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The packet header is fixed and consists in a synchronization byte (0xA5), followed by a unused (possibly RFU) zero byte. After these two hardcoded bytes, there's a varint representing the payload size, which may use one byte or two, depending on the how big the payload is.+
For packets with less than 64 bytes:+! Size+For packets with up to 16383 bytes:+
−! Size (1)+
Packets are sent using IrDA-SIR (using [[ir:USER]]), with a 8N1 encoding (eight data bits, one stop bit, without parity). Each one is formed by a 2-byte header, a varint with the payload size, an obfuscated payload, and trailing error detection byte.
Packets are sent using IrDA-SIR (using [[ir:USER]]), with a 8N1 encoding (eight data bits, one stop bit, without parity). Each one is formed by a 2-byte header, a varint with the payload size, an obfuscated payload, and trailing error detection byte.
−== Packet header ==
+== Layer 1 - framing format ==
−Frames are encoded using two different yet very simmilar formats, depending on how large the payload to be transmitted is:
−* For payloads with less than 64 bytes, the third byte represents the payload size.
* For payloads with less than 64 bytes, the third byte represents the payload size.
* For packets with up to 16383 bytes, the size is split in two bytes, with the third byte being the upper 6 bits of the payload size, OR'd with 0x40, and the fourth being the lower eight bits of the payload size
* For packets with up to 16383 bytes, the size is split in two bytes, with the third byte being the upper 6 bits of the payload size, OR'd with 0x40, and the fourth being the lower eight bits of the payload size
−{| class="wikitable" style="margin: 0 auto; text-align: center;"
−{| class="wikitable" border="1"
+|+IR framing format - short frame
−! Sync
+|-
−! RFU
+! style="border-top: none" | Byte
−! style="width:10%;"| 7
+! style="width:10%;"| 6
+! style="width:10%;"| 5
+! style="width:10%;"| 4
+! style="width:10%;"| 3
+! style="width:10%;"| 2
+! style="width:10%;"| 1
+! style="width:10%;"| 0
+|-
+! 0x00
+| colspan="8"|Synchronization byte (<code>0xA5</code>)
+|-
+! 0x01
+| colspan="8"|Reserved for future use (<code>0x00</code>)
+|-
+! 0x02
+| RFU (<code>0</code>)
+| Short frame (<code>0</code>)
+| colspan="6"|Payload size
|-
|-
−| 0xA5
+! 0x03
−| 0x00
+| colspan="8"|Payload byte 0
−| size
+|-
+| colspan="9"|...
+|-
+! 0x03+n-1
+| colspan="8"|Payload byte n-1
+|-
+! 0x03+n
+| colspan="8"|[[CRC-8-CCITT]] computer over whole packet
|}
|}
−{| class="wikitable" border="1"
+{| class="wikitable" style="margin: 0 auto; text-align: center;"
−! Sync
+|+IR framing format - long frame
−! RFU
+|-
−! style="border-top: none" | Byte
−! Size (2)
+! style="width:10%;"| 7
+! style="width:10%;"| 6
+! style="width:10%;"| 5
+! style="width:10%;"| 4
+! style="width:10%;"| 3
+! style="width:10%;"| 2
+! style="width:10%;"| 1
+! style="width:10%;"| 0
+|-
+! 0x00
+| colspan="8"|Synchronization byte (<code>0xA5</code>)
+|-
+! 0x01
+| colspan="8"|Reserved for future use (<code>0x00</code>)
+|-
+! 0x02
+| RFU (<code>0</code>)
+| Long frame (<code>1</code>)
+| colspan="6"|Payload size (upper 6 bits)
+|-
+! 0x03
+| colspan="8"|Payload size (lower 8 bits)
+|-
+! 0x04
+| colspan="8"|Payload byte 0
|-
|-
−| 0xA5
+| colspan="9"|...
−| 0x00
+|-
−| (size >> 8) <nowiki>|</nowiki> 0x40
+! 0x04+n-1
−| size & 0xFF
+| colspan="8"|Payload byte n-1
+|-
+! 0x04+n
+| colspan="8"|[[CRC-8-CCITT]] computer over whole packet
|}
|}
+
+=== Header ===
+The packet header is fixed and consists in a synchronization byte (0xA5), followed by a unused (possibly RFU) zero byte. After these two hardcoded bytes, there's a varint representing the payload size, which may use one byte or two, depending on the how big the payload is.
In C:
In C:
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}</nowiki>
}</nowiki>
−== Payload ==
+=== Payload ===
The payload is obfuscated using a XOR-based encryption. In C:
The payload is obfuscated using a XOR-based encryption. In C:
<nowiki>void payloadObfuscate(const void * voidplain, void * voidcipher, size_t size) {
<nowiki>void payloadObfuscate(const void * voidplain, void * voidcipher, size_t size) {
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}</nowiki>
}</nowiki>
−== Error detection ==
+=== Error detection ===
The trailing error detection byte is calculated using [[CRC-8-CCITT]] <b>over the whole packet</b> (both the header and the payload)
The trailing error detection byte is calculated using [[CRC-8-CCITT]] <b>over the whole packet</b> (both the header and the payload)