ACK prioritization

In today’s asymmetric internet environment up and download ratios can be as poor as 1 to 11. While this works well for situations where downloads predominate, it can cause terrible side effects on an otherwise healthy network when an upload saturates the connection, starving the downloads of needed ACK packets.

Ratios worse than 11 to 1 appear to touch closely to the theoretical limit of ~23 to 1 that TCP/IP invokes in it’s control stream.
This is why, in part, worse ratios haven’t appeared in vendor offerings, as downloads are effectively throttled by upload bandwidth.

Since ack packets are very small (less than 72 bytes in ipv4 and less than 140 bytes in IPv6, depending on encapsulation), shaping methods that depend more on packets than bytes tend to suffer. 23 (ipv4) ack packets can fit into the same amount of buffer space as a single upload packet.

It is helpful from a policing perspective to look more at bytes than packets, for uploads in an asymmetric network, to determine what packets to best ‘shoot’. Even then, the side effects of shooting an upload packet instead of 23 ack packets tend to be more beneficial than often realized.

Lastly, no shaping system takes into account the up/download ratio in its decision making particularly well.

Several tries exist - notably, wondershaper, which pioneered the concept of doing ACK prioritization for interactive ssh traffic back in 2002.

  • Note also that IPv6 changes the ack equation markedly - an encapsulated IPv6 6in4 ack packet can be as large as 140 (reference needed)
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