tcp hostcache and ip fastforward for review

Mike Silbersack silby at
Mon Nov 10 11:45:55 PST 2003

On Mon, 10 Nov 2003, Andre Oppermann wrote:

> >     - Ensures that a cached entry isn't added until the 3WHS is completed.
> >
> > This should help make synfloods with random source addresses less
> > damaging.
> The cache will only be updated if the tcp connection is being closed.
> All updates are done in tcp_drop. The T/TCP updates have to be done
> inline during connection setup. I've converted all places which
> updated the T/TCP rtmetrics in routing table with updates to the
> hostcache.

Good, that's exactly how it should work.

> > Would it be possible to provide a way for netstat to view the host cache
> > table?  I think that it would be useful.
> At the moment is visible via "sysctl -a net.inet.tcp.hostcache.list".
> Syncache ain't visible via netstat either. So far you had to use
> route get x.x.x.x to see the rtmetrics for a (host-)route. So I'm
> sure whether netstat is the right place for it. But I can do that
> in a second step.

Ok, that should be good enough for now.

> The actually solves the problem. Let me explain in more detail. When
> we get so many small packets per second the CPU will become pretty
> saturated. Depending on how much data is sent it can go on for minutes
> or hours. This code jumps in there and disconnects the within a second.
> Of course someone can immediatly reconnect and do it again. But that
> needs the 3WHS again and gives some delay. In the end this code is
> like the ICMP rate limiter code. It there to migitate a problem to
> manageable level, not to make it go away.

Ok, so the problem is that the sockbuf chain keeps getting longer, causing
the delay to grow as more fragments pile in... I see now.  I drop my
objection to it.

Mike "Silby" Silbersack

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