Technical data

Troubleshooting Techniques and Tools
1.2 Isolating Problems
1.2.5.4 Restrictions
The following restrictions apply to using tcpdump on OpenVMS:
• Copy-all mode is on by default on OpenVMS.
• Promiscuous mode is not available, so tracing must be issued on either the
source or destination host.
• Only Ethernet native tracing on is supported on OpenVMS.
• Only one user may trace at a time on OpenVMS using either
tcpdump or
tcptrace .
• Name server inverse queries are not dumped correctly: The (empty) question
section is displayed rather than real query in the answer section.
• A packet trace that crosses a daylight saving time change produces skewed
time stamps (the time change is ignored).
1.2.5.5 Reducing Discarded Packets
When packets are copied by the TCP/IP kernel, it places them into a ring buffer
that is emptied by
tcpdump . If packets are received fast enough, the ring will fill
up and the TCP/IP kernel discards (drops) packets until tcpdump has caught up.
Because tcpdump has not seen these dropped packets, it cannot tell whether they
were relevant to the requested trace.
If the option
-B is used, tcpdump indicates when the drops occur by issuing a
BUFFERSFULL error. This can be useful if the drops occur outside the sequence
being analyzed.
There are several methods for reducing the number of packet drops:
• Specify a more detailed filter in the tcpdump command.
• Trace to a file instead of SYS$OUTPUT using -w filename . For best results,
use a disk with little activity or a RAM disk.
• Increase the number of buffers in the ring using -b buffers . The default
for Alpha systems is 400. For VAX systems, the default is 50. The processes
working set quota (WSQUOTA) may need to be increased for larger numbers
than the default.
• Increase the default process priority of the process that issued the
tcpdump
command.
1.2.6 Monitoring Socket Activity
TCP/IP Services provides a call tracing facility that can be used to help
characterize and debug the use of the sockets API for many applications.
To enable tracing, define the TCPIP$SOCKET_TRACE logical name. The logical
name accepts the following arguments:
• 1 or 0
Specify 1 to enable socket tracing, or 0 to disable socket tracing. When the
logical name is set to 1, the output from the trace is displayed interactively.
For example:
$ DEFINE TCPIP$SOCKET_TRACE 1
• Log file name
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