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Re: Anycast 101
On 17-dec-04, at 19:43, Paul Vixie wrote:
i don't think iljitsch is in a position to teach an "anycast 101" class.
If anyone feels they can do better, please step up...
here's my evidence:
note-- harald asked us to move this thread off of ietf@, so i've done that.Hey! I missed this one. I'm on dnsop but it's pretty low on my to-read list.
Unfortunately, your evidence contains its share of errors so I'm not sure if you should be teaching the class either.
... It's possible for bad things to happen if:
1. some DNS server is anycast (TLD servers are worse than roots because the
#1 and #2 are normal, even though fragmented udp isn't very common nowadays.Nope. Consider:
Where the anycast instances exchange routing information using BGP.
If there is no special BGP configuration in effect, the ISPrtr1 will prefer the path to anycast instance A and 2 to B, because the external path takes precedence over a same length path that's learned over iBGP.
The current Cisco multipath BGP rules require the whole AS path to be the same (which would be the case in this diagram if both anycast instances use the same AS number), but older IOSes only require the next hop AS and the path length to be the same.
Now the question is: how do we deal with this? I don't think removing
as i said the other day, "all power tools can kill." if you turn on PPLBYes, per packet load balancing will cause reordering, and if that's an issue you shouldn't use it. But if with pplb packets end up at two different hosts, that's not the fault of the people who invented per packet load balancing or the people who turned it on, but the fault of the people giving the same address to two different hosts.
A better solution would be to give network operators something that enables them to make sure load balancing doesn't happen for anycasted destinations. A good way to do this would be having an "anycast" or "don't load balance" community in BGP, or publication of a list of ASes and/or prefixes that shouldn't be load balanced because the destinations are anycast.
since PPLB won't affect BGP (since BGP is not multipath by default), this isIf the uncommon network setup exists, and pplb is turned on, the problem can manifest itself. The fact that someone had to turn on a feature that's turned off by default is immaterial. (There is no BGP by default to begin with.)
and they would know that PPLB is basically a link bundling technology used
It doesn't make much sense to have multiple links terminate on the same
i don't even know what conversation we're in any more. why does it matterThere is no requirement that all packets between two hosts follow the same path. So people who pplb have the IP architecture at their side, unlike those who implement anycast. So a little less blaming the victim would be in order. (Well, if there are any victims, because all of this happening is pretty unlikely.)