tcp hostcache and ip fastforward for review

Andre Oppermann oppermann at
Mon Nov 10 01:40:13 PST 2003

Jonathan Mini wrote:
> On Nov 9, 2003, at 2:47 PM, Andre Oppermann wrote:
> > Jonathan Mini wrote:
> >>
> >> On Nov 9, 2003, at 8:19 AM, Andre Oppermann wrote:
> >>
> >>>   - DoS attack 2: make MSS very low on local side of connection
> >>>     and send maaaany small packet to remote host. For every packet
> >>>     (eg. 2 bytes payload) a sowakeup is done to the listening
> >>>     process. Consumes a lot of CPU there.
> >>>
> >>
> >> This sounds as if it might be worthwhile to add a delay to
> >> the TF_NODELAY case for receive processing as well.
> >
> > Unfortunatly it is not that easy. We can't just do that unconditionally
> > to all connections. It would probably break or delay many things. You
> > never know how much data is outstanding and whether it's just this
> > packet with 2 bytes outstanding...
> This would be disastrous to the performance of interactive
> sockets, however theoretically those connections have
> NODELAY set. My above comment is a bit confusing: I meant the
> "non TF_NODELAY" case, that is when Nagling is enabled.
> In this situation, you would be delay an sowakeup until
> either a timeout or SO_RCVLOWAT-set value was reached.  The normal
> SO_RCVLOWAT case delays until SO_RCVTIMEO is reached.  I suppose
> the application could simulate this with a large SO_RCVLOWAT and a
> small SO_RCVTIMEO, but I was wondering about the effects of such a
> change as part of !TF_NODELAY.

To do this we need another callout to do the eventual wakeup if
no further packet arrive within whatever/HZ. In addition it probably
would make FreeBSD look bad in network benchmarks since this causes
the connection RTT to go up.

All in all I don't think it is worth adding this complexity.

> Sadly, there's this PSH bit in the TCP header that's completely
> unreliable and could be used for scenarios like this.
> > As an application aware of this problematic you have currently two
> > options: use accept filters (FreeBSD only) or set SO_RCVLOWAT to some
> > higher value than the default 1 byte. Only the first one is workable
> > if you don't know what and how much the clients send to you. Relying
> > on the application to activate any such option to prevent this kind
> > of DoS is unfortunatly whishful thinking.
> I was not suggesting that we use this to counter an attack, only asking
> if it might be a worthwhile performance optimization to consider.
> This is an unlikely case (many small packets sent to a non-interactive
> application), so I can't see the improvement as being globally useful.

No, I don't think it is a worthwhile opimization. If the application
wants to do it, it can do so already via socket options. Normally
an application needs such a delay feature to be specific to it's
message types. Like with http accept filter.

> > The code I've put in here simply caps off the extreme cases. It
> > counts all packets and bytes in any given second and computes the
> > average payload size per packet. If that is less than we have defined
> > for minmss it will reset and drop the connection. However it will only
> > start to compute the average if there are more than 1'000 packets per
> > second on the same tcp connection. I've chosen this quite high value
> > to never disconnect any ligitimate connection which just happens to
> > send many small packets. In my tests I've seen telnet/ssh sending
> > close to 100 small packets per second (some large copy-pasting and
> > cat'ing of many small files). Probably 500 packets per second is a
> > better cut-off value but I just want to be sure to never hit a false
> > positive.
> This is actually a small value for TCP connections which are being
> used to forward messages, especially on gigabit links.
> Heavily-intensive
> web applications that are using small HTTP requests (pipelined inside a
> persistent connection) to update small manipulations of state are
> a good example of this.  I wouldn't be surprised to see chatter
> between SQL servers follow similar patterns.  Applications which
> use XML-based messaging often send several small packets per message,
> which is unfortunate.

Do you think such applications manage to send 1000 packets per second
with less than 256 bytes payload per packet? I think the network code
would collect some data to form a larger packet (unless TCP_NODELAY

> On the other hand, I'm used to looking at proxies, which are not
> the general case.  This is why the limits are tunable, after all. =)

Is there way you could monitor such connections and compile some
statistics how many small packets per second are sent? I could adjust
the patch to just report the fact instead of dropping the connection.
Could do it for 4.9-R too, it's fairly easy.


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