TCP协议深度解析

// TCP connect系统调用
int tcp_v4_connect(struct sock *sk, struct sockaddr *uaddr, int addr_len)
{
    struct sockaddr_in *usin = (struct sockaddr_in *)uaddr;
    struct inet_sock *inet = inet_sk(sk);
    struct tcp_sock *tp = tcp_sk(sk);
    __be16 orig_sport, orig_dport;
    __be32 daddr, nexthop;
    int err;
    
    if (addr_len < sizeof(struct sockaddr_in))
        return -EINVAL;
    
    // 获取目标地址和端口
    daddr = usin->sin_addr.s_addr;
    nexthop = daddr;
    
    // 分配本地端口
    if (!inet->inet_sport) {
        err = inet_hash_connect(&tcp_death_row, sk);
        if (err)
            return err;
    }
    
    // 设置TCP选项
    tp->rx_opt.mss_clamp = TCP_MSS_DEFAULT;
    
    // 发送SYN包
    err = tcp_connect(sk);
    
    return err;
}

// 发送SYN包
int tcp_connect(struct sock *sk)
{
    struct tcp_sock *tp = tcp_sk(sk);
    struct sk_buff *buff;
    int err;
    
    // 初始化序列号
    tp->write_seq++;
    tp->snd_nxt = tp->write_seq;
    tp->pushed_seq = tp->write_seq;
    
    // 分配SKB
    buff = sk_stream_alloc_skb(sk, 0, sk->sk_allocation, true);
    if (unlikely(!buff))
        return -ENOBUFS;
    
    // 设置SYN标志
    tcp_init_nondata_skb(buff, tp->write_seq++, TCPHDR_SYN);
    
    // 发送SYN包
    err = tcp_transmit_skb(sk, buff, 1, sk->sk_allocation);
    
    // 设置重传定时器
    inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
                             inet_csk(sk)->icsk_rto, TCP_RTO_MAX);
    
    // 进入SYN_SENT状态
    tcp_set_state(sk, TCP_SYN_SENT);
    
    return err;
}

// TCP数据接收主函数
int tcp_v4_rcv(struct sk_buff *skb)
{
    const struct iphdr *iph;
    const struct tcphdr *th;
    struct sock *sk;
    int ret;
    
    // 获取TCP头部
    th = tcp_hdr(skb);
    
    // 查找socket
    sk = __inet_lookup_skb(&tcp_hashinfo, skb, th->source, th->dest);
    if (!sk)
        goto no_tcp_socket;
    
    // 已建立连接的处理
    if (sk->sk_state == TCP_ESTABLISHED) {
        ret = tcp_rcv_established(sk, skb, th);
        return ret;
    }
    
    // 其他状态处理
    ret = tcp_v4_do_rcv(sk, skb);
    return ret;
}

// 已建立连接的数据接收
int tcp_rcv_established(struct sock *sk, struct sk_buff *skb)
{
    struct tcp_sock *tp = tcp_sk(sk);
    
    // 快速路径：顺序数据且无特殊情况
    if (tp->rcv_nxt == TCP_SKB_CB(skb)->seq &&
        !tcp_paws_discard(sk, skb)) {
        
        // 将数据加入接收队列
        __skb_queue_tail(&sk->sk_receive_queue, skb);
        tp->rcv_nxt = TCP_SKB_CB(skb)->end_seq;
        
        // 发送ACK
        tcp_send_ack(sk);
        
        // 唤醒等待的进程
        sk->sk_data_ready(sk);
        
        return 0;
    }
    
    // 慢速路径：处理乱序、重传等情况
    tcp_data_queue(sk, skb);
    tcp_ack_snd_check(sk);
    
    return 0;
}

// 拥塞控制接口
struct tcp_congestion_ops {
    void (*init)(struct sock *sk);
    void (*release)(struct sock *sk);
    u32 (*ssthresh)(struct sock *sk);
    void (*cong_avoid)(struct sock *sk, u32 ack, u32 acked);
    void (*set_state)(struct sock *sk, u8 new_state);
    void (*cwnd_event)(struct sock *sk, enum tcp_ca_event ev);
    const char *name;
};

// CUBIC拥塞控制算法
static void cubictcp_cong_avoid(struct sock *sk, u32 ack, u32 acked)
{
    struct tcp_sock *tp = tcp_sk(sk);
    struct cubictcp *ca = inet_csk_ca(sk);
    
    if (!tcp_is_cwnd_limited(sk))
        return;
    
    if (tcp_in_slow_start(tp)) {
        // 慢启动阶段
        acked = tcp_slow_start(tp, acked);
        if (!acked)
            return;
    }
    
    // 拥塞避免阶段
    bictcp_update(ca, tp->snd_cwnd, acked);
    tcp_cong_avoid_ai(tp, ca->cnt, acked);
}

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

#define PORT 8080
#define BACKLOG 128
#define BUFFER_SIZE 1024

int main() {
    int listen_fd, conn_fd;
    struct sockaddr_in server_addr, client_addr;
    socklen_t client_len;
    char buffer[BUFFER_SIZE];
    
    // 创建socket
    listen_fd = socket(AF_INET, SOCK_STREAM, 0);
    if (listen_fd < 0) {
        perror("socket");
        exit(1);
    }
    
    // 设置地址重用
    int opt = 1;
    setsockopt(listen_fd, SOL_SOCKET, SO_REUSEADDR, &opt, sizeof(opt));
    
    // 绑定地址
    memset(&server_addr, 0, sizeof(server_addr));
    server_addr.sin_family = AF_INET;
    server_addr.sin_addr.s_addr = INADDR_ANY;
    server_addr.sin_port = htons(PORT);
    
    if (bind(listen_fd, (struct sockaddr*)&server_addr, sizeof(server_addr)) < 0) {
        perror("bind");
        exit(1);
    }
    
    // 监听
    if (listen(listen_fd, BACKLOG) < 0) {
        perror("listen");
        exit(1);
    }
    
    printf("服务器启动，监听端口 %d\n", PORT);
    
    // 接受连接
    while (1) {
        client_len = sizeof(client_addr);
        conn_fd = accept(listen_fd, (struct sockaddr*)&client_addr, &client_len);
        if (conn_fd < 0) {
            perror("accept");
            continue;
        }
        
        printf("接受连接: %s:%d\n", 
               inet_ntoa(client_addr.sin_addr),
               ntohs(client_addr.sin_port));
        
        // 接收数据
        int n = recv(conn_fd, buffer, BUFFER_SIZE - 1, 0);
        if (n > 0) {
            buffer[n] = '\0';
            printf("接收到: %s\n", buffer);
            
            // 回复数据
            send(conn_fd, buffer, n, 0);
        }
        
        close(conn_fd);
    }
    
    close(listen_fd);
    return 0;
}

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

#define SERVER_IP "127.0.0.1"
#define SERVER_PORT 8080
#define BUFFER_SIZE 1024

int main() {
    int sock_fd;
    struct sockaddr_in server_addr;
    char buffer[BUFFER_SIZE];
    
    // 创建socket
    sock_fd = socket(AF_INET, SOCK_STREAM, 0);
    if (sock_fd < 0) {
        perror("socket");
        exit(1);
    }
    
    // 设置服务器地址
    memset(&server_addr, 0, sizeof(server_addr));
    server_addr.sin_family = AF_INET;
    server_addr.sin_port = htons(SERVER_PORT);
    inet_pton(AF_INET, SERVER_IP, &server_addr.sin_addr);
    
    // 连接服务器
    if (connect(sock_fd, (struct sockaddr*)&server_addr, sizeof(server_addr)) < 0) {
        perror("connect");
        exit(1);
    }
    
    printf("连接到服务器 %s:%d\n", SERVER_IP, SERVER_PORT);
    
    // 发送数据
    const char *msg = "Hello, TCP!";
    send(sock_fd, msg, strlen(msg), 0);
    printf("发送: %s\n", msg);
    
    // 接收响应
    int n = recv(sock_fd, buffer, BUFFER_SIZE - 1, 0);
    if (n > 0) {
        buffer[n] = '\0';
        printf("接收: %s\n", buffer);
    }
    
    close(sock_fd);
    return 0;
}