---
title: Header and Trailer
seo:
  title: Header and Trailer | FIX API
  description: Shared FIX header and trailer fields for session and application
    messages in the market-data and order-entry APIs.
sidebar:
  label: Header and Trailer
---
Every session-level and application-level message in both FIX APIs uses the same header and trailer. The message body between them contains the fields listed on that message's reference page.

See [FIX Message Layouts](/reference/fix-message-layouts) for ordering and presence rules.

## Header

| Field | Type | Presence | Behavior |
| --- | --- | --- | --- |
| `BeginString(8)` | [`STRING`](/reference/fix-data-types#string) | Required | <span style="overflow-wrap:anywhere">The session protocol version is BeginString(8)=FIXT.1.1 for both message levels.</span> |
| `BodyLength(9)` | [`LENGTH`](/reference/fix-data-types#length) | Required |  |
| `MsgType(35)` | [`STRING`](/reference/fix-data-types#string) | Required |  |
| `SenderCompID(49)` | [`STRING`](/reference/fix-data-types#string) | Required |  |
| `TargetCompID(56)` | [`STRING`](/reference/fix-data-types#string) | Required |  |
| `MsgSeqNum(34)` | [`SEQNUM`](/reference/fix-data-types#seqnum) | Required |  |
| `PossDupFlag(43)` | [`BOOLEAN`](/reference/fix-data-types#boolean) | Optional |  |
| `PossResend(97)` | [`BOOLEAN`](/reference/fix-data-types#boolean) | Optional |  |
| `SendingTime(52)` | [`UTCTIMESTAMP`](/reference/fix-data-types#utctimestamp) | Required |  |
| `OrigSendingTime(122)` | [`UTCTIMESTAMP`](/reference/fix-data-types#utctimestamp) | Conditional | <span style="overflow-wrap:anywhere">Sent by the exchange on retransmissions and sequence-reset gap fills. Carries the original sending time.</span> |

## Trailer

| Field | Type | Presence | Behavior |
| --- | --- | --- | --- |
| `CheckSum(10)` | [`STRING`](/reference/fix-data-types#string) | Required |  |