A question about IPSec implementation..

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Hi, all:
  Recently I found a paragraph of codes about IPSec replay prevention 
that confused me a lot. Could you shed some light on me?

  line 2370 to line 2407 in ipsec.c deal with the replay window update.

/    if (seq > replay->lastseq) {
        /* seq is larger than lastseq. */
        diff = seq - replay->lastseq;

        /* new larger sequence number */
        if (diff < wsizeb) {
            /* In window */
            /* set bit for this packet */
            vshiftl(replay->bitmap, diff, replay->wsize);
            replay->bitmap[frlast] |= 1;
        } else {
            /* this packet has a "way larger" */
            bzero(replay->bitmap, replay->wsize);
            replay->bitmap[frlast] = 1;
        }
        replay->lastseq = seq;

        /* larger is good */
    } else {
...../

  When the receiving sequence number larger than the maintained last 
largest one, it will do /vshiftl/ and then /switch on the last bit of 
the bitmap/. What I am wondering here is: is the current receiving 
sequence number necessarily the last bit after doing /vshiftl/? Why to 
do /vshiftl/?

Thanks for your time.

BR,

Yi-Wen