9. Limitations

Happy Eyeballs will handle initial connection failures at the TCP/IP layer; however, other failures or performance issues may still affect the chosen connection.

9.1. Path Maximum Transmission Unit Discovery

Since Happy Eyeballs is only active during the initial handshake and TCP does not pass the initial handshake, issues related to MTU can be masked and go unnoticed during Happy Eyeballs. Solving this issue is out of scope of this document. One solution is to use "Packetization Layer Path MTU Discovery" [RFC4821].

9.2. Application Layer

If the DNS returns multiple addresses for different application servers, the application itself may not be operational and functional on all of them. Common examples include Transport Layer Security (TLS) and HTTP.

9.3. Hiding Operational Issues

It has been observed in practice that Happy Eyeballs can hide issues in networks. For example, if a misconfiguration causes IPv6 to consistently fail on a given network while IPv4 is still functional, Happy Eyeballs may impair the operator's ability to notice the issue. It is recommended that network operators deploy external means of monitoring to ensure functionality of all address families.