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NAME | DESCRIPTION | SEE ALSO | COLOPHON |
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PCAP-TSTAMP(7) Miscellaneous Information Manual PCAP-TSTAMP(7)
pcap-tstamp - packet time stamps in libpcap
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.
pcap(3PCAP)
This page is part of the libpcap (packet capture library) project.
Information about the project can be found at
⟨http://www.tcpdump.org/⟩. If you have a bug report for this
manual page, see ⟨http://www.tcpdump.org/#patches⟩. This page was
obtained from the project's upstream Git repository
⟨https://github.com/the-tcpdump-group/libpcap.git⟩ on 2025-08-11.
(At that time, the date of the most recent commit that was found
in the repository was 2025-08-10.) If you discover any rendering
problems in this HTML version of the page, or you believe there is
a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
man-pages@man7.org
1 October 2024 PCAP-TSTAMP(7)
Pages that refer to this page: pcap(3pcap), pcap_free_tstamp_types(3pcap), pcap_get_tstamp_precision(3pcap), pcap_list_tstamp_types(3pcap), pcap_set_tstamp_precision(3pcap), pcap_set_tstamp_type(3pcap)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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