6to4 is a good idea in its concept. Still, it brings its lot of problems.
These are mainly due to the fact that a common network prefix, 2002::/16, is advertised on the IPv6 Internet by all the ISPs doing 6to4 :
As you can see in the previous diagram, the 2002::/16 prefix is advertised on the IPv6 Internet by two differents ASes, AS3215 and AS6453 (and probably hundreds of others, if not thousands). It means that routers on the v6 Internet, when wanting to throw packets to any network beginning with 2002:: will have plenty of gateways to choose from.
That simple fact will give birth to this situation :
In this diagram, you can see that the outgoing flow coming from one customer of Orange will be going to the v6 Internet through the Orange backbone, thus allowing that ISP to offer a certain Quality of Service.
What about the return traffic, now coming back from the v6 Internet?
What you can see here, is that since the v6 routers have plenty of choices among many 6to4 gateways, they will take the closest one, the one presenting the shortest path. In this case, the return traffic to the Orange customer will be sent through the TATA 6to4 gateway.
The situation is now this one :
– we have an asymetric trafic
– TATA is carrying transit traffic for another ISP, when TATA could just be a stub AS
– Orange can’t really control the trafic destined to its customer, nor apply a preference or QoS to it
– the availability of the TATA 6to4 gateway is not guaranteed, so it’s likely that some of the return traffic will be lost.
This last point is highlighted by an RIPE article (RIPE is the European RIR) titled : “6to4 : how bad is it really?” and showing the lacks in this protocol. A study has been conducted over a week and showed that on average, 15% of the return traffic was lost due to the poor quality of some of the 6to4 gateways, something that an ISP cannot control for its customers :
The author of the article suggests some solutions to solve the problem. The main one is to go Dual-Stack on the backbone, which is clearly the best solution. Still, for those ISPs who want a quick 6to4-like solution to connect their customers to the v6 Internet right now, and who have not yet migrated their backbone to IPv6, a simple though efficient solution exist : 6RD.