> ## 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`) |

<Tabs>
  <Tab title="Outgoing">
    Send the Logon message to initiate the session:

    | Tag  | Field        | Value                          | Notes |
    | ---- | ------------ | ------------------------------ | ----- |
    | `49` | SenderCompID | Your client ID                 |       |
    | `56` | TargetCompID | `ITBIT_PROD` / `ITBIT_SANDBOX` |       |
  </Tab>

  <Tab title="Response">
    Paxos responds with a matching Logon message:

    | Tag  | Field        | Value                          | Notes |
    | ---- | ------------ | ------------------------------ | ----- |
    | `49` | SenderCompID | `ITBIT_PROD` / `ITBIT_SANDBOX` |       |
    | `56` | TargetCompID | Your client ID                 |       |
  </Tab>
</Tabs>

***

## Logon with Reset Sequence Number Flag

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

<Tip>
  Implement a [sequence number management](/guides/crypto-brokerage/fix/best-practices#sequence-number-management) strategy to promote a stable trading environment.
</Tip>

### 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 |

<Tabs>
  <Tab title="Outgoing">
    Send the Logon with **ResetSeqNumFlag (`141`)** = `Y`.
  </Tab>

  <Tab title="Response">
    Paxos responds with a matching Logon that also includes **ResetSeqNumFlag (`141`)** = `Y`.
  </Tab>
</Tabs>

***

## 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          |       |

<Tabs>
  <Tab title="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) |
  </Tab>

  <Tab title="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   |
  </Tab>
</Tabs>

***

## 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 |

<Tabs>
  <Tab title="Outgoing">
    Send a Heartbeat when no message has been sent within the **HeartBtInt** interval, or in response to a Test Request.
  </Tab>

  <Tab title="Response">
    Paxos responds to Test Requests with a Heartbeat containing the matching **TestReqID (`112`)**.
  </Tab>
</Tabs>

***

## 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.

<Warning>
  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).
</Warning>

### 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 |