Computer NetworksThe Socket API (Project 1) & Traceroute (HW 1)

(§1.3.4, 6.1.2-6.1.4)

Originally By David Wetherall (djw@), Modified By Qian Yan (qiany7@)

About Me

I’m Anir! (Anirban)

o 5th year masters student

I love low level systems programming and distributed systems

This is my 2nd quarter as a TA

Interned at CDK Global, Facbeookand VMware in the past

Currently doing research in the Systems Lab

Computer Networks 2

About Me

Michael (Chenyang)

Q: What do I like?

A: Scuba Diving + music

Q: What kind of music do you like?

A: British ones, like those made by

The Stone Roses

Q: What song do you listen to

recently?

A: Step into My World by Hurricane

#1Computer Networks 3

Administrivia

Projects (15 + 15 + 15)%

Groups up to 3

Any language. Provide build instructions. We can only help with C/Java/Python

Homework (10%)

Textbook questions + traceroute/wireshark

Individual

Pop quiz (5%)

5 quizzes. Lowest score dropped

Midterm (15%)

Final (25%)

Computer Networks 4

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

host

appapp

host

???

Traceroute

Widely used command-line tool to let hosts peek inside

the network

On all OSes (tracert on Windows)

Developed by Van Jacobson ~1987

Uses a network-network interface (IP) in ways we

will explain later

Computer Networks 6

: Credit: Wikipedia (public domain)

Van Jacobson

Traceroute

Probes successive hops to find network

path

TTL: time-to-live

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. . .

Local

HostRemote

Host

Traceroute

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. . .

Local

HostRemote

Host

1 hop2 hops

3 hops N-1 hopsN hops

Using Traceroute

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Using Traceroute (2)

ISP names and places are educated guesses

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. . .

My computer www.uw.edu

(www1.cac.washington.edu)

tde

3 hops

Telefonica

4 hops

Level3

6 hopspnw-gigapop

1 hop

UW

3 hops

NYC San Jose Seattle

UW

Home

1 hop

100 ms180 ms

>200 ms

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

Computer Networks11

host

appapp

host

network

Project 1

Simple Client

Send requests to attu server

Wait for a reply

Extract the information from the reply

Continue…

Simple Server

Server handles the Client requests

Multi-threaded

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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 …

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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

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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

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Socket

Port 1

Socket

Port 2

Socket API (3)

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Primitive Meaning

SOCKET Create a new communication endpoint

BIND Associate a local address (port) with a socket

LISTENAnnounce 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

https://docs.oracle.com/javase/8/docs/api/java/net/Socket.html

https://docs.oracle.com/javase/8/docs/api/java/net/ServerSocket.html

Using Sockets

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Client (host 1) Server (host 2)Time

Using Sockets (2)

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Client (host 1) Server (host 2)Time

request

reply

disconnect

1 1

2

3

44

connect

Using Sockets (3)

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Client (host 1) Server (host 2)Time

5: connect*

1: socket2: (bind)

1: socket

3: (listen)

9: send

6: recv*

4: accept*

7: send

8: recv*

10: close 10: close

request

reply

disconnect

connect

*= call blocks

Client Program (outline)

socket() // make socket

getaddrinfo() // server and port name

// www.example.com:80

connect() // connect to server [block]

…

send() // send request

recv() // await reply [block]

… // do something with data!

close() // done, disconnect

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Server Program (outline)

socket() // make socket

getaddrinfo() // for port on this host

bind() // associate port with socket

listen() // prepare to accept connections

accept() // wait for a connection [block]

…

recv() // wait for request

…

send() // send the reply

close() // eventually disconnect

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Java Examples with Socket & ServerSocket

Server

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• http://cs.lmu.edu/~ray/notes/javanetexampl

es/

• https://docs.oracle.com/javase/tutorial/net

working/datagrams/clientServer.html

• https://docs.oracle.com/javase/tutorial/net

working/sockets/index.html

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();

END

Computer Networks 23

© 2013 D. Wetherall

Slide material from: TANENBAUM, ANDREW S.; WETHERALL, DAVID J., COMPUTER NETWORKS, 5th

Edition, © 2011. Electronically reproduced by permission of Pearson Education, Inc., Upper Saddle

River, New Jersey