Re: Anycast 101

[Joe Shen <joe_hznm () yahoo com sg>]

I don't think PPLB is compatible with anycast esp. in
situation when we consider end-to-end communication
with multiple packets. 

As PPLB may derive to out-of-sequence  between TCP
pacekets & different DNS server destination of the
same UDP stream,  it will broke anycast DNS service in
some situation. So, if TCP based DNS requests is
considered, flow-based load balancing should be
considered which is total differnt from PPLB.

Joe


 --- Iljitsch van Beijnum <iljitsch () muada com> wrote: 
 
> On 18-dec-04, at 22:31, Paul Vixie wrote:
>
> > > i would be interested in hearing from anybody
> else who thinks that
> > > turning on pplb in a eyeball-centric isp that has
> multiple upstream
> > > paths is a reasonable thing to do, even if there
> were no anycast
> > > services deployed anywhere in the world.
>
> > so far, no takers.  i've heard from rfc-writers
> who say pplb was never
> > meant to be used the way Iljitsch is describing
> it, and i've heard from
> > equipment vendors who say their customers don't do
> that and that if 
> > some
> > customer did that and asked for support the
> response would be "don't do
> > that!", and i've heard from network operators who
> say they would never 
> > do
> > that, and i've heard from customers of network
> operators who did that 
> > with
> > notable bad effects.
>
> > but so far nobody has said "yes, what Iljitsch is
> describing should 
> > work."
>
> Apparently you also didn't get any pointers to RFCs
> or other 
> authoritative sources that say "each and every
> packet injected into the 
> internet must be delivered in sequence".
>
> You feel you get to decide what other people should
> and shouldn't do. I 
> find that dangerous. As long as there is no standard
> or law that says 
> something can't be done, people are free to do it.
>
> Apart from that, I'm not convinced per packet load
> balancing is as bad 
> as people keep saying. In the absense of any
> research that I know of, 
> my position is that per packet load balancing does
> have potential 
> adverse effects, so per destination load balancing
> is preferred, but if 
> there is a reason why pdlb doesn't fit the bill,
> pplb is a reasonable 
> choice.
>
> > let me summarize.  Iljitsch says that pplb is
> incompatible with 
> > anycast,
>
> No. What I'm saying in general is that anycast isn't
> 100% problem free, 
> so:
>
> 1. There should always be non-anycast alternatives
> 2. It would be good if we had a way (= BGP
> community) to make sure that 
> anycasted routes aren't load balanced across
>
> I don't think either of these is unreasonable.
>
> > since a pplb-using access router at the inner edge
> of an ISP could hear
> > two different IGP routes to some destination,
> which ended up taking
> > different exits from the ISP and thus different
> BGP paths.
>
> I'm not even sure if I understand this sentence, but
> it sure doesn't 
> look like something I said. What I said was, that if
> you inject packets 
> towards an anycasted address into two different
> routers within a 
> certain AS, there is a very real possibility these
> two packets will end 
> up at different anycast instances. I'm on very firm
> ground here as this 
> follows directly from the BGP path selection rules.
> (Although in real 
> life this wouldn't happen too often because
> customers tend to connect 
> to two routers in the same or neighboring pops.)
>
> > whereas pplb
> > would normally only operate on equal-cost paths,
> the BGP->IGP path 
> > would
> > hide the variance in BGP paths and make these
> "paths" eligible for 
> > pplb.
>
> Again: huh?
>
> > i've said that pplb is only useful for turning two
> OC3's into an "OC6" 
> > (or
> > similar circuit bundling where a pair of routers
> has multiple 
> > connections
> > to eachother) and that even in this case, packet
> reordering is likely 
> > to
> > occur, which will make tcp-flow performance suffer
> across this "link".
>
> But would the TCP performance over this "OC6 link"
> be better than that 
> over a single OC3 link? That's the real question.
>
> > i have also said that turning pplb on across
> non-parallel links, such 
> > as to
> > multiple providers or through multiple tunnels or
> whatever, would 
> > pretty
> > much guaranty that a word rhyming with "massive
> suckage" would occur.  
> > and
> > i've made these claims independent of anycast --
> that is, life will be 
> > bad
> > if you use pplb outside its intended purpose, even
> if nobody anywhere 
> > was
> > using anycast.
>
> Your argument is that since it's a bad idea to do
> this, nobody will, so 
> making it even worse is ok. My argument is that even
> though it's a bad 
> idea, some people will do it we shouldn't
> unnecessarily make things 
> worse and/or make a reasonable effort to repair the
> damage.
>
> > loath though i am to treat a "preponderance of
> assertion" as equivilent
> > to "proof", i see no alternative on this issue. 
> noone is defending 
> > the use
> > case Iljitsch is proposing.  noone is even saying
> "i tried that and it 
> > was
> > OK".  lots of people are saying various things
> like "don't do that!" 
> > and
> > "are you crazy?"
>
> And we all know that when you tell people not to do
> something they 
> don't, and there are no crazy people connected to
> the net.
>
>  

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