README revision 214478
1@(#) $Header: /tcpdump/master/tcpdump/README,v 1.68 2008-12-15 00:05:27 guy Exp $ (LBL) 2 3TCPDUMP 4.x.y 4Now maintained by "The Tcpdump Group" 5See www.tcpdump.org 6 7Please send inquiries/comments/reports to: 8 tcpdump-workers@lists.tcpdump.org 9 10Anonymous Git is available via: 11 git clone git://bpf.tcpdump.org/tcpdump 12 13Version 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 16Please submit patches against the master copy to the tcpdump project on 17sourceforge.net. 18 19formerly from Lawrence Berkeley National Laboratory 20 Network Research Group <tcpdump@ee.lbl.gov> 21 ftp://ftp.ee.lbl.gov/tcpdump.tar.Z (3.4) 22 23This directory contains source code for tcpdump, a tool for network 24monitoring and data acquisition. This software was originally 25developed by the Network Research Group at the Lawrence Berkeley 26National Laboratory. The original distribution is available via 27anonymous ftp to ftp.ee.lbl.gov, in tcpdump.tar.Z. More recent 28development is performed at tcpdump.org, http://www.tcpdump.org/ 29 30Tcpdump uses libpcap, a system-independent interface for user-level 31packet capture. Before building tcpdump, you must first retrieve and 32build libpcap, also originally from LBL and now being maintained by 33tcpdump.org; see http://www.tcpdump.org/ . 34 35Once libpcap is built (either install it or make sure it's in 36../libpcap), you can build tcpdump using the procedure in the INSTALL 37file. 38 39The program is loosely based on SMI's "etherfind" although none of the 40etherfind code remains. It was originally written by Van Jacobson as 41part of an ongoing research project to investigate and improve tcp and 42internet gateway performance. The parts of the program originally 43taken from Sun's etherfind were later re-written by Steven McCanne of 44LBL. To insure that there would be no vestige of proprietary code in 45tcpdump, Steve wrote these pieces from the specification given by the 46manual entry, with no access to the source of tcpdump or etherfind. 47 48Over the past few years, tcpdump has been steadily improved by the 49excellent contributions from the Internet community (just browse 50through the CHANGES file). We are grateful for all the input. 51 52Richard Stevens gives an excellent treatment of the Internet protocols 53in his book ``TCP/IP Illustrated, Volume 1''. If you want to learn more 54about tcpdump and how to interpret its output, pick up this book. 55 56Some tools for viewing and analyzing tcpdump trace files are available 57from the Internet Traffic Archive: 58 59 http://www.acm.org/sigcomm/ITA/ 60 61Another tool that tcpdump users might find useful is tcpslice: 62 63 ftp://ftp.ee.lbl.gov/tcpslice.tar.Z 64 65It is a program that can be used to extract portions of tcpdump binary 66trace files. See the above distribution for further details and 67documentation. 68 69Problems, bugs, questions, desirable enhancements, etc. should be sent 70to the address "tcpdump-workers@lists.tcpdump.org". Bugs, support 71requests, and feature requests may also be submitted on the SourceForge 72site for tcpdump at 73 74 http://sourceforge.net/projects/tcpdump/ 75 76Source code contributions, etc. should be sent to the email address 77submitted as patches on the SourceForge site for tcpdump. 78 79Current versions can be found at www.tcpdump.org, or the SourceForge 80site for tcpdump. 81 82 - The TCPdump team 83 84original text by: Steve McCanne, Craig Leres, Van Jacobson 85 86------------------------------------- 87This directory also contains some short awk programs intended as 88examples of ways to reduce tcpdump data when you're tracking 89particular network problems: 90 91send-ack.awk 92 Simplifies the tcpdump trace for an ftp (or other unidirectional 93 tcp transfer). Since we assume that one host only sends and 94 the other only acks, all address information is left off and 95 we just note if the packet is a "send" or an "ack". 96 97 There is one output line per line of the original trace. 98 Field 1 is the packet time in decimal seconds, relative 99 to the start of the conversation. Field 2 is delta-time 100 from last packet. Field 3 is packet type/direction. 101 "Send" means data going from sender to receiver, "ack" 102 means an ack going from the receiver to the sender. A 103 preceding "*" indicates that the data is a retransmission. 104 A preceding "-" indicates a hole in the sequence space 105 (i.e., missing packet(s)), a "#" means an odd-size (not max 106 seg size) packet. Field 4 has the packet flags 107 (same format as raw trace). Field 5 is the sequence 108 number (start seq. num for sender, next expected seq number 109 for acks). The number in parens following an ack is 110 the delta-time from the first send of the packet to the 111 ack. A number in parens following a send is the 112 delta-time from the first send of the packet to the 113 current send (on duplicate packets only). Duplicate 114 sends or acks have a number in square brackets showing 115 the number of duplicates so far. 116 117 Here is a short sample from near the start of an ftp: 118 3.00 0.20 send . 512 119 3.20 0.20 ack . 1024 (0.20) 120 3.20 0.00 send P 1024 121 3.40 0.20 ack . 1536 (0.20) 122 3.80 0.40 * send . 0 (3.80) [2] 123 3.82 0.02 * ack . 1536 (0.62) [2] 124 Three seconds into the conversation, bytes 512 through 1023 125 were sent. 200ms later they were acked. Shortly thereafter 126 bytes 1024-1535 were sent and again acked after 200ms. 127 Then, for no apparent reason, 0-511 is retransmitted, 3.8 128 seconds after its initial send (the round trip time for this 129 ftp was 1sec, +-500ms). Since the receiver is expecting 130 1536, 1536 is re-acked when 0 arrives. 131 132packetdat.awk 133 Computes chunk summary data for an ftp (or similar 134 unidirectional tcp transfer). [A "chunk" refers to 135 a chunk of the sequence space -- essentially the packet 136 sequence number divided by the max segment size.] 137 138 A summary line is printed showing the number of chunks, 139 the number of packets it took to send that many chunks 140 (if there are no lost or duplicated packets, the number 141 of packets should equal the number of chunks) and the 142 number of acks. 143 144 Following the summary line is one line of information 145 per chunk. The line contains eight fields: 146 1 - the chunk number 147 2 - the start sequence number for this chunk 148 3 - time of first send 149 4 - time of last send 150 5 - time of first ack 151 6 - time of last ack 152 7 - number of times chunk was sent 153 8 - number of times chunk was acked 154 (all times are in decimal seconds, relative to the start 155 of the conversation.) 156 157 As an example, here is the first part of the output for 158 an ftp trace: 159 160 # 134 chunks. 536 packets sent. 508 acks. 161 1 1 0.00 5.80 0.20 0.20 4 1 162 2 513 0.28 6.20 0.40 0.40 4 1 163 3 1025 1.16 6.32 1.20 1.20 4 1 164 4 1561 1.86 15.00 2.00 2.00 6 1 165 5 2049 2.16 15.44 2.20 2.20 5 1 166 6 2585 2.64 16.44 2.80 2.80 5 1 167 7 3073 3.00 16.66 3.20 3.20 4 1 168 8 3609 3.20 17.24 3.40 5.82 4 11 169 9 4097 6.02 6.58 6.20 6.80 2 5 170 171 This says that 134 chunks were transferred (about 70K 172 since the average packet size was 512 bytes). It took 173 536 packets to transfer the data (i.e., on the average 174 each chunk was transmitted four times). Looking at, 175 say, chunk 4, we see it represents the 512 bytes of 176 sequence space from 1561 to 2048. It was first sent 177 1.86 seconds into the conversation. It was last 178 sent 15 seconds into the conversation and was sent 179 a total of 6 times (i.e., it was retransmitted every 180 2 seconds on the average). It was acked once, 140ms 181 after it first arrived. 182 183stime.awk 184atime.awk 185 Output one line per send or ack, respectively, in the form 186 <time> <seq. number> 187 where <time> is the time in seconds since the start of the 188 transfer and <seq. number> is the sequence number being sent 189 or acked. I typically plot this data looking for suspicious 190 patterns. 191 192 193The problem I was looking at was the bulk-data-transfer 194throughput of medium delay network paths (1-6 sec. round trip 195time) under typical DARPA Internet conditions. The trace of the 196ftp transfer of a large file was used as the raw data source. 197The method was: 198 199 - On a local host (but not the Sun running tcpdump), connect to 200 the remote ftp. 201 202 - On the monitor Sun, start the trace going. E.g., 203 tcpdump host local-host and remote-host and port ftp-data >tracefile 204 205 - On local, do either a get or put of a large file (~500KB), 206 preferably to the null device (to minimize effects like 207 closing the receive window while waiting for a disk write). 208 209 - When transfer is finished, stop tcpdump. Use awk to make up 210 two files of summary data (maxsize is the maximum packet size, 211 tracedata is the file of tcpdump tracedata): 212 awk -f send-ack.awk packetsize=avgsize tracedata >sa 213 awk -f packetdat.awk packetsize=avgsize tracedata >pd 214 215 - While the summary data files are printing, take a look at 216 how the transfer behaved: 217 awk -f stime.awk tracedata | xgraph 218 (90% of what you learn seems to happen in this step). 219 220 - Do all of the above steps several times, both directions, 221 at different times of day, with different protocol 222 implementations on the other end. 223 224 - Using one of the Unix data analysis packages (in my case, 225 S and Gary Perlman's Unix|Stat), spend a few months staring 226 at the data. 227 228 - Change something in the local protocol implementation and 229 redo the steps above. 230 231 - Once a week, tell your funding agent that you're discovering 232 wonderful things and you'll write up that research report 233 "real soon now". 234