include IPv4 Option padding and type packing/unpacking
This commit is contained in:
brent saner
320
README.html
320
README.html
@@ -559,7 +559,7 @@ pre.rouge .gs {
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<div class="details">
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<span id="author" class="author">Brent Saner</span><br>
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<span id="email" class="email"><a href="mailto:bts@square-r00t.net">bts@square-r00t.net</a></span><br>
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<span id="revdate">Last rendered 2025-08-30 17:05:09 -0400</span>
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<span id="revdate">Last rendered 2025-08-31 00:29:24 -0400</span>
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</div>
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<div id="toc" class="toc2">
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<div id="toctitle">Table of Contents</div>
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@@ -625,7 +625,19 @@ pre.rouge .gs {
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<li><a href="#xtra_bitpacked_vihl">8.1.1. IP Version, IHL (IPv4)</a></li>
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<li><a href="#xtra_bitpacked_de">8.1.2. DSCP, ECN (IPv4)</a></li>
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<li><a href="#xtra_bitpacked_frag">8.1.3. Flags, Fragmentation Offset (IPv4)</a></li>
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<li><a href="#xtra_bitpacked_vtf">8.1.4. IP Version, Traffic Class, Flow Label (IPv6)</a></li>
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<li><a href="#xtra_bitpacked_ip4opt_t">8.1.4. IPv4 Option Type</a></li>
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<li><a href="#xtra_bitpacked_vtf">8.1.5. IP Version, Traffic Class, Flow Label (IPv6)</a></li>
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</ul>
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</li>
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<li><a href="#xtra_pad">8.2. Padded Fields</a>
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<ul class="sectlevel3">
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<li><a href="#xtra_pad_v4opts">8.2.1. IPv4 Options</a>
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<ul class="sectlevel4">
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<li><a href="#xtra_pad_v4opts_t">8.2.1.1. Type</a></li>
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<li><a href="#xtra_pad_v4opts_l">8.2.1.2. Length</a></li>
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<li><a href="#xtra_pad_v4opts_v">8.2.1.3. Value</a></li>
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</ul>
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</li>
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</ul>
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</li>
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</ul>
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@@ -1700,7 +1712,7 @@ If using the online version, <em>Part I-2</em> is summarized/starts <a href="htt
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<summary class="title">Example in Go</summary>
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<div class="content">
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<div class="listingblock">
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<div class="title"><code>hasflag.go</code></div>
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<div class="title"><code>examples/hasflag.go</code></div>
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<div class="content">
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<pre class="rouge highlight"><code data-lang="go"><span class="k">package</span> <span class="n">main</span>
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@@ -2079,7 +2091,7 @@ Its source <em>must</em> be the MAC/PHYS address of the network interface it is
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</dd>
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<dt class="hdlist1">IP:O</dt>
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<dd>
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<p>Options (Optional) (See the <a href="https://www.iana.org/assignments/ip-parameters/ip-parameters.xhtml" target="_blank" rel="noopener">IANA Registered IP Parameters</a> for details) <em>(Variable Length)</em></p>
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<p>Options + Null Padding (Optional) <em>(Variable Length, see <a href="#xtra_pad_v4opts">Addendum</a>)</em></p>
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</dd>
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</dl>
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</div>
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@@ -2439,12 +2451,13 @@ IPv6 does not require a header checksum.</p>
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<summary class="title">Example in Go</summary>
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<div class="content">
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<div class="listingblock">
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<div class="title"><code>examples/vihl.go</code></div>
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<div class="content">
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<pre class="rouge highlight"><code data-lang="go"><span class="k">package</span> <span class="n">main</span>
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<span class="k">import</span> <span class="p">(</span>
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<span class="s">`fmt`</span>
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<span class="s">`log`</span>
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<span class="s">"fmt"</span>
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<span class="s">"log"</span>
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<span class="s">"strconv"</span>
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<span class="p">)</span>
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@@ -2573,12 +2586,13 @@ IPv6 does not require a header checksum.</p>
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<summary class="title">Example in Go</summary>
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<div class="content">
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<div class="listingblock">
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<div class="title"><code>examples/de.go</code></div>
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<div class="content">
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<pre class="rouge highlight"><code data-lang="go"><span class="k">package</span> <span class="n">main</span>
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<span class="k">import</span> <span class="p">(</span>
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<span class="s">`fmt`</span>
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<span class="s">`log`</span>
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<span class="s">"fmt"</span>
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<span class="s">"log"</span>
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<span class="s">"strconv"</span>
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<span class="p">)</span>
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@@ -2665,12 +2679,13 @@ IPv6 does not require a header checksum.</p>
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<summary class="title">Example in Go</summary>
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<div class="content">
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<div class="listingblock">
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<div class="title"><code>examples/frag.go</code></div>
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<div class="content">
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<pre class="rouge highlight"><code data-lang="go"><span class="k">package</span> <span class="n">main</span>
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<span class="k">import</span> <span class="p">(</span>
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<span class="s">`fmt`</span>
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<span class="s">`log`</span>
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<span class="s">"fmt"</span>
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<span class="s">"log"</span>
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<span class="s">"strconv"</span>
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<span class="p">)</span>
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@@ -2747,7 +2762,147 @@ IPv6 does not require a header checksum.</p>
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</details>
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</div>
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<div class="sect3">
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<h4 id="xtra_bitpacked_vtf"><a class="link" href="#xtra_bitpacked_vtf">8.1.4. IP Version, Traffic Class, Flow Label (IPv6)</a></h4>
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<h4 id="xtra_bitpacked_ip4opt_t"><a class="link" href="#xtra_bitpacked_ip4opt_t">8.1.4. IPv4 Option Type</a></h4>
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<div class="paragraph">
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<p>The <strong>Type</strong> field of an <a href="#xtra_pad_v4opts">IPv4 option</a> is a single byte that consists of:</p>
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</div>
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<div class="ulist">
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<ul>
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<li>
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<p>A "Copy" flag <em>(1 Bit)</em></p>
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</li>
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<li>
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<p>A "Class" flag <em>(2 Bits)</em></p>
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</li>
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<li>
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<p>An Option "Number" <em>(5 Bits)</em></p>
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</li>
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</ul>
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</div>
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<details>
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<summary class="title">Example in Go</summary>
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<div class="content">
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<div class="listingblock">
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<div class="title"><code>examples/v4optstype.go</code></div>
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<div class="content">
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<pre class="rouge highlight"><code data-lang="go"><span class="k">package</span> <span class="n">main</span>
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<span class="k">import</span> <span class="p">(</span>
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<span class="s">"fmt"</span>
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<span class="s">"log"</span>
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<span class="s">"strconv"</span>
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<span class="p">)</span>
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<span class="k">const</span> <span class="p">(</span>
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<span class="n">ClassControl</span> <span class="kt">uint8</span> <span class="o">=</span> <span class="no">iota</span>
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<span class="n">ClassReserved</span>
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<span class="n">ClassDebug</span>
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<span class="n">ClassReserved2</span>
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<span class="p">)</span>
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<span class="k">var</span> <span class="p">(</span>
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<span class="n">classStr</span> <span class="k">map</span><span class="p">[</span><span class="kt">uint8</span><span class="p">]</span><span class="kt">string</span> <span class="o">=</span> <span class="k">map</span><span class="p">[</span><span class="kt">uint8</span><span class="p">]</span><span class="kt">string</span><span class="p">{</span>
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<span class="n">ClassControl</span><span class="o">:</span> <span class="s">"Control"</span><span class="p">,</span>
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<span class="n">ClassReserved</span><span class="o">:</span> <span class="s">"Reserved (1)"</span><span class="p">,</span>
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<span class="n">ClassDebug</span><span class="o">:</span> <span class="s">"Debugging/Measurement"</span><span class="p">,</span>
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<span class="n">ClassReserved2</span><span class="o">:</span> <span class="s">"Reserved (2)"</span><span class="p">,</span>
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<span class="p">}</span>
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<span class="p">)</span>
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<span class="k">const</span> <span class="p">(</span>
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<span class="c">/// This is the same type from examples/v4optspad.go.</span>
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<span class="n">ip4OptTypBits</span> <span class="kt">string</span> <span class="o">=</span> <span class="s">"10010100"</span> <span class="c">// [1001 0100], or 148 (0x94)</span>
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<span class="p">)</span>
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<span class="k">var</span> <span class="p">(</span>
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<span class="n">v4TypCpyOffset</span> <span class="kt">uint8</span> <span class="o">=</span> <span class="m">0x80</span> <span class="c">// 128</span>
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<span class="n">v4TypCpyMask</span> <span class="kt">uint8</span> <span class="o">=</span> <span class="m">0x01</span> <span class="c">// Mask to 1 bit</span>
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<span class="n">v4TypCpyPos</span> <span class="kt">uint8</span> <span class="o">=</span> <span class="m">8</span> <span class="o">-</span> <span class="n">v4TypCpyMask</span> <span class="c">// 7 or 0x07 (8 (bits) - mask = Shifted to 7th bit)</span>
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<span class="n">v4TypClsOffset</span> <span class="kt">uint8</span> <span class="o">=</span> <span class="m">0x60</span> <span class="c">// 96</span>
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<span class="n">v4TypClsMask</span> <span class="kt">uint8</span> <span class="o">=</span> <span class="m">0x05</span> <span class="c">// mask to 5 bits</span>
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<span class="n">v4TypClsPos</span> <span class="kt">uint8</span> <span class="o">=</span> <span class="m">8</span> <span class="o">-</span> <span class="n">v4TypClsMask</span>
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<span class="n">v4TypNumOffset</span> <span class="kt">uint8</span> <span class="o">=</span> <span class="m">0x1f</span> <span class="c">// 31</span>
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<span class="p">)</span>
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<span class="k">func</span> <span class="n">ToV4OptTyp</span><span class="p">(</span><span class="n">copied</span><span class="p">,</span> <span class="n">class</span><span class="p">,</span> <span class="n">num</span> <span class="kt">uint8</span><span class="p">)</span> <span class="p">(</span><span class="n">typ</span> <span class="kt">uint8</span><span class="p">)</span> <span class="p">{</span>
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<span class="n">typ</span> <span class="o">=</span> <span class="p">((</span><span class="n">copied</span> <span class="o">&</span> <span class="n">v4TypCpyMask</span><span class="p">)</span> <span class="o"><<</span> <span class="n">v4TypCpyPos</span><span class="p">)</span> <span class="o">|</span> <span class="p">((</span><span class="n">class</span> <span class="o">&</span> <span class="n">v4TypClsMask</span><span class="p">)</span> <span class="o"><<</span> <span class="n">v4TypClsPos</span><span class="p">)</span> <span class="o">|</span> <span class="p">(</span><span class="n">num</span> <span class="o">&</span> <span class="n">v4TypNumOffset</span><span class="p">)</span>
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<span class="k">return</span>
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<span class="p">}</span>
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<span class="k">func</span> <span class="n">FromV4OptTyp</span><span class="p">(</span><span class="n">typ</span> <span class="kt">uint8</span><span class="p">)</span> <span class="p">(</span><span class="n">copied</span><span class="p">,</span> <span class="n">class</span><span class="p">,</span> <span class="n">num</span> <span class="kt">uint8</span><span class="p">)</span> <span class="p">{</span>
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<span class="n">copied</span> <span class="o">=</span> <span class="p">(</span><span class="n">typ</span> <span class="o">&</span> <span class="n">v4TypCpyOffset</span><span class="p">)</span> <span class="o">>></span> <span class="n">v4TypCpyPos</span>
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<span class="n">class</span> <span class="o">=</span> <span class="p">(</span><span class="n">typ</span> <span class="o">&</span> <span class="n">v4TypClsOffset</span><span class="p">)</span> <span class="o">>></span> <span class="n">v4TypClsPos</span>
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<span class="n">num</span> <span class="o">=</span> <span class="p">(</span><span class="n">typ</span> <span class="o">&</span> <span class="n">v4TypNumOffset</span><span class="p">)</span>
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<span class="k">return</span>
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<span class="p">}</span>
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<span class="k">func</span> <span class="n">main</span><span class="p">()</span> <span class="p">{</span>
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<span class="k">var</span> <span class="n">err</span> <span class="kt">error</span>
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<span class="k">var</span> <span class="n">u64</span> <span class="kt">uint64</span>
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<span class="k">var</span> <span class="n">typ</span> <span class="kt">uint8</span>
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<span class="k">var</span> <span class="n">cpd</span> <span class="kt">uint8</span>
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<span class="k">var</span> <span class="n">cls</span> <span class="kt">uint8</span>
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<span class="k">var</span> <span class="n">num</span> <span class="kt">uint8</span>
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<span class="c">// Given a type of ip4OptTypBits (see const at top)...</span>
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<span class="k">if</span> <span class="n">u64</span><span class="p">,</span> <span class="n">err</span> <span class="o">=</span> <span class="n">strconv</span><span class="o">.</span><span class="n">ParseUint</span><span class="p">(</span><span class="n">ip4OptTypBits</span><span class="p">,</span> <span class="m">2</span><span class="p">,</span> <span class="m">8</span><span class="p">);</span> <span class="n">err</span> <span class="o">!=</span> <span class="no">nil</span> <span class="p">{</span>
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<span class="n">log</span><span class="o">.</span><span class="n">Panicln</span><span class="p">(</span><span class="n">err</span><span class="p">)</span>
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<span class="p">}</span>
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<span class="n">typ</span> <span class="o">=</span> <span class="kt">uint8</span><span class="p">(</span><span class="n">u64</span><span class="p">)</span>
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<span class="c">// Prints:</span>
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<span class="c">/*
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Type is: 148 (0x0094)
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*/</span>
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<span class="n">fmt</span><span class="o">.</span><span class="n">Printf</span><span class="p">(</span><span class="s">"Type is:</span><span class="se">\t</span><span class="s">%d (%#04x)</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span> <span class="n">typ</span><span class="p">,</span> <span class="n">typ</span><span class="p">)</span>
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<span class="n">cpd</span><span class="p">,</span> <span class="n">cls</span><span class="p">,</span> <span class="n">num</span> <span class="o">=</span> <span class="n">FromV4OptTyp</span><span class="p">(</span><span class="n">typ</span><span class="p">)</span>
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<span class="c">// Prints:</span>
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<span class="c">/*
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Copied: 1 (0x0001)
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Class: 0 (0x0000)
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Number: 20 (0x0014)
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*/</span>
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<span class="n">fmt</span><span class="o">.</span><span class="n">Printf</span><span class="p">(</span>
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<span class="s">"Copied:</span><span class="se">\t\t</span><span class="s">%d %#04x)</span><span class="se">\n</span><span class="s">"</span><span class="o">+</span>
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<span class="s">"Class:</span><span class="se">\t\t</span><span class="s">%d (%#04x)</span><span class="se">\n</span><span class="s">"</span><span class="o">+</span>
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<span class="s">"Number:</span><span class="se">\t\t</span><span class="s">%d (%#04x)</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span>
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<span class="n">cpd</span><span class="p">,</span> <span class="n">cpd</span><span class="p">,</span>
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<span class="n">cls</span><span class="p">,</span> <span class="n">cls</span><span class="p">,</span>
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<span class="n">num</span><span class="p">,</span> <span class="n">num</span><span class="p">,</span>
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<span class="p">)</span>
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<span class="n">typ</span> <span class="o">=</span> <span class="n">ToV4OptTyp</span><span class="p">(</span><span class="n">cpd</span><span class="p">,</span> <span class="n">cls</span><span class="p">,</span> <span class="n">num</span><span class="p">)</span>
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<span class="c">// Prints:</span>
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<span class="c">/*
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Confirmed Type: 148 (0x94)
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*/</span>
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<span class="n">fmt</span><span class="o">.</span><span class="n">Printf</span><span class="p">(</span><span class="s">"Confirmed Type:</span><span class="se">\t</span><span class="s">%d (%#02x)</span><span class="se">\n\n</span><span class="s">"</span><span class="p">,</span> <span class="n">typ</span><span class="p">,</span> <span class="n">typ</span><span class="p">)</span>
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<span class="n">fmt</span><span class="o">.</span><span class="n">Println</span><span class="p">(</span><span class="s">"Class Name:"</span><span class="p">)</span>
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<span class="c">// Prints:</span>
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<span class="c">/*
|
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Control
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*/</span>
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<span class="k">for</span> <span class="n">c</span><span class="p">,</span> <span class="n">cNm</span> <span class="o">:=</span> <span class="k">range</span> <span class="n">classStr</span> <span class="p">{</span>
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<span class="k">if</span> <span class="n">c</span> <span class="o">==</span> <span class="n">cls</span> <span class="p">{</span>
|
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<span class="n">fmt</span><span class="o">.</span><span class="n">Println</span><span class="p">(</span><span class="n">cNm</span><span class="p">)</span>
|
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<span class="p">}</span>
|
||||
<span class="p">}</span>
|
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<span class="p">}</span></code></pre>
|
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</div>
|
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</div>
|
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</div>
|
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</details>
|
||||
</div>
|
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<div class="sect3">
|
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<h4 id="xtra_bitpacked_vtf"><a class="link" href="#xtra_bitpacked_vtf">8.1.5. IP Version, Traffic Class, Flow Label (IPv6)</a></h4>
|
||||
<div class="paragraph">
|
||||
<p>IPv6 thankfully only has one bitpacking in the header. Unfortunately, it’s a triple-whammy.</p>
|
||||
</div>
|
||||
@@ -2774,12 +2929,13 @@ IPv6 does not require a header checksum.</p>
|
||||
<summary class="title">Example in Go</summary>
|
||||
<div class="content">
|
||||
<div class="listingblock">
|
||||
<div class="title"><code>examples/vtf.go</code></div>
|
||||
<div class="content">
|
||||
<pre class="rouge highlight"><code data-lang="go"><span class="k">package</span> <span class="n">main</span>
|
||||
|
||||
<span class="k">import</span> <span class="p">(</span>
|
||||
<span class="s">`fmt`</span>
|
||||
<span class="s">`log`</span>
|
||||
<span class="s">"fmt"</span>
|
||||
<span class="s">"log"</span>
|
||||
<span class="s">"strconv"</span>
|
||||
<span class="p">)</span>
|
||||
|
||||
@@ -2866,12 +3022,148 @@ IPv6 does not require a header checksum.</p>
|
||||
</details>
|
||||
</div>
|
||||
</div>
|
||||
<div class="sect2">
|
||||
<h3 id="xtra_pad"><a class="link" href="#xtra_pad">8.2. Padded Fields</a></h3>
|
||||
<div class="sect3">
|
||||
<h4 id="xtra_pad_v4opts"><a class="link" href="#xtra_pad_v4opts">8.2.1. IPv4 Options</a></h4>
|
||||
<div class="paragraph">
|
||||
<p>The IPv4 options, if specified, <strong>must</strong> align to a 32-bit/4-byte multiple size.
|
||||
(In other words, its total length of bytes <strong>must</strong> be cleanly divisible by 4;
|
||||
or said another way its total length of bits must be cleanly divisible by 32).</p>
|
||||
</div>
|
||||
<div class="paragraph">
|
||||
<p>It uses null-byte (<code>0x00</code>) padding to achieve this.</p>
|
||||
</div>
|
||||
<details>
|
||||
<summary class="title">Example in Go</summary>
|
||||
<div class="content">
|
||||
<div class="listingblock">
|
||||
<div class="title"><code>examples/v4optspad.go</code></div>
|
||||
<div class="content">
|
||||
<pre class="rouge highlight"><code data-lang="go"><span class="k">package</span> <span class="n">main</span>
|
||||
|
||||
<span class="k">import</span> <span class="p">(</span>
|
||||
<span class="s">"bytes"</span>
|
||||
<span class="s">"fmt"</span>
|
||||
<span class="p">)</span>
|
||||
|
||||
<span class="k">const</span> <span class="p">(</span>
|
||||
<span class="n">padVal</span> <span class="kt">uint8</span> <span class="o">=</span> <span class="m">0x00</span> <span class="c">// Padded with NUL/null-bytes</span>
|
||||
<span class="n">alignLen</span> <span class="kt">int</span> <span class="o">=</span> <span class="m">4</span> <span class="c">// 4-Byte alignment</span>
|
||||
<span class="p">)</span>
|
||||
|
||||
<span class="k">var</span> <span class="p">(</span>
|
||||
<span class="c">// See the examples/v4opts.go for how these bytes are constructed.</span>
|
||||
<span class="n">opts</span> <span class="p">[]</span><span class="kt">byte</span> <span class="o">=</span> <span class="p">[]</span><span class="kt">byte</span><span class="p">{</span>
|
||||
<span class="c">// This is an example.</span>
|
||||
<span class="c">// Option 1</span>
|
||||
<span class="m">0x94</span><span class="p">,</span> <span class="c">// Type 148, RTRALT (Router Alert) (RFC 2113)</span>
|
||||
<span class="m">0x04</span><span class="p">,</span> <span class="c">// Length (4 bytes)</span>
|
||||
<span class="m">0x00</span><span class="p">,</span> <span class="m">0x00</span><span class="p">,</span> <span class="c">// "Router shall examine packet"</span>
|
||||
<span class="c">// EOOL</span>
|
||||
<span class="m">0x00</span><span class="p">,</span>
|
||||
<span class="c">// Padding will go here.</span>
|
||||
<span class="p">}</span>
|
||||
<span class="p">)</span>
|
||||
|
||||
<span class="k">func</span> <span class="n">main</span><span class="p">()</span> <span class="p">{</span>
|
||||
|
||||
<span class="k">var</span> <span class="n">optLen</span> <span class="kt">int</span>
|
||||
<span class="k">var</span> <span class="n">padLen</span> <span class="kt">int</span>
|
||||
<span class="k">var</span> <span class="n">pad</span> <span class="p">[]</span><span class="kt">byte</span>
|
||||
|
||||
<span class="n">optLen</span> <span class="o">=</span> <span class="nb">len</span><span class="p">(</span><span class="n">opts</span><span class="p">)</span>
|
||||
|
||||
<span class="n">fmt</span><span class="o">.</span><span class="n">Println</span><span class="p">(</span><span class="s">"Before padding:"</span><span class="p">)</span>
|
||||
<span class="c">// Prints:</span>
|
||||
<span class="c">/*
|
||||
0x9404000000
|
||||
(Length: 5)
|
||||
*/</span>
|
||||
<span class="n">fmt</span><span class="o">.</span><span class="n">Printf</span><span class="p">(</span><span class="s">"%#02x</span><span class="se">\n</span><span class="s">(Length: %d)</span><span class="se">\n\n</span><span class="s">"</span><span class="p">,</span> <span class="n">opts</span><span class="p">,</span> <span class="n">optLen</span><span class="p">)</span>
|
||||
|
||||
<span class="c">/*
|
||||
The remainder of the current length divided by
|
||||
alignLen (4) (modulo) is subtracted from the alignLen
|
||||
to determine how much must be added to reach the next
|
||||
"boundary". It's then modulo'd *again* to rule out
|
||||
currently being on an alignment bounary.
|
||||
*/</span>
|
||||
<span class="n">padLen</span> <span class="o">=</span> <span class="p">(</span><span class="n">alignLen</span> <span class="o">-</span> <span class="p">(</span><span class="n">optLen</span> <span class="o">%</span> <span class="n">alignLen</span><span class="p">))</span> <span class="o">%</span> <span class="n">alignLen</span>
|
||||
<span class="c">// Prints:</span>
|
||||
<span class="c">/*
|
||||
Pad length needed: 3
|
||||
*/</span>
|
||||
<span class="n">fmt</span><span class="o">.</span><span class="n">Printf</span><span class="p">(</span><span class="s">"Pad length needed:</span><span class="se">\t</span><span class="s">%d</span><span class="se">\n\n</span><span class="s">"</span><span class="p">,</span> <span class="n">padLen</span><span class="p">)</span>
|
||||
<span class="n">pad</span> <span class="o">=</span> <span class="n">bytes</span><span class="o">.</span><span class="n">Repeat</span><span class="p">([]</span><span class="kt">byte</span><span class="p">{</span><span class="n">padVal</span><span class="p">},</span> <span class="n">padLen</span><span class="p">)</span>
|
||||
|
||||
<span class="n">opts</span> <span class="o">=</span> <span class="nb">append</span><span class="p">(</span><span class="n">opts</span><span class="p">,</span> <span class="n">pad</span><span class="o">...</span><span class="p">)</span>
|
||||
|
||||
<span class="c">// Alternatively, this can be implemented with a loop, though it's likely less efficient:</span>
|
||||
<span class="c">/*
|
||||
for len(opts) % alignLen != 0 {}
|
||||
opts = append(opts, padVal)
|
||||
}
|
||||
*/</span>
|
||||
|
||||
<span class="c">// Prints:</span>
|
||||
<span class="c">/*
|
||||
Padded:
|
||||
0x9404000000000000
|
||||
*/</span>
|
||||
<span class="n">fmt</span><span class="o">.</span><span class="n">Printf</span><span class="p">(</span><span class="s">"Padded:</span><span class="se">\n</span><span class="s">%#x</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span> <span class="n">opts</span><span class="p">)</span>
|
||||
<span class="p">}</span></code></pre>
|
||||
</div>
|
||||
</div>
|
||||
</div>
|
||||
</details>
|
||||
<div class="paragraph">
|
||||
<p>For more extensive details, see <a href="https://datatracker.ietf.org/doc/html/rfc791#section-3.1" target="_blank" rel="noopener">RFC 791 § 3.1</a>
|
||||
and the <a href="https://www.iana.org/assignments/ip-parameters/ip-parameters.xhtml" target="_blank" rel="noopener">IANA Registered IP Paramaeters</a>
|
||||
(the <strong>Value</strong> column is what should be used as the <strong>Type</strong>, see <a href="#xtra_bitpacked_ip4opt_t">IPv4 Option Type</a>.</p>
|
||||
</div>
|
||||
<div class="paragraph">
|
||||
<p>Each option (with two exceptions) is a TLV (<a href="#xtra_pad_v4opts_t">type</a> / <a href="#xtra_pad_v4opts_l">length</a> / <a href="#xtra_pad_v4opts_v">value</a>) structured field.</p>
|
||||
</div>
|
||||
<div class="paragraph">
|
||||
<p>The two exceptions for the above are <code>EOOL</code> (<code>0x00</code>) and <code>NOP</code> (<code>0x01</code>), both of which occupy only a single byte — they are <strong>unvalued</strong>, so they have no length or value field.</p>
|
||||
</div>
|
||||
<div class="sect4">
|
||||
<h5 id="xtra_pad_v4opts_t"><a class="link" href="#xtra_pad_v4opts_t">8.2.1.1. Type</a></h5>
|
||||
<div class="paragraph">
|
||||
<p>See <a href="#xtra_bitpacked_ip4opt_t">IPv4 Option Type</a> for details.</p>
|
||||
</div>
|
||||
</div>
|
||||
<div class="sect4">
|
||||
<h5 id="xtra_pad_v4opts_l"><a class="link" href="#xtra_pad_v4opts_l">8.2.1.2. Length</a></h5>
|
||||
<div class="paragraph">
|
||||
<p>The <strong>Length</strong> is the length of this <em>entire</em> option as an 8-bit unsigned integer.</p>
|
||||
</div>
|
||||
<div class="paragraph">
|
||||
<p>In other words it includes the length of the <a href="#xtra_pad_v4opts_t">Type</a> field, the <a href="#xtra_pad_v4opts_l">Length</a> field (this field),
|
||||
<strong>and</strong> the <a href="#xtra_pad_v4opts_v">Value</a> field together.</p>
|
||||
</div>
|
||||
<div class="paragraph">
|
||||
<p>This is not present for <code>EOOL</code> (<code>0x00</code>) and <code>NOP</code> (<code>0x01</code>) (or any other "non-valued" options).</p>
|
||||
</div>
|
||||
</div>
|
||||
<div class="sect4">
|
||||
<h5 id="xtra_pad_v4opts_v"><a class="link" href="#xtra_pad_v4opts_v">8.2.1.3. Value</a></h5>
|
||||
<div class="paragraph">
|
||||
<p>Value is the option’s value. Its <a href="#xtra_pad_v4opts_l">Length</a> is variable depending on the <a href="#xtra_pad_v4opts_t">Type</a>.</p>
|
||||
</div>
|
||||
<div class="paragraph">
|
||||
<p>This is not present for <code>EOOL</code> (<code>0x00</code>) and <code>NOP</code> (<code>0x01</code>) (or any other "non-valued" options).</p>
|
||||
</div>
|
||||
</div>
|
||||
</div>
|
||||
</div>
|
||||
</div>
|
||||
</div>
|
||||
</div>
|
||||
<div id="footer">
|
||||
<div id="footer-text">
|
||||
Last updated 2025-08-30 16:39:39 -0400
|
||||
Last updated 2025-08-31 00:25:52 -0400
|
||||
</div>
|
||||
</div>
|
||||
</body>
|
||||
|
||||
Reference in New Issue
Block a user