man > pcap-tstamp(7)

PCAP-TSTAMP(7)                    Miscellaneous Information Manual                    PCAP-TSTAMP(7)

NAME
       pcap-tstamp - packet time stamps in libpcap

DESCRIPTION
       When capturing traffic, each packet is given a time stamp representing, for incoming packets,
       the  arrival  time  of  the  packet  and,  for outgoing packets, the transmission time of the
       packet.  This time is an approximation of the arrival or transmission time.  If  it  is  sup‐
       plied  by  the operating system running on the host on which the capture is being done, there
       are several reasons why it might not precisely represent the arrival or transmission time:

              if the time stamp is applied to the packet when  the  networking  stack  receives  the
              packet,  the networking stack might not see the packet until an interrupt is delivered
              for the packet or a timer event causes the networking device driver to poll for  pack‐
              ets,  and the time stamp might not be applied until the packet has had some processing
              done by other code in the networking stack, so there might be a significant delay  be‐
              tween  the  time when the last bit of the packet is received by the capture device and
              when the networking stack time-stamps the packet;

              the timer used to generate the time stamps might have low resolution, for example,  it
              might  be a timer updated once per host operating system timer tick, with the host op‐
              erating system timer ticking once every few milliseconds;

              a high-resolution timer might use a counter that runs  at  a  rate  dependent  on  the
              processor  clock  speed,  and  that clock speed might be adjusted upwards or downwards
              over time and the timer might not be able to compensate for all those adjustments;

              the host operating system's clock might be adjusted over time to match a time standard
              to which the host is being synchronized, which might be done  by  temporarily  slowing
              down or speeding up the clock or by making a single adjustment;

              different CPU cores on a multi-core or multi-processor system might be running at dif‐
              ferent  speeds,  or  might  not  have time counters all synchronized, so packets time-
              stamped by different cores might not have consistent time stamps;

              some time sources, such as those that supply POSIX "seconds since the Epoch" time,  do
              not  count  leap  seconds, meaning that the seconds portion (tv_sec) of the time stamp
              might not be incremented for a leap second, so that the fraction-of-a-second  part  of
              the  time stamp might roll over past zero but the second part would not change, or the
              clock might run slightly more slowly for a period before the leap second.

       For these reasons, time differences between packet time stamps  will  not  necessarily  accu‐
       rately reflect the time differences between the receipt or transmission times of the packets.

       In  addition,  packets time-stamped by different cores might be time-stamped in one order and
       added to the queue of packets for libpcap to read in another order, so time stamps might  not
       be monotonically increasing.

       Some capture devices on some platforms can provide time stamps for packets; those time stamps
       are usually high-resolution time stamps, and are usually applied to the packet when the first
       or  last  bit  of the packet arrives, and are thus more accurate than time stamps provided by
       the host operating system.  Those time stamps might not, however, be  synchronized  with  the
       host  operating  system's  clock,  so that, for example, the time stamp of a packet might not
       correspond to the time stamp of an event on the host triggered by the arrival of that packet.
       If they are synchronized with the host operating system's clock, some of  the  issues  listed
       above  with  time  stamps supplied by the host operating system may also apply to time stamps
       supplied by the capture device.

       Depending on the capture device and the software on the host, libpcap might  allow  different
       types  of  time  stamp to be used.  The pcap_list_tstamp_types(3PCAP) routine provides, for a
       packet capture handle created by pcap_create(3PCAP)  but  not  yet  activated  by  pcap_acti‐
       vate(3PCAP), a list of time stamp types supported by the capture device for that handle.  The
       list  might  be empty, in which case no choice of time stamp type is offered for that capture
       device.  If the list is not empty, the pcap_set_tstamp_type(3PCAP) routine can be used  after
       a  pcap_create()  call and before a pcap_activate() call to specify the type of time stamp to
       be used on the device.  The time stamp types are listed here; the first value is the  #define
       to use in code, the second value is the value returned by pcap_tstamp_type_val_to_name(3PCAP)
       and accepted by pcap_tstamp_type_name_to_val(3PCAP).

            PCAP_TSTAMP_HOST - host
                 Time  stamp provided by the host on which the capture is being done.  The precision
                 of this time stamp is unspecified; it might or might not be synchronized  with  the
                 host operating system's clock.

            PCAP_TSTAMP_HOST_LOWPREC - host_lowprec
                 Time stamp provided by the host on which the capture is being done.  This is a low-
                 precision time stamp, synchronized with the host operating system's clock.

            PCAP_TSTAMP_HOST_HIPREC - host_hiprec
                 Time  stamp  provided  by  the  host on which the capture is being done.  This is a
                 high-precision time stamp, synchronized with the host operating system's clock.  It
                 might be more expensive to fetch than PCAP_TSTAMP_HOST_LOWPREC.

            PCAP_TSTAMP_HOST_HIPREC_UNSYNCED - host_hiprec_unsynced
                 Time  stamp  provided  by  the  host on which the capture is being done.  This is a
                 high-precision time stamp, not synchronized with the host operating system's clock.
                 It might be more expensive to fetch than PCAP_TSTAMP_HOST_LOWPREC.

            PCAP_TSTAMP_ADAPTER - adapter
                 Time stamp provided by the network adapter on which  the  capture  is  being  done.
                 This  is a high-precision time stamp, synchronized with the host operating system's
                 clock.

            PCAP_TSTAMP_ADAPTER_UNSYNCED - adapter_unsynced
                 Time stamp provided by the network adapter on which  the  capture  is  being  done.
                 This is a high-precision time stamp; it is not synchronized with the host operating
                 system's clock.

       Time  stamps  synchronized with the system clock can go backwards, as the system clock can go
       backwards. If a clock is not in sync with the system clock, that could be because the  system
       clock  isn't  keeping  accurate time, because the other clock isn't keeping accurate time, or
       both.

       Host-provided time stamps generally correspond to the time when the time-stamping  code  sees
       the  packet;  this  could  be  some unknown amount of time after the first or last bit of the
       packet is received by the network adapter, due to batching of interrupts for packet  arrival,
       queueing delays, etc..

       By  default,  when performing a live capture or reading from a savefile, time stamps are sup‐
       plied as seconds since January 1, 1970, 00:00:00 UTC, and  microseconds  since  that  seconds
       value,  even if higher-resolution time stamps are available from the capture device or in the
       savefile.  If, when reading a savefile, the time stamps in the file have a higher  resolution
       than one microsecond, the additional digits of resolution are discarded.

       The pcap_set_tstamp_precision(3PCAP) routine can be used after a pcap_create() call and after
       a  pcap_activate()  call  to specify the resolution of the time stamps to get for the device.
       If  the  hardware  or  software  cannot  supply   a   higher-resolution   time   stamp,   the
       pcap_set_tstamp_precision() call will fail, and the time stamps supplied after the pcap_acti‐
       vate() call will have microsecond resolution.

       When    opening    a   savefile,   the   pcap_open_offline_with_tstamp_precision(3PCAP)   and
       pcap_fopen_offline_with_tstamp_precision(3PCAP) routines can be used to specify  the  resolu‐
       tion  of  time  stamps  to be read from the file; if the time stamps in the file have a lower
       resolution, the fraction-of-a-second portion of the time stamps will be scaled to the  speci‐
       fied resolution.

       The  pcap_get_tstamp_precision(3PCAP) routine returns the resolution of time stamps that will
       be supplied; when capturing packets, this does not reflect the actual precision of  the  time
       stamp  supplied  by  the hardware or operating system and, when reading a savefile, this does
       not indicate the actual precision of time stamps in the file.

SEE ALSO
       pcap(3PCAP)

                                            14 July 2020                              PCAP-TSTAMP(7)