On Tue, 2 Nov 2004 09:54:44 +0200
Michael Vittrup Larsen <michael.vittrup.larsen@xxxxxxxxxxxx> wrote:
> On Monday 01 November 2004 18:20, Stephen Hemminger wrote:
> > > * It is probably a good strategy to set 'tcp_rover_next' such that
> > > the next search is resumed from the previous port found to be free.
> > > (similar to the old algorithm). I don't see this in your patch,
> > > but of course I could have missed it.
> > It was intentional since it would require holding a lock around the search.
> > The tradeoff is better SMP performance in the sparsely filled port space
> > (more typical) vs. better UP performance in the case of a mostly full port
> > space.
> I think a typical scenario is many short-lived (e.g. minutes) TCP
> few long-lived (e.g. hours) connections and an ephemeral port wrap-around
> probably also in hours - at least a long time compared to the life-time of
> the short-lived connections.
But because of the hashing most ports will be scattered all over the port space,
because they come from different hosts.
> This would result in a closely spaced 'group' of ports being occupied
> somewhere in the ephemeral port range, and 'tcp_rover_next' would point at
> the uppermost extreme of this group and thus always guarantee a free port on
> first try (collisions will only happen with long-lived connections). If you
> don't update 'tcp_rover_next', and this somehow gets to lag behind this
> 'group' of ports (say point at the lower extreme) you will need to search
> through this group first before you enter the unoccupied port space.
Also, Linux TCP will reuse ports if (saddr, daddr, sport) are different.
Look at __tcp_v4_check_established. This means that the ports actually have
to be in use with real connections to the same host.
> Your scheme works initially because you do not lag behind the free port
> but eventually you will, and I think this will result in less optimal
> performance compared to the old behaviour.
Free port space should be evenly distributed because of the hash function.
> Since updating the 'tcp_rover_next' practically always result in a free port
> on first try, I think SMP performance will not suffer even though the lock
> was held all through the port search (except when the port space is very
But by not having a global lock on port allocation, different cpu's can be
searching different hash trees. This would matter under Dos attack with
> And yes, I do use Linux exclusively, so I do care :-))
> >From a statistically point of view, if the connection life-times are
> distributed from zero to infinite (theoretical scenario), it does not matter
> what starting point you use. However, soon as life-times are not uniformly
> distributed, this kind of search algorithm will benefit from good starting
> point defining where the probability of used vs. unused port drop from high
> to low.
> The BSD solution with a pure random rover suffers similarly, especially when
> the port space becomes crowded.
> > > * connect_port_offset() does not (at least from an algorithm point
> > > of view) need to return an u32, an u16 is sufficient.
> > If it is truncated to u16, then compiler has to take extra effort to
> > truncate is unnecessary given later modulo operation.
> I agree (in fact thats what I argued in the draft) - it probably depends on
> your platform - you are assuming a 32-bit platform I guess.