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.

       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.

       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() and accepted by pcap_tstamp_name_to_val().

            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 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.

SEE ALSO

       pcap_set_tstamp_type(3PCAP),     pcap_list_tstamp_types(3PCAP),      pcap_tstamp_type_val_to_name(3PCAP),
       pcap_tstamp_name_to_val(3PCAP)

                                                 22 August 2010                                   PCAP-TSTAMP(7)