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Re: [tcpm] TLP questions

Praveen Balasubramanian <pravb@microsoft.com> Wed, 09 May 2018 18:30 UTC

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From: Praveen Balasubramanian <pravb@microsoft.com>
To: Yuchung Cheng <ycheng@google.com>, Neal Cardwell <ncardwell@google.com>
CC: "tcpm@ietf.org" <tcpm@ietf.org>
Thread-Topic: TLP questions
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Date: Wed, 09 May 2018 18:29:49 +0000
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Archived-At: <https://mailarchive.ietf.org/arch/msg/tcpm/UqsXB_ei7ZWMM_wxSHYSZIxA9Ec>
Subject: Re: [tcpm] TLP questions
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Including tcpm as this can be a review of the TLP portions of draft-ietf-tcpm-rack-03.

From: Praveen Balasubramanian
Sent: Tuesday, May 8, 2018 6:13 PM
To: Yuchung Cheng <ycheng@google.com>; Neal Cardwell <ncardwell@google.com>
Subject: TLP questions

Hey folks, I have some questions on draft-ietf-tcpm-rack-03 mainly around TLP.


  1.  "Open state: the sender's loss recovery state machine is in its  normal, default state: there are no SACKed sequence ranges in the  SACK scoreboard, and neither fast recovery, timeout-based recovery, nor ECN-based cwnd reduction are underway. "

Open state is defined and then never used. I assume you require that TLP be scheduled only for connections in open state? I wonder why there is the requirement on "no SACKed sequence ranges in the  SACK scoreboard".

  1.  PTO += 2ms

If we already are being conservative by waiting 2*SRTT, then how is adding 2 msec going to help? Was this added due to a real world issue?

  1.  "If a previously unsent segment exists AND
         the receive window allows new data to be sent:
           Transmit that new segment
           FlightSize += SMSS
       Else:
           Retransmit the last segment"
This needs to be crisper about what is meant by "previously unsent" and "last segment". For example the sender could have sent a large amount of data and then taken a full RTO. In this case if PTO fires, do "previously unsent" and "last segment" refer to MSS size segments straddling just before and  after SND.NXT? OR do they straddle around the largest sent sequence number ever in the connection lifetime?

  1.  "On each incoming ACK, the sender should cancel any existing loss  probe timer."

Even on duplicate ACKs?

Thanks

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