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UDP 数据包丢失

tag-icon • tag-icon networking proc udp
UDP 数据包丢失

据我所知，UDP数据包的接收流程是

  1. 检查 udp 标头是否有错误
  2. 将目的地与套接字匹配
  3. 如果没有这样的套接字，则发送错误消息
  4. 将数据包放入适当的套接字接收队列
  5. 唤醒进程等待该套接字的数据

但在上述阶段，什么才算/proc/net/udp是下降呢？上述任何一步失败是否构成掉落？或者仅当接收队列/缓冲区已满时才执行？

答案1

在 Linux 5.4.66 中，伪文件/proc/net/udp由以下函数生成net/ipv4/udp.c:

int udp4_seq_show(struct seq_file *seq, void *v)
{
        seq_setwidth(seq, 127);
        if (v == SEQ_START_TOKEN)
                seq_puts(seq, "  sl  local_address rem_address   st tx_queue "
                           "rx_queue tr tm->when retrnsmt   uid  timeout "
                           "inode ref pointer drops");
        else {
                struct udp_iter_state *state = seq->private;

                udp4_format_sock(v, seq, state->bucket);
        }
        seq_pad(seq, '\n');
        return 0;
}

这udp4_format_sock()在同一个文件中调用：

static void udp4_format_sock(struct sock *sp, struct seq_file *f,
                int bucket)
{
        struct inet_sock *inet = inet_sk(sp);
        __be32 dest = inet->inet_daddr;
        __be32 src  = inet->inet_rcv_saddr;
        __u16 destp       = ntohs(inet->inet_dport);
        __u16 srcp        = ntohs(inet->inet_sport);

        seq_printf(f, "%5d: %08X:%04X %08X:%04X"
                " %02X %08X:%08X %02X:%08lX %08X %5u %8d %lu %d %pK %u",
                bucket, src, srcp, dest, destp, sp->sk_state,
                sk_wmem_alloc_get(sp),
                udp_rqueue_get(sp),
                0, 0L, 0,
                from_kuid_munged(seq_user_ns(f), sock_i_uid(sp)),
                0, sock_i_ino(sp),
                refcount_read(&sp->sk_refcnt), sp,
                atomic_read(&sp->sk_drops));
}

请注意，该字段的值drops来自atomic_read(&sp->sk_drops).该值使用以下方法递增atomic_inc(&sk->sk_drops)在几个地方使用时会增加。

  1. 接收队列已满

    int __udp_enqueue_schedule_skb(struct sock *sk, struct sk_buff *skb)
{
         ...
        /* try to avoid the costly atomic add/sub pair when the receive
         * queue is full; always allow at least a packet
         */
        rmem = atomic_read(&sk->sk_rmem_alloc);
        if (rmem > sk->sk_rcvbuf)
                 goto drop;
       ...
drop:
        atomic_inc(&sk->sk_drops);

  2. 错误的校验和帧

    static struct sk_buff *__first_packet_length(struct sock *sk,
                                             struct sk_buff_head *rcvq,
                                             int *total)
{
        struct sk_buff *skb;

        while ((skb = skb_peek(rcvq)) != NULL) {
                if (udp_lib_checksum_complete(skb)) {
                        ...
                        atomic_inc(&sk->sk_drops);

  3. 无法阅读

    int udp_recvmsg(struct sock *sk, struct msghdr *msg, size_t len, int noblock,
                int flags, int *addr_len)
{
        ...
        if (checksum_valid || udp_skb_csum_unnecessary(skb)) {
                if (udp_skb_is_linear(skb))
                        err = copy_linear_skb(skb, copied, off, &msg->msg_iter);
                else
                        err = skb_copy_datagram_msg(skb, off, msg, copied);
        } else {
                err = skb_copy_and_csum_datagram_msg(skb, off, msg);

                if (err == -EINVAL)
                        goto csum_copy_err;
        }

        if (unlikely(err)) {
                if (!peeking) {
                        atomic_inc(&sk->sk_drops);

  4. 接收期间失败

    static int udp_queue_rcv_one_skb(struct sock *sk, struct sk_buff *skb)
{
        ...
drop:
        ...
        atomic_inc(&sk->sk_drops);

  5. 尝试克隆消息时多播处理期间失败

    static int __udp4_lib_mcast_deliver(struct net *net, struct sk_buff *skb,
                                    struct udphdr  *uh,
                                    __be32 saddr, __be32 daddr,
                                    struct udp_table *udptable,
                                    int proto)
{
        ...
        sk_for_each_entry_offset_rcu(sk, node, &hslot->head, offset) {
                ...
                nskb = skb_clone(skb, GFP_ATOMIC);

                if (unlikely(!nskb)) {
                        atomic_inc(&sk->sk_drops);

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