Hi there, As part of our ongoing research effort to understand the discrepancies among Linux, macOS (FreeBSD), and Windows. We discover a violation of the way Linux hanldes incoming TCP packet, specifically ACK number validation.
According to RFC 793, "If the ACK is a duplicate (SEG.ACK < SND.UNA), it can be ignored. If the ACK acknowledges something not yet sent (SEG.ACK > SND.NXT) then send an ACK, drop the segment, and return". In RFC 5961, the first sentence is changed (for more stringent ACK number validation) but the second sentence remains the same. Clearly, when the ACK number of the incoming packet is larger than SND.NXT, we are supposed to send back an ACK. However, Linux currently chooses to silently discard the packet without any reply. We have checked macOS implementation which adheres to the specification. I'd love to hear any thoughts on this. Best, -Zhiyun
