QUIC server response bigger than allowed x3 byte limit

[josephnoir]

I performed a handshake with https://cloudflare-quic.com. The reply from the server is bigger than the allowed x3 limit of the client payload. Here's a screenshot of the first packets, the complete PCAP trace is attached in the zip.

Only looking at the UDP payload sizes, the initial packet from the client has 1337 bytes. This would give the server 4011 bytes to answer. However, it sends a total of 5406 bytes, which is 1395 more than allowed by the specification.

A big chunk of the overhead stems form the padding in the two Initial responses. Here, the server sends two datagrams instead of coalescing the ACK and CRYPTO packet into a single one. But even if only a single datagram would be sent, the sever would still go over the limit.

I tried looking through the code to identify the specific locations, but haven't been successful so far.

firefox-cloudflare-quic-handshake.pcapng.zip

[josephnoir]

Any updates or interest to fix this?

[LPardue]

This question relates to a specific deployment of QUIC related to https://cloudflare-quic.com. We communicated directly with the team that looked into this and the findings were summarized in https://dl.acm.org/doi/10.1145/3555050.3569123. Since the question was answered elsewhere, I'll close this one.