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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;

              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(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(3) and accepted by
       pcap_tstamp_type_name_to_val(3).

            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(3) 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(3) and
       pcap_fopen_offline_with_tstamp_precision(3) 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(3) 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(3)

                                 14 July 2020                   pcap-tstamp(7)

libpcap 1.10.5 - Generated Sat Aug 31 09:46:30 CDT 2024
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