Update ECN (RFC3168) and Add Accurate ECN (RFC9768) feature #232
Description
The current virtio specification on RFC3168 ECN is incorrect because it states that the VIRTIO_NET_HDR_GSO_ECN gso_type flag indicates that TCP has IP-ECN set. Instead, this flag shall indicate that the TCP CWR flag is set and that the TCP CWR flag is cleared starting from the second segment to comply with RFC3168.
Recently, the Accurate ECN (RFC9768-to-be) was added, which shall never attempt to clear the TCP CWR flag because it is part of the ACE counter used to accumulate how many packets are being marked. Therefore, a fallback shall be applied by setting NETIF_F_GSO_ACCECN to ensure that the CWR flag should not be cleared within the aggregated segment.
spec patch: https://lore.kernel.org/virtio-comment/[email protected]/