| Offset | Size (bytes) | Field | Meaning |
| -----: | ------------ | ------------------- | -------------------------------------------------------------------------------- |
| 0 | 4 | **Protocol ID** | ASCII **"TRTP"** (0x54 52 54 50). Distinguishes Hotline from other TCP services. |
| 4 | 4 | **Sub-protocol ID** | Application-specific tag (e.g. "CHAT", "FILE"). Can be 0. |
| 8 | 2 | **Version** | Currently **0x0001**. A server should refuse versions it does not understand. |
| 10 | 2 | **Sub-version** | Application-defined; often used for build/revision numbers. |