Re: [quicwg/base-drafts] Proposal for adding ECN support to QUIC. (#1372)

[mirjak <[e-mail]>]

mirjak commented on this pull request.



> @@ -2887,6 +3033,95 @@ by a client in protected packets, because it is certain that the server is able
 to decipher the packet.
 
 
+## ACK_ECN Frame {#frame-ack-ecn}
+
+A QUIC connection MUST keep counters for each ECN codepoint, recording
+the number of packets that were received with the corresponding ECN
+codepoint in the IP header. If the header is not readable from the
+application, the codepoint 00 (Not-ECT) MUST be assumed. If any packet
+are duplicated by the network then only the value of the ECN field of
+the packet copy first received SHALL be included in the counters, the ECN
+field value for a duplicate SHALL be ignored. This to prevent the

No, that is not true. The network marks a packet as congested because it is congested and want to signal this to the endpoint. The network does not distinguish between retransmission or duplicates or anything because all these packets need the same amount of network resources and therefore cause congestion. RFC8085 even says 
"A receiver that enables the use of ECN for a UDP port MUST check
      the ECN field at the receiver for each UDP datagram that it
      receives on this port." 
and 
"This MUST report the
      presence of datagrams received with a CE-mark by providing a
      mechanism to feed this congestion information back to the sending
      application."
You cannot just ignore an CE because it's on a retransmission or a duplicated packet!

The problem with duplicates it that a packet could have been duplicated before or after it was CE marked. If it gets duplicated after it was marked, this is not a real congestion indication and if counted overstates the congestion. However, if it gets marked after it was duplicated, it is an indication of congestion. As you don't know when it got marked, you need to be conservative and account for it.

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