Re: [quicwg/base-drafts] Packet number echo with fixed-length, 32-bit packet and echo numbers (#393)

[janaiyengar <[e-mail]>]

janaiyengar commented on this pull request.



> -packet number.  Once any packet has been acknowledged, subsequent packets can
-use a shorter packet number encoding.
+
+### Packet Number Echo {#packet-number-echo}
+
+If present, the Packet Number Echo field contains the least significant 32 bits
+of the highest packet number seen in the opposite direction before this packet
+was sent. This allows devices along the path to estimate round-trip times and
+observe indications of loss, for passive performance measurement of QUIC flows
+equivalent to present techniques using TCP sequence and acknowledgement numbers
+and/or timestamps.
+
+The Packet Number Echo field SHOULD be present on 1-RTT packets containing at
+least one ACK frame (see {{frame-ack}}). For efficiency's sake, it MAY be
+omitted if a 1-RTT packet containing an ACK frame has already been sent during a
+time interval equal to the sender's current estimate of the RTT.

or alternatively, you could remove the largest_acked out of the ACK frame and require this on all packets that carry an ACK frame.

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