> ## Documentation Index
> Fetch the complete documentation index at: https://docs.vos.verolabs.co/llms.txt
> Use this file to discover all available pages before exploring further.

# Session messages

> FIX session message types and body fields for logon, heartbeat, resend, reject, sequence reset, and logout

Session messages control authentication, liveness, sequence recovery, and orderly disconnect.
All session messages use the standard FIX header and trailer.

## Session message list

| Message       | `MsgType (35)` | Direction                            | Purpose                                                                         |
| ------------- | -------------- | ------------------------------------ | ------------------------------------------------------------------------------- |
| Logon         | `A`            | Client to service; service to client | Opens a FIX session and negotiates heartbeat and sequence behavior.             |
| Heartbeat     | `0`            | Both directions                      | Confirms that the FIX connection is alive.                                      |
| TestRequest   | `1`            | Both directions                      | Requests an immediate heartbeat response when a peer appears idle.              |
| ResendRequest | `2`            | Both directions                      | Requests retransmission for a missing sequence range.                           |
| Reject        | `3`            | Service to client                    | Rejects a session-level message or invalid session condition.                   |
| SequenceReset | `4`            | Both directions                      | Advances sequence state or fills gaps for messages that are not resent.         |
| Logout        | `5`            | Both directions                      | Closes the FIX session or reports a session condition that requires disconnect. |

## Logon fields

`Logon (35=A)` opens a session.

| Tag   | Name              | Req | Type    | Description                                                                               |
| ----- | ----------------- | --- | ------- | ----------------------------------------------------------------------------------------- |
| `98`  | `EncryptMethod`   | Y   | Int     | Encryption method. Use `0` for none unless a different value is assigned for the session. |
| `108` | `HeartBtInt`      | Y   | Int     | Heartbeat interval in seconds. Must be greater than `15` and less than `100`.             |
| `141` | `ResetSeqNumFlag` | N   | Boolean | `Y` resets sequence numbers for the session. `N` continues the previous sequence state.   |

## Heartbeat fields

`Heartbeat (35=0)` contains no required body fields in this contract.
When replying to `TestRequest (35=1)`, include the received `TestReqID (112)`.

## Test request fields

| Tag   | Name        | Req | Type   | Description                                                |
| ----- | ----------- | --- | ------ | ---------------------------------------------------------- |
| `112` | `TestReqID` | Y   | String | Identifier that the peer echoes in the heartbeat response. |

## Resend request fields

| Tag  | Name         | Req | Type | Description                                                                                   |
| ---- | ------------ | --- | ---- | --------------------------------------------------------------------------------------------- |
| `7`  | `BeginSeqNo` | Y   | Int  | First sequence number requested.                                                              |
| `16` | `EndSeqNo`   | Y   | Int  | Last sequence number requested. Use `0` to request all messages from `BeginSeqNo (7)` onward. |

## Reject fields

| Tag   | Name                  | Req | Type   | Description                                      |
| ----- | --------------------- | --- | ------ | ------------------------------------------------ |
| `45`  | `RefSeqNum`           | Y   | Int    | Sequence number of the rejected message.         |
| `58`  | `Text`                | N   | String | Human-readable reject detail.                    |
| `371` | `RefTagID`            | N   | Int    | FIX tag that caused the reject, when applicable. |
| `372` | `RefMsgType`          | Y   | String | `MsgType (35)` of the rejected message.          |
| `373` | `SessionRejectReason` | N   | Int    | FIX session reject reason code.                  |

## Sequence reset fields

| Tag   | Name          | Req | Type    | Description                                                          |
| ----- | ------------- | --- | ------- | -------------------------------------------------------------------- |
| `36`  | `NewSeqNo`    | Y   | Int     | Next sequence number expected after the reset or gap fill.           |
| `123` | `GapFillFlag` | N   | Boolean | `Y` indicates a gap fill. Omitted or `N` indicates a sequence reset. |

## Logout fields

| Tag  | Name   | Req | Type   | Description                                                                                                          |
| ---- | ------ | --- | ------ | -------------------------------------------------------------------------------------------------------------------- |
| `58` | `Text` | N   | String | Logout detail. If the value states `MsgSeqNum too low, expecting X but received Y`, relogon with `MsgSeqNum (34)=X`. |