RFC 3390 (rfc3390) - Page 3 of 15
Increasing TCP's Initial Window
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RFC 3390 Increasing TCP's Initial Window October 2002 permit the lowest possible window size in the case of severe congestion). 2. Implementation Issues When larger initial windows are implemented along with Path MTU Discovery [RFC 1191], and the MSS being used is found to be too large, the congestion window `cwnd' SHOULD be reduced to prevent large bursts of smaller segments. Specifically, `cwnd' SHOULD be reduced by the ratio of the old segment size to the new segment size. When larger initial windows are implemented along with Path MTU Discovery [RFC 1191], alternatives are to set the "Don't Fragment" (DF) bit in all segments in the initial window, or to set the "Don't Fragment" (DF) bit in one of the segments. It is an open question as to which of these two alternatives is best; we would hope that implementation experiences will shed light on this question. In the first case of setting the DF bit in all segments, if the initial packets are too large, then all of the initial packets will be dropped in the network. In the second case of setting the DF bit in only one segment, if the initial packets are too large, then all but one of the initial packets will be fragmented in the network. When the second case is followed, setting the DF bit in the last segment in the initial window provides the least chance for needless retransmissions when the initial segment size is found to be too large, because it minimizes the chances of duplicate ACKs triggering a Fast Retransmit. However, more attention needs to be paid to the interaction between larger initial windows and Path MTU Discovery. The larger initial window specified in this document is not intended as encouragement for web browsers to open multiple simultaneous TCP connections, all with large initial windows. When web browsers open simultaneous TCP connections to the same destination, they are working against TCP's congestion control mechanisms [FF99], regardless of the size of the initial window. Combining this behavior with larger initial windows further increases the unfairness to other traffic in the network. We suggest the use of HTTP/1.1 [RFC 2068] (persistent TCP connections and pipelining) as a way to achieve better performance of web transfers. 3. Advantages of Larger Initial Windows 1. When the initial window is one segment, a receiver employing delayed ACKs [RFC 1122] 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, and possibly up to 500 msec [RFC 1122]). Allman, et. al. Standards Track
