Packet Switching zPacket is a unit of data that can be transferred at one time. zPacket includes a header and the data. zPackets are of variable sizes with a maximum size limit. zAll data is transferred across the Internet in packets.

Packet Switching (contd.) zPacket switching is the process of routing communications traffic in the form of data packets from a sender to the (correct) receiver. zLANs and WANs use packet switching. zPacket switching avoids delays; short messages need not wait for long transfers to complete.

Packet Switching (contd.) zPacket switching permits multiple pairs of computers to communicate simultaneously in a shared network. zComputers take turns sending packets. zInterface hardware handles sharing automatically.

Packet Switching (contd.) zEach device connected to a network contains a small computer that handles the communication. zNetwork software on the sender side divides data into packets. zNetwork software on the receiver side reassembles the packets.

Internet - A Network of Networks zInterconnection of multiple packet switched networks. zMultiple packet switching technologies - ydifferent speed, distances, & cost characteristics zInternet provides ya mechanism to interconnect arbitrary networks ysoftware to transfer data across the connections

Routers zComputers with special purpose software that interconnects networks. zForward packets from one computer to another. zRouting is the process of selecting a network over which to send a packet. zRouter can interconnect diverse technologies, for example, a LAN to a WAN.

Backbone Network & Sites zA major WAN to which other networks attach is called a backbone network. yExample: vBNS - very high-speed Backbone Network System (by MCI in 1995). zSites reached by the backbone are called backbone sites. zAt each backbone site, a router connects a LAN to the backbone.

Wide Area Backbone LAN at site 1 LAN at site 2 LAN at site 3 LAN at site 4 LAN at site 5 LAN at site 6 LAN at site 7 Router net at 6 net at 7 net at 2net at 3 Source: Comer, D.E. (1994), The Internet Book, Second Edition, Prentice-Hall, Inc., New Jersey.

IP - Internet Protocol zA Communication Protocol is an agreement that specifies a common language two computers use to exchange messages. zA protocol specifies yexact format & meaning of each message yconditions under which a message can be sent yhow a computer should respond to a message

IP - Internet Protocol (contd.) zIP specifies how a packet must be formed, and how a router must forward it to its destination. zIP software is needed on a computer connected to the Internet. zIP software is memory resident. zInternet packets are called IP datagrams.

IP - Internet Protocol (contd.) zIP transforms a collection of networks and routers into a seamless communication system. zIP makes the complex internal physical structure of the Internet transparent to the end user.

Internal Structure f e d c b a Source: Comer, D.E. (1994), The Internet Book, Second Edition, Prentice-Hall, Inc., New Jersey. Network a Router connecting networks a and b Computer attached to network d

IP Datagrams & Network Packets zIP datagram defines a standard format for all Internet packets. zIP datagram travels inside a network packet. zIP datagram is data within a network. zRouter creates a new network packet for transmission across another network. zDatagram is processed by software on the destination computer.

Internet or IP Address zIP address is a unique number assigned to a computer. zComputer stores an IP address in 4 bytes. zIP address is displayed as 4 decimal numbers separated by periods, x zIP addresses are not random; prefix is the same for all computers on the same network.

Routing Example de a f bc C3 C2 C1 Computer C3 Computer C1 Router 1 Router 2 Router 3 Router 5 Router 4 Computer C2 Source: Comer, D.E. (1994), The Internet Book, Second Edition, Prentice-Hall, Inc., New Jersey.

TCP - Transmission Control Protocol zIn addition to IP software, most of the computers that connect to the Internet also run TCP software. zTCP/IP - an entire set of Internet communication protocols. zTCP enables two computer programs to communicate across the Internet - connect, exchange data, and disconnect.

TCP (contd.) zTCP makes the Internet reliable. yChecks for lost datagrams that were discarded by routers. yPuts incoming datagrams in the order they were sent. yChecks for duplicate datagrams. zTCP handles the problem of lost datagrams by using timers and acknowledgements.

TCP (contd.) zTCP software on the receiver side sends an acknowledgement back to the sender. zTCP on the sender side starts a timer when it sends data. If an ack. arrives before the timer expires then the TCP cancels the timer, else it retransmits another copy. zTCP’s timer mechanism adapts to “distance” of destination and Internet traffic delays.

Names for Computers zUsers prefer alphabetic names to IP addresses. zEach computer on the Internet must have a unique name. zNames with many parts yFull Name = local name + organization suffix xE.g. = www + okstate.edu yFull Name = computer name + dept. name + organization suffix xE.g. cimctr.inden.okstate.edu

Names for Computers (contd.) zDomain Name System (DNS) is a software that translates a domain name to an IP address. zDNS uses the client/server approach; application program contacts the domain name server to get the IP address. zDomain name servers store names of computers at only one company or enterprise.

Names for Computers (contd.) zComputer name lookup is automatic. yApplication asks a local domain name server and if needed this server then contacts a remote domain name server. zThere is no correspondence between the parts of a domain name and the parts of its IP address.

Reasons for Internet’s success zIP provides flexibility yAccommodates many types of hardware - WAN or LAN, high-speed or slow, wired or wireless, etc. zTCP/IP standards specify how to send IP datagrams on each type of network. zTCP provides reliability yAutomatically adapts to Internet conditions yMakes reliable communication possible even during periods of congestion.