如何找到我可以在不分段的情况下发送的最大 UDP 数据包? [英] How to find the largest UDP packet I can send without fragmenting?
问题描述
我需要知道我可以发送到另一台计算机而没有碎片的最大 UDP 数据包是多少.
I need to know what the largest UDP packet I can send to another computer is without fragmentation.
此大小通常称为 MTU(最大传输单元).据说，在两台计算机之间，将有许多路由器和调制解调器，它们可能具有不同的 MTU.
This size is commonly known as the MTU (Maximum Transmission Unit). Supposedly, between 2 computers, will be many routers and modems that may have different MTUs.
我读到 Windows 中的 TCP 实现会自动找到路径中的最大 MTU.
I read that the TCP implementation in windows automatically finds the maximum MTU in a path.
我也在试验，我发现从我的电脑到服务器的最大 MTU 是 57712 字节 + 标头.上面的任何东西都被丢弃了.我的电脑在局域网上，MTU 不是应该在 1500 字节左右吗?
I was also experimenting, and I found out that the maximum MTU from my computer to a server was 57712 bytes+header. Anything above that was discarded. My computer is on a LAN, isn't the MTU supposed to be around 1500 bytes?
推荐答案
以下内容没有直接回答您的问题，但您可能会觉得它很有趣；它说 IP 数据包可以分解/重新组装，因此大于底层媒体(例如 1500 字节以太网)的限制:解决 GRE 和 IPSEC 的 IP 碎片、MTU、MSS 和 PMTUD 问题
The following doesn't answer your question directly but you might find it interesting; it says that IP packets can be disassembled/reassembled, and therefore bigger than limit on the underling media (e.g. 1500-byte Ethernet): Resolve IP Fragmentation, MTU, MSS, and PMTUD Issues with GRE and IPSEC
有关此主题的更多信息:
- Re: UDP fragmentation says you should use ICMP instead of UDP to discover MTU
- Path MTU Discovery says that a TCP connection might include implicit MTU negotiation via ICMP
我不知道如何通过 Windows 上的 API 生成 ICMP:曾经提出过这样的 API，并且引起了争议，因为人们认为这将使编写实现拒绝服务功能的软件变得容易大量 ICMP 消息.
I don't know about generating ICMP via an API on Windows: at one time such an API was proposed, and was controversial because people argued that would make it easy to write software that implements denial-of-service functionality by generating a flood of ICMP messages.
不，看起来它是实现了:参见例如Winsock程序员常见问题解答示例:Ping:原始套接字方法.
No, it looks like it is implemented: see for example Winsock Programmer's FAQ Examples: Ping: Raw Sockets Method.
因此，要发现 MTU，请生成带有不分段"标志的 ping 数据包.
So, to discover MTU, generate ping packets with the 'do not fragment' flag.
也许还有比这更简单的 API，我不知道；但我希望我已经让你了解底层协议[s].
Maybe there's an easier API than this, I don't know; but I hope I've given you to understand the underlying protocol[s].
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