1. CS 1652 Jack Lange University of Pittsburgh
The slides are adapted from the publisher’s material
All material copyright
J.F Kurose and K.W. Ross, All Rights Reserved 1

2. Socket programming
Goal: learn how to build client/server application that communicate using sockets
Socket API
introduced in BSD4.1 UNIX, 1981
explicitly created, used, released by apps
client/server paradigm
two types of transport service via socket API:
unreliable datagram
reliable, byte stream- oriented
a host-local,
application-created,
OS-controlled interface (a “door”) into which
application process can both send and
receive messages to/from another application process
socket
2: Application Layer 22

3. Socket-programming using TCP
Socket: a door between application process and end-end-transport protocol (UDP or TCP) TCP service: reliable transfer of bytes from one process to another
Internet
controlled
by OS
controlled by
app developer
transport
application
physical
link
network
process
socket
2: Application Layer 23

4. Socket programming with TCP
Client must contact server
server process must first be running
server must have created socket (door) that welcomes client’s contact
Client contacts server by:
creating client-local TCP socket
specifying IP address, port number of server process
When client creates socket: client TCP establishes connection to server TCP
When contacted by client, server TCP creates new socket for server process to communicate with client
allows server to talk with multiple clients
source port numbers used to distinguish clients (more in Chap 3)
TCP provides reliable, in-order
transfer of bytes (“pipe”)
between client and server
application viewpoint
2: Application Layer 24

5. Client/server socket interaction: TCP
Server (running on hostid)
Client
create socket,
port=x, for
incoming request:
listenSocket =
socket()
create socket,
connect to hostid, port=x
clientSocket =
socket()
TCP
connection setup
connect to hostid, port=x
connect(clientSocket, dst)
close
connectionSocket
clientSocket
wait for incoming
connection request
connectionSocket =
accept(listenSocket)
read reply from
clientSocket
send request using
read request from
connectionSocket
write reply to
2: Application Layer 25

6. Stream jargon
A stream is a sequence of characters that flow into or out of a process.
An input stream is attached to some input source for the process, e.g., keyboard or socket.
An output stream is attached to an output source, e.g., monitor or socket.
fgets(), fputs(), getchar(), putchar()
Client
process
client TCP socket
2: Application Layer 26

7. Socket programming with TCP
Example client-server app:
1) client reads line from standard input, sends to server via socket
2) server reads line from socket
3) server converts line to uppercase, sends back to client
4) client reads, prints modified line from socket
2: Application Layer 27

8. Example: C client (TCP)
int main(const int argc, const char** argv) {
int s, port, len, res;
char buf[MAX_LINE];
struct hostent *hp;
struct sockaddr_in saddr;
if (argc < 3 || ((port = atoi(argv[2])) <= 0 || port > 65535)){
// error processing; }
if ((s = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP)) < 0) {
if ((hp = gethostbyname(argv[1])) == NULL) {
saddr.sin_family = AF_INET;
memcpy(&saddr.sin_addr.s_addr, hp->h_addr, hp->length);
saddr.sin_port = htons(port);
if (connect(s, (struct sockaddr *)&addr, sizeof(addr)) < 0) {
parameter
checking
create
socket
name lookup
connect to server
2: Application Layer 28

9. Example: C client (TCP)
Read a line
from stdin
if (fgets(buf, sizeof(buf), stdin) == NULL) {
// error processing; }
if ((res = write(s, buf, len)) <= 0) {
if ((res = read(s, buf, sizeof(buf)-1) <= 0) {
buf[res] = 0;
printf(“received: %s”, buf);
close(s);
return 0;
Send the line
to server
Receive the line
from server
Print out the line
from server
Close socket
2: Application Layer 29

10. Example: C server (TCP)
int main(const int argc, const char** argv) {
int i, s, c, len, pos, res, port;
char buf[MAX_LINE];
struct sockaddr_in saddr;
if (argc < 2 || ((port = atoi(argv[1])) <= 0 || port > 65535)) {
// error processing; }
if ((s = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP)) < 0) {
memset(&saddr, 0, sizeof(saddr));
saddr.sin_family = AF_INET;
saddr.sin_addr.s_addr = INADDR_ANY;
saddr.sin_port = htons(port);
if (bind(s, (struct sockaddr *)&saddr, sizeof(saddr)) < 0) {
if (listen(s, 32) < 0) {
Create
socket
Reserve the port
on interfaces
Set as a server socket
Create a client
connection queue
2: Application Layer 30

11. Example: C server (TCP), cont
Accept a new client connection
while ((c = accept(s, NULL, NULL)) >= 0) {
if ((len = read(c, buf, sizeof(buf) -1)) <= 0) {
// error processing; }
buf[len] = 0;
for (i = 0; i < len; i++) {
if (islower(buf[i])) {
buf[i] = toupper(buf[i]);
if ((res = write(c, buf, len)) <= 0) {
close(c);
close(s);
return(0);
Read in line
from socket
Write out line
to socket
End of while loop,
loop back and wait for
another client connection
2: Application Layer 31

12. Example: What’s not covered?
Error handling
read() == 0 ?
Socket buffer issue
A kernel-level buffer to hold the user-level content before sending/receiving
Two buffers per socket: send/receive buffers
Implication: write(s, buf, len) < len
What about read?
Feel free to reuse the code for assignment 1
Read socket programming primer linked at the course page
2: Application Layer 32

13. Socket programming with UDP
UDP: no “connection” between client and server
no handshaking
sender explicitly attaches IP address and port of destination to each packet
server must extract IP address, port of sender from received packet
UDP: transmitted data may be received out of order, or lost
application viewpoint
UDP provides unreliable transfer
of groups of bytes (“datagrams”)
between client and server
2: Application Layer 33

14. Client/server socket interaction: UDP
Server (running on hostid)
create socket,
clientSocket = DatagramSocket()
Client
Create datagram with server IP and port;
send datagram via clientSocket
create socket on port X.
serverSocket = DatagramSocket()
read datagram from
serverSocket
Close clientSocket
Read datagram from clientSocket
write reply to
serverSocket
specifying client address,
port number
2: Application Layer 34

15. Example: C client (UDP)
process
Input: receives packet (recall thatTCP received “byte stream”)
Output: sends packet (recall
that TCP sent “byte stream”)
client UDP socket
2: Application Layer 35

16. Example: C client (UDP)
int main(const int argc, const char** argv) {
int s, port, len, res, fromlen;
char buf[MAX_LINE];
struct hostent *hp;
struct sockaddr_in saddr, raddr;
if (argc < 3 || ((port = atoi(argv[2])) <= 0 || port > 65535)) {
// error processing; }
if ((s = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)) < 0) {
// error processing;
if ((hp = gethostbyname(argv[1])) == NULL) {
memset(&saddr, 0, sizeof(saddr));
saddr.sin_family = AF_INET;
memcpy(&saddr.sin_addr.s_addr, hp->h_addr, hp->h_length);
saddr.sin_port = htons(port);
Create
client socket
Translate
hostname to IP
address using DNS
2: Application Layer 36

17. Example: C client (UDP), cont.
if (fgets(buf, sizeof(buf), stdin) == NULL) {
// error processing; }
len = strlen(buf);
if ((res = sendto(s, buf, len, 0,
(struct sockaddr *)&saddr,
sizeof(saddr))) < 0) {
fromlen = sizeof(raddr);
if ((res = recvfrom(s, buf, sizeof(buf) - 1, 0,
(struct sockaddr *)&raddr,
&fromlen) < 0) {
buf[res] = 0;
printf(“received: %s”, buf);
close(s);
return 0;
Create datagram
Send datagram
to server
Read datagram
from server
2: Application Layer 37

18. Example: C server (UDP)
int main(const int argc, const char** argv) {
int i, s, len, res, port, fromlen;
char buf[MAX_LINE];
struct sockaddr_in saddr, claddr;
if (argc < 2 || ((port = atoi(argv[1])) <= 0 || port > 65535)) {
// error processing; }
if ((s = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)) < 0) {
memset(&saddr, 0, sizeof(saddr));
saddr.sin_family = AF_INET;
saddr.sin_addr.s_addr = INADDR_ANY;
saddr.sin_port = htons(port);
if (bind(s, (struct sockaddr *)&saddr, sizeof(saddr)) < 0) {
Create
a socket
Reserve a port
to receive
datagram
2: Application Layer 38

19. Example: C server (UDP), cont
while (1) {
fromlen = sizeof(claddr);
if ((len = recvfrom(s, buf, sizeof(buf) - 1, 0,
(struct sockaddr *)&claddr,
&fromlen)) <= 0) {
// error processing; }
buf[len] = 0;
for (i = 0; i < len; i++) {
if (islower(buf[i])) {
buf[i] = toupper(buf[i]);
if ((res = sendto(s, buf, len, 0,
fromlen)) <= 0) {
close(s);
return(0);
Receive
datagram
Get IP addr
port #, of
sender
Create and send
datagram to client
End of while loop,
loop back and wait for
another datagram
2: Application Layer 39

20. Summary Application architectures Internet transport service model
client-server
P2P
Hybrid
Internet transport service model
TCP: connection-oriented, reliable
UDP: unreliable, datagrams
Socket programming
socket(), read()/send(), write()/recv(), sendto(), recvfrom(), close()
2: Application Layer 40