Re: [quicwg/base-drafts] Public Flags to Aid Troubleshooting (#279)

[martinduke <[e-mail]>]

> BLOCKED frames are rare, but tend to be isolated to a small number of services, which either are slow at consuming data or serve clients with much higher than average bandwidth. As such, they serve as an excellent monitoring and debugging tool if you understand the full context.
> 
> If this was available in QUIC, what use cases would you expect it to help with?

Are you asking me to defend BLOCKED frames, or the header bit? In the use case where BLOCKED would be helpful, the bit would be helpful for monitoring performance of provider networks. The BLOCKED _frame_ makes window-limitation more obvious, but if you have the decrypted payload, you can also glean this from WINDOW_UPDATE frames.

I can see three possibilites:
- window-limited connections are a common occurrence in QUIC: in this case, the header bit is essential to have any idea what is a real network performance problem and what comes from client configuration. In some cases, it might lead to a mobile software update that fixes the config.
- window-limited connections are extremely rare: it is not worth it to take a bit in the header. In the extreme limit, there is no need for flow control at all because QUIC endpoints have ample memory resources and reduced HOL blocking.
- window-limited connections are "sort of" rare: in which case we ought to have flow control and BLOCKED, but it's not worth a valuable public header bit. I _think_ this is where you believe we are.

Obviously, I have no real-world QUIC deployment experience, so I can't state with authority where we are on this spectrum, and if current experience is relevant to a future with IoT devices, etc. All I can say is that in TCP the flow control information is very valuable to have, and it's not hard to see how certain applications and implementations in QUIC might have the same problems.

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