CONTRIBUTION TO T1 STANDARDS PROJECT. On Shared Risk Link Groups for diversity and risk assessment Sudheer Dharanikota, Raj Jain Nayna Networks Inc.

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1 Bulder, CO., March 26-28, 2001 / CONTRIBUTION TO T1 STANDARDS PROJECT TITLE SOURCE PROJECT On Shared Risk Link Grups fr diversity and risk assessment Sudheer Dharanikta, Raj Jain Nayna Netwrks Inc. Riad Hartani Dimitri Papadimitriu Yng Xue, Curtis Brwnmiller CONTACT Optical Hierarchical Interfaces Caspian Netwrks Inc. Alcatel WrldCm 2400 N. Glenville Dr. Richardsn, TX Raj Jain Nayna Netwrks Inc. 157 Tpaz Street Milpitas, CA (408) x 309 ABSTRACT Shared Risk Link Grup (SRLG) is a well-knwn cncept in the Telecmmunicatins wrld. In this dcument we prvide discussin n hw this cncept can be used fr diverse path cmputatin and risk assessment. The requirements we extract ut f this discussin are mapped t the SRLG encding and prtcl requirements. Key wrds: SRLG, Diversity, MPLS, Risk assessment Ntice This Dcument has been prepared t assist Standards Cmmittee T1X1. It is ffered t the cmmittee as a basis fr discussin and is nt a binding prpsal n Nayna Netwrks Inc., Caspian Netwrks Inc., Alcatel, r WrldCm. Infrmatin presented in this dcument may be subject t change after mre study. Nayna Netwrks Inc., Caspian Netwrks Inc., Alcatel, and WrldCm specifically reserve the right t add t, amend, r t withdraw the statements cntained herein. 1 * CONTACT: Curtis Brwnmiller, Curtis.Brwnmiller@wcm.cm, ,

2 Bulder, CO., March 26-28, 2001 / Intrductin Gruping f links, which share the same risk, is the cncept used fr many years in the telecmmunicatins industry. Links, which share the same risk, are gruped tgether t have the same SRLG (Shared Risk Link Grup) value. In this dcument we revisit the SRLG cncept: - with the new develpments in the data wrld namely MPLS (Multi Prtcl Label Switching) technlgy, - with the gal f assessing the risk assciated with a path, - with the gal f prviding cnstraint-based diverse paths, and - extend the definitin f SRLG t accmmdate lgical tplgies which share the same risk. The fllwing are the gals f this wrk: - Diversity: Capture the link-level and the lgical-level diversity. Summarizable SRLG ntatin, which prvides ease f and faster cmputatin. - Risk: Assign capabilities t the SRLGs, which in turn help in the risk assessment. Facilitate path cmputatin against a given risk type. - Other: Lcalizatin f the SRLG allcatin. This dcument is rganized as fllws: - Discussin n diversity in sectin 2, - Discussin n risk assessment in sectin 3, - Discussin n the requirements n this cncept fr dynamic and static prvisining in sectin 4, and - Cnclusins and references in the fllwing sectins. 2 Diversity and SRLG A brief intrductin is prvided t the need fr diversity t set the stage fr the discussin. Traffic engineering in IP (Internet Prtcl) netwrks is achieved using MPLS technlgy as an verlay n the IP netwrks. With the success f the MPLS TE cncept, they are being applied in the Optical Dmains as well. By intrducing the cnnectin-rientedness in the IP technlgy, we lse the autmatic recvery f the data paths in case f failures. Hence, diverse paths are established between the surce and destinatin MPLS ndes fr achieving path resiliency. The diversity requirements in the transprt netwrks have sme different requirements ver that in the ruter (Layer 3 and Layer 2 switching) netwrks. They are mainly: - Transprt netwrks prvide elabrate prtectin and restratin mechanisms, - Transprt tplgies are nt always mesh tplgies, as assumed by the ruter netwrks, and - Transprt-level prtectin and restratin mechanisms are nt cnsidered in the diverse path cmputatin by the MPLS technlgy (at least till nw). 2

3 Bulder, CO., March 26-28, 2001 / Regin 2 Regin 1 Regin 4 Regin 3 Zne 1 Zne 3 Zne 4 Regin 3 Zne 2 Zne 5 Figure 1 Lgical tplgy representatin using SRLG Diversity in the ruter wrld is achieved as fllws: - Request: Given the (physical and lgical) tplgy, link capabilities, and cnstraints calculate 1 t N diverse path between tw pints in the netwrk. - Input: Tplgy: Physical tplgy is always meshed. Areas and autnmus systems achieve lgical tplgy. Capability: Only link capabilities are prpagated. Cnstraints: Inclusive requirements: Such as a preferred links (clr). 3

4 Bulder, CO., March 26-28, 2001 / Exclusive requirements: Such as aviding a nde (nde-level diversity, linklevel diversity). Limiting requirements: Such as bandwidth. - Output: Paths available r nt. If paths are available then prvide the strict r lse paths. Strict paths can be prvided if the physical tplgy is knwn between the surce and destinatin. Lse paths are prvided if the end-t-end physical tplgy is nt knwn (r fr ther reasns). Diversity in the transprt wrld is achieved as fllws: - Request: Given the (physical and lgical) tplgy, link capabilities, dmain capabilities and cnstraints calculate 1 t N diverse path between tw pints in the netwrk. - Input: Tplgy: Physical tplgy is flexible (Rings, Meshes, Ring-Mesh inter cnnected). Dmains (r islands) are used t achieve lgical tplgy. Nte: These dmains may nt map nt areas and autnmus systems. Capability: Bth link (r span) and dmain capabilities need t be prpagated. Cnstraints: Inclusive requirements: Such as preferred links and preferred link, nde and dmain capabilities. Exclusive requirements: Such as aviding a link, nde r dmain. Limiting requirements: Such as bandwidth. - Output: Paths available r nt. Discussin: If paths are available then prvide the strict r lse paths. Strict paths can be prvided if the physical tplgy is knwn between the surce and destinatin. Lse paths are prvided if the end-t-end physical tplgy is nt knwn r cannt be interpreted such as in ring tplgies (r fr ther reasns). The fllwing bservatins can be made between the diverse path setup mechanisms in the ruter wrld and in the transprt wrld: - Sharing risk is nt nly the prperty f the links, but it can be extended t the ndes and the dmains (frm exclusive cnstraints). That an SRLG can be assciated with the links, ndes and dmains. - Als, fr example, dmain capabilities can be assciated with the dmain SRLG t achieve inclusive cnstraints. Fr example a BLSR ring can have an SRLG with the ring capabilities assciated with it. As a precursr t achieve such cnstraints we prpse t extend the SRLG ntin t represent the lgical tplgies als, as shwn in Figure 1. As shwn in the figure, a lgical tplgy can be cnstructed based n the regins, znes and finally ndes. By assigning SRLGs in a hierarchical fashin (t a regin, a zne and ndes), we can capture the capabilities, and risks assciated with them. An extensive discussin n this subject is prvided in [IETF-SRLG]. 4

5 Bulder, CO., March 26-28, 2001 / Nte 1: A dmain may be mapped nt a regin r a zne. Nte 2: Here we can debate the name SRLG, which has the term Link in it t say this is applicable nly fr the link. A typical trans-atlantic fiber can cntain 100s f SRLGs. Jhn Strand, AT&T. Cnsidering the abve practical knwledge f the real wrld scenaris, it is essential (t reduce the cmputatin time) t reduce the number f SRLGs by sme means f encding. Als, we need t bserve that the amunt f cnfiguratin can be reduced, frm 100s n a link fr the 100s f links, by distributing it. This pses the fllwing requirements: - Need a mechanism t encde (and hence summarizable) SRLG t grup represent a link r nde r dmain wide individual SRLGs. - Need t mdify the path cmputatin algrithms (such as CSPF) fr accmmdating the new encding scheme. - Need t enhance the path cmputatin mechanisms t wrk with the lgical tplgies (r dmains) - Need t prpagate the lgical tplgies (r dmains) via the ruting prtcls. 3 Risk assessment and SRLG Risk (the cmplementary f availability) assessment is defined as the evaluatin f the ptential risk assciated t the inclusin f a given resurce (this resurce belngs t a given resurce type lcated within a given lgical structure such as a gegraphical lcatin) in a given path. A brief discussin fr the mtivatin f the risk assessment capabilities f SRLG is prvided here. Cnsider the fllwing example, where the client device makes the fllwing requests t the ptical netwrk: - Request (either thrugh signaling prtcls r using an SLA) fr a persistent cnnectin with % (widely knwn 5 9s) f availability r equally a dwn time less than X minutes per year. - Request a high-prtectin fr a prtin f the traffic (at the expense f mre charging) cmpared t ther lw-pririty traffic. Such requirements will be translated int cnstraints in the path cmputatin. Such cnstraints can be gruped int path selectin cnstraints and path characterizatin cnstraints. - Path selectin cnstraints: These typically dictate which physical path shuld be taken t achieve the availability requirements f the client. These requirements are typically the lgical and physical diversity. - Path characterizatin cnstraints: 5

6 Bulder, CO., March 26-28, 2001 / Path characterizatin requirements typically dictate the prtectin mechanisms as requested by the client. This can be achieved in the frm f ptical rings, meshed prtectin mechanisms, etc. These cnstraints can be used in using the link, nde, and dmain capabilities as discussed in the previus sectin n diversity. The cmpnents that need frmalizatin in this example are: - Step 1. Specificatin f the user requirements (such as the example abve) - Step 2. Cnfiguring the netwrk that helps in assessing the features such as the availability - Step 3. Prpagating the abve-cnfigured infrmatin. - Step 4. Using the abve-prpagated infrmatin. Step 1 f specifying the requirements is nt in the scpe f this dcument. Steps 2-4 are discussed belw. Discussin: A simple way t achieve risk assessment is by assciating a cnditinal risk value t each f the SRLGs, as discussed in [IETF-SRLG]. Als by assciating a weight factr t the SRLG, we can increase the chice f selecting specific SRLGs. This call fr cnfiguring: - Risk factr per SRLG - Weight factr per SRLG The abve values in additin t the SRLG capabilities, as discussed befre can be prpagated via ruting prtcls. These ruting requirements are discussed in the fllwing sectin. With the help f the abve tw cnfiguratin parameters, using typical CSPF algrithm t cmpute the path can be extended t assess the risk assciated with it. Fr example, if a path is traversing thrugh SRLGs 1, 3, 5, then we can infer the risk with the path as (Risk 1 x Risk 3 x Risk 5). 4 Requirements and extensins The fllwing requirements can be derived frm the abve discussins: - Encding: Lgical and physical structure: A lgical and physical encding f SRLGs shuld be prpsed t reduce the number f SRLGs frm the scalability pint-f-view. This als helps in hiding the tplgy in many cases and helps in lse path cmputatin. - Capability assignment Dmain, nde, and link capability: Dmain capability such as transprt netwrk level diversity prvided by the dmain can be assciated with the SRLG. This helps in deciding the lgical and physical tplgies f the chice fr the path. Risk assessment parameters: Risks can be assciated with bth the lgical and physical SRLGs t assess the pssible risk assciate t a path. Preferential rute selectin: By assciating weights t the SRLGs, ne can make the autmatic path selectin algrithms t chse certain SRLG path. - Ruting prtcl extensins t prpagate them 6

7 Bulder, CO., March 26-28, 2001 / Given the regin-level and zne-level decmpsitin f the physical tplgy f the ptical netwrk, the link semantics shuld be extended t accmmdate the inter-regin and inter-znal links. Mrever, this cncept helps in cnstructing the lgical-level tplgies at the regin-level and zne-level abstractin, which in turn can be used in the SRLG summarizatin and lse-path cmputatin. Prpagate these additinal (regin and znal) links using the IGP ruting prtcls fr intra- and inter-area ruting purpses. T reduce the amunt f the flded infrmatin and hence lightpath rute cmputatin cmplexity, the flding scpe f the infrmatin prpagatin is extended t accmmdate regin-level and zne-level. Prpagate the capability, risk assessment, and preferential rute selectin parameters per SRLG. - CSPF t use this path Extensins: Extend the cnstraint-based path cmputatin t accmmdate the extensins due t the abve mechanisms. - Encding: Lgical and physical structure (refer t [IETF-SRLG]) - Capability assignment [IETF-SRLG-PROTO-EXT] Dmain capability Nde capability Link capability Cnditinal prbability Weight f selectin - Ruting prtcl extensins t prpagate them (SRLG, Capability) prpagatin, which means new TE infrmatin - CSPF t use this path Path cmputatin extensin with ptimizatin n risk 5 Cnclusins In this dcument, we demnstrated SRLG uses in diverse path cmputatin and risk assessments. Discussed relevant example t derive the requirements t the encding mechanisms, capability assignment, CSPF algrithms and ruting prtcl extensins. 6 References [IETF-SRLG] D. Papadimitriu et al., Inference f Shared Risk Link Grups, draft-many-inferencesrlg-00.txt, IETF draft (wrk in prgress). [IETF-SRLG-PROTO-EXT] S. Dharanikta et al., SRLG related prtcl extensins fr diversity and risk assessment, IETF draft (wrk in prgress). 7