Linux内核UDP接收时间戳

Question

我一直在阅读linux内核的network timestamping文档，有一些东西我不太清楚。

SO_TIMESTAMPNS提供的时间戳是在哪里生成的？在硬件中还是在内核中？如果是这样，那么一旦新数据包的中断被引发，它就会生成吗？

SO_TIMESTAMPING还应允许生成硬件时间戳。是否所有NIC都支持此功能？How How SO_TIMESTAMPING with options SOF_TIMESTAMPING_RX_HARDWARE and SO_TIMESTAMPNS？在这种情况下，硬件时间戳是指系统时钟还是指NIC时钟？在第二种情况下，如何检索NIC时钟来计算已用时间？

Answer 1

用于软件时间戳的套接字属性是SO_TIMESTAMPNS。此套接字属性返回系统时钟的时间。它不是在硬件中生成的，而是在软件中处理中断时系统时间的快照。我们可以通过不是套接字有效负载一部分的辅助数据(CMSG)访问此时间戳，使用：

int level, type;
struct cmsghdr *cm;
struct timespec *ts = NULL;
for (cm = CMSG_FIRSTHDR(&msg); cm != NULL; cm = CMSG_NXTHDR(&msg, cm))
{
    level = cm->cmsg_level;
    type  = cm->cmsg_type;
    if (SOL_SOCKET == level && SO_TIMESTAMPNS == type) {
        ts = (struct timespec *) CMSG_DATA(cm);
        printf("SW TIMESTAMP %ld.%09ld\n", (long)ts[0].tv_sec, (long)ts[0].tv_nsec);
    }
}

SO_TIMESTAMPING套接字选项提供了许多不同的标志，其中一些是，

SOF_TIMESTAMPING_TX_HARDWARE // Transmit timestamp generated in hardware by NIC clock
SOF_TIMESTAMPING_RX_HARDWARE // Receive  timestamp generated in hardware by NIC clock
SOF_TIMESTAMPING_TX_SOFTWARE // Transmit timestamp generated in kernel driver by NIC clock
SOF_TIMESTAMPING_RX_SOFTWARE // Receive  timestamp generated in kernel driver by NIC clock

并非所有网络接口卡(NIC)都支持此套接字选项。目前，许多以太网卡都支持SO_TIMESTAMPING。为了找出特定的接口驱动程序是否支持SO_TIMESTAMPING，请使用：

ethtool -T ethX // where X corresponds to your particular interface

这将返回ethX支持的用于时间戳的所有套接字属性。

要使用特定NIC提供的硬件时间戳功能，请使用以下代码：

int flags;
flags   = SOF_TIMESTAMPING_TX_HARDWARE
            | SOF_TIMESTAMPING_RX_HARDWARE 
            | SOF_TIMESTAMPING_TX_SOFTWARE
            | SOF_TIMESTAMPING_RX_SOFTWARE 
            | SOF_TIMESTAMPING_RAW_HARDWARE;
    if (setsockopt(sd, SOL_SOCKET, SO_TIMESTAMPING, &flags, sizeof(flags)) < 0)
        printf("ERROR: setsockopt SO_TIMESTAMPING\n");

int level, type;
struct cmsghdr *cm;
struct timespec *ts = NULL;
for (cm = CMSG_FIRSTHDR(&msg); cm != NULL; cm = CMSG_NXTHDR(&msg, cm))
{
     if (SOL_SOCKET == level && SO_TIMESTAMPING == type) {
        ts = (struct timespec *) CMSG_DATA(cm);
        printf("HW TIMESTAMP %ld.%09ld\n", (long)ts[2].tv_sec, (long)ts[2].tv_nsec);
      }
}