vjcompress.c

经典的ppp程序

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

    if ((deltaS = ntohl(th->th_seq) - ntohl(oth->th_seq)) > 0) {
	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;

	/* (fall through) */

    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;
	*vjhdrp = (cp += hlen);
	*cp++ = changes | NEW_C;
	*cp++ = cs->cs_id;
    } else {
	hlen -= deltaS + 3;
	*vjhdrp = (cp += hlen);
	*cp++ = changes;
    }
    *cp++ = deltaA >> 8;
    *cp++ = deltaA;
    BCOPY(new_seq, cp, deltaS);
    INCR(vjs_compressed);
    return (TYPE_COMPRESSED_TCP);

    /*
     * Update connection state cs & send uncompressed packet (that is,
     * 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);
}

/*
 * Called when we may have missed a packet.
 */
void
vj_uncompress_err(comp)
    struct vjcompress *comp;
{
    comp->flags |= VJF_TOSS;
    INCR(vjs_errorin);
}

/*
 * "Uncompress" a packet of type TYPE_UNCOMPRESSED_TCP.
 */
int
vj_uncompress_uncomp(buf, buflen, comp)
    u_char *buf;
    int buflen;
    struct vjcompress *comp;
{
    register u_int hlen;
    register struct cstate *cs;
    register struct ip *ip;

    ip = (struct ip *) buf;
    hlen = getip_hl(*ip) << 2;
    if (ip->ip_p >= MAX_STATES
	|| hlen + sizeof(struct tcphdr) > buflen
	|| (hlen += getth_off(*((struct tcphdr *)&((char *)ip)[hlen])) << 2)
	    > buflen
	|| hlen > MAX_HDR) {
	comp->flags |= VJF_TOSS;
	INCR(vjs_errorin);
	return (0);
    }
    cs = &comp->rstate[comp->last_recv = ip->ip_p];
    comp->flags &=~ VJF_TOSS;
    ip->ip_p = IPPROTO_TCP;
    BCOPY(ip, &cs->cs_ip, hlen);
    cs->cs_hlen = hlen;
    INCR(vjs_uncompressedin);
    return (1);
}

/*
 * Uncompress a packet of type TYPE_COMPRESSED_TCP.
 * The packet starts at buf and is of total length total_len.
 * The first buflen bytes are at buf; this must include the entire
 * compressed 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
vj_uncompress_tcp(buf, buflen, total_len, comp, hdrp, hlenp)
    u_char *buf;
    int buflen, total_len;
    struct vjcompress *comp;
    u_char **hdrp;
    u_int *hlenp;
{
    register u_char *cp;
    register u_int hlen, changes;
    register struct tcphdr *th;
    register struct cstate *cs;
    register u_short *bp;
    register u_int vjlen;
    register u_int32_t tmp;

    INCR(vjs_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 &=~ VJF_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 & VJF_TOSS) {
	    INCR(vjs_tossed);
	    return (-1);
	}
    }
    cs = &comp->rstate[comp->last_recv];
    hlen = getip_hl(cs->cs_ip) << 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_int32_t i = ntohs(cs->cs_ip.ip_len) - cs->cs_hlen;
	    /* some compilers can't nest inline assembler.. */
	    tmp = ntohl(th->th_ack) + i;
	    th->th_ack = htonl(tmp);
	    tmp = ntohl(th->th_seq) + i;
	    th->th_seq = htonl(tmp);
	}
	break;

    case SPECIAL_D:
	/* some compilers can't nest inline assembler.. */
	tmp = ntohl(th->th_seq) + ntohs(cs->cs_ip.ip_len) - cs->cs_hlen;
	th->th_seq = htonl(tmp);
	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 = ntohs(cs->cs_ip.ip_id) + 1;
	cs->cs_ip.ip_id = htons(cs->cs_ip.ip_id);
    }

    /*
     * 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_short *) &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 |= VJF_TOSS;
    INCR(vjs_errorin);
    return (-1);
}