- PJON (Padded Jittering Operative Network) Protocol specification: v3.0
- Acknowledge specification: v1.0
- Dynamic addressing specification: v2.0
- PJON known protocols: list
- PJDL (Padded Jittering Data Link) specification: PJDL v2.0 - PJDLR v2.0 - PJDLS v2.0
- TSDL (Tardy Serial Data Link) specification: TSDL v2.0
- SFSP (Secure Frame Separation Protocol) specification: SFSP v1.0
/*
Milan, Italy
Originally published: 24/11/2017
Latest revision: 26/1/2018
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 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 uses a more cautious approach, appending at the end of the frame an 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 uses a xor based flag obfuscation method to avoid the occurrence of flags within a frame. Its procedure is lightweight and more efficient if compared with the flag 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.
Frame transmission starts with START
flag, followed by data bytes. When START
, END
or ESC
flag occurs in data, it is prepended with a ESC
flag and its value is xored with ESC
flag to avoid the presence of START
, END
or ESC
flag in data. 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 xored with ESC flag
|
1 Flags inside data are escaped
START: 149 - 10010101 - 0x95 - �
END: 234 - 11101010 - 0xea - ê
ESC: 187 - 10111011 - 0xBB - »
Receiver identifies the start of a frame when a START
flag is received. If ESC
flag occurs during reception, it is ignored and the next byte xored with ESC
flag to get back its original value; any value out START
, END
or ESC
flag 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
flag 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 expecting to read the last byte of a packet and that is not followed by an END
flag, a parsing error occurred and reception is discarded.