usr/src/sys/netinet/ip.h

/* ip.h 1.4 81/10/28 */

struct ip {
        u_char  ip_hl:4,                /* header length */
                ip_v:4;                 /* version */
        u_char  ip_tos;                 /* type of service */
#define ip_mff  ip_tos                  /* more fragments flag (input) */
/* by rights, ip_len should be a u_short, but this makes operations */
/* on it very dangerous as comparisons become unsigned and comparing */
/* against negative numbers then fails... we don't expect any > 32767 */
/* byte packets, so pragmatically delcare it to be a short */
        short   ip_len;                 /* total length */
        u_short ip_id;                  /* identification */
/* ip_off should also, by rights, be u_short, ala ip_len */
        short   ip_off;                 /* fragment offset field */
#define ip_df 0x4000                    /* dont fragment flag */
#define ip_mf 0x2000                    /* more fragments flag (output) */
        u_char  ip_ttl;                 /* time to live */
        u_char  ip_p;                   /* protocol */
        u_short ip_sum;                 /* checksum */
#define ip_end  ip_sum                  /* fragment end */
        union {
                struct socket ip_s;     /* source address */
                struct ip *ip_nxt;      /* next fragment */
        } I_sun;
#define ip_src  I_sun.ip_s
#define ip_next I_sun.ip_nxt
        union {
                struct socket ip_d;     /* destination address */
                struct ip *ip_prv;      /* prev fragment */
        } I_dun;
#define ip_dst  I_dun.ip_d
#define ip_prev I_dun.ip_prv
};

/*
 * Ip reassembly queue.
 */
struct ipq {
        struct  ip iqx;         /* dummy ip element for top of list */
        struct  ipq *iq_next;   /* -> next chain on q */
        struct  ipq *iq_prev;   /* -> prev chain on q */
        struct  ip iqh;         /* fragment header */
};

#define IPVERSION       4               /* internet protocol version number */
#define IPLOLINK        155             /* internet link numbers */
#define IPHILINK        158
#define IPLINK          IPLOLINK
#define MAXTTL          255             /* maximum time to live (seconds) */

#define ip_bswap(p) { \
        p->ip_len = ntohs(p->ip_len); \
        p->ip_id = ntohs(p->ip_id); \
        p->ip_off = ntohs(p->ip_off); }