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FIX 4.4 Connection: Logon Process

The Logon process is the first step in establishing a FIX session between two counterparties (client and server).\ It ensures that both sides agree on session parameters and synchronize message sequence numbers.

Logon Message (MsgType = A)

A Logon message must be the first message sent by each side when initiating a FIX session.

Example

8=FIX.4.4|9=75|35=A|34=1|49=CLIENT1|56=BROKER1|52=20250912-15:40:00.000|
98=0|108=30|141=Y|553=100000|554=Secret123|10=128|

Key Fields

  • 35=A -- MsgType = Logon\
  • 34 -- MsgSeqNum, first message in the session (typically 1)\
  • 49 -- SenderCompID (client ID)\
  • 56 -- TargetCompID (server/broker ID)\
  • 52 -- SendingTime\
  • 98 -- EncryptMethod (0 = None, 1 = PKCS, etc.)\
  • 108 -- HeartBtInt (heartbeat interval in seconds)\
  • 141 -- ResetSeqNumFlag (Y = reset sequence numbers, N = continue)\
  • 553 -- Username (required by brokers)\
  • 554 -- Password (required by brokers)

Logon Workflow

  1. Initiator (client) sends Logon message with session parameters.\
  2. Acceptor (server/broker) replies with its own Logon message.\
  3. If both sides accept, the session is established.\
  4. Heartbeats (35=0) begin at intervals defined by 108=HeartBtInt.\
  5. Trading messages (orders, executions, etc.) can be exchanged.

Session Management Considerations

  • Both sides must maintain message sequence numbers (34).\
  • If numbers are out of sync, the Resend Request (2) is used.\
  • Heartbeats ensure connectivity; if not received within timeout, the session is terminated.\
  • To close the session properly, the Logout (5) message is used.

Example Logon Exchange

Client → Server

8=FIX.4.4|9=75|35=A|34=1|49=CLIENT1|56=BROKER1|52=20250912-15:40:00.000|98=0|108=30|141=Y|10=128|

Server → Client

8=FIX.4.4|9=75|35=A|34=1|49=BROKER1|56=CLIENT1|52=20250912-15:40:00.500|98=0|108=30|141=Y|10=129|

At this point, the FIX session is active and ready for message exchange.

Best Practices

  • Always reset sequence numbers (141=Y) at the start of a new day/session.\
  • Ensure accurate system time synchronization (NTP).\
  • Protect credentials (553/554) -- use secure transport if possible.\
  • Log all session messages for audit and troubleshooting.