Re: [nwcrg] [LOOPS] IETF106: LOOPS side meeting on Tuesday 08:30, Orchard Room

Michael Welzl <> Tue, 17 December 2019 07:23 UTC

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From: Michael Welzl <>
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Date: Tue, 17 Dec 2019 08:23:09 +0100
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Subject: Re: [nwcrg] [LOOPS] IETF106: LOOPS side meeting on Tuesday 08:30, Orchard Room
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I would say it depends on how much loss we’re optimizing for (and the LOOPS vs. e2e RTT difference; lots of trade-offs here).

One point in favor of the simpler one-retransmission approach: just like a link layer doing ARQ, for its retransmissions, LOOPS will inflate the RTT measurements seen by transport protocols. How bad that really is depends on the transport protocol (e.g., TCP’s RTO is super conservative anyway), on the LOOPS vs. e2e RTT, on how often LOOPS retries…   I’d say there’s reason to hope that retransmitting once is relatively “safe” in that sense.


> On Dec 17, 2019, at 5:14 AM, Liyizhou <> wrote:
> Hi Carsten and all,
> Thanks for putting all these together. 
> I would like to have a follow-up discussion regarding single retransmission vs. multiple retransmission.
> There is a potential choice to design a single retransmission mode *only* mechanism. The reason we might want to do it is the design is easier comparing with a generic retransmission mechanism allowing both single or multiple retransmission by parameter configuration.
> For example,  LOOPS egress can mark a lost packet directly as "received" after it feedbacks a few ACKs to indicate a lost packet. Ingress removes the packet from its cache after a single retransmission. Once the retransmitted packet reaches the egress, egress simply forwards it. Such a mechanism only works for single retransmission.  
> In this way, egress has a simple logic to decide when to stop sending the indication about a "lost" packet.  And the ingress does not need a logic to control whether and when 2nd and later retransmissions should be performed.
> In multiple retransmission, it has to be more carefully designed to sync up when the egress gives up asking for retransmission and  when the ingress can remove the packet from its cache since no more retransmission is required.  As LOOPS does not provide 100% reliability, normally egress side should be the one determines when to give up asking for retransmission. Then egress needs to know the local forward RTT if multiple retransmission is expected.
> So single retransmission *only* can be worth considering. It is a design choice more than an implementation choice to me. 
> If there is a simple and efficient way to unify both, I would be more than happy to learn.  
> Thanks,
> Yizhou
> -----Original Message-----
> From: nwcrg [] On Behalf Of Carsten Bormann
> Sent: Monday, December 16, 2019 4:22 AM
> To:
> Cc:
> Subject: Re: [nwcrg] [LOOPS] IETF106: LOOPS side meeting on Tuesday 08:30, Orchard Room
> Thanks again everyone for the participation at the LOOPS side meeting and in our slot at the NWCRG meeting.
> I have compiled more detailed notes, with links to/copies of the meeting materials, at
> Please do have a look at the notes, specifically [1] and [2], and send corrections and additions.  Also, there were a few items in the NWCRG discussion where there may be results that we already can point to; as Marie-Jose said, please send them to both mailing lists.
> Grüße, Carsten
> [1]:
> [2]:
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