lwip/proj/unixsim/apps/fs/os.html
2002-10-19 12:59:30 +00:00

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<head><title>lwIP - A Lightweight TCP/IP Stack - OS vs non-OS</title></head>
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<center><h1>lwIP - A Lightweight TCP/IP Stack</h1></center>
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<td><center><h2>OS vs non-OS</h2></center>
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<h2>Using lwIP with or without an operating system</h2>
<p align="justify">
There has been a few questions about how lwIP can be used in a
standalone environment (i.e., an environment without a multi-threaded
operating system) lately. The purpose of this document is to describe
how lwIP is designed to be used with and without a multi-threaded
operating system.
</p>
<center><img src="os.png"></center>
<h3>The lwIP single-threaded core</h3>
<p align="justify">
The core of lwIP consists of the actual implementations of the IP,
ICMP, UDP, and TCP protocols, as well as the support functions such as
buffer and memory management. The core components are the only ones
that are needed when lwIP is to be run in a single-threaded (non-OS)
environment.
</p>
<p align="justify">
The core components can be viewed as a software library which has the
following interface:
</p>
<ul>
<li><tt>ip_input(pbuf, netif)</tt>: Takes an IP packet and the incoming network
interface as arguments and does the TCP/IP processing for the packet.
<li><tt>tcp_tmr()</tt>: Should be called every 100 ms. Does all TCP
timer processing such as doing retransmissions.
</ul>
<p align="justify">
Because none of the core functions ever needs to block when run in a
single-threaded environment, the <tt>sys_arch</tt> (the operating
system abstraction layer) does not need to implement locking
semaphores or mailboxes. In future versions of lwIP, the dependancy of
the <tt>sys_arch</tt> will be removed in the single-threaded case.
</p>
<p align="justify">
A simple main loop for a single-threaded system might look
like this:
</p>
<pre>
while(1) {
if(poll_driver(netif) == PACKET_READY) {
pbuf = get_packet(netif);
ip_input(pbuf, netif);
}
if(clock() - last_time == 100 * CLOCK_MS) {
tcp_tmr();
last_time = clock();
}
}
</pre>
<h3>lwIP in a multi-threaded system</h3>
<p align="justify">
lwIP is designed to be able to be run in a multi-threaded system with
applications running in concurrent threads. The model used in this
case is that all TCP/IP processing is done in a single thread. The
application thread communicate with the TCP/IP thread using the
sequential API.
</p>
<center><img src="threads.png"></center>
<p align="justify">
The inter-thread communication is implemented in the two files
<tt>api_lib.c</tt> and <tt>api_msg.c</tt>. The former contains the
functions used by the application programs and the latter implements
the TCP/IP stack interface. A third file, <tt>tcpip.c</tt>, handles
incoming packets and timer events as described in the previous
section.
</p>
<p align="justify">
When run in a multi-threaded environment, incoming packets are handled
by the function <tt>tcpip_input()</tt>, which takes the same arguments
as the <tt>ip_input()</tt> function. The difference between the two
functions is that the <tt>tcpip_input()</tt> function does not process
the incoming packet immediately. It merely puts the packet on a queue
which later is drained by the TCP/IP thread.
</p>
<p align="justify">
When being run in a multi-threaded system, timer events are taken care
of internally in <tt>tcpip.c</tt>.
</p>
<p align="right">
<font size="-1"><i>
$Date: 2002/10/19 13:00:03 $
</i></font>
</p>
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&nbsp;
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<hr>
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<hr>
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<td>
<div align="right">
<a href="http://www.sics.se/~adam/">Adam Dunkels</a></div>
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