【Linux网络】TCP全连接队列

TCP 相关实验

理解 listen 的第二个参数

• 基于刚才封装的 TcpSocket 实现以下测试代码
• 对于服务器, listen 的第二个参数设置为 1, 并且不调用 accept
• 测试代码链接

test_server.cc

#include "tcp_socket.hpp"int main(int argc, char* argv[]) {if (argc != 3) {printf("Usage ./test_server [ip] [port]\n");return 1;}TcpSocket sock;bool ret = sock.Bind(argv[1], atoi(argv[2]));if (!ret) {return 1;}ret = sock.Listen(2);if (!ret) {return 1;}// 客户端不进行 acceptwhile (1) {sleep(1);}return 0;
}

test_client.cc

#include "tcp_socket.hpp"int main(int argc, char* argv[]) {if (argc != 3) {printf("Usage ./test_client [ip] [port]\n");return 1;}TcpSocket sock;bool ret = sock.Connect(argv[1], atoi(argv[2]));if (ret) {printf("connect ok\n");} else {printf("connect failed\n");}while (1) {sleep(1);}return 0;
}

此时启动 3 个客户端同时连接服务器, 用 netstat 查看服务器状态, 一切正常.
但是启动第四个客户端时, 发现服务器对于第四个连接的状态存在问题了

tcp        3      0 0.0.0.0:9090            0.0.0.0:*               LISTEN      9084/./test_server
tcp        0      0 127.0.0.1:9090          127.0.0.1:48178         SYN_RECV    -
tcp        0      0 127.0.0.1:9090          127.0.0.1:48176         ESTABLISHED -
tcp        0      0 127.0.0.1:48178         127.0.0.1:9090          ESTABLISHED 9140/./test_client
tcp        0      0 127.0.0.1:48174         127.0.0.1:9090          ESTABLISHED 9087/./test_client
tcp        0      0 127.0.0.1:48176         127.0.0.1:9090          ESTABLISHED 9088/./test_client
tcp        0      0 127.0.0.1:48172         127.0.0.1:9090          ESTABLISHED 9086/./test_client
tcp        0      0 127.0.0.1:9090          127.0.0.1:48174         ESTABLISHED -
tcp        0      0 127.0.0.1:9090          127.0.0.1:48172         ESTABLISHED -

客户端状态正常, 但是服务器端出现了 SYN_RECV 状态, 而不是 ESTABLISHED 状态
这是因为, Linux 内核协议栈为一个 tcp 连接管理使用两个队列:

1. 半连接队列 (用来保存处于 SYN_SENT 和 SYN_RECV 状态的请求)
2. 全连接队列 (accpetd 队列) (用来保存处于 established 状态, 但是应用层没有调用 accept 取走的请求)
   而全连接队列的长度会受到 listen 第二个参数的影响.
   全连接队列满了的时候, 就无法继续让当前连接的状态进入 established 状态了.
   这个队列的长度通过上述实验可知, 是 listen 的第二个参数 + 1.

使用 TCP dump 进行抓包，分析 TCP 过程

我们代码中故意在 close(sockfd)那里留了一个问题
TCPDump 是一款强大的网络分析工具, 主要用于捕获和分析网络上传输的数据包。

安装 tcpdump
tcpdump 通常已经预装在大多数 Linux 发行版中。如果没有安装, 可以使用包管理器进行安装。例如 Ubuntu, 可以使用以下命令安装:

sudo apt - get update
sudo apt - get install tcpdump

在 Red Hat 或 CentOS 系统中, 可以使用以下命令:

sudo yum install tcpdump

常见使用

1. 捕获所有网络接口上的 TCP 报文
   使用以下命令可以捕获所有网络接口上传输的 TCP 报文:

$ sudo tcpdump -i any tcp

注意: -i any 指定捕获所有网络接口上的数据包, tcp 指定捕获 TCP 协议的数据, -i 可以理解为 interface 的意思
2. 捕获指定网络接口上的 TCP 报文
如果你只想捕获某个特定网络接口 (如 eth0) 上的 TCP 报文, 可以使用以下命令:

$ sudo tcpdump -i eth0 tcp

eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500inet 172.18.45.153  netmask 255.255.192.0  broadcast 172.18.63.255inet6 fe80::216:3eff:fe03:059b  prefixlen 64  scopeid 0x20<link>ether 00:16:3e:03:95:9b  txqueuelen 1000  (Ethernet)RX packets 34378927  bytes 6954263239 (6.9 GB)RX errors 0  dropped 0  overruns 0  frame 0TX packets 34274797  bytes 6954263239 (6.9 GB)TX errors 0  dropped 0  overruns 0  carrier 0  collisions 0

3. 捕获特定源或目的 IP 地址的 TCP 报文
   使用 host 关键字可以指定源或目的 IP 地址。例如, 要捕获源 IP 地址为 192.168.1.100 的 TCP 报文, 可以使用以下命令:

$ sudo tcpdump src host 192.168.1.100 and tcp

要捕获目的 IP 地址为 192.168.1.200 的 TCP 报文, 可以使用以下命令:

$ sudo tcpdump dst host 192.168.1.200 and tcp

同时指定源和目的 IP 地址, 可以使用 and 关键字连接两个条件:

$ sudo tcpdump src host 192.168.1.100 and dst host 192.168.1.200 and tcp

4. 捕获特定端口的 TCP 报文
   使用 port 关键字可以指定端口号。例如, 要捕获端口号为 80 的 TCP 报文 (通常是 HTTP 请求), 可以使用以下命令:

$ sudo tcpdump port 80 and tcp

5. 保存捕获的数据包到文件
   使用 -w 选项可以将捕获的数据包保存到文件中, 以便后续分析。例如:

$ sudo tcpdump -i eth0 port 80 -w data.pcap

这将把捕获到的 HTTP 流量保存到名为 data.pcap 的文件中。

• 了解: pcap 后缀的文件通常与 PCAP (Packet Capture) 文件格式相关, 这是一种用于捕获网络数据包的文件格式

6. 从文件中读取数据包进行分析
   使用 -r 选项可以从文件中读取数据包进行分析。例如:

tcpdump -r data.pcap

这将读取 data.pcap 文件中的数据包并进行分析。

注意事项

• 使用 tcpdump 时, 请确保你有足够的权限来捕获网络接口上的数据。通常, 你需要以 root 用户身份运行 tcpdump。
• 使用 tcpdump 的时候, 有些主机名会被云服务器解释成为随机的主机名, 如果不想要, 就用-n 选项
• 主机观察三次握手的第三次握手, 不占序号

socket源码分析

相关源码

struct socket {socket_state		state;unsigned long		flags;const struct proto_ops	*ops;struct fasync_struct	*fasync_list;struct file		*file;struct sock		*sk;wait_queue_head_t	wait;short			type;
};

我们知道。在Linux系统下，一切皆文件。所以socket是如何与文件描述符相关联的？

struct task_struct {volatile long state;	/* -1 unrunnable, 0 runnable, >0 stopped */void *stack;atomic_t usage;unsigned int flags;	/* per process flags, defined below */unsigned int ptrace;int lock_depth;		/* BKL lock depth */#ifdef CONFIG_SMP
#ifdef __ARCH_WANT_UNLOCKED_CTXSWint oncpu;
#endif
#endifint prio, static_prio, normal_prio;const struct sched_class *sched_class;struct sched_entity se;struct sched_rt_entity rt;#ifdef CONFIG_PREEMPT_NOTIFIERS/* list of struct preempt_notifier: */struct hlist_head preempt_notifiers;
#endif/** fpu_counter contains the number of consecutive context switches* that the FPU is used. If this is over a threshold, the lazy fpu* saving becomes unlazy to save the trap. This is an unsigned char* so that after 256 times the counter wraps and the behavior turns* lazy again; this to deal with bursty apps that only use FPU for* a short time*/unsigned char fpu_counter;s8 oomkilladj; /* OOM kill score adjustment (bit shift). */
#ifdef CONFIG_BLK_DEV_IO_TRACEunsigned int btrace_seq;
#endifunsigned int policy;cpumask_t cpus_allowed;#ifdef CONFIG_PREEMPT_RCUint rcu_read_lock_nesting;int rcu_flipctr_idx;
#endif /* #ifdef CONFIG_PREEMPT_RCU */#if defined(CONFIG_SCHEDSTATS) || defined(CONFIG_TASK_DELAY_ACCT)struct sched_info sched_info;
#endifstruct list_head tasks;/** ptrace_list/ptrace_children forms the list of my children* that were stolen by a ptracer.*/struct list_head ptrace_children;struct list_head ptrace_list;struct mm_struct *mm, *active_mm;/* task state */struct linux_binfmt *binfmt;int exit_state;int exit_code, exit_signal;int pdeath_signal;  /*  The signal sent when the parent dies  *//* ??? */unsigned int personality;unsigned did_exec:1;pid_t pid;pid_t tgid;#ifdef CONFIG_CC_STACKPROTECTOR/* Canary value for the -fstack-protector gcc feature */unsigned long stack_canary;
#endif/* * pointers to (original) parent process, youngest child, younger sibling,* older sibling, respectively.  (p->father can be replaced with * p->parent->pid)*/struct task_struct *real_parent; /* real parent process (when being debugged) */struct task_struct *parent;	/* parent process *//** children/sibling forms the list of my children plus the* tasks I'm ptracing.*/struct list_head children;	/* list of my children */struct list_head sibling;	/* linkage in my parent's children list */struct task_struct *group_leader;	/* threadgroup leader *//* PID/PID hash table linkage. */struct pid_link pids[PIDTYPE_MAX];struct list_head thread_group;struct completion *vfork_done;		/* for vfork() */int __user *set_child_tid;		/* CLONE_CHILD_SETTID */int __user *clear_child_tid;		/* CLONE_CHILD_CLEARTID */unsigned int rt_priority;cputime_t utime, stime, utimescaled, stimescaled;cputime_t gtime;cputime_t prev_utime, prev_stime;unsigned long nvcsw, nivcsw; /* context switch counts */struct timespec start_time; 		/* monotonic time */struct timespec real_start_time;	/* boot based time */
/* mm fault and swap info: this can arguably be seen as either mm-specific or thread-specific */unsigned long min_flt, maj_flt;cputime_t it_prof_expires, it_virt_expires;unsigned long long it_sched_expires;struct list_head cpu_timers[3];/* process credentials */uid_t uid,euid,suid,fsuid;gid_t gid,egid,sgid,fsgid;struct group_info *group_info;kernel_cap_t   cap_effective, cap_inheritable, cap_permitted, cap_bset;unsigned securebits;struct user_struct *user;
#ifdef CONFIG_KEYSstruct key *request_key_auth;	/* assumed request_key authority */struct key *thread_keyring;	/* keyring private to this thread */unsigned char jit_keyring;	/* default keyring to attach requested keys to */
#endifchar comm[TASK_COMM_LEN]; /* executable name excluding path- access with [gs]et_task_comm (which lockit with task_lock())- initialized normally by flush_old_exec */
/* file system info */int link_count, total_link_count;
#ifdef CONFIG_SYSVIPC
/* ipc stuff */struct sysv_sem sysvsem;
#endif
#ifdef CONFIG_DETECT_SOFTLOCKUP
/* hung task detection */unsigned long last_switch_timestamp;unsigned long last_switch_count;
#endif
/* CPU-specific state of this task */struct thread_struct thread;
/* filesystem information */struct fs_struct *fs;
/* open file information */struct files_struct *files;       //重点！！！！！
/* namespaces */struct nsproxy *nsproxy;
/* signal handlers */struct signal_struct *signal;struct sighand_struct *sighand;sigset_t blocked, real_blocked;sigset_t saved_sigmask;	/* restored if set_restore_sigmask() was used */struct sigpending pending;unsigned long sas_ss_sp;size_t sas_ss_size;int (*notifier)(void *priv);void *notifier_data;sigset_t *notifier_mask;
#ifdef CONFIG_SECURITYvoid *security;
#endifstruct audit_context *audit_context;
#ifdef CONFIG_AUDITSYSCALLuid_t loginuid;unsigned int sessionid;
#endifseccomp_t seccomp;/* Thread group tracking */u32 parent_exec_id;u32 self_exec_id;
/* Protection of (de-)allocation: mm, files, fs, tty, keyrings */spinlock_t alloc_lock;/* Protection of the PI data structures: */spinlock_t pi_lock;#ifdef CONFIG_RT_MUTEXES/* PI waiters blocked on a rt_mutex held by this task */struct plist_head pi_waiters;/* Deadlock detection and priority inheritance handling */struct rt_mutex_waiter *pi_blocked_on;
#endif#ifdef CONFIG_DEBUG_MUTEXES/* mutex deadlock detection */struct mutex_waiter *blocked_on;
#endif
#ifdef CONFIG_TRACE_IRQFLAGSunsigned int irq_events;int hardirqs_enabled;unsigned long hardirq_enable_ip;unsigned int hardirq_enable_event;unsigned long hardirq_disable_ip;unsigned int hardirq_disable_event;int softirqs_enabled;unsigned long softirq_disable_ip;unsigned int softirq_disable_event;unsigned long softirq_enable_ip;unsigned int softirq_enable_event;int hardirq_context;int softirq_context;
#endif
#ifdef CONFIG_LOCKDEP
# define MAX_LOCK_DEPTH 48ULu64 curr_chain_key;int lockdep_depth;struct held_lock held_locks[MAX_LOCK_DEPTH];unsigned int lockdep_recursion;
#endif/* journalling filesystem info */void *journal_info;/* stacked block device info */struct bio *bio_list, **bio_tail;/* VM state */struct reclaim_state *reclaim_state;struct backing_dev_info *backing_dev_info;struct io_context *io_context;unsigned long ptrace_message;siginfo_t *last_siginfo; /* For ptrace use.  */
#ifdef CONFIG_TASK_XACCT
/* i/o counters(bytes read/written, #syscalls */u64 rchar, wchar, syscr, syscw;
#endifstruct task_io_accounting ioac;
#if defined(CONFIG_TASK_XACCT)u64 acct_rss_mem1;	/* accumulated rss usage */u64 acct_vm_mem1;	/* accumulated virtual memory usage */cputime_t acct_stimexpd;/* stime since last update */
#endif
#ifdef CONFIG_NUMAstruct mempolicy *mempolicy;short il_next;
#endif
#ifdef CONFIG_CPUSETSnodemask_t mems_allowed;int cpuset_mems_generation;int cpuset_mem_spread_rotor;
#endif
#ifdef CONFIG_CGROUPS/* Control Group info protected by css_set_lock */struct css_set *cgroups;/* cg_list protected by css_set_lock and tsk->alloc_lock */struct list_head cg_list;
#endif
#ifdef CONFIG_FUTEXstruct robust_list_head __user *robust_list;
#ifdef CONFIG_COMPATstruct compat_robust_list_head __user *compat_robust_list;
#endifstruct list_head pi_state_list;struct futex_pi_state *pi_state_cache;
#endifatomic_t fs_excl;	/* holding fs exclusive resources */struct rcu_head rcu;/** cache last used pipe for splice*/struct pipe_inode_info *splice_pipe;
#ifdef	CONFIG_TASK_DELAY_ACCTstruct task_delay_info *delays;
#endif
#ifdef CONFIG_FAULT_INJECTIONint make_it_fail;
#endifstruct prop_local_single dirties;
#ifdef CONFIG_LATENCYTOPint latency_record_count;struct latency_record latency_record[LT_SAVECOUNT];
#endif
};/** Priority of a process goes from 0..MAX_PRIO-1, valid RT* priority is 0..MAX_RT_PRIO-1, and SCHED_NORMAL/SCHED_BATCH* tasks are in the range MAX_RT_PRIO..MAX_PRIO-1. Priority* values are inverted: lower p->prio value means higher priority.** The MAX_USER_RT_PRIO value allows the actual maximum* RT priority to be separate from the value exported to* user-space.  This allows kernel threads to set their* priority to a value higher than any user task. Note:* MAX_RT_PRIO must not be smaller than MAX_USER_RT_PRIO.*/

struct files_struct {/** read mostly part*/atomic_t count;struct fdtable *fdt;struct fdtable fdtab;/** written part on a separate cache line in SMP*/spinlock_t file_lock ____cacheline_aligned_in_smp;int next_fd;struct embedded_fd_set close_on_exec_init;struct embedded_fd_set open_fds_init;struct file * fd_array[NR_OPEN_DEFAULT];
};

struct file {/** fu_list becomes invalid after file_free is called and queued via* fu_rcuhead for RCU freeing*/union {struct list_head	fu_list;struct rcu_head 	fu_rcuhead;} f_u;struct path		f_path;
#define f_dentry	f_path.dentry
#define f_vfsmnt	f_path.mntconst struct file_operations	*f_op;atomic_t		f_count;unsigned int 		f_flags;mode_t			f_mode;loff_t			f_pos;struct fown_struct	f_owner;unsigned int		f_uid, f_gid;struct file_ra_state	f_ra;u64			f_version;
#ifdef CONFIG_SECURITYvoid			*f_security;
#endif/* needed for tty driver, and maybe others */void			*private_data;                        //重点！！！！#ifdef CONFIG_EPOLL/* Used by fs/eventpoll.c to link all the hooks to this file */struct list_head	f_ep_links;spinlock_t		f_ep_lock;
#endif /* #ifdef CONFIG_EPOLL */struct address_space	*f_mapping;
#ifdef CONFIG_DEBUG_WRITECOUNTunsigned long f_mnt_write_state;
#endif
};

struct file 中成员void*private_data; ，将会指向struct socket；同时struct socket中的struct file *file同时也会指向管理他的文件描述符。

struct tcp_sock {/* inet_connection_sock has to be the first member of tcp_sock */struct inet_connection_sock	inet_conn;                           //重点！！！！！u16	tcp_header_len;	/* Bytes of tcp header to send		*/u16	xmit_size_goal;	/* Goal for segmenting output packets	*//**	Header prediction flags*	0x5?10 << 16 + snd_wnd in net byte order*/__be32	pred_flags;/**	RFC793 variables by their proper names. This means you can*	read the code and the spec side by side (and laugh ...)*	See RFC793 and RFC1122. The RFC writes these in capitals.*/u32	rcv_nxt;	/* What we want to receive next 	*/u32	copied_seq;	/* Head of yet unread data		*/u32	rcv_wup;	/* rcv_nxt on last window update sent	*/u32	snd_nxt;	/* Next sequence we send		*/u32	snd_una;	/* First byte we want an ack for	*/u32	snd_sml;	/* Last byte of the most recently transmitted small packet */u32	rcv_tstamp;	/* timestamp of last received ACK (for keepalives) */u32	lsndtime;	/* timestamp of last sent data packet (for restart window) *//* Data for direct copy to user */struct {struct sk_buff_head	prequeue;struct task_struct	*task;struct iovec		*iov;int			memory;int			len;
#ifdef CONFIG_NET_DMA/* members for async copy */struct dma_chan		*dma_chan;int			wakeup;struct dma_pinned_list	*pinned_list;dma_cookie_t		dma_cookie;
#endif} ucopy;u32	snd_wl1;	/* Sequence for window update		*/u32	snd_wnd;	/* The window we expect to receive	*/u32	max_window;	/* Maximal window ever seen from peer	*/u32	mss_cache;	/* Cached effective mss, not including SACKS */u32	window_clamp;	/* Maximal window to advertise		*/u32	rcv_ssthresh;	/* Current window clamp			*/u32	frto_highmark;	/* snd_nxt when RTO occurred */u8	reordering;	/* Packet reordering metric.		*/u8	frto_counter;	/* Number of new acks after RTO */u8	nonagle;	/* Disable Nagle algorithm?             */u8	keepalive_probes; /* num of allowed keep alive probes	*//* RTT measurement */u32	srtt;		/* smoothed round trip time << 3	*/u32	mdev;		/* medium deviation			*/u32	mdev_max;	/* maximal mdev for the last rtt period	*/u32	rttvar;		/* smoothed mdev_max			*/u32	rtt_seq;	/* sequence number to update rttvar	*/u32	packets_out;	/* Packets which are "in flight"	*/u32	retrans_out;	/* Retransmitted packets out		*/
/**      Options received (usually on last packet, some only on SYN packets).*/struct tcp_options_received rx_opt;/**	Slow start and congestion control (see also Nagle, and Karn & Partridge)*/u32	snd_ssthresh;	/* Slow start size threshold		*/u32	snd_cwnd;	/* Sending congestion window		*/u32	snd_cwnd_cnt;	/* Linear increase counter		*/u32	snd_cwnd_clamp; /* Do not allow snd_cwnd to grow above this */u32	snd_cwnd_used;u32	snd_cwnd_stamp;struct sk_buff_head	out_of_order_queue; /* Out of order segments go here */u32	rcv_wnd;	/* Current receiver window		*/u32	write_seq;	/* Tail(+1) of data held in tcp send buffer */u32	pushed_seq;	/* Last pushed seq, required to talk to windows *//*	SACKs data	*/struct tcp_sack_block duplicate_sack[1]; /* D-SACK block */struct tcp_sack_block selective_acks[4]; /* The SACKS themselves*/struct tcp_sack_block recv_sack_cache[4];struct sk_buff *highest_sack;   /* highest skb with SACK received* (validity guaranteed only if* sacked_out > 0)*//* from STCP, retrans queue hinting */struct sk_buff* lost_skb_hint;struct sk_buff *scoreboard_skb_hint;struct sk_buff *retransmit_skb_hint;struct sk_buff *forward_skb_hint;int     lost_cnt_hint;int     retransmit_cnt_hint;u32	lost_retrans_low;	/* Sent seq after any rxmit (lowest) */u16	advmss;		/* Advertised MSS			*/u32	prior_ssthresh; /* ssthresh saved at recovery start	*/u32	lost_out;	/* Lost packets			*/u32	sacked_out;	/* SACK'd packets			*/u32	fackets_out;	/* FACK'd packets			*/u32	high_seq;	/* snd_nxt at onset of congestion	*/u32	retrans_stamp;	/* Timestamp of the last retransmit,* also used in SYN-SENT to remember stamp of* the first SYN. */u32	undo_marker;	/* tracking retrans started here. */int	undo_retrans;	/* number of undoable retransmissions. */u32	urg_seq;	/* Seq of received urgent pointer */u16	urg_data;	/* Saved octet of OOB data and control flags */u8	urg_mode;	/* In urgent mode		*/u8	ecn_flags;	/* ECN status bits.			*/u32	snd_up;		/* Urgent pointer		*/u32	total_retrans;	/* Total retransmits for entire connection */u32	bytes_acked;	/* Appropriate Byte Counting - RFC3465 */unsigned int		keepalive_time;	  /* time before keep alive takes place */unsigned int		keepalive_intvl;  /* time interval between keep alive probes */int			linger2;unsigned long last_synq_overflow; u32	tso_deferred;/* Receiver side RTT estimation */struct {u32	rtt;u32	seq;u32	time;} rcv_rtt_est;/* Receiver queue space */struct {int	space;u32	seq;u32	time;} rcvq_space;/* TCP-specific MTU probe information. */struct {u32		  probe_seq_start;u32		  probe_seq_end;} mtu_probe;#ifdef CONFIG_TCP_MD5SIG
/* TCP AF-Specific parts; only used by MD5 Signature support so far */struct tcp_sock_af_ops	*af_specific;/* TCP MD5 Signagure Option information */struct tcp_md5sig_info	*md5sig_info;
#endif
};

inet_connection_sock中request_sock_queue，既是全连接队列

struct inet_connection_sock {/* inet_sock has to be the first member! */struct inet_sock	  icsk_inet;struct request_sock_queue icsk_accept_queue;struct inet_bind_bucket	  *icsk_bind_hash;unsigned long		  icsk_timeout;struct timer_list	  icsk_retransmit_timer;struct timer_list	  icsk_delack_timer;__u32			  icsk_rto;__u32			  icsk_pmtu_cookie;const struct tcp_congestion_ops *icsk_ca_ops;const struct inet_connection_sock_af_ops *icsk_af_ops;unsigned int		  (*icsk_sync_mss)(struct sock *sk, u32 pmtu);__u8			  icsk_ca_state;__u8			  icsk_retransmits;__u8			  icsk_pending;__u8			  icsk_backoff;__u8			  icsk_syn_retries;__u8			  icsk_probes_out;__u16			  icsk_ext_hdr_len;struct {__u8		  pending;	 /* ACK is pending			   */__u8		  quick;	 /* Scheduled number of quick acks	   */__u8		  pingpong;	 /* The session is interactive		   */__u8		  blocked;	 /* Delayed ACK was blocked by socket lock */__u32		  ato;		 /* Predicted tick of soft clock	   */unsigned long	  timeout;	 /* Currently scheduled timeout		   */__u32		  lrcvtime;	 /* timestamp of last received data packet */__u16		  last_seg_size; /* Size of last incoming segment	   */__u16		  rcv_mss;	 /* MSS used for delayed ACK decisions	   */ } icsk_ack;struct {int		  enabled;/* Range of MTUs to search */int		  search_high;int		  search_low;/* Information on the current probe. */int		  probe_size;} icsk_mtup;u32			  icsk_ca_priv[16];
#define ICSK_CA_PRIV_SIZE	(16 * sizeof(u32))
};

inet_sock首个成员struct sock sk

struct inet_sock {/* sk and pinet6 has to be the first two members of inet_sock */struct sock		sk;                     
#if defined(CONFIG_IPV6) || defined(CONFIG_IPV6_MODULE)struct ipv6_pinfo	*pinet6;
#endif/* Socket demultiplex comparisons on incoming packets. */__be32			daddr;__be32			rcv_saddr;__be16			dport;__u16			num;__be32			saddr;__s16			uc_ttl;__u16			cmsg_flags;struct ip_options	*opt;__be16			sport;__u16			id;__u8			tos;__u8			mc_ttl;__u8			pmtudisc;__u8			recverr:1,is_icsk:1,freebind:1,hdrincl:1,mc_loop:1;int			mc_index;__be32			mc_addr;struct ip_mc_socklist	*mc_list;struct {unsigned int		flags;unsigned int		fragsize;struct ip_options	*opt;struct dst_entry	*dst;int			length; /* Total length of all frames */__be32			addr;struct flowi		fl;} cork;
};