#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <netinet/tcp.h>   // For TCP_MAXSEG

#define PORT 9090
#define BUFFER_SIZE 1024

int main() {
    int server_fd, client_fd;
    struct sockaddr_in server_addr, client_addr;
    socklen_t addr_len = sizeof(client_addr);

    char buffer[BUFFER_SIZE];
    int mss;

    // Create socket
    if ((server_fd = socket(AF_INET, SOCK_STREAM, 0)) < 0) {
        perror("Socket creation failed");
        exit(EXIT_FAILURE);
    }

    // Bind socket
    server_addr.sin_family = AF_INET;
    server_addr.sin_addr.s_addr = INADDR_ANY;
    server_addr.sin_port = htons(PORT);

    if (bind(server_fd, (struct sockaddr *)&server_addr, sizeof(server_addr)) < 0) {
        perror("Bind failed");
        close(server_fd);
        exit(EXIT_FAILURE);
    }

    // Listen
    if (listen(server_fd, 3) < 0) {
        perror("Listen failed");
        close(server_fd);
        exit(EXIT_FAILURE);
    }

    printf("Server listening on port %d...\n", PORT);

    // Accept client
    if ((client_fd = accept(server_fd, (struct sockaddr *)&client_addr, &addr_len)) < 0) {
        perror("Accept failed");
        close(server_fd);
        exit(EXIT_FAILURE);
    }

    // Get TCP_MAXSEG value
    socklen_t optlen = sizeof(mss);
    if (getsockopt(client_fd, IPPROTO_TCP, TCP_MAXSEG, &mss, &optlen) == 0) {
        printf("Server: Default TCP_MAXSEG (MSS) = %d bytes\n", mss);
    }

    // Change MSS (optional, e.g., force smaller MSS)
    int new_mss = 600;
    if (setsockopt(client_fd, IPPROTO_TCP, TCP_MAXSEG, &new_mss, sizeof(new_mss)) == 0) {
        printf("Server: TCP_MAXSEG set to %d bytes\n", new_mss);
    }

    // Read data from client
    int n = read(client_fd, buffer, sizeof(buffer));
    buffer[n] = '\0';
    printf("Server received: %s\n", buffer);

    // Send reply
    char *msg = "Hello from Server";
    send(client_fd, msg, strlen(msg), 0);

    close(client_fd);
    close(server_fd);
    return 0;
}