source: rtems/cpukit/libnetworking/net/slcompress.c @ 1cb54d1f

/*-
 * Copyright (c) 1989, 1993, 1994
 *      The Regents of the University of California.  All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 4. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 *      @(#)slcompress.c        8.2 (Berkeley) 4/16/94
 * $FreeBSD: src/sys/net/slcompress.c,v 1.19 2004/04/07 20:46:12 imp Exp $
 */

/*
 * Routines to compress and uncompess tcp packets (for transmission
 * over low speed serial lines.
 *
 * Van Jacobson (van@helios.ee.lbl.gov), Dec 31, 1989:
 *      - Initial distribution.
 *
 */

#include <stdint.h>

#include <sys/param.h>
#include <sys/mbuf.h>
#include <sys/systm.h>

#include <netinet/in.h>
#include <netinet/in_systm.h>
#include <netinet/ip.h>
#include <netinet/tcp.h>

#include <net/slcompress.h>

#ifndef SL_NO_STATS
#define INCR(counter) ++comp->counter;
#else
#define INCR(counter)
#endif

#define BCMP(p1, p2, n) bcmp((void *)(p1), (void *)(p2), (int)(n))
#define BCOPY(p1, p2, n) bcopy((void *)(p1), (void *)(p2), (int)(n))

void
sl_compress_init(struct slcompress *comp, int max_state)
{
        register u_int i;
        register struct cstate *tstate = comp->tstate;

        if (max_state == -1) {
                max_state = MAX_STATES - 1;
                bzero((char *)comp, sizeof(*comp));
        } else {
                /* Don't reset statistics */
                bzero((char *)comp->tstate, sizeof(comp->tstate));
                bzero((char *)comp->rstate, sizeof(comp->rstate));
        }
        for (i = max_state; i > 0; --i) {
                tstate[i].cs_id = i;
                tstate[i].cs_next = &tstate[i - 1];
        }
        tstate[0].cs_next = &tstate[max_state];
        tstate[0].cs_id = 0;
        comp->last_cs = &tstate[0];
        comp->last_recv = 255;
        comp->last_xmit = 255;
        comp->flags = SLF_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_int16_t)(n) >= 256) { \
                *cp++ = 0; \
                cp[1] = (n); \
                cp[0] = (n) >> 8; \
                cp += 2; \
        } else { \
                *cp++ = (n); \
        } \
}
#define ENCODEZ(n) { \
        if ((u_int16_t)(n) >= 256 || (u_int16_t)(n) == 0) { \
                *cp++ = 0; \
                cp[1] = (n); \
                cp[0] = (n) >> 8; \
                cp += 2; \
        } else { \
                *cp++ = (n); \
        } \
}

#define DECODEL(f) { \
        if (*cp == 0) {\
                (f) = htonl(ntohl(f) + ((cp[1] << 8) | cp[2])); \
                cp += 3; \
        } else { \
                (f) = htonl(ntohl(f) + (u_int32_t)*cp++); \
        } \
}

#define DECODES(f) { \
        if (*cp == 0) {\
                (f) = htons(ntohs(f) + ((cp[1] << 8) | cp[2])); \
                cp += 3; \
        } else { \
                (f) = htons(ntohs(f) + (u_int32_t)*cp++); \
        } \
}

#define DECODEU(f) { \
        if (*cp == 0) {\
                (f) = htons((cp[1] << 8) | cp[2]); \
                cp += 3; \
        } else { \
                (f) = htons((u_int32_t)*cp++); \
        } \
}

/*
 * Attempt to compress an outgoing TCP packet and return the type of
 * the result.  The caller must have already verified that the protocol
 * is TCP.  The first mbuf must contain the complete IP and TCP headers,
 * and "ip" must be == mtod(m, struct ip *).  "comp" supplies the
 * compression state, and "compress_cid" tells us whether it is OK
 * to leave out the CID field when feasible.
 *
 * The caller is responsible for adjusting m->m_pkthdr.len upon return,
 * if m is an M_PKTHDR mbuf.
 */
u_int
sl_compress_tcp(struct mbuf *m, struct ip *ip, struct slcompress *comp,
        int compress_cid)
{
        register struct cstate *cs = comp->last_cs->cs_next;
        register u_int hlen = ip->ip_hl;
        register struct tcphdr *oth;
        register struct tcphdr *th;
        register u_int deltaS, deltaA;
        register u_int changes = 0;
        u_char new_seq[16];
        register u_char *cp = new_seq;

        /*
         * 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).  (We assume that the caller has already made sure the
         * packet is IP proto TCP).
         */
        if ((ip->ip_off & htons(0x3fff)) || m->m_len < 40)
                return (TYPE_IP);

        th = (struct tcphdr *)&((int32_t *)ip)[hlen];
        if ((th->th_flags & (TH_SYN|TH_FIN|TH_RST|TH_ACK)) != TH_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(sls_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 ||
            *(int32_t *)th != ((int32_t *)&cs->cs_ip)[cs->cs_ip.ip_hl]) {
                /*
                 * 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(sls_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
                            && *(int32_t *)th ==
                            ((int32_t *)&cs->cs_ip)[cs->cs_ip.ip_hl])
                                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(sls_misses)
                comp->last_cs = lcs;
                hlen += th->th_off;
                hlen <<= 2;
                if (hlen > m->m_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;
                }
        }

        /*
         * 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'.
         */
        oth = (struct tcphdr *)&((int32_t *)&cs->cs_ip)[hlen];
        deltaS = hlen;
        hlen += th->th_off;
        hlen <<= 2;
        if (hlen > m->m_len)
            return TYPE_IP;

        if (((u_int16_t *)ip)[0] != ((u_int16_t *)&cs->cs_ip)[0] ||
            ((u_int16_t *)ip)[3] != ((u_int16_t *)&cs->cs_ip)[3] ||
            ((u_int16_t *)ip)[4] != ((u_int16_t *)&cs->cs_ip)[4] ||
            th->th_off != oth->th_off ||
            (deltaS > 5 &&
             BCMP(ip + 1, &cs->cs_ip + 1, (deltaS - 5) << 2)) ||
            (th->th_off > 5 &&
             BCMP(th + 1, oth + 1, (th->th_off - 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 & TH_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;

        deltaS = (u_int16_t)(ntohs(th->th_win) - ntohs(oth->th_win));
        if (deltaS) {
                ENCODE(deltaS);
                changes |= NEW_W;
        }

        deltaA = ntohl(th->th_ack) - ntohl(oth->th_ack);
        if (deltaA) {
                if (deltaA > 0xffff)
                        goto uncompressed;
                ENCODE(deltaA);
                changes |= NEW_A;
        }

        deltaS = ntohl(th->th_seq) - ntohl(oth->th_seq);
        if (deltaS) {
                if (deltaS > 0xffff)
                        goto uncompressed;
                ENCODE(deltaS);
                changes |= NEW_S;
        }

        switch(changes) {

        case 0:
                /*
                 * Nothing changed. If this packet contains data and the
                 * last one didn't, this is probably a data packet following
                 * an ack (normal on an interactive connection) and we send
                 * it compressed.  Otherwise it's probably a retransmit,
                 * retransmitted ack or window probe.  Send it uncompressed
                 * in case the other side missed the compressed version.
                 */
                if (ip->ip_len != cs->cs_ip.ip_len &&
                    ntohs(cs->cs_ip.ip_len) == hlen)
                        break;

                /* FALLTHROUGH */

        case SPECIAL_I:
        case SPECIAL_D:
                /*
                 * actual changes match one of our special case encodings --
                 * send packet uncompressed.
                 */
                goto uncompressed;

        case NEW_S|NEW_A:
                if (deltaS == deltaA &&
                    deltaS == ntohs(cs->cs_ip.ip_len) - hlen) {
                        /* special case for echoed terminal traffic */
                        changes = SPECIAL_I;
                        cp = new_seq;
                }
                break;

        case NEW_S:
                if (deltaS == ntohs(cs->cs_ip.ip_len) - hlen) {
                        /* special case for data xfer */
                        changes = SPECIAL_D;
                        cp = new_seq;
                }
                break;
        }

        deltaS = ntohs(ip->ip_id) - ntohs(cs->cs_ip.ip_id);
        if (deltaS != 1) {
                ENCODEZ(deltaS);
                changes |= NEW_I;
        }
        if (th->th_flags & TH_PUSH)
                changes |= TCP_PUSH_BIT;
        /*
         * Grab the cksum before we overwrite it below.  Then update our
         * state with this packet's header.
         */
        deltaA = ntohs(th->th_sum);
        BCOPY(ip, &cs->cs_ip, hlen);

        /*
         * We want to use the original packet as our compressed packet.
         * (cp - new_seq) is the number of bytes we need for compressed
         * sequence numbers.  In addition we need one byte for the change
         * mask, one for the connection id and two for the tcp checksum.
         * So, (cp - new_seq) + 4 bytes of header are needed.  hlen is how
         * many bytes of the original packet to toss so subtract the two to
         * get the new packet size.
         */
        deltaS = cp - new_seq;
        cp = (u_char *)ip;
        if (compress_cid == 0 || comp->last_xmit != cs->cs_id) {
                comp->last_xmit = cs->cs_id;
                hlen -= deltaS + 4;
                cp += hlen;
                *cp++ = changes | NEW_C;
                *cp++ = cs->cs_id;
        } else {
                hlen -= deltaS + 3;
                cp += hlen;
                *cp++ = changes;
        }
        m->m_len -= hlen;
        m->m_data += hlen;
        *cp++ = deltaA >> 8;
        *cp++ = deltaA;
        BCOPY(new_seq, cp, deltaS);
        INCR(sls_compressed)
        return (TYPE_COMPRESSED_TCP);

        /*
         * Update connection state cs & send uncompressed packet ('uncompressed'
         * means a regular ip/tcp packet but with the 'conversation id' we hope
         * to use on future compressed packets in the protocol field).
         */
uncompressed:
        BCOPY(ip, &cs->cs_ip, hlen);
        ip->ip_p = cs->cs_id;
        comp->last_xmit = cs->cs_id;
        return (TYPE_UNCOMPRESSED_TCP);
}


int
sl_uncompress_tcp(u_char **bufp, int len, u_int type, struct slcompress *comp)
{
        u_char *hdr, *cp;
        u_int hlen;
        int vjlen;

        cp = bufp? *bufp: NULL;
        vjlen = sl_uncompress_tcp_core(cp, len, len, type, comp, &hdr, &hlen);
        if (vjlen < 0)
                return (0);     /* error */
        if (vjlen == 0)
                return (len);   /* was uncompressed already */

        cp += vjlen;
        len -= vjlen;

        /*
         * At this point, cp points to the first byte of data in the
         * packet.  If we're not aligned on a 4-byte boundary, copy the
         * data down so the ip & tcp headers will be aligned.  Then back up
         * cp by the tcp/ip header length to make room for the reconstructed
         * header (we assume the packet we were handed has enough space to
         * prepend 128 bytes of header).
         */
        if ((intptr_t)cp & 3) {
                if (len > 0)
                        BCOPY(cp, ((intptr_t)cp &~ 3), len);
                cp = (u_char *)((intptr_t)cp &~ 3);
        }
        cp -= hlen;
        len += hlen;
        BCOPY(hdr, cp, hlen);

        *bufp = cp;
        return (len);
}

/*
 * Uncompress a packet of total length total_len.  The first buflen
 * bytes are at buf; this must include the entire (compressed or
 * uncompressed) TCP/IP header.  This procedure returns the length
 * of the VJ header, with a pointer to the uncompressed IP header
 * in *hdrp and its length in *hlenp.
 */
int
sl_uncompress_tcp_core(u_char *buf, int buflen, int total_len,
        u_int type, struct slcompress *comp,
        u_char **hdrp, u_int *hlenp)
{
        register u_char *cp;
        register uint32_t hlen, changes;
        register struct tcphdr *th;
        register struct cstate *cs;
        register struct ip *ip;
        register u_int16_t *bp;
        register u_int vjlen;

        switch (type) {

        case TYPE_UNCOMPRESSED_TCP:
                ip = (struct ip *) buf;
                if (ip->ip_p >= MAX_STATES)
                        goto bad;
                cs = &comp->rstate[comp->last_recv = ip->ip_p];
                comp->flags &=~ SLF_TOSS;
                ip->ip_p = IPPROTO_TCP;
                /*
                 * Calculate the size of the TCP/IP header and make sure that
                 * we don't overflow the space we have available for it.
                 */
                hlen = ip->ip_hl << 2;
                if (hlen + sizeof(struct tcphdr) > buflen)
                        goto bad;
                hlen += ((struct tcphdr *)&((char *)ip)[hlen])->th_off << 2;
                if (hlen > MAX_HDR || hlen > buflen)
                        goto bad;
                BCOPY(ip, &cs->cs_ip, hlen);
                cs->cs_hlen = hlen;
                INCR(sls_uncompressedin)
                *hdrp = (u_char *) &cs->cs_ip;
                *hlenp = hlen;
                return (0);

        default:
                goto bad;

        case TYPE_COMPRESSED_TCP:
                break;
        }
        /* We've got a compressed packet. */
        INCR(sls_compressedin)
        cp = buf;
        changes = *cp++;
        if (changes & NEW_C) {
                /* Make sure the state index is in range, then grab the state.
                 * If we have a good state index, clear the 'discard' flag. */
                if (*cp >= MAX_STATES)
                        goto bad;

                comp->flags &=~ SLF_TOSS;
                comp->last_recv = *cp++;
        } else {
                /* this packet has an implicit state index.  If we've
                 * had a line error since the last time we got an
                 * explicit state index, we have to toss the packet. */
                if (comp->flags & SLF_TOSS) {
                        INCR(sls_tossed)
                        return (-1);
                }
        }
        cs = &comp->rstate[comp->last_recv];
        hlen = cs->cs_ip.ip_hl << 2;
        th = (struct tcphdr *)&((u_char *)&cs->cs_ip)[hlen];
        th->th_sum = htons((*cp << 8) | cp[1]);
        cp += 2;
        if (changes & TCP_PUSH_BIT)
                th->th_flags |= TH_PUSH;
        else
                th->th_flags &=~ TH_PUSH;

        switch (changes & SPECIALS_MASK) {
        case SPECIAL_I:
                {
                register u_int i = ntohs(cs->cs_ip.ip_len) - cs->cs_hlen;
                th->th_ack = htonl(ntohl(th->th_ack) + i);
                th->th_seq = htonl(ntohl(th->th_seq) + i);
                }
                break;

        case SPECIAL_D:
                th->th_seq = htonl(ntohl(th->th_seq) + ntohs(cs->cs_ip.ip_len)
                                   - cs->cs_hlen);
                break;

        default:
                if (changes & NEW_U) {
                        th->th_flags |= TH_URG;
                        DECODEU(th->th_urp)
                } else
                        th->th_flags &=~ TH_URG;
                if (changes & NEW_W)
                        DECODES(th->th_win)
                if (changes & NEW_A)
                        DECODEL(th->th_ack)
                if (changes & NEW_S)
                        DECODEL(th->th_seq)
                break;
        }
        if (changes & NEW_I) {
                DECODES(cs->cs_ip.ip_id)
        } else
                cs->cs_ip.ip_id = htons(ntohs(cs->cs_ip.ip_id) + 1);

        /*
         * At this point, cp points to the first byte of data in the
         * packet.  Fill in the IP total length and update the IP
         * header checksum.
         */
        vjlen = cp - buf;
        buflen -= vjlen;
        if (buflen < 0)
                /* we must have dropped some characters (crc should detect
                 * this but the old slip framing won't) */
                goto bad;

        total_len += cs->cs_hlen - vjlen;
        cs->cs_ip.ip_len = htons(total_len);

        /* recompute the ip header checksum */
        bp = (u_int16_t *) &cs->cs_ip;
        cs->cs_ip.ip_sum = 0;
                for (changes = 0; hlen > 0; hlen -= 2)
                        changes += *bp++;
                changes = (changes & 0xffff) + (changes >> 16);
                changes = (changes & 0xffff) + (changes >> 16);
        cs->cs_ip.ip_sum = ~ changes;

        *hdrp = (u_char *) &cs->cs_ip;
        *hlenp = cs->cs_hlen;
        return vjlen;

bad:
        comp->flags |= SLF_TOSS;
        INCR(sls_errorin)
        return (-1);
}