Re: [quicwg/base-drafts] max_packet_size in 0-RTT (#3447)

Martin Thomson <> Fri, 21 February 2020 04:31 UTC

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From: Martin Thomson <>
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Subject: Re: [quicwg/base-drafts] max_packet_size in 0-RTT (#3447)
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I don't like @kazuho's proposed change.  The point of this is to express a limitation of the endpoint.  If the endpoint only has X bytes of memory for receiving packets, then it can use this limit.  Absent this, we can't ever hope to have big endpoints talking to little ones.  We will end up with middleboxes (hello CoAP).

If the limit exists at a server and the client ignores it for 0-RTT, then the effect is that 0-RTT doesn't get through.  I would rather just say that client's are expected to remember it and then if they don't they can wear the consequences.  But that can be a protocol violation.

The problem here is that we now have protection for Initial packets and no way to signal that there is a limit.  Our only real protection there is that the limit manifests in much the same way as having a lower MTU because Initial packets don't get through.

This matters especially if clients start remembering or inferring path MTU.  I don't remember us ever expressly allowing this sort of thing, but it seems like a reasonable thing that might happen when probing can be quite costly and wasteful.

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