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| <section id="config-net-guardsize"> |
| <h1>CONFIG_NET_GUARDSIZE<a class="headerlink" href="#config-net-guardsize" title="Permalink to this heading"></a></h1> |
| <section id="global-option-for-all-drivers"> |
| <h2>Global Option for All Drivers<a class="headerlink" href="#global-option-for-all-drivers" title="Permalink to this heading"></a></h2> |
| <p><code class="docutils literal notranslate"><span class="pre">CONFIG_NET_GUARD_SIZE</span></code> is global option. It is added to the allocated size of |
| each driver packet buffer. Currently it is a very small value, defaulting to only |
| two bytes. So it is not a memory hog and should be added to the packetsize for |
| all drivers for commonality. But why?</p> |
| <p>It should (eventually) be larger and common for all drivers. We need to look at |
| how it is used today and how it might be used tomorrow. There is a probably a lot |
| more involved than you might be initially considering.</p> |
| </section> |
| <section id="packet-receipt"> |
| <h2>Packet Receipt<a class="headerlink" href="#packet-receipt" title="Permalink to this heading"></a></h2> |
| <p>For packet receipt, it is necessary for some hardware, but not for others. Often |
| the hardware will DMA a 2 byte FCS at the end of the packet or possibly other |
| hardware-specific info. But that is only part of the whole story. |
| <code class="docutils literal notranslate"><span class="pre">CONFIG_NET_GUARDSIZE</span></code> is not just for hardware packet receipt.</p> |
| </section> |
| <section id="packet-transmission"> |
| <h2>Packet Transmission<a class="headerlink" href="#packet-transmission" title="Permalink to this heading"></a></h2> |
| <p>There are several issues for packet transmission. These are less well defined |
| and need further study, but we need to keep all of the driver packet definitions |
| in place until we understand how we are going to handle these things:</p> |
| <ul> |
| <li><p>Memory Overrun Bugs</p> |
| <p>There was in the past, a bug that caused write past the end of the buffer by |
| a couple of bytes during TX message formatting. I don’t know if that bug still |
| exists, but the minimum, two-byte <code class="docutils literal notranslate"><span class="pre">CONFIG_NET_GUARDSIZE</span></code> was sufficient to |
| eliminate the bug. That is why it has the name GUARD: Its primary purpose is |
| to protect from overrunning the packet buffer and corrupting the following memory.</p> |
| <p>I do no know if we have any such bugs today. Perhaps they still do? |
| Perhaps they do not? Having such a guard is a good thing for reliability in |
| any case.</p> |
| </li> |
| <li><p>Variable size IP/TCP headers</p> |
| <p>There is a limitation in the way IP packets are formatted now. Basically they |
| are formatted like this:</p> |
| <blockquote> |
| <div><ol class="arabic simple"> |
| <li><p>When the packet is received a pointer to the location of the payload is |
| set (d_appdata). This is an offset into the packet buffer For TCP, that |
| accounts for the MAC/Ethernet header, the minimum IPv4/IPv6 header size, |
| and the minimum TCP header size.</p></li> |
| <li><p>The TCP payload is written at that location,</p></li> |
| <li><p>The correctly sized IPv4/IPv6 headers and the correctly sized TCP header |
| are added below the payload, and finally</p></li> |
| <li><p>The MAC/Ethernet header as added.</p></li> |
| </ol> |
| </div></blockquote> |
| <p>The start offset of the packet in the packet is no longer zero, but some |
| variable offset into the packet buffer. That new start offset would have |
| to be passed to driver in order to send the packet.</p> |
| <p>The key to making this all work is:</p> |
| <blockquote> |
| <div><ul class="simple"> |
| <li><p>Keep <code class="docutils literal notranslate"><span class="pre">CONFIG_NET_GUARDSIZE</span></code> in all driver buffers, and</p></li> |
| <li><p>Set the <code class="docutils literal notranslate"><span class="pre">CONFIG_NET_GUARDSIZE</span></code> to the maximum size of IPv4/IPv6 and TCP options |
| (depending on which IP version is enabled and if TCP is enabled)</p></li> |
| <li><p>Extend the driver interface to accept data offset into the driver’s packet buffer.</p></li> |
| </ul> |
| </div></blockquote> |
| </li> |
| <li><p>Variable MSS</p> |
| <p>Closely related to this is the MSS which is the maximum size of the payload. |
| Currently that is a constant because it assumes the minimum header lengths. |
| It should be variable, depending on the actual header sizes.</p> |
| </li> |
| </ul> |
| </section> |
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