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 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;
- 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 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 precision 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 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-precision 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)