QUIC / HTTP3 transport level privacy notes

[nothingmuch]

QUIC has the nice property that it can use multiple connection IDs (which are exchanged over an encrypted channel) and supports connection migration for multipath. With connection migration, two asymmetric QUIC + HTTP3 connections can be merged to the same (set of) UDP sockets, making it impossible to rely on UDP ports to identify application level traffic direction (With HTTP 1.1, 2 and HTTP 3 without connection migration HTTP requests and responses would be distinguishable as their traffic is identified with different connections and with different ports).

Therefore for Relay-Directory hybrid nodes communicating over QUIC and migrating both their connections as described, especially if they pad their QUIC packets to a consistent size and generate additional QUIC cover traffic, can hide the direction of relayed traffic from a global passive adversary. This is is especially useful in the the onion/mixnet setting, since async and sync HTTP requests can no longer be distinguished making the use of reply blocks undetectable to a GPA. tquic.net is a rust quic impl that seems capable of such connection migration.

It's not clear that QUIC + HTTP3 can materially impact client-to-relay transport layer privacy, but it can definitely provide UX improvements due to the improved handshaking over HTTP2.