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 network‐
ing 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 sig‐
nificant 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.
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.
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 sec‐
ond value is the value returned by pcap_tstamp_type_val_to_name() and accepted by pcap_tstamp_name_to_val().
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_ADAPTER_UNSYNCED - adapter_unsynced
Time stamp provided by the network adapter on which the capture is being done. This is a high-pre‐
cision time stamp; it is not synchronized with the host operating system's clock.
SEE ALSO
pcap_set_tstamp_type(3PCAP), pcap_list_tstamp_types(3PCAP), pcap_tstamp_type_val_to_name(3PCAP),
pcap_tstamp_name_to_val(3PCAP)
22 August 2010 PCAP-TSTAMP(7)
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