diff --git a/protocol/03_encoding.md b/protocol/03_encoding.md
index 1ca7d6f..2323351 100644
--- a/protocol/03_encoding.md
+++ b/protocol/03_encoding.md
@@ -1,7 +1,23 @@
 # Encoding
 
 ## Requirements
+- The protocols must use JavaScript Object Notation (JSON).
+- The JSON string must be UTF-8 encoded.
+- ...
 
 ## Format
+### Header
+| Type  | Name      | Description                                      |
+|-------|-----------|--------------------------------------------------|
+| int_? | Length    | Message length (including or excluding?) header. | 
+| int_? | Checksum  | ...                                              |
+TODO:
+- Define int sizes. The length cannot exceed the uint16 range. However, it may be more convenient to use int32 in languages that do not support unsigned types.
+- Select a checksum algorithm.
+
+### Data
+| Type    | Name | Description                             |
+|---------|------|-----------------------------------------|
+| String  | Data | JSON message encoded as a UTF-8 string. |
 
 ## Example