HP-UX TCP/IP Performance White Paper, March 2008

0 embryonic connections dropped

4985775 segments updated rtt (of 4985775 attempts)

0 retransmit timeouts

0 connections dropped by rexmit timeout

0 persist timeouts

2 keepalive timeouts

0 keepalive probe sent

0 connections dropped by keepalive

0 connect requests dropped due to full queue

0 connect requests dropped due to no listener

0 suspect connect requests dropped due to aging

0 suspect connect requests dropped due to rate

Retransmitted segments are an indication of loss, delay, or reordering of segments on the network, and will

have a strong negative effect on TCP throughput.

Important fields for the netstat –p udp command include the bad checksums and socket

overflows fields, which should have low values. For example:

# netstat -p udp

udp:

0 incomplete headers

11 bad checksums

3318 socket overflows

The previous example shows a value of 3318 in the socket overflows field, which indicates that the

UDP socket buffer may be too small.

You can tune the default UDP socket buffer size larger using the following ndd command:

# ndd -set /dev/sockets socket_udp_rcvbuf_default 65535

If you have UDP applications that need larger socket buffer sizes, HP recommends that you set the socket

buffer sizes using setsockopt() instead of setting the system-wide default to a larger value.

Regardless, it should be noted that simply increasing the size of the socket buffer only helps if the length of

the overload condition is short and the burst of traffic is less than the size of the socket buffer. Increasing the

socket buffer size will not help if the overload is sustained, or if the overload is caused by a slow or

unresponsive application that is not receiving data from the connection at a rate at least equal to the rate

from the network.

There are two most common ways to identify the overflowing UDP socket:

• ndd –get /dev/ip ip_udp_status lists the UDP fanout table and one of the columns is

“overflows”. If the UDP port is long-lived, this will tell which UDP socket is experiencing the

overflows.

• Collect a network trace during one of the overflows and look for an ICMP SOURCE QUENCH

packet. If the SOURCE QUENCH was generated because of a UDP socket overflow, the SOURCE

QUENCH packet will tell you which UDP socket is overflowing.