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Versions: 00 01 02

CCAMP Working Group                    Dimitri Papadimitriou  (Alcatel)
Category: Internet Draft                       Fabrice Poppe  (Alcatel)
Expires: May 2003                        Sudheer Dharanikota  (Consult)
                                                Riad Hartani  (Caspian)
                                                    Raj Jain    (Nayna)
                                                   Jim Jones  (Alcatel)
                                       Senthil Venkatachalam    (OPNet)
                                                    Yong Xue (WorldCom)

                                                          November 2002



            Shared Risk Link Groups Encoding and Processing

          draft-papadimitriou-ccamp-srlg-processing-01.txt (*)




Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that
   other groups may also distribute working documents as Internet-
   Drafts. Internet-Drafts are draft documents valid for a maximum of
   six months and may be updated, replaced, or obsoleted by other
   documents at any time. It is inappropriate to use Internet-Drafts as
   reference material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   (*) Formerly draft-many-inference-srlg-02.txt

Abstract

   The Shared Risk Link Group (SRLG) concept introduced in [IPO-FRM]
   and extended in [CCAMP-SRG] is considered as one of the most
   important criteria concerning the constraint-based path computation
   of optical channel routes. By applying the SRLG disjointness
   criteria to the constraint-based path computation, one can select a
   route taking into account both resource and logical structure
   disjointness. That implies a lower probability of simultaneous
   optical channel failure.

   This document describes the various physical and logical resource
   types considered in the SRLG concept. The proposed method focuses on

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   the inference of SRLG information between the network physical and
   logical layers. 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 Shared Risk Link Group (SRLG) concept
   when considering the disjointness for the constraint-based path
   computation of optical channel routes. In the optical domain, the
   SRLG concept is used for deriving a path, which is disjoint with
   respect to physical (or passive) and logical (or active) resources.
   [CROCH] describes algorithms that can use the SRLG information
   inferred using the mechanisms described in this document, in order
   to determine survivable logical topologies on top of a given
   physical topology.

   The corresponding requirements have already been described in [IPO-
   IMP] while considering physical network topology and associated
   risks. In the scope of this document, these requirements can be
   summarized as follows:
   1. The SRLG encoding mechanism should reduce the path computation
      complexity.
   2. The SRLG information flooding should be scoped to reduce the
      amount of information that is exchanged 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 described in
   [GMPLS-OSPF] and [GMPLS-ISIS] does not provide:
   1. The relationship between physical (or logical) resources and
      between physical and logical resources. For example, a fiber
      could be part of a sequence of fiber segments or an optical
      channel may cover several fibers links.
   2. The risk assessment during path computation implying the
      assignment of a conditional failure probability to the SRLGs.
   3. The analysis of the specifics aspects of constraint-based path
      computation and path re-optimization taking SRLG information into
      account.

   This document proposes among others 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 these
   resources to the lower (passive) physical layers. The result of the
   computation also enables to determine the risk associated to each of
   the SRLGs, that is, the probability that two entities (both using
   network resources belonging to a given SRLG) go simultaneously down
   if one of them goes down (thus defining a conditional failure
   probability).


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   The remainder of this memo is organized as follows. In section 3, we
   present the hierarchical model of the network resources and the
   corresponding SRLG encoding. In section 4, we discuss the use of the
   proposed approach for risk assessment during path computation and
   selection. Considerations on topology summarization and path
   disjointness with respect to SRLG are proposed in section 5.

2. Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in
   this document are to be interpreted as described in RFC-2119 [1].

   The reader is assumed to be familiar to the terminology and concepts
   detailed in [GMPLS-RTG], [GMPLS-OSPF] and [GMPLS-ISIS].

3. Hierarchical Model

   The proposed approach is based on a representation of the network
   resources in terms of a hierarchy. This hierarchical structure is
   related to the fiber topology (or more generally the physical
   resources) of the optical network and the channels built on top of
   this physical topology.

   The encoding of the SRLG could be either mapped on this hierarchical
   model or simply use a flat encoding scheme. Difference between both
   encoding relies on the extended usage of the SRLG identifiers 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 (in
   the SRLG Sub-TLV), as described in [GMPLS-OSPF] and [GMPLS-ISIS] is
   associated with each link (i.e. the SRLG Sub-TLV is defined as a
   Sub-TLV of the TE Link TLV). This results in a linear, unordered and
   non-structured information from which the underlying structure
   cannot be deduced.

   Consequently, either a type field indicating the type of network
   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 is that
   a hierarchical encoding (since depending on the physical layer which
   is static by definition) needs an additional mapping structure in
   order to keep the relationship with link identifiers.

3.1 Network Resource Hierarchy

   The network (physical and logical) resource model considered in the
   inference of the Shared Risk Link Groups (SRLGs) is based on
   premises detailed in [IPO-FRM] and [IPO-IMP]. The network resource
   hierarchy includes the logical resources built on top of the
   physical resource hierarchy belonging to the optical network.

   The concepts around network resource hierarchy are based on the
   following logical resource definitions:

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   - Sub-Channel: a dedicated container included within a given optical
     channel uniquely identifies a sub-channel.

   - Channel(*): an optical channel is uniquely identified by its
     corresponding and dedicated wavelength (sub-)interface identifier.

   (*) when conversion or amplification is non-collocated with the
   corresponding interface capability, the passive components are
   logically embedded into the fiber link (see below definition).

   These resources are built on top of the physical resource hierarchy
   composed by the following physical resources considered as a common
   denominator of most Optical Transport Network (OTN) environments:

   - Fiber Link: a fiber connects two node ports communicating through
     one or more optical channel if their interfaces support (Dense)
     Wavelength Division Multiplexing û (D)WDM, also a fiber link can
     be decomposed as a sequence of fiber spans separated by optical
     amplifiers

   - Fiber Sub-segment: a group of fiber links, any fiber link of a
     sub-segment start and terminates at the same location

   - Fiber Segment: a group of fiber sub-segments, any fiber sub-
     segment of segment start and terminates at the same location

   Moreover, one can consider sequences of fiber (sub-)segment starting
   and terminating at the same node and defined them as fiber trunks
   (also referred to as cable or fiber duct).

   Thus, the approach developed here extends the definition and
   concepts given in [IPO-FRM] and [IPO-IMP] by enabling fiber
   topologies not strictly limited to point-to-point physical inter-
   connections between nodes.

   As represented in Figure 1, the fiber trunk going from location N[1]
   to location N[3] is composed by the fiber segments A and B and the
   fiber trunk from location N[1] to location N[2] includes the fiber
   segments A, C and D.

     Location N[1]                                     Location N[3]

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

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

     Location N[2]                                     Location N[4]

                  Figure 1. Example of 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 N[2] to location N[4]
   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 N[1] to location N[2] includes the fiber sub-segments A[n],
   C[1] and D[1].

3.3 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 and
   segment) 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
   l.

3.3.1 SRLG Properties

   The SRLG properties can be summarized as follows:

   1) A link belongs to more than one SRLG if and only if it crosses
   (at least) one of the resources covered by each of these SRLGs

   For instance: fiber link l belongs to the SRLG s1 and s2, if and
   only 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




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   For instance: fiber links l1 and l2 belong to SRLG s3 (segment A)
   while l1 belongs to SRLG s1 (since it covers sub-segment A[1]) and
   l2 to SRLG s4 (since it covers sub-segment D[1])

3.4.2 SRLG Disjointness

   To define the LSP SRLG disjointness notion, we first introduce the
   following definition: an LSP (i.e. sequence of links) covers an SRLG
   if and only if it crosses one of the links belonging to that SRLG.
   For instance: LSP p1 covers SRLG s1 (since it crosses link l1)

   LSP SRLG disjointness can then be defined as follows.

   1) Two LSPs are disjoint with respect to an SRLG s1 if and only if
   at most one of them covers this SRLG.

   For instance: LSPs p1 and p2 are disjoint with respect to SRLG s1
   since only p1 covers SRLG s1.

   2) Two LSPs are disjoint with respect to a set of SRLG S if and only
   if the sets of SRLGs they individually 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 Computational Model Capabilities

   This section briefly describes guidelines for an SRLG computational
   model described in appendix A and based on the above definitions.
   The main features of this model are:

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

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

   - CSPF computation including the relationship between physical
     resources and topological structures. For instance:
     . a fiber link can be part of given fiber trunk
     . a fiber cable passing through an earthquake region

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

   - 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 physical and topological hierarchies
   - Reduction of the path (i.e. explicit route) optimality

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3.6 SRLG Encoding

   Using the above definitions and properties, the objective of the
   hierarchical encoding is to achieve aggregation of the SRLG
   Identifiers on top of the (passive) 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.

   The proposed SRLG sub-TLV of the TE Link top level TLV includes the
   following SRLG Identifier (64-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     |                    Weight                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           Identifier                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Within the SRLG Identifier field (64 bits), the Type field (8 bits)
   defines the resource type (also generically referred to as the link
   type) to which the Identifier (32 bits) integer value refers. The
   weight field (24 bits) assigned on a per-SRLG basis along with the
   Identifier is defined in Section 4.

   The following Type field values are currently defined for physical
   resources, any other value being reserved:

        Type                     Value
        --------------------    ------
        Reserved                 0x00
        Fiber Trunk              0x10
        Fiber Segment            0x20
        Fiber Sub-segment        0x30
        Fiber Link               0x40
        Node(*)                  0xFF

   (*) represents a non-GMPLS capable switching elements

   Logical resources such as optical channels and TDM circuits (or
   optical sub-channels) can take one of the following values:

        Type                     Value
        -------------------     ------
        Optical Channel          0x50
        Optical Sub-Channel      0x60

   Since a given physical or logical resource (for instance a fiber
   link) can belong to more than one SRLG, the SRLG Identifier
   structure is defined in the general case as a list of SRLG
   Identifier (n x 64-bit):

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       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     |                    Weight                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           Identifier                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      Type     |                    Weight                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           Identifier                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      //                           ...                               //
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      Type     |                    Weight                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           Identifier                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Therefore, though we propose a linear encoding, the aggregation of
   the SRLG is still possible. This encoding also enables to perform
   summarization (which is not equivalent to aggregation) at the
   boundaries of structures defining the spatial coverage of an SRLG
   Identifier list while overcoming the drawbacks of full hierarchical
   encoding schemes. In this context, the weight value follows the
   rules defined in the Section 4.

4. Risk Assessment

   Risk assessment is defined as the quantification process of the
   potential risk associated to the inclusion of a given resource
   (belonging to a given resource type) located within a given logical
   structure such as a geographical location for a given logical
   network resource such as an 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 % percent of
     availability or equivalently a downtime less than X minutes per
     year.

   - Request for protection for a portion of the traffic (at the
     expense of more charging) compared to other portion of the
     traffic left unprotected and considered as extra traffic by the
     server.




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   Such requirements will be translated into path specific requests.
   Such path specific requests 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 request. These
   requirements are typically related to the logical and physical
   diversity as mentioned in Section 3.

   2. Path characterization requirements

   Path characterization requirements typically dictate the recovery
   mechanisms as specified by the client connection request. This can
   be achieved in the form of section and/or path recovery but also
   depending on the ring or meshed topology of the optical network.
   However, these considerations are out of the scope of this document.

   The components that need formalization in this example are:
   - Step 1. Specification of the client requirements (service level).
   - Step 2. Configuration of the network that helps in assessing the
             service specifics such as the availability.
   - Step 3. Propagation of the above-configured information.
   - Step 4. Processing of 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 following sections where
   we elaborate on the risk assessment during path selection.

4.2 Risk Assessment

   Risk (the complementary of availability) assessment is defined as
   the evaluation of the potential risk associated to the inclusion for
   a given path of a specific network resource (i.e. belonging to a
   given resource type). Given that an SRLG Identifier list is used to
   encode the group of logical and physical resources, if a mechanism
   is devised to assign the risk of failure associated with the
   corresponding resource, the availability of the path using these
   resources can be computed. This, in order to meet the connection
   availability as requested by the client request.

   A simple approach is to assign the conditional failure probability
   to each of the SRLG Identifiers. This information is encoded in the
   weight (24 bits) field along with the SRLG information as defined in
   Section 3.3. The associated weights can be used to either increase
   or decrease the potential usage of the resource (i.e. inclusion into
   the selected route). The Weight field is defined as a 24-bit length
   encoded integer number (maximum value 2^24 - 1) to be divided by
   (2^24 û 1) and multiplied by 100 to get the corresponding value in
   percents. For instance a conditional failure probability of 0.99999
   corresponds to a weight of 16.777.047 and 0.00005 to 839.


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   For instance, we can associate a conditional failure probability of
   25% (weight = 4.194.304) to any fiber sub-segment. It means that by
   selecting two (or more than two) different optical channel paths
   including the same SRLG identifier with respect to fiber sub-segment
   failure, if one of these connections fails, then the probability
   that the other connection fails is 25%.

   The failure probability of a fiber can also depend on the length of
   the fiber. In addition, a fiber can pass across different regions
   with different failure probabilities. In this case, we need to
   consider an aggregated failure probability per fiber taking into
   account each of the failure probabilities of its sub-components.

   For instance, if we refer to our previous example and by considering
   1. a conditional failure probability of 5% is associated to any
      fiber link
   2. a conditional failure probability of 1% is associated to any
      fiber segment

   Then by selecting two different optical channels included within the
   same SRLG with respect to fiber segment failure, we obtain a
   simultaneous connection failure probability of 1%. Consequently,
   when a protected connection service is requested, by choosing fiber
   segment path disjointness, the simultaneous connection failure
   probability is also of 1%. However, if two optical channels flowing
   through the same fiber link are selected, then we have a probability
   of 5% that both optical channels fail simultaneously.

   More generically, if one assigns a conditional failure probability
   c[i] to each SRLG Identifier i, the resulting conditional failure
   probability C for a logical resource (a connection, for instance)
   using these N resources is given by:

   C = 1 - (1 û c[1]) x (1 û c[2]) x . . . x (1 û c[N])

   When all the conditional failure probabilities c[i] are small (i.e.
   c[i] << 1) C can be approximated by (1 - Sum[i]c[i]).

4.3 Risk Assessment Application

   The relationship between the availability of the service requested
   by the client (i.e. a working and a protection connection) and the
   conditional failure probabilities of the physical resource elements
   included within the corresponding paths can be described as follows.

   If we consider for instance 1) a conditional failure probability of
   1% if fiber links are selected within the same fiber trunk 2) a
   conditional failure probability of 1% if fiber links are selected
   within the same fiber segment and 3) the corresponding failure are
   independent events.

   Then the availability of a connection whose backup is established
   over fiber links (in same fiber segment) and within the same fiber

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   trunks as the primary connection itself is approximately 98% (by
   using the above formula).

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

5. Applications

   The SRLG method developed here results in applications related to
   the CSPF explicit route computation and the SRLG identifier sets
   summarization. The latter enables in turn, intra- and inter-area
   diverse path computation. For that purpose we first extend the SRLG
   concept for logical resources such as optical channels and optical
   sub-channels (i.e. TDM connections).

5.1 Extension to the Logical Resources

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

   1.   Given the physical decomposition of the optical network
        topology, the SRLG encoding can be hierarchically structured.
        The hierarchical encoding helps in constructing the logical
        topology abstraction, which in turn can be used in SRLG
        summarization and loose-path computation. The link semantic may
        also be extended to accommodate virtual links.

   2.   Propagate the SRLG information related to these logical
        (structures and resources) links, for instance (a set of)
        optical channels using IGP flooding for intra- and inter-area
        routing purposes.

   3.   Reduce the amount of the flooded information and hence explicit
        route computation optimality and extend the flooding scope of
        the propagated information to accommodate logical resources
        (i.e. optical channels and TDM circuits).

5.2 SRLG Information Flooding and Summarization

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

   Nevertheless, the propagation of SRLG information will be necessary
   whenever a new link is added or an existing link is removed.
   Initially, it is assumed that the failure probabilities of the
   various resources are (statically) configured. However, it is

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   envisioned that after a certain running period, the failure
   probability associated to the SRLG and propagated along with the
   SRLG sub-TLV itself (as described in Section 3.3) will change over
   time and thus give rise to a dynamic metric.

5.3 Bottom-Up Computation of SRLG Relationships

   Once the traffic engineering (topology-related) link information is
   received by the node, the corresponding SRLG relationships can be
   computed on a regular basis.

   The fiber trunk SRLG relationships are used to compute the fiber
   segment SRLG relationships, which in turn are used to compute the
   fiber sub-segment SRLG relationships and finally the fiber SRLG
   relationships. To each SRLG relationship (at each level), which
   defines the membership of a resource to a particular SRLG, we
   associate the conditional failure probability between two resources
   belonging to this level (for instance, between two fibers that have
   a common fiber sub-segment).

5.4 Topology Summarization and Route Computation

   A direct application of this model is the Constraint-based Shortest
   Path First (CSPF) algorithm used for explicit route computation
   (i.e. traffic-engineered LSP creation) to maximize the connection
   disjointness and so decrease their common failure probability. Given
   an existing set of connections across the network, the objective is
   to compute, for a newly requested connection, an explicit route
   across the optical network topology such that this connection is
   diversely routed from an existing given set of connections.

   The diversity requirement is a routing constraint, that can be
   expressed in terms of a conditional failure probability with respect
   to an existing (set of) connections and more generally any kind of
   resources. Hence, in addition to the other traffic-engineering
   constraints, the diversity constraint requires that the conditional
   failure probability does not exceed a given threshold. The CSPF
   algorithm needs to be updated to take the routing diversity
   constraint into account.

   The SRLG concept generates another dimension to the existing
   constraint-based path computation methods traditionally used in
   hierarchical networks. The SRLG constraints comes in addition to the
   common traffic-engineering constraints such as bandwidth
   availability, link metrics and TE attributes (see [OSPF-TE]). The
   specificity of the routing diversity constraint requires the use of
   a path computation algorithm that provides 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 recovery strategies.



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   The specific aspect of complete and partial multi-path disjointness
   is related to the fact that a path may with respect to SRLG be fully
   or partially disjoint from a given set of path. In brief, one speaks
   about SRLG partial or full multi-path disjointness. This means that
   a connection may be disjoint from another but only to some extent
   with respect to some risk factors. Thus when referring to path
   disjointness with respect to SRLG one may also include the ratio of
   disjointness with respect to various risk type assigned to their
   component resources. Consider for instance the LSPs p1 and p2 using
   j1 and j2 resources respectively, m of these resources having an
   SRLG in common, with m =< j1 and m =< j2, then [(j1-m) + (j2-
   m)]/(j1+j2) provides the disjointness ratio. For instance, if j1=13,
   j2=7 and m=1, this ratio equals 0.90, thus corresponding path p1 and
   p2 are SRLG disjoint at 90%.

   As such, this feature differentiates path disjointness constraint
   from any other constraint commonly considered in path computation.

6. Scalability Considerations

6.1 OSPF Scalability Considerations

   A node SHOULD minimize the amount of routing information it floods.
   Each time a SRLG sub-TLV is configured or removed that information
   shall be flooded (not necessarily immediately) to all nodes in the
   routing domain. This results in updating an existing LSA or flushing
   an existing LSA. Removing an LSA from the (TE) Link State DataBase
   (TE LSDB) can be accomplished in OSPF by prematurely aging the LSA.
   The LSA is re-flooded with an LSA age equal to MaxAge. Each node
   receiving an existing LSA with MaxAge removes it from its link state
   database.

   Also, the usage of OSPF implies each LSA must be refreshed
   periodically (when the LSA age field reaches the LSRefreshTime, see
   [RFC-2328]) to avoid age timeout and removal from the link state
   database. This periodical LSA flooding and processing applies
   particularly to the TE link capability sub-TLVs defined in this
   document since their variation period is expected to be much larger
   than the LSRefreshTime.

   An Opaque LSA has a length field of 16 bits indicating the length of
   the LSA, including the header. Thus, the length of OSPF packets can
   be up to 65535 octets (including the IP header). Moreover, an OSPF
   packet can contain several (Opaque) LSAs. OSPF relies if necessary
   on the IP fragmentation to transmit large packets without any loss
   of functionality. However this is not recommended and it is
   suggested to split packets that are too large into several smaller
   packets.

   Therefore, the proposed SRLG sub-TLVs defined in this document (and
   included in the top level TE Link TLV) MUST not exceed the maximum
   OSPF packet size. This limits the number of SRLG identifiers that a
   sub-TLV can include to approximately 125 (also this number largely

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   depends on the other sub-TLVs included in the corresponding TE LSA
   constraining to take for the sake of this application a conservative
   approach).

6.2 IS-IS Scalability Considerations

   TBD.

7. Compatibility Considerations

7.1 OSPF Compatibility Considerations

   There should be no interoperability issues with GMPLS-capable nodes
   that do not implement the proposed SRLG extensions since the sub-
   TLVs proposed in this document is optional and thus will be silently
   ignored.

   The result of having such nodes that do not implement the proposed
   SRLG extensions is largely detailed in this memo. However, SRLG
   constraint paths can still be calculated using the SRLG sub-TLVs as
   proposed in [GMPLS-OSPF].

   The present memo mandates these sub-TLVs to be mutually exclusive
   i.e. they MUST never appear simultaneously as sub-TLV of the same
   top level Link TLV [OSPF].

7.2 IS-IS Compatibility Considerations

   TBD.

8. Security Considerations

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

9. References

   [CROCH]      O.Crochat, J.-Y. Le Boudec and O. Gerstel, "Protection
                Interoperability for WDM Optical Networks", IEEE/ACM
                Transactions on Networking, Vol. 8, No. 3, June 2000,
                pp. 384-395.

   [IEEE-ORL]   J.Strand et al., æIssues for Routing in the Optical
                LayerÆ, IEEE Communication Magazine, Volume 39, Number
                2, FebÆ01.

   [GMPLS-ISIS] K.Kompella et al., æISIS Extensions in Support of
                Generalized MPLSÆ, Internet Draft, Work in Progress,
                draft-ietf-isis-gmpls-extensions-14.txt.

   [GMPLS-OSPF] K.Kompella et al., æOSPF Extensions in Support of
                Generalized MPLSÆ, Internet Draft, Work in Progress,
                draft-ietf-ccamp-ospf-gmpls-extensions-08.txt.

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   [GMPLS-RTG]  K.Kompella et al., æRouting Extensions in Support of
                Generalized MPLS,Æ Internet Draft, Work in Progress,
                draft-ietf-ccamp-gmpls-routing-05.txt.

   [IPO-FRM]    J.Luciani et al., æIP over Optical Networks A
                FrameworkÆ, Internet Draft, Work in progress, draft-
                ietf-ipo-framework-03.txt.

   [IPO-IMP]    J.Strand et al., æImpairments And Other Constraints On
                Optical Layer RoutingÆ, Internet Draft, Work in
                progress, draft-ietf-ipo-impairments-02.txt.

   [MPLS-BDL]   K.Kompella et al., æLink Bundling in MPLS Traffic
                EngineeringÆ, Internet Draft, Work in progress, draft-
                ietf-mpls-bundle-04.txt.

   [OSPF-TE]    D.Katz, D.Yeung and K.Kompella, ôTraffic Engineering
                Extensions to OSPFö, draft-katz-yeung-ospf-traffic-
                08.txt, Internet Draft, Work in Progress.

   [RFC-2328]   J.Moy, RFC 2328, ôOSPF Version 2ö, STD 54, IETF
                Standard Track, April 1998.

   [RFC-2370]   R.Coltun, RFC 2370, Standard Track, "The OSPF Opaque
                LSA Option", July 1998.

10. Acknowledgments

   The authors would like to thank Bernard Sales, Emmanuel Desmet and
   Gert Grammel for their constructive comments, David Griffith for its
   active contribution and Bart Rousseau for its revision efforts.

11. 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 (Alcatel)
   Francis Wellesplein, 1
   B-2018 Antwerpen, Belgium
   Phone: +32 3 240-8006
   Email: fabrice.poppe@alcatel.be

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


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   Senthil Venkatachalam (OPNet)
   Email: svenkatachalam@opnet.com

   Sudheer Dharanikota (Consulting)
   Email: sudheer@ieee.org

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

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

   Yong Xue (WorldCom)
   Ashburn, VA, USA
   Phone: +1 703 886-5358
   Email: yong.xue@wcom.com

Appendix A: SRLG Computational Model

A.1 SRLG Concept and Example

   The present model is intended to be used to automate the discovery
   of the Shared Risk Link Groups (SRLGs) at a given layer for a given
   physical resource type.

   Note that a typical resource type can be a fiber, a fiber sub-
   segment, a fiber segment or a fiber trunk. SRLG definition and
   properties were described in Section 3.

   Example:

   The following example referring to Figure 5 (for the physical
   network topology) offers some clarification. Let assume that
   - N1, N2, N3, and N4 represent locations that are linked by the
     fiber sub-segments,
   - A, B, C, D and E be fiber segments,
   - and F1 (ACD), F2 (AB), F3 (BCD) and F4 (DE) are fiber links routed
     over the fiber segment topology.


         N1            N2
         |             |
         |             |                          F1
         |A            |D                 N1 ------------ N2
         |             |                  |               |
         |             |                  |               |
         |      C      |                  |               |

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         x-------------x                  |F2             |F4
         |             |                  |               |
         |             |                  |               |
         |             |                  |       F3      |
         |B            |E                 N3 ------------ N4
         |             |
         |             |
         N3            N4

   Figure 4. Correlation between Fiber segment and Fiber link topology

   In such a physical topology the obvious SRLGs are the following:
   - {F1, F2} both going down when segment A breaks
   - {F1, F3} both going down when segment C breaks
   - {F1, F4} both going down when segment D breaks
   - {F2, F3} both going down when segment B breaks
   - {F3, F4} both going down when segment E breaks

   These five SRLGs can be replaced by two SRLGs, S1 = {F1, F2, F3} and
   S2 = {F1, F3, F4}, where S1 and S2 constitute the minimum edge
   covering with cliques of the Shared Risk Relationship (SRR) graph
   that can be drawn between F1, F2, F3, F4 (see Figure 5). A clique of
   a graph G is a sub-graph of G in which every two nodes are connected
   by an edge. This decomposition is unique. If there was an additional
   dependency between F2 and F4, there would be a unique SRLG, S = {F1,
   F2, F3, F4}.

                            F1 ------- F4
                            |  \        |
                            |   \       |
                            |    \      |
                            |     \     |
                            |      \    |
                            |       \   |
                            |        \  |
                            F2 ------- F3

            Figure 5. SRR Graph between fiber link and segment

   Although R1 = F1-F2-F3 and R2 = F4 are diverse path between
   locations N2 and N4 in the fiber topology (link and node
   disjointness), they are not diverse with respect to SRLGs. This is
   because both R1 and R2 cover SRLG S2, which contains F1, F3 (part of
   R1) and F4 (part of R2). SRLGs are thus a way of formalizing the
   propagation of (link-based) risk dependencies from server to client
   layers.

   The rules guiding the definition of minimum set of SRLGs for more
   complex physical network topologies will be addressed in a future
   version of this document.

A.2 Risk Type


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   As specified up to now, the current approach considers that each of
   the network resource may experience one or more failure type(s). The
   same applies to geographical locations: a given location might
   experience one or more failure types. Moreover, by applying the SRLG
   properties, a network resource failure can potentially cover more
   than one geographical location. Consequently, some heuristics must
   be introduced to keep the SRLG computational complexity limited.

   In order to limit the computational complexity, we define the
   following heuristics when considering the SRLG computation with
   respect to the type of risk:

   1. The set of risk types associated to network resources corresponds
      exactly to the set of resource type failure.
      - So, for instance, the risk type associated to a fiber segment
        is a fiber segment failure. The same principle applies for
        other network resources such as fiber link, fiber sub-segment
        and fiber trunk. Consequently, the current approach does not
        consider a finest granularity for the network resource failure
        than the one referred by their type.

   2. A risk type associated to a geographical structure covers exactly
      the region where it is defined. Moreover, a geographical failure
      is limited to a given location and does not contaminate the
      neighboring locations or generate another failure
      - For instance, we consider that an earthquake covers exactly
        one region or one area and that such a failure does not
        generate a hurricane impacting the neighboring locations. So,
        there is no correlation between geographical failures.

   3. Each of the network resources covers exactly one geographical
      logical structure.
      - Consequently, when a geographical failure occurs, it
        generates a failure impacting the entire network resources
        included within the corresponding location. Hence, there is
        an ON/OFF relationship between geographical and network
        resource failures.

   Consequently, when considering network resources, the risk type
   associated to an SRLG is defined as the potential failure of one (or
   more than one) instance belonging to a given resource type or the
   potential failure of any of its hierarchical resource dependence.

   Thus, we define the concept of SRLG with respect to a given resource
   type and subsequently to the Risk Type (RT) to which this resource
   type refers. Moreover, for a given resource type (or class), each of
   the resources are identified by a Resource Identifier (RID). Since
   each of these resource classes corresponds to an SRLG class, we can
   assign an identifier to each of the SRLG classes members and define
   their value as the SRLG identifier.

   Moreover, by applying the defined heuristics above, the SRLG
   identifiers can be grouped together by taking into account their

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   geographical location. The latter is encoded by identifying the
   region identifier (region ID) including the resource identifiers to
   which the SRLG refers.

A.3 Calculation of Shared Risk Link Groups

   In the calculation method, shared-risk(RID i, RID j, RT) is TRUE
   only if the resource identifiers RID i and RID j belong to the same
   SRLG with respect to the risk type RT. The risk types considered
   here are related the fiber segment, the fiber sub-segment and the
   fiber link risk failure.

   A recursive calculation of shared-risk proceeds as follows:

   shared-risk(RID i, RID j, RT) =
        at-risk(RID i, RT)
                and at-risk(RID j, RT)
                and (RID i = RID j
                        or (exists RID k, RID l
                        such that
                                depends-on(RID i, RID k)
                                and depends-on(RID j, RID l)
                                and shared-risk(RID k, RID l, RT)))

   In this calculation:
   - at-risk(RID i, RT) is TRUE only if RID is susceptible to a risk of
     type RT, either directly, or indirectly, through the failure of
     one of the resources it depends on.
   - depends-on(RID i, RID j) is TRUE only if RID i fails as soon as
     RID j fails.

   If we refer to the example detailed in section 1.1, then shared-
   risk(F1, F2, [fiber segment failure]) = TRUE because depends-on(F1,
   A) = TRUE, depends-on(F2, A) = TRUE and at-risk(A, [fiber segment
   failure]) = TRUE, the latter simply because A is a fiber segment.

A.4 Practical Method for SRLG Calculation

   The recursive formula presented in the previous section does not
   directly lead to an efficient algorithm. ItÆs top-down nature
   illustrates nicely the recursive nature of the SRLG concept, but the
   calculation of the SRLGs in a top-down fashion would be totally
   inefficient, entailing the calculation of the same SRLGs in lower
   network layers over and over again.

   A more efficient algorithm can be obtained from a bottom-up
   calculation. Figure 6 illustrates this by using the example we
   introduced in the Section A.1 and in by introducing the concept of
   Shared Risk Relationship Graph (SRR) which defines the membership of
   a resource belonging to the same SRLG.


             F1 ---------- F4

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             |  \      ^   |
             |   \     |   |
             |    \    |   |        Fiber SRR Graph
         --->|     \   |   |<---
        |    |      \  |   |    |   where F1=ACD, F2=AB, F3=BCE, F4=DE
        |    |      ^\ |   |    |
        |    |      | \|   |    |
        |    |      |  |   |    |
        |    |      |  |\  |    |
        |    |      |  | \ |    |
        |    F2 ----|--|-- F3   |
        |        ^  |  |        |
        |        |  |  |        |
      +++++++++++++++++++++++++++++
        |        |  |  |        |
        |        |  |  |        |
         --- A   |  |   -- D    |
                 |  |           |
                 |  |           |
                 |  C           |   Fiber Segment SRR Graph
                 |              |
                 |              |
             B --          E ---


       Figure 6. Bottom-up calculation of Shared Risk Relationships

   For the calculation of a set of SRLGs, we need to calculate a Shared
   Risk Relationship (SRR) graph. The bottom-up calculation of the
   fiber SRR graph proceeds as follows:

   - Step 1. For each fiber segment, there is an SRR between every two
     fibers contained in that segment (vertical arrows in Figure 6.)

   - Step 2. For every SRR between two fiber segments, there is an SRR
     between every two fibers contained in either of the two fiber
     segments.

   In the previous example, there are no SRRs between fiber segments,
   and the calculation stops after Step 1.

A.5 Application of the Model

   The model is intended to be used to automate the discovery of the
   SRLGs at a given layer for a given risk type (RT).

   The dependencies may be confined to one layer, e.g. the dependency
   of an optical link on a node (for instance, a DWDM system) to which
   it is connected, when the RT = [Node failure]. Dependencies may also
   extend over layer boundaries, e.g. the dependency of a TDM circuit
   (or sub-channel) in an SDH network established by using an optical
   channel (or wavelength) through the optical network that is the
   server layer of the SDH network, when RT = [fiber failure].

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   Let two optical network resources RID i and RID j within the same
   layer share a common risk of type RT. Let this risk type be tied to
   a lower layer, referred to as the risk layer. To enable this layer
   to infer shared-risk(RID i, RID j, RT), its server layer should
   advertise the following information:

     shared-risk(component_1, component_2, RT)

   where component_1 are services of the server layer on which RID i
   relies and component_2 are services of the serving layer on which
   RID j relies, respectively.

   If the server layer is not the risk layer, the latter has to infer
   this knowledge from what its server layer is advertising. If shared
   risk relationships are not advertised, client layers should at least
   be able to query from their server layer the shared risk
   relationships between the services they receive.

   Some dependencies do not lend themselves easily to automatic
   discovery. For instance, it is hardly imaginable that the process of
   finding out through which fiber segments a fiber goes can be
   automated. This means that part of the image of depends-on (RID i,
   RID j) will have to be provided æmanuallyÆ by the operator or be at
   least statically configured into a centralized repository.

   More formally, an efficient calculation of shared risk link
   relationships relies on two things:

   - In the lowest network layer with elements susceptible to the risk
     type RT that is considered, every network element RID j
     susceptible to the risk RT constitutes an SRR on its own, that is,
     (RID j, RID j) satisfies the recursive formula;

   - Every SRR that has been discovered in one network layer leads to
     SRRs in the next higher network layer. In particular, two next
     higher layer network elements (RID i, RID j) depending on lower
     layer network elements that have an SRR satisfy the recursive
     formula. In order to allow an efficient calculation of the shared
     risk relationships in the next higher layer (e.g. the fiber
     layer), the shared risk relationships that were discovered in
     lower layers (e.g. the fiber segment layer) are stored in SRR
     graphs. This way, the recalculation of lower layer shared risk
     relationships can be avoided.










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