Session Messages

Logon, logout, heartbeat, replay, and reject message formats and fields.

Logon

Initiate a FIX session with a Logon message — MsgType (35) set to A. Set EncryptMethod (98) to 0 and HeartBtInt (108) to the desired interval (default: 30 seconds).

Message Format

Standard FIX Fields

Tag	Field	When to Include	Notes
8	BeginString	Always	Standard FIX header
9	BodyLength	Always	Standard FIX header
35	MsgType	Always	A (Logon)
49	SenderCompID	Always	Standard FIX header
56	TargetCompID	Always	Standard FIX header
34	MsgSeqNum	Always	Standard FIX header
52	SendingTime	Always	Standard FIX header

Standard FIX Trailer

Tag	Field	When to Include	Notes
10	CheckSum	Always

Logon Fields

Tag	Field	When to Include	Notes
98	EncryptMethod	Always	Must be 0
108	HeartBtInt	Always	Heartbeat interval in seconds (default: 30)

Outgoing

Send the Logon message to initiate the session:

Tag	Field	Value	Notes
49	SenderCompID	Your client ID
56	TargetCompID	ITBIT

Response

Paxos responds with a matching Logon message:

Tag	Field	Value	Notes
49	SenderCompID	ITBIT
56	TargetCompID	Your client ID

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Logon with Reset Sequence Number Flag

Reset sequence numbers on both sides by sending a Logon with ResetSeqNumFlag (141) set to Y.

Implement a sequence number management strategy to promote a stable trading environment.

Message Format

Uses the same Standard FIX Fields and Logon Fields as a regular Logon, plus:

Tag	Field	When to Include	Notes
141	ResetSeqNumFlag	Always	Y — reset sequence numbers on both sides

Outgoing

Send the Logon with ResetSeqNumFlag (141) = Y.

Response

Paxos responds with a matching Logon that also includes ResetSeqNumFlag (141) = Y.

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Replay Messages

Request retransmission of messages with a Resend Request — MsgType (35) set to 2. Use this when a gap in sequence numbers is detected, a message is lost, or higher-than-expected sequence numbers arrive from the counterparty.

Resend Scenarios

Scenario	BeginSeqNo (7)	EndSeqNo (16)
Single message	N	N (same value)
Range of messages	First in range	Last in range
All subsequent messages	N	0

Message Format

Standard FIX Fields

Tag	Field	When to Include	Notes
8	BeginString	Always	Standard FIX header
9	BodyLength	Always	Standard FIX header
35	MsgType	Always	2 (Resend Request)
49	SenderCompID	Always	Standard FIX header
56	TargetCompID	Always	Standard FIX header
34	MsgSeqNum	Always	Standard FIX header
52	SendingTime	Always	Standard FIX header

Standard FIX Trailer

Tag	Field	When to Include	Notes
10	CheckSum	Always

Outgoing

Required

Tag	Field	When to Include	Notes
7	BeginSeqNo	Always	First message sequence number in range
16	EndSeqNo	Always	Last message sequence number in range (0 for all subsequent)

Response

Paxos resends the requested messages sequentially. Resent messages include:

Required

Tag	Field	When Present	Notes
7	BeginSeqNo	Always	First message sequence number in range
16	EndSeqNo	Always	Last message sequence number in range

Conditional

Tag	Field	When Present	Notes
43	PossDupFlag	Retransmitted messages	Y — indicates possible retransmission
122	OrigSendingTime	Retransmitted messages	Original time of message transmission

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Heartbeat

Monitor the FIX connection with Heartbeat messages — MsgType (35) set to 0. The default HeartBtInt (108) interval is 30 seconds.

Key Behavior

• Reset the heartbeat timer after every transmitted message, not just heartbeats.
• If no data is received within the heartbeat interval, send a Test Request (MsgType 1) to verify the connection.
• Include TestReqID (112) to match Test Requests with Heartbeat responses. Any string can be used (a timestamp is recommended).
• If no Heartbeat response arrives in a reasonable time, consider the connection lost.

Message Format

Standard FIX Fields

Tag	Field	When to Include	Notes
8	BeginString	Always	Standard FIX header
9	BodyLength	Always	Standard FIX header
35	MsgType	Always	0 (Heartbeat)
49	SenderCompID	Always	Standard FIX header
56	TargetCompID	Always	Standard FIX header
34	MsgSeqNum	Always	Standard FIX header
52	SendingTime	Always	Standard FIX header

Standard FIX Trailer

Tag	Field	When to Include	Notes
10	CheckSum	Always

Heartbeat Fields

Tag	Field	When to Include	Notes
112	TestReqID	In response to a Test Request	Echoes the TestReqID from the Test Request

Outgoing

Send a Heartbeat when no message has been sent within the HeartBtInt interval, or in response to a Test Request.

Response

Paxos responds to Test Requests with a Heartbeat containing the matching TestReqID (112).

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Reject

A Reject message — MsgType (35) set to 3 — is sent when a message cannot be processed due to a session-level rule violation.

Message Format – Response Message

Standard FIX Fields

Tag	Field	When Present	Notes
8	BeginString	Always	Standard FIX header
9	BodyLength	Always	Standard FIX header
35	MsgType	Always	3 (Reject)
49	SenderCompID	Always	Standard FIX header
56	TargetCompID	Always	Standard FIX header
34	MsgSeqNum	Always	Standard FIX header
52	SendingTime	Always	Standard FIX header

Standard FIX Trailer

Tag	Field	When Present	Notes
10	CheckSum	Always

Reject Fields

Always Present

Tag	Field	When Present	Notes
45	RefSeqNum	Always	Sequence number of the rejected message
58	Text	Always	Human-readable reason for the reject
373	SessionRejectReason	Always	Reject reason code

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Logout

A Logout message — MsgType (35) set to 5 — initiates or confirms FIX session termination.

Disconnection without exchanging Logout messages indicates an abnormal condition. An unsolicited Logout may indicate a connection or account issue — Text (58) contains a human-readable message. If you cannot reconnect with ResetSeqNumFlag (141) set to Y, contact Support.

Message Format – Incoming Message

Standard FIX Fields

Tag	Field	When Present	Notes
8	BeginString	Always	Standard FIX header
9	BodyLength	Always	Standard FIX header
35	MsgType	Always	5 (Logout)
49	SenderCompID	Always	Standard FIX header
56	TargetCompID	Always	Standard FIX header
34	MsgSeqNum	Always	Standard FIX header
52	SendingTime	Always	Standard FIX header

Standard FIX Trailer

Tag	Field	When Present	Notes
10	CheckSum	Always

Logout Fields

Always Present

Tag	Field	When Present	Notes
58	Text	Always	Human-readable message indicating any issues