huschi.net

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 approxi-
       mation 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  net-
              working 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 reso-
              lution, 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.
       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 exam-
       ple, 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.
       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 cap-
       ture 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; it might or might
                 not  be  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 cap-
                 ture 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 cap-
                 ture is being done.  This is a high-precision time stamp;  it
                 is not synchronized with the host operating system's clock.
       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-resolu-
       tion time stamps are available from the capture device or in the  save-
       file.   If, when reading a savefile, the time stamps in the file have a
       higher resolution than one microsecond, the additional digits of  reso-
       lution 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 res-
       olution  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 sup-
       plied 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-sec-
       ond  portion of the time stamps will be scaled to the specified resolu-
       tion.
       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)
                                 8 March 2015                   PCAP-TSTAMP(7)