Chapter 16 2 Distributed System Structures Chapter 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 Issues Operating System Concepts 17 2 Silberschatz Galvin and Gagne 2005 Network 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 Operating System Concepts 17 3 Silberschatz Galvin and Gagne 2005 Fully 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 Operating System Concepts 17 4 Silberschatz Galvin and Gagne 2005 Rings 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 Operating System Concepts 17 5 Silberschatz Galvin and Gagne 2005 Communication 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 Contention Operating System Concepts 17 6 Silberschatz Galvin and Gagne 2005 Naming 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 Operating System Concepts 17 7 Silberschatz Galvin and Gagne 2005 DNS 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 Operating System Concepts 17 8 Silberschatz Galvin and Gagne 2005 Routing Strategies How are messages sent If there s only one path then the path is clear But this is often not the case and we have many options Normally each site has a routing table which points to sites along the way that may be used in transmitting a message These tables are often updated as sites go down and are changed Fixed routing a specific path is fixed ahead of time generally a shortest path is preferred Cannot change the path despite potentially better ones If path goes down communication is lost Virtual routing a specific path is established for a session Later sessions would likely have a different path Since fixed path is determined maintained at session time different routes will be selected for different sessions This is a more reliable routing mechanism Dynamic routing Path determined only when a message is sent Message may go from site to site to site usually advancing to site with least traffic Path may not be direct but it ll get there Unix uses fixed routing for simple networks dynamic for the rest Operating System Concepts 17 9 Silberschatz Galvin and Gagne 2005 Routing Strategies Comparisons Gateways Sometimes we only need to know how to route to a gateway These usually connect a local network to other networks and the Internet So here we might use a fixed route to the gateway server realizing that the gateway