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Diffstat (limited to 'Documentation/networking/timestamping.rst')
| -rw-r--r-- | Documentation/networking/timestamping.rst | 32 |
1 files changed, 31 insertions, 1 deletions
diff --git a/Documentation/networking/timestamping.rst b/Documentation/networking/timestamping.rst index be4eb124205709..f17c01834a1230 100644 --- a/Documentation/networking/timestamping.rst +++ b/Documentation/networking/timestamping.rst @@ -179,7 +179,8 @@ SOF_TIMESTAMPING_OPT_ID: identifier and returns that along with the timestamp. The identifier is derived from a per-socket u32 counter (that wraps). For datagram sockets, the counter increments with each sent packet. For stream - sockets, it increments with every byte. + sockets, it increments with every byte. For stream sockets, also set + SOF_TIMESTAMPING_OPT_ID_TCP, see the section below. The counter starts at zero. It is initialized the first time that the socket option is enabled. It is reset each time the option is @@ -192,6 +193,35 @@ SOF_TIMESTAMPING_OPT_ID: among all possibly concurrently outstanding timestamp requests for that socket. +SOF_TIMESTAMPING_OPT_ID_TCP: + Pass this modifier along with SOF_TIMESTAMPING_OPT_ID for new TCP + timestamping applications. SOF_TIMESTAMPING_OPT_ID defines how the + counter increments for stream sockets, but its starting point is + not entirely trivial. This option fixes that. + + For stream sockets, if SOF_TIMESTAMPING_OPT_ID is set, this should + always be set too. On datagram sockets the option has no effect. + + A reasonable expectation is that the counter is reset to zero with + the system call, so that a subsequent write() of N bytes generates + a timestamp with counter N-1. SOF_TIMESTAMPING_OPT_ID_TCP + implements this behavior under all conditions. + + SOF_TIMESTAMPING_OPT_ID without modifier often reports the same, + especially when the socket option is set when no data is in + transmission. If data is being transmitted, it may be off by the + length of the output queue (SIOCOUTQ). + + The difference is due to being based on snd_una versus write_seq. + snd_una is the offset in the stream acknowledged by the peer. This + depends on factors outside of process control, such as network RTT. + write_seq is the last byte written by the process. This offset is + not affected by external inputs. + + The difference is subtle and unlikely to be noticed when configured + at initial socket creation, when no data is queued or sent. But + SOF_TIMESTAMPING_OPT_ID_TCP behavior is more robust regardless of + when the socket option is set. SOF_TIMESTAMPING_OPT_CMSG: Support recv() cmsg for all timestamped packets. Control messages |