Subject: [quicwg/base-drafts] Retransmitting ACKs may cause misleading RTT samples (#2748)

From: ianswett <[email-masked]>
Date: Wed, 22 May 2019 07:04:00 -0700

Even though a sender is encouraged to assemble frames containing up-to-date information every time it sends a packet, it is not forbidden to retransmit copies of frames from lost packets. A receiver MUST accept packets containing an outdated frame, such as a MAX_DATA frame carrying a smaller maximum data than one found in an older packet.
https://github.com/quicwg/base-drafts/pull/2312/files

If a packet containing an ACK is lost and the payload is retransmitted with a new packet number AND no ACKs are received prior to the retransmitted ACK frame arriving, the peer will calculate and RTT sample that is 2 RTTs + loss recovery delay.

As such, I'd suggest we add a warning that retransmitting ACKs can cause RTT inflation and is heavily discouraged.

There may be other issues with retransmitting ACKs, but this is the first that comes to mind.

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