MTU是1500，为什么IPv6的第一个分片长度是1496？

Question

ping6 ipv6Address -s 1500

此命令将产生一个片段。通过wireshark观察，第一个分片数据包的长度为1496（来自第3层）。由于 MTU 是 1500，我对这个 1496 的长度感到困惑。它从何而来？

Answer 1

这只是我自己关于 IPv6 MTU 的参考。我需要一些问题来添加它 - 我选择了你。

首先是我们的以太网数据包，通常我们会忽略它：

+-------------------------+
| Ethernet - 16 bytes     |
|  6 byte destination MAC |
|  6 byte source MAC      |
|  4 byte type            |
+-------------------------+
|                         |
/  1,500 byte payload     /
|                         |
+-------------------------+

现在我们添加 IPv4 或 IPv6 标头：

IPv4                           IPv6
+-------------------------+    +-------------------------+
| Ethernet - 12 bytes     |    | Ethernet - 12 bytes     |
|  6 byte destination MAC |    |  6 byte destination MAC |
|  6 byte source MAC      |    |  6 byte source MAC      |
|  4 byte type            |    |  4 byte type            |
+-------------------------+    +-------------------------+
| IPv4 - 20 bytes         |    | IPv6 - 40 bytes         |
|  10  byte control flags |    |  6 byte control flags   |
|  2 byte total length    |    |  2 byte payload length  |
|  4 byte source addr     |    |  16 byte source addr    |
|  4 byte dest addr       |    |                         |
+-------------------------+    |  16 byte dest addr      |
|                         |    |                         |
|                         |    +-------------------------|
|                         |    |                         |
/ 1,480 payload           /    / 1,460 payload           /
|                         |    |                         |
+-------------------------+    +-------------------------+

接下来我们添加一个 PPPoE 连接的现实，它从 PPPoE PADS 标记的数据包中取出 8 个字节：

+-------------------------+
| Ethernet - 12 bytes     |
|  6 byte destination MAC |
|  6 byte source MAC      |
|  4 byte type            |
+-------------------------+
| PPPoE PADS - 8 bytes    |
+-------------------------+
|                         |
/ 1,492 payload           /
|                         |
+-------------------------+

这减少了可用于 IPv4 和 IPv6 流量的 MTU：

IPv4                           IPv6
+-------------------------+    +-------------------------+
| Ethernet - 12 bytes     |    | Ethernet - 12 bytes     |
|  6 byte destination MAC |    |  6 byte destination MAC |
|  6 byte source MAC      |    |  6 byte source MAC      |
|  4 byte type            |    |  4 byte type            |
+-------------------------+    +-------------------------+
| PPPoE PADS - 8 bytes    |    | PPPoE PADS - 8 bytes    |
+-------------------------+    +-------------------------+
| IPv4 - 20 bytes         |    | IPv6 - 40 bytes         |
|  10  byte control flags |    |  6 byte control flags   |
|  2 byte total length    |    |  2 byte payload length  |
|  4 byte source addr     |    |  16 byte source addr    |
|  4 byte dest addr       |    |                         |
+-------------------------+    |  16 byte dest addr      |
|                         |    |                         |
|                         |    +-------------------------|
|                         |    |                         |
/ 1,472 payload           /    / 1,452 payload           /
|                         |    |                         |
+-------------------------+    +-------------------------+

然后，就我而言，我正在使用Hurricane Electric的6in4 隧道，因为我的 ISP 不提供本地 IPv6 连接。这意味着我的 IPv6 流量包含在 IPv4 数据包中：

IPv4                           IPv6
+-------------------------+    +-------------------------+
| Ethernet - 12 bytes     |    | Ethernet - 12 bytes     |
|  6 byte destination MAC |    |  6 byte destination MAC |
|  6 byte source MAC      |    |  6 byte source MAC      |
|  4 byte type            |    |  4 byte type            |
+-------------------------+    +-------------------------+
| PPPoE PADS - 8 bytes    |    | PPPoE PADS - 8 bytes    |
+-------------------------+    +-------------------------+
| IPv4 - 20 bytes         |    | IPv4 - 20 bytes         |
|  10  byte control flags |    |  10  byte control flags |
|  2 byte total length    |    |  2 byte total length    |
|  4 byte source addr     |    |  4 byte source addr     |
|  4 byte dest addr       |    |  4 byte dest addr       |
+-------------------------+    +-------------------------+
|                         |    | IPv6 - 40 bytes         |
|                         |    |  6 byte control flags   |
|                         |    |  2 byte payload length  |
|                         |    |  16 byte source addr    |
|                         |    |                         |
|                         |    |  16 byte dest addr      |
|                         |    |                         |
|                         |    +-------------------------|
|                         |    |                         |
/ 1,472 payload           /    / 1,432 payload           /
|                         |    |                         |
+-------------------------+    +-------------------------+

但重要的数字是MTU，它是 IPv4 或 IPv6 自身可用的大小（包括其标头）：

IPv4                           IPv6
+-------------------------+    +-------------------------+
| Ethernet - 12 bytes     |    | Ethernet - 12 bytes     |
|  6 byte destination MAC |    |  6 byte destination MAC |
|  6 byte source MAC      |    |  6 byte source MAC      |
|  4 byte type            |    |  4 byte type            |
+-------------------------+    +-------------------------+
| PPPoE PADS - 8 bytes    |    | PPPoE PADS - 8 bytes    |
+-------------------------+    +-------------------------+
|                         |    | IPv4 - 20 bytes         |
/ MTU = 1492              /    |  10  byte control flags |
|                         |    |  2 byte total length    |
+-------------------------+    |  4 byte source addr     |
                               |  4 byte dest addr       |
                               +-------------------------+
                               |                         |
                               / MTU = 1472              |
                               |                         |
                               +-------------------------+

接下来是关于有效载荷大小的混淆，在使用不分段标志ping 某人时使用。ICMP 在 IP 上运行，并添加自己的头开销：

IPv4                           IPv6
+-------------------------+    +-------------------------+
| Ethernet - 12 bytes     |    | Ethernet - 12 bytes     |
|  6 byte destination MAC |    |  6 byte destination MAC |
|  6 byte source MAC      |    |  6 byte source MAC      |
|  4 byte type            |    |  4 byte type            |
+-------------------------+    +-------------------------+
| IPv4 - 20 bytes         |    | IPv6 - 40 bytes         |
|  10  byte control flags |    |  6 byte control flags   |
|  2 byte total length    |    |  2 byte payload length  |
|  4 byte source addr     |    |  16 byte source addr    |
|  4 byte dest addr       |    |                         |
+-------------------------+    |  16 byte dest addr      |
| ICMP - 8 bytes          |    |                         |
+-------------------------+    +-------------------------|    
|                         |    | ICMP - 8 bytes          |
|                         |    +-------------------------+
|                         |    |                         |
/ 1,472 ping payload      /    / 1,452 ping payload      /
|                         |    |                         |
+-------------------------+    +-------------------------+

注意：这就是为什么您可以1,452在需要分段之前使用字节有效负载进行ping操作。

就我而言，使用 PPPoE PADS 标签的开销：

IPv4                           IPv6
+-------------------------+    +-------------------------+
| Ethernet - 12 bytes     |    | Ethernet - 12 bytes     |
|  6 byte destination MAC |    |  6 byte destination MAC |
|  6 byte source MAC      |    |  6 byte source MAC      |
|  4 byte type            |    |  4 byte type            |
+-------------------------+    +-------------------------+
| PPPoE PADS - 8 bytes    |    | PPPoE PADS - 8 bytes    |
+-------------------------+    +-------------------------+
| IPv4 - 20 bytes         |    | IPv6 - 40 bytes         |
|  10  byte control flags |    |  6 byte control flags   |
|  2 byte total length    |    |  2 byte payload length  |
|  4 byte source addr     |    |  16 byte source addr    |
|  4 byte dest addr       |    |                         |
+-------------------------+    |  16 byte dest addr      |
| ICMP - 8 bytes          |    |                         |
+-------------------------+    +-------------------------|    
|                         |    | ICMP - 8 bytes          |
|                         |    +-------------------------+
|                         |    |                         |
/ 1,464 ping payload      /    / 1,444 ping payload      /
|                         |    |                         |
+-------------------------+    +-------------------------+

我的额外开销6in4：

IPv4                           IPv6
+-------------------------+    +-------------------------+
| Ethernet - 12 bytes     |    | Ethernet - 12 bytes     |
|  6 byte destination MAC |    |  6 byte destination MAC |
|  6 byte source MAC      |    |  6 byte source MAC      |
|  4 byte type            |    |  4 byte type            |
+-------------------------+    +-------------------------+
| PPPoE PADS - 8 bytes    |    | PPPoE PADS - 8 bytes    |
+-------------------------+    +-------------------------+
| IPv4 - 20 bytes         |    | IPv4 - 20 bytes         |
|  10  byte control flags |    |  10  byte control flags |
|  2 byte total length    |    |  2 byte total length    |
|  4 byte source addr     |    |  4 byte source addr     |
|  4 byte dest addr       |    |  4 byte dest addr       |
+-------------------------+    +-------------------------+
| ICMP - 8 bytes          |    | IPv6 - 40 bytes         |
+-------------------------+    |  6 byte control flags   |    
|                         |    |  2 byte payload length  |
|                         |    |  16 byte source addr    |
|                         |    |                         |
|                         |    |  16 byte dest addr      |
|                         |    |                         |
|                         |    +-------------------------|
|                         |    | ICMP - 8 bytes          |
|                         |    +-------------------------+
|                         |    |                         |
/ 1,464 ping payload      /    / 1,424 ping payload      /
|                         |    |                         |
+-------------------------+    +-------------------------+

您会明白为什么我只能使用 1,424 字节的有效负载进行 ping 操作。