In this paper the all-terminal reliability and the two-terminal reliability models of rings using the Spatial Reuse Protocol (SRP) are developed. Moreover the interconnection of SRP-rings is considered

Schupke, D. A.

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Association for Information SystemsAIS Electronic Library (AISeL)AMCIS 2000 Proceedings Americas Conference on Information Systems(AMCIS)2000Reliability Models of SRP RingsD. A. SchupkeMunich University of Technology, dominic.schupke@ei.tum.deFollow this and additional works at: http://aisel.aisnet.org/amcis2000This material is brought to you by the Americas Conference on Information Systems (AMCIS) at AIS Electronic Library (AISeL). It has been acceptedfor inclusion in AMCIS 2000 Proceedings by an authorized administrator of AIS Electronic Library (AISeL). For more information, please contactelibrary@aisnet.org.Recommended CitationSchupke, D. A., "Reliability Models of SRP Rings" (2000). AMCIS 2000 Proceedings. 276.http://aisel.aisnet.org/amcis2000/276Reliability Models of SRP Rings D. A. Schupke Institute of Communication Networks Munich University of Technology 80290 Munich, Germany Tel.: +49 89 289-23511, Fax: -63511 Email: Dominic. Schupke¢ei. rum. de Research-in-Progress Paper Keywords -  Reliability, Availability, SRP, Spatial Reuse Protocol, Ring Networks Abstract  In this paper the all-terminal reliability and the two-terminal reliability models of rings using the Spatial Reuse Protocol (SRP) are developed. Moreover the interconnection f SRP-rings is considered. 1 Int roduct ion  The Spatial Reuse Protocol (SRP) has recently been introduced as a MAC layer protocol for ring based media [1, 2]. The protocol carries both IP and ATM client traffic and supports ring network redundancy similar to bidirectional self-healing rings (BSHRs) of SDH/SONET standards. Since each network element is subject to failures or disruptions, the network reliability is an important parameter for the employment of such a ring. This also emerges for today's IP networks [3], which may run over SRP. It should be noted that the major concern of reliability considerations is not motivated by the discard of packets at the SRP-buffers caused by network congestion, which are momentary effects (partly used for flow-control as in TCP) and should be negligible in a well dimensioned network. The overall network reliability can be used for the comparison of different design alternatives, e.g. between SRP-rings and rings operated with other protocols, or the dimensioning alternatives of rings in multiple-rings networks (e.g. size of ring-nodes). The reliability for an end-to-end connection also comes to front as a Quality of Service (QoS) parameter, which can be offered to customers (SRP-clients) or can be used for the QoS-routing in traffic engineered networks. The reliability models of SONET/SDH BSHRs have been determined in [4, 5]. The reliability models of SRP-rings differ from these, since SRP-rings interconnect hosts and allow for bypassing the hosts (e.g. during a host's software failure) in the "pass-thru mode" [1]. Thus the SRP nodes are organized ifferently in contrast o SONET/SDH nodes. In this paper we develop the reliability models of SRP-rings and moreover consider the interconnection f SRP-rings. It should be noted that the models are applicable both on repairable and non-repairable systems, where strictly speaking in former ones we deal with '~reliabilities" and in latter ones with "availabilities." 2 Network  Mode l  We describe a SRP-ring with n nodes by the network model in figure 1. A symmetric 545network model is used, where each element (e.g. each host) has the same reliability. , L • Figure 1: Generic model of a SRP-ring. The nodes (with reliability Rn) comprise both a host (Rh) and the SRP forwarding device (R f). The SRP forwarding device is responsible for terminating the links, interfacing the host and performing all SRP-protocol processing (framing, buffering, etc.), see [1]. The host may be bypassed in the "pass-thru mode" [1] by the forwarding device. Two adjacent SRP-nodes are interconnected by a bidirectional point-to-point link (e.g. SONET/SDH or fiber link), which is modeled by two unidirectional links (Rt) for each direction both guided through a segment (Rs). The SRP-ring uses the Intelligent Protection Switching (IPS) protocol in combination with an automated topology discovery mechanism. For instance refer to the four-node ring with an end-to-end connection in figure 2 (a). A link failure as in (b) causes a (bidirectional) protection switching and the ring reaches the so-called "wrap state." A subsequent topology discovery as in (c) may reroute connections according to the novel topology. (a) (b) (¢} Figure 2: (a) four-node SRP-ring with an end-to- end connection. Protection switching (b) and topol- ogy discovery (c) after a failure. In this paper we consider single failures at a time only, even if the IPS protocol can handle multiple failures. We should also note that in the states (b) and (c) new routes and thus new bandwidth demands are present in the ring. Therefore the ring has to be dimensioned to cope with these states. This can be done somewhat similar to SONET/SDH BSHRs, where half of the capacity bandwidth is used for working traffic and the other half is used for protection (spare capacity). A more advanced approach would calculate the routes and dimensioning requirements by evaluating a set of likely failure scenarios. As the SRP-ring does not provide synchronous multiplexing of working and protection traffic, a further margin may be introduced for possible packet discards in SRP-buffers (e.g. due to the packet clumping effect). 3 Reliability Models of the SRP-Ring For the SRP-ring reliability model we employ the all-terminal reliability and the two-terminal reliability (which are defined similar to [4]). 3.1  A l l -Termina l  Re l iab i l i ty  The all-terminal reliability (Rau) is defined as the reliability that all hosts on the ring are operating and can communicate with 546each other (network operator's viewpoint). ]~vo mutually exclusive vents contribute to the all-terminal reliability. (1) All parts of the network are operational. rl = (RnRhRfRsR2)  n (1) (2) At least one (unidirectional) ink in a segment or the segment itself failed, while all other elements of the ring are operating. Thus the ring wrap function is performed. r2 = n(RnRIRh)U(RsR2)  n-1 × [Rs(1 - R~) + (1 - Rs)] (2) The all-terminal reliability is the sum of equations (1) and (2): Ran = r l  + r~ (3) 3.2 Two-Termina l  Re l iab i l i ty  The two-terminal reliability (Rs-t,riu9) is defined as the reliability that two given hosts on the ring can communicate with each other, independent of the states of the other parts of the network (user's viewpoint). Three mutually exclusive vents contribute to the two-terminal reliability, where the first two are contained in section 3.1 excluding all non-terminal hosts (i.e. divide the all-terminal reliability by u-2 R h ). The third event is described here. (3) At one node (which is not one of the terminal nodes) the ring is disconnected since the node's forwarding part failed or the node itself failed, all forwarding devices, all segments and links except for the failed node's adjacent segments and links and the terminal hosts are operating. Thus the ring wrap is performed. r3=(n-2)  n-1 2 (RnRI )  Rh ~(RsR2) n-2 t x[Rn(1 - Rf)  + (1 - R~)] (4) The two-terminal reliability is obtained via equations (1), (2), and (4) and independent of the considered terminal nodes: rl -t- r2 R~-t,~9 - R~_ ~h + r3 (5) 4 Ring Interconnection In networks with multiple rings, SRP client data traffic (e.g. IP traffic) may be guided through a segment more than once, if some links of both rings are guided through one segment (i.e. ring overlap). In this case the failure events of the rings are not mutually exclusive anymore, however, the above reliability models provide an approximation. Assuming a ring interconnection as in figure 3, the failure events for the node reliability are also not mutually exclusive, since both rings share the node. Concerning the all-terminal reliability, again the reliability models provide an approximation. But for the two-terminal reliability an exact calculation scheme can be determined. IR Ring 1 F~ing 2 Figure 3: Single interconnection between rings. We assume that there is only one possible path between rings. In this case we can calculate the two-terminal reliability of nodes residing on different rings via the product of the sub-paths' reliabilities corresponding to each ring. Denote m as the number of rings the path of the two terminals is routed. Then the two-terminal reliability Rs-t can be 547(6) R Example :  Consider the two-terminal reliability when deciding between one 16 node ring and two interconnected 8 node rings (single homing at one node) to realize a 16 node network. We neglect hat by these decision options the 16 node ring would have at least one node in common. Assume that each reliability is equal Rn = Rh = Rf  = Rl = 0.99999 (equivalent to 5 minutes outage per year), except for the segments' reliability which is Rs = 0.9993 (equivalent to 6 hours outage per year). This could represent ametropolitan area network example. Then by (5) the 16 node ring with Rs-t(n = 16) = 0.9998753918 (equivalent to 66 minutes outage per year) is slightly more reliable than the 8 node rings for interring connectivities which have via (6) Rs-t(n = 8, m = 2) = 0.9998700213 (equivalent to 68 minutes outage per year) but is considerably less reliable for intraring connectivities which have by (5) Rs-t(n = 8: m = 1) = 0.9999250092 (equivalent to 40 minutes outage per year). Thus it may be desirable to partition the network in two rings while trying to minimize the interring traffic. If two rings are interconnected more than once, we obtain a higher reliability than above, e.g. like in the dual homing case in figure 4. The SRP's client layer can reroute the traffic upon a node failure. The node failure events in equation (4) then become mutually dependent and are for future research. calculated as: Rs-t ---- l~hl~n f i  Rs-t,ringi i=1 RhRn Ring 1 =. - Ring 2 Figure 4: Double interconnection between r ings .  5 Conc lus ions  In this paper we determined the reliability models for a SRP-ring and considered the case of the single interconnection of multiple SRP-rings. The models are the basis for a further reliability analysis. References [1] D. Tsiang and G. Suwala. The Cisco SRP MAC Layer Protocol. Internet Draft: d ra f t - t s iang-s rp -01 ,  March 2000. [2] M. Jaeger, H.-M. Foisel, H.-M. Westphal, J. Chawki, K. Ovsthus, and J.-C. Bischoff. Evaluation of Network Architectures for the Integration of IP over Optical Networks. In 2nd Int. Workshop on Design of Reliable Comm. Networks (DRCN 2000), April 2000. [3] A. Autenrieth and A. Kirstaedter. Fault Tolerance and Resilience Issues in 548IP-Based Networks. In 2nd lnt. Workshop on Design of Reliable Comm. Networks (DRCN 2000), April 2000. [4] J. Lee. Reliability Models of a Class of Self-Healing Rings. Microelectronics and Reliability, 37(8):1179-1183, August 1997. [5] D.A. Schupke. Reliability Models of SDH/SONET Self-Healing Rings over unprotected non-WDM and WDM Networks. In 8th Int. Conf. on Telecomm. Systems, Nashville, TN, USA, March 2000. 549

Reliability Models of SRP Rings

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Reliability Models of SRP Rings

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