The Application Layer

Application Services

(Telnet, FTP, e-mail, WWW)

Reliable Stream

Transport (TCP)

Connectionless Packet Delivery Service

(IP)

Unreliable Transport

Service (UDP)

The Client-Server Model

• Commonly used model for application interaction over a TCP/IP internet– Server: a program that offers a service that can

be reached over a network• Examples: web server, file server

– Client: a program that requests a service from a server

• Examples: ping, browser

The Client-Server Model (cont)

Example: UDP Echo Server

• Server process attaches to port 7 (echo port)

• While (TRUE) do– Wait for a user datagram to arrive– Swap the source and destination IP addresses

and port numbers– Return the user datagram to the original sender

Example: UDP Echo Client

• Obtain an unused UDP port

• Create a user datagram– Source and destination IP addresses– Source and destination port numbers

• Send the user datagram to the echo server

• Await the server’s reply

Differences BetweenClients and Servers

Client Server- Requests service - Provides service- Usually terminates after

using a server a finitenumber of times

- Usually accepts requestsand provides responsesindefinitely

- Obtains an arbitrary,unused port forcommunication

- Listens for requests on awell-known, reserved port

Servers - Master and Slaves

• Can receive multiple concurrent requests

• Each request can take considerable time to process

• Most complex servers have two parts:– A single master program that accepts new

requests– A set of slaves that are responsible for handling

individual requests

The Master-Slave Server Algorithm

• Master:– Open a well-known port for requests

– While (TRUE) do• Waits for a client request

• Start an independent, concurrent slave to handle the request

• Slave:– Process the request given by the master

– Send reply to the client

– Exit

Servers - Must be well Written!

• Servers are usually more complex and harder to write than clients:– Handle multiple, concurrent requests– Enforce authorization and protection– Must handle errors/malformed requests

• The Oracle of Bacon at Virginia

• The Internet worm

Alternatives to the Client-Server Model

• Consider ARP:– Follows the client-server model– Demand driven

• Consider NNTP (Network News Transport Protocol):– Does not follow the client-server model– Uses precollection

Alternatives to the Client-Server Model

• Precollection– Advantages

• Speed

• Mask network/hardware failures

– Disadvantages• Uses processor time and network bandwidth

whether anyone uses the service or not

How Programmers use TCP/IP

• Example: UNIX– Application programs interact with the

operating system by making system calls– Standard I/O operations:

• Open• Read/write• Close

– Network I/O: has own system calls and library routines

Using UNIX Sockets forNetwork I/O

• Socket = abstraction of the port concept:– Application programs request that the operating

system create a socket when one is needed– O.S. returns a small integer (socket descriptor)

that the program uses to reference the socket– Application program can then use read and

write system calls on the socket– Program closes the socket when finished using

it

Creating a Socket

• The socket system call:int sd; // socket descriptorint pf; // protocol family (one of PF_INET,

// PF_PUP, PF_APPLETALK, // PF_UNIX, etc.)

int type; // type of service (one of SOCK_RAW, // SOCK_DGRAM,

SOCK_STREAM)int protocol; // specific protocol in pf

sd = socket(pf, type, protocol) // create a new socket

Binding a Socket to a Source Address

• Some programs (mainly servers) will want a specific (reserved) address

• Some programs (mainly clients) don’t care what address they use and will allow the O.S. to choose one

• Uses the bind system call

Binding a Socket to a Source Address (cont)

• The bind system call:int sd; // socket descriptor

struct sockaddr addr; // structure specifying source // address

int len; // length (in bytes) of

// addr struct

bind(sd,addr,len) // bind socket to source address

Binding a Socket to a Source Address (cont)

• The sockaddr struct:

• Address family field: determines the format of the remaining address octets

0 16 31

ADDRESS FAMILY ADDRESS OCTETS 0-1

ADDRESS OCTETS 2-5

ADDRESS OCTETS 6-9

ADDRESS OCTETS 10-13

Binding a Socket to an Internet Source Address

• The sockaddr_in struct for PF_INET:0 16 31

ADDRESS FAMILY(2) PROTOCOL PORT

IP ADDRESS

UNUSED (ZERO)

UNUSED (ZERO)

Binding a Socket to an Internet Source Address (cont)

• The bind system call:int sd; // socket descriptor

struct sockaddr_in addr; // structure specifying source // address

int len; // length (in bytes) of

// addr struct

bind(sd,addr,len) // bind socket to source IP // address

Connecting a Socket to a Destination Address

• The connect system call:int sd; // socket descriptor

struct sockaddr addr; // structure specifying dest addr

int len; // length (in bytes) of

// addr struct

connect(sd,addr,len) // connect socket to// dest address

• Can also use a sockaddr_in struct for dest address

Sending Data Through a Socket

• The write system call:

int sd; // socket descriptor

void *buffer; // address of the data to be sent

int len; // number of bytes to send

write(sd,buffer,len) // send data through socket

Receiving Data Through a Socket

• The read system call:

int sd; // socket descriptor

void *buffer; // address in memory at which // to store the data

int len; // number of bytes to receive

read(sd,buffer,len) // receive data through socket

Closing a Socket

• The close system call:

int sd; // socket descriptor

close(sd) // close socket

Obtaining Local Socket Addresses

• The getsockname system call:int sd; // socket descriptor

struct sockaddr *addr; // address structure to be filled

int *len; // pointer to integer that will // contain the length of

the // address

getsockname(sd, addr, len); // obtain local socket address

Obtaining and SettingSocket Options

• The getsockopt system call:int sd; // socket descriptorint level; // option for socket or protocol?int optionid; // which specific option?void *optionval; // where to place the requested

// valueint *len; // length of the optionval

getsockopt(sd, level, optionid, optionval, len); // obtain

// socket opt

Obtaining and SettingSocket Options (cont)

• Values for level (from <netinet/in.h>):

– SOL_SOCKET option for the socket

– IPPROTO_IP option for IP

– IPPROTO_ICMP option for ICMP

– IPPROTO_TCP option for TCP

– IPPROTO_UDP option for UDP

Obtaining and SettingSocket Options (cont)

• Values for optionid (from <sys/socket.h>):– SO_TYPE socket type

– SO_SNDBUF send buffer size

– SO_RCVBUF receive buffer size

– SO_DEBUG debugging info available?

– SO_ACCEPTCONN socket listening enabled?

– SO_BROADCAST broadcast supported?

– SO_REUSEADDR address reuse allowed?

– SO_KEEPALIVE keep alive after close?

Obtaining and SettingSocket Options (cont)

• The setsockopt system call:int sd; // socket descriptor

int level; // option for socket or protocol?

int optionid; // which specific option?

void *optionval; // option value

int *len; // length of the option value

setsockopt(sd, level, optionid, optionval, len); // set option // value

Socket Options for Servers

• The listen system call:int sd; // socket descriptor

int length; // length of request queue

listen(sd, length) // set socket request queue

// length

• Can only be used for SOCK_STREAM sockets

Servers: Accepting Connections

• The accept system call:int sd; // socket descriptorstruct sockaddr *name; // address of clientint *len; // length of address struct

newsock = accept(sd, addr, len) // accept connection

• A new socket is created that connects to the client• Server handles request, sends reply, closes newsock

Servers That ProvideMultiple Services

• The select system call:int ndesc; // check descriptors 0...ndesc-1void *indesc; // descriptors to check for inputvoid *outdesc; // descriptors to check for outputvoid *excdesc; // descriptors to check for exceptionsint *timeout; // how long to wait for a connection

nready = select(ndesc, indesc, outdesc, excdesc, timeout)// determine which descriptors

are // ready for I/O

Servers That ProvideMultiple Services (cont)

• The select system call:

nready = select(ndesc, indesc, outdesc, excdesc, timeout)

• Returns the number of descriptors from the specified set that are ready for I/O

• A process can use select to communicate over more than one socket at a time

• Typically each socket provides a distinct service

Miscellaneous (Useful)System Calls

• The gethostname system call:

char *name; // buffer to store nameint length; // size of buffer in

bytes

gethostname(name, length) // get name of host

• Defined in <netdb.h> include file• Process can learn host it’s running on

Miscellaneous (Useful)System Calls (cont)

• The network byte order conversion routines:– Network-to-host (short), ntohs, convert a short

int from network byte order to host byte order– Network-to-host (long), ntohl, convert a long

int from network byte order to host byte order– Host-to-network (short), htons, convert a short

int from network byte order to host byte order– Host-to-network (long), htonl, convert a long

int from network byte order to host byte order

Miscellaneous (Useful)System Calls (cont)

• Testing htons:#include <stdio.h>#include <string.h>#include <netdb.h>

main(int argc, char **argv) { if (argc != 2) exit(-1); printf("%d\n",atoi(argv[1])); printf("%d\n",htons(atoi(argv[1])));

}

Miscellaneous (Useful)System Calls (cont)

• The gethostbyname system call:

struct hostent *h; // hostent structure

char *name; // host name

h = gethostbyname(name); // fill in hostent with // info about

name

Miscellaneous (Useful)System Calls (cont)

• The hostent structure (defined in <netdb.h>):

struct hostent {

char *h_name; /* official name of host */

char **h_aliases; /* alias list */

int h_addrtype; /* host address type */

int h_length; /* length of address */

char **h_addr_list; /* list of addresses from

name server */

Datagram Socket Example: Receiver

#include <sys/types.h>#include <sys/socket.h>#include <netinet/in.h>#include <stdio.h>main() { int sock, len; struct sockaddr_in name; char buf [1024]; sock = socket(AF_INET, SOCK_DGRAM, 0); if (sock < 0) exit(-1); name.sin_family = AF_INET; name.sin_addr.s_addr = INADDR_ANY; name.sin_port = 0; if (bind(sock, (struct sockaddr *) &name, sizeof(name))) exit(-1); len = sizeof(name); if (getsockname(sock, (struct sockaddr *) &name, &len)) exit(-1); printf("Receiver listening on port %d\n",ntohs(name.sin_port)); if (read(sock, buf, 1024) < 0) exit(-1); printf("%s\n",buf); close(sock);}

Datagram Socket Example: Sender

#include <sys/types.h>#include <sys/socket.h>#include <netinet/in.h>#include <netdb.h>#include <stdio.h>#include <string.h>main(int argc, char **argv) { int sock; struct sockaddr_in name; struct hostent *hp, *gethostbyname(); if (argc != 3) exit(-1); sock = socket(AF_INET, SOCK_DGRAM, 0); if (sock < 0) exit(-1); hp = gethostbyname("prime.cs.ohiou.edu"); if (hp == 0) exit(-1); bcopy(hp->h_addr, &name.sin_addr, hp->h_length); name.sin_family = AF_INET; name.sin_port = htons(atoi(argv[1])); if (connect(sock, (struct sockaddr *) &name, sizeof(name))) exit(-1); if (write(sock, argv[2], (strlen(argv[2])+1)) < 0) exit(-1); close(sock);}

Summary

• The client-server model is widely-used for application interaction over a TCP/IP internet– Server: a program that offers a service that can be reached over a

network• Examples: web server, file server

– Client: a program that requests a service from a server• Examples: ping, browser

• Application programs typically interact with the networking software by using sockets and system calls:– Socket – create a socket– Bind – bind a socket to a port– Read/write – get/put data in a socket– Etc.