Update BUILDFORAP2.md

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+++ b/BUILDFORAP2.md
@@ -5,8 +5,9 @@ Experimental Build Instructions for AirPlay 2
 * Initially, at least, leave the settings in the configuration file at default except as noted below.
 * At the time of writing, May 2, 2021, everything is on the latest version of the software -- macOS 11..3, iOS 14.5, Raspberry Pi OS 5.10.17-v7l+ (Buster), Ubuntu 20.04 -- fully updated.
 * At present, Shairport Sync will compile *but not work* on Raspbian Stretch (thanks JD Smith).
-* AirPlay 2 uses a timing system based on the Precision Timing Protocol (PTP). Shairport Sync relies on a program called `nqptp` ("Not Quite PTP") to monitor PTP signals. This program uses ports 319 and 320 and replaces any PTP service you have on the computer.
-  In addition, `nqptp` must run with `root` privileges. (FYI, most computers do not have a PTP clock running -- instead, they may use an NTP service to keep the system clock synchronised with world time.)
+* AirPlay 2 seems to use a timing system based on the [IEEE-1588 Precision Timing Protocol (PTP)](https://standards.ieee.org/standard/1588-2008.html). Shairport Sync relies on a program called `nqptp` ("Not Quite PTP") to monitor PTP signals. This program uses ports 319 and 320 and replaces any PTP service you have on the computer.
+  In addition, `nqptp` must run with `root` privileges.  
+ (FYI, most computers do not have a PTP clock running -- instead, they may use an NTP service to keep the system clock synchronised with world time.)
 * When Shairport Sync is updated, you should check and update `nqptp` *before* building the update.
 
 Installation Instructions