TCP Client–Server
Examples
Stevens의 UNP를 중심으로
TCP Client-Server
- Getting Started
Simple TCP Client and Server
Client: simplec.c
Server: simples.c
Simple Daytime Client
26 bytes can be returned,
but always need to code
in read loop !! Why?
Simple Daytime Client – over IPv6
Only runs on IPv6,
but not runs on IPv4
New application should be
written in a manner of
protocol independence
Error Handling: Wrapper Functions – in
files wrap….c
Standard Error Functions – lib/error.c
Standard Error Functions – lib/error.c
Useful UNP Library Functions(1)
Useful UNP Library Functions(2)
Echo Client-Server
- with Concurrent Processes
TCP Echo Client
tcpcliserv/tcpcli01.c:
stdin
stdout
lib/str_cli.c:
fgets
fputs
TCP
Client
writen
readline
TCP
Server
Server는 동시에 여러 Client를 지원해야 한다!
1.
Server는 동시에 여러 개의 client의 connection request
SYN)를 받아들여야 한다.
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
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2.
Listening socket에 connection completion queue 가 있다
listen(listenfd, …): queue의 크기를 선언하고, listening
socket으로 만듬
connfd = accept(listenfd, …): queue에서 connection이 완료된
connected socket을 가져옴
Server는 연결된 여러 개의 client가 보낸 데이터
(request)를 지체없이 처리해서 response해야 한다.
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Iterative Server

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연결 후 client의 request를 차례로 반복적으로(loop을 돌면서) 처리
Client의 연결시간이 길면 지원하기 곤란
Concurrent Server


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Concurrent processes를 이용
Multi-thread 이용
기타 다른 방법: I/O Multiplexing, Non-blocking
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()
TCP
Server
(Parent)
TCP Echo Server
tcpcliserv/tcpserv01.c:
fork
writen
TCP
Client
readline
writen
TCP
Client
readline
lib/str_echo.c:
readline
writen
TCP
Server
(Child)
readline
writen
fork
TCP
Server
(Child)
Problems in this Echo Server

정상 종료후에도 child는 zombie process로 남아 있음

UNIX에서는 child process가 끝나도 parent가 child의 termination status
등의 정보를 전달받을 때 까지 zombie process로 남음


When a child process is terminated, the signal SIGCHLD is
delivered to the parent automatically.

Solution



최대 process개수를 초과하게 되면 process를 더 이상 fork할 수 없게 됨 
새로운 명령을 내릴 수 없음 (먹통)
The parent process should receive the SIGCHLD signal
and get the exit status of child process (wait() or waitpid())
+ handling interrupted system call (due to the signal handling)
비정상 종료(Abnormal Termination) 경우도 처리해야
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Client host crashes
Client process crashes
Network connectivity is lost, and so on
Posix Signal Handling

Signal(software interrupt) can be sent asynchronously
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
by one process to another process(or to itself)
by the kernel to a process
Signal disposition (or actions)

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catch: call a signal handler
ignore: SIG_IGN

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default:SIG_DFL


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
SIGKILL and SIGSTOP cannot be caught nor ignored
Normally, the process receiving a signal is terminated
Ignored for SIGCHLD and SIGURG
signal() vs Posix sigaction()
Posix Signal Semantics

Once a signal handler is installed, it remains installed



Classical UNIX signal is not…
While a signal handler is executing, the signal being delivered is
blocked
Signals are not queued(delivered only one time)  may lost
Correct TCP Echo Server
tcpcliserv/tcpserv04.c:
with Concurrent Processes
Data Format Exchange

Potential problems in exchanging data structure
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Solutions
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
different format of binary numbers: big endian, little
endian
different implementation of C data type: e.g) long, int,
short
different implementation packing and alignment of
structures
pass all the numeric data as text string
define and use common exchange format - e.g) RPC
TCP 상의 application protocol은 통상 PDU를
사용하지 않고 textual syntax를 사용함.
Why?
Echo Client-Server
- with I/O Multiplexing
Filling the Pipe: Echo C/S

In stop-and-wait mode


stdin
stdout

response time = RTT(round-trip
time) + server’s processing
time(=0)
batch mode로(file을 stdin으로
redirection해서) 1000 line을
보내면 1000 x response time
fgets
fputs
TCP
Client
writen
readline
TCP
Server
Continuous Tx

Fill the pipe
Blocking I/O Model
I/O Multiplexing Model
Blocking I/O versus I/O Multiplexing
Blocking I/O
I/O Multiplexing
read()
select()
read()
read()
Ready!
read()
Ready!
read()
Usage of I/O Multiplexing

Client



handles an interactive input and a socket
handles multiple sockets at the same time
Server

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
handles both a listening socket and its connected
socket
handles both TCP and UDP
handles multiple services and perhaps multiple
protocols (e.g., inetd daemon)
Select Functions
Socket API가
아님
#include <sys/time.h>
/* UNIX */
#include <sys/select.h>
/* UNIX */
#include <unistd.h>
/* UNIX */
#include <winsock2.h>
/* Windows */
int select(int maxfdp1, fd_set *readset, fd_set *writeset, fd_set
*exceptset, const struct timeval *timeout);
Returns: count of ready descriptors if positive, 0 on timeout, -1 on error
Read Write Exception
set
set
set

Wait for any one of multiple events
to occur and wake up the process only when
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
one or more of these events occurs
or, a specified amount of time has passed




wait forever: timeout = NULL
wait up to a fixed amount of time
polling: do not wait at all: timer value = 0
ready ?
read write exception
handling
readset, writeset, exceptset after select returns may be changed

Need to set them again for testing file descriptors ready
How to Manipulate Descriptor Sets

Descriptor sets(fd_set): array of integers(FD_SETSIZE)

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
if fdset == NULL, no interest on the condition
caution: value result arguments
Macros
Conditions for Descriptor Ready

A socket is ready for reading (readable)
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A socket is ready for writing (writable)
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
data in socket receive buffer >= low-water mark SO_RCVLOWAT(==1, default)
read-half of connection is closed (TCP has received a FIN)
listening socket and # of completed connections > 0
socket error is pending
available space in socket send buffer >= low-water mark SO_SNDLOWAT ( ==
2048, default (TCP, UDP))
write-half connection is closed: write() will generate SIGPIPE and return error
EPIPE
A socket using nonblocking connect has completed the connection, or the connect
has failed
socket error is pending
Socket has an exception condition pending if


there exists out-of-band data
still at out-of-band mark
Echo client - using I/O Multiplexing
EoF on input  close socket 남은 reply를 socket에서 read 불가능
UNP
Echo client (using shutdown)
UNP
TCP Echo Server - I/O Multiplexing
TCP Echo Server – I/O Multiplexing
UNP
Echo Client-Server
- with Threads
Introduction

Process

Overhead in process
creation(fork)




Thread: light-weight
process

memory is copied
all descriptor are
duplicated

IPC required to pass
information between
parent and child
processes after fork


All threads within a
process share the same
global memory
raises synchronization
and mutual exclusion
problems
10 - 100 times faster than
process creation
Many different thread
implementation

Pthread: Posix thread
Shared and Own data in Threads

Shared information

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process instructions
most data
open files (e.g.,
descriptor)
signal handlers and signal
dispositions
current working directory
user and group IDs

Thread’s own data
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thread ID
set of registers
stack
errno
signal masks
priority
Thread Creation and Termination

Thread attributes
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priority
initial stack size
daemon thread or not: detached or joinable thread
default: NULL
Terminating a thread

implicit termination


when thread starting function returns
explicit termination


pthread_exit
exit
A Simpler Version of str_cli
using Threads
UNP
TCP Echo Server using Threads
threads/tcpserv01.c
threads/tcpserv02.c – more portable
UNP
Non-determinism causes time-dependent errors

Non-determinism (Race Condition)





in C
int ndone = 0;
/* shared variable */
Thread A:
Thread B:
ndone++;
done--;
Compiled output (ASM)
LOAD ndone
INCR
LOAD ndone
DECR
STORE ndone
STORE ndone
Result ??
An instruction is atomic (indivisible), but a statement may be
divisible !!
Shared variables should be used in mutually exclusive
manner

Critical region: code region accessing share variable
Mutual Exclusion
Shared data!
Critical region
For shared data
count
Condition Variables
unlock
lock
mutex
Critical section
wait
condition
Shared variable

wait for condition and wake up one thread

wait for condition and wake up all thread
signal
Example: Condition Variables
Interprocess Communication
via UNIX Domain Protocols
UNIX Domain Sockets

A UNIX IPC method

Absolute pathname is used instead of protocol address and port
number

Stream socket: similar to TCP socket
Datagram socket: similar to UDP socket




An unreliable datagram service that preserves record boundary

may be discarded (receiver가 빨리 읽어내지 못하면)
Normally, used for passing descriptor
Usage of UNIX domain socket



Twice as fast as a TCP socket on the same host
Used when passing descriptors between processes on the same
host (using sendmsg(), recvmsg());
Provides client’s credentials (UID, GID) to the server
UNIX Domain Stream Client/Server
unixdomain/unixstrcli01.c
unixdomain/unixstrserv01.c
UNP
UNIX Domain Datagram Protocol
unixdomai/unixdgcli01.c
unixdomai/unixdgserv01.c
UNP
Binding client address (pathname) is necessary
to identify client