Re: [quicwg/base-drafts] introduce a version alias mechanism (#2573)

"Philipp S. Tiesel" <> Wed, 10 April 2019 07:51 UTC

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From: "Philipp S. Tiesel" <>
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Subject: Re: [quicwg/base-drafts] introduce a version alias mechanism (#2573)
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philsbln commented on this pull request.

> @@ -4115,6 +4130,25 @@ preferred_address (0x000d):
 {: #fig-preferred-address title="Preferred Address format"}
+version_aliases (0x000e):
+: A list of version numbers that the server accepts as an alias for the
+  currently used versions. This transport parameter is only sent by the server.
+  Every version alias contains a lifetime in milliseconds. The alias is only valid
+  for that lifetime, clients MUST NOT use it after expiry.

I don't consider this a fully effective downgrade-prevention mechanism, but it provides a mechanism for the client to detect on-path version tapering: If the client gets a version negotiation packet in return to an alias version and receives the same version alias that failed again, it can be sure someone is doing a downgrade attack (or the server is horribly broken).

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