[devinplatt]

I was curious about this given what happened with SDCH.

Here is what Wikipedia[0] says

> Due to the diffing results and the data being compressed with the same coding, SDCH dictionaries aged relatively quickly and compression density became quickly worse than with the usual non-dictionary compression such as GZip. This created extra effort in production to keep the dictionaries fresh and reduced its applicability. Modern dictionary coding such as Shared Brotli has a more effective solution for this that fixes the dictionary aging problem.

This new proposal uses Brotli.

[0]: https://en.m.wikipedia.org/wiki/SDCH

[patrickmeenan]

SDCH was removed when SPECTRE became a thing (CRIME/BREACH) because it was open to side-channel attacks.

Yes, it had other problems, not the least of which was that it would block the processing of a response while a client fetched the dictionary, but the side-channel attacks were what killed it.

The compression dictionary transport work addresses all of the known issues that we had with SDCH and we're cautiously optimistic that this will be around for a long time.