Installing and Administering LAN/9000 Software

Chapter 4 69
Troubleshooting LAN/9000
Troubleshooting Q & A
may also be in the upper layer software (ftp or telnet).
Also, it is possible that too little memory is allocated to hold fragmented
messages in the IP layer. IP messages may be fragmented into smaller
parts when the message is sent through the system. The fragments must
be held in memory for some time so that the entire message can be
reassembled because the fragments arrive at the destination at different
times and possibly out of order. Normally, fragmentation reassembly
memory is limited arbitrarily so that incomplete messages do not
consume all of memory, which could cripple the system. During stressful
networking activity, some fragments might never be delivered because
they are typically dropped in transit, for example, due to a collision or
resource limitations on an intermediate system. However, fragments
might also not be delivered (“dropped”) if there is insufficient
fragmentation reassembly memory on the destination system during
periods of high network activity. This can degrade performance due to
retransmissions of data. If the problem is due to a high number of
fragments dropped after time-out (see the output from the command
netstat -sp ip), you might want to increase the size of the
fragmentation reassembly memory by changing the
ip_reass_mem_limit value using the ndd command. (The default is 2
MB for the system.) Enter the command /usr/bin/ndd -h to display
ndd(1M) parameters and their use.
Deferred transmissions/collisions: Why is there a significant
increase in the number of deferred transmissions and collisions on my
network?
Answer: On IEEE802.3/Ethernet networks, a collision occurs when two
or more stations try to transmit data simultaneously. A deferred
transmission occurs if the network is busy when a station attempts to
transmit data. The number of collisions and deferred transmissions on a
node is directly related to the network load. As the network load
increases, the number of collisions and deferred transmissions also
increase.
When high-performance systems are placed on a LAN with
lower-performance systems (HP or non-HP systems), it is possible for the
high-performance systems to use a higher percentage of the LAN
bandwidth with network traffic intensive applications.
High-performance systems generate network traffic at a 10Mbits/s link
rate, and lower-performance systems cannot match this rate. Heavily
loaded LAN networks can result in lower throughput performance on
lower-performance systems.