Originally Posted by Poodlestrike
Wasn't ME3's model wildly successful for them? They'll probably take the same tack.
Mass Relays reduce the entire corridor of space between them to negative mass, thus allowing for effectively instant transportation between them. Non-Relay FTL is achieved by folding space around the ship using Eezo, which is how you got around between relays in ME3. You're right that the distance involved is incredibly massive, though; if they really make it in 600 years, they're probably going to have either some revolutionary new on-ship FTL (unlikely) or they somehow harness a Relay to shoot them most of the distance (more likely).
Oooh I like that last option. Could even play some tpe of story role.