netbsd-1.2/slcompress.c

   1 /*      $NetBSD: slcompress.c,v 1.17 1997/05/17 21:12:10 christos Exp $   */
   2 /*      Id: slcompress.c,v 1.3 1996/05/24 07:04:47 paulus Exp   */
   3
   4 /*
   5  * Copyright (c) 1989, 1993, 1994
   6  *      The Regents of the University of California.  All rights reserved.
   7  *
   8  * Redistribution and use in source and binary forms, with or without
   9  * modification, are permitted provided that the following conditions
  10  * are met:
  11  * 1. Redistributions of source code must retain the above copyright
  12  *    notice, this list of conditions and the following disclaimer.
  13  * 2. Redistributions in binary form must reproduce the above copyright
  14  *    notice, this list of conditions and the following disclaimer in the
  15  *    documentation and/or other materials provided with the distribution.
  16  * 3. All advertising materials mentioning features or use of this software
  17  *    must display the following acknowledgement:
  18  *      This product includes software developed by the University of
  19  *      California, Berkeley and its contributors.
  20  * 4. Neither the name of the University nor the names of its contributors
  21  *    may be used to endorse or promote products derived from this software
  22  *    without specific prior written permission.
  23  *
  24  * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
  25  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  26  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  27  * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
  28  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  29  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  30  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  31  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  32  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  33  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  34  * SUCH DAMAGE.
  35  *
  36  *      @(#)slcompress.c        8.2 (Berkeley) 4/16/94
  37  */
  38
  39 /*
  40  * Routines to compress and uncompess tcp packets (for transmission
  41  * over low speed serial lines.
  42  *
  43  * Van Jacobson (van@helios.ee.lbl.gov), Dec 31, 1989:
  44  *      - Initial distribution.
  45  */
  46
  47 #include <sys/param.h>
  48 #include <sys/mbuf.h>
  49 #include <sys/systm.h>
  50
  51 #include <netinet/in.h>
  52 #include <netinet/in_systm.h>
  53 #include <netinet/ip.h>
  54 #include <netinet/tcp.h>
  55
  56 #include <net/slcompress.h>
  57
  58 #ifndef SL_NO_STATS
  59 #define INCR(counter) ++comp->counter;
  60 #else
  61 #define INCR(counter)
  62 #endif
  63
  64 #define BCMP(p1, p2, n) bcmp((char *)(p1), (char *)(p2), (int)(n))
  65 #define BCOPY(p1, p2, n) bcopy((char *)(p1), (char *)(p2), (int)(n))
  66 #ifndef _KERNEL
  67 #define ovbcopy bcopy
  68 #endif
  69
  70
  71 void
  72 sl_compress_init(comp)
  73         struct slcompress *comp;
  74 {
  75         register u_int i;
  76         register struct cstate *tstate = comp->tstate;
  77
  78         bzero((char *)comp, sizeof(*comp));
  79         for (i = MAX_STATES - 1; i > 0; --i) {
  80                 tstate[i].cs_id = i;
  81                 tstate[i].cs_next = &tstate[i - 1];
  82         }
  83         tstate[0].cs_next = &tstate[MAX_STATES - 1];
  84         tstate[0].cs_id = 0;
  85         comp->last_cs = &tstate[0];
  86         comp->last_recv = 255;
  87         comp->last_xmit = 255;
  88         comp->flags = SLF_TOSS;
  89 }
  90
  91
  92 /*
  93  * Like sl_compress_init, but we get to specify the maximum connection
  94  * ID to use on transmission.
  95  */
  96 void
  97 sl_compress_setup(comp, max_state)
  98         struct slcompress *comp;
  99         int max_state;
 100 {
 101         register u_int i;
 102         register struct cstate *tstate = comp->tstate;
 103
 104         if (max_state == -1) {
 105                 max_state = MAX_STATES - 1;
 106                 bzero((char *)comp, sizeof(*comp));
 107         } else {
 108                 /* Don't reset statistics */
 109                 bzero((char *)comp->tstate, sizeof(comp->tstate));
 110                 bzero((char *)comp->rstate, sizeof(comp->rstate));
 111         }
 112         for (i = max_state; i > 0; --i) {
 113                 tstate[i].cs_id = i;
 114                 tstate[i].cs_next = &tstate[i - 1];
 115         }
 116         tstate[0].cs_next = &tstate[max_state];
 117         tstate[0].cs_id = 0;
 118         comp->last_cs = &tstate[0];
 119         comp->last_recv = 255;
 120         comp->last_xmit = 255;
 121         comp->flags = SLF_TOSS;
 122 }
 123
 124
 125 /* ENCODE encodes a number that is known to be non-zero.  ENCODEZ
 126  * checks for zero (since zero has to be encoded in the long, 3 byte
 127  * form).
 128  */
 129 #define ENCODE(n) { \
 130         if ((u_int16_t)(n) >= 256) { \
 131                 *cp++ = 0; \
 132                 cp[1] = (n); \
 133                 cp[0] = (n) >> 8; \
 134                 cp += 2; \
 135         } else { \
 136                 *cp++ = (n); \
 137         } \
 138 }
 139 #define ENCODEZ(n) { \
 140         if ((u_int16_t)(n) >= 256 || (u_int16_t)(n) == 0) { \
 141                 *cp++ = 0; \
 142                 cp[1] = (n); \
 143                 cp[0] = (n) >> 8; \
 144                 cp += 2; \
 145         } else { \
 146                 *cp++ = (n); \
 147         } \
 148 }
 149
 150 #define DECODEL(f) { \
 151         if (*cp == 0) {\
 152                 (f) = htonl(ntohl(f) + ((cp[1] << 8) | cp[2])); \
 153                 cp += 3; \
 154         } else { \
 155                 (f) = htonl(ntohl(f) + (u_int32_t)*cp++); \
 156         } \
 157 }
 158
 159 #define DECODES(f) { \
 160         if (*cp == 0) {\
 161                 (f) = htons(ntohs(f) + ((cp[1] << 8) | cp[2])); \
 162                 cp += 3; \
 163         } else { \
 164                 (f) = htons(ntohs(f) + (u_int32_t)*cp++); \
 165         } \
 166 }
 167
 168 #define DECODEU(f) { \
 169         if (*cp == 0) {\
 170                 (f) = htons((cp[1] << 8) | cp[2]); \
 171                 cp += 3; \
 172         } else { \
 173                 (f) = htons((u_int32_t)*cp++); \
 174         } \
 175 }
 176
 177 u_int
 178 sl_compress_tcp(m, ip, comp, compress_cid)
 179         struct mbuf *m;
 180         register struct ip *ip;
 181         struct slcompress *comp;
 182         int compress_cid;
 183 {
 184         register struct cstate *cs = comp->last_cs->cs_next;
 185         register u_int hlen = ip->ip_hl;
 186         register struct tcphdr *oth;
 187         register struct tcphdr *th;
 188         register u_int deltaS, deltaA;
 189         register u_int changes = 0;
 190         u_char new_seq[16];
 191         register u_char *cp = new_seq;
 192
 193         /*
 194          * Bail if this is an IP fragment or if the TCP packet isn't
 195          * `compressible' (i.e., ACK isn't set or some other control bit is
 196          * set).  (We assume that the caller has already made sure the
 197          * packet is IP proto TCP).
 198          */
 199         if ((ip->ip_off & htons(0x3fff)) || m->m_len < 40)
 200                 return (TYPE_IP);
 201
 202         th = (struct tcphdr *)&((int32_t *)ip)[hlen];
 203         if ((th->th_flags & (TH_SYN|TH_FIN|TH_RST|TH_ACK)) != TH_ACK)
 204                 return (TYPE_IP);
 205         /*
 206          * Packet is compressible -- we're going to send either a
 207          * COMPRESSED_TCP or UNCOMPRESSED_TCP packet.  Either way we need
 208          * to locate (or create) the connection state.  Special case the
 209          * most recently used connection since it's most likely to be used
 210          * again & we don't have to do any reordering if it's used.
 211          */
 212         INCR(sls_packets)
 213         if (ip->ip_src.s_addr != cs->cs_ip.ip_src.s_addr ||
 214             ip->ip_dst.s_addr != cs->cs_ip.ip_dst.s_addr ||
 215             *(int32_t *)th != ((int32_t *)&cs->cs_ip)[cs->cs_ip.ip_hl]) {
 216                 /*
 217                  * Wasn't the first -- search for it.
 218                  *
 219                  * States are kept in a circularly linked list with
 220                  * last_cs pointing to the end of the list.  The
 221                  * list is kept in lru order by moving a state to the
 222                  * head of the list whenever it is referenced.  Since
 223                  * the list is short and, empirically, the connection
 224                  * we want is almost always near the front, we locate
 225                  * states via linear search.  If we don't find a state
 226                  * for the datagram, the oldest state is (re-)used.
 227                  */
 228                 register struct cstate *lcs;
 229                 register struct cstate *lastcs = comp->last_cs;
 230
 231                 do {
 232                         lcs = cs; cs = cs->cs_next;
 233                         INCR(sls_searches)
 234                         if (ip->ip_src.s_addr == cs->cs_ip.ip_src.s_addr
 235                             && ip->ip_dst.s_addr == cs->cs_ip.ip_dst.s_addr
 236                             && *(int32_t *)th ==
 237                             ((int32_t *)&cs->cs_ip)[cs->cs_ip.ip_hl])
 238                                 goto found;
 239                 } while (cs != lastcs);
 240
 241                 /*
 242                  * Didn't find it -- re-use oldest cstate.  Send an
 243                  * uncompressed packet that tells the other side what
 244                  * connection number we're using for this conversation.
 245                  * Note that since the state list is circular, the oldest
 246                  * state points to the newest and we only need to set
 247                  * last_cs to update the lru linkage.
 248                  */
 249                 INCR(sls_misses)
 250                 comp->last_cs = lcs;
 251                 hlen += th->th_off;
 252                 hlen <<= 2;
 253                 goto uncompressed;
 254
 255         found:
 256                 /*
 257                  * Found it -- move to the front on the connection list.
 258                  */
 259                 if (cs == lastcs)
 260                         comp->last_cs = lcs;
 261                 else {
 262                         lcs->cs_next = cs->cs_next;
 263                         cs->cs_next = lastcs->cs_next;
 264                         lastcs->cs_next = cs;
 265                 }
 266         }
 267
 268         /*
 269          * Make sure that only what we expect to change changed. The first
 270          * line of the `if' checks the IP protocol version, header length &
 271          * type of service.  The 2nd line checks the "Don't fragment" bit.
 272          * The 3rd line checks the time-to-live and protocol (the protocol
 273          * check is unnecessary but costless).  The 4th line checks the TCP
 274          * header length.  The 5th line checks IP options, if any.  The 6th
 275          * line checks TCP options, if any.  If any of these things are
 276          * different between the previous & current datagram, we send the
 277          * current datagram `uncompressed'.
 278          */
 279         oth = (struct tcphdr *)&((int32_t *)&cs->cs_ip)[hlen];
 280         deltaS = hlen;
 281         hlen += th->th_off;
 282         hlen <<= 2;
 283
 284         if (((u_int16_t *)ip)[0] != ((u_int16_t *)&cs->cs_ip)[0] ||
 285             ((u_int16_t *)ip)[3] != ((u_int16_t *)&cs->cs_ip)[3] ||
 286             ((u_int16_t *)ip)[4] != ((u_int16_t *)&cs->cs_ip)[4] ||
 287             th->th_off != oth->th_off ||
 288             (deltaS > 5 &&
 289              BCMP(ip + 1, &cs->cs_ip + 1, (deltaS - 5) << 2)) ||
 290             (th->th_off > 5 &&
 291              BCMP(th + 1, oth + 1, (th->th_off - 5) << 2)))
 292                 goto uncompressed;
 293
 294         /*
 295          * Figure out which of the changing fields changed.  The
 296          * receiver expects changes in the order: urgent, window,
 297          * ack, seq (the order minimizes the number of temporaries
 298          * needed in this section of code).
 299          */
 300         if (th->th_flags & TH_URG) {
 301                 deltaS = ntohs(th->th_urp);
 302                 ENCODEZ(deltaS);
 303                 changes |= NEW_U;
 304         } else if (th->th_urp != oth->th_urp)
 305                 /* argh! URG not set but urp changed -- a sensible
 306                  * implementation should never do this but RFC793
 307                  * doesn't prohibit the change so we have to deal
 308                  * with it. */
 309                  goto uncompressed;
 310
 311         deltaS = (u_int16_t)(ntohs(th->th_win) - ntohs(oth->th_win));
 312         if (deltaS) {
 313                 ENCODE(deltaS);
 314                 changes |= NEW_W;
 315         }
 316
 317         deltaA = ntohl(th->th_ack) - ntohl(oth->th_ack);
 318         if (deltaA) {
 319                 if (deltaA > 0xffff)
 320                         goto uncompressed;
 321                 ENCODE(deltaA);
 322                 changes |= NEW_A;
 323         }
 324
 325         deltaS = ntohl(th->th_seq) - ntohl(oth->th_seq);
 326         if (deltaS) {
 327                 if (deltaS > 0xffff)
 328                         goto uncompressed;
 329                 ENCODE(deltaS);
 330                 changes |= NEW_S;
 331         }
 332
 333         switch(changes) {
 334
 335         case 0:
 336                 /*
 337                  * Nothing changed. If this packet contains data and the
 338                  * last one didn't, this is probably a data packet following
 339                  * an ack (normal on an interactive connection) and we send
 340                  * it compressed.  Otherwise it's probably a retransmit,
 341                  * retransmitted ack or window probe.  Send it uncompressed
 342                  * in case the other side missed the compressed version.
 343                  */
 344                 if (ip->ip_len != cs->cs_ip.ip_len &&
 345                     ntohs(cs->cs_ip.ip_len) == hlen)
 346                         break;
 347
 348                 /* (fall through) */
 349
 350         case SPECIAL_I:
 351         case SPECIAL_D:
 352                 /*
 353                  * actual changes match one of our special case encodings --
 354                  * send packet uncompressed.
 355                  */
 356                 goto uncompressed;
 357
 358         case NEW_S|NEW_A:
 359                 if (deltaS == deltaA &&
 360                     deltaS == ntohs(cs->cs_ip.ip_len) - hlen) {
 361                         /* special case for echoed terminal traffic */
 362                         changes = SPECIAL_I;
 363                         cp = new_seq;
 364                 }
 365                 break;
 366
 367         case NEW_S:
 368                 if (deltaS == ntohs(cs->cs_ip.ip_len) - hlen) {
 369                         /* special case for data xfer */
 370                         changes = SPECIAL_D;
 371                         cp = new_seq;
 372                 }
 373                 break;
 374         }
 375
 376         deltaS = ntohs(ip->ip_id) - ntohs(cs->cs_ip.ip_id);
 377         if (deltaS != 1) {
 378                 ENCODEZ(deltaS);
 379                 changes |= NEW_I;
 380         }
 381         if (th->th_flags & TH_PUSH)
 382                 changes |= TCP_PUSH_BIT;
 383         /*
 384          * Grab the cksum before we overwrite it below.  Then update our
 385          * state with this packet's header.
 386          */
 387         deltaA = ntohs(th->th_sum);
 388         BCOPY(ip, &cs->cs_ip, hlen);
 389
 390         /*
 391          * We want to use the original packet as our compressed packet.
 392          * (cp - new_seq) is the number of bytes we need for compressed
 393          * sequence numbers.  In addition we need one byte for the change
 394          * mask, one for the connection id and two for the tcp checksum.
 395          * So, (cp - new_seq) + 4 bytes of header are needed.  hlen is how
 396          * many bytes of the original packet to toss so subtract the two to
 397          * get the new packet size.
 398          */
 399         deltaS = cp - new_seq;
 400         cp = (u_char *)ip;
 401         if (compress_cid == 0 || comp->last_xmit != cs->cs_id) {
 402                 comp->last_xmit = cs->cs_id;
 403                 hlen -= deltaS + 4;
 404                 cp += hlen;
 405                 *cp++ = changes | NEW_C;
 406                 *cp++ = cs->cs_id;
 407         } else {
 408                 hlen -= deltaS + 3;
 409                 cp += hlen;
 410                 *cp++ = changes;
 411         }
 412         m->m_len -= hlen;
 413         m->m_data += hlen;
 414         *cp++ = deltaA >> 8;
 415         *cp++ = deltaA;
 416         BCOPY(new_seq, cp, deltaS);
 417         INCR(sls_compressed)
 418         return (TYPE_COMPRESSED_TCP);
 419
 420         /*
 421          * Update connection state cs & send uncompressed packet ('uncompressed'
 422          * means a regular ip/tcp packet but with the 'conversation id' we hope
 423          * to use on future compressed packets in the protocol field).
 424          */
 425 uncompressed:
 426         BCOPY(ip, &cs->cs_ip, hlen);
 427         ip->ip_p = cs->cs_id;
 428         comp->last_xmit = cs->cs_id;
 429         return (TYPE_UNCOMPRESSED_TCP);
 430 }
 431
 432
 433 int
 434 sl_uncompress_tcp(bufp, len, type, comp)
 435         u_char **bufp;
 436         int len;
 437         u_int type;
 438         struct slcompress *comp;
 439 {
 440         u_char *hdr, *cp;
 441         int hlen, vjlen;
 442
 443         cp = bufp? *bufp: NULL;
 444         vjlen = sl_uncompress_tcp_core(cp, len, len, type, comp, &hdr, &hlen);
 445         if (vjlen < 0)
 446                 return (0);     /* error */
 447         if (vjlen == 0)
 448                 return (len);   /* was uncompressed already */
 449
 450         cp += vjlen;
 451         len -= vjlen;
 452
 453         /*
 454          * At this point, cp points to the first byte of data in the
 455          * packet.  If we're not aligned on a 4-byte boundary, copy the
 456          * data down so the ip & tcp headers will be aligned.  Then back up
 457          * cp by the tcp/ip header length to make room for the reconstructed
 458          * header (we assume the packet we were handed has enough space to
 459          * prepend 128 bytes of header).
 460          */
 461         if ((long)cp & 3) {
 462                 if (len > 0)
 463                         (void) ovbcopy(cp, (caddr_t)((long)cp &~ 3), len);
 464                 cp = (u_char *)((long)cp &~ 3);
 465         }
 466         cp -= hlen;
 467         len += hlen;
 468         BCOPY(hdr, cp, hlen);
 469
 470         *bufp = cp;
 471         return (len);
 472 }
 473
 474 /*
 475  * Uncompress a packet of total length total_len.  The first buflen
 476  * bytes are at buf; this must include the entire (compressed or
 477  * uncompressed) TCP/IP header.  This procedure returns the length
 478  * of the VJ header, with a pointer to the uncompressed IP header
 479  * in *hdrp and its length in *hlenp.
 480  */
 481 int
 482 sl_uncompress_tcp_core(buf, buflen, total_len, type, comp, hdrp, hlenp)
 483         u_char *buf;
 484         int buflen, total_len;
 485         u_int type;
 486         struct slcompress *comp;
 487         u_char **hdrp;
 488         u_int *hlenp;
 489 {
 490         register u_char *cp;
 491         register u_int hlen, changes;
 492         register struct tcphdr *th;
 493         register struct cstate *cs;
 494         register struct ip *ip;
 495         register u_int16_t *bp;
 496         register u_int vjlen;
 497
 498         switch (type) {
 499
 500         case TYPE_UNCOMPRESSED_TCP:
 501                 ip = (struct ip *) buf;
 502                 if (ip->ip_p >= MAX_STATES)
 503                         goto bad;
 504                 cs = &comp->rstate[comp->last_recv = ip->ip_p];
 505                 comp->flags &=~ SLF_TOSS;
 506                 ip->ip_p = IPPROTO_TCP;
 507                 /*
 508                  * Calculate the size of the TCP/IP header and make sure that
 509                  * we don't overflow the space we have available for it.
 510                  */
 511                 hlen = ip->ip_hl << 2;
 512                 if (hlen + sizeof(struct tcphdr) > buflen)
 513                         goto bad;
 514                 hlen += ((struct tcphdr *)&((char *)ip)[hlen])->th_off << 2;
 515                 if (hlen > MAX_HDR || hlen > buflen)
 516                         goto bad;
 517                 BCOPY(ip, &cs->cs_ip, hlen);
 518                 cs->cs_hlen = hlen;
 519                 INCR(sls_uncompressedin)
 520                 *hdrp = (u_char *) &cs->cs_ip;
 521                 *hlenp = hlen;
 522                 return (0);
 523
 524         default:
 525                 goto bad;
 526
 527         case TYPE_COMPRESSED_TCP:
 528                 break;
 529         }
 530         /* We've got a compressed packet. */
 531         INCR(sls_compressedin)
 532         cp = buf;
 533         changes = *cp++;
 534         if (changes & NEW_C) {
 535                 /* Make sure the state index is in range, then grab the state.
 536                  * If we have a good state index, clear the 'discard' flag. */
 537                 if (*cp >= MAX_STATES)
 538                         goto bad;
 539
 540                 comp->flags &=~ SLF_TOSS;
 541                 comp->last_recv = *cp++;
 542         } else {
 543                 /* this packet has an implicit state index.  If we've
 544                  * had a line error since the last time we got an
 545                  * explicit state index, we have to toss the packet. */
 546                 if (comp->flags & SLF_TOSS) {
 547                         INCR(sls_tossed)
 548                         return (-1);
 549                 }
 550         }
 551         cs = &comp->rstate[comp->last_recv];
 552         hlen = cs->cs_ip.ip_hl << 2;
 553         th = (struct tcphdr *)&((u_char *)&cs->cs_ip)[hlen];
 554         th->th_sum = htons((*cp << 8) | cp[1]);
 555         cp += 2;
 556         if (changes & TCP_PUSH_BIT)
 557                 th->th_flags |= TH_PUSH;
 558         else
 559                 th->th_flags &=~ TH_PUSH;
 560
 561         switch (changes & SPECIALS_MASK) {
 562         case SPECIAL_I:
 563                 {
 564                 register u_int i = ntohs(cs->cs_ip.ip_len) - cs->cs_hlen;
 565                 th->th_ack = htonl(ntohl(th->th_ack) + i);
 566                 th->th_seq = htonl(ntohl(th->th_seq) + i);
 567                 }
 568                 break;
 569
 570         case SPECIAL_D:
 571                 th->th_seq = htonl(ntohl(th->th_seq) + ntohs(cs->cs_ip.ip_len)
 572                                    - cs->cs_hlen);
 573                 break;
 574
 575         default:
 576                 if (changes & NEW_U) {
 577                         th->th_flags |= TH_URG;
 578                         DECODEU(th->th_urp)
 579                 } else
 580                         th->th_flags &=~ TH_URG;
 581                 if (changes & NEW_W)
 582                         DECODES(th->th_win)
 583                 if (changes & NEW_A)
 584                         DECODEL(th->th_ack)
 585                 if (changes & NEW_S)
 586                         DECODEL(th->th_seq)
 587                 break;
 588         }
 589         if (changes & NEW_I) {
 590                 DECODES(cs->cs_ip.ip_id)
 591         } else
 592                 cs->cs_ip.ip_id = htons(ntohs(cs->cs_ip.ip_id) + 1);
 593
 594         /*
 595          * At this point, cp points to the first byte of data in the
 596          * packet.  Fill in the IP total length and update the IP
 597          * header checksum.
 598          */
 599         vjlen = cp - buf;
 600         buflen -= vjlen;
 601         if (buflen < 0)
 602                 /* we must have dropped some characters (crc should detect
 603                  * this but the old slip framing won't) */
 604                 goto bad;
 605
 606         total_len += cs->cs_hlen - vjlen;
 607         cs->cs_ip.ip_len = htons(total_len);
 608
 609         /* recompute the ip header checksum */
 610         bp = (u_int16_t *) &cs->cs_ip;
 611         cs->cs_ip.ip_sum = 0;
 612         for (changes = 0; hlen > 0; hlen -= 2)
 613                 changes += *bp++;
 614         changes = (changes & 0xffff) + (changes >> 16);
 615         changes = (changes & 0xffff) + (changes >> 16);
 616         cs->cs_ip.ip_sum = ~ changes;
 617
 618         *hdrp = (u_char *) &cs->cs_ip;
 619         *hlenp = cs->cs_hlen;
 620         return vjlen;
 621
 622 bad:
 623         comp->flags |= SLF_TOSS;
 624         INCR(sls_errorin)
 625         return (-1);
 626 }