Proposal

Support HPACK dynamic table sizes other than the default, specifically:

• Add ability to configure the max dynamic table size for HPACK encoders/decoders in both the http2 server and client
• Add correct handling of sent/received SETTINGS_HEADER_TABLE_SIZE

Context

HTTP/2 uses HPACK for header compression. HPACK is a stateful compression algorithm where both the encoder and decoder reference a static table (defined in the RFC) and maintain a dynamic table. This dynamic table defaults to 4096 bytes and has header fields added/removed in FIFO order over the course of the connection lifetime.

SETTINGS_HEADER_TABLE_SIZE is an h2 setting used to adjust the size of the HPACK dynamic table. The decoder communicates the largest dynamic table size it's willing to support via SETTINGS_HEADER_TABLE_SIZE, then the encoder on the other end of the connection chooses what size it wants to use (up to the decoder's max) and communicates that back to the decoder via a dynamic header table size update instruction. Thus, there are two relevant client settings: the allowable size we want to advertise for our decoder, and the max size we want to use for our encoder.

Use Case

The main motivation is to allow increasing the dynamic table size for http2 connections that multiplex disparate requests, specifically in proxies and load balancers. The default size of 4096 works well for connections servicing a single client, but when multiple clients are bundled together on a reverse proxy's shared upstream connection and sharing the same decoder/encoder, the small table size and churning hurts the HPACK compression ratio, increases bytes on the wire and increases latency.

References

• Previous discussion about this here x/net/http2: missing SETTINGS_HEADER_TABLE_SIZE support #29356
• Current CL here https://go.dev/cl/435899
• HTTP/2 RFC section on SETTINGS_HEADER_TABLE_SIZE
• HPACK RFC