关于TCP Delay ACK的概念我就不多说了，到处复制粘贴标准文献以及别人的文章只能让本文篇幅加长而降低被阅读完的欲望，再者这也不是什么论文，附录参考文献几乎很少有人去看，所以我把这些都略过了。

和风吹的干皮鞋，吹的断愁绪吗？

写完本文后的补充：

序言

Delay ACK的实质

正文

关于Delay ACK的触发声明

一个场景

#undef UNICODE

#define WIN32_LEAN_AND_MEAN

#include <windows.h>
#include <winsock2.h>
#include <ws2tcpip.h>
#include <stdlib.h>
#include <stdio.h>

// Need to link with Ws2_32.lib
#pragma comment (lib, "Ws2_32.lib")

#define DEFAULT_PORT "8080"

int __cdecl main(void) 
{
    WSADATA wsaData;
    int iResult;

    SOCKET ListenSocket = INVALID_SOCKET;
    SOCKET ClientSocket = INVALID_SOCKET;

    struct addrinfo *result = NULL;
    struct addrinfo hints;

    int iSendResult;

    // Initialize Winsock
    iResult = WSAStartup(MAKEWORD(2,2), &wsaData);
    if (iResult != 0) {
        printf("WSAStartup failed with error: %d\n", iResult);
        return 1;
    }

    ZeroMemory(&hints, sizeof(hints));
    hints.ai_family = AF_INET;
    hints.ai_socktype = SOCK_STREAM;
    hints.ai_protocol = IPPROTO_TCP;
    hints.ai_flags = AI_PASSIVE;

    // Resolve the server address and port
    iResult = getaddrinfo(NULL, DEFAULT_PORT, &hints, &result);
    if ( iResult != 0 ) {
        printf("getaddrinfo failed with error: %d\n", iResult);
        WSACleanup();
        return 1;
    }

    // Create a SOCKET for connecting to server
    ListenSocket = socket(result->ai_family, result->ai_socktype, result->ai_protocol);
    if (ListenSocket == INVALID_SOCKET) {
        printf("socket failed with error: %ld\n", WSAGetLastError());
        freeaddrinfo(result);
        WSACleanup();
        return 1;
    }

    // Setup the TCP listening socket
    iResult = bind( ListenSocket, result->ai_addr, (int)result->ai_addrlen);
    if (iResult == SOCKET_ERROR) {
        printf("bind failed with error: %d\n", WSAGetLastError());
        freeaddrinfo(result);
        closesocket(ListenSocket);
        WSACleanup();
        return 1;
    }

    freeaddrinfo(result);

    iResult = listen(ListenSocket, SOMAXCONN);
    if (iResult == SOCKET_ERROR) {
        printf("listen failed with error: %d\n", WSAGetLastError());
        closesocket(ListenSocket);
        WSACleanup();
        return 1;
    }

    // Accept a client socket
    ClientSocket = accept(ListenSocket, NULL, NULL);
    if (ClientSocket == INVALID_SOCKET) {
        printf("accept failed with error: %d\n", WSAGetLastError());
        closesocket(ListenSocket);
        WSACleanup();
        return 1;
    }
    int v = 1;
	//setsockopt(ClientSocket, IPPROTO_TCP, TCP_QUICKACK, &v, 4);
    // No longer need server socket
    closesocket(ListenSocket);

    // Receive until the peer shuts down the connection
    do {
		char buff1[1] = {0};
		char buff2[999] = {0};
		char buff3[1000] = {0};
		char buff4[2000] = {0};
		iSendResult = send( ClientSocket, buff1, sizeof(buff1), 0 );
        iResult = recv(ClientSocket, buff4, 700, 0);
		iResult = recv(ClientSocket, buff4, 300, 0);
		iResult = recv(ClientSocket, buff4, 1000, 0);
        iSendResult = send( ClientSocket, buff2, sizeof(buff2), 0 );
        iResult = recv(ClientSocket, buff4, 700, 0);
        iResult = recv(ClientSocket, buff4, 300, 0);
        iResult = recv(ClientSocket, buff4, 700, 0);
        iResult = recv(ClientSocket, buff4, 300, 0);
        iSendResult = send( ClientSocket, buff3, sizeof(buff3), 0 );
        iResult = recv(ClientSocket, buff4, 700, 0);
        iResult = recv(ClientSocket, buff4, 300, 0);
        iResult = recv(ClientSocket, buff4, 700, 0);
        iResult = recv(ClientSocket, buff4, 300, 0);
        iResult = recv(ClientSocket, buff4, 700, 0);
        iResult = recv(ClientSocket, buff4, 300, 0);
        iResult = recv(ClientSocket, buff4, 700, 0);
        iResult = recv(ClientSocket, buff4, 300, 0);
    } while (0);
    
	sleep(10);
    // shutdown the connection since we‘re done
    iResult = shutdown(ClientSocket, SD_SEND);
    if (iResult == SOCKET_ERROR) {
        printf("shutdown failed with error: %d\n", WSAGetLastError());
        closesocket(ClientSocket);
        WSACleanup();
        return 1;
    }

    // cleanup
    closesocket(ClientSocket);
    WSACleanup();

    return 0;
}

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>          /* See NOTES */
#include <sys/socket.h>
#include<arpa/inet.h>
#include<string.h>
#include <netinet/in.h>
#include <netinet/tcp.h>

#define DEFAULT_PORT "8080"

int main(void) 
{
    int iResult;

    int ListenSocket;
    int ClientSocket;

    struct sockaddr_in server;

    int iSendResult;
    char recvbuf[2000];
    int recvbuflen = 2000;
    

    memset(&server, 0, sizeof(server));
    server.sin_family = AF_INET;
    server.sin_addr.s_addr = INADDR_ANY;
    server.sin_port = htons(8080);

    // Create a SOCKET for connecting to server
    ListenSocket = socket(AF_INET , SOCK_STREAM , 0);
    if (ListenSocket == -1) {
        return 1;
    }

    // Setup the TCP listening socket
    iResult = bind( ListenSocket, (struct sockaddr *)&server , sizeof(server));
    if (iResult < 0) {
        perror("bind");
        close(ListenSocket);
        return 1;
    }


    iResult = listen(ListenSocket, 3);
    if (iResult == -1) {
        close(ListenSocket);
        return 1;
    }

    // Accept a client socket
    ClientSocket = accept(ListenSocket, NULL, NULL);
    if (ClientSocket == -1) {
        close(ListenSocket);
        return 1;
    }
    int v = 0;
    setsockopt(ClientSocket, IPPROTO_TCP, TCP_QUICKACK, &v, 4);
    // No longer need server socket
    close(ListenSocket);

    // Receive until the peer shuts down the connection
    do {
        char buff1[1] = {0};
        char buff2[999] = {0};
        char buff3[1000] = {0};
        char buff4[2000] = {0};
        iSendResult = send( ClientSocket, buff1, sizeof(buff1), 0 );
        iResult = recv(ClientSocket, buff4, 700, 0);
        iResult = recv(ClientSocket, buff4, 300, 0);
        iResult = recv(ClientSocket, buff4, 1000, 0);
        iSendResult = send( ClientSocket, buff2, sizeof(buff2), 0 );
        iResult = recv(ClientSocket, buff4, 700, 0);
        iResult = recv(ClientSocket, buff4, 300, 0);
        iResult = recv(ClientSocket, buff4, 700, 0);
        iResult = recv(ClientSocket, buff4, 300, 0);
        iSendResult = send( ClientSocket, buff3, sizeof(buff3), 0 );
        iResult = recv(ClientSocket, buff4, 700, 0);
        iResult = recv(ClientSocket, buff4, 300, 0);
        iResult = recv(ClientSocket, buff4, 700, 0);
        iResult = recv(ClientSocket, buff4, 300, 0);
        iResult = recv(ClientSocket, buff4, 700, 0);
        iResult = recv(ClientSocket, buff4, 300, 0);
        iResult = recv(ClientSocket, buff4, 700, 0);
        iResult = recv(ClientSocket, buff4, 300, 0);
    } while (0);

    sleep(10);
    // cleanup
    close(ClientSocket);

    return 0;
}

#include <stdio.h>
#include <stdlib.h>
#include <string.h> 
#include <unistd.h> 
#include <sys/types.h> 
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>

#define DEFAULT_PORT 8080

int main(int argc, char** argv)
{
    int cPort = DEFAULT_PORT;
    int cClient = 0;
    int cLen = 0;
    struct sockaddr_in cli;
    char cbuf[4096] = {0};
    char buff1[700] = {0};
    char buff2[300] = {0};
    char buff3[1000] = {0};
    
    if(argc < 2)
    {
        printf("Uasge: client[server IP address]\n");
        return -1;
    }
    
    memset(cbuf, 0, sizeof(cbuf));
    
    cli.sin_family = AF_INET;
    cli.sin_port = htons(cPort);
    cli.sin_addr.s_addr = inet_addr(argv[1]);
    
    cClient = socket(AF_INET, SOCK_STREAM, 0);
    if(cClient < 0) {
        printf("socket() failure!\n");
        return -1; 
    }
    
    if(connect(cClient, (struct sockaddr*)&cli, sizeof(cli)) < 0) {
        printf("connect() failure!\n");
        return -1;
    }
    
    cLen = recv(cClient, cbuf, 1,0);    
    cLen = send(cClient, buff1, sizeof(buff1), 0);
    sleep(1);
    cLen = send(cClient, buff2, sizeof(buff2), 0);
    cLen = send(cClient, buff3, sizeof(buff3), 0);

    cLen = recv(cClient, cbuf, 999, 0);    
    
    cLen = send(cClient, buff1, sizeof(buff1), 0);
    cLen = send(cClient, buff2, sizeof(buff2), 0);
    sleep(1);
    cLen = send(cClient, buff1, sizeof(buff1), 0);
    cLen = send(cClient, buff2, sizeof(buff2), 0);
    
    cLen = recv(cClient, cbuf, 1000, 0);    
    
    cLen = send(cClient, buff1, sizeof(buff1), 0);
    cLen = send(cClient, buff2, sizeof(buff2), 0);
    sleep(1);
    cLen = send(cClient, buff1, sizeof(buff1), 0);
    cLen = send(cClient, buff2, sizeof(buff2), 0);
    sleep(1);
    cLen = send(cClient, buff1, sizeof(buff1), 0);
    cLen = send(cClient, buff2, sizeof(buff2), 0);
    cLen = send(cClient, buff1, sizeof(buff1), 0);
    cLen = send(cClient, buff2, sizeof(buff2), 0);
    
    close(cClient);
    
    return 0;
}

我给出该场景的连接拓扑：

        想要解释表象的背后原理首先要摸清表象本身。很多人比如温州皮鞋厂老板太关注实现细节，上去就去看代码，这是不对的！

场景表现的分析

        然而相同的代码在Linux中表现却是另一番景象。它好像并不受某一个参数或者某个socket选项的影响(正如TCP_QUICKACK的manual里所述，其实如果你在服务器端的accept调用后添加设置TCP_QUICKACK选项，情况也是一样的。)。也就是说，Linux中的Delay ACK不是配置参数和socket选项决定的，而是自适应的，在这个自适应机制的范围内，系统依然提供了一个临时的开启/关闭Delay ACK的开关，即TCP_QUICKACK选项！

以上场景的packetdrill代码

0.000 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
0.000 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
0.000 bind(3, ..., ...) = 0
0.000 listen(3, 1) = 0
0.000...0.280 accept(3, ..., ...) = 4

0.100 < S 0:0(0) win 32792 <mss 1460, sackOK, nop, nop, nop, wscale 7>
0.100 > S. 0:0(0) ack 1 <...>

0.280 < . 1:1(0) ack 1 win 100
//0.300 setsockopt(4, IPPROTO_TCP, TCP_QUICKACK, [0], 4) = 0

0.300 write(4, ..., 1) = 1
0.300 < P. 1:701(700) ack 2 win 16425
1.300 < P. 701:1001(300) ack 2 win 16425
1.300 < P. 1001:2001(1000) ack 2 win 16425

1.300 write(4, ..., 999) = 999

1.300 %{ print tcpi_rcv_mss }%
1.300 < P. 2001:2701(700) ack 1001 win 16421
1.300 < P. 2701:3001(300) ack 1001 win 16421
2.300 < P. 3001:3701(700) ack 1001 win 16421
2.300 < P. 3701:4001(300) ack 1001 win 16421

2.300 write(4, ..., 1000) = 1000
2.300 < P. 4001:4701(700) ack 2001 win 16425
2.300 < P. 4701:5001(300) ack 2001 win 16425
3.300 < P. 5001:5701(700) ack 2001 win 16425
3.300 < P. 5701:6001(300) ack 2001 win 16425
4.300 < P. 6001:6701(700) ack 2001 win 16425
4.300 < P. 6701:7001(300) ack 2001 win 16425
4.300 < P. 7001:7701(700) ack 2001 win 16425
4.300 < P. 7701:8001(300) ack 2001 win 16425

10.300 write(4, ..., 13) = 13

解析自适应Delay ACK    

这个意义如下图所示：

我们可以看出，在TCP双向数据传输是一来一回的场景下，Delay ACK可以省去所有的纯ACK段的发送。典型的比如远程终端登录(需要回显的如telnet，ssh之类)。

关于QUICK计数器

自适应Delay ACK的Oneshot特性

自适应Delay ACK的另一些细节

1.慢启动的问题

2.分配纯ACK失败后Delay ACK对其的接管

buff = alloc_skb(MAX_TCP_HEADER, GFP_ATOMIC);
if (buff == NULL) {
    inet_csk_schedule_ack(sk);
    inet_csk(sk)->icsk_ack.ato = TCP_ATO_MIN;
    inet_csk_reset_xmit_timer(sk, ICSK_TIME_DACK,
                  TCP_DELACK_MAX, TCP_RTO_MAX);
    return;
}

代码细节

1.tcp_enter_quickack_mode

2.if (skb_queue_empty(&tp->out_of_order_queue))

3.tcp_delack_timer

4.tcp_event_data_sent

if ((u32)(now - icsk->icsk_ack.lrcvtime) < icsk->icsk_ack.ato)
    icsk->icsk_ack.pingpong = 1;

5.tcp_event_data_recv

6.tcp_event_ack_sent

结语

TCP之Delay ACK在Linux和Windows上实现的异同-Linux的自适应ACK

原文：http://blog.csdn.net/dog250/article/details/52664508