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feat: use Cubic CC by default #2295

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feat: use Cubic CC by default #2295

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mxinden
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@mxinden mxinden commented Dec 21, 2024

Firefox uses Cubic by default:

- name: network.http.http3.cc_algorithm
  type: RelaxedAtomicUint32
  value: 1
  mirror: always
  rust: true

https://searchfox.org/mozilla-central/rev/f9517009d8a4946dbdd3acd72a31dc34fca79586/modules/libpref/init/StaticPrefList.yaml

This commit updates Neqo to use Cubic instead of New Reno by default.

@mxinden
feat: use Cubic CC by default
cc2837b 
Firefox uses Cubic by default:

``` yaml
- name: network.http.http3.cc_algorithm
  type: RelaxedAtomicUint32
  value: 1
  mirror: always
  rust: true
```

https://searchfox.org/mozilla-central/rev/f9517009d8a4946dbdd3acd72a31dc34fca79586/modules/libpref/init/StaticPrefList.yaml

This commit updates Neqo to use Cubic instead of New Reno by default.
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Failed Interop Tests

QUIC Interop Runner, client vs. server, differences relative to bb45c74.

neqo-latest as client

neqo-latest as server

All results

Succeeded Interop Tests

QUIC Interop Runner, client vs. server

neqo-latest as client

neqo-latest as server

Unsupported Interop Tests

QUIC Interop Runner, client vs. server

neqo-latest as client

neqo-latest as server

@mxinden
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mxinden commented Jan 5, 2025 •
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Cubic does not seem to grow the cwnd as fast as New Reno in congestion avoidance phase. More particularly, while our New Reno implementation increases its cwnd in each iteration by the SMSS, our Cubic implementation only does so in every second iteration.

neqo/neqo-transport/src/connection/tests/cc.rs

Lines 286 to 287 in 922d266

expected_cwnd += client.plpmtu();
assert_eq!(cwnd(&client), expected_cwnd);

That is surprising to me. I expected Cubic to grow faster than New Reno in the beginning, given its concave increase early in congestion avoidance.

I will give this more thought.

@larseggert
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Cubic does not seem to grow the cwnd as fast as New Reno in congestion avoidance phase.

That seems like a bug: https://datatracker.ietf.org/doc/html/rfc9438#section-1-5

Specifically, CUBIC may increase the congestion window more aggressively than Reno during the congestion avoidance phase. According to [RFC5681], during congestion avoidance, the sender must not increment cwnd by more than Sender Maximum Segment Size (SMSS) bytes once per round-trip time (RTT), whereas CUBIC may increase cwnd much more aggressively.

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@KershawChang KershawChang Awaiting requested review from KershawChang KershawChang will be requested when the pull request is marked ready for review KershawChang is a code owner

@martinthomson martinthomson Awaiting requested review from martinthomson martinthomson will be requested when the pull request is marked ready for review martinthomson is a code owner

@larseggert larseggert Awaiting requested review from larseggert larseggert will be requested when the pull request is marked ready for review larseggert is a code owner

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@mxinden @larseggert