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Re: [quicwg/base-drafts] Do not allow ICMP PTB to increase PMTU (#2161)

Martin Thomson <notifications@github.com> Fri, 14 December 2018 04:59 UTC

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Subject: Re: [quicwg/base-drafts] Do not allow ICMP PTB to increase PMTU (#2161)
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martinthomson requested changes on this pull request.



> @@ -3139,7 +3139,8 @@ attacks that successfully guess the IP address 3-tuple and reduce the PMTU to a
 bandwidth-inefficient value.
 
 An endpoint MUST ignore an ICMP message that claims the PMTU has decreased below
-1280 bytes.
+1280 bytes.  And endpoint MUST ignore an ICMP message that attempts to increase
+PMTU, unless the ICMP message was received in response to a PMTU probe packet.

Not sure about the "unless" part here.

Say I expect PMTU to be 1280.  Then I send a probe for 2048.  I get back an ICMP PTB that says 1500.  This would allow the PMTU to be set to 1500, but I think that you want to probe at 1500 before increasing anything.

Thus, I would instead say that an ICMP message that claims a larger PMTU than the confirmed PMTU MUST be confirmed (by sending a packet at that size and having it acknowledged) before the PMTU can be increased.

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