[quicwg/base-drafts] Changing the Default QUIC ACK Policy (#3529)

[Gorry Fairhurst <[e-mail]>]

As currently specified, the default QUIC ACK policy models TCP’s ACK policy, but consumes significantly more return path capacity. This can impact resource usage of the return channel, can constrain forward throughput, and can impact the performance of other sessions sharing an uplink (draft-fairhurst-quic-ack-scaling). This was not discussed in Zurich - where discussion moved instead to the separate topic of draft-iyengar-quic-delayed-ack.

In many current network segments (e.g., cable networks, mobile cellular, WiFi and other radio tech), the effects of asymetry are presently mitigated by network devices RFC3449 that rely upon observing TCP headers.  This results in ossification problems, but does practically have beneit. It reduces the TCP ACK rate over the constrained link, it reduces consumption of "radio" transmission opportunities, and reduces traffic volume from ACKs. These mitigations are not possible, nor desirable for QUIC. However, the default performance of QUIC  is therefore much worse than for TCP.

In QUIC, the default ACK Ratio has to set by the endpoints implementing the protocol  because the sender and receiver have no information about the "cost" of sending an ACK using a link along an asymmetric path.  These endpoints can also choose a non-default value to enable a new CC, scale to higher rates, etc (e.g. draft-iyengar-quic-delayed-ack).

We have shown there can be significant benefit from an ACK Ratio of 1:10 and this is expected to alos have benefit for higher levels of asymmetry. The proposed change from 1:2 to 1:10 is predicated on  QUIC's design decisions: starting a connection with an Initial Window of 10 is acceptable, QUIC uses PTO as the loss detection method (a predicate to issue #3357), and QUIC has some form of pacing at the sender, at least to mitigate IW bursts. 




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