Re: [quicwg/base-drafts] Updated ICMP PMTU section (#1412)

[ianswett <[e-mail]>]

ianswett commented on this pull request.

This looks good, but I'd like someone who's more of an expert in ICMP to read it, so I'm adding @martinduke 

> @@ -3228,6 +3261,18 @@ increases in the size of probe packets. As QUIC probe packets need not contain
 application data, aggressive increases in probe size carry fewer consequences.
 
 
+## Responding to ICMP "Unreachable" messages {#icmp-unreach}
+
+When a QUIC endpoint receives an ICMP "Unreachable" message during a handshake,
+the response SHOULD be identical to receiving an ICMP TPB message that announces
+a Path MTU smaller than 1280 octets (see {{icmp-pmtu}}).
+
+When an ICMP "Unreachable" message is received after the handshake, the QUIC
+endpoint should send a PATH_CHALLENGE frame ({{frame-path-challenge}}).  Sending
+PATH_CHALLENGE frames on the same path due to ICMP "Unreachable" messages should
+be rate limited.

To what?  How about one outstanding PATH_CHALLENGE at once?

>  
-As a result, endpoints that implement PMTUD in IPv4 SHOULD take steps to
-mitigate this risk. For instance, an application could:
+The minimum required validation of ICMP messages with an on-path proof invoves
+verifying that the message was sent by this endpoint with at least 1-2^32
+probability and it is still outstanding (not acknowledged and not deemed lost).
+If a QUIC endpoint does not perform this minimum validation, it SHOULD treat the
+packet as an ICMP message without an on-path proof.
+
+As noted in {{?RFC5927}}, using ICMP messages without an on-path proof exposes
+the protocol implementation to off-path attacks and requires mitigations.
+
+Even ICMP messages without an on-path proof SHOULD undergo some validation, such
+as:
 
 * Set the IPv4 Don't Fragment (DF) bit on a small proportion of packets, so that

I thought we were recommending setting DF on all packets?

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