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Re: RTT sampling on retransmissions

Ian Swett <ianswett@google.com> Sat, 21 July 2018 16:45 UTC

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From: Ian Swett <ianswett@google.com>
Date: Sat, 21 Jul 2018 12:44:56 -0400
Message-ID: <CAKcm_gMt5ezBHSHkCBp+QBjQZgkdwXg3puwY9+ZBx_seY2h0Hg@mail.gmail.com>
Subject: Re: RTT sampling on retransmissions
To: aep@exys.org
Cc: IETF QUIC WG <quic@ietf.org>
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QUIC doesn't retransmits packets.  If data needs to be retransmitted
because it's lost, it's sent in a new packet with a new packet number.  The
new packet number prevents issues with ambiguity.

See recovery section 2(particularly 2.1.2) for more details:
https://tools.ietf.org/html/draft-ietf-quic-recovery-13#section-2



On Sat, Jul 21, 2018 at 10:23 AM Arvid Picciani <aep@exys.org> wrote:

> Hi,
>
> UpdateRtt in
> https://tools.ietf.org/html/draft-ietf-quic-recovery-13#section-3.5.5
> does seem to calculate the RTT from an ack on any packet, even if the
> packet was resent.
>
> 1. In a TLP event The oldest packet is resent and the sent_time reset
> to the current time.
> 2. Eventually that packet is acked, because the network was just slow.
> 3. RTT is calculated from the last sent_time (which was the time it was
> resent).
> 4. RTT is way too small now, leading to lots of timeouts.
>
>
> what am i missing here?
>
> https://tools.ietf.org/html/rfc6298 section 2 specifically says that
> RTT must NOT be calculated from retransmissions. Is a "retransmitted"
> bit in the quic-recovery left as exercise to the reader or should
> retransmissions actually never be passed to OnPacketSent?
>
> /b/
>
>

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