A.1 Definitions and State Data

   #define MAX_STATES 16   /* must be >2 and <255 */
   #define MAX_HDR 128     /* max TCP+IP hdr length (by protocol def) */

   /* packet types */
   #define TYPE_IP 0x40
   #define TYPE_UNCOMPRESSED_TCP 0x70
   #define TYPE_COMPRESSED_TCP 0x80
   #define TYPE_ERROR 0x00 /* this is not a type that ever appears on
                            * the wire.  The receive framer uses it to
                            * tell the decompressor there was a packet
                            * transmission error. */
   /*
    * Bits in first octet of compressed packet
    */

   /* flag bits for what changed in a packet */

   #define NEW_C  0x40
   #define NEW_I  0x20
   #define TCP_PUSH_BIT 0x10

   #define NEW_S  0x08
   #define NEW_A  0x04
   #define NEW_W  0x02
   #define NEW_U  0x01

   /* reserved, special-case values of above */
   #define SPECIAL_I (NEW_S|NEW_W|NEW_U)        /* echoed interactive traffic */
   #define SPECIAL_D (NEW_S|NEW_A|NEW_W|NEW_U)  /* unidirectional data */
   #define SPECIALS_MASK (NEW_S|NEW_A|NEW_W|NEW_U)


   /*
    * "state" data for each active tcp conversation on the wire.  This is
    * basically a copy of the entire IP/TCP header from the last packet together
    * with a small identifier the transmit & receive ends of the line use to
    * locate saved header.
    */
   struct cstate {
        struct cstate *cs_next;  /* next most recently used cstate (xmit only) */
        u_short cs_hlen;         /* size of hdr (receive only) */
        u_char cs_id;            /* connection # associated with this state */
        u_char cs_filler;
        union {
             char hdr[MAX_HDR];
             struct ip csu_ip;   /* ip/tcp hdr from most recent packet */
        } slcs_u;
   };
   #define cs_ip slcs_u.csu_ip
   #define cs_hdr slcs_u.csu_hdr

   /*
    * all the state data for one serial line (we need one of these per line).
    */
   struct slcompress {
        struct cstate *last_cs;            /* most recently used tstate */
        u_char last_recv;                  /* last rcvd conn. id */
        u_char last_xmit;                  /* last sent conn. id */
        u_short flags;
        struct cstate tstate[MAX_STATES];  /* xmit connection states */
        struct cstate rstate[MAX_STATES];  /* receive connection states */
   };

   /* flag values */
   #define SLF_TOSS 1       /* tossing rcvd frames because of input err */

   /*
    * The following macros are used to encode and decode numbers.  They all
    * assume that `cp' points to a buffer where the next byte encoded (decoded)
    * is to be stored (retrieved).  Since the decode routines do arithmetic,
    * they have to convert from and to network byte order.
    */

   /*
    * ENCODE encodes a number that is known to be non-zero.  ENCODEZ checks for
    * zero (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); \
        } \
   }

   /*
    * DECODEL takes the (compressed) change at byte cp and adds it to the
    * current value of packet field 'f' (which must be a 4-byte (long) integer
    * in network byte order).  DECODES does the same for a 2-byte (short) field.
    * DECODEU takes the change at cp and stuffs it into the (short) field f.
    * 'cp' is updated to point to the next field in the compressed header.
    */
   #define DECODEL(f) { \
        if (*cp == 0) {\
             (f) = htonl(ntohl(f) + ((cp[1] << 8) | cp[2])); \
             cp += 3; \
        } else { \
             (f) = htonl(ntohl(f) + (u_long)*cp++); \
        } \
   }
   #define DECODES(f) { \
        if (*cp == 0) {\
             (f) = htons(ntohs(f) + ((cp[1] << 8) | cp[2])); \
             cp += 3; \
        } else { \
             (f) = htons(ntohs(f) + (u_long)*cp++); \
        } \
   }
   #define DECODEU(f) { \
        if (*cp == 0) {\
             (f) = htons((cp[1] << 8) | cp[2]); \
             cp += 3; \
        } else { \
             (f) = htons((u_long)*cp++); \
        } \
   }