Linux的BSD和INET  socket层分别对应于ISO的表示层和会话层，其中有两个比较重要的数据结构：

struct socket

{

socket_statestate;

unsigned longflags;

struct proto_ops*ops;

struct inode*inode;

struct fasync_struct*fasync_list;/* Asynchronous wake up list*/

struct file*file;/* File back pointer for gc*/

struct sock*sk;

wait_queue_head_twait;

shorttype;

unsigned charpasscred;

};

其中state表示socket当前的状态，file和inode指向文件系统的文件描述符和节点信息。每个文件描述符都要对应一个inode节点，其中file->f_inode指向inode节点。

struct proto_ops *ops是一个L6层的处理函数，pops数组将在sock_register中初始化，对应于不同的作用域将被初始化为不同的值。fasync_list用于异步唤醒，对应的函数是sock_wake_async()。

而struct sock *sk 就是我们要说的另外一个非常重要的数据结构，相对于socket只应用于L6层，sock贯穿于L2～L5层并且做过各层之间贯穿的一个纽带。

struct sock {

/* Socket demultiplex comparisons on incoming packets. */

__u32daddr;/* Foreign IPv4 addr*/

__u32rcv_saddr;/* Bound local IPv4 addr*/

__u16dport;/* Destination port*/

unsigned shortnum;/* Local port*/

intbound_dev_if;/* Bound device index if != 0*/

/* Main hash linkage for various protocol lookup tables. */

struct sock*next;

struct sock**pprev;

struct sock*bind_next;

struct sock**bind_pprev;

volatile unsigned charstate,/* Connection state*/

zapped;/* In ax25 & ipx means not linked*/

__u16sport;/* Source port*/

unsigned shortfamily;/* Address family*/

unsigned charreuse;/* SO_REUSEADDR setting*/

unsigned charshutdown;

atomic_trefcnt;/* Reference count*/

socket_lock_tlock;/* Synchronizer...*/

intrcvbuf;/* Size of receive buffer in bytes*/

wait_queue_head_t*sleep;/* Sock wait queue*/

struct dst_entry*dst_cache;/* Destination cache*/

rwlock_tdst_lock;

atomic_trmem_alloc;/* Receive queue bytes committed*/

struct sk_buff_headreceive_queue;/* Incoming packets*/

atomic_twmem_alloc;/* Transmit queue bytes committed*/

struct sk_buff_headwrite_queue;/* Packet sending queue*/

atomic_tomem_alloc;/* "o" is "option" or "other" */

intwmem_queued;/* Persistent queue size */

intforward_alloc;/* Space allocated forward. */

__u32saddr;/* Sending source*/

unsigned intallocation;/* Allocation mode*/

intsndbuf;/* Size of send buffer in bytes*/

struct sock*prev;

/* Not all are volatile, but some are, so we might as well say they all are.

* XXX Make this a flag word -DaveM

*/

volatile chardead,

done,

urginline,

keepopen,

linger,

destroy,

no_check,

broadcast,

bsdism;

unsigned chardebug;

unsigned charrcvtstamp;

unsigned charuse_write_queue;

unsigned charuserlocks;

/* Hole of 3 bytes. Try to pack. */

introute_caps;

intproc;

unsigned long lingertime;

inthashent;

struct sock*pair;

/* The backlog queue is special, it is always used with

* the per-socket spinlock held and requires low latency

* access. Therefore we special case it's implementation.

*/

struct {

struct sk_buff *head;

struct sk_buff *tail;

} backlog;

rwlock_tcallback_lock;

/* Error queue, rarely used. */

struct sk_buff_headerror_queue;

struct proto*prot;

#if defined(CONFIG_IPV6) || defined (CONFIG_IPV6_MODULE)

union {

struct ipv6_pinfoaf_inet6;

} net_pinfo;

#endif

union {

struct tcp_optaf_tcp;

#if defined(CONFIG_INET) || defined (CONFIG_INET_MODULE)

struct raw_opttp_raw4;

#endif

#if defined(CONFIG_IPV6) || defined (CONFIG_IPV6_MODULE)

struct raw6_opttp_raw;

#endif /* CONFIG_IPV6 */

#if defined(CONFIG_SPX) || defined (CONFIG_SPX_MODULE)

struct spx_optaf_spx;

#endif /* CONFIG_SPX */

} tp_pinfo;

interr, err_soft;/* Soft holds errors that don't

cause failure but are the cause

of a persistent failure not just

'timed out' */

unsigned shortack_backlog;

unsigned shortmax_ack_backlog;

__u32priority;

unsigned shorttype;

unsigned charlocalroute;/* Route locally only */

unsigned charprotocol;

struct ucredpeercred;

intrcvlowat;

longrcvtimeo;

longsndtimeo;

#ifdef CONFIG_FILTER

/* Socket Filtering Instructions */

struct sk_filter *filter;

#endif /* CONFIG_FILTER */

/* This is where all the private (optional) areas that don't

* overlap will eventually live.

*/

union {

void *destruct_hook;

struct unix_optaf_unix;

#if defined(CONFIG_INET) || defined (CONFIG_INET_MODULE)

struct inet_opt af_inet;

#endif

#if defined(CONFIG_ATALK) || defined(CONFIG_ATALK_MODULE)

struct atalk_sockaf_at;

#endif

#if defined(CONFIG_IPX) || defined(CONFIG_IPX_MODULE)

struct ipx_optaf_ipx;

#endif

#if defined (CONFIG_DECNET) || defined(CONFIG_DECNET_MODULE)

struct dn_scp dn;

#endif

#if defined (CONFIG_PACKET) || defined(CONFIG_PACKET_MODULE)

struct packet_opt*af_packet;

#endif

#if defined(CONFIG_X25) || defined(CONFIG_X25_MODULE)

x25_cb*x25;

#endif

#if defined(CONFIG_AX25) || defined(CONFIG_AX25_MODULE)

ax25_cb*ax25;

#endif

#if defined(CONFIG_NETROM) || defined(CONFIG_NETROM_MODULE)

nr_cb*nr;

#endif

#if defined(CONFIG_ROSE) || defined(CONFIG_ROSE_MODULE)

rose_cb*rose;

#endif

#if defined(CONFIG_PPPOE) || defined(CONFIG_PPPOE_MODULE)

struct pppox_opt*pppox;

#endif

struct netlink_opt*af_netlink;

#if defined(CONFIG_ECONET) || defined(CONFIG_ECONET_MODULE)

struct econet_opt*af_econet;

#endif

#if defined(CONFIG_ATM) || defined(CONFIG_ATM_MODULE)

struct atm_vcc*af_atm;

#endif

#if defined(CONFIG_IRDA) || defined(CONFIG_IRDA_MODULE)

struct irda_sock *irda;

#endif

#if defined(CONFIG_WAN_ROUTER) || defined(CONFIG_WAN_ROUTER_MODULE)

struct wanpipe_opt *af_wanpipe;

#endif

} protinfo;

/* This part is used for the timeout functions. */

struct timer_listtimer;/* This is the sock cleanup timer. */

struct timevalstamp;

/* Identd and reporting IO signals */

struct socket*socket;

/* RPC layer private data */

void*user_data;

/* Callbacks */

void(*state_change)(struct sock *sk);

void(*data_ready)(struct sock *sk,int bytes);

void(*write_space)(struct sock *sk);

void(*error_report)(struct sock *sk);

int(*backlog_rcv) (struct sock *sk,

struct sk_buff *skb);

void (*destruct)(struct sock *sk);

};

write_queue表示待发送到队列，send_head和send_tail表示队列数据包已经发送出去尚未收到应答；receive_queue为读队列其不同于back_log在于后者缓冲从网络层传上来的数据包然后移至receive_queue队列，如果此时此时缓冲区太小则直接丢弃从back_log队列上摘下的数据包。