While we're talking, this 'requirement' means you will never have a reliable system. It is better to not have this as a design goal. The software ought to be built assuming that the connection can be temporarily interrupted. The internet is unreliable (for very many reasons) and the software must take this into account.
Authentication over the unreliable internet is a very well known problem for internet-scale applications. Generally they have a time-limited session token of some sort. Once you have authenticated you remain authenticated until your session token expires. Of course, servers can expire a token at any time so they still have control, it just means the client doesn't need continuous connectivity.
Before expiration the client software can either kick you off (old style and hostile) or attempt to re-authenticate on your behalf in the background. For example you could have a 1 hour session token that tried to re-authenticate in the background after 30 minutes and re-tried every 5 minutes or so until successful or the 1 hour session was up. Plenty of 'enterprise' applications are built to work this way (I know, I've written some).
Using sessions also takes the load off the authentication servers. Meaning they can be *very cheap* virtual machines - which allows you to have more of them for redundancy.
Of course, for reliability you'll have several authentication servers in geographically diverse areas (eg. California, Singapore, Frankfurt). If one server cannot be reached then the others are tried (in a DNS-like fashion). Not only will this give you reliability (no more 'players can't connect because the single master server is down' stuff) but it also allows you to pull authentication servers down for maintenance or upgrades while the others stay up. The geographic diversity is necessary because undersea cables can be snagged or significant earthquakes can happen (California!) taking a whole area out. So you need to spread your servers around a bit.
Doing this is very reliable, cheap, and good for players (they can always connect no matter what Internet storms there may be). Rather than provisioning your own servers/VMs you can also use services like Amazon's EC2 which is pretty cheap and scales well with variable load.
This post is not intended to tell ED how to run their business, I'm just passing on what I've learned about how other outfits with international customers run theirs.