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

# Derivatives SBE Header Message

## Message Header

Each message begins with the following SBE message header:

| Field | Name                | Type   | Length | Description                                                                       |
| :---- | :------------------ | :----- | :----- | :-------------------------------------------------------------------------------- |
| 1     | protocolId          | uint8  | 1      | Constant (= `0xF1`)                                                               |
| 2     | flags               | uint8  | 1      | Bitset of flags.` 0x01 = resend`                                                  |
| 3     | messageLength       | uint16 | 2      | Total length of message including this header and body.                           |
| 4     | sequenceNumber      | uint32 | 4      | Sequence number of message.                                                       |
| 5     | lastProcessedSeqNum | uint32 | 4      | Sequence number of last message received/processed by the sender of this message. |
| 6     | reserved            |        | 4      | Padding                                                                           |
| 7     | sendTimeEpochNanos  | int64  | 8      | Sending time in nanoseconds since epoch.                                          |
| 8     | blockLength         | uint16 | 2      | Length of message root block in bytes, before variable data commences.            |
| 9     | templateId          | uint16 | 2      | Message type Id                                                                   |
| 10    | schemaId            | uint16 | 2      | Message schema Id (`1100` for admin/session messages, `1101` for orders)          |
| 11    | version             | uint16 | 2      | Message version number                                                            |