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

Colin Perkins <csp@csperkins.org> Tue, 17 December 2019 11:58 UTC

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From: Colin Perkins <csp@csperkins.org>
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Date: Tue, 17 Dec 2019 11:32:53 +0000
Cc: Carsten Bormann <cabo@tzi.org>, "loops@ietf.org" <loops@ietf.org>, "nwcrg@irtf.org" <nwcrg@irtf.org>
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To: Liyizhou <liyizhou@huawei.com>
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Subject: Re: [nwcrg] [LOOPS] IETF106: LOOPS side meeting on Tuesday 08:30, Orchard Room
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Hi,

Is not the ingress logic the same for single or multiple retransmissions? The ingress node will maintain a queue of packets that have been sent and can be retransmitted. Packets expire from the queue if an ACK for their sequence number is received, or after a certain amount of time. Packets are retransmitted if a retransmission request arrives before they expire. You need the timeout to expire packets in case ACKs are lost, so whether the ingress supports single or multiple retransmissions is just a case of configuring the timeout.

If you somehow communicate that timeout to the egress node, then it can make a local decision on whether to make one or more retransmission requests for a lost packet. This would give the egress flexibility to pick an appropriate retransmission request strategy without needing to standardise what that strategy is. LOOPS would define the mechanism and give guidance on choice of the ingress expiration timeout. 

Colin



> On 17 Dec 2019, at 04:14, Liyizhou <liyizhou@huawei.com> 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 [mailto:nwcrg-bounces@irtf.org] On Behalf Of Carsten Bormann
> Sent: Monday, December 16, 2019 4:22 AM
> To: loops@ietf.org
> Cc: nwcrg@irtf.org
> 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
> 
> https://github.com/loops-wg/ietf106
> 
> 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]: https://github.com/loops-wg/ietf106/blob/master/ietf106-loops-side-meeting.md
> [2]: https://github.com/loops-wg/ietf106/blob/master/ietf106-nwcrg-loops.md
> 
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-- 
Colin Perkins
https://csperkins.org/