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.