1<html><head><meta http-equiv="Content-Type" content="text/html; charset=ANSI_X3.4-1968"><title>Packet alignment</title><meta name="generator" content="DocBook XSL Stylesheets V1.78.1"><link rel="home" href="index.html" title="The 802.11 subsystems &#8211; for kernel developers"><link rel="up" href="rx-tx.html" title="Chapter&#160;4.&#160;Receive and transmit processing"><link rel="prev" href="bk02pt01ch04s02.html" title="Frame format"><link rel="next" href="bk02pt01ch04s04.html" title="Calling into mac80211 from interrupts"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Packet alignment</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="bk02pt01ch04s02.html">Prev</a>&#160;</td><th width="60%" align="center">Chapter&#160;4.&#160;Receive and transmit processing</th><td width="20%" align="right">&#160;<a accesskey="n" href="bk02pt01ch04s04.html">Next</a></td></tr></table><hr></div><div class="sect1"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="idp1113576620"></a>Packet alignment</h2></div></div></div><p>
2   </p><p>
3   Drivers always need to pass packets that are aligned to two-byte boundaries
4   to the stack.
5   </p><p>
6   Additionally, should, if possible, align the payload data in a way that
7   guarantees that the contained IP header is aligned to a four-byte
8   boundary. In the case of regular frames, this simply means aligning the
9   payload to a four-byte boundary (because either the IP header is directly
10   contained, or IV/RFC1042 headers that have a length divisible by four are
11   in front of it).  If the payload data is not properly aligned and the
12   architecture doesn't support efficient unaligned operations, mac80211
13   will align the data.
14   </p><p>
15   With A-MSDU frames, however, the payload data address must yield two modulo
16   four because there are 14-byte 802.3 headers within the A-MSDU frames that
17   push the IP header further back to a multiple of four again. Thankfully, the
18   specs were sane enough this time around to require padding each A-MSDU
19   subframe to a length that is a multiple of four.
20   </p><p>
21   Padding like Atheros hardware adds which is between the 802.11 header and
22   the payload is not supported, the driver is required to move the 802.11
23   header to be directly in front of the payload in that case.
24</p></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="bk02pt01ch04s02.html">Prev</a>&#160;</td><td width="20%" align="center"><a accesskey="u" href="rx-tx.html">Up</a></td><td width="40%" align="right">&#160;<a accesskey="n" href="bk02pt01ch04s04.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Frame format&#160;</td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top">&#160;Calling into mac80211 from interrupts</td></tr></table></div></body></html>
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