The Tcpdump Group git mirrors - tcpdump/blob - README

   1 @(#) $Header: /tcpdump/master/tcpdump/README,v 1.68 2008-12-15 00:05:27 guy Exp $ (LBL)
   2
   3 TCPDUMP 4.x.y
   4 Now maintained by "The Tcpdump Group"
   5 See             www.tcpdump.org
   6
   7 Please send inquiries/comments/reports to:
   8         tcpdump-workers@lists.tcpdump.org
   9
  10 Anonymous Git is available via:
  11         git clone git://bpf.tcpdump.org/tcpdump
  12
  13 Version 4.x.y of TCPDUMP can be retrieved with the CVS tag "tcpdump_4_xrely":
  14         cvs -d :pserver:cvs.tcpdump.org:/tcpdump/master checkout -r tcpdump_4_xrely tcpdump
  15
  16 Please submit patches by forking the branch on GitHub at
  17
  18         http://github.com/the-tcpdump-group/tcpdump/tree/master
  19
  20 and issuing a pull request.
  21
  22 formerly from   Lawrence Berkeley National Laboratory
  23                 Network Research Group <tcpdump@ee.lbl.gov>
  24                 ftp://ftp.ee.lbl.gov/tcpdump.tar.Z (3.4)
  25
  26 This directory contains source code for tcpdump, a tool for network
  27 monitoring and data acquisition.  This software was originally
  28 developed by the Network Research Group at the Lawrence Berkeley
  29 National Laboratory.  The original distribution is available via
  30 anonymous ftp to ftp.ee.lbl.gov, in tcpdump.tar.Z.  More recent
  31 development is performed at tcpdump.org, http://www.tcpdump.org/
  32
  33 Tcpdump uses libpcap, a system-independent interface for user-level
  34 packet capture.  Before building tcpdump, you must first retrieve and
  35 build libpcap, also originally from LBL and now being maintained by
  36 tcpdump.org; see http://www.tcpdump.org/ .
  37
  38 Once libpcap is built (either install it or make sure it's in
  39 ../libpcap), you can build tcpdump using the procedure in the INSTALL
  40 file.
  41
  42 The program is loosely based on SMI's "etherfind" although none of the
  43 etherfind code remains.  It was originally written by Van Jacobson as
  44 part of an ongoing research project to investigate and improve tcp and
  45 internet gateway performance.  The parts of the program originally
  46 taken from Sun's etherfind were later re-written by Steven McCanne of
  47 LBL.  To insure that there would be no vestige of proprietary code in
  48 tcpdump, Steve wrote these pieces from the specification given by the
  49 manual entry, with no access to the source of tcpdump or etherfind.
  50
  51 Over the past few years, tcpdump has been steadily improved by the
  52 excellent contributions from the Internet community (just browse
  53 through the CHANGES file).  We are grateful for all the input.
  54
  55 Richard Stevens gives an excellent treatment of the Internet protocols
  56 in his book ``TCP/IP Illustrated, Volume 1''. If you want to learn more
  57 about tcpdump and how to interpret its output, pick up this book.
  58
  59 Some tools for viewing and analyzing tcpdump trace files are available
  60 from the Internet Traffic Archive:
  61
  62         http://www.acm.org/sigcomm/ITA/
  63
  64 Another tool that tcpdump users might find useful is tcpslice:
  65
  66         ftp://ftp.ee.lbl.gov/tcpslice.tar.Z
  67
  68 It is a program that can be used to extract portions of tcpdump binary
  69 trace files. See the above distribution for further details and
  70 documentation.
  71
  72 Problems, bugs, questions, desirable enhancements, etc. should be sent
  73 to the address "tcpdump-workers@lists.tcpdump.org".  Bugs, support
  74 requests, and feature requests may also be submitted on the GitHub issue
  75 tracker for tcpdump at
  76
  77         https://github.com/the-tcpdump-group/tcpdump/issues
  78
  79 Source code contributions, etc. should be sent to the email address
  80 above or submitted by forking the branch on GitHub at
  81
  82         http://github.com/the-tcpdump-group/tcpdump/tree/master
  83
  84 and issuing a pull request.
  85
  86 Current versions can be found at www.tcpdump.org.
  87
  88  - The TCPdump team
  89
  90 original text by: Steve McCanne, Craig Leres, Van Jacobson
  91
  92 -------------------------------------
  93 This directory also contains some short awk programs intended as
  94 examples of ways to reduce tcpdump data when you're tracking
  95 particular network problems:
  96
  97 send-ack.awk
  98         Simplifies the tcpdump trace for an ftp (or other unidirectional
  99         tcp transfer).  Since we assume that one host only sends and
 100         the other only acks, all address information is left off and
 101         we just note if the packet is a "send" or an "ack".
 102
 103         There is one output line per line of the original trace.
 104         Field 1 is the packet time in decimal seconds, relative
 105         to the start of the conversation.  Field 2 is delta-time
 106         from last packet.  Field 3 is packet type/direction.
 107         "Send" means data going from sender to receiver, "ack"
 108         means an ack going from the receiver to the sender.  A
 109         preceding "*" indicates that the data is a retransmission.
 110         A preceding "-" indicates a hole in the sequence space
 111         (i.e., missing packet(s)), a "#" means an odd-size (not max
 112         seg size) packet.  Field 4 has the packet flags
 113         (same format as raw trace).  Field 5 is the sequence
 114         number (start seq. num for sender, next expected seq number
 115         for acks).  The number in parens following an ack is
 116         the delta-time from the first send of the packet to the
 117         ack.  A number in parens following a send is the
 118         delta-time from the first send of the packet to the
 119         current send (on duplicate packets only).  Duplicate
 120         sends or acks have a number in square brackets showing
 121         the number of duplicates so far.
 122
 123         Here is a short sample from near the start of an ftp:
 124                 3.00    0.20   send . 512
 125                 3.20    0.20    ack . 1024  (0.20)
 126                 3.20    0.00   send P 1024
 127                 3.40    0.20    ack . 1536  (0.20)
 128                 3.80    0.40 * send . 0  (3.80) [2]
 129                 3.82    0.02 *  ack . 1536  (0.62) [2]
 130         Three seconds into the conversation, bytes 512 through 1023
 131         were sent.  200ms later they were acked.  Shortly thereafter
 132         bytes 1024-1535 were sent and again acked after 200ms.
 133         Then, for no apparent reason, 0-511 is retransmitted, 3.8
 134         seconds after its initial send (the round trip time for this
 135         ftp was 1sec, +-500ms).  Since the receiver is expecting
 136         1536, 1536 is re-acked when 0 arrives.
 137
 138 packetdat.awk
 139         Computes chunk summary data for an ftp (or similar
 140         unidirectional tcp transfer). [A "chunk" refers to
 141         a chunk of the sequence space -- essentially the packet
 142         sequence number divided by the max segment size.]
 143
 144         A summary line is printed showing the number of chunks,
 145         the number of packets it took to send that many chunks
 146         (if there are no lost or duplicated packets, the number
 147         of packets should equal the number of chunks) and the
 148         number of acks.
 149
 150         Following the summary line is one line of information
 151         per chunk.  The line contains eight fields:
 152            1 - the chunk number
 153            2 - the start sequence number for this chunk
 154            3 - time of first send
 155            4 - time of last send
 156            5 - time of first ack
 157            6 - time of last ack
 158            7 - number of times chunk was sent
 159            8 - number of times chunk was acked
 160         (all times are in decimal seconds, relative to the start
 161         of the conversation.)
 162
 163         As an example, here is the first part of the output for
 164         an ftp trace:
 165
 166         # 134 chunks.  536 packets sent.  508 acks.
 167         1       1       0.00    5.80    0.20    0.20    4       1
 168         2       513     0.28    6.20    0.40    0.40    4       1
 169         3       1025    1.16    6.32    1.20    1.20    4       1
 170         4       1561    1.86    15.00   2.00    2.00    6       1
 171         5       2049    2.16    15.44   2.20    2.20    5       1
 172         6       2585    2.64    16.44   2.80    2.80    5       1
 173         7       3073    3.00    16.66   3.20    3.20    4       1
 174         8       3609    3.20    17.24   3.40    5.82    4       11
 175         9       4097    6.02    6.58    6.20    6.80    2       5
 176
 177         This says that 134 chunks were transferred (about 70K
 178         since the average packet size was 512 bytes).  It took
 179         536 packets to transfer the data (i.e., on the average
 180         each chunk was transmitted four times).  Looking at,
 181         say, chunk 4, we see it represents the 512 bytes of
 182         sequence space from 1561 to 2048.  It was first sent
 183         1.86 seconds into the conversation.  It was last
 184         sent 15 seconds into the conversation and was sent
 185         a total of 6 times (i.e., it was retransmitted every
 186         2 seconds on the average).  It was acked once, 140ms
 187         after it first arrived.
 188
 189 stime.awk
 190 atime.awk
 191         Output one line per send or ack, respectively, in the form
 192                 <time> <seq. number>
 193         where <time> is the time in seconds since the start of the
 194         transfer and <seq. number> is the sequence number being sent
 195         or acked.  I typically plot this data looking for suspicious
 196         patterns.
 197
 198
 199 The problem I was looking at was the bulk-data-transfer
 200 throughput of medium delay network paths (1-6 sec.  round trip
 201 time) under typical DARPA Internet conditions.  The trace of the
 202 ftp transfer of a large file was used as the raw data source.
 203 The method was:
 204
 205   - On a local host (but not the Sun running tcpdump), connect to
 206     the remote ftp.
 207
 208   - On the monitor Sun, start the trace going.  E.g.,
 209       tcpdump host local-host and remote-host and port ftp-data >tracefile
 210
 211   - On local, do either a get or put of a large file (~500KB),
 212     preferably to the null device (to minimize effects like
 213     closing the receive window while waiting for a disk write).
 214
 215   - When transfer is finished, stop tcpdump.  Use awk to make up
 216     two files of summary data (maxsize is the maximum packet size,
 217     tracedata is the file of tcpdump tracedata):
 218       awk -f send-ack.awk packetsize=avgsize tracedata >sa
 219       awk -f packetdat.awk packetsize=avgsize tracedata >pd
 220
 221   - While the summary data files are printing, take a look at
 222     how the transfer behaved:
 223       awk -f stime.awk tracedata | xgraph
 224     (90% of what you learn seems to happen in this step).
 225
 226   - Do all of the above steps several times, both directions,
 227     at different times of day, with different protocol
 228     implementations on the other end.
 229
 230   - Using one of the Unix data analysis packages (in my case,
 231     S and Gary Perlman's Unix|Stat), spend a few months staring
 232     at the data.
 233
 234   - Change something in the local protocol implementation and
 235     redo the steps above.
 236
 237   - Once a week, tell your funding agent that you're discovering
 238     wonderful things and you'll write up that research report
 239     "real soon now".