Re: [Dots] Improve controllability and predictability of keepalives

Carsten Bormann <cabo@tzi.org> Wed, 20 November 2019 06:19 UTC

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From: Carsten Bormann <cabo@tzi.org>
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Date: Wed, 20 Nov 2019 14:19:32 +0800
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Subject: Re: [Dots] Improve controllability and predictability of keepalives
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>> Thank you.
>> I still have a few questions, but I think this looks very good.
>> 
>> 4.4 says:
>>           Mitigation requests MUST NOT be delayed
>> 	   because of other congestion control checks.
>> What might those be?
> 
> [Med] A too low probing rate for example as indicated in the sentence right after that excerpt. There is a risk that mitigation requests are delayed (or that heartbeats are not sent which may lead to session liveness checks).   
> 
> 
>> (You don’t want to be in a situation where you send a ton of mitigation
>> requests that are mostly dropped at the tail of a queue.)
>> 
> 
> [Med] Yes, we don't that. The authoritative rule is as follows: 
> 
>   Requests marked by the DOTS client as Non-confirmable messages are
>   sent at regular intervals until a response is received from the DOTS
>   server.  If the DOTS client cannot maintain an RTT estimate, it MUST
>   NOT send more than one Non-confirmable request every 3 seconds, and
>   SHOULD use an even less aggressive rate whenever possible (case 2 in
>   Section 3.1.3 of [RFC8085]).  

And if it can?

> 
> 
>> Before Figure 27, it says:
>> 
>>           The PUT request used for DOTS heartbeat MUST NOT have a
>> 	   'cuid', 'cdid,' or 'mid' Uri-Path.  Such PUT requests MUST NOT be
>> 
>> 	   relayed by DOTS gateways.
>> 
>> Why?  (And what should the DOTS gateways do instead?)
> 
> [Med] Session loss detect and recovery is local to an agent and its immediate peer. Heartbeats are thus not relayed. 

Ah, I was confused.
Where is “DOTS gateway” defined?
I’m assuming this is not like a CoAP Proxy?
It would need to expose the equivalent to /hb.

> We can update as follows: 
> 
> "This procedure occurs between a DOTS agent and its immediate peer DOTS agent. As such, this GET request MUST NOT be relayed by a DOTS gateway. The PUT request used for DOTS heartbeat MUST NOT have a ‘cuid', 'cdid,' or 'mid' Uri-Path."

This doesn’t quite address the above, though.

Grüße, Carsten