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.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(3PCAP)

and accepted by

.BR pcap_tstamp_type_name_to_val(3PCAP) .

.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

pcap(3PCAP)