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

4.3 Tuning Servers in Wireless Networks

Cellular mobile wireless networks have characteristics of a long latency, large bandwidth-delay product,

and volatile delays. Because cellular wireless networks have a long latency of a few hundreds of

milliseconds to a few seconds, their bandwidth-delay product is large, especially for the 3G wireless and

beyond. Therefore, the TCP window size needs to be set sufficiently large in a wireless network

environment. For efficient TCP communications over wireless networks, HP-UX provides the following

features:

• TCP Window Size and Window Scale Option

• Large Initial Congestion Window

• Limited Transmit

• SACK

• Smoothed RTO Algorithm

• F-RTO

This section describes Smoothed RTO Algorithm and F-RTO and their configuration. Refer to Chapter 2 for

the other four TCP performance features as they are covered under the “Out of the box performance

features”. These features are available in HP-UX 11i v3. Many of these new features were delivered in

ARPA Transport patches for 11i v1 and v2. Refer to Table 2 (at the end of Appendix B) for more

information.

4.3.1 Smoothed RTO Algorithm

The round trip times (RTT) of cellular mobile wireless networks are highly variable, and therefore they are

prone to produce spurious retransmission timeouts. HP-UX has an enhanced TCP Retransmit Timeout (RTO)

algorithm so that it adapts quickly even in the volatile environment of wireless networks. As a result, it

provides a much better probability of avoiding costly spurious retransmissions without sacrificing the

responsiveness to real packet loss. Besides that, the initial value and the lower limit for the RTO timeout are

configurable.

The initial value for the RTO timeout is configured by the ndd tunable tcp_rexmit_interval_initial.

When TCP first sends a segment to the remote, it has no history about the round-trip times. So, it will use

tcp_rexmit_interval_initial as a “best guess” for its first retransmit timeout setting. Setting this

value too low will result in spurious retransmissions and could result in poor initial connection performance.

If this value is set too high, TCP may not be able to send many retransmissions before it reaches the

tcp_ip_abort_interval or tcp_ip_abort_cinterval and could result in a spurious connection

abort.

The lower limit for the RTO timeout is configured by the ndd tunable tcp_rexmit_interval_min.

Setting this value too low could result in spurious TCP retransmissions, which decrease throughput by

keeping the TCP congestion window artificially small.

4.3.2 Forward-Retransmission Timeout (F-RTO)

Cellular mobile wireless networks have volatile behavior in the round trip times (RTT). Due to handovers

and interferences such as tunnels, they often exhibit spikes that are several times larger than a typical RTT.

Some spurious retransmission timeouts are inevitable. The F-RTO algorithm is an HP-UX enhancement that

can help TCP handle spikes in RTTs.

Spurious retransmission timeouts are costly because they incur unnecessary retransmissions and also keep

the congestion window (cwnd) small. By detecting spurious timeouts with the F-RTO, HP-UX effectively