4. Sorting Addresses

Before attempting to connect to any of the resolved destination addresses, the client should define the order in which to start the attempts. Once the order has been defined, the client can use a simple algorithm for racing each option after a short delay (see Section 5). It is important that the ordered list involve all addresses from both families that have been received by this point, as this allows the client to get the racing effect of Happy Eyeballs for the entire list, not just the first IPv4 and first IPv6 addresses.

First, the client MUST sort the addresses received up to this point using Destination Address Selection ([RFC6724], Section 6).

If the client is stateful and has a history of expected round-trip times (RTTs) for the routes to access each address, it SHOULD add a Destination Address Selection rule between rules 8 and 9 that prefers addresses with lower RTTs. If the client keeps track of which addresses it used in the past, it SHOULD add another Destination Address Selection rule between the RTT rule and rule 9, which prefers used addresses over unused ones. This helps servers that use the client's IP address during authentication, as is the case for TCP Fast Open [RFC7413] and some Hypertext Transport Protocol (HTTP) cookies. This historical data MUST NOT be used across different network interfaces and SHOULD be flushed whenever a device changes the network to which it is attached.

Next, the client SHOULD modify the ordered list to interleave address families. Whichever address family is first in the list should be followed by an address of the other address family; that is, if the first address in the sorted list is IPv6, then the first IPv4 address should be moved up in the list to be second in the list. An implementation MAY want to favor one address family more by allowing multiple addresses of that family to be attempted before trying the other family. The number of contiguous addresses of the first address family will be referred to as the "First Address Family Count" and can be a configurable value. This is performed to avoid waiting through a long list of addresses from a given address family if connectivity over that address family is impaired.

Note that the address selection described in this section only applies to destination addresses; Source Address Selection ([RFC6724], Section 5) is performed once per destination address and is out of scope of this document.