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Network Working Group                                       G. BernsteinRequest for Comments: 4257                             Grotto NetworkingCategory: Informational                                        E. Mannie                                                                Perceval                                                               V. Sharma                                                          Metanoia, Inc.                                                                 E. Gray                                                Marconi Corporation, plc                                                           December 2005Framework for Generalized Multi-Protocol LabelSwitching (GMPLS)-based Control of Synchronous DigitalHierarchy/Synchronous Optical Networking (SDH/SONET) NetworksStatus 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.Bernstein, et al.            Informational                      [Page 1]

RFC 4257            GMPLS based Control of SDH/SONET       December 2005Table of Contents1. Introduction ....................................................31.1. MPLS Overview ..............................................31.2. SDH/SONET Overview .........................................5      1.3. The Current State of Circuit Establishment in           SDH/SONET Networks .........................................71.3.1. Administrative Tasks ................................81.3.2. Manual Operations ...................................81.3.3. Planning Tool Operation .............................81.3.4. Circuit Provisioning ................................81.4. Centralized Approach versus Distributed Approach ...........91.4.1. Topology Discovery and Resource Dissemination ......101.4.2. Path Computation (Route Determination) .............101.4.3. Connection Establishment (Provisioning) ............101.5. Why SDH/SONET Will Not Disappear Tomorrow .................122. GMPLS Applied to SDH/SONET .....................................132.1. Controlling the SDH/SONET Multiplex .......................132.2. SDH/SONET LSR and LSP Terminology .........................143. Decomposition of the GMPLS Circuit-Switching Problem Space .....144. GMPLS Routing for SDH/SONET ....................................154.1. Switching Capabilities ....................................164.1.1. Switching Granularity ..............................164.1.2. Signal Concatenation Capabilities ..................174.1.3. SDH/SONET Transparency .............................194.2. Protection ................................................204.3. Available Capacity Advertisement ..........................234.4. Path Computation ..........................................245. LSP Provisioning/Signaling for SDH/SONET .......................255.1. What Do We Label in SDH/SONET?  Frames or Circuits? .......255.2. Label Structure in SDH/SONET ..............................265.3. Signaling Elements ........................................276. Summary and Conclusions ........................................297. Security Considerations ........................................298. Acknowledgements ...............................................309. Informative References .........................................3110. Acronyms ......................................................33Bernstein, et al.            Informational                      [Page 2]

RFC 4257            GMPLS based Control of SDH/SONET       December 20051. 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.Bernstein, et al.            Informational                      [Page 3]

RFC 4257            GMPLS based Control of SDH/SONET       December 2005   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.Bernstein, et al.            Informational                      [Page 4]

RFC 4257            GMPLS based Control of SDH/SONET       December 20051.2.  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 parentBernstein, et al.            Informational                      [Page 5]

RFC 4257            GMPLS based Control of SDH/SONET       December 2005   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/T1Bernstein, et al.            Informational                      [Page 6]

RFC 4257            GMPLS based Control of SDH/SONET       December 2005               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.Bernstein, et al.            Informational                      [Page 7]

RFC 4257            GMPLS based Control of SDH/SONET       December 20051.3.1.  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 provisioningBernstein, et al.            Informational                      [Page 8]

RFC 4257            GMPLS based Control of SDH/SONET       December 2005   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:Bernstein, et al.            Informational                      [Page 9]

RFC 4257            GMPLS based Control of SDH/SONET       December 20051.4.1.  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.  ConnectionBernstein, et al.            Informational                     [Page 10]

RFC 4257            GMPLS based Control of SDH/SONET       December 2005   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, messagesBernstein, et al.            Informational                     [Page 11]

RFC 4257            GMPLS based Control of SDH/SONET       December 2005       Planning (optimization)           Planning is a background       harder to achieve                 centralized activity       Easier future integration       with other control plane       layers1.5.  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 inRFC1619/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 IPBernstein, et al.            Informational                     [Page 12]

RFC 4257            GMPLS based Control of SDH/SONET       December 2005   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/SONET2.1.  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 haveBernstein, et al.            Informational                     [Page 13]

RFC 4257            GMPLS based Control of SDH/SONET       December 2005   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.Bernstein, et al.            Informational                     [Page 14]

RFC 4257            GMPLS based Control of SDH/SONET       December 2005   (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.Bernstein, et al.            Informational                     [Page 15]

RFC 4257            GMPLS based Control of SDH/SONET       December 2005   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 beBernstein, et al.            Informational                     [Page 16]

RFC 4257            GMPLS based Control of SDH/SONET       December 2005   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 (seeSection 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.Bernstein, et al.            Informational                     [Page 17]

RFC 4257            GMPLS based Control of SDH/SONET       December 2005   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.Bernstein, et al.            Informational                     [Page 18]

RFC 4257            GMPLS based Control of SDH/SONET       December 2005      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/s4.1.3.  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.Bernstein, et al.            Informational                     [Page 19]

RFC 4257            GMPLS based Control of SDH/SONET       December 2005      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.Bernstein, et al.            Informational                     [Page 20]

RFC 4257            GMPLS based Control of SDH/SONET       December 2005      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 makeBernstein, et al.            Informational                     [Page 21]

RFC 4257            GMPLS based Control of SDH/SONET       December 2005   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 andBernstein, et al.            Informational                     [Page 22]

RFC 4257            GMPLS based Control of SDH/SONET       December 2005   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 particularlyBernstein, et al.            Informational                     [Page 23]

RFC 4257            GMPLS based Control of SDH/SONET       December 2005   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.Bernstein, et al.            Informational                     [Page 24]

RFC 4257            GMPLS based Control of SDH/SONET       December 20055.  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 inSection 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   (seeSection 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.Bernstein, et al.            Informational                     [Page 25]

RFC 4257            GMPLS based Control of SDH/SONET       December 2005   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 inSection 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 eachBernstein, et al.            Informational                     [Page 26]

RFC 4257            GMPLS based Control of SDH/SONET       December 2005   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.Bernstein, et al.            Informational                     [Page 27]

RFC 4257            GMPLS based Control of SDH/SONET       December 2005   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]).Bernstein, et al.            Informational                     [Page 28]

RFC 4257            GMPLS based Control of SDH/SONET       December 20056.  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.Bernstein, et al.            Informational                     [Page 29]

RFC 4257            GMPLS based Control of SDH/SONET       December 2005   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.Bernstein, et al.            Informational                     [Page 30]

RFC 4257            GMPLS based Control of SDH/SONET       December 20059.  Informative References   In the ITU references below, please seehttp://www.itu.int for   availability of ITU documents.  For ANSI references, please see the   Library available throughhttp://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.Bernstein, et al.            Informational                     [Page 31]

RFC 4257            GMPLS based Control of SDH/SONET       December 2005   [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.Bernstein, et al.            Informational                     [Page 32]

RFC 4257            GMPLS based Control of SDH/SONET       December 200510.  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 MultiplexingBernstein, et al.            Informational                     [Page 33]

RFC 4257            GMPLS based Control of SDH/SONET       December 2005Author'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.comBernstein, et al.            Informational                     [Page 34]

RFC 4257            GMPLS based Control of SDH/SONET       December 2005Full Copyright Statement   Copyright (C) The Internet Society (2005).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   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.Intellectual Property   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 inBCP 78 andBCP 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 athttp://www.ietf.org/ipr.   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.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Bernstein, et al.            Informational                     [Page 35]