netdev - Re: [RFC] TCP illinois max rtt aging

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]

Date:	Fri, 7 Dec 2007 20:27:13 +0200 (EET)
From:	"Ilpo Järvinen" <ilpo.jarvinen@...sinki.fi>
To:	David Miller <davem@...emloft.net>
cc:	lachlan.andrew@...il.com, Netdev <netdev@...r.kernel.org>,
	quetchen@...tech.edu
Subject: Re: [RFC] TCP illinois max rtt aging

On Fri, 7 Dec 2007, Ilpo Järvinen wrote:

> On Fri, 7 Dec 2007, David Miller wrote:
> 
> > From: "Ilpo_Järvinen" <ilpo.jarvinen@...sinki.fi>
> > Date: Fri, 7 Dec 2007 13:05:46 +0200 (EET)
> > 
> > > I guess if you get a large cumulative ACK, the amount of processing is 
> > > still overwhelming (added DaveM if he has some idea how to combat it).
> > > 
> > > Even a simple scenario (this isn't anything fancy at all, will occur all 
> > > the time): Just one loss => rest skbs grow one by one into a single 
> > > very large SACK block (and we do that efficiently for sure) => then the 
> > > fast retransmit gets delivered and a cumulative ACK for whole orig_window 
> > > arrives => clean_rtx_queue has to do a lot of processing. In this case we 
> > > could optimize RB-tree cleanup away (by just blanking it all) but still 
> > > getting rid of all those skbs is going to take a larger moment than I'd 
> > > like to see.
> > > 
> > > That tree blanking could be extended to cover anything which ACK more than 
> > > half of the tree by just replacing the root (and dealing with potential 
> > > recolorization of the root).
> > 
> > Yes, it's the classic problem.  But it ought to be at least
> > partially masked when TSO is in use, because we'll only process
> > a handful of SKBs.  The more effectively TSO batches, the
> > less work clean_rtx_queue() will do.
> 
> No, that's not what is going to happen, TSO won't help at all
> because one-by-one SACKs will fragment every single one of them
> (see tcp_match_skb_to_sack) :-(. ...So we're back in non-TSO
> case, or am I missing something?

Hmm... this could be solved though by postponing the fragmentation of a 
partially sacked skb when the first sack block can (is likely) to still 
grow and remove the need for fragmentation. Has some implications to 
packet processing, increases burstiness a bit & tcp_max_burst kicks in too 
easily.

-- 
 i.