The FIX Session Layer: Logon, Heartbeat, Logout

FIX (Financial Information eXchange) messages come in two flavors that get bundled together but do very different jobs. Application messages carry the actual trading content — new orders, cancels, execution reports. Session messages exist purely to keep the connection between two counterparties alive and synchronized, and never touch a matching engine. The session layer is the plumbing that makes the application layer trustworthy.

The three messages that bookend every session

A FIX connection starts with a Logon message, where both sides confirm protocol version, exchange credentials, and session parameters like the heartbeat interval. Once logged on, both sides periodically send a Heartbeat message — a simple "I'm still here" — at the agreed interval, say every 30 seconds, even if there's no trading activity to report. If one side doesn't hear a heartbeat (or any message) within roughly 1.2× the interval, it sends a Test Request, demanding an immediate reply; if that goes unanswered too, the connection is assumed dead and torn down. At the end of the day, or when a firm wants to disconnect cleanly, a Logout message is exchanged so both sides know the session ended deliberately rather than by network failure.

This matters because a silently dropped TCP connection looks identical to a slow network from the outside — without heartbeats, neither side would know whether the counterparty is just quiet or has actually vanished, and an order sent into a dead connection can sit unacknowledged with the sender unsure if it was received.

Why this is a separate layer at all

Keeping session management distinct from trading content means a firm can build (or buy) one FIX engine that handles logon, heartbeats, and reconnection generically, then plug in different order-entry logic per venue on top. It also means the two problems get solved independently: sequence numbers and message integrity (covered separately) live in the session layer, while things like order tags and execution report fields live in the application layer. A broken heartbeat is purely a connectivity problem to diagnose; a rejected order is purely a business-logic problem — the split makes troubleshooting far more tractable than if both were tangled into one message stream.