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SFSP-frame-separation-specification-v1.0.md

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Network layer

  • PJON (Padded Jittering Operative Network) Protocol specification: v3.0
  • Acknowledge specification: v1.0
  • Dynamic addressing specification: v2.0
  • PJON known protocols: list

Data link layer

/*
Milan, Italy
Originally published: 24/11/2017
SFSP v1.0
Invented by Giovanni Blu Mitolo,
released into the public domain

Related work: https://github.com/gioblu/PJON/
Compliant implementations: PJON v10.0 and following
*/

SFSP (Secure Frame Separation Protocol) v1.0

SFSP has been specified to obtain reliable frame separation using byte stuffing and XOR based flags obfuscation. It is similar to SLIP and its variations, although a more cautious approach has been applied, appending at the end of the frame a END flag to exclude a set of vulnerabilities in case of externally induced error. Its overhead is 1.33 times higher if compared to SLIP's but it is more reliable thanks to the presence of the END flag. SFSP's XOR based flags obfuscation method is lightweight and more efficient if compared with the flags transposition method used by SLIP that requires more memory and operations to be executed. The suggested sampling and transmission strategy is byte-by-byte. SFSP is designed to support fast on the fly encoding and decoding implementations with no need of additional buffers.

SFSP frame transmission

Frame transmission starts with START flag, followed by data bytes. When START, END or ESC occurs in data, it is prepended with ESC and its value is XOR-ed with ESC to enforce the absence of START or END in the data stream. END is appended at the end of the frame.

           ______________________________
          |  DATA 1-65535 bytes          |
 _______  |______  _____  _______  ______|  _____
| START | | BYTE || ESC || START || BYTE | | END |
|-------| |------||-----||-------||------| |-----|
|  149  | |  23  || 76  ||149^ESC||  52  | | 234 |
|_______| |______||_____||_______||______| |_____|
                     |     |
                     |  2 START flag is XOR-ed with ESC flag
                   1 Flags inside data are escaped

START:  149 - 10010101 - 0x95 - �
END:    234 - 11101010 - 0xea - ê
ESC:    187 - 10111011 - 0xBB - »

SFSP frame reception

Receiver identifies the start of a frame when a START is received. If ESC occurs during reception, it is ignored and the next byte XOR-ed with ESC to get back its original value; any value out ESC, START or END causes the receiver to discard the frame and be ready to receive the next one nominally. The end of the frame is detected when an unescaped END is received. The END flag can be used to verify that both frame and packet end are coincident: if an abstraction of higher level is requesting to read the last byte of a frame and that is not followed by END a parsing error occurred at one or both levels and the frame is discarded.