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PCAP-TSTAMP(7)                                                  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 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  transmis-
       sion 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  signifi-
              cant  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-preci-
                 sion 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-preci-
                 sion 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)