TCP Congestion Control

  • Congestion occurs when total arrival rate from all packet flows exceeds R over a sustained(维持) period of time
  • Buffers(缓冲) at multiplexer will fill and packets will be lost

Phases of Congestion Behavior

  1. Light traffic

    • Arrival Rate << R
    • Low delay
    • Can accommodate(容纳) more
  2. Knee (congestion onset)
    • Arrival rate approaches R
    • Delay increases rapidly
    • Throughput(吞吐量) begins to saturate(饱和)
  3. Congestion collapse
    • Arrival rate > R
    • Large delays, packet loss
    • Useful application throughput drops

Congestion Window

  • Desired operating point: just before knee
  • TCP sender maintains a congestion window (cwnd) to control congestion at intermediate(中间的) routers
  • Effective window is minimum of congestion window and advertised window(广播窗口)
  • Problem: senders does not know what its “fair” share of available bandwidth should be
  • Solution:
    • adapt dynamically to available BW
    • Senders probe(探查) the network by increasing cwnd
    • When congestion detected, senders reduce rate
    • Ideally, sending rate stabilizes(稳定) near optimal(最优) point

Congestion Window (Cont.)

  • How does the TCP congestion algorithm change congestion window dynamically according to the most up-to-date state of the network?
  • At light traffic: each segment is ACKed quickly
    • Increase cwnd aggresively
  • At knee: segment ACKs arrive, but more slowly
    • Slow down increase in cwnd
  • At congestion: segments encounter large delays, timeout, segments are dropped in router buffers
    • Reduce transmission rate, then probe again

TCP Congestion Control (1): Slow Start

  • Slow start: increase congestion window size by one segment upon receiving an ACK from receiver

    • initialized at 2 segments; usually 1 segment
    • used at start of data transfer
    • congestion window increases exponentially(指数)

TCP Congestion Control (2): Congestion Avoidance

  • Algorithm progressively(逐步) sets a congestion threshold(门槛)

    • When cwnd > threshold, slow down rate at which cwnd is increased
  • Increase congestion window size by one segment per round-trip-time (RTT)
    • Each time an** ACK arrives, cwnd is increased by 1/cwnd**
    • In one RTT, all ccwnd segments are sent, so total increase in cwnd is cwnd x 1/cwnd = 1
    • cwnd grows linearly with time

TCP Congestion Control (3): Congestion

  • Congestion is detected upon timeout or receipt of duplicate ACKs
  • Assume current cwnd corresponds to available bandwidth
  • Adjust congestion threshold = ½ x current cwnd
  • Reset cwnd to 1
  • Go back to slow-start
  • Over several cycles expect to converge(收敛) to congestion threshold equal to about ½ the available bandwidth

Fast Retransmit & Fast Recovery

  • Congestion causes many segments to be dropped
  • Burt if only a single segment is dropped, then subsequent(随后的) segments trigger duplicate ACKs before timeout
  • Can avoid large decrease in cwnd as follows:
    • When three duplicate ACKs arrive before timeout expires(期满), retransmit lost segment immediately
    • Reset congestion threshold to ½ cwnd
    • Reset cwnd to congestion threshold + 3 to account for the three segments that triggered duplicate ACKs
    • Remain in congestion avoidance phase
    • In absence of timeouts, cwnd will oscillate(振动) around optimal value

TCP Congestion Control: Fast Retransmit & Fast Recovery

TCP Congestion Control的更多相关文章

  1. Network | TCP congestion control

    拥塞控制算法:1. 加性增.乘性减:2. 慢启动:3. 对超时事件作出反应: 整体过程如下: 慢启动->到达阈值->加性增(窗口+1个MSS), 这个阶段叫拥塞避免(CA)->3个冗 ...

  2. TCP Flow Control and Data Transfer

    TCP Flow Control TCP Data Transfer Selective Repeat ARQ with Positive ACK Window slides a byte basis ...

  3. Google Congestion Control介绍

    随着网络带宽的日益增加和便携式设备,如智能手机或平板电脑处理能力的增强,基于互联网的实时通信已经成为热点. 虽然视频会议已商用了多年,特别是SKYPE这样的视频应用在互联网上已有10年时间,但针对实时 ...

  4. java基础 UDP通信 user datagram protocol 用户数据豆协议 TCP transmission control protocol 传输控制协议 多线程TCP

    无连接通信 UDP 客户端 package com.swift.test; import java.io.IOException; import java.net.DatagramPacket; im ...

  5. Congestion Avoidance in TCP

    Congestion Avoidance in TCP Consequence of lack of congestion control When a popular resource is sha ...

  6. Internet History,Technology,and Security -Transport Control Protocol(TCP)(Week6)

    Week6 Technology: Transport Control Protocol(TCP) Welcome to Week 6 of IHTS. We are in our second we ...

  7. TCP的那些事儿(下)

    TCP的那些事儿(下) 这篇文章是下篇,所以如果你对TCP不熟悉的话,还请你先看看上篇<TCP的那些事儿(上)> 上篇中,我们介绍了TCP的协议头.状态机.数据重传中的东西.但是TCP要解 ...

  8. [转载] tcp那些事2

    原文: http://coolshell.cn/articles/11609.html 这篇文章是下篇,所以如果你对TCP不熟悉的话,还请你先看看上篇<TCP的那些事儿(上)> 上篇中,我 ...

  9. IP, TCP, and HTTP--reference

    IP, TCP, and HTTP Issue #10 Syncing Data, March 2014 By Daniel Eggert When an app communicates with ...

随机推荐

  1. javascript之Array()数组函数讲解

    Array()是一个用来构建数组的内建构造器函数.数组主要由如下三种创建方式: array = new Array() array = new Array([size]) array = new Ar ...

  2. python 需求文件requirements.txt的创建及使用

    在虚拟环境中使用pip生成: (venv) $ pip freeze >requirements.txt 当需要创建这个虚拟环境的完全副本,可以创建一个新的虚拟环境,并在其上运行以下命令: (v ...

  3. 为My97DatePicker日期插件设置默认日期

    datepicker.zip 为My97DatePicker日期插件设置默认日期,开始日期为系统日期的前一个月,结束日期为系统日期: 开始日期不能大于结束日期,且都不能大于今天: 开始日期-maxDa ...

  4. 139.00.006 Git学习-标签管理Tag

    @(139 - Environment Settings | 环境配置) 一.Why 发布一个版本时,我们通常先在版本库中打一个标签(tag),这样,就唯一确定了打标签时刻的版本.将来无论什么时候,取 ...

  5. 任务九:使用HTML/CSS实现一个复杂页面

    任务目的 通过实现一个较为复杂的页面,加深对于HTML,CSS的实战能力 实践代码的复用.优化 任务描述 通过HTML及CSS实现设计稿 设计稿PSD文件(点击下载),效果如 效果图(点击打开) 整个 ...

  6. Incorrect string value: '\xE8\xAF\xAD\xE6\x96\x87' for column 'name' at row 1

    报错的原因就是在执行插入时对Name这个字段被赋予了错误的字符串值:’\xE4\xB8\xAD\xE6\x96\x87’ 实际上就函数里面的变量接收到的值编码格式跟它定义的不一致. 使用navicat ...

  7. 微软发布Azure Stack第一个技术预览版

    为了提升商业灵敏度和加快创新步伐,各个企业都在迅速地转向云服务.在微软,我们已经见到微软智能云Azure的飞速发展和使用,每月我们都有近十万的新增订阅量.然而,我们也了解到还有很多企业在完全移到公有云 ...

  8. c#编程指南(五) 扩展方法(Extension Method)

    C# 3.0就引入的新特性,扩展方法可以很大的增加你代码的优美度,扩展方法提供你扩展.NET Framewoke类的扩展途径,书写和规则也简单的要命. 编写扩展方法有下面几个要求: 第一:扩展方法所在 ...

  9. DevExpress源码编译总结 z

    本篇文章内容包括基础知识(GAC.程序集强签名.友元程序集).编译过程.注册GAC.添加工具箱.多语言支持.运行时和设计时调试 源码地址 链接:http://pan.baidu.com/s/1eQm1 ...

  10. Angular2 前端代码规范

    不要重置对象的引用!(重置只应该在组件或服务的初始化时) why:会使页面产生闪烁 不要给图片绑定一个空的值或空的图片引用(如果值从服务器异步过来,那应该在初始化时给它一个默认值)why:会触发一个4 ...