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 NIST Yong Xue UUNet November 2001 Inference of Shared Risk Link Groups draft-many-inference-srlg-02.txt 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 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. D.Papadimitriou et al. - Expires May 2002 1 draft-many-inference-srlg-02.txt November 2001 Abstract 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 complexity. 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 account. 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 D.Papadimitriou et al. Internet Draft - Expires May 2002 2 draft-many-inference-srlg-02.txt November 2001 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", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in 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 deduced. 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 D.Papadimitriou et al. Internet Draft - Expires May 2002 3 draft-many-inference-srlg-02.txt November 2001 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 definitions: - 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 sub-segment. - Fiber Segment: a fiber segment includes a collection of fiber sub- segments. - 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 + | | | | + + | | | | + D.Papadimitriou et al. Internet Draft - Expires May 2002 4 draft-many-inference-srlg-02.txt November 2001 + | | | | + 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. D.Papadimitriou et al. Internet Draft - Expires May 2002 5 draft-many-inference-srlg-02.txt November 2001 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 D.Papadimitriou et al. Internet Draft - Expires May 2002 6 draft-many-inference-srlg-02.txt November 2001 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 trunk÷ included in a specific geographical region (Paris, Channel, etc.) - a fiber cable passing through earthquake÷ region like Japan or California - Provide Risk assessment during path computation implying allocation of conditional failure probabilities with SRLGs D.Papadimitriou et al. Internet Draft - Expires May 2002 7 draft-many-inference-srlg-02.txt November 2001 - 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 hierarchies - 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 Region. 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 link÷ type) to which the Identifier defined as a 24-bit integer value. The following resource types (i.e. link÷ type) are currently defined: Type Value D.Papadimitriou et al. Internet Draft - Expires May 2002 8 draft-many-inference-srlg-02.txt November 2001 ----------------- ----- 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 D.Papadimitriou et al. Internet Draft - Expires May 2002 9 draft-many-inference-srlg-02.txt November 2001 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 year. - 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 document. As an example for this discussion we elaborate on the risk assessment for a selected path. 4.2 Quantifying the Risk Assessment D.Papadimitriou et al. Internet Draft - Expires May 2002 10 draft-many-inference-srlg-02.txt November 2001 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 that: 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 D.Papadimitriou et al. Internet Draft - Expires May 2002 11 draft-many-inference-srlg-02.txt November 2001 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 1) 3. a conditional failure probability of 1% if fiber links are selected within the same fiber trunk (and located within the zone 2) 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 history÷ 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 D.Papadimitriou et al. Internet Draft - Expires May 2002 12 draft-many-inference-srlg-02.txt November 2001 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 time. 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 D.Papadimitriou et al. Internet Draft - Expires May 2002 13 draft-many-inference-srlg-02.txt November 2001 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 2001. D.Papadimitriou et al. Internet Draft - Expires May 2002 14 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 revision. 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) Alcatel 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.jones1@usa.alcatel.com Senthil Venkatachalam Alcatel 45195 Business Court, Suite 400 Dulles, VA 20166, USA Phone: +1 703 654-8635 Email: senthil.venkatachalam@usa.alcatel.com Sudheer Dharanikota D.Papadimitriou et al. Internet Draft - Expires May 2002 15 draft-many-inference-srlg-02.txt November 2001 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 UUNET/WorldCom Ashburn, VA, USA Phone: +1 703 886-5358 Email: yxue@uu.net D.Papadimitriou et al. Internet Draft - Expires May 2002 16 draft-many-inference-srlg-02.txt November 2001 Full Copyright Statement "Copyright (C) The Internet Society (date). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. 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