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function [ out ] = GetNTP( com, arg, dbg )
% GetNTP - Query time from NTP server
%
% socket = GetNTP( 'open', 'hostname' )
% pkg = GetNTP( 'read', socket[, dbg ] )
% GetNTP( 'close', socket )
%
% GetNTP is a very simple pnet based NTP client to query time from an NTP
% server over the network.
%
% A pnet socket has to be created with the open command first. The read
% command returns a structure with the fields timestamps, delay and rtt.
% pkg.timestamps is a vector of four timestamps reflecting client send
% time, server receive time, server transmit time, and client receive time.
% pkg.delay is the network delay as specified by the NTP specification.
% pkg.rtt is the round-trip-time.
%
% If dbg is true the NTP header is decoded completely and added to the
% returned structure including the NTP server's reference timestamp for
% debugging. Additionally server receive and transmit timestamps are
% printed to the console in human readable format.
%
% Note that any argument checking and error handling was omitted
% intentionally to improve timing with NetStation synchronization.
%
% Reference: Mills, D. L. (2006). Network Time Protocol Version 4
% Reference and Implementation Guide. Retrieved May 21, 2017 from
% https://www.eecis.udel.edu/~mills/database/reports/ntp4/ntp4.pdf
%
% Author: Andreas Widmann, University of Leipzig, 2017
% History:
% 2017-05-25 AW Written.
% 2017-07-19 AW Reformatted help text.
% Copyright (C) 2017 Andreas Widmann, University of Leipzig, widmann@uni-leipzig.de
%
% MIT license:
%
% Permission is hereby granted, free of charge, to any person obtaining a
% copy of this software and associated documentation files (the
% "Software"), to deal in the Software without restriction, including
% without limitation the rights to use, copy, modify, merge, publish,
% distribute, sublicense, and/or sell copies of the Software, and to permit
% persons to whom the Software is furnished to do so, subject to the
% following conditions:
%
% The above copyright notice and this permission notice shall be included
% in all copies or substantial portions of the Software.
%
% THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
% OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
% MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN
% NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
% DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
% OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE
% USE OR OTHER DEALINGS IN THE SOFTWARE.
switch com
case 'open'
out = pnet( 'udpsocket', 3333 );
pnet( out, 'setreadtimeout', 1 );
pnet( out, 'udpconnect', arg, 123 );
case 'close'
pnet( arg, 'close' );
case 'read'
% Timing sensitive
msg = uint8( [ 27 zeros(1, 47) ] );
pnet( arg, 'write', msg);
out.timestamps( 1 ) = GetSecs;
pnet( arg, 'writepacket' );
pkgsize = pnet( arg, 'readpacket' );
out.timestamps( 4 ) = GetSecs;
% Timing insensitive
if pkgsize < 48
error( 'No NTP package received.' );
end
% Header
lvm = pnet( arg, 'read', 1, 'uint8');
% Decode header only for potential KoD (LI == 3) package or on request
if lvm > 191 || ( nargin > 2 && dbg )
lvm = dec2bin( lvm, 8 );
out.LI = bin2dec( lvm( 1:2 ) );
out.VN = bin2dec( lvm( 3:5 ) );
out.Mode = bin2dec( lvm( 6:8 ) );
out.stratum = pnet( arg, 'read', 1, 'uint8');
out.poll = pnet( arg, 'read', 1, 'int8');
out.prec = pnet( arg, 'read', 1, 'int8');
% Root delay and dispersion; signed fixed point; decoding to be implemented, possibly
out.root = pnet( arg, 'read', 2, 'uint32');
id = pnet( arg, 'read', 4, 'uint8');
if out.stratum == 0 || out.stratum == 1
out.id = char( id );
else
out.id = sprintf( '%d.%d.%d.%d', id );
end
if out.LI == 3 && out.stratum == 0
error( 'KoD package received. Reason: %s.', out.id );
end
else
pnet( arg, 'read', 15, 'uint8');
end
% Timestamps
timestamps = double( pnet( arg, 'read', 8, 'uint32') );
timestamps = timestamps( :, 1:2:7 ) + timestamps( :, 2:2:8 ) / 2 ^ 32;
% Receive and transmit timestamps
out.timestamps( 2:3 ) = timestamps( 3:4 );
out.delay = ( out.timestamps( 4 ) - out.timestamps( 1 ) ) - ( out.timestamps( 3 ) - out.timestamps( 2 ) );
out.rtt = out.timestamps( 4 ) - out.timestamps( 1 );
if nargin > 2 && dbg
% Reference timestamp
out.reference = timestamps( 1 );
for iTimestamp = 2:3
% From modified NetStation.m by Justin and Mario:
matlabDateNum = datenum( [ 1900 1 1 0 0 out.timestamps( iTimestamp ) ] );
datestr( matlabDateNum, 'yyyy-mm-dd HH:MM:SS.FFF' )
end
end
end
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