Re: [v6ops] Flow Label Load Balancing

[Alexander Azimov <[e-mail]>]

Hi Fernando,

Stating that FL change during TCP session lifetime is a bug - is a bit
harsh.

It is a fantastic idea to change the FL value if RTO or SYN_RTO happens
in a controlled environment.
These are very specific TCP timeouts, that provide enough guarantee that
there will be no out-of-order packets, though your packets will reach
the destination even in case of an outage inside your network. Zero
influence on your services in case of the network outage - doesn't sound
like a bug for me.

The problem is with the current Linux (though I haven't checked other OSes)
defaults. Specifically, with the default behavior after RTO event.

вт, 24 нояб. 2020 г. в 22:06, Fernando Gont <[e-mail]>:

> 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:
> >
> >   *
> https://lore.kernel.org/netdev/[e-mail]/
> >   *
> https://lore.kernel.org/netdev/[e-mail]/
> >   *
> https://lore.kernel.org/netdev/[e-mail]/
> >
> >
> > 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.
>
> Thanks,
> --
> Fernando Gont
> e-mail: [e-mail] || [e-mail]
> PGP Fingerprint: 7809 84F5 322E 45C7 F1C9 3945 96EE A9EF D076 FFF1
>
>
>
>

-- 
Best regards,
Alexander Azimov