[quicwg/base-drafts] Endpoints cannot verify padding (#4253)

Kazuho Oku <notifications@github.com> Tue, 20 October 2020 01:24 UTC

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Date: Mon, 19 Oct 2020 18:24:28 -0700
From: Kazuho Oku <notifications@github.com>
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Subject: [quicwg/base-drafts] Endpoints cannot verify padding (#4253)
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In #4183, we added the following sentence, but I think it does not make sense.
> When a client receives an ack-eliciting Initial packet that is carried in a UDP datagram with a payload that is less than 1200 bytes, that client MAY close the connection by sending a CONNECTION_CLOSE frame.

Generally speaking, a receiver of a _datagram_ cannot verify if the sender sent a full-sized packet.

That is because a datagram as a whole is not authenticated. Instead, we authenticate each of the QUIC packets that constitute the datagram.

Therefore, if the sender sends a datagram consisting of multiple QUIC packet, it is (almost always) possible for a MITM to take one of the QUIC packets, coalesce that with a few octets, and forward that _datagram_. That forwarded _datagram_ would be smaller than the original datagram.

Note also that a sender can send coalesced packet after the handshake too; see [section 14.4.1 of the transport draft](https://quicwg.org/base-drafts/draft-ietf-quic-transport.html#name-pmtu-probes-containing-sour).

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