Linux素以優秀的網絡管理能力而著稱，linux為何具有這麼高的效率？我們從網絡設備的管理說起。
Linux為何要對網絡設備單獨管理呢？這是因為。協議棧很多地方都會涉及到網絡設備。小至IP地址的設置。大至IP路由的更新。都離不開高效的網絡設備管理。將網絡設備單獨管理可以提高效率！
每個網絡設備，在linux中都會對應一個數據結構,net_device。 就從這個結構說起
Linux 2。6。21中，對net_device定義如下：
struct net_device
{
//設備的名稱，例如常見的“eth0”等
char name[IFNAMSIZ];
//共享內存的起始，結束地址
unsigned long mem_end; /* shared mem end */
unsigned long mem_start; /* shared mem start */
//網絡設備的I/O基地址
unsigned long base_addr; /* device I/O address */
//被賦予的中斷號
unsigned int irq; /* device IRQ number */
//在多端口設備上使用哪一個端口
unsigned char if_port; /* Selectable AUI, TP,..*/
//為設備分配的DMA通道
unsigned char dma; /* DMA channel */
//設備的狀態
unsigned long state;
// 下一個net_device
struct net_device *next;
//初始化函數。
int (*init)(struct net_device *dev);
struct net_device *next_sched;
/* Interface index. Unique device identifier */
//設備在內核中對應的序號
int ifindex;
int iflink;
//獲得接口狀態的函數指針
struct net_device_stats* (*get_stats)(struct net_device *dev);
struct iw_statistics* (*get_wireless_stats)(struct net_device *dev);
struct iw_handler_def * wireless_handlers;
struct ethtool_ops *ethtool_ops;
//傳輸狀態。檢查傳輸是否被鎖住
unsigned long trans_start; /* Time (in jiffies) of last Tx */
//最使使用的時間
unsigned long last_rx; /* Time of last Rx */
//接口標志
unsigned short flags; /* interface flags (a la BSD) */
unsigned short gflags;
unsigned short priv_flags; /* Like 'flags' but invisible to userspace. */
unsigned short unused_alignment_fixer; /* Because we need priv_flags,
* and we want to be 32-bit aligned.
*/
unsigned mtu; /* interface MTU value */
unsigned short type; /* interface hardware type */
unsigned short hard_header_len; /* hardware hdr length */
void *priv; /* pointer to private data */
struct net_device *master; /* Pointer to master device of a group,
* which this device is member of.
*/
/* Interface address info. */
unsigned char broadcast[MAX_ADDR_LEN]; /* hw bcast add */
unsigned char dev_addr[MAX_ADDR_LEN]; /* hw address */
unsigned char addr_len; /* hardware address length */
struct dev_mc_list *mc_list; /* Multicast mac addresses */
int mc_count; /* Number of installed mcasts */
int promiscuity;
int allmulti;
int watchdog_timeo;
struct timer_list watchdog_timer;
/* Protocol specific pointers */
void *atalk_ptr; /* AppleTalk link */
void *ip_ptr; /* IPv4 specific data */
void *dn_ptr; /* DECnet specific data */
void *ip6_ptr; /* IPv6 specific data */
void *ec_ptr; /* Econet specific data */
void *ax25_ptr; /* AX.25 specific data */
struct list_head poll_list; /* Link to poll list */
int quota;
int weight;
struct Qdisc *qdisc;
struct Qdisc *qdisc_sleeping;
struct Qdisc *qdisc_ingress;
struct list_head qdisc_list;
unsigned long tx_queue_len; /* Max frames per queue allowed */
/* ingress path synchronizer */
spinlock_t ingress_lock;
/* hard_start_xmit synchronizer */
spinlock_t xmit_lock;
/* cpu id of processor entered to hard_start_xmit or -1,
if nobody entered there.
*/
int xmit_lock_owner;
/* device queue lock */
spinlock_t queue_lock;
/* Number of references to this device */
atomic_t refcnt;
/* delayed register/unregister */
struct list_head todo_list;
/* device name hash chain */
struct hlist_node name_hlist;
/* device index hash chain */
struct hlist_node index_hlist;
