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Increasing TCP Receive Window for Apple Macintosh

Caveat: The following information has not been tested by this author. USE AT YOUR OWN RISK.

TCP Receive Window can be adjusted with the "tcp_rwin_mss_multiplier" setting of the OT Advanced

Tuner from Sustainable Softworks. This author suggests a starting value of 20. You may need to

experiment to find your own optimum setting(s). For more information, see:

"More on TCP/IP Tuning" (Sustainable Softworks)

"How to Download and Browse at the Same Time" (Sustainable Softworks)

Note: This author has no connection to Sustainable Softworks.

[Jump to Contents]

Why TCP Receive Window Matters

TCP is a packet-based protocol where data is transmitted in variable-sized blocks, typically with a

maximum size of 500-1500 characters (usually 1500 characters for Cable Modem or DSL). Two

important characteristics of the TCP protocol:

Packet Acknowledgments

In order to insure delivery of each packet, the receiver must acknowledge successful receipt by

sending a special acknowledgment packet to the sender. If the sender does not receive the

acknowledgment packet within a certain time limit, it assumes the packet has been lost and

retransmits it (up to a retransmission limit).

Receive Window

If each data packet had to be acknowledged before another could be sent, then performance could

suffer due to the delay time needed for the data packet to reach the receiver plus the time needed

for the acknowledgment packet to get back to the sender. To avoid this delay, the sender is allowed

to keep transmitting data packets prior to receiving acknowledgments up to a maximum "window"

size advertised by the receiver, normally large enough for several packets. The larger the window,

the more packets that can be sent before needing an acknowledgment; however, larger windows can

require more packets to be retransmitted when a transmission error occurs. Hence, the receive

window size needs to be large enough to keep data flowing continuously, but not excessively large.

The TCP Receive Window has a default value of only about 8K bytes in Windows 95/98/NT, and about

16K bytes in Windows Me/2000/XP, which is adequate for relatively slow dialup modems and for

high-speed networks with relatively low latency (e.g., less than 20 milliseconds). Increasing the TCP

Receive Window above the default settings (e.g., to 32-63K) can substantially improve throughput on

high-speed (e.g., Cable Modem or DSL) connections where there is higher latency (e.g., 100-200

milliseconds), as is often the case on the Internet, particularly over long network paths. (Increasing the

TCP Receive Window will usually not have an adverse effect on other connections.)

As an example, consider the case of downloading a file at 100 kilobytes per second from a remote server

over a Cable Modem or DSL connection. The default TCP Receive Window of about 8K bytes will be

consumed in only about 80 milliseconds, which is often less than the round-trip latency on the Internet. At

this point the sender has to stop sending until an acknowledgment that data was received comes back from

the receiver. With a TCP Receive Window of 32K bytes, the sender can continue for as long as 325

milliseconds without an acknowledgment, which should permit uninterrupted data flow even when

latency is 100-200 milliseconds or more. (With a TCP Receive Window of 63K bytes, the sender can

continue for as long as 650 milliseconds.)

See animations in TCP Receive Window Illustration.

The following table can be used to determine the minimum TCP Receive Window size needed for given

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