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FIX 4.4: Connection and Logon

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.

Current ScaleTrade Example

8=FIX.4.4|9=116|35=A|34=1|49=CLIENT_1|52=20260408-20:33:33|56=SERVER|
98=0|108=30|141=Y|553=304|554=<fix-slot-password>|10=050|

Key Fields

  • 35=A -- MsgType = Logon\
  • 34 -- MsgSeqNum, first message in the session (typically 1)\
  • 49 -- SenderCompID (FIX slot code, for example CLIENT_1)\
  • 56 -- TargetCompID (server CompID, usually SERVER)\
  • 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 (must equal account login)\
  • 554 -- Password (must equal current FIX slot password)

Logon Workflow in Current Server

  1. Client sends Logon using:
    • 49 = slot_code
    • 56 = server sender_comp_id
    • 553 = login
    • 554 = slot password\
  2. Server validates the request through FixManager.\
  3. If accepted, the server replies with its own Logon.\
  4. Runtime FIX session becomes active.\
  5. The slot runtime state is moved to logged_on.\
  6. Trading and market-data messages 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.

Validation Rules

The current server accepts Logon only if all conditions are satisfied:

  • slot exists
  • slot is assigned to an account
  • slot is enabled
  • slot is license-enabled
  • slot rules allow logon
  • Username == login
  • Password == current slot password

Example Logon Exchange

Client → Server

8=FIX.4.4|9=116|35=A|34=1|49=CLIENT_1|56=SERVER|52=20260408-20:33:33|
98=0|108=30|141=Y|553=304|554=<fix-slot-password>|10=050|

Server → Client

8=FIX.4.4|9=75|35=A|34=1|49=SERVER|52=20260408-20:33:33.374|56=CLIENT_1|
98=0|108=30|141=Y|10=094|

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

Best Practices

  • Use the FIX slot code as SenderCompID, not the server CompID.\
  • Always reset sequence numbers (141=Y) at the start of a new day/session unless your engine manages continuation intentionally.\
  • Ensure accurate system time synchronization (NTP).\
  • Protect credentials (553/554) -- use secure transport if possible.\
  • Log all session messages for audit and troubleshooting.