rfc1977.txt

改文件中包含了三个协议

        if (i != db->seqno++) {
                freemsg(cmsg);
                if (db->debug)
                        printf("bsd_decomp%d: bad sequence number 0x%x"
                               " instead of 0x%x\n",
                               db->unit, i, db->seqno-1);
                return 0;
        }




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RFC 1977                    PPP BSD Compress                 August 1996


        /* Guess how much memory we will need.  Assume this packet was
         * compressed by at least 1.5X regardless of the recent ratio.
         */
        if (db->ratio > (RATIO_SCALE*3)/2)
                explen = (msgdsize(cmsg)*db->ratio)/RATIO_SCALE;
        else
                explen = (msgdsize(cmsg)*3)/2;
        if (explen > db->mru)
                explen = db->mru;

        dmsg = dmsg1 = allocb(explen+PPP_BUF_HEAD_INFO, BPRI_HI);
        if (!dmsg1) {
                freemsg(cmsg);
                return 0;
        }

        wptr = dmsg1->b_wptr;

        ((struct ppp_buf*)wptr)->type = BEEP_FRAME;
        /* the protocol field must be compressed */
        ((struct ppp_buf*)wptr)->proto = 0;
        wptr += PPP_BUF_HEAD_PROTO+1;

        rptr9 = cmsg->b_wptr;
        db->bytes_out += rptr9-rptr;
        wptr0 = wptr;
        explen = dmsg1->b_datap->db_lim - wptr;
        oldcode = CLEAR;
        for (;;) {
                if (rptr >= rptr9) {
                        bp = cmsg;
                        cmsg = cmsg->b_cont;
                        freeb(bp);
                        if (!cmsg)      /* quit at end of message */
                                break;
                        rptr = cmsg->b_rptr;
                        rptr9 = cmsg->b_wptr;
                        db->bytes_out += rptr9-rptr;
                        continue;       /* handle 0-length buffers */
                }

                /* Accumulate bytes until we have a complete code.
                 * Then get the next code, relying on the 32-bit,
                 * unsigned accum to mask the result.
                 */
                bitno -= 8;
                accum |= *rptr++ << bitno;
                if (tgtbitno < bitno)



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RFC 1977                    PPP BSD Compress                 August 1996


                        continue;
                incode = accum >> tgtbitno;
                accum <<= n_bits;
                bitno += n_bits;

                if (incode == CLEAR) {
                        /* The dictionary must only be cleared at
                         * the end of a packet.  But there could be an
                         * empty message block at the end.
                         */
                        if (rptr != rptr9
                            || cmsg->b_cont != 0) {
                                cmsg->b_rptr = rptr;
                                i = msgdsize(cmsg);
                                if (i != 0) {
                                        freemsg(dmsg);
                                        freemsg(cmsg);
                                        if (db->debug)
                                                printf("bsd_decomp%d: "
                                                       "bad CLEAR\n",
                                                       db->unit);
                                        return 0;
                                }
                        }
                        pf_bsd_clear(db);
                        freemsg(cmsg);
                        wptr0 = wptr;
                        break;
                }

                /* Special case for KwKwK string. */
                if (incode > max_ent) {
                        if (incode > max_ent+2
                            || incode > db->maxmaxcode
                            || oldcode == CLEAR) {
                                freemsg(dmsg);
                                freemsg(cmsg);
                                if (db->debug)
                                   printf("bsd_decomp%d: bad code %x\n",
                                          db->unit, incode);
                                return 0;
                        }
                        i = db->lens[oldcode];
                        /* do not write past end of buf */
                        explen -= i+1;
                        if (explen < 0) {
                                db->undershoot -= explen;
                                db->in_count += wptr-wptr0;



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RFC 1977                    PPP BSD Compress                 August 1996


                                dmsg1->b_wptr = wptr;
                                CK_WPTR(dmsg1);
                                explen = MAX(64,i+1);
                                bp = allocb(explen, BPRI_HI);
                                if (!bp) {
                                        freemsg(cmsg);
                                        freemsg(dmsg);
                                        return 0;
                                }
                                dmsg1->b_cont = bp;
                                dmsg1 = bp;
                                wptr0 = wptr = dmsg1->b_wptr;
                               explen=dmsg1->b_datap->db_lim-wptr-(i+1);
                        }
                        p = (wptr += i);
                        *wptr++ = finchar;
                        finchar = oldcode;
                } else {
                        i = db->lens[finchar = incode];
                        explen -= i;
                        if (explen < 0) {
                                db->undershoot -= explen;
                                db->in_count += wptr-wptr0;
                                dmsg1->b_wptr = wptr;
                                CK_WPTR(dmsg1);
                                explen = MAX(64,i);
                                bp = allocb(explen, BPRI_HI);
                                if (!bp) {
                                        freemsg(dmsg);
                                        freemsg(cmsg);
                                        return 0;
                                }
                                dmsg1->b_cont = bp;
                                dmsg1 = bp;
                                wptr0 = wptr = dmsg1->b_wptr;
                                explen = dmsg1->b_datap->db_lim-wptr-i;
                        }
                        p = (wptr += i);
                }

                /* decode code and install in decompressed buffer */
                while (finchar > LAST) {
                        dictp = &db->dict[db->dict[finchar].cptr];
                        *--p = dictp->f.hs.suffix;
                        finchar = dictp->f.hs.prefix;
                }
                *--p = finchar;




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RFC 1977                    PPP BSD Compress                 August 1996


                /* If not first code in a packet, and
                 * if not out of code space, then allocate a new code.
                 *
                 * Keep the hash table correct so it can be used
                 * with uncompressed packets.
                 */
                if (oldcode != CLEAR
                    && max_ent < db->maxmaxcode) {
                        struct bsd_dict *dictp2;
                        __uint32_t fcode;
                        int hval, disp;

                        fcode = BSD_KEY(oldcode,finchar);
                        hval = BSD_HASH(oldcode,finchar,db->hshift);
                        dictp = &db->dict[hval];
                        /* look for a free hash table entry */
                        if (dictp->codem1 < max_ent) {
                                disp = (hval == 0) ? 1 : hval;
                                do {
                                        hval += disp;
                                        if (hval >= db->hsize)
                                                hval -= db->hsize;
                                        dictp = &db->dict[hval];
                                } while (dictp->codem1 < max_ent);
                        }

                        /* Invalidate previous hash table entry
                         * assigned this code, and then take it over
                         */
                        dictp2 = &db->dict[max_ent+1];
                        if (db->dict[dictp2->cptr].codem1 == max_ent) {
                                db->dict[dictp2->cptr].codem1=BADCODEM1;
                        }
                        dictp2->cptr = hval;
                        dictp->codem1 = max_ent;
                        dictp->f.fcode = fcode;

                        db->max_ent = ++max_ent;
                        db->lens[max_ent] = db->lens[oldcode]+1;

                        /* Expand code size if needed.
                         */
                        if (max_ent >= MAXCODE(n_bits)
                            && max_ent < db->maxmaxcode) {
                                db->n_bits = ++n_bits;
                                tgtbitno = 32-n_bits;
                        }
                }



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RFC 1977                    PPP BSD Compress                 August 1996


                oldcode = incode;
        }

        db->in_count += wptr-wptr0;
        dmsg1->b_wptr = wptr;
        CK_WPTR(dmsg1);

        db->overshoot += explen;

        /* Keep the checkpoint right so that incompressible packets
         * clear the dictionary at the right times.
         */
        if (pf_bsd_check(db)
            && db->debug) {
                printf("bsd_decomp%d: peer should have "
                       "cleared dictionary\n", db->unit);
        }

        return dmsg;
}

Security Considerations

   Security issues are not discussed in this memo.

References

   [1]   Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51,
         RFC 1661, July 1994.

   [2]   Rand, D., "The PPP Compression Control Protocol (CCP)", RFC
         1962, June 1996.

   [3]   Simpson, W., "PPP LCP Extensions", RFC 1570, January 1994.

   [4]   Simpson, W., "PPP in HDLC-like Framing", STD 51, RFC 1662,
         July 1994.

Acknowledgments

   William Simpson provided and supported the very valuable idea of not
   using any additional header bytes for incompressible packets.









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RFC 1977                    PPP BSD Compress                 August 1996


Chair's Address

   The working group can be contacted via the current chair:

   Karl Fox
   Ascend Communications
   3518 Riverside Drive, Suite 101
   Columbus, Ohio 43221

   EMail: [e-mail]

Author's Address

   Questions about this memo can also be directed to:

   Vernon Schryver
   2482 Lee Hill Drive
   Boulder, Colorado 80302

   EMail: [e-mail]































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