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CSCE 515:
Computer Network
Programming
------ Sockets
Wenyuan Xu
Department of Computer Science and Engineering University of South Carolina
9/8/2008 CSCE515 – Computer Network Programming
TCP UDP
IP
Process Layer
Transport Layer
Network Layer
Data-Link Layer
Process Process
ICMP, ARP & RARP
9/8/2008 CSCE515 – Computer Network Programming
Network API
API - Application Programming Interface
API is a set of functionality/services delivered by a programming system.
Network API
The services ( often provided by the operating system) that provide the interface between application and protocol software.
9/8/2008 CSCE515 – Computer Network Programming
Network API
OSI model (^) protocol suiteInternet Application Presentation Session Transport Network Data link Physical
TCP IPv4, IPv
Application
Data link Physical
UDP
processorUser
kernel
Applicationdetails
Communicationsdetails
Network API wish list
Generic Programming Interface.
Support multiple communication protocol suites (families).
Address (endpoint) representation independence.
Provide special services for Client and Server?
Support for message oriented and connection oriented communication. Work with existing I/O services (when this makes sense). Operating System independence
TCP/IP
TCP/IP does not include an API definition.
There are a variety of APIs for use with
TCP/IP:
Sockets by Berkeley
XTI (X/Open Transport Interface) by AT&T
Winsock - Windows Sockets API by Microsoft
MacTCP / Open Transport by Apple
9/8/2008 CSCE515 – Computer Network Programming
Client-Server Model
One side of communication is client, and the other side is server Server waits for a client request to arrive Server processes the client request and sends the response back to the client Iterative or concurrent
Server
Client 1 Client 2 Client 3
9/8/2008 CSCE515 – Computer Network Programming
Functions needed:
Specify local and remote communication
endpoints
Initiate a connection
Wait for incoming connection
Send and receive data
Terminate a connection gracefully
Error handling
9/8/2008 CSCE515 – Computer Network Programming
Berkeley Sockets
A socket is an abstract representation of a communication endpoint.
Generic:
support for multiple protocol families.
address representation independence
Sockets (obviously) have special needs:
establishing a connection
specifying communication endpoint addresses
Sockets work with Unix I/O services just like files, pipes & FIFOs 9/8/2008 CSCE515 – Computer Network Programming
Elements of a Socket
Each socket can be uniquely identified by
Source IP address
Source port number
Destination IP address
Destination port number
An end-to-end protocol (TCP or UDP)
Types of Sockets
Two different types of sockets
Stream sockets
Datagram sockets
Stream Sockets
Also known as connection-oriented socket Use TCP Provide reliable, connected networking service Error free; no out-of-order packets Applications: telnet, ssh, http
9/8/2008 CSCE515 – Computer Network Programming
socket()
The socket() system call returns a
socket descriptor (small integer) or -1 on
error.
socket() allocates resources needed for
a communication endpoint - but it does not
deal with endpoint addressing.
9/8/2008 CSCE515 – Computer Network Programming
Specifying an Endpoint Address
Remember that the sockets API is generic
There must be a generic way to specify
endpoint addresses.
TCP/IP requires an IP address and a port
number for each endpoint address.
9/8/2008 CSCE515 – Computer Network Programming
bind()
calling bind() assigns the address
specified by the sockaddr structure to the
socket descriptor.
bind( mysock, (struct sockaddr) &myaddr, sizeof(myaddr) );*
9/8/2008 CSCE515 – Computer Network Programming
Necessary Background Information: POSIX data types
int8_t signed 8bit int
uint8_t unsigned 8 bit int
int16_t signed 16 bit int
uint16_t unsigned 16 bit int
int32_t signed 32 bit int
uint32_t unsigned 32 bit int
More POSIX data types
sa_family_t address family
socklen_t length of struct
in_addr_t IPv4 address
in_port_t IP port number
Generic socket addresses
struct sockaddr { uint8_t sa_len; sa_family_t sa_family; char sa_data[14]; };
sa_family specifies the address type. sa_data specifies the address value.
Used by kernel
9/8/2008 CSCE515 – Computer Network Programming
sockaddr
An address that will allow me to use sockets to communicate with you. address type AF_CSCE address values: Dean 1 Sayan 6 Devon 2 Yuliya 7 Samuel 3 Razvan 8 Shamik 4 Mythri 9 Henry 5 Femitolu 10
9/8/2008 CSCE515 – Computer Network Programming
AF_CSCE
Initializing a sockaddr structure to point
to Henry :
struct sockaddr henry;
henry.sa_family = AF_CSCE515; henry.sa_data[0] = 5;
9/8/2008 CSCE515 – Computer Network Programming
AF_INET
For AF_CSCE515 we only needed 1 byte
to specify the address.
For AF_INET we need:
16 bit port number
32 bit IP address
IPv4 only!
9/8/2008 CSCE515 – Computer Network Programming
struct sockaddr_in (IPv4)
struct sockaddr_in { uint8_t sin_len; sa_family_t sin_family; in_port_t sin_port; struct in_addr sin_addr; char sin_zero[8]; }; A special kind of sockaddr structure
struct in_addr
struct in_addr { in_addr_t s_addr; };
in_addr just provides a name for the ‘C’ type associated with IP addresses.
Byte Ordering
Different computer architectures use
different byte ordering to represent
multibyte values.
16 bit integer:
Low Byte High Byte
High Byte Low Byte
Address AAddress A Address A+1Address A+
9/8/2008 CSCE515 – Computer Network Programming
sa_lensa_len sin_len sin_len sa_familysa_family
sa_datasa_data
AF_INET
sin_port
sin_addr
sin_zero
sockaddrsockaddr^ sockaddr_insockaddr_in
9/8/2008 CSCE515 – Computer Network Programming
Assigning an address to a socket
The bind() system call is used to assign
an address to an existing socket.
*int bind( int sockfd, const struct sockaddr myaddr, int addrlen);
bind returns 0 if successful or -1 on error.
const!
9/8/2008 CSCE515 – Computer Network Programming
bind() Example
int mysock,err; struct sockaddr_in myaddr;
mysock = socket(PF_INET,SOCK_STREAM,0); myaddr.sin_family = AF_INET; myaddr.sin_port = htons( portnum ); myaddr.sin_addr = htonl( ipaddress);
*err=bind(mysock, (sockaddr ) &myaddr, sizeof(myaddr));
Why no htons/htosl?Why no htons/htosl?Why no htons/htosl?
9/8/2008 CSCE515 – Computer Network Programming
Uses for bind()
There are a number of uses for bind() :
Server would like to bind to a well known address (port number).
Client can bind to a specific port.
Client can ask the OS to assign any available port number.
Port schmo - who cares?
Clients typically don’t care what port they are assigned.
When you call bind you can tell it to assign you any available port:
myaddr.port = htons(0);
1-1024: reserved port (assigned by privileged processes)
Why htons? 0 is 1 byteWhy htons? 0 is 1 byte
What is my IP address?
How can you find out what your IP address is so you can tell bind()?
There is no realistic way for you to know the right IP address to give bind() - what if the computer has multiple network interfaces?
specify the IP address as: INADDR_ANY , this tells the OS to take care of things. myaddr.sin_addr.s_addr = htonl( INADDR_ANY );
1 byte, Why htonl?
9/8/2008 CSCE515 – Computer Network Programming
IPv4 Address Conversion
**int inet_aton( char , struct in_addr );
Convert ASCII dotted-decimal IP address to
network byte order 32 bit value. Returns 1
on success, 0 on failure.
*char inet_ntoa(struct in_addr);
Convert network byte ordered value to
ASCII dotted-decimal (a string).
9/8/2008 CSCE515 – Computer Network Programming
Client-Server Communication (TCP)
socket() bind() listen() accept()
read()
write()
read() close()
socket() connect() write()
read() close()
TCP Client
TCP Server well-known port
blocks until connection from client
process request
connection establishment data(request)
data(reply) end-of-file notification
int socket(int family, int type, int protocol); int bind(int sockfd, struct sockaddr *my_addr, int addrlen); int listen(int sockfd, int backlog); int connect(int sockfd, struct sockaddr *serv_addr, int addrlen);int accept(int sockfd, void *addr, int *addrlen);
int close(int sockfd); int close(int sockfd);
int socket(int family, int type, int protocol);
9/8/2008 CSCE515 – Computer Network Programming
Other socket system calls
General Use
read()
write()
close()
- • ConnectionConnection--orientedoriented
(TCP)(TCP)
- – connect()connect()
- – listen()listen()
- – accept()accept()
- Connectionless (UDP) • Connectionless (UDP)
- – send()send()
- –^ recvrecv()() 9/8/2008 2007 CSCE515 – Computer Network ProgrammingCSCE515 – Computer Network Programming
Assignment & Next time
Reading:
UNP1, 3**
Socket Programming FAQ
Next Lecture:
TCP Details
