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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 pack-
ets, 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 deliv-
ered 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 signifi-
cant delay between the time when the last bit of the packet is received by the cap-
ture 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 stan-
dard 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 synchro-
nized 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 differ-
ent 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) rou-
tine 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() 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 preci-
sion 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; it might or might not be 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 sys-
tem'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 oper-
ating 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)
Generated by $Id: phpMan.php,v 4.55 2007/09/05 04:42:51 chedong Exp $ Author: Che Dong
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