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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