On Fri, Aug 18, 2017 at 3:27 PM, David Miller <da...@davemloft.net> wrote: > From: Martin KaFai Lau <ka...@fb.com> > Date: Fri, 18 Aug 2017 13:51:36 -0700 > >> It seems like that middle box specifically drops TCP_RST if it >> does not know anything about this flow. Since the flowlabel of the TCP_RST >> (sent in TW state) is always different, it always lands to a different middle >> box. All of these TCP_RST cannot be delivered. > > This really is illegal behavior. The flow label is not a flow _KEY_ > by any definition whatsoever. > > Flow labels are an optimization, not a determinant for flow matching > particularly for proper TCP state processing. > > I'd rather you invest all of this energy getting that vendor to fix > their kit. > We're now seeing several router vendors recommending people to not use flow labels for ECMP hashing. This is precisely because when a flow label changes, network devices that maintain state (firewalls, NAT, load balancers) can't deal with packets being rerouted so connections are dropped. Unfortunately, the need for packets of a flow to always follow the same path has become an implicit requirement that I think we need follow at least as the default behavior.
Martin: is there any change you could resurrect these patches? In order to solve the general problem of making routing consistent, I believe we want to keep sk_tx_hash consistent for the connection from which a consistent flow label can be derived. To avoid the overhead of a hash field in sk_common, maybe we could initially set a connection hash to a five-tuple hash for a flow instead of a random value? So in TW state the consistent hash can be computed on the fly. Tom
