??? Traceroute. app. app. host. host. Apps talk to other apps with no real idea of what is inside the network

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2 About Me Esther Jang 3rd year PhD student Information and Communications Technology for Development Community Cellular Network deployments My goal is to get better at teaching. Computer Networks 2

3 Traceroute Apps talk to other apps with no real idea of what is inside the network This is good! But you may be curious Peeking inside the Network with Traceroute app host??? host app

4 Traceroute 4

5 Traceroute Probes successive hops to find network path TTL: time-to-live Local Host Computer Networks 5... Remote Host

6 Traceroute Local Host 1 hop 2 hops... 3 hops Computer Networks 6 N-1 hops N hops Remote Host

7 Using Traceroute Computer Networks 7

8 Using Traceroute (2) ISP names and places are educated guesses Home 1 hop tde 3 hops Telefonica 4 hops Level3 6 hops pnw-gigapop 1 hop UW 3 hops My computer ms. UW 180 ms >200 ms NYC San Jose Seattle (www1.cac.washington.edu) Computer Networks 8

9 Client-server interaction 9

10 Network-Application Interface Defines how apps use the network Application Layer APIs Lets apps talk to each other hides the other layers of the network app app network Computer Networks host host 10

11 Project 1 Simple Client Send requests to server Wait for a reply Extract the information from the reply Continue Simple Server Server handles the Client requests Multi-threaded Computer Networks 11

12 Project 1 This is the basis for many apps! File transfer: send name, get file ( 6.1.4) Web browsing: send URL, get page Echo: send message, get it back Let s see how to write this app Computer Networks 12

13 Socket API (Generalized) Simple application-layer abstractions (APIs) to use the network The network service API used to write all Internet applications Part of all major OSes and languages; originally Berkeley (Unix) ~1983 Two kinds of sockets Streams (TCP): reliably send a stream of bytes Datagrams (UDP): unreliably send separate messages Computer Networks 13

14 Socket API (2) Sockets let apps attach to the local network at different ports Ports are used by OS to distinguish services/apps using internet Socket Port 1 Socket Port 2 Computer Networks 14

15 Socket API (3) Primitive Meaning SOCKET BIND LISTEN Create a new communication endpoint Associate a local address (port) with a socket Announce willingness to accept connections; (give queue size) ACCEPT Passively establish an incoming connection CONNECT Actively attempt to establish a connection SEND Send some data over the connection RECEIVE Receive some data from the connection CLOSE Release the connection Computer Networks

16 Using Sockets Client (host 1) Time Server (host 2) Computer Networks 16

17 Using Sockets (2) Client (host 1) Time Server (host 2) 1 connect 1 2 request 4 reply disconnect 3 4 Computer Networks 17

18 Using Sockets (3) Client (host 1) Time Server (host 2) 1: socket 1: socket 2: (bind) connect 3: (listen) 4: accept* 5: connect* request 6: recv* 7: send 8: recv* reply 9: send disconnect 10: close 10: close *= call blocks Computer Networks 18

19 Client Program (outline) socket() // make socket getaddrinfo() // server and port name // connect() // connect to server [block] send() // send request recv() // await reply [block] // do something with data! close() // done, disconnect Computer Networks 19

20 Server Program (outline) socket() getaddrinfo() bind() listen() accept() recv() send() close() // make socket // for port on this host // associate port with socket // prepare to accept connections // wait for a connection [block] // wait for request // send the reply // eventually disconnect Computer Networks 20

21 Java Examples with Socket & ServerSocket Server Client ServerSocket listener = new ServerSocket(9090); try { while (true) { Socket socket = listener.accept(); try { socket.getinputstream(); } finally { socket.close(); } } } finally { listener.close(); } Socket socket = new Socket(server, 9090); out = new PrintWriter(socket.getOutputStream(), true); socket.close(); es/ working/datagrams/clientserver.html working/sockets/index.html Computer Networks 21

22 END 2013 D. Wetherall Slide material from: TANENBAUM, ANDREW S.; WETHERALL, DAVID J., COMPUTER NETWORKS, 5th Edition, Electronically reproduced by permission of Pearson Education, Inc., Upper Saddle River, New Jersey Computer Networks 22