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.TH PCAP-TSTAMP 7 "21 December 2013"
.SH NAME
pcap-tstamp \- packet time stamps in libpcap
.SH 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:
.IP
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;
.IP
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;
.IP
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;
.IP
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;
.IP
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.
.LP
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.
.LP
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.
.LP
Depending on the capture device and the software on the host, libpcap
might allow different types of time stamp to be used.  The
.BR pcap_list_tstamp_types (3PCAP)
routine provides, for a packet capture handle created by
.BR pcap_create (3PCAP)
but not yet activated by
.BR 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
.BR pcap_set_tstamp_type (3PCAP)
routine can be used after a
.B pcap_create()
call and before a
.B 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
.B pcap_tstamp_type_val_to_name()
and accepted by
.BR pcap_tstamp_type_name_to_val() .
.RS 5
.TP 5
.BR 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.
.TP 5
.BR 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.
.TP 5
.BR 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
.BR PCAP_TSTAMP_HOST_LOWPREC .
.TP 5
.BR 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.
.TP 5
.BR 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.
.RE
.SH SEE ALSO
pcap_set_tstamp_type(3PCAP),
pcap_list_tstamp_types(3PCAP),
pcap_tstamp_type_val_to_name(3PCAP),
pcap_tstamp_type_name_to_val(3PCAP)