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Network Working Group                  G. Bernstein (Grotto Networking)
Internet Draft                                E. Mannie (InterAir Link)
Category: Informational                      V. Sharma (Metanoia, Inc.)
                                       E. Gray (Marconi Communications)
Expires August 2005                                       February 2005


        Framework for GMPLS-based Control of SDH/SONET Networks
               <draft-ietf-ccamp-sdhsonet-control-05.txt>


   Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of section 3 of RFC 3667 [1] and Section 6 of RFC 3668 [2].

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   Abstract

   Generalized MPLS (GMPLS) is a suite of protocol extensions to MPLS
   (Multi-Protocol Label Switching) to make it generally applicable, to
   include - for example - control of non packet-based switching, and
   particularly, optical switching.  One consideration is to use GMPLS
   protocols to upgrade the control plane of optical transport networks.
   This document illustrates this process by describing those extensions
   to GMPLS protocols that are aimed at controlling Synchronous Digital
   Hierarchy (SDH) or Synchronous Optical Networking (SONET) networks.
   SDH/SONET networks make good examples of this process for a variety
   of reasons.  This document high-lights extensions to GMPLS-related
   routing protocols to disseminate information needed in transport path
   computation and network operations, together with (G)MPLS protocol
   extensions required for the provisioning of transport circuits. New
   capabilities that an GMPLS control plane would bring to SDH/SONET
   networks, such as new restoration methods and multi-layer circuit
   establishment, are also discussed.


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   1.   Introduction ................................................3
    1.1.  MPLS Overview .............................................3
    1.2.  SDH/SONET Overview ........................................4
    1.3.  The Current State of Circuit Establishment in SDH/SONET
          Networks ..................................................7
      1.3.1.  Administrative Tasks ..................................7
      1.3.2.  Manual Operations .....................................7
      1.3.3.  Planning Tool Operation ...............................7
      1.3.4.  Circuit Provisioning ..................................8
    1.4.  Centralized Approach versus Distributed Approach ..........8
      1.4.1.  Topology Discovery and Resource Dissemination .........9
      1.4.2.  Path Computation (Route Determination).................9
      1.4.3.  Connection Establishment (Provisioning)...............10
    1.5.  Why SDH/SONET will not Disappear Tomorrow ................11
   2.   GMPLS Applied to SDH/SONET .................................12
    2.1.  Controlling the SDH/SONET Multiplex ......................12
    2.2.  SDH/SONET LSR and LSP Terminology ........................13
   3.   Decomposition of the GMPLS Circuit-Switching Problem Space .13
   4.   GMPLS Routing for SDH/SONET ................................14
    4.1.  Switching Capabilities ...................................15
      4.1.1.  Switching Granularity ................................15
      4.1.2.  Signal Concatenation Capabilities ....................16
      4.1.3.  SDH/SONET Transparency ...............................17
    4.2.  Protection ...............................................18
    4.3.  Available Capacity Advertisement .........................21
    4.4.  Path Computation .........................................22
   5.   LSP Provisioning/Signaling for SDH/SONET ...................22
    5.1.  What do we Label in SDH/SONET? Frames or Circuits?........23
    5.2.  Label Structure in SDH/SONET .............................24
    5.3.  Signaling Elements .......................................24
   6.   Summary and Conclusions ....................................26
   7.   Security Considerations ....................................27
   8.   Acknowledgments ............................................27
   9.   Author's Addresses .........................................27
   10.  References .................................................28
    10.1.   Normative References ...................................28
    10.2.   Informative References .................................28
   11.  Intellectual Property Statement ............................29
   12.  Disclaimer .................................................30
   13.  Copyright Statement ........................................30
   14. IANA Considerations .........................................30
   15. Acronyms ....................................................30
   16. Acknowledgement .............................................31











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1. Introduction

   The CCAMP Working Group of the IETF has the goal of 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 GMPLS effort is, in effect, extending IP/MPLS 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 IP/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
   SDH/SONET 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
   GMPLS can be extended to the SDH/SONET 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.

1.1.  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
   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.



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   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 or RSVP-TE is required. Between two adjacent
   LSRs, an LSP is locally identified by a fixed length identifier
   called a label, which is only significant between those two LSRs.
   A signaling protocol is used for inter-node communication to assign
   and maintain 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 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 SDH/SONET network these operations do not occur in quite
   the same way.

1.2.  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.


   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.


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   ITU-T (G.707) [4] includes both the European ETSI SDH hierarchy and
   the USA ANSI SONET hierarchy [5].  The ETSI SDH and SONET standards
   regarding frame structures and higher-order multiplexing are the
   same. There are some regional differences in terminology, on the use
   of some overhead bytes, and lower-order multiplexing. Interworking
   between the two lower-order hierarchies 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 SONET 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/STS 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 SDH/SONET multiplex is shown in
   Figure 1. In addition, we show reference points in this figure that
   are explained in later sections.

   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 GMPLS-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.


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             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
   STM-0<------------AU-3<---VC-3<-- I ---------------------I
                              ^      I
                              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.


   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
   hence this relieves, when possible, an end system of any inverse
   multiplexing bonding processes. Different types of concatenations
   are defined in SDH/SONET.

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   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.


1.3.  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.

1.3.1.  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] can be used.

1.3.2.  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.

1.3.3.  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
   require a significant running time, sometimes on the order of days.




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   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.

1.3.4.  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.

1.4.  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 SDH/SONET
   networks does not preclude either model, although GMPLS 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 NMS 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 NMS 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:

1.4.1.  Topology Discovery and Resource Dissemination

   Currently an NMS maintains a consistent view of all the networking
   layers under its 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 disseminate 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.

1.4.2.  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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1.4.3.  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

       Packet-based control plane        Management plane like TMN or
       (like GMPLS or PNNI) useful?      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.



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       High scalability                  Limited scalability: #nodes,
       (hierarchical)                    links, circuits, messages

       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.


1.5.  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 carrier's 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 SDH/SONET), i.e., IP
   over PPP (in HDLC-like format) over SDH/SONET.  SDH and SONET are


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

   Any encapsulation of IP over WDM should, in the data plane, 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). In
   the case where associated signaling is used (that is the control and
   data plane topologies are congruent) the encapsulation should also
   provide the capacity to transport signaling, routing and management
   messages, in order to control the optical switches. Rather SDH and
   SONET cover all these aspects natively, except FEC, which tends to
   be supported in a proprietary way. (We note, however, that
   associated signaling is not a requirement for the GMPLS-based
   control of SDH/SONET networks. Rather, it is just one option. Non
   associated signaling, as would happen with an out-of-band control
   plane network is another equally valid option.)

   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.


2. GMPLS Applied to SDH/SONET

2.1.  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 SPEs in SONET do not
   have individual names, although they can be referred to simply as
   VT-N SPEs. We will refer to them by identifying the structure that
   contains them, namely STS-1, VT-6, VT-3, VT-2 and VT-1.5.



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   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-
   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 GMPLS. SDH also
   defines virtual (non-contiguous) concatenation of TU- 2s, however
   in that case each constituent VC-2 is switched individually.

2.2.  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.

3. Decomposition of the GMPLS Circuit-Switching Problem Space

   Although those familiar with GMPLS 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

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   (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
   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.

4.  GMPLS Routing for SDH/SONET

   Modern SDH/SONET transport networks excel at interoperability in the
   performance monitoring (PM) and fault management (FM) areas [10],
   [11]. They do not, however, interoperate 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 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 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


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   [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
   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.

   Indeed, from the carriers' perspective, the up-to-date dissemination
   of all link properties is essential and desired, and the use of a
   link-state routing protocol to distribute this information provides
   timely and efficient delivery.  If GMPLS-based networks got to the
   point that bandwidth updates happen very frequently, it makes sense,
   from an efficiency point of view, to separate them out for update.
   This situation is not yet seen in actual networks; however, if GMPLS
   signaling is put into widespread use then the need could arise.


4.1.  Switching Capabilities

   The main switching capabilities that characterize a SDH/SONET 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.

4.1.1.  Switching Granularity

   From references [4], [5] and the overview section on SDH/SONET we
   see that there are a number of different signals that compose the
   SDH/SONET 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 SDH/SONET 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, STS-3c SPE
         Order



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   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 5.1.2 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]) covers both higher order and
   lower order signals.

4.1.2.  Signal Concatenation Capabilities

   As stated in the SDH/SONET 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,
   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.



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   Standard and flexible concatenations are network services, while
   virtual concatenation is a SDH/SONET end-system service approved by
   the Committee T1 of ANSI [5] and the ITU-T [4].  The essence of this
   service is to have SDH/SONET end systems "glue" together the VCs or
   SPEs of separate signals rather than requiring that the 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 STS-1-2v for the
   efficient transport of 100 Mbps Ethernet traffic. Note that this
   inverse multiplexing process (or virtual concatenation) can be
   significantly easier to implement with SDH/SONET than packet switched
   circuits, because ensuring that timing and in-order frame delivery is
   preserved may be simpler to establish using SDH/SONET rather than
   packet switched circuits, where more sophisticated techniques may be
   needed.

   Since virtual concatenation is provided by end systems, it is
   compatible with existing SDH/SONET 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

     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


4.1.3.  SDH/SONET Transparency

   The purposed of SDH/SONET 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


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   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 pipe multiplexing (that is, multiplexing of 50 Mbps or 150
   Mbps chunks), 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 SDH/SONET transparency.

      Table 4. SDH/SONET 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.

      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 modify/terminate
                                any of the SDH/SONET overhead bits.

4.2.  Protection

   SONET and SDH networks offer a variety of protection options at both
   the SONET line (SDH multiplex section) and SDH/SONET 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. Likewise, standardized SDH multiplex
   section protection techniques include linear 1+1 and 1:N automatic p
   protection switching and both two-fiber and four-fiber bi-directional
   MS-SPRings (Multiplex Section-Shared Protection Rings).

   At the path layer, SDH offers SNCP (sub-network connection
   protection) ring protection.


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   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 GMPLS layer protection or restoration mechanisms.

      Table 5. Common SDH/SONET 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)

       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


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   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,
                           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], [18]
   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


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   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.


4.3.  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 SDH/SONET 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.

   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


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   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 SDH/SONET. 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, reporting of bandwidth particular to signal types
   rather than as a single aggregate bit rate may be desirable. For
   details on why this may be the case, we refer the reader to ITU-T
   publications G.7715.1 [19] and to Chapter 12 of [20].

4.4.  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
   use during restoration situations. Such an approach is in line with
   the use of GMPLS for traffic engineering, but is much different than
   typical OSPF or IS-IS usage where all nodes must run the same
   routing algorithm.


5. 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,


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  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 3, 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 SDH/SONET LSPs.

5.1. What do we Label in SDH/SONET? Frames or Circuits?

   GMPLS 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. The H1-H2-H3
   Au-n pointer bytes in the SDH overhead 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 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) [17].

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   A SDH/SONET-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 SDH/SONET
   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.

5.2.  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 SDH/SONET 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 6.1, 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
   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 [18].

5.3.  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)?


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   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 [18], each of which refers
   to a different network layer.

   1. GENERALIZED_LABEL REQUEST (as in [6], [7]), which contains three
      parts: LSP Encoding Type, Switching Type, and G-PID.

   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 certain link selection constraints, which
   control, at each hop, the selection of the underlying link that is
   used to transport this LSP.

   The third 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.

   2. SONET/SDH TRAFFIC_PARAMETERS (as in [18], Section 2.1) used as a
      SENDER_TSPEC/FLOWSPEC, which contains 7 parts: Signal Type,
      (Requested Contiguous Concatenation (RCC), Number of Contiguous
      Components (NCC), Number of Virtual Components (NVC)), Multiplier
      (MT), Transparency, and Profile.

   The Signal Type indicates the type of elementary signal comprising
   the LSP, while the remaining fields indicate transforms that can be
   applied to the basic signal to build the final signal that
   corresponds to the LSP actually being requested. For instance (see
   [18] for details):

          - Contiguous concatenation (by using the RCC and NCC
           fields) can be optionally applied on the Elementary Signal,
           resulting in a contiguously concatenated signal.

          - Then, virtual concatenation (by using the NVC field) can be
          optionally applied on the Elementary Signal resulting in
           a virtually concatenated signal.

          - Third, some transparency (by using the Transparency field)
           can be optionally specified when requesting a frame as
           signal rather than an SPE or VC based signal.

          - Fourth, a multiplication (by using the Multiplier field)
          can be optionally applied either directly on the Elementary
          Signal, or on the contiguously concatenated signal obtained
          from the first phase, or on the virtually concatenated signal


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          obtained from the second phase, or on these signals combined
          with some transparency.

   Transparency indicates precisely which fields in these overheads
   must be delivered unmodified at the other end of the LSP. An ingress
   LSR requesting transparency will pass these overhead fields that
   must be delivered to the egress LSR without any change. From the
   ingress and egress LSRs point of views, these fields must be seen as
   unmodified.

   Transparency is not applied at the interfaces with the initiating
   and terminating LSRs, but is only applied between intermediate LSRs.

   The transparency field is used to request an LSP that supports the
   requested transparency type; it may also be used to setup the
   transparency process to be applied at each intermediate LSR.

   Finally, the profile field is intended particular capabilities that
   must be supported for the LSP, for example monitoring capabilities.
   No standard profile is currently defined, however.

   3. UPSTREAM_LABEL for Bi-directional LSP's (as in [6], [7]).

   4. Local Link Selection e.g. IF_ID_RSVP_HOP Object (as in [7]).


6. Summary and Conclusions

   We provided a detailed account of the issues involved in applying
   generalized GMPLS-based control (GMPLS) to TDM networks.

   We began with a brief overview of GMPLS 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 IP/MPLS applied to SDH/SONET
   networks, where we considered why such an application makes sense,
   and reviewed some GMPLS terminology as applied to TDM networks.

   We considered the two main areas of application of IP/MPLS methods
   to TDM networks, namely routing and signaling, and discussed how
   Generalized MPLS routing and signaling are used in the context of
   TDM networks. We reviewed in detail the switching capabilities of
   TDM equipment, and the requirement to 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


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   the signaling elements involved when setting up an 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.

7.  Security Considerations

   This document describes the framework for GMPLS extensions for use
   in SDH/SONET control. As such, it introduces no new security issues
   with respect to GMPLS specifications.  GMPLS protocol specifications
   should identify and address security issues specific to protocol.

   Among the considerations that should be addressed by GMPLS protocol
   specifications, are any security vulnerabilities that are introduced
   by specific GMPLS extensions added in each specification.


8. 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 document, and for his helpful comments
   and feedback, and to Dimitri Papadimitriou for his review on behalf
   of the Routing Area Directorate, which provided many useful inputs
   to help update the document to conform to the standards evolutions
   since this document passed last call.


9. Author's Addresses

   Greg Bernstein
   Grotto Networking
   Phone:  +1 510 573-2237
   E-mail:  gregb@grotto-networking.com

   Eric Mannie
   InterAir Link
   Phone:  +32 2 790 34 25
   E-mail:  eric_mannie@hotmail.com

   Vishal Sharma
   Metanoia, Inc.
   888 Villa Street, Suite 200B
   Mountain View, CA 94041
   Phone:  +1 408 530 8313
   Email: v.sharma@ieee.org

   Eric Gray
   Marconi Communications
   E-mail:  Eric.Gray@Marconi.com


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10. References

10.1. Normative References

   [1]  Bradner, S., "IETF Rights in Contributions" BCP 78, RFC 3667,
        February, 2004.

   [2]  Bradner, S., "Intellectual Property Rights in IETF Technology",
        BCP 79, RFC 3668, February, 2004.


10.2. Informative References

   In the ITU references below, please see http://www.itu.int for
   availability of ITU documents. For ANSI references, please see
   the Library available through http://www.ansi.org.

   [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, March
        1996.

   [5]  ANSI T1.105-1995, Synchronous Optical Network (SONET) Basic
        Description including Multiplex Structure, Rates, and Formats,
        American National Standards Institute.

   [6]  Berger, L. (Editor), "Generalized MPLS - Signaling Functional
        Description," RFC 3471, January 2003.

   [7]  Berger, L. (Editor), "Generalized MPLS Signaling - RSVP-TE
        Extensions," RFC 3473, January 2003.

   [8]  Omitted.

   [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.

   [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, July 1995.

   [12] Kompella, K., et al, "Routing Extensions in Support of
        Generalized MPLS," Internet Draft, Work-in-Progress,
        draft-ietf-ccamp-gmpls-routing-09.txt, October 2003.




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   [13] Kompella, K., et al, "OSPF Extensions in Support of Generalized
        MPLS," Internet Draft, Work-in-Progress,
        draft-ietf-ccamp-ospf-extensions-12.txt, October 2003.


   [14] Kompella, K., et al, "IS-IS Extensions in Support of
        Generalized MPLS," Internet Draft, Work-in-Progress,
        draft-ietf-isis-gmpls-extensions-16.txt, August 2002.

   [15] Bernstein, G., Sharma, V., Ong, L., "Inter-domain Optical
        Routing, " 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-ietf-mpls-bundle-04.txt, July 2002.

   [17] Kompella, K., Rekhter, Y., "LSP Hierarchy with Generalized
        MPLS-TE", Internet Draft, Work-in-Progress,
        draft-ietf-mpls-lsp-hierarchy-08.txt,  February 2002.

   [18] Mannie, E. (Editor), "GMPLS Extensions for SONET and SDH
        Control", Internet Draft, Work-in-Progress,
        draft-ietf-ccamp-gmpls-sonet-sdh-08.txt, February 2003.

   [19] G.7715.1, ASON Routing Architecture and Requirements for
        Link-State Protocols, International Telecommunications Union,
        February 2004.

   [20] Bernstein, G., Rajagopalan, R., and Saha, D., "Optical Network
        Control: Protocols, Architectures, and Standards,"
        Addison-Wesley, July 2003.

11. Intellectual Property Statement


   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use
   of such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository
   at http://www.ietf.org/ipr.



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   The IETF invites any interested party to bring to its attention
   any copyrights, patents or patent applications, or other
   proprietary rights that may cover technology that may be required
   to implement this standard.  Please address the information to the
   IETF at ietf-ipr@ietf.org.


12. Disclaimer

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.


13. Copyright Statement

   Copyright (C) The Internet Society (2004).  This document is
   subject to the rights, licenses and restrictions contained in BCP
   78, and except as set forth therein, the authors retain all their
   rights.


14. IANA Considerations

   There are no IANA considerations that apply to this document.


15. Acronyms

   ANSI     - American National Standards Institute
   APS      - Automatic Protection Switching
   ATM      - Asynchronous Transfer Mode
   BLSR     - Bi-directional Line Switch Ring
   CPE      - Customer Premise Equipment
   DLCI     - Data Link Connection Identifier
   ETSI     - European Telecommunication Standards Institute
   FEC      - Forwarding Equivalency Class
   GMPLS    - Generalized MPLS
   IP       - Internet Protocol
   IS-IS    - Intermediate System to Intermediate System (RP)
   LDP      - Label Distribution Protocol
   LSP      - Label Switched Path
   LSR      - Label Switching Router
   MPLS     - Multi-Protocol Label Switching
   NMS      - Network Management System
   OSPF     - Open Shortest Path First (RP)
   PNNI     - Private Network Node Interface
   PPP      - Point to Point Protocol


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   QoS      - Quality of Service
   RP       - Routing Protocol
   RSVP     - ReSerVation Protocol
   SDH      - Synchronous Digital Hierarchy
   SNMP     - Simple Network Management Protocol
   SONET    - Synchronous Optical NETworking
   SPE      - SONET Payload Envelope
   STM      - Synchronous Transport Module (or Terminal Multiplexer)
   STS      - Synchronous Transport Signal
   TDM      - Time Division Multiplexer
   TE       - Traffic Engineering
   TMN      - Telecommunication Management Network
   UPSR     - Uni-directional Path Switch Ring
   VC       - Virtual Container (SDH) or Virtual Circuit
   VCI      - Virtual Circuit Identifier (ATM)
   VPI      - Virtual Path Identifier (ATM)
   VT       - Virtual Tributary
   WDM      - Wave-length Division Multiplexing


16. Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.






























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