Chapter 16-2 Distributed System StructuresChapter 16 Distributed System StructuresNetwork TopologyFully & Partially Connected Networks and TreesRings and Star Network TopologiesCommunication StructureNaming and Name Resolution - DNSDNS Naming ResolutionRouting StrategiesRouting Strategies – ComparisonsPacket StrategiesConnection StrategiesCircuit, Message, Packet SwitchingContentionContention using CSMA/CDContention using Token Passing===== Ethernet =====Communication ProtocolsLayers in HardwareLayers – in SoftwareSlide 21RobustnessFailures – Detection, Reconfiguration, Recovery.Slide 24Slide 25Design IssuesDesign Issues – TransparencyDesign Issues – Fault ToleranceDesign Issues – ScalabilityDesign Issues – Scalability - moreDesign Issues – Scalability – still moreDesign Issues – Scalability – ClusteringEnd Chapter 16.2Chapter 16-2 Distributed System Chapter 16-2 Distributed System StructuresStructures17.2Silberschatz, Galvin and Gagne ©2005Operating System ConceptsChapter 16 Distributed System StructuresChapter 16 Distributed System StructuresChapter 16.1BackgroundMotivationTypes of Distributed Operating SystemsNetwork StructureChapter 16.2Network TopologyCommunication StructureCommunication ProtocolsRobustnessDesign Issues17.3Silberschatz, Galvin and Gagne ©2005Operating System ConceptsNetwork TopologyWhen we speak of topology, we are speaking of physical connections.Each of the types I will present differ in installation cost (cost of linking up the sites), communication costs (amount of time / money it takes to send a message from node A to node B, and availability, essentially the ability to use the topology in the face of a downed links or sites.Some topologies have all nodes directly connected to every other node, some have only ‘some’ nodes directly connected and others indirectly connected, some topologies look like trees, stars, and rings.Each has advantages and disadvantages.17.4Silberschatz, Galvin and Gagne ©2005Operating System ConceptsFully & Partially Connected Networks and TreesFully Connected: Here, every node is connected to every other node.Adv: no switching or broadcasting is needed.Dis: as the number of nodes increases, cost rises dramatically!Good for small network.Partially Connected:Adv: Clearly, installation costs are lower since not all nodes are connected to every node – only some.Dis: for nodes that wish to communicate and are not directly connected, messages must be routed through communication links, which, of course, raises the cost.Trees:Adv: installation and communication costs are low but the very nature of a tree implies that there is only one path to a node.Dis: If this path ‘goes down’ we have the network ‘partitioned.’Partitioning refers to the situation where the network is broken into two (or more) subsystems that cannot communicate between themselves.17.5Silberschatz, Galvin and Gagne ©2005Operating System ConceptsRings and Star Network TopologiesRings:Adv: higher degree of reliability, Dis: but communication costs are high because a message may need to travel through a number of links before it arrives at its destination.Adv: Better availability than the tree – not likely to result in a partition…Adv: At least two links must go down for a partition to occur.Star:Failure of any link results in a partition, but a partition may be only a single site.Adv: low communication costs, because every node is at most two nodes away from the target node, butDis: the central site is critical. If it goes down, the entire network is down.17.6Silberschatz, Galvin and Gagne ©2005Operating System ConceptsCommunication StructureNeed to look away from some of the physical aspects of networking to the internal workings of communications.While this might appear to have become a course in communications, understanding of these topics is absolutely essential to understanding how distributed operating systems work.So, we will look at common issues that a communications network must address:Naming and name resolutionRouting StrategiesPacket StrategiesConnection Strategies, andContention17.7Silberschatz, Galvin and Gagne ©2005Operating System ConceptsNaming and Name Resolution - DNSHow do two processes locate each other in order to communicate?Processes need to be able to reference each other by a name.So, within a computer system, each process has a process identifier.Processes on remote systems are identified by a <host name, identifier>‘host name’ is unique within a network – usually alphanumeric; ‘identifier’ may be a process id or other unique number at host site.But computers like numbers, so we try to bind names to a host-id that describes the destination system to the networking hardware.Nowadays we distribute names among systems on the network, and the network must use a protocol to distribute and receive the information.We call this the domain-name system (DNS).17.8Silberschatz, Galvin and Gagne ©2005Operating System ConceptsDNS Naming ResolutionDNS specifies naming structure of the host as well as name-to-address resolution Component separated by periods.Hosts (on the Internet) - logically addressed w/multi-part name: More specific to more general.We know there are several popular domains: .com, .org, .mil, .gov,… and countries.Each component has a name server, which is only a process on a system. Name servers accept a name, return address of name server responsible for that nameThe location of the name server for domain .edu is known and is issued a request for the address of the name server for csuchico.edu.The domain name server returns the address of the host on which the csuchico.edu name server resides.This name server is sent a request for the name server of cs.chico.edu. Address retnd.Then a request to this name server for broggio.cs.csuchico.edu returns an Internet address host-id for that host, such as 137.62.37.20.In practice using local caches makes this process quick. The .edu name server would have csuchico.edu in its cache and would inform the sending process that it could resolve two parts of the address, then returns a pointer to the cs.csuchico.edu name server.17.9Silberschatz, Galvin and Gagne ©2005Operating System ConceptsRouting StrategiesHow are