# pcap-tstamp

## 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**(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**(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,
                     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**(3PCAP) routine can be used after a
           **pcap_create**() call and before 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**(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-second portion of the time stamps will be scaled to
           the specified resolution.
    
           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.
    


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