Post on 01-Jan-2016
description
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An abstract interface provided to the application programmer File descriptor, allows apps to read/write to
the network Allows to processes on remotely
connected computers to talk to each other
What is a socket?
SOCK_STREAM TCP connection
oriented, bidirectional
reliable, in-order delivery
Two types of sockets
SOCK_DGRAM
UDP
no connection
unreliable delivery, no guarantee on the order
can send/receive
CPSC 441 - Application Layer 4
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
process
TCP withbuffers,
variables
socket
controlled byapplicationdeveloper
controlled byoperating
system
host orserver
process
TCP withbuffers,
variables
socket
controlled byapplicationdeveloper
controlled byoperatingsystem
host orserver
internet
CPSC 441 - Application Layer 5
Socket programming with TCPClient 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
TCP provides reliable, in-order transfer of bytes (“pipe”) between client and server
application viewpoint
Used to address processes on a host 0-1023 is usually reserved for known
service 1024-49151 are registered. They are
used for multiple purposes 49152-65535 are private ports
Ports
TransportLayer
FTP Server
Web Server
21 80
NetworkLayer
DLL/Physical
Socket Programming - Flow
socket()
connect()
send()
recv()
close()
..
.
socket()
bind()
listen()
accept()
recv()
send()
close()
wait for connection request from next client
Client Server
int s_listen = socket(family, type, protocol);
family: AF_INET specifies Ipv4 type: SOCK_STREAM, SOCK_DGRAM protocol: 0 (pseudo, IP ). See /etc/protocols
socket()
connect() int connect(int sockfd, const struct sockaddr
*servaddr, socklen_t addrlen); Connect to server. sockfd is socket descriptor from socket() servaddr is a pointer to a structure with:
port number and IP address must be specified (unlike bind())
addrlen is length of structure client doesn’t need bind()
OS will pick ephemeral port
returns socket descriptor if ok, -1 on error
CPSC 441 - Application Layer 10
bind(s_listen, localAdd, addLength) Server specifies which port and address it will
be listening to s_listen: our listening socket descriptor localAdd: socket address structure addLength: length of localAdd
bind()
struct in_addr { in_addr_t s_addr; /* 32-bit IPv4 addresses */};
struct sockaddr_in { u_char sin_len; /* length of address */ u_char sin_family; /* family of address, Ex. AF_INET */ u_short sin_port; /* Protocol (TCP/UDP) Port num */ struct in_addr sin_addr; /* IPv4 address (above) */ char sin_zero[8]; /* set to zero, used for padding */};
Socket Address Structure
Declare address structure struct sockaddr_in sockAdd;
Set family sockAdd.sin_family = AF_INET;
Set IP address (2 ways)//specify address to listen to inet_pton(AF_INET, “127.0.0.1”, &sockAdd.sin_addr.s_addr)
//listen to any local address sockAdd.sin_addr.s_addr = htonl(INADDR_ANY)
Set port sockAdd.sin_port = htons(9999);
Address Structure
int status = listen(s_listen, queuelength); status: -1 if error, 0 otherwise s_listen: socket descriptor queuelength: Number of clients that can “wait” for a
connection listen is non-blocking: returns immediately
listen()
int s_new = accept(s_listen, &clientAddress, &addLength); s_new: new socket for communication with client s_listen: the listening socket clientAddress: struct sockaddr, address of client addLength: size of client address structure accept is blocking: waits for connection before returning
accept()
int send(int s_new, const void *buf, int len, int flags);
• s_new – socket descriptor• buf – pointer to buffer• len – size of buffer• flags – can be safely set to 0
int recv(int s_new, void *buf, int len, unsigned int flags);
• similar to send• buf holds the data to be transferred
Talking
fork() is a C system call used to spawn child processes Execution for both child and parent process
continues at the next instruction fork() returns
• 0 if this is the child process• PID (>0) of the child process if this is the parent• <0 if fork() fails
Used to keep listening on socket and talking on another socket
System calls - fork()
Socket Programming, Dan Rubinstein, http://www1.cs.columbia.edu/~danr/courses/6761/Fall00/intro/6761-1b-sockets.ppt
15-441 Socket Programming, www.cs.cmu.edu/afs/cs/academic/class/15441-f01/www/lectures/lecture03.ppt
Network Programming, Geoff Kuenning, www.cs.hmc.edu/~geoff/classes/hmc.cs105.200701/slides/class21_net2.ppt
Socket Programming, Abhinav Jain, www.cs.purdue.edu/homes/jain8/cs422/pso3.ppt
References