12b15cb3dSCy Schubert<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> 22b15cb3dSCy Schubert<html> 32b15cb3dSCy Schubert<head> 42b15cb3dSCy Schubert<meta http-equiv="content-type" content="text/html;charset=iso-8859-1"> 52b15cb3dSCy Schubert<meta name="generator" content="HTML Tidy, see www.w3.org"> 62b15cb3dSCy Schubert<title>NTP Interleaved Modes</title> 72b15cb3dSCy Schubert<link href="scripts/style.css" type="text/css" rel="stylesheet"> 82b15cb3dSCy Schubert</head> 92b15cb3dSCy Schubert<body> 102b15cb3dSCy Schubert<h3>NTP Interleaved Modes </h3> 112b15cb3dSCy Schubert<img src="pic/pogo4.gif" alt="gif" align="left"><a href="http://www.eecis.udel.edu/%7emills/pictures.html">from <i>Pogo</i>, Walt Kelly</a> 122b15cb3dSCy Schubert<p>You need a little magic.</p> 132b15cb3dSCy Schubert<p>Last update: 14*4990d495SXin LI <!-- #BeginDate format:En2m -->6-Feb-2016 07:17<!-- #EndDate --> 152b15cb3dSCy Schubert UTC</p> 162b15cb3dSCy Schubert<br clear="left"> 172b15cb3dSCy Schubert<hr> 18*4990d495SXin LI<p>In the protocol described in the NTP specification and reference implementation up to now, the transmit timestamp, which is captured before the message digest is computed and the packet queued for output, is properly called as a <em>softstamp</em>. The receive timestamp, which is captured after the input driver interrupt routine and before the packet is queued for input, is properly called a <em>drivestamp</em>. For enhanced accuracy it is desirable to capture the transmit timestamp as close to the wire as possible; for example, after the output driver interrupt routine.</p> 192b15cb3dSCy Schubert<p> In other words, we would like to replace the transmit softstamp with a drivestamp, but the problem is the transmit drivestamp is available only after the packet has been sent. A solution for this problem is the two-step or interleaved protocol described on this page and included in the the current reference implementation. In interleaved modes the transmit drivestamp for one packet is actually carried in the immediately following packet. The trick, however, is to implement the interleaved protocol without changing the NTP packet header format, without compromising backwards compatibility and without compromising the error recovery properties.</p> 20*4990d495SXin LI<p> The reference implementation captures a softstamp before the message digest routine and a drivestamp after the output interrupt routine. In this design the latter timestamp can be considered most accurate, as it avoids the various queuing and transmission latencies. The difference between the two timestamps, which is called the interleaved or output delay, varies from 16 μs (microseconds) for a dual-core Pentium running FreeBSD 6.1 to 1100 μs (microseconds) for a Sun Blade 1500 running Solaris 10.</p> 212b15cb3dSCy Schubert<p>Interleaved mode can be used only in NTP symmetric and broadcast modes. 222b15cb3dSCy Schubert It is activated by the <tt>xleave</tt> option with the <tt>peer</tt> or <tt>broadcast</tt> configuration 232b15cb3dSCy Schubertcommands. A broadcast server configured for interleaved mode is transparent to ordinary broadcast clients, so both ordinary and interleaved broadcast clients can use the same packets. An interleaved symmetric active peer automatically switches to ordinary symmetric mode if the other peer is not capable of operation in interleaved mode.</p> 242b15cb3dSCy Schubert<p>As demonstrated in the white paper <a href="http://www.eecis.udel.edu/~mills/onwire.html">Analysis and Simulation of the NTP On-Wire Protocols</a>, the interleaved modes have the same resistance to lost packets, duplicate packets, packets crossed in flight and protocol restarts as the ordinary modes. An application of the interleaved symmetric mode in space missions is presented in the white paper <a href="http://www.eecis.udel.edu/~mills/proximity.html">Time Synchronization for Space Data Links</a>.</p> 252b15cb3dSCy Schubert<hr> 262b15cb3dSCy Schubert<div align="center"> <img src="pic/pogo1a.gif" alt="gif"> </div> 272b15cb3dSCy Schubert<br> 282b15cb3dSCy Schubert<script type="text/javascript" language="javascript" src="scripts/footer.txt"></script> 292b15cb3dSCy Schubert</body> 302b15cb3dSCy Schubert</html> 31
