@(#) $Header: /tcpdump/master/tcpdump/README,v 1.66.2.1 2008-05-30 01:38:20 guy Exp $ (LBL)TCPDUMP 3.9Now maintained by "The Tcpdump Group"See 		www.tcpdump.orgPlease send inquiries/comments/reports to:	tcpdump-workers@lists.tcpdump.orgAnonymous CVS is available via:	cvs -d :pserver:cvs.tcpdump.org:/tcpdump/master login	(password "anoncvs")	cvs -d :pserver:cvs.tcpdump.org:/tcpdump/master checkout tcpdumpVersion 3.9 of TCPDUMP can be retrieved with the CVS tag "tcpdump_3_9rel1":	cvs -d :pserver:cvs.tcpdump.org:/tcpdump/master checkout -r tcpdump_3_9rel1 tcpdumpPlease submit patches against the master copy to the tcpdump project onsourceforge.net.formerly from 	Lawrence Berkeley National Laboratory		Network Research Group <tcpdump@ee.lbl.gov>		ftp://ftp.ee.lbl.gov/tcpdump.tar.Z (3.4)This directory contains source code for tcpdump, a tool for networkmonitoring and data acquisition.  This software was originallydeveloped by the Network Research Group at the Lawrence BerkeleyNational Laboratory.  The original distribution is available viaanonymous ftp to ftp.ee.lbl.gov, in tcpdump.tar.Z.  More recentdevelopment is performed at tcpdump.org, http://www.tcpdump.org/Tcpdump uses libpcap, a system-independent interface for user-levelpacket capture.  Before building tcpdump, you must first retrieve andbuild libpcap, also originally from LBL and now being maintained bytcpdump.org; see http://www.tcpdump.org/ .Once libpcap is built (either install it or make sure it's in../libpcap), you can build tcpdump using the procedure in the INSTALLfile.The program is loosely based on SMI's "etherfind" although none of theetherfind code remains.  It was originally written by Van Jacobson aspart of an ongoing research project to investigate and improve tcp andinternet gateway performance.  The parts of the program originallytaken from Sun's etherfind were later re-written by Steven McCanne ofLBL.  To insure that there would be no vestige of proprietary code intcpdump, Steve wrote these pieces from the specification given by themanual entry, with no access to the source of tcpdump or etherfind.Over the past few years, tcpdump has been steadily improved by theexcellent contributions from the Internet community (just browsethrough the CHANGES file).  We are grateful for all the input.Richard Stevens gives an excellent treatment of the Internet protocolsin his book ``TCP/IP Illustrated, Volume 1''. If you want to learn moreabout tcpdump and how to interpret its output, pick up this book.Some tools for viewing and analyzing tcpdump trace files are availablefrom the Internet Traffic Archive:	http://www.acm.org/sigcomm/ITA/Another tool that tcpdump users might find useful is tcpslice:	ftp://ftp.ee.lbl.gov/tcpslice.tar.ZIt is a program that can be used to extract portions of tcpdump binarytrace files. See the above distribution for further details anddocumentation.Problems, bugs, questions, desirable enhancements, etc. should be sentto the address "tcpdump-workers@lists.tcpdump.org".  Bugs, supportrequests, and feature requests may also be submitted on the SourceForgesite for tcpdump at	http://sourceforge.net/projects/tcpdump/Source code contributions, etc. should be sent to the email addresssubmitted as patches on the SourceForge site for tcpdump.Current versions can be found at www.tcpdump.org, or the SourceForgesite for tcpdump. - The TCPdump teamoriginal text by: Steve McCanne, Craig Leres, Van Jacobson-------------------------------------This directory also contains some short awk programs intended asexamples of ways to reduce tcpdump data when you're trackingparticular network problems:send-ack.awk	Simplifies the tcpdump trace for an ftp (or other unidirectional	tcp transfer).  Since we assume that one host only sends and	the other only acks, all address information is left off and	we just note if the packet is a "send" or an "ack".	There is one output line per line of the original trace.	Field 1 is the packet time in decimal seconds, relative	to the start of the conversation.  Field 2 is delta-time	from last packet.  Field 3 is packet type/direction.	"Send" means data going from sender to receiver, "ack"	means an ack going from the receiver to the sender.  A	preceding "*" indicates that the data is a retransmission.	A preceding "-" indicates a hole in the sequence space	(i.e., missing packet(s)), a "#" means an odd-size (not max	seg size) packet.  Field 4 has the packet flags	(same format as raw trace).  Field 5 is the sequence	number (start seq. num for sender, next expected seq number	for acks).  The number in parens following an ack is	the delta-time from the first send of the packet to the	ack.  A number in parens following a send is the	delta-time from the first send of the packet to the	current send (on duplicate packets only).  Duplicate	sends or acks have a number in square brackets showing	the number of duplicates so far.	Here is a short sample from near the start of an ftp:		3.00    0.20   send . 512		3.20    0.20    ack . 1024  (0.20)		3.20    0.00   send P 1024		3.40    0.20    ack . 1536  (0.20)		3.80    0.40 * send . 0  (3.80) [2]		3.82    0.02 *  ack . 1536  (0.62) [2]	Three seconds into the conversation, bytes 512 through 1023	were sent.  200ms later they were acked.  Shortly thereafter	bytes 1024-1535 were sent and again acked after 200ms.	Then, for no apparent reason, 0-511 is retransmitted, 3.8	seconds after its initial send (the round trip time for this	ftp was 1sec, +-500ms).  Since the receiver is expecting	1536, 1536 is re-acked when 0 arrives.packetdat.awk	Computes chunk summary data for an ftp (or similar	unidirectional tcp transfer). [A "chunk" refers to	a chunk of the sequence space -- essentially the packet	sequence number divided by the max segment size.]	A summary line is printed showing the number of chunks,	the number of packets it took to send that many chunks	(if there are no lost or duplicated packets, the number	of packets should equal the number of chunks) and the	number of acks.	Following the summary line is one line of information	per chunk.  The line contains eight fields:	   1 - the chunk number	   2 - the start sequence number for this chunk	   3 - time of first send	   4 - time of last send	   5 - time of first ack	   6 - time of last ack	   7 - number of times chunk was sent	   8 - number of times chunk was acked	(all times are in decimal seconds, relative to the start	of the conversation.)	As an example, here is the first part of the output for	an ftp trace:	# 134 chunks.  536 packets sent.  508 acks.	1       1       0.00    5.80    0.20    0.20    4       1	2       513     0.28    6.20    0.40    0.40    4       1	3       1025    1.16    6.32    1.20    1.20    4       1	4       1561    1.86    15.00   2.00    2.00    6       1	5       2049    2.16    15.44   2.20    2.20    5       1	6       2585    2.64    16.44   2.80    2.80    5       1	7       3073    3.00    16.66   3.20    3.20    4       1	8       3609    3.20    17.24   3.40    5.82    4       11	9       4097    6.02    6.58    6.20    6.80    2       5	This says that 134 chunks were transferred (about 70K	since the average packet size was 512 bytes).  It took	536 packets to transfer the data (i.e., on the average	each chunk was transmitted four times).  Looking at,	say, chunk 4, we see it represents the 512 bytes of	sequence space from 1561 to 2048.  It was first sent	1.86 seconds into the conversation.  It was last	sent 15 seconds into the conversation and was sent	a total of 6 times (i.e., it was retransmitted every	2 seconds on the average).  It was acked once, 140ms	after it first arrived.stime.awkatime.awk	Output one line per send or ack, respectively, in the form		<time> <seq. number>	where <time> is the time in seconds since the start of the	transfer and <seq. number> is the sequence number being sent	or acked.  I typically plot this data looking for suspicious	patterns.The problem I was looking at was the bulk-data-transferthroughput of medium delay network paths (1-6 sec.  round triptime) under typical DARPA Internet conditions.  The trace of theftp transfer of a large file was used as the raw data source.The method was:  - On a local host (but not the Sun running tcpdump), connect to    the remote ftp.  - On the monitor Sun, start the trace going.  E.g.,      tcpdump host local-host and remote-host and port ftp-data >tracefile  - On local, do either a get or put of a large file (~500KB),    preferably to the null device (to minimize effects like    closing the receive window while waiting for a disk write).  - When transfer is finished, stop tcpdump.  Use awk to make up    two files of summary data (maxsize is the maximum packet size,    tracedata is the file of tcpdump tracedata):      awk -f send-ack.awk packetsize=avgsize tracedata >sa      awk -f packetdat.awk packetsize=avgsize tracedata >pd  - While the summary data files are printing, take a look at    how the transfer behaved:      awk -f stime.awk tracedata | xgraph    (90% of what you learn seems to happen in this step).  - Do all of the above steps several times, both directions,    at different times of day, with different protocol    implementations on the other end.  - Using one of the Unix data analysis packages (in my case,    S and Gary Perlman's Unix|Stat), spend a few months staring    at the data.  - Change something in the local protocol implementation and    redo the steps above.  - Once a week, tell your funding agent that you're discovering    wonderful things and you'll write up that research report    "real soon now".