Re: [v6ops] Flow Label Load Balancing

Fernando Gont <> Tue, 24 November 2020 19:06 UTC

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To: Alexander Azimov <>, tcpm <>,
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From: Fernando Gont <>
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Date: Tue, 24 Nov 2020 16:05:50 -0300
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Subject: Re: [v6ops] Flow Label Load Balancing
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On 19/11/20 07:48, Alexander Azimov wrote:
> Dear colleagues,
> I have added in the cc both v6op and tcpm for a reason and let me 
> explain why.
> It's clear that we are moving forward with load balancing that uses flow 
> label (FL). And the pressure will increase with SRv6 adoption. But at 
> the moment wide adoption of FL-based load-balancing may create 
> significant issues for TCP Anycast services.
> RFC6437 suggests putting hash from 5-tuple into FL value. And as far as 
> I know, there is no document that updates this behavior. This 
> description is perfectly fine, but what is implemented in the Linux 
> kernel is different: FL is carrying hash from 5-tuple with an additional 
> seed, and this seed is randomly changed after each RTO/SYN_RTO event. 

Changing the FL upon RTO is a bug.

I guess/assume that when you say SYN-RTO, you really mean "user 
timeout", rather than RTO. If you don't, then that's also a bug.
If you do, I fail to understand what's the reason for wanting the FL to 
change in that case, because as a result of port randomization, it 0s 
unlikely that the same four-tuple is employed for the next connection retry.

> Here are related patches:
>   *
>   *
>   *
> This is a great thing by the way because in the data center 
> environment with multiple equal paths it gives a way to have 
> pseudo-multipath TCP which jumps between paths in case of an outage. 
> There might be interest to writedown an informational document for this.

That's a bad idea, since specs-wise the Flow-Label is not guaranteed to 
remain unchanged from source to destination. If you want to ahve 
multiple paths, then you should implement that in routing.

> I wonder what you think is a proper solution:
>   * Making FL related RTO change as knob instead of default behavior;
>   * Adding negotiation behavior in TCP;
>   * Something else?

Just make the FL a function of the connection "identifier". And keeo it 
constant for the lifetime of that conenction.

Fernando Gont
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