[Mageia-discuss] network balancing by default

[Maarten Vanraes]

Op maandag 25 oktober 2010 08:41:01 schreef Luca Berra:
> On Mon, Oct 25, 2010 at 12:00:46AM +0200, Maarten Vanraes wrote:
> >Op zondag 24 oktober 2010 22:39:29 schreef Luca Berra:
> >> On Sun, Oct 24, 2010 at 11:43:28AM +0200, Maarten Vanraes wrote:
> >> >I would propose the following:
> >First off, the timing of this proposal is probably too soon, i just wanted
> >to get it out there, in case i forgot later.
> 
> open an enhancement on initscripts :P

imho, this in itself is wrong; i want network-scripts to be split off from 
initscripts; especially if we're going to use systemd later on.

> >> >A.) by default, add for every interface, a little advanced routing
> >> >which makes packets return from the same way they came.
> >> >This usually is only useful with incoming packets, but can still be
> >> >useful if laptops have for example 2 gateways because the wifi is
> >> >still on and the cable is too. That would mean that from both
> >> >interfaces it'd be possible to use ssh or vnc or whatever.
> >> 
> >> this is possible with incoming packets, but, how do you select the
> >> source of a new one?
> >
> >this step is only for the replies of incoming packets and never has any
> >effect on new outgoing packets; this step doesn't change anything for new
> >outgoing packets. and this can even be used on interfaces that aren't
> >used as default gateway.
> 
> i did not understood the second and third sentence in A.), then.
> 
> anyways i believe A is useful and can be implemented without any issue


it will not conflict with current situation.


> >possible problems:
> >A) interface down
> >B) DHCP expired
> >C) gateway down
> >D) further routing down
> >E) DNS down
> >
> >A is trivial, so we'll just skip that one.
> >
> >B seems easy to do too; however, reusing the last DHCP lease could still
> >be usefull, it might well be only a dhcp failure; we should try with the
> >current lease if possible.
> 
> if it is expired you should not. doing this will result in duplicate
> ips.

ok.

> >E is a bit of an extra (it's not really routing, but a DNS that's down
> >(does not answer) could well be eliminated (not sure if this should be
> >done separately or not)) OTOH, failure of the recursive DNS of the ISP
> >seems to be somewhat frequent in my experience.
> 
> so a connectivity issue will leave users without dns?


more the other way around; in the event of dns failure; the dns of the other 
gateway could be used. if it would be a routing issue to the DNS (and others), 
then other rules could be triggered (C+D)


> >C+D are tricky: D is even a bit of a grey area; my ISP frequently has a
> >few routes broken. icmp can definately not be relied on in all cases. and
> >even if you ping your gateway, you don't know if it goes any further.
> >
> >This could be circumvented by putting known servers that actually echo
> >icmp in a list and ping those. but for that matter, it doesn't have to be
> >icmp; we could easily have a list of public services that can be
> >connected to. but is this really what we want?
> >
> >We could even just monitor how much packets are unreplied to per interface
> >and choose that.
> >
> >Or we could try to have each retry of unreplied packet go through the next
> >default route.
> >
> >Or we could just not handle that (like it is now).
> 
> +1
> you are considering the only scenario of a home user. doing some things
> you propose above would prevent using mageia in any medium sized
> network. (i.e. i could not use my mageia laptop at work)

I don't see what you mean by this. i list 4 options; knowing full well that 
some of those options are not usefull by default. also, this is only required 
if more than one default gateway is active; which is a small percentage in 
itself. (my personal favourite is having it sent to the other default gateway 
after failure; or seeing which has more unreplied packets; and then check some 
public services)

> >remember that right now only A(+B) is used; and having balanced default
> >routes would probably mean that there is 50% packet loss, instead of 100%
> >in most cases.
> 
> which may be worse.
> if nothing works the user will try switching to a different connection
> if stuff do not work at random the user will not know what to do.

it could be worse, depending on the type of person.

> btw, the assumption about 50% is flawed, i don't know if it is an
> oversimplification or a failure to understand how load balancing over
> multiple network links work in practice.
> it is not round-robin, it is route-based (on ip hash)
> the result of a failure upstream will result in the user being able to,
> say, watch some videos on youtube, but not update her fb profile, or
> worse.

i meant on average in total, depending on what kind of balancing is used.

> >also remember that if the metrics are the same for some reason, you will
> >get much stranger things when both are working perfectly.
> 
> L.
> 
> btw, there is no need to cc me on discussions, in fact it breaks my
> filters.

sorry,