netdev - A buggy behavior for Linux TCP Reno and HTCP

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Date:   Tue, 18 Jul 2017 16:36:08 -0500
From:   Wei Sun <unlcsewsun@...il.com>
To:     netdev@...r.kernel.org
Subject: A buggy behavior for Linux TCP Reno and HTCP

Hi there,

We find a buggy behavior when using Linux TCP Reno and HTCP in low
bandwidth or highly congested network environments.

In a simple word, their undo functions may mistakenly double the cwnd,
leading to a more aggressive behavior in a highly congested scenario.

The detailed reason:

The current reno undo function assumes cwnd halving (and thus doubles
the cwnd), but it doesn't consider a corner case condition that
ssthresh is at least 2.

e.g.,
                         cwnd              ssth
An initial state:     2                    5
A spurious loss:   1                    2
Undo:                   4                    5

Here the cwnd after undo is two times as that before undo. Attached is
a simple script to reproduce it.

A similar reason for HTCP, so we recommend to store the cwnd on loss
in .ssthresh implementation and restore it again in .undo_cwnd for TCP
Reno and HTCP implementations.

Thanks

Download attachment "undo-2-1-4.pkt" of type "application/octet-stream" (2102 bytes)