plucky (7) pcap-tstamp.7.gz

Provided by: libpcap0.8t64_1.10.5-2ubuntu1_amd64 bug

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 supplied 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 packets, 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 between 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 operating 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  different
              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  accurately  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_activate(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 supplied 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_activate() 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 resolution 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 specified 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)