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Versions: 00 01 02 03 04 05 RFC 4257
CCAMP G. Bernstein (Ciena)
Internet Draft E. Mannie (KPNQwest)
Document: <draft-ietf-ccamp-sdhsonet- V. Sharma (Metanoia, Inc.)
control-01.txt>
Category: Informational
Expires November 2002 May 2002
Framework for GMPLS-based Control of SDH/SONET Networks
<draft-ietf-ccamp-sdhsonet-control-01.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026 [1].
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1. Abstract
The suite of protocols that defines Multi-Protocol Label Switching
(MPLS) is in the process of enhancement to generalize its
applicability to the control of non-packet based switching, that is,
optical switching. One area of prime consideration is to use these
generalized MPLS (GMPLS) protocols in upgrading the control plane of
optical transport networks. This document illustrates this process
by describing those extensions to MPLS protocols that are directed
towards controlling SONET/SDH networks. SONET/SDH networks make
very good examples of this process since they possess a rich
multiplex structure, a variety of protection/restoration options,
are well defined, and are widely deployed. The document discusses
extensions to MPLS routing protocols to disseminate information
needed in transport path computation and network operations,
together with the extensions to MPLS label distribution protocols
needed for the provisioning of transport circuits. New capabilities
that an MPLS control plane would bring to SONET/SDH networks, such
as new restoration methods and multi-layer circuit establishment,
are also discussed.
2. Conventions used in this document
Bernstein, Mannie, Sharma Informational - November 2002 1
GMPLS based Control of SDH/SONET May 2002
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 [2].
3. Introduction
The CCAMP Working Group of the IETF is currently working on
extending MPLS [3] protocols to support multiple network layers and
new services. This extended MPLS, which was initially known as
Multi-Protocol Lambda Switching, is now better referred to as
Generalized MPLS (or GMPLS). The authors of this work are among the
co-authors of the GMPLS specifications, and focus mainly on those
aspects of GMPLS that relate to the control of SDH/SONET networks.
The GMPLS effort is, in effect, extending IP technology to control
and manage lower layers. Using the same framework and similar
signaling and routing protocols to control multiple layers can not
only reduce the overall complexity of designing, deploying and
maintaining networks, but can also make it possible to operate two
contiguous layers by using either an overlay model, a peer model, or
an integrated model. The benefits of using a peer or an overlay
model between the IP layer and its underlying layer(s) will have to
be clarified and evaluated in the future. In the mean time, GMPLS
could be used for controlling each layer independently.
The goal of this work is to highlight how GMPLS could be used to
dynamically establish, maintain, and tear down SDH/SONET circuits.
The objective of using these extended MPLS protocols is to provide
at least the same kinds of SDH/SONET services as are provided today,
but using signaling instead of provisioning via centralized
management to establish those services. This will allow operators to
propose new services, and will allow clients to create SONET/SDH
paths on-demand, in real-time, through the provider network. We
first review the essential properties of SDH/SONET networks and
their operations, and we show how the label concept in MPLS can be
extended to the SONET/SDH case. We then look at important
information to be disseminated by a link state routing protocol and
look at the important signal attributes that need to be conveyed by
a label distribution protocol. Finally, we look at some outstanding
issues and future possibilities.
3.1. MPLS Overview
A major advantage of the MPLS architecture [3] for use as a general
network control plane is its clear separation between the forwarding
(or data) plane, the signaling (or connection control) plane, and
the routing (or topology discovery/resource status) plane. This
allows the work on MPLS extensions to focus on the forwarding and
signaling planes, while allowing well-known IP routing protocols to
be reused in the routing plane. This clear separation also allows
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GMPLS based Control of SDH/SONET May 2002
for MPLS to be used to control networks that do not have a packet-
based forwarding plane.
An MPLS network consists of MPLS nodes called Label Switch Routers
(LSRs) connected via circuits called Label Switched Paths (LSPs). An
LSP is unidirectional and could be of several different types such
as point-to-point, point-to-multipoint, and multipoint-to-point.
Border LSRs in an MPLS network act either as ingress or egress LSRs
depending on the direction of the traffic being forwarded.
Each LSP is associated with a Fowarding Equivalence Class (FEC),
which may be thought of as a set of packets that receive identical
forwarding treatment at an LSR. The simplest example of an FEC might
be the set of destination addresses lying in a given address range.
All packets that have a destination address lying within this
address range are forwarded identically at each LSR configured with
that FEC.
To establish an LSP, a signaling protocol (or label distribution
protocol) such as LDP/CR-LDP or RSVP-TE is required. Between two
adjacent LSRs, an LSP is locally identified by a short, fixed length
identifier called a label, which is only significant between those
two LSRs. The signaling protocol is responsible for the inter-node
communication that assigns and maintains these labels.
When a packet enters an MPLS-based packet network, it is classified
according to its FEC and, possibly, additional rules, which together
determine the LSP along which the packet must be sent. For this
purpose, the ingress LSR attaches an appropriate label to the
packet, and forwards the packet to the next hop. The label may be
attached to a packet in different ways. For example, it may be in
the form of a header encapsulating the packet (the "shim" header) or
it may be written in the VPI/VCI field (or DLCI field) of the layer
2 encapsulation of the packet. In case of SDH/SONET networks, we
will see that a label is simply associated with a segment of a
circuit, and is mainly used in the signaling plane to identify this
segment (e.g. a time-slot) between two adjacent nodes.
When a packet reaches a core packet LSR, this LSR uses the label as
an index into a forwarding table to determine the next hop and the
corresponding outgoing label (and, possibly, the QoS treatment to be
given to the packet), writes the new label into the packet, and
forwards the packet to the next hop. When the packet reaches the
egress LSR, the label is removed and the packet is forwarded using
appropriate forwarding, such as normal IP forwarding. We will see
that for a SONET/SDH network these operations do not occur in quite
the same way.
3.2. SDH/SONET Overview
There are currently two different multiplexing technologies in use
in optical networks: wavelength division multiplexing (WDM) and time
division multiplexing (TDM). This work focuses on TDM technology.
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SDH and SONET are two TDM standards widely used by operators to
transport and multiplex different tributary signals over optical
links, thus creating a multiplexing structure, which we call the
SDH/SONET multiplex. SDH, which was developed by the ETSI and later
standardized by the ITU-T [4], is now used worldwide, while SONET,
which was standardized by the ANSI [5], is mainly used in the US.
However, these two standards have several similarities, and to some
extent SONET can be viewed as a subset of SDH. Internetworking
between the two is possible using gateways.
The fundamental signal in SDH is the STM-1 that operates at a rate
of about 155 Mbps, while the fundamental signal in SONET is the STS-
1 that operates at a rate of about 51 Mbps. These two signals are
made of contiguous frames that consist of transport overhead
(header) and payload. To solve synchronization issues, the actual
data is not transported directly in the payload but rather in
another internal frame that is allowed to float over two successive
SDH/SONET payloads. This internal frame is named a Virtual Container
(VC) in SDH and a Synchronous Payload Envelope (SPE) in SONET.
The SDH/SONET architecture identifies three different layers, each
of which corresponds to one level of communication between SDH/SONET
equipment. These are, starting with the lowest, the regenerator
section/section layer, the multiplex section/line layer, and (at the
top) the path layer. Each of these layers in turn has its own
overhead (header). The transport overhead of a SDH/SONET frame is
mainly sub-divided in two parts that contain the regenerator
section/section overhead and the multiplex section/line overhead. In
addition, a pointer (in the form of the H1, H2 and H3 bytes)
indicates the beginning of the VC/SPE in the payload of the overall
STM/SDH frame.
The VC/SPE itself is made up of a header (the path overhead) and a
payload. This payload can be further subdivided into sub-elements
(signals) in a fairly complex way. In the case of SDH, the STM-1
frame may contain either one VC-4 or three multiplexed VC-3s. The
SONET multiplex is a pure tree, while the SDH multiplex is not a
pure tree, since it contains a node that can be attached to two
parent nodes. The structure of the SONET/SDH multiplex is shown in
Figure 1. In addition, we show reference points in this figure that
are explained in later sections.
xN x1
STM-N<----AUG<----AU-4<--VC4<------------------------------C-4 E4
^ ^
Ix3 Ix3
I I x1
I -----TUG-3<----TU-3<---VC-3<---I
I ^ C-3 DS3/E3
-------AU-3<---VC-3<-- I ---------------------I
^ I
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GMPLS based Control of SDH/SONET May 2002
Ix7 Ix7
I I x1
-----TUG-2<---TU-2<---VC-2<---C-2 DS2/T2
^ ^
I I x3
I I----TU-12<---VC-12<--C-12 E1
I
I x4
I-------TU-11<---VC-11<--C-11 DS1/T1
xN
STS-N<-------------------SPE<------------------------------DS3/T3
^
Ix7
I x1
I---VT-Group<---VT-6<----SPE DS2/T2
^ ^ ^
I I I x2
I I I-----VT-3<----SPE DS1C
I I
I I x3
I I--------VT-2<----SPE E1
I
I x4
I-----------VT-1.5<--SPE DS1/T1
Figure 1. SDH and SONET multiplexing structure and typical PDH
payload signals.
The leaves of these multiplex structures are time slots (positions)
of different sizes that can contain tributary signals. These
tributary signals (e.g. E1, E3, etc) are mapped into the leaves
using standardized mapping rules. In general, a tributary signal
does not fill a time slot completely, and the mapping rules define
precisely how to fill it.
What is important for the MPLS-based control of SDH/SONET circuits
is to identify the elements that can be switched from an input
multiplex on one interface to an output multiplex on another
interface. The only elements that can be switched are those that can
be re-aligned via a pointer, i.e. a VC-x in the case of SDH and a
SPE in the case of SONET.
An STM-N/STS-N signal is formed from N x STM-1/STS-1 signals via
byte interleaving. The VCs/SPEs in the N interleaved frames are
independent and float according to their own clocking. To transport
tributary signals in excess of the basic STM-1/STS-1 signal rates,
the VCs/SPEs can be concatenated, i.e., glued together. In this case
their relationship with respect to each other is fixed in time and
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GMPLS based Control of SDH/SONET May 2002
hence this relieves, when possible, an end system of any inverse
multiplexing bonding processes. Different types of concatenations
are defined in SDH/SONET.
For example, standard SONET concatenation allows the concatenation
of M x STS-1 signals within an STS-N signal with M <= N, and M = 3,
12, 48, 192,...). The SPEs of these M x STS-1s can be concatenated
to form an STS-Mc. The STS-Mc notation is short hand for describing
an STS-M signal whose SPEs have been concatenated.
3.3. The Current State of Circuit Establishment in SDH/SONET Networks
In present day SDH and SONET networks, the networks are primarily
statically configured. When a client of an operator requests a
point-to-point circuit, the request sets in motion a process that
can last for several weeks or more. This process is composed of a
chain of shorter administrative and technical tasks, some of which
can be fully automated, resulting in significant improvements in
provisioning time and in operational savings. In the best case, the
entire process can be fully automated allowing, for example,
customer premise equipment (CPE) to contact a SDH/SONET switch to
request a circuit. Currently, the provisioning process involves the
following tasks.
3.3.1. Administrative Tasks
The administrative tasks represent a significant part of the
provisioning time. Most of them can be automated using IT
applications, e.g., a client still has to fill a form to request a
circuit. This form can be filled via a Web-based application and can
be automatically processed by the operator. A further enhancement is
to allow the client's equipment to coordinate with the operator's
network directly and request the desired circuit. This could be
achieved through a signaling protocol at the interface between the
client equipment and an operator switch, i.e., at the UNI, where
GMPLS signaling [6], [7], [8] can be used.
3.3.2. Manual Operations
Another significant part of the time may be consumed by manual
operations that involve installing the right interface in the CPE
and installing the right cable or fiber between the CPE and the
operator switch. This time can be especially significant when a
client is in a different time zone than the operator's main office.
This first-time connection time is frequently accounted for in the
overall establishment time.
3.3.3. Planning Tool Operation
Another portion of the time is consumed by planning tools that run
simulations using heuristic algorithms to find an optimized
placement for the required circuits. These planning tools can
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require a significant running time, sometimes on the order of days.
These simulations are, in general, executed for a set of demands for
circuits, i.e., a batch mode, to improve the optimality of network
resource usage and other parameters. Today, we do not really have a
means to reduce this simulation time. On the contrary, to support
fast, on-line, circuit establishment, this phase may be invoked more
frequently, i.e., we will not "batch up" as many connection
requests before we plan out the corresponding circuits. This means
that the network may need to be re-optimized periodically, implying
that the signaling should support re-optimization with minimum
impact to existing services.
3.3.4. Circuit Provisioning
Once the first three steps discussed above have been completed, the
operator must provision the circuits using the outputs of the
planning process. The time required for provisioning varies greatly.
It can be fairly short, on the order of a few minutes, if the
operators already have tools that help them to do the provisioning
over heterogeneous equipment. Otherwise, the process can take days.
Developing these tools for each new piece of equipment and each
vendor is a significant burden on the service provider. A
standardized interface for provisioning, such as GMPLS signaling,
could significantly reduce or eliminate this development burden. In
general, provisioning is a batched activity, i.e., a few times per
week an operator provisions a set of circuits. GMPLS will reduce
this provisioning time from a few minutes to a few seconds and could
help to transform this periodic process into a real-time process.
When a circuit is provisioned, it is not delivered directly to a
client. Rather, the operator first tests its performance and
behavior and if successful, delivers the circuit to the client. This
testing phase lasts, in general, for up to 24 hours. The operator
installs test equipment at each end and uses pre-defined test
streams to verify performance. If successful, the circuit is
officially accepted by the client. To speed up the verification
(sometimes known as "proving") process, it would be necessary to
support some form of automated performance testing.
3.4. Centralized Approach versus Distributed Approach
Whether a centralized approach or a distributed approach will be
used to control SDH/SONET networks is an open question, since each
approach has its merits. The application of GMPLS to SONET/SDH
networks does not preclude either model, although MPLS is itself a
distributed technology.
The basic tradeoff between the centralized and distributed
approaches is that of complexity of the network elements versus that
of the network management system (NMS). Since adding functionality
to existing SDH/SONET network elements may not be possible, a
centralized approach may be needed in some cases. The main issue
facing centralized control via an NMS is one of scalability. For
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GMPLS based Control of SDH/SONET May 2002
instance, this approach may be limited in the number of network
elements that can be managed (e.g., one thousand). It is, therefore,
quite common for operators to deploy several NMSÆs in parallel at
the Network Management Layer, each managing a different zone. In
that case, however, a Service Management Layer must be built on the
top of several individual NMSÆs to take care of end-to-end on-demand
services. On the other hand, in a complex and/or dense network,
restoration could be faster with a distributed approach than with a
centralized approach.
Let's now look at how the major control plane functional components
are handled via the centralized and distributed approaches:
3.4.1. Topology Discovery and Resource Dissemination
Currently NMS's maintain a consistent view of all the networking
layers under their purview. This can include the physical topology,
such as information about fibers and ducts. Since most of this
information is entered manually, it remains error prone.
A link state GMPLS routing protocol, on the other hand, could
perform automatic topology discovery and dissemination the topology
as well as resource status. This information would be available to
all nodes in the network, and hence also the NMS. Hence one can
look at a continuum of functionality between manually provisioned
topology information (of which there will always be some) and fully
automated discovery and dissemination as in a link state protocol.
Note that, unlike the IP datagram case, a link state routing
protocol applied to the SDH/SONET network does not have any service
impacting implications. This is because in the SDH/SONET case, the
circuit is source-routed (so there can be no loops), and no traffic
is transmitted until a circuit has been established, and an
acknowledgement received at the source.
3.4.2. Path Computation (Route Determination)
In the SDH/SONET case, unlike the IP datagram case, there is no need
for network elements to all perform the same path calculation [9].
In addition, path determination is an area for vendors to provide a
potentially significant value addition in terms of network
efficiency, reliability, and service differentiation. In this sense,
a centralized approach to path computation may be easier to operate
and upgrade. For example, new features such as new types of path
diversity or new optimization algorithms can be introduced with a
simple NMS software upgrade. On the other hand, updating switches
with new path computation software is a more complicated task. In
addition, many of the algorithms can be fairly computationally
intensive and may be completely unsuitable for the embedded
processing environment available on most switches. In restoration
scenarios, the ability to perform a reasonably sophisticated level
of path computation on the network element can be particularly
useful for restoring traffic during major network faults.
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3.4.3. Connection Establishment (provisioning)
The actual setting up of circuits, i.e., a coupled collection of
cross connects across a network, can be done either via the NMS
setting up individual cross connects or via a "soft permanent LSP"
(SPLSP) type approach. In the SPLSP approach, the NMS may just kick
off the connection at the "ingress" switch with GMPLS signaling
setting up the connection from that point onward. Connection
establishment is the trickiest part to distribute, however, since
errors in the connection setup/tear down process are service
impacting.
The table below compares the two approaches to connection
establishment.
Distributed approach Centralized approach
Control plane like MPLS or Management plane like TMN or
PNNI SNMP
Do we really need it? Being Always needed! Already there,
added/specified by several proven and understood.
standardization bodies
High survivability (e.g. in Potential single point(s) of
case of partition) failure
Distributed load Bottleneck: #requests and
actions to/from NMS
Individual local routing Centralized routing decision,
decision can be done per block of
requests
Routing scalable as for the Assumes a few big
Internet administrative domains
Complex to change routing Very easy local upgrade (non-
protocol/algorithm intrusive)
Requires enhanced routing Better consistency
protocol (traffic
engineering)
Ideal for inter-domain Not inter-domain friendly
Suitable for very dynamic For less dynamic demands
demands (longer lived)
Probably faster to restore, Probably slower to restore,but
but more difficult to have could effect reliable
reliable restoration. restoration.
High scalability Limited scalability: #nodes,
(hierarchical) links, circuits, messages
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Planning (optimization) Planning is a background
harder to achieve centralized activity
Easier future integration
with other control plane
layers
Table 1. Qualitative comparison between centralized and distributed
approaches.
3.5. Why SDH/SONET will not Disappear Tomorrow
As IP traffic becomes the dominant traffic transported over the
transport infrastructure, it is useful to compare the statistical
multiplexing of IP with the time division multiplexing of SDH and
SONET.
Consider, for instance, a scenario where IP over WDM is used
everywhere and lambdas are optically switched. In such a case, a
carrier's carrier would sell dynamically controlled lambdas with
each customers building their own IP backbones over these lambdas.
This simple model implies that a carrier would sell lambdas instead
of bandwidth. The carrierÆs goal will be to maximize the number of
wavelengths/lambdas per fiber, with each customer having to fully
support the cost for each end-to-end lambda whether or not the
wavelength is fully utilized. Although, in the near future, we may
have technology to support up to several hundred lambdas per fiber,
a world where lambdas are so cheap and abundant that every
individual customer buys them, from one point to any other point,
appears an unlikely scenario today.
More realistically, there is still room for a multiplexing
technology that provides circuits with a lower granularity than a
wavelength. (Not everyone needs a minimum of 10 Gbps or 40 Gbps per
circuit, and IP does not yet support all telecom applications in
bulk efficiently.)
SDH and SONET possess a rich multiplexing hierarchy that permits
fairly fine granularity and that provides a very cheap and simple
physical separation of the transported traffic between circuits,
i.e., QoS. Moreover, even IP datagrams cannot be transported
directly over a wavelength. A framing or encapsulation is always
required to delimit IP datagrams. The Total Length field of an IP
header cannot be trusted to find the start of a new datagram, since
it could be corrupted and would result in a loss of synchronization.
The typical framing used today for IP over DWDM is defined in
RFC1619/RFC2615 and known as POS (Packet Over SONET/SDH), i.e., IP
over PPP (in HDLC-like format) over SDH/SONET. SDH and SONET are
actually efficient encapsulations for IP. For instance, with an
average IP datagram length of 350 octets, an IP over GBE
encapsulation using an 8B/10B encoding results in 28% overhead, an
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GMPLS based Control of SDH/SONET May 2002
IP/ATM/SDH encapsulation results in 22% overhead and an IP/PPP/SDH
encapsulation results in only 6% overhead. (New simplified
encapsulations could reduce this overhead to as low as 3%, but the
gain is not huge compared to SDH and SONET, which have other
benefits as well.)
Any encapsulation of IP over WDM should at least provide error
monitoring capabilities (to detect signal degradation), error
correction capabilities, such as FEC (Forward Error Correction) that
are particularly needed for ultra long haul transmission, sufficient
timing information, to allow robust synchronization (that is, to
detect the beginning of a packet), and capacity to transport
signaling, routing and management messages, in order to control the
optical switches. SDH and SONET cover all these aspects natively,
except FEC, which tends to be supported in a proprietary way.
Since IP encapsulated in SDH/SONET is efficient and widely used, the
only real difference between an IP over WDM network and an IP over
SDH over WDM network is the layers at which the switching or
forwarding can take place. In the first case, it can take place at
the IP and optical layers. In the second case, it can take place at
the IP, SDH/SONET, and optical layers.
Almost all transmission networks today are based on SDH or SONET. A
client is connected either directly through an SDH or SONET
interface or through a PDH interface, the PDH signal being
transported between the ingress and the egress interfaces over SDH
or SONET. What we are arguing here is that it makes sense to do
switching or forwarding at all these layers.
4. GMPLS Applied to SDH/SONET
4.1. Controlling the SDH/SONET Multiplex
Controlling the SDH/SONET multiplex implies deciding which of the
different switchable components of the SDH/SONET multiplex do we
wish to control using GMPLS. Essentially, every SDH/SONET element
that is referenced by a pointer can be switched. These component
signals are the VC-4, VC-3, VC-2, VC-12 and VC-11 in the SDH case;
and the VT and STS SPEs in the SONET case. The SONET case is a bit
difficult to explain since, unlike in SDH, SPEs in SONET do not have
individual names. We will refer to them by identifying the structure
that contains them, namely the STS-1, VT-6, VT-3, VT-2 and VT-1.5.
The STS-1 SPE corresponds to a VC-3, a VT-6 SPE corresponds to a VC-
2, a VT-2 SPE corresponds to a VC-12, and a VT-1.5 SPE corresponds
to a VC-11. The SONET VT-3 SPE has no correspondence in SDH, however
SDH's VC-4 corresponds to SONET's STS-3c SPE.
In addition, it is possible to concatenate some of the structures
that contain these elements to build larger elements. For instance,
SDH allows the concatenation of X contiguous AU-4s to build a VC-4-
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Xc and of m contiguous TU-2s to build a VC-2-mc. In that case, a VC-
4-Xc or a VC-2-mc can be switched and controlled by MPLS. Note that
SDH also defines virtual (non-contiguous) concatenation of TU- 2s,
but in that case each constituent VC-2 is switched individually.
4.2. SDH/SONET LSR and LSP Terminology
Let a SDH or SONET Terminal Multiplexer (TM), Add-Drop Multiplexer
(ADM) or cross-connect (i.e. a switch) be called an SDH/SONET LSR. A
SDH/SONET path or circuit between two SDH/SONET LSRs now becomes a
GMPLS LSP. An SDH/SONET LSP is a logical connection between the
point at which a tributary signal (client layer) is adapted into its
virtual container, and the point at which it is extracted from its
virtual container.
To establish such an LSP, a signaling protocol is required to
configure the input interface, switch fabric, and output interface
of each SDH/SONET LSR along the path. An SDH/SONET LSP can be point-
to-point or point-to-multipoint, but not multipoint-to-point, since
no merging is possible with SDH/SONET signals.
To facilitate the signaling and setup of SDH/SONET circuits, an
SDH/SONET LSR must, therefore, identify each possible signal
individually per interface, since each signal corresponds to a
potential LSP that can be established through the SDH/SONET LSR. It
turns out, however, that not all SDH signals correspond to an LSP
and therefore not all of them need be identified. In fact, only
those signals that can be switched need identification.
5. Decomposition of the MPLS Circuit-Switching Problem Space
Although those familiar with MPLS may be familiar with its
application in a variety of application areas, e.g., ATM, Frame
Relay, and so on, here we quickly review its decomposition when
applied to the optical switching problem space.
(i) Information needed to compute paths must be made globally
available throughout the network. Since this is done via the link
state routing protocol, any information of this nature must either
be in the existing link state advertisements (LSAs) or the LSAs must
be supplemented to convey this information. For example, if it is
desirable to offer different levels of service in a network based on
whether a circuit is routed over SDH/SONET lines that are ring
protected versus being routed over those that are not ring protected
(differentiation based on reliability), the type of protection on a
SDH/SONET line would be an important topological parameter that
would have to be distributed via the link state routing protocol.
(ii) Information that is only needed between two "adjacent" switches
for the purposes of connection establishment is appropriate for
distribution via one of the label distribution protocols. In fact,
this information can be thought of as the "virtual" label. For
example, in SONET networks, when distributing information to
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switches concerning an end-to-end STS-1 path traversing a network,
it is critical that adjacent switches agree on the multiplex entry
used by this STS-1 (but this information is only of local
significance between those two switches). Hence, the multiplex entry
number in this case can be used as a virtual label. Note that the
label is virtual, in that it is not appended to the payload in any
way, but it is still a label in the sense that it uniquely
identifies the signal locally on the link between the two switches.
(iii) Information that all switches in the path need to know about a
circuit will also be distributed via the label distribution
protocol. Examples of such information include bandwidth, priority,
and preemption for instance.
(iv) Information intended only for end systems of the connection.
Some of the payload type information in may fall into this category.
6. MPLS Routing for SDH/SONET
Modern transport networks based on SONET/SDH excel at
interoperability in the performance monitoring (PM) and fault
management (FM) areas [10], [11]. They do not, however, inter-
operate in the areas of topology discovery or resource status.
Although link state routing protocols, such as IS-IS and OSPF, have
been used for some time in the IP world to compute destination-based
next hops for routes (without routing loops), they are particularly
valuable for providing timely topology and network status
information in a distributed manner, i.e., at any network node. If
resource utilization information is disseminated along with the link
status (as was done in ATM's PNNI routing protocol) then a very
complete picture of network status is available to a network
operator for use in planning, provisioning and operations.
The information needed to compute the path a connection will take
through a network is important to distribute via the routing
protocol. In the optical TDM case, this information includes, but
is not limited to: the available capacity of the network links, the
switching and termination capabilities of the nodes and interfaces,
and the protection properties of the link. This is what is being
proposed in the GMPLS extensions to IP routing protocols [12], [13],
[14].
When applying routing to circuit switched networks it is useful to
compare and contrast this situation with the datagram routing case
[15]. In the case of routing datagrams, all routes on all nodes
must be calculated exactly the same to avoid loops and "black
holes". In circuit switching, this is not the case since routes are
established per circuit and are fixed for that circuit. Hence,
unlike the datagram case, routing is not service impacting in the
circuit switched case. This is helpful, because, to accommodate the
optical layer, routing protocols need to be supplemented with new
information, much more than the datagram case. This information is
also likely to be used in different ways for implementing different
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GMPLS based Control of SDH/SONET May 2002
user services. Due to the increase in information transferred in
the routing protocol, it may be useful to separate the relatively
static parameters concerning a link from those that may be subject
to frequent changes. The current GMPLS routing extensions
[12],[13],[14] do not make such a separation, however.
6.1. Switching Capabilities
The main switching capabilities that characterize a SONET/SDH end
system and thus need to be advertised via the link state routing
protocol are: the switching granularity, supported forms of
concatenation, and the level of transparency.
6.1.1. Switching Granularity
From references [4], [5] and the overview section on SONET/SDH we
see that there are a number of different signals that compose the
SONET/SDH hierarchies. Those signals that are referenced via a
pointer, i.e., the VCs in SDH and the SPEs in SONET are those that
will actually be switched within a SONET/SDH network. These signals
are subdivided into lower order signals and higher order signals as
shown in Table 2.
Table 2. SDH/SONET switched signal groupings.
Signal Type SDH SONET
Lower Order VC-11, VC-12, VC-2 VT-1.5 SPE, VT-2 SPE,
VT-3 SPE, VT-6 SPE
Higher VC-3, VC-4 STS-1 SPE
Order
Manufacturers today differ in the types of switching capabilities
their systems support. Many manufacturers today switch signals
starting at VC-4 for SDH or STS-1 for SONET (i.e. the basic frame)
and above (see Section 6.1.2 on concatenation), but they do not
switch lower order signals. Some of them only allow the switching of
entire aggregates (concatenated or not) of signals such as 16 VC-4s,
i.e. a complete STM-16, and nothing finer. Some go down to the VC-3
level for SDH. Finally, some offer highly integrated switches that
switch at the VC-3/STS-1 level down to lower order signals such as
VC-12s. In order to cover the needs of all manufacturers and
operators, GMPLS signaling [6],[7],[8] covers both higher order and
lower order signals.
6.1.2. Signal Concatenation Capabilities
As stated in the SONET/SDH overview, to transport tributary signals
with rates in excess of the basic STM-1/STS-1 signal, the VCs/SPEs
can be concatenated, i.e., glued together. Different types of
concatenations are defined: contiguous standard concatenation,
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arbitrary concatenation, and virtual concatenation with different
rules concerning their size, placement, and binding.
Standard SONET concatenation allows the concatenation of M x STS-1
signals within an STS-N signal with M <= N, and M = 3, 12, 48, 192,
...). The SPEs of these M x STS-1s can be concatenated to form an
STS-Mc. The STS-Mc notation is short hand for describing an STS-M
signal whose SPEs have been concatenated. The multiplexing
procedures for SDH and SONET are given in references [4] and [5],
respectively. Constraints are imposed on the size of STS-Mc signals,
i.e., they must be a multiple of 3, and on their starting location
and interleaving.
This has the following advantages: (a) restriction to multiples of 3
helps with SDH compatibility (there is no STS-1 equivalent signal in
SDH); (b) the restriction to multiples of 3 reduces the number of
connection types; (c) the restriction on the placement and
interleaving could allow more compact representation of the "label";
The major disadvantages of these restrictions are:
(a) Limited flexibility in bandwidth assignment (somewhat inhibits
finer grained traffic engineering). (b) The lack of flexibility in
starting time slots for STS-Mc signals and in their interleaving
(where the rest of the signal gets put in terms of STS-1 slot
numbers) leads to the requirement for re-grooming (due to bandwidth
fragmentation).
Due to these disadvantages some SONET framer manufacturers now
support "flexible" or arbitrary concatenation, i.e., no restrictions
on the size of an STS-Mc (as long as M <= N) and no constraints on
the STS-1 timeslots used to convey it, i.e., the signals can use any
combination of available time slots.
Standard and flexible concatenations are network services, while
virtual concatenation is a SONET/SDH end-system service recently
approved by the committee T1 of ANSI and ITU-T. The essence of this
service is to have SONET/SDH end systems "glue" together the VCs or
SPEs of separate signals rather than requiring that he signals be
carried through the network as a single unit. In one example of
virtual concatenation, two end systems supporting this feature could
essentially "inverse multiplex" two STS-1s into a virtual STS-2c for
the efficient transport of 100 Mbps Ethernet traffic. Note that this
inverse multiplexing process can be significantly easier to
implement with SONET/SDH signals rather than packets. Since virtual
concatenation is provided by end systems, it is compatible with
existing SONET/SDH networks. Virtual concatenation is defined for
both higher order signals and low order signals. Table 3 shows the
nomenclature and capacity for several lower-order virtually
concatenated signals contained within different higher-order
signals.
Table 3 Capacity of Virtually Concatenated VTn-Xv (9/G.707)
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Carried In X Capacity In steps
of
VT1.5/ STS-1/VC-3 1 to 28 1600kbit/s to 1600kbit/s
VC-11-Xv 44800kbit/s
VT2/ STS-1/VC-3 1 to 21 2176kbit/s to 2176kbit/s
VC-12-Xv 45696kbit/s
VT1.5/ STS-3c/VC-4 1 to 64 1600kbit/s to 1600kbit/s
VC-11-Xv 102400kbit/s
VT2/ STS-3c/VC-4 1 to 63 2176kbit/s to 2176kbit/s
VC-12-Xv 137088kbit/s
6.1.3. SDH/SONET Transparency
The purposed of SONET/SDH is to carry its payload signals in a
transparent manner. This can include some of the layers of SONET
itself. For example, situations where the path overhead can never be
touched, since it actually belongs to the client. This was another
reason for not coding an explicit label in the SDH/SONET path
overhead. It may be useful to transport, multiplex and/or switch
lower layers of the SONET signal transparently.
As mentioned in the introduction, SONET overhead is broken into
three layers: Section, Line and Path. Each of these layers is
concerned with fault and performance monitoring. The Section
overhead is primarily concerned with framing, while the Line
overhead is primarily concerned with multiplexing and protection. To
perform multiplexing, a SONET network element should be line
terminating. However, not all SONET multiplexers/switches perform
SONET pointer adjustments on all the STS-1s contained within a
higher order SONET signal passing through them. Alternatively, if
they perform pointer adjustments, they do not terminate the line
overhead. For example, a multiplexer may take four SONET STS-48
signals and multiplex them onto an STS-192 without performing
standard line pointer adjustments on the individual STS-1s. This
can be looked at as a service since it may be desirable to pass
SONET signals, like an STS-12 or STS-48, with some level of
transparency through a network and still take advantage of TDM
technology. Transparent multiplexing and switching can also be
viewed as a constraint, since some multiplexers and switches may not
switch with as fine a granularity as others. Table 4 summarizes the
levels of SONET/SDH transparency.
Table 4. SONET/SDH transparency types and their properties.
Transparency Type Comments
Path Layer (or Line Standard higher order SONET path
Terminating) switching. Line overhead is terminated
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or modified.
Line Level (or Section Preserves line overhead and switches
Terminating) the entire line multiplex as a whole.
Section overhead is terminated or
modified.
Section layer Preserves all section overhead,
Basically does not touch any of the
SONET/SDH bits.
6.2. Protection
SONET and SDH networks offer a variety of protection options at both
the SONET line (SDH multiplex section) and SONET/SDH path level
[10],[11]. Standardized SONET line level protection techniques
include: Linear 1+1 and linear 1:N automatic protection switching
(APS) and both two-fiber and four-fiber bi-directional line switched
rings (BLSRs). At the path layer, SONET offers uni-directional path
switched ring protection. Both ring and 1:N line protection also
allow for "extra traffic" to be carried over the protection line
when that line is not being used, i.e., when it is not carrying
traffic for a failed working line. These protection methods are
summarized in Table 5. It should be noted that these protection
methods are completely separate from any MPLS layer protection or
restoration mechanisms.
Table 5. Common SONET/SDH protection mechanisms.
Protection Type Extra Comments
Traffic
Optionally
Supported
1+1 No Requires no coordination
Unidirectional between the two ends of the
circuit. Dedicated
protection line.
1+1 Bi- No Coordination via K byte
directional protocol. Lines must be
consistently configured.
Dedicated protection line.
1:1 Yes Dedicated protection.
1:N Yes One Protection line shared
by N working lines
4F-BLSR (4 Yes Dedicated protection, with
fiber bi- alternative ring path.
directional
line switched
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ring)
2F-BLSR (2 Yes Dedicated protection, with
fiber bi- alternative ring path
directional
line switched
ring)
UPSR (uni- No Dedicated protection via
directional alternative ring path.
path switched Typically used in access
ring) networks.
It may be desirable to route some connections over lines that
support protection of a given type, while others may be routed over
unprotected lines, or as "extra traffic" over protection lines.
Also, to assist in the configuration of these various protection
methods it can be extremely valuable to advertise the link
protection attributes in the routing protocol, as is done in the
current GMPLS routing protocols. For example, suppose that a 1:N
protection group is being configured via two nodes. One must make
sure that the lines are "numbered the same" with respect to both
ends of the connection or else the APS (K1/K2 byte) protocol will
not correctly operate.
Table 6. Parameters defining protection mechanisms.
Protection Comments
Related Link
Information
Protection Type Indicates which of the protection types
delineated in Table 5.
Protection Indicates which of several protection
Group Id groups (linear or ring) that a node belongs
to. Must be unique for all groups that a
node participates in
Working line Important in 1:N case and to differentiate
number between working and protection lines
Protection line Used to indicate if the line is a
number protection line.
Extra Traffic Yes or No
Supported
Layer If this protection parameter is specific to
SONET then this parameter is unneeded,
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otherwise it would indicate the signal
layer that the protection is applied.
An open issue concerning protection is the extent of information
regarding protection that must be disseminated. The contents of
Table 6 represent one extreme while a simple enumerated list of:
Extra-Traffic/Protection line, Unprotected, Shared (1:N)/Working
line, Dedicated (1:1, 1+1)/Working Line, Enhanced (Ring) /Working
Line, represents the other.
There is also a potential implication for link bundling [16], that
is, for each link, the routing protocol could advertise whether that
link is a working or protection link and possibly some parameters
from Table 6. A possible drawback of this scheme is that the routing
protocol would be burdened with advertising properties even for
those protection links in the network that could not, in fact, be
used for routing working traffic, e.g., dedicated protection links.
An alternative method would be to bundle the working and protection
links together, and advertise the bundle instead. Now, for each
bundled link, the protocol would have to advertise the amount of
bandwidth available on its working links, as well as the amount of
bandwidth available on those protection links within the bundle that
were capable of carrying "extra traffic." This would reduce the
amount of information to be advertised. An issue here would be to
decide which types of working and protection links to bundle
together. For instance, it might be preferable to bundle working
links (and their corresponding protection links) that are "shared"
protected separately from working links that are "dedicated"
protected.
6.3. Available Capacity Advertisement
Each SDH/SONET LSR must maintain an internal table per interface
that indicates each signal in the multiplex structure that is
allocated at that interface. This internal table is the most
complete and accurate view of the link usage and available capacity.
For use in path computation, this information needs to be advertised
in some way to all others SONET/SDH LSRs in the same domain. There
is a trade off to be reached concerning: the amount of detail in the
available capacity information to be reported via a link state
routing protocol, the frequency or conditions under which this
information is updated, the percentage of connection establishments
that are unsuccessful on their first attempt due to the granularity
of the advertised information, and the extent to which network
resources can be optimized. There are different levels of
summarization that are being considered today for the available
capacity information. At one extreme, all signals that are allocated
on an interface could be advertised, while at the other extreme, a
single aggregated value of the available bandwidth per link could be
advertised.
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Consider first the relatively simple structure of SONET and its most
common current and planned usage. DS1s and DS3s are the signals most
often carried within a SONET STS-1. Either a single DS3 occupies
the STS-1 or up to 28 DS1s (4 each within the 7 VT groups) are
carried within the STS-1. With a reasonable VT1.5 placement
algorithm within each node it may be possible to just report on
aggregate bandwidth usage in terms of number of whole STS-1s
(dedicated to DS3s) used and the number of STS-1s dedicated to
carrying DS1s allocated for this purpose. This way a network
optimization program could try to determine the optimal placement of
DS3s and DS1s to minimize wasted bandwidth due to half-empty STS-1s
at various places within the transport network. Similarly consider
the set of super rate SONET signals (STS-Nc). If the links between
the two switches support flexible concatenation then the reporting
is particularly straightforward since any of the STS-1s within an
STS-M can be used to comprise the transported STS- Nc. However, if
only standard concatenation is supported then reporting gets
trickier since there are constraints on where the STS-1s can be
placed. SDH has still more options and constraints, hence it is not
yet clear which is the best way to advertise bandwidth resource
availability/usage in SONET/SDH. At present, the GMPLS routing
protocol extensions define minimum and maximum values for available
bandwidth, which allows a remote node to make some deductions about
the amount of capacity available at a remote link and the types of
signals it can accommodate. However, due to the multiplexed nature
of the signals, the authors are of the opinion that reporting of
bandwidth particular to signal types rather than as a single
aggregate bit rate is probably very desirable.
6.4. Path Computation
Although a link state routing protocol can be used to obtain network
topology and resource information, this does not imply the use of an
"open shortest path first" route [9]. The path must be open in the
sense that the links must be capable of supporting the desired
signal type and that capacity must be available to carry the
signal. Other constraints may include hop count, total delay
(mostly propagation), and underlying protection. In addition, it may
be desirable to route traffic in order to optimize overall network
capacity, or reliability, or some combination of the two. Dikstra's
algorithm computes the shortest path with respect to link weights
for a single connection at a time. This can be much different than
the paths that would be selected in response to a request to set up
a batch of connections between a set of endpoints in order to
optimize network link utilization. One can think of this along the
lines of global or local optimization of the network in time.
Due to the complexity of some of the connection routing algorithms
(high dimensionality, non-linear integer programming problems) and
various criteria by which one may optimize a network, it may not be
possible or desirable to run these algorithms on network nodes.
However, it may still be desirable to have some basic path
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computation ability running on the network nodes, particularly for
use during restoration situations. Such an approach is in line with
the use of MPLS for traffic engineering, but is much different than
typical OSPF or IS-IS usage where all nodes must run the same
routing algorithm.
7. LSP Provisioning/Signaling for SDH/SONET
Traditionally, end-to-end circuit connections in SDH/SONET networks
have been set up via network management systems (NMSs), which issue
commands (usually under the control of a human operator) to the
various network elements involved in the circuit, via an equipment
vendor's element management system (EMS). Very little multi-vendor
interoperability has been achieved via management systems. Hence,
end-to-end circuits in a multi-vendor environment typically require
the use of multiple management systems and the infamous
configuration via "yellow sticky notes". As discussed in Section 2,
a common signaling protocol, such as RSVP with TE extensions or CR-
LDP appropriately extended for circuit switching applications, could
therefore help to solve these interoperability problems. In this
section, we examine the various components involved in the automated
provisioning of SONET/SDH LSPs.
7.1.1. What do we Label in SDH/SONET? Frames or Circuits?
MPLS was initially introduced to control asynchronous technologies
like IP, where a label was attached to each individual block of
data, such as an IP packet or a Frame Relay frame. SONET and SDH,
however, are synchronous technologies that define a multiplexing
structure (see Section 3), which we referred to as the SDH (or
SONET) multiplex. This multiplex involves a hierarchy of signals,
lower order signals embedded within successive higher order ones
(see Fig. 1). Thus, depending on its level in the hierarchy, each
signal consists of frames that repeat periodically, with a certain
number of byte time slots per frame.
The question then arises: is it these frames that we label in GMPLS?
It will be seen in what follows that each SONET or SDH "frame"
need not have its own label, nor is it necessary to switch frames
individually. Rather, the unit that is switched is a "flow"
comprised of a continuous sequence of time slots that appear at a
given position in a frame. That is, we switch an individual SONET or
SDH signal, and a label associated with each given signal.
For instance, the payload of an SDH STM-1 frame does not fully
contain a complete unit of user data. In fact, the user data is
contained in a virtual container (VC) that is allowed to float over
two contiguous frames for synchronization purposes. A pointer in the
Section Overhead (SOH) indicates the beginning of the VC in the
payload. Thus, frames are now inter-related, since each consecutive
pair may share a common virtual container. From the point of view of
GMPLS, therefore, it is not the successive frames that are treated
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GMPLS based Control of SDH/SONET May 2002
independently or labeled, but rather the entire user signal. An
identical argument applies to SONET.
Observe also that the GMPLS signaling used to control the SDH/SONET
multiplex must honor its hierarchy. In other words, the SDH/SONET
layer should not be viewed as homogeneous and flat, because this
would limit the scope of the services that SDH/SONET can provide.
Instead, GMPLS tunnels should be used to dynamically and
hierarchically control the SDH/SONET multiplex. For example, one
unstructured VC-4 LSP may be established between two nodes, and
later lower order LSPs (e.g. VC-12) may be created within that
higher order LSP. This VC-4 LSP can, in fact, be established
between two non-adjacent internal nodes in an SDH network, and later
advertised by a routing protocol as a new (virtual) link called a
Forwarding Adjacency (FA).
A SONET/SDH-LSR will have to identify each possible signal
individually per interface to fulfill the GMPLS operations. In order
to stay transparent the LSR obviously should not touch the SONET/SDH
overheads; this is why an explicit label is not encoded in the
SDH/SONET overheads. Rather, a label is associated with each
individual signal. This approach is similar to the one considered
for lambda switching, except that it is more complex, since SONET
and SDH define a richer multiplexing structure. Therefore a label
is associated with each signal, and is locally unique for each
signal at each interface. This signal could, and will most probably,
occupy different time-slots at different interfaces.
7.2. Label Structure in SDH/SONET
The signaling protocol used to establish an SDH/SONET LSP must have
specific information elements in it to map a label to the particular
signal type that it represents, and to the position of that signal
in the SONET/SDH multiplex. As we will see shortly, with a
carefully chosen label structure, the label itself can be made to
function as this information element.
In general, there are two ways to assign labels for signals between
neighboring SDH/SONET LSRs. One way is for the labels to be
allocated completely independently of any SDH/SONET semantics; e.g.
labels could just be unstructured 16 or 32 bit numbers. In that
case, in the absence of appropriate binding information, a label
gives no visible information about the flow that it represents. From
a management and debugging point of view, therefore, it becomes
difficult to match a label with the corresponding signal, since , as
we saw in Section 7.1.1, the label is not coded in the SDH/SONET
overhead of the signal.
Another way is to use the well-defined and finite structure of the
SDH/SONET multiplexing tree to devise a signal numbering scheme that
makes use of the multiplex as a naming tree, and assigns each
multiplex entry a unique associated value. This allows the unique
identification of each multiplex entry (signal) in terms of its type
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GMPLS based Control of SDH/SONET May 2002
and position in the multiplex tree. By using this multiplex entry
value itself as the label, we automatically add SDH/SONET semantics
to the label! Thus, simply by examining the label, one can now
directly deduce the signal that it represents, as well as its
position in the SDH/SONET multiplex. We refer to this as
multiplex-based labeling. This is the idea that was incorporated in
the GMPLS signaling specifications for SDH/SONET [17].
7.3. Signaling Elements
In the preceding sections, we defined the meaning of a SDH/SONET
label and specified its structure. A question that arises naturally
at this point is the following. In an LSP or connection setup
request, how do we specify the signal for which we want to establish
a path (and for which we desire a label)?
Clearly, information that is required to completely specify the
desired signal and its characteristics must be transferred via the
label distribution protocol, so that the switches along the path can
be configured to correctly handle and switch the signal. This
information is specified in three parts [17], each of which refers
to a different network layer.
The first specifies the nature/type of the LSP or the desired
SDH/SONET channel, in terms of the particular signal (or collection
of signals) within the SDH/SONET multiplex that the LSP represents,
and is used by all the nodes along the path of the LSP.
The second specifies the payload carried by the LSP or SDH/SONET
channel, in terms of the termination and adaptation functions
required at the end points, and is used by the source and
destination nodes of the LSP.
The third specifies certain link selection constraints, which
control, at each hop, the selection of the underlying link that is
used to transport this LSP.
8. Summary and Conclusions
We provided a detailed account of the issues involved in applying
MPLS-based control to TDM networks.
We began with a brief overview of MPLS and SDH/SONET networks,
discussing current circuit establishment in TDM networks, and
arguing why SDH/SONET technologies will not be "outdated" in the
foreseeable future. Next, we looked at MPLS applied to SDH/SONET
networks, where we considered why such an application makes sense,
and reviewed some MPLS terminology as applied to TDM networks. We
considered the two main areas of application of MPLS methods to TDM
networks, namely routing and signaling. We reviewed in detail the
switching capabilities of TDM equipment, and the requirement to
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GMPLS based Control of SDH/SONET May 2002
learn about the protection capabilities of underlying links, and how
these influence the available capacity advertisement in TDM
networks. We focused briefly on path computation methods, pointing
out that these were not subject to standardization. We then examined
optical path provisioning or signaling, considering the issue of
what constitutes an appropriate label for TDM circuits and how this
label should be structured, and we focused on the importance of
hierarchical label allocation in a TDM network. Finally, we reviewed
the signaling elements involved when setting up an optical TDM
circuit, focusing on the nature of the LSP, the type of payload it
carries, and the characteristics of the links that the LSP wishes to
use at each hop along its path for achieving a certain reliability.
9. Security Considerations
This draft raises no new security issues in the MPLS specifications.
10.Acknowledgments
We acknowledge all the participants of the MPLS and CCAMP WGs, whose
constant enquiry about GMPLS issues in TDM networks motivated the
writing of this document, and whose questions helped shape its
contents. Also, thanks to Kireeti Kompella for his careful reading
of the last version of this draft, and for his helpful comments and
feedback.
11.Author's Addresses
Greg Bernstein
Ciena Corporation
10480 Ridgeview Court
Cupertino, CA 94014
Phone: +1 510 573-2237
E-mail: greg@ciena.com
Eric Mannie
KPNQwest
Terhulpsesteenweg 6A
1560 Hoeilaart - Belgium
Phone: +32 2 658 56 52
Mobile: +32 496 58 56 52
Fax: +32 2 658 51 18
E-mail: eric.mannie@kpnqwest.com
Vishal Sharma
Metanoia, Inc.
305 Elan Village Lane, Unit 121
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Phone: +1 408 955 0910
Email: v.sharma@ieee.org
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GMPLS based Control of SDH/SONET May 2002
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12.References
[1] Bradner, S., "The Internet Standards Process -- Revision 3", BCP
9, RFC 2026, October 1996.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] Rosen, E., Viswanathan, A., and Callon, R., "Multiprotocol Label
Switching Architecture", RFC 3031, January 2001.
[4] G.707, Network Node Interface for the Synchronous Digital
Hierarchy (SDH), International Telecommunication Union, 03/96.
[5] Synchronous Optical Network (SONET) Basic Description including
Multiplex Structure, Rates, and Formats, ANSI T1.105-1995.
[6] Berger, L. (Editor), "Generalized MPLS - - Signaling Functional
Description," Internet Draft, Work in Progress, draft-ietf-mpls-
generalized-signaling-08.txt, April 2002.
[7] Berger, L. (Editor), "Generalized MPLS Signaling - - RSVP-TE
Extensions," Internet Draft, Work in Progress, draft-ietf-mpls-
generalized-rsvp-te-07.txt, April 2002.
[8] Berger, L. (Editor), "Generalized MPLS Signaling - - CR-LDP
Extensions," Internet Draft, Work in Progress, draft-ietf-mpls-
generalized-cr-ldp-06.txt, April 2002.
[9] Bernstein, G., Yates, J., Saha, D., "IP-Centric Control and
Management of Optical Transport Networks," IEEE Communications
Mag., Vol. 40, Issue 10, October 2000.
Bernstein, Mannie, Sharma Informational- Expires August 2002 25
GMPLS based Control of SDH/SONET May 2002
[10] ANSI T1.105.01-1995, Synchronous Optical Network (SONET)
Automatic Protection Switching, American National Standards
Institute.
[11] G.841, Types and Characteristics of SDH Network Protection
Architectures, ITU-T, 07/95.
[12] Kompella, K., et al, "Routing Extensions in Support of
Generalize MPLS, " Internet Draft, Work-in-Progress, draft-ietf-
ccamp-gmpls-routing-04.txt, April 2002.
[13] Kompella, K., et al, "OSPF Extensions in Support of Generalize
MPLS," Internet Draft, Work-in-Progress, draft-ietf-ccamp-ospf-
extensions-07.txt, May 2002.
[14] Kompella, K., et al, "IS-IS Extensions in Support of Generalize
MPLS," Internet Draft, Work-in-Progress, draft-ietf-isis-gmpls-
extensions-12.txt, May 2002.
[15] Bernstein, G., Sharma, V., Ong, L., ææInter-domain Optical
Routing,ÆÆ OSA J. of Optical Networking, vol. 1, no. 2, pp. 80-92.
[16] Kompella, K., Rekhter, Y., and Berger, L., "Link Bundling in
MPLS Traffic Engineering", Internet Draft, Work-in-Progress,
draft-kompella-mpls-bundle-05.txt, Feb. 2001.
[17] Mannie, E. (Editor), "GMPLS Extensions for SONET and SDH
Control", Internet Draft, Work-in-Progress, draft-ietf-ccamp-
gmpls-sonet-sdh-04.txt, April 2002.
Bernstein, Mannie, Sharma Informational- Expires August 2002 26
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