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Advanced Programming Rabie A. Ramadan [email protected] Network Programming An Overview 2 Computer Network A computer network is an interconnected collection of autonomous computers. Network topology A topology is a way of “laying out” the network. Topologies can be either physical or logical. Physical topologies describe how the cables are run. Logical topologies describe how the network messages travel Network topology (cont.) Bus (can be both logical and physical) Star (physical only) Ring (can be both logical and physical) Mesh (can be both logical and physical) Network topology (cont.) Bus A bus is the simplest physical topology. It consists of a single cable that runs to every workstation This topology uses the least amount of cabling, but also covers the shortest amount of distance. Network topology (cont.) It is difficult to add a workstation Have to completely reroute the cable and possibly run two additional lengths of it. If any one of the cables breaks, the entire network is disrupted. Therefore, it is very expensive to maintain. Network topology (cont.) Star Topology A physical star topology branches each network device off a central device called a hub, making it very easy to add a new workstation. Also, if any workstation goes down it does not affect the entire network. Network topology (cont.) Star topologies are more expensive to install than bus networks, • There are several more cables that need to be installed, plus the cost of the hubs that are needed. Network topology (cont.) Ring Each computer connects to two other computers, joining them in a circle creating a unidirectional path where messages move workstation to workstation. Network topology (cont.) The ring makes it difficult to add new computers. Unlike a star topology network, the ring topology network will go down if one entity is removed from the ring. Physical ring topology systems don’t exist much anymore, mainly because the hardware involved was fairly expensive and the fault tolerance was very low. Network topology (cont.) Mesh The mesh topology is the simplest logical topology in terms of data flow, but it is the most complex in terms of physical design. In this physical topology, each device is connected to every other device Network topology (cont.) The physical mesh topology is very expensive to install and maintain. Cables must be run from each device to every other device. The advantage you gain from it is its high fault tolerance. There will always be a way of getting the data from source to destination. Network topology (cont.) Advantages and Disadvantages of Network Topologies Topology Advantages Disadvantages Bus Cheap. Easy to install. Difficult to reconfigure. Break in bus disables entire network. Star Cheap. Easy to install. Easy to reconfigure. Fault tolerant. More expensive than bus. Ring Efficient. Easy to install. Reconfiguration difficult. Very expensive. Mesh Simplest. Most fault tolerant. Reconfiguration extremely difficult. Extremely expensive. Very complex. Computer Network A network includes: • Special purpose hardware devices that: • Interconnect transmission media • Control transmission of data • Run protocol software • Protocol software that: • Encodes and formats data • Detects and corrects problems encountered during transmission Network Hardware Repeaters Repeaters are very simple devices. Allow a cabling system to extend beyond its maximum allowed length by amplifying the network voltages so they travel farther. Repeaters are nothing more than amplifiers and, as such, are very inexpensive. Network Hardware Hubs Hubs are devices used to link several computers together. They repeat any signal that comes in on one port and copy it to the other ports (a process that is also called broadcasting). There are two types of hubs: active and passive. • • Passive hubs simply connect all ports together electrically and are usually not powered. Active hubs use electronics to amplify and clean up the signal before it is broadcast to the other ports. In the category of active hubs, there is also a class called “intelligent” hubs, which are hubs that can be remotely managed on the network. Network Hardware Hubs Network Hardware Bridges They join similar topologies and are used to divide network segments. For example, with 200 people on one Ethernet segment, the performance will be mediocre, because of the design of Ethernet and the number of workstations that are fighting to transmit. If you divide the segment into two segments of 100 workstations each, the traffic will be much lower on either side and performance will increase. They are more intelligent than repeaters but are unable to move data across multiple networks simultaneously. Unlike repeaters, bridges can filter out noise. The main disadvantage to bridges is that they can’t connect dissimilar network types or perform intelligent path selection. For that function, you would need a router. Network Hardware Bridges Network Hardware Routers Routers are highly intelligent devices that connect multiple network types and determine the best path for sending data. The advantage of using a router over a bridge is that routers can determine the best path that data can take to get to its destination. Like bridges, they can segment large networks and can filter out noise. However, they are slower than bridges because they are more intelligent devices; Network Hardware Switch Just like a hub A network switch conserves network bandwidth and offers generally better performance than a hub. A vital difference between a hub and a switch is that all the nodes connected to a hub share the bandwidth among themselves, while a device connected to a switch port has the full bandwidth all to itself. For example, if 10 nodes are communicating using a hub on a 10-Mbps network, then each node may only get a portion of the 10 Mbps if other nodes on the hub want to communicate as well. . But with a switch, each node could possibly communicate at the full 10 Mbps. Network Hardware Switch Addressing and Routing Address: byte-string that identifies a node Routing: process of forwarding messages to the destination node based on its address Types of addresses • usually unique • unicast: node-specific • broadcast: all nodes on the network • multicast: some subset of nodes on the network IP Addresses and Classes IP Addresses and Classes IP Addresses and Classes IP Addresses and Classes 28 Subnet Mask 29 Network Architecture A network architecture is a set of layers and protocols used to reduce network design complexity. The TCP/IP Protocol Suite (also called the Internet Architecture) is an important example of a network architecture. The OSI (Open Systems Interconnection) 7-Layer Reference Model [ISO,1984] is a guide that specifies what each layer should do, but not how each layer is implemented. ISO/OSI Reference Model ISO 7-Layer Reference Model End host End host Application Application Presentation Presentation Session Session Transport Transport Network Network Network Network Data link Data link Data link Data link Physical Physical Physical Physical One or more nodes within the network TCP/IP Model 33 Internet Model 34 Protocols A protocol is a set of rules of communication. Protocols are the building blocks of a network architecture. Term “protocol” is overloaded • specification of peer-to-peer interface • module that implements this interface Network Programming A network allows arbitrary applications to communicate. However, a network programmer doesn’t need to know the details of all lower-level network technologies. Network facilities are accessed through an Application Programming Interface (API); e.g., a Service Interface. Basic Paradigm for Communication Most network applications can be divided into two pieces: a client and a server. A Web browser (a client) communicate with a Web server. A Telnet client that we use to log in to a remote host. A user who needs access to data located at remote server. Basic Paradigm for Communication Establish contact (connection). Exchange information (bi-directional). Terminate contact. Client-Server Paradigm Server waits for client to request a connection. Client contacts server to establish a connection. Client sends request. Server sends reply. Client and/or server terminate connection. Two types of Communication Connection-oriented • Setup the link before communication. • Similar to the phone call. We need the phone number and receiver. Connectionless • No link needed to be set up before communication. • Similar to send a letter. We need the address and receiver. TCP and UDP TCP (Transmission Control Protocol) is a connection-oriented protocol. UDP (User Datagram Protocol) is connectionless (UDP) protocol. Ports Identifying the ultimate destination IP addresses identify hosts Host has many applications Ports (16-bit identifier) Application Port WWW 80 E-mail 25 Telnet 23 192.18.22.13 Sockets A socket is defined as an endpoint for communication. Concatenation of IP address and port A socket pair (local IP address, local port, foreign IP address, foreign port) uniquely identifies a communication. The socket 161.25.19.8:1625 refers to port 1625 on host 161.25.19.8 Sockets 44 Sockets and Ports socket any port agreed port socket message client server other ports Internet address = 138.37.94.248 Internet address = 138.37.88.249 TCP Socket Bind() binds the socket to the specified address. The address parameter specifies the local component of the address, e.g. IP address and UDP/TCP port UDP Socket Types of Sockets A stream socket uses TCP for connection-oriented communication. It is also called a TCP socket. A datagram socket uses UDP for connectionless communication. It is also called a UDP socket. A raw socket uses IP directly. It is used in such applications as in ICMP protocol ICMP: Internet Control Message Protocol ICMP: Internet Control Message Protocol • ICMP handles error and control information between gateways and hosts. • The messages are transmitted using IP datagrams. • They are normally generated and processed by the TCP/IP software itself, not user processes. UNIX TCP Communication Normally, a server would first listen and accept a connection and then fork a new process to communicate with the client. It uses the listen operation to listen for connections on a socket. int listen (int sockfd, int backlog) : - Sockfd a socket descriptor created by the socket call. - The backlog parameter defines the maximum length the queue of pending connections may grow to. UNIX TCP Communication The server uses the accept system call to accept connection requested by a client. After a connection has been established, both processes may then use the write (send) and read (recv) operations to send and receive messages. Establishing a TCP Connection Passive Open Server process does a ‘passive’ open on a port It waits for a client to connect At this stage there is no Internet network traffic Establishing a TCP Connection Active Open client process usually on a different machine performs an ‘active’ open on the port Example - Programming Server Initialization: • • • socket - create socket bind – bind socket to the local address listen - associate socket with incoming requests Loop: • • • accept - accept incoming connection recv - receive message from client send - send message to client Termination: • close - close connection socket Example - Programming Client Initialization: • • • gethostbyname - look up server socket - create socket connect - connect to server port Transmission: • • send – send message to server recv - receive message from server Termination: • close - close socket Windows Sockets Similar to UNIX, only the interface is different We call it Windows Socket or WinSock Fits above the transport layer It is not a Protocol -- Located in a dynamic linked library (DLL). 56 Windows Sockets • Windows places a single copy of socket functions and TCP/IP code in memory. • All applications share these functions/code 57 Steps in creating Clients in Java 58 Steps in creating Clients in Java 59 60 61 62 63 64 65 66 67 68 69 70 References http://www.sockets.com/winsock.htm http://tangentsoft.net/wskfaq/ Core Web Programming book Chapter 17. 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