Re: [quicwg/base-drafts] amplification attack using Retry and VN triggered by coalesced Initial packets (#2259)

[Marten Seemann <[e-mail]>]

> We need to maintain state while parsing the datagram (to not let each of enclosed packets generate a Retry)

State is exactly what I'm concerned about. I'd like to avoid allocating state for 67 QUIC packets due to a single datagram. With my proposed requirement, you can at most have 4 QUIC packets per datagram.
Not sending more than a single Retry or a single VN packet per datagram gets trivial if we introduce this requirement, since you already detect duplicate Initials when parsing, not just when handling packets.

> Therefore, I do not see benefit in disallowing coalescence, while the downside would be that it makes difficult to stitch a fake QUIC packet in front of the datagram to help PMTUD (as of -17, we do not have "reserved" packet types).

I haven't implemented PMTUD yet, and as far as I can see, the PMTU section doesn't say anything about coalesced packets. To increase the packet size you're supposed to fill packets with PING or PADDING frames. I assume you don't want to do this, and you want to use coalesced packets instead. What's the advantage of that approach? It seems like you just add a bunch of corner case to your code (since then there's a range of packet sizes you can't reach, because you have to add a whole QUIC header).

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