Subject: [tcpm] dctcp: alpha may not reach 0 in current linux implementation

From: Florian Westphal <[email-masked]>
Date: Mon, 19 Oct 2015 15:58:59 +0200

Quoting from a patch[1] from Andrew Shewmaker:

============
If alpha is strictly reduced by alpha >> dctcp_shift_g and if alpha is less
than 1 << dctcp_shift_g, then alpha may never reach zero.
[..]
The effect isn't noticeable in this case below cwnd=137, but
could gradually drive uncongested flows with leftover alpha down to
cwnd=137. A larger dctcp_shift_g would have a greater effect.
===========

So this doesn't appear to be a problem in practice since cwnd=137 is quite big.

>From looking at current ietf draft alpha should be allowed to reach 0 if there
is no congestion so it seems this patch fits what the dctcp draft recommends wrt.
alpha update in section 3.3.

Nevertheless, some of the other implementors might want to look into this
bogus cwnd reduction too.

[1] http://patchwork.ozlabs.org/patch/532144/