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

Appendix A: Annotated output of netstat –s (TCP, UDP, IP,

ICMP)

This chapter contains the annotated output of the netstat –s command run on an HP-UX 11i system. The

following annotated output is from a test system for illustration only. There may not be any particular

problem or issue as indicated by the output.

TCP:

The TCP statistics can be retrieved by themselves with the command netstat -p tcp.

tcp:

612755429 packets sent

This is the total number of packets sent by TCP since boot. A "TCP packet" is referred to as a "segment."

This value includes both data and control segments. A control segment could be a standalone ACK or

window update, or a SYNchronize, FINished, or ReSeT segment.

414543546 data packets (3533061789 bytes)

This is the number of TCP segments sent which carried data, and the total quantity of data they carried. If

one divides the octets (bytes) by the number of segments (packets) one has some idea of the average

quantity of data per segment. In general, TCP will move data more efficiently the greater the amount of

data per segment is, with an upper limit of the Maximum Segment Size (MSS). This is not always possible

if applications are naturally making small sends, or if the TCP_NODELAY option is set unnecessarily,

preventing TCP from aggregating multiple small sends from the application into a single large segment. For

some applications (eg Telnet, ssh) which carry user-interactive traffic (eg keystrokes) the average segment

size will necessarily be quite small.

The average segment size could also be small because remote systems are advertising small values for the

MSS (related ndd: tcp_mss_def, tcp_mss_min) or because this system is configured to not use Path

MTU Discovery (related ndd: ip_pmtu_strategy, tcp_ignore_path_mtu).

6579 data packets (5914789 bytes) retransmitted

This is the number of TCP data segments which were retransmitted. These retransmissions could have come

from a TCP retransmission timer expiring (ndd: tcp_rexmit_interval_min,

tcp_rexmit_interval_max, tcp_rexmit_interval_initial), or from a “fast retransmission”,

which is triggered by the arrival of multiple duplicate ACKs. In either case, 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.

As with initial data segment transmission, one can arrive at an average retransmitted segment size by

dividing the octet count by the segment count.

As far as a range of acceptable values for the number of retransmitted segments, it depends entirely on the

conditions. Retransmission rate of as little as 1% can have a considerable effect on overall throughput. It

can also have a noticeable effect on perceived responsiveness. Consider for example, the case where one

is simply typing at an ssh session. Consider also that the average round-trip time is something like 10