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<BLOCKQUOTE
style="PADDING-RIGHT: 0px; PADDING-LEFT: 5px; MARGIN-LEFT: 5px; BORDER-LEFT: #000000 2px solid; MARGIN-RIGHT: 0px">
<DIV style="FONT: 10pt arial">----- Original Message ----- </DIV>
<DIV
style="BACKGROUND: #e4e4e4; FONT: 10pt arial; font-color: black"><B>From:</B>
<A title=jhunter@aptcodecs.com href="mailto:jhunter@aptcodecs.com">James
Hunter</A> </DIV>
<DIV style="FONT: 10pt arial"><B>To:</B> <A title=winpcap-users@winpcap.org
href="mailto:winpcap-users@winpcap.org">winpcap-users@winpcap.org</A> </DIV>
<DIV style="FONT: 10pt arial"><B>Sent:</B> Tuesday, May 19, 2009 8:33 AM</DIV>
<DIV style="FONT: 10pt arial"><B>Subject:</B> [Winpcap-users] pcap_loop
callback timing problem..</DIV>
<DIV><BR></DIV>
<DIV class=Section1>
<P class=MsoNormal>Hi,<o:p></o:p></P>
<P class=MsoNormal><o:p> </o:p></P>
<P class=MsoNormal>I`m creating an application that can sniff packets from a
network interface real-time, whilst repeating only certain packets to a
different network interface.<o:p></o:p></P>
<P class=MsoNormal><o:p> </o:p></P>
<P class=MsoNormal>I have found that the callback is being called multiple
times in a burst style, rather than as packets arrive. If the network sends
packets every 100ms, I will receive 10 callbacks immediately after each other
– with each burst of 10 callbacks spaced about a second apart..<o:p></o:p></P>
<P class=MsoNormal><o:p> </o:p></P></DIV></BLOCKQUOTE>
<P class=MsoNormal><o:p><FONT face="Courier New" size=2>This is the default
behavior of WinPcap. WinPcap is a capture engine, so it favors efficiency and
performance over low latency in receiving packets. Basically packets are stored
in the kernel and brought to user mode in batches, checking these 2
conditions</FONT></o:p></P>
<P class=MsoNormal><o:p><FONT face="Courier New" size=2>1. a timeout occurs (the
timeout that you set in pcap_openlive)</FONT></o:p></P>
<P class=MsoNormal><o:p><FONT face="Courier New" size=2>2. at least mintocopy
bytes are present in the kernel buffer (by default it's 16k if you use
pcap_openlive)</FONT></o:p></P>
<P class=MsoNormal><o:p><FONT face="Courier New" size=2>(whatever happens
first).</FONT></o:p></P>
<P class=MsoNormal><o:p><FONT face="Courier New" size=2></FONT></o:p> </P>
<P class=MsoNormal><o:p><FONT face="Courier New" size=2>You can lower this
latency by playing with mintocopy (use pcap_setmintocopy to change it) or with
the timeout, remembering that the lower those values are, the lower the latency
in receiving packets is, *obviously* with a performance hit.</FONT></o:p></P>
<P class=MsoNormal><o:p><FONT face="Courier New" size=2></FONT></o:p> </P>
<P class=MsoNormal><o:p><FONT face="Courier New" size=2>Hope it
helps</FONT></o:p></P>
<P class=MsoNormal><o:p><FONT face="Courier New" size=2>GV</FONT></o:p></P>
<BLOCKQUOTE
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<P class=MsoNormal><o:p><FONT face="Courier New"
size=2></FONT></o:p> </P>
<P class=MsoNormal>This is causing problems at the far end which expects a
constant flow of packets, is there anything I can do get the callback to
trigger as and when packets arrive?<o:p></o:p></P>
<P class=MsoNormal><o:p> </o:p></P>
<P class=MsoNormal>Regards,<o:p></o:p></P>
<P class=MsoNormal><o:p> </o:p></P>
<P class=MsoNormal>James<o:p></o:p></P>
<P class=MsoNormal><o:p> </o:p></P>
<P>
<HR>
<P></P>_______________________________________________<BR>Winpcap-users
mailing
list<BR>Winpcap-users@winpcap.org<BR>https://www.winpcap.org/mailman/listinfo/winpcap-users<BR></BLOCKQUOTE></BODY></HTML>