1PCAP-TSTAMP(7)         Miscellaneous Information Manual         PCAP-TSTAMP(7)


6       pcap-tstamp - packet time stamps in libpcap


9       When capturing traffic, each packet is given a time stamp representing,
10       for incoming packets, the arrival time of the packet and, for  outgoing
11       packets, the transmission time of the packet.  This time is an approxi‐
12       mation of the arrival or transmission time.  If it is supplied  by  the
13       operating  system  running  on  the  host on which the capture is being
14       done, there are several reasons why it might  not  precisely  represent
15       the arrival or transmission time:
17              if  the  time stamp is applied to the packet when the networking
18              stack receives the packet, the networking stack  might  not  see
19              the  packet  until an interrupt is delivered for the packet or a
20              timer event causes the networking  device  driver  to  poll  for
21              packets,  and  the  time  stamp  might  not be applied until the
22              packet has had some processing done by other code  in  the  net‐
23              working stack, so there might be a significant delay between the
24              time when the last bit of the packet is received by the  capture
25              device and when the networking stack time-stamps the packet;
27              the  timer used to generate the time stamps might have low reso‐
28              lution, for example, it might be a timer updated once  per  host
29              operating  system  timer  tick,  with  the host operating system
30              timer ticking once every few milliseconds;
32              a high-resolution timer might use a counter that runs at a  rate
33              dependent  on  the  processor  clock speed, and that clock speed
34              might be adjusted upwards or downwards over time and  the  timer
35              might not be able to compensate for all those adjustments;
37              the host operating system's clock might be adjusted over time to
38              match a time standard to which the host is  being  synchronized,
39              which  might  be done by temporarily slowing down or speeding up
40              the clock or by making a single adjustment;
42              different CPU cores on a multi-core  or  multi-processor  system
43              might  be  running  at  different speeds, or might not have time
44              counters all synchronized, so packets time-stamped by  different
45              cores might not have consistent time stamps.
47       In  addition,  packets  time-stamped  by different cores might be time-
48       stamped in one order and added to the queue of packets for  libpcap  to
49       read  in  another  order,  so  time  stamps  might not be monotonically
50       increasing.
52       Some capture devices on some platforms  can  provide  time  stamps  for
53       packets; those time stamps are usually high-resolution time stamps, and
54       are usually applied to the packet when the first or  last  bit  of  the
55       packet arrives, and are thus more accurate than time stamps provided by
56       the host operating system.  Those time stamps might  not,  however,  be
57       synchronized with the host operating system's clock, so that, for exam‐
58       ple, the time stamp of a packet might not correspond to the time  stamp
59       of an event on the host triggered by the arrival of that packet.
61       Depending  on  the capture device and the software on the host, libpcap
62       might  allow  different  types  of  time  stamp  to   be   used.    The
63       pcap_list_tstamp_types(3PCAP)  routine  provides,  for a packet capture
64       handle  created  by  pcap_create(3PCAP)  but  not  yet   activated   by
65       pcap_activate(3PCAP),  a list of time stamp types supported by the cap‐
66       ture device for that handle.  The list might be empty, in which case no
67       choice  of  time stamp type is offered for that capture device.  If the
68       list is not empty, the pcap_set_tstamp_type(3PCAP) routine can be  used
69       after a pcap_create() call and before a pcap_activate() call to specify
70       the type of time stamp to be used on the device.  The time stamp  types
71       are  listed  here;  the  first value is the #define to use in code, the
72       second value is the value  returned  by  pcap_tstamp_type_val_to_name()
73       and accepted by pcap_tstamp_type_name_to_val().
75            PCAP_TSTAMP_HOST - host
76                 Time stamp provided by the host on which the capture is being
77                 done.  The precision of this time stamp  is  unspecified;  it
78                 might  or  might  not be synchronized with the host operating
79                 system's clock.
81            PCAP_TSTAMP_HOST_LOWPREC - host_lowprec
82                 Time stamp provided by the host on which the capture is being
83                 done.   This is a low-precision time stamp, synchronized with
84                 the host operating system's clock.
86            PCAP_TSTAMP_HOST_HIPREC - host_hiprec
87                 Time stamp provided by the host on which the capture is being
88                 done.  This is a high-precision time stamp; it might or might
89                 not be synchronized with the host operating  system's  clock.
90                 It    might    be    more    expensive    to    fetch    than
91                 PCAP_TSTAMP_HOST_LOWPREC.
93            PCAP_TSTAMP_ADAPTER - adapter
94                 Time stamp provided by the network adapter on which the  cap‐
95                 ture  is  being  done.   This is a high-precision time stamp,
96                 synchronized with the host operating system's clock.
98            PCAP_TSTAMP_ADAPTER_UNSYNCED - adapter_unsynced
99                 Time stamp provided by the network adapter on which the  cap‐
100                 ture  is being done.  This is a high-precision time stamp; it
101                 is not synchronized with the host operating system's clock.
103       By default, when performing a live capture or reading from a  savefile,
104       time  stamps  are  supplied  as seconds since January 1, 1970, 00:00:00
105       UTC, and microseconds since that seconds value, even if  higher-resolu‐
106       tion  time stamps are available from the capture device or in the save‐
107       file.  If, when reading a savefile, the time stamps in the file have  a
108       higher  resolution than one microsecond, the additional digits of reso‐
109       lution are discarded.
111       The  pcap_set_tstamp_precision(3PCAP)  routine  can  be  used  after  a
112       pcap_create() call and after a pcap_activate() call to specify the res‐
113       olution of the time stamps to get for the device.  If the  hardware  or
114       software   cannot   supply   a   higher-resolution   time   stamp,  the
115       pcap_set_tstamp_precision() call will fail, and the  time  stamps  sup‐
116       plied after the pcap_activate() call will have microsecond resolution.
118       When   opening  a  savefile,  the  pcap_open_offline_with_tstamp_preci‐
119       sion(3PCAP)  and  pcap_fopen_offline_with_tstamp_precision(3PCAP)  rou‐
120       tines  can  be used to specify the resolution of time stamps to be read
121       from the file; if the time stamps in the file have a lower  resolution,
122       the  fraction-of-a-second  portion of the time stamps will be scaled to
123       the specified resolution.
125       The pcap_get_tstamp_precision(3PCAP) routine returns the resolution  of
126       time  stamps  that  will be supplied; when capturing packets, this does
127       not reflect the actual precision of the  time  stamp  supplied  by  the
128       hardware  or  operating  system and, when reading a savefile, this does
129       not indicate the actual precision of time stamps in the file.


132       pcap_set_tstamp_type(3PCAP), pcap_list_tstamp_types(3PCAP),
133       pcap_tstamp_type_val_to_name(3PCAP),
134       pcap_tstamp_type_name_to_val(3PCAP), pcap_set_tstamp_precision(3PCAP),
135       pcap_open_offline_with_tstamp_precision(3PCAP),
136       pcap_fopen_offline_with_tstamp_precision(3PCAP),
137       pcap_get_tstamp_precision(3PCAP)
141                                 8 March 2015                   PCAP-TSTAMP(7)