FIX 4.1 : Logon <A> message

Structure | Sample Message | Related Messages


The Logon <A> message authenticates a user establishing a connection to a remote system. The Logon <A> message must be the first message sent by the application requesting to initiate a FIX session.

The HeartBtInt <108> field is used to declare the timeout interval for generating heartbeats (same value used by both sides). The HeartBtInt <108> value should be agreed upon by the two firms and specified by the Logon <A> initiator and echoed back by the Logon <A> acceptor.

Upon receipt of a Logon <A> message, the session acceptor will authenticate the party requesting connection and issue a Logon <A> message as acknowledgment that the connection request has been accepted. The acknowledgment Logon <A> can also be used by the initiator to validate that the connection was established with the correct party.

successful-logonSuccessful Logon

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

The confirmation Logon <A> 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 <A> message with a new key. This is only valid for encryption protocols that allow for key negotiation. (See the FIX Web Site's Application notes for more information on a method for encryption and key passing.)


Tag Field Name Req'd Comments
<MessageHeader> Y MsgType <35> = A
98 EncryptMethod Y

(Always unencrypted)

108 HeartBtInt Y
95 RawDataLength N

Required for some authentication methods

96 RawData N

Required for some authentication methods

141 ResetSeqNumFlag N

Indicates both sides of a FIX session should reset sequence numbers

<MessageTrailer> Y


Sample Message

The '^' character is used to represent SOH character.


Related Messages