Network Working Group           Peter Ashwood-Smith (Nortel Networks)
Internet Draft                        Daniel Awduche (Movaz Networks)
Expiration date: August 2001         Ayan Banerjee (Calient Networks)
                                  Debashis Basak (Accelight Networks)
                                          Lou Berger (Movaz Networks)
                                   Greg Bernstein (Ciena Corporation)
                                        John Drake (Calient Networks)
                                        Yanhe Fan (Axiowave Networks)
                                          Don Fedyk (Nortel Networks)
                                               Gert Grammel (Alcatel)
                                  Kireeti Kompella (Juniper Networks)
                                     Alan Kullberg (NetPlane Systems)
                                  Jonathan P. Lang (Calient Networks)
                                            Fong Liaw (Zaffire, Inc.)
                                      Dimitri Papadimitriou (Alcatel)
                                 Dimitrios Pendarakis (Tellium, Inc.)
                                     Bala Rajagopalan (Tellium, Inc.)
                                     Yakov Rekhter (Juniper Networks)
                                        Debanjan Saha (Tellium, Inc.)
                                        Hal Sandick (Nortel Networks)
                                     Vishal Sharma (Jasmine Networks)
                                       George Swallow (Cisco Systems)
                                           Z. Bo Tang (Tellium, Inc.)
                                              John Yu (Zaffire, Inc.)
                                           Alex Zinin (Cisco Systems)

                                         Eric Mannie (Ebone) - Editor

                                                        February 2001

    Generalized Multi-Protocol Label Switching (GMPLS) Architecture


Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026 [1].

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that
   other groups may also distribute working documents as Internet-

   Internet-Drafts are draft documents valid for a maximum of six
   months and may be updated, replaced, or obsoleted by other documents
   at any time. It is inappropriate to use Internet- Drafts as
   reference material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at

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   The list of Internet-Draft Shadow Directories can be accessed at

1. Abstract

   Future data and transmission networks will consist of elements such
   as routers, switches, DWDM systems, Add-Drop Multiplexors (ADMs),
   photonic cross-connects (PXCs) or optical cross-connects (OXCs), etc
   that will use Generalized MPLS (GMPLS) to dynamically provision
   resources and to provide network survivability using protection and
   restoration techniques.

   This document describes the architecture of GMPLS. GMPLS extends
   MPLS to encompass time-division (e.g. SDH/SONET, PDH, G.709),
   wavelength (lambdas) and spatial switching (e.g. incoming port or
   fiber to outgoing port or fiber). The main focus of GMPLS is on the
   control plane of these various layers since each of them can use
   totally different data or forwarding planes. The intention is to
   cover both the signaling and the routing part of that control plane.

2. Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   this document are to be interpreted as described in RFC-2119 [2].

3. Introduction

   The architecture presented in this document covers the main building
   blocks needed to build a consistent control plane for multiple
   switching layers. It does not restrict the way that these layers
   work together. Different models can be applied: e.g. overlay,
   augmented or integrated. Moreover, each pair of contiguous layer may
   work jointly in a different way. It results that a number of
   combinations are possible, at the discretion of manufacturers and

   This document generalizes the MPLS architecture [MPLS-ARCH], and in
   some cases can differ slightly from that architecture since non
   packet-based forwarding planes are now considered. It is not the
   intention of this document to describe concepts already described in
   the current MPLS architecture. The goal is to describe specific
   concepts of GMPLS.

   However, some of the concepts described hereafter are not described
   in the current MPLS architecture and are applicable to both MPLS and
   GMPLS, i.e. link bundling, unnumbered links and LSP hierarchy. Since
   they raised from the GMPLS needs and since they are of paramount
   importance for an operational GMPLS network, they will be introduced

   The following sections will first introduce GMPLS. Then the specific
   GMPLS building blocks will be presented and we will explain how they

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   can be combined together. Details about these separate building
   blocks can be found in the corresponding documents.

3.1. Acronyms & abbreviations

   ABR          Area Border Router
   AS           Autonomous System
   ASBR         Autonomous System Boundary Router
   BGP          Border Gateway Protocol
   CR-LDP       Constraint-based Routing LDP
   CSPF         Constraint-based Shortest Path First
   DWDM         Dense Wavelength Division Multiplexing
   FA           Forwarding Adjacency
   GMPLS        Generalized Multi-Protocol Label Switching
   IGP          Interior Gateway Protocol
   LDP          Label Distribution Protocol
   LMP          Link Management Protocol
   LSA          Link State Advertisement
   LSR          Label Switching Router
   LSP          Label Switched Path
   MIB          Management Information Base
   MPLS         Multi-Protocol Label Switching
   RSVP         ReSource reserVation Protocol
   SDH          Synchronous Digital Hierarchy
   STM(-N)      Synchronous Transport Module (-N)
   STS(-N)      Synchronous Transport Signal-Level N (SONET)
   TE           Traffic Engineering

3.2. Multiple Types of Switching and Forwarding Hierarchies

   Generalized MPLS differs from traditional MPLS in that it supports
   multiple types of switching, i.e. the addition of support for TDM,
   lambda, and fiber (port) switching. The support for the additional
   types of switching has driven generalized MPLS to extend certain
   base functions of traditional MPLS and, in some cases, to add
   functionality.  These changes and additions impact basic LSP
   properties, how labels are requested and communicated, the
   unidirectional nature of LSPs, how errors are propagated, and
   information provided for synchronizing the ingress and egress LSRs.

   The MPLS architecture [MPLS-ARCH] was defined to support the
   forwarding of data based on a label. In this architecture, Label
   Switching Routers (LSRs) were assumed to have a forwarding plane
   that is capable of (a) recognizing either packet or cell boundaries,
   and (b) being able to process either packet headers (for LSRs
   capable of recognizing packet boundaries) or cell headers (for LSRs
   capable of recognizing cell boundaries).

   This original architecture is here extended to include LSRs whose
   forwarding plane recognizes neither packet, nor cell boundaries, and
   therefore, can't forward data based on the information carried in
   either packet or cell headers. Specifically, such LSRs include
   devices where the forwarding decision is based on time slots,
   wavelengths, or physical ports. So, the new set of LSRs, or more

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   precisely interfaces on these LSRs, can be subdivided into the
   following classes:

      1. Packet-Switch Capable (PSC) interfaces:

   Interfaces that recognize packet/cell boundaries and can forward
   data based on the content of the packet/cell header. Examples
   include interfaces on routers that forward data based on the content
   of the "shim" header, interfaces on ATM-LSRs that forward data based
   on the ATM VPI/VCI.

      2. Time-Division Multiplex Capable (TDM) interfaces:

   Interfaces that forward data based on the data's time slot in a
   repeating cycle.  An example of such an interface is an interface on
   a SDH/SONET Cross-Connect (XC), Terminal Multiplexer (TM) or Add-
   Drop Multiplexer (ADM). Other examples are an interface implementing
   G.709 (the digital wrapper), or a PDH interface.

      3. Lambda Switch Capable (LSC) interfaces:

   Interfaces that forward data based on the wavelength on which the
   data is received.  An example of such an interface is an interface
   on an Optical Cross-Connect that can operate at the level of an
   individual wavelength. Another example is an interface that can
   operate at the level of a group of wavelengths, i.e. a waveband.

      4. Fiber-Switch Capable (FSC) interfaces:

   Interfaces that forward data based on a position of the data in the
   real world physical spaces.  An example of such an interface is an
   interface on a photonic Cross-Connect that can operate at the level
   of a single (or multiple) fibers.

   A circuit can be established only between, or through, interfaces of
   the same type. Depending on the particular technology being used for
   each interface, different circuit names can be used, e.g. SDH
   circuit, optical trail, light path etc. In the context of GMPLS, all
   these circuits are referenced by a common name: Label Switched Path

   The concept of nested LSP (LSP within LSP) already available in the
   traditional MPLS allows here to build a forwarding hierarchy, i.e. a
   hierarchy of LSPs. This hierarchy of LSPs can occur on the same
   interface, or between different interfaces.

   It can occur on the same interface if this interface is capable of
   multiplexing several LSPs from the same technology (layer), e.g. a
   lower order SDH/SONET LSP (VC-12) nested in a higher order SDH/SONET
   LSP (VC-4). Several levels of signal (LSP) nesting are defined in
   the SDH/SONET multiplexing hierarchy.

   The nesting can also occur between interfaces. At the top of the
   hierarchy are FSC interfaces, followed by LSC interfaces, followed

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   by TDM interfaces, followed by PSC interfaces. This way, an LSP that
   starts and ends on a PSC interface can be nested (together with
   other LSPs) into an LSP that starts and ends on a TDM interface.
   This LSP, in turn, can be nested (together with other LSPs) into an
   LSP that starts and ends on an LSC interface, which in turn can be
   nested (together with other LSPs) into an LSP that starts and ends
   on a FSC interface.

3.3. Extension of the MPLS Control Plane

   The establishment of LSPs that span only Packet Switch Capable (PSC)
   interfaces is defined for the original MPLS and/or MPLS-TE control
   planes. GMPLS extends these control planes to support each of the
   four classes of interfaces (i.e. layers) defined in the previous

   Note that the GMPLS control plane supports as well an overlay model,
   an augmented model or an integrated model. The benefits of using an
   augmented or integrated model will have to be clarified and
   evaluated in the future. In the mean time, GMPLS is very suitable
   for controlling each layer completely independently. This elegant
   approach will facilitate the future deployment of other models.

   The GMPLS control plane is made of several building blocks that will
   be described in more details in the following sections. These
   building blocks are indeed well-known IETF signaling and routing
   protocols that have been extended and/or modified. They use IPv4
   and/or IPv6 addresses. Only one new specialized protocol was
   required to support the operations of GMPLS, a signaling protocol
   for link management [LMP].

   GMPLS is indeed based on the Traffic Engineering (TE) extensions to
   MPLS, a.k.a. MPLS-TE. This is because most of the technologies that
   can be used below the PSC level require some traffic engineering.
   The placement of LSPs at these levels needs in general to take
   several constraints into consideration (such as bandwidth,
   protection capability, etc) and to bypass the legacy Shortest-Path
   First (SPF) algorithm. Note however that this is not mandatory and
   that in some cases an SPF routing could be applied.

   In order for such a constrained-based SPF routing of LSPs to happen,
   the nodes performing LSP establishment need more information about
   the links in the network than standard intra-domain routing
   protocols provide. These TE attributes are distributed using the
   transport mechanisms already available in IGPs and are taken into
   consideration by the LSP routing algorithm. Optimization of the LSP
   trajectories may also require some external simulations using
   heuristics that serve as input for the actual path calculation and
   LSP establishment process.

   Extensions to traditional routing protocols and algorithms are
   needed to uniformly encode and carry TE link information, and
   explicit routes (e.g. source routes) are required in the signaling.
   In addition, the signaling must now be capable of transporting the

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   required circuit (LSP) parameters such as the bandwidth, the type of
   signal, the desired protection, the position in a particular
   multiplex, etc. Most of these extensions have already been defined
   for PSC (IP) traffic engineering with MPLS. GMPLS mainly adds
   additional extensions for TDM, LSC and FSC traffic engineering, by
   staying as generic as possible. Only a very few elements are
   technology specific.

   Thus, GMPLS extends the two signaling protocols defined for MPLS-TE
   signaling, i.e. RSVP-TE and CR-LDP. However, GMPLS does not specify
   which one of these two signaling protocols must be used. It is the
   role of manufacturers and operators to evaluate the two possible
   solutions for their own interest.

   Since GMPLS is based on RSVP-TE and CR-LDP, it mandates a
   downstream-on-demand label allocation and distribution, with an
   ingress initiated ordered control. A liberal label retention is
   normally used, but a conservative label retention mode could be
   used. There is no restriction on the label allocation strategy, it
   can be request driven (obvious for circuit switching technologies),
   traffic/data driven, or even topology driven. There is no
   restriction neither on the route selection, explicit routing is
   normally used (strict or loose) but an hop-by-hop routing could be
   used as well.

   GMPLS extends also two traditional intra-domain routing protocols
   already extended for TE, i.e. OSPF-TE and IS-IS-TE. However, if
   explicit routing is used, the routing algorithms used by these
   protocols don't need to be standardized anymore since they are now
   used to compute explicit routes only, and are thus not used anymore
   for hop-by-hop routing. Extensions for inter-domain routing (e.g.
   BGP) are for further study.

   The use of technologies like DWDM (Dense Wavelength Division
   Multiplexing) implies that we can now have a very large number of
   parallel links between two directly adjacent nodes (hundreds of
   wavelengths, or even thousands of wavelengths if multiple fibers are
   used). Such a large number of links was not originally considered
   for an IP or MPLS control plane. Some slight adaptations of that
   control plane are thus required if we want to reuse it in the GMPLS

   For instance, the traditional IP routing model assumes the
   establishment of a routing adjacency over each link connecting two
   adjacent nodes. Having such a large number of adjacencies is not
   scalable at all. Each node needs to maintain each of its adjacencies
   one by one, and link state routing information must be flooded in
   the topology for each link.

   To solve this issue the concept of bundling was introduced.
   Moreover, the manual configuration and control of these links, even
   if they are unnumbered, becomes totally impractical. The Link
   Management Protocol (LMP) was specified to solve these problems.

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   LMP runs between data-plane adjacent nodes and is used for both link
   provisioning and fault isolation. LMP was defined in the context of
   GMPLS, but was specified independently of the GMPLS signaling
   specification. It results that LMP can be reused in other contexts,
   with non-GMPLS signaling protocols as well.

   A unique feature of LMP is that it is able to isolate faults in both
   opaque and transparent networks, independent of the encoding scheme
   used for the data. LMP will be used to verify connectivity between
   nodes; and isolate link, fiber, or channel failures within the

   The MPLS signaling and routing protocols require at least one bi-
   directional control channel to communicate even if two adjacent node
   are connected by unidirectional links. Several control channels can
   be used. LMP can be used to establish, maintain and manage these
   control channels.

   GMPLS does not specify how these control channels must be
   implemented, but GMPLS requires IP to transport the signaling and
   routing protocols over them. Control channels can be either in-band
   or out-of-band, and several solutions can be used to carry IP. Note
   also that one type of LMP message is used in-band in the data plane
   and may not be transported over IP, but this is a particular case,
   needed to verify connectivity in the data plane.

3.4. Key Differences Between MPLS-TE and GMPLS

   Some key differences between MPLS-TE and GMPLS are highlighted in
   the following. Some of them are key advantages of GMPLS to control
   non-PSC layers.

   - In MPLS-TE, links traversed by an LSP can include an intermix of
   links with heterogeneous label encoding (e.g. links between routers,
   links between routers and ATM-LSRs, and links between ATM-LSRs.
   GMPLS extends this by including links where the label is encoded as
   a time slot, or a wavelength, or a position in the real world
   physical space.

   - In MPLS-TE, an LSP that carries IP has to start and end on a
   router. GMPLS extends this by requiring an LSP to start and end on
   similar type of LSRs.

   - The type of a payload that can be carried in GMPLS by an LSP is
   extended to allow such payloads as SONET/SDH, 1 or 10Gb Ethernet,

   - For non-PSC interfaces, bandwidth allocation for an LSP can be
   performed only in discrete units.

   - It is expected to have (much) fewer labels on non-PSC links than
   on PSC links.

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   - The use of Forwarding Adjacencies (FA), provides a mechanism that
   may improve bandwidth utilization, when bandwidth allocation can be
   performed only in discrete units, as well as a mechanism to
   aggregate forwarding state, thus allowing the number of required
   labels to be reduced

   - GMPLS allows for a label to be suggested by an upstream node to
   reduce the setup latency. This suggestion may be overridden by a
   downstream node but, in some cases, at the cost of higher LSP setup

   - GMPLS extends on the notion of restricting the range of labels
   that may be selected by a downstream node. In GMPLS, an ingress or
   other upstream node may restrict the labels that may be used by an
   LSP along either a single hop or along the whole LSP path.

   - While traditional TE-based (and even LDP-based) LSPs are
   unidirectional, GMPLS supports the establishment of bi-directional

   - GMPLS supports the termination of an LSP on a specific egress
   port, i.e. the port selection at the destination side.

   - GMPLS with RSVP-TE supports an RSVP specific mechanism for rapid
   failure notification.

4. Routing and addressing model

   GMPLS is based on the IP routing and addressing models. This assumes
   that IPv4 and/or IPv6 addresses are used to identify interfaces and
   that traditional (distributed) IP routing protocols are also reused.
   Indeed, the discovery of the topology and the resource state of all
   links in a routing domain is achieved via these routing protocols.

   Since control and data planes are de-coupled in GMPLS, one cannot do
   anymore the assumption that control-plane neighbors (i.e. IGP-learnt
   neighbors) are data-plane neighbors, hence mechanisms like LMP are
   needed to associate TE links with neighboring nodes.

   IP addresses are not used only to identify interfaces of IP hosts
   and routers, but more generally to identify any PSC and non-PSC
   interfaces. Similarly IP routing protocols are not used only to find
   routes for IP datagrams but also to find routes for non-PSC circuits
   by using a CSPF algorithm instead of legacy SPF.

   However, some additional mechanisms are needed to increase the
   scalability of these models and to deal with specific traffic
   engineering requirements of non-PSC layers. These mechanisms will be
   introduced in the following.

   Re-using existing IP routing protocols allows for non-PSC layers to
   take advantages of all the valuable developments that toke place
   since years for IP routing, in particular in the context of link-
   state routing and policy routing.

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   Each particular non-PSC layer can be seen as a set of Autonomous
   Systems (ASs) interconnected in an arbitrary way. Similarly to the
   traditional IP routing, each AS is managed by a single
   administrative authority. For instance, an AS can be an SDH/SONET
   network operated by a given carrier. The set of interconnected ASs
   being an SDH/SONET Internetwork.

   Exchange of routing information between ASs can be done via an
   inter-domain routing protocol like BGP-4. There is obviously a huge
   value of re-using well-known policy routing facilities provided by
   BGP in a non-PSC context. Extensions for BGP traffic engineering in
   the context of non-PSC layers are for further study.

   Each AS can be subdivided in different routing domains, and each can
   run a different intra-domain routing protocol. In turn, each
   routing-domain can be divided in areas.

   A routing domain is made of GMPLS nodes. These nodes can be either
   edge nodes (i.e. hosts, ingress LSRs or egress LSRs), or internal
   LSRs. An example of non-PSC host is an SDH/SONET Terminal
   Multiplexer (TM). Another example, is an SDH/SONET interface card
   within an IP router or ATM switch.

   Note that traffic engineering in the intra-domain requires the use
   of link-state routing protocols like OSPF or IS-IS.

   GMPLS defines extensions to these protocols. These extensions are
   needed to disseminate specific non-PSC static and dynamic
   characteristics related to nodes and links. The current focus is on
   intra-area traffic engineering. However, inter-area traffic
   engineering is also under investigation.

4.1 Addressing of PSC and non-PSC layers

   The fact that IPv4 and/or IPv6 addresses are used doesn't imply at
   all that they should be allocated in the same addressing space than
   public IPv4 and/or IPv6 addresses used for the Internet. Each layer
   could have a different addressing authority responsible for address
   allocation and re-using the full addressing space, completely

   Private IP addresses can be used if they don't require to be
   exchanged with any other operator, public IP addresses are otherwise
   required. Of course, if an integrated model is used, two layers
   could share the same addressing space.

   Note that there is a benefit of using public IPv4 and/or IPv6
   Internet addresses for non-PSC layers if an integrated model with
   the IP layer is foreseen.

   If we consider the scalability enhancements proposed in the next
   section, the IPv4 (32 bits) and the IPv6 (128 bits) addressing
   spaces are both more than sufficient to accommodate any non-PSC

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   layer. We can reasonably expect to have much less non-PSC devices
   (e.g. SDH/SONET nodes) than we have today IP hosts and routers.

   Other kinds of addressing schemes (e.g. NSAP) are not considered
   here since this would imply a modification of the already existing
   signaling and routing protocols that uses IPv4 and/or IPv6
   addresses. This would be incompatible to our objectives of re-using
   existing IP protocols.

4.2 GMPLS scalability enhancements

   Non-PSC layers introduce new constraints on the IP addressing and
   routing models since several hundreds of parallel physical links
   (e.g. wavelengths) can now connect two nodes. Most of the carriers
   already have today several tenths of wavelengths per fiber between
   two nodes. New generation of DWDM systems will allow several
   hundreds of wavelengths.

   It becomes rather impractical to associate an IP address to each end
   of each physical link, to represent each link as a separate routing
   adjacency, and to advertise link states for each of these links. For
   that purpose, GMPLS enhances the MPLS routing and addressing models
   to increase their scalability.

   Two optional mechanisms can be used to increase the scalability of
   the addressing and the routing: unnumbered links and link bundling.
   These two mechanisms can also be combined. They require extensions
   to signaling (RSVP-TE and CR-LDP) and routing (OSPF-TE and IS-IS-TE)

4.3 Extensions to IP TE routing protocols

   Traditionally, a TE link is advertised as an adjunct to a "regular"
   OSPF or IS-IS link, i.e., an adjacency is brought up on the link,
   and when the link is up, both the regular IGP properties of the link
   (basically, the SPF metric) and the TE properties of the link are
   then advertised.

   However, GMPLS challenges this notion in three ways:

   - first, links that are non-PSC may yet have TE properties; however,
   an OSPF adjacency cannot be brought up directly on such links.

   - second, an LSP can be advertised as a point-to-point TE link in
   the routing protocol, i.e. as a Forwarding Adjacency (FA); thus, an
   advertised TE link need no longer be between two OSPF neighbors.
   Forwarding Adjacencies (FA) are further described in a separate

   - third, a number of links may be advertised as a single TE link
   (e.g. for improved scalability), so again, there is no longer a one-
   to-one association of a regular adjacency and a TE link.

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   Thus we have a more general notion of a TE link. A TE link is a
   logical link that has TE properties, some of which may be configured
   on the advertising LSR, others which may be obtained from other LSRs
   by means of some protocol, and yet others which may be deduced from
   the component(s) of the TE link.

   An important TE property of a TE link is related to the bandwidth
   accounting for that link. GMPLS will define different accounting
   rules for different non-PSC layers. Generic bandwidth attributes are
   however defined by the TE routing extensions and by GMPLS, such as
   the unreserved bandwidth, the maximum reservable bandwidth, the
   maximum LSP bandwidth.

   It is expected in a dynamic environment to have frequent changes of
   bandwidth accounting information. A flexible policy for triggering
   link state updates based on bandwidth thresholds and link dampening
   mechanism can be implemented.

   TE properties associated with a link should also capture protection
   and restoration related characteristics. For instance, shared
   protection can be elegantly combined with bundling. Protection and
   restoration are mainly generic mechanisms also applicable to MPLS.
   It is expected that they will first be developed for MPLS and later
   on generalized to GMPLS.

   A TE link between a pair of LSRs doesn't imply the existence of an
   IGP adjacency between these LSRs. A TE link must also have some
   means by which the advertising LSR can know of its liveness (e.g. by
   using LMP hellos). When an LSR knows that a TE link is up, and can
   determine the TE link's TE properties, the LSR may then advertise
   that link to its GMPLS enhanced OSPF or IS-IS neighbors using the TE
   objects/TLVs. We call the interfaces over which GMPLS enhanced OSPF
   or ISIS adjacencies are established "control channels".

5. Unnumbered links

   Unnumbered links (or interfaces) are links(or interfaces) that do
   not have IP addresses. Using such links involves two capabilities:
   (a) the ability to carry (TE) information about unnumbered links in
   IGP TE extensions (ISIS or OSPF), and (b) the ability to specify
   unnumbered links in MPLS TE signaling.

   The former is covered in ISIS-TE and OSPF-TE. The later requires
   extensions to RSVP-TE and CR-LDP since MPLS-TE signaling doesn't
   provide support for unnumbered links. GMPLS defines simple
   extensions to indicate an unnumbered link in the Explicit Route and
   Record Route Objects/TLVs of these protocols, using a new Interface
   ID object/TLV.

   Since unnumbered links are not identified by an IP address, then for
   the purpose of MPLS TE each end need some other identifier, local to
   the LSR to which the link belongs. Note that links are directed,
   i.e., a link l is from some LSR A to some other LSR B. LSR A chooses
   the interface identifier for link l, we call this the "outgoing

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   interface identifier from LSR A's point of view". If there is a
   reverse link from LSR B to LSR A, B chooses the outgoing interface
   identifier for the reverse link. There is no a priori relationship
   between the two interface identifiers. Both ends must also agree on
   each of these identifiers.

5.1 Unnumbered Forwarding Adjacencies

   If an LSR that originates an LSP advertises this LSP as an
   unnumbered FA in IS-IS or OSPF, the LSR must allocate an Interface
   ID to that FA. If the LSP is bi-directional, the tail-end LSR
   advertises the reverse LSP as an unnumbered FA, the tail-end LSR
   must allocate an Interface ID to the reverse FA.

   Signaling has been enhanced to carry the Interface IDs. When an LSP
   is created which will be advertised as an FA, the head-end LSR
   assigns an Interface ID and includes it in the signaling request.
   The tail-end LSR responds by assigning and including an Interface ID
   in the signaling response.

6. Link bundling

   When a pair of LSRs is connected by multiple links, it is possible
   to advertise several (or all) of these links as a single link into
   OSPF and/or IS-IS. This process is called link bundling, or just
   bundling. The resulting logical link is called a bundled link as its
   physical links are called component links.

   The purpose of link bundling is to improve routing scalability by
   reducing the amount of information that has to be handled by OSPF
   and/or IS-IS. This reduction is accomplished by performing
   information aggregation/abstraction. As with any other information
   aggregation/abstraction, this results in losing some of the
   information. To limit the amount of losses one need to restrict the
   type of the information that can be aggregated/abstracted.

6.1 Restrictions on bundling

   The following restrictions are required for GMPLS. All component
   links in a bundle must begin and end on the same pair of LSRs, and
   share some common characteristics: they must have the same type
   (e.g. point-to-point), the same TE metric, the same set of resource
   classes, and the same multiplexing capabilities. An FA may be a
   component link; in fact, a bundle can consist of a mix of point-to-
   point links and FAs.

6.2 Routing considerations for bundling

   A bundled link is just another kind of TE link such as those defined
   by OSPF-TE or IS-IS-TE. The liveness of the bundled link is
   determined by the liveness of each of the component links within the
   bundled link. The liveness of a component link can be determined by
   any of several means: IS-IS or OSPF hellos over the component link,

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   or RSVP Hello, or LMP hellos, or from layer 1 or layer 2

   Once a bundled link is determined to be alive, it can be advertised
   as a TE link and the TE information can be flooded.  If IS-IS/OSPF
   hellos are run over the component links, IS-IS/OSPF flooding can be
   restricted to just one of the component links.

   Note that advertising a (bundled) TE link between a pair of LSRs
   doesn't imply that there is an IGP adjacency between these LSRs that
   is associated with just that link. In fact, in certain cases a TE
   link between a pair of LSRs could be advertised even if there is no
   IGP adjacency at all between the LSR (e.g. when the TE link is an

   Bandwidth accounting must be clearly defined since an abstraction is
   done. Bandwidth information is an important part of a bundle
   advertisement. Some attributes can be sums of component
   characteristics such as the unreserved bandwidth and the maximum
   reservable bandwidth. A GMPLS node with bundled links must apply
   admission control on a per-component link basis.

6.3 Signaling considerations

   Typically, an LSP's explicit route (contained in an ERO) will choose
   the bundled link to be used for the LSP, but not the component
   link(s), since information about the bundled link is flooded, but
   information about the component links is kept local to the LSR.

   The choice of the component link to use is always made by an
   upstream node. If the LSP is bidirectional, the upstream node
   chooses a component link in each direction.

   Three mechanisms for indicating this choice to the downstream node
   are possible.

   - Mechanism 1: Implicit Indication

   This mechanism requires that each component link has a dedicated
   signaling channel. The upstream node tells the receiver which
   component link to use by sending the message over the chosen
   component link's dedicated signaling channel.

   - Mechanism 2: Explicit Indication by IP Address

   This mechanism requires that each component link has a unique remote
   IP address. The upstream node can either send messages addressed to
   the remote IP address for the component link or encapsulate messages
   in an IP header whose destination address is the remote IP address.
   This mechanism does not require each component link to have its own
   control channel. In fact, it doesn't even require the whole
   (bundled) link to have its own control channel.

   - Mechanism 3: Explicit Indication by Component Interface ID

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   With this mechanism, each component link in unnumbered and is
   assigned a unique Interface Identifier. These identifiers are
   exchanged by the two LSRs at each end of the bundled link. The
   choice of a component link is indicated by an upstream node by
   including the corresponding identifier in signaling messages.
   Discovering Interface Identifiers at bootstrap may be accomplished
   by configuration, by means of a protocol such as LMP (preferred
   solution), or by means of RSVP/CR-LDP (especially in the case where
   a component link is a Forwarding Adjacency). New objects are needed
   to indicate Interface Identifiers in signaling, GMPLS defines one
   Upstream Interface ID object/TLV and one Downstream Interface ID

6.4 Unnumbered Bundled Link

   A bundled link may itself be numbered or unnumbered independent of
   whether the component links are numbered or not. This affects how
   the bundled link is advertised in IS-IS/OSPF, and the format of LSP
   EROs that traverse the bundled link. Furthermore, unnumbered
   Interface Identifiers for all unnumbered outgoing links of a given
   LSR (whether component links, Forwarding Adjacencies or bundled
   links) MUST be unique in the context of that LSR.

7. UNI and NNI

   The interface between an edge GMPLS node and a GMPLS LSR on the
   network side may be referred to as a User to Network Interface
   (UNI), while the interface between two network side LSRs may be
   referred to as a Network to Network Interface (NNI).

   GMPLS does not specify separately a UNI and an NNI. Edge nodes are
   connected to LSRs on the network side, and these LSRs are in turn
   connected between them. Of course, the behavior of an edge node is
   not exactly the same as the behavior of an LSR on the network side.
   Note also, that an edge node may run a routing protocol, however it
   is expected that in most of the cases it will not (see also section
   7.2 and the section about signaling with an explicit route).

   Conceptually, a difference between UNI and NNI make sense either if
   both interface uses completely different protocols, or if they use
   the same protocols but with some outstanding differences. In the
   first case, separate protocols are often defined successively, with
   more or less success.

   The GMPLS approach consisted in building a consistent model from day
   one, considering both the UNI and NNI interfaces at the same time.
   For that purpose a very few specific UNI particularities have been
   ignored in a first time. GMPLS is being enhanced to support such
   particularities at the UNI by some other standardization bodies,
   like the OIF.

7.1 OIF UNI versus GMPLS

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   The current OIF UNI specification [OIF-UNI] defines an interface
   between a client SDH/SONET equipment and an SDH/SONET network, each
   belonging to a distinct administrative authority. The OIF UNI
   defines additional mechanisms on the top of GMPLS for the UNI

   For instance, the OIF service discovery procedure is a precursor to
   obtaining UNI services. Service discovery allows a client to
   determine the static parameters of the interconnection with the
   network, including the UNI signaling protocols, the transparency
   levels as well as the protection level supported by the network.

   Moreover, the only additional mechanism covered by the OIF UNI is
   the address allocation process. The corresponding mechanism is
   tightly related to the link bundle mechanism as described in LMP
   using LinkSummary (and include an address allocation request) and
   LinkSummaryAck (and include an address allocation response)

   Since the current OIF UNI interface does not cover photonic
   networks, G.709 Digital Wrapper, etc, it is a sub-set of the GMPLS

7.2 Routing at the UNI

   This section discusses the selection of an explicit route by an edge
   node. The selection of the first LSR by an edge node connected to
   multiple LSRs is part of that problem.

   An edge node (host or LSR) can participate more or less deeply in
   the GMPLS routing. Four different routing models can be supported at
   the UNI: configuration based, partial peering, silent listening and
   full peering.

   - Configuration based: this routing model requires the manual or
   automatic configuration of an edge node with a list of neighbor LSRs
   sorted by preference order. Automatic configuration can be achieved
   using DHCP for instance. No routing information is exchanged at the
   UNI, except maybe the ordered list of LSRs. The only routing
   information used by the edge node is that list. The edge node sends
   by default an LSP request to the preferred LSR. ICMP redirects could
   be send by this LSR to redirect some LSP requests to another LSR
   connected to the edge node. GMPLS does not preclude that model.

   - Partial peering: limited routing information (mainly reachability)
   can be exchanged across the UNI using some extensions in the
   signaling plane. The reachability information exchanged at the UNI
   may be used to initiate edge node specific routing decision over the
   network. GMPLS does not have any capability to support this model

   - Silent listening: the edge node can silently listen to routing
   protocols and take routing decisions based on the information
   obtained. An edge node receives the full routing information,
   including traffic engineering extensions. One LSR should forward

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   transparently all routing pdus to the edge node. An edge node can
   now compute a complete explicit route taking into consideration all
   the end-to-end routing information. GMPLS does not preclude this

   - Full peering: In addition to silent listening, the edge node
   participates within the routing, establish adjacencies with its
   neighbors and advertises LSAs. This is useful only if there are
   benefits for edge nodes to advertise themselves traffic engineering
   information. GMPLS does not preclude this model.

8. Link Management

   In the context of GMPLS, a pair of nodes (e.g., a photonic switch)
   may be connected by tenths of fibers, and each fiber may be used to
   transmit hundreds of wavelengths if DWDM is used. Furthermore,
   multiple fibers and/or multiple wavelengths may be combined into one
   or more bundled links as explained previously.

   Dealing with hundreds or thousands of individual or bundled links
   between two nodes requires the help of some signaling tools. In
   addition, at least one control channel must be established and
   maintained between a node pair, possibly, using some of these links.

   Link management is a collection of useful functionality between
   adjacent nodes that provide different local services such as control
   channel management, link connectivity verification, link property
   correlation, and fault isolation. A Link Management Protocol (LMP)
   has been defined to fulfill these operations. LMP was initiated in
   the context of GMPLS but is indeed a generic toolbox that can be
   also used in other contexts.

   Control channel management and link connectivity verification are
   mandatory mechanisms of LMP. Link property correlation and fault
   isolation are optional.

8.1 Control channel

   Control plane communications between neighboring nodes need a bi-
   directional control channel. The control channel can be used to
   exchange MPLS control-plane information such as signaling, routing
   and management information.

   In GMPLS, the control channel(s) between two adjacent nodes is no
   longer required to use the same physical medium as the data-bearing
   links between those nodes. For example, a control channel could use
   a separate wavelength or fiber, an Ethernet link, or an IP tunnel
   through a separate management network. A consequence of allowing the
   control channel(s) between two nodes to be physically diverse from
   the associated data-bearing links is that the health of a control
   channel does not necessarily correlate to the health of the data-
   bearing links, and vice-versa. Therefore, new mechanisms must be
   developed to manage links, both in terms of link provisioning and
   fault isolation.

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   It is essential that a control channel is always available, and in
   the event of a control channel failure, an alternate (or backup)
   control channel must be made available to reestablish communication
   with the neighboring node.

   If a primary control channel cannot be established, then an
   alternate control channel should be tried. Of course, alternate
   control channels should be pre-configured, however, coordinating the
   switchover of the control channel to an alternate channel is still
   an important issue.

   Specifically, if the control channel fails but the node is still
   operational (i.e., the data-bearing links are still passing user
   data), then both the local and remote nodes should switch to an
   alternate control channel. If the bi-directional control channel is
   implemented using two separate unidirectional channels, and only one
   direction of the control channel has failed, both the local and
   remote nodes need to understand that the control channel has failed
   so that they can coordinate a switchover. LMP provides a graceful
   switchover from one control channel to the other.

8.2 Control channel management

   Once a control channel is configured between two neighboring nodes,
   a Hello protocol will be used to establish and maintain connectivity
   between the nodes and to detect failures. The Hello protocol of LMP
   is intended to be a lightweight keep-alive mechanism that will react
   to control channel failures rapidly so that IGP Hellos are not lost
   and the associated link-state adjacencies are not removed

   The Hello protocol consists of two phases: a negotiation phase and a
   keep-alive phase. The negotiation phase allows negotiation of some
   basic Hello protocol parameters, like the Hello frequency. The keep-
   alive phase consists of a fast lightweight Hello message exchange.

   The failure of a control channel can also be detected by lower
   layers (e.g., SONET/SDH) since control channels are electrically
   terminated at each node.

8.3 Control channel interfaces

   LMP functions to maintain logical control channels between a pair of
   nodes via control channel interfaces. Each control channel interface
   hides a set of control channels and which of these is actually used
   to transport the messages and how this is achieved. This isolate
   signaling, routing and management from the actual control channel

   LMP does not specify how control channels are implemented, however
   it states that messages transported over a control channel must be
   IP encoded. Furthermore, since the messages are IP encoded, the link
   level encoding is not part of LMP.

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   LMP associates (possibly multiple) link bundles with a control
   channel. Multiple control channels may then be configured and
   associated with a control channel interface. One control channel is
   actually used while the others are backup control channels sorted by
   preference order. The control channel interface is announced into
   the IGP domain so that messages can be routed to that interface. The
   associations between the control channels and the control channel
   interface are purely a local matter.

   The control channel of a link bundle can be either explicitly
   configured or automatically selected, however, GMPLS currently
   assume that the control channel is explicitly configured. Once a
   link bundle is associated with a control channel, it follows the
   failover of that control channel. The association of the control
   channel to the control channel interface is configured or
   automatically bootstrapped and is a local issue.

   Between any two adjacent nodes (from the perspective of link
   bundles) there may be multiple active control channel interfaces,
   and these control channel interfaces are used for LMP, routing, and
   signaling messages. For purposes of flooding routing messages, LMP
   messages, and signaling messages, any of the active control channel
   interfaces may be used.

8.4 Link property correlation

   A link property exchange mechanism allows to dynamically change some
   link characteristics. It allows for instance to add data-bearing
   links to a link bundle, change a link's protection mechanism, change
   port identifiers, or change component identifiers in a bundle. This
   mechanism is supported by an exchange of link summary messages.

8.5 Link connectivity verification

   Link connectivity verification is an optional procedure that may be
   used to verify the physical connectivity of data-bearing links (e.g.
   component links of a bundle)as well as to exchange the link
   identifiers that will be further used in the RSVP-TE and CR-LDP

   The use of this procedure is negotiated as part of the configuration
   exchange that take place during the negotiation phase of the Hello
   protocol. If enabled, the procedure is done initially when a link
   bundle is first established, and subsequently, on a periodic basis
   for all free component links of a link bundle.

   Ping-type Test messages are exchanged over each of the data-bearing
   links specified in the bundled link. It should be noted that all LMP
   messages except for the Test message are exchanged over the control
   channel and that Hello messages continue to be exchanged over the
   control channel during the data-bearing link verification process.
   The Test message is sent over the data-bearing link that is being
   verified. Data-bearing links are tested in the transmit direction as

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   they are uni-directional, and as such, it may be possible for both
   nodes to exchange the Test messages simultaneously.

   Before exchanging these test messages, the node that initiates the
   verification indicates to the adjacent node that it will begin
   sending test messages across the data-bearing links of a particular
   bundled link. It indicates also the number of data-bearing links
   that are to be verified; the interval at which the test messages
   will be sent; the encoding scheme, the transport mechanism that are
   supported, and data rate for Test messages; and, in the case where
   the data-bearing links correspond to fibers, the wavelength over
   which the Test messages will be transmitted. The transport mechanism
   is negotiated between the two nodes. Furthermore, the local and
   remote bundle identifiers are transmitted at this time to perform
   the data-bearing link association with the bundle identifiers.

   A unique characteristic of photonic switches (all-optical) is that
   the data being transmitted over a data-bearing link is not
   terminated at the switch, but instead passes through transparently.
   This characteristic of PXCs poses a challenge for validating the
   connectivity of the data-bearing links.

   Therefore, to ensure proper verification of data-bearing link
   connectivity in that case, we require that until the links are
   allocated, it must be possible to terminate them locally. There is
   no requirement that all data-bearing links be terminated
   simultaneously, but at a minimum, the data-bearing links must be
   able to be terminated one at a time. Furthermore, we assume that the
   nodal architecture is designed so that messages can be sent and
   received over any data-bearing link. Note that this requirement is
   trivial for a digital switch since each data-bearing link is
   received electronically before being forwarded to the next switch.
   This is an additional requirement for photonic switches.

8.6 Fault localization

   Fault localization or isolation is an important requirement from the
   operational point of view. When a failure occurs an operator needs
   to know where exactly it happened. It can also be used to support
   some specific local protection/restoration mechanisms. Logically,
   fault localization can occur only after a fault is detected.

   Fault detection must be handled at the layer closest to the failure;
   for optical networks, this is the physical (optical) layer. One
   measure of fault detection at the physical layer is simply detecting
   loss of light (LOL). Other techniques for monitoring optical signals
   are still being developed and are for further study. However, it
   should be clear that the mechanism used to locate the failure is
   independent of the mechanism used to detect the failure, but simply
   relies on the fact that a failure is detected.

   In new technologies such as transparent photonic switching currently
   no method is defined to locate a fault, and the mechanism by which

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   the fault information is propagated must be sent ôout of bandö (via
   the control plane).

   Fault localization is an optional LMP procedure that is used to
   rapidly locate link failures. The use of this procedure is also
   negotiated as part of the configuration exchange that take place
   during the negotiation phase of the Hello protocol. As before, we
   assume each link has a bi-directional control channel that is always
   available for inter-node communication and that the control channel
   spans a single hop between two neighboring nodes.

   The mechanism used to rapidly isolate link failures is designed to
   work for unidirectional LSPs, and can be easily extended to work for
   bi-directional LSPs.

   If data-bearing links fail between two photonic switches, the power
   monitoring system in all of the downstream nodes will detect LOL and
   indicate a failure. To correlate multiple failures between a pair of
   nodes, a monitoring window can be used in each node to determine if
   a single data-bearing link has failed or if multiple data-bearing
   links have failed. As part of the fault localization, a downstream
   node that detects data-bearing link failures will send a channel
   fail message to its upstream neighbor (bundling together the
   notification of all of the failed data-bearing links).

   An upstream node that receives the channel fail message will
   correlate the failure to see if there is a failure on the
   corresponding input and output ports for the LSP()s using this/these
   link(s). If there is also a failure on the input port(s) of the
   upstream node, the node will return a message to the downstream node
   (bundling together the notification of all the data-bearing links),
   indicating that it too has detected a failure. If, however, the
   fault is clear in the upstream node (e.g., there is no LOL on the
   corresponding input channels), then the upstream node will have
   localized the failure and will return a specific message to the
   downstream node. Once the failure has been localized, the signaling
   protocols can be used to initiate span or path
   protection/restoration procedures.

9. Generalized Signaling

   The GMPLS signaling extends certain base functions of the RSVP-TE
   and CR-LDP signaling and, in some cases, add functionality. These
   changes and additions impact basic LSP properties, how labels are
   requested and communicated, the unidirectional nature of LSPs, how
   errors are propagated, and information provided for synchronizing
   the ingress and egress.

   The GMPLS signaling specification is available in three parts:

      1. A signaling functional description [GMPLS-SIG].
      2. RSVP-TE extensions [GMPLS-RSVP-TE].
      3. CR-LDP extensions [GMPLS-CR-LDP].

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   The following MPLS profile applies to GMPLS:

      - Downstream-on-demand label allocation and distribution.
      - Ingress initiated ordered control.
      - Liberal (typical), or conservative (could) label retention
      - Request, traffic/data, or topology driven label allocation
      - Explicit routing (typical), or hop-by-hop routing (could).

   The GMPLS signaling defines the following new building blocks on the
   top of MPLS-TE:

      1. A new label request format to encompass non-PSC
      2. Labels for non-PSC interfaces, generically known as
         Generalized Label.
      3. Waveband switching support.
      4. Label suggestion by the upstream for optimization purposes
         (e.g. latency).
      5. Label restriction by the upstream to support some optical
      6. Bi-directional LSP establishment with contention
      7. Rapid failure notification to ingress node.
      8. Explicit routing with explicit label control for a fine
         degree of control.

   These building blocks will be described in mode details in the
   following. A complete specification can be found in the
   corresponding documents.

   Note that GMPLS is highly generic and optional. Only building blocks
   1 and 2 are mandatory, and only within the specific format that is
   needed. Typically building blocks 6 and 8 should be implemented.
   Building blocks 3, 4, 5 and 7 are optional.

   A typical SDH/SONET switching network would implement building
   blocks: 1 (but the SDH/SONET format), 2 (the SDH/SONET label), 6 and
   8. It could implement another format of label in case of link
   bundling. Building block 7 is optional since the
   protection/restoration can be achieved using SDH/SONET overhead

   A typical wavelength switching network would implement building
   blocks: 1 (but the wavelength label), 2 (the generic format), 4, 5,
   6, 7 and 8. Building block 3 is only needed in the particular case
   of waveband switching.

   A typical fiber switching network would implement building blocks: 1
   (but the port label), 2 (the generic format), 6, 7 and 8.

   A typical MPLS-IP network would not implement any of these building
   blocks, since the absence of building block 1 would indicate regular

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   MPLS-IP. Note however that building block 1 can be used to signal
   MPLS-IP as well. In that case, the MPLS-IP network can benefit from
   the link protection type (not available in CR-LDP, some very basic
   form being available in RSVP-TE). Building block 2 is here a regular
   MPLS label and no new label format is required.

   GMPLS does not specify any profile for RSVP-TE and CR-LDP
   implementations that have to support GMPLS - except for what is
   directly related to GMPLS procedures. It is to the manufacturer to
   decide which are the optional elements and procedures of RSVP-TE and
   CR-LDP that need to be implemented. Some optional MPLS-TE elements
   can be useful for non-PSC layers, for instance the setup and holding
   priorities that are inherited from MPLS-TE.

9.1. Overview: How to Request an LSP

   A non-PSC LSP is established by sending a PATH/Label Request message
   downstream to the destination. This message contains a Generalized
   Label Request with the type of LSP (i.e. the layer concerned), its
   payload type and the requested local protection per link. An
   Explicit Route (ERO) is also normally added to the message, but this
   can be added and/or completed by the first/default LSR.

   The requested bandwidth is encoded in the RSVP-TE SENDER_TSPEC and
   FLOWSPEC objects, or in the CR-LDP Traffic Parameters TLV. The end-
   to-end protection type is for further study. In case of SDH/SONET
   concatenation, the requested bandwidth is the total bandwidth and a
   field in the Generalized Label Request allows to know the number of

   Specific parameters for a given technology are given in the
   Generalized Label Request, such as the type of concatenation and/or
   transparency for a SDH/SONET LSP.

   If the LSP is a bi-directional LSP, an Upstream Label is also
   specified in the Path/Label request message. This label will be the
   one to use in the downstream to upstream direction.

   Additionally, a Suggested Label, a Label Set and a Waveband Label
   can also be included in the message. Other operations are defined in

   The downstream node will send back a Resv/Label Mapping message
   including one Generalized Label object/TLV that can contain several
   Generalized Labels. For instance, if a concatenated SDH/SONET signal
   is requested, several labels can be returned.

   In case of SDH/SONET virtual concatenation, a list of labels is
   returned. Each label identifying one element of the virtual
   concatenated signal. This limits virtual concatenation to remain
   within a single (component) link.

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   In case of any type of SDH/SONET contiguous concatenation, only one
   label is returned. That label is the lowest signal of the contiguous
   concatenated signal (given an order specified in [GMPLS-SIG].

   In case of SDH/SONET bundling, i.e. co-routing of circuits of the
   same type but without concatenation, the explicit list of all
   signals that take part in the bundling is returned.

9.2. Generalized Label Request

   The Generalized Label Request is a new object/TLV to be added in an
   RSVP-TE Path message instead of the regular Label Request, or in a
   CR-LDP Request message in addition to the already existing TLVs.
   Only one label request can be used per message, so a single LSP can
   be requested at a time per signaling message.

   The Generalized Label Request gives some major characteristics
   (parameters) required to support the LSP being requested, such as
   the LSP encoding type, the LSP payload type, the desired link

   GMPLS defines a generic Generalized Label Request, and in addition
   it can define specialized Generalized Label Requests, if and only if
   there are specific characteristics that cannot be signaled by the
   generic request, i.e. specific characteristics.

   Currently, only one specific Generalized Label Request is defined,
   for SDH/SONET. The SDH/SONET Generalized Label Request indicates the
   same generic characteristics as the generic request but includes in
   addition the requested SDH/SONET concatenation and transparency (if

   Note that it is expected than a specific Generalized Label Request
   will be defined in the future for photonic (all optical) switching.

   The characteristics described hereafter are generic to all

      - The LSP encoding type.
      - The LSP payload type.
      - The link protection type.

   The LSP encoding type indicates the type of technology (e.g.
   Ethernet, SDH, SONET, fiber, etc) to which this requested LSP
   corresponds. It represents the nature of the LSP, and not the nature
   of the links that the LSP traverses. A link may support a set of
   encoding formats, where support means that a link is able to carry
   and switch a signal of one or more of these encoding formats
   depending on the resource availability and capacity of the link.

   For example, consider an LSP signaled with "photonic" encoding. It
   is expected that such an LSP would be supported with no electrical
   conversion and no knowledge of the modulation and speed by the

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   transit nodes. Some other formats (electrical) require other
   knowledge such as the bandwidth.

   The LSP payload type identifies the payload carried by an LSP, i.e.
   the client layer of that LSP. This must be interpreted according to
   the technology encoding type of the LSP and is used by the nodes at
   the endpoints of the LSP to know to which client layer a request is

   The link protection type indicates the desired local link protection
   for each link of an LSP. If a particular protection type, i.e., 1+1,
   or 1:N, is requested, then a connection request is processed only if
   the desired protection type can be honored. Note that GMPLS
   advertises the protection capabilities of a link in the routing
   protocols. Path computation algorithms may take this information
   into account when computing paths for setting up LSPs.

9.3. Generalized Label

   The Generalized Label extends the traditional MPLS label by allowing
   the representation of not only labels which identify and travel in-
   band with associated data packets, but also (virtual) labels which
   identify time-slots, wavelengths, or space division multiplexed

   For example, the Generalized Label may identify (a) a single fiber
   in a bundle, (b) a single waveband within fiber, (c) a single
   wavelength within a waveband (or fiber), or (d) a time-slot within a
   wavelength (or fiber).  It may also be a generic MPLS label, a Frame
   Relay label, or an ATM label (VCI/VPI). The format of a label can be
   as simple as an integer value such as a wavelength label or can be
   more elaborated such as an SDH/SONET label.

   SDH and SONET define each a multiplexing structure. These
   multiplexing structures will be used as naming trees to create
   unique labels. Such a label will identify the type of a particular
   signal (time-slot) and its exact position in a multiplexing
   structure (both are related). Since the SONET multiplexing structure
   may be seen as a subset of the SDH multiplexing structure, the same
   format of label is used for SDH and SONET.

   Since the nodes sending and receiving the Generalized Label know
   what kinds of link they are using, the Generalized Label does not
   identify its type, instead the nodes are expected to know from the
   context what type of label to expect.

   A Generalized Label only carries a single level of label, i.e., it
   is non-hierarchical. When nested LSPs are used, each LSP must be
   established separately and has its own label at each local interface
   between two nodes at its level.

9.4. Waveband Switching

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   A special case of wavelength switching is waveband switching. A
   waveband represents a set of contiguous wavelengths which can be
   switched together to a new waveband. For optimization reasons it may
   be desirable for an photonic cross-connect to optically switch
   multiple wavelengths as a unit. This may reduce the distortion on
   the individual wavelengths and may allow tighter separation of the
   individual wavelengths. A Waveband label is defined to support this
   special case.

   Waveband switching naturally introduces another level of label
   hierarchy and as such the waveband is treated the same way all other
   upper layer labels are treated. As far as the MPLS protocols are
   concerned there is little difference between a waveband label and a
   wavelength label except that semantically the waveband can be
   subdivided into wavelengths whereas the wavelength can only be
   subdivided into time or statistically multiplexed labels.

9.5. Label Suggestion by the Upstream

   GMPLS allows for a label to be suggested by an upstream node. This
   suggestion may be overridden by a downstream node but, in some
   cases, at the cost of higher LSP setup time. The suggested label is
   valuable when establishing LSPs through certain kinds of optical
   equipment where there may be a lengthy (in electrical terms) delay
   in configuring the switching fabric. For example micro mirrors may
   have to be elevated or moved, and this physical motion and
   subsequent damping takes time.  If the labels and hence switching
   fabric are configured in the reverse direction (the norm) the
   MAPPING/Resv message may need to be delayed by 10's of milliseconds
   per hop in order to establish a usable forwarding path. It can also
   be important for restoration purposes where alternate LSPs may need
   to be rapidly established as a result of network failures.

9.6. Label Restriction by the Upstream

   An upstream node can optionally restrict (limit) the choice of label
   of a downstream node to a set of acceptable labels. This restriction
   is done by giving a list of inclusive (acceptable) or exclusive
   (unacceptable) labels in a Label Set. If not applied, all labels
   from the valid label range may be used. There are four cases where a
   label restriction is useful in the "optical" domain.

   The first case is where the end equipment is only capable of
   transmitting and receiving on a small specific set of

   The second case is where there is a sequence of interfaces which
   cannot support wavelength conversion and require the same wavelength
   be used end-to-end over a sequence of hops, or even an entire path.

   The third case is where it is desirable to limit the amount of
   wavelength conversion being performed to reduce the distortion on
   the optical signals.

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   The last case is where two ends of a link support different sets of

   The receiver of a Label Set must restrict its choice of labels to
   one which is in the Label Set. A Label Set may be present across
   multiple hops. In this case each node generates it's own outgoing
   Label Set, possibly based on the incoming Label Set and the node's
   hardware capabilities. This case is expected to be the norm for
   nodes with conversion incapable interfaces.

9.7. Bi-directional LSP

   GMPLS allows establishment of bi-directional LSPs. A bi-directional
   LSP has the same traffic engineering requirements including fate
   sharing, protection and restoration, LSRs, and resource requirements
   (e.g., latency and jitter) in each direction. In the remainder of
   this section, the term "initiator" is used to refer to a node that
   starts the establishment of an LSP and the term "terminator" is used
   to refer to the node that is the target of the LSP. For a bi-
   directional LSPs, there is only one initiator and one terminator.

   Normally to establish a bi-directional LSP when using [RSVP-TE] or
   [CR-LDP] two unidirectional paths must be independently established.
   This approach has the following disadvantages:

   1. The latency to establish the bi-directional LSP is equal to one
   round trip signaling time plus one initiator-terminator signaling
   transit delay. This not only extends the setup latency for
   successful LSP establishment, but it extends the worst-case latency
   for discovering an unsuccessful LSP to as much as two times the
   initiator-terminator transit delay. These delays are particularly
   significant for LSPs that are established for restoration purposes.

   2. The control overhead is twice that of a unidirectional LSP. This
   is because separate control messages (e.g. Path and Resv) must be
   generated for both segments of the bi-directional LSP.

   3. Because the resources are established in separate segments, route
   selection is complicated. There is also additional potential race
   for conditions in assignment of resources, which decreases the
   overall probability of successfully establishing the bi-directional

   4. It is more difficult to provide a clean interface for SDH/SONET
   equipment that may rely on bi-directional hop-by-hop paths for
   protection switching. Note that existing SDH/SONET gear transmits
   the control information in-band with the data.

   5. Bi-directional optical LSPs (or lightpaths) are seen as a
   requirement for many optical networking service providers.

   With bi-directional LSPs both the downstream and upstream data
   paths, i.e. from initiator to terminator and terminator to
   initiator, are established using a single set of signaling messages.

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   This reduces the setup latency to essentially one initiator-
   terminator round trip time plus processing time, and limits the
   control overhead to the same number of messages as a unidirectional

   For bi-directional LSPs, two labels must be allocated. Bi-
   directional LSP setup is indicated by the presence of an Upstream
   Label in the appropriate signaling message.

9.8. Bi-directional LSP Contention Resolution

   Contention for labels may occur between two bi-directional LSP setup
   requests traveling in opposite directions. This contention occurs
   when both sides allocate the same resources (ports) at effectively
   the same time. The GMPLS signaling defines a procedure to resolve
   that contention, basically the node with the higher node ID will win
   the contention. To reduce the probability of contention, some
   mechanisms are also suggested.

9.9. Rapid Notification of Failure

   GMPLS defines three signaling extensions for RSVP-TE that enable
   expedited notification of failures and other events to nodes
   responsible for restoring failed LSPs, and modify error handling.
   For CR-LDP there is not currently a similar mechanism.

   The first extension, identifies where event notifications are to be
   sent. The second, provides for general expedited event notification.
   Such extensions can be used by fast restoration mechanisms.

   The final extension is an RSVP optimization to allow the faster
   removal of intermediate states in some cases.

9.10. Explicit Routing and Explicit Label Control

   The path taken by an LSP can be controlled more or less precisely by
   using an explicit route. Typically, the node at the head-end of an
   LSP finds a more or less precise explicit route and builds an
   Explicit Route Object (ERO) that contains that route. Possibly, the
   edge node don't build any ERO, and just transmit a signaling request
   to a default neighbor LSR (as IP hosts today). For instance, an
   explicit route could be added to a signaling message by the first
   switching node, on behalf of the edge node. Note also that an
   explicit route is altered by intermediate LSRs during its
   progression towards the destination.

   The ERO is originally defined by MPLS-TE as a list of abstract nodes
   (i.e. groups of nodes) along the explicit route. Each abstract node
   can be an IPv4 address prefix, an IPv6 address prefix, or an AS
   number. This capability allows the generator of the explicit route
   to have imperfect information about the details of the path. In the
   simplest case, an abstract node can be a full IP address that
   identify a specific node (called a simple abstract node).

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   MPLS-TE allows strict and loose abstract nodes. The path between a
   strict node and its preceding node must include only network nodes
   from the strict node and its preceding abstract node. The path
   between a loose node and its preceding node may include other
   network nodes that are not part of the strict node or its preceding
   abstract node.

   This ERO was extended to include interface numbers as abstract nodes
   to support unnumbered interfaces; and further extended by GMPLS to
   include labels as abstract nodes. Having labels in an explicit route
   is an important feature that allows to control the placement of an
   LSP with a very fine granularity. This is more likely to be used for
   non-PSC links.

   In particular, the explicit label control in the ERO allows to
   terminate an LSP on a particular outgoing port to an egress node.

   This can also be used when it is desirable to "splice" two LSPs
   together, i.e. where the tail of the first LSP would be "spliced"
   into the head of the second LSP.

   Another use is when an optimization algorithm is used for an
   SDH/SONET network. This algorithm can provide very detailed explicit
   routes, including the label (time-slot) to use on a link, in order
   to minimize the external fragmentation of the SDH/SONET multiplex on
   the corresponding interface.

   Another use is when the label indicates a particular component in a
   bundle in order to stay diverse with other components of that
   bundle, i.e. to control the usage of components in a bundle for
   different LSPs.

9.11 LSP modification and LSP re-routing

   LSP modification and re-routing are two features already available
   in MPLS-TE. GMPLS does not add anything new. Elegant re-routing is
   possible with the concept of "make-before-break" whereby an old path
   is still used while a new path is set up by avoiding double
   reservation of resources. Then, the node performing the re-routing
   can swap on the new path and close the old path. This feature is
   supported with RSVP-TE (using shared explicit filters) and CR-LDP
   (using the action indicator flag).

   LSP modification consists in changing some LSP parameters, but
   normally without changing the route. It is supported using the same
   mechanism as re-routing. However, the semantic of LSP modification
   will differ from one technology to the other. For instance, further
   studies are required to understand the impact of dynamically
   changing some SDH/SONET circuit characteristics such as the
   bandwidth, the protection type, the transparency, the concatenation,

9.12. Route recording

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   In order to improve the reliability and the manageability of the LSP
   being established, the concept of the route recording was introduced
   in RSVP-TE to function as:

   - First, a loop detection mechanism to discover L3 routing loops, or
   loops inherent in the explicit route (this mechanism is strictly
   exclusive with the use of explicit routing objects).

   - Second, a route recording mechanism collects up-to-date detailed
   path information on a hop-by-hop basis during the LSP setup process.
   This mechanism provides valuable information to the source and
   destination nodes. Any intermediate routing change at setup time, in
   case of loose explicit routing, will be reported.

   - Third, a recorded route can be used as input for an explicit
   route. This is useful if a source node receives the recorded route
   from a destination node and applies it as an explicit route in order
   to "pin down the path".

   Within the GMPLS architecture only the second and third functions
   are mainly applicable for non-PSC layers.

10. Forwarding Adjacencies (FA)

   To improve scalability of MPLS TE (and thus GMPLS) it may be useful
   to aggregate multiple LSPs inside a bigger LSP. Intermediate nodes
   see the external LSP only, they don't have to maintain forwarding
   states for each internal LSP, less signaling messages need to be
   exchanged and the external LSP can be somehow protected instead (or
   in addition) to the internal LSPs. This can considerably increase
   the scalability of the signaling.

   The aggregation is accomplished by (a) an LSR creating a TE LSP, (b)
   the LSR forming a forwarding adjacency out of that LSP (advertising
   this LSP as a link into ISIS/OSPF), (c) allowing other LSRs to use
   forwarding adjacencies for their path computation, and (d) nesting
   of LSPs originated by other LSRs into that LSP (e.g. by using the
   label stack construct in the case of IP).

   An LSR may (under its local configuration control) announce an LSP
   as a link into ISIS/OSPF.  When this link is advertised into the
   same instance of ISIS/OSPF as the one that determines the route
   taken by the LSP, we call such a link a "forwarding adjacency" (FA).
   We refer to the LSP as the "forwarding adjacency LSP", or just FA-
   LSP.  Note that since the advertised entity is a link in ISIS/OSPF,
   both the end point LSRs of the FA-LSP must belong to the same ISIS
   level/OSPF area.

   In general, creation/termination of a FA and its FA-LSP could be
   driven either by mechanisms outside of MPLS (e.g., via configuration
   control on the LSR at the head-end of the adjacency), or by
   mechanisms within MPLS (e.g., as a result of the LSR at the head-end
   of the adjacency receiving LSP setup requests originated by some
   other LSRs).

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   ISIS/OSPF floods the information about FAs just as it floods the
   information about any other links.  As a result of this flooding, an
   LSR has in its link state database the information about not just
   conventional links, but FAs as well.

   An LSR, when performing path computation, uses not just conventional
   links, but FAs as well.  Once a path is computed, the LSR uses RSVP-
   TE/CR-LDP for establishing label binding along the path. FAs needs
   simple extensions to signaling and routing protocols.

   Forwarding adjacencies may be represented as either unnumbered or
   numbered links. A FA can also be a bundle of LSPs between two nodes.

   When a FA is created dynamically, its TE attributes are inherited
   from the TE LSP which induced its creation. Note that the bandwidth
   of the FA-LSP must be at least as big as the LSP that induced it,
   but may be bigger if only discrete bandwidths are available for the
   FA-LSP. In general, for dynamically provisioned forwarding
   adjacencies, a policy-based mechanism may be needed to associate
   attributes to forwarding adjacencies.

10.1 Routing and Forwarding Adjacencies

   A FA advertisement could contain the information about the path
   taken by the FA-LSP associated with that FA. This information may be
   used for path calculation by other LSRs. This information is carried
   in a new OSPF and IS-IS TLV called the Path TLV.

   It is possible that the underlying path information might change
   over time, via configuration updates, or dynamic route
   modifications, resulting in the change of that TLV.

   If forwarding adjacencies are bundled (via link bundling), and if
   the resulting bundled link carries a Path TLV, the underlying path
   followed by each of the FA-LSPs that form the component links must
   be the same.

   It is expected that forwarding adjacencies will not be used for
   establishing ISIS/OSPF peering relation between the routers at the
   ends of the adjacency.

10.2. Signaling aspects

   For the purpose of processing the ERO in a Path/Request message of
   an LSP that is to be tunneled over a forwarding adjacency, an LSR at
   the head-end of the FA-LSP views the LSR at the tail of that FA-LSP
   as adjacent (one IP hop away).

10.3 Cascading of Forwarding Adjacencies

   With an integrated model several layers are controlled using the
   same routing and signaling protocols. A network may then have links
   with different multiplexing/demultiplexing capabilities. For

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   example, a node may be able to multiplex/demultiplex individual
   packets on a given link, and may be able to multiplex/demultiplex
   channels within a SONET payload on other links.

   A new OSPF and IS-IS TLV has been defined to advertise the
   multiplexing capability of each interface: PSC, TDM, LSC or FSC. The
   information carried in this TLV is used to construct LSP regions,
   and determine regions' boundaries.

   Path computation may take into account region boundaries when
   computing a path for an LSP. For example, path computation may
   restrict the path taken by an LSP to only the links whose
   multiplexing/demultiplexing capability is PSC. When an LSP need to
   cross a region boundary, it can trigger the establishment of an FA
   at the underlying layer. This can trigger a cascading of FAs between
   layers with the following obvious order: TDM, then LSC, and then
   finally FSC.

11. Security considerations

   GMPLS introduces no new security considerations to the current MPLS-
   TE signaling (RSVP-TE, CR-LDP) and routing protocols (OSPF-TE, IS-

12. Acknowledgements

   This draft is the work of numerous authors and consists of a
   composition of a number of previous drafts in this area.

   Many thanks to Ben Mack-Crane (Tellabs) for all the useful SDH/SONET
   discussions that we had together. Thanks also to Pedro Falcao
   (Ebone) and Michael Moelants (Ebone) for their SDH/SONET and optical
   technical advice and support. Finally, many thanks also to Krishna
   Mitra (Calient) and Curtis Villamizar (Avici).

   A list of the drafts from which material and ideas were incorporated

   1. draft-ietf-mpls-generalized-signaling-01.txt
   Generalized MPLS - Signaling Functional Description

   2. draft-ietf-mpls-generalized-rsvp-te-00.txt
   Generalized MPLS Signaling - RSVP-TE Extensions

   3. draft-ietf-mpls-generalized-cr-ldp-00.txt
   Generalized MPLS Signaling - CR-LDP Extensions

   4. draft-ietf-mpls-lmp-01.txt
   Link Management Protocol (LMP)

   5. draft-ietf-mpls-lsp-hierarchy-01.txt
   LSP Hierarchy with MPLS TE

   6. draft-ietf-mpls-rsvp-unnum-00.txt

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   Signalling Unnumbered Links in RSVP-TE

   7. draft-ietf-mpls-crldp-unnum-00.txt
   Signalling Unnumbered Links in CR-LDP

   8. draft-kompella-mpls-bundle-04.txt
   Link Bundling in MPLS Traffic Engineering

   9. draft-kompella-ospf-gmpls-extensions-00.txt
   OSPF Extensions in Support of Generalized MPLS

   10. draft-ietf-isis-gmpls-extensions-01.txt
   IS-IS Extensions in Support of Generalized MPLS

13. References


14. Author's Addresses

   Peter Ashwood-Smith                Fong Liaw
   Nortel Networks Corp.              Zaffire Inc.
   P.O. Box 3511 Station C,           2630 Orchard Parkway
   Ottawa, ON K1Y 4H7                 San Jose, CA 95134
   Canada                             USA
   Phone: +1 613 763 4534             Email:

   Daniel O. Awduche                  Eric Mannie (editor)
   Movaz Networks                     Ebone (GTS)
   7296 Jones Branch Drive            Terhulpsesteenweg 6A
   Suite 615                          1560 Hoeilaart
   McLean, VA 22102                   Belgium
   USA                                Phone: +32 2 658 56 52
   Phone: +1 703 847-7350             Email:

   Ayan Banerjee                      Dimitri Papadimitriou
   Calient Networks                   Alcatel - IPO NSG
   5853 Rue Ferrari                   Francis Wellesplein, 1
   San Jose, CA 95138                 B-2018 Antwerpen
   USA                                Belgium
   Phone: +1 408 972-3645             Phone: +32 3 240-84-91
   Email:       Email:

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   Debashis Basak                     Dimitrios Pendarakis
   Accelight Networks                 Tellium, Inc.
   70 Abele Road, Bldg.1200           2 Crescent Place
   Bridgeville, PA 15017              P.O. Box 901
   USA                                Oceanport, NJ 07757-0901
   Phone: +1 412 220-2102 (ext115)    USA
   email:        Email:

   Lou Berger                         Bala Rajagopalan
   Movaz Networks, Inc.               Tellium, Inc.
   7926 Jones Branch Drive            2 Crescent Place
   Suite 615                          P.O. Box 901
   MCLean VA, 22102                   Oceanport, NJ 07757-0901
   USA                                USA
   Phone: +1 703 847-1801             Phone: +1 732 923 4237
   Email:           Email:

   Greg Bernstein                     Yakov Rekhter
   Ciena Corporation                  Juniper
   10480 Ridgeview Court              Email:
   Cupertino, CA 94014
   Phone: +1 408 366 4713

   John Drake                         Hal Sandick
   Calient Networks                   Nortel Networks
   5853 Rue Ferrari                   Email:
   San Jose, CA 95138       
   Phone: +1 408 972 3720

   Yanhe Fan                          Debanjan Saha
   Axiowave Networks, Inc.            Tellium Optical Systems
   100 Nickerson Road                 2 Crescent Place
   Marlborough, MA 01752              Oceanport, NJ 07757-0901
   USA                                USA
   Phone: +1 508 460 6969 Ext. 627    Phone: +1 732 923 4264
   Email:           Email:

   Don Fedyk                          Vishal Sharma
   Nortel Networks Corp.              Jasmine Networks, Inc.
   600 Technology Park Drive          3061 Zanker Road, Suite B
   Billerica, MA 01821                San Jose, CA 95134
   USA                                USA
   Phone: +1-978-288-4506             Phone: +1 408 895 5030
   Email:                             Email:

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  draft-many-gmpls-architecture-00.txt                    Feb 2001

   Gert Grammel                       George Swallow
   Alcatel                            Cisco Systems, Inc.
   Italy                              250 Apollo Drive
   Email:                             Chelmsford, MA 01824      USA
                                      Phone: +1 978 244 8143

   Kireeti Kompella                   Z. Bo Tang
   Juniper Networks, Inc.             Tellium, Inc.
   1194 N. Mathilda Ave.              2 Crescent Place
   Sunnyvale, CA 94089                P.O. Box 901
   USA                                Oceanport, NJ 07757-0901
   Email:         USA
                                      Phone: +1 732 923 4231

   Alan Kullberg                      John Yu
   NetPlane Systems, Inc.             Zaffire Inc.
   888 Washington                     2630 Orchard Parkway
   St.Dedham, MA 02026                San Jose, CA 95134
   USA                                USA
   Phone: +1 781 251-5319             Email:

   Jonathan P. Lang                   Alex Zinin
   Calient Networks                   Cisco Systems
   25 Castilian                       150 W. Tasman Dr.
   Goleta, CA 93117                   San Jose, CA 95134
   Email:         Email:

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