Lecture 14
Sockets I
Outline
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Sockets
Socket Domain and Type
Socket System Calls
Stream Socket Operation
Datagram Socket Operation
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Sockets
• Method of bidirectional interprocess communication (IPC)
• Can be on same host (computer)
• Or different hosts connected by network
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Sockets in Client-Server Applications
• Each application creates a socket
• Socket is the apparatus that allows communication
• Both applications require a socket
• Server binds socket to well-known address
• Allows client to locate server
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Socket Creation
• Socket created with socket() system call
fd = socket( domain, type, protocol)
• Returns a file descriptor referring to socket
• In this class and in book, protocol always set to 0
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Outline
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Sockets
Socket Domain and Type
Socket System Calls
Stream Socket Operation
Datagram Socket Operation
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Socket Domain
fd = socket( domain, type, protocol)
• Sockets exist in a communication domain
• Domain determines:
• Method of identification (i.e., format of socket’s “address”)
• Range of communications (same vs. different hosts)
• Linux supports 3 different domains:
• AF_UNIX, AF_INET, and AF_INET6
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Socket Domain
• Linux supports 3 different domains:
1. UNIX domain (AF_UNIX): allows communication between
applications on same host
Address = pathname
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Socket Domain
• Linux supports 3 different domains:
2. IPv4 domain (AF_INET): allows communication between
application on hosts connected with Internet Protocol v4
Address = 32-bit IPv4 address + 16-port number
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Socket Domain
• Linux supports 3 different domains:
3. IPv6 domain (AF_INET6): allows communication between
application on hosts connected with Internet Protocol v6
Address= 128-bit IPv6 address + 16-bit port number
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Socket Type
fd = socket( domain, type, protocol)
• Two types of sockets
1. Stream
2. Datagram
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Stream Sockets
• Reliable, bidirectional, byte-stream communication channel
• Similar to having two pipes for IPC
• Connection oriented socket
• Operates in connected pairs
• Can only be connected to one “peer” at a time
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Datagram Sockets
• Allows data to be exchanged in “datagram” messages
• Message boundaries are preserved
• Data transmission is NOT reliable
• Messages may arrive out of order
• Message may get duplicated
• Messages may not arrive at all
• Connectionless socket
• Does NOT need to be connected to another socket
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Internet Domain
• Stream sockets employ TCP (transmission control protocol)
• Datagram sockets employ UDP (user datagram protocol)
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Outline
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Sockets
Socket Domain and Type
Socket System Calls
Stream Socket Operation
Datagram Socket Operation
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Socket System Calls
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socket() – creates a new socket
bind() – binds socket to address
listen() – allows stream socket to accept incoming connections
accept() – accepts connection from peer on listening stream
connect() – establishes connection with other socket
• Socket I/O – uses read(), write(), and socket-specific calls:
send() recv(), etc.
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socket()
int socket( int domain, int type, int protocol );
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#include <sys/socket.h>
Returns file descriptor of socket on success
Returns -1 on error
Creates a new socket
domain : domain of socket ( AF_UNIX, AF_INET, or AF_INET6 )
type : type of socket ( SOCK_STREAM or SOCK_DGRAM )
protocol : set to 0
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bind()
int bind( int socketfd, const struct sockaddr *addr,
socklen_t addrlen );
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#include <sys/socket.h>
Returns 0 on success
Returns -1 on error
Binds a socket to an address
sockfd : file descriptor of socket obtained from socket()
addr : pointer to structure specifying address to bind
addrlen : size of structure to be passed
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struct sockaddr
• Generic socket address structure
• Size depends on domain
• Typical contents:
sa_family_t
char
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sa_family;
//address family (AF_*)
sa_data[14]; //socket address
Outline
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Sockets
Socket Domain and Type
Socket System Calls
Stream Socket Operation
Datagram Socket Operation
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Stream Socket Operation (Telephone Analogy)
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Install telephone
Known telephone number
Telephone is turned on
Other phone dials number
Answer phone
Hang up phone
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=> socket() to create socket
=> bind() socket to address
=> listen() for incoming sockets
=> connect()
=> listener accept() socket
=> close() socket
“passive” socket
(server)
socket()
bind()
“active” socket
(client)
listen()
socket()
accept()
connect()
read()
write()
blocks until
client
connects
write()
close()
multiple
communications
read()
close()
listen()
int listen( int socketfd, int backlog );
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#include <sys/socket.h>
Returns 0 on success
Returns -1 on error
“Listen” for incoming socket connection
Marks socketfd as “passive”
Socket used to accept connections from “active” sockets
backlog : limits the number of pending connections
• pending – client calls connect() before server calls accept()
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accept()
int accept( int socketfd, struct sockaddr *addr,
socklen_t *addrlen);
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#include <sys/socket.h>
Returns a file descriptor on success
Returns -1 on error
Accepts an incoming connection on the listening stream socket
referred to by socketfd
• accept() creates a NEW socket
• Listening socket remains open (can be used to accept more connections)
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accept()
int accept( int socketfd, struct sockaddr *addr,
socklen_t *addrlen);
• Remaining arguments return address of peer socket
• addr points to structure used to return socket address
• addrlen points to an integer that has been initialized to the size
of the buffer pointed to be addr
• If we don’t care about addresses, set both to NULL
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connect()
int connect( int socketfd, const struct sockaddr *addr,
socklen_t addrlen );
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#include <sys/socket.h>
Returns 0 on success
Returns -1 on error
Connects the active socket referred to by socketfd to the
listening socket specified by addr and addrlen
• addr and addrlen specified in same way as bind()
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connect()
int connect( int socketfd, const struct sockaddr *addr,
socklen_t addrlen );
• If connect() fails:
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3.
Close the socket
Create a new socket
Reattempt to connect
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I/O on Stream Sockets
• Pair of connected stream sockets provides bidirectional
communication
• Use read() and write() system calls on socket FDs
• If socket closed with close(), read end receives EOF
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Outline
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Sockets
Socket Domain and Type
Socket System Calls
Stream Socket Operation
Datagram Socket Operation
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Datagram Socket Operation (Postal System Analogy)
• Set up mailbox
=> socket()
• Known address
=> server bind() socket to address
• Send letter (with address)
=> sendto()
• Receive letter
=> recvfrom()
• Remove mailbox
=> close()
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Datagram Socket Operation (Postal System Analogy)
• Just like postal system, when multiple letters (datagrams) sent
from on to another:
• No guarantee that letters will arrive in order sent
• No guarantee that letter(s) will arrive at all
• Unlike post system: datagrams can arrive more than once
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Datagram Socket Operation
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sendto()
ssize_t sendto(
int sockfd,
const void *buffer,
size_t length,
int flags,
const struct sockaddr *dest,
socklen_t addrlen
);
• #include <sys/socket.h>
• Returns number of bytes sent; -1 on error
• First 3 arguments same as write() system call
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sendto()
ssize_t sendto(
int sockfd,
const void *buffer,
size_t length,
int flags,
const struct sockaddr *dest,
socklen_t addrlen
);
• flags : bit mask controlling socket-specific I/O features
• dest & addrlen – specify address of destination
• Same as connect()
• Can be set to NULL
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recvfrom()
ssize_t recvfrom(
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int sockfd,
void *buffer,
size_t length,
int flags,
struct sockaddr *src,
socklen_t *addrlen
#include <sys/socket.h>
Returns number of bytes read; -1 on error
Retrieves exactly one message
First 3 arguments same as read() system call
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);
recvfrom()
ssize_t recvfrom(
int sockfd,
void *buffer,
size_t length,
int flags,
struct sockaddr *src,
socklen_t *addrlen );
• flags : bit mask controlling socket-specific I/O features
• src & addrlen – gets filled with address of sender
• Same as accept()
• Can be set to NULL
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connect() with Datagram Sockets
• When connect() called on datagram socket, kernel records
address of socket’s peer
• Known as a “connected datagram socket”
• If connect() has NOT been called yet:
• “unconnected datagram socket”
• Advantage of “connected datagram socket”: simplifies I/O calls
when transmitting data
• No need to supply dest_addr and addrlen arguments
• Use write() instead
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