Re: Expiring flows in an XDP based router

On Sun, May 18, 2025 at 7:19=E2=80=AFPM Toke H=C3=B8iland-J=C3=B8rgensen <t=
[email protected]> wrote:
>
> Edvard Fagerholm <[email protected]> writes:
>
> > Hi,
> >
> > I'm working on an in-house router and I'm looking for some advice on
> > whether to try to implement some functionality directly in an XDP
> > program or instead use AF_XDP sockets with the additional flexibility
> > provided by userspace.
> >
> > We basically have a flow table, which contains entries of the form:
> >
> >   (source ip, source port) -> (action, timestamp)
> >
> > Every time we see a packet belonging to a flow, the timestamp is
> > updated and if the flow has been inactive for e.g. 10 seconds, the
> > flow is considered inactive and should be deleted. Actions are of the
> > form:
> >
> >   "forward packet to a.b.c.d port X using source port Y"
> >
> > The challenge that I have is how to clean up expired flows. Built-in
> > options would be BPF_MAP_TYPE_LRU_PERCPU_HASH. However, dropping an
> > active flow would be unacceptable.
> >
> > I'm looking at at most 10k new flow entries being added per second per
> > router with a maximum number of concurrent flows at around 256k. Each
> > flow sends a packet at least every 5 seconds, but most every 50ms.
> > Does this allow me to tune the table size in such a way that no active
> > flows can be evicted? If not, are there any other reasonable
> > approaches for cleaning up the flows?
>
> Well, hard to tell with only the number of additions. I mean, 256.000 +
> 10.000 * 10 seconds means you'd need on the order of 356k entries in the
> map, assuming flows expire at the same rate as they are added. Do you
> assume any such guarantee?

We have average session lengths over 30 seconds and a maximum of 256k
concurrents the way the servers are set up. The first number is a
conservative lower
bound and the latter a conservative upper bound, so from there you get 10k =
per
second as a conservative upper bound too. We see spikes sometimes during
updates with polling clients, but they stay below this too.

I read the description of the LRU implementation in the Cilium docs. For an
element to be expired, it would need to be put at the head of the active li=
st
and propagate all the way down towards the tail until being moved over to
the head of the inactive list. Then propagate down the inactive list
and be cleaned
up.

If one could prove that being propagated in the lists to being cleaned
up takes longer
than the maximum interval every flow sees packets, then that's good enough.
Alternatively, if the table is always larger than the maximum number of flo=
ws by
some constant factor and one can prove some invariant that the kernel alway=
s
recycles enough "real" inactive flows before accidentally recycling an
active flow,
then that would also be enough.

However, I don't think I'm quite comfortable with such an approach,
since it relies
on implementation details of the data structure that are not part of
the promised API.

On the other hand, given the number of routers and load balancers using XDP=
,
I was kinda hoping there was some well-known robust solution that is also
performant.

The box in question is basically doing DDoS protection, so it needs to
be able to
handle traffic at line rate. I'm also trying to avoid doing any
expensive processing
that takes a long time, which could cause NIC buffers to fill up and
cause packet
loss. Our current header based solution spends a constant amount of time pe=
r
packet and is stateless.

> Alternatively, you could have your XDP program start a BPF timer and
> perform the periodic cleanup from that. There will be some overhead from
> walking the map, and a potential for racy behaviour if the same flow is
> expired and re-added frequently, but otherwise this should be quite
> doable.

I would need to look into timers. Never had to deal with these before in XD=
P.

> In any case, you'd need to size your map so that it doesn't fill up
> before flows expire. So whether to use an LRU map is a case of whether
> overflowing the map will result in overwriting existing entries, or
> blocking the addition of new ones.

We basically need both:

1. Table should be large enough to never block connections.
2. The error of the approximate LRU algorithm needs to be precise
enough to never recycle active flows.

Best,
Edvard