CCAMP Working Group                    G. Bernstein (Ciena) (Grotto Networking)
Internet Draft                                E. Mannie (KPNQwest) (InterAir Link)
Document: <draft-ietf-ccamp-sdhsonet-        V. Sharma (Metanoia, Inc.)
control-01.txt>
control-02.txt>
Category: Informational
Expires November 2002                                          May 2002 August 2003                                       February 2003

        Framework for GMPLS-based Control of SDH/SONET Networks
               <draft-ietf-ccamp-sdhsonet-control-01.txt>
               <draft-ietf-ccamp-sdhsonet-control-02.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

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                   GMPLS based Control of SDH/SONET           May 2002      February 2003

   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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   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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                              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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   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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   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.

     3.4.3.     Connection Establishment (provisioning)

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     3.4.3.     Connection Establishment (provisioning)      February 2003

   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

       Planning (optimization)           Planning is a background

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       Planning (optimization)           Planning is a background      February 2003

       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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   IP/ATM/SDH encapsulation results in 22% overhead and an IP/PPP/SDH
   encapsulation results in only 6% overhead. (New simplified

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   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

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   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

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   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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   user services.  Due to the increase in information transferred in
   the routing protocol, it may be useful to separate the relatively

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   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.

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   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)

                  Carried In      X           Capacity       In steps
                                                              of

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                  Carried In      X           Capacity       In steps
                                                              of      February 2003

     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
                                or modified.

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                                or modified.      February 2003

      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
       ring)

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       ring)      February 2003

       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.

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   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
   computation ability running on the network nodes, particularly for

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   computation ability running on the network nodes, particularly for      February 2003

   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 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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   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      February 2003

   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      February 2003

   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
   Grotto Networking

   Phone:  +1 510 573-2237
   E-mail:  greg@ciena.com

   Eric Mannie
   KPNQwest
   Terhulpsesteenweg 6A
   1560 Hoeilaart - Belgium
   InterAir Link

   Phone:  +32 2 658 56 52
   Mobile: +32 496 58 56 52
   Fax:    +32 2 658 51 18 790 34 25
   E-mail:  eric.mannie@kpnqwest.com  eric_mannie@hotmail.com

   Vishal Sharma
   Metanoia, Inc.
   305 Elan Village Lane,
   1600 Villa Street, Unit 121
   San Jose, 352
   Mountain View, CA 95134 94041

   Phone:  +1 408 955 0910 650 386 6723
   Email: v.sharma@ieee.org

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                   GMPLS based Control of SDH/SONET           May 2002      February 2003

   Full Copyright Statement

   "Copyright (C) The Internet Society (date). All Rights Reserved.
   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implmentation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph
   are included on all such copies and derivative works. However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into

12.References

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   [2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
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   [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
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   [6] Berger, L. (Editor), "Generalized MPLS -                                              - Signaling Functional
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   [7] Berger, L. (Editor), "Generalized MPLS Signaling -                                                        - RSVP-TE
      Extensions," Internet Draft, Work in Progress, draft-ietf-mpls-
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   [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. RFC 3473, January 2003.

   [9] Bernstein, G.,  Yates, J., Saha, D.,  "IP-Centric Control and
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      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)
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Bernstein, Mannie, Sharma   Expires August 2003                     25
                   GMPLS based Control of SDH/SONET      February 2003

   [11] G.841, Types and Characteristics of SDH Network Protection
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   [12] Kompella, K., et al, "Routing Extensions in Support of
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   [13] Kompella, K., et al, "OSPF Extensions in Support of Generalize
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   [14] Kompella, K., et al, "IS-IS Extensions in Support of Generalize
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   [15] Bernstein, G., Sharma, V., Ong, L., ææInter-domain Optical
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   [16] Kompella, K., Rekhter, Y., and Berger, L., "Link Bundling in
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   [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
      gmpls-sonet-sdh-07.txt, October 2002.

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