
time. Specifically, while setting up an optical path, an OXC A may
select output port i which is connected to input port j of the
"next" OXC B. Concurrently, OXC B may select output port j for
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setting up a different optical path, where the "next" OXC is A. This
results in a "collision". Similarly, when WDM functionality is built
into OXCs, a collision occurs when adjacent OXCs choose directly
connected output ports and the same wavelength for two different
optical paths. There are two ways to deal with such collisions.
First, collisions may be detected and the involved paths may be torn
down and re-established. Or, collisions may be avoided altogether.
8.6.2 Failure Recovery
The impact of transient partial failures must be minimized in an
optical network. Specifically, optical paths that are not directly
affected by a failure must not be torn down due to the failure. For
example, the control processor in an OXC may fail, affecting
signaling and other internodal control communication. Similarly,
the control channel between OXCs may be affected temporarily by a
failure. These failure may not affect already established optical
paths passing through the OXC fabric. The detection of such failures
by adjacent nodes, for example, through a keepalive mechanism
between signaling peers, must not result in these optical paths
being torn down.
It is likely that when the above failures occur, a backup processor
or a backup control channel will be activated. The signaling
protocol must be designed such that it is resilient to transient
failures. During failure recovery, it is desirable to recover local
state at the concerned OXC with least disruption to existing optical
paths.
8.6.3 Restoration
Signaling for restoration has two distict phases. There is a
reservation phase in which capacity for the protection path is
established. Then, there is an activation phase in which the
back-up path is actually put in service. The former phase typically
is not subject to strict time constraints, while the latter is.
Signaling to establish a "1+1" back-up path is relatively straight-
forward. This signaling is very similar to signaling used for
establishing the primary path. Signaling to establish a shared back-
up path is a little bit different. Here, each OXC must understand
which back-up paths can share resources. The signaling message must
itself indicate shared reservation. The sharing rule is as described
in Section 8.4: back-up paths corresponding to physically diverse
primary paths may share the same network resources. It is
therefore necessary for the signaling message to carry adequate
information that allows an OXC to verify that back-up paths that
share a certain resources are allowed to do so.
Under both 1+1 and shared protection, the activation phase has two
parts: propagation of failure information to the source OXC from the
point of failure, and activation of the back-up path. The signaling
for these two phases must be very fast in order to realize response
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times in the order of tens of milliseconds. When optical links are
SONET-based, in-band signals may be used, resulting in quick
response. With out-of-band control, it is necessary to consider
fast signaling over the control channel using very short IP packets
and prioritized processing. While it is possible to use RSVP or CR-
LDP for activating protection paths, these protocols do not provide
any means to give priority to restoration signaling as opposed to
signaling for provisioning. For instance, it is possible for a
restoration-related RSVP message to be queued behind a number of
provisioning messages thereby delaying restoration. It is therefore
necessary to develop a definition of QoS for restoration signaling
and incorporate mechanisms in existing signaling protocols to
achieve this. Or, a new signaling protocol may be developed
exclusively for activating protection paths during restoration.
8.7 Optical Internetworking
Ideally, a set of interconnected optical sub-networks must be
functionally similar to a single optical sub-network. Thus, it must
be possible to dynamically provision and restore lightpaths across
optical sub-networks. Therefore:
o A standard scheme for uniquely identifying lightpath end-points in
different sub-networks is required.
o A protocol is required for determining reachability of end-points
across sub-networks.
o A standard signaling protocol is required for provisioning
lightpaths across sub-networks.
o A standard procedure is required for the restoration of lightpaths
across sub-networks.
o It should be possible to apply proprietary provisioning and
restoration procedures for the segment of a lightpath passing
through a given sub-net.
The IP-centric control architecture for optical sub-networks can be
extended to satisfy the functional requirements of optical
internetworking. Routing and signaling interaction between optical
sub-networks can be standardized across the interface IF2 (Figure
1). For the joint control and management of the network, an
integration of the sub-network management systems is required. The
functionality provided across IF2 is as follows.
8.7.1 Neighbor Discovery
NDP, as described in Section 8.2, can be used for this. Indeed, a
single protocol should be standardized for neighbor discovery within
and across sub-networks.
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8.7.2 Addressing and Routing Model
The addressing mechanisms described in Section 8.1 can be used to
identify OXCs, ports, channels and sub-channels in each sub-network.
It is essential that the OXC IP addresses are unique network-wide.
Provisioning an end-to-end lightpath across multiple sub-networks
involves the establishment of path segments in each sub-network
sequentially. Thus, a path segment is established from the source
OXC to a border OXC in the source sub-network. From this border OXC,
signaling across IF2 is used to establish a path segment to a border
OXC in the next sub-network. Provisioning then continues in the next
sub-network and so on until the destination OXC is reached.
A version of BGP may be used to determine the routes to destinations
across sub-networks. Using exterior BGP, adjacent border OXCs in
different sub-networks can exchange reachability of OXCs and other
external IP endpoints (border routers). Using interior BGP, the same
information is propagated from one border OXC to others in the same
sub-network. Thus, every border OXC eventually learns of all IP
addresses reachable across different neighboring sub-networks. These
addresses may be propagated to other OXCs within the sub-network
thereby allowing them to select appropriate border OXCs as exit
points for external destinations. To support VPONs, the external
reachability information should include VPON identifiers.
8.7.3 Restoration
It is likely that proprietary restoration schemes may be implemented
within optical sub-networks. It is therefore necessary to consider a
two-level restoration mechanism. Path failures within an optical
sub-network should be handled using procedures specific to the
sub-network. If this fails, end-to-end restoration across sub-
networks should be invoked. The border OXC that is the ingress to a
sub-network can act as the source for restoration procedures within
a sub-network. The signaling for invoking end-to-end restoration
across IF2 is similar to the signaling described in Section 8.6.3.
The computation of the back-up path for end-to-end restoration may
be based on various criteria. It is assumed that the back-up path is
computed by the source OXC, and signaled using standard methods.
9. Other Issues
9.1 WDM and TDM in the same network element
A practical assumption would be that if SONET (or some other TDM
mechanism that is capable partitioning the bandwidth of a
wavelength) is used, then TDM is leveraged as an additional method
to differentiate between "flows." In such cases, wavelengths and
time intervals (sub-channels) within a wavelength become analogous
to labels (as noted in [1]) which can be used to make switching
decisions. This would be somewhat akin to using VPI (e.g.,
wavelength) and VCI (e.g., TDM sub-channel) in ATM networks. More
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generally, this will be akin to label stacking and to LSP nesting
within the context of Multi-Protocol Lambda Switching [1].
9.2 Wavelength converters
Some form of wavelength conversion may exist at some switching
elements. This however may not be case in some pure optical
switching elements. A switching element is essentially anything
more sophisticated than a simple repeater, that is capable of
switching and converting a wavelength Lambda(k) from an input port
to a wavelength Lambda(l) on an output port. In this display, it
is not necessarily the case that Lambda(k) = Lambda(l), nor is it
necessarily the case that the data carried on Lambda(k) is switched
through the device without being examined or modified.
It is not necessary to have a wavelength converter at every
switching element. A number of studies have attempted to address
the issue of the value of wavelength conversion in an optical
network. Such studies typically use the blocking probability (the
probability that a lightpath cannot be established because the
requisite wavelengths are not available) as a metric to adjudicate
the effectiveness of wavelength conversion. The IP over optical
architecture must take into account hybrid networks with some OXCs
capable of wavelength conversion and others incapable of this.
9.3 Service provider peering points
There are proposed inter-network interconnect models which allow
certain types of peering relationships to occur at the optical
layer. This is consistent with the need to support optical layer
services independent of higher layers payloads. In the context of IP
over optical networks, peering relationships between different trust
domains will eventually have to occur at the IP layer, on IP routing
elements, even though non-IP paths may exist between the peering
routers.
9.4 Rate of LP setups
Dynamic establishment of optical channel trails and lightpaths is
quite desirable in IP over optical networks, especially when such
instantiations are driven by a stable traffic engineering control
system, or in response to authenticated and authorized requests from
client.
However, there are many proposals suggesting the use of dynamic,
data-driven shortcut-LP setups in IP over optical networks. The
arguments put forth in such proposals are quite reminiscent of
similar discussions regarding ATM deployment in the core of IP
networks. Deployment of highly dynamic data driven shortcuts within
core networks has not been widely adopted by carriers and ISPs for a
number of reasons: possible CPU overhead in core network elements,
complexity of proposed solutions, stability concerns, and lack of
true economic drivers for this type of service. This draft assumes
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that this paradigm will not change and that highly dynamic, data-
driven shortcut LP setups are for future investigation.
Instead, the optical channel trails and LPs that are expected to be
widely used at the initial phases in the evolution of IP over
optical networks will include the following:
o Dynamic connections for control plane traffic and default path
routed data traffic,
o Establishment and re-arrangement of arbitrary virtual topologies
over rings and other physical layer topologies.
o Use of stable traffic engineering control systems to engineer LP
connections to enhance network performance, either for explicit
demand based QoS reasons or for load balancing).
Other issues surrounding dynamic connection setup within the core
center around resource usage at the edge of the optical domain.
One potential issue pertains to the number of flows that can be
processed by an ingress or egress network element either because of
aggregate bandwidth limitations or because of a limitation on the
number of flows (e.g., LPs) that can be processed concurrently.
Another possible short term reason for dynamic shortcut LP setup
would be to quickly pre-provisioned paths based on some criteria
(TOD, CEO wants a high BW reliable connection, etc.). In this
scenario, a set of paths is pre-provisioned, but not actually
instantiated until the customer initiates an authenticated and
authorized setup requests which is consistent with existing
agreements between the provider and the customer. In a sense, the
provider may have already agreed to supply this service, but will
only instantiate it by setting up a lightpath when the customer
submits an explicit request.
9.5 Distributed vs. centralized cut through path calculation
One model of shortcut path calculation is to have a centralized
mechanism (perhaps simply a suitably equipped high end PC) which has
complete knowledge of the physical topology, the available
wavelengths, and where applicable relevant time domain information.
In this centralized model, the centralized resource acts a server or
bandwidth broker. A corresponding client will reside on each network
element that can source or sink an LP. The source client would
query the server in order to set up an LP from the source to the
destination. The server would then check to see if such an LP can
be established based on prevailing conditions. Furthermore,
depending on the specifics of the model, the server may either setup
the LP on behalf of the client or provide the necessary information
to the client or to some other entity to allow the LP to
instantiated (e.g., the information may consist of a set of WLPs
to be used at each hop within the trust domain). Another option may
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be for the server to indicate that it is not possible to setup an LP
to the egress point but that it might be possible to bypass a
certain number of nodes and terminate the shortcut at a routed hop
which is closer to the destination than the source node is.
The second possibility is a distributed model in which all nodes
maintain a synchronized topology database, and advertise topology
state information to maintain and refresh the database. A
constraint-based routing entity on each node may then use the
information in the topology database and other relevant details to
compute appropriate paths through the optical domain. Once a path is
computed, a signaling protocol such as RSVP can be used to
instantiate the LP.
9.6 Wavelength and TDM signaling
It will be assumed that there exists some default communication
mechanism between routers prior to using any of the routing and
signaling mechanisms. If a ring topology exists then this
default mechanism would most likely be pre-configured (e.g., a
default communication WL between routers or perhaps routers and
ADMs). If a switched infrastructure exists then it is likely
that some dynamic routing protocol would exist or at the very
least some NM interface would need to exist in order to statically
connect each router with its appropriate peer within the trust
domain.
10. Evolution Path for IP over Optical Architectures
The architectural models described in Section 7 imply a certain
degree of implementation complexity. Specifically, the overlay
model was described as the least complex for near term deployment
and the peer model the most complex. Nevertheless, each model has
certain advantages and this raises the question as to the evolution
path for IP over optical network architectures.
It is quite likely that initial deployments will be based on the
overlay or inter-domain model, and the final evolution will be to
the peer model. Implementation agreements based on the domain
services model (Section 6) are being developed in bodies such as the
Optical Interworking Forum (OIF) and the Optical Domain Services
Interconnect (ODSI). These agreements are aimed at near term
deployment and are expected to be the pre-cursors to the peer model
architecture.
11. Security Considerations
TBD.
12. Summary and Conclusions
The objective of this draft was to define a framework for IP over
optical networks, considering the service models, routing and
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signaling issues. There are a diversity of choices, as described
in this draft, for IP-optical interconnection, service models
and protocol mechanisms. The approach advocated in this draft
was to allow different service models and proprietary enhancements
in optical networks, and define complementary signaling and
routing mechanisms that would support these.
13. References
1. D. Awduche, Y. Rekhter, J. Drake, R. Coltun, "Multi-Protocol
Lambda Switching: Combining MPLS Traffic Engineering Control
With Optical Crossconnects," draft-awduche-mpls-te-optical-
01.txt, Work in Progress.
2. B. Rajagopalan, D. Saha, B. Tang, and K. Bala, "Signaling
Framework for Automated Establishment and Restoration of Paths in
Optical Mesh Networks," draft-rstb-optical-signaling-framework-
01.txt, Work in progress.
3. G. Bernstein, R. Coltun, J. Moy, and A. Sodder, "Optical Domain
Services Interconnect (ODSI) Functional Specification,"
www.odsi-coalition.com, March, 2000.
4. K. Kompella et al, "Extensions to IS-IS/OSPF and RSVP in support
of MPL(ambda)S," draft-kompella-mpls-optical-00.txt, Work in
Progress.
5. D. Basak, D. Awduche, J. Drake, and Y. Rekhter, "Multi-protocol
Lambda Switching: Issues in Combining MPLS Traffic Engineering
Control With Optical Crossconnects," Internet Draft, Work in
Progress, February 2000.
6. Y. Rekhter and T. Li, "A Border Gateway Protocol 4 (BGP4)",
RFC 1771, March, 1995.
7. T. Bates, R. Chandra, D. Katz, and Y. Rekhter, "Multiprotocol
Extensions for BGP-4," RFC 2283, Feb., 1998.
8. E. Rosen and Y. Rekhter, "BGP/MPLS VPNs", RFC 2547, March, 1999.
9. D. Saha, B. Rajagopalan and B. Tang, "RSVP Extensions for
Signaling Optical Paths", draft-saha-rsvp-optical-signaling-
00.txt, Work in progress.
10. K. Kompella and Y. Rekhter, "Link Bundling in MPLS Traffic
Engineering," draft-kompella-mpls-bundle-01.txt, Work in
progress.
11. B. Rajagopalan and D. Saha, "Link Bundling Considerations in
Optical Networks," draft-rs-optical-bundling-01.txt, Work in
progress.
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12. S. Chaudhuri, G. Hjalmtysson, and J. Yates, "Control of
Lightpaths in an Optical Network," draft-chaudhuri-ip-olxc-
control-00.txt, Work in progress.
13. J. Lang, et al., "Link Management Protocol," draft-lang-mpls-lmp-
01.txt, Work in progress.
14. E. Crawley, R. Nair, B. Rajagopalan and H. Sandick, "A Framework
for QoS-based Routing in the Internet," RFC 2386, August, 1998.
15. S. Ramamurthy, Z. Bogdanowicz, S. Samieian, et al., "Capacity
Performance of Dynamic Provisioning in Optical Networks", to
appear in J. of Lightwave Technology.
16. D. Awduche, L. Berger, Der-Hwa Gan, T. Li, G. Swallow,
V. Srinivasan, "RSVP-TE: Extensions to RSVP for LSP Tunnels,"
draft-ietf-mpls-rsvp-lsp-tunnel-05.txt, Work in progress.
17. J. Suurballe, "Disjoint Paths in a Network," Networks, vol. 4,
1974.
18. B. Doshi, S. Dravida, P. Harshavardhana, et. al, "Optical Network
Design and Restoration," Bell Labs Technical Journal, Jan-March,
1999.
19. B. Jamoussi, Ed., "Constraint-Based LSP Setup using LDP,"
draft-ietf-mpls-cr-ldp-03.txt, Work in progress.
20. J. Luciani et al, "NBMA Next Hop Resolution Protocol (NHRP),"
RFC-2332, April 1998.
21. P. Ashwood-Smith et. al, "Generalized MPLS - Signaling Functional
Description", Internet Draft, Work in Progress.
12. Acknowledgments
We would like to thank Zouheir Mansourati and Ian Duncan of Nortel
Networks for their comments on this draft.
13. Author's Addresses
Bala Rajagopalan James V. Luciani
Tellium, Inc. TollBridge Technologies
2 Crescent Place P,O. Box 1010
P.O. Box 901 Concord, MA 01742
Oceanport, NJ 07757-0901 Email: james_luciani@mindspring.com
Email: braja@tellium.com
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Daniel O. Awduche Brad Cain, Bilel Jamoussi
UUNET (MCI Worldcom) Nortel Networks
Loudoun County Parkway 600 Tech Park
Ashburn, VA 20247 Billerica, MA 01821
Phone: 703-886-5277 Phone: 978-288-4734
Email: awduche@uu.net Email: bcain@nortelnetworks.com
jamoussi@nortelnetworks.com
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