[quicwg/base-drafts] Stateless Reset packet sizes should not depend on the maximum connection ID length (#2869)

[David Schinazi <[e-mail]>]

Currently, the spec recommends sending Stateless Reset packets of at least 39 bytes. The number is the minimum packet size when using the maximum connection ID length of 18:
- type byte: 1
- client connection ID: 18
- packet number + payload: 4
- auth tag: 16

39 = 1 + 18 + 4 + 16.

The intention there was to make Stateless Resets harder to distinguish from regular server-to-client traffic on a given QUIC connection.

However, when client connection IDs are in use, Stateless Resets are inherently distinguishable from regular traffic because they do not carry the previously-used client connection ID. One might say that they could be confused for a change in connection ID, which is true. But let's assume we just changed from an 18 byte client connection ID to a 4-byte connection ID (which is specifically allowed by the spec), then that new packet is still equally indistinguishable from a Stateless Reset. It is true that a client connection ID length change from 18 to 4 is unlikely, but that's not a problem. We just need it to be possible for this to be a valid packet to observers without keys.

So I propose that instead of having a minimum required size for stateless resets of 21 and a recommended minimum of 39, we can just say the minimum required size is 25. The important value here is the minimum non-zero connection ID length, not the maximum connection ID length.

At the end of the day, a Stateless Reset will stick out to observers who will notice that the client stops sending on that particular connection. The current value of 39 is harming the protocol by increasing the odds of servers not sending Stateless Resets which will cause timeouts on clients instead of failing fast. This is a tradeoff, and we should strive to avoid timeouts instead of trying to hide a packet that can't be hidden.

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