TCP Window Scaling options - Large Windows

(Last modified: 09Aug2005)

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fact

Novell NetWare 6.5

Novell NetWare 6.0

TCP Large Window Option

TCP Maximum Initial Window

RFC 1323

RFC 2414

goal

TCP Window Scaling options - Large Windows

fix

The TCP Large Window Option:  TCP Large Window Option = On  (Default)  The Large Windows option facilitates the advertisements of windows larger than 2**16.  This is useful in high bandwidth long delay networks such as satellite links. It defines an implicit scale factor, which is used to scale up the window size value found in a received TCP segment to obtain the true window size.  With this scaling, receive window advertisements can go up to 1 GB. This Large Window option is negotiated when establishing the connection.
 
TCP Maximum Initial Window option:  TCP Maximum Initial Window = 2-4 (default 4 TCP segments)  This is the maximum number of packets to use for the initial TCP window.  Although normally set to 4 per RFC2414, it can be reduced to 2 or 3 if needed for performance reasons.

Advantages of Larger Initial Windows:

1.  When the initial window is one segment, a receiver employing delayed ACK's (most TCPIP stacks) is forced to wait for a timeout before generating an ACK.  With an initial window of at least two segments, the receiver will generate an ACK after the second data segment arrives.  This eliminates the wait on the timeout (often up to 200 msec).

2.  For connections transmitting only a small amount of data, a larger initial window reduces the transmission time (assuming at most moderate segment drop rates).  For many email and web page transfers that are less than 4K bytes, the larger initial window would reduce the data transfer time to a single RTT.

3.  For connections that will be able to use large congestion windows, this modification eliminates up to three RTTs and a delayed ACK timeout during the initial slow-start phase.  This would be of particular benefit for high-bandwidth large-propagation-delay TCP connections, such as those over satellite links.

The default advertised TCP window from a NetWare server is 65535

RFC1323 - see section 2.2

RFC2414 - Increasing TCP's Initial Window

NetWare TCPIP Performance Troubleshooting and Tuning Guide

Novell TCP/IP Administration Guide - Large Window Section

document

Document Title:	TCP Window Scaling options - Large Windows
Document ID:	10098555
Solution ID:	NOVL103042
Creation Date:	09Aug2005
Modified Date:	09Aug2005
Novell Product Class:	Netware

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