Re: [v6ops] Flow Label Load Balancing

[Tom Herbert <[e-mail]>]

Joel,

Is there an RFC that clearly and unambiguously states that a host MUST
use the same flow label for the lifetime _and_ clearly defines exactly
what a flow is with respect to such a requirement (for instance, how
would you define a flow and enforce such a requirement in UDP? IPsec?
other encapsulations?). If there is such a requirement then we'll
change the code to be conformant.

Tom

On Wed, Nov 25, 2020 at 12:39 PM Joel M. Halpern <[e-mail]> wrote:
>
> This kind of thing is why, as I understand it, MPTCP has discovery
> mechanisms ot know if both sides use it, and can select alternative
> addresses for communication.
>
> Trying to guess flow labels that might avoid a problem because it might
> be an ECMP problem, is just flailing about.  Not a good design for
> operational protocols.
>
> And in general, designing protocols around "I know exactly what is going
> on"  (the requirement for what you describe that goes well beyond just
> "limited domains") is also a recipe for failure.
>
> The Flow Label RFCs are actually very explicit that a flow label is
> supposed to be stable for the life of the flow.  Otherwise, it isn't a
> flow label.
>
> Yours,
> Joel
>
> On 11/25/2020 2:35 PM, Tom Herbert wrote:
> > Hi Fernando, comments in line...
> >
> > On Wed, Nov 25, 2020 at 12:13 AM Fernando Gont <[e-mail]> wrote:
> >>
> >> Hi, Tom,
> >>
> >> On 24/11/20 16:43, Tom Herbert wrote:
> >> [....]
> >>> Modulating the flow label is a means to affect the routing of packets
> >>> through the network that uses flow labels as input to the ECMP hash.
> >>
> >> What's the point?
> >>
> >> 1) You cannot tell *if* the FL is being used.
> >>
> > Generally true, but in a limited domain this information could be
> > discerned. I'd note that it's also generally true that we don't know
> > if there is a load balancer or stateful firewall in the path that
> > requires consistent routing, but in a limited domain we could know
> > that also.
> >
> >> 2) Changing the FL does not necessarily mean that packets will employ a
> >> different link.
> >
> > It's an opportunistic mechanism. If a connection is failing and we get
> > a better path that fixes it by simply changing the flow label then
> > what's the harm?
> >
> >>
> >> 3) If the network is failing, shouldn't you handle this via routing?
> >>
> > Sure, but then that requires an out of band feedback loop from a TCP
> > implementation to the network infrastructure to indicate there is a
> > problem and then the network needs to respond. That's significant
> > infrastructure and higher reaction time than doing something in TCP
> > and IP. Think of modulating the flow label is an inexpensive form of
> > source routing within a limited domain that doesn't need any
> > infrastructure or heavyweight protocols or something like segment
> > routing.
> >
> >>
> >>
> >>> The basic idea is that the flow label associated with a connection is
> >>> randomly changed when the stack observes that the connection is
> >>> failing (e.g. and an RTO). There is nothing in the specs that prevents
> >>> this since the source is at liberty to set the flow label as it sees
> >>> fit.
> >>
> >> The FL is expected to remain constant for the life of a flow. A
> >> retransmitted packet is part of the same flow as the
> >> originally-transmitted packet. So this seems to be contradicting the
> >> very specification of the FL.
> >>
> >> For instance, If a RTO for a flow causes the FL to change, then one may
> >> possibly argue that the FL is not naming/labeling what is said/expected
> >> to be anming/labeling.
> >
> > Specifically, RFC6437 states:
> >
> > "It is therefore RECOMMENDED that source hosts support the flow label
> > by setting the flow label field for all packets of a given flow to the
> > same value chosen from an approximation to a discrete uniform
> > distribution."
> >
> > So that is clearly a just recommendation, and not a requirement (and
> > definitely not a MUST). Furthermore, RFC6437 states:
> >
> > "A forwarding node MUST either leave a non-zero flow label value
> > unchanged or change it only for compelling operational security
> > reasons as described in Section 6.1."
> >
> > So there's no guarantee in the protocol specs that flow labels are
> > consistent for the life of the connection, which means that the
> > network cannot assume that and thus it would be incorrect if the
> > network tried to enforce flow label consistency as a protocol
> > requirement. As I said, it is prudent to try to be consistent with
> > flow labels and the default behavior in Linux should be changed,
> > however I do not believe there's a valid claim of non-conformance that
> > motivates removal of the feature that is already deployed.
> >
> > Tom
> >
> >
> >
> >
> >>
> >>
> >>
> >>> The feature is useful in large datacenter networks, like
> >>> pparently Facebook where the patches originate, since information
> >>> discerned by TCP can opportunistically be applied to route selection.
> >>> The practical issue is that there are stateful devices like firewalls
> >>> that require consistent routing in the network in which case changing
> >>> the flow label can confuse them. As I mentioned, the original intent
> >>> was that the flow label randomization feature should be opt-in instead
> >>> of on by default.
> >>
> >> So... where is the "source" of the packet that would be "modulating" the FL?
> >>
> >> Thanks,
> >> --
> >> Fernando Gont
> >> e-mail: [e-mail] || [e-mail]
> >> PGP Fingerprint: 7809 84F5 322E 45C7 F1C9 3945 96EE A9EF D076 FFF1
> >>
> >>
> >>
> >
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