So I've never tried to figure out how things break, just observed that they do - first many many years ago (close to 15ish) - between my wifi connected laptop at home and my university server in the same city. I've kept an INVALID->DROP rule in all my firewalls since then and not had problems. I vaguely recall seeing delayed packets when I debugged it back then. See for example: https://github.com/moby/libnetwork/issues/1090 for others running into this. Now we've hit an issue at work where a network misconfiguration has asymmetric one way pathing with a result that some packets were getting *massively* delayed, and it's been causing user firewalls to generate tcp resets for 'too old' 'already ack'ed' packets (ie. dups). While this is of course a misconfig, and it shouldn't happen, in practice it sometimes simply does. All it takes is for a packet to get into a long queue, and the network path to shift (immediately after it) to a less congested path. Due to bufferbloat those long queues can take seconds to drain and exceed path rtt by orders of magnitude. I *think* what happens is: A non-final tcp packet gets massively delayed, the packet past that makes it through to the receive, and triggers an ACK with SACK, which makes it back to the sender and triggers a retransmit and the connections keeps on making forward progress, then eventually the delayed packet arrives and it's no longer considered valid and triggers a tcp reset. Massively of course depends on the rtt and retransmit aggressiveness. Here's my attempt to demonstrate what I believe the problem to be: (on a freshly booted clean/empty/idle fedora 31 vm) iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT iptables -A INPUT -m state --state INVALID -j DROP modprobe ifb ip link set dev ifb0 up tc qdisc add dev ifb0 root netem reorder 99% 0% delay 10s tc qdisc add dev eth0 clsact tc filter add dev eth0 ingress u32 match u32 0 0 action mirred egress redirect dev ifb0 wget -O /dev/null https://git.kernel.org/torvalds/t/linux-5.7-rc4.tar.gz iptables-save -c ... /dev/null [ <=> ] 169.58M 2.93MB/s in 45s 2020-05-09 10:35:44 (3.81 MB/s) - ‘/dev/null’ saved [177819073] ... [31750:181080717] -A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT [244:1403178] -A INPUT -m state --state INVALID -j DROP Now if I reboot, and run the same script, except instead of the INVALID/DROP rule I do iptables -A INPUT -p tcp -j REJECT --reject-with tcp-reset then the download never finishes (it hangs after 15MB @ 2MB/s and eventually times out). [4170:16758894] -A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT [37:147454] -A INPUT -p tcp -j REJECT --reject-with tcp-reset (arguably since this is a VM, and thus NAT'ed by my host, and then again by the real ipv4 NAT, the setup isn't entirely clear, but I hope it makes my point: INVALID state needs to be dropped, not rejected)
