netdev - Re: SACK scoreboard

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <20080108.144456.173014334.davem@davemloft.net>
Date:	Tue, 08 Jan 2008 14:44:56 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	jheffner@....edu
Cc:	ilpo.jarvinen@...sinki.fi, lachlan.andrew@...il.com,
	netdev@...r.kernel.org, quetchen@...tech.edu
Subject: Re: SACK scoreboard

From: John Heffner <jheffner@....edu>
Date: Tue, 08 Jan 2008 11:51:53 -0500

> I haven't thought about this too hard, but can we approximate this by 
> moving scaked data into a sacked queue, then if something bad happens 
> merge this back into the retransmit queue?

That defeats the impetus for the change.

We want to free up the data, say, 2 packets at a time as
ACKs come in.  The key goal is smooth liberation of
retransmit queue packets over time.

The big problem is that recovery from even a single packet loss in a
window makes us run kfree_skb() for a all the packets in a full
window's worth of data when recovery completes.

If we just move such packets to a seperate list, we still have to
iterate over all of them when the cumulative ACK arrives.

This problem, that retransmit queue liberation is not smooth, is the
biggest flaw in how SACK is specified.  I mean, consider Ilpo's
mentioned case of 500,000 packet windows.  The issue cannot be
ignored.  SACK is clearly broken.

You speak of a path in Linux where we can reneg on SACKs, but I doubt
it really ever runs because of how aggressive the queue collapser is.
Alexey even has a comment there:

	 * This must not ever occur. */

To be honest this code sits here because it was written before the
queue collapser was coded up.

Really, find me a box where the LINUX_MIB_OFOPRUNED or
LINUX_MIB_RECVPRUNED counters are anything other than zero.

So this is a non-issue and I did consider it before proposing that we
redefine SACK.
--
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html