Session layer

The FIX API uses FIX 4.2 over TCP/IP. Each FIX message contains a standard header, a message body, and a standard trailer. Fields use the tag=value<SOH> format, where <SOH> is ASCII 0x01.

Standard header

Tag	Name	Req	Type	Description
`8`	`BeginString`	Y	String	FIX version. Use `FIX.4.2`. This field must be the first field in the message.
`9`	`BodyLength`	Y	Length	Message body length in bytes, calculated from the field after `BodyLength (9)` through the delimiter before `CheckSum (10)`.
`34`	`MsgSeqNum`	Y	SeqNum	Sequence number for the current message direction.
`35`	`MsgType`	Y	String	FIX message type.
`43`	`PossDupFlag`	N	Boolean	Set to `Y` when resending a message that may duplicate a previous transmission.
`49`	`SenderCompID`	Y	String	Sender identifier assigned for the FIX session.
`52`	`SendingTime`	Y	UTCTimestamp	Time when the message is sent.
`56`	`TargetCompID`	Y	String	Target identifier assigned for the FIX session.
`122`	`OrigSendingTime`	N	UTCTimestamp	Original send time for a resent message. Required when `PossDupFlag (43)=Y`.

Standard trailer

Tag	Name	Req	Type	Description
`10`	`CheckSum`	Y	String	Three-digit modulo 256 checksum for the message. This field must be the final field.

Sequence numbers

Each side maintains its own outbound sequence number. MsgSeqNum (34) starts at 1 for a reset session and increments by 1 for every session and application message sent in that direction. If a sequence gap is detected, the receiving side requests retransmission with ResendRequest (35=2). If a sequence number is lower than expected and is not a permitted duplicate, the service rejects the session state and requires a corrected logon.

Logon and reconnect behavior

Scenario	Client behavior	Service behavior
First logon for the trading day	Send `Logon (35=A)` with `MsgSeqNum (34)=1`.	Accepts the session when credentials, identifiers, and sequence state are valid.
Normal relogon	Send `ResetSeqNumFlag (141)=N` and `MsgSeqNum (34)` equal to the last client-sent sequence plus `1`.	Accepts the logon when the sequence matches.
Client sequence is higher than expected	Send the current client sequence.	Accepts the logon, then sends `ResendRequest (35=2)` for the missing range.
Client sequence is lower than expected	Relogon with the expected value returned in `Logout (35=5)` text.	Sends `Logout (35=5)` with text in the form `MsgSeqNum too low, expecting X but received Y`.
Service sequence is higher than client expected	Send `ResendRequest (35=2)` for the missing service messages.	Resends the requested application messages or sends gap fills for non-resendable messages.

Heartbeat rules

Rule	Requirement
Heartbeat interval	`HeartBtInt (108)` must be greater than `15` seconds and less than `100` seconds. The recommended value is `30` seconds.
Idle connection check	If no application message is received within the heartbeat interval, send `TestRequest (35=1)`.
Test response	The peer must respond with `Heartbeat (35=0)` containing the received `TestReqID (112)` when applicable.
Disconnect threshold	If no message is received for two heartbeat intervals, close the TCP connection.

Resend and gap fill

Case	Required behavior
Service requests client messages	The service sends `ResendRequest (35=2)` with `BeginSeqNo (7)` equal to the expected sequence and `EndSeqNo (16)=0` when requesting all messages from that point onward.
Client requests service messages	The client sends `ResendRequest (35=2)` with `BeginSeqNo (7)` equal to the expected sequence and `EndSeqNo (16)` equal to the last missing sequence, or `0` for all messages from that point onward.
Resent application message	Set `PossDupFlag (43)=Y` and include `OrigSendingTime (122)`. Preserve the original application payload.
One non-resendable message	Send `SequenceReset (35=4)` with `MsgSeqNum (34)=x`, `NewSeqNo (36)=x+1`, and `GapFillFlag (123)=Y`.
Non-resendable range	Send `SequenceReset (35=4)` with `MsgSeqNum (34)=x`, `NewSeqNo (36)=y+1`, and `GapFillFlag (123)=Y` for range `x` through `y`.

Endpoint failover

Primary and secondary endpoints do not share FIX session state. Use the secondary endpoint only when the primary endpoint cannot be reconnected after 30 seconds and the switch is authorized for the session.

Switch	Sequence behavior	Order behavior
Reconnect to primary endpoint	Send `ResetSeqNumFlag (141)=N` and continue with the next expected sequence number.	Existing orders continue on the same route.
Switch to secondary endpoint	Send `ResetSeqNumFlag (141)=Y` and `MsgSeqNum (34)=1`.	Orders routed before the switch are not automatically cancelled and do not update through the secondary session. Confirm or cancel active orders before switching when operationally required.

Logout

To close a FIX session, the initiating side sends Logout (35=5). The receiving side replies with Logout (35=5), then the TCP connection closes.

Overview

Integrations

Authentication

REST API

Streaming API

FIX API

Standard header

Standard trailer

Sequence numbers

Logon and reconnect behavior

Heartbeat rules

Resend and gap fill

Endpoint failover

Logout

​Standard header

​Standard trailer

​Sequence numbers

​Logon and reconnect behavior

​Heartbeat rules

​Resend and gap fill

​Endpoint failover

​Logout

Standard header

Standard trailer

Sequence numbers

Logon and reconnect behavior

Heartbeat rules

Resend and gap fill

Endpoint failover

Logout