On Tue, 07 Jan 2003 03:16:38 -0300, Werner Almesberger said:
> Valdis.Kletnieks@vt.edu wrote:
> > is currently busticated for gigabit and higher (it takes *hours* without a
> > packet drop to get the window open *all* the way
>
> Don't use 2.0.21 for gigabit traffic :-)
>
> (2.0.21 and earlier initialized sstresh to zero, which would indeed
> cause the behaviour you're describing.)
That's not the problem. The problem is that TCP slow-start itself (and some of
the related congestion control stuff) has some issues scaling to the very high
end. Floyd (of RFC3168 fame) has done some work in the area:
from http://www.ietf.org/internet-drafts/draft-floyd-tcp-highspeed-01.txt
Abstract
This document proposes HighSpeed TCP, a modification to TCP's
congestion control mechanism for use with TCP connections with large
congestion windows. The congestion control mechanisms of the current
Standard TCP constrains the congestion windows that can be achieved
by TCP in realistic environments. For example, for a Standard TCP
connection with 1500-byte packets and a 100 ms round-trip time,
achieving a steady-state throughput of 10 Gbps would require an
average congestion window of 83,333 segments, and a packet drop rate
of at most one congestion event every 5,000,000,000 packet (or
equivalently, at most one congestion event every 1 2/3 hours). This
is widely acknowledged as an unrealistic constraint. To address this
limitation of TCP, this document proposes HighSpeed TCP, and solicits
experimentation and feedback from the wider community.
Also http://www.ietf.org/internet-drafts/draft-floyd-tcp-slowstart-01.txt
Abstract
This note proposes a modification for TCP's slow-start for use with
TCP connections with large congestion windows. For TCP connections
that are able to use congestion windows of thousands (or tens of
thousands) of MSS-sized segments (for MSS the sender's MAXIMUM
SEGMENT SIZE), the current slow-start procedure can result in
increasing the congestion window by thousands of segments in a single
round-trip time. Such an increase can easily result in thousands of
packets being dropped in one round-trip time. This is often counter-
productive for the TCP flow itself, and is also hard on the rest of
the traffic sharing the congested link. This note proposes Limited
Slow-Start as an optional mechanism for limiting the number of
segments by which the congestion window is increased for one window
of data during slow-start, in order to improve performance for TCP
connections with large congestion windows.
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