Technical data

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Troubleshooting Techniques and Tools

1.2 Isolating Problems

• window

The number of bytes of receive buffer space available from the other direction

on this connection.

•

urgent

Indicates there is urgent data in the packet.

• options

The TCP options enclosed in angle brackets. For example:

The

src

dst

, and

flags

ﬁelds are always present. The other ﬁelds depend on the

contents of the packet’s TCP protocol header and are output only if appropriate.

Examples

The following example shows the opening portion of an RLOGIN session from

host rtsg to host csam:

rtsg.1023 > csam.login: S 768512:768512(0) win 4096 <mss 1024>

csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win 4096 <mss 1024>

rtsg.1023 > csam.login: . ack 1 win 4096

rtsg.1023 > csam.login: P 1:2(1) ack 1 win 4096

csam.login > rtsg.1023: . ack 2 win 4096

rtsg.1023 > csam.login: P 2:21(19) ack 1 win 4096

csam.login > rtsg.1023: P 1:2(1) ack 21 win 4077

csam.login > rtsg.1023: P 2:3(1) ack 21 win 4077 urg 1

csam.login > rtsg.1023: P 3:4(1) ack 21 win 4077 urg 1

The example shows the following sequence of communication:

• The ﬁrst line indicates that TCP port 1023 on system rtsg sent a packet

to port login on host csam. The S indicates that the SYN ﬂag was set.

The packet sequence number was 768512 and it contained no data. (The

notation is ﬁrst:last(nbytes), which means sequence numbers ﬁrst up to but

not including last, which is nbytes bytes of user data.) There was no piggy-

backed ack, the available receive window was 4096 bytes and there was a

max-segment-size

option requesting an

mss

of 1024 bytes.

• Host csam replies with a similar packet except that it includes a piggy-backed

ack for the SYN sent by rtsg.

• Host rtsg then sends an ack reply to the SYN sent by csam. The period (.)

means no ﬂags were set. The packet contained no data, so there is no data

sequence number. Note that the ack sequence number is a small integer (1).

The ﬁrst time

tcpdump

sees a TCP conversation, it displays the sequence

number from the packet.

• On subsequent packets of the conversation, the difference between the current

packet’s sequence number and this initial sequence number is displayed.

Thus, sequence numbers after the ﬁrst can be interpreted as relative byte

positions in the conversation’s data stream (with the ﬁrst data byte each

direction being 1). The -S option overrides this feature, causing the original

sequence numbers to be output.

• The sixth line indicates that host rtsg sends host csam 19 bytes of data (bytes

2 through 20 in the rtsg-to-csam side of the conversation). The PUSH ﬂag is

set in the packet.

Troubleshooting Techniques and Tools 1–17