//由tcp、sctp使用
//skb：封包描述符                                                                                                                                           
//ipfragok： sctp使用的标志，指明是否可以分段
int ip_queue_xmit(struct sk_buff *skb, int ipfragok)
{
    struct sock *sk = skb->sk;
    struct inet_sock *inet = inet_sk(sk); //要通过的套接字
    struct ip_options_rcu *inet_opt = NULL;
    struct rtable *rt; 
    struct iphdr *iph;
    int res; 

    /* Skip all of this if the packet is already routed,
     * f.e. by something like SCTP.
     */
    rcu_read_lock();
    rt = skb_rtable(skb); //如果缓冲区已经设置了正确的路由信息，就不需要查找路由表了
    if (rt != NULL)
        goto packet_routed;

    /* Make sure we can route this packet. */
    rt = (struct rtable *)__sk_dst_check(sk, 0);
    inet_opt = rcu_dereference(inet->inet_opt); //选项初始化
    if (rt == NULL) {
        __be32 daddr;

        /* Use correct destination address if we have options. */
        daddr = inet->daddr;
        if (inet_opt && inet_opt->opt.srr)
            daddr = inet_opt->opt.faddr;
        {
            struct flowi fl = { .oif = sk->sk_bound_dev_if,
                        .mark = sk->sk_mark,
                        .nl_u = { .ip4_u =
                              { .daddr = daddr,
                            .saddr = inet->saddr,
                            .tos = RT_CONN_FLAGS(sk) } },
                        .proto = sk->sk_protocol,
                        .flags = inet_sk_flowi_flags(sk),
                        .uli_u = { .ports =
                               { .sport = inet->sport,
                             .dport = inet->dport } } };

            /* If this fails, retransmit mechanism of transport layer will
             * keep trying until route appears or the connection times
             * itself out.
             */
            security_sk_classify_flow(sk, &fl);
            if (ip_route_output_flow(sock_net(sk), &rt, &fl, sk, 0))
                goto no_route;
        }
        sk_setup_caps(sk, &rt->u.dst);
    }
    skb_dst_set(skb, dst_clone(&rt->u.dst));

packet_routed:
    if (inet_opt && inet_opt->opt.is_strictroute && rt->rt_dst != rt->rt_gateway)
        goto no_route;

    /* OK, we know where to send it, allocate and build IP header. */
    //把skb-》data往回移动，使其指向ip报头（而不是数据段）
    skb_push(skb, sizeof(struct iphdr) + (inet_opt ? inet_opt->opt.optlen : 0));
    skb_reset_network_header(skb);

    /* 构建ip报头*/
    iph = ip_hdr(skb); 
    *((__be16 *)iph) = htons((4 << 12) | (5 << 8) | (inet->tos & 0xff));
    if (ip_dont_fragment(sk, &rt->u.dst) && !ipfragok)
        iph->frag_off = htons(IP_DF);
    else
        iph->frag_off = 0;
    iph->ttl      = ip_select_ttl(inet, &rt->u.dst);
    iph->protocol = sk->sk_protocol;
    iph->saddr    = rt->rt_src;
    iph->daddr    = rt->rt_dst;
    /* Transport layer set skb->h.foo itself. */

    if (inet_opt && inet_opt->opt.optlen) {
        iph->ihl += inet_opt->opt.optlen >> 2;
        ip_options_build(skb, &inet_opt->opt, inet->daddr, rt, 0);
    }

    ip_select_ident_more(iph, &rt->u.dst, sk,
                 (skb_shinfo(skb)->gso_segs ?: 1) - 1);

    skb->priority = sk->sk_priority;
    skb->mark = sk->sk_mark;
    res = ip_local_out(skb);                                                                                                                                
    rcu_read_unlock();
    return res;

no_route:
    rcu_read_unlock();
    IP_INC_STATS(sock_net(sk), IPSTATS_MIB_OUTNOROUTES);
    kfree_skb(skb);
    return -EHOSTUNREACH;
}