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22 .TH PCAP-TSTAMP 5 "8 March 2015"
23 .SH NAME
24 pcap-tstamp \- packet time stamps in libpcap
25 .SH DESCRIPTION
26 When capturing traffic, each packet is given a time stamp representing,
27 for incoming packets, the arrival time of the packet and, for outgoing
28 packets, the transmission time of the packet. This time is an
29 approximation of the arrival or transmission time. If it is supplied by
30 the operating system running on the host on which the capture is being
31 done, there are several reasons why it might not precisely represent the
32 arrival or transmission time:
33 .IP
34 if the time stamp is applied to the packet when the networking stack
35 receives the packet, the networking stack might not see the packet until
36 an interrupt is delivered for the packet or a timer event causes the
37 networking device driver to poll for packets, and the time stamp might
38 not be applied until the packet has had some processing done by other
39 code in the networking stack, so there might be a significant delay
40 between the time when the last bit of the packet is received by the
41 capture device and when the networking stack time-stamps the packet;
42 .IP
43 the timer used to generate the time stamps might have low resolution,
44 for example, it might be a timer updated once per host operating system
45 timer tick, with the host operating system timer ticking once every few
46 milliseconds;
47 .IP
48 a high-resolution timer might use a counter that runs at a rate
49 dependent on the processor clock speed, and that clock speed might be
50 adjusted upwards or downwards over time and the timer might not be able
51 to compensate for all those adjustments;
52 .IP
53 the host operating system's clock might be adjusted over time to match a
54 time standard to which the host is being synchronized, which might be
55 done by temporarily slowing down or speeding up the clock or by making a
56 single adjustment;
57 .IP
58 different CPU cores on a multi-core or multi-processor system might be
59 running at different speeds, or might not have time counters all
60 synchronized, so packets time-stamped by different cores might not have
61 consistent time stamps.
62 .LP
63 In addition, packets time-stamped by different cores might be
64 time-stamped in one order and added to the queue of packets for libpcap
65 to read in another order, so time stamps might not be monotonically
66 increasing.
67 .LP
68 Some capture devices on some platforms can provide time stamps for
69 packets; those time stamps are usually high-resolution time stamps, and
70 are usually applied to the packet when the first or last bit of the
71 packet arrives, and are thus more accurate than time stamps provided by
72 the host operating system. Those time stamps might not, however, be
73 synchronized with the host operating system's clock, so that, for
74 example, the time stamp of a packet might not correspond to the time
75 stamp of an event on the host triggered by the arrival of that packet.
76 .LP
77 Depending on the capture device and the software on the host, libpcap
78 might allow different types of time stamp to be used. The
79 .BR pcap_list_tstamp_types (3PCAP)
80 routine provides, for a packet capture handle created by
81 .BR pcap_create (3PCAP)
82 but not yet activated by
83 .BR pcap_activate (3PCAP),
84 a list of time stamp types supported by the capture device for that
85 handle.
86 The list might be empty, in which case no choice of time stamp type is
87 offered for that capture device. If the list is not empty, the
88 .BR pcap_set_tstamp_type (3PCAP)
89 routine can be used after a
90 .B pcap_create()
91 call and before a
92 .B pcap_activate()
93 call to specify the type of time stamp to be used on the device.
94 The time stamp types are listed here; the first value is the #define to
95 use in code, the second value is the value returned by
96 .B pcap_tstamp_type_val_to_name()
97 and accepted by
98 .BR pcap_tstamp_type_name_to_val() .
99 .RS 5
100 .TP 5
101 .BR PCAP_TSTAMP_HOST " - " host
102 Time stamp provided by the host on which the capture is being done. The
103 precision of this time stamp is unspecified; it might or might not be
104 synchronized with the host operating system's clock.
105 .TP 5
106 .BR PCAP_TSTAMP_HOST_LOWPREC " - " host_lowprec
107 Time stamp provided by the host on which the capture is being done.
108 This is a low-precision time stamp, synchronized with the host operating
109 system's clock.
110 .TP 5
111 .BR PCAP_TSTAMP_HOST_HIPREC " - " host_hiprec
112 Time stamp provided by the host on which the capture is being done.
113 This is a high-precision time stamp; it might or might not be
114 synchronized with the host operating system's clock. It might be more
115 expensive to fetch than
116 .BR PCAP_TSTAMP_HOST_LOWPREC .
117 .TP 5
118 .BR PCAP_TSTAMP_ADAPTER " - " adapter
119 Time stamp provided by the network adapter on which the capture is being
120 done. This is a high-precision time stamp, synchronized with the host
121 operating system's clock.
122 .TP 5
123 .BR PCAP_TSTAMP_ADAPTER_UNSYNCED " - " adapter_unsynced
124 Time stamp provided by the network adapter on which the capture is being
125 done. This is a high-precision time stamp; it is not synchronized with
126 the host operating system's clock.
127 .RE
128 .LP
129 By default, when performing a live capture or reading from a savefile,
130 time stamps are supplied as seconds since January 1, 1970, 00:00:00 UTC,
131 and microseconds since that seconds value, even if higher-resolution
132 time stamps are available from the capture device or in the savefile.
133 If, when reading a savefile, the time stamps in the file have a higher
134 resolution than one microsecond, the additional digits of resolution are
135 discarded.
136 .LP
137 The
138 .BR pcap_set_tstamp_precision (3PCAP)
139 routine can be used after a
140 .B pcap_create()
141 call and after a
142 .B pcap_activate()
143 call to specify the resolution of the time stamps to get for the device.
144 If the hardware or software cannot supply a higher-resolution time
145 stamp, the
146 .B pcap_set_tstamp_precision()
147 call will fail, and the time stamps supplied after the
148 .B pcap_activate()
149 call will have microsecond resolution.
150 .LP
151 When opening a savefile, the
152 .BR pcap_open_offline_with_tstamp_precision (3PCAP)
153 and
154 .BR pcap_fopen_offline_with_tstamp_precision (3PCAP)
155 routines can be used to specify the resolution of time stamps to be read
156 from the file; if the time stamps in the file have a lower resolution,
157 the fraction-of-a-second portion of the time stamps will be scaled to
158 the specified resolution.
159 .LP
160 The
161 .BR pcap_get_tstamp_precision (3PCAP)
162 routine returns the resolution of time stamps that will be supplied;
163 when capturing packets, this does not reflect the actual precision of
164 the time stamp supplied by the hardware or operating system and, when
165 reading a savefile, this does not indicate the actual precision of time
166 stamps in the file.
167 .SH SEE ALSO
168 pcap_set_tstamp_type(3PCAP),
169 pcap_list_tstamp_types(3PCAP),
170 pcap_tstamp_type_val_to_name(3PCAP),
171 pcap_tstamp_type_name_to_val(3PCAP),
172 pcap_set_tstamp_precision(3PCAP),
173 pcap_open_offline_with_tstamp_precision(3PCAP),
174 pcap_fopen_offline_with_tstamp_precision(3PCAP),
175 pcap_get_tstamp_precision(3PCAP)