网络数据结构简介

最近在看tcp/ip v2，发现看完前几章之后，脑子里面一坨。。。呵呵哒，惨不忍睹啊，不过还是有点小收获，在这里跟各位看官分享一下网络相关的各个数据结构以及之间关系，拍砖的请自备工具。。。

俗话说，一图胜千言（也有人说，没图你说个xx。。。），先上图：

今天这篇文章的任务就是将上面的这张图给大家说清楚：

我将从水平和垂直两个维度来讲解这张图片：

第一行，表示的是整个机器上面，所有的网络硬件在内核当中的数据结构，在系统初始化期间，分别为每个网络设备分配一个独立的ifnet{}结构（其中，以太网卡貌似支持热插拔检测），每个ifnet{}结构的下面，连接着位于其上的一个或多个协议地址，ifnet{}的各个字段如下所示：

struct ifnet {
    char    *if_name;        /* name, e.g. ``en'' or ``lo'' */
    struct    ifnet *if_next;        /* all struct ifnets are chained */
    struct    ifaddr *if_addrlist;    /* linked list of addresses per if */
    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 */
    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 *, int, 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 */
};

它使用if_next指针将系统中所有检测到的interface连接起来，组成一个单向链表，链表的头指针由全局变量ifnet表示，每一个ifnet{}结构中，有一个if_addrlist指针，分别对应，位于该interface上的所有协议的地址列表（如链路层，网络层等等）；if_netaddrs指向一个数组，数组中的每一个元素，指向一个链路层地址结构if_addr{}, 其定义如下：

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 */
    int    ifa_metric;        /* cost of going out this interface */
#ifdef notdef
    struct    rtentry *ifa_rt;    /* XXXX for ROUTETOIF ????? */
#endif
};

在往下，就是in_ifaddrs指针了，它指向一个单向链表，链表中的每一项就是该主机对应的一个网络地址，且每一个网络地址必须对应一个ifnet{}结构，每一个项由一个in_ifaddr{}机构表示，如下图所示：

struct in_ifaddr {
    struct    ifaddr ia_ifa;        /* protocol-independent info */
#define    ia_ifp        ia_ifa.ifa_ifp
#define ia_flags    ia_ifa.ifa_flags
                    /* ia_{,sub}net{,mask} in host order */
    u_long    ia_net;            /* network number of interface */
    u_long    ia_netmask;        /* mask of net part */
    u_long    ia_subnet;        /* subnet number, including net */
    u_long    ia_subnetmask;        /* mask of subnet part */
    struct    in_addr ia_netbroadcast; /* to recognize net broadcasts */
    struct    in_ifaddr *ia_next;    /* next in list of internet addresses */
    struct    sockaddr_in ia_addr;    /* reserve space for interface name */
    struct    sockaddr_in ia_dstaddr; /* reserve space for broadcast addr */
#define    ia_broadaddr    ia_dstaddr
    struct    sockaddr_in ia_sockmask; /* reserve space for general netmask */
    struct    in_multi *ia_multiaddrs; /* list of multicast addresses */
};

目前为止，就读明白这些，有了新进展，一定会分享的哈~~