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Extension Message

The extension message (#20) is a flexible all-purpose message that is used for features that were not in the original protocol.

An extension message starts with the message type 20 (one byte), followed by the message sub-type (one byte), and then followed by message sub-type specific data.

Message sub-type 0 is the extended handshake which is sent in the early stages of the peer connection, immediately after the bitfield.

The other sub-type numbers are assigned dynamically using a map contained in the extended handshake.