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<TITLE>3 Design Thoughts</TITLE>
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<A NAME="CHILD_LINKS"><STRONG>Subsections</STRONG></A>

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<LI><A NAME="tex2html60"
  HREF="node3.html#SECTION00031000000000000000"><SPAN CLASS="arabic">3</SPAN>.<SPAN CLASS="arabic">1</SPAN> <SPAN ID="hue96">Sending and Receiving traffic</SPAN></A>
<LI><A NAME="tex2html61"
  HREF="node3.html#SECTION00032000000000000000"><SPAN CLASS="arabic">3</SPAN>.<SPAN CLASS="arabic">2</SPAN> <SPAN ID="hue120">Handling Multiple Connections</SPAN></A>
<LI><A NAME="tex2html62"
  HREF="node3.html#SECTION00033000000000000000"><SPAN CLASS="arabic">3</SPAN>.<SPAN CLASS="arabic">3</SPAN> <SPAN ID="hue129">Data Synchronization</SPAN></A>
<LI><A NAME="tex2html63"
  HREF="node3.html#SECTION00034000000000000000"><SPAN CLASS="arabic">3</SPAN>.<SPAN CLASS="arabic">4</SPAN> <SPAN ID="hue134">TCP/IP</SPAN></A>
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<H1><A NAME="SECTION00030000000000000000">
<SPAN CLASS="arabic">3</SPAN> <SPAN ID="hue94">Design Thoughts</SPAN></A>
</H1>

<P>

<H2><A NAME="SECTION00031000000000000000">
<SPAN CLASS="arabic">3</SPAN>.<SPAN CLASS="arabic">1</SPAN> <SPAN ID="hue96">Sending and Receiving traffic</SPAN></A>
</H2>

<P>
<SPAN ID="hue98">Flowreplay must be able to process multiple connections
to one or more devices. There are two options:</SPAN>
<P>

<OL>
<LI><SPAN ID="hue101">Use sockets</SPAN><A NAME="tex2html2"
  HREF="#foot371"><SUP><SPAN CLASS="arabic">2</SPAN></SUP></A> <SPAN ID="hue105">to send and receive data</SPAN>
</LI>
<LI><SPAN ID="hue107">Use libpcap</SPAN><A NAME="tex2html3"
  HREF="#foot372"><SUP><SPAN CLASS="arabic">3</SPAN></SUP></A> <SPAN ID="hue111">to receive packets and libnet</SPAN><A NAME="tex2html4"
  HREF="#foot373"><SUP><SPAN CLASS="arabic">4</SPAN></SUP></A> <SPAN ID="hue115">to send packets</SPAN>
</LI>
</OL>
<SPAN ID="hue118">Although using libpcap/libnet would allow more simultaneous
connections and greater flexibility, there would be a very high complexity
cost associated with it. With that in mind, I've decided to use sockets
to send and receive data.</SPAN>
<P>

<H2><A NAME="SECTION00032000000000000000">
<SPAN CLASS="arabic">3</SPAN>.<SPAN CLASS="arabic">2</SPAN> <SPAN ID="hue120">Handling Multiple Connections</SPAN></A>
</H2>

<P>
<SPAN ID="hue122">Because a pcap file can contain multiple simultaneous
flows, we need to be able to support that too. The biggest problem
with this is reading packet data in a different order then stored
in the pcap file. </SPAN>
<P>
<SPAN ID="hue124">Reading and writing to multiple sockets is easy
with select() or poll(), however a pcap file has it's data stored
serially, but we need to access it randomly. There are a number of
possible solutions for this such as caching packets in RAM where they
can be accessed more randomly, creating an index of the packets in
the pcap file, or converting the pcap file to another format altogether.
Alternatively, I've started looking at libpcapnav</SPAN><A NAME="tex2html5"
  HREF="#foot125"><SUP><SPAN CLASS="arabic">5</SPAN></SUP></A> <SPAN ID="hue127">as an alternate means to navigate a pcap file and
process packets out of order.</SPAN>
<P>

<H2><A NAME="SECTION00033000000000000000">
<SPAN CLASS="arabic">3</SPAN>.<SPAN CLASS="arabic">3</SPAN> <SPAN ID="hue129">Data Synchronization</SPAN></A>
</H2>

<P>
<SPAN ID="hue376">Knowing when to start sending client traffic in
response to the server will be &#34;tricky&#34;. Without
understanding the actual protocol involved, probably the best general
solution is waiting for a given period of time after no more data
from the server has been received. Not sure what to do if the client
traffic doesn't elicit a response from the server (implement some
kind of timeout?). This will be the basis for the default plug-in.</SPAN>
<P>

<H2><A NAME="SECTION00034000000000000000">
<SPAN CLASS="arabic">3</SPAN>.<SPAN CLASS="arabic">4</SPAN> <SPAN ID="hue134">TCP/IP</SPAN></A>
</H2>

<P>
<SPAN ID="hue136">Dealing with IP fragmentation and TCP stream reassembly
will be another really complex problem. We're basically talking about
implementing a significant portion of a TCP/IP stack. One thought
is to use libnids</SPAN><A NAME="tex2html6"
  HREF="#foot404"><SUP><SPAN CLASS="arabic">6</SPAN></SUP></A> <SPAN ID="hue140">which basically implements a Linux 2.0.37 TCP/IP
stack in user-space. Other solutions include porting a TCP/IP stack
from Open/Net/FreeBSD or writing our own custom stack from scratch.</SPAN>
<P>
<BR><HR><H4>Footnotes</H4>
<DL>
<DT><A NAME="foot371">...&nbsp;</A><A
 HREF="node3.html#tex2html2"><SUP><SPAN CLASS="arabic">2</SPAN></SUP></A></DT>
<DD><SPAN ID="hue103">socket(2)</SPAN>

</DD>
<DT><A NAME="foot372">...&nbsp;</A><A
 HREF="node3.html#tex2html3"><SUP><SPAN CLASS="arabic">3</SPAN></SUP></A></DT>
<DD><SPAN ID="hue109">http://www.tcpdump.org/</SPAN>

</DD>
<DT><A NAME="foot373">...&nbsp;</A><A
 HREF="node3.html#tex2html4"><SUP><SPAN CLASS="arabic">4</SPAN></SUP></A></DT>
<DD><SPAN ID="hue113">http://www.packetfactory.net/projects/libnet/</SPAN>

</DD>
<DT><A NAME="foot125">...&nbsp;</A><A
 HREF="node3.html#tex2html5"><SUP><SPAN CLASS="arabic">5</SPAN></SUP></A></DT>
<DD>http://netdude.sourceforge.net/

</DD>
<DT><A NAME="foot404">...&nbsp;</A><A
 HREF="node3.html#tex2html6"><SUP><SPAN CLASS="arabic">6</SPAN></SUP></A></DT>
<DD><SPAN ID="hue378">http://www.avet.com.pl/~nergal/libnids/</SPAN>

</DD>
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<ADDRESS>
Aaron Turner
2006-07-17
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