Imagine you are a business that wants to have redundant connections to the Internet. Given the importance of an active Internet connection for many businesses this is a reasonable thing for an IT shop or business owner to ask for. One could also consider the serious home gamer who can’t risk being cut off as another use case.
Let’s dig into the technical options for achieving Internet redundancy.
The first and most obvious path would be to purchase a router that has two WAN ports and ordering Internet service from two different providers. Bam, you are ready to go right? Well… not really.
The way this typically works is that the router will choose one of the two Internet connections for a given outbound connection. The policy could be always use connection A until it fails or be more dynamic and take some advantage of both connections at the same time. The problem with this approach is that because the traffic will be NATed towards each Internet provider, there is no way to fail a given connection from one Internet provider to another. So the failure of one of the Internet connections means that your voice call, SQL or game connection will die, probably after some annoyingly lengthy TCP or application level timeout expires. If the site is strictly doing short outbound connections such as the case with HTTP 1.1 traffic this isn’t such a big deal.
So the ‘get two standard Internet connections and a dual port WAN router’ approach sort of works. Let’s call it partially redundant.
How do we get to true redundancy that can survive a connection failure without dropping connections? To this we need the site’s network to be reachable through multiple paths. The standard way to do this is to obtain IP address space from one of the service providers or get provider independent IP space from one of the registries (such as ARIN). Given that IPv4 addresses are in short supply this isn’t a trivial task. The conditions that have to be met to get address space are well out of the reach of small and medium businesses. Even when the barriers can be met, it’s still archaic to have to do a bunch of paper work with a third party for something that is so obviously needed.
The real kicker is that the lack of IP space is only part of the problem. IPv6’s huge 128-bit address space doesn’t really help at all because to use both paths, the site or home’s IP prefix needs to exist in the global routing table. That is, every core router on the Internet needs an entry that tells it how to reach this newly announced chunk of address space. The specialized memory (CAM) used by these routers isn’t cheap so there is a strong incentive within the Internet operations community to keep this kind of redundancy out of the reach of everyone except other ISPs and large businesses.
So the simple option doesn’t really solve the problem and ‘true’ redundancy isn’t possible for most businesses. What about something over the top?
Consider a router that is connected to multiple standard Internet connections. It could maintain two tunnels, one over each connection, towards another router somewhere else on the Internet. To the rest of the Internet, this second router is where the business is connected to the Internet. If one of the site’s Internet connections fails, the routers can simply continue passing packets over the remaining live tunnel thereby maintaining connectivity to the end site. From an end user’s perspective, this solution mostly works but let’s think about the downsides. We’ve essentially made our site’s redundancy dependent on the tunnel termination router and its Internet connectivity whereas without this we are just at the mercy of the ISP’s network. Also, unless the end site obtains its own address space, this approach has all downsides of the first approach except the NAT related problems occur at the tunnel termination router instead of being on-site. Finally, if the site can get its own address space, why do the tunnel approach at all?
I should note, because someone will point it out in the comments, that for very large organizations it’s possible to get layer two connectivity to each site and essentially build their own internal internet. If they have enough public IP space they can achieve redundancy to the end site for connections with hosts on the public Internet. With private IP space, they can achieve redundancy for connections within their own network. Without public IP space, even these networks suffer from the NAT related failure modes.
To summarize, if you aren’t a very large business, there is no way to get true Internet connection redundancy with the current Internet protocols. That’s kinda sad.
Part 2: Redundant Connections to a Single Host
Part 3: Detecting Failure