Subject: Re: [PATCH 1/3] Support arbitrary initial TCP timestamps

From: Glenn Griffin <[email-masked]>
Date: Tue, 19 Feb 2008 10:02:38 -0800

> > Adding yet another member to the already bloated tcp_sock structure to
> > implement this is too high a cost.
> 
> Yes, I was worried that would be deemed too high of a cost, but it was
> the most efficient way I could think to accomplish what I wanted.
> 
> > I would instead prefer that there be some global random number
> > calculated when the first TCP socket is created, and use that as a
> > global offset.  You can even recompute it every few hours if you
> > like.
> 
> That would work fine if my mine purpose was to randomize the tcp
> timestamp to mitigate the leak in information regarding uptime, but
> despite the brief description, that's only a side effect of what I
> intended to do.  What I wanted was a way to be able to choose an initial
> tcp timestamp for a particular connection that was not tied directly to
> jiffies.
> 
> The two patches following this show my intended use case.  I intend to
> enhance syncookie support to allow it to support advanced tcp options
> (sack and window scaling).  Normally syncookies encode the bare minimum
> state of a connection in the ISN they choose, but the 32bit ISN isn't
> enough to encode advanced tcp options so you are left with a working but
> crippled tcp stack during a synflood attack.  If in addition to choosing
> an ISN we are able to choose an initial tcp timestamp, we are then able
> to encode an additional 32 bits of information that can contain more of
> the advanced tcp options.

Perhaps I should clarify.  I don't see a way to implement the additional
syncookie enhancements without storing an offset in some type of
per-socket structure.  Given that, is it still deemed too high of a cost?
Is enhancing syncookies not deemed important enough to warrant the
additional 4 bytes?  What if there was an additional config variable to
support arbitrary/random tcp timestamps, and then syncookies only used
the additional options when that setting was chosen?  Is there some
possiblity I'm missing that could get this feature in a form suitable
for inclusion?  Thanks.

--Glenn