Long time lurker here. Hefty read follows - apologies. Can't blame anyone for TL;DR.
On 11 November 2015 at 02:07, Alexander List <[email protected]> wrote: > > On Wednesday, November 11, 2015 03:15 AM, Don Latham wrote: > Indeed. HFT firms have been working on their own "advanced" timekeeping > using PTP and GPS and what not, but it seems the MIFID-II directive and > equivalents in other markets force the entire industry to up their > systems from NTP to PTP... It's all nice and simple when you have a GPSDO in your lab and nice, steady 10 MHz, and that's that. So far MIFID-II sets a precedent. The tightest US requirement is currently 50 ms to UTC (time error, not timestamp granularity), but the US is bound to follow EU at some point. PTP has been pretty much a standard for bigger trading firms for 5+ years. I first saw it in 2007. Until now, it has only been used by traders to gain advantage in establishing data arrival times, and exchanges used it to maintain timestamp consistency between different components of a trading system. Now it becomes a legal requirement for both. They were all screaming and shouting, but the EU settled on 100 us from UTC (only for firms with low latency systems). Initially they suggested 1 us! NTP is on the way out, but it's not the protocol's fault, it's the implementations' fault. Long poll intervals, small amount of samples and no hardware timestamping. PTP resolves all of those in its very core definitions - and implementations follow. Your clock may be stable, but if you're polling every 16 seconds and packet delay variation of the network stack outweighs your path delays, and you have no accurate transmission timestamp, no Allan Intercept will help. Suboptimal measurement, and you have crap in, crap out. Hardware can do small nanoseconds, the problem is getting the time to the operating system where the application lives. People have to realise that they are not driving actual crystals, just software models describing OS clocks, that only use assumed known frequency as a starting point. It is advanced timekeeping, considering scale, network conditions, monitoring, holdover, mitigating GPS vulnerabilities, complex GPS distribution into data centre bunkers, calibration, etc. OK, it's not exactly China Mobile with 700,000 base stations using PTP, but it's pretty advanced in my humble opinion. With software-only PTP, with good filtering you get low microseconds, albeit somewhat noisy. With a PTP timestamping NIC you can get sub-150 ns between OS and NIC, and the NIC can definitely get sub-500 ns to reference, assuming little or no delay asymmetries, or use of PTP transparent clocks across the whole path. 100 us is easy if you architect it right, the Finance community just has a lot to learn. For many engineers from the "IP generation", phase and frequency concepts are completely foreign, they mix definitions and generally get confused. NTP you could just leave running, syncing to something, and it gave you a number. With PTP, suddenly every small detail affecting phase offset has to be understood and accounted for, because your target is smaller. It also works the other way round: explaining the problems of time sync in enterprise networks to frequency / telecom people (vendors) is a tough task. I have been doing this for a few years now, and today they mostly get it. Now they have to explain "their" stuff back to the users and everything will be fine (ha-ha). Before the legal requirement is cut down to 10 us, hardware timestamping will be available on every NIC (mostly is today for new systems), and sync will be much less challenging. Solving the "Last two inch problem" ( (c) John Eidson), that is building computer systems that are time-aware and time- correct by design, will be the last task on the road to ubiquitous sync. Intel have already laid the foundations allowing to relate the TSC counter to PTP time, and there is the PTM functionality in PCI Express 3.x+ spec (basically PTP over PCIE). Finally, there is Synchronous Ethernet, providing tight frequency lock between network ports - add PTP for phase and time, and you've got White Rabbit (picosecond accuracy). The funniest thing about MIFID-II, or rather ESMA, the regulatory body, is that they question the traceability of GPS time to UTC! Perhaps because the EU has Galileo. Apparently. [disclaimer: personal opinions only] Thanks, Wojciech -- - Wojciech Owczarek _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
