Computer Operating System - Lecture 15: Network structures - Nguyen Thanh Son

Nội dung text: Computer Operating System - Lecture 15: Network structures - Nguyen Thanh Son

1. Chapter’s Content  Background  Topology  Network Types  Communication  Communication Protocol  Robustness  Design Strategies BK TP.HCM 07-Feb-17 Faculty of Computer Science & Engineering 2
2. Motivation  Resource sharing  sharing and printing files at remote sites  processing information in a distributed database  using remote specialized hardware devices  Computation speedup – load sharing  Reliability – detect and recover from site failure, function transfer, reintegrate failed site  Communication – message passing BK TP.HCM 07-Feb-17 Faculty of Computer Science & Engineering 4
3. Distributed-Operating Systems  Users not aware of multiplicity of machines. Access to remote resources similar to access to local resources.  Data Migration – transfer data by transferring entire file, or transferring only those portions of the file necessary for the immediate task.  Computation Migration – transfer the computation, rather than the data, across the system. BK TP.HCM 07-Feb-17 Faculty of Computer Science & Engineering 6
4. Topology  Sites in the system can be physically connected in a variety of ways; they are compared with respect to the following criteria:  Basic cost. How expensive is it to link the various sites in the system?  Communication cost. How long does it take to send a message from site A to site B?  Reliability. If a link or a site in the system fails, can the remaining sites still communicate with each other?  The various topologies are depicted as graphs whose nodes correspond to sites. An edge from node A to node B corresponds to a direct connection between the two sites.  The following six items depict various network BK topologies. TP.HCM 07-Feb-17 Faculty of Computer Science & Engineering 8
5. Network Types  Local-Area Network (LAN) – designed to cover small geographical area.  Multiaccess bus, ring, or star network.  Speed 10 megabits/second, or higher.  Broadcast is fast and cheap.  Nodes:  usually workstations and/or personal computers  a few (usually one or two) mainframes. BK TP.HCM 07-Feb-17 Faculty of Computer Science & Engineering 10
6. Network Types (Cont.)  Wide-Area Network (WAN) – links geographically separated sites.  Point-to-point connections over long-haul lines (often leased from a phone company).  Speed 100 kilobits/second.  Broadcast usually requires multiple messages.  Nodes:  usually a high percentage of mainframes BK TP.HCM 07-Feb-17 Faculty of Computer Science & Engineering 12
7. Communication The design of a communication network must address four basic issues:  Naming and name resolution: How do two processes locate each other to communicate?  Routing strategies. How are messages sent through the network?  Connection strategies. How do two processes send a sequence of messages?  Contention. The network is a shared resource, so how do we resolve conflicting demands BK for its use? TP.HCM 07-Feb-17 Faculty of Computer Science & Engineering 14
8. Routing Strategies  Fixed routing. A path from A to B is specified in advance; path changes only if a hardware failure disables it.  Since the shortest path is usually chosen, communication costs are minimized.  Fixed routing cannot adapt to load changes.  Ensures that messages will be delivered in the order in which they were sent.  Virtual circuit. A path from A to B is fixed for the duration of one session. Different sessions involving messages from A to B may have different paths.  Partial remedy to adapting to load changes.  Ensures that messages will be delivered in the order in which BK TP.HCM they were sent. 07-Feb-17 Faculty of Computer Science & Engineering 16
9. Connection Strategies  Circuit switching. A permanent physical link is established for the duration of the communication (i.e., telephone system).  Message switching. A temporary link is established for the duration of one message transfer (i.e., post-office mailing system).  Packet switching. Messages of variable length are divided into fixed-length packets which are sent to the destination. Each packet may take a different path through the network. The packets must be reassembled into messages as they arrive.  Circuit switching requires setup time, but incurs less overhead for shipping each message, and may waste network bandwidth. Message and packet switching require less setup time, but incur more overhead per message. BK TP.HCM 07-Feb-17 Faculty of Computer Science & Engineering 18
10. Contention (Cont.)  Token passing. A unique message type, known as a token, continuously circulates in the system (usually a ring structure). A site that wants to transmit information must wait until the token arrives. When the site completes its round of message passing, it retransmits the token. A token-passing scheme is used by the IBM and Apollo systems.  Message slots. A number of fixed-length message slots continuously circulate in the system (usually a ring structure). Since a slot can contain only fixed-sized messages, a single logical message may have to be broken down into a number of smaller packets, each of which is sent in a separate slot. This scheme has been adopted in the experimental Cambridge Digital Communication Ring BK TP.HCM 07-Feb-17 Faculty of Computer Science & Engineering 20
11. Communication Protocol (Cont.)  Transport layer – responsible for low-level network access and for message transfer between clients, including partitioning messages into packets, maintaining packet order, controlling flow, and generating physical addresses.  Session layer – implements sessions, or process-to- process communications protocols.  Presentation layer – resolves the differences in formats among the various sites in the network, including character conversions, and half duplex/full duplex (echoing).  Application layer – interacts directly with the users’ deals with file transfer, remote-login protocols and electronic mail, as well as schemas for distributed databases. BK TP.HCM 07-Feb-17 Faculty of Computer Science & Engineering 22
12. The ISO Protocol Layer BK TP.HCM 07-Feb-17 Faculty of Computer Science & Engineering 24
13. The TCP/IP Protocol Layers BK TP.HCM 07-Feb-17 Faculty of Computer Science & Engineering 26
14. Failure Detection  Detecting hardware failure is difficult.  To detect a link failure, a handshaking protocol can be used.  Assume Site A and Site B have established a link. At fixed intervals, each site will exchange an I-am-up message indicating that they are up and running.  If Site A does not receive a message within the fixed interval, it assumes either (a) the other site is not up or (b) the message was lost.  Site A can now send an Are-you-up? message to Site B.  If Site A does not receive a reply, it can repeat the message or try an alternate route to Site B. BK TP.HCM 07-Feb-17 Faculty of Computer Science & Engineering 28
15. Reconfiguration  When Site A determines a failure has occurred, it must reconfigure the system: 1. If the link from A to B has failed, this must be broadcast to every site in the system. 2. If a site has failed, every other site must also be notified indicating that the services offered by the failed site are no longer available.  When the link or the site becomes available again, this information must again be broadcast to all other sites. BK TP.HCM 07-Feb-17 Faculty of Computer Science & Engineering 30
16. Networking Example  The transmission of a network packet between hosts on an Ethernet network.  Every host has a unique IP address and a corresponding Ethernet (MAC) address.  Communication requires both addresses.  Domain Name Service (DNS) can be used to acquire IP addresses.  Address Resolution Protocol (ARP) is used to map MAC addresses to IP addresses.  If the hosts are on the same network, ARP can be used. If the hosts are on different networks, the sending host will send the packet to a router which routes the packet to the destination network. BK TP.HCM 07-Feb-17 Faculty of Computer Science & Engineering 32