[Mageia-discuss] network balancing by default

[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

>> >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

>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.

>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?

>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)

>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.

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.

>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.

-- 
Luca Berra -- bluca at vodka.it