Network time representation in the network stack
API Reference
- group net_time
Defines
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NET_TIME_MAX
The largest positive time value that can be represented by net_time_t.
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NET_TIME_MIN
The smallest negative time value that can be represented by net_time_t.
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NET_TIME_SEC_MAX
The largest positive number of seconds that can be safely represented by net_time_t.
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NET_TIME_SEC_MIN
The smallest negative number of seconds that can be safely represented by net_time_t.
Typedefs
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typedef int64_t net_time_t
Any occurrence of net_time_t specifies a concept of nanosecond resolution scalar time span, future (positive) or past (negative) relative time or absolute timestamp referred to some local network uptime reference clock that does not wrap during uptime and is - in a certain, well-defined sense - common to all local network interfaces, sometimes even to remote interfaces on the same network.
This type is EXPERIMENTAL. Usage is currently restricted to representation of time within the network subsystem.
Timed network protocols (PTP, TDMA, …) usually require several local or remote interfaces to share a common notion of elapsed time within well-defined tolerances. Network uptime therefore differs from time represented by a single hardware counter peripheral in that it will need to be represented in several distinct hardware peripherals with different frequencies, accuracy and precision. To co-operate, these hardware counters will have to be “syntonized” or “disciplined” (i.e. frequency and phase locked) with respect to a common local or remote network reference time signal. Be aware that while syntonized clocks share the same frequency and phase, they do not usually share the same epoch (zero-point).
This also explains why network time, if represented as a cycle value of some specific hardware counter, will never be “precise” but only can be “good
enough” with respect to the tolerances (resolution, drift, jitter) required by a given network protocol. All counter peripherals involved in a timed network protocol must comply with these tolerances.
Please use specific cycle/tick counter values rather than net_time_t whenever possible especially when referring to the kernel system clock or values of any single counter peripheral.
net_time_t cannot represent general clocks referred to an arbitrary epoch as it only covers roughly +/- ~290 years. It also cannot be used to represent time according to a more complex timescale (e.g. including leap seconds, time adjustments, complex calendars or time zones). In these cases you may use timespec (C11, POSIX.1-2001), timeval (POSIX.1-2001) or broken down time as in tm (C90). The advantage of net_time_t over these structured time representations is lower memory footprint, faster and simpler scalar arithmetic and easier conversion from/to low-level hardware counter values. Also net_time_t can be used in the network stack as well as in applications while POSIX concepts cannot. Converting net_time_t from/to structured time representations is possible in a limited way but - except for timespec - requires concepts that must be implemented by higher-level APIs. Utility functions converting from/to timespec will be provided as part of the net_time_t API as and when needed.
If you want to represent more coarse grained scalar time in network applications, use time_t (C99, POSIX.1-2001) which is specified to represent seconds or suseconds_t (POSIX.1-2001) for microsecond resolution. Kernel k_ticks_t and cycles (both specific to Zephyr) have an unspecified resolution but are useful to represent kernel timer values and implement high resolution spinning.
If you need even finer grained time resolution, you may want to look at (g)PTP concepts, see net_ptp_extended_time.
The reason why we don’t use int64_t directly to represent scalar nanosecond resolution times in the network stack is that it has been shown in the past that fields using generic type will often not be used correctly (e.g. with the wrong resolution or to represent underspecified concepts of time with unclear syntonization semantics).
Any API that exposes or consumes net_time_t values SHALL ensure that it maintains the specified contract including all protocol specific tolerances and therefore clients can rely on common semantics of this type. This makes times coming from different hardware peripherals and even from different network nodes comparable within well-defined limits and therefore net_time_t is the ideal intermediate building block for timed network protocols.
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NET_TIME_MAX