178 lines
8.0 KiB
Plaintext
178 lines
8.0 KiB
Plaintext
.\"
|
|
.\" Copyright (c) 1987, 1988, 1989, 1990, 1991, 1992, 1994, 1995, 1996, 1997
|
|
.\" The Regents of the University of California. All rights reserved.
|
|
.\" All rights reserved.
|
|
.\"
|
|
.\" Redistribution and use in source and binary forms, with or without
|
|
.\" modification, are permitted provided that: (1) source code distributions
|
|
.\" retain the above copyright notice and this paragraph in its entirety, (2)
|
|
.\" distributions including binary code include the above copyright notice and
|
|
.\" this paragraph in its entirety in the documentation or other materials
|
|
.\" provided with the distribution, and (3) all advertising materials mentioning
|
|
.\" features or use of this software display the following acknowledgement:
|
|
.\" ``This product includes software developed by the University of California,
|
|
.\" Lawrence Berkeley Laboratory and its contributors.'' Neither the name of
|
|
.\" the University nor the names of its contributors may be used to endorse
|
|
.\" or promote products derived from this software without specific prior
|
|
.\" written permission.
|
|
.\" THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR IMPLIED
|
|
.\" WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF
|
|
.\" MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
|
|
.\"
|
|
.TH PCAP-TSTAMP @MAN_MISC_INFO@ "8 March 2015"
|
|
.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
|
|
.LP
|
|
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.
|
|
.LP
|
|
The
|
|
.BR pcap_set_tstamp_precision (3PCAP)
|
|
routine can be used after a
|
|
.B pcap_create()
|
|
call and after a
|
|
.B 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
|
|
.B pcap_set_tstamp_precision()
|
|
call will fail, and the time stamps supplied after the
|
|
.B pcap_activate()
|
|
call will have microsecond resolution.
|
|
.LP
|
|
When opening a savefile, the
|
|
.BR pcap_open_offline_with_tstamp_precision (3PCAP)
|
|
and
|
|
.BR 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.
|
|
.LP
|
|
The
|
|
.BR 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.
|
|
.SH SEE ALSO
|
|
.na
|
|
pcap_set_tstamp_type(3PCAP),
|
|
pcap_list_tstamp_types(3PCAP),
|
|
pcap_tstamp_type_val_to_name(3PCAP),
|
|
pcap_tstamp_type_name_to_val(3PCAP),
|
|
pcap_set_tstamp_precision(3PCAP),
|
|
pcap_open_offline_with_tstamp_precision(3PCAP),
|
|
\%pcap_fopen_offline_with_tstamp_precision(3PCAP),
|
|
\%pcap_get_tstamp_precision(3PCAP)
|
|
.ad
|