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;
90 /* ENCODE encodes a number that is known to be non-zero. ENCODEZ
91 * checks for zero (since zero has to be encoded in the long, 3 byte
95 if ((u_int16_t)(n) >= 256) { \
104 #define ENCODEZ(n) { \
105 if ((u_int16_t)(n) >= 256 || (u_int16_t)(n) == 0) { \
115 #define DECODEL(f) { \
117 (f) = htonl(ntohl(f) + ((cp[1] << 8) | cp[2])); \
120 (f) = htonl(ntohl(f) + (u_int32_t)*cp++); \
124 #define DECODES(f) { \
126 (f) = htons(ntohs(f) + ((cp[1] << 8) | cp[2])); \
129 (f) = htons(ntohs(f) + (u_int32_t)*cp++); \
133 #define DECODEU(f) { \
135 (f) = htons((cp[1] << 8) | cp[2]); \
138 (f) = htons((u_int32_t)*cp++); \
143 * Attempt to compress an outgoing TCP packet and return the type of
144 * the result. The caller must have already verified that the protocol
145 * is TCP. The first mbuf must contain the complete IP and TCP headers,
146 * and "ip" must be == mtod(m, struct ip *). "comp" supplies the
147 * compression state, and "compress_cid" tells us whether it is OK
148 * to leave out the CID field when feasible.
150 * The caller is responsible for adjusting m->m_pkthdr.len upon return,
151 * if m is an M_PKTHDR mbuf.
154 sl_compress_tcp(struct mbuf *m, struct ip *ip, struct slcompress *comp,
157 struct cstate *cs = comp->last_cs->cs_next;
158 u_int hlen = ip->ip_hl;
161 u_int deltaS, deltaA;
164 u_char *cp = new_seq;
167 * Bail if this is an IP fragment or if the TCP packet isn't
168 * `compressible' (i.e., ACK isn't set or some other control bit is
169 * set). (We assume that the caller has already made sure the
170 * packet is IP proto TCP).
172 if ((ip->ip_off & htons(0x3fff)) || m->m_len < 40)
175 th = (struct tcphdr *)&((int32_t *)ip)[hlen];
176 if ((th->th_flags & (TH_SYN|TH_FIN|TH_RST|TH_ACK)) != TH_ACK)
179 * Packet is compressible -- we're going to send either a
180 * COMPRESSED_TCP or UNCOMPRESSED_TCP packet. Either way we need
181 * to locate (or create) the connection state. Special case the
182 * most recently used connection since it's most likely to be used
183 * again & we don't have to do any reordering if it's used.
186 if (ip->ip_src.s_addr != cs->cs_ip.ip_src.s_addr ||
187 ip->ip_dst.s_addr != cs->cs_ip.ip_dst.s_addr ||
188 *(int32_t *)th != ((int32_t *)&cs->cs_ip)[cs->cs_ip.ip_hl]) {
190 * Wasn't the first -- search for it.
192 * States are kept in a circularly linked list with
193 * last_cs pointing to the end of the list. The
194 * list is kept in lru order by moving a state to the
195 * head of the list whenever it is referenced. Since
196 * the list is short and, empirically, the connection
197 * we want is almost always near the front, we locate
198 * states via linear search. If we don't find a state
199 * for the datagram, the oldest state is (re-)used.
202 struct cstate *lastcs = comp->last_cs;
205 lcs = cs; cs = cs->cs_next;
207 if (ip->ip_src.s_addr == cs->cs_ip.ip_src.s_addr
208 && ip->ip_dst.s_addr == cs->cs_ip.ip_dst.s_addr
210 ((int32_t *)&cs->cs_ip)[cs->cs_ip.ip_hl])
212 } while (cs != lastcs);
215 * Didn't find it -- re-use oldest cstate. Send an
216 * uncompressed packet that tells the other side what
217 * connection number we're using for this conversation.
218 * Note that since the state list is circular, the oldest
219 * state points to the newest and we only need to set
220 * last_cs to update the lru linkage.
232 * Found it -- move to the front on the connection list.
237 lcs->cs_next = cs->cs_next;
238 cs->cs_next = lastcs->cs_next;
239 lastcs->cs_next = cs;
244 * Make sure that only what we expect to change changed. The first
245 * line of the `if' checks the IP protocol version, header length &
246 * type of service. The 2nd line checks the "Don't fragment" bit.
247 * The 3rd line checks the time-to-live and protocol (the protocol
248 * check is unnecessary but costless). The 4th line checks the TCP
249 * header length. The 5th line checks IP options, if any. The 6th
250 * line checks TCP options, if any. If any of these things are
251 * different between the previous & current datagram, we send the
252 * current datagram `uncompressed'.
254 oth = (struct tcphdr *)&((int32_t *)&cs->cs_ip)[hlen];
261 if (((u_int16_t *)ip)[0] != ((u_int16_t *)&cs->cs_ip)[0] ||
262 ((u_int16_t *)ip)[3] != ((u_int16_t *)&cs->cs_ip)[3] ||
263 ((u_int16_t *)ip)[4] != ((u_int16_t *)&cs->cs_ip)[4] ||
264 th->th_off != oth->th_off ||
266 BCMP(ip + 1, &cs->cs_ip + 1, (deltaS - 5) << 2)) ||
268 BCMP(th + 1, oth + 1, (th->th_off - 5) << 2)))
272 * Figure out which of the changing fields changed. The
273 * receiver expects changes in the order: urgent, window,
274 * ack, seq (the order minimizes the number of temporaries
275 * needed in this section of code).
277 if (th->th_flags & TH_URG) {
278 deltaS = ntohs(th->th_urp);
281 } else if (th->th_urp != oth->th_urp)
282 /* argh! URG not set but urp changed -- a sensible
283 * implementation should never do this but RFC793
284 * doesn't prohibit the change so we have to deal
288 deltaS = (u_int16_t)(ntohs(th->th_win) - ntohs(oth->th_win));
294 deltaA = ntohl(th->th_ack) - ntohl(oth->th_ack);
302 deltaS = ntohl(th->th_seq) - ntohl(oth->th_seq);
313 * Nothing changed. If this packet contains data and the
314 * last one didn't, this is probably a data packet following
315 * an ack (normal on an interactive connection) and we send
316 * it compressed. Otherwise it's probably a retransmit,
317 * retransmitted ack or window probe. Send it uncompressed
318 * in case the other side missed the compressed version.
320 if (ip->ip_len != cs->cs_ip.ip_len &&
321 ntohs(cs->cs_ip.ip_len) == hlen)
329 * actual changes match one of our special case encodings --
330 * send packet uncompressed.
335 if (deltaS == deltaA &&
336 deltaS == ntohs(cs->cs_ip.ip_len) - hlen) {
337 /* special case for echoed terminal traffic */
344 if (deltaS == ntohs(cs->cs_ip.ip_len) - hlen) {
345 /* special case for data xfer */
352 deltaS = ntohs(ip->ip_id) - ntohs(cs->cs_ip.ip_id);
357 if (th->th_flags & TH_PUSH)
358 changes |= TCP_PUSH_BIT;
360 * Grab the cksum before we overwrite it below. Then update our
361 * state with this packet's header.
363 deltaA = ntohs(th->th_sum);
364 BCOPY(ip, &cs->cs_ip, hlen);
367 * We want to use the original packet as our compressed packet.
368 * (cp - new_seq) is the number of bytes we need for compressed
369 * sequence numbers. In addition we need one byte for the change
370 * mask, one for the connection id and two for the tcp checksum.
371 * So, (cp - new_seq) + 4 bytes of header are needed. hlen is how
372 * many bytes of the original packet to toss so subtract the two to
373 * get the new packet size.
375 deltaS = cp - new_seq;
377 if (compress_cid == 0 || comp->last_xmit != cs->cs_id) {
378 comp->last_xmit = cs->cs_id;
381 *cp++ = changes | NEW_C;
392 BCOPY(new_seq, cp, deltaS);
394 return (TYPE_COMPRESSED_TCP);
397 * Update connection state cs & send uncompressed packet ('uncompressed'
398 * means a regular ip/tcp packet but with the 'conversation id' we hope
399 * to use on future compressed packets in the protocol field).
402 BCOPY(ip, &cs->cs_ip, hlen);
403 ip->ip_p = cs->cs_id;
404 comp->last_xmit = cs->cs_id;
405 return (TYPE_UNCOMPRESSED_TCP);
409 sl_uncompress_tcp(u_char **bufp, int len, u_int type, struct slcompress *comp)
414 cp = bufp? *bufp: NULL;
415 vjlen = sl_uncompress_tcp_core(cp, len, len, type, comp, &hdr, &hlen);
417 return (0); /* error */
419 return (len); /* was uncompressed already */
425 * At this point, cp points to the first byte of data in the
426 * packet. If we're not aligned on a 4-byte boundary, copy the
427 * data down so the ip & tcp headers will be aligned. Then back up
428 * cp by the tcp/ip header length to make room for the reconstructed
429 * header (we assume the packet we were handed has enough space to
430 * prepend 128 bytes of header).
432 if ((intptr_t)cp & 3) {
434 BCOPY(cp, ((intptr_t)cp &~ 3), len);
435 cp = (u_char *)((intptr_t)cp &~ 3);
439 BCOPY(hdr, cp, hlen);
446 * Uncompress a packet of total length total_len. The first buflen
447 * bytes are at buf; this must include the entire (compressed or
448 * uncompressed) TCP/IP header. This procedure returns the length
449 * of the VJ header, with a pointer to the uncompressed IP header
450 * in *hdrp and its length in *hlenp.
453 sl_uncompress_tcp_core(u_char *buf, int buflen, int total_len, u_int type,
454 struct slcompress *comp, u_char **hdrp, u_int *hlenp)
465 case TYPE_UNCOMPRESSED_TCP:
466 ip = (struct ip *) buf;
467 if (ip->ip_p >= MAX_STATES)
469 cs = &comp->rstate[comp->last_recv = ip->ip_p];
470 comp->flags &=~ SLF_TOSS;
471 ip->ip_p = IPPROTO_TCP;
473 * Calculate the size of the TCP/IP header and make sure that
474 * we don't overflow the space we have available for it.
476 hlen = ip->ip_hl << 2;
477 if (hlen + sizeof(struct tcphdr) > buflen)
479 hlen += ((struct tcphdr *)&((char *)ip)[hlen])->th_off << 2;
480 if (hlen > MAX_HDR || hlen > buflen)
482 BCOPY(ip, &cs->cs_ip, hlen);
484 INCR(sls_uncompressedin)
485 *hdrp = (u_char *) &cs->cs_ip;
492 case TYPE_COMPRESSED_TCP:
495 /* We've got a compressed packet. */
496 INCR(sls_compressedin)
499 if (changes & NEW_C) {
500 /* Make sure the state index is in range, then grab the state.
501 * If we have a good state index, clear the 'discard' flag. */
502 if (*cp >= MAX_STATES)
505 comp->flags &=~ SLF_TOSS;
506 comp->last_recv = *cp++;
508 /* this packet has an implicit state index. If we've
509 * had a line error since the last time we got an
510 * explicit state index, we have to toss the packet. */
511 if (comp->flags & SLF_TOSS) {
516 cs = &comp->rstate[comp->last_recv];
517 hlen = cs->cs_ip.ip_hl << 2;
518 th = (struct tcphdr *)&((u_char *)&cs->cs_ip)[hlen];
519 th->th_sum = htons((*cp << 8) | cp[1]);
521 if (changes & TCP_PUSH_BIT)
522 th->th_flags |= TH_PUSH;
524 th->th_flags &=~ TH_PUSH;
526 switch (changes & SPECIALS_MASK) {
529 u_int i = ntohs(cs->cs_ip.ip_len) - cs->cs_hlen;
530 th->th_ack = htonl(ntohl(th->th_ack) + i);
531 th->th_seq = htonl(ntohl(th->th_seq) + i);
536 th->th_seq = htonl(ntohl(th->th_seq) + ntohs(cs->cs_ip.ip_len)
541 if (changes & NEW_U) {
542 th->th_flags |= TH_URG;
545 th->th_flags &=~ TH_URG;
554 if (changes & NEW_I) {
555 DECODES(cs->cs_ip.ip_id)
557 cs->cs_ip.ip_id = htons(ntohs(cs->cs_ip.ip_id) + 1);
560 * At this point, cp points to the first byte of data in the
561 * packet. Fill in the IP total length and update the IP
567 /* we must have dropped some characters (crc should detect
568 * this but the old slip framing won't) */
571 total_len += cs->cs_hlen - vjlen;
572 cs->cs_ip.ip_len = htons(total_len);
574 /* recompute the ip header checksum */
575 bp = (u_int16_t *) &cs->cs_ip;
576 cs->cs_ip.ip_sum = 0;
577 for (changes = 0; hlen > 0; hlen -= 2)
579 changes = (changes & 0xffff) + (changes >> 16);
580 changes = (changes & 0xffff) + (changes >> 16);
581 cs->cs_ip.ip_sum = ~ changes;
583 *hdrp = (u_char *) &cs->cs_ip;
584 *hlenp = cs->cs_hlen;
588 comp->flags |= SLF_TOSS;