Re: [v6ops] Flow Label Load Balancing

Fernando Gont <fernando@gont.com.ar> Fri, 27 November 2020 20:47 UTC

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To: Tom Herbert <tom@herbertland.com>, Brian E Carpenter <brian.e.carpenter@gmail.com>
Cc: tcpm <tcpm@ietf.org>, IPv6 Operations <v6ops@ietf.org>
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From: Fernando Gont <fernando@gont.com.ar>
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Date: Fri, 27 Nov 2020 17:47:21 -0300
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Subject: Re: [v6ops] Flow Label Load Balancing
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Tom,

On 25/11/20 21:15, Tom Herbert wrote:
> On Wed, Nov 25, 2020 at 3:33 PM Brian E Carpenter
> <brian.e.carpenter@gmail.com> wrote:
>>
>> I'm not Joel, but I did once spend some time grepping RFCs to find out whether
>> "flow" or "microflow" was the preferred term. In RFC2474, which is normative,
>> we have:
>>
>>     Microflow: a single instance of an application-to-application flow of
>>     packets which is identified by source address, destination address,
>>     protocol id, and source port, destination port (where applicable).
>>
>> But in the flow label work we explicitly avoided being that precise, and
>> did not use the term "microflow". There might be some load balancing
>> scenarios where you want a broader definition, even including bidirectional
>> flows. There are expired drafts on that topic:
>> draft-tarreau-extend-flow-label-balancing
>> draft-wang-6man-flow-label-reflection
>>
> Brian,
> 
> Random Packet Spraying
> (https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.297.529&rep=rep1&type=pdf)
> is an interesting idea where packets for a single connection are
> purposely distributed across multiple paths for load distribution. Per
> packet randomized flow labels with flow label aware ECMP makes this
> quite easy to do without requiring any special support in switches
> like you'd need with IPv4. I'm not necessarily advocating this, but it
> does highlight one potential use case of having a flow label that
> doesn;t have rigidly defined requirements on the host.

You don't need to randomize the FL of packets corresponding to a flow to 
get this --- simply do round-robin at the router that wants load-sharing 
over different links.

OTOH, if you do randomize the FL of each packet, then routers that want 
to avoid packet reordering simply can't.

Thanks,
-- 
Fernando Gont
e-mail: fernando@gont.com.ar || fgont@si6networks.com
PGP Fingerprint: 7809 84F5 322E 45C7 F1C9 3945 96EE A9EF D076 FFF1