sys/net/slcompress.c

   1 /*      $NetBSD: slcompress.c,v 1.37 2009/04/18 14:58:05 tsutsui 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. Neither the name of the University nor the names of its contributors
  17  *    may be used to endorse or promote products derived from this software
  18  *    without specific prior written permission.
  19  *
  20  * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
  21  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  22  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  23  * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
  24  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  25  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  26  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  27  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  28  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  29  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  30  * SUCH DAMAGE.
  31  *
  32  *      @(#)slcompress.c        8.2 (Berkeley) 4/16/94
  33  */
  34
  35 /*
  36  * Routines to compress and uncompess tcp packets (for transmission
  37  * over low speed serial lines.
  38  *
  39  * Van Jacobson (van@helios.ee.lbl.gov), Dec 31, 1989:
  40  *      - Initial distribution.
  41  */
  42
  43 #include <sys/cdefs.h>
  44 __KERNEL_RCSID(0, "$NetBSD: slcompress.c,v 1.37 2009/04/18 14:58:05 tsutsui Exp $");
  45
  46 #include "opt_inet.h"
  47 #ifdef INET
  48 #include <sys/param.h>
  49 #include <sys/mbuf.h>
  50 #include <sys/systm.h>
  51
  52 #include <netinet/in.h>
  53 #include <netinet/in_systm.h>
  54 #include <netinet/ip.h>
  55 #include <netinet/tcp.h>
  56
  57 #include <net/slcompress.h>
  58
  59 #ifndef SL_NO_STATS
  60 #define INCR(counter) ++comp->counter;
  61 #else
  62 #define INCR(counter)
  63 #endif
  64
  65
  66 void
  67 sl_compress_init(struct slcompress *comp)
  68 {
  69         u_int i;
  70         struct cstate *tstate = comp->tstate;
  71
  72         memset(comp, 0, sizeof(*comp));
  73         for (i = MAX_STATES - 1; i > 0; --i) {
  74                 tstate[i].cs_id = i;
  75                 tstate[i].cs_next = &tstate[i - 1];
  76         }
  77         tstate[0].cs_next = &tstate[MAX_STATES - 1];
  78         tstate[0].cs_id = 0;
  79         comp->last_cs = &tstate[0];
  80         comp->last_recv = 255;
  81         comp->last_xmit = 255;
  82         comp->flags = SLF_TOSS;
  83 }
  84
  85
  86 /*
  87  * Like sl_compress_init, but we get to specify the maximum connection
  88  * ID to use on transmission.
  89  */
  90 void
  91 sl_compress_setup(struct slcompress *comp, int max_state)
  92 {
  93         u_int i;
  94         struct cstate *tstate = comp->tstate;
  95
  96         if (max_state == -1) {
  97                 max_state = MAX_STATES - 1;
  98                 memset(comp, 0, sizeof(*comp));
  99         } else {
 100                 /* Don't reset statistics */
 101                 memset(comp->tstate, 0, sizeof(comp->tstate));
 102                 memset(comp->rstate, 0, sizeof(comp->rstate));
 103         }
 104         for (i = max_state; i > 0; --i) {
 105                 tstate[i].cs_id = i;
 106                 tstate[i].cs_next = &tstate[i - 1];
 107         }
 108         tstate[0].cs_next = &tstate[max_state];
 109         tstate[0].cs_id = 0;
 110         comp->last_cs = &tstate[0];
 111         comp->last_recv = 255;
 112         comp->last_xmit = 255;
 113         comp->flags = SLF_TOSS;
 114 }
 115
 116
 117 /* ENCODE encodes a number that is known to be non-zero.  ENCODEZ
 118  * checks for zero (since zero has to be encoded in the long, 3 byte
 119  * form).
 120  */
 121 #define ENCODE(n) { \
 122         if ((uint16_t)(n) >= 256) { \
 123                 *cp++ = 0; \
 124                 cp[1] = (n); \
 125                 cp[0] = (n) >> 8; \
 126                 cp += 2; \
 127         } else { \
 128                 *cp++ = (n); \
 129         } \
 130 }
 131 #define ENCODEZ(n) { \
 132         if ((uint16_t)(n) >= 256 || (uint16_t)(n) == 0) { \
 133                 *cp++ = 0; \
 134                 cp[1] = (n); \
 135                 cp[0] = (n) >> 8; \
 136                 cp += 2; \
 137         } else { \
 138                 *cp++ = (n); \
 139         } \
 140 }
 141
 142 #define DECODEL(f) { \
 143         if (*cp == 0) {\
 144                 (f) = htonl(ntohl(f) + ((cp[1] << 8) | cp[2])); \
 145                 cp += 3; \
 146         } else { \
 147                 (f) = htonl(ntohl(f) + (uint32_t)*cp++); \
 148         } \
 149 }
 150
 151 #define DECODES(f) { \
 152         if (*cp == 0) {\
 153                 (f) = htons(ntohs(f) + ((cp[1] << 8) | cp[2])); \
 154                 cp += 3; \
 155         } else { \
 156                 (f) = htons(ntohs(f) + (uint32_t)*cp++); \
 157         } \
 158 }
 159
 160 #define DECODEU(f) { \
 161         if (*cp == 0) {\
 162                 (f) = htons((cp[1] << 8) | cp[2]); \
 163                 cp += 3; \
 164         } else { \
 165                 (f) = htons((uint32_t)*cp++); \
 166         } \
 167 }
 168
 169 u_int
 170 sl_compress_tcp(struct mbuf *m, struct ip *ip, struct slcompress *comp,
 171     int compress_cid)
 172 {
 173         struct cstate *cs = comp->last_cs->cs_next;
 174         u_int hlen = ip->ip_hl;
 175         struct tcphdr *oth;
 176         struct tcphdr *th;
 177         u_int deltaS, deltaA;
 178         u_int changes = 0;
 179         u_char new_seq[16];
 180         u_char *cp = new_seq;
 181
 182         /*
 183          * Bail if this is an IP fragment or if the TCP packet isn't
 184          * `compressible' (i.e., ACK isn't set or some other control bit is
 185          * set).  (We assume that the caller has already made sure the
 186          * packet is IP proto TCP).
 187          */
 188         if ((ip->ip_off & htons(0x3fff)) || m->m_len < 40)
 189                 return (TYPE_IP);
 190
 191         th = (struct tcphdr *)&((int32_t *)ip)[hlen];
 192         if ((th->th_flags & (TH_SYN|TH_FIN|TH_RST|TH_ACK)) != TH_ACK)
 193                 return (TYPE_IP);
 194         /*
 195          * Packet is compressible -- we're going to send either a
 196          * COMPRESSED_TCP or UNCOMPRESSED_TCP packet.  Either way we need
 197          * to locate (or create) the connection state.  Special case the
 198          * most recently used connection since it's most likely to be used
 199          * again & we don't have to do any reordering if it's used.
 200          */
 201         INCR(sls_packets)
 202         if (ip->ip_src.s_addr != cs->cs_ip.ip_src.s_addr ||
 203             ip->ip_dst.s_addr != cs->cs_ip.ip_dst.s_addr ||
 204             *(int32_t *)th != ((int32_t *)&cs->cs_ip)[cs->cs_ip.ip_hl]) {
 205                 /*
 206                  * Wasn't the first -- search for it.
 207                  *
 208                  * States are kept in a circularly linked list with
 209                  * last_cs pointing to the end of the list.  The
 210                  * list is kept in lru order by moving a state to the
 211                  * head of the list whenever it is referenced.  Since
 212                  * the list is short and, empirically, the connection
 213                  * we want is almost always near the front, we locate
 214                  * states via linear search.  If we don't find a state
 215                  * for the datagram, the oldest state is (re-)used.
 216                  */
 217                 struct cstate *lcs;
 218                 struct cstate *lastcs = comp->last_cs;
 219
 220                 do {
 221                         lcs = cs; cs = cs->cs_next;
 222                         INCR(sls_searches)
 223                         if (ip->ip_src.s_addr == cs->cs_ip.ip_src.s_addr
 224                             && ip->ip_dst.s_addr == cs->cs_ip.ip_dst.s_addr
 225                             && *(int32_t *)th ==
 226                             ((int32_t *)&cs->cs_ip)[cs->cs_ip.ip_hl])
 227                                 goto found;
 228                 } while (cs != lastcs);
 229
 230                 /*
 231                  * Didn't find it -- re-use oldest cstate.  Send an
 232                  * uncompressed packet that tells the other side what
 233                  * connection number we're using for this conversation.
 234                  * Note that since the state list is circular, the oldest
 235                  * state points to the newest and we only need to set
 236                  * last_cs to update the lru linkage.
 237                  */
 238                 INCR(sls_misses)
 239                 comp->last_cs = lcs;
 240                 hlen += th->th_off;
 241                 hlen <<= 2;
 242                 if (hlen > m->m_len)
 243                         return (TYPE_IP);
 244                 goto uncompressed;
 245
 246         found:
 247                 /*
 248                  * Found it -- move to the front on the connection list.
 249                  */
 250                 if (cs == lastcs)
 251                         comp->last_cs = lcs;
 252                 else {
 253                         lcs->cs_next = cs->cs_next;
 254                         cs->cs_next = lastcs->cs_next;
 255                         lastcs->cs_next = cs;
 256                 }
 257         }
 258
 259         /*
 260          * Make sure that only what we expect to change changed. The first
 261          * line of the `if' checks the IP protocol version, header length &
 262          * type of service.  The 2nd line checks the "Don't fragment" bit.
 263          * The 3rd line checks the time-to-live and protocol (the protocol
 264          * check is unnecessary but costless).  The 4th line checks the TCP
 265          * header length.  The 5th line checks IP options, if any.  The 6th
 266          * line checks TCP options, if any.  If any of these things are
 267          * different between the previous & current datagram, we send the
 268          * current datagram `uncompressed'.
 269          */
 270         oth = (struct tcphdr *)&((int32_t *)&cs->cs_ip)[hlen];
 271         deltaS = hlen;
 272         hlen += th->th_off;
 273         hlen <<= 2;
 274         if (hlen > m->m_len)
 275                 return (TYPE_IP);
 276
 277         if (((uint16_t *)ip)[0] != ((uint16_t *)&cs->cs_ip)[0] ||
 278             ((uint16_t *)ip)[3] != ((uint16_t *)&cs->cs_ip)[3] ||
 279             ((uint16_t *)ip)[4] != ((uint16_t *)&cs->cs_ip)[4] ||
 280             th->th_off != oth->th_off ||
 281             (deltaS > 5 &&
 282              memcmp(ip + 1, &cs->cs_ip + 1, (deltaS - 5) << 2)) ||
 283             (th->th_off > 5 &&
 284              memcmp(th + 1, oth + 1, (th->th_off - 5) << 2)))
 285                 goto uncompressed;
 286
 287         /*
 288          * Figure out which of the changing fields changed.  The
 289          * receiver expects changes in the order: urgent, window,
 290          * ack, seq (the order minimizes the number of temporaries
 291          * needed in this section of code).
 292          */
 293         if (th->th_flags & TH_URG) {
 294                 deltaS = ntohs(th->th_urp);
 295                 ENCODEZ(deltaS);
 296                 changes |= NEW_U;
 297         } else if (th->th_urp != oth->th_urp)
 298                 /* argh! URG not set but urp changed -- a sensible
 299                  * implementation should never do this but RFC793
 300                  * doesn't prohibit the change so we have to deal
 301                  * with it. */
 302                  goto uncompressed;
 303
 304         deltaS = (uint16_t)(ntohs(th->th_win) - ntohs(oth->th_win));
 305         if (deltaS) {
 306                 ENCODE(deltaS);
 307                 changes |= NEW_W;
 308         }
 309
 310         deltaA = ntohl(th->th_ack) - ntohl(oth->th_ack);
 311         if (deltaA) {
 312                 if (deltaA > 0xffff)
 313                         goto uncompressed;
 314                 ENCODE(deltaA);
 315                 changes |= NEW_A;
 316         }
 317
 318         deltaS = ntohl(th->th_seq) - ntohl(oth->th_seq);
 319         if (deltaS) {
 320                 if (deltaS > 0xffff)
 321                         goto uncompressed;
 322                 ENCODE(deltaS);
 323                 changes |= NEW_S;
 324         }
 325
 326         switch (changes) {
 327
 328         case 0:
 329                 /*
 330                  * Nothing changed. If this packet contains data and the
 331                  * last one didn't, this is probably a data packet following
 332                  * an ack (normal on an interactive connection) and we send
 333                  * it compressed.  Otherwise it's probably a retransmit,
 334                  * retransmitted ack or window probe.  Send it uncompressed
 335                  * in case the other side missed the compressed version.
 336                  */
 337                 if (ip->ip_len != cs->cs_ip.ip_len &&
 338                     ntohs(cs->cs_ip.ip_len) == hlen)
 339                         break;
 340
 341                 /* (fall through) */
 342
 343         case SPECIAL_I:
 344         case SPECIAL_D:
 345                 /*
 346                  * actual changes match one of our special case encodings --
 347                  * send packet uncompressed.
 348                  */
 349                 goto uncompressed;
 350
 351         case NEW_S|NEW_A:
 352                 if (deltaS == deltaA &&
 353                     deltaS == ntohs(cs->cs_ip.ip_len) - hlen) {
 354                         /* special case for echoed terminal traffic */
 355                         changes = SPECIAL_I;
 356                         cp = new_seq;
 357                 }
 358                 break;
 359
 360         case NEW_S:
 361                 if (deltaS == ntohs(cs->cs_ip.ip_len) - hlen) {
 362                         /* special case for data xfer */
 363                         changes = SPECIAL_D;
 364                         cp = new_seq;
 365                 }
 366                 break;
 367         }
 368
 369         deltaS = ntohs(ip->ip_id) - ntohs(cs->cs_ip.ip_id);
 370         if (deltaS != 1) {
 371                 ENCODEZ(deltaS);
 372                 changes |= NEW_I;
 373         }
 374         if (th->th_flags & TH_PUSH)
 375                 changes |= TCP_PUSH_BIT;
 376         /*
 377          * Grab the cksum before we overwrite it below.  Then update our
 378          * state with this packet's header.
 379          */
 380         deltaA = ntohs(th->th_sum);
 381         memcpy(&cs->cs_ip, ip, hlen);
 382
 383         /*
 384          * We want to use the original packet as our compressed packet.
 385          * (cp - new_seq) is the number of bytes we need for compressed
 386          * sequence numbers.  In addition we need one byte for the change
 387          * mask, one for the connection id and two for the tcp checksum.
 388          * So, (cp - new_seq) + 4 bytes of header are needed.  hlen is how
 389          * many bytes of the original packet to toss so subtract the two to
 390          * get the new packet size.
 391          */
 392         deltaS = cp - new_seq;
 393         cp = (u_char *)ip;
 394         if (compress_cid == 0 || comp->last_xmit != cs->cs_id) {
 395                 comp->last_xmit = cs->cs_id;
 396                 hlen -= deltaS + 4;
 397                 cp += hlen;
 398                 *cp++ = changes | NEW_C;
 399                 *cp++ = cs->cs_id;
 400         } else {
 401                 hlen -= deltaS + 3;
 402                 cp += hlen;
 403                 *cp++ = changes;
 404         }
 405         m->m_len -= hlen;
 406         m->m_data += hlen;
 407         *cp++ = deltaA >> 8;
 408         *cp++ = deltaA;
 409         memcpy(cp, new_seq, deltaS);
 410         INCR(sls_compressed)
 411         return (TYPE_COMPRESSED_TCP);
 412
 413         /*
 414          * Update connection state cs & send uncompressed packet ('uncompressed'
 415          * means a regular ip/tcp packet but with the 'conversation id' we hope
 416          * to use on future compressed packets in the protocol field).
 417          */
 418 uncompressed:
 419         memcpy(&cs->cs_ip, ip, hlen);
 420         ip->ip_p = cs->cs_id;
 421         comp->last_xmit = cs->cs_id;
 422         return (TYPE_UNCOMPRESSED_TCP);
 423 }
 424
 425
 426 int
 427 sl_uncompress_tcp(u_char **bufp, int len, u_int type, struct slcompress *comp)
 428 {
 429         u_char *hdr, *cp;
 430         int vjlen;
 431         u_int hlen;
 432
 433         cp = bufp ? *bufp : NULL;
 434         vjlen = sl_uncompress_tcp_core(cp, len, len, type, comp, &hdr, &hlen);
 435         if (vjlen < 0)
 436                 return (0);     /* error */
 437         if (vjlen == 0)
 438                 return (len);   /* was uncompressed already */
 439
 440         cp += vjlen;
 441         len -= vjlen;
 442
 443         /*
 444          * At this point, cp points to the first byte of data in the
 445          * packet.  If we're not aligned on a 4-byte boundary, copy the
 446          * data down so the ip & tcp headers will be aligned.  Then back up
 447          * cp by the tcp/ip header length to make room for the reconstructed
 448          * header (we assume the packet we were handed has enough space to
 449          * prepend 128 bytes of header).
 450          */
 451         if ((long)cp & 3) {
 452                 if (len > 0)
 453                         memmove((void *)((long)cp &~ 3), cp, len);
 454                 cp = (u_char *)((long)cp &~ 3);
 455         }
 456         cp -= hlen;
 457         len += hlen;
 458         memcpy(cp, hdr, hlen);
 459
 460         *bufp = cp;
 461         return (len);
 462 }
 463
 464 /*
 465  * Uncompress a packet of total length total_len.  The first buflen
 466  * bytes are at buf; this must include the entire (compressed or
 467  * uncompressed) TCP/IP header.  This procedure returns the length
 468  * of the VJ header, with a pointer to the uncompressed IP header
 469  * in *hdrp and its length in *hlenp.
 470  */
 471 int
 472 sl_uncompress_tcp_core(u_char *buf, int buflen, int total_len, u_int type,
 473     struct slcompress *comp, u_char **hdrp, u_int *hlenp)
 474 {
 475         u_char *cp;
 476         u_int hlen, changes;
 477         struct tcphdr *th;
 478         struct cstate *cs;
 479         struct ip *ip;
 480         uint16_t *bp;
 481         u_int vjlen;
 482
 483         switch (type) {
 484
 485         case TYPE_UNCOMPRESSED_TCP:
 486                 if (buf == NULL)
 487                         goto bad;
 488                 ip = (struct ip *) buf;
 489                 if (ip->ip_p >= MAX_STATES)
 490                         goto bad;
 491                 cs = &comp->rstate[comp->last_recv = ip->ip_p];
 492                 comp->flags &=~ SLF_TOSS;
 493                 ip->ip_p = IPPROTO_TCP;
 494                 /*
 495                  * Calculate the size of the TCP/IP header and make sure that
 496                  * we don't overflow the space we have available for it.
 497                  */
 498                 hlen = ip->ip_hl << 2;
 499                 if (hlen + sizeof(struct tcphdr) > buflen)
 500                         goto bad;
 501                 hlen += ((struct tcphdr *)&((char *)ip)[hlen])->th_off << 2;
 502                 if (hlen > MAX_HDR || hlen > buflen)
 503                         goto bad;
 504                 memcpy(&cs->cs_ip, ip, hlen);
 505                 cs->cs_hlen = hlen;
 506                 INCR(sls_uncompressedin)
 507                 *hdrp = (u_char *) &cs->cs_ip;
 508                 *hlenp = hlen;
 509                 return (0);
 510
 511         default:
 512                 goto bad;
 513
 514         case TYPE_COMPRESSED_TCP:
 515                 break;
 516         }
 517         /* We've got a compressed packet. */
 518         INCR(sls_compressedin)
 519         if (buf == NULL)
 520                 goto bad;
 521         cp = buf;
 522         changes = *cp++;
 523         if (changes & NEW_C) {
 524                 /* Make sure the state index is in range, then grab the state.
 525                  * If we have a good state index, clear the 'discard' flag. */
 526                 if (*cp >= MAX_STATES)
 527                         goto bad;
 528
 529                 comp->flags &=~ SLF_TOSS;
 530                 comp->last_recv = *cp++;
 531         } else {
 532                 /* this packet has an implicit state index.  If we've
 533                  * had a line error since the last time we got an
 534                  * explicit state index, we have to toss the packet. */
 535                 if (comp->flags & SLF_TOSS) {
 536                         INCR(sls_tossed)
 537                         return (-1);
 538                 }
 539         }
 540         cs = &comp->rstate[comp->last_recv];
 541         hlen = cs->cs_ip.ip_hl << 2;
 542         th = (struct tcphdr *)&((u_char *)&cs->cs_ip)[hlen];
 543         th->th_sum = htons((*cp << 8) | cp[1]);
 544         cp += 2;
 545         if (changes & TCP_PUSH_BIT)
 546                 th->th_flags |= TH_PUSH;
 547         else
 548                 th->th_flags &=~ TH_PUSH;
 549
 550         switch (changes & SPECIALS_MASK) {
 551         case SPECIAL_I:
 552                 {
 553                 u_int i = ntohs(cs->cs_ip.ip_len) - cs->cs_hlen;
 554                 th->th_ack = htonl(ntohl(th->th_ack) + i);
 555                 th->th_seq = htonl(ntohl(th->th_seq) + i);
 556                 }
 557                 break;
 558
 559         case SPECIAL_D:
 560                 th->th_seq = htonl(ntohl(th->th_seq) + ntohs(cs->cs_ip.ip_len)
 561                                    - cs->cs_hlen);
 562                 break;
 563
 564         default:
 565                 if (changes & NEW_U) {
 566                         th->th_flags |= TH_URG;
 567                         DECODEU(th->th_urp)
 568                 } else
 569                         th->th_flags &=~ TH_URG;
 570                 if (changes & NEW_W)
 571                         DECODES(th->th_win)
 572                 if (changes & NEW_A)
 573                         DECODEL(th->th_ack)
 574                 if (changes & NEW_S)
 575                         DECODEL(th->th_seq)
 576                 break;
 577         }
 578         if (changes & NEW_I) {
 579                 DECODES(cs->cs_ip.ip_id)
 580         } else
 581                 cs->cs_ip.ip_id = htons(ntohs(cs->cs_ip.ip_id) + 1);
 582
 583         /*
 584          * At this point, cp points to the first byte of data in the
 585          * packet.  Fill in the IP total length and update the IP
 586          * header checksum.
 587          */
 588         vjlen = cp - buf;
 589         buflen -= vjlen;
 590         if (buflen < 0)
 591                 /* we must have dropped some characters (crc should detect
 592                  * this but the old slip framing won't) */
 593                 goto bad;
 594
 595         total_len += cs->cs_hlen - vjlen;
 596         cs->cs_ip.ip_len = htons(total_len);
 597
 598         /* recompute the ip header checksum */
 599         bp = (uint16_t *) &cs->cs_ip;
 600         cs->cs_ip.ip_sum = 0;
 601         for (changes = 0; hlen > 0; hlen -= 2)
 602                 changes += *bp++;
 603         changes = (changes & 0xffff) + (changes >> 16);
 604         changes = (changes & 0xffff) + (changes >> 16);
 605         cs->cs_ip.ip_sum = ~ changes;
 606
 607         *hdrp = (u_char *) &cs->cs_ip;
 608         *hlenp = cs->cs_hlen;
 609         return vjlen;
 610
 611 bad:
 612         comp->flags |= SLF_TOSS;
 613         INCR(sls_errorin)
 614         return (-1);
 615 }
 616 #endif