[quicwg/base-drafts] Initial header table size of QPACK (#1767)

[Kazuho Oku <[e-mail]>]

Unlike HPACK (quoted below), QPACK does not require an encoder to emit Dynamic Table Size Update as its first instruction.

> This dynamic table size update MUST occur at the beginning of the first header block following the change to the dynamic table size. In HTTP/2, this follows a settings acknowledgment (see Section 6.5.3 of [HTTP2]).
> [RFC 7541; section 4.2](https://httpwg.org/specs/rfc7541.html#maximum.table.size)

This means that the peers might disagree on the table size, unless we require the encoder to set the table size to the value advertised by the peers SETTINGS frame. But I wouldn't assume that we would want to require that; for example some encoders might want to stick to 4KB even when the peer's SETTINGS frame says the table can be as large as 1MB.

Considering these aspects, I think that we should require the QPACK encoder to send Dynamic Table Size Update as its first instruction, at least when the SETTING frame advertises a non-default value.

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