Internet Draft
IPO and MPLS N. Chandhok
Ohio State University
A. Durresi
Ohio State University
R. Jagannathan,
Ohio State University
R. Jain
Nayna Networks
S. Seetharaman
Ohio State University
K. Vinodkrishnan
Ohio State University
Internet Draft
Document: draft-osu-ipo-mpls-issues-00 .txt July 2000
Category: Informational
IP over Optical Networks: A Summary of Issues
Status of this Memo
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Abstract
This draft presents a summary of issues related to transmission of
IP packets over optical networks. This is a compilation of many
drafts presented so far in IETF. The goal is to create a common
document, which by including all the views and proposals will serve
as a better reference point for further discussion. The novelty of
this draft is that we try to cover all the main areas of integration
and deployment of IP and optical networks including architecture,
routing, signaling, management, and survivability.
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Several existing and proposed network architectures are discussed.
The two-layer model, which aims at a tighter integration between IP
and optical layers, offers a series of important advantages over the
current multi-layer architecture. The benefits include more
flexibility in handling higher capacity networks, better network
scalability, more efficient operations and better traffic
engineering.
Multiprotocol Label Switching (MPLS) has been proposed as the
integrating structure between IP and optical layers. Routing in the
non-optical and optical parts of the hybrid IP network needs to be
coordinated. Several models have been proposed including overlay,
augmented, and peer-to-peer models. These models and the required
enhancements to IP routing protocols, such as, OSPF and IS-IS are
provided.
Control in the IP over Optical networks is facilitated by MPLS
control plane. Each node consists of an integrated IP router and
optical layer crossconnect (OLXC). The interaction between the
router and OLXC layers is defined. Signaling among various nodes is
achieved using CR-LDP and RSVP protocols.
The management functionality in optical networks is still being
developed. The issues of link initialization and performance
monitoring are summarized in this document.
With the introduction of IP in telecommunications networks, there is
tremendous focus on reliability and availability of the new IP-
optical hybrid infrastructures. Automated establishment and
restoration of end to end paths in such networks require
standardized signaling and routing mechanisms. Layering models that
facilitate fault restoration are discussed. A better integration
between IP and optical will provide opportunities to implement a
better fault restoration.
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 .
Contents:
1. Overview
1.1 Introduction
1.2 Network Models
2. Optical Switch Architecture
2.1 Isomorphic Relations Between OXCs And LSRs
2.2 Distinctions Between OXCs And LSRs
2.3 Isomorphic Relations between LSPs And Lightpaths
2.4 Distinction Between LSPs And Lightpaths
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2.5 General Requirements For The OXC Control Plane
2.5.1 Overview of The MPLS Traffic Engineering Control
2.5.2 OXC Enhancements to Support MPLS Control Plane
2.5.3 MPLS Control Plane Enhancements
2.6 MPLS Traffic Engineering Control Plane With OXCs
3. Routing in Optical Networks
3.1 Models for IP-Optical Network Interaction
3.1.1 Overlay model
3.1.2 Integrated/Augmented model
3.1.3 Peer model
3.2 The Lightpath routing solution
3.2.1 What is an IGP?
3.2.2 How does MPLS come into the picture?
3.2.3 Lightpath Selection
3.3 IS-IS/OSPF enhancements
3.3.1 Link Type
3.3.2 Link Media Type (LMT)
3.3.3 Link ID
3.3.4 Local Interface IP address
3.3.5 Remote Interface IP address.
3.3.6 TE Metric
3.3.7 Path TLV
3.3.8 Shared Risk Link Group TLV
3.4 Control Channels, Data Channels and IP links
3.4.1 Excluding Data Traffic from Control Channels
3.4.2 Forwarding Adjacencies
3.4.3 Two way Connectivity
3.4.4 Optical LSAs
3.5 Open Questions
4. Control
4.1 MPLS Control Plane
4.2 Addressing
4.3 Path Setup
4.3.1 Basic Path Setup Procedure
4.3.2 CR-LDP Extensions for Path Setup
4.3.3 RSVP Extensions for Path Setup
4.4 Resource Discovery and Maintenance
4.5 Configuration Control Using GSMP
4.6 Resource Discovery Using NHRP
5. Optical Network Management
5.1 Link Initialization
5.1.1 Control Channel Management
5.1.2 Verifying Link Connectivity
5.1.3 Fault Localization
5.2 Optical Performance Monitoring (OPM)
6. Fault restoration in Optical networks
6.1 Layering
6.1.1 Layer 1 Protection
6.1.2 Layer 0 Protection
6.2 MPLS in Protection
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6.2.1 Motivations
6.2.2 Goals
6.3 Protection options
6.3.1 Dynamic Protection
6.3.2 Pre-negotiated Protection
6.3.3 End to end repair
6.3.4 Local repair
6.3.5 Link protection
6.3.6 Path protection
6.3.7 Revertive Mode
6.3.8 Non-revertive Mode
6.3.9 1+1 Protection
6.3.10 1:1, 1:n, and n:m Protection
6.3.11 Recovery Granularity
6.4. Failure detection
6.5 Failure Notification
6.5.1 Reverse Notification Tree (RNT)
6.6. Timing
6.6.1 Protection Switching Interval Timer
6.6.2 Inter-FIS Packet Timer
6.6.3 Maximum FIS Duration Timer
6.6.4 Protection switching Dampening Timer
6.6.5 Liveness Message Send interval
6.6.6 Failure Indication Hold-off Timer
6.6.7 Lost Liveness Message Threshold
6.7 Signaling Requirements related to restoration
6.8 RSVP/CR-LDP Support for Restoration
6.8.1 Proposed Extensions for Protection Paths
6.9 Fast restoration of MPLS LSPs
6.9.1 L1/L2/L3 Integration
6.9.2 An example
6.10 LMP's Fault localization mechanism
7. Security Considerations
8. Acronyms
9. Terminology
10. References
11. Author's Addresses
1. Overview
1.1. Introduction
Challenges presented by the exponential growth of the Internet have
resulted in the intense demand for broadband services. In
satisfying the increasing demand for bandwidth, optical network
technologies represent an unique opportunity because of their almost
unlimited potential bandwidth.
Recent developments in wavelength-division multiplexing (WDM)
technology have dramatically increased the traffic capacities of
optical networks. Research is ongoing to introduce more intelligence
in the control plane of the optical transport systems, which will
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make them more survivable, flexible, controllable and open for
traffic engineering. Some of the essential desirable attributes of
optical transport networks include real-time provisioning of optical
channel trails, providing capabilities that enhance network
survivability, providing interoperability functionality between
vendor-specific optical sub-networks, and enabling protection and
restoration capabilities in operational contexts. The research
efforts now are focusing on the efficient internetworking of higher
layers, primarily IP with WDM layer.
Along with this WDM network, IP networks, SONET networks, ATM
backbones shall all coexist. Various standardization bodies have
been involved in determining an architectural framework for the
interoperability of all these systems.
One approach for sending IP traffic on WDM networks would use a
multi-layered architecture comprising of IP/MPLS layer over ATM over
SONET over WDM. If an appropriate interface is designed to provide
access to the optical network, multiple higher layer protocols can
request lightpaths to peers connected across the optical network.
This architecture has 4 management layers. One can also use a
packet over SONET approach, doing away with the ATM layer, by
putting IP/PPP/HDLC into SONET framing. This architecture has 3
management layers. A few problems of such multi layered
architectures have been studied.
+---------------+
| |
| IP / MPLS |
| |
+---------------+ +---------------+
| | | |
| ATM | | IP/ MPLS |
| | | |
+---------------+ +---------------+ +-------------+
| | | | | |
| SONET | | SONET | | IP / MPLS |
| | | | | |
+---------------+ +---------------+ +-------------+
| | | | | |
| WDM | | WDM | | WDM |
| | | | | |
+---------------+ +---------------+ +-------------+
(4 LAYERS) (3 LAYERS) (2 LAYERS)
Fig.1: Layering Architectures Possible
The fact that it supports multiple protocols, will increase
complexity for IP-WDM integration because of various edge-
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interworkings required to route, map and protect client signals
across WDM subnetworks. The existence of separate optical layer
protocols will increase management costs for service providers.
One of the main goals of the integration architecture is to make
optical channel provisioning driven by IP data paths and traffic
engineering mechanisms. This will require a tight cooperation of
routing and resource management protocols at the two layers. The
multi-layered protocols architecture can complicate the timely flow
of the possibly large amount of topological and resource
information.
Another problem is with respect to survivability. There are various
proposals stating that the optical layer itself should provide
restoration/protection capabilities of some form. This will require
careful coordination with the mechanisms of the higher layers such
as the SONET Automatic Protection Switching (APS) and the IP re-
routing strategies. Hold-off timers have been proposed to inhibit
higher layers backup mechanisms.
Problems can also arise from the high level of multiplexing done.
The optical fiber links contain a large number of higher layer flows
such as SONET / SDH, IP flows or ATM VCs. Since these have their
own mechanisms, a flooding of alarm messages can take place.
Hence, a much closer IP/WDM integration is required. The
discussions, henceforth in this document, shall be of such an
architecture. There exist, clouds of IP networks, clouds of WDM
networks. Transfer of packets from a source IP router to a
destination is required. How the combination does signaling to find
an optimal path, route the packet, and ensure survivability are the
topics of discussion.
Multi-Protocol Label Switching (MPLS) for IP packets is believed to
be the best integrating structure between IP and WDM. MPLS brings
two main advantages. First, it can be used as a powerful instrument
for traffic engineering. Second, it fits naturally to WDM when
wavelengths are used as labels. This extension of the MPLS is
called the Multi-protocol lambda switching.
This document starts off with a description of the optical network
model. Section 2 describes the correspondence between the optical
network model and the MPLS architecture and how it can bring about
the inter-working. Section 3 is on routing in this architecture.
It also describes 3 models for looking at the IP cloud and the
Optical cloud namely the Overlay model, the augmented model and the
peer model. Sections 4 and 5 are on control, signaling and
management, respectively. Section 6 is on restoration. Acronyms and
glossary are defined in Sections 8 and 9.
1.2 Network Model
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The network model consists of IP routers attached to an optical core
network. The optical network consists of multiple optical
crossconnects (OXCs) interconnected by optical links. Each OXC is
capable of switching a data stream using a switching function,
controlled by appropriately configuring a crossconnect table. Thus,
in this document, the term OXC is used to denote the hybrid node
consisting of switching element referred to as optical layer
crossconnect (OLXC) and a control plane. The switching within the
OXC can be accomplished either in the electrical domain, or in the
optical domain. In this network model, a switched lightpath is
established between IP routers. Designing an IP-based control plane
should include designing standard signaling and routing protocols
for coherent end-to-end provisioning and restoration of lightpaths
across multiple optical sub-networks, and determining IP
reachability and seamless establishment of paths from one IP end-
point to another over an optical core network.
Several standards organizations and interoperability forums have
initiated work items to study the requirements and architectures for
reconfigurable optical networks, under-scoring the importance of
versatile networking capabilities in the optical domain. ITU-T
recommendation G.872, for example, defines a functional architecture
for an optical transport network (OTN) that supports the transport
of digital client signals. It defines OTN as "a transport network
bounded by optical channel access points". The architecture of
G.872's OTN is based on a layered structure, which includes:
(a) an optical channel (OCh) layer network : The optical channel
layer network supports end-to-end networking of optical channel
trails providing functionalities like routing, monitoring, grooming,
and protection and restoration of optical channels.
(b) 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.
(c) an optical transmission section (OTS) layer network : This
provides functionality for transmission of the optical signals on
optical media of different types.
To realize the functions in the OCh layer, an optical crossconnect
with rearrangeable switch fabrics and a control plane will be
critical. In the existing IP-centric data network domain, the
functionalities of the OCh layer are performed by the MPLS traffic
engineering control plane. Thus, there is a similarity between the
IP/MPLS over WDM and the ITU recommendation.
In the following section, we stress on the relations that exist
between the all-optical crossconnects of the optical networks and
the label switch routers of the MPLS networks and identify how the
control plane model of MPLS traffic engineering (TE) can be applied
to that of optical transport networks. Before we propose a control
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plane model for the optical networks based on the MPLS control plane
traffic engineering, we discuss how the similarities can help to
expose the reusable software artifacts from the MPLS traffic
engineering control plane model. Consider an IP-centric hybrid
optical internetworking environment, which consist of both LSRs and
OXCs. Let us assume that OXCs are programmable and support
wavelength conversion.
2. Optical Switch Architecture
Multiprotocol Label Switching is a switching method in which a label
field in the incoming packets is used to determine the next hop. At
each hop, the incoming label is replaced by another label that is
used at the next hop. The path thus realized is called a Label
Switched Path (LSP). Each LSP has a set of criteria associated with
it, which describes the traffic that traverses the LSP. This set of
criteria groups the incoming traffic into classed called _Forwarding
Equivalence Classes (FECs)._ LSPs are setup using signaling
protocols like RSVP or CR-LDP. A device that can classify traffic
into FECs is called a label edge router (LER) while the devices
which base their forwarding decision only on the basis of the
incoming labels (and ports) are called Label Switched Routers
(LSRs).
Here we consider a hybrid, IP-centric optical internetworking
environment consisting of both label switching routers (LSRs) and
OXCs. The OXCs are programmable and support wavelength conversion
and translation. It is important here to enumerate the relations
and distinctions between OXCs and LSRs to expose the reusable
software artifacts from the MPLS traffic engineering control plane
model. Both OXCs and LSRs emphasize problem decomposition by
architecturally decoupling the control plane from the data plane.
2.1 Isomorphic Relations between OXCs And LSRs
While an LSR's data plane uses the label swapping paradigm to
transfer a labeled packet from an input port to an output port, the
data plane of an OXC uses a switch matrix to provision an optical
channel trail from an input port to an output port. An LSR performs
label switching by establishing a relation between an tuple and an tuple.
Similarly, OXC provisions optical channel trail by establishing a
relation between an tuple and an
tuple. The functions of the
control plane of both LSRs and OXCs include resource discovery,
distributed routing control, and connection management. LSR's
control plane is used to discover, distribute, and maintain relevant
state information related to the MPLS network, and to instantiate
and maintain label switched paths (LSPs) under various MPLS traffic
engineering rules and policies. OXC's control plane is used to
discover, distribute, and maintain relevant state information
associated with the OTN, and to establish and maintain optical
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channel trails under various optical internetworking traffic
engineering rules and policies [Awuduche99].
2.2 Distinctions Between OXCs And LSRs
Current generation of OXCs and LSRs differ in certain
characteristics. While LSRs are datagram devices which can perform
certain packet level operations in the data plane, OXCs cannot. It
cannot perform packet level processing in the data plane.
Conceptually another difference is there, which is that the
forwarding information is carried explicitly in LSRs as part of the
labels appended to the data packets, while in the OXCs switching
information is not appended to the data packet, rather it is implied
from the wavelength or the optical channel.
2.3 Isomorphic Relations Between Explicit LSPs And Optical Channel
Trails
Both the explicit LSPs and optical channel trails (OCTs) exhibit
certain commonalties. For example, both of them are the
abstractions of unidirectional, point-to-point virtual path
connections. An explicit LSP provides a parameterized packet
forwarding path (traffic-trunk) between an ingress LSR and an
egress LSR, while an optical channel trail provides a optical
channel between two endpoints for the transport of digital client
signals [Awuduche99]. Another commonality is that the payload
carried by both LSPs and optical trails are transparent along their
respective paths. They can be parameterized to stipulate their
performance, behavioral, and survivability requirements from the
network. Paths that satisfy some demands and policy requirements
subject to some constraints imposed by the operational environment
can be selected using constraint-based routing scheme. There are
certain similarities in the allocation of labels to LSPs and in the
allocation of wavelengths to optical trails.
2.4 Distinction between LSPs and Optical Channel Trails
There is one major distinction between LSPs and OCTs in that LSPs
support label stacking, but the concept similar to label stacking,
ie, wavelength stacking doesn't exist in the optical domain at this
time.
2.5 General Requirements for the OXC Control Plane
This section has some of the requirements for the OXC control plane
emphasizing on the routing components of these requirements. Some
of the key aspects to these requirements are:(a) to expedite the
capability to establish optical channel trails.(b) to support
traffic engineering functions.(c) to support various protection and
restoration schemes. Since the historical implementation of the
"control plane" of optical transport networks via network management
has detrimental effects like slow restoration, preclusion of
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distributed dynamic routing control, etc., motivation is to improve
the responsiveness of the optical transport network and to increase
the level of interoperability within and between service provider
networks.
In the following sections, we summarize the enhancements that are
required in the OXCs to support the MPLS TE as well as the changes
required in the MPLS control plane to adapt to the OXCs. The next
section gives a brief overview of MPLS traffic engineering.
2.5.1 Overview Of The MPLS Traffic Engineering Control
In this section, we discuss the components of the MPLS traffic
engineering control plane model, which include the following modules
[Awuduche99]:
(a) Resource discovery.
(b) State information dissemination to distribute relevant
information concerning the state of the network. The state of the
network includes topology and resource availability information.
This can be accomplished by extending conventional interior gateway
protocols (IGPs) to carry additional information in their link state
advertisements.
(c) Path selection that is used to select an appropriate route
through the MPLS network for explicit routing. It is implemented by
introducing the concept of constraint-based routing which is used to
compute paths that satisfy certain constraints, including
constraints imposed by the operational environment.
(d) Path management, which includes label distribution, path
placement, path maintenance, and path revocation. These functions
are implemented through a signaling protocol, such as the RSVP
extensions or through CR-LDP.The above components of the MPLS
traffic engineering control plane are separable, and independent of
each other, and hence it allows an MPLS control plane to be
implemented using a composition of best of breed modules.
2.5.2 Required Enhancements To OXCs To Support MPLS Control Plane
This section discusses some of the enhancements to OXCs to support
MPL(ambda)S.(a) There should be a mechanism to exchange control
information between OXCs, and between OXCs and other LSRs. This can
be accomplished in-band or quasi-in-band using the same links that
are used to carry data-plane traffic, or out-of-band via a separate
network.(b) An OXC must be able to provide the MPLS traffic
engineering control plane with pertinent information regarding the
state of individual fibers attached to that OXC, as well the state
of individual lightpaths or optical channel trails within each
fiber.(c) Even when an edge LSR does not have WDM capabilities, it
should still have the capability to exchange control information
with the OXCs in the domain.
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2.5.3 Required Enhancements To Current MPLS Control Plane
This section discusses the enhancements that are to be made in the
MPLS control plane to support MPL(ambda)S [Basak99].
An MPLS domain may consist of links with different properties
depending upon the type of network elements at the endpoints of the
links. Within the context of MPL(ambda)S, the properties of a link
consisting of a fiber with WDM that interconnects two OXCs are
different form that of a SONET link that interconnects two LSRs. As
an example, a conventional LSP cannot be terminated on a link
connected to a pure OXC. However, a conventional LSP can be
certainly be terminated on a link connected to a frame-based LSR.
These differences should be taken into account when performing path
computations to determine an explicit route for an LSP. It is also
feasible to have the capability to restrict the path of some LSPs to
links with certain characteristics. Path computation algorithms may
then take this information into account when computing paths LSPs.
There must be procedures to associate control channels to bearer
channels between a pair of adjacent OXCs, if there exists multiple
control channels and bearer channels between them. Procedures are
required to demultiplex the control traffic for different bearer
channels if a control channel is associated with multiple bearer
channels. Procedures are also needed to activate and deactivate
bearer channels, to identify the bearer channels associated with any
given physical link, to identify spare bearer channels for
protection purposes, and to identify impaired bearer channels,
particularly, in the situation where the physical links carrying the
bearer channel are not impaired.
Signaling protocols (RSVP and CR-LDP) need to be extended with
objects that can provide sufficient details to establish
reconfiguration parameters for OXC switch elements. IGP should be
extended to carry information about the physical diversity of the
fibers. IGP should be able to distribute information regarding the
allocatable bandwidth granularity of any particular link.
2.6 Synthesizing The MPLS Traffic Engineering Control Plane With OXCs
In IP-centric optical interworking systems, given that both OXCs and
LSRs require control planes, one option would be to have two
separate and independent control planes [Awuduche99]. Another
option is to develop a uniform control plane that can be used for
both LSRs and OXCs. This option of having a uniform control plane
will eliminate the administrative complexity of managing hybrid
optical internetworking systems with separate, dissimilar control
and operational semantics. Specialization may be introduced in the
control plane, as necessary, to account for inherent peculiarities
of the underlying technologies and networking contexts. A single
control plane would be able to span both routers and OXCs. In such
an environment a LSP could traverse an intermix of routers and OXCs,
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or could span just routers, or just OXCs. This offers the potential
for real bandwidth-on-demand networking, in which an IP router may
dynamically request bandwidth services from the optical transport
network.
To bootstrap the system, OXCs must be able to exchange control
information. One way to support this is to pre-configure a
dedicated control wavelength between each pair of adjacent OXCs, or
between an OXC and a router, and to use this wavelength as a
supervisory channel for exchange of control traffic. Another
possibility would be to construct a dedicated out-of-band IP network
for the distribution of control traffic.
Though an OXC equipped with an MPLS traffic engineering control
plane would resemble a Label Switching Router, there are some
important distinctions and limitations. The distinction concerns
the fact that there are no analogs of label merging in the optical
domain, which implies that an OXC cannot merge several wavelengths
into one wavelength. Another major distinction is that an OXC
cannot perform the equivalent of label push and pop operation in the
optical domain. This is due to lack of the concept of pushing and
popping wavelengths is infeasible with contemporary commercial
optical technologies. Finally, there is another important
distinction, which is concerned with the granularity of resource
allocation. An MPLS router operating in the electrical domain can
potentially support an arbitrary number of LSPs with arbitrary
bandwidth reservation granularities, whereas an OXC can only support
a relatively small number of optical channel trails, each of which
will have coarse discrete bandwidth granularities.
3. Routing in Optical Networks
The optical network model considered in this draft consists of
multiple Optical Crossconnects (OXCs) interconnected by optical
links in a general topology (referred to as an "optical mesh
network"). Each OXC is assumed to be capable of switching a data
stream from a given input port to a given output port. This
switching function is controlled by appropriately configuring a
crossconnect table. Conceptually, the crossconnect table consists
of entries of the form , indicating
that data stream entering input port i will be switched to output
port j. An "lightpath" from an ingress port in an OXC to an egress
port in a remote OXC is established by setting up suitable
crossconnects 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. Lightpaths 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.
It is assumed that one or more control channels exist between
neighboring OXCs for signaling purposes.
3.1 Models for IP-Optical Network Interaction
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Some of the proposed models for interaction between IP and optical
components in a hybrid network are [Luciani00]:
(1) Overlay model
(2) Integrated / Augmented model
(3) Peer model.
The key consideration in deciding which model is whether there is a
single/separate monolithic routing and signaling protocol spanning
the IP and the Optical domains. If there are separate instances of
routing protocols running for each domain then 1) What is the
interface defined between the two protocol instances? 2) What kind
of information can be leaked from one protocol instance to the
other? 3) Would one label switching protocol run on both domains?
If that were to be the case then how would labels map to
wavelengths? Also, how would IP QoS parameters be mapped into the
optical domain?
3.1.1 Overlay model
Under the overlay model, IP is more or less independent of the
optical subnetwork. That is IP acts as a client to the Optical
domain. In this scenario, the optical network provides point to
point connection to the IP domain. The IP/ MPLS routing protocols
are independent of the routing and signaling protocols of the
optical layer. The overlay model may be divided into 2 parts:
a) Static overlay model: In this model path endpoints are specified
through a network management system (NMS) though the paths may be
laid out statically by the NMS or dynamically by the network
elements. This would be similar to ATM permanent virtual circuits
(PVCs) and ATM Soft PVCs (SPVCs).
b) Signaled overlay model: The path end-points are specified through
signaling via a User to Network Interface (UNI). Paths must be laid
out dynamically since they are specified by signaling. This is
similar to ATM switched virtual circuits (SVCs). The Optical Domain
Services Interoperability (ODSI) forum and Optical Internetworking
Forum (OIF) are also defining similar standards for the Optical UNI.
In these models user devices which reside on the edge of the optical
network can signal and request bandwidth dynamically. These models
use IP/optical layering. Endpoints are specified using a port
number/IP address tuple. PPP is used for service discovery wherein a
user device can discover whether it can use ODSI or OIF protocols to
connect to an optical port. Unlike MPLS there are no labels to be
setup. The resulting bandwidth connection will look like a leased
line.
3.1.2 The Integrated/ Augmented model
In the integrated model, the MPLS/IP layers act as peers of the
optical transport network, such that a single routing protocol
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instance runs over both the IP/MPLS and optical domains. A common
IGP like OSPF or IS-IS, with appropriate extensions will be used to
distribute topology information. Also this model assumes a common
address space for the optical and IP domain. In the augmented
model, there are actually separate routing instances in the IP and
optical domains but information from one routing instance is leaked
into the other routing instance. For example IP addresses could be
assigned to optical network elements and carried by optical routing
protocols to allow reachability information to be shared with the IP
domain to support some degree of automated discovery.
3.1.3 The peer model
The peer model is somewhat similar to the integrated model in that
the IP reachability information might be passed around within the
optical routing protocol but the actual flow will be terminated at
the edge of the optical network and will only be reestablished upon
reaching a non-peer capable node at the edge of the optical domain
or at the edge of the domain, which implements both the peer and the
overlay models.
3.2 The Lightpath routing solution
3.2.1 What is an IGP?
An IGP is a interior gateway routing protocol. Examples of IGPs
would be OSPF and IS-IS. IGPs are used to exchange state
information within a specified administrative domain and for
topology discovery. This exchange of information inside the domain
is done by advertising the Link state information periodically.
Please refer to [OSPF] and [IS-IS] for more details.
3.2.2 How does MPLS fit into the picture?
While the idea of bandwidth-on-demand is certainly not new, existing
networks do not support instantaneous service provisioning. Current
provisioning of bandwidth is painstakingly static. Activation of
large pipes of bandwidth takes anything from weeks to months. The
imminent introduction of photonic switches in the transport networks
opens new perspectives. Combining the bandwidth provisioning
capabilities of photonic switches with the traffic engineering
capabilities of MPLS, will allow routers and ATM switches to request
bandwidth where and when they need it.
+--------+ +--------+ +--------+
| |<----->|Path | |OSPF |
| CR-LDP | +-->|Selector| | |
| | | | | |TE EXT |
+--------+ | +---+----+ +---|OPT EXT |
| | | +--------+
| v |
| +--------+ | +--------+
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+--------+ | |TE | | |IS-IS |
| | | |Database| | | |
| RSVP |<--+ | | | |TE EXT |
| | | |<--+---|OPT EXT |
+--------+ +--------+ +--------+
Fig.2: Lightpath Selection
3.2.3 Lightpath Selection
The lightpath routing system is based on the MPLS Constraint based
routing model. Fig.2 illustrates lightpath selection. These
systems use CR-LDP or RSVP to signal MPLS paths. These protocols
can source route by consulting a traffic engineering database which
is maintained along with the IGP database. This information is
carried opaquely by the IGP for constraint based routing. If RSVP
or CR-LDP is used solely for label provisioning, the IP router
functionality must be present at every label switch hop along the
way. Once the label has been provisioned by the protocol then at
each hop the traffic is switched using the native capabilities of
the device to the eventual egress LSR. To exchange information
using IGP protocols like OSPF and IS-IS, certain extensions need to
be made to both of these to support MPL(ambda) switching.
3.3 IS-IS/ OSPF enhancements
OSPF defined in [OSPF] and IS-IS defined in [IS-IS] are the commonly
deployed routing protocols in large networks. OSPF/IS-IS have been
extended to include traffic engineering capability [Katz99],
[ISISTE]. There is a need to add the optical link state
advertisement (LSA) to OSPF/IS-IS to support lightpath routing
computation. The optical LSA would include a number of new elements,
called TLVs (type-length-value) because of the way they are coded.
The following sections describe some of the proposed TLVs.
3.3.1 Link Type
A network may have a link with many different characteristics. A
link type TLV allows to identify a particular type of link. One way
to describe the links would be [WANG00]:
a) Service transparent: Service transparent is a point-to-point
physical link.
b) Service aware: A service aware link is a point-to-point logical
optical link.
Another way of classifying the links is based on the types of end
nodes [Kompella00-a]. Nodes that can switch individual packets are
called packet switch capable (PSC). Nodes that can transmit/receive
SONET payloads are called time division multiplex (TDM) capable.
Nodes that can switch individual wavelengths are called lambda
switch capable (LSC). Finally, nodes that switch entire contents of
one fiber into other are called fiber switch capable (FSC).
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Links can be either physical (one hop) links or logical links
consisting of multiple hop connections. Logical links are called
_Forwarding Adjacencies (FAs)._ This leads to the following types of
links:
a) PSC links end (terminate or egress) on PSC nodes. Depending
upon the hierarchy of LSPs tunneled within LSPs, several
different types of PSC links can be defined.
b) TDM links end on TDM nodes and carry SONET/SDH payloads.
c) LSC links end on LSC nodes and consist of wavelengths.
d) FSC links end on FSC nodes and consist of fibers.
e) Forwarding Adjacency PSC (FA-PSC) links are forwarding
adjacencies whose egress nodes are packet switching.
f) FA-TDM, FA-LSC, and FA-LSP are forwarding adjacencies whose
egress nodes are TDM, LSC, and LSP capable, respectively.
3.3.2 Link Media Type (LMT)/ Link resource
A link may support a set of media types depending on resource
availability and capacity of link. Such TLVs may have two fields of
which the first one defines the media type, and the second field
defines the lowest priority at which the media is available
[Kompella00-a]. Link Media Types present a new constraint for LSP
path computation. Specifically when a LSP is setup and it includes
one or more subsequences of links which carry the LMT TLV then for
all the links within each subsequence the encoding has to be the
same and the bandwidth has to be at least the LSP's specified
bandwidth. The total classified bandwidth available over one link
can be classified using a resource component TLV [WANG00]. This TLV
represents a group of lambdas with the same line encoding rate and
total current available bandwidth over these lambdas. This TLV
describes all lambdas that can be used on this link in this
direction grouped by encoding protocol. There is one resource
component per encoding type per fiber. If multiple fibers are used
per link there will be a resource component per fiber to support
fiber bundling.
3.3.3 Link ID
An identifier which identifies the optical link exactly as the point
to point case for TE extensions.
3.3.4 Local Interface IP address
The interface address may be omitted in which case it defaults to
the router address of the local node.
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3.3.5 Remote Interface IP address
This address may be specified as an IP address on the remote node or
the router address of the remote node .
3.3.6 Traffic Engineering (TE) Metric
This metric value can be assigned for path selection.
3.3.7 Path sub-TLV
When an LSP advertises a forwarding adjacency into an IGP, it may be
desirable to carry the information about the path taken by this
adjacency. This information may be used for path calculation by
other LSRs.
3.3.8 Shared Risk Link group TLV
A set of links may constitute a 'shared risk link group' (SRLG) if
they share a resource whose failure may affect all links in the set.
An example would be two fibers in the same conduit. Also, a fiber
may be part of more than one SRLG.
3.4 Control Channels, Data Channels, and IP Links
A pair of OXCs are said to be neighbors from the MPLS point of view
if they are connected by one or more logical or physical channels.
If several fibers share the same TE characteristic then a single
control channel would suffice for all of them. From the IGP point
of view this control channel along with all its fibers form a single
IP link. Sometimes fibers may need to be divided into sets that
share the same TE characteristic. Corresponding to each such set,
there must be a logical control channel to form an IP link. All of
the multiple logical control channels could be realized via one
common control channel. When an adjacency is established over a
logical control channel that is part of an IP link formed by the
channel and a set of fibers, this link is announced into IS- IS/OSPF
as a "normal" link; the fiber characteristics are represented as TE
parameters of that link. If there are more than one fibers in the
set, the set is announced using bundling techniques discussed in
[Kompella00-b].
3.4.1 Excluding data traffic from control channels
The control channels between OXCs or between an OXC and a router are
generally meant for low bandwidth control traffic. These control
channels are advertised as normal IP links. However if regular
traffic is forwarded on these links the channel capacity will soon
be exhausted. To avoid this, if we assume that data traffic is sent
over BGP destinations and control traffic is sent to IGP
destinations. Ways to do this are discussed in [KOMPELLA00-a].
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3.4.2 Forwarding adjacencies
An LSR at the head of an LSP may advertise this LSP as a link into a
link state IGP. When this LSP is advertised into the same instance
of the IGP as the one that determines the route taken this adjacency
then we call such a link a "forwarding adjacency". Such an LSP is
referred to as a "forwarding adjacency LSP" or just FA-LSP.
Forwarding adjacencies may be statically provisioned or created
dynamically. Forwarding adjacencies are by definition
unidirectional.
When a forwarding adjacency is statically provisioned, the
parameters that can be configured are: the head-end address, the
tail-end address, bandwidth, and resource color constraints. The
path taken by the FA-LSP can be computed by the Constrained Shortest
Path Formulation (CSPF) mechanism or MPLS TE or by explicit
configuration. When a forwarding adjacency is created dynamically
its parameters are inherited by the LSP which induced its creation.
Note that the bandwidth of the FA-LSP must be at least as big as the
LSP that induced it.
When an FA-LSP is advertised into IS-IS/OSPF, the link type
associated with this LSP is the link type of the last link in the
FA-LSP. Some of the attributes of this link can be derived from the
FA-LSP but others need to be configured. Configuration of the
attributes of statically provisioned FAs is straightforward, but for
dynamically provisioned FAs a policy-based mechanism may be needed.
The link media type of the FA is the most restrictive of the link
media types of the component links of the forwarding adjacency. FAs
will not be used to establish peering relationships between routers
at the end of the adjacencies but will only be used for CSPF
computation.
3.4.3 Two way connectivity
CSPF shouldn't perform any two way connectivity check on links used
by CSPF. This is because some of the links are unidirectional and
may be associated with forwarding adjacencies.
3.4.4 Optical LSAs
There needs to be a way of controlling the protocol overhead
introduced by optical LSAs. One way to do this is to make sure that
a Link State Advertisement happens only when there is a significant
change in the value of metrics since the last advertisement. A
definition of significant change is when the difference between the
currently available bandwidth and last advertised bandwidth crosses
a threshold[WANG00]. The frequency of these updates can be decreased
dramatically using event driven feedback.
3.5 Some Open Questions
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Some issues that have not been resolved so far are: How to ensure
that end-to-end information is propagated across as an optical
network? How to accommodate proprietary optimizations within optical
sub-networks for provisioning and restoration of lightpaths? Whether
dynamic and precomputed information can be used and if so what is
the interaction between them? What QOS related parameters need to be
defined? How to ensure fault tolerant operation at protocol level
when hardware does not support fault tolerance? How to address
scalability issues? What additional modifications are required to
support a network for routing control traffic?
4. Signaling & Control
Signaling means to intimate any particular element of certain
characteristics or services. This section discusses a few of the
signaling procedures. It is assumed that there exists some default
communication mechanism between routers prior to using any of the
routing and signaling mechanisms.
4.1 MPLS Control Plane
A candidate system architecture for an OXC equipped with an
MPLS control plane model is shown in Fig.3
--------------------------------
| OXC WITH MPLS CONTROL PLANE |
| |
| ------------------- |
| | | |
| | MPLS Control Plane| |
| | | |
| ------------------- |
| | |
| ------------------- |
| | | |
| |Control Adaptation | |
| ------------------- |
| | OXC Switch | |
| | Controller | |
| ------------------- |
| | |
| ------------------- |
| | | |
| | OXC Switch Fabric | |
| | OXC Data Plane | |
| ------------------- |
| |
--------------------------------
Fig.3: OXC Architecture
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The salient feature of the network architecture is that every node
in the network consists of an IP router and a reconfigurable OLXC.
The IP router is responsible for all non-local management functions,
including the management of optical resources, configuration and
capacity management, addressing, routing, traffic engineering,
topology discovery, exception handling and restoration. In general,
the router may be traffic bearing as proposed in, or it may function
purely as a controller for the optical network and carry no IP data
traffic. Although the IP router performs all management and control
functions, lightpaths may carry arbitrary types of traffic.
+-------------------------------+
| |
| Router module |
| |
| |
| | | |
| | Primitives | |
| | /|\ | | |
| | | | | |
+--+---------+----+----------+--+ Control
Channel | | | | | |
----------+--+ | | +--+---------
----------+---------- | \|/ ----------+---------
----------+---------- ----------+---------
----------+---------- ----------+---------
----------+---------- ----------+---------
| OLXC |
| |
| |
+-------------------------------+
Fig.4: Control Plane Architecture
The IP router implements the necessary IP protocols and uses IP for
signaling to establish lightpaths. Specifically, optical resource
management requires resource availability per link to be propagated,
implying link state protocols such as OSPF. On each link within the
network, one channel is assigned as the default routed (one hop)
lightpath. The routed lightpath provides router to router
connectivity over this link. These routed lightpaths reflect (and
are thus identical to) the physical topology. The assignment of
this default lightpath is by convention, e.g. the 'first' channel.
All traffic using this lightpath is IP traffic and is forwarded by
the router. All control messages are sent in-band on a routed
lightpath as regular IP datagrams, potentially mixed with other data
but with the highest forwarding priority. It is assumed multiple
channels on each link, a fraction of which is reserved at any given
time for restoration. The default-routed lightpath is restored on
one of these channels. Therefore, we can assume that as long as the
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link is functional, there is a default routed lightpath on that
link.
The IP router communicates with the OLXC device through a logical
interface. The interface defines a set of basic primitives to
configure the OLXC, and to enable the OLXC to convey information to
the router. The mediation device translates the logical primitives
to and from the proprietary controls of the OLXC. Ideally, this
interface is both explicit and open. We recognize that a particular
realization may integrate the router and the OLXC into a single box
and use a proprietary interface implementation. Fig.4 illustrates
this implementation.
The following interface primitives are examples of a proposal for
communication between the router and the OLXC within a node:
a) connect(input link, input channel, output link, output channel):
Commands sent from the router to the OLXC requesting that the OLXC
crossconnect input channel on the input link to the output channel
on the output link.
b) disconnect(input link, input channel, output link, output
channel): Command sent from the router to the OLXC requesting that
it disconnect the output channel on the output link from the
connected input channel on the input link.
c) bridge(input link, input channel, output link, output channel):
Command sent from the router controller to the OLXC requesting the
bridging of a connected input channel on input link to another
output channel on output link.
d) switch(old input link, old input channel, new input link, new
input channel, output link, output channel): Switch output port from
the currently connected input channel on the input link to the new
input channel on the new input link. The switch primitive is
equivalent to atomically implementing a disconnect(old input
channel, old input link, output channel, output link) followed by a
connect(new input link, new input channel, output link, output
channel).
e) alarm(exception, object):
Command sent from the OLXC to the router informing it of a failure
detected by the OLXC. The object represents the element for which
the failure has been detected.
For all of the above interfaces, the end of the connection can also
be a drop port.
4.2 Addressing
Every network addressable element must have an IP address.
Typically these elements include each node and every optical link
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and IP router port. When it is desirable to have the ability to
address individual optical channels those are assigned IP addresses
as well. The IP addresses must be globally unique if the element is
globally addressable. Otherwise domain unique addresses suffice. A
client must also have an IP address by which it is identified.
However, optical lightpaths could potentially be established between
devices that do not support IP (i.e., are not IP aware), and
consequently do not have IP addresses. This could be handled either
by assigning an IP address to the device, or by assigning an address
to the OLXC port to which the device is attached. Whether or not a
client is IP aware can be discovered by the network using
traditional IP mechanisms.
4.3 Path setup
This section describes a protocol proposed for setting up an end-to-
end lightpath for a channel. This proposal uses the concept of
softness of lightpaths. This implies that the lightpath expires
unless refreshed periodically bu the source. The first-hop router
must periodically resend the lightpath setup request. If the state
of a lightpath expires at a particular node, the state is locally
removed and all resources allocated to the lightpath are reclaimed.
4.3.1 Basic Path Setup Procedure
Techniques for link provisioning depend upon whether the OXCs do or
do not have wavelength conversion. Both these cases are discussed
below.
4.3.1.1 Network with Wavelength Converters
In an optical network with wavelength conversion, channel allocation
can be performed independently on different links along a route. A
lightpath request from a source is received by the first-hop router.
(The term router here denotes the routing entity in the optical
nodes or OXCs) A sample format for the setup request has been
defined in [Chaudhuri00]. The first-hop router creates a lightpath
setup message and sends it towards the destination of the lightpath
where it is received by the last-hop router. The lightpath setup is
sent from the first-hop router on the default routed lightpath as
the payload of a normal IP packet with router alert. A router alert
ensures that the packet is processed by every router in the path. A
channel is allocated for the lightpath on the downstream link at
every node traversed by the setup. The identifier of the allocated
channel is written to the setup message.
Note that the lightpath is established over the links traversed by
the lightpath setup packet. After a channel has been allocated at a
node, the router communicates with the OLXC to reconfigure the OLXC
to provide the desired connectivity. After processing the setup,
the destination (or the last-hop router) returns an acknowledgement
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to the source. The acknowledgment indicates that a channel has been
allocated on each hop of the lightpath. It does not, however,
confirm that the lightpath has been successfully implemented (or
configured).
If no channel is available on some link, the setup fails, and a
message is returned to the first-hop router informing it that the
lightpath cannot be established. If the setup fails, the first-hop
router issues a release message to release resources allocated for
the partially constructed lightpath. Upon failure, the first-hop
router may attempt to establish the lightpath over an alternate
route, before giving up on satisfying the original user request.
The first-hop router is obligated to establish the complete path.
Only if it fails on all possible routes does it give a failure
notification to the true source.
4.3.1.2 Network without wavelength converters
However, if wavelength converters are not available, then a common
wavelength must be located on each link along the entire route,
which requires some degree of coordination between different nodes
in choosing an appropriate wavelength.
Sections of a network that do not have wavelength converters are
thus referred to as being wavelength continuous. A common
wavelength must be chosen on each link along a wavelength continuous
section of a lightpath. Whatever wavelength is chosen on the first
link defines the wavelength allocation along the rest of the
section. A wavelength assignment algorithm must thus be used to
choose this wavelength. Wavelength selection within the network
must be performed within a subset of client wavelengths.
Optical non-linearities, chromatic dispersion, amplifier spontaneous
emission and other factors together may limit the scalability of an
all-optical network. Routing in such networks will then have to
take into account noise accumulation and dispersion to ensure that
lightpaths are established with adequate signal qualities. Hence,
all routes become geographically constrained so that they will have
adequate signal quality, and physical layer attributes can be
ignored during routing and wavelength assignment.
One approach to provisioning in a network without wavelength
converters would be to propagate information throughout the network
about the state of every wavelength on every link in the network.
However, the state required and the overhead involved in maintaining
this information would be excessive. By not propagating individual
wavelength availability information around the network, we must
select a route and wavelength upon which to establish a new
lightpath, without detailed knowledge of wavelength availability.
A probe message can be used to determine available wavelengths along
wavelength continuous routes. A vector of the same size as the
number of wavelengths on the first link is sent out to each node in
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turn along the desired route. This vector represents wavelength
availability, and is set at the first node to the wavelength
availability on the first link along the wavelength continuous
section. If a wavelength on a link is not available or does not
exist, then this is noted in the wavelength availability vector
(i.e. the wavelength is set to being unavailable ). Once the
entire route has been traversed, the wavelength availability vector
will denote the wavelengths that are available on every link along
the route. The vector is returned to the source OXC, and a
wavelength is chosen from amongst the available wavelengths using an
arbitrary wavelength assignment scheme, such as first-fit.
The construction of a bi-directional lightpath differs from the
construction of a uni-directional lightpath above only in that upon
receiving the setup request, the last-hop router returns the setup
message using the reverse of the explicit route of the forward path.
Both directions of a bi-directional lightpath share the same
characteristics, i.e., set of nodes, bandwidth and restoration
requirements. For more general bi-directional connectivity, a user
simply requests multiple individual lightpaths.
A lightpath must be removed when it is no longer required. To
achieve this, an explicit release request is sent by the first-hop
router along the lightpath route. Each router in the path processes
the release message by releasing the resources allocated to the
lightpath, and removing the associated state. It is worth noting
that the release message is an optimization and need not be sent
reliably, as if it is lost or never issued (e.g., due to customer
premise equipment failure) the softness of the lightpath state
ensures that it will eventually expire and be released.
4.3.2 CR-LDP Extensions for Path Setup
Label Distribution Protocol (LDP) is defined for distribution of
labels inside one MPLS domain. CR-LDP is the contraint-based
extesion of LDP. One of the most important services that may be
offered using MPLS in general and CR-LDP in particular is support
for constraint-based routing of lightpaths across the routed
network. Constraint-based routing offers the opportunity to extend
the information used to setup paths beyond what is available for the
routing protocol. For instance, an LSP can be setup based on
explicit route constraints, QoS constraints, and other constraints.
Constraint-based routing (CR) is a mechanism used to meet traffic
engineering requirements that have been proposed.
Automated establishment of lightpaths involves setting up the
crossconnect table entries in the appropriate OLXCs in a coordinated
manner such that the desired physical path is realized. The request
to establish an lightpath may arise either from a router (or some
other device) connected to the OXCs or from a management system.
Such a request must identify the ingress and the egress OXC as
endpoints of the lightpath. In addition, it may also optionally
specify the input and output ports, wavelengths, and TDM channels.
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The request may also include bandwidth parameters and channel type,
reliability parameters, restoration options, setup and holding
priorities for the path etc. On receipt of the request, the ingress
node computes a suitable route for the requested path, following
applicable policies and constraints. Once the route has been
computed, the ingress node invokes CR-LDP to set up the path.
In optical networks, label mapping corresponds to the assignment of
input or output ports for paths by optical switches and the
communication of this information to the appropriate neighbors.
A Label Request message is used by an upstream LSR to request a
label binding from the downstream LSR for a specified FEC and CR-
LSP. In optical networks, a Label Request message may be used by
the upstream OXC to request a port (and wavelength) assignment from
the downstream OXC for the lightpath being established. Using
downstream-on-demand and ordered control mode, a Label Request
message is initially generated at the ingress OXC and is propagated
to the egress OXC. Also, a protocol is required to determine the
port mappings.
To incorporate the above mentioned constraints, the following
extensions to current version of CR-LDP have been proposed:
* Inclusion of Signaling Port ID: This field specifies ports to be
assigned for setting up the path. Such a "label" (wavelength) must
be assigned in a coordinated manner by a pair of adjacent OXCs,
since the "label" at one OXC is tied to a specific "label" at a
neighboring OXC based on physical connectivity.
* Signaling Optical Switched Path Identifier: This field identifies
the lightpath being established. This provides the flexibility of
establishing LSPs on the top of an lightpath already setup.
* Signaling the two end points of the path being set up: These
fields indicate the two end-points at the port level of the
lightpath. The port selected for the egress node is propagated to
the egress node.
* Signaling requirements for both span and path protection: This
field signals the protection levels required for both span (or
local) and path protection. Examples of span (or local) protection
include SONET 1+1 and 1:N APS. Examples of path protection include
various levels regarding how an alternate path is shared such as in
a style of 1+1 or 1:N analogous to span protection.
* Recording the precise route of the path being established: This is
done by letting each OXC insert its node ID and the both output and
input port selected for the path in the Label Mapping message. The
message received by the ingress OXC will have the complete route at
the port level. This information is useful for network management
functions.
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4.3.3 Extensions to RSVP for Path Setup
Resource reSerVation Protocol (RSVP) is a unicast and multicast
signaling protocol, designed to install and maintain reservation
state information at each routing engine along a path [Luciani00].
The key characteristics of RSVP are that it is simplex, receiver-
oriented and soft. It makes reservations for unidirectional data
flows. The receiver of a data flow generally initiates and maintains
the resource reservation used for that flow. It maintains "soft"
state in routing engines. The _path_ messages are propagated from
the source towards potential receipients. The receivers interested
in communicating with the source send the _Resv_ messages.
The following extensions to RSVP have been proposed to support path
setup [Jonathan00]:
- Reduction of trail establishment latency
- Establishment of bi-directional lightpaths
- Fast failure notification
- Bundling of notifications
These extensions are described below.
4.3.3.1 Reduction Of Trail Establishment Latency
Currently due to receiver-oriented nature of RSVP, the internal
configuration of an OXC in the downstream direction cannot be
initiated until it receives the Resv message from the downstream
node. The ability to begin configuring an OXC before receiving a
Label Object in the Resv message can provide a significant reduction
in the setup latency, especially in OXCs with non-negligible
configuration time. To accomplish this, a new approach has been
proposed in which an upstream OXC suggest a (fiber, lambda) label
for the downstream node to use by including the suggested Label
object in the Label Request object of the Path message. The Label
object will contain the downstream node's Label for the bearer
channel, which can be obtained through the Link Management Protocol
(LMP). This will allow the upstream OXC to begin its internal
configuration before receiving the Resv message from the downstream
node.
4.3.3.2 Establishment Of Bi-directional Lightpaths
In the new approach that is proposed, a Label Object is added to the
Path message in the downstream direction. In this way, the upstream
direction of the bi-directional path is established on the first
pass from the source to destination, reducing the latency of the
reservation process. For bi-directional trails, if a label
suggestion is also used, there will be two Labels in the Path
message: the upstream Label in the Label object and the suggested
Label in the Label_Request object.
4.3.3.3 Failure Notification
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A new RSVP message, called the Notify message, can be used to notify
RSVP nodes when failures occur. The Notify message will be
transmitted with the router alert option turned off so that
intermediate nodes will not process or modify the message, but only
perform standard IP forwarding of the message.
4.3.3.4 Bundling of Notifications
Another extension to RSVP has been recently proposed to allow the
use of bundle messages in order to reduce the overall message-
handling load. An RSVP bundle message consists of a bundle header
followed by a body consisting of a variable number of standard RSVP
messages. Support for the bundle message is optional, and
currently, bundle messages can only be sent to adjacent RSVP nodes.
In order to effectively restore a network to a stable state, nodes
that are running restoration algorithms should consider as many
failed trails as possible before making restoration decisions. To
improve performance and ensure that the nodes are provided with as
many of the affected paths as possible, it is useful to include the
entire set of Notify messages in a single bundle message and send it
to the responsible RSVP node directly, without message processing by
the intermediate RSVP nodes. This can be accomplished by addressing
the bundle message to the source RSVP nodes and turning off the
router alert option in the IP header. Intermediate RSVP nodes then
should perform standard IP forwarding of this message.
4.4 Resource Discovery and Maintenance
Topology information is distributed and maintained using standard
routing algorithms, e.g., OSPF and IS-IS. On boot, each network
node goes through neighbor discovery. By combining neighbor
discovery with local configuration, each node creates an inventory
of local resources and resource hierarchies, namely: channels,
channel capacity, wavelengths, and links.
For optical networks, the following information need to be stored at
each node and propagated throughout the network as OSPF link-state
information:
- Representation of the current network topology and the link states
(which reflect the wavelength availability). This can be achieved
by associating with the link state,
- total number of active channels
- number of allocated non-preemptable channels
- number of allocated preemptable channels
- number of reserved protection channels
- Optional physical layer parameters for each link. These
parameters are not expected to be required in a network with 3R
signal regeneration, but may be used in all-optical networks.
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All of the above information is obtained via OSPF updates, and is
propagated throughout the network. In networks with OXCs without
wavelength converters, decisions at the first-hop router are made
without knowledge of wavelength availability. This is done to
reduce the state information that needs to be propagated within the
network.
4.5 Configuration Control using GSMP
In a general mesh network where the OXCs do not participate in
topology distribution protocols, General Switch Management Protocol
(GSMP) is used to communicate crossconnect information. This
ensures that the OXCs on the lightpath maintain appropriate
databases. The first hop router having complete knowledge of LP, L2
and L3 topology acts as the "controller" to the OXCs in the
lightpath.
GSMP is a master-slave protocol [GSMP]. The controller issues
request messages to the switch. Each request message indicates
whether a response is required from the switch (and contains a
transaction identifier to enable the response to be associated with
the request). The switch replies with a response message indicating
either a successful result or a failure. There are six classes of
GSMP request-response message:
Connection Management
Reservation Management
Port Management
State and Statistics
Configuration, and
Quality of Service
The switch may also generate asynchronous Event messages to inform
the controller of asynchronous events.
4.6 Resource Discovery Using NHRP
The Next Hop Resolution Protocol (NHRP) allows a source station (a
host or router), wishing to communicate over a Non-Broadcast, Multi-
Access (NBMA) subnetwork, to determine the internetworking layer
addresses and NBMA addresses of suitable "NBMA next hops" toward a
destination station [NHRP]. A subnetwork can be non-broadcast
either because it technically doesn't support broadcasting (e.g., an
X.25 subnetwork) or because broadcasting is not feasible for one
reason or another (e.g., a Switched Multi-megabit Data Service
multicast group or an extended Ethernet would be too large).
If the destination is connected to the NBMA subnetwork, then the
NBMA next hop is the destination station itself. Otherwise, the
NBMA next hop is the egress router from the NBMA subnetwork that is
"nearest" to the destination station. NHRP is intended for use in a
multiprotocol internetworking layer environment over NBMA
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subnetworks. NHRP functions are performed by two types of logical
entities:
Next Hop Server (NHS) - implemented in routers
Next Hop Client (NHC) - implemented in routers or NBMA-attached
hosts.
In short, NHRP may be applied as a resource discovery to find the
egress OXC in an optical network. To request this information, 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. The extension would include enough information at each
hop (including the source and destination)
* to uniquely identify which wavelength.
* to use when bypassing each routed/forwarded hop and which port
that the request was received on.
Essentially a shortcut is setup from ingress to egress using this
protocol.
5. Optical Network Management
The management functionality in all-optical networks are still in
the rudimentary phase. Management in a system refers to set of
functionalities like performance monitoring, link initialization and
other network diagnostics to verify safe and continued operation of
the network. The wavelengths in the optical domain will require
routing, add/drop, and protection functions, which can only be
achieved through the implementation of network-wide management and
monitoring capabilities. Current proposals for link initialization
and performance monitoring are summarized below.
5.1 Link Initialization
The links between OXCs will carry a number of user bearer channels
and possibly one or more associated control channels. This section
describes a link management protocol (LMP) that can be run between
neighboring OXCs and can be used for both link provisioning and
fault isolation. A unique feature of LMP is that it is able to
isolate faults independent of the encoding scheme used for the
bearer channels. LMP will be used to maintain control channel
connectivity, verify bearer channel connectivity, and isolate link,
fiber, or channel failures within the optical network.
5.1.1 Control Channel Management
For LMP, it is essential that a control channel is always available
for a link, and in the event of a control channel failure, an
alternate (or backup) control channel must be made available to
reestablish communication with the neighboring OXC. If the control
channel cannot be established on the primary (fiber, wavelength)
pair, then a backup control channel should be tried. The control
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channel of a link can be either explicitly configured or
automatically selected. The control channel can be used to exchange:
a)MPLS control-plane information such as link provisioning and fault
isolation information (implemented using a messaging protocol such
as LMP, proposed in this section),
b)path management and label distribution information (implemented
using a signaling protocol such as RSVP-TE or CR-LDP), and
c)topology and state distribution information (implemented using
traffic engineering extended protocols such as OSPF and IS-IS).
Once a control channel is configured between two OXCs, a Hello protocol
can be used to establish and maintain connectivity between the OXCs and
to detect link failures. The Hello protocol of LMP is intended to be a
lightweight keep-alive mechanism that will react to control channel
failures rapidly. A protocol similar to the HDLC frame exchange is
used to continue the handshake. [Lang00]
5.1.2 Verifying link connectivity
In this section, we describe the mechanism used to verify the
physical connectivity of the bearer channels. This will be done
initially when a link is established, and subsequently, on a
periodic basis for all free bearer channels on the link. To ensure
proper verification of bearer channel connectivity, it is required
that until the bearer channels are allocated, they must be opaque.
As part of the link verification protocol, the control channel is
first verified, and connectivity maintained, using the Hello
protocol discussed in Section 5.1.1. Once the control channel has
been established between the two OXCs, bearer channel connectivity
is verified by exchanging Ping-type Test messages over all of the
bearer channels specified in the link. It should be noted that all
messages except for the Test message are exchanged over the control
channel and that Hello messages continue to be exchanged over the
control channel during the bearer channel verification process. The
Test message is sent over the bearer channel that is being verified.
Bearer channels are tested in the transmit direction as they are
uni-directional, and as such, it may be possible for both OXCs to
exchange the Test messages simultaneously. [Lang00]
5.1.3 Fault localization
Fault detection is delegated to the physical layer (i.e., loss of
light or optical monitoring of the data) instead of the layer 2 or
layer 3. Hence, detection must be handled at the layer closest to
the failure; for optical networks, this is the physical (optical)
layer. One measure of fault detection at the physical layer is
simply detecting loss of light (LOL). Other techniques for
monitoring optical signals are still being developed.
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A link connecting two OXCs consists of a control channel and a
number of bearer channels. If bearer channels fail between two
OXCs, a mechanism must be used to rapidly locate the failure so
that appropriate protection/restoration mechanisms can be initiated.
This is discussed further in Section 6.10.
5.2 Optical Performance Monitoring (OPM)
Current-generation DWDM networks are monitored, managed and
protected within the digital domain, using SONET and its associated
support systems. However, to leverage the full potential of
wavelength-based networking, the provisioning, switching, management
and monitoring functions have to move from the digital to the
optical domain.
The information generated by the performance monitoring operation
can be used to ensure safe operation of the optical network. In
addition to verifying the service level provided by the network to
the user, performance monitoring is also necessary to ensure that
the users of the network comply with the requirements that were
negotiated between them and the network operator. For example, one
function may be to monitor the wavelength and power levels of
signals being input to the network to ensure that they meet the
requirements imposed by the network. Current performance monitoring
in optical networks requires termination of a channel at an optical-
electrical-optical conversion point to detect bits related to BER of
the payload or frame (e.g., SONET LTE monitoring). However, while
these bits indicate if errors have occurred, they do not supply
channel-performance data. This makes it very difficult to assess
the actual cause of the degraded performance.
Fast and accurate determination of the various performance measures
of a wavelength channel implies that measurements have to be done
while leaving it in optical format. One possible way of achieving
this is by tapping a portion of the optical power from the main
channel using a low loss tap of about 1%. In this scenario, the
most basic form of monitoring will utilize a power-averaging
receiver to detect loss of signal at the optical power tap point.
Existing DWDM systems use optical time-domain reflectometers to
measure the parameters of the optical links.
Another problem lies in determining the threshold values for the
various parameters at which alarms must be declared. Very often
these values depend on the bit rate on the channel and should
ideally be set depending on the bit rate. In addition, since a
signal is not terminated at an intermediate node, if a wavelength
fails, all nodes along the path downstream of the failed wavelength
could trigger an alarm. This can lead to a large number of alarms
for a single failure, and makes it somewhat more complicated to
determine the cause of the alarm (alarm correlation). A list of
such optical parameters to be monitored periodically have been
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proposed [Ceuppens00]. Optical cross talk, dispersion, insertion
loss are key parameters to name a few.
Care needs to be taken in exchanging these performance parameters.
The vast majority of existing telecommunication networks uses
framing and data formatting overhead as the means to communicate
between network elements and management systems. It is worth
mentioning that while the signaling is used to communicate all
monitoring results, the monitoring itself is done on the actual data
channel, or some range of bandwidth around the channel. Therefore,
all network elements must be guaranteed to pass this bandwidth in
order for monitoring to happen at any point in the network.
One of the options being considered for transmitting the information
is the framing and formatting bits of the SONET interface. But, it
hampers transparency. It is clear that truly transparent and open
photonic networks can only be built with transparent signaling
support. The MPLS control plane architecture suggested can be
extended beyond simple bandwidth provisioning to include optical
performance monitoring.
6. Fault restoration in Optical networks
Telecom networks have traditionally been designed with rapid fault
detection, rapid fault isolation and recovery. With the introduction
of IP and WDM in these networks, these features need to be provided
in the IP and WDM layers also. Automated establishment and
restoration of end-to-end paths in such networks requires
standardized signaling, routing, and restoration mechanisms.
6.1 Layering
Clearly the layering and architecture for service restoration is a
major component for IP to optical internetworking. This section
summarizes some schemes, which aid in optical protection at the
lower layers, SONET and Optical.
6.1.1 SONET Protection
The SONET standards specify an end-to-end two-way availability
objective of 99.98% for inter office applications (0.02%
unavailability or 105 minutes/year maximum down time) and 99.99 %
for loop transport between the central office and the customer's
premises. To conform to these standards, failure/restoration times
have to be short. For both, point-to-point and ring systems,
automatic protection switching (APS) is used, the network performs
failure restoration in tens of milliseconds (approximately 50
milliseconds).
Architectures composed of SONET add-drop multiplexers (ADMs)
interconnected in a ring provide an a method of APS that allows
facilities to be shared while protecting traffic within an
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acceptable restoration time. There are 2 possible ring
architectures:
* UPSR: Unidirectional path switched ring architecture is a 1+1
single-ended, unidirectional, SONET path layer dedicated protection
architecture. The nodes are connected in a ring configuration with
one fiber pair connecting adjacent nodes. One fiber on a link is
used as the working and other is protection. They operate in
opposite directions. So there is a working ring in one direction
and a protection ring in the opposite direction. The optical signal
is sent on both outgoing fibers. The receiver compares the 2
signals and selects the better of the two based on signal quality.
This transmission on both fibers is called 1+1 protection.
* BLSR: In bidirectional line switched ring architecture, a
bidirectional connection between 2 nodes traverses the same
intermediate nodes and links in opposite directions. In contrast to
the UPSR, where the protection capacity is dedicated, the BLSR
shares protection capacity among all spans on the ring. They are
also called Shared Protection ring (SPRing) architectures. In BLSR
architecture, switching is coordinated by the nodes on either side
of a failure in the ring, so that a signaling protocol is required
to perform a line switch and to restore the network. These
architectures are more difficult to operate than UPSRs where no
signaling is required.
6.1.2 Optical layer Protection
The concept of SONET ring architectures can be extended to WDM self-
healing optical rings (SHRs). As in SONET, WDM SHRs can be either
path switched or line switched. In recent testbed experiments,
lithium niobate protection switches have been used to achieve 10
micro-seconds restoration times in WDM Shared protection Rings.
Multiwavelength systems add extra complexity to the restoration
problem. Under these circumstances, a simple ring architecture may
not suffice. Hence, arbitrary mesh architectures become important.
Usually, for such architectures, restoration is usually performed
after evaluation at the higher layer. But this takes a lot of time.
Optical protection techniques for mesh architectures have also been
proposed. They operate on a line rather than path protection basis.
The fundamental unit being protected is a transmission link rather
than an end to end connection. The methodology is a generalization
of that is used in SPRings. The system requires 100% redundancy.
Fault recovery decisions are made locally in a distributed fashion
and independent of the state of activity of the network. The
implementation uses simple and reliable protection switches in each
network node so that protection is accomplished without significant
processing, transmission, and propagation delays.
Thus, the main advantage of lower layer mechanisms is the fast
restoration. When we talk of the IP/MPLS over WDM architecture, we
may seal off SONET APS protection from the discussion and the WDM
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optical layer can provide the same kind of restoration capabilities
at the lower layer. Thus there has to be interaction only between
the MPLS and optical layers.
6.2 MPLS in Protection
Although the current routing algorithms are very robust and
survivable, the amount of time they take to recover from a failure
can be significant, on the order of several seconds or minutes,
causing serious disruption of service in the interim. This is
unacceptable to many organizations that aim to provide a highly
reliable service, and thus require recovery times on the order of
tens of milliseconds.
Since MPLS binds packets to a route (or path) via the labels, it is
imperative that MPLS be able to provide protection and restoration
of traffic. In fact, a protection priority could be used as a
differentiating mechanism for premium services that require high
reliability.
6.2.1 Motivations
The need for MPLS layer protection and for open standards in
protection arises because of the following:
1. Layer 3 or IP rerouting may be too slow for a core MPLS network
that needs to support high reliability/availability.
2. Layer 0 (for example, optical layer) or Layer 1 (for example,
SONET) mechanisms may be limited to ring topologies and may not
include mesh protection.
3. Layer 0 or Layer 1 mechanisms may have no visibility into higher
layer operations. Thus, while they may provide link protection for
example, they cannot easily provide MPLS path protection.
4. Establishing interoperability of protection mechanisms between
multi-vendor LSRs in core MPLS networks is urgently required to
enable the adoption of MPLS as a viable core transport technology.
6.2.2 Goals
Based on our motivations, we can state some goals for MPLS based
protection:
1. MPLS-based recovery mechanisms should facilitate fast (10As of
ms) recovery times.
2. MPLS-based recovery techniques should be applicable for
protection of traffic at various granularities. For example, it
should be possible to specify MPLS- based recovery for a portion of
the traffic on an individual path, for all traffic on an individual
path, or for all traffic on a group of paths.
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3. MPLS-based recovery techniques may be applicable for an entire
end-to-end path or for segments of an end-to-end path.
4. MPLS-based recovery mechanisms should be able to take into
consideration the recovery actions of other layers.
5. MPLS-based recovery actions should avoid network-layering
violations. That is, defects in MPLS-based mechanisms should not
trigger lower layer protection switching.
6. MPLS-based recovery mechanisms should minimize the loss of data
and packet reordering during recovery operations.
7. MPLS-based recovery mechanisms should minimize the state
overhead incurred for each recovery path maintained.
8. MPLS-based recovery mechanisms should be able to preserve the
constraints on traffic after switchover, if desired. That is, if
desired, the recovery path should meet the resource requirements of,
and achieve the same performance characteristics, as the working
path.
6.3 Protection options
6.3.1 Dynamic Protection
These protection mechanisms dynamically create protection entities
for restoring traffic, based upon failure information, bandwidth
allocation and optimized reroute assignment. Thus, upon detecting
failure, the LSPs crossing a failed link or LSR are broken at the
point of failure and reestablished using signaling. These methods
may increase resource utilization because capacity or bandwidth is
not reserved beforehand and because it is available for use by other
(possibly lower priority) traffic, when the protection path does not
require this capacity. They may, however, require longer
restoration times, since it is difficult to instantaneously switch
over to a protection entity, following the detection of a failure.
6.3.2 Pre-negotiated Protection
These are dedicated protection mechanisms, where for each working
path there exists a pre-established protection path, which is node
and link disjoint with the primary/working path, but may merge with
other working paths that are disjoint with the primary. The
resources (bandwidth, buffers, processing) on the backup entity may
be either pre-determined and reserved beforehand (and unused), or
may be allocated dynamically by displacing lower priority traffic
that was allowed to use them in the absence of a failure on the
working path.
6.3.3 End-to-end Repair
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In end-to-end repair, upon detection of a failure on the primary
path, an alternate or backup path is re-established starting at the
source. Thus, protection is always activated on an end-to-end
basis, irrespective of where along a working path a failure occurs.
This method might be slower than the local repair method discussed
below, since the failure information has to propagate all the way
back to the source before a protection switch is accomplished.
6.3.4 Local Repair
In local repair, upon detecting a failure on the primary path, an
alternate path is re-established starting from the point of failure.
Thus protection is activated by each LSR along the path in a
distributed fashion on an as-needed basis. While this method has an
advantage in terms of the time taken to react to a fault, it
introduces the complication that every LSR along a working path may
now have to function as a path splitting LSR (PSL).
6.3.5 Link Protection
The intent is to protect against a single link failure. For
example, the protection path may be configured to route around
certain links deemed to be potentially risky. If static
configuration is used, several protection paths may be
preconfigured, depending on the specific link failure that each
protects against. Alternatively, if dynamic configuration is used,
upon the occurrence of a failure on the working path, the protection
path is rebuilt such that it detours around the failed link.
6.3.6 Path Protection
The intention is to protect against any link or node failure on the
entire working path. This has the advantage of protecting against
multiple simultaneous failures on the working path, and possibly
being more bandwidth efficient than link protection.
6.3.7 Revertive Mode
In the revertive mode of operation, the traffic is automatically
restored to the working path once repairs have been affected, and
the PSL(s) are informed that the working path is up. This is
useful, since once traffic is switched to the protection path it is,
in general, unprotected. Thus, revertive switching ensures that the
traffic remains unprotected only for the shortest amount of time.
This could have the disadvantage, however, of producing oscillation
of traffic in the network, by altering link loads.
6.3.8 Non-revertive Mode
In the non-revertive mode of operation, traffic once switched to the
protection path is not automatically restored to the working path,
even if the working path is repaired. Thus, some form of
administrative intervention is needed to invoke the restoration
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action. The advantage is that only one protection switch is needed
per working path. A disadvantage is that the protection path
remains unprotected until administrative action (or manual
reconfiguration) is taken to either restore the traffic back to the
working path or to configure a backup path for the protection path.
6.3.9 1+1 Protection
In 1+1 protection, the resources (bandwidth, buffers, processing
capacity) on the backup path are fully reserved to carry only
working traffic. In MPLS, this bandwidth may be considered wasted.
Alternately, this bandwidth could be used to transmit an exact copy
of the working traffic, with a selection between the traffic on the
working and protection paths being made at the path merge LSR (PML).
6.3.10 1:1, 1:n, and n:m Protection
In 1:1 protection, the resources (bandwidth, buffers, and processing
capacity) allocated on the protection path are fully available to
preemptable low priority traffic when the protection path is not in
use by the working traffic. In other words, in 1:1 protection, the
working traffic normally travels only on the working path, and is
switched to the protection path only when the working entity is
unavailable. Once the protection switch is initiated, all the low
priority traffic being carried on the protection path is discarded
to free resources for the working traffic. This method affords a
way to make efficient use of the backup path, since resources on the
protection path are not locked and can be used by other traffic when
the backup path is not being used to carry working traffic.
Similarly, in 1:n protection, up to n working paths are protected
using only one backup path, while in m:n protection, up to n working
paths are protected using up to m backup paths.
6.3.11 Recovery Granularity
Another dimension of recovery considers the amount of traffic
requiring protection. This may range from a fraction of a path to a
bundle of paths.
6.3.11.1 Selective Traffic Recovery
This option allows for the protection of a fraction of traffic
within the same path. The portion of the traffic on an individual
path that requires protection is called a protected traffic portion
(PTP). A single path may carry different classes of traffic, with
different protection requirements. The protected portion of this
traffic may be identified by its class, as for example, via the EXP
bits in the MPLS shim header or via the cell loss priority (CLP) bit
in the ATM header.
6.3.11.2 Bundling
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Bundling is a technique used to group multiple working paths
together in order to recover them simultaneously. The logical
bundling of multiple working paths requiring protection, each of
which is routed identically between a PSL and a PML, is called a
protected path group (PPG). When a fault occurs on the working path
carrying the PPG, the PPG as a whole can be protected either by
being switched to a bypass tunnel or by being switched to a recovery
path.
6.4. Failure detection
Loss of Signal (LOS) is a lower layer impairment that arises when a
signal is not detected at an interface, for example, a SONET LOS.
In this case, enough time should be provided for the lower layer to
detect LOS and take corrective action.
A Loss of Function (LF) is declared when the link probing mechanism
fails. An example of a probing mechanism is the Liveness message
that is exchanged periodically along the working path between peer
LSRs. A LF is detected when a certain number k of consecutive
Liveness messages are either not received from a peer LSR or are
received in error.
A Loss of Packets (LOP) occurs when there is excessive discarding of
packets at an LSR interface, either due to label mismatches or due
to time-to-live (TTL) errors. LOP due to label mismatch may be
detected simply by counting the number of packets dropped at an
interface because an incoming label did not match any label in the
forwarding table. Likewise, LOP due to invalid TTL may be detected
by counting the number of packets that were dropped at an interface
because the TTL decrements to zero.
6.5 Failure Notification
Protection switching relies on rapid notification of failures. Once
a failure is detected, the node that detected the failure must send
out a notification of the failure by transmitting a fault indication
signal (FIS) to those of its upstream LSRs that were sending traffic
on the working path that is affected by the failure. This
notification is relayed hop-by-hop by each subsequent LSR to its
upstream neighbor, until it eventually reaches a PSL.
The PSL is the LSR that originates both the working and protection
paths, and the LSR that is the termination point of both the FIS and
the fault restoration signal (FRS). Note that the PSL need not be
the origin of the working LSP.
The PML is the LSR that terminates both the working path and its
corresponding protection path. Depending on whether or not the PML
is a destination, it may either pass the traffic on to the higher
layers or may merge the incoming traffic on to a single outgoing
LSR. Thus, the PML need not be the destination of the working LSP.
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An LSR that is neither a PSL nor a PML is called an intermediate
LSR. The intermediate LSR could be either on the working or the
protection path, and could be a merging LSR (without being a PML).
6.5.1 Reverse Notification Tree (RNT)
Since the LSPs are unidirectional entities and protection requires
the notification of failures, the failure indication and the failure
recovery notification both need to travel along a reverse path of
the working path from the point of failure back to the PSL(s). When
label merging occurs, the working paths converge to form a
multipoint-to-point tree, with the PSLs as the leaves and the PML as
the root. The reverse notification tree is a point-multipoint tree
rooted at the PML along which the FIS and the FRS travel, and which
is an exact mirror image of the converged working paths
The establishment of the protection path requires identification of
the working path, and hence the protection domain. In most cases,
the working path and its corresponding protection path would be
specified via administrative configuration, and would be established
between the two nodes at the boundaries of the protection domain
(the PSL and PML) via explicit (or source) routing using LDP , RSVP,
signaling (alternatively, using manual configuration).
The RNT is used for propagating the FIS and the FRS, and can be
created very easily by a simple extension to the LSP setup process.
During the establishment of the working path, the signaling message
carries with it the identity (address) of the upstream node that
sent it. Each LSR along the path simply remembers the identity of
its immediately prior upstream neighbor on each incoming link. The
node then creates an inverse crossconnect table that for each
protected outgoing LSP maintains a list of the incoming LSPs that
merge into that outgoing LSP, together with the identity of the
upstream node that each incoming LSP comes from. Upon receiving an
FIS, an LSR extracts the labels contained in it (which are the
labels of the protected LSPs that use the outgoing link that the FIS
was received on) consults its inverse crossconnect table to
determine the identity of the upstream nodes that the protected LSPs
come from, and creates and transmits an FIS to each of them.
6.6. Timing
There are a number of timing parameters that need to be specified
for proper restoration of the IP/WDM networks. Some of these are
described below.
6.6.1 Protection Switching Interval Timer T1:
Controls the maximum duration within which a protection switch must
be accomplished, following the detection of a failure.
6.6.2 Inter-FIS Packet Timer T2:
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Interval at which successive FIS packets are transmitted by a LSR to
its upstream neighbor.
6.6.3 Maximum FIS duration timer T3:
Max. time for which FIS packets are transmitted by an LSR to is
upstream peer.
6.6.4 Protection switching dampening timer T4:
Time interval between receipt of a protection switch trigger and the
initiation of the protection switch. The purpose of this timer is
to minimize misordering of packets at a PML following a protection
(restoration) switch from the working (backup) to the backup
(working) path. This is because packets buffered on the working
(backup) path may continue to arrive at the PML even as working
traffic begins to arrive on the protection (working) path.
Therefore, forcing the PSL to hold off the protection (or
restoration) switching action, gives the buffers on the working
(protection) path time to clear before data on the protection
(working) path begins to arrive.
6.6.5 Liveness Message Send interval T5:
Interval at which successive Liveness messages are sent by an LSR to
peer LSRs that have a working path (and RNT) through this LSR.
6.6.6 Failure Indication Hold-off Timer T6:
Interval between the detection of a failure at an LSR, and the
generation of the first FIS message, to allow time for lower layer
protection to take effect.
6.6.7 Lost Liveness Message Threshold K:
No. of Liveness messages that can be lost before an LSR will
declare LF and generate the FIS.
For proper operation, it is required that T1 >> T2 > T3 and T1> T4
6.7. Signaling Requirements related to restoration
Signaling mechanisms for optical networks should be tailored to the
needs of optical networking.
Some signaling requirements directed towards restoration in optical
networks are:
1. Signaling mechanisms must minimize the need for manual
configuration of relevant information, such as local topology.
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2. Lightpaths are fixed bandwidth pipes. There is no need to
convey complex traffic characterization or other QoS parameters in
signaling messages. On the other hand, new service related
parameters such as restoration priority, protection scheme desired,
etc., may have to be conveyed.
3. Signaling for path establishment must be quick and reliable. It
is especially important to minimize signaling delays during
restoration.
4. Lightpaths are typically bidirectional. Both directions of the
path should generally be established along the same physical route.
5. OXCs are subject to high reliability requirements. A transient
failure that does not affect the data plane of the established paths
should not result in these paths being torn down.
6. Restoration schemes in mesh networks rely on sharing backup path
among many primary paths. Signaling protocols should support this
feature.
7. The interaction between path establishment signaling and
automatic protection schemes must be well-defined to avoid false
restoration attempts during path set-up or tear down.
6.8 RSVP/CR-LDP Support for Restoration
Special requirements for protecting and restoring lightpaths and the
extensions to RSVP and CR-LDP have been identified. Some of the
proposed extensions are as follows:
a. A new SESSION_ATTRIBUTE object has been proposed, which indicates
whether the path is unidirectional/bi-directional,
primary/backup. Local protection 1+1 or1:N can also be specified.
b. Setup Priority: The priority of the session with respect to
taking resources. The Setup Priority is used in deciding whether
this session can preempt another session.
Holding Priority: The priority of the session with respect to
holding resources. Holding Priority is used in deciding whether
this session can be preempted by another session.
Note that for the shared backup paths the crossconnects can not be
setup during the signaling for the backup path since multiple backup
paths may share the same resource and can over-subscribe it. The
idea behind shared backups is to make soft reservations and to claim
the resource only when it is required.
6.9 Fast restoration of MPLS LSPs
Fast recovery in MPLS is hampered by the fact that detecting an LSP
failure at the ingress LSR can take a long time. After a break in
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an LSP hop, Notification messages are propagated along the LSP
intermediate nodes back to the ingress LSR.
The fastest detection occurs at the local end of a link failure.
Schemes that try to mend connections at the point of failure are
known as "local repair" schemes.
A problem with single L2 link failure is that multiple LSPs can be
affected and many (hundreds) ingress points must be informed. Just
as a single L2 failure can affect multiple LSPs, a single L1 failure
can affect multiple L2 links.
As noted earlier, L1 failure detection is fast due to physical
methods (loss of light, loss of carrier signal). This is an
attractive property. Further, in a TDM, optical mux (SONET), or
optical cross connect network, when a link fails all of the paths
(at that layer) which use the link go down.
Unlike higher layers, the endpoints of those paths detect the
failure quickly because the signaling of the failure is very fast
(e.g., AIS signals in SONET) and because the signaling is sent to
each channel of the failed link. So in L1 networks, the detection
of a failed connection is fast and scales well for all connections
on the failed link.
A key to the solution for fast detection is the alignment of L1, L2,
and L3 capabilities into a single node. This architecture and its
impacts on the ability to detect LSP failure are now described.
6.9.1 L1/L2/L3 Integration
As was noted earlier, in MPLS LSRs, the alignment of the L3 and L2
topology brings some advantages in the speed at which the network
can react to a link failure. This integration is extended to
encompass L1 components in order to realize further speed
advantages.
An L1/L2/L3 switch is defined as an LSR combined with an L1 cross
connect switch. This could be a SONET Add/Drop Mux, an optical
cross connect, or traditional TDM switch. The integrated switch is
able to originate and terminate IP traffic from the L1 cross
connects. Conceptually, this is done over dedicated L1 channels
between the L1 cross connect and the pure IP router function of the
integrated switch.
Two L1/L2/L3 nodes are connected by a physical L1 link. A channel
in that link is used as a router-router IP link. For example, an
OC-3 channel of an OC-48 link with PPP over SONET for the framing.
This is analogous to the L2 control channel between two MPLS
switches connected over an ATM interface.
A key difference between this type of network and L2/L3 networks,
which are overlayed on L1 networks, is that the L1/L2/L3 network
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does not have any L1 paths, which act as router-router links. In an
integrated network, the L3 routing protocol has a view of both the
L2 and L1 topology since those layers are now aligned.
Here, in an L1/L2/L3 network, an L1 path has an LSR at every cross
connect point. To use an L1 path, treat it as if were an LSP, or
overlay an LSP onto this path. That is, consider the L1 path as a
cut-through. When an incoming IP packet is matched to a Forwarding
Equivalence Class associated with this L1 cut- through, the IP
forwarding table entry points to the start of this L1 path. As with
L2 cut-throughs, an L2 header is added. The packet is sent to this
path and is then L1 switched until it reaches the end of the path.
At the termination point, the packet could be L2 switched or L3
forwarded.
6.9.2 An example
Using L1 cut-throughs in an L1/L2/L3 network enables fast detection
of LSP failure. Consider two LSPs that are L1 cut-throughs:
LSR1-LSR2-LSR3-LSR4 and
LSR5-LSR2-LSR3-LSR6
If L1 link LSR2-LSR3 goes down, all nodes in both LSRs can detect
the path failure based on L1 physical methods. For example, loss of
light (Alarm Indication Signal in SONET) or carrier signal (TDM).
In particular, the LSP endpoints can determine that the LSP is down
much faster than the protocol based method in LDP of Notification
messages which is processed at each LSR on the paths back to the
ingress and egress. For example, propagation of the physical
failure is about 5 microseconds per kilometer.
Not only is the failure detection fast, but it scales for all LSPs
that are affected by a single L1 failure. In the example above, two
LSPs are notified, but if there were 192 paths in an OC192 link,
then all of their endpoints could detect the link failure within a
short period of time (a few milliseconds).
When an LSP failure is detected, the LSR can reroute the traffic to
a backup LSP. This backup LSP could be pre-defined to be link
disjoint from the primary LSP, and could also be set up in advance.
To avoid wasting dedicated bandwidth (i.e., a dedicated backup L1
cut-through), the backup LSP for the L1 cut-through could be an LSP
created over L2 connections which share bandwidth (e.g., ATM UBR
VC).
Assuming that a backup LSP is already set up, restoration of a
failed LSP that is overlayed on an L1 cut-through could be
implemented with similar performance to SONET Line and Ring
restoration.
For LSR which provide L3 connectionless forwarding, traffic from the
failed LSP could also be immediately handled by L3 forwarding if a
backup path LSP is not provided.
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6.10 LMP's Fault localization mechanism
If bearer channels fail between two OXCs, the power monitoring
system in all of the downstream nodes will detect loss of light
(LOL) and indicate a failure. As part of the fault localization, a
monitoring window can be used in each node to determine if a single
bearer channel has failed or if multiple bearer channels have
failed.
As part of the fault localization, a downstream node that detects
bearer channel failures across a link will send a Channel_Fail
message to its upstream neighbor (bundling together the notification
of all of the failed bearer channels) and the node will put the
ports associated with the failed bearer channels into the standby
state. An upstream node that receives the Channel_Fail message will
correlate the failure to see if there is a failure on the
corresponding input and output ports for the optical trail(s). If
there is also a failure on the input channel(s) of the upstream
node, the node will return a Channel_Fail_Ack message to the
downstream node (bundling together the notification of all the
channels), indicating that it too has detected a failure. If,
however, the fault is CLEAR in the upstream node (i.e., there is no
LOL on the corresponding input channels), then the upstream node
will have localized the failure and will return a Channel_Fail_Nack
message to the downstream node, and initiate protection/restoration
procedures. The protection channels may be preconfigured or they
may be dynamically selected by the OXC on the transmit side.
In Fig. 5, a sample network is shown where four OXCs are connected
in a linear array configuration. The control channels are bi-
directional and are labeled with a "c". All optical trails are uni-
directional going left to right. In the example there is a failure
on a single bearer channel between OXC2 and OXC3. Both OXC3 and
OXC4 will detect the failure and each node will send a Channel_Fail
message to the corresponding upstream node (OXC3 will send a message
to OXC2 and OXC4 will send a message to OXC3). When OXC3 receives
the Channel_Fail message from OXC4, it will correlate the failure
and return a Channel_Fail_Ack message back to OXC4. Upon receipt of
the Channel_Fail_Ack message, OXC4 will move the associated ports
into a standby state. When OXC2 receives the Channel_Fail message
from OXC3, it will correlate the failure, verify that it is CLEAR,
localize the failure to the bearer channel between OXC2 and OXC3,
and send a Channel_Fail_Nack message back to OXC3.
+-------+ +-------+ +-------+ +-------+
+ OXC 1 + + OXC 2 + + OXC 3 + + OXC 4 +
+ +-- c ---+ +-- c ---+ +-- c ---+ +
----+-------+--------+---\ + + + + +
+ + + \--+---##---+--\ + + +
+ + + + (A) + \ + + +
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IP over Optical Networks: Summary of Issues July 2000
----+--\ + + + + \--+--------+-------+--->
+ \ + + /--+--------+---\ + + +
+ \--+--------+---/ + + \--+--------+-------+--->
+ + + + + + + +
+-------+ +-------+ +-------+ +-------+
Fig.5 : Example - We show one type of bearer channel failures
(indicated by ## in the figure): a single bearer channel fails
between two OXCs
7. Security Considerations
This document raises no new security issues for MPL(ambda) Switching
implementation over optical networks. Security considerations are
for future study.
8. Acronyms
CR-LPD - Constraint-Based Routing Setup using LDP
CSPF - Constraint Shortest Path First
FIS - Failure Indication Signal
FRS - Failure Recovery Signal
GSMP - General Switch Management Protocol
LDP - Label Distribution Protocol
LF - Link Failure
LMP - Link Management Protocol
LOP - Loss of Packets
LOS - Loss Of Signal
LP - Lightpath
LSR - Label Switched Router
MPLS - Multi-Protocol Lambda Switching
MTG - MPLS Traffic Group
NBMA - Non-Broadcast Multi-Access
NHRP - Next Hop Resolution Protocol
OCT - Optical Channel Trail
OLXC - Optical layer crossconnect
OPM - Opical Performance Monitoring
OSPF - Open Shortest Path First
OTN - Optical Transport Network
OXC - Optical Crossconnect
PML - Protection Merge LSR
PMTG - Protected MPLS Traffic Group
PMTP - Protected MPLS Traffic Portion
PSL - Protection Switch LSR
PXC - Photonic Crossconnect
RNT - Reverse Notification Tree
RSVP - Resource reSerVation Protocol
SPRing - Shared Protection ring
TLV - Type Length Value
9. Terminology
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IP over Optical Networks: Summary of Issues July 2000
Channel:
A channel is a uni-directional optical tributary connecting two
OLXCs. Multiple channels are multiplexed optically at the WDM
system. One direction of an OC- 48/192 connecting two immediately
neighboring OLXCs is an example of a channel. A channel can
generally be associated with a specific wavelength in the WDM
system. A single wavelength may transport multiple channels
multiplexed in the time domain.
Downstream node:
In an uni-directional lightpath, this is the next node closer to
destination.
Failure Indication Signal:
A signal that indicates that a failure has been detected at a peer
LSR. It consists of a sequence of failure indication packets
transmitted by a downstream LSR to an upstream LSR repeatedly. It
is relayed by each intermediate LSR to its upstream neighbor, until
it reaches an LSR that is setup to perform a protection switch.
Failure Recovery Signal:
A signal that indicates that a failure along the path of an LSP has
been repaired. It consists of a sequence of recovery indication
packets that are transmitted by a downstream LSR to its upstream
LSR, repeatedly. Again, like the failure indication signal, it is
relayed by each intermediate LSR to its upstream neighbor, until is
reaches the LSR that performed the original protection switch.
First-hop router:
The first router within the domain of concern along the lightpath
route. If the source is a router in the network, it is also its own
first-hop router.
Intermediate LSR:
LSR on the working or protection path that is neither a PSL nor a
PML.
Last-hop router:
The last router within the domain of concern along the lightpath
route. If the destination is a router in the network, it is also
its own last-hop router.
Lightpath:
This denotes an Optical Channel Trail in the context of this
document.
Link Failure:
A link failure is defined as the failure of the link probing
mechanism, and is indicative of the failure of either the underlying
physical link between adjacent LSRs or a neighbor LSR itself. (In
case of a bidirectional link implemented as two unidirectional
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IP over Optical Networks: Summary of Issues July 2000
links, it could mean that either one or both unidirectional links
are damaged.)
Liveness Message:
A message exchanged periodically between two adjacent LSRs that
serves as a link probing mechanism. It provides an integrity check
of the forward and the backward directions of the link between the
two LSRs as well as a check of neighbor aliveness.
Loss of Signal:
A lower layer impairment that occurs when a signal is not detectedat
an interface. This may be communicated to the MPLS layer by the
lower layer.
Loss of Packet:
An MPLS layer impairment that is local to the LSR and consists of
excessive discarding of packets at an interface, either due to label
mismatch or due to TTL errors. Working or Active LSP established to
carry traffic from a source LSR to a destination LSR under normal
conditions, that is, in the absence of failures. In other words, a
working LSP is an LSP that contains streams that require protection.
MPLS Traffic Group:
A logical bundling of multiple, working LSPs, each of which is
routed identically between a PSL and a PML. Thus, each LSP in a
trafficgroup shares the same redundant routing between the PSL and
the PML.
MPLS Protection Domain:
The set of LSRs over which a working path and its corresponding
protection path are routed. The protection domain is denoted as:
(working path, protection path).
Non-revertive:
A switching option in which streams are not automatically switched
back from a protection path to its corresponding working path upon
the restoration of the working path to a fault-free condition.
Opaque:
Used to denote a bearer channel characteristic where it is capable
of being terminated.
Optical Channel Trail:
The elementary abstraction of optical layer connectivity between two
end points is a uni-directional Optical Channel Trail. A Optical
Channel Trail is a fixed bandwidth connection between two network
elements established via the OLXCs. A bi-directional Optical
Channel Trail consists of two associated Optical Channel Trails in
opposite directions routed over the same set of nodes.
Optical layer crossconnect (OLXC):
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IP over Optical Networks: Summary of Issues July 2000
A switching element which connects an optical channel from an input
port to an output port. The switching fabric in an OLXC may be
either electronic or optical.
Protected MPLS Traffic Group (PMTG):
An MPLS traffic group that requires protection.
Protected MPLS Traffic Portion:
The portion of the traffic on an individual LSP that requires
protection. A single LSP may carry different classes of traffic,
with different protection requirements. The protected portion of
this traffic may be identified by its class, as for example, via the
EXP bits in the MPLS shim header or via the priority bit in the ATM
header.
Protection Merge LSR:
LSR that terminates both a working path and its corresponding
protection path, and either merges their traffic into a single
outgoing LSP, or, if it is itself the destination, passes the
traffic on to the higher layer protocols.
Protection Switch LSR:
LSR that is the origin of both the working path and its
corresponding protection path. Upon learning of a failure, either
via the FIS or via its own detection mechanism, the protection
switch LSR switches protected traffic from the working path to the
corresponding backup path.
Protection or Backup LSP (or Protection or Backup Path):
A LSP established to carry the traffic of a working path (or paths)
following a failure on the working path (or on one of the working
paths, if more than one) and a subsequent protection switch by the
PSL. A protection LSP may protect either a segment of a working LSP
(or a segment of a PMTG) or an entire working LSP (or PMTG). A
protection path is denoted by the sequence of LSRs that it
traverses.
Reverse Notification Tree:
A point-to-multipoint tree that is rooted at a PML and follows
theexact reverse path of the multipoint-to-point tree formed by
merging ofworking paths (due to label merging). The reverse
notification tree allows the FIS to travel along its branches
towards the PSLs, which perform the protection switch.
Revertive:
A switching option in which streams are automatically switched
backfrom the protection path to the working path upon the
restoration of the working path to a fault-free condition.
Soft state:
It has an associated time-to-live, and expires and may be discarded
once that time is passed. To avoid expiration the state must be
periodically refreshed. To reduce the overhead of the state
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IP over Optical Networks: Summary of Issues July 2000
maintenance, the expiration period may be increased exponentially
over time to a predefined maximum. This way the longer a state has
survived the fewer the number of refresh messages that are required.
Traffic Trunk:
An 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.
Upstream node:
In an uni-directional lightpath, this is the node closer to the
source.
Working or Active Path:
The portion of a working LSP that requires protection. (A working
path can be a segment of an LSP (or a segment of a PMTG) or a
complete LSP (or PMTG).) The working path is denoted by the sequence
of LSRs that it tranverses.
10. References
[Awduche99] D. Awduche, Y. Rekhter, J. Drake, R. Coltun, "Multi-
Protocol Lambda Switching: Combining MPLS Traffic Engineering
Control With Optical Crossconnects", Internet Draft draft-awduche-
mpls-te-optical-01.txt, Work in Progress, November 1999.
[Basak99] Debashis Basak, D. Awduche, J. Drake, Y. Rekhter,
"Multi-Protocol Lambda Switching: Issues in Combining MPLS Traffic
Engineering Control With Optical Crossconnects", Internet Draft
draft-basak-mpls-oxc-issues-01 .txt, Work in Progress, February 2000.
[Ceuppens00] L. Ceuppens, et. al. "Performance Monitoring in
Photonic Networks in support of MPL(ambda)S," Internet Draft draft-
ceuppens-mpls-optical-00.txt, March 2000.
[Chaudhuri00] S. Chaudhuri, et. al. "Control of Lightpaths in an
Optical Network," Internet Draft draft-chaudhuri-ip-olxc-control-
00.txt, February 2000.
[CRLDP] B. Jamoussi, et. al. "Constraint-Based LSP Setup using
LDP," Internet Draft draft-ietf-mpls-cr-ldp-03 .txt, February 1999.
[GSMP] A. Doria, et. al. "General Switch Management Protocol
V3,"Internet Draft draft-ietf-gsmp-05 .txt, April 2000.
[ISIS] ISO 10589, "Intermediate System to Intermediate System Intra-
Domain Routing Exchange Protocol for use in Conjunction with the
Protocol for Providing the Connectionless-mode Network Service".
[ISISTE] Henk Smit, Tony Li, "IS-IS extensions for Traffic
Engineering," Internet Draft, draft-ietf-isis-traffic-01 .txt, work
Various Authors Informational _ Expires January 2001 49
IP over Optical Networks: Summary of Issues July 2000
in progress, May 1999
[Jonathan00] J.P. Lang, K. Mitra, J. Drake,, "Extensions to RSVP
for Optical Networking", Internet Draft draft-lang-mpls-rsvp-oxc-
00.txt, Work in Progress, March 2000.
[Jamoussi99] L. Andersson, A. Fredette, B. Jamoussi, et al.,
"Constraint- Based LSP Setup using LDP", Internet Draft draft-tang-
crldp-optical-00.txt, Work in Progress, January 1999.
[Katz99] Katz, D. and Yeung, D., "Traffic Engineering Extensions to
OSPF," Internet Draft dtaft-katz-yeung-traffic-01.txt, Work in
progress, October 1999.
[KOMPELLA00-a] K. Kompella et. al., "Extensions to IS-IS/OSPF and
RSVP in support of MPL(ambda)S", Internet Draft draft-kompella-
mpls-optical-00.txt, Work in Progress, February 2000 .
[Kompella00-b] Kompella, K., Rekhter, Y., "Link Bundling in MPLS
Traffic Engineering", draft-kompella-mpls-bundle-00 .txt, February
2000.
[Lang00] J.P. Lang, "Link Management Protocol (LMP)," Internet
Draft draft-lang- mpls-lmp-00.txt, March 2000.
[Luciani00] J. Luciani, B. Rajagopalan, D. Awuduche, B. Cain,
Bilel Jamoussi, "IP Over Optical Networks - A Framework", Internet
Draft draft-ip-optical-framework-00 .txt, Work in Progress, March
2000.
[NHRP] Luciani, et. al. "NBMA Next Hop Resolution Protocol
(NHRP)," RFC 2332 , April 1998.
[ODSI00] G.Bernstein et. al., " Optical Domain Service Interconnect
(ODSI) Functional Specification", ODSI Coalition, April 2000.
[OSPF] Moy, J., _OSPF Version 2, RFC 1583 , March 1994
[Tang00] Z.B. Tang et. al. "Extensions to CR-LDP for Path
Establishment in Optical Networks," Internet Draft draft-tang-crldp-
optical-00.txt, March 2000
[WANG] G.Wang et. al., "Extensions to OSPF/IS-IS for Optical
Routing", Internet Draft draft-wang-ospf-isis-lambda-te-routing-
00.txt, Work in Progress, March 2000.
11. Author's Addresses
Various Authors Informational _ Expires January 2001 50
IP over Optical Networks: Summary of Issues July 2000
N. Chandhok, A. Durresi, R. Jagannathan, S. Seetharaman, K.
vinodkrishnan
Department of Computer and Information Science
The Ohio State University
2015 Neil Avenue, Columbus, OH 43210-1277, USA
Phone: (614)-292-3989
Email: {chandhok, durresi, rjaganna, seethara, vinodkri}@cis.ohio-
state.edu
Raj Jain
Nayna Networks, Inc.
157 Topaz Street
Milpitas, CA 95035
Phone: (408)-956-8000X309
Email: raj@nayna.com
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Various Authors Informational _ Expires January 2001 51