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CSCE 515:
Computer Network Programming
------ TCP Programming
Wenyuan Xu
Department of Computer Science and Engineering University of South Carolina
9/15/2008 CSCE515 – Computer Network Programming
TCP
TCP provides the end-to-end reliable
connection that IP alone cannot support
The TCP protocol
Segment format
Connection Creation
Flow control
Congestion control
Connection termination
9/15/2008 CSCE515 – Computer Network Programming
TCP Segment Format
0 15 16 31
20 bytes
destination port number
TCP checksum urgent pointer
option (if any)
source port number
window size
sequence number
acknowledgment number header length reserved
UR G
AC K
PS H
RS T
SY N
FI N
data (if any)
9/15/2008 CSCE515 – Computer Network Programming
Client Server
SYN
ISN= X
SYN
ISN= X
SYN
ISN= Y ACK= X +
SYN
ISN= Y ACK= X +
ACK= ACK= YY +1+
time
TCP Connection Establishment
– Three-way handshake
“I want to talk, and I’m starting with byte number X+1 ”.
“OK, I’m here and I’ll talk. My first byte will be called number Y+1, and I know your first byte will be number X+1” “Got it - you start at byte number Y+1”.
TCP Data and ACK
Once the connection is established, data
can be sent.
Each data segment includes a sequence
number identifying the first byte in the
segment.
Each ACK segment includes a request
number indicating what data has been
received. (bytes instead of packets)
Important Information in TCP/IP packet headers
Send
N SEQ
Recv
ACK WIN
Number of bytes in packet (N)
ACK bit set
Sequence number of next expected byte (ACK)
Sequence number of first data byte in packet (SEQ)
Window size at the receiver (WIN)
Contained in IP header Contained in TCP header
9/15/2008 CSCE515 – Computer Network Programming
TCP Flow Control
Sender
Application does a 2K write
Application reads 2k
Sender is blocked
2K SEQ=0^ empty
receiver 0 4K
recv’s buffer
2K
ACK = 2048 WIN = 2048 Application does a 3K write 2K SEQ= Full ACK = 4096 WIN = 0
ACK = 4096 WIN = 2048
1k (^) SEQ=
Sender may send up to 2k
2K
1K 2K
9/15/2008 CSCE515 – Computer Network Programming
FIN
Either end of the connection can initiate
termination.
A FIN is sent, which means the application
is done sending data.
The FIN is ACK’d.
The other end must now send a FIN.
That FIN must be ACK’d.
9/15/2008 CSCE515 – Computer Network Programming
App1 App
FIN
SN= X
FIN
SN= X
ACK= ACK= XX +1+
ACK= ACK= YY +1+
FIN
SN= Y
FIN
SN= Y
“I have no more data for you” FIN_WAIT_
“ OK, I understand you are done sending .” CLOSE_WAIT
“OK - Now I’m also done sending data”. LAST_ACK
“Over and Out, Goodbye” TIME_WAIT
TCP Termination
FIN_WAIT_
CLOSED
9/15/2008 CSCE515 – Computer Network Programming
TCP Sockets Programming
Creating a passive mode (server) socket.
Establishing an application-level
connection.
send/receive data.
Terminating a connection.
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);
Creating a TCP socket
int socket(int family,int type,int proto);
int sock;
sock = socket( AF_INET,
SOCK_STREAM,
if (sock<0) { /* ERROR */ }
9/15/2008 CSCE515 – Computer Network Programming
Binding to well known address
int mysock;
struct sockaddr_in myaddr;
mysock = socket(AF_INET,SOCK_STREAM,0);
myaddr.sin_family = AF_INET;
myaddr.sin_port = htons( 80 );
myaddr.sin_addr = htonl( INADDR_ANY );
bind(mysock, (struct sockaddr *) &myaddr,
sizeof(myaddr));
#include <sys/socket.h> int bind( int sockfd, const struct sockaddr * myaddr, socklen_t addrlen); bind( ) return 0 if OK, -1 on error 9/15/2008 CSCE515 – Computer Network Programming
getsockname()
struct socketaddr_in ss; socklen_t len;
*myaddr.sin_port = htons( 0 ); myaddr.sin_addr = htonl( INADDR_ANY ); bind(mysock, (struct sockaddr ) &myaddr, sizeof(myaddr));
getsockname(mysock, &ss, &len)
#include <sys/socket.h> int getsockname(int sockfd, struct sockaddr * localaddr, socklen_t * addrlen); bind( ) return 0 if OK, -1 on error
9/15/2008 CSCE515 – Computer Network Programming
listen()
int listen( int sockfd, int backlog);
sockfd is the TCP socket (already bound to an
address)
backlog is the number of incoming
connections the kernel should be able to
keep track of (queue for us).
listen() returns -1 on error (otherwise 0).
9/15/2008 CSCE515 – Computer Network Programming
listen()
Server
TCP
3-way handshakecomplete
accept
arrivingSYN
Completed connection queueESTABLISHED state
Incomplete connection queueSYN_RCVD state
Sum of both queuescannot exceed backlog
Accepting an incoming connection.
Once we call listen(), the O.S. will
queue incoming connections
Handles the 3-way handshake
Queues up multiple connections.
When our application is ready to handle a
new connection, we need to ask the O.S.
for the next connection.
accept()
int accept(int sockfd,
struct sockaddr* cliaddr,
socklen_t *addrlen);
sockfd is the passive mode TCP socket.
cliaddr is a pointer to allocated space.
addrlen is a value-result argument
must be set to the size of cliaddr
on return, will be set to be the number of
used bytes in cliaddr.
9/15/2008 CSCE515 – Computer Network Programming
accept() return value
accept() returns a new socket descriptor
(small positive integer) or -1 on error.
After accept returns a new socket descriptor,
I/O can be done using the read() and
write() system calls.
read() and write() operate a little
differently on sockets (vs. file operation)!
9/15/2008 CSCE515 – Computer Network Programming
Terminating a TCP connection
Either end of the connection can call the
close() system call.
If the other end has closed the connection,
and there is no buffered data, reading from
a TCP socket returns 0 to indicate EOF.
9/15/2008 CSCE515 – Computer Network Programming
fork()
In Unix the way to create a new process is the
fork() system call.
fork() is called once but it returns twice
Return value:
0: return in the child
Non-0: the PID of the newly created process
9/15/2008 CSCE515 – Computer Network Programming
Client/Server
before call to accept return
Client (129.1.1.200) server
connect()
connection request listenfd
129.1.1.200:1500 (^) 65.1.1.
*:
listenfd=socket(…) bind(listenfd…) listen(listenfd,LISTENQ); For( ; ;) { connfd = accept(listenfd, …); if ( (pid = fork())==0) { close(listendf); doit(connfd); close(connfd); exit(0); } close(connfd);
Client/Server
After call to accept return
Client (129.1.1.200) server
connect()
listenfd connection request
{129.1.1.200:1500, 65.1.1.200:80} (^) 65.1.1.
*: connfd
listenfd=socket(…) bind(listenfd…) listen(listenfd,LISTENQ); For( ; ;) { connfd = accept(listenfd, …); if ( (pid = fork())==0) { close(listendf); doit(connfd); close(connfd); exit(0); } close(connfd);
Client/Server
After call to accept return
Client (129.1.1.200) Server (parent)
connect()
listenfd connection request
{129.1.1.200:1500, 65.1.1.200:80} (^) 65.1.1.
*: connfd
listenfd=socket(…) bind(listenfd…) listen(listenfd,LISTENQ); For( ; ;) { connfd = accept(listenfd, …); if ( (pid = fork())==0) { close(listendf); doit(connfd); close(connfd); exit(0); } close(connfd);
*listenfd : connfd
Server (child)
fork
