接口层

struct ifnet {
    char *if_name;              /* name, e.g. ``en'' or ``lo'' */ (1)
    struct ifnet *if_next;      /* all struct ifnets are chained */
    struct ifaddr *if_addrlist; /* linked list of addresses per if */ (2)
    int if_pcount;              /* number of promiscuous listeners */
    caddr_t if_bpf;             /* packet filter structure */
    u_short if_index;           /* numeric abbreviation for this if  */
    short if_unit;              /* sub-unit for lower level driver */
    short if_timer;             /* time 'til if_watchdog called */ (3)
    short if_flags;             /* up/down, broadcast, etc. */
    struct if_data {
        /* generic interface information */
        u_char ifi_type;               /* ethernet, tokenring, etc */
        u_char ifi_addrlen;            /* media address length */
        u_char ifi_hdrlen;             /* media header length */
        u_long ifi_mtu;                /* maximum transmission unit */
        u_long ifi_metric;             /* routing metric (external only) */
        u_long ifi_baudrate;           /* linespeed */
                                       /* volatile statistics */
        u_long ifi_ipackets;           /* packets received on interface */
        u_long ifi_ierrors;            /* input errors on interface */
        u_long ifi_opackets;           /* packets sent on interface */
        u_long ifi_oerrors;            /* output errors on interface */
        u_long ifi_collisions;         /* collisions on csma interfaces */
        u_long ifi_ibytes;             /* total number of octets received */
        u_long ifi_obytes;             /* total number of octets sent */
        u_long ifi_imcasts;            /* packets received via multicast */
        u_long ifi_omcasts;            /* packets sent via multicast */
        u_long ifi_iqdrops;            /* dropped on input, this interface */
        u_long ifi_noproto;            /* destined for unsupported protocol */
        struct timeval ifi_lastchange; /* last updated */
    } if_data;
    /* procedure handles */
    int(*if_init) /* init routine */
        __P((int));
    int(*if_output) /* output routine (enqueue) */
        __P((struct ifnet *, struct mbuf *, struct sockaddr *, struct rtentry *));
    int(*if_start) /* initiate output routine */
        __P((struct ifnet *));
    int(*if_done)              /* output complete routine */
        __P((struct ifnet *)); /* (XXX not used; fake prototype) */
    int(*if_ioctl)             /* ioctl routine */
        __P((struct ifnet *, u_long, caddr_t));
    int(*if_reset)
        __P((int));   /* new autoconfig will permit removal */
    int(*if_watchdog) /* timer routine */
        __P((int));
    struct ifqueue {
        struct mbuf *ifq_head;
        struct mbuf *ifq_tail;
        int ifq_len;
        int ifq_maxlen;
        int ifq_drops;
    } if_snd; /* output queue * (4)
};
#define if_mtu if_data.ifi_mtu (5)
#define if_type if_data.ifi_type (6)
#define if_addrlen if_data.ifi_addrlen
#define if_hdrlen if_data.ifi_hdrlen
#define if_metric if_data.ifi_metric (7)
#define if_baudrate if_data.ifi_baudrate
#define if_ipackets if_data.ifi_ipackets
#define if_ierrors if_data.ifi_ierrors
#define if_opackets if_data.ifi_opackets
#define if_oerrors if_data.ifi_oerrors
#define if_collisions if_data.ifi_collisions
#define if_ibytes if_data.ifi_ibytes
#define if_obytes if_data.ifi_obytes
#define if_imcasts if_data.ifi_imcasts
#define if_omcasts if_data.ifi_omcasts
#define if_iqdrops if_data.ifi_iqdrops
#define if_noproto if_data.ifi_noproto
#define if_lastchange if_data.ifi_lastchange

struct ifaddr {
    struct sockaddr *ifa_addr;    /* address of interface */
    struct sockaddr *ifa_dstaddr; /* other end of p-to-p link */
#define ifa_broadaddr ifa_dstaddr /* broadcast address interface */
    struct sockaddr *ifa_netmask; /* used to determine subnet */
    struct ifnet *ifa_ifp;        /* back-pointer to interface */
    struct ifaddr *ifa_next;      /* next address for interface */
    void (*ifa_rtrequest)();      /* check or clean routes (+ or -)'d */
    u_short ifa_flags;            /* mostly rt_flags for cloning */
    short ifa_refcnt;             /* extra to malloc for link info */ (1)
    int ifa_metric;               /* cost of going out this interface */
#ifdef notdef
    struct rtentry *ifa_rt; /* XXXX for ROUTETOIF ????? */
#endif
};

/*
 * Structure used by kernel to store most
 * addresses.
 */
struct sockaddr {
    u_char sa_len;    /* total length */ (1)
    u_char sa_family; /* address family */ (2)
    char sa_data[14]; /* actually longer; address value */ (3)
};

void main(void *framep) {
    // ...
    cpu_startup(); (1)
    // ...

    /* Attach pseudo-devices. */
    for (pdev = pdevinit; pdev->pdev_attach != NULL; pdev++) (2)
        (*pdev->pdev_attach)(pdev->pdev_count);

    /*
     * Initialize protocols.  Block reception of incoming packets
     * until everything is ready.
     */
    s = splimp();
    ifinit(); (3)
    domaininit(); (4)
    splx(s);

    // ...
    /* The scheduler is an infinite loop. */
    scheduler();
    /* NOTREACHED */
}

#define LOMTU (1024 + 512)

void loopattach(int n) {
    struct ifnet *ifp = &loif;

    ifp->if_name = "lo";
    ifp->if_mtu = LOMTU; (1)
    ifp->if_flags = IFF_LOOPBACK | IFF_MULTICAST; (2)
    ifp->if_ioctl = loioctl;
    ifp->if_output = looutput;
    ifp->if_type = IFT_LOOP;
    ifp->if_hdrlen = 0; (3)
    ifp->if_addrlen = 0; (4)
    if_attach(ifp);
    bpfattach(&ifp->if_bpf, ifp, DLT_NULL, sizeof(u_int)); (5)
}

struct ifaddr **ifnet_addrs = NULL;

void if_attach(struct ifnet *ifp) {
    unsigned socksize, ifasize;
    int namelen, unitlen, masklen, ether_output();
    char workbuf[12], *unitname;
    register struct ifnet **p = &ifnet;
    register struct sockaddr_dl *sdl;
    register struct ifaddr *ifa;
    static int if_indexlim = 8;
    extern void link_rtrequest();

    while (*p)
        p = &((*p)->if_next);
    *p = ifp;
    ifp->if_index = ++if_index;
    if (ifnet_addrs == NULL || if_index >= if_indexlim) { (1)
        unsigned n = (if_indexlim <<= 1) * sizeof(ifa);
        struct ifaddr **q = (struct ifaddr **)malloc(n, M_IFADDR, M_WAITOK);
        if (ifnet_addrs) {
            bcopy((caddr_t)ifnet_addrs, (caddr_t)q, n / 2);
            free((caddr_t)ifnet_addrs, M_IFADDR);
        }
        ifnet_addrs = q;
    }
    /*
     * create a Link Level name for this device
     */
    unitname = sprint_d((u_int)ifp->if_unit, workbuf, sizeof(workbuf)); (2)
    namelen = strlen(ifp->if_name);
    unitlen = strlen(unitname);
#define _offsetof(t, m) ((int)((caddr_t) & ((t *)0)->m))
    masklen = _offsetof(struct sockaddr_dl, sdl_data[0]) +
              unitlen + namelen;
    socksize = masklen + ifp->if_addrlen;
#define ROUNDUP(a) (1 + (((a)-1) | (sizeof(long) - 1)))
    socksize = ROUNDUP(socksize);
    if (socksize < sizeof(*sdl))
        socksize = sizeof(*sdl);
    ifasize = sizeof(*ifa) + 2 * socksize;
    if (ifa = (struct ifaddr *)malloc(ifasize, M_IFADDR, M_WAITOK)) { (3)
        bzero((caddr_t)ifa, ifasize);
        sdl = (struct sockaddr_dl *)(ifa + 1);
        sdl->sdl_len = socksize;
        sdl->sdl_family = AF_LINK;
        bcopy(ifp->if_name, sdl->sdl_data, namelen); (4)
        bcopy(unitname, namelen + (caddr_t)sdl->sdl_data, unitlen);
        sdl->sdl_nlen = (namelen += unitlen);
        sdl->sdl_index = ifp->if_index;
        sdl->sdl_type = ifp->if_type;
        ifnet_addrs[if_index - 1] = ifa;
        ifa->ifa_ifp = ifp;
        ifa->ifa_next = ifp->if_addrlist;
        ifa->ifa_rtrequest = link_rtrequest; (5)
        ifp->if_addrlist = ifa;
        ifa->ifa_addr = (struct sockaddr *)sdl;
        sdl = (struct sockaddr_dl *)(socksize + (caddr_t)sdl);
        ifa->ifa_netmask = (struct sockaddr *)sdl;
        sdl->sdl_len = masklen;
        while (namelen != 0)
            sdl->sdl_data[--namelen] = 0xff;
    }
    /* XXX -- Temporary fix before changing 10 ethernet drivers */
    if (ifp->if_output == ether_output)
        ether_ifattach(ifp);
}

void ifinit() {
    register struct ifnet *ifp;

    for (ifp = ifnet; ifp; ifp = ifp->if_next)
        if (ifp->if_snd.ifq_maxlen == 0) (1)
            ifp->if_snd.ifq_maxlen = ifqmaxlen;
    if_slowtimo(0); (2)
}