On 2-okt-04, at 18:25, Paul Vixie wrote:
nycast has worked very well. both inter-AS and intra-AS. the fact that
a not-clueful-enough engineer *could* build a non-working topology using
anycast and PPLB as ingredients, does not mean that anycast or PPLB are
bad. it means you have to be clueful-enough before you use either tool.
(and remember kids, all power tools can kill.)
It's not as simple as that. It's possible for bad things to happen if:
1. some DNS server is anycast (TLD servers are worse than roots because the root zone is so small)
2. fragmented UDP packets or TCP are used as a transport
3. a network is built such that packets entering it through router X may prefer a different external link towards a certain destination than packet entering it through router Y
4. a customer of this network is connected to two different routers
5. the customer enables per packet load balancing
All of these steps happen in the real world, and are in and of themselves not examples of bad engineering. However, the end result can be reduced connectivity to one or more anycasted DNS servers under some circumstances.
(See my message to dnsop from yesterday http://darkwing.uoregon.edu/~llynch/dnsop/msg03105.html for more info, reformat using a non proportional font if necessary.)
Now the question is: how do we deal with this? I don't think removing anycast wholesale makes sense and/or is feasible. Same thing for declaring per packet load balancing an evil practice. 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.
and they would know that PPLB is basically a link bundling technology used
when all members of the PPLB group start and end in the same router-pair;
It doesn't make much sense to have multiple links terminate on the same router on both ends as then both these routers become single points of failure. Often, the end sending out most traffic will have the links terminate on one router (so load balancing is possible) while the other ends of the links terminate on two or more routers.
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