/*
 * Routines to compress and uncompess tcp packets (for transmission
 * over low speed serial lines.
 *
 * Copyright (c) 1989 Regents of the University of California.
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms are permitted
 * provided that the above copyright notice and this paragraph are
 * duplicated in all such forms and that any documentation,
 * advertising materials, and other materials related to such
 * distribution and use acknowledge that the software was developed
 * by the University of California, Berkeley.  The name of the
 * University may not be used to endorse or promote products derived
 * from this software without specific prior written permission.
 * THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR
 * IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED
 * WARRANTIES OF MERCHANTIBILITY AND FITNESS FOR A PARTICULAR PURPOSE.
 *
 *	Van Jacobson (van@helios.ee.lbl.gov), Dec 31, 1989:
 *	- Initial distribution.
 *
 * Modified June 1993 by Paul Mackerras, paulus@cs.anu.edu.au,
 * so that the entire packet being decompressed doesn't have
 * to be in contiguous memory (just the compressed header).
 *
 * Modified March 1998 by Guy Lancaster, glanca@gesn.com,
 * for a 16 bit processor.
 */

#include "pppconf.h"
#include "ppp.h"
#include "vj.h"
#include "pppdebug.h"


#if VJ_SUPPORT > 0

#ifdef LINK_STATS
#define INCR(counter) ++comp->stats.counter
#else
#define INCR(counter)
#endif

#if defined(NO_CHAR_BITFIELDS)
#define getip_hl(base)	((base).ip_hl_v&0xf)
#define getth_off(base)	(((base).th_x2_off&0xf0)>>4)
#else
#define getip_hl(base)	((base).ip_hl)
#define getth_off(base)	((base).th_off)
#endif

void vj_compress_init(struct vjcompress *comp)
{
	register u_int i;
	register struct cstate *tstate = comp->tstate;
	
#if MAX_SLOTS == 0
	memset((char *)comp, 0, sizeof(*comp));
#endif
	comp->maxSlotIndex = MAX_SLOTS - 1;
	comp->compressSlot = 0;		/* Disable slot ID compression by default. */
	for (i = MAX_SLOTS - 1; i > 0; --i) {
		tstate[i].cs_id = i;
		tstate[i].cs_next = &tstate[i - 1];
	}
	tstate[0].cs_next = &tstate[MAX_SLOTS - 1];
	tstate[0].cs_id = 0;
	comp->last_cs = &tstate[0];
	comp->last_recv = 255;
	comp->last_xmit = 255;
	comp->flags = VJF_TOSS;
}


/* ENCODE encodes a number that is known to be non-zero.  ENCODEZ
 * checks for zero (since zero has to be encoded in the long, 3 byte
 * form).
 */
#define ENCODE(n) { \
	if ((u_short)(n) >= 256) { \
		*cp++ = 0; \
		cp[1] = (n); \
		cp[0] = (n) >> 8; \
		cp += 2; \
	} else { \
		*cp++ = (n); \
	} \
}
#define ENCODEZ(n) { \
	if ((u_short)(n) >= 256 || (u_short)(n) == 0) { \
		*cp++ = 0; \
		cp[1] = (n); \
		cp[0] = (n) >> 8; \
		cp += 2; \
	} else { \
		*cp++ = (n); \
	} \
}

#define DECODEL(f) { \
	if (*cp == 0) {\
		u32_t tmp = ntohl(f) + ((cp[1] << 8) | cp[2]); \
		(f) = htonl(tmp); \
		cp += 3; \
	} else { \
		u32_t tmp = ntohl(f) + (u32_t)*cp++; \
		(f) = htonl(tmp); \
	} \
}

#define DECODES(f) { \
	if (*cp == 0) {\
		u_short tmp = ntohs(f) + (((u_short)cp[1] << 8) | cp[2]); \
		(f) = htons(tmp); \
		cp += 3; \
	} else { \
		u_short tmp = ntohs(f) + (u_short)*cp++; \
		(f) = htons(tmp); \
	} \
}

#define DECODEU(f) { \
	if (*cp == 0) {\
		(f) = htons(((u_short)cp[1] << 8) | cp[2]); \
		cp += 3; \
	} else { \
		(f) = htons((u_short)*cp++); \
	} \
}

/*
 * vj_compress_tcp - Attempt to do Van Jacobsen header compression on a
 * packet.  This assumes that nb and comp are not null and that the first
 * buffer of the chain contains a valid IP header.
 * Return the VJ type code indicating whether or not the packet was
 * compressed.
 */
u_int vj_compress_tcp(
	struct vjcompress *comp,
	struct pbuf *pb
)
{
	register struct ip *ip = (struct ip *)pb->payload;
	register struct cstate *cs = comp->last_cs->cs_next;
	register u_short hlen = getip_hl(*ip);
	register struct tcphdr *oth;
	register struct tcphdr *th;
	register u_short deltaS, deltaA;
	register u_long deltaL;
	register u_int changes = 0;
	u_char new_seq[16];
	register u_char *cp = new_seq;

	/*	
	 * Check that the packet is IP proto TCP.
	 */
	if (ip->ip_p != IPPROTO_TCP)
		return (TYPE_IP);
		
	/*
	 * Bail if this is an IP fragment or if the TCP packet isn't
	 * `compressible' (i.e., ACK isn't set or some other control bit is
	 * set).  
	 */
	if ((ip->ip_off & htons(0x3fff)) || pb->tot_len < 40)
		return (TYPE_IP);
	th = (struct tcphdr *)&((long *)ip)[hlen];
	if ((th->th_flags & (TCP_SYN|TCP_FIN|TCP_RST|TCP_ACK)) != TCP_ACK)
		return (TYPE_IP);
		
	/*
	 * Packet is compressible -- we're going to send either a
	 * COMPRESSED_TCP or UNCOMPRESSED_TCP packet.  Either way we need
	 * to locate (or create) the connection state.  Special case the
	 * most recently used connection since it's most likely to be used
	 * again & we don't have to do any reordering if it's used.
	 */
	INCR(vjs_packets);
	if (ip->ip_src.s_addr != cs->cs_ip.ip_src.s_addr 
			|| ip->ip_dst.s_addr != cs->cs_ip.ip_dst.s_addr 
			|| *(long *)th != ((long *)&cs->cs_ip)[getip_hl(cs->cs_ip)]) {
		/*
		 * Wasn't the first -- search for it.
		 *
		 * States are kept in a circularly linked list with
		 * last_cs pointing to the end of the list.  The
		 * list is kept in lru order by moving a state to the
		 * head of the list whenever it is referenced.  Since
		 * the list is short and, empirically, the connection
		 * we want is almost always near the front, we locate
		 * states via linear search.  If we don't find a state
		 * for the datagram, the oldest state is (re-)used.
		 */
		register struct cstate *lcs;
		register struct cstate *lastcs = comp->last_cs;
		
		do {
			lcs = cs; cs = cs->cs_next;
			INCR(vjs_searches);
			if (ip->ip_src.s_addr == cs->cs_ip.ip_src.s_addr
					&& ip->ip_dst.s_addr == cs->cs_ip.ip_dst.s_addr
					&& *(long *)th == ((long *)&cs->cs_ip)[getip_hl(cs->cs_ip)])
				goto found;
		} while (cs != lastcs);
		
		/*
		 * Didn't find it -- re-use oldest cstate.  Send an
		 * uncompressed packet that tells the other side what
		 * connection number we're using for this conversation.
		 * Note that since the state list is circular, the oldest
		 * state points to the newest and we only need to set
		 * last_cs to update the lru linkage.
		 */
		INCR(vjs_misses);
		comp->last_cs = lcs;
		hlen += getth_off(*th);
		hlen <<= 2;
		/* Check that the IP/TCP headers are contained in the first buffer. */
		if (hlen > pb->len)
			return (TYPE_IP);
		goto uncompressed;
		
		found:
		/*
		 * Found it -- move to the front on the connection list.
		 */
		if (cs == lastcs)
			comp->last_cs = lcs;
		else {
			lcs->cs_next = cs->cs_next;
			cs->cs_next = lastcs->cs_next;
			lastcs->cs_next = cs;
		}
	}
	
	oth = (struct tcphdr *)&((long *)&cs->cs_ip)[hlen];
	deltaS = hlen;
	hlen += getth_off(*th);
	hlen <<= 2;
	/* Check that the IP/TCP headers are contained in the first buffer. */
	if (hlen > pb->len) {
		PPPDEBUG((LOG_INFO, "vj_compress_tcp: header len %d spans buffers\n", 
					hlen));
		return (TYPE_IP);
	}
	
	/*
	 * Make sure that only what we expect to change changed. The first
	 * line of the `if' checks the IP protocol version, header length &
	 * type of service.  The 2nd line checks the "Don't fragment" bit.
	 * The 3rd line checks the time-to-live and protocol (the protocol
	 * check is unnecessary but costless).  The 4th line checks the TCP
	 * header length.  The 5th line checks IP options, if any.  The 6th
	 * line checks TCP options, if any.  If any of these things are
	 * different between the previous & current datagram, we send the
	 * current datagram `uncompressed'.
	 */
	if (((u_short *)ip)[0] != ((u_short *)&cs->cs_ip)[0] 
			|| ((u_short *)ip)[3] != ((u_short *)&cs->cs_ip)[3] 
			|| ((u_short *)ip)[4] != ((u_short *)&cs->cs_ip)[4] 
			|| getth_off(*th) != getth_off(*oth) 
			|| (deltaS > 5 && BCMP(ip + 1, &cs->cs_ip + 1, (deltaS - 5) << 2)) 
			|| (getth_off(*th) > 5 && BCMP(th + 1, oth + 1, (getth_off(*th) - 5) << 2)))
		goto uncompressed;
	
	/*
	 * Figure out which of the changing fields changed.  The
	 * receiver expects changes in the order: urgent, window,
	 * ack, seq (the order minimizes the number of temporaries
	 * needed in this section of code).
	 */
	if (th->th_flags & TCP_URG) {
		deltaS = ntohs(th->th_urp);
		ENCODEZ(deltaS);
		changes |= NEW_U;
	} else if (th->th_urp != oth->th_urp)
		/* argh! URG not set but urp changed -- a sensible
		 * implementation should never do this but RFC793
		 * doesn't prohibit the change so we have to deal
		 * with it. */
		goto uncompressed;
	
	if ((deltaS = (u_short)(ntohs(th->th_win) - ntohs(oth->th_win))) != 0) {
		ENCODE(deltaS);
		changes |= NEW_W;
	}
	
	if ((deltaL = ntohl(th->th_ack) - ntohl(oth->th_ack)) != 0) {
		if (deltaL > 0xffff)
			goto uncompressed;
		deltaA = (u_short)deltaL;
		ENCODE(deltaA);
		changes |= NEW_A;
	}
	
	if ((deltaL = ntohl(th->th_seq) - ntohl(oth->th_seq)) != 0) {
		if (deltaL > 0xffff)
			goto uncompressed;
		deltaS = (u_short)deltaL;
		ENCODE(deltaS);
		changes |= NEW_S;
	}