2 * SPDX-License-Identifier: BSD-3-Clause
4 * Copyright (c) 1989, 1993, 1994
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31 * @(#)slcompress.c 8.2 (Berkeley) 4/16/94
36 * Routines to compress and uncompess tcp packets (for transmission
37 * over low speed serial lines.
39 * Van Jacobson (van@helios.ee.lbl.gov), Dec 31, 1989:
40 * - Initial distribution.
44 #include <sys/param.h>
46 #include <sys/systm.h>
48 #include <netinet/in.h>
49 #include <netinet/in_systm.h>
50 #include <netinet/ip.h>
51 #include <netinet/tcp.h>
53 #include <net/slcompress.h>
56 #define INCR(counter) ++comp->counter;
61 #define BCMP(p1, p2, n) bcmp((void *)(p1), (void *)(p2), (int)(n))
62 #define BCOPY(p1, p2, n) bcopy((void *)(p1), (void *)(p2), (int)(n))
65 sl_compress_init(struct slcompress *comp, int max_state)
68 struct cstate *tstate = comp->tstate;
70 if (max_state == -1) {
71 max_state = MAX_STATES - 1;
72 bzero((char *)comp, sizeof(*comp));
74 /* Don't reset statistics */
75 bzero((char *)comp->tstate, sizeof(comp->tstate));
76 bzero((char *)comp->rstate, sizeof(comp->rstate));
78 for (i = max_state; i > 0; --i) {
80 tstate[i].cs_next = &tstate[i - 1];
82 tstate[0].cs_next = &tstate[max_state];
84 comp->last_cs = &tstate[0];
85 comp->last_recv = 255;
86 comp->last_xmit = 255;
87 comp->flags = SLF_TOSS;
91 /* ENCODE encodes a number that is known to be non-zero. ENCODEZ
92 * checks for zero (since zero has to be encoded in the long, 3 byte
96 if ((u_int16_t)(n) >= 256) { \
105 #define ENCODEZ(n) { \
106 if ((u_int16_t)(n) >= 256 || (u_int16_t)(n) == 0) { \
116 #define DECODEL(f) { \
118 (f) = htonl(ntohl(f) + ((cp[1] << 8) | cp[2])); \
121 (f) = htonl(ntohl(f) + (u_int32_t)*cp++); \
125 #define DECODES(f) { \
127 (f) = htons(ntohs(f) + ((cp[1] << 8) | cp[2])); \
130 (f) = htons(ntohs(f) + (u_int32_t)*cp++); \
134 #define DECODEU(f) { \
136 (f) = htons((cp[1] << 8) | cp[2]); \
139 (f) = htons((u_int32_t)*cp++); \
144 * Attempt to compress an outgoing TCP packet and return the type of
145 * the result. The caller must have already verified that the protocol
146 * is TCP. The first mbuf must contain the complete IP and TCP headers,
147 * and "ip" must be == mtod(m, struct ip *). "comp" supplies the
148 * compression state, and "compress_cid" tells us whether it is OK
149 * to leave out the CID field when feasible.
151 * The caller is responsible for adjusting m->m_pkthdr.len upon return,
152 * if m is an M_PKTHDR mbuf.
155 sl_compress_tcp(struct mbuf *m, struct ip *ip, struct slcompress *comp,
158 struct cstate *cs = comp->last_cs->cs_next;
159 u_int hlen = ip->ip_hl;
162 u_int deltaS, deltaA;
165 u_char *cp = new_seq;
168 * Bail if this is an IP fragment or if the TCP packet isn't
169 * `compressible' (i.e., ACK isn't set or some other control bit is
170 * set). (We assume that the caller has already made sure the
171 * packet is IP proto TCP).
173 if ((ip->ip_off & htons(0x3fff)) || m->m_len < 40)
176 th = (struct tcphdr *)&((int32_t *)ip)[hlen];
177 if ((th->th_flags & (TH_SYN|TH_FIN|TH_RST|TH_ACK)) != TH_ACK)
180 * Packet is compressible -- we're going to send either a
181 * COMPRESSED_TCP or UNCOMPRESSED_TCP packet. Either way we need
182 * to locate (or create) the connection state. Special case the
183 * most recently used connection since it's most likely to be used
184 * again & we don't have to do any reordering if it's used.
187 if (ip->ip_src.s_addr != cs->cs_ip.ip_src.s_addr ||
188 ip->ip_dst.s_addr != cs->cs_ip.ip_dst.s_addr ||
189 *(int32_t *)th != ((int32_t *)&cs->cs_ip)[cs->cs_ip.ip_hl]) {
191 * Wasn't the first -- search for it.
193 * States are kept in a circularly linked list with
194 * last_cs pointing to the end of the list. The
195 * list is kept in lru order by moving a state to the
196 * head of the list whenever it is referenced. Since
197 * the list is short and, empirically, the connection
198 * we want is almost always near the front, we locate
199 * states via linear search. If we don't find a state
200 * for the datagram, the oldest state is (re-)used.
203 struct cstate *lastcs = comp->last_cs;
206 lcs = cs; cs = cs->cs_next;
208 if (ip->ip_src.s_addr == cs->cs_ip.ip_src.s_addr
209 && ip->ip_dst.s_addr == cs->cs_ip.ip_dst.s_addr
211 ((int32_t *)&cs->cs_ip)[cs->cs_ip.ip_hl])
213 } while (cs != lastcs);
216 * Didn't find it -- re-use oldest cstate. Send an
217 * uncompressed packet that tells the other side what
218 * connection number we're using for this conversation.
219 * Note that since the state list is circular, the oldest
220 * state points to the newest and we only need to set
221 * last_cs to update the lru linkage.
233 * Found it -- move to the front on the connection list.
238 lcs->cs_next = cs->cs_next;
239 cs->cs_next = lastcs->cs_next;
240 lastcs->cs_next = cs;
245 * Make sure that only what we expect to change changed. The first
246 * line of the `if' checks the IP protocol version, header length &
247 * type of service. The 2nd line checks the "Don't fragment" bit.
248 * The 3rd line checks the time-to-live and protocol (the protocol
249 * check is unnecessary but costless). The 4th line checks the TCP
250 * header length. The 5th line checks IP options, if any. The 6th
251 * line checks TCP options, if any. If any of these things are
252 * different between the previous & current datagram, we send the
253 * current datagram `uncompressed'.
255 oth = (struct tcphdr *)&((int32_t *)&cs->cs_ip)[hlen];
262 if (((u_int16_t *)ip)[0] != ((u_int16_t *)&cs->cs_ip)[0] ||
263 ((u_int16_t *)ip)[3] != ((u_int16_t *)&cs->cs_ip)[3] ||
264 ((u_int16_t *)ip)[4] != ((u_int16_t *)&cs->cs_ip)[4] ||
265 th->th_off != oth->th_off ||
267 BCMP(ip + 1, &cs->cs_ip + 1, (deltaS - 5) << 2)) ||
269 BCMP(th + 1, oth + 1, (th->th_off - 5) << 2)))
273 * Figure out which of the changing fields changed. The
274 * receiver expects changes in the order: urgent, window,
275 * ack, seq (the order minimizes the number of temporaries
276 * needed in this section of code).
278 if (th->th_flags & TH_URG) {
279 deltaS = ntohs(th->th_urp);
282 } else if (th->th_urp != oth->th_urp)
283 /* argh! URG not set but urp changed -- a sensible
284 * implementation should never do this but RFC793
285 * doesn't prohibit the change so we have to deal
289 deltaS = (u_int16_t)(ntohs(th->th_win) - ntohs(oth->th_win));
295 deltaA = ntohl(th->th_ack) - ntohl(oth->th_ack);
303 deltaS = ntohl(th->th_seq) - ntohl(oth->th_seq);
315 * Nothing changed. If this packet contains data and the
316 * last one didn't, this is probably a data packet following
317 * an ack (normal on an interactive connection) and we send
318 * it compressed. Otherwise it's probably a retransmit,
319 * retransmitted ack or window probe. Send it uncompressed
320 * in case the other side missed the compressed version.
322 if (ip->ip_len != cs->cs_ip.ip_len &&
323 ntohs(cs->cs_ip.ip_len) == hlen)
331 * actual changes match one of our special case encodings --
332 * send packet uncompressed.
337 if (deltaS == deltaA &&
338 deltaS == ntohs(cs->cs_ip.ip_len) - hlen) {
339 /* special case for echoed terminal traffic */
346 if (deltaS == ntohs(cs->cs_ip.ip_len) - hlen) {
347 /* special case for data xfer */
354 deltaS = ntohs(ip->ip_id) - ntohs(cs->cs_ip.ip_id);
359 if (th->th_flags & TH_PUSH)
360 changes |= TCP_PUSH_BIT;
362 * Grab the cksum before we overwrite it below. Then update our
363 * state with this packet's header.
365 deltaA = ntohs(th->th_sum);
366 BCOPY(ip, &cs->cs_ip, hlen);
369 * We want to use the original packet as our compressed packet.
370 * (cp - new_seq) is the number of bytes we need for compressed
371 * sequence numbers. In addition we need one byte for the change
372 * mask, one for the connection id and two for the tcp checksum.
373 * So, (cp - new_seq) + 4 bytes of header are needed. hlen is how
374 * many bytes of the original packet to toss so subtract the two to
375 * get the new packet size.
377 deltaS = cp - new_seq;
379 if (compress_cid == 0 || comp->last_xmit != cs->cs_id) {
380 comp->last_xmit = cs->cs_id;
383 *cp++ = changes | NEW_C;
394 BCOPY(new_seq, cp, deltaS);
396 return (TYPE_COMPRESSED_TCP);
399 * Update connection state cs & send uncompressed packet ('uncompressed'
400 * means a regular ip/tcp packet but with the 'conversation id' we hope
401 * to use on future compressed packets in the protocol field).
404 BCOPY(ip, &cs->cs_ip, hlen);
405 ip->ip_p = cs->cs_id;
406 comp->last_xmit = cs->cs_id;
407 return (TYPE_UNCOMPRESSED_TCP);
412 sl_uncompress_tcp(u_char **bufp, int len, u_int type, struct slcompress *comp)
417 cp = bufp? *bufp: NULL;
418 vjlen = sl_uncompress_tcp_core(cp, len, len, type, comp, &hdr, &hlen);
420 return (0); /* error */
422 return (len); /* was uncompressed already */
428 * At this point, cp points to the first byte of data in the
429 * packet. If we're not aligned on a 4-byte boundary, copy the
430 * data down so the ip & tcp headers will be aligned. Then back up
431 * cp by the tcp/ip header length to make room for the reconstructed
432 * header (we assume the packet we were handed has enough space to
433 * prepend 128 bytes of header).
435 if ((intptr_t)cp & 3) {
437 BCOPY(cp, ((intptr_t)cp &~ 3), len);
438 cp = (u_char *)((intptr_t)cp &~ 3);
442 BCOPY(hdr, cp, hlen);
449 * Uncompress a packet of total length total_len. The first buflen
450 * bytes are at buf; this must include the entire (compressed or
451 * uncompressed) TCP/IP header. This procedure returns the length
452 * of the VJ header, with a pointer to the uncompressed IP header
453 * in *hdrp and its length in *hlenp.
456 sl_uncompress_tcp_core(u_char *buf, int buflen, int total_len, u_int type,
457 struct slcompress *comp, u_char **hdrp, u_int *hlenp)
469 case TYPE_UNCOMPRESSED_TCP:
470 ip = (struct ip *) buf;
471 if (ip->ip_p >= MAX_STATES)
473 cs = &comp->rstate[comp->last_recv = ip->ip_p];
474 comp->flags &=~ SLF_TOSS;
475 ip->ip_p = IPPROTO_TCP;
477 * Calculate the size of the TCP/IP header and make sure that
478 * we don't overflow the space we have available for it.
480 hlen = ip->ip_hl << 2;
481 if (hlen + sizeof(struct tcphdr) > buflen)
483 hlen += ((struct tcphdr *)&((char *)ip)[hlen])->th_off << 2;
484 if (hlen > MAX_HDR || hlen > buflen)
486 BCOPY(ip, &cs->cs_ip, hlen);
488 INCR(sls_uncompressedin)
489 *hdrp = (u_char *) &cs->cs_ip;
496 case TYPE_COMPRESSED_TCP:
499 /* We've got a compressed packet. */
500 INCR(sls_compressedin)
503 if (changes & NEW_C) {
504 /* Make sure the state index is in range, then grab the state.
505 * If we have a good state index, clear the 'discard' flag. */
506 if (*cp >= MAX_STATES)
509 comp->flags &=~ SLF_TOSS;
510 comp->last_recv = *cp++;
512 /* this packet has an implicit state index. If we've
513 * had a line error since the last time we got an
514 * explicit state index, we have to toss the packet. */
515 if (comp->flags & SLF_TOSS) {
520 cs = &comp->rstate[comp->last_recv];
521 hlen = cs->cs_ip.ip_hl << 2;
522 th = (struct tcphdr *)&((u_char *)&cs->cs_ip)[hlen];
523 th->th_sum = htons((*cp << 8) | cp[1]);
525 if (changes & TCP_PUSH_BIT)
526 th->th_flags |= TH_PUSH;
528 th->th_flags &=~ TH_PUSH;
530 switch (changes & SPECIALS_MASK) {
533 u_int i = ntohs(cs->cs_ip.ip_len) - cs->cs_hlen;
534 th->th_ack = htonl(ntohl(th->th_ack) + i);
535 th->th_seq = htonl(ntohl(th->th_seq) + i);
540 th->th_seq = htonl(ntohl(th->th_seq) + ntohs(cs->cs_ip.ip_len)
545 if (changes & NEW_U) {
546 th->th_flags |= TH_URG;
549 th->th_flags &=~ TH_URG;
558 if (changes & NEW_I) {
559 DECODES(cs->cs_ip.ip_id)
561 cs->cs_ip.ip_id = htons(ntohs(cs->cs_ip.ip_id) + 1);
564 * At this point, cp points to the first byte of data in the
565 * packet. Fill in the IP total length and update the IP
571 /* we must have dropped some characters (crc should detect
572 * this but the old slip framing won't) */
575 total_len += cs->cs_hlen - vjlen;
576 cs->cs_ip.ip_len = htons(total_len);
578 /* recompute the ip header checksum */
579 bp = (u_int16_t *) &cs->cs_ip;
580 cs->cs_ip.ip_sum = 0;
581 for (changes = 0; hlen > 0; hlen -= 2)
583 changes = (changes & 0xffff) + (changes >> 16);
584 changes = (changes & 0xffff) + (changes >> 16);
585 cs->cs_ip.ip_sum = ~ changes;
587 *hdrp = (u_char *) &cs->cs_ip;
588 *hlenp = cs->cs_hlen;
592 comp->flags |= SLF_TOSS;