5. Connection Attempts

Once the list of addresses received up to this point has been constructed, the client will attempt to make connections. In order to avoid unreasonable network load, connection attempts SHOULD NOT be made simultaneously. Instead, one connection attempt to a single address is started first, followed by the others in the list, one at a time. Starting a new connection attempt does not affect previous attempts, as multiple connection attempts may occur in parallel. Once one of the connection attempts succeeds (generally when the TCP handshake completes), all other connections attempts that have not yet succeeded SHOULD be canceled. Any address that was not yet attempted as a connection SHOULD be ignored. At that time, the asynchronous DNS query MAY be canceled as new addresses will not be used for this connection. However, the DNS client resolver SHOULD still process DNS replies from the network for a short period of time (recommended to be 1 second), as they will populate the DNS cache and can be used for subsequent connections.

A simple implementation can have a fixed delay for how long to wait before starting the next connection attempt. This delay is referred to as the "Connection Attempt Delay". One recommended value for a default delay is 250 milliseconds. A more nuanced implementation's delay should correspond to the time when the previous attempt is sending its second TCP SYN, based on the TCP's retransmission timer [RFC6298]. If the client has historical RTT data gathered from other connections to the same host or prefix, it can use this information to influence its delay. Note that this algorithm should only try to approximate the time of the first SYN retransmission, and not any further retransmissions that may be influenced by exponential timer back off.

The Connection Attempt Delay MUST have a lower bound, especially if it is computed using historical data. More specifically, a subsequent connection MUST NOT be started within 10 milliseconds of the previous attempt. The recommended minimum value is 100 milliseconds, which is referred to as the "Minimum Connection Attempt Delay". This minimum value is required to avoid congestion collapse in the presence of high packet-loss rates. The Connection Attempt Delay SHOULD have an upper bound, referred to as the "Maximum Connection Attempt Delay". The current recommended value is 2 seconds.