Network Programming — TCP Sockets

Internet-এর নিচের স্তর আপনার হাতে

~55 min Advanced 12 practice problems Live code

1. Why Sockets?

HTTP server, database driver, Slack app, game client — সবগুলোরই নিচে C socket, TCP/UDP। একবার সকেট শিখলে Internet কীভাবে কাজ করে সেটা স্পষ্ট হয়ে যাবে।

2. TCP Server Skeleton

echo_server.c

#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <sys/socket.h>
#include <netinet/in.h>

int main(void) {
    int srv = socket(AF_INET, SOCK_STREAM, 0);
    if (srv < 0) { perror("socket"); return 1; }

    int yes = 1;
    setsockopt(srv, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof yes);

    struct sockaddr_in addr = {0};
    addr.sin_family      = AF_INET;
    addr.sin_port        = htons(8080);
    addr.sin_addr.s_addr = INADDR_ANY;

    bind(srv, (struct sockaddr *)&addr, sizeof addr);
    listen(srv, 10);
    printf("listening on :8080\n");

    while (1) {
        int c = accept(srv, NULL, NULL);
        if (c < 0) continue;
        char buf[1024];
        int n = read(c, buf, sizeof buf);
        if (n > 0) write(c, buf, n);    // echo
        close(c);
    }
    return 0;
}

Online runner-এ network listen blocked/sandboxed থাকতে পারে। নিজের machine-এ compile করে telnet localhost 8080 দিয়ে test করুন।

3. TCP Client

client.c

#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>

int main(void) {
    int s = socket(AF_INET, SOCK_STREAM, 0);

    struct sockaddr_in a = {0};
    a.sin_family = AF_INET;
    a.sin_port   = htons(8080);
    inet_pton(AF_INET, "127.0.0.1", &a.sin_addr);

    connect(s, (struct sockaddr *)&a, sizeof a);
    const char *msg = "Hello, server!";
    write(s, msg, strlen(msg));

    char buf[128] = {0};
    int n = read(s, buf, sizeof buf - 1);
    if (n > 0) printf("server said: %s\n", buf);
    close(s);
    return 0;
}

4. Concurrency Models

Fork per connection — সহজ, heavy at scale।
Thread per connection — fork-এর চেয়ে হালকা, তবু limit।
select / poll — এক thread-ই অনেক socket handle।
epoll (Linux), kqueue (BSD), IOCP (Windows) — modern high-scale।
Event loops (libuv, libevent) — উপরের layer।

5. Minimal HTTP/1.0 Server (excerpt)

const char *resp =
    "HTTP/1.0 200 OK\r\n"
    "Content-Type: text/plain\r\n"
    "Content-Length: 13\r\n"
    "\r\n"
    "Hello, world!";
write(c, resp, strlen(resp));

Browser-এ http://localhost:8080 খুললে আপনার C সার্ভার সেই পেজ দেখাবে। Milestone moment।

6. Practice Problems

TCP echo server + matching client.
Echo server ও client।

✨ Show Answer
Sections 2 ও 3-ই reference। এক terminal-এ server, অন্যটায় client চালান।
Send an int between client & server (htonl / ntohl).
Int পাঠান (htonl/ntohl)।
✨ Show Answer
```
uint32_t n = htonl(42);
write(s, &n, 4);
// other side:
read(s, &n, 4);
int x = ntohl(n);
```
Network byte order = big-endian; x86 little-endian। htonl/ntohl platform-independent।
Chat server supporting multiple clients (fork per connection).
Fork-per-client chat server।

✨ Show Answer
accept-এর পর fork; child communicate করে, parent আবার accept-এ ফিরে যায়। SIGCHLD handler দিয়ে zombie reap।
Chat server using select.
Select-based chat server।

✨ Show Answer
fd_set-এ সব active fd; select(maxfd+1, &rfds, NULL, NULL, NULL); ready fd loop-এ accept বা read handle।
Minimal HTTP/1.0 server returning static files.
Static file server।

✨ Show Answer
Request-এর path parse → fopen → file size → HTTP header (Content-Length) + body write। 404 handle।
Handle simple GET with path parsing.
GET parse।

✨ Show Answer
sscanf(buf, "GET %1023s HTTP/1.%d", path, &ver); root-এ /-কে index.html-এ map করুন।
Add Content-Type detection by extension.
Extension-ভিত্তিক Content-Type।

✨ Show Answer
Map: .html → text/html, .css → text/css, .js → application/javascript, .png → image/png ইত্যাদি।
Gracefully handle SIGPIPE (broken pipe).
SIGPIPE handle।

✨ Show Answer
Client disconnect-এর পর write করলে SIGPIPE signal → default kill। Solution: signal(SIGPIPE, SIG_IGN); বা send(..., MSG_NOSIGNAL) ব্যবহার।
Measure requests/sec your server handles.
Requests/sec measure।

✨ Show Answer
ab -n 10000 -c 100 http://localhost:8080/ (Apache Bench) বা wrk -t4 -c100 -d10s।
Port to epoll on Linux and compare.
epoll port।

✨ Show Answer
epoll_create1, epoll_ctl দিয়ে fd register, epoll_wait-এ events। thousands of connection-এ select-এর চেয়ে significantly দ্রুত (O(1) per event vs O(n))।
UDP client/server echoing datagrams.
UDP echo।

✨ Show Answer
SOCK_DGRAM। recvfrom/sendto — connectionless। Server bind; client sendto। Simple, no handshake।
Explain why TCP is reliable and UDP is not.
TCP reliable, UDP না — কেন?

✨ Show Answer
TCP-এ sequence number, ACK, retransmission, ordering এবং flow control — তাই reliable, ordered, duplicate-free stream। UDP-এ কিছুই নেই — fast, connectionless, lossy — voice/gaming-এ উপযোগী।

Glossary (শব্দকোষ)

Term	Meaning	বাংলায়
Socket	Endpoint of a network connection (a file descriptor).	Network connection-এর endpoint।
TCP	Reliable, ordered, connection-oriented stream protocol.	Reliable, ordered, connection-ভিত্তিক protocol।
UDP	Lightweight connectionless datagram protocol.	হালকা, connectionless datagram protocol।
IP Address	Numeric host identifier on the network.	হোস্ট চেনানোর সংখ্যাগত ঠিকানা।
Port	16-bit number selecting a service on a host.	Host-এর কোন service — তা বাছাইয়ের 16-bit সংখ্যা।
Client / Server	One initiates the connection (client); the other listens (server).	একজন connect করে (client), একজন শোনে (server)।
`socket()`	Creates a new socket file descriptor.	নতুন socket FD তৈরি।
`bind()`	Binds a socket to a local address and port.	Local address+port-এ socket বাঁধা।
`listen()`	Marks a socket as ready to accept connections.	Connection accept-এর জন্য সক্রিয় করা।
`accept()`	Pulls a new client connection off the queue.	Queue থেকে client-connection নেয়।
`connect()`	Client-side call to start a connection.	Client-side connection শুরু।
`send` / `recv`	Transmit/receive bytes on a socket.	Socket-এ byte পাঠানো/পড়া।
`htons` / `ntohl`	Convert between host and network byte order.	Host ও network byte-order রূপান্তর।
`sockaddr`	Struct holding address+port for socket APIs.	Address+port ধারণকারী struct।
Three-way Handshake	SYN → SYN-ACK → ACK that establishes a TCP connection.	TCP connection স্থাপনের তিন ধাপ।
`epoll` / `select`	Multiplex many sockets in one thread.	এক thread-এ অনেক socket একসাথে দেখা।

Summary — Module 39

socket → bind → listen → accept → read/write → close। Client: socket → connect → read/write → close। Scale-এর জন্য epoll। আপনার প্রথম HTTP সার্ভার একটি rite of passage।

Next Module → Capstone — একটি বাস্তব সিস্টেম।

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