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 approxi-
mation 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 net-
working 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 reso-
lution, 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 in-
cremented 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 in-
creasing.
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 exam-
ple, 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_ac-
tivate(3PCAP), a list of time stamp types supported by the capture de-
vice 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 ex-
pensive to fetch than PCAP_TSTAMP_HOST_LOWPREC.
PCAP_TSTAMP_ADAPTER - adapter
Time stamp provided by the network adapter on which the cap-
ture 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 cap-
ture 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 sys-
tem clock, that could be because the system clock isn't keeping accu-
rate 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-resolu-
tion time stamps are available from the capture device or in the save-
file. If, when reading a savefile, the time stamps in the file have a
higher resolution than one microsecond, the additional digits of reso-
lution are discarded.
The pcap_set_tstamp_precision(3PCAP) routine can be used after a
pcap_create() call and after a pcap_activate() call to specify the res-
olution 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 sup-
plied 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-sec-
ond portion of the time stamps will be scaled to the specified resolu-
tion.
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.
SEE ALSO
pcap(3PCAP)
14 July 2020 PCAP-TSTAMP(7)
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