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Re: Anycast 101

  • From: Joe Shen
  • Date: Fri Dec 17 05:24:42 2004

My question:

I noticed that people always talked about BGP  when
they talked about anycast dns server farm. 

But, is there any problem or anything must be taken
care about when anycast is employed within a DNS
server farm within MAN? 

What I mean is, if we want to employ anycast in a
cache server farm which is located within a big OSPF
network, is there anything problemetic ? or should we
consider anycast only when root server is to be
installed ?

Some people said, it's not needed to set up anycast in
MAN because DNS system in such situation is very small
( less than 10 SUN servers ).  



 --- Iljitsch van Beijnum <[email protected]> wrote: 
> I got some messages from people who weren't exactly
> clear on how 
> anycast works and fails. So let me try to explain...
> In IPv6, there are three ways to address a packet:
> one-to-one 
> (unicast), one-to-many (multicast), or one-to-any
> (anycast). Like 
> multicast addresses, anycast addresses are shared by
> a group of 
> systems, but a packet addressed to the group address
> is only delivered 
> to a single member of the group. IPv6 has "round
> robin ARP" 
> functionality that allows anycast to work on local
> subnets.
> Anycast DNS is a very different beast. Unlike IPv6,
> IPv4 has no 
> specific support for anycast, and the point here is
> to distribute the 
> group address very widely, rather than over a single
> subnet anyway. So 
> what happens is that a BGP announcement that covers
> the service address 
> is sourced in different locations, and each location
> is basically 
> configured to think it's the "owner" of the address.
> The idea is that BGP will see the different paths
> towards the different 
> anycast instances, and select the best one. Now note
> that the only real 
> benefit of doing this is reducing the network
> distance between the 
> users and the service. (Some people cite DoS
> benefits but DoSsers play 
> the distribution game too, and they're much better
> at it.)
> Anycast is now deployed for a significant number of
> root and gtld 
> servers. Before anycast, most of those servers were
> located in the US, 
> and most of the rest of the world suffered
> significant latency in 
> querying them. Due to limitations in the DNS
> protocol, it's not 
> possible to increase the number of authoritative DNS
> servers for a zone 
> beyond around 13. With anycast, a much larger part
> of the world now has 
> regional access to the root and com and net zones,
> and probably many 
> more that I don't know about.
> However, there are some issues. The first one is
> that different packets 
> can end up at different anycast instances. This can
> happen when BGP 
> reconverges after some network event (or after an
> anycast instance goes 
> offline and stops announcing the anycast prefix),
> but under some very 
> specific circumstances it can also happen with per
> packet load 
> balancing. Most DNS traffic consists of single
> packets, but the DNS 
> also uses TCP for queries sometimes, and when
> intermediate MTUs are 
> small there may be fragmentation.
> Another issue is the increased risk of fait sharing.
> In the old root 
> setup, it was very unlikely for a non-single homed
> network to see all 
> the root DNS servers behind the same next hop
> address. With anycast, 
> this is much more likely to happen. The pathological
> case is one where 
> a small network connects to one or more transit
> networks and has 
> local/regional peering, and then sees an anycast
> instance for all root 
> servers over peering. If then something bad happens
> to the peering 
> connection (peering router melts down, a peer pulls
> an AS7007, peering 
> fabric goes down, or worse, starts flapping), all
> the anycasted 
> addresses become unreachable at the same time.
> Obviously this won't happen to the degree of
> unreachability in practice 
> (well, unless there are only two addresses that are
> both anycast for a 
> certain TLD, then your milage may vary), but even if
> 5 or 8 or 12 
> addresses become unreachable the timeouts get bad
> enough for users to 
> notice.
> The 64000 ms timeout query is: at what point do the
> downsides listed 
> above (along with troubleshooting hell) start to
> overtake the benefit 
> of better latency? I think the answer lies in the
> answers to these 
> three questions:
> - How good is BGP in selecting the lowest latency
> path?
> - How fast is BGP convergence?
> - Which percentage of queries go to the first or
> fastest server in the 
> list?

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