North American Network Operators Group Date Prev | Date Next | Date Index | Thread Index | Author Index | Historical Re: Anycast 101
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 <[email protected]> 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. > > __________________________________________________ Do You Yahoo!? Log on to Messenger with your mobile phone! http://sg.messenger.yahoo.com
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