getsockopt的TCP层实现剖析

应用层

NAME

getsockopt - get options on sockets

SYNOPSIS

#include <sys/types.h>

#include <sys/socket.h>

int getsockopt (int s, int level, int optname, void *optval, socklen_t *optlen);

调用关系

函数的调用关系图如下：

数据结构

struct tcp_sock {
    ...
    u32 mss_cache; /* Cached effective mss, not including SACKS */
    u8 nonagle : 4, /* Disable Nagle algorithm? */
          thin_lto : 1, /* Use linear timeouts for thin streams */
          thin_dupack : 1, /* Fast retransmit on first dupack */
          unused : 2;
    unsigned int keepalive_time; /* time before keep alive takes place */
    unsigned int keepalive_intvl; /* time interval between keep alive probes */
    u8 keepalive_probes; /* num of allowed keep alive probes */
    int linger2;
    u8 ecn_flags; /* ECN status bits. */
    u32 rcv_tstamp; /* timestamp of last received ACK (for keepalives) */
    u32 lsndtime; /* timestamp of last sent data packet (for restart window) */

    u32 window_clamp; /* Maximal window to advertise，本端通告窗口最大值 */
    u32 rcv_ssthresh; /* Current window clamp */
 
    u16 advmss; /* Advertised MSS，本端的通告MSS */
    u32 total_retrans; /* Total retransmits for entire connection */
    ...
}

struct tcp_options_received {
    ...
    u16 saw_tstamp : 1, /* Saw TIMESTAMP on last packet */
             tstamp_ok : 1, /* TIMESTAMP seen on SYN packet */
             dsack : 1, /* D-SACK is scheduled */
             wscale_ok : 1, /* Wscale seen on SYN packet */
             sack_ok : 4, /* SACK seen on SYN packet */
             snd_wscale : 4, /* Window scaling received from sender */
             rcv_wscale : 4, /* Window scaling to send to receiver */
    ...
}

/* for TCP_INFO socket option */
#define TCPI_OPT_TIMESTAMPS 1
#define TCPI_OPT_SACK 2
#define TCPI_OPT_WSCALE 4
#define TCPI_OPT_ECN 8

struct tcp_info {
    __u8 tcpi_state; /* TCP状态 */
    __u8 tcpi_ca_state; /* TCP拥塞状态 */
    __u8 tcpi_retransmits; /* 超时重传的次数 */
    __u8 tcpi_probes; /* 持续定时器或保活定时器发送且未确认的段数*/
    __u8 tcpi_backoff; /* 退避指数 */
    __u8 tcpi_options; /* 时间戳选项、SACK选项、窗口扩大选项、ECN选项是否启用*/
    __u8 tcpi_snd_wscale : 4, tcpi_rcv_wscale : 4; /* 发送、接收的窗口扩大因子*/

    __u32 tcpi_rto; /* 超时时间，单位为微秒*/
    __u32 tcpi_ato; /* 延时确认的估值，单位为微秒*/
    __u32 tcpi_snd_mss; /* 本端的MSS */
    __u32 tcpi_rcv_mss; /* 对端的MSS */

    __u32 tcpi_unacked; /* 未确认的数据段数，或者current listen backlog */
    __u32 tcpi_sacked; /* SACKed的数据段数，或者listen backlog set in listen() */
    __u32 tcpi_lost; /* 丢失且未恢复的数据段数 */
    __u32 tcpi_retrans; /* 重传且未确认的数据段数 */
    __u32 tcpi_fackets; /* FACKed的数据段数 */

    /* Times. 单位为毫秒 */
    __u32 tcpi_last_data_sent; /* 最近一次发送数据包在多久之前 */
    __u32 tcpi_last_ack_sent;  /* 不能用。Not remembered, sorry. */
    __u32 tcpi_last_data_recv; /* 最近一次接收数据包在多久之前 */
    __u32 tcpi_last_ack_recv; /* 最近一次接收ACK包在多久之前 */

    /* Metrics. */
    __u32 tcpi_pmtu; /* 最后一次更新的路径MTU */
    __u32 tcpi_rcv_ssthresh; /* current window clamp，rcv_wnd的阈值 */
    __u32 tcpi_rtt; /* 平滑的RTT，单位为微秒 */
    __u32 tcpi_rttvar; /* 四分之一mdev，单位为微秒v */
    __u32 tcpi_snd_ssthresh; /* 慢启动阈值 */
    __u32 tcpi_snd_cwnd; /* 拥塞窗口 */
    __u32 tcpi_advmss; /* 本端能接受的MSS上限，在建立连接时用来通告对端 */
    __u32 tcpi_reordering; /* 没有丢包时，可以重新排序的数据段数 */

    __u32 tcpi_rcv_rtt; /* 作为接收端，测出的RTT值，单位为微秒*/
    __u32 tcpi_rcv_space;  /* 当前接收缓存的大小 */

    __u32 tcpi_total_retrans; /* 本连接的总重传个数 */
};

函数实现

内核版本：2.6.37

int tcp_getsockopt (struct sock *sk, int level, int optname, char __user *optval, int __user *optlen)
{
    struct inet_connection_sock *icsk = inet_csk(sk);

    if (level != SOL_TCP)
        return icsk->icsk_af_ops->getsockopt(sk, level, optname, optval, optlen);

    return do_tcp_getsockopt(sk, level, optname, optval, optlen);
}

static int do_tcp_getsockopt (struct sock *sk, int level, int optname, char __user *optval, int __user *optlen)
{
    struct inet_connection_sock *icsk = inet_csk(sk);
    struct tcp_sock *tp = tcp_sk(sk);
    int val, len;

    if (get_user(len, optlen))
        return -EFAULT;

    len = min_t(unsigned int, len, sizeof(int)); 
    if (len < 0) 
        return -EINVAL;
 
    switch (optname) {
        case TCP_MAXSEG:
            val = tp->mss_cache; /* 默认为1460 */
            if (!val && ((1 << sk->sk_state) & (TCPF_CLOSE) | TCPF_LISTEN))
                val = tp->rx_opt.user_mss; /* mss requested by user in ioctl */
            break;

        case TCP_NODELAY:
            val = !! (tp->nonagle & TCP_NAGLE_OFF); /* Nagle默认使用，故默认值为0 */
            break;

        case TCP_CORK:
            val = !! (tp->nonagle & TCP_NAGLE_CORK); /* 默认值为0 */
            break;

        case TCP_KEEPIDLE:
            val = keepalive_time_when(tp) / HZ; /* 默认为7200s */
            break;

        case TCP_KEEPINTVL:
            val = keepalive_intvl_when(tp) / HZ; /* 默认为75s */
            break;

        case TCP_KEEPCNT:
            val = keepalive_probes(tp); /* 默认为9 */
            break;

        case TCP_SYNCNT:
            val = icsk->icsk_syn_retries ? : sysctl_tcp_syn_retries; /* 默认为5 */
            break;

        case TCP_LINGER2:
            val = tp->linger2;
            if (val >= 0)
                val = (val ? : sysctl_tcp_fin_timeout) / HZ; /* 默认为60s */

        case TCP_DEFER_ACCEPT:
            ...

        case TCP_WINDOW_CLAMP:
            val = tp->window_clamp;
            break;

        case TCP_INFO: {
            struct tcp_info info;

            if (get_user(len, optlen))
                return -EFAULT;
 
            tcp_get_info(sk, &info); /* 获取TCP连接的详细信息！*/
 
            len = min_t(unsigned int, len, sizeof(info));

            if (put_user(len, optlen))
                return -EFAULT;

            if (copy_to_user(optval, &info, len))
                 return -EFAULT;

            return 0;
        }

        case TCP_QUICKACK:
            val = ! icsk->icsk_ack.pingpong; /* 快速确认模式 */
            break;

        case TCP_CONGESTION:
            if (get_user(len, optlen))
                return -EFAULT;

            len = min_t(unsigned int, len, TCP_CA_NAME_MAX); /* 16 Bytes */

            if (put_user(len, optlen)
                return -EFAULT;

            if (copy_to_user(optval, icsk->icsk_ca_ops->name, len)) /* 默认为cubic */
                return -EFAULT;

            return 0;

        case TCP_COOKIE_TRANSACTIONS :
            ...

        case TCP_THIN_LINEAR_TIMEOUTS:
            val = tp->thin_lto; /* 默认为0 */
            break;

        case TCP_THIN_DUPACK:
            val = tp->thin_dupack; /* 默认为0 */
            break;

        case TCP_USER_TIMEOUT:
            val = jiffies_to_msecs(icsk->icsk_user_timeout);
            break;

        default:
            return -ENOPROTOOPT;
    }

    if (put_user(len, optlen))
        return -EFAULT;

    if (copy_to_user(optval, &val, len))
        return -EFAULT;

    return 0;
}

static inline int keepalive_time_when(const struct tcp_sock *tp)
{
    return tp->keepalive_time ? : sysctl_tcp_keepalive_time; 
}

static inline int keepalive_intvl_when(const struct tcp_sock *tp)
{
    return tp->keepalive_intvl ? : sysctl_tcp_keepalive_intvl;
}

static inline int keepalive_probes(const struct tcp_sock *tp)
{
    return tp->keepalive_probes ? : sysctl_tcp_keepalive_probes;
}

TCP_INFO

/* Return information about state of tcp endpoint in API format. */

void tcp_get_info(struct sock *sk, struct tcp_info *info)
{
    struct tcp_sock *tp = tcp_sk(sk);
    const struct inet_connection_sock *icsk = inet_csk(sk);
    u32 now = tcp_time_stamp;
 
    memset(info, 0, sizeof(*info));
    
    info->tcpi_state = sk->sk_state; /* TCP状态 */
    info->tcpi_ca_state = icsk_ca_state; /* TCP拥塞状态 */
    info->tcpi_retransmits = icsk->icsk_retransmits; /* Number of unrecovered [RTO] timeouts */
    info->tcpi_probes = icsk->icsk_probes_out; /* unanswered 0 window probes */
    info->tcpi_backoff = icsk->icsk_backoff; /* 退避指数 */

    if (tp->rx_opt.tstamp_ok) /* TIMESTAMP seen on SYN packet */
        info->tcpi_options |= TCPI_OPT_TIMESTAMPS; /* 时间戳选项使用与否 */
    if (tcp_is_sack(tp))
        info->tcpi_options |= TCPI_OPT_SACK; /* SACK选项使用与否 */
    if (tp->rx_opt.wscale_ok) { /* Wscale seen on SYN packet */
        info->tcpi_options |= TCPI_OPT_WSCALE; /* 窗口扩大选项使用与否 */
        info->tcpi_snd_wscale = tp->rx_opt.snd_wscale; /* 发送窗口扩大因子 */
        info->tcpi_rcv_wscale = tp->rx_opt.rcv_wscale; /* 接收窗口扩大因子 */
    }

    if (tp->ecn_flags & TCP_ECN_OK)
        info->tcpi_options |= TCPI_OPT_ECN; /* ECN选项使用与否 */

    info->tcpi_rto = jiffies_to_usecs(icsk->icsk_rto); /* RTO，单位微秒 */
    info->tcpi_ato = jiffies_to_usecs(icsk->icsk_ack.ato); /* Predicted tick of soft clock */
    info->tcpi_snd_mss = tp->mss_cache; /* 本端MSS */
    info->tcpi_rcv_mss = icsk->icsk_ack.rcv_mss; /* 对端MSS */

    if (sk->sk_state == TCP_LISTEN) {
        info->tcpi_unacked = sk->sk_ack_backlog; /* current listen backlog */
        info->tcpi_sacked = sk->sk_max_ack_backlog; /* listen backlog set in listen() */

    } else {
        info->tcpi_unacked = tp->packets_out; /* 未确认的数据包数 */
        info->tcpi_sacked = tp->sacked_out; /* SACKed的数据包数 */
    }

    info->tcpi_lost = tp->lost_out; /* 丢失的数据包数 */
    info->tcpi_retrans = tp->retrans_out; /* 重传的数据包数 */
    info->tcpi_fackets = tp->fackets_out; /* FACKed的数据包数 */
 
    info->tcpi_last_data_sent = jiffies_to_msecs(now - tp->lsndtime); /* 最近一次发数据包的时间间隔 */
    info->tcpi_last_data_recv = jiffies_to_msecs(now - icsk->icsk_ack.lrcvtime); /* 最近一次收数据包的时间间隔 */
    info->tcpi_last_ack_recv = jiffies_to_msecs(now - tp->rcv_tstamp); /* 最近一次收ACK包的时间间隔 */
 
    info->tcpi_pmtu = icsk->icsk_pmtu_cookie; /* Last pmtu seen by socket */
    info->tcpi_rcv_ssthresh = tp->rcv_ssthresh; /* current window clamp*/
    info->tcpi_rtt = jiffies_to_usecs(tp->srtt) >> 3; /* 平滑的RTT */
    info->tcpi_rttvar = jiffies_to_usecs(tp->mdev) >> 2; /* 四分之一mdev */

    info->tcpi_snd_ssthresh = tp->snd_ssthresh;
    info->tcpi_snd_cwnd = tp->snd_cwnd;
    info->tcpi_advmss = tp->advmss; /* 本端能接受的MSS上限，在建立连接时用来通告对端 */
    info->tcpi_reordering = tp->reordering; /* 没有丢包时，可以重新排序的数据段数 */
 
    info->tcpi_rcv_rtt = jiffies_to_usecs(tp->rcv_rtt_est.rtt) >> 3; /* 作为接收端，测出的RTT值 */
    info->tcpi_rcv_space = tp->rcvq_space.space; /* 接收缓存的大小 */

    info->tcpi_total_retrans = tp->total_retrans; /* 本连接的总重传个数 */
}

Author

zhangskd @ csdn blog