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| <section id="ntpclient-ntp-client"> |
| <h1><code class="docutils literal notranslate"><span class="pre">ntpclient</span></code> NTP client<a class="headerlink" href="#ntpclient-ntp-client" title="Permalink to this heading"></a></h1> |
| <p>The NTP (Network Time Protocol) client is a network utility that synchronizes |
| the system clock with time servers over the Internet. This implementation |
| provides a minimal but functional NTP client for NuttX.</p> |
| <section id="what-is-ntp"> |
| <h2>What is NTP?<a class="headerlink" href="#what-is-ntp" title="Permalink to this heading"></a></h2> |
| <p>The Network Time Protocol (NTP) is a networking protocol designed to |
| synchronize clocks of computer systems over packet-switched, variable-latency |
| data networks. NTP is one of the oldest Internet protocols still in use, |
| originally designed by David L. Mills of the University of Delaware.</p> |
| <p>Key features of NTP:</p> |
| <ul class="simple"> |
| <li><p><strong>High Precision</strong>: NTP can achieve sub-millisecond accuracy on local area |
| networks and typically 10-100 millisecond accuracy over the Internet</p></li> |
| <li><p><strong>Robust Algorithm</strong>: Uses sophisticated algorithms to filter out network |
| jitter and select the best time sources</p></li> |
| <li><p><strong>Hierarchical Structure</strong>: Uses a stratum system where stratum 0 devices |
| are atomic clocks, stratum 1 servers sync with stratum 0, and so on</p></li> |
| <li><p><strong>Fault Tolerance</strong>: Can handle multiple time sources and automatically |
| switch between them</p></li> |
| </ul> |
| </section> |
| <section id="ntp-protocol-overview"> |
| <h2>NTP Protocol Overview<a class="headerlink" href="#ntp-protocol-overview" title="Permalink to this heading"></a></h2> |
| <p>NTP uses UDP port 123 and follows a client-server model. The protocol |
| exchanges timestamps to calculate:</p> |
| <ul class="simple"> |
| <li><p><strong>Offset</strong>: The difference between the client’s clock and the server’s clock</p></li> |
| <li><p><strong>Delay</strong>: The round-trip network delay</p></li> |
| <li><p><strong>Dispersion</strong>: The maximum error due to clock frequency tolerance</p></li> |
| </ul> |
| <p>The NTP packet format (version 3) includes:</p> |
| <ul class="simple"> |
| <li><p><strong>Leap Indicator</strong>: Warns of an impending leap second</p></li> |
| <li><p><strong>Version Number</strong>: NTP version (3 in this implementation)</p></li> |
| <li><p><strong>Mode</strong>: Client, server, broadcast, etc.</p></li> |
| <li><p><strong>Stratum</strong>: Clock level (0-15)</p></li> |
| <li><p><strong>Poll Interval</strong>: Maximum interval between successive messages</p></li> |
| <li><p><strong>Precision</strong>: Clock precision</p></li> |
| <li><p><strong>Root Delay/Dispersion</strong>: Total delay and dispersion to the reference clock</p></li> |
| <li><p><strong>Reference Identifier</strong>: Identifies the reference source</p></li> |
| <li><p><strong>Reference Timestamp</strong>: Time when the system clock was last set</p></li> |
| <li><p><strong>Originate Timestamp</strong>: Time when the request departed the client</p></li> |
| <li><p><strong>Receive Timestamp</strong>: Time when the request arrived at the server</p></li> |
| <li><p><strong>Transmit Timestamp</strong>: Time when the reply departed the server</p></li> |
| </ul> |
| </section> |
| <section id="implementation-details"> |
| <h2>Implementation Details<a class="headerlink" href="#implementation-details" title="Permalink to this heading"></a></h2> |
| <p>The NuttX NTP client implementation consists of several key components:</p> |
| <section id="source-code-structure"> |
| <h3>Source Code Structure<a class="headerlink" href="#source-code-structure" title="Permalink to this heading"></a></h3> |
| <p><strong>ntpclient.c</strong> - Main implementation file containing:</p> |
| <ul class="simple"> |
| <li><p><strong>Daemon Management</strong>: Functions to start, stop, and manage the NTP daemon</p></li> |
| <li><p><strong>Time Synchronization</strong>: Core algorithms for calculating clock offset and delay</p></li> |
| <li><p><strong>Network Communication</strong>: UDP socket handling and NTP packet exchange</p></li> |
| <li><p><strong>Sample Collection</strong>: Gathering multiple time samples for statistical filtering</p></li> |
| </ul> |
| <p><strong>ntpv3.h</strong> - NTP version 3 packet format definitions:</p> |
| <ul class="simple"> |
| <li><p><strong>ntp_datagram_s</strong>: Complete NTP packet structure</p></li> |
| <li><p><strong>ntp_timestamp_s</strong>: 64-bit NTP timestamp format</p></li> |
| <li><p><strong>Protocol Constants</strong>: NTP version, modes, and stratum definitions</p></li> |
| </ul> |
| </section> |
| <section id="key-functions"> |
| <h3>Key Functions<a class="headerlink" href="#key-functions" title="Permalink to this heading"></a></h3> |
| <ul class="simple"> |
| <li><p><strong>ntpc_start_with_list()</strong>: Starts the NTP daemon with a list of servers</p></li> |
| <li><p><strong>ntpc_start()</strong>: Starts the NTP daemon with default configuration</p></li> |
| <li><p><strong>ntpc_stop()</strong>: Stops the running NTP daemon</p></li> |
| <li><p><strong>ntpc_status()</strong>: Retrieves current synchronization status and samples</p></li> |
| <li><p><strong>ntpc_daemon()</strong>: Main daemon loop that:</p> |
| <ul> |
| <li><p>Connects to configured NTP servers</p></li> |
| <li><p>Sends NTP requests and processes responses</p></li> |
| <li><p>Calculates clock offset and delay</p></li> |
| <li><p>Applies time corrections to the system clock</p></li> |
| <li><p>Continues polling at configured intervals</p></li> |
| </ul> |
| </li> |
| <li><p><strong>ntpc_get_ntp_sample()</strong>: Performs a single NTP transaction:</p> |
| <ul> |
| <li><p>Creates UDP socket to NTP server</p></li> |
| <li><p>Sends NTP request packet with current timestamp</p></li> |
| <li><p>Receives and validates NTP response</p></li> |
| <li><p>Calculates offset and delay using NTP algorithms</p></li> |
| </ul> |
| </li> |
| <li><p><strong>ntpc_calculate_offset()</strong>: Implements the NTP clock filter algorithm</p> |
| <ul> |
| <li><p>Uses four timestamps, calculates offset and delay</p></li> |
| <li><p>Applies statistical filtering to reduce jitter</p></li> |
| </ul> |
| </li> |
| <li><p><strong>ntpc_settime()</strong>: Applies time correction to system clock:</p> |
| <ul> |
| <li><p>Uses calculated offset to adjust system time</p></li> |
| <li><p>Handles both positive and negative time adjustments</p></li> |
| <li><p>Maintains monotonic clock consistency</p></li> |
| </ul> |
| </li> |
| </ul> |
| </section> |
| </section> |
| <section id="configuration-options"> |
| <h2>Configuration Options<a class="headerlink" href="#configuration-options" title="Permalink to this heading"></a></h2> |
| <p>The NTP client can be configured through Kconfig options:</p> |
| <ul class="simple"> |
| <li><p><strong>CONFIG_NETUTILS_NTPCLIENT_SERVER</strong>: List of NTP server hostnames</p></li> |
| <li><p><strong>CONFIG_NETUTILS_NTPCLIENT_PORTNO</strong>: NTP server port (default: 123)</p></li> |
| <li><p><strong>CONFIG_NETUTILS_NTPCLIENT_STACKSIZE</strong>: Daemon task stack size</p></li> |
| <li><p><strong>CONFIG_NETUTILS_NTPCLIENT_SERVERPRIO</strong>: Daemon task priority</p></li> |
| <li><p><strong>CONFIG_NETUTILS_NTPCLIENT_STAY_ON</strong>: Keep polling continuously</p></li> |
| <li><p><strong>CONFIG_NETUTILS_NTPCLIENT_POLLDELAYSEC</strong>: Polling interval in seconds</p></li> |
| <li><p><strong>CONFIG_NETUTILS_NTPCLIENT_NUM_SAMPLES</strong>: Number of samples for filtering</p></li> |
| <li><p><strong>CONFIG_NETUTILS_NTPCLIENT_TIMEOUT_MS</strong>: Network timeout in milliseconds</p></li> |
| </ul> |
| </section> |
| <section id="usage"> |
| <h2>Usage<a class="headerlink" href="#usage" title="Permalink to this heading"></a></h2> |
| <p>The NTP client is typically used through the system commands:</p> |
| <div class="admonition note"> |
| <p class="admonition-title">Note</p> |
| <p>The NTP client functionality requires enabling the <code class="code docutils literal notranslate"><span class="pre">SYSTEM_NTPC</span></code> option in your configuration. |
| Make sure to select this option in menuconfig or your Kconfig fragment before building.</p> |
| </div> |
| <ul class="simple"> |
| <li><p><strong>ntpcstart</strong>: Start the NTP daemon</p></li> |
| <li><p><strong>ntpcstop</strong>: Stop the NTP daemon</p></li> |
| <li><p><strong>ntpcstatus</strong>: Display synchronization status</p></li> |
| </ul> |
| <p>Example workflow:</p> |
| <ol class="arabic simple"> |
| <li><p>Configure network connectivity</p></li> |
| <li><p>Start NTP client: <code class="docutils literal notranslate"><span class="pre">ntpcstart</span></code></p></li> |
| <li><p>Check status: <code class="docutils literal notranslate"><span class="pre">ntpcstatus</span></code></p></li> |
| <li><p>Verify time: <code class="docutils literal notranslate"><span class="pre">date</span></code> command</p></li> |
| <li><p>Stop when needed: <code class="docutils literal notranslate"><span class="pre">ntpcstop</span></code></p></li> |
| </ol> |
| <p>The client will automatically: |
| - Connect to configured NTP servers |
| - Exchange time information |
| - Calculate and apply clock corrections |
| - Continue periodic synchronization</p> |
| </section> |
| <section id="limitations"> |
| <h2>Limitations<a class="headerlink" href="#limitations" title="Permalink to this heading"></a></h2> |
| <p>This is a minimal NTP client implementation with some limitations:</p> |
| <ul class="simple"> |
| <li><p><strong>No Authentication</strong>: Does not support NTP authentication (MD5/SHA1)</p></li> |
| <li><p><strong>Basic Filtering</strong>: Uses simple statistical filtering, not full NTP algorithms</p></li> |
| <li><p><strong>Single Reference</strong>: Does not implement full NTP reference clock selection</p></li> |
| <li><p><strong>No Leap Seconds</strong>: Does not handle leap second announcements</p></li> |
| <li><p><strong>Limited Error Handling</strong>: Basic error recovery and retry mechanisms</p></li> |
| </ul> |
| <p>Despite these limitations, the implementation provides sufficient accuracy |
| for most embedded applications requiring network time synchronization.</p> |
| </section> |
| <section id="dependencies"> |
| <h2>Dependencies<a class="headerlink" href="#dependencies" title="Permalink to this heading"></a></h2> |
| <p>The NTP client requires:</p> |
| <ul class="simple"> |
| <li><p><strong>CONFIG_NET</strong>: Network support</p></li> |
| <li><p><strong>CONFIG_NET_UDP</strong>: UDP protocol support</p></li> |
| <li><p><strong>CONFIG_NET_SOCKOPTS</strong>: Socket options support</p></li> |
| <li><p><strong>CONFIG_LIBC_NETDB</strong>: DNS resolution (recommended)</p></li> |
| <li><p><strong>CONFIG_HAVE_LONG_LONG</strong>: 64-bit integer support</p></li> |
| </ul> |
| <p>For best results, ensure: |
| - Stable network connectivity |
| - Access to reliable NTP servers |
| - Sufficient system resources for daemon operation</p> |
| </section> |
| </section> |
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