Re: [quicwg/base-drafts] DPLPMTU merge tweaks (#3702)

Martin Thomson <> Tue, 09 June 2020 07:10 UTC

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Date: Tue, 09 Jun 2020 00:10:57 -0700
From: Martin Thomson <>
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Subject: Re: [quicwg/base-drafts] DPLPMTU merge tweaks (#3702)
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@martinthomson commented on this pull request.

> -However, prior to learning the value of the transport parameter, endpoints risk
-datagrams being lost if they send packets larger than 1200 bytes.
+({{transport-parameter-definitions}}) might act as an additional limit on the
+maximum packet size. A sender can avoid exceeding this limit, once the value is
+known.  However, prior to learning the value of the transport parameter,
+endpoints risk datagrams being lost if they send packets larger than the
+smallest allowed maximum packet size of 1200 bytes.
+A client MUST expand the payload of all UDP datagrams carrying Initial packets
+to at least the smallest allowed maximum packet size (1200 bytes) by adding
+PADDING frames to the Initial packet or by coalescing the Initial packet; see
+{{packet-coalesce}}.  Sending a UDP datagram of this size ensures that the
+network path from the client to the server supports a reasonable Path Maximum
+Transmission Unit (PMTU).  This also helps reduce the amplitude of amplification
+attacks caused by server responses toward an unverified client address; see
 Datagrams containing Initial packets MAY exceed 1200 bytes if the client

I think we caught this in #3742.

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