Leon-Garcia & Widjaja: Communication Networks Copyright ©2000 The McGraw Hill Companies The user clicks on a link to indicate which document is to be retrieved. The browser must determine the address that contains the document. It does this by sending a query to its local name server. Once the address is known the browser establishes a connection to the specified machine, usually a TCP connection. In order for the connection to be successful, the specified machine must be ready to accept TCP connections. The browser runs a client version of HTTP, which issues a request specifying both the name of the document and the possible document formats it can handle. The machine that contains the requested document runs a server version of HTTP. It reacts to the HTTP request by sending an HTTP response which contains the desired document in the appropriate format. The TCP connection is then closed and the user may view the document Figure Introduces topic

Leon-Garcia & Widjaja: Communication Networks Copyright ©2000 The McGraw Hill Companies HTTP server HTTP client Request Response Figure 2.1

Leon-Garcia & Widjaja: Communication Networks Copyright ©2000 The McGraw Hill Companies HTTP server HTTP client TCP GET 80, # #, 80 STATUS Port 80 Ephemeral Port # Figure 2.2

Leon-Garcia & Widjaja: Communication Networks Copyright ©2000 The McGraw Hill Companies n entity n-PDUs n entity Figure 2.3

Leon-Garcia & Widjaja: Communication Networks Copyright ©2000 The McGraw Hill Companies n+1 entity n-SAP n+1 entity n-SAP n entity n-SDU H H n-PDU Figure 2.4

Leon-Garcia & Widjaja: Communication Networks Copyright ©2000 The McGraw Hill Companies n-SDU n-PDU n-SDU n-PDU Segmentation Reassembly n-PDU n-SDU n-PDU Blocking Unblocking n-SDU (a) (b) Figure 2.5

Leon-Garcia & Widjaja: Communication Networks Copyright ©2000 The McGraw Hill Companies Application Layer Presentation Layer Session Layer Transport Layer Network Layer Data Link Layer Physical Layer Application Layer Presentation Layer Session Layer Transport Layer Network Layer Data Link Layer Physical Layer Network Layer Electrical and/or Optical Signals Application A Application B Data Link Layer Physical Layer Network Layer Data Link Layer Physical Layer Communication Network Figure 2.6

Leon-Garcia & Widjaja: Communication Networks Copyright ©2000 The McGraw Hill Companies C PS C C C C PS = packet switch C = computer Figure 2.7

Leon-Garcia & Widjaja: Communication Networks Copyright ©2000 The McGraw Hill Companies G G G G G net 1 net 2 net 3 net 4 net 5 G = gateway/router G Figure 2.8

Leon-Garcia & Widjaja: Communication Networks Copyright ©2000 The McGraw Hill Companies Application Layer Presentation Layer Session Layer Transport Layer Network Layer Data Link Layer Physical Layer Application Layer Presentation Layer Session Layer Transport Layer Network Layer Data Link Layer Physical Layer Application A Application B data ah ph sh th nh dh bits dt Figure 2.9

Leon-Garcia & Widjaja: Communication Networks Copyright ©2000 The McGraw Hill Companies Application Layer Transport Layer Internet Layer Network Interface Application Layer Transport Layer Internet Layer Network Interface (a) (b) Figure 2.10

Leon-Garcia & Widjaja: Communication Networks Copyright ©2000 The McGraw Hill Companies Application Transport Internet Network Interface Application Transport Internet Network Interface Internet Network Interface Network 1 Network 2 Machine A Machine B Router/Gateway Figure 2.11

Leon-Garcia & Widjaja: Communication Networks Copyright ©2000 The McGraw Hill Companies HTTP SMTP RTP TCP UDP IP Network Interface 1 Network Interface 3 Network Interface 2 DNS Figure 2.12

Leon-Garcia & Widjaja: Communication Networks Copyright ©2000 The McGraw Hill Companies Net Interface IP TCP HTTP Net Interface IP Net Interface IP TCP HTTP EthernetPPP Router router (1,1) s (1,2) w (2,1) (1,3) r (2,2) PPP Ethernet (a) (b) Server PC Figure 2.13

Leon-Garcia & Widjaja: Communication Networks Copyright ©2000 The McGraw Hill Companies IP Header Header contains source and destination physical addresses; network protocol type Frame Check Sequence Ethernet Header Figure 2.14

Leon-Garcia & Widjaja: Communication Networks Copyright ©2000 The McGraw Hill Companies HTTP Request TCP Header Header contains source and destination port numbers Header contains source and destination IP addresses; transport protocol type IP Header Header contains source and destination physical addresses; network protocol type Frame Check Sequence Ethernet Header Figure 2.15

Leon-Garcia & Widjaja: Communication Networks Copyright ©2000 The McGraw Hill Companies Application 1 Socket socket interface user kernel Application 2 user kernel Underlying communication Protocols Communications network Socket socket interface Figure 2.16

Leon-Garcia & Widjaja: Communication Networks Copyright ©2000 The McGraw Hill Companies Figure 2.17

Leon-Garcia & Widjaja: Communication Networks Copyright ©2000 The McGraw Hill Companies socket() bind() sendto() close() socket() bind() recvfrom() sendto() close() blocks until server receives data from client data Server Client recvfrom() Figure 2.18

Leon-Garcia & Widjaja: Communication Networks Copyright ©2000 The McGraw Hill Companies User Interface User PI User DTP PI = Protocol interpreter DTP = Data transfer process User FTP Server PI Server DTP Server FTP Control Connection Data Connection Figure 2.19

Leon-Garcia & Widjaja: Communication Networks Copyright ©2000 The McGraw Hill Companies C:\WINDOWS>ping nal.toronto.edu Pinging nal.toronto.edu [ ] with 32 bytes of data: Reply from : bytes=32 time=118ms TTL=243 C:\WINDOWS> Figure 2.20