1 @(#) $Header: /tcpdump/master/tcpdump/README,v 1.54 1999-10-07 23:47:09 mcr Exp $ (LBL)
4 Lawrence Berkeley National Laboratory
7 ftp://ftp.ee.lbl.gov/tcpdump.tar.Z
9 This directory contains source code for tcpdump, a tool for network
10 monitoring and data acquisition. The original distribution is
11 available via anonymous ftp to ftp.ee.lbl.gov, in tcpdump.tar.Z.
13 Tcpdump now uses libpcap, a system-independent interface for user-level
14 packet capture. Before building tcpdump, you must first retrieve and
15 build libpcap, also from LBL, in:
17 ftp://ftp.ee.lbl.gov/libpcap.tar.Z
19 Once libpcap is built (either install it or make sure it's in
20 ../libpcap), you can build tcpdump using the procedure in the INSTALL
23 The program is loosely based on SMI's "etherfind" although none of the
24 etherfind code remains. It was originally written by Van Jacobson as
25 part of an ongoing research project to investigate and improve tcp and
26 internet gateway performance. The parts of the program originally
27 taken from Sun's etherfind were later re-written by Steven McCanne of
28 LBL. To insure that there would be no vestige of proprietary code in
29 tcpdump, Steve wrote these pieces from the specification given by the
30 manual entry, with no access to the source of tcpdump or etherfind.
32 Over the past few years, tcpdump has been steadily improved by the
33 excellent contributions from the Internet community (just browse
34 through the CHANGES file). We are grateful for all the input.
36 Richard Stevens gives an excellent treatment of the Internet protocols
37 in his book ``TCP/IP Illustrated, Volume 1''. If you want to learn more
38 about tcpdump and how to interpret its output, pick up this book.
40 Some tools for viewing and analyzing tcpdump trace files are available
41 from the Internet Traffic Archive:
43 http://www.acm.org/sigcomm/ITA/
45 Another tool that tcpdump users might find useful is tcpslice:
47 ftp://ftp.ee.lbl.gov/tcpslice.tar.Z
49 It is a program that can be used to extract portions of tcpdump binary
50 trace files. See the above distribution for further details and
53 Problems, bugs, questions, desirable enhancements, source code
54 contributions, etc., should be sent to the email address
60 -------------------------------------
61 This directory also contains some short awk programs intended as
62 examples of ways to reduce tcpdump data when you're tracking
63 particular network problems:
66 Simplifies the tcpdump trace for an ftp (or other unidirectional
67 tcp transfer). Since we assume that one host only sends and
68 the other only acks, all address information is left off and
69 we just note if the packet is a "send" or an "ack".
71 There is one output line per line of the original trace.
72 Field 1 is the packet time in decimal seconds, relative
73 to the start of the conversation. Field 2 is delta-time
74 from last packet. Field 3 is packet type/direction.
75 "Send" means data going from sender to receiver, "ack"
76 means an ack going from the receiver to the sender. A
77 preceding "*" indicates that the data is a retransmission.
78 A preceding "-" indicates a hole in the sequence space
79 (i.e., missing packet(s)), a "#" means an odd-size (not max
80 seg size) packet. Field 4 has the packet flags
81 (same format as raw trace). Field 5 is the sequence
82 number (start seq. num for sender, next expected seq number
83 for acks). The number in parens following an ack is
84 the delta-time from the first send of the packet to the
85 ack. A number in parens following a send is the
86 delta-time from the first send of the packet to the
87 current send (on duplicate packets only). Duplicate
88 sends or acks have a number in square brackets showing
89 the number of duplicates so far.
91 Here is a short sample from near the start of an ftp:
93 3.20 0.20 ack . 1024 (0.20)
95 3.40 0.20 ack . 1536 (0.20)
96 3.80 0.40 * send . 0 (3.80) [2]
97 3.82 0.02 * ack . 1536 (0.62) [2]
98 Three seconds into the conversation, bytes 512 through 1023
99 were sent. 200ms later they were acked. Shortly thereafter
100 bytes 1024-1535 were sent and again acked after 200ms.
101 Then, for no apparent reason, 0-511 is retransmitted, 3.8
102 seconds after its initial send (the round trip time for this
103 ftp was 1sec, +-500ms). Since the receiver is expecting
104 1536, 1536 is re-acked when 0 arrives.
107 Computes chunk summary data for an ftp (or similar
108 unidirectional tcp transfer). [A "chunk" refers to
109 a chunk of the sequence space -- essentially the packet
110 sequence number divided by the max segment size.]
112 A summary line is printed showing the number of chunks,
113 the number of packets it took to send that many chunks
114 (if there are no lost or duplicated packets, the number
115 of packets should equal the number of chunks) and the
118 Following the summary line is one line of information
119 per chunk. The line contains eight fields:
121 2 - the start sequence number for this chunk
122 3 - time of first send
123 4 - time of last send
124 5 - time of first ack
126 7 - number of times chunk was sent
127 8 - number of times chunk was acked
128 (all times are in decimal seconds, relative to the start
129 of the conversation.)
131 As an example, here is the first part of the output for
134 # 134 chunks. 536 packets sent. 508 acks.
135 1 1 0.00 5.80 0.20 0.20 4 1
136 2 513 0.28 6.20 0.40 0.40 4 1
137 3 1025 1.16 6.32 1.20 1.20 4 1
138 4 1561 1.86 15.00 2.00 2.00 6 1
139 5 2049 2.16 15.44 2.20 2.20 5 1
140 6 2585 2.64 16.44 2.80 2.80 5 1
141 7 3073 3.00 16.66 3.20 3.20 4 1
142 8 3609 3.20 17.24 3.40 5.82 4 11
143 9 4097 6.02 6.58 6.20 6.80 2 5
145 This says that 134 chunks were transferred (about 70K
146 since the average packet size was 512 bytes). It took
147 536 packets to transfer the data (i.e., on the average
148 each chunk was transmitted four times). Looking at,
149 say, chunk 4, we see it represents the 512 bytes of
150 sequence space from 1561 to 2048. It was first sent
151 1.86 seconds into the conversation. It was last
152 sent 15 seconds into the conversation and was sent
153 a total of 6 times (i.e., it was retransmitted every
154 2 seconds on the average). It was acked once, 140ms
155 after it first arrived.
159 Output one line per send or ack, respectively, in the form
161 where <time> is the time in seconds since the start of the
162 transfer and <seq. number> is the sequence number being sent
163 or acked. I typically plot this data looking for suspicious
167 The problem I was looking at was the bulk-data-transfer
168 throughput of medium delay network paths (1-6 sec. round trip
169 time) under typical DARPA Internet conditions. The trace of the
170 ftp transfer of a large file was used as the raw data source.
173 - On a local host (but not the Sun running tcpdump), connect to
176 - On the monitor Sun, start the trace going. E.g.,
177 tcpdump host local-host and remote-host and port ftp-data >tracefile
179 - On local, do either a get or put of a large file (~500KB),
180 preferably to the null device (to minimize effects like
181 closing the receive window while waiting for a disk write).
183 - When transfer is finished, stop tcpdump. Use awk to make up
184 two files of summary data (maxsize is the maximum packet size,
185 tracedata is the file of tcpdump tracedata):
186 awk -f send-ack.awk packetsize=avgsize tracedata >sa
187 awk -f packetdat.awk packetsize=avgsize tracedata >pd
189 - While the summary data files are printing, take a look at
190 how the transfer behaved:
191 awk -f stime.awk tracedata | xgraph
192 (90% of what you learn seems to happen in this step).
194 - Do all of the above steps several times, both directions,
195 at different times of day, with different protocol
196 implementations on the other end.
198 - Using one of the Unix data analysis packages (in my case,
199 S and Gary Perlman's Unix|Stat), spend a few months staring
202 - Change something in the local protocol implementation and
203 redo the steps above.
205 - Once a week, tell your funding agent that you're discovering
206 wonderful things and you'll write up that research report
projects / tcpdump / blob