分析过linux-2.6.32内核源码后，发现通过设置此选项，我们可以让内核协议栈在接受到一个网络帧时为其打上时间戳，并将此时间戳作为一笔附加数据，与网络帧数据一起递交到上层协议。

    netif_receive_skb()，linux内核协议栈中的关键函数，通常在网卡驱动程序poll函数（RX中断处理函数会调度poll函数，详情参考最新内核机制NAPI）的最后一步调用，占们用来处理网络帧，并将网络帧递交至上层协议，而netif_receive_skb函数第一件要做的事就是调用net_timestamp，为当前收到的网络帧打时间戳（net_timestamp函数里会判断是否已经使能了网络时间戳功能，即netstamp_need），并将此时间戳作为一笔SCM_TIMESTAMP类型的附加数据插入sk_buff（即cmsg）。

    上层代码如果要获取内核协议栈为网络帧打的时间戳，就需要拿到附加数据，很显然，我们要拿的是SCM_TIMESTAMP类型的附加数据。

    我们要在收到的报文中遍历附加数据（可能有很多笔附加数据），可以使用CMSG_FIRSTHDR()与CMSG_NXTHDR()宏在附加数据对象中进行遍历，if(cmsg->cmsg_level == SOL_SOCKET && cmsg->cmsg_type == SCM_TIMESTAMP)条件一旦成立，就表明已经找到了SCM_TIMESTAMP类型的附加数据，那就是之前内核协议栈为这一帧网络报文打上的时间戳，也就是收到此网络报文的时间。

    这个特性在PTP协议中非常有用，要做网络时间同步，必须有办法知道网络报文收到的时间，如果没有硬件时间戳（精密PHY），上层应用程序就需要使用此特性获取网络帧收到时的时间戳，或者自己编写内核模块代码接入底层协议栈，加盖软件时间戳。

The existing interfaces for getting network packages time stamped are:

* SO_TIMESTAMP
  Generate time stamp for each incoming packet using the (not necessarily
  monotonous!) system time. Result is returned via recv_msg() in a
  control message as timeval_r(usec resolution).

* SO_TIMESTAMPNS
  Same time stamping mechanism as SO_TIMESTAMP, but returns result as
  timespec (nsec resolution).

* IP_MULTICAST_LOOP + SO_TIMESTAMP[NS]
  Only for multicasts: approximate send time stamp by receiving the looped
  packet and using its receive time stamp.

The following interface complements the existing ones: receive time
stamps can be generated and returned for arbitrary packets and much
closer to the point where the packet is really sent. Time stamps can
be generated in software (as before) or in hardware (if the hardware
has such a feature).

SO_TIMESTAMPING:

Instructs the socket layer which kind of information is wanted. The
parameter is an integer with some of the following bits set. Setting
other bits is an error and doesn't change the current state.

SOF_TIMESTAMPING_TX_HARDWARE:  try to obtain send time stamp in hardware
SOF_TIMESTAMPING_TX_SOFTWARE:  if SOF_TIMESTAMPING_TX_HARDWARE is off or
                               fails, then do it in software
SOF_TIMESTAMPING_RX_HARDWARE:  return the original, unmodified time stamp
                               as generated by the hardware
SOF_TIMESTAMPING_RX_SOFTWARE:  if SOF_TIMESTAMPING_RX_HARDWARE is off or
                               fails, then do it in software
SOF_TIMESTAMPING_RAW_HARDWARE: return original raw hardware time stamp
SOF_TIMESTAMPING_SYS_HARDWARE: return hardware time stamp transformed to
                               the system time base
SOF_TIMESTAMPING_SOFTWARE:     return system time stamp generated in
                               software

SOF_TIMESTAMPING_TX/RX determine how time stamps are generated.
SOF_TIMESTAMPING_RAW/SYS determine how they are reported in the
following control message:

struct scm_timestamping {
 struct timespec systime;
 struct timespec hwtimetrans;
 struct timespec hwtimeraw;
};

recvmsg() can be used to get this control message for regular incoming
packets. For send time stamps the outgoing packet is looped back to
the socket's error queue with the send time stamp(s) attached. It can
be received with recvmsg(flags=MSG_ERRQUEUE). The call returns the
original outgoing packet data including all headers preprended down to
and including the link layer, the scm_timestamping control message and
a sock_extended_err control message with ee_errno==ENOMSG and
ee_origin==SO_EE_ORIGIN_TIMESTAMPING. A socket with such a pending
bounced packet is ready for reading as far as select() is concerned.
If the outgoing packet has to be fragmented, then only the first
fragment is time stamped and returned to the sending socket.

All three values correspond to the same event in time, but were
generated in different ways. Each of these values may be empty (= all
zero), in which case no such value was available. If the application
is not interested in some of these values, they can be left blank to
avoid the potential overhead of calculating them.

systime is the value of the system time at that moment. This
corresponds to the value also returned via SO_TIMESTAMP[NS]. If the
time stamp was generated by hardware, then this field is
empty. Otherwise it is filled in if SOF_TIMESTAMPING_SOFTWARE is
set.

hwtimeraw is the original hardware time stamp. Filled in if
SOF_TIMESTAMPING_RAW_HARDWARE is set. No assumptions about its
relation to system time should be made.

hwtimetrans is the hardware time stamp transformed so that it
corresponds as good as possible to system time. This correlation is
not perfect; as a consequence, sorting packets received via different
NICs by their hwtimetrans may differ from the order in which they were
received. hwtimetrans may be non-monotonic even for the same NIC.
Filled in if SOF_TIMESTAMPING_SYS_HARDWARE is set. Requires support
by the network device and will be empty without that support.

SIOCSHWTSTAMP:

Hardware time stamping must also be initialized for each device driver
that is expected to do hardware time stamping. The parameter is defined in
/include/linux/net_tstamp.h as:

struct hwtstamp_config {
 int flags; 
 int tx_type; 
 int rx_filter; 
};

Desired behavior is passed into the kernel and to a specific device by
calling ioctl(SIOCSHWTSTAMP) with a pointer to a struct ifreq whose
ifr_data points to a struct hwtstamp_config. The tx_type and
rx_filter are hints to the driver what it is expected to do. If
the requested fine-grained filtering for incoming packets is not
supported, the driver may time stamp more than just the requested types
of packets.

A driver which supports hardware time stamping shall update the struct
with the actual, possibly more permissive configuration. If the
requested packets cannot be time stamped, then nothing should be
changed and ERANGE shall be returned (in contrast to EINVAL, which
indicates that SIOCSHWTSTAMP is not supported at all).

Only a processes with admin rights may change the configuration. User
space is responsible to ensure that multiple processes don't interfere
with each other and that the settings are reset.

enum {
 
 HWTSTAMP_TX_OFF,

 
 HWTSTAMP_TX_ON,
};

enum {
 
 HWTSTAMP_FILTER_NONE,

 
 HWTSTAMP_FILTER_ALL,

 
 HWTSTAMP_FILTER_SOME,

 
 HWTSTAMP_FILTER_PTP_V1_L4_EVENT,

 
};

DEVICE IMPLEMENTATION

A driver which supports hardware time stamping must support the
SIOCSHWTSTAMP ioctl and update the supplied struct hwtstamp_config with
the actual values as described in the section on SIOCSHWTSTAMP.

Time stamps for received packets must be stored in the skb. To get a pointer
to the shared time stamp structure of the skb call skb_hwtstamps(). Then
set the time stamps in the structure:

struct skb_shared_hwtstamps {
 
 ktime_t hwtstamp;
 ktime_t syststamp;
};

Time stamps for outgoing packets are to be generated as follows:
- In hard_start_xmit(), check if skb_tx(skb)->hardware is set no-zero.
  If yes, then the driver is expected to do hardware time stamping.
- If this is possible for the skb and requested, then declare
  that the driver is doing the time stamping by setting the field
  skb_tx(skb)->in_progress non-zero. You might want to keep a pointer
  to the associated skb for the next step and not free the skb. A driver
  not supporting hardware time stamping doesn't do that. A driver must
  never touch sk_buff::tstamp! It is used to store software generated
  time stamps by the network subsystem.
- As soon as the driver has sent the packet and/or obtained a
  hardware time stamp for it, it passes the time stamp back by
  calling skb_hwtstamp_tx() with the original skb, the raw
  hardware time stamp. skb_hwtstamp_tx() clones the original skb and
  adds the timestamps, therefore the original skb has to be freed now.
  If obtaining the hardware time stamp somehow fails, then the driver
  should not fall back to software time stamping. The rationale is that
  this would occur at a later time in the processing pipeline than other
  software time stamping and therefore could lead to unexpected deltas
  between time stamps.
- If the driver did not call set skb_tx(skb)->in_progress, then
  dev_hard_start_xmit() checks whether software time stamping
  is wanted as fallback and potentially generates the time stamp.