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
35 * Routines to compress and uncompess tcp packets (for transmission
36 * over low speed serial lines.
38 * Van Jacobson (van@helios.ee.lbl.gov), Dec 31, 1989:
39 * - Initial distribution.
43 #include <sys/param.h>
45 #include <sys/systm.h>
47 #include <netinet/in.h>
48 #include <netinet/in_systm.h>
49 #include <netinet/ip.h>
50 #include <netinet/tcp.h>
52 #include <net/slcompress.h>
55 #define INCR(counter) ++comp->counter;
60 #define BCMP(p1, p2, n) bcmp((void *)(p1), (void *)(p2), (int)(n))
61 #define BCOPY(p1, p2, n) bcopy((void *)(p1), (void *)(p2), (int)(n))
64 sl_compress_init(struct slcompress *comp, int max_state)
67 struct cstate *tstate = comp->tstate;
69 if (max_state == -1) {
70 max_state = MAX_STATES - 1;
71 bzero((char *)comp, sizeof(*comp));
73 /* Don't reset statistics */
74 bzero((char *)comp->tstate, sizeof(comp->tstate));
75 bzero((char *)comp->rstate, sizeof(comp->rstate));
77 for (i = max_state; i > 0; --i) {
79 tstate[i].cs_next = &tstate[i - 1];
81 tstate[0].cs_next = &tstate[max_state];
83 comp->last_cs = &tstate[0];
84 comp->last_recv = 255;
85 comp->last_xmit = 255;
86 comp->flags = SLF_TOSS;
89 /* ENCODE encodes a number that is known to be non-zero. ENCODEZ
90 * checks for zero (since zero has to be encoded in the long, 3 byte
94 if ((u_int16_t)(n) >= 256) { \
103 #define ENCODEZ(n) { \
104 if ((u_int16_t)(n) >= 256 || (u_int16_t)(n) == 0) { \
114 #define DECODEL(f) { \
116 (f) = htonl(ntohl(f) + ((cp[1] << 8) | cp[2])); \
119 (f) = htonl(ntohl(f) + (u_int32_t)*cp++); \
123 #define DECODES(f) { \
125 (f) = htons(ntohs(f) + ((cp[1] << 8) | cp[2])); \
128 (f) = htons(ntohs(f) + (u_int32_t)*cp++); \
132 #define DECODEU(f) { \
134 (f) = htons((cp[1] << 8) | cp[2]); \
137 (f) = htons((u_int32_t)*cp++); \
142 * Attempt to compress an outgoing TCP packet and return the type of
143 * the result. The caller must have already verified that the protocol
144 * is TCP. The first mbuf must contain the complete IP and TCP headers,
145 * and "ip" must be == mtod(m, struct ip *). "comp" supplies the
146 * compression state, and "compress_cid" tells us whether it is OK
147 * to leave out the CID field when feasible.
149 * The caller is responsible for adjusting m->m_pkthdr.len upon return,
150 * if m is an M_PKTHDR mbuf.
153 sl_compress_tcp(struct mbuf *m, struct ip *ip, struct slcompress *comp,
156 struct cstate *cs = comp->last_cs->cs_next;
157 u_int hlen = ip->ip_hl;
160 u_int deltaS, deltaA;
163 u_char *cp = new_seq;
166 * Bail if this is an IP fragment or if the TCP packet isn't
167 * `compressible' (i.e., ACK isn't set or some other control bit is
168 * set). (We assume that the caller has already made sure the
169 * packet is IP proto TCP).
171 if ((ip->ip_off & htons(0x3fff)) || m->m_len < 40)
174 th = (struct tcphdr *)&((int32_t *)ip)[hlen];
175 if ((th->th_flags & (TH_SYN|TH_FIN|TH_RST|TH_ACK)) != TH_ACK)
178 * Packet is compressible -- we're going to send either a
179 * COMPRESSED_TCP or UNCOMPRESSED_TCP packet. Either way we need
180 * to locate (or create) the connection state. Special case the
181 * most recently used connection since it's most likely to be used
182 * again & we don't have to do any reordering if it's used.
185 if (ip->ip_src.s_addr != cs->cs_ip.ip_src.s_addr ||
186 ip->ip_dst.s_addr != cs->cs_ip.ip_dst.s_addr ||
187 *(int32_t *)th != ((int32_t *)&cs->cs_ip)[cs->cs_ip.ip_hl]) {
189 * Wasn't the first -- search for it.
191 * States are kept in a circularly linked list with
192 * last_cs pointing to the end of the list. The
193 * list is kept in lru order by moving a state to the
194 * head of the list whenever it is referenced. Since
195 * the list is short and, empirically, the connection
196 * we want is almost always near the front, we locate
197 * states via linear search. If we don't find a state
198 * for the datagram, the oldest state is (re-)used.
201 struct cstate *lastcs = comp->last_cs;
204 lcs = cs; cs = cs->cs_next;
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
209 ((int32_t *)&cs->cs_ip)[cs->cs_ip.ip_hl])
211 } while (cs != lastcs);
214 * Didn't find it -- re-use oldest cstate. Send an
215 * uncompressed packet that tells the other side what
216 * connection number we're using for this conversation.
217 * Note that since the state list is circular, the oldest
218 * state points to the newest and we only need to set
219 * last_cs to update the lru linkage.
231 * Found it -- move to the front on the connection list.
236 lcs->cs_next = cs->cs_next;
237 cs->cs_next = lastcs->cs_next;
238 lastcs->cs_next = cs;
243 * Make sure that only what we expect to change changed. The first
244 * line of the `if' checks the IP protocol version, header length &
245 * type of service. The 2nd line checks the "Don't fragment" bit.
246 * The 3rd line checks the time-to-live and protocol (the protocol
247 * check is unnecessary but costless). The 4th line checks the TCP
248 * header length. The 5th line checks IP options, if any. The 6th
249 * line checks TCP options, if any. If any of these things are
250 * different between the previous & current datagram, we send the
251 * current datagram `uncompressed'.
253 oth = (struct tcphdr *)&((int32_t *)&cs->cs_ip)[hlen];
260 if (((u_int16_t *)ip)[0] != ((u_int16_t *)&cs->cs_ip)[0] ||
261 ((u_int16_t *)ip)[3] != ((u_int16_t *)&cs->cs_ip)[3] ||
262 ((u_int16_t *)ip)[4] != ((u_int16_t *)&cs->cs_ip)[4] ||
263 th->th_off != oth->th_off ||
265 BCMP(ip + 1, &cs->cs_ip + 1, (deltaS - 5) << 2)) ||
267 BCMP(th + 1, oth + 1, (th->th_off - 5) << 2)))
271 * Figure out which of the changing fields changed. The
272 * receiver expects changes in the order: urgent, window,
273 * ack, seq (the order minimizes the number of temporaries
274 * needed in this section of code).
276 if (th->th_flags & TH_URG) {
277 deltaS = ntohs(th->th_urp);
280 } else if (th->th_urp != oth->th_urp)
281 /* argh! URG not set but urp changed -- a sensible
282 * implementation should never do this but RFC793
283 * doesn't prohibit the change so we have to deal
287 deltaS = (u_int16_t)(ntohs(th->th_win) - ntohs(oth->th_win));
293 deltaA = ntohl(th->th_ack) - ntohl(oth->th_ack);
301 deltaS = ntohl(th->th_seq) - ntohl(oth->th_seq);
312 * Nothing changed. If this packet contains data and the
313 * last one didn't, this is probably a data packet following
314 * an ack (normal on an interactive connection) and we send
315 * it compressed. Otherwise it's probably a retransmit,
316 * retransmitted ack or window probe. Send it uncompressed
317 * in case the other side missed the compressed version.
319 if (ip->ip_len != cs->cs_ip.ip_len &&
320 ntohs(cs->cs_ip.ip_len) == hlen)
328 * actual changes match one of our special case encodings --
329 * send packet uncompressed.
334 if (deltaS == deltaA &&
335 deltaS == ntohs(cs->cs_ip.ip_len) - hlen) {
336 /* special case for echoed terminal traffic */
343 if (deltaS == ntohs(cs->cs_ip.ip_len) - hlen) {
344 /* special case for data xfer */
351 deltaS = ntohs(ip->ip_id) - ntohs(cs->cs_ip.ip_id);
356 if (th->th_flags & TH_PUSH)
357 changes |= TCP_PUSH_BIT;
359 * Grab the cksum before we overwrite it below. Then update our
360 * state with this packet's header.
362 deltaA = ntohs(th->th_sum);
363 BCOPY(ip, &cs->cs_ip, hlen);
366 * We want to use the original packet as our compressed packet.
367 * (cp - new_seq) is the number of bytes we need for compressed
368 * sequence numbers. In addition we need one byte for the change
369 * mask, one for the connection id and two for the tcp checksum.
370 * So, (cp - new_seq) + 4 bytes of header are needed. hlen is how
371 * many bytes of the original packet to toss so subtract the two to
372 * get the new packet size.
374 deltaS = cp - new_seq;
376 if (compress_cid == 0 || comp->last_xmit != cs->cs_id) {
377 comp->last_xmit = cs->cs_id;
380 *cp++ = changes | NEW_C;
391 BCOPY(new_seq, cp, deltaS);
393 return (TYPE_COMPRESSED_TCP);
396 * Update connection state cs & send uncompressed packet ('uncompressed'
397 * means a regular ip/tcp packet but with the 'conversation id' we hope
398 * to use on future compressed packets in the protocol field).
401 BCOPY(ip, &cs->cs_ip, hlen);
402 ip->ip_p = cs->cs_id;
403 comp->last_xmit = cs->cs_id;
404 return (TYPE_UNCOMPRESSED_TCP);
408 sl_uncompress_tcp(u_char **bufp, int len, u_int type, struct slcompress *comp)
413 cp = bufp? *bufp: NULL;
414 vjlen = sl_uncompress_tcp_core(cp, len, len, type, comp, &hdr, &hlen);
416 return (0); /* error */
418 return (len); /* was uncompressed already */
424 * At this point, cp points to the first byte of data in the
425 * packet. If we're not aligned on a 4-byte boundary, copy the
426 * data down so the ip & tcp headers will be aligned. Then back up
427 * cp by the tcp/ip header length to make room for the reconstructed
428 * header (we assume the packet we were handed has enough space to
429 * prepend 128 bytes of header).
431 if ((intptr_t)cp & 3) {
433 BCOPY(cp, ((intptr_t)cp &~ 3), len);
434 cp = (u_char *)((intptr_t)cp &~ 3);
438 BCOPY(hdr, cp, hlen);
445 * Uncompress a packet of total length total_len. The first buflen
446 * bytes are at buf; this must include the entire (compressed or
447 * uncompressed) TCP/IP header. This procedure returns the length
448 * of the VJ header, with a pointer to the uncompressed IP header
449 * in *hdrp and its length in *hlenp.
452 sl_uncompress_tcp_core(u_char *buf, int buflen, int total_len, u_int type,
453 struct slcompress *comp, u_char **hdrp, u_int *hlenp)
464 case TYPE_UNCOMPRESSED_TCP:
465 ip = (struct ip *) buf;
466 if (ip->ip_p >= MAX_STATES)
468 cs = &comp->rstate[comp->last_recv = ip->ip_p];
469 comp->flags &=~ SLF_TOSS;
470 ip->ip_p = IPPROTO_TCP;
472 * Calculate the size of the TCP/IP header and make sure that
473 * we don't overflow the space we have available for it.
475 hlen = ip->ip_hl << 2;
476 if (hlen + sizeof(struct tcphdr) > buflen)
478 hlen += ((struct tcphdr *)&((char *)ip)[hlen])->th_off << 2;
479 if (hlen > MAX_HDR || hlen > buflen)
481 BCOPY(ip, &cs->cs_ip, hlen);
483 INCR(sls_uncompressedin)
484 *hdrp = (u_char *) &cs->cs_ip;
491 case TYPE_COMPRESSED_TCP:
494 /* We've got a compressed packet. */
495 INCR(sls_compressedin)
498 if (changes & NEW_C) {
499 /* Make sure the state index is in range, then grab the state.
500 * If we have a good state index, clear the 'discard' flag. */
501 if (*cp >= MAX_STATES)
504 comp->flags &=~ SLF_TOSS;
505 comp->last_recv = *cp++;
507 /* this packet has an implicit state index. If we've
508 * had a line error since the last time we got an
509 * explicit state index, we have to toss the packet. */
510 if (comp->flags & SLF_TOSS) {
515 cs = &comp->rstate[comp->last_recv];
516 hlen = cs->cs_ip.ip_hl << 2;
517 th = (struct tcphdr *)&((u_char *)&cs->cs_ip)[hlen];
518 th->th_sum = htons((*cp << 8) | cp[1]);
520 if (changes & TCP_PUSH_BIT)
521 th->th_flags |= TH_PUSH;
523 th->th_flags &=~ TH_PUSH;
525 switch (changes & SPECIALS_MASK) {
528 u_int i = ntohs(cs->cs_ip.ip_len) - cs->cs_hlen;
529 th->th_ack = htonl(ntohl(th->th_ack) + i);
530 th->th_seq = htonl(ntohl(th->th_seq) + i);
535 th->th_seq = htonl(ntohl(th->th_seq) + ntohs(cs->cs_ip.ip_len)
540 if (changes & NEW_U) {
541 th->th_flags |= TH_URG;
544 th->th_flags &=~ TH_URG;
553 if (changes & NEW_I) {
554 DECODES(cs->cs_ip.ip_id)
556 cs->cs_ip.ip_id = htons(ntohs(cs->cs_ip.ip_id) + 1);
559 * At this point, cp points to the first byte of data in the
560 * packet. Fill in the IP total length and update the IP
566 /* we must have dropped some characters (crc should detect
567 * this but the old slip framing won't) */
570 total_len += cs->cs_hlen - vjlen;
571 cs->cs_ip.ip_len = htons(total_len);
573 /* recompute the ip header checksum */
574 bp = (u_int16_t *) &cs->cs_ip;
575 cs->cs_ip.ip_sum = 0;
576 for (changes = 0; hlen > 0; hlen -= 2)
578 changes = (changes & 0xffff) + (changes >> 16);
579 changes = (changes & 0xffff) + (changes >> 16);
580 cs->cs_ip.ip_sum = ~ changes;
582 *hdrp = (u_char *) &cs->cs_ip;
583 *hlenp = cs->cs_hlen;
587 comp->flags |= SLF_TOSS;