I would appreciate any donations. Wishlist or send e-mail type donations to maekawa AT daemon-systems.org.

Thank you.


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 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(3) routine provides, for a packet capture handle
       created by pcap_create(3) but not yet activated by pcap_activate(3), 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(3) 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(3), pcap_list_tstamp_types(3),
       pcap_tstamp_type_val_to_name(3), pcap_tstamp_name_to_val(3)



                                22 August 2010                  PCAP-TSTAMP(7)