MTU

  • 什么是MTU以及MTU的作用
    • 为什么是1500？
  • MTU是哪一层网络的概念？
  • 假设取一个更大的值
  • 既然大了不行，那设置小一点可以么？
  • 在我玩游戏的时候，为什么把MTU改成1480就不卡了？
    • PPPoE
  • 最根本原因

什么是MTU以及MTU的作用

  • MTU（Maximum Transmission Unit）中文名：最大传输单元。
  • MTU的作用

举一个最简单的场景，你在家用自己的笔记本上网，用的是路由器，路由器连接电信网络，然后访问了www.qq.com，从你的笔记本出发的一个以太网数据帧总共经过了以下路径

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#sequencenumber{fill:#333}#mermaid-svg-iwcEgLiUHiSibWbo #crosshead path{fill:#333;stroke:#333}#mermaid-svg-iwcEgLiUHiSibWbo .messageText{fill:#333;stroke:#333}#mermaid-svg-iwcEgLiUHiSibWbo .labelBox{stroke:#ccf;fill:#ECECFF}#mermaid-svg-iwcEgLiUHiSibWbo .labelText,#mermaid-svg-iwcEgLiUHiSibWbo .labelText>tspan{fill:#000;stroke:none}#mermaid-svg-iwcEgLiUHiSibWbo .loopText,#mermaid-svg-iwcEgLiUHiSibWbo .loopText>tspan{fill:#000;stroke:none}#mermaid-svg-iwcEgLiUHiSibWbo .loopLine{stroke-width:2px;stroke-dasharray:2, 2;stroke:#ccf;fill:#ccf}#mermaid-svg-iwcEgLiUHiSibWbo .note{stroke:#aa3;fill:#fff5ad}#mermaid-svg-iwcEgLiUHiSibWbo .noteText,#mermaid-svg-iwcEgLiUHiSibWbo .noteText>tspan{fill:#000;stroke:none}#mermaid-svg-iwcEgLiUHiSibWbo .activation0{fill:#f4f4f4;stroke:#666}#mermaid-svg-iwcEgLiUHiSibWbo .activation1{fill:#f4f4f4;stroke:#666}#mermaid-svg-iwcEgLiUHiSibWbo .activation2{fill:#f4f4f4;stroke:#666}#mermaid-svg-iwcEgLiUHiSibWbo .mermaid-main-font{font-family:"trebuchet ms", 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.statediagram-cluster.statediagram-cluster-alt .inner{fill:#e0e0e0}#mermaid-svg-iwcEgLiUHiSibWbo .statediagram-cluster .inner{rx:0;ry:0}#mermaid-svg-iwcEgLiUHiSibWbo .statediagram-state rect.basic{rx:5px;ry:5px}#mermaid-svg-iwcEgLiUHiSibWbo .statediagram-state rect.divider{stroke-dasharray:10,10;fill:#efefef}#mermaid-svg-iwcEgLiUHiSibWbo .note-edge{stroke-dasharray:5}#mermaid-svg-iwcEgLiUHiSibWbo .statediagram-note rect{fill:#fff5ad;stroke:#aa3;stroke-width:1px;rx:0;ry:0}:root{--mermaid-font-family: '"trebuchet ms", verdana, arial';--mermaid-font-family: "Comic Sans MS", "Comic Sans", cursive}#mermaid-svg-iwcEgLiUHiSibWbo .error-icon{fill:#522}#mermaid-svg-iwcEgLiUHiSibWbo .error-text{fill:#522;stroke:#522}#mermaid-svg-iwcEgLiUHiSibWbo .edge-thickness-normal{stroke-width:2px}#mermaid-svg-iwcEgLiUHiSibWbo .edge-thickness-thick{stroke-width:3.5px}#mermaid-svg-iwcEgLiUHiSibWbo .edge-pattern-solid{stroke-dasharray:0}#mermaid-svg-iwcEgLiUHiSibWbo .edge-pattern-dashed{stroke-dasharray:3}#mermaid-svg-iwcEgLiUHiSibWbo .edge-pattern-dotted{stroke-dasharray:2}#mermaid-svg-iwcEgLiUHiSibWbo .marker{fill:#333}#mermaid-svg-iwcEgLiUHiSibWbo .marker.cross{stroke:#333}:root { --mermaid-font-family: "trebuchet ms", verdana, arial;}#mermaid-svg-iwcEgLiUHiSibWbo {color: rgba(0, 0, 0, 0.75);font: ;}

笔记本
路由器
电信机房
服务器

其中，每个节点都有一个MTU值，如下：

#mermaid-svg-ntXfbm9qa6yW18Ms .label{font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family);fill:#333;color:#333}#mermaid-svg-ntXfbm9qa6yW18Ms .label text{fill:#333}#mermaid-svg-ntXfbm9qa6yW18Ms .node rect,#mermaid-svg-ntXfbm9qa6yW18Ms .node circle,#mermaid-svg-ntXfbm9qa6yW18Ms .node ellipse,#mermaid-svg-ntXfbm9qa6yW18Ms .node polygon,#mermaid-svg-ntXfbm9qa6yW18Ms .node path{fill:#ECECFF;stroke:#9370db;stroke-width:1px}#mermaid-svg-ntXfbm9qa6yW18Ms .node .label{text-align:center;fill:#333}#mermaid-svg-ntXfbm9qa6yW18Ms .node.clickable{cursor:pointer}#mermaid-svg-ntXfbm9qa6yW18Ms .arrowheadPath{fill:#333}#mermaid-svg-ntXfbm9qa6yW18Ms .edgePath .path{stroke:#333;stroke-width:1.5px}#mermaid-svg-ntXfbm9qa6yW18Ms .flowchart-link{stroke:#333;fill:none}#mermaid-svg-ntXfbm9qa6yW18Ms .edgeLabel{background-color:#e8e8e8;text-align:center}#mermaid-svg-ntXfbm9qa6yW18Ms .edgeLabel rect{opacity:0.9}#mermaid-svg-ntXfbm9qa6yW18Ms .edgeLabel 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1500
笔记本
1500
路由器

电信机房
1500
服务器

假设现在我把笔记本的MTU最大值设置成了1700，然后发送了一个超大的ip数据包（2000），这时候在以外网传输的时候会被拆成2个包，一个1700，一个300，然后加上头信息进行传输。

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1700
笔记本
1500
路由器

电信机房
1500
服务器

路由器接收到了一个1700的帧，发现大于自己设置的最大值：1500，如果IP包DF标志位为1，也就是不允许分包，那么路由器直接就把这个包丢弃了，根本就不会到达电信机房，也就到不了服务器了，所以，到这里我们就会发现，MTU其实就是在每一个节点的管控值，只要是大于这个值的数据帧，要么选择分片，要么直接丢弃。

为什么是1500？

  • 其实一个标准的以太网数据帧大小是：1518，头信息有14字节，尾部校验和FCS占了4字节，所以真正留给上层协议传输数据的大小就是：1518 - 14 - 4 = 1500

MTU是哪一层网络的概念？

  • 从表格中可以看到，在7层网络协议中，MTU是数据链路层的概念。MTU限制的是数据链路层的payload，也就是上层协议的大小，例如IP，ICMP等。
OSI中的层 功能 TCP/IP协议族
应用层 文件传输，电子邮件，文件服务，虚拟终端 TFTP，HTTP，SNMP，FTP，SMTP，DNS，Telnet
表示层 数据格式化，代码转换，数据加密 没有协议
会话层 解除或建立与别的接点的联系 没有协议
传输层 提供端对端的接口 TCP，UDP
网络层 为数据包选择路由 IP，ICMP，RIP，OSPF，BGP，IGMP
数据链路层 传输有地址的帧以及错误检测功能 SLIP，CSLIP，PPP，ARP，RARP，MTU
物理层 以二进制数据形式在物理媒体上传输数据 ISO2110，IEEE802，IEEE802.2

假设取一个更大的值

  • 假设MTU值和IP数据包大小一致，一个IP数据包的大小是：65535，那么加上以太网帧头和为，一个以太网帧的大小就是：65535 + 14 + 4 = 65553，看起来似乎很完美，发送方也不需要拆包，接收方也不需要重组。

  • 那么假设我们现在的带宽是：100Mbps，因为以太网帧是传输中的最小可识别单元，再往下就是0101所对应的光信号了，所以我们的一条带宽同时只能发送一个以太网帧。如果同时发送多个，那么对端就无法重组成一个以太网帧了，在100Mbps的带宽中（假设中间没有损耗），我们计算一下发送这一帧需要的时间：

  • ( 65553 * 8 ) / ( 100 * 1024 * 1024 ) ≈ 0.005(s)
    在100M网络下传输一帧就需要5ms，也就是说这5ms其他进程发送不了任何数据。如果是早先的电话拨号，网速只有2M的情况下：

  • ( 65553 * 8 ) / ( 2 * 1024 * 1024 ) ≈ 0.100(s)
    100ms，这简直是噩梦。其实这就像红绿灯，时间要设置合理，交替通行，不然同一个方向如果一直是绿灯，那么另一个方向就要堵成翔了。

既然大了不行，那设置小一点可以么？

  • 假设MTU值设置为100，那么单个帧传输的时间，在2Mbps带宽下需要：( 100 * 8 ) / ( 2 * 1024 * 1024 ) * 1000 ≈ 5(ms) 时间上已经能接受了，问题在于，不管MTU设置为多少，以太网头帧尾大小是固定的，都是14 + 4，所以在MTU为100的时候，一个以太网帧的传输效率为： ( 100 - 14 - 4 ) / 100 = 82%写成公式就是：( T - 14 - 4 ) / T，当T趋于无穷大的时候，效率接近100%，也就是MTU的值越大，传输效率最高，但是基于上一点传输时间的问题，来个折中的选择吧，既然头加尾是18，那就凑个整来个1500，总大小就是1518，传输效率：1500 / 1518 = 98.8%。100Mbps传输时间：( 1518 * 8 ) / ( 100 * 1024 * 1024 ) * 1000 = 0.11(ms)。2Mbps传输时间：( 1518 * 8 ) / ( 2 * 1024 * 1024 ) * 1000 = 5.79(ms)
    总体上时间都还能接受

在我玩游戏的时候，为什么把MTU改成1480就不卡了？

  • 路由器默认值大多都是1500，理论上是没有问题的，那为什么我玩游戏的时候改成1480才能流畅呢？原因在于当时我使用的是ADSL上网的方式，ADSL使用的PPPoE协议。

PPPoE

  • PPPoE协议介于以太网和IP之间，协议分为两部分，PPP( Point to Point Protocol )和oE( over Ethernet )，也就是以太网上的PPP协议，而PPPoE协议头信息为
  • 这里总共是48位，也就是6个字节，另外2个字节是PPP协议的ID号，占用两个字节，所以在PPPoE环境下，最佳MTU值应该是：1500 - 4 - 2 = 1492。

最根本原因

  • 问题就出在路由器拨号，如果是PC拨号，那么PC会进行PPPoE的封装，会按照MTU:1492来进行以太网帧的封装，即使通过路由器，路由器这时候也只是转发而已，不会进行拆包。
    而当用路由器拨号时，PC并不知道路由器的通信方式，会以网卡的设置，默认1500的MTU来进行以太网帧的封装，到达路由器时，由于路由器需要进行PPPoE协议的封装，加上8字节的头信息，这样一来，就必须进行拆包，路由器把这一帧的内容拆成两帧发送，一帧是1492，一帧是8，然后分别加上PPPoE的头进行发送。
    平时玩游戏不卡，是因为数据量路由器还处理得过来，而当短时间数据量过大时候，由于短时间数据量过大，路由器处理不过来，就会发生丢包卡顿的情况，也就掉线了。
    这里面提到的1480，是因为尽量设小一点，避免二次拨号带来的又一次PPPoE的封装

什么是MTU？为什么把MTU改成1480游戏就不卡了？带你详细了解MTU相关推荐

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