> ## Documentation Index
> Fetch the complete documentation index at: https://docs.paxos.com/llms.txt
> Use this file to discover all available pages before exploring further.
# 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`) |
Send the Logon message to initiate the session:
| Tag | Field | Value | Notes |
| - | - | - | - |
| `49` | SenderCompID | Your client ID | |
| `56` | TargetCompID | `ITBIT_PROD` / `ITBIT_SANDBOX` | |
Paxos responds with a matching Logon message:
| Tag | Field | Value | Notes |
| - | - | - | - |
| `49` | SenderCompID | `ITBIT_PROD` / `ITBIT_SANDBOX` | |
| `56` | TargetCompID | Your client ID | |
***
## 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](/guides/crypto-brokerage/fix/best-practices#sequence-number-management) strategy to promote a stable trading environment.
### Message Format
Uses the same [Standard FIX Fields](#standard-fix-fields) and [Logon Fields](#logon-fields) as a regular Logon, plus:
| Tag | Field | When to Include | Notes |
| - | - | - | - |
| `141` | ResetSeqNumFlag | Always | `Y` — reset sequence numbers on both sides |
Send the Logon with **ResetSeqNumFlag (`141`)** = `Y`.
Paxos responds with a matching Logon that also includes **ResetSeqNumFlag (`141`)** = `Y`.
***
## 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 | |
**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) |
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 |
***
## 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 |
Send a Heartbeat when no message has been sent within the **HeartBtInt** interval, or in response to a Test Request.
Paxos responds to Test Requests with a Heartbeat containing the matching **TestReqID (`112`)**.
***
## 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 |
***
## 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](https://support.paxos.com).
### 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 |