xref: /original-bsd/sys/net/slcompress.c (revision 0d42da1b)
1 /*-
2  * Copyright (c) 1989, 1993, 1994
3  *	The Regents of the University of California.  All rights reserved.
4  *
5  * %sccs.include.redist.c%
6  *
7  *	@(#)slcompress.c	8.2 (Berkeley) 04/16/94
8  */
9 
10 /*
11  * Routines to compress and uncompess tcp packets (for transmission
12  * over low speed serial lines.
13  *
14  * Van Jacobson (van@helios.ee.lbl.gov), Dec 31, 1989:
15  *	- Initial distribution.
16  *
17  * static char rcsid[] =
18  * "$Header: slcompress.c,v 1.19 89/12/31 08:52:59 van Exp $";
19  */
20 
21 #include <sys/param.h>
22 #include <sys/mbuf.h>
23 
24 #include <netinet/in.h>
25 #include <netinet/in_systm.h>
26 #include <netinet/ip.h>
27 #include <netinet/tcp.h>
28 
29 #include <net/slcompress.h>
30 
31 #ifndef SL_NO_STATS
32 #define INCR(counter) ++comp->counter;
33 #else
34 #define INCR(counter)
35 #endif
36 
37 #define BCMP(p1, p2, n) bcmp((char *)(p1), (char *)(p2), (int)(n))
38 #define BCOPY(p1, p2, n) bcopy((char *)(p1), (char *)(p2), (int)(n))
39 #ifndef KERNEL
40 #define ovbcopy bcopy
41 #endif
42 
43 void
sl_compress_init(comp)44 sl_compress_init(comp)
45 	struct slcompress *comp;
46 {
47 	register u_int i;
48 	register struct cstate *tstate = comp->tstate;
49 
50 	bzero((char *)comp, sizeof(*comp));
51 	for (i = MAX_STATES - 1; i > 0; --i) {
52 		tstate[i].cs_id = i;
53 		tstate[i].cs_next = &tstate[i - 1];
54 	}
55 	tstate[0].cs_next = &tstate[MAX_STATES - 1];
56 	tstate[0].cs_id = 0;
57 	comp->last_cs = &tstate[0];
58 	comp->last_recv = 255;
59 	comp->last_xmit = 255;
60 	comp->flags = SLF_TOSS;
61 }
62 
63 
64 /* ENCODE encodes a number that is known to be non-zero.  ENCODEZ
65  * checks for zero (since zero has to be encoded in the long, 3 byte
66  * form).
67  */
68 #define ENCODE(n) { \
69 	if ((u_short)(n) >= 256) { \
70 		*cp++ = 0; \
71 		cp[1] = (n); \
72 		cp[0] = (n) >> 8; \
73 		cp += 2; \
74 	} else { \
75 		*cp++ = (n); \
76 	} \
77 }
78 #define ENCODEZ(n) { \
79 	if ((u_short)(n) >= 256 || (u_short)(n) == 0) { \
80 		*cp++ = 0; \
81 		cp[1] = (n); \
82 		cp[0] = (n) >> 8; \
83 		cp += 2; \
84 	} else { \
85 		*cp++ = (n); \
86 	} \
87 }
88 
89 #define DECODEL(f) { \
90 	if (*cp == 0) {\
91 		(f) = htonl(ntohl(f) + ((cp[1] << 8) | cp[2])); \
92 		cp += 3; \
93 	} else { \
94 		(f) = htonl(ntohl(f) + (u_long)*cp++); \
95 	} \
96 }
97 
98 #define DECODES(f) { \
99 	if (*cp == 0) {\
100 		(f) = htons(ntohs(f) + ((cp[1] << 8) | cp[2])); \
101 		cp += 3; \
102 	} else { \
103 		(f) = htons(ntohs(f) + (u_long)*cp++); \
104 	} \
105 }
106 
107 #define DECODEU(f) { \
108 	if (*cp == 0) {\
109 		(f) = htons((cp[1] << 8) | cp[2]); \
110 		cp += 3; \
111 	} else { \
112 		(f) = htons((u_long)*cp++); \
113 	} \
114 }
115 
116 u_int
sl_compress_tcp(m,ip,comp,compress_cid)117 sl_compress_tcp(m, ip, comp, compress_cid)
118 	struct mbuf *m;
119 	register struct ip *ip;
120 	struct slcompress *comp;
121 	int compress_cid;
122 {
123 	register struct cstate *cs = comp->last_cs->cs_next;
124 	register u_int hlen = ip->ip_hl;
125 	register struct tcphdr *oth;
126 	register struct tcphdr *th;
127 	register u_int deltaS, deltaA;
128 	register u_int changes = 0;
129 	u_char new_seq[16];
130 	register u_char *cp = new_seq;
131 
132 	/*
133 	 * Bail if this is an IP fragment or if the TCP packet isn't
134 	 * `compressible' (i.e., ACK isn't set or some other control bit is
135 	 * set).  (We assume that the caller has already made sure the
136 	 * packet is IP proto TCP).
137 	 */
138 	if ((ip->ip_off & htons(0x3fff)) || m->m_len < 40)
139 		return (TYPE_IP);
140 
141 	th = (struct tcphdr *)&((int *)ip)[hlen];
142 	if ((th->th_flags & (TH_SYN|TH_FIN|TH_RST|TH_ACK)) != TH_ACK)
143 		return (TYPE_IP);
144 	/*
145 	 * Packet is compressible -- we're going to send either a
146 	 * COMPRESSED_TCP or UNCOMPRESSED_TCP packet.  Either way we need
147 	 * to locate (or create) the connection state.  Special case the
148 	 * most recently used connection since it's most likely to be used
149 	 * again & we don't have to do any reordering if it's used.
150 	 */
151 	INCR(sls_packets)
152 	if (ip->ip_src.s_addr != cs->cs_ip.ip_src.s_addr ||
153 	    ip->ip_dst.s_addr != cs->cs_ip.ip_dst.s_addr ||
154 	    *(int *)th != ((int *)&cs->cs_ip)[cs->cs_ip.ip_hl]) {
155 		/*
156 		 * Wasn't the first -- search for it.
157 		 *
158 		 * States are kept in a circularly linked list with
159 		 * last_cs pointing to the end of the list.  The
160 		 * list is kept in lru order by moving a state to the
161 		 * head of the list whenever it is referenced.  Since
162 		 * the list is short and, empirically, the connection
163 		 * we want is almost always near the front, we locate
164 		 * states via linear search.  If we don't find a state
165 		 * for the datagram, the oldest state is (re-)used.
166 		 */
167 		register struct cstate *lcs;
168 		register struct cstate *lastcs = comp->last_cs;
169 
170 		do {
171 			lcs = cs; cs = cs->cs_next;
172 			INCR(sls_searches)
173 			if (ip->ip_src.s_addr == cs->cs_ip.ip_src.s_addr
174 			    && ip->ip_dst.s_addr == cs->cs_ip.ip_dst.s_addr
175 			    && *(int *)th == ((int *)&cs->cs_ip)[cs->cs_ip.ip_hl])
176 				goto found;
177 		} while (cs != lastcs);
178 
179 		/*
180 		 * Didn't find it -- re-use oldest cstate.  Send an
181 		 * uncompressed packet that tells the other side what
182 		 * connection number we're using for this conversation.
183 		 * Note that since the state list is circular, the oldest
184 		 * state points to the newest and we only need to set
185 		 * last_cs to update the lru linkage.
186 		 */
187 		INCR(sls_misses)
188 		comp->last_cs = lcs;
189 		hlen += th->th_off;
190 		hlen <<= 2;
191 		goto uncompressed;
192 
193 	found:
194 		/*
195 		 * Found it -- move to the front on the connection list.
196 		 */
197 		if (cs == lastcs)
198 			comp->last_cs = lcs;
199 		else {
200 			lcs->cs_next = cs->cs_next;
201 			cs->cs_next = lastcs->cs_next;
202 			lastcs->cs_next = cs;
203 		}
204 	}
205 
206 	/*
207 	 * Make sure that only what we expect to change changed. The first
208 	 * line of the `if' checks the IP protocol version, header length &
209 	 * type of service.  The 2nd line checks the "Don't fragment" bit.
210 	 * The 3rd line checks the time-to-live and protocol (the protocol
211 	 * check is unnecessary but costless).  The 4th line checks the TCP
212 	 * header length.  The 5th line checks IP options, if any.  The 6th
213 	 * line checks TCP options, if any.  If any of these things are
214 	 * different between the previous & current datagram, we send the
215 	 * current datagram `uncompressed'.
216 	 */
217 	oth = (struct tcphdr *)&((int *)&cs->cs_ip)[hlen];
218 	deltaS = hlen;
219 	hlen += th->th_off;
220 	hlen <<= 2;
221 
222 	if (((u_short *)ip)[0] != ((u_short *)&cs->cs_ip)[0] ||
223 	    ((u_short *)ip)[3] != ((u_short *)&cs->cs_ip)[3] ||
224 	    ((u_short *)ip)[4] != ((u_short *)&cs->cs_ip)[4] ||
225 	    th->th_off != oth->th_off ||
226 	    (deltaS > 5 &&
227 	     BCMP(ip + 1, &cs->cs_ip + 1, (deltaS - 5) << 2)) ||
228 	    (th->th_off > 5 &&
229 	     BCMP(th + 1, oth + 1, (th->th_off - 5) << 2)))
230 		goto uncompressed;
231 
232 	/*
233 	 * Figure out which of the changing fields changed.  The
234 	 * receiver expects changes in the order: urgent, window,
235 	 * ack, seq (the order minimizes the number of temporaries
236 	 * needed in this section of code).
237 	 */
238 	if (th->th_flags & TH_URG) {
239 		deltaS = ntohs(th->th_urp);
240 		ENCODEZ(deltaS);
241 		changes |= NEW_U;
242 	} else if (th->th_urp != oth->th_urp)
243 		/* argh! URG not set but urp changed -- a sensible
244 		 * implementation should never do this but RFC793
245 		 * doesn't prohibit the change so we have to deal
246 		 * with it. */
247 		 goto uncompressed;
248 
249 	if (deltaS = (u_short)(ntohs(th->th_win) - ntohs(oth->th_win))) {
250 		ENCODE(deltaS);
251 		changes |= NEW_W;
252 	}
253 
254 	if (deltaA = ntohl(th->th_ack) - ntohl(oth->th_ack)) {
255 		if (deltaA > 0xffff)
256 			goto uncompressed;
257 		ENCODE(deltaA);
258 		changes |= NEW_A;
259 	}
260 
261 	if (deltaS = ntohl(th->th_seq) - ntohl(oth->th_seq)) {
262 		if (deltaS > 0xffff)
263 			goto uncompressed;
264 		ENCODE(deltaS);
265 		changes |= NEW_S;
266 	}
267 
268 	switch(changes) {
269 
270 	case 0:
271 		/*
272 		 * Nothing changed. If this packet contains data and the
273 		 * last one didn't, this is probably a data packet following
274 		 * an ack (normal on an interactive connection) and we send
275 		 * it compressed.  Otherwise it's probably a retransmit,
276 		 * retransmitted ack or window probe.  Send it uncompressed
277 		 * in case the other side missed the compressed version.
278 		 */
279 		if (ip->ip_len != cs->cs_ip.ip_len &&
280 		    ntohs(cs->cs_ip.ip_len) == hlen)
281 			break;
282 
283 		/* (fall through) */
284 
285 	case SPECIAL_I:
286 	case SPECIAL_D:
287 		/*
288 		 * actual changes match one of our special case encodings --
289 		 * send packet uncompressed.
290 		 */
291 		goto uncompressed;
292 
293 	case NEW_S|NEW_A:
294 		if (deltaS == deltaA &&
295 		    deltaS == ntohs(cs->cs_ip.ip_len) - hlen) {
296 			/* special case for echoed terminal traffic */
297 			changes = SPECIAL_I;
298 			cp = new_seq;
299 		}
300 		break;
301 
302 	case NEW_S:
303 		if (deltaS == ntohs(cs->cs_ip.ip_len) - hlen) {
304 			/* special case for data xfer */
305 			changes = SPECIAL_D;
306 			cp = new_seq;
307 		}
308 		break;
309 	}
310 
311 	deltaS = ntohs(ip->ip_id) - ntohs(cs->cs_ip.ip_id);
312 	if (deltaS != 1) {
313 		ENCODEZ(deltaS);
314 		changes |= NEW_I;
315 	}
316 	if (th->th_flags & TH_PUSH)
317 		changes |= TCP_PUSH_BIT;
318 	/*
319 	 * Grab the cksum before we overwrite it below.  Then update our
320 	 * state with this packet's header.
321 	 */
322 	deltaA = ntohs(th->th_sum);
323 	BCOPY(ip, &cs->cs_ip, hlen);
324 
325 	/*
326 	 * We want to use the original packet as our compressed packet.
327 	 * (cp - new_seq) is the number of bytes we need for compressed
328 	 * sequence numbers.  In addition we need one byte for the change
329 	 * mask, one for the connection id and two for the tcp checksum.
330 	 * So, (cp - new_seq) + 4 bytes of header are needed.  hlen is how
331 	 * many bytes of the original packet to toss so subtract the two to
332 	 * get the new packet size.
333 	 */
334 	deltaS = cp - new_seq;
335 	cp = (u_char *)ip;
336 	if (compress_cid == 0 || comp->last_xmit != cs->cs_id) {
337 		comp->last_xmit = cs->cs_id;
338 		hlen -= deltaS + 4;
339 		cp += hlen;
340 		*cp++ = changes | NEW_C;
341 		*cp++ = cs->cs_id;
342 	} else {
343 		hlen -= deltaS + 3;
344 		cp += hlen;
345 		*cp++ = changes;
346 	}
347 	m->m_len -= hlen;
348 	m->m_data += hlen;
349 	*cp++ = deltaA >> 8;
350 	*cp++ = deltaA;
351 	BCOPY(new_seq, cp, deltaS);
352 	INCR(sls_compressed)
353 	return (TYPE_COMPRESSED_TCP);
354 
355 	/*
356 	 * Update connection state cs & send uncompressed packet ('uncompressed'
357 	 * means a regular ip/tcp packet but with the 'conversation id' we hope
358 	 * to use on future compressed packets in the protocol field).
359 	 */
360 uncompressed:
361 	BCOPY(ip, &cs->cs_ip, hlen);
362 	ip->ip_p = cs->cs_id;
363 	comp->last_xmit = cs->cs_id;
364 	return (TYPE_UNCOMPRESSED_TCP);
365 }
366 
367 
368 int
sl_uncompress_tcp(bufp,len,type,comp)369 sl_uncompress_tcp(bufp, len, type, comp)
370 	u_char **bufp;
371 	int len;
372 	u_int type;
373 	struct slcompress *comp;
374 {
375 	register u_char *cp;
376 	register u_int hlen, changes;
377 	register struct tcphdr *th;
378 	register struct cstate *cs;
379 	register struct ip *ip;
380 
381 	switch (type) {
382 
383 	case TYPE_UNCOMPRESSED_TCP:
384 		ip = (struct ip *) *bufp;
385 		if (ip->ip_p >= MAX_STATES)
386 			goto bad;
387 		cs = &comp->rstate[comp->last_recv = ip->ip_p];
388 		comp->flags &=~ SLF_TOSS;
389 		ip->ip_p = IPPROTO_TCP;
390 		hlen = ip->ip_hl;
391 		hlen += ((struct tcphdr *)&((int *)ip)[hlen])->th_off;
392 		hlen <<= 2;
393 		BCOPY(ip, &cs->cs_ip, hlen);
394 		cs->cs_ip.ip_sum = 0;
395 		cs->cs_hlen = hlen;
396 		INCR(sls_uncompressedin)
397 		return (len);
398 
399 	default:
400 		goto bad;
401 
402 	case TYPE_COMPRESSED_TCP:
403 		break;
404 	}
405 	/* We've got a compressed packet. */
406 	INCR(sls_compressedin)
407 	cp = *bufp;
408 	changes = *cp++;
409 	if (changes & NEW_C) {
410 		/* Make sure the state index is in range, then grab the state.
411 		 * If we have a good state index, clear the 'discard' flag. */
412 		if (*cp >= MAX_STATES)
413 			goto bad;
414 
415 		comp->flags &=~ SLF_TOSS;
416 		comp->last_recv = *cp++;
417 	} else {
418 		/* this packet has an implicit state index.  If we've
419 		 * had a line error since the last time we got an
420 		 * explicit state index, we have to toss the packet. */
421 		if (comp->flags & SLF_TOSS) {
422 			INCR(sls_tossed)
423 			return (0);
424 		}
425 	}
426 	cs = &comp->rstate[comp->last_recv];
427 	hlen = cs->cs_ip.ip_hl << 2;
428 	th = (struct tcphdr *)&((u_char *)&cs->cs_ip)[hlen];
429 	th->th_sum = htons((*cp << 8) | cp[1]);
430 	cp += 2;
431 	if (changes & TCP_PUSH_BIT)
432 		th->th_flags |= TH_PUSH;
433 	else
434 		th->th_flags &=~ TH_PUSH;
435 
436 	switch (changes & SPECIALS_MASK) {
437 	case SPECIAL_I:
438 		{
439 		register u_int i = ntohs(cs->cs_ip.ip_len) - cs->cs_hlen;
440 		th->th_ack = htonl(ntohl(th->th_ack) + i);
441 		th->th_seq = htonl(ntohl(th->th_seq) + i);
442 		}
443 		break;
444 
445 	case SPECIAL_D:
446 		th->th_seq = htonl(ntohl(th->th_seq) + ntohs(cs->cs_ip.ip_len)
447 				   - cs->cs_hlen);
448 		break;
449 
450 	default:
451 		if (changes & NEW_U) {
452 			th->th_flags |= TH_URG;
453 			DECODEU(th->th_urp)
454 		} else
455 			th->th_flags &=~ TH_URG;
456 		if (changes & NEW_W)
457 			DECODES(th->th_win)
458 		if (changes & NEW_A)
459 			DECODEL(th->th_ack)
460 		if (changes & NEW_S)
461 			DECODEL(th->th_seq)
462 		break;
463 	}
464 	if (changes & NEW_I) {
465 		DECODES(cs->cs_ip.ip_id)
466 	} else
467 		cs->cs_ip.ip_id = htons(ntohs(cs->cs_ip.ip_id) + 1);
468 
469 	/*
470 	 * At this point, cp points to the first byte of data in the
471 	 * packet.  If we're not aligned on a 4-byte boundary, copy the
472 	 * data down so the ip & tcp headers will be aligned.  Then back up
473 	 * cp by the tcp/ip header length to make room for the reconstructed
474 	 * header (we assume the packet we were handed has enough space to
475 	 * prepend 128 bytes of header).  Adjust the length to account for
476 	 * the new header & fill in the IP total length.
477 	 */
478 	len -= (cp - *bufp);
479 	if (len < 0)
480 		/* we must have dropped some characters (crc should detect
481 		 * this but the old slip framing won't) */
482 		goto bad;
483 
484 	if ((int)cp & 3) {
485 		if (len > 0)
486 			(void) ovbcopy(cp, (caddr_t)((int)cp &~ 3), len);
487 		cp = (u_char *)((int)cp &~ 3);
488 	}
489 	cp -= cs->cs_hlen;
490 	len += cs->cs_hlen;
491 	cs->cs_ip.ip_len = htons(len);
492 	BCOPY(&cs->cs_ip, cp, cs->cs_hlen);
493 	*bufp = cp;
494 
495 	/* recompute the ip header checksum */
496 	{
497 		register u_short *bp = (u_short *)cp;
498 		for (changes = 0; hlen > 0; hlen -= 2)
499 			changes += *bp++;
500 		changes = (changes & 0xffff) + (changes >> 16);
501 		changes = (changes & 0xffff) + (changes >> 16);
502 		((struct ip *)cp)->ip_sum = ~ changes;
503 	}
504 	return (len);
505 bad:
506 	comp->flags |= SLF_TOSS;
507 	INCR(sls_errorin)
508 	return (0);
509 }
510