Elementary TCP
Sockets
Chap 4
Socket functions for TCP client/server
Socket Function

Socket: create a socket

Socket descriptor is a file descriptor in UNIX systems,
but is not a file descriptor in Windows
Connect Function

Connect: establish a connection with a server


Kernel chooses both an ephemeral port and the source IP addr
Error returns with errno

No response to SYN  connection timeout (ETIMEDOUT)





send one SYN, another 6 sec later, and another 24 sec later
If no response after total of 75 sec, the error is returned
Response is RST  no process on the server port (ECONNREFUSED)
ICMP unreachable error message  save msg and retry to connect
host unreachable (EHOSTUNREACH or ENETUNREACH)
The socket is no longer usable and must be closed
Bind Function

Bind: assign a local protocol address to a socket



If port # = 0, the kernel chooses a port (ephemeral port)
If wildcard address(INADDR_ANY), the kernel chooses IP addr
Example: IPv4
struct sockaddr_in servaddr;
sockfd = socket(AF_INET, SOCK_STREAM, 0);
bzero(&servaddr, sizeof(servaddr));
seraddr.sin_family = AF_INET;
seraddr.sin_addr.s_addr = htonl(INADDR_ANY);/* wild card */
seraddr.sin_port = htons(13);
/* daytime server */
bind(sockfd, (struct sockaddr *) &servaddr, sizeof(servaddr));

Example: IPv6
struct sockaddr_in6 servaddr;
seraddr.sin6_addr.s_addr =in6addr_any;
/* wild card */
Listen Function(1)

Listen



convert an unconnected socket into passive socket
and define connection queue size (backlog) for this listening
socket
CLOSED  LISTEN
Listen Function(2)

Max number of queued connections is
implementation-dependent
Accept Function

Accept a connection request: return the next
completed connection from the connection queue

blocked if the completed
connection queue is
empty
If the identity of client is
not interested,
use NULL pointer instead
of cliaddr, addrlen

Daytime TCP Server
Send and Receive Functions

Send: Put n-byte message (stored in buff) into the TCP send buffer
int write(int sockfd, const char *buff, int nbytes);
int send(int sockfd, const char *buff, int nbytes, int flags);
Both return: number of bytes written if OK, -1 on error


Blocked if not enough space in TCP send buffer
Recv: Get a message(<= bufsize) from the TCP receive buffer and
save it into buff
int read(int sockfd, char *buff, int bufsize);
int recv(int sockfd, char *buff, int bufsize, int flags);
Both return: number of bytes read if OK,
0 if received FIN and no more data
-1 on error


Blocked until TCP
receive buffer becomes
non-empty
n-byte message를 send()
했다고 해서, 수신측에서
recv()할 때 n-byte를
받는 게 아니다.
Close Function

Close a socket and terminate a TCP connection
But, TCP will try to send any data that is already
queued in the send buffer,
 Then normal connection termination sequence
take places only if reference count == 0



다른 프로세스가 socket을 share하고 있다면, FIN을
보내지 않는다.
To send a FIN explicitly, use shutdown
Finding Protocol Addresses from
sockets

Get the local and foreign protocol address

When is getsockname function required ?





client가 connect후 자신의 IP addr/port #를
알고자 할 때
0인 port #로 bind한 후 assign된 local port
#를 알고자 할 때
socket의 address family를 알고자 할 때
server가 wildcard IP addr를 bind하여
connection이 이루어진 후(accept가
return된 후) local IP addr를 알고자 할 때
When is getpeername function required ?

server가 accept후 child process가
fork되고 exec되면 client를 알 수 없음 (예,
inetd)
Daytime Iterative Server
UNIX Process Creation

Fork: create a new process

Exec: replace current process image with the new
executable file
Typical Concurrent Servers
Server
Before accept
listenfd
After return
from accept
listenfd
connfd
After fork return
listenfd
connfd
fork
After close sockets
Client
connection request
connection
connection
connect()
connect()
connect()
listenfd
connfd
listenfd
connfd
connect()
Summary
12.106.32.254
192.168.42.1
206.168.112.219
203.253.70.44
local
?
?
foreign
?
?
local
206.168.112.219
1500
foreign
12.106.32.254
21
local
?
?
foreign
?
?
local
0
21
foreign
?
?
Listening socket (=3)
local
0
21
foreign
?
?
Connected socket (=4)
local
12.106.32.254
21
TCP segment
FIN
foreign
206.168.112.219
1500