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Internet Working Group                                 D. Papadimitriou
Internet Draft                                                 F. Poppe
Document: draft-many-inference-srlg-02.txt                     J. Jones
Category: Internet Draft                               S. Venkatachalam
Expires: May 2002                                               Alcatel

                                                         S. Dharanikota
                                                                R. Jain
                                                         Nayna Networks

                                                             R. Hartani
                                                       Caspian Networks

                                                            D. Griffith

                                                               Yong Xue

                                                          November 2001

                  Inference of Shared Risk Link Groups


Status of this Memo

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

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026 except that the right to
   produce derivative works is not granted.

   Internet-Drafts are working documents of the Internet Engineering
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   The list of current Internet-Drafts can be accessed at

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   The Shared Risk Link Group (SRLG) concept introduced in [IPO-Frame]
   is considered as one of the most important criteria concerning the
   constrained-based path computation of optical channel routes. By
   applying the SRLG constraint criteria to the constrained-based path
   computation, one can select a route taking into account resource and
   logical structure disjointness that implies a lower probability of
   simultaneous lightpath failure. This contribution describes the
   various physical and logical resource types considered in the SRLG
   concept. The proposed model focuses on the inference of SRLG
   information between the network physical layers as well as logical
   structures such as geographical locations. The main applications of
   the proposed model are related to the Constraint-based Shortest Path
   First (CSPF) algorithm for optical channel route computation and the
   aggregation of the SRLG information flooded throughout traffic
   engineering extensions of the IGP routing protocols (such as OSPF
   and IS-IS).

1. Introduction

   Many proposals include the SRLG concept when considering the
   disjointness of the constraint-based path computation for optical
   channel routes. In optical domains this concept of SRLG is used for
   deriving a path, which is disjoint from the physical resource and
   logical topology point-of-view. The SRLG concept and the
   corresponding requirements have already been described in [IPO-IMP]
   while considering physical network topology and associated risks.
   Within the scope of this document, these requirements can be
   summarized as follows:
   1. The SRLG encoding mechanism should reduce the path computation
   2. The SRLG information flooding should be scoped to reduce the
      amount of information that is sent across domains.
   3. The SRLG encoding should accommodate the physical and logical
      restrictions imposed on the diversity requirements.

   However, the definition of SRLG in the current format as described
   in [GMPLS-OSPF] and [GMPLS-ISIS] does not provide:
   1. The relationship between logical structures or physical resources
      For example, a fiber could be part of a sequence of fiber
      segments, which is included in a given geographical region.
   2. The risk assessment during path computation implying the
      allocation of a conditional failure probabilities with the SRLGs
   3. The analysis of the specifications of constraint-based path
      computation and path re-optimization taking SRLG information into

   The model described in this document proposes a technique to compute
   the SRLG with respect to a given risk type. This is achieved by
   identifying for a given physical layer the resources belonging to an
   SRLG. The proposed model also permits to compute the dependencies of

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   these resources on the resources belonging to lower physical layers.
   The result of the computation also enables to determine the risk
   associated to each of the SRLGs.

   The remainder of this memo is organized as follows. In section 3, we
   present the hierarchical model of the resources and the
   corresponding SRLG encoding. In section 4, we discuss the use of
   such a model for the risk assessment for the path computation.
   Future work is proposed in section 5, which is followed by
   references in section 6. Appendix 1 provides an elaborate discussion
   on the inference of SRLGs.

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 [1].

3. Hierarchical Model

   The model described in this proposal includes two hierarchies
   defined as follows:

   - Physical hierarchy, which is related to the fiber topology (more
     generally the physical resources) of the optical network including
     the wavelengths built on top of this physical topology.

   - Logical hierarchy, which is related to the geographical topology
     of the network.

   Between these two hierarchies, the nodes such as Optical Cross-
   Connect (OXC) and Photonic Cross-Connect (PXC) constitute the
   boundary layer. Each of these concepts is elaborated in the
   following sections.

   The encoding of the SRLG could be either mapped on this hierarchical
   model or simply use a flat encoding scheme. Both methods seam
   feasible. Difference between both approaches relies on the extended
   usage of the SRLGs in the context of diverse route computation (i.e.
   path disjointness). Since a link can belong to more than one SRLG,
   an SRLG identifier list (i.e. the SRLG Sub-TLV), as described in
   [GMPLS-OSPF] and [GMPLS-ISIS] is associated with the link to which
   this link belongs (i.e. the SRLG Sub-TLV is defined as a Sub-TLV of
   the Link TLV). This results in a linear, unordered and non-
   structured information from which the underlying structure cannot be

   Consequently, either a type field indicating the type of resource
   (or logical structure) to which this SRLG identifier refers extends
   the flat encoding scheme or the encoding itself translates the
   underlying hierarchical structure. Worth mentioning here that an
   hierarchical encoding (since depending on the physical layer which
   is by definition static) needs an additional mapping structure in

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   order to keep the relationship with link identifiers. Nevertheless,
   the computational model developed in Appendix 1 does not depend on
   the encoding scheme.

3.1 Physical Hierarchy (or Network Resource Hierarchy)

   The network (physical) resource model considered in the inference of
   the Shared Risk Link Groups (SRLGs) is based on concepts detailed in
   [IPO-FRAME] and [IPO-IMP]. The concepts around network resource
   hierarchy developed within this document are based on the following
   - Sub-Channel: a dedicated container included within a given channel
     uniquely identifies a sub-channel
   - Channel (or wavelength): a channel is uniquely identified by a
     dedicated wavelength (i.e. lambda)
   - Fiber Link: a fiber connects two node ports communicating through
     one optical channel or more than one optical channel if the node
     interfaces support Wavelength Division Multiplexing (WDM).
   - Fiber Sub-segment: grouping of several fiber links forms a fiber
   - Fiber Segment: a fiber segment includes a collection of fiber sub-
   - Fiber Trunks: a fiber trunk is a sequence of fiber segments,
     including one or more fiber segments starting and terminating at
     the same node.

   The model developed extends the definition given within [IPO-IMP]
   and [IPO-FRAME] by enabling †fiber topologyË non-limited to point-
   to-point node connections. Physical resources considered within this
   model are a common denominator of most Optical Transport Network
   (OTN) environments.

   As represented in Figure 1, the fiber trunk from the location N1 to
   the location N3 is composed by the fiber segments A and B and the
   fiber trunk from the location N1 to the location N2 includes the
   fiber segment A, C and D.

    Location N1                                          Location N3

   === . . . ====== Fiber                     Fiber ====== . .   ====
   === . . . ====== Fiber                     Fiber ====== . . . ====
   Sub-Segment A[1]                                  Sub-Segment B[1]
     ------------------------------   -----------------------------
   === . . . ====== Fiber          | |        Fiber ====== . . . ====
   === . . . ====== Fiber          | |        Fiber ====== . . . ====
     -------------------------     | |     -------------------------
    +++++++++++++++++++++++++ |    | |    | +++++++++++++++++++++++++
      Segment A             + |    | |    | +         Segment B
                            + |    | |    | +
                            + |    | |    | +

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                            + |    | |    | + Segment C
                            + |    | |    | +
                            + |    | |    | +
      Segment D             + |    | |    | +         Segment E
    +++++++++++++++++++++++++ |    | |    | +++++++++++++++++++++++++
     -------------------------     | |     -------------------------
   === . . . ====== Fiber          | |        Fiber ====== . . . ====
   === . . . ====== Fiber          | |        Fiber ====== . . . ====
     ------------------------------   ------------------------------
   Sub-Segment D[1]                                  Sub-Segment E[1]
   === . . . ====== Fiber                     Fiber ====== . . . ====
   === . . . ====== Fiber                     Fiber ====== . . . ====
   Sub-Segment D[n]                                  Sub-Segment E[n]

    Location N2                                          Location N4

              Figure 1. An example for the physical topology

   In this figure, the Segment A is composed by the fiber sub-segments
   A[1], A[2], ..., A[I], ..., A[n]. The same terminology applies for
   the segments B, C, D and E.

   Consequently, the fiber trunk from location N2 to location N4
   includes the sub-segments D[2] to D[n] and their corresponding sub-
   segments within the segment E: E[2] to E[n]. The fiber trunk from
   location N1 to location N2 includes the fiber sub-segments A[n],
   C[1] and D[1].

3.2 Geographical Hierarchy (or Logical Hierarchy)

   Concerning the geographical hierarchy, the SRLG model developed in
   this document, includes the following definitions going from the
   less to the most extended logical structure partitioning of the area
   covered by the optical network (as shown in Figure 2.)

   - Node: a node is a single device or active element included within
     the optical network; a node could be an Optical Cross-Connect
     (OXC) or a Photonic Cross-Connect (PXC). Exit points of a node are
     defined as the node ports.

   - Zone: a zone includes one or more nodes whose location is limited
     to a confined area for the sake of maintainability. Zones have a
     fixed number of exit points and are non-overlapping meaning that a
     given node belongs to only one zone.

   - Region: a region includes one or more zones whose location covers
     the individual locations of each of the area composing this
     region. Regions have a fixed number of exit points and are non-
     overlapping meaning that a given zone belongs to only one region.

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   Hence, a region could include one or more than one non-overlapping
   zone each of these zones could include one or generally more than
   one node.

   |                                           Region 2            |
   |  +--------------------------+  +---------------------------+  |
   |  |                          |  |                 Zone 2    |  |
   |  |                          |  | +----------+ +----------+ |  |
   |  |                          |  | |          | |  A----B  | |  |
   |  |        Region 1          |  | |  Zone 1  | |  |    |  | |  |
   |  |                          |  | |          | |  C----D  | |  |
   |  |                          |  | +----------+ +----------+ |  |
   |  |                          |  |                           |  |
   |  +--------------------------+  +---------------------------+  |
   |                                                               |
   |                 +---------------------------+                 |
   |                 |                           |                 |
   |                 | +----------+ +----------+ |                 |
   |                 | |          | |          | |                 |
   |                 | |  Zone 3  | |  Zone 4  | |                 |
   |                 | |          | |          | |                 |
   |                 | +----------+ +----------+ |                 |
   |                 |          Region 3         |                 |
   |                 +---------------------------+                 |
   |                                                               |

               Figure 2. An example for the logical topology

   Note: A zone could correspond to an IGP area such as an OSPF area,
   and a region to an OSPF Autonomous System (or BGP Autonomous
   Systems). However, the model does not exclude network topologies
   where the SRLG geographical hierarchy does not map the routing
   hierarchical topology.

3.4 SRLG Definition and Properties

   A SRLG is defined as the set of links or optical lines sharing a
   common physical resource (including fiber links/sub-segment/
   segment/trunk) i.e. sharing a common risk. For instance, a set of
   links L belongs to the same SRLG S, if established over the same
   fiber link F.

3.4.1 SRLG Properties

   The SRLG properties can be summarized as follows:

   1) A link belong to more than one SRLG if and only if it crosses one
   of the resources covered by each of these sets

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   For instance: link l belongs to the SRLG s1 and s2, if it crosses
   the fiber sub-segment A[1] and B[1]

   2) Two links belonging to the same SRLG can belong individually to
   other (one or more) SRLGs

   For instance: link l1 and link l2 belongs to SRLG s3 (segment A)
   while l1 belongs to SRLG s1 (since covering sub-segment A[1]) and l2
   to SRLG s4 (since covering sub-segment D[1])

3.4.2 LSP SRLG Disjointess

   The LSP SRLG disjointness concept is based on the following
   postulate: an LSP (i.e. sequence of links) cover an SRLG if and only
   if it crosses one of the links belonging to that SRLG. For instance:
   LSP p1 covering SRLG s1 (since including link l1)

   Therefore, the LSP SRLG disjointness can be defined as follows: two
   LSPs are disjoint with respect to an SRLG s1 if and only if none of
   them covers simultaneously this SRLG. For instance: LSPs p1 and p2
   are disjoint with respect to SRLG s1 since only p1 covers SRLG s1

   While the LSP SRLG (set) disjointness is defined when two lightpaths
   are disjoint with respect to a set of SRLGs S if and only if the
   sets of SRLGs they cover are completely disjoint. For instance: LSP
   p1 and p3 are disjoint with respect to set of SRLG S = {s1, s2, s3}
   since only p1 covers SRLG set S.

3.5 SRLG Computational Model

   This section briefly describes the guidelines for an SRLG
   Computational Model based on the above definition. The main features
   of this model are:

   - Support Constraint-based Shortest Path First (CSPF) algorithm for
     lightpath explicit route (or path) computation by considering
     physical SRLG disjointness with respect to one (or more than one)
     risk type

   - Encompass hierarchical dependencies between physical resources
     (inference of SRLG sets using bottom-up relational computation)

   - CSPF computation including the relationship between physical
     resources and topological structures. For instance:
     - a fiber link can be part of Ÿtrunk÷ included in a specific
       geographical region (Paris, Channel, etc.)
     - a fiber cable passing through Ÿearthquake÷ region like Japan or

   - Provide Risk assessment during path computation implying
     allocation of conditional failure probabilities with SRLGs

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   - SRLG information flooding well-scoped to reduce the amount of
     link-state advertisements by using summarization

   Consequently, the above features suggest an SRLG encoding mechanism
   that enables:
   - Accommodation of the resources covered by physical and topological
   - Reduction of the optical channel path (explicit route)
     computational complexity

3.6 Hierarchical SRLG encoding

   Using the above definitions and properties, the objective of the
   hierarchical encoding is to achieve aggregation (i.e. summarization)
   of the SRLG Identifiers at the boundary of geographical structures
   defined logically on top of the optical network topology. For this
   purpose, we propose a linear encoding scheme including a type field.
   This provides abstraction of the physical layer structure and should
   facilitate the management of the SRLG Identifiers.

   Consequently, the detailed encoding of an SRLG includes:

   1. SRLG Location (32-bit field)

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      |   Region ID   |     Zone ID     |      Reserved (16-bit)      |

   The SRLG Location field identifies the logical structure into which
   the common resource(s) defining the SRLG are included. For
   simplicity, we say that the SRLG Location field identifies the
   location of the SRLG.

   The Location field includes the Region ID (8-bit) which identifies a
   Region and the Zone ID (8-bit) identifying a Zone belonging to this

   2. SRLG Identifier (32-bit field)

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      |      Type     |                  Identifier                   |

   Within the SRLG Identifier, the Type field defines the resource type
   (i.e. the Ÿlink÷ type) to which the Identifier defined as a 24-bit
   integer value. The following resource types (i.e. Ÿlink÷ type) are
   currently defined:

        Type                    Value

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        -----------------       -----
        Reserved                0x00
        Fiber Trunk             0x01
        Fiber Segment           0x02
        Fiber Sub-segment       0x03
        Fiber Link              0x04

   Logical resources such as optical channels and TDM circuits (or
   optical sub-channels) can be also defined as described in Section 3:

        Type                    Value
        -------------------     -----
        Optical Channel         0x05
        Optical Sub-Channel     0x06

   Since a given resource (for instance a fiber link) can belong to
   more than one SRLG, the SRLG Identifier structure is defined in the
   most general case as a list of SRLG Identifier (n x 32-bit):

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      |      Type     |                  Identifier                   |
      |      Type     |                  Identifier                   |
      |                                                               |
      //                           ...                               //
      |                                                               |
      |      Type     |                  Identifier                   |

   Therefore, though we propose a linear encoding, the summarization of
   the SRLG (at the logical structure boundaries) is still possible
   since the SRLG identifiers are structured as follows:
   - An SRLG Location field (32 bits): Region (8 bits) + Zone (8
     bits) + Unspecified (16 bits)
   - An SRLG Identifier field (32 bits): Type (8 bits) +
     Identifier (24 bits)

   This encoding enables one to perform summarization at the boundaries
   of logical structures defining the spatial coverage of an SRLG
   Identifier List while overcoming the drawbacks of full hierarchical
   encoding scheme.

   Note: the proposed encoding does not include the conditional failure
   probability as defined in section 4.2

4. Risk Assessment

   Risk assessment is defined as the quantification process of the
   potential risk associated to the inclusion of a given resource (this

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   resource belongs to a given resource type located within a given
   logical structure such as a geographical location) in a given
   optical channel.

4.1 Rationale for Risk Assessment

   Consider the following example, where the client device makes the
   following connection requests to the optical network:

   - Request for a persistent connection with 99.999 % (well known 5
     9s) of availability or equally a down time less than X minutes per

   - Request a high-protection for a portion of the traffic (at the
     expense of more charging) compared to other low-priority traffic.

   Such requirements will be translated into path specific request.
   Such path specific request can be grouped into path selection
   requirements and path characterization requirements.

   1. Path selection requirements

   These typically dictate which physical path should be taken to
   achieve the availability requirements of the client. These
   requirements are typically the logical and physical diversity as
   mentioned in the hierarchical encoding section (see section 3).

   2. Path characterization requirements

   Path characterization requirements typically dictate the protection
   mechanisms as specified by the client connection request. This can
   be achieved in the form of optical ringed protection, meshed
   protection mechanisms, or combination of both linear and ringed
   protection. However, these are out of the scope of this document.

   The components that need formalization in this example are:
   - Step 1. Specification of the user requirements (such as the
             example above)
   - Step 2. Configuring the network that helps in assessing the
             features such as the availability
   - Step 3. Propagating the above-configured information.
   - Step 4. Using the above-propagated information.

   Step 1 of specifying the requirements is not in the scope of this
   document. Steps 2 to 4 are discussed in the remainder of this

   As an example for this discussion we elaborate on the risk
   assessment for a selected path.

4.2 Quantifying the Risk Assessment

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   Risk (the complementary of availability) assessment is defined as
   the evaluation of the potential risk associated to the inclusion of
   a specific resource (this resource belongs to a given resource type
   located within a given logical structure such as a geographical
   location) in a given path.

   Given that an SRLG Identifier list is used to encode the group of
   logical or physical resources, if a mechanism is devised to assign
   the risk associated with the corresponding resource, we can
   calculate the availability of the corresponding path. This, in order
   to meet the connection availability as requested by the client.

   A simple approach is to assign the conditional failure probability
   with each of the SRLG Identifier. This information can be encoded as
   an optional parameter along with the SRLG information as defined in
   Section 3.3. In addition, weights can be associated to each of the
   SRLG to either increase or decrease the potential usage of the
   resource (i.e. inclusion into the selected route).

   In this approach the configurable parameters are:
   - SRLG Resource and SRLG Location Identifiers
   - Conditional failure probability per SRLG
   - Weight for the selection of the SRLG

   As mentioned above, the resource failure probability is defined as a
   conditional probability. For instance, we can associate a
   conditional failure probability of 25% to any fiber sub-segment
   located within the same zone. It means that by selecting two (or
   more than two) different optical channel routes including the same
   SRLG identifier with respect to fiber sub-segment failure, if one of
   these lightpaths fails, then the probability that the other
   lightpath fails is 25%.

   Moreover, the failure probability of a fiber can also depend on the
   zone into which the fiber is located as well as the length of the
   fiber. In addition, a fiber can pass across different zones with
   different failure probabilities. In this case, we need to consider
   an aggregated failure probability per fiber taking into account each
   of the failure probability of the sub-components.

   For instance, if we refer to our previous example and by considering
   1. a conditional failure probability of 50% is associated to any
      fiber link
   2. a conditional failure probability of 1% to any fiber segment
      located within the same zone

   Then by selecting two different optical channels included within the
   same SRLG with respect to fiber segment failure (S1, for instance),
   we obtain a simultaneous lightpath failure probability of 1%.
   Consequently, if the client asks for a protected path, by choosing
   fiber segment path disjointness, the simultaneous lightpath failure
   probability is also of 1%. However, choose two optical channels

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   flowing through the same fiber (r1, for instance), then we have a
   probability of 50% that both optical channels fail simultaneously.

4.3 Risk Assessment Application

   Up to now we didnËt define the association between the high
   availability of the path and SRLG conditional failure probability. A
   simple way to define the relationship is to consider the
   availability of the service requested by the client (i.e. a working
   and a protected path from the provider point of view) and
   conditional failure probability of the sequence of physical resource
   elements included within the corresponding paths. So if we consider,
   1. a path whose source is located is zone 1 and whose destination in
      zone 2 (same region)
   2. a conditional failure probability of 1% if fiber links are
      selected within the same fiber trunk (and located within the zone
   3. a conditional failure probability of 1% if fiber links are
      selected within the same fiber trunk (and located within the zone
   4. the conditional failure probabilities are independent and
      weighted equally

   Then, the availability of the service concerning the fiber link
   availability is of 98% since in this specific case conditional
   failure probabilities are additive.

   Note that currently, the initial conditional failure probability
   value need to be statically encoded; however, based on the Ÿhistory÷
   of the failures these values could be dynamically re-evaluated. The
   corresponding mechanism still needs to be specified and left for
   further study.

5. SRLG Inference Model Application

   The SRLG Inference Model applications are related to the CSPF
   lightpath route computation and the SRLG identifier sets
   summarization in order to enable intra- and inter-area diverse
   routing. For that purpose we first extend the SRLG concept for
   logical resources such as optical channels and optical sub-channels
   (i.e. TDM circuits).

5.1 Extension of the SRLG Concept to Logical Structures and Resources

   The SRLG concept can be extended to logical-level structures and
   resources by taking into account the following purposes:

   1. Given the physical and geographical-level decomposition of the
      optical network topology, the SRLG encoding can be hierarchically
      structured. The hierarchical encoding helps in constructing the
      logical-level topological abstraction, which in turn can be used
      in the SRLG summarization and loose-path computation. The link
      semantics could be also extended to accommodate the inter-region

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      and inter-zonal links.

   2. Propagate these additional logical-level (structures and
      resources) links using the IGP routing protocols for intra- and
      inter-area routing purposes.

   3. To reduce the amount of the flooded information and hence
      lightpath route computation complexity, the flooding scope of the
      information propagation is extended to accommodate logical
      structures (i.e. region and zone) and logical resources (i.e.
      optical channels and TDM circuits).

5.2 Propagation SRLG Information

   The SRLG of each link (i.e. physical and logical resources) is
   encoded as described in Section 3.3, and this information is
   propagated once at configuration between the various nodes using the
   traffic engineering extensions to the IGP routing protocols such as
   OSPF [GMPLS-OSPF] and IS-IS [GMPLS-ISIS]. After this initial SRLG
   identifier exchange, corresponding values do not change over the

   This propagation of SRLG information will be necessary whenever a
   new link is added or an existing link is removed. Initially the
   probability of failure of the various resources are assumed to be
   configured; it is envisioned that at some later time, the
   probability of failure of the SRLG will be propagated along with the
   SRLG itself (as described in Section 3.3).

5.3 Bottom-Up Computation of the SRR Relations

   Once the traffic-engineering topological information is received by
   the node, the Shared Risk Relationship (SRR) graph can be calculated
   on a regular basis, using the bottom up method described in [SRLG-
   RTG]. The fiber trunk SRR is used to compute the fiber segment SRR,
   which in turn is then used to compute the fiber sub-segment SRR
   until the fiber SRR computation is achieved. To the SRR which
   defines the membership of a resource belonging to the same SRLG set,
   we associate at each resource level (for instance, with this fiber
   SRR), the conditional failure probability between two elements
   belonging to this level (for instance, between two fibers).

5.4 Summarization in Topology and Resource Distribution

   By combining recursively several dependency graphs of known
   structures into a higher-level dependency graph, the number of SRLG
   sets and the number of element they include can be further reduced
   (i.e. the SRLG identifier information is aggregated). Consequently,
   the applications of the extended model will also cover the reduction
   of the SRLG advertisements in the Topology and Resource Distribution
   running instance (i.e. the traffic engineering extensions to the
   link-state advertisements of the IGP protocol). In turn, this

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   improvement will reduce the CSPF algorithm complexity for optical
   channel path calculation (i.e. engineered lightpath setup).

5.5 CSPF Route Computation

   Applications of this model are directly related to the Constraint-
   based Shortest Path First (CSPF) algorithm used for lightpath route
   computation (i.e. traffic-engineered lightpath creation) to maximize
   the lightpath disjointness and so decrease their common failure
   probability. Given an existing set of lightpaths across the network,
   the objective is thus to compute a route across the optical network
   topology for a newly requested lightpath such that this lightpath is
   diversely routed from a given set of existing lightpaths.

   The diversity requirement is a routing constraint, and is expressed
   as the conditional failure probability of a requested lightpath with
   respect to the failure of an existing (set of) lightpath. Hence, in
   addition to the other traffic-engineering constraints, the diversity
   constraint requires that the conditional failure probability not
   exceed a given threshold. Therefore, the CSPF algorithm needs to be
   updated to take the routing diversity constraint into account.

   Moreover, the SRLG concept generates another dimension to the
   existing constraint-based path computation methods traditionally
   used in MPLS-TE based hierarchical networks. The SRLG constraints
   provide an additional dimension to the common traffic-engineering
   constraints such as bandwidth availability, link metrics and other
   parameters. The routing diversity constraint specificity requires
   the use of more appropriate path computation algorithms that provide
   not only complete multi-path disjointness but also partial multi-
   path disjointness with respect to various risk factors. In a similar
   way, appropriate mechanisms should also be used in order to perform
   path re-optimization following various restoration strategies.

6. Security Considerations

   Security considerations related to SRLG Inference model and its
   applications are left for further study.

7. References

   1. [GMPLS-OSPF] K.Kompella et al., †OSPF Extensions in Support of
   Generalized MPLSË, Internet Draft, Work in Progress, draft-ietf-
   ccamp-ospf-gmpls-extensions-00.txt, September 2001.

   2. [GMPLS-ISIS] K.Kompella et al., †ISIS Extensions in Support of
   Generalized MPLSË, Internet Draft, Work in Progress, draft-ietf-
   isis-gmpls-extensions-04.txt, September 2001.

   3. [IEEE-ORL] John Strand et al., †Issues for Routing in the Optical
   LayerË, IEEE Communication Magazine, Volume 39, Number 2, February

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draft-many-inference-srlg-02.txt                         November 2001

   4. [IPO-FRAME] J. Luciani et al., †IP over Optical Networks A
   FrameworkË, Internet Draft, Work in progress, draft-many-ip-optical-
   framework-03.txt, March 2001.

   5. [IPO-IMP] J. Strand, A.Chiu et al., †Impairments And Other
   Constraints On Optical Layer RoutingË, Internet Draft, Work in
   progress, draft-ietf-ipo-impairments-00.txt, May 2001.

   6. [MPLS-BUNDLE] K.Kompella et al., †Link Bundling in MPLS Traffic
   EngineeringË, Internet Draft, Work in progress, draft-kompella-mpls-
   bundle-05.txt, March 2001.

   7. [SRLG-RTG] F.Poppe et al., †SRLG and RoutingË, Paper under

8. Acknowledgments

   The authors would like to thank Bernard Sales, Emmanuel Desmet, Hans
   De Neve, Fabrice Poppe and Gert Grammel for their constructive
   comments and input.

9. Author's Addresses

   Dimitri Papadimitriou (Editor)
   Francis Wellesplein, 1
   B-2018 Antwerpen, Belgium
   Phone: +32 3 240-8491
   Email: dimitri.papadimitriou@alcatel.be

   Fabrice Poppe
   Francis Wellesplein, 1
   B-2018 Antwerpen, Belgium
   Phone: +32 3 240-8006
   Email: fabrice.poppe@alcatel.be

   Jim Jones
   3400 W. Plano Parkway,
   Plano, TX 75075, USA
   Phone: +1 972 519-2744
   Email: jim.d.jones1@usa.alcatel.com

   Senthil Venkatachalam
   45195 Business Court, Suite 400
   Dulles, VA 20166, USA
   Phone: +1 703 654-8635
   Email: senthil.venkatachalam@usa.alcatel.com

   Sudheer Dharanikota

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   Nayna Networks
   157 Topaz St.,
   Milpitas, CA 95035, USA
   Phone: +1 408 956-8000X357
   Email: sudheer@nayna.com

   Raj Jain
   Nayna Networks
   157 Topaz St.,
   Milpitas, CA 95035, USA
   Phone: +1 408 956-8000X309
   Email: raj@nayna.com

   David W. Griffith
   National Institute of Standards and Technology (NIST)
   100 Bureau Drive, Stop 8920
   Gaithersburg, MD 20899-8920, USA
   Phone: +1 301 975-3512
   Email: david.griffith@nist.gov

   Riad Hartani
   Caspian Networks
   170 Baytech Drive,
   San Jose, CA 95134, USA
   Phone: +1 408 382-5216
   Email: riad@caspiannetworks.com

   Yong Xue
   Ashburn, VA, USA
   Phone: +1 703 886-5358
   Email: yxue@uu.net

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