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

Gorry Fairhurst <> Thu, 28 May 2020 03:06 UTC

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Date: Wed, 27 May 2020 20:06:44 -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.

> -used for routing.
+packets are likely to require that the connection ID be included in
+PMTUD/DPLPMTUD probe packets to route any resulting ICMP messages
+({{icmp-pmtud}}) back to the correct endpoint.  However, only long header
+packets ({{long-header}}) contain source connection IDs, and long header packets
+are not decrypted or acknowledged by the peer once the handshake is complete.
+One way to construct a PMTU probe is to coalesce (see {{packet-coalesce}}) a
+packet with a long header, such as a Handshake or 0-RTT packet
+({{long-header}}), with a short header packet in a single UDP datagram.  If the
+UDP datagram reaches the endpoint, the packet with the long header will be
+ignored, but the short header packet will be acknowledged.  If the UDP datagram
+causes an ICMP message to be sent, the first part of the datagram will be quoted
+in that message.  If the source connection ID is within the quoted portion of
+the UDP datagram, that could be used for routing.

:-). (Knowing that needs a measurement survey) However, the implications are you wouldn't be able to rely on ICMP to help you. Well that's no new news there are many reasons why you may not see a PTB message, that's why DPLPMTUD was made.

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