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Willem de Bruijn8fe2f762014-08-31 21:27:47 -04001
21. Control Interfaces
3
4The interfaces for receiving network packages timestamps are:
Patrick Ohlycb9eff02009-02-12 05:03:36 +00005
6* SO_TIMESTAMP
Willem de Bruijn8fe2f762014-08-31 21:27:47 -04007 Generates a timestamp for each incoming packet in (not necessarily
8 monotonic) system time. Reports the timestamp via recvmsg() in a
9 control message as struct timeval (usec resolution).
Patrick Ohlycb9eff02009-02-12 05:03:36 +000010
11* SO_TIMESTAMPNS
Willem de Bruijn8fe2f762014-08-31 21:27:47 -040012 Same timestamping mechanism as SO_TIMESTAMP, but reports the
13 timestamp as struct timespec (nsec resolution).
Patrick Ohlycb9eff02009-02-12 05:03:36 +000014
15* IP_MULTICAST_LOOP + SO_TIMESTAMP[NS]
Willem de Bruijn8fe2f762014-08-31 21:27:47 -040016 Only for multicast:approximate transmit timestamp obtained by
17 reading the looped packet receive timestamp.
Patrick Ohlycb9eff02009-02-12 05:03:36 +000018
Willem de Bruijn8fe2f762014-08-31 21:27:47 -040019* SO_TIMESTAMPING
20 Generates timestamps on reception, transmission or both. Supports
21 multiple timestamp sources, including hardware. Supports generating
22 timestamps for stream sockets.
Patrick Ohlycb9eff02009-02-12 05:03:36 +000023
Patrick Ohlycb9eff02009-02-12 05:03:36 +000024
Willem de Bruijn8fe2f762014-08-31 21:27:47 -0400251.1 SO_TIMESTAMP:
Patrick Ohlycb9eff02009-02-12 05:03:36 +000026
Willem de Bruijn8fe2f762014-08-31 21:27:47 -040027This socket option enables timestamping of datagrams on the reception
28path. Because the destination socket, if any, is not known early in
29the network stack, the feature has to be enabled for all packets. The
30same is true for all early receive timestamp options.
Patrick Ohlycb9eff02009-02-12 05:03:36 +000031
Willem de Bruijn8fe2f762014-08-31 21:27:47 -040032For interface details, see `man 7 socket`.
33
34
351.2 SO_TIMESTAMPNS:
36
37This option is identical to SO_TIMESTAMP except for the returned data type.
38Its struct timespec allows for higher resolution (ns) timestamps than the
39timeval of SO_TIMESTAMP (ms).
40
41
421.3 SO_TIMESTAMPING:
43
44Supports multiple types of timestamp requests. As a result, this
45socket option takes a bitmap of flags, not a boolean. In
46
47 err = setsockopt(fd, SOL_SOCKET, SO_TIMESTAMPING, (void *) val, &val);
48
49val is an integer with any of the following bits set. Setting other
50bit returns EINVAL and does not change the current state.
51
52
531.3.1 Timestamp Generation
54
55Some bits are requests to the stack to try to generate timestamps. Any
56combination of them is valid. Changes to these bits apply to newly
57created packets, not to packets already in the stack. As a result, it
58is possible to selectively request timestamps for a subset of packets
59(e.g., for sampling) by embedding an send() call within two setsockopt
60calls, one to enable timestamp generation and one to disable it.
61Timestamps may also be generated for reasons other than being
62requested by a particular socket, such as when receive timestamping is
63enabled system wide, as explained earlier.
64
65SOF_TIMESTAMPING_RX_HARDWARE:
66 Request rx timestamps generated by the network adapter.
67
68SOF_TIMESTAMPING_RX_SOFTWARE:
69 Request rx timestamps when data enters the kernel. These timestamps
70 are generated just after a device driver hands a packet to the
71 kernel receive stack.
72
73SOF_TIMESTAMPING_TX_HARDWARE:
74 Request tx timestamps generated by the network adapter.
75
76SOF_TIMESTAMPING_TX_SOFTWARE:
77 Request tx timestamps when data leaves the kernel. These timestamps
78 are generated in the device driver as close as possible, but always
79 prior to, passing the packet to the network interface. Hence, they
80 require driver support and may not be available for all devices.
81
82SOF_TIMESTAMPING_TX_SCHED:
83 Request tx timestamps prior to entering the packet scheduler. Kernel
84 transmit latency is, if long, often dominated by queuing delay. The
85 difference between this timestamp and one taken at
86 SOF_TIMESTAMPING_TX_SOFTWARE will expose this latency independent
87 of protocol processing. The latency incurred in protocol
88 processing, if any, can be computed by subtracting a userspace
89 timestamp taken immediately before send() from this timestamp. On
90 machines with virtual devices where a transmitted packet travels
91 through multiple devices and, hence, multiple packet schedulers,
92 a timestamp is generated at each layer. This allows for fine
93 grained measurement of queuing delay.
94
95SOF_TIMESTAMPING_TX_ACK:
96 Request tx timestamps when all data in the send buffer has been
97 acknowledged. This only makes sense for reliable protocols. It is
98 currently only implemented for TCP. For that protocol, it may
99 over-report measurement, because the timestamp is generated when all
100 data up to and including the buffer at send() was acknowledged: the
101 cumulative acknowledgment. The mechanism ignores SACK and FACK.
102
103
1041.3.2 Timestamp Reporting
Andrew Lutomirskiadca4762014-03-04 17:24:10 -0800105
106The other three bits control which timestamps will be reported in a
Willem de Bruijn8fe2f762014-08-31 21:27:47 -0400107generated control message. Changes to the bits take immediate
108effect at the timestamp reporting locations in the stack. Timestamps
109are only reported for packets that also have the relevant timestamp
110generation request set.
Andrew Lutomirskiadca4762014-03-04 17:24:10 -0800111
Willem de Bruijn8fe2f762014-08-31 21:27:47 -0400112SOF_TIMESTAMPING_SOFTWARE:
113 Report any software timestamps when available.
Andrew Lutomirskiadca4762014-03-04 17:24:10 -0800114
Willem de Bruijn8fe2f762014-08-31 21:27:47 -0400115SOF_TIMESTAMPING_SYS_HARDWARE:
116 This option is deprecated and ignored.
Andrew Lutomirskiadca4762014-03-04 17:24:10 -0800117
Willem de Bruijn8fe2f762014-08-31 21:27:47 -0400118SOF_TIMESTAMPING_RAW_HARDWARE:
119 Report hardware timestamps as generated by
120 SOF_TIMESTAMPING_TX_HARDWARE when available.
121
122
1231.3.3 Timestamp Options
124
125The interface supports one option
126
127SOF_TIMESTAMPING_OPT_ID:
128
129 Generate a unique identifier along with each packet. A process can
130 have multiple concurrent timestamping requests outstanding. Packets
131 can be reordered in the transmit path, for instance in the packet
132 scheduler. In that case timestamps will be queued onto the error
133 queue out of order from the original send() calls. This option
134 embeds a counter that is incremented at send() time, to order
135 timestamps within a flow.
136
137 This option is implemented only for transmit timestamps. There, the
138 timestamp is always looped along with a struct sock_extended_err.
139 The option modifies field ee_info to pass an id that is unique
140 among all possibly concurrently outstanding timestamp requests for
141 that socket. In practice, it is a monotonically increasing u32
142 (that wraps).
143
144 In datagram sockets, the counter increments on each send call. In
145 stream sockets, it increments with every byte.
146
147
1481.4 Bytestream Timestamps
149
150The SO_TIMESTAMPING interface supports timestamping of bytes in a
151bytestream. Each request is interpreted as a request for when the
152entire contents of the buffer has passed a timestamping point. That
153is, for streams option SOF_TIMESTAMPING_TX_SOFTWARE will record
154when all bytes have reached the device driver, regardless of how
155many packets the data has been converted into.
156
157In general, bytestreams have no natural delimiters and therefore
158correlating a timestamp with data is non-trivial. A range of bytes
159may be split across segments, any segments may be merged (possibly
160coalescing sections of previously segmented buffers associated with
161independent send() calls). Segments can be reordered and the same
162byte range can coexist in multiple segments for protocols that
163implement retransmissions.
164
165It is essential that all timestamps implement the same semantics,
166regardless of these possible transformations, as otherwise they are
167incomparable. Handling "rare" corner cases differently from the
168simple case (a 1:1 mapping from buffer to skb) is insufficient
169because performance debugging often needs to focus on such outliers.
170
171In practice, timestamps can be correlated with segments of a
172bytestream consistently, if both semantics of the timestamp and the
173timing of measurement are chosen correctly. This challenge is no
174different from deciding on a strategy for IP fragmentation. There, the
175definition is that only the first fragment is timestamped. For
176bytestreams, we chose that a timestamp is generated only when all
177bytes have passed a point. SOF_TIMESTAMPING_TX_ACK as defined is easy to
178implement and reason about. An implementation that has to take into
179account SACK would be more complex due to possible transmission holes
180and out of order arrival.
181
182On the host, TCP can also break the simple 1:1 mapping from buffer to
183skbuff as a result of Nagle, cork, autocork, segmentation and GSO. The
184implementation ensures correctness in all cases by tracking the
185individual last byte passed to send(), even if it is no longer the
186last byte after an skbuff extend or merge operation. It stores the
187relevant sequence number in skb_shinfo(skb)->tskey. Because an skbuff
188has only one such field, only one timestamp can be generated.
189
190In rare cases, a timestamp request can be missed if two requests are
191collapsed onto the same skb. A process can detect this situation by
192enabling SOF_TIMESTAMPING_OPT_ID and comparing the byte offset at
193send time with the value returned for each timestamp. It can prevent
194the situation by always flushing the TCP stack in between requests,
195for instance by enabling TCP_NODELAY and disabling TCP_CORK and
196autocork.
197
198These precautions ensure that the timestamp is generated only when all
199bytes have passed a timestamp point, assuming that the network stack
200itself does not reorder the segments. The stack indeed tries to avoid
201reordering. The one exception is under administrator control: it is
202possible to construct a packet scheduler configuration that delays
203segments from the same stream differently. Such a setup would be
204unusual.
205
206
2072 Data Interfaces
208
209Timestamps are read using the ancillary data feature of recvmsg().
210See `man 3 cmsg` for details of this interface. The socket manual
211page (`man 7 socket`) describes how timestamps generated with
212SO_TIMESTAMP and SO_TIMESTAMPNS records can be retrieved.
213
214
2152.1 SCM_TIMESTAMPING records
216
217These timestamps are returned in a control message with cmsg_level
218SOL_SOCKET, cmsg_type SCM_TIMESTAMPING, and payload of type
Patrick Loschmidt69298692010-04-07 21:52:07 -0700219
220struct scm_timestamping {
Willem de Bruijn8fe2f762014-08-31 21:27:47 -0400221 struct timespec ts[3];
Patrick Loschmidt69298692010-04-07 21:52:07 -0700222};
Patrick Ohlycb9eff02009-02-12 05:03:36 +0000223
Willem de Bruijn8fe2f762014-08-31 21:27:47 -0400224The structure can return up to three timestamps. This is a legacy
225feature. Only one field is non-zero at any time. Most timestamps
226are passed in ts[0]. Hardware timestamps are passed in ts[2].
Patrick Ohlycb9eff02009-02-12 05:03:36 +0000227
Willem de Bruijn8fe2f762014-08-31 21:27:47 -0400228ts[1] used to hold hardware timestamps converted to system time.
229Instead, expose the hardware clock device on the NIC directly as
230a HW PTP clock source, to allow time conversion in userspace and
231optionally synchronize system time with a userspace PTP stack such
232as linuxptp. For the PTP clock API, see Documentation/ptp/ptp.txt.
Patrick Ohlycb9eff02009-02-12 05:03:36 +0000233
Willem de Bruijn8fe2f762014-08-31 21:27:47 -04002342.1.1 Transmit timestamps with MSG_ERRQUEUE
Patrick Ohlycb9eff02009-02-12 05:03:36 +0000235
Willem de Bruijn8fe2f762014-08-31 21:27:47 -0400236For transmit timestamps the outgoing packet is looped back to the
237socket's error queue with the send timestamp(s) attached. A process
238receives the timestamps by calling recvmsg() with flag MSG_ERRQUEUE
239set and with a msg_control buffer sufficiently large to receive the
240relevant metadata structures. The recvmsg call returns the original
241outgoing data packet with two ancillary messages attached.
Patrick Ohlycb9eff02009-02-12 05:03:36 +0000242
Willem de Bruijn8fe2f762014-08-31 21:27:47 -0400243A message of cm_level SOL_IP(V6) and cm_type IP(V6)_RECVERR
244embeds a struct sock_extended_err. This defines the error type. For
245timestamps, the ee_errno field is ENOMSG. The other ancillary message
246will have cm_level SOL_SOCKET and cm_type SCM_TIMESTAMPING. This
247embeds the struct scm_timestamping.
Patrick Ohlycb9eff02009-02-12 05:03:36 +0000248
249
Willem de Bruijn8fe2f762014-08-31 21:27:47 -04002502.1.1.2 Timestamp types
251
252The semantics of the three struct timespec are defined by field
253ee_info in the extended error structure. It contains a value of
254type SCM_TSTAMP_* to define the actual timestamp passed in
255scm_timestamping.
256
257The SCM_TSTAMP_* types are 1:1 matches to the SOF_TIMESTAMPING_*
258control fields discussed previously, with one exception. For legacy
259reasons, SCM_TSTAMP_SND is equal to zero and can be set for both
260SOF_TIMESTAMPING_TX_HARDWARE and SOF_TIMESTAMPING_TX_SOFTWARE. It
261is the first if ts[2] is non-zero, the second otherwise, in which
262case the timestamp is stored in ts[0].
263
264
2652.1.1.3 Fragmentation
266
267Fragmentation of outgoing datagrams is rare, but is possible, e.g., by
268explicitly disabling PMTU discovery. If an outgoing packet is fragmented,
269then only the first fragment is timestamped and returned to the sending
270socket.
271
272
2732.1.1.4 Packet Payload
274
275The calling application is often not interested in receiving the whole
276packet payload that it passed to the stack originally: the socket
277error queue mechanism is just a method to piggyback the timestamp on.
278In this case, the application can choose to read datagrams with a
279smaller buffer, possibly even of length 0. The payload is truncated
280accordingly. Until the process calls recvmsg() on the error queue,
281however, the full packet is queued, taking up budget from SO_RCVBUF.
282
283
2842.1.1.5 Blocking Read
285
286Reading from the error queue is always a non-blocking operation. To
287block waiting on a timestamp, use poll or select. poll() will return
288POLLERR in pollfd.revents if any data is ready on the error queue.
289There is no need to pass this flag in pollfd.events. This flag is
290ignored on request. See also `man 2 poll`.
291
292
2932.1.2 Receive timestamps
294
295On reception, there is no reason to read from the socket error queue.
296The SCM_TIMESTAMPING ancillary data is sent along with the packet data
297on a normal recvmsg(). Since this is not a socket error, it is not
298accompanied by a message SOL_IP(V6)/IP(V6)_RECVERROR. In this case,
299the meaning of the three fields in struct scm_timestamping is
300implicitly defined. ts[0] holds a software timestamp if set, ts[1]
301is again deprecated and ts[2] holds a hardware timestamp if set.
302
303
3043. Hardware Timestamping configuration: SIOCSHWTSTAMP and SIOCGHWTSTAMP
Patrick Ohlycb9eff02009-02-12 05:03:36 +0000305
306Hardware time stamping must also be initialized for each device driver
Patrick Loschmidt69298692010-04-07 21:52:07 -0700307that is expected to do hardware time stamping. The parameter is defined in
308/include/linux/net_tstamp.h as:
Patrick Ohlycb9eff02009-02-12 05:03:36 +0000309
310struct hwtstamp_config {
Patrick Loschmidt69298692010-04-07 21:52:07 -0700311 int flags; /* no flags defined right now, must be zero */
312 int tx_type; /* HWTSTAMP_TX_* */
313 int rx_filter; /* HWTSTAMP_FILTER_* */
Patrick Ohlycb9eff02009-02-12 05:03:36 +0000314};
315
316Desired behavior is passed into the kernel and to a specific device by
317calling ioctl(SIOCSHWTSTAMP) with a pointer to a struct ifreq whose
318ifr_data points to a struct hwtstamp_config. The tx_type and
319rx_filter are hints to the driver what it is expected to do. If
320the requested fine-grained filtering for incoming packets is not
321supported, the driver may time stamp more than just the requested types
322of packets.
323
324A driver which supports hardware time stamping shall update the struct
325with the actual, possibly more permissive configuration. If the
326requested packets cannot be time stamped, then nothing should be
327changed and ERANGE shall be returned (in contrast to EINVAL, which
328indicates that SIOCSHWTSTAMP is not supported at all).
329
330Only a processes with admin rights may change the configuration. User
331space is responsible to ensure that multiple processes don't interfere
332with each other and that the settings are reset.
333
Ben Hutchingsfd468c72013-11-14 01:19:29 +0000334Any process can read the actual configuration by passing this
335structure to ioctl(SIOCGHWTSTAMP) in the same way. However, this has
336not been implemented in all drivers.
337
Patrick Ohlycb9eff02009-02-12 05:03:36 +0000338/* possible values for hwtstamp_config->tx_type */
339enum {
340 /*
341 * no outgoing packet will need hardware time stamping;
342 * should a packet arrive which asks for it, no hardware
343 * time stamping will be done
344 */
345 HWTSTAMP_TX_OFF,
346
347 /*
348 * enables hardware time stamping for outgoing packets;
349 * the sender of the packet decides which are to be
350 * time stamped by setting SOF_TIMESTAMPING_TX_SOFTWARE
351 * before sending the packet
352 */
353 HWTSTAMP_TX_ON,
354};
355
356/* possible values for hwtstamp_config->rx_filter */
357enum {
358 /* time stamp no incoming packet at all */
359 HWTSTAMP_FILTER_NONE,
360
361 /* time stamp any incoming packet */
362 HWTSTAMP_FILTER_ALL,
363
Patrick Loschmidt69298692010-04-07 21:52:07 -0700364 /* return value: time stamp all packets requested plus some others */
365 HWTSTAMP_FILTER_SOME,
Patrick Ohlycb9eff02009-02-12 05:03:36 +0000366
367 /* PTP v1, UDP, any kind of event packet */
368 HWTSTAMP_FILTER_PTP_V1_L4_EVENT,
369
Patrick Loschmidt69298692010-04-07 21:52:07 -0700370 /* for the complete list of values, please check
371 * the include file /include/linux/net_tstamp.h
372 */
Patrick Ohlycb9eff02009-02-12 05:03:36 +0000373};
374
Willem de Bruijn8fe2f762014-08-31 21:27:47 -04003753.1 Hardware Timestamping Implementation: Device Drivers
Patrick Ohlycb9eff02009-02-12 05:03:36 +0000376
377A driver which supports hardware time stamping must support the
Patrick Loschmidt69298692010-04-07 21:52:07 -0700378SIOCSHWTSTAMP ioctl and update the supplied struct hwtstamp_config with
Ben Hutchingsfd468c72013-11-14 01:19:29 +0000379the actual values as described in the section on SIOCSHWTSTAMP. It
380should also support SIOCGHWTSTAMP.
Patrick Loschmidt69298692010-04-07 21:52:07 -0700381
382Time stamps for received packets must be stored in the skb. To get a pointer
383to the shared time stamp structure of the skb call skb_hwtstamps(). Then
384set the time stamps in the structure:
385
386struct skb_shared_hwtstamps {
387 /* hardware time stamp transformed into duration
388 * since arbitrary point in time
389 */
390 ktime_t hwtstamp;
Patrick Loschmidt69298692010-04-07 21:52:07 -0700391};
Patrick Ohlycb9eff02009-02-12 05:03:36 +0000392
393Time stamps for outgoing packets are to be generated as follows:
Oliver Hartkopp2244d072010-08-17 08:59:14 +0000394- In hard_start_xmit(), check if (skb_shinfo(skb)->tx_flags & SKBTX_HW_TSTAMP)
395 is set no-zero. If yes, then the driver is expected to do hardware time
396 stamping.
Patrick Ohlycb9eff02009-02-12 05:03:36 +0000397- If this is possible for the skb and requested, then declare
Oliver Hartkopp2244d072010-08-17 08:59:14 +0000398 that the driver is doing the time stamping by setting the flag
399 SKBTX_IN_PROGRESS in skb_shinfo(skb)->tx_flags , e.g. with
400
401 skb_shinfo(skb)->tx_flags |= SKBTX_IN_PROGRESS;
402
403 You might want to keep a pointer to the associated skb for the next step
404 and not free the skb. A driver not supporting hardware time stamping doesn't
405 do that. A driver must never touch sk_buff::tstamp! It is used to store
406 software generated time stamps by the network subsystem.
Jakub Kicinski59cb89e2014-03-16 20:32:48 +0100407- Driver should call skb_tx_timestamp() as close to passing sk_buff to hardware
408 as possible. skb_tx_timestamp() provides a software time stamp if requested
409 and hardware timestamping is not possible (SKBTX_IN_PROGRESS not set).
Patrick Ohlycb9eff02009-02-12 05:03:36 +0000410- As soon as the driver has sent the packet and/or obtained a
411 hardware time stamp for it, it passes the time stamp back by
412 calling skb_hwtstamp_tx() with the original skb, the raw
Patrick Loschmidt69298692010-04-07 21:52:07 -0700413 hardware time stamp. skb_hwtstamp_tx() clones the original skb and
414 adds the timestamps, therefore the original skb has to be freed now.
415 If obtaining the hardware time stamp somehow fails, then the driver
416 should not fall back to software time stamping. The rationale is that
417 this would occur at a later time in the processing pipeline than other
418 software time stamping and therefore could lead to unexpected deltas
419 between time stamps.