socket example1 berkeley sockets details for everyday use carsten griwodz

Socket example 1

Berkeley SocketsDetails for everyday use

Carsten Griwodz

Socket example 2

Data types

Basic data types Have a certain number of bits Not very important in platform-dependent

non-networked applications Not very important in platform-dependent

networked applications Very important in platform-independent

networked applications

Socket example 3

Data types

Basic data types C data types

char unsigned char - 8 bitsshort unsigned short - 16 bitsint unsigned int - 32 (or 64) bitslong unsigned long - 32 or 64 bitslong long unsigned long long - 64 (or 128) bits

Socket API portable typessize_t – 32 bits unsigned, 0 – 2^32-1ssize_t – 32 bits signed, -2^31 – 2^31-1

Socket example 4

IP addresses and hostnames

IPv4 host address When written on paper, it looks like 129.240.71.213

• ”dotted decimal notation” Represents a 32 bit address Binary in bits

• 10000001 11110000 01000111 11010101 Hexadecimal in bytes

• 0x81 0xf0 0x47 0xd5 One 4 byte int on x86, StrongARM, XScale, …

• 0xd547f081 One 4 byte int on PowerPC, POWER, Sparc, …

• 0x81f047d5 In network byte order

• 0x81f047d5

Socket example 5


On x86 etc. ntohl(0x81f047d5) == 0xd547f081

On PowerPC etc. ntohl(0x81f047d5) == 0x81f047d5

Socket example 6


IPv4 host address Corresponds to one network interface card

(NIC) IPv4 network address

Looks like 129.240.69.0/24• Refers to add addresses that that the same first

24 bits• 10000001 11110000 01000111 11010101• 129.240.69.213 is in that network

Socket example 7


IPv4 networks Institutes and companies own network address ranges

• e.g. 129.240.0.0/16 - UiO• e.g. 9.0.0.0/8 – IBM

Institutes and companies assign addresses to their computers• Fixed addresses• Temporary addresses

0 network host

10 network host

110 network host

1110 multicast address

Class A addresses

Class B addresses

Class C addresses

Class D or multicast addresses

1.0.0.0/8 - 127.0.0.0/8

128.0.0.0/16 - 191.255.0.0/16

192.0.0.0/24 -223.255.255.0/24

224.0.0.0 -239.255.255.255

Socket example 8


IPv4 networks Institutes and companies own network address ranges

• e.g. 129.240.0.0/16 - UiO• e.g. 9.0.0.0/8 – IBM

Institutes and companies assign addresses to their computers

• Fixed addresses• Temporary addresses

They can also create subnets• IFI has subnets of UiO’s address space• E.g. 129.250.69.0/24• 129.250.69.0 can’t be used for a computer, it’s the

network’s address• 129.250.69.255 can’t be used for a computer, it’s an

address for all computers in the network, the broadcast address

Socket example 9


These are many addresses Why do we need IPv6?

Most IPv4 addresses have owners• No matter whether the addresses are needed• Most in the US, using 1%• Several in Europe• Really tight in Asia, only assigned when need is proven

IPv6 addresses 128 bits In text

• 2FFF:80:0:0:0:0:94:1 – 8 times 16 bits• Hard to remember own address

001 13 bit top level 32 bit next level 16 bit site level 64 bit interface id

Socket example 10


Hostnames More exactly fully qualified host names Look like niu.ifi.uio.no

• Host niu• In subdomain ifi, Institutt for Informatik• In domain uio, Universitet i Oslo• In top level domain no, Norway

Who decided this? .no - IANA gave it to Uninett .uio - Uninett gave it to UiO .ifi - USIT, UiO’s drift, gave it to IFI niu - IFI drift gave it to the machine

Socket example 11

Name resolution

Gethostbyname Takes a hostname Returns information

about that hostname Including its IP

address

How?

struct hostent *hostp; struct sockaddr_in serveraddr; int sock;

/* Look in DNS for the IP address of the name */ if ((hostp = gethostbyname(machine)) == 0) { fprintf(stderr,“Unknown machine %s\n",machine); exit(1); }

bzero((void *) &serveraddr, sizeof(serveraddr)); serveraddr.sin_family = AF_INET; memcpy(&serveraddr.sin_addr, hostp->h_addr, hostp->h_length); serveraddr.sin_port = htons(port_number);

Socket example 12

Name resolution



address

How? First:

Look into /etc/hosts But only for few,

special, well-known hosts

## list of statically known hosts#127.0.0.1 localhost129.27.2.1 argul.ifi.uio.no argul129.27.2.21 fileserver.ifi.uio.no fileserver

Socket example 13

Name resolution



address

How? Then:

Using DNS the Domain Name

System

a121.ifi.uio.no

ifi.uio.no

Root name server

gaia.cs.umass.edu

dns.umass.edu

1

2 34

5

6

Socket example 14

Name resolution



address

Return value Pointer tostruct hostent

Contains more than just an IP address

• Other names• All addresses

#define h_addr h_addr_list[0]

struct hostent{ /* official hostname */ char* h_name; /* alias names of the host * entry NULL indicates end of the list */ char **h_aliases; /* host address type, e.g. AF_INET */ int h_addrtype; /* length of each address */ int h_length; /* list of addresses, primary is 0th entry * entry 0 indicates end of the list */ char** h_addr_list;};

Socket example 15

Name resolution

Finding the own hostname Command line

ifconfig –a (Unix) ipconfig /a (Windows) Gives you all IP addresses Typically 2:

• 127.0.0.1 localhost• and the actual one

Inside a program Difficult without a

connected socket With a connected socket:

• struct hostent* getsockname(int sock);

• Similar to gethostbyname

eth0 Link encap:Ethernet HWaddr 00:C0:4F:A3:0C:D3 inet addr:129.240.70.96 Bcast:129.240.71.255 Mask:255.255.248.0 UP BROADCAST NOTRAILERS RUNNING MULTICAST MTU:1500 Metric:1 RX packets:40543011 errors:65 dropped:0 overruns:203 frame:65 TX packets:9222810 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:100 RX bytes:3360933562 (3205.2 Mb) TX bytes:2321704496 (2214.1 Mb) Interrupt:19 Base address:0xfcc0

lo Link encap:Local Loopback inet addr:127.0.0.1 Mask:255.0.0.0 UP LOOPBACK RUNNING MTU:16436 Metric:1 RX packets:26429 errors:0 dropped:0 overruns:0 frame:0 TX packets:26429 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:0 RX bytes:5877172 (5.6 Mb) TX bytes:5877172 (5.6 Mb)

Socket example 16

Netmasks

Help your computer find the way All computers with addresses in the same network can be

reached directly• If you know your own computer’s address, e.g. 129.240.70.96• … and the relevant bits of your network, e.g. /24• … you know that all computers with addresses 129.240.70.?

are in the same network For each computer with another address, a member of the

own network (computer or router) must be found that can send the data into the right direction

• This is sometimes wrong• Understanding of network addresses becomes important

Tell your computer its network address For computers with a fixed IP address: set up once For computers with a dynamic address: set up together

with address

Socket example 17

Netmasks

The computer Is not told directly that it’s on the network 129.240.70.0/24 Instead it is told that it’s address is 129.240.70.97 And that its netmask is 255.255.255.0 – these are 24 bits It figures the network address out from this

In ifconfig or ipconfig Lists hostname

• E.g. 129.240.70.97 Lists netmask

• e.g. 255.255.255.0• Or 0xffffff00• Or the highest 24 bits set

Implies that• 129.240.70.97• is part of the subnet 129.240.70.97/24• which is the same as 129.240.70.0/24

This answer from ifconfig implies also that• All computers in the net 129.240.70.0/24 are reachable directly• Without router

Socket example 18

Binding TCP client sockets to ports Main reasons to do this

Use a specific network card

Other potential reasons Firewalls may restrict

open ports Server demands

connection from a priviledged port

• 0 – 1023 Associate virtual circuit

tags with source ports

int boundconnect( char* servername, int port, int ownport ){ … if ((sock = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP)) < 0) { … } bzero(&ownaddr, sizeof(ownaddr)); ownaddr.sin_family = AF_INET; ownaddr.sin_addr.s_addr = INADDR_ANY; ownaddr.sin_port = htons(ownport); if (bind(sock, (struct sockaddr *)&ownaddr, sizeof(ownaddr)) < 0) { … }

if ((hostp = gethostbyname(machine)) == 0) { … } bzero(&serveraddr, sizeof(serveraddr)); serveraddr.sin_family = AF_INET; memcpy(&serveraddr.sin_addr, hostp->h_addr, hostp->h_length); serveraddr.sin_port = htons(port_number); if (connect(sock, (struct sockaddr *)&serveraddr, sizeof serveraddr) < 0) { … } return sock;}

Socket example 19

Connecting TCP socket

CLOSED

LISTEN

SYN_REVD

ESTABLISHED

CLOSE_WAIT

LAST_ACK

CLOSED

SYN_SENT

ESTABLISHED

FIN_WAIT_1

FIN_WAIT_2

TIME_WAIT

socketbind

listen

socket

connect

Send SYN

Rcvd SYN, ACKSend ACKread

write

Send FINRecv ACK

Recv FIN,Send ACK

Wait30 sec

close

Recv SYN,Send ACKand SYN

Recv ACKreadwriteRecv FIN,

Send ACK

Send FIN

Recv ACK

Client Server

Socket example 20

TCP Handshake

LISTEN

SYN_REVD

ESTABLISHED

CLOSED

SYN_SENT

ESTABLISHED

connect

Send SYN

Rcvd SYN, ACKSend ACK

Recv SYN,Send ACKand SYN

Recv ACK

Client Server

Socket example 21

Connecting UDP sockets

Datagram service UDP Usually unconnected connect can be used

Has only local meaning Local UDP ”remembers” a target socket

Advantage Reading and writing is simpler Possible to figure best packet size

Disadvantage Can use unconnected sockets to send to several

receivers Connected sockets only to one

Socket example 22

Reading and writing function calls int read( int sock, void* buffer, int n)

sock must be connected int recv( int sock, void* buffer, int n,

int options ) sock must be connected Sometimes useful option MSG_PEEK: read packet but leave

it in the queue int recvfrom( int sock, void* buffer, int n,

int options, struct sockaddr* src, int* srclen ) Meant for unconnected sockets If sock is connected, src is identical to sockaddr from

accept

Socket example 23

Reading and writing function calls int write( int sock, void* buffer, int n)

sock must be connected int send( int sock, void* buffer, int n,

int options ) sock must be connected

int sendto( int sock, void* buffer, int n, int options, struct sockaddr* dest, int destlen ) Meant for unconnected sockets If sock is connected, dest address must refer to the other

side

Socket example 24

Select

Complicated at first But very useful Can wait for activity on

many sockets New connections on

request sockets New data on connected

sockets Closing of a connected

socket Ready-to-send on a

socket Can wait for user input Can wait for timeouts

int select( int max_fd, fd_set* read_set, fd_set* write_set, fd_set* except_set, struct timeval* timeout );

Socket example 25

Select

For servers Serve many clients at

once Handle clients that

close connections, clients that crash, …

For the chat example Wait for data from

chat partner Wait for typing of the

user


Socket example 26

Select

read_set Arriving connect

requests Arriving data Closing sockets

write_set Non often used Non-blocking send is

finished

except_set Hardly ever used sendto(.,.,.,MSG_OOB)


Socket example 27

Select

Using only the read_set is typical

Clear the read_set Must be done every time

Put all sockets into the read set

Find the highest socket number, add 1

NULL timeout Means forever

Call select waits for arriving data

void wait_for_all(int clientsock[], int clients){ fd_set read_set; int i,act,top=0;

FD_ZERO(&read_set); for( i=0; i<clients; i++ ) { FD_SET(clientsock[i],&read_set); top = MAX(top,clientsock[i]); }

act = select( top+1, &read_set, NULL, NULL, NULL); …}

Socket example 28

Select

select can also wait only for a timeout Without sockets

Timeout parameter NULL means wait

forever Timeval {5,0} means

wait 5 seconds Timeval {0,0} means

don’t wait at all

void wait_some_time(int sec, int usec){ struct timeval timeout; timeout.tv_sec = sec; timeout.tv_usec = usec;

act = select( 0, NULL, NULL, NULL, &timeout ); …}

Socket example 29

Safe writing

You never know when something strange happens on your TCP connection

Safe writing is necessary Code from last lecture

Considers closing of the socket

int safewrite(int so, char buf[], int l){ int i; if (i=write(so, buf, l)==0) { printf("Can't write to socket, connection is closed" ); exit(1); } return i;}

Socket example 30

Safer writing

Not all data may be sent at once TCP takes as much as it wants to Try to send again

• If there was no error

Socket example 31

Safer writingint safewrite( int sock, char buffer[], int len ){ int ij=0; do { i = write( sock, &buffer[j], len ); if( i == -1 ) { perror( “Error writing to socket” ); exit( 1 ); } else if( i == 0 ) { printf( “The socket has been closed, quitting\n” ); exit( 1 ); } len = len – i; j = j + i; } while( len > 0 ); return 1;}

Socket example 32

Safer writing

Not all data may be sent at once TCP takes as much as it

wants to Try to send again

• If there was no error• If there was no

important error

Socket example 33

Safer writing else if( i == 0 ) { printf( “The socket has been closed, quitting\n” ); exit( 1 ); } else if( i > 0 ) { len = len – i; j = j + i; } } while( len > 0 ); return 1;}

int safewrite( int sock, char buffer[], int len ){ int I,j=0; do { i = write( sock, &buffer[j], len ); if( i == -1 ) { switch( errno ) { case EINTR : /* do nothing, no problem */ break; default: perror( “Error writing to socket” ); exit( 1 ); } }

Socket example 34

Safer writing

Not all data may be sent at once

TCP takes as much as it wants to

Try to send again• If there was no error• If there was no important

error You may receive a signal

These are alarm signals in case of emergencies

Pressing CTRL-C raises a signal

• SIGKILL An interrupt that ends your

program if you don’t handle it Server that use TCP

• Get SIGPIPE often• When the receiver crashes in

the middle of a write

Socket example 35

Safer writing default: perror( “Error writing to socket” ); exit( 1 ); } } else if( i == 0 ) { printf( “The socket has been closed, quitting\n” ); exit( 1 ); } else if( i > 0 ) { len = len – i; j = j + i; } } while( len > 0 ); return 1;}

int main( ){ … signal( SIGPIPE, SIG_IGN ); /* I don’t want to do much */ …}

int safewrite( int sock, char buffer[], int len ){ int I, j = 0; do { i = write( sock, &buffer[j], len ); if( i == -1 ) { switch( errno ) { case EINTR : /* do nothing, no problem */ break; case EPIPE : /* do nothing, will write 0 next round */ break;

Socket example 36

Safer writing

Not all data may be sent at once

TCP takes as much as it wants to

Try to send again• If there was no error• If there was no important

error You may receive a signal

These are alarm signals in case of emergencies

Pressing CTRL-C raises a signal

• SIGKILL An interrupt that ends your

program if you don’t handle it Server that use TCP

• Get SIGPIPE often• When the receiver crashes in

the middle of a write

Slow clients block the server Important if your server has

other things to do For example sending data

to many clients at once• Send only the data that

you can send immediately• Send more data when the

connection is ready to take more

Set the socket in non-blocking mode …

… and use select

Socket example 37

Safer writing default: perror( “Error writing to socket” ); return -1; } } else if( i == 0 ) { printf( “The socket has been closed, quitting\n” ); return -1; } else if( i > 0 ) { len = len – i; j = j + i; } return i;}

int main( ){ … signal( SIGPIPE, SIG_IGN ); /* I don’t want to do much */ …}

int safewrite( int sock, char buffer[], int len ){ int I, j = 0; i = write( sock, &buffer[j], len ); if( i == -1 ) { switch( errno ) { case EINTR : /* do nothing, no problem */ break; case EPIPE : /* do nothing, will write 0 next round */ break;

Socket example 38

Safer writing FD_ZERO( &writable ); for( i=0; i<nsock; i++ ) { FD_SET( sock[i], &writable ); max=MAX(sock[i],max); } retval = select( max+1, NULL, &writable, NULL, NULL ); if( retval > 0 ) { for( i=0; i<nsock; i++ ) { if( FD_ISSET( sock[i], &writable ) ) { retval = safewrite( sock[i], &buf[w[i]], l[i] ); … l[i] = l[i] – retval; /* if no error but not finished */ w[i] = w[i] + retval; if( l[i] > 0 ) again = 1; } } } } free( w );}

int createASocket( char machine[], int port_number ){ … sock[i] = TCPClientSocket(machine, port_number); int retval = fcntl( sock[i], F_SETFL, O_NONBLOCK); if( retval == -1 ) { perror( “Could not set socket into non-blocking mode” ); } …}

void safeWriteToMany( int sock[], int nsock, char buf [], int l[]){ int i, retval, again=1, max=0; int* w; fd_set writable; w = (int*)malloc( nsock ); memset( w, 0, sizeof(int)*nsock ); while( again == 1 ) { again = 0;

Socket example 39

Socket options

Two ways to set socket options setsockopt( int sock, int level,

int optname, void* optvalue, int optlen )

• Options for reading and writing all types of sockets

• Often used insetsockopt( request_sock, SOL_SOCKET, SO_REUSEADDR, &i, sizeof(i));

fcntl( int sock, int optname, long optvalue );

• Options for reading and writing all types of sockets, files, pipes, user input, graphical output

• Often used in fcntl( sock[i], F_SETFL, O_NONBLOCK);

Socket example 40

Summary

Names and addresses IP addresses Fully qualified hostnames Domain Name System

• How to use it

Connections in TCP and UDP Functions

The select function call Several functions for reading and writing

Writing really safe To one To many

socket example1 berkeley sockets details for everyday use carsten griwodz

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