[ntp:questions] IEEE 1588 (PTP) at the nanosecond level?

Terje Mathisen terje.mathisen at tmsw.no
Thu Mar 20 08:01:01 UTC 2014


Hal Murray wrote:
> In article <53298269.6000500 at SystematicSw.ab.ca>,
>   Brian Inglis <Brian.Inglis at SystematicSw.ab.ca> writes:
>
>> Something like a Thunderbolt GPSDO feeding data, PPS, and 10MHz clock to a
>> chip executing instructions at some clock multiple and handling interrupts
>> in a deterministic time to feed the PTP GM?
>
> Why use interrupts?  Suppose you put a clock and ethernet in a FPGA.
> How much extra logic does it take to process NTP packets?  Everything
> but the simple case gets punted off to a real CPU.

Having PTP hw support in the network card would seem to be sufficient:

Incoming packets are timestamped by the network card, so when the host 
OS retrieves them, both the packet timestamp and the current PTP time 
can be queried, giving an exact offset from the current OS time.

I.e. this gives you sub-us OS-level timestamping of incoming packets.

If you then also have the capability to query the network card/os queues 
and only pass off outgoing packets when the transmit buffer is empty, 
you should be able to calculate and enter into the response packet an 
outgoing timestamp which is very accurate.

If you want even better performance you'll have to move to full PTP, 
including hw support in all intermediate switches/routers.

Terje

-- 
- <Terje.Mathisen at tmsw.no>
"almost all programming can be viewed as an exercise in caching"



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