Bytes that arrive with no frame open are taken as the body of a frame whose opening FLAG was missed, so they close at the next FLAG as one typically undecodable frame and the scanner realigns. RNS’s FLAG data FLAG FLAG data FLAG layout would otherwise let a mid-frame join lock the decoder a half-frame out of phase permanently.
XOR mask applied after ESC to recover the original byte and at encode time to produce the escaped byte. The escaped form of FLAG (0x5E) and ESC (0x5D) can never collide with another FLAG or ESC in the byte stream.