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

Gorry Fairhurst <> Fri, 05 June 2020 13:05 UTC

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Date: Fri, 05 Jun 2020 06:05:56 -0700
From: Gorry Fairhurst <>
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Subject: Re: [quicwg/base-drafts] DPLPMTU merge tweaks (#3702)
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@gorryfair 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

Needs to be 1200B of payload? (We need to be careful about which layer these numbers are describing, since the datagram itself will be larger.

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