Re: [tcpm] RFC 5925 SNE algorithm concern

Hello,

+1 for below algorithm.

The earlier improved algorithm I suggested did not handle middle-range
retransmissions correctly, this one does. It also passes other tests I
have (which found the original issues).

Thank you,
--
 Juhamatti

On Wed, 6 May 2020 at 22:36, Joseph Touch <touch@strayalpha.com> wrote:
>
> 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
>   }
> }
>
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