This message is used to establish a FIX connection. The Logon message must be the first message sent by the application attempting to initiating a FIX session.

Upon receipt of a Logon message, the session acceptor will authenticate the requester and respond with a Logon message as acknowledgment that the connection request has been accepted. The acknowledgment Logon can be used by the initiator for validation purposes.

The session acceptor must be prepared to immediately begin processing messages after receipt of the Logon. The session initiator can choose to begin transmission of FIX messages before receipt of the confirmation Logon, however it is recommended that normal message delivery wait until after the return Logon is received.

The confirmation Logon can be used for encryption key negotiation. If a session key is deemed to be weak, a stronger session key can be suggested by returning a Logon message with a new key. This is only valid for encryption protocols that allow for key negotiation. See the FIX website's Application notes for more information on a method for encryption and key passing.

The Logon message can be used to specify the MaxMessageSize (383) supported, which can be used to control fragmentation rules for very large messages which support fragmentation. The Logon message can also be used to specify which MsgType (35)s are supported.

The HeartBtInt (108) field is used to declare the interval for generating heartbeats. See Heartbeat <35=0>.