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Jul 20 2017

IPv6 anycast possibly done better

Anycasting IP space has become quite a meme in the networking world in the last few years, with it being used sparsely in the past for UDP based services like DNS. Now it’s being used for TCP based services too, meaning that all the services that use TCP are now also being served on Anycast IP ranges. (ie, HTTP, or other TCP based applications, like games)

Before anycast was used in this way providers who had services in multiple locations would use a practice called GeoDNS, where the DNS server would send you to the (hopefully) correct and nearby unicast address to the customer making the DNS request.

You can see that Google does this currently for their root domain, here is an example of a DNS lookup for it in Japan:

ben@jp:~$ dig google.com +short

And in same query in the EU:

ben@it:~$ dig google.com +short

These IP addresses take the system to a nearby server of Google’s, the reasoning for this is better performance/reliability (the packets do not have to move as far around the earth) and load balancing (lots of smaller server deployments, rather than a very large server deployment)

At the time of writing, my blog is behind Cloudflare, a service that uses anycast to serve their traffic. This means wherever you look up my blog you (provided there is no tampering from your ISP) get the same IP addresses:

ben@it:~$ dig blog.benjojo.co.uk +short

ben@jp:~$ dig blog.benjojo.co.uk +short

The idea is, rather than having the DNS server pick where the client goes, you use the routing layer of the internet to guide traffic to the closest location. This also means that if a failure was to happen to a set of servers, traffic would re-route to the next place where that IP address is serviced. (in practice, the routing layer has many complicated failure models, just like GeoDNS has)

This also means that no single deployment is not directly targetable, you cannot force a client in Italy to go to Japan by given it a set of addresses, the routing layer of the client ISP (and their upstream ISPs) are the ones that make that choice. This means that traffic floods are spread over all the servers (assuming the sources of the flood are distributed around the world)

However there are deployment concerns with anycast. Traffic can be hard to attract into the right places, as it is the client’s ISP that decides where the traffic goes, making the need to use traffic engineering necessary (often with BGP community strings to prevent announcement to some networks/regions)

Below is a RIPE Atlas traceroute measurement of an unnamed anycast deployment; you can see traffic is not quite contained in country quite a lot of the time, with some traffic in the USA going to Japan, even a trace going from the USA, EU, India, and then Singapore!

Bad anycast routing

This is often caused by a mix up of upstream providers in a region, if you do not have direct access to Tier 1 ISP then you easily end up with large amounts of traffic going to the wrong place because of the large routing attraction those networks have.

Sometimes it is either not possible or economical to have all of your server deployments backed behind the same set of Tier 1 ISPs, and this will mean that you will have a much harder time keeping traffic in regions.

A CDN called Fastly made this pretty awesome talk last year that shows a newer way to deploy IPv6 networks, that uses both GeoDNS and Anycast concepts, the video is here below, but given it’s 23 mins (Skipping to 11 mins gets to the point faster), I will summarise it as well:

Altitude 2016: Addressing IPv6 from Fastly on Vimeo.

Archive.org Link in case the above iframe does not work

João Taveira explains that Fastly was a unicast (ie, GeoDNS) network in the beginning, that grew to have a anycast offering due to some clients requiring anycast. The interesting part is where they end up, they begin to do a model called backing anycast. IPv6 makes backing anycast easy because of the sheer amount of IP addresses a service provider can use.

The idea is this, because a network operator will be allocated around a /32 or larger in IPv6 address space, this allows them to have 16 bits of individual prefixes they can announce. (In my example, the AAAA’s are what is announceable on a /32, the 0000’s are too small to announce on the routing table)

What a IPv6 block looks like

This means you could announce two prefixes per PoP, the /32 “super” block, and and the deployment unicast specific /48, you can use GeoDNS to ensure a client ends up in the correct place. If that prefix is attacked, you can drop the prefix, and traffic falls back to the super block, that is hopefully spread out enough that your service can absorb the attack, all while the /48 you dropped is likely still serving the traffic from the same place, assuming that the super block ended up in the same deployment.

Traffic flows inside a ipv6 block, traffic follows a more specific route

This means you can fix the 99th percentile of clients that are not going to the correct PoP’s on anycast, while also not being as vulnerable to traffic floods and other issues involved with unicast only models. You may even decide not to send a specific IP address on GeoDNS all the time. Instead send the majority of your traffic to the super block by default assuming good faith that the internet routing layer will take care of it.

But João argues. Why stop at that? Since you can now control clients routing to you, while maintaining the optional ability to protect the network from attack, you can could divide it up much finer and by provider!

My proposal on how a /32 bock should be split up

In my example, I have kept in mind some things:

In practice failover works like this:

Step 1, Everything working as normal

Assuming a client is being sent to a specific deployment on a specific link, and something happens to that link and it is taken offline, traffic stays within deployment! Ensuring connections do not break!

Step 2, The deployment link prefix is withdrawn, but traffic stays in deployment

However, let’s say something requires the deployment to stop serving traffic. (for example attack or system maintenance) When you withdraw the prefixes of the deployment, traffic will failover to the nearest deployment within region!

Step 3, The whole deployent prefixes are withdrawn, traffic stays in continent

This reduces the unpredictable redirection of traffic, ensuring the network works better when not all of the deployments are online.

To observe and recreate this idea I used Vultr (Full disclosure: Affiliate link to help me run more experiments like this cheaper), a VPS host that allows their customers to have automated BGP sessions to build a cheap (75$ per month) anycast network.

To save time and avoiding writing 15 different bird configs, I used their “Startup Script” feature to build the BGP config in real time (using imperfect bash) on boot, this script produces a config that looks like this:

router id;
protocol kernel {
	scan time 20;
	export none;
	import none;
protocol device {
	scan time 10;
protocol static
	route 2a07:1500:1000::/36 via 2001:19f0:ac01:5ba:5400:00ff:fe7d:1fdd;
	route 2a07:1500:1300::/40 via 2001:19f0:ac01:5ba:5400:00ff:fe7d:1fdd;
	route 2a07:1500:1310::/44 via 2001:19f0:ac01:5ba:5400:00ff:fe7d:1fdd;
	route 2a07:1500:1311::/48 via 2001:19f0:ac01:5ba:5400:00ff:fe7d:1fdd;
	route 2a07:1500:1312::/48 via 2001:19f0:ac01:5ba:5400:00ff:fe7d:1fdd;
protocol bgp vultr
    local as 206924;
    source address 2001:19f0:ac01:5ba:5400:00ff:fe7d:1fdd;
    import none;
    graceful restart on;
    multihop 2;
    neighbor 2001:19f0:ffff::1 as 64515;
    password "xxxxxxx";
    export filter {
        if (net = 2a07:1500:1311::/48) then {
        if (net = 2a07:1500:1312::/48) then {
        if (net = 2a07:1500:131B::/48) then {

The config produced uses the Vultr BGP communities (A) to allow the per link (upstream providers) prefixes to work

Using RIPE Atlas you can see (darker blue = higher latency), the anycast roll out by default is actually pretty good, but there are a few details worth pointing out where it has gone wrong.

a gif showing the latency of probes as deployments are put online

If you look close, you will notice that the addition of London (LHR) deployment actually made the west coast ping worse! And the deployment of more PoPs on the west coast did not make a massive improvement (though it did for Canada)

the same thing as above but focusing on North America

To prevent this from happening, we can use DNS to ensure those clients go to the correct coast:

Comparison in latency between DNS Backed Anycast and Normal Anycast

Here it makes quite a big difference in sorting out those cases of mis-routing.

This takes the latency histogram (produced using mmhistogram) from:

Values min:0.54 avg:72.35 med=47.76 max:287.74 dev:61.90 count:448
 value |-------------------------------------------------- count
     0 |                                                 * 5
     1 |                                                 * 5
     2 |                                                ** 6
     4 |                                              **** 12
     8 |                               ******************* 53
    16 |           *************************************** 104
    32 |                              ******************** 54
    64 |************************************************** 133
   128 |                       *************************** 72
   256 |                                                 * 4

To a surprisingly low:

Values min:0.54 avg:32.51 med=21.32 max:219.32 dev:33.56 count:455
 value |-------------------------------------------------- count
     0 |                                                ** 9
     1 |                                                 * 6
     2 |                                                ** 10
     4 |                                          ******** 31
     8 |                          ************************ 86
    16 |************************************************** 173
    32 |                          ************************ 84
    64 |                                     ************* 45
   128 |                                               *** 11
   256 |                                                   0

Or as a graph:

Comparison of latency histogram between the two methods

Overall, if you do not have the resources to ensure that your providers are consistent over all regions, this is a great way to ensure that your traffic follows a (hopefully) logical path.

The only downside of this approach is that it would be cost prohibitive to apply on IPv4, as the amount of IP addresses required are very high and the world is in a tad of a shortage of IPv4.

Hopefully you found this interesting, I hope to have more networking content around soon, you can use the blog’s RSS, or my twitter to ensure you know when that happens!