1PCAP-TSTAMP(7) Miscellaneous Information Manual PCAP-TSTAMP(7)
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6 pcap-tstamp - packet time stamps in libpcap
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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:
16 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;
26 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;
31 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;
36 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;
41 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;
46 some time sources, such as those that supply POSIX "seconds
48 since the Epoch" time, do not count leap seconds, meaning that
49 the seconds portion (tv_sec) of the time stamp might not be
50 incremented for a leap second, so that the fraction-of-a-second
51 part of the time stamp might roll over past zero but the second
52 part would not change, or the clock might run slightly more
53 slowly for a period before the leap second.
54 For these reasons, time differences between packet time stamps will not
56 necessarily accurately reflect the time differences between the receipt
57 or transmission times of the packets.
58 In addition, packets time-stamped by different cores might be time-
60 stamped in one order and added to the queue of packets for libpcap to
61 read in another order, so time stamps might not be monotonically
62 increasing.
63 Some capture devices on some platforms can provide time stamps for
65 packets; those time stamps are usually high-resolution time stamps, and
66 are usually applied to the packet when the first or last bit of the
67 packet arrives, and are thus more accurate than time stamps provided by
68 the host operating system. Those time stamps might not, however, be
69 synchronized with the host operating system's clock, so that, for exam‐
70 ple, the time stamp of a packet might not correspond to the time stamp
71 of an event on the host triggered by the arrival of that packet. If
72 they are synchronized with the host operating system's clock, some of
73 the issues listed above with time stamps supplied by the host operating
74 system may also apply to time stamps supplied by the capture device.
75 Depending on the capture device and the software on the host, libpcap
77 might allow different types of time stamp to be used. The
78 pcap_list_tstamp_types(3PCAP) routine provides, for a packet capture
79 handle created by pcap_create(3PCAP) but not yet activated by
80 pcap_activate(3PCAP), a list of time stamp types supported by the cap‐
81 ture device for that handle. The list might be empty, in which case no
82 choice of time stamp type is offered for that capture device. If the
83 list is not empty, the pcap_set_tstamp_type(3PCAP) routine can be used
84 after a pcap_create() call and before a pcap_activate() call to specify
85 the type of time stamp to be used on the device. The time stamp types
86 are listed here; the first value is the #define to use in code, the
87 second value is the value returned by
88 pcap_tstamp_type_val_to_name(3PCAP) and accepted by
89 pcap_tstamp_type_name_to_val(3PCAP).
90 PCAP_TSTAMP_HOST - host
92 Time stamp provided by the host on which the capture is being
93 done. The precision of this time stamp is unspecified; it
94 might or might not be synchronized with the host operating
95 system's clock.
96 PCAP_TSTAMP_HOST_LOWPREC - host_lowprec
98 Time stamp provided by the host on which the capture is being
99 done. This is a low-precision time stamp, synchronized with
100 the host operating system's clock.
101 PCAP_TSTAMP_HOST_HIPREC - host_hiprec
103 Time stamp provided by the host on which the capture is being
104 done. This is a high-precision time stamp, synchronized with
105 the host operating system's clock. It might be more expensive
106 to fetch than PCAP_TSTAMP_HOST_LOWPREC.
107 PCAP_TSTAMP_HOST_HIPREC_UNSYNCED - host_hiprec_unsynced
109 Time stamp provided by the host on which the capture is being
110 done. This is a high-precision time stamp, not synchronized
111 with the host operating system's clock. It might be more
112 expensive to fetch than PCAP_TSTAMP_HOST_LOWPREC.
113 PCAP_TSTAMP_ADAPTER - adapter
115 Time stamp provided by the network adapter on which the cap‐
116 ture is being done. This is a high-precision time stamp,
117 synchronized with the host operating system's clock.
118 PCAP_TSTAMP_ADAPTER_UNSYNCED - adapter_unsynced
120 Time stamp provided by the network adapter on which the cap‐
121 ture is being done. This is a high-precision time stamp; it
122 is not synchronized with the host operating system's clock.
123 Time stamps synchronized with the system clock can go backwards, as the
125 system clock can go backwards. If a clock is not in sync with the sys‐
126 tem clock, that could be because the system clock isn't keeping accu‐
127 rate time, because the other clock isn't keeping accurate time, or
128 both.
129 Host-provided time stamps generally correspond to the time when the
131 time-stamping code sees the packet; this could be some unknown amount
132 of time after the first or last bit of the packet is received by the
133 network adapter, due to batching of interrupts for packet arrival,
134 queueing delays, etc..
135 By default, when performing a live capture or reading from a savefile,
137 time stamps are supplied as seconds since January 1, 1970, 00:00:00
138 UTC, and microseconds since that seconds value, even if higher-resolu‐
139 tion time stamps are available from the capture device or in the save‐
140 file. If, when reading a savefile, the time stamps in the file have a
141 higher resolution than one microsecond, the additional digits of reso‐
142 lution are discarded.
143 The pcap_set_tstamp_precision(3PCAP) routine can be used after a
145 pcap_create() call and after a pcap_activate() call to specify the res‐
146 olution of the time stamps to get for the device. If the hardware or
147 software cannot supply a higher-resolution time stamp, the
148 pcap_set_tstamp_precision() call will fail, and the time stamps sup‐
149 plied after the pcap_activate() call will have microsecond resolution.
150 When opening a savefile, the
152 pcap_open_offline_with_tstamp_precision(3PCAP) and
153 pcap_fopen_offline_with_tstamp_precision(3PCAP) routines can be used to
154 specify the resolution of time stamps to be read from the file; if the
155 time stamps in the file have a lower resolution, the fraction-of-a-sec‐
156 ond portion of the time stamps will be scaled to the specified resolu‐
157 tion.
158 The pcap_get_tstamp_precision(3PCAP) routine returns the resolution of
160 time stamps that will be supplied; when capturing packets, this does
161 not reflect the actual precision of the time stamp supplied by the
162 hardware or operating system and, when reading a savefile, this does
163 not indicate the actual precision of time stamps in the file.
164
166 pcap(3PCAP)
167 14 July 2020 PCAP-TSTAMP(7)