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authorGeorge V. Neville-Neil <gnn@FreeBSD.org>2012-01-31 18:24:25 +0000
committerGeorge V. Neville-Neil <gnn@FreeBSD.org>2012-01-31 18:24:25 +0000
commit5a0615f5c40ce036a7e13cfffe00927f845ad563 (patch)
tree5764c271f89293572dc3cb25e3818e0295eb67f0 /pcap-tstamp.manmisc.in
parent59f06eaa08f055b60a0ee4e9694cce270b4bb6d8 (diff)
downloadsrc-vendor/libpcap/1.2.1.tar.gz
src-vendor/libpcap/1.2.1.zip
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+.\"
+.\" 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@ "22 August 2010"
+.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_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
+.SH 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)