问题描述
我需要知道我可以发送到另一台计算机的最大UDP数据包没有碎片.
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(最大传输单位).假设在两台计算机之间，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.
我了解到Windows中的TCP实现会自动在路径中找到最大MTU.
I read that the TCP implementation in windows automatically finds the maximum MTU in a path.
我也在进行实验，发现从计算机到服务器的最大MTU为57712字节+头.高于此的任何东西都将被丢弃.我的计算机在局域网上，MTU是否应该为1500字节左右?
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
以下内容无法直接回答您的问题，但您可能会发现它很有趣；它说IP数据包可以分解/重新组装，因此大于基础媒体(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:一次提出了这样的API，并引起了争议，因为人们认为这将使编写通过生成拒绝服务功能的软件变得容易.大量的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，它似乎已实现 :例如，参见 Winsock程序员的常见问题解答示例: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，请生成带有请勿分段"标志的ping数据包.
So, to discover MTU, generate ping packets with the 'do not fragment' flag.
也许我不知道，有比这更简单的API了；但我希望我已经让您了解了基础协议.
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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