Re: [tcpm] RFC 5925 SNE algorithm concern

[Joseph Touch <[e-mail]>]

Here’s a full version, with everything including initialization and definitions. 

// compute the distance from x to y assuming they are in the same SNE
#define DIST(x,y) (((x)<(y))?((y)-(x)):((x)-(y)))
#define HALF      (0x80000000)              // shorthand used in the code below

// variables, with inits where required
unsigned long RCV_SNE = 0;                  // high-watermark SNE, init per RFC5925
int           RCV_SNE_FLAG = 1;             // set during first half of rollover
unsigned long RCV_PREV_SEQ;                 // high-watermark SEQ, init by ISN
unsigned long SEG_SEQ;                      // input - received SEQ
unsigned long SNE;                          // output - SNE corresponding to received SEQ

if (SYN == 1) {                             // SYN or SYN-ACK initialization
  RCV_PREV_SEQ = RECV_ISN;
}                                           // after SYN or SYN-ACK
SNE = RCV_SNE;                              // use current SNE (as default)
if (DIST(SEG_SEQ,RCV_PREV_SEQ) < HALF) {    // both in same SNE range?
  if ((SEG_SEQ >= HALF) 
      && (RCV_PREV_SEQ < HALF)) {           // jumps fwd over N/2?
    RCV_SNE_FLAG = 0;                       // reset wrap increment flag
  }
  RCV_PREV_SEQ = MAX(SEG_SEQ,RCV_PREV_SEQ); // move prev forward if needed
} else {                                    // both in diff SNE ranges
  if (SEG_SEQ < HALF) {                     // jumps forward over zero?
    RCV_PREV_SEQ = SEG_SEQ;                 // update prev
    if (RCV_SNE_FLAG == 0) {.               // first jump over zero? (wrap)
      RCV_SNE_FLAG = 1;                     // set flag so we increment once
      RCV_SNE = RCV_SNE + 1;                // increment window
      SNE = RCV_SNE;                        // use updated SNE value
    }
  } else {                                  // jump backward over zero
    SNE = RCV_SNE - 1;                      // use pre-rollover SNE value
  }
}