Страницы справочника OS X

Эта страница руководства для  версии 10.9 Mac OS X

Читать страницы руководства

PCAP-TSTAMP(7)                                                                                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 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 han-dle handle
       dle 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_acti-vate() pcap_activate()
       vate()  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-preci-sion low-precision
                 sion 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-preci-sion high-precision
                 sion  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)



                                               22 August 2010                                 PCAP-TSTAMP(7)