S101

The S101 protocol is used for framing, allowing the Ember encoded data to be transmitted via any transport layer.

Control bytes

The following control bytes are used in the S101 protocol:

Name	Value	Description
BOF	0xFE	Begin of Frame
EOF	0xFF	End of Frame
CE	0xFD	Character escape
XOR	0x20	XOR value for Character escape
BOFNE | Invalid	0xF8	Within a variant 1 packet, all bytes above or equal to this value must be escaped. Outside of a frame, this byte indicates a variant 2 packet which uses the Non-Escaping version of S101.

Messaging

The first byte of the payload in S101 messages is the slot identifier, which is usually set to 0x00. The second byte contains the message type, which is set 0x0E for Ember. The third byte determines the command. The fourth byte determines the version, which is currently set to 0x01.

Value	Command
0x00	EmBER Packet
0x01	Keep Alive Request
0x02	Keep Alive Response

The meaning of the remaining bytes depends on the specified command.

EmBER packet

When the command field is set to 0x00, the payload contains an encoded Ember packet.

Flags

The upper three bits of the flags are used to indicate partitioned ember packets.

Value	Meaning
0xC0	Single-packet message
0x80	First multi-packet message
0x40	Last multi-packet message
0x20	Empty packet
0x00	A packet within a multi-packet message

DTD

The DTD byte defines the "Design Type Document" that is being used. This value must be set to one, which means that the Glow specification is used.

App Bytes

This field contains the number of bytes that follow before the payload begins.

The current version uses two application bytes, which contain the version number of the Glow DTD.

Payload

The payload contains the BER-encoded data.