bsd/net/slcompress.c

   1 /*
   2  * Copyright (c) 2000 Apple Computer, Inc. All rights reserved.
   3  *
   4  * @APPLE_LICENSE_HEADER_START@
   5  *
   6  * The contents of this file constitute Original Code as defined in and
   7  * are subject to the Apple Public Source License Version 1.1 (the
   8  * "License").  You may not use this file except in compliance with the
   9  * License.  Please obtain a copy of the License at
  10  * http://www.apple.com/publicsource and read it before using this file.
  11  *
  12  * This Original Code and all software distributed under the License are
  13  * distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY KIND, EITHER
  14  * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
  15  * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
  16  * FITNESS FOR A PARTICULAR PURPOSE OR NON-INFRINGEMENT.  Please see the
  17  * License for the specific language governing rights and limitations
  18  * under the License.
  19  *
  20  * @APPLE_LICENSE_HEADER_END@
  21  */
  22 /*-
  23  * Copyright (c) 1989, 1993, 1994
  24  *      The Regents of the University of California.  All rights reserved.
  25  *
  26  * Redistribution and use in source and binary forms, with or without
  27  * modification, are permitted provided that the following conditions
  28  * are met:
  29  * 1. Redistributions of source code must retain the above copyright
  30  *    notice, this list of conditions and the following disclaimer.
  31  * 2. Redistributions in binary form must reproduce the above copyright
  32  *    notice, this list of conditions and the following disclaimer in the
  33  *    documentation and/or other materials provided with the distribution.
  34  * 3. All advertising materials mentioning features or use of this software
  35  *    must display the following acknowledgement:
  36  *      This product includes software developed by the University of
  37  *      California, Berkeley and its contributors.
  38  * 4. Neither the name of the University nor the names of its contributors
  39  *    may be used to endorse or promote products derived from this software
  40  *    without specific prior written permission.
  41  *
  42  * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
  43  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  44  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  45  * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
  46  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  47  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  48  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  49  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  50  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  51  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  52  * SUCH DAMAGE.
  53  *
  54  *      @(#)slcompress.c        8.2 (Berkeley) 4/16/94
  55  */
  56
  57 /*
  58  * Routines to compress and uncompess tcp packets (for transmission
  59  * over low speed serial lines.
  60  *
  61  * Van Jacobson (van@helios.ee.lbl.gov), Dec 31, 1989:
  62  *      - Initial distribution.
  63  *
  64  */
  65
  66 #include <sys/param.h>
  67 #include <sys/mbuf.h>
  68 #include <sys/systm.h>
  69
  70 #include <netinet/in.h>
  71 #include <netinet/in_systm.h>
  72 #include <netinet/ip.h>
  73 #include <netinet/tcp.h>
  74
  75 #include <net/slcompress.h>
  76
  77 #ifndef SL_NO_STATS
  78 #define INCR(counter) ++comp->counter;
  79 #else
  80 #define INCR(counter)
  81 #endif
  82
  83 #define BCMP(p1, p2, n) bcmp((char *)(p1), (char *)(p2), (int)(n))
  84 #define BCOPY(p1, p2, n) bcopy((char *)(p1), (char *)(p2), (int)(n))
  85 #ifndef KERNEL
  86 #define ovbcopy bcopy
  87 #endif
  88
  89 void
  90 sl_compress_init(comp, max_state)
  91         struct slcompress *comp;
  92         int max_state;
  93 {
  94         register u_int i;
  95         register struct cstate *tstate = comp->tstate;
  96
  97         if (max_state == -1) {
  98                 max_state = MAX_STATES - 1;
  99                 bzero((char *)comp, sizeof(*comp));
 100         } else {
 101                 /* Don't reset statistics */
 102                 bzero((char *)comp->tstate, sizeof(comp->tstate));
 103                 bzero((char *)comp->rstate, sizeof(comp->rstate));
 104         }
 105         for (i = max_state; i > 0; --i) {
 106                 tstate[i].cs_id = i;
 107                 tstate[i].cs_next = &tstate[i - 1];
 108         }
 109         tstate[0].cs_next = &tstate[max_state];
 110         tstate[0].cs_id = 0;
 111         comp->last_cs = &tstate[0];
 112         comp->last_recv = 255;
 113         comp->last_xmit = 255;
 114         comp->flags = SLF_TOSS;
 115 }
 116
 117
 118 /* ENCODE encodes a number that is known to be non-zero.  ENCODEZ
 119  * checks for zero (since zero has to be encoded in the long, 3 byte
 120  * form).
 121  */
 122 #define ENCODE(n) { \
 123         if ((u_int16_t)(n) >= 256) { \
 124                 *cp++ = 0; \
 125                 cp[1] = (n); \
 126                 cp[0] = (n) >> 8; \
 127                 cp += 2; \
 128         } else { \
 129                 *cp++ = (n); \
 130         } \
 131 }
 132 #define ENCODEZ(n) { \
 133         if ((u_int16_t)(n) >= 256 || (u_int16_t)(n) == 0) { \
 134                 *cp++ = 0; \
 135                 cp[1] = (n); \
 136                 cp[0] = (n) >> 8; \
 137                 cp += 2; \
 138         } else { \
 139                 *cp++ = (n); \
 140         } \
 141 }
 142
 143 #define DECODEL(f) { \
 144         if (*cp == 0) {\
 145                 (f) = htonl(ntohl(f) + ((cp[1] << 8) | cp[2])); \
 146                 cp += 3; \
 147         } else { \
 148                 (f) = htonl(ntohl(f) + (u_int32_t)*cp++); \
 149         } \
 150 }
 151
 152 #define DECODES(f) { \
 153         if (*cp == 0) {\
 154                 (f) = htons(ntohs(f) + ((cp[1] << 8) | cp[2])); \
 155                 cp += 3; \
 156         } else { \
 157                 (f) = htons(ntohs(f) + (u_int32_t)*cp++); \
 158         } \
 159 }
 160
 161 #define DECODEU(f) { \
 162         if (*cp == 0) {\
 163                 (f) = htons((cp[1] << 8) | cp[2]); \
 164                 cp += 3; \
 165         } else { \
 166                 (f) = htons((u_int32_t)*cp++); \
 167         } \
 168 }
 169
 170 u_int
 171 sl_compress_tcp(m, ip, comp, compress_cid)
 172         struct mbuf *m;
 173         register struct ip *ip;
 174         struct slcompress *comp;
 175         int compress_cid;
 176 {
 177         register struct cstate *cs = comp->last_cs->cs_next;
 178         register u_int hlen = ip->ip_hl;
 179         register struct tcphdr *oth;
 180         register struct tcphdr *th;
 181         register u_int deltaS, deltaA;
 182         register u_int changes = 0;
 183         u_char new_seq[16];
 184         register u_char *cp = new_seq;
 185
 186         /*
 187          * Bail if this is an IP fragment or if the TCP packet isn't
 188          * `compressible' (i.e., ACK isn't set or some other control bit is
 189          * set).  (We assume that the caller has already made sure the
 190          * packet is IP proto TCP).
 191          */
 192         if ((ip->ip_off & htons(0x3fff)) || m->m_len < 40)
 193                 return (TYPE_IP);
 194
 195         th = (struct tcphdr *)&((int32_t *)ip)[hlen];
 196         if ((th->th_flags & (TH_SYN|TH_FIN|TH_RST|TH_ACK)) != TH_ACK)
 197                 return (TYPE_IP);
 198         /*
 199          * Packet is compressible -- we're going to send either a
 200          * COMPRESSED_TCP or UNCOMPRESSED_TCP packet.  Either way we need
 201          * to locate (or create) the connection state.  Special case the
 202          * most recently used connection since it's most likely to be used
 203          * again & we don't have to do any reordering if it's used.
 204          */
 205         INCR(sls_packets)
 206         if (ip->ip_src.s_addr != cs->cs_ip.ip_src.s_addr ||
 207             ip->ip_dst.s_addr != cs->cs_ip.ip_dst.s_addr ||
 208             *(int32_t *)th != ((int32_t *)&cs->cs_ip)[cs->cs_ip.ip_hl]) {
 209                 /*
 210                  * Wasn't the first -- search for it.
 211                  *
 212                  * States are kept in a circularly linked list with
 213                  * last_cs pointing to the end of the list.  The
 214                  * list is kept in lru order by moving a state to the
 215                  * head of the list whenever it is referenced.  Since
 216                  * the list is short and, empirically, the connection
 217                  * we want is almost always near the front, we locate
 218                  * states via linear search.  If we don't find a state
 219                  * for the datagram, the oldest state is (re-)used.
 220                  */
 221                 register struct cstate *lcs;
 222                 register struct cstate *lastcs = comp->last_cs;
 223
 224                 do {
 225                         lcs = cs; cs = cs->cs_next;
 226                         INCR(sls_searches)
 227                         if (ip->ip_src.s_addr == cs->cs_ip.ip_src.s_addr
 228                             && ip->ip_dst.s_addr == cs->cs_ip.ip_dst.s_addr
 229                             && *(int32_t *)th ==
 230                             ((int32_t *)&cs->cs_ip)[cs->cs_ip.ip_hl])
 231                                 goto found;
 232                 } while (cs != lastcs);
 233
 234                 /*
 235                  * Didn't find it -- re-use oldest cstate.  Send an
 236                  * uncompressed packet that tells the other side what
 237                  * connection number we're using for this conversation.
 238                  * Note that since the state list is circular, the oldest
 239                  * state points to the newest and we only need to set
 240                  * last_cs to update the lru linkage.
 241                  */
 242                 INCR(sls_misses)
 243                 comp->last_cs = lcs;
 244                 hlen += th->th_off;
 245                 hlen <<= 2;
 246                 if (hlen > m->m_len)
 247                     return TYPE_IP;
 248                 goto uncompressed;
 249
 250         found:
 251                 /*
 252                  * Found it -- move to the front on the connection list.
 253                  */
 254                 if (cs == lastcs)
 255                         comp->last_cs = lcs;
 256                 else {
 257                         lcs->cs_next = cs->cs_next;
 258                         cs->cs_next = lastcs->cs_next;
 259                         lastcs->cs_next = cs;
 260                 }
 261         }
 262
 263         /*
 264          * Make sure that only what we expect to change changed. The first
 265          * line of the `if' checks the IP protocol version, header length &
 266          * type of service.  The 2nd line checks the "Don't fragment" bit.
 267          * The 3rd line checks the time-to-live and protocol (the protocol
 268          * check is unnecessary but costless).  The 4th line checks the TCP
 269          * header length.  The 5th line checks IP options, if any.  The 6th
 270          * line checks TCP options, if any.  If any of these things are
 271          * different between the previous & current datagram, we send the
 272          * current datagram `uncompressed'.
 273          */
 274         oth = (struct tcphdr *)&((int32_t *)&cs->cs_ip)[hlen];
 275         deltaS = hlen;
 276         hlen += th->th_off;
 277         hlen <<= 2;
 278         if (hlen > m->m_len)
 279             return TYPE_IP;
 280
 281         if (((u_int16_t *)ip)[0] != ((u_int16_t *)&cs->cs_ip)[0] ||
 282             ((u_int16_t *)ip)[3] != ((u_int16_t *)&cs->cs_ip)[3] ||
 283             ((u_int16_t *)ip)[4] != ((u_int16_t *)&cs->cs_ip)[4] ||
 284             th->th_off != oth->th_off ||
 285             (deltaS > 5 &&
 286              BCMP(ip + 1, &cs->cs_ip + 1, (deltaS - 5) << 2)) ||
 287             (th->th_off > 5 &&
 288              BCMP(th + 1, oth + 1, (th->th_off - 5) << 2)))
 289                 goto uncompressed;
 290
 291         /*
 292          * Figure out which of the changing fields changed.  The
 293          * receiver expects changes in the order: urgent, window,
 294          * ack, seq (the order minimizes the number of temporaries
 295          * needed in this section of code).
 296          */
 297         if (th->th_flags & TH_URG) {
 298                 deltaS = ntohs(th->th_urp);
 299                 ENCODEZ(deltaS);
 300                 changes |= NEW_U;
 301         } else if (th->th_urp != oth->th_urp)
 302                 /* argh! URG not set but urp changed -- a sensible
 303                  * implementation should never do this but RFC793
 304                  * doesn't prohibit the change so we have to deal
 305                  * with it. */
 306                  goto uncompressed;
 307
 308         deltaS = (u_int16_t)(ntohs(th->th_win) - ntohs(oth->th_win));
 309         if (deltaS) {
 310                 ENCODE(deltaS);
 311                 changes |= NEW_W;
 312         }
 313
 314         deltaA = ntohl(th->th_ack) - ntohl(oth->th_ack);
 315         if (deltaA) {
 316                 if (deltaA > 0xffff)
 317                         goto uncompressed;
 318                 ENCODE(deltaA);
 319                 changes |= NEW_A;
 320         }
 321
 322         deltaS = ntohl(th->th_seq) - ntohl(oth->th_seq);
 323         if (deltaS) {
 324                 if (deltaS > 0xffff)
 325                         goto uncompressed;
 326                 ENCODE(deltaS);
 327                 changes |= NEW_S;
 328         }
 329
 330         switch(changes) {
 331
 332         case 0:
 333                 /*
 334                  * Nothing changed. If this packet contains data and the
 335                  * last one didn't, this is probably a data packet following
 336                  * an ack (normal on an interactive connection) and we send
 337                  * it compressed.  Otherwise it's probably a retransmit,
 338                  * retransmitted ack or window probe.  Send it uncompressed
 339                  * in case the other side missed the compressed version.
 340                  */
 341                 if (ip->ip_len != cs->cs_ip.ip_len &&
 342                     ntohs(cs->cs_ip.ip_len) == hlen)
 343                         break;
 344
 345                 /* (fall through) */
 346
 347         case SPECIAL_I:
 348         case SPECIAL_D:
 349                 /*
 350                  * actual changes match one of our special case encodings --
 351                  * send packet uncompressed.
 352                  */
 353                 goto uncompressed;
 354
 355         case NEW_S|NEW_A:
 356                 if (deltaS == deltaA &&
 357                     deltaS == ntohs(cs->cs_ip.ip_len) - hlen) {
 358                         /* special case for echoed terminal traffic */
 359                         changes = SPECIAL_I;
 360                         cp = new_seq;
 361                 }
 362                 break;
 363
 364         case NEW_S:
 365                 if (deltaS == ntohs(cs->cs_ip.ip_len) - hlen) {
 366                         /* special case for data xfer */
 367                         changes = SPECIAL_D;
 368                         cp = new_seq;
 369                 }
 370                 break;
 371         }
 372
 373         deltaS = ntohs(ip->ip_id) - ntohs(cs->cs_ip.ip_id);
 374         if (deltaS != 1) {
 375                 ENCODEZ(deltaS);
 376                 changes |= NEW_I;
 377         }
 378         if (th->th_flags & TH_PUSH)
 379                 changes |= TCP_PUSH_BIT;
 380         /*
 381          * Grab the cksum before we overwrite it below.  Then update our
 382          * state with this packet's header.
 383          */
 384         deltaA = ntohs(th->th_sum);
 385         BCOPY(ip, &cs->cs_ip, hlen);
 386
 387         /*
 388          * We want to use the original packet as our compressed packet.
 389          * (cp - new_seq) is the number of bytes we need for compressed
 390          * sequence numbers.  In addition we need one byte for the change
 391          * mask, one for the connection id and two for the tcp checksum.
 392          * So, (cp - new_seq) + 4 bytes of header are needed.  hlen is how
 393          * many bytes of the original packet to toss so subtract the two to
 394          * get the new packet size.
 395          */
 396         deltaS = cp - new_seq;
 397         cp = (u_char *)ip;
 398         if (compress_cid == 0 || comp->last_xmit != cs->cs_id) {
 399                 comp->last_xmit = cs->cs_id;
 400                 hlen -= deltaS + 4;
 401                 cp += hlen;
 402                 *cp++ = changes | NEW_C;
 403                 *cp++ = cs->cs_id;
 404         } else {
 405                 hlen -= deltaS + 3;
 406                 cp += hlen;
 407                 *cp++ = changes;
 408         }
 409         m->m_len -= hlen;
 410         m->m_data += hlen;
 411         *cp++ = deltaA >> 8;
 412         *cp++ = deltaA;
 413         BCOPY(new_seq, cp, deltaS);
 414         INCR(sls_compressed)
 415         return (TYPE_COMPRESSED_TCP);
 416
 417         /*
 418          * Update connection state cs & send uncompressed packet ('uncompressed'
 419          * means a regular ip/tcp packet but with the 'conversation id' we hope
 420          * to use on future compressed packets in the protocol field).
 421          */
 422 uncompressed:
 423         BCOPY(ip, &cs->cs_ip, hlen);
 424         ip->ip_p = cs->cs_id;
 425         comp->last_xmit = cs->cs_id;
 426         return (TYPE_UNCOMPRESSED_TCP);
 427 }
 428
 429
 430 int
 431 sl_uncompress_tcp(bufp, len, type, comp)
 432         u_char **bufp;
 433         int len;
 434         u_int type;
 435         struct slcompress *comp;
 436 {
 437         u_char *hdr, *cp;
 438         int hlen, vjlen;
 439
 440         cp = bufp? *bufp: NULL;
 441         vjlen = sl_uncompress_tcp_core(cp, len, len, type, comp, &hdr, &hlen);
 442         if (vjlen < 0)
 443                 return (0);     /* error */
 444         if (vjlen == 0)
 445                 return (len);   /* was uncompressed already */
 446
 447         cp += vjlen;
 448         len -= vjlen;
 449
 450         /*
 451          * At this point, cp points to the first byte of data in the
 452          * packet.  If we're not aligned on a 4-byte boundary, copy the
 453          * data down so the ip & tcp headers will be aligned.  Then back up
 454          * cp by the tcp/ip header length to make room for the reconstructed
 455          * header (we assume the packet we were handed has enough space to
 456          * prepend 128 bytes of header).
 457          */
 458         if ((intptr_t)cp & 3) {
 459                 if (len > 0)
 460                         (void) ovbcopy(cp, (caddr_t)((intptr_t)cp &~ 3), len);
 461                 cp = (u_char *)((intptr_t)cp &~ 3);
 462         }
 463         cp -= hlen;
 464         len += hlen;
 465         BCOPY(hdr, cp, hlen);
 466
 467         *bufp = cp;
 468         return (len);
 469 }
 470
 471 /*
 472  * Uncompress a packet of total length total_len.  The first buflen
 473  * bytes are at buf; this must include the entire (compressed or
 474  * uncompressed) TCP/IP header.  This procedure returns the length
 475  * of the VJ header, with a pointer to the uncompressed IP header
 476  * in *hdrp and its length in *hlenp.
 477  */
 478 int
 479 sl_uncompress_tcp_core(buf, buflen, total_len, type, comp, hdrp, hlenp)
 480         u_char *buf;
 481         int buflen, total_len;
 482         u_int type;
 483         struct slcompress *comp;
 484         u_char **hdrp;
 485         u_int *hlenp;
 486 {
 487         register u_char *cp;
 488         register u_int hlen, changes;
 489         register struct tcphdr *th;
 490         register struct cstate *cs;
 491         register struct ip *ip;
 492         register u_int16_t *bp;
 493         register u_int vjlen;
 494
 495         switch (type) {
 496
 497         case TYPE_UNCOMPRESSED_TCP:
 498                 ip = (struct ip *) buf;
 499                 if (ip->ip_p >= MAX_STATES)
 500                         goto bad;
 501                 cs = &comp->rstate[comp->last_recv = ip->ip_p];
 502                 comp->flags &=~ SLF_TOSS;
 503                 ip->ip_p = IPPROTO_TCP;
 504                 /*
 505                  * Calculate the size of the TCP/IP header and make sure that
 506                  * we don't overflow the space we have available for it.
 507                  */
 508                 hlen = ip->ip_hl << 2;
 509                 if (hlen + sizeof(struct tcphdr) > buflen)
 510                         goto bad;
 511                 hlen += ((struct tcphdr *)&((char *)ip)[hlen])->th_off << 2;
 512                 if (hlen > MAX_HDR || hlen > buflen)
 513                         goto bad;
 514                 BCOPY(ip, &cs->cs_ip, hlen);
 515                 cs->cs_hlen = hlen;
 516                 INCR(sls_uncompressedin)
 517                 *hdrp = (u_char *) &cs->cs_ip;
 518                 *hlenp = hlen;
 519                 return (0);
 520
 521         default:
 522                 goto bad;
 523
 524         case TYPE_COMPRESSED_TCP:
 525                 break;
 526         }
 527         /* We've got a compressed packet. */
 528         INCR(sls_compressedin)
 529         cp = buf;
 530         changes = *cp++;
 531         if (changes & NEW_C) {
 532                 /* Make sure the state index is in range, then grab the state.
 533                  * If we have a good state index, clear the 'discard' flag. */
 534                 if (*cp >= MAX_STATES)
 535                         goto bad;
 536
 537                 comp->flags &=~ SLF_TOSS;
 538                 comp->last_recv = *cp++;
 539         } else {
 540                 /* this packet has an implicit state index.  If we've
 541                  * had a line error since the last time we got an
 542                  * explicit state index, we have to toss the packet. */
 543                 if (comp->flags & SLF_TOSS) {
 544                         INCR(sls_tossed)
 545                         return (-1);
 546                 }
 547         }
 548         cs = &comp->rstate[comp->last_recv];
 549         hlen = cs->cs_ip.ip_hl << 2;
 550         th = (struct tcphdr *)&((u_char *)&cs->cs_ip)[hlen];
 551         th->th_sum = htons((*cp << 8) | cp[1]);
 552         cp += 2;
 553         if (changes & TCP_PUSH_BIT)
 554                 th->th_flags |= TH_PUSH;
 555         else
 556                 th->th_flags &=~ TH_PUSH;
 557
 558         switch (changes & SPECIALS_MASK) {
 559         case SPECIAL_I:
 560                 {
 561                 register u_int i = ntohs(cs->cs_ip.ip_len) - cs->cs_hlen;
 562                 th->th_ack = htonl(ntohl(th->th_ack) + i);
 563                 th->th_seq = htonl(ntohl(th->th_seq) + i);
 564                 }
 565                 break;
 566
 567         case SPECIAL_D:
 568                 th->th_seq = htonl(ntohl(th->th_seq) + ntohs(cs->cs_ip.ip_len)
 569                                    - cs->cs_hlen);
 570                 break;
 571
 572         default:
 573                 if (changes & NEW_U) {
 574                         th->th_flags |= TH_URG;
 575                         DECODEU(th->th_urp)
 576                 } else
 577                         th->th_flags &=~ TH_URG;
 578                 if (changes & NEW_W)
 579                         DECODES(th->th_win)
 580                 if (changes & NEW_A)
 581                         DECODEL(th->th_ack)
 582                 if (changes & NEW_S)
 583                         DECODEL(th->th_seq)
 584                 break;
 585         }
 586         if (changes & NEW_I) {
 587                 DECODES(cs->cs_ip.ip_id)
 588         } else
 589                 cs->cs_ip.ip_id = htons(ntohs(cs->cs_ip.ip_id) + 1);
 590
 591         /*
 592          * At this point, cp points to the first byte of data in the
 593          * packet.  Fill in the IP total length and update the IP
 594          * header checksum.
 595          */
 596         vjlen = cp - buf;
 597         buflen -= vjlen;
 598         if (buflen < 0)
 599                 /* we must have dropped some characters (crc should detect
 600                  * this but the old slip framing won't) */
 601                 goto bad;
 602
 603         total_len += cs->cs_hlen - vjlen;
 604         cs->cs_ip.ip_len = htons(total_len);
 605
 606         /* recompute the ip header checksum */
 607         bp = (u_int16_t *) &cs->cs_ip;
 608         cs->cs_ip.ip_sum = 0;
 609                 for (changes = 0; hlen > 0; hlen -= 2)
 610                         changes += *bp++;
 611                 changes = (changes & 0xffff) + (changes >> 16);
 612                 changes = (changes & 0xffff) + (changes >> 16);
 613         cs->cs_ip.ip_sum = ~ changes;
 614
 615         *hdrp = (u_char *) &cs->cs_ip;
 616         *hlenp = cs->cs_hlen;
 617         return vjlen;
 618
 619 bad:
 620         comp->flags |= SLF_TOSS;
 621         INCR(sls_errorin)
 622         return (-1);
 623 }