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RFC 4257

Framework for Generalized Multi-Protocol Label Switching (GMPLS)-based Control of Synchronous Digital Hierarchy/Synchronous Optical Networking (SDH/SONET) Networks

Pages: 35
Informational

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Network Working Group                                       G. Bernstein
Request for Comments: 4257                             Grotto Networking
Category: Informational                                        E. Mannie
                                                                Perceval
                                                               V. Sharma
                                                          Metanoia, Inc.
                                                                 E. Gray
                                                Marconi Corporation, plc
                                                           December 2005


             Framework for Generalized Multi-Protocol Label
         Switching (GMPLS)-based Control of Synchronous Digital
     Hierarchy/Synchronous Optical Networking (SDH/SONET) Networks

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

Generalized Multi-Protocol Label Switching (GMPLS) is a suite of protocol extensions to MPLS 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 highlights 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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Table of Contents

1. Introduction ....................................................3 1.1. MPLS Overview ..............................................3 1.2. SDH/SONET Overview .........................................5 1.3. The Current State of Circuit Establishment in SDH/SONET Networks .........................................7 1.3.1. Administrative Tasks ................................8 1.3.2. Manual Operations ...................................8 1.3.3. Planning Tool Operation .............................8 1.3.4. Circuit Provisioning ................................8 1.4. Centralized Approach versus Distributed Approach ...........9 1.4.1. Topology Discovery and Resource Dissemination ......10 1.4.2. Path Computation (Route Determination) .............10 1.4.3. Connection Establishment (Provisioning) ............10 1.5. Why SDH/SONET Will Not Disappear Tomorrow .................12 2. GMPLS Applied to SDH/SONET .....................................13 2.1. Controlling the SDH/SONET Multiplex .......................13 2.2. SDH/SONET LSR and LSP Terminology .........................14 3. Decomposition of the GMPLS Circuit-Switching Problem Space .....14 4. GMPLS Routing for SDH/SONET ....................................15 4.1. Switching Capabilities ....................................16 4.1.1. Switching Granularity ..............................16 4.1.2. Signal Concatenation Capabilities ..................17 4.1.3. SDH/SONET Transparency .............................19 4.2. Protection ................................................20 4.3. Available Capacity Advertisement ..........................23 4.4. Path Computation ..........................................24 5. LSP Provisioning/Signaling for SDH/SONET .......................25 5.1. What Do We Label in SDH/SONET? Frames or Circuits? .......25 5.2. Label Structure in SDH/SONET ..............................26 5.3. Signaling Elements ........................................27 6. Summary and Conclusions ........................................29 7. Security Considerations ........................................29 8. Acknowledgements ...............................................30 9. Informative References .........................................31 10. Acronyms ......................................................33
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1. Introduction

The CCAMP Working Group of the IETF has the goal of extending MPLS [1] 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 [1] 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.
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   An MPLS network consists of MPLS nodes called Label Switch Routers
   (LSRs) connected via Label Switched Paths (LSPs).  An LSP is uni-
   directional 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 as either ingress or egress LSRs, depending on
   the direction of the traffic being forwarded.

   Each LSP is associated with a Forwarding 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 an SDH/SONET network these operations do not occur in quite
   the same way.
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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. ITU-T (G.707) [2] includes both the European Telecommunications Standards Institute (ETSI) SDH hierarchy and the USA ANSI SONET hierarchy [3]. 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 an 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
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   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.

             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
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               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
   Plesiochronous Digital Hierarchy (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;
   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.

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 an SDH/SONET switch to request a circuit. Currently, the provisioning process involves the following tasks.
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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 [4], [5] 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. 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
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   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, 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 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:
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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 [6]. 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.

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
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   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.

   Table 1.  Qualitative comparison between centralized and distributed
   approaches.

       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.

       High scalability                  Limited scalability: #nodes,
       (hierarchical)                    links, circuits, messages
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       Planning (optimization)           Planning is a background
       harder to achieve                 centralized activity

       Easier future integration
       with other control plane
       layers

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 Dense WDM (DWDM) is defined in RFC1619/RFC2615 and is known as POS (Packet Over SDH/SONET), i.e., IP over PPP (in High-Level Data Link Control (HDLC)-like format) over SDH/SONET. SDH and SONET are actually efficient encapsulations for IP. For instance, with an average IP
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   datagram length of 350 octets, an IP over Gigabit Ethernet (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 the following: 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; and 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,
   where 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 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
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   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.

   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 an SDH or SONET Terminal Multiplexer (TM), Add-Drop Multiplexer (ADM), or cross-connect (i.e., a switch) be called an SDH/SONET LSR. An 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.
Top   ToC   RFC4257 - Page 15
   (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 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.

   (iv) Information intended only for end systems of the connection.
   Some of the payload type information 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 [7], [8]. 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.
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   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 [9], [10],
   [11].

   When applying routing to circuit switched networks, it is useful to
   compare and contrast this situation with the datagram routing case
   [12].  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, as compared to 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.  However, the current GMPLS routing extensions [9],
   [10], [11] do not make such a separation.

   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 an 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 [2], [3], 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) will actually be
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   switched within an 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

   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., down 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 ([4], [5]) 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, STS-Mc. The STS-Mc notation is shorthand for describing an STS-M signal whose SPEs have been concatenated. The multiplexing procedures for SDH and SONET are given in references [2] and [3], 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.
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   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.  That is, they support
   concatenation with 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 an SDH/SONET end-system service approved by
   the Committee T1 of ANSI [3] and the ITU-T [2].  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 an 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.
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      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. An example of this is a situation 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 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.
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      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 [7], [8]. 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. 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.
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      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
   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
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   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
   (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 [13], [15]
   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
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   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 other 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
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   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
   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
   [16] and to Chapter 12 of [17].

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 [6]. 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.
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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, 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.
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   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) [14].

   An 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
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   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 [15].

5.3. Signaling Elements

In the preceding sections, we defined the meaning of an 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 [15], each of which refers to a different network layer. 1. GENERALIZED_LABEL REQUEST (as in [4], [5]), 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 [15], 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.
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   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
   [15] 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 a 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 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 to specify particular
   capabilities that must be supported for the LSP, for example
   monitoring capabilities.  However, no standard profile is currently
   defined.

   3. UPSTREAM_LABEL for Bi-directional LSP's (as in [4], [5]).

   4. Local Link Selection, e.g., IF_ID_RSVP_HOP Object (as in [5]).
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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 the signaling elements involved when setting up a 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

The use of a control plane to provision connectivity through a SONET/SDH network shifts the security burden significantly from the management plane to the control plane. Before the introduction of a control plane, the communications that had to be secured were between the management stations (Element Management Systems or Network Management Systems) and each network element that participated in the network connection. After the introduction of the control plane, the only management plane communication that needs to be secured is that to the head-end (ingress) network node as the end-to-end service is requested. On the other hand, the control plane introduces a new requirement to secure signaling and routing communications between adjacent nodes in the network plane.
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   The security risk from impersonated management stations is
   significantly reduced by the use of a control plane.  In particular,
   where unsecure versions of network management protocols such as SNMP
   versions 1 and 2 were popular configuration tools in transport
   networks, the use of a control plane may significantly reduce the
   security risk of malicious and false assignment of network resources
   that could cause the interception or disruption of data traffic.

   On the other hand, the control plane may increase the number of
   security relationships that each network node must maintain.  Instead
   of a single security relationship with its management element, each
   network node must now maintain a security relationship with each of
   its signaling and routing neighbors in the control plane.

   There is a strong requirement for signaling and control plane
   exchanges to be secured, and any protocols proposed for this purpose
   must be capable of secure message exchanges.  This is already the
   case for the existing GMPLS routing and signaling protocols.

8. Acknowledgements

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.
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9. 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. [1] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol Label Switching Architecture", RFC 3031, January 2001. [2] G.707, Network Node Interface for the Synchronous Digital Hierarchy (SDH), International Telecommunication Union, March 1996. [3] ANSI T1.105-1995, Synchronous Optical Network (SONET) Basic Description including Multiplex Structure, Rates, and Formats, American National Standards Institute. [4] Berger, L., "Generalized Multi-Protocol Label Switching (GMPLS) Signaling Functional Description", RFC 3471, January 2003. [5] Berger, L., "Generalized Multi-Protocol Label Switching (GMPLS) Signaling Resource ReserVation Protocol-Traffic Engineering (RSVP-TE) Extensions", RFC 3473, January 2003. [6] Bernstein, G., Yates, J., Saha, D., "IP-Centric Control and Management of Optical Transport Networks," IEEE Communications Mag., Vol. 40, Issue 10, October 2000. [7] ANSI T1.105.01-1995, Synchronous Optical Network (SONET) Automatic Protection Switching, American National Standards Institute. [8] G.841, Types and Characteristics of SDH Network Protection Architectures, ITU-T, July 1995. [9] Kompella, K., Ed. and Y. Rekhter, Ed., "Routing Extensions in Support of Generalized Multi-Protocol Label Switching (GMPLS)", RFC 4202, October 2005. [10] Kompella, K., Ed. and Y. Rekhter, Ed., "OSPF Extensions in Support of Generalized Multi-Protocol Label Switching (GMPLS)", RFC 4203, October 2005. [11] Kompella, K., Ed. and Y. Rekhter, Ed., "Intermediate System to Intermediate System (IS-IS) Extensions in Support of Generalized Multi-Protocol Label Switching (GMPLS)", RFC 4205, October 2005.
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   [12] Bernstein, G., Sharma, V., Ong, L., "Inter-domain Optical
        Routing," OSA J. of Optical Networking, vol. 1, no. 2, pp.  80-
        92.

   [13] Kompella, K., Rekhter, Y. and L. Berger, "Link Bundling in MPLS
        Traffic Engineering (TE)", RFC 4201, October 2005.

   [14] Kompella, K. and Y. Rekhter, "Label Switched Paths (LSP)
        Hierarchy with Generalized Multi-Protocol Label Switching
        (GMPLS) Traffic Engineering (TE)", RFC 4206, October 2005.

   [15] Mannie, E. and D. Papadimitriou, "Generalized Multi-Protocol
        Label Switching (GMPLS) Extensions for Synchronous Optical
        Network (SONET) and Synchronous Digital Hierarchy (SDH)
        Control", RFC 3946, October 2004.

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

   [17] Bernstein, G., Rajagopalan, R., and Saha, D., "Optical Network
        Control: Protocols, Architectures, and Standards," Addison-
        Wesley, July 2003.
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10. 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 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 - Wavelength-Division Multiplexing
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Author's Addresses

Greg Bernstein Grotto Networking Phone: +1 510 573-2237 EMail: gregb@grotto-networking.com Eric Mannie Perceval Rue Tenbosch, 9 1000 Brussels Belgium Phone: +32-2-6409194 EMail: eric.mannie@perceval.net Vishal Sharma Metanoia, Inc. 888 Villa Street, Suite 500 Mountain View, CA 94041 Phone: +1 650 641 0082 Email: v.sharma@ieee.org Eric Gray Marconi Corporation, plc 900 Chelmsford Street Lowell, MA 01851 USA Phone: +1 978 275 7470 EMail: Eric.Gray@Marconi.com
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