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+Network Working Group                                     J. Strand, Ed.
+Request for Comments: 4054                                  A. Chiu, Ed.
+Category: Informational                                             AT&T
+                                                                May 2005
+
+
+      Impairments and Other Constraints on Optical Layer Routing
+
+Status of This Memo
+
+   This memo provides information for the Internet community.  It does
+   not specify an Internet standard of any kind.  Distribution of this
+   memo is unlimited.
+
+Copyright Notice
+
+   Copyright (C) The Internet Society (2005).
+
+Abstract
+
+   Optical networking poses a number challenges for Generalized Multi-
+   Protocol Label Switching (GMPLS).  Fundamentally, optical technology
+   is an analog rather than digital technology whereby the optical layer
+   is lowest in the transport hierarchy and hence has an intimate
+   relationship with the physical geography of the network.  This
+   contribution surveys some of the aspects of optical networks that
+   impact routing and identifies possible GMPLS responses for each:  (1)
+   Constraints arising from the design of new software controllable
+   network elements, (2) Constraints in a single all-optical domain
+   without wavelength conversion, (3) Complications arising in more
+   complex networks incorporating both all-optical and opaque
+   architectures, and (4) Impacts of diversity constraints.
+
+Table of Contents
+
+   1.  Introduction .................................................  2
+   2.  Sub-IP Area Summary and Justification of Work ................  3
+   3.  Reconfigurable Network Elements ..............................  3
+       3.1.  Technology Background ..................................  3
+       3.2.  Implications for Routing ...............................  6
+   4.  Wavelength Routed All-Optical Networks .......................  6
+       4.1.  Problem Formulation ....................................  7
+       4.2.  Polarization Mode Dispersion (PMD) .....................  8
+       4.3.  Amplifier Spontaneous Emission .........................  9
+       4.4.  Approximating the Effects of Some Other
+             Impairments Constraints ................................ 10
+       4.5.  Other Impairment Considerations ........................ 13
+
+
+
+
+Strand & Chiu                Informational                      [Page 1]
+
+RFC 4054                 Optical Layer Routing                  May 2005
+
+
+       4.6.  An Alternative Approach - Using Maximum
+             Distance as the Only Constraint ........................ 13
+       4.7.  Other Considerations ................................... 15
+       4.8.  Implications for Routing and Control Plane Design ...... 15
+   5.  More Complex Networks ........................................ 17
+   6.  Diversity .................................................... 19
+       6.1.  Background on Diversity ................................ 19
+       6.2.  Implications for Routing ............................... 23
+   7.  Security Considerations ...................................... 23
+   8.  Acknowledgements ............................................. 24
+   9.  References ................................................... 25
+       9.1.  Normative References ................................... 25
+       9.2.  Informative References ................................. 26
+   10. Contributing Authors ......................................... 26
+
+1.  Introduction
+
+   Generalized Multi-Protocol Label Switching (GMPLS) [Mannie04] aims to
+   extend MPLS to encompass a number of transport architectures,
+   including optical networks that incorporate a number of all-optical
+   and opto-electronic elements, such as optical cross-connects with
+   both optical and electrical fabrics, transponders, and optical add-
+   drop multiplexers.  Optical networking poses a number of challenges
+   for GMPLS.  Fundamentally, optical technology is an analog rather
+   than digital technology whereby the optical layer is lowest in the
+   transport hierarchy and hence has an intimate relationship with the
+   physical geography of the network.
+
+   GMPLS already has incorporated extensions to deal with some of the
+   unique aspects of the optical layer.  This contribution surveys some
+   of the aspects of optical networks that impact routing and identifies
+   possible GMPLS responses for each.  Routing constraints and/or
+   complications arising from the design of network elements, the
+   accumulation of signal impairments, and the need to guarantee the
+   physical diversity of some circuits are discussed.
+
+   Since the purpose of this document is to further the specification of
+   GMPLS, alternative approaches to controlling an optical network are
+   not discussed.  For discussions of some broader issues, see
+   [Gerstel2000] and [Strand02].
+
+   The organization of the contribution is as follows:
+
+   -  Section 2 is a section requested by the sub-IP Area management for
+      all new documents.  It explains how this document fits into the
+      Area and into the IPO WG, and why it is appropriate for these
+      groups.
+
+
+
+
+Strand & Chiu                Informational                      [Page 2]
+
+RFC 4054                 Optical Layer Routing                  May 2005
+
+
+   -  Section 3 describes constraints arising from the design of new
+      software controllable network elements.
+
+   -  Section 4 addresses the constraints in a single all-optical domain
+      without wavelength conversion.
+
+   -  Section 5 extends the discussion to more complex networks and
+      incorporates both all-optical and opaque architectures.
+
+   -  Section 6 discusses the impacts of diversity constraints.
+
+   -  Section 7 deals with security requirements.
+
+   -  Section 8 contains acknowledgments.
+
+   -  Section 9 contains references.
+
+   -  Section 10 contains contributing authors' addresses.
+
+2.  Sub-IP Area Summary and Justification of Work
+
+   This document merges and extends two previous expired Internet-Drafts
+   that were made IPO working group documents to form a basis for a
+   design team at the Minneapolis IETF meeting, where it was also
+   requested that they be merged to create a requirements document for
+   the WG.
+
+   In the larger sub-IP Area structure, this merged document describes
+   specific characteristics of optical technology and the requirements
+   they place on routing and path selection.  It is appropriate for the
+   IPO working group because the material is specific to optical
+   networks.  It identifies and documents the characteristics of the
+   optical transport network that are important for selecting paths for
+   optical channels, which is a work area for the IPO WG.  The material
+   covered is directly aimed at establishing a framework and
+   requirements for routing in an optical network.
+
+3.  Reconfigurable Network Elements
+
+3.1.  Technology Background
+
+   Control plane architectural discussions (e.g., [Awduche99]) usually
+   assume that the only software reconfigurable network element is an
+   optical layer cross-connect (OLXC).  There are however other software
+   reconfigurable elements on the horizon, specifically tunable lasers
+   and receivers and reconfigurable optical add-drop multiplexers
+
+
+
+
+
+Strand & Chiu                Informational                      [Page 3]
+
+RFC 4054                 Optical Layer Routing                  May 2005
+
+
+   (OADM).  These elements are illustrated in the following simple
+   example, which is modeled on announced Optical Transport System (OTS)
+   products:
+
+               +                                       +
+   ---+---+    |\                                     /|    +---+---
+   ---| A |----|D|          X              Y         |D|----| A |---
+   ---+---+    |W|     +--------+     +--------+     |W|    +---+---
+        :      |D|-----|  OADM  |-----|  OADM  |-----|D|      :
+   ---+---+    |M|     +--------+     +--------+     |M|    +---+---
+   ---| A |----| |      |      |       |      |      | |----| A |---
+   ---+---+    |/       |      |       |      |       \|    +---+---
+               +      +---+  +---+   +---+  +---+      +
+                D     | A |  | A |   | A |  | A |     E
+                      +---+  +---+   +---+  +---+
+                       | |    | |     | |    | |
+
+       Figure 3-1: An OTS With OADMs - Functional Architecture
+
+   In Fig. 3-1, the part that is on the inner side of all boxes labeled
+   "A" defines an all-optical subnetwork.  From a routing perspective
+   two aspects are critical:
+
+   -  Adaptation: These are the functions done at the edges of the
+      subnetwork that transform the incoming optical channel into the
+      physical wavelength to be transported through the subnetwork.
+
+   -  Connectivity: This defines which pairs of edge Adaptation
+      functions can be interconnected through the subnetwork.
+
+   In Fig. 3-1, D and E are DWDMs and X and Y are OADMs.  The boxes
+   labeled "A" are adaptation functions.  They map one or more input
+   optical channels assumed to be standard short reach signals into a
+   long reach (LR) wavelength or wavelength group that will pass
+   transparently to a distant adaptation function.  Adaptation
+   functionality that affects routing includes:
+
+   -  Multiplexing: Either electrical or optical TDM may be used to
+      combine the input channels into a single wavelength.  This is done
+      to increase effective capacity:  A typical DWDM might be able to
+      handle 100 2.5 Gb/sec signals (250 Gb/sec total) or 50 10 Gb/sec
+      (500 Gb/sec total); combining the 2.5 Gb/sec signals together thus
+      effectively doubles capacity.  After multiplexing the combined
+      signal must be routed as a group to the distant adaptation
+      function.
+
+
+
+
+
+
+Strand & Chiu                Informational                      [Page 4]
+
+RFC 4054                 Optical Layer Routing                  May 2005
+
+
+   -  Adaptation Grouping: In this technique, groups of k (e.g., 4)
+      wavelengths are managed as a group within the system and must be
+      added/dropped as a group.  We will call such a group an
+      "adaptation grouping".  Examples include so called "wave group"
+      and "waveband" [Passmore01].  Groupings on the same system may
+      differ in basics such as wavelength spacing, which constrain the
+      type of channels that can be accommodated.
+
+   -  Laser Tunability: The lasers producing the LR wavelengths may have
+      a fixed frequency, may be tunable over a limited range, or may be
+      tunable over the entire range of wavelengths supported by the
+      DWDM.  Tunability speeds may also vary.
+
+   Connectivity between adaptation functions may also be limited:
+
+   -  As pointed out above, TDM multiplexing and/or adaptation grouping
+      by the adaptation function forces groups of input channels to be
+      delivered together to the same distant adaptation function.
+
+   -  Only adaptation functions whose lasers/receivers are tunable to
+      compatible frequencies can be connected.
+
+   -  The switching capability of the OADMs may also be constrained.
+
+   For example:
+
+   o  There may be some wavelengths that can not be dropped at all.
+
+   o  There may be a fixed relationship between the frequency dropped
+      and the physical port on the OADM to which it is dropped.
+
+   o  OADM physical design may put an upper bound on the number of
+      adaptation groupings dropped at any single OADM.
+
+   For a fixed configuration of the OADMs and adaptation functions
+   connectivity will be fixed: Each input port will essentially be
+   hard-wired to some specific distant port.  However this connectivity
+   can be changed by changing the configurations of the OADMs and
+   adaptation functions.  For example, an additional adaptation grouping
+   might be dropped at an OADM or a tunable laser retuned.  In each case
+   the port-to-port connectivity is changed.
+
+   These capabilities can be expected to be under software control.
+   Today the control would rest in the vendor-supplied Element
+   Management system (EMS), which in turn would be controlled by the
+   operator's OSes.  However in principle the EMS could participate in
+   the GMPLS routing process.
+
+
+
+
+Strand & Chiu                Informational                      [Page 5]
+
+RFC 4054                 Optical Layer Routing                  May 2005
+
+
+3.2.  Implications for Routing
+
+   An OTS of the sort discussed in Sec. 3.1 is essentially a
+   geographically distributed but blocking cross-connect system.  The
+   specific port connectivity is dependent on the vendor design and also
+   on exactly what line cards have been deployed.
+
+   One way for GMPLS to deal with this architecture would be to view the
+   port connectivity as externally determined.  In this case the links
+   known to GMPLS would be groups of identically routed wavebands.  If
+   these were reconfigured by the external EMS the resulting
+   connectivity changes would need to be detected and advertised within
+   GMPLS.  If the topology shown in Fig. 3-1 became a tree or a mesh
+   instead of the linear topology shown, the connectivity changes could
+   result in Shared Risk Link Group (SRLG - see Section 6.2) changes.
+
+   Alternatively, GMPLS could attempt to directly control this port
+   connectivity.  The state information needed to do this is likely to
+   be voluminous and vendor specific.
+
+4.  Wavelength Routed All-Optical Networks
+
+   The optical networks deployed until recently may be called "opaque"
+   ([Tkach98]): each link is optically isolated by transponders doing
+   O/E/O conversions.  They provide regeneration with retiming and
+   reshaping, also called 3R, which eliminates transparency to bit rates
+   and frame format.  These transponders are quite expensive and their
+   lack of transparency also constrains the rapid introduction of new
+   services.  Thus there are strong motivators to introduce "domains of
+   transparency" - all-optical subnetworks - larger than an OTS.
+
+   The routing of lightpaths through an all-optical network has received
+   extensive attention.  (See [Yates99] or [Ramaswami98]).  When
+   discussing routing in an all-optical network it is usually assumed
+   that all routes have adequate signal quality.  This may be ensured by
+   limiting all-optical networks to subnetworks of limited geographic
+   size that are optically isolated from other parts of the optical
+   layer by transponders.  This approach is very practical and has been
+   applied to date, e.g., when determining the maximum length of an
+   Optical Transport System (OTS).  Furthermore operational
+   considerations like fault isolation also make limiting the size of
+   domains of transparency attractive.
+
+   There are however reasons to consider contained domains of
+   transparency in which not all routes have adequate signal quality.
+   From a demand perspective, maximum bit rates have rapidly increased
+   from DS3 to OC-192 and soon OC-768 (40 Gb/sec).  As bit rates
+   increase it is necessary to increase power.  This makes impairments
+
+
+
+Strand & Chiu                Informational                      [Page 6]
+
+RFC 4054                 Optical Layer Routing                  May 2005
+
+
+   and nonlinearities more troublesome.  From a supply perspective,
+   optical technology is advancing very rapidly, making ever-larger
+   domains possible.  In this section, we assume that these
+   considerations will lead to the deployment of a domain of
+   transparency that is too large to ensure that all potential routes
+   have adequate signal quality for all circuits.  Our goal is to
+   understand the impacts of the various types of impairments in this
+   environment.
+
+   Note that, as we describe later in the section, there are many types
+   of physical impairments.  Which of these needs to be dealt with
+   explicitly when performing on-line distributed routing will vary
+   considerably and will depend on many variables, including:
+
+   -  Equipment vendor design choices,
+   -  Fiber characteristics,
+   -  Service characteristics (e.g., circuit speeds),
+   -  Network size,
+   -  Network operator engineering and deployment strategies.
+
+   For example, a metropolitan network that does not intend to support
+   bit rates above 2.5 Gb/sec may not be constrained by any of these
+   impairments, while a continental or international network that wished
+   to minimize O/E/O regeneration investment and support 40 Gb/sec
+   connections might have to explicitly consider many of them.  Also, a
+   network operator may reduce or even eliminate their constraint set by
+   building a relatively small domain of transparency to ensure that all
+   the paths are feasible, or by using some proprietary tools based on
+   rules from the OTS vendor to pre-qualify paths between node pairs and
+   put them in a table that can be accessed each time a routing decision
+   has to be made through that domain.
+
+4.1.  Problem Formulation
+
+   We consider a single domain of transparency without wavelength
+   translation.  Additionally, due to the proprietary nature of DWDM
+   transmission technology, we assume that the domain is either single
+   vendor or architected using a single coherent design, particularly
+   with regard to the management of impairments.
+
+   We wish to route a unidirectional circuit from ingress client node X
+   to egress client node Y.  At both X and Y, the circuit goes through
+   an O/E/O conversion that optically isolates the portion within our
+   domain.  We assume that we know the bit rate of the circuit.  Also,
+   we assume that the adaptation function at X may apply some Forward
+   Error Correction (FEC) method to the circuit.  We also assume we know
+   the launch power of the laser at X.
+
+
+
+
+Strand & Chiu                Informational                      [Page 7]
+
+RFC 4054                 Optical Layer Routing                  May 2005
+
+
+   Impairments can be classified into two categories, linear and
+   nonlinear.  (See [Tkach98] or [Kaminow02] for more on impairment
+   constraints.)  Linear effects are independent of signal power and
+   affect wavelengths individually.  Amplifier spontaneous emission
+   (ASE), polarization mode dispersion (PMD), and chromatic dispersion
+   are examples.  Nonlinearities are significantly more complex: they
+   generate not only impairments on each channel, but also crosstalk
+   between channels.
+
+   In the remainder of this section we first outline how two key linear
+   impairments (PMD and ASE) might be handled by a set of analytical
+   formulae as additional constraints on routing.  We next discuss how
+   the remaining constraints might be approached.  Finally we take a
+   broader perspective and discuss the implications of such constraints
+   on control plane architecture and also on broader constrained domain
+   of transparency architecture issues.
+
+4.2.  Polarization Mode Dispersion (PMD)
+
+   For a transparent fiber segment, the general PMD requirement is that
+   the time-average differential group delay (DGD) between two
+   orthogonal state of polarizations should be less than some fraction a
+   of the bit duration, T=1/B, where B is the bit rate.  The value of
+   the parameter a depends on three major factors: 1) margin allocated
+   to PMD, e.g., 1dB; 2) targeted outage probability, e.g., 4x10-5, and
+   3) sensitivity of the receiver to DGD.  A typical value for a is 10%
+   [ITU].  More aggressive designs to compensate for PMD may allow
+   values higher than 10%.  (This would be a system parameter dependent
+   on the system design.  It would need to be known to the routing
+   process.)
+
+   The PMD parameter (Dpmd) is measured in pico-seconds (ps) per
+   sqrt(km).  The square of the PMD in a fiber span, denoted as span-
+   PMD-square is then given by the product of Dpmd**2 and the span
+   length.  (A fiber span in a transparent network refers to a segment
+   between two optical amplifiers.)  If Dpmd is constant, this results
+   in a upper bound on the maximum length of an M-fiber-span transparent
+   segment, which is inversely proportional to the square of the product
+   of bit rate and Dpmd (the detailed equation is omitted due to the
+   format constraint - see [Strand01] for details).
+
+   For older fibers with a typical PMD parameter of 0.5 picoseconds per
+   square root of km, based on the constraint, the maximum length of the
+   transparent segment should not exceed 400km and 25km for bit rates of
+   10Gb/s and 40Gb/s, respectively.  Due to recent advances in fiber
+   technology, the PMD-limited distance has increased dramatically.  For
+   newer fibers with a PMD parameter of 0.1 picosecond per square root
+   of km, the maximum length of the transparent segment (without PMD
+
+
+
+Strand & Chiu                Informational                      [Page 8]
+
+RFC 4054                 Optical Layer Routing                  May 2005
+
+
+   compensation) is limited to 10000km and 625km for bit rates of 10Gb/s
+   and 40Gb/, respectively.  Still lower values of PMD are attainable in
+   commercially available fiber today, and the PMD limit can be further
+   extended if a larger value of the parameter a (ratio of DGD to the
+   bit period) can be tolerated.  In general, the PMD requirement is not
+   an issue for most types of fibers at 10Gb/s or lower bit rate.  But
+   it will become an issue at bit rates of 40Gb/s and higher.
+
+   If the PMD parameter varies between spans, a slightly more
+   complicated equation results (see [Strand01]), but in any event the
+   only link dependent information needed by the routing algorithm is
+   the square of the link PMD, denoted as link-PMD-square.  It is the
+   sum of the span-PMD-square of all spans on the link.
+
+   Note that when one has some viable PMD compensation devices and
+   deploy them ubiquitously on all routes with potential PMD issues in
+   the network, then the PMD constraint disappears from the routing
+   perspective.
+
+4.3.  Amplifier Spontaneous Emission
+
+   ASE degrades the optical signal to noise ratio (OSNR).  An acceptable
+   optical SNR level (SNRmin), which depends on the bit rate,
+   transmitter-receiver technology (e.g., FEC), and margins allocated
+   for the impairments, needs to be maintained at the receiver.  In
+   order to satisfy this requirement, vendors often provide some general
+   engineering rule in terms of maximum length of the transparent
+   segment and number of spans.  For example, current transmission
+   systems are often limited to up to 6 spans each 80km long.  For
+   larger transparent domains, more detailed OSNR computations will be
+   needed to determine whether the OSNR level through a domain of
+   transparency is acceptable.  This would provide flexibility in
+   provisioning or restoring a lightpath through a transparent
+   subnetwork.
+
+   Assume that the average optical power launched at the transmitter is
+   P.  The lightpath from the transmitter to the receiver goes through M
+   optical amplifiers, with each introducing some noise power.  Unity
+   gain can be used at all amplifier sites to maintain constant signal
+   power at the input of each span to minimize noise power and
+   nonlinearity.  A constraint on the maximum number of spans can be
+   obtained [Kaminow97] which is proportional to P and inversely
+   proportional to SNRmin, optical bandwidth B, amplifier gain G-1 and
+   spontaneous emission factor n of the optical amplifier, assuming all
+   spans have identical gain and noise figure.  (Again, the detailed
+   equation is omitted due to the format constraint - see [Strand01] for
+   details.)  Let's take a typical example.  Assuming P=4dBm,
+   SNRmin=20dB with FEC, B=12.5GHz, n=2.5, G=25dB, based on the
+
+
+
+Strand & Chiu                Informational                      [Page 9]
+
+RFC 4054                 Optical Layer Routing                  May 2005
+
+
+   constraint, the maximum number of spans is at most 10.  However, if
+   FEC is not used and the requirement on SNRmin becomes 25dB, the
+   maximum number of spans drops down to 3.
+
+   For ASE the only link-dependent information needed by the routing
+   algorithm is the noise of the link, denoted as link-noise, which is
+   the sum of the noise of all spans on the link.  Hence the constraint
+   on ASE becomes that the aggregate noise of the transparent segment
+   which is the sum of the link-noise of all links can not exceed
+   P/SNRmin.
+
+4.4.  Approximating the Effects of Some Other Impairment Constraints
+
+   There are a number of other impairment constraints that we believe
+   could be approximated with a domain-wide margin on the OSNR, plus in
+   some cases a constraint on the total number of networking elements
+   (OXC or OADM) along the path.  Most impairments generated at OXCs or
+   OADMs, including polarization dependent loss, coherent crosstalk, and
+   effective passband width, could be dealt with using this approach.
+   In principle, impairments generated at the nodes can be bounded by
+   system engineering rules because the node elements can be designed
+   and specified in a uniform manner.  This approach is not feasible
+   with PMD and noise because neither can be uniformly specified.
+   Instead, they depend on node spacing and the characteristics of the
+   installed fiber plant, neither of which are likely to be under the
+   system designer's control.
+
+   Examples of the constraints we propose to approximate with a domain-
+   wide margin are given in the remaining paragraphs in this section.
+   It should be kept in mind that as optical transport technology
+   evolves it may become necessary to include some of these impairments
+   explicitly in the routing process.  Other impairments not mentioned
+   here at all may also become sufficiently important to require
+   incorporation either explicitly or via a domain-wide margin.
+
+   Other Polarization Dependent Impairments
+      Other polarization-dependent effects besides PMD influence system
+      performance.  For example, many components have polarization-
+      dependent loss (PDL) [Ramaswami98], which accumulates in a system
+      with many components on the transmission path.  The state of
+      polarization fluctuates with time and its distribution is very
+      important also.  It is generally required that the total PDL on
+      the path be maintained within some acceptable limit, potentially
+      by using some compensation technology for relatively long
+      transmission systems, plus a small built-in margin in OSNR.  Since
+      the total PDL increases with the number of components in the data
+      path, it must be taken into account by the system vendor when
+      determining the maximum allowable number of spans.
+
+
+
+Strand & Chiu                Informational                     [Page 10]
+
+RFC 4054                 Optical Layer Routing                  May 2005
+
+
+   Chromatic Dispersion
+      In general this impairment can be adequately (but not optimally)
+      compensated for on a per-link basis, and/or at system initial
+      setup time.  Today most deployed compensation devices are based on
+      Dispersion Compensation Fiber (DCF).  DCF provides per fiber
+      compensation by means of a spool of fiber with a CD coefficient
+      opposite to the fiber.  Due to the imperfect matching between the
+      CD slope of the fiber and the DCF some lambdas can be over
+      compensated while others can be under compensated.  Moreover DCF
+      modules may only be available in fixed lengths of compensating
+      fiber; this means that sometimes it is impossible to find a DCF
+      module that exactly compensates the CD introduced by the fiber.
+      These effects introduce what is known as residual CD.  Residual CD
+      varies with the frequency of the wavelength.  Knowing the
+      characteristics of both of the fiber and the DCF modules along the
+      path, this can be calculated with a sufficient degree of
+      precision.  However this is a very challenging task.  In fact the
+      per-wavelength residual dispersion needs to be combined with other
+      information in the system (e.g., types fibers to figure out the
+      amount of nonlinearities) to obtain the net effect of CD either by
+      simulation or by some analytical approximation.  It appears that
+      the routing/control plane should not be burdened by such a large
+      set of information while it can be handled at the system design
+      level.  Therefore it will be assumed until proven otherwise that
+      residual dispersion should not be reported.  For high bit rates,
+      dynamic dispersion compensation may be required at the receiver to
+      clean up any residual dispersion.
+
+   Crosstalk
+      Optical crosstalk refers to the effect of other signals on the
+      desired signal.  It includes both coherent (i.e., intrachannel)
+      crosstalk and incoherent (i.e., interchannel) crosstalk.  Main
+      contributors of crosstalk are the OADM and OXC sites that use a
+      DWDM multiplexer/demultiplexer (MUX/DEMUX) pair.  For a relatively
+      sparse network where the number of OADM/OXC nodes on a path is
+      low, crosstalk can be treated with a low margin in OSNR without
+      being a binding constraint.  But for some relatively dense
+      networks where crosstalk might become a binding constraint, one
+      needs to propagate the per-link crosstalk information to make sure
+      that the end-to-end path crosstalk which is the sum of the
+      crosstalks on all the corresponding links to be within some limit,
+      e.g., -25dB threshold with 1dB penalty ([Goldstein94]).  Another
+      way to treat it without having to propagate per-link crosstalk
+      information is to have the system evaluate what the maximum number
+      of OADM/OXC nodes that has a MUX/DEMUX pair for the worst route in
+      the transparent domain for a low built-in margin.  The latter one
+      should work well where all the OXC/OADM nodes have similar level
+      of crosstalk.
+
+
+
+Strand & Chiu                Informational                     [Page 11]
+
+RFC 4054                 Optical Layer Routing                  May 2005
+
+
+   Effective Passband
+      As more and more DWDM components are cascaded, the effective
+      passband narrows.  The number of filters along the link, their
+      passband width and their shape will determine the end-to-end
+      effective passband.  In general, this is a system design issue,
+      i.e., the system is designed with certain maximum bit rate using
+      the proper modulation format and filter spacing.  For linear
+      systems, the filter effect can be turned into a constraint on the
+      maximum number of narrow filters with the condition that filters
+      in the systems are at least as wide as the one in the receiver.
+      Because traffic at lower bit rates can tolerate a narrower
+      passband, the maximum allowable number of narrow filters will
+      increase as the bit rate decreases.
+
+   Nonlinear Impairments
+      It seems unlikely that these can be dealt with explicitly in a
+      routing algorithm because they lead to constraints that can couple
+      routes together and lead to complex dependencies, e.g., on the
+      order in which specific fiber types are traversed [Kaminow97].
+      Note that different fiber types (standard single mode fiber,
+      dispersion shifted fiber, dispersion compensated fiber, etc.) have
+      very different effects from nonlinear impairments.  A full
+      treatment of the nonlinear constraints would likely require very
+      detailed knowledge of the physical infrastructure, including
+      measured dispersion values for each span, fiber core area and
+      composition, as well as knowledge of subsystem details such as
+      dispersion compensation technology.  This information would need
+      to be combined with knowledge of the current loading of optical
+      signals on the links of interest to determine the level of
+      nonlinear impairment.  Alternatively, one could assume that
+      nonlinear impairments are bounded and result in X dB margin in the
+      required OSNR level for a given bit rate, where X for performance
+      reasons would be limited to 1 or 2 dB, consequently setting a
+      limit on the maximum number of spans.  For the approach described
+      here to be useful, it is desirable for this span length limit to
+      be longer than that imposed by the constraints which can be
+      treated explicitly.  When designing a DWDM transport system, there
+      are tradeoffs between signal power launched at the transmitter,
+      span length, and nonlinear effects on BER that need to be
+      considered jointly.  Here, we assume that an X dB margin is
+      obtained after the transport system has been designed with a fixed
+      signal power and maximum span length for a given bit rate.  Note
+      that OTSs can be designed in very different ways, in linear,
+      pseudo-linear, or nonlinear environments.  The X-dB margin
+      approach may be valid for some but not for others.  However, it is
+      likely that there is an advantage in designing systems that are
+
+
+
+
+
+Strand & Chiu                Informational                     [Page 12]
+
+RFC 4054                 Optical Layer Routing                  May 2005
+
+
+      less aggressive with respect to nonlinearities, and therefore
+      somewhat sub-optimal, in exchange for improved scalability,
+      simplicity and flexibility in routing and control plane design.
+
+4.5.  Other Impairment Considerations
+
+   There are many other types of impairments that can degrade
+   performance.  In this section, we briefly mention one other type of
+   impairment, which we propose be dealt with by either the system
+   designer or by the transmission engineers at the time the system is
+   installed.  If dealt with successfully in this manner they should not
+   need to be considered in the dynamic routing process.
+
+   Gain Nonuniformity and Gain Transients For simple noise estimates to
+   be of use, the amplifiers must be gain-flattened and must have
+   automatic gain control (AGC).  Furthermore, each link should have
+   dynamic gain equalization (DGE) to optimize power levels each time
+   wavelengths are added or dropped.  Variable optical attenuators on
+   the output ports of an OXC or OADM can be used for this purpose, and
+   in-line devices are starting to become commercially available.
+   Optical channel monitors are also required to provide feedback to the
+   DGEs.  AGC must be done rapidly if signal degradation after a
+   protection switch or link failure is to be avoided.
+
+   Note that the impairments considered here are treated more or less
+   independently.  By considering them jointly and varying the tradeoffs
+   between the effects from different components may allow more routes
+   to be feasible.  If that is desirable or the system is designed such
+   that certain impairments (e.g., nonlinearities) need to be considered
+   by a centralized process, then distributed routing is not the one to
+   use.
+
+4.6.  An Alternative Approach - Using Maximum Distance as the Only
+      Constraint
+
+   Today, carriers often use maximum distance to engineer point-to-point
+   OTS systems given a fixed per-span length based on the OSNR
+   constraint for a given bit rate.  They may desire to keep the same
+   engineering rule when they move to all-optical networks.  Here, we
+   discuss the assumptions that need to be satisfied to keep this
+   approach viable and how to treat the network elements between two
+   adjacent links.
+
+   In order to use the maximum distance for a given bit rate to meet an
+   OSNR constraint as the only binding constraint, the operators need to
+   satisfy the following constraints in their all-optical networks:
+
+
+
+
+
+Strand & Chiu                Informational                     [Page 13]
+
+RFC 4054                 Optical Layer Routing                  May 2005
+
+
+   -  All the other non-OSNR constraints described in the previous
+      subsections are not binding factors as long as the maximum
+      distance constraint is met.
+
+   -  Specifically for PMD, this means that the whole all-optical
+      network is built on top of sufficiently low-PMD fiber such that
+      the upper bound on the mean aggregate path DGD is always satisfied
+      for any path that does not exceed the maximum distance, or PMD
+      compensation devices might be used for routes with high-PMD
+      fibers.
+
+   -  In terms of the ASE/OSNR constraint, in order to convert the ASE
+      constraint into a distance constraint directly, the network needs
+      to have a fixed fiber distance D for each span (so that ASE can be
+      directly mapped by the gain of the amplifier which equals to the
+      loss of the previous fiber span), e.g., 80km spacing which is
+      commonly chosen by carriers.  However, when spans have variable
+      lengths, certain adjustment and compromise need to be made in
+      order to avoid treating ASE explicitly as in section 4.3.  These
+      include: 1) Unless a certain mechanism is built in the OTS to take
+      advantage of shorter spans, spans shorter than a typical span
+      length D need to be treated as a span of length D instead of with
+      its real length.  2) Spans that are longer than D would have a
+      higher average span loss.  In general, the maximum system reach
+      decreases when the average span loss increases.  Thus, in order to
+      accommodate longer spans in the network, the maximum distance
+      upper bound has to be set with respect to the average span loss of
+      the worst path in the network.  This sub-optimality may be
+      acceptable for some networks if the variance is not too large, but
+      may be too conservative for others.
+
+   If these assumptions are satisfied, the second issue we need to
+   address is how to treat a transparent network element (e.g., MEMS-
+   based switch) between two adjacent links in terms of a distance
+   constraint since it also introduces an insertion loss.  If the
+   network element cannot somehow compensate for this OSNR degradation,
+   one approach is to convert each network element into an equivalent
+   length of fiber based on its loss/ASE contribution.  Hence, in
+   general, introducing a set of transparent network elements would
+   effectively result in reducing the overall actual transmission
+   distance between the OEO edges.
+
+   With this approach, the link-specific state information is link-
+   distance, the length of a link.  It equals the distance sum of all
+   fiber spans on the link and the equivalent length of fiber for the
+   network element(s) on the link.  The constraint is that the sum of
+
+
+
+
+
+Strand & Chiu                Informational                     [Page 14]
+
+RFC 4054                 Optical Layer Routing                  May 2005
+
+
+   all the link-distance over all links of a path should be less than
+   the maximum-path-distance, the upper bound of all paths.
+
+4.7.  Other Considerations
+
+   Routing in an all-optical network without wavelength conversion
+   raises several additional issues:
+
+   -  Since the route selected must have the chosen wavelength available
+      on all links, this information needs to be considered in the
+      routing process.  One approach is to propagate information
+      throughout the network about the state of every wavelength on
+      every link in the network.  However, the state required and the
+      overhead involved in processing and maintaining this information
+      is proportional to the total number of links (thus, number of
+      nodes squared), maximum number of wavelengths (which keeps
+      doubling every couple of years), and the frequency of wavelength
+      availability changes, which can be very high.  Instead
+      [Hjalmtysson00], proposes an alternative method which probes along
+      a chosen path to determine which wavelengths (if any) are
+      available.  This would require a significant addition to the
+      routing logic normally used in OSPF.  Others have proposed
+      simultaneously probing along multiple paths.
+
+   -  Choosing a path first and then a wavelength along the path is
+      known to give adequate results in simple topologies such as rings
+      and trees ([Yates99]).  This does not appear to be true in large
+      mesh networks under realistic provisioning scenarios, however.
+      Instead significantly better results are achieved if wavelength
+      and route are chosen simultaneously ([Strand01b]).  This approach
+      would however also have a significant effect on OSPF.
+
+4.8.  Implications For Routing and Control Plane Design
+
+   If distributed routing is desired, additional state information will
+   be required by the routing to deal with the impairments described in
+   Sections 4.2 - 4.4:
+
+   -  As mentioned earlier, an operator who wants to avoid having to
+      provide impairment-related parameters to the control plane may
+      elect not to deal with them at the routing level, instead treating
+      them at the system design and planning level if that is a viable
+      approach for their network.  In this approach the operator can
+      pre-qualify all or a set of feasible end-to-end optical paths
+      through the domain of transparency for each bit rate.  This
+      approach may work well with relatively small and sparse networks,
+      but it may not be scalable for large and dense networks where the
+      number of feasible paths can be very large.
+
+
+
+Strand & Chiu                Informational                     [Page 15]
+
+RFC 4054                 Optical Layer Routing                  May 2005
+
+
+   -  If the optical paths are not pre-qualified, additional link-
+      specific state information will be required by the routing
+      algorithm for each type of impairment that has the potential of
+      being limiting for some routes.  Note that for one operator, PMD
+      might be the only limiting constraint while for another, ASE might
+      be the only one, or it could be both plus some other constraints
+      considered in this document.  Some networks might not be limited
+      by any of these constraints.
+
+   -  For an operator needing to deal explicitly with these constraints,
+      the link-dependent information identified above for PMD is link-
+      PMD-square which is the square of the total PMD on a link.  For
+      ASE the link-dependent information identified is link-noise which
+      is the total noise on a link.  Other link-dependent information
+      includes link-span-length which is the total number of spans on a
+      link, link-crosstalk or OADM-OXC-number which is the total
+      crosstalk or the number of OADM/OXC nodes on a link, respectively,
+      and filter-number which is the number of narrow filters on a link.
+      When the alternative distance-only approach is chosen, the link-
+      specific information is link-distance.
+
+   -  In addition to the link-specific information, bounds on each of
+      the impairments need to be quantified.  Since these bounds are
+      determined by the system designer's impairment allocations, these
+      will be system dependent.  For PMD, the constraint is that the sum
+      of the link-PMD-square of all links on the transparent segment is
+      less than the square of (a/B) where B is the bit rate.  Hence, the
+      required information is the parameter "a".  For ASE, the
+      constraint is that the sum of the link-noise of all links is no
+      larger than P/SNRmin.  Thus, the information needed include the
+      launch power P and OSNR requirement SNRmin.  The minimum
+      acceptable OSNR, in turn, depends on the strength of the FEC being
+      used and the margins reserved for other types of impairments.
+      Other bounds include the maximum span length of the transmission
+      system, the maximum path crosstalk or the maximum number of
+      OADM/OXC nodes, and the maximum number of narrow filters, all are
+      bit rate dependent.  With the alternative distance-only approach,
+      the upper bound is the maximum-path-distance.  In single-vendor
+      "islands" some of these parameters may be available in a local or
+      EMS database and would not need to be advertised
+
+   -  It is likely that the physical layer parameters do not change
+      value rapidly and could be stored in some database; however these
+      are physical layer parameters that today are frequently not known
+      at the granularity required.  If the ingress node of a lightpath
+      does path selection these parameters would need to be available at
+      this node.
+
+
+
+
+Strand & Chiu                Informational                     [Page 16]
+
+RFC 4054                 Optical Layer Routing                  May 2005
+
+
+   -  The specific constraints required in a given situation will depend
+      on the design and engineering of the domain of transparency; for
+      example it will be essential to know whether chromatic dispersion
+      has been dealt with on a per-link basis, and whether the domain is
+      operating in a linear or nonlinear regime.
+
+   -  As optical transport technology evolves, the set of constraints
+      that will need to be considered either explicitly or via a
+      domain-wide margin may change.  The routing and control plane
+      design should therefore be as open as possible, allowing
+      parameters to be included as necessary.
+
+   -  In the absence of wavelength conversion, the necessity of finding
+      a single wavelength that is available on all links introduces the
+      need to either advertise detailed information on wavelength
+      availability, which probably doesn't scale, or have some mechanism
+      for probing potential routes with or without crankback to
+      determine wavelength availability.  Choosing the route first, and
+      then the wavelength, may not yield acceptable utilization levels
+      in mesh-type networks.
+
+5.  More Complex Networks
+
+   Mixing optical equipment in a single domain of transparency that has
+   not been explicitly designed to interwork is beyond the scope of this
+   document.  This includes most multi-vendor all-optical networks.
+
+   An optical network composed of multiple domains of transparency
+   optically isolated from each other by O/E/O devices (transponders) is
+   more plausible.  A network composed of both "opaque" (optically
+   isolated) OLXCs and one or more all-optical "islands" isolated by
+   transponders is of particular interest because this is most likely
+   how all-optical technologies (such as that described in Sec. 2) are
+   going to be introduced.  (We use the term "island" in this discussion
+   rather than a term like "domain" or "area" because these terms are
+   associated with specific approaches like BGP or OSPF.)
+
+   We consider the complexities raised by these alternatives now.
+
+   The first requirement for routing in a multi-island network is that
+   the routing process needs to know the extent of each island.  There
+   are several reasons for this:
+
+   -  When entering or leaving an all-optical island, the regeneration
+      process cleans up the optical impairments discussed in Sec. 3.
+
+   -  Each all-optical island may have its own bounds on each
+      impairment.
+
+
+
+Strand & Chiu                Informational                     [Page 17]
+
+RFC 4054                 Optical Layer Routing                  May 2005
+
+
+   -  The routing process needs to be sensitive to the costs associated
+      with "island-hopping".
+
+   This last point needs elaboration.  It is extremely important to
+   realize that, at least in the short to intermediate term, the
+   resources committed by a single routing decision can be very
+   significant: The equipment tied up by a single coast-to-coast OC-192
+   can easily have a first cost of $10**6, and the holding times on a
+   circuit once established is likely to be measured in months.
+   Carriers will expect the routing algorithms used to be sensitive to
+   these costs.  Simplistic measures of cost such as the number of
+   "hops" are not likely to be acceptable.
+
+   Taking the case of an all-optical island consisting of an "ultra
+   long-haul" system like that in Fig. 3-1 embedded in an OEO network of
+   electrical fabric OLXCs as an example: It is likely that the ULH
+   system will be relatively expensive for short hops but relatively
+   economical for longer distances.  It is therefore likely to be
+   deployed as a sort of "express backbone".  In this scenario a carrier
+   is likely to expect the routing algorithm to balance OEO costs
+   against the additional costs associated with ULH technology and route
+   circuitously to make maximum use of the backbone where appropriate.
+   Note that the metrics used to do this must be consistent throughout
+   the routing domain if this expectation is to be met.
+
+   The first-order implications for GMPLS seem to be:
+
+   -  Information about island boundaries needs to be advertised.
+
+   -  The routing algorithm needs to be sensitive to island transitions
+      and to the connectivity limitations and impairment constraints
+      particular to each island.
+
+   -  The cost function used in routing must allow the balancing of
+      transponder costs, OXC and OADM costs, and line haul costs across
+      the entire routing domain.
+
+   Several distributed approaches to multi-island routing seem worth
+   investigating:
+
+   -  Advertise the internal topology and constraints of each island
+      globally; let the ingress node compute an end-to-end strict
+      explicit route sensitive to all constraints and wavelength
+      availabilities.  In this approach the routing algorithm used by
+      the ingress node must be able to deal with the details of routing
+      within each island.
+
+
+
+
+
+Strand & Chiu                Informational                     [Page 18]
+
+RFC 4054                 Optical Layer Routing                  May 2005
+
+
+   -  Have the EMS or control plane of each island determine and
+      advertise the connectivity between its boundary nodes together
+      with additional information such as costs and the bit rates and
+      formats supported.  As the spare capacity situation changes,
+      updates would be advertised.  In this approach impairment
+      constraints are handled within each island and impairment-related
+      parameters need not be advertised outside of the island.  The
+      ingress node would then do a loose explicit route and leave the
+      routing and wavelength selection within each island to the island.
+
+   -  Have the ingress node send out probes or queries to nearby gateway
+      nodes or to an NMS to get routing guidance.
+
+6.  Diversity
+
+6.1.  Background on Diversity
+
+   "Diversity" is a relationship between lightpaths.  Two lightpaths are
+   said to be diverse if they have no single point of failure.  In
+   traditional telephony the dominant transport failure mode is a
+   failure in the interoffice plant, such as a fiber cut inflicted by a
+   backhoe.
+
+   Why is diversity a unique problem that needs to be considered for
+   optical networks?  Traditionally, data network operators have relied
+   on their private line providers to ensure diversity and so have not
+   had to deal directly with the problem.  GMPLS makes the complexities
+   handled by the private line provisioning process, including
+   diversity, part of the common control plane and so visible to all.
+
+   To determine whether two lightpath routings are diverse it is
+   necessary to identify single points of failure in the interoffice
+   plant.  To do so we will use the following terms: A fiber cable is a
+   uniform group of fibers contained in a sheath.  An Optical Transport
+   System will occupy fibers in a sequence of fiber cables.  Each fiber
+   cable will be placed in a sequence of conduits - buried honeycomb
+   structures through which fiber cables may be pulled - or buried in a
+   right of way (ROW).  A ROW is land in which the network operator has
+   the right to install his conduit or fiber cable.  It is worth noting
+   that for economic reasons, ROWs are frequently obtained from
+   railroads, pipeline companies, or thruways.  It is frequently the
+   case that several carriers may lease ROW from the same source; this
+   makes it common to have a number of carriers' fiber cables in close
+   proximity to each other.  Similarly, in a metropolitan network,
+   several carriers might be leasing duct space in the same RBOC
+   conduit.  There are also "carrier's carriers" - optical networks
+   which provide fibers to multiple carriers, all of whom could be
+   affected by a single failure in the "carrier's carrier" network.  In
+
+
+
+Strand & Chiu                Informational                     [Page 19]
+
+RFC 4054                 Optical Layer Routing                  May 2005
+
+
+   a typical intercity facility network there might be on the order of
+   100 offices that are candidates for OLXCs.  To represent the inter-
+   office fiber network accurately a network with an order of magnitude
+   more nodes is required.  In addition to Optical Amplifier (OA) sites,
+   these additional nodes include:
+
+   -  Places where fiber cables enter/leave a conduit or right of way;
+
+   -  Locations where fiber cables cross; Locations where fiber splices
+      are used to interchange fibers between fiber cables.
+
+   An example of the first might be:
+
+                                    A                 B
+      A-------------B                 \             /
+                                        \         /
+                                          X-----Y
+                                        /         \
+      C-------------D                 /             \
+                                    C                 D
+
+      (a) Fiber Cable Topology      (b) Right-Of-Way/Conduit Topology
+
+             Figure 6-1:  Fiber Cable vs. ROW Topologies
+
+   Here the A-B fiber cable would be physically routed A-X-Y-B and the
+   C-D cable would be physically routed C-X-Y-D.  This topology might
+   arise because of some physical bottleneck: X-Y might be the Lincoln
+   Tunnel, for example, or the Bay Bridge.
+
+   Fiber route crossing (the second case) is really a special case of
+   this, where X and Y coincide.  In this case the crossing point may
+   not even be a manhole; the fiber routes might just be buried at
+   different depths.
+
+   Fiber splicing (the third case) often occurs when a major fiber route
+   passes near to a small office.  To avoid the expense and additional
+   transmission loss only a small number of fibers are spliced out of
+   the major route into a smaller route going to the small office.  This
+   might well occur in a manhole or hut.  An example is shown in Fig.
+   6-2(a), where A-X-B is the major route, X the manhole, and C the
+   smaller office.  The actual fiber topology would then look like Fig.
+   6-2(b), where there would typically be many more A-B fibers than A-C
+   or C-B fibers, and where A-C and C-B might have different numbers of
+   fibers.  (One of the latter might even be missing.)
+
+
+
+
+
+
+Strand & Chiu                Informational                     [Page 20]
+
+RFC 4054                 Optical Layer Routing                  May 2005
+
+
+                      C                             C
+                      |                           /   \
+                      |                         /       \
+                      |                       /           \
+               A------X------B              A---------------B
+
+               (a) Fiber Cable Topology     (b) Fiber Topology
+
+                 Figure 6-2.  Fiber Cable vs Fiber Topologies
+
+   The imminent deployment of ultra-long (>1000 km) Optical Transport
+   Systems introduces a further complexity: Two OTSes could interact a
+   number of times.  To make up a hypothetical example: A New York -
+   Atlanta OTS and a Philadelphia - Orlando OTS might ride on the same
+   right of way for x miles in Maryland and then again for y miles in
+   Georgia.  They might also cross at Raleigh or some other intermediate
+   node without sharing right of way.
+
+   Diversity is often equated to routing two lightpaths between a single
+   pair of points, or different pairs of points so that no single route
+   failure will disrupt them both.  This is too simplistic, for a number
+   of reasons:
+
+   -  A sophisticated client of an optical network will want to derive
+      diversity needs from his/her end customers' availability
+      requirements.  These often lead to more complex diversity
+      requirements than simply providing diversity between two
+      lightpaths.  For example, a common requirement is that no single
+      failure should isolate a node or nodes.  If a node A has single
+      lightpaths to nodes B and C, this requires A-B and A-C to be
+      diverse.  In real applications, a large data network with N
+      lightpaths between its routers might describe their needs in an
+      NxN matrix, where (i,j) defines whether lightpaths i and j must be
+      diverse.
+
+   -  Two circuits that might be considered diverse for one application
+      might not be considered diverse for in another situation.
+      Diversity is usually thought of as a reaction to interoffice route
+      failures.  High reliability applications may require other types
+      of failures to be taken into account.  Some examples:
+
+      o  Office Outages: Although less frequent than route failures,
+         fires, power outages, and floods do occur.  Many network
+         managers require that diverse routes have no (intermediate)
+         nodes in common.  In other cases an intermediate node might be
+         acceptable as long as there is power diversity within the
+         office.
+
+
+
+
+Strand & Chiu                Informational                     [Page 21]
+
+RFC 4054                 Optical Layer Routing                  May 2005
+
+
+      o  Shared Rings: Many applications are willing to allow "diverse"
+         circuits to share a SONET ring-protected link; presumably they
+         would allow the same for optical layer rings.
+
+      o  Disasters: Earthquakes and floods can cause failures over an
+         extended area.  Defense Department circuits might need to be
+         routed with nuclear damage radii taken into account.
+
+   -  Conversely, some networks may be willing to take somewhat larger
+      risks.  Taking route failures as an example: Such a network might
+      be willing to consider two fiber cables in heavy duty concrete
+      conduit as having a low enough chance of simultaneous failure to
+      be considered "diverse".  They might also be willing to view two
+      fiber cables buried on opposite sides of a railroad track as being
+      diverse because there is minimal danger of a single backhoe
+      disrupting them both even though a bad train wreck might
+      jeopardize them both.  A network seeking N mutually diverse paths
+      from an office with less than N diverse ROWs will need to live
+      with some level of compromise in the immediate vicinity of the
+      office.
+
+   These considerations strongly suggest that the routing algorithm
+   should be sensitive to the types of threat considered unacceptable by
+   the requester.  Note that the impairment constraints described in the
+   previous section may eliminate some of the long circuitous routes
+   sometimes needed to provide diversity.  This would make it harder to
+   find many diverse paths through an all-optical network than an opaque
+   one.
+
+   [Hjalmtysson00] introduced the term "Shared Risk Link Group" (SRLG)
+   to describe the relationship between two non-diverse links.  The
+   above examples and discussion given at the start of this section
+   suggests that an SRLG should be characterized by 2 parameters:
+
+   -  Type of Compromise: Examples would be shared fiber cable, shared
+      conduit, shared ROW, shared optical ring, shared office without
+      power sharing, etc.)
+
+   -  Extent of Compromise:  For compromised outside plant, this would
+      be the length of the sharing.
+
+   A CSPF algorithm could then penalize a diversity compromise by an
+   amount dependent on these two parameters.
+
+
+
+
+
+
+
+
+Strand & Chiu                Informational                     [Page 22]
+
+RFC 4054                 Optical Layer Routing                  May 2005
+
+
+   Two links could be related by many SRLGs.  (AT&T's experience
+   indicates that a link may belong to over 100 SRLGs, each
+   corresponding to a separate fiber group.)  Each SRLG might relate a
+   single link to many other links.  For the optical layer, similar
+   situations can be expected where a link is an ultra-long OTS.
+
+   The mapping between links and different types of SRLGs is in general
+   defined by network operators based on the definition of each SRLG
+   type.  Since SRLG information is not yet ready to be discoverable by
+   a network element and does not change dynamically, it need not be
+   advertised with other resource availability information by network
+   elements.  It could be configured in some central database and be
+   distributed to or retrieved by the nodes, or advertised by network
+   elements at the topology discovery stage.
+
+6.2.  Implications For Routing
+
+   Dealing with diversity is an unavoidable requirement for routing in
+   the optical layer.  It requires dealing with constraints in the
+   routing process, but most importantly requires additional state
+   information (e.g., the SRLG relationships).  The routings of any
+   existing circuits from which the new circuit must be diverse must
+   also be available to the routing process.
+
+   At present SRLG information cannot be self-discovered.  Indeed, in a
+   large network it is very difficult to maintain accurate SRLG
+   information.  The problem becomes particularly daunting whenever
+   multiple administrative domains are involved, for instance after the
+   acquisition of one network by another, because there normally is a
+   likelihood that there are diversity violations between the domains.
+   It is very unlikely that diversity relationships between carriers
+   will be known any time in the near future.
+
+   Considerable variation in what different customers will mean by
+   acceptable diversity should be anticipated.  Consequently we suggest
+   that an SRLG should be defined as follows: (i) It is a relationship
+   between two or more links, and (ii) it is characterized by two
+   parameters, the type of compromise (shared conduit, shared ROW,
+   shared optical ring, etc.) and the extent of the compromise (e.g.,
+   the number of miles over which the compromise persisted).  This will
+   allow the SRLGs appropriate to a particular routing request to be
+   easily identified.
+
+7.  Security Considerations
+
+   We are assuming OEO interfaces to the domain(s) covered by our
+   discussion (see, e.g., Sec. 4.1 above).  If this assumption were to
+   be relaxed and externally generated optical signals allowed into the
+
+
+
+Strand & Chiu                Informational                     [Page 23]
+
+RFC 4054                 Optical Layer Routing                  May 2005
+
+
+   domain, network security issues would arise.  Specifically,
+   unauthorized usage in the form of signals at improper wavelengths or
+   with power levels or impairments inconsistent with those assumed by
+   the domain would be possible.  With OEO interfaces, these types of
+   layer one threats should be controllable.
+
+   A key layer one security issue is resilience in the face of physical
+   attack.  Diversity, as describe in Sec. 6, is a part of the solution.
+   However, it is ineffective if there is not sufficient spare capacity
+   available to make the network whole after an attack.  Several major
+   related issues are:
+
+   -  Defining the threat: If, for example, an electro-magnetic
+      interference (EMI) burst is an in-scope threat, then (in the
+      terminology of Sec. 6) all of the links sufficiently close
+      together to be disrupted by such a burst must be included in a
+      single SRLG.  Similarly for other threats: For each in-scope
+      threat, SRLGs must be defined so that all links vulnerable to a
+      single incident of the threat must be grouped together in a single
+      SRLG.
+
+   -  Allocating responsibility for responding to a layer one failure
+      between the various layers (especially the optical and IP layers):
+      This must be clearly specified to avoid churning and unnecessary
+      service interruptions.
+
+   The whole proposed process depends on the integrity of the impairment
+   characterization information (PMD parameters, etc.) and also the SRLG
+   definitions.  Security of this information, both when stored and when
+   distributed, is essential.
+
+   This document does not address control plane issues, and so control-
+   plane security is out of scope.  IPO control plane security
+   considerations are discussed in [Rajagopalam04].  Security
+   considerations for GMPLS, a likely control plane candidate, are
+   discussed in [Mannie04].
+
+8.  Acknowledgments
+
+   This document has benefited from discussions with Michael Eiselt,
+   Jonathan Lang, Mark Shtaif, Jennifer Yates, Dongmei Wang, Guangzhi
+   Li, Robert Doverspike, Albert Greenberg, Jim Maloney, John Jacob,
+   Katie Hall, Diego Caviglia, D. Papadimitriou, O. Audouin, J. P.
+   Faure, L. Noirie, and with our OIF colleagues.
+
+
+
+
+
+
+
+Strand & Chiu                Informational                     [Page 24]
+
+RFC 4054                 Optical Layer Routing                  May 2005
+
+
+9.  References
+
+9.1.  Normative References
+
+   [Goldstein94]   Goldstein, E. L., Eskildsen, L., and Elrefaie, A. F.,
+                   Performance Implications of Component Crosstalk in
+                   Transparent Lightwave Networks", IEEE Photonics
+                   Technology Letters, Vol.6, No.5, May 1994.
+
+   [Hjalmtysson00] Gsli Hjalmtysson, Jennifer Yates, Sid Chaudhuri and
+                   Albert Greenberg, "Smart Routers - Simple Optics: An
+                   Architecture for the Optical Internet, IEEE/OSA
+                   Journal of Lightwave Technology, December 2000, Vo
+                   18, Issue 12, Dec. 2000, pp. 1880-1891.
+
+   [ITU]           ITU-T Doc. G.663, Optical Fibers and Amplifiers,
+                   Section II.4.1.2.
+
+   [Kaminow97]     Kaminow, I. P. and Koch, T. L., editors, Optical
+                   Fiber Telecommunications IIIA, Academic Press, 1997.
+
+   [Mannie04]      Mannie, E., Ed., "Generalized Multi-Protocol Label
+                   Switching (GMPLS) Architecture", RFC 3945, October
+                   2004.
+
+   [Rajagopalam04]  Rajagopalan, B., Luciani, J., and D. Awduche, "IP
+                   over Optical Networks: A Framework", RFC 3717, March
+                   2004.
+
+   [Strand01]      Strand, J., Chiu, A., and R. Tkach, "Issues for
+                   Routing in the Optical Layer", IEEE Communications
+                   Magazine, Feb. 2001, vol. 39 No. 2, pp. 81-88.
+
+   [Strand01b]     Strand, J., Doverspike, R., and G. Li, "Importance of
+                   Wavelength Conversion In An Optical Network", Optical
+                   Networks Magazine, May/June 2001, pp. 33-44.
+
+   [Yates99]       Yates, J. M., Rumsewicz, M. P., and J. P. R. Lacey,
+                   "Wavelength Converters in Dynamically-Reconfigurable
+                   WDM Networks", IEEE Communications Surveys, 2Q1999
+                   (online at
+                   www.comsoc.org/pubs/surveys/2q99issue/yates.html).
+
+
+
+
+
+
+
+
+
+Strand & Chiu                Informational                     [Page 25]
+
+RFC 4054                 Optical Layer Routing                  May 2005
+
+
+9.2.  Informative References
+
+   [Awduche99]     Awduche, D. O., Rekhter, Y., Drake, J., R. and
+                   Coltun, "Multi-Protocol Lambda Switching: Combining
+                   MPLS Traffic Engineering Control With Optical
+                   Crossconnects", Work in Progress.
+
+   [Gerstel2000]   Gorstel, O., "Optical Layer Signaling: How Much Is
+                   Really Needed?" IEEE Communications Magazine, vol. 38
+                   no. 10, Oct. 2000, pp. 154-160
+
+   [Kaminow02]     Ivan P. Kaminow and Tingye Li (editors), "Optical
+                   Fiber Communications IV: Systems and Impairments",
+                   Elsevier Press, 2002.
+
+   [Passmore01]    Passmore, D., "Managing Fatter Pipes," Business
+                   Communications Review, August 2001, pp. 20-21.
+
+   [Ramaswami98]   Ramaswami, R. and K. N. Sivarajan, Optical Networks:
+                   A Practical Perspective, Morgan Kaufmann Publishers,
+                   1998.
+
+   [Strand02]      John Strand, "Optical Network Architecture
+                   Evolution", in [Kaminow02].
+
+   [Tkach98]       Tkach, R., Goldstein, E., Nagel, J., and J. Strand,
+                   "Fundamental Limits of Optical Transparency", Optical
+                   Fiber Communication Conf., Feb. 1998, pp. 161-162.
+
+10.  Contributing Authors
+
+   This document was a collective work of a number of people. The text
+   and content of this document was contributed by the editors and the
+   co-authors listed below.
+
+   Ayan Banerjee
+   Calient Networks
+   6620 Via Del Oro
+   San Jose, CA 95119
+   EMail: abanerjee@calient.net
+
+
+   Prof. Dan Blumenthal
+   Eng. Science Bldg., Room 2221F
+   Department of Electrical and Computer Engineering
+   University of California
+   Santa Barbara, CA 93106-9560
+   EMail: danb@ece.ucsb.edu
+
+
+
+Strand & Chiu                Informational                     [Page 26]
+
+RFC 4054                 Optical Layer Routing                  May 2005
+
+
+   Dr. John Drake
+   Boeing
+   2260 E Imperial Highway
+   El Segundo, Ca 90245
+   EMail: John.E.Drake2@boeing.com
+
+
+   Andre Fredette
+   Hatteras Networks
+   PO Box 110025
+   Research Triangle Park, NC 27709
+   EMail: afredette@hatterasnetworks.com
+
+
+   Change Nan Froberg's reach info to:
+   Dr. Nan Froberg
+   Photonic Systems, Inc.
+   900 Middlesex Turnpike, Bldg #5
+   Billerica, MA 01821
+   EMail: nfroberg@photonicsinc.com
+
+
+   Dr. Taha Landolsi
+   King Fahd University
+   KFUPM Mail Box 1026
+   Dhahran 31261, Saudi Arabia
+   EMail: landolsi@kfupm.edu.sa
+
+
+   James V. Luciani
+   900 Chelmsford St.
+   Lowell, MA 01851
+   EMail: james_luciani@mindspring.com
+
+
+   Dr. Robert Tkach
+   32 Carriage House Lane
+   Little Silver, NJ 07739
+   908 246 5048
+   EMail: tkach@ieee.org
+
+
+
+
+
+
+
+
+
+
+
+Strand & Chiu                Informational                     [Page 27]
+
+RFC 4054                 Optical Layer Routing                  May 2005
+
+
+   Yong Xue
+   Dr. Yong Xue
+   DoD/DISA
+   5600 Columbia Pike
+   Falls Church VA 22041
+   EMail: yong.xue@disa.mil
+
+Editors' Addresses
+
+   Angela Chiu
+   AT&T Labs
+   200 Laurel Ave., Rm A5-1F13
+   Middletown, NJ 07748
+
+   Phone: (732) 420-9061
+   EMail: chiu@research.att.com
+
+
+   John Strand
+   AT&T Labs
+   200 Laurel Ave., Rm A5-1D33
+   Middletown, NJ 07748
+
+   Phone: (732) 420-9036
+   EMail: jls@research.att.com
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Strand & Chiu                Informational                     [Page 28]
+
+RFC 4054                 Optical Layer Routing                  May 2005
+
+
+Full Copyright Statement
+
+   Copyright (C) The Internet Society (2005).
+
+   This document is subject to the rights, licenses and restrictions
+   contained in BCP 78, and except as set forth therein, the authors
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+
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+Acknowledgement
+
+   Funding for the RFC Editor function is currently provided by the
+   Internet Society.
+
+
+
+
+
+
+
+Strand & Chiu                Informational                     [Page 29]
+