1. TCP Client-Server Example
Chap 1 Foundation
TCP Client-Server Example
Chap 5

2. TCP Echo Client tcpcliserv/tcpcli01.c: lib/str_cli.c: TCP Client
Server
stdin
stdout
fgets
fputs
writen
readline

3. TCP Echo Server tcpcliserv/tcpserv01.c: lib/str_echo.c: (Parent) TCP
Client
Server
(Child)
writen
readline
(Parent)
fork
connect accept
tcpcliserv/tcpserv01.c:
lib/str_echo.c:

4. Problems in Echo Server
Normal Termination후에 zombie process로 남아 있음
Zombie state of process in UNIX
To maintain information about the child for the parent fetch at some later time such as child process ID, termination status, and information on the resource utilization
Solution
When a child process is terminated, signal SIGCHLD is delivered to the parent
Abnormal Termination 경우도 처리해야
Client host crashes
Client process crashes
Network connectivity is lost, and so on

5. Posix Signal Handling
Signal(software interrupt) can be sent asynchronously
by one process to another process(or to itself)
by the kernel to a process
Signal disposition (or actions)
catch: call a signal handler
ignore: SIG_IGN
SIGKILL and SIGSTOP cannot be caught nor ignored
default: SIG_DFL
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

6. Handling Zombies using signal
When the parent process captures SIGCHLD signals, wait until termination status is delivered
In main program of the server
signal(SIGCHLD, sig_child); …
/* Signal handler */
void
sig_chld(int signo) {
pid_t pid;
int stat;
pid = wait(&stat);
printf("child %d terminated\n", pid);
return; }

7. Problems still remain Problem-1: Problem-2:
Parent가 slow system call(accept)에서 blocked 됐을 때 signal이 들어오면 accept는 EINTR error로 return하게 됨
Parent aborted !
Problem-2:
5개의 client가 거의 동시에 terminate
5개의 client가 동시에 SIGCHLD를 server의 child에게 보냄
하나의 SIGCHLD가 도착한 후 나머지는 저장(queueing)되지 않을 수 있음
여전히 zombie가 남아 있음

8. Correct Solution to Problem-1
Handling interrupted system calls

9. Correct Solution to Problem-2 using waitpid()
pid: PID that we want to wait
-1 to wait for the first of children to terminate
WNOHANG option: not to block if there are no terminated children

10. TCP Echo Server handling Zombie Problem

11. Handling Abnormal Termination: Connection aborted before call accept
due to the fact that client process killed or client host crashed ???
When accept is called,
SVR4 returns error EPROTO
Posix.1g returns error ECONNABORTED
Berkeley-derived implementation handle completely within kernel
Solution: call accept again
See lib/wrapsock.c
lib/Wrapsock.c:

12. Handling Abnormal Termination: Termination of Server Process
Case 1: client blocked for reading from socket
Server: process killed  close socket send FIN
Client: readline() returns 0  normal termination
Case 2: client blocked for reading from stdin
Client: User input  writen()
Server: send RST
Client: readline() returns error (ECONNRESET)
Socket과 Stdin 두개의 descriptor가 ready될 조건을 알아내어 별개로 처리 가능해야  use I/O Multiplexing (select(), poll())
When a process is terminated or killed
Calls close()
TCP sends FIN
When TCP receives a segment towards terminated process
(i.e. invalid destination port #)
 TCP responds RST

13. Abnormal Termination: Crashing or Shutdown of Server Host
Crashing of Server Host
ETIMEOUT error return after retransmitting 12 times(9 min)
or EHOSTUNREACH (or ENETUNREACH) error returns with ICMP message
데이터를 보내지 않고도 connection이 끊어졌음을 알 수는 없는가 ?
초 단위로 더 빨리 알아낼 방법은 없는가?
Use SO_KEEPALIVE socket option
Construct heartbeat mechanism using OOB data (TCP URG flag)
Crashing and rebooting of server host
loose connection information responds RST to the request form client  readlline() in client returns error ECONNRESET
Shutdown of server host
SIGKILL  socket closed  send FIN 
same as in “Termination of Server Process”

14. Client’s and Server’s Perspective

15. Data Format Potential problems in exchanging data structure Solutions
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
Solutions
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?