Week10 Sockets

8/8/2019 Week10 Sockets

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1

Communication

Sockets (Haviland Ch. 10)


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2

Simple Web Request

Karens Home Page

-Courses-kites


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The request

How do we tell the server what wewant?

How do we even find the server? How do the server and browser talk to

each other?


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How do we find the server?

Every computer on the Internet has anInternet address.

Called an IP address (Internet Protocol) An IP address is 4 numbers separated

by dots.

www.tkf.toronto.on.ca = 207.245.2.3


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Domain Name Serversbrowser

localname server

rootname server ca

name server

onname server

torontoname server

www.tkf.

toronto.on.ca?

caserver

onserver

torontoserver

207.245.2.3

207.245.2.3

www.tkf.toronto.on.ca?





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This is getting complicated!

browser

localname server

rootname server

caname server

onname server

torontoname server

server

Number of

messages?

10-12


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Actually, its worse than that

The web page for www.tkf.toronto.on.ca doesntreally live at 207.245.2.3

So we need to go look up www.kites.org

GET / HTTP/1.1Connection: Keep-AliveHost:www.tkf.toronto.on.ca

request

HTTP/1.1 301 Moved PermanentlyDate: Tues, 23 July 2002Server: Apache/1.3.22(unix)Location: http://www.kites.orgContent-Type: text/html

reply


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Now what?

Okay, we have the address. What do we do with it? Lets look at how two computers

communicate.

HTTP is a high-level protocol HTTP is specific to the web. Computers communicate for other

reasons.


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10

Protocols

Computers use several layers ofgeneral protocols to communicate.

To understand why these layers areimportant, think about how a company

sends you an invoice for a purchase.


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TCP/IP

Transmission Control Protocol.Tells us how to package up the data.

source address dest. address

bytes ack port

data


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3-way

handshakeSYN

Heres some data

Got it

TCP Connection

Hi 24.197.0.67Connection port 80? Hi 128.100.27.9

Lets talk

okay

Send me a file

Heres some more

Got itIm done

Im done too

ack

ack fin

fin


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TCP: Three-way handshakeclient server

socket

connect

(blocks)

connect

returns

socket,bind,listen

accept(blocks)

accept returns

SYN J

SYN K, ack J+1

ack K+1

sequence

number = J

sequence

number = K


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read()

write()

close()

socket()

connect()

write()

read()

close()

block until connection

from client

Connection establishment

(3-way handshake)

data transfer

end-of-file notification

socket()

bind()

listen()

accept()

TCP ServerTCP Client


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16

Details011001110010010010001000111110100010111

101010001111010101100110010110101111001011011

101010001111010101100110010110101111001011011

101010001111010101100110010110101111001011011

101010001111010101100110010110101111001011011

IP

TCP

IP IP IP

To24.197.0.67

To24.197.0.67

To24.197.0.67

To24.197.0.67

makepackets

Each TCP packet is given a header-sequence number-checksum

put in anIP envelopewith anotherheader


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When something goes wrong

A packet might not arrivetraffic overloadbit corruption

Receiver asks for missing packets to beresent.

Want to send data as fast as possible. But sending too fast wastes resources.


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TCP Congestion Control Window-based:

some number of packets allowed to besent and not ackd

as successful acks arrive, grow windowif packet loss is detected, cut window size

window

ackd sent but not ackd


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TCP Congestion Control

Time

Wind

ow

siz

e

packet losses


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The Big Picture

Client-Server model: a client process wants to talk to aserver process

Client must find server - DNS lookup Client must find process on server - ports Finally establish a connection so two processes can talk

Client

Process

Server

Process

network


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21

Routing

browser

www.tkf.toronto.on.ca

sf-cs27.gw.

utoronto.ca

mcl-gpb.gw.utoronto.ca

fe6-3.gwy2-tor.bb.attcanada.ca

pos10-1.peera-toroon.ip.

grouptelecom.net

border.onet.on.ca

gate-gateway.gw.utoronto.ca

srp2-0.core2-tor.bb.attcanada.ca

pos8-0-0.hcap2-tor.bb.attcanada.ca

atm1-0-101.adsl1-tor.bb.attcanada.ca

206.222.73.174


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Plus the redirect (20 hops)

browser

gar3-p370.attga.ip.att.net

gbr6-p40.attga.ip.att.net

www.kites.org

sf-cs27.gw.utoronto.ca

mcl-gpb.gw.utoronto.ca

if-10-0.core1.Scarborough.Teleglobe.net

if-3-2.core1.Toronto2.Teleglobe.net

border.onet.on.ca

gate-gateway.gw.utoronto.ca

??

if-7-0.core1.Chicago3.Teleglobe.net

??

bed-va-0-105.cablenet-va.com

??

12.122.10.29

tbr1-p012201.

n54ny.ip.att.net

tbr2-p013601.

wswdc.ip.att.net

12.122.10.74

12.124.65.22

if-7-0.core1.Chicago3.Teleglobe.net


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At least 5 different cities

my office St. LouisMissouri

CSLab

U of T

Scarborough.ON

Toronto,ON

Toronto,ON

U of T

??

Chicago

??

St. LouisMissouri

??

??

Washington,

DC

Washington,

DC

??

??

Atlanta,Georgia

Atlanta,Georgia

Chicago


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Putting it together

browser

local

name server

rootname serverca

name server

onname server

torontoname server

localname server

rootname server

orgname server

www.tkf.toronto.on.ca

sf-cs27.gw.

utoronto.ca

mcl-gpb.gw.

utoronto.ca

fe6-3.gwy2-tor.

bb.attcanada.ca

pos10-1.peera-

toroon.ip.grouptelecom.net

border.onet.on.ca

gate-gateway.gw

.utoronto.ca

srp2-0.core2-tor.bb.attcanada.ca

pos8-0-0.hcap2-tor.

bb.attcanada.ca

atm1-0-101.adsl1-tor.

bb.attcanada.ca

206.222.73.174

gar3-p370.attga.

ip.att.net

gbr6-p40.attga.

ip.att.net

www.kites.org

sf-cs27.gw.

utoronto.ca

mcl-gpb.gw.

utoronto.ca

if-10-0.core1.

Scarborough.Teleglobe.net

if-3-2.core1.Toronto2.

Teleglobe.netborder.onet.on.ca

gate-gateway.gw

.utoronto.ca

??

if-7-0.core1.Chicago3.

Teleglobe.net

??bed-va-0-105.

cablenet-va.com

??

12.122.10.29

tbr1-p012201.

n54ny.ip.att.net

tbr2-p013601.

wswdc.ip.att.net

12.122.10.74

12.124.65.22

if-7-0.core1.Chicago3.

Teleglobe.net


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It depends on the size of the web pagewe retrieve.

If the web page is 100 Kbytes (small!) itwill be broken up into ~80 IP packets.

How many messages?

10 (DNS) +

22 (Connect with toronto.on.ca) +6 (DNS) ++ 80 * 20 hops = 1638 messages!


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Sockets

One form of communication betweenprocesses.

Similar to pipes, except sockets can beused between processes on different

machines.

Use file descriptors to refer to sockets. Built on top of TCP layer


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TCP: Three-way handshakeclient server

socket

connect

(blocks)

connect

returns

socket,bind,listen

accept(blocks)

accept returns

SYN J

SYN K, ack J+1

ack K+1

sequence

number = J

sequence

number = K


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read()

write()

close()

socket()

connect()

write()

read()

close()


from client


(3-way handshake)

data transfer


socket()

bind()

listen()

accept()



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Connection-OrientedServer

Create a socket: socket()

Assign a name to a socket: bind() Establish a queue for connections: listen() Get a connection from the queue: accept()!Client

Create a socket: socket() Initiate a connection: connect()!


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Socket Types Two main categories of sockets

UNIX domain: both processes on the samemachine

INET domain: processes on different machines Three main types of sockets:

SOCK_STREAM: the one we will useSOCK_DGRAM: for connectionless socketsSOCK_RAW


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Addresses and Ports A socket pairis the two endpoints of the

connection.

An endpoint is identified by an IP address anda port.

IPv4 addresses are 4 8-bit numbers: 128.100.31.200 = werewolf 128.100.31.201 = seawolf 128.100.31.202 = skywolf

Ports because multiple processes can communicate

with a single machine we need another identifier.

try nslookup


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read()

write()

close()

socket()

connect()

write()

read()

close()


from client


(3-way handshake)

data transfer


socket()

bind()

listen()

accept()



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Server sideint socket(int family, int type,

int protocol);

family specifies protocol family:PF_INET IPv4PF_LOCAL Unix domain

typeSOCK_STREAM, SOCK_DGRAM, SOCK_RAW

protocol set to 0 except for RAW sockets

returns a socket descriptor


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bind to a nameint bind(int sockfd,

const struct sockaddr *servaddr,

socklen_t addrlen);

sockfd returned by socket struct sockaddr_in {short sin_family; /*PF_INET */

u_short sin_port;

struct in_addr sin_addr;

char sin_zero[8]; /*filling*/}; sin_addr can be set to INADDR_ANY to communicate

on any network interface.


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Set up queue in kernelint listen(int sockfd, int backlog)

after calling listen, a socket is ready toaccept connections

prepares a queue in the kernel where partiallycompleted connections wait to be accepted.

backlog is the maximum number of partiallycompleted connections that the kernel shouldqueue.


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Complete the connectionint accept(int sockfd,

struct sockaddr *cliaddr,

socklen_t *addrlen);

blocks waiting for a connection (from thequeue)

returns a new descriptor which refers to theTCP connection with the client

sockfd is the listening socketcliaddr is the address of the client reads and writes on the connection will use

the socket returned by accept


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Client sidesocket() same as server, to say how we are

going to talk

int connect(int sockfd,

const struct sockaddr *servaddr,

socklen_t addrlen);

the kernel will choose a dynamic port and sourceIP address.

returns 0 on success and -1 on failure settingerrno.

initiates the three-way handshake.


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inetserver.cstruct sockaddr_in peer;!struct sockaddr_in self; !int soc, ns, k;!int peer_len = sizeof(peer);!self.sin_family = PF_INET;!self.sin_port = htons(PORT); !self.sin_addr.s_addr = INADDR_ANY;

! bzero(&(self.sin_zero), 8);!peer.sin_family = PF_INET;!/* set up listening socket soc */!soc = socket(PF_INET, SOCK_STREAM, 0);!if (soc < 0) { !perror("server:socket"); exit(1);!

}!if (bind(soc, (struct sockaddr *)&self, sizeof(self))== -1){!perror("server:bind"); close(soc); exit(1); !

}! listen(soc, 1);!... !

full code forthis example is

on the

webpage


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inetserver.c (concluded)/* ... repeated from previous slide ...!soc = socket(PF_INET, SOCK_STREAM, 0);!

bind(soc, (struct sockaddr *)&self, sizeof(self))== -1){!perror("server:bind"); close(soc); exit(1); !

}! listen(soc, 1);!... and now continuing ... */

!/* accept connection request */!ns = accept(soc, (struct sockaddr *)&peer, &peer_len); !if (ns < 0) { !perror("server:accept"); close(soc); exit(1);!

}!/* data transfer on connected socket ns */!k = read(ns, buf, sizeof(buf));!printf("SERVER RECEIVED: %s\n", buf);!

write(ns, buf, k);! close(ns); ! close(soc);!


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Byte order Big-endian

Little-endian

Intel is little-endian, and Sparc is big-endian

00 01 64 C1

A A+1 A+2 A+3

91,329 =

00 01 64 C1

A+3 A+2 A+1 A

91,329 =


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Network byte order To communicate between machines with

unknown or different endian-ness we

convert numbers to network byte order (big-endian) before we send them.

There are functions provided to do this:unsigned long htonl(unsigned long)!unsigned short htons(unsigned short)!unsigned long ntohl(unsigned long)!unsigned short ntohs(unsigned short)!


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Sending and Receiving Dataread and write calls work on sockets, but

sometimes we want more control

ssize_t send(int fd, const void *buf,size_t len, int flags);

works like write ifflags==0 flags: MSG_OOB, MSG_DONTROUTE, MSG_DONTWAIT

ssize_t recv(int fd, void *buf,size_t len, int flags); flags: MSG_OOB, MSG_WAITALL, MSG_PEEK

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Week10 Sockets