Re: [quicwg/base-drafts] 3+ and 4+ in the QPACK wire format are complex and inefficient (#3561)

Mike Bishop <notifications@github.com> Tue, 31 March 2020 18:02 UTC

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Subject: Re: [quicwg/base-drafts] 3+ and 4+ in the QPACK wire format are complex and inefficient (#3561)
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Let's look at the 3+ and 4+ cases:

- Literal with Post-Base Name Reference.  A 3+ can reference up to 6 values before spilling into a second byte, and the expectation is that there will usually be very few post-base entries to reference.  I would expect this to be single-byte much of the time.
- Literal without Name Reference.  I agree that there are very few names that won't require a second byte, so this doesn't come out any better most of the time.  There are also very few names that will require a third byte, so it's also no worse.
- Indexed with Post-Base Index.  A 4+ can reference up to 14 values in a single byte.  Here again, we expect there to be few Post-Base entries, so I would expect this to be single-byte most of the time.
- Literal with Name Reference.  We designed the static table's order to fill the first 14 slots with names of fields that you often won't to add to the dynamic table.  (That's why so many of the first 14 entries have an empty value.)  My expectation, which could be proven wrong by more data, is that this fits in a single byte often enough to have some benefit.

As to the complexity, I don't see it.  As soon as you have to write an encoder/decoder function that takes the prefix length as an argument, no prefix length is more complex than any other.

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