Re: [tcpm] comments on draft-kang-tcpm-accurate-data-scheduling-by-server-00

Christoph Paasch <cpaasch@apple.com> Thu, 23 July 2020 03:10 UTC

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From: Christoph Paasch <cpaasch@apple.com>
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Date: Wed, 22 Jul 2020 20:10:41 -0700
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Cc: Yoshifumi Nishida <nsd.ietf@gmail.com>, "tcpm@ietf.org Extensions" <tcpm@ietf.org>, Liangqiandeng <liangqiandeng@huawei.com>
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To: Kangjiao <kangjiao@huawei.com>
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Subject: Re: [tcpm] comments on draft-kang-tcpm-accurate-data-scheduling-by-server-00
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Hello,

> On Jul 22, 2020, at 7:01 PM, Kangjiao <kangjiao@huawei.com> wrote:
> 
> Hi Christoph and Yoshi,
> 
> If I understand right, you both point to the same question whether reusing the MP_PRIO can finish accurate data scheduling for server. I think MP_PRIO may be not enough even if setting all the "non-target" subflows to backup ones. I can give a scenario:
> 
> Six subflows have been established between client and server that are <IP_C1, IP_S1>, <IP_C2, IP_S1>, <IP_C1, IP_S2>, <IP_C2, IP_S2>, <IP_C1, IP_S3> and <IP_C2, IP_S3>. The client has IP_C1 and IP_C2. The server has IP_S1, IP_S2 and IP_S3.
> 
> Server tool detects that KPI of IP_S2 is better now so the server wants to switch data traffic from <IP_C1, IP_S1> to the destination IP_S2. If we set <IP_C1, IP_S1> as "backup", it is not guaranteed that all the load over <IP_C1, IP_S1> can be diverted to IP_S2 because of the ongoing suflows towards IP_S3.

But we could simply also set <IP_C1,IP_S3> ans <IP_C2,IP_S3> as backup as well, no?

Now, all the traffic will be steered towards S2.

Christoph


> 
> So I think MP_PRIO can reduce the chance of data transmission on a specific subflow but it cannot convey the info of target network interface from server to client.
> 
> Thanks
> Jiao
> 
> -----Original Message-----
> From: Christoph Paasch [mailto:cpaasch@apple.com] 
> Sent: Wednesday, July 22, 2020 12:00 AM
> To: Kangjiao <kangjiao@huawei.com>
> Cc: Yoshifumi Nishida <nsd.ietf@gmail.com>om>; tcpm@ietf.org Extensions <tcpm@ietf.org>rg>; Liangqiandeng <liangqiandeng@huawei.com>
> Subject: Re: [tcpm] comments on draft-kang-tcpm-accurate-data-scheduling-by-server-00
> 
> Hello,
> 
> as an overall comment, I agree that one of the difficulties around multipath is how the hosts can communicate to the peers the desired scheduling mechanisms. Currently there is no reliable way besides backup-bits which is a "hard" switch. However, often scheduling is much more fine-grained as shown in a number of the schedulers described in https://tools.ietf.org/html/draft-bonaventure-iccrg-schedulers-00.
> 
> It's a bit similar to TCP which can't pick the peer's congestion-control and for example if one wants the peer to use a background-CC the host needs to resort to tuning the receive-window a la draft-bagnulo-iccrg-rledbat-00.
> 
> So, having a way to communicate the scheduling "requirements" to the peer (and let the peer match these requirements with its local policies), would be good!
> 
> 
> One more comment inline:
> 
>> On 07/20/20 - 03:54, Kangjiao wrote:
>> Hi Yoshi,
>> 
>> Thanks for your suggestions. We clarify the issues as below:
>> 
>> 1: One thing I'm not very clear is why we cannot use MP_PRIO for the 
>> use cases described in the draft. I believe the draft should describe 
>> the cases where existing features cannot fulfill the requirements more 
>> specifically.
>> 
>> KJ: The new MP_Navigation Option is used for the server to indicate 
>> destination network interface to client for which server wants to use 
>> for traffic switching. For my understanding, MP_PRIO is used to signal 
>> a change in priority of subflows to the peer. In application, MP_PRIO 
>> can reduce the chance of data transmission on a specific subflow but 
>> it cannot tell its peer which network interface is the destination from server side.
>> For example, if there are multiple subflows with high priority from 
>> difference network interfaces, client receiving MP_PRIO does not know 
>> which is the target one.
> 
> For this particular scenario the server could send the MP_PRIO with backup-bit set on all subflows that the client should not use. Thus implicitly the high-priority network-interfaces can then be used by the client, right?
> 
> 
> Christoph
> 
>> 2: Clients also have their own constraints. (e.g. policy or routing) 
>> So, even though servers send a navigation request, they might not 
>> follow it. I think this point should be clarified.
>> 
>> KJ: If the mechanism of accurate-data-scheduling-by-server is 
>> deployed, the principle is that the server takes precedence.
>> 
>> 3: What's the meaning of 'r', 'E', 'B' flags in Section 4.1?
>> 
>> KJ:  For the protocol design, the definition of ’r’, ‘E’ and ’B’ are 
>> as
>> following: Flag ‘r’: reserved for future usage.  Flag ‘E’: exists to 
>> provide reliability for this option (like that in ”ADD_ADDR”).  Flag ’B’:
>> indicates whether the subflow over which the option is received is a 
>> backup one (that is compatiable with the value by MP_PRIO).
>> 
>> But we are thinking whether these fields are necessary and should be 
>> set as mandatory.
>> 
>> Sincerely, Jiao From: tcpm [mailto:tcpm-bounces@ietf.org] On Behalf Of 
>> Yoshifumi Nishida Sent: Sunday, July 19, 2020 7:26 PM To: 
>> tcpm@ietf.org Extensions <tcpm@ietf.org> Subject: [tcpm] comments on
>> draft-kang-tcpm-accurate-data-scheduling-by-server-00
>> 
>> Hi, I've read draft-kang-tcpm-accurate-data-scheduling-by-server-00.  
>> I think this is an interesting topic for mptcp, but I think it would 
>> be better to clarify the following points.
>> 
>> 1: One thing I'm not very clear is why we cannot use MP_PRIO for the 
>> use cases described in the draft. I believe the draft should describe 
>> the cases where existing features cannot fulfill the requirements more 
>> specifically.
>> 
>> 2: Clients also have their own constraints. (e.g. policy or routing) 
>> So, even though servers send a navigation request, they might not 
>> follow it. I think this point should be clarified.
>> 
>> 3: What's the meaning of 'r', 'E', 'B' flags in Section 4.1?
>> 
>> Thanks, -- Yoshi
> 
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