在使用的IP标志的Do not Fragment位？ [英] Where is the Don't Fragment Bit of the IP Flags used?

问题描述

我很想知道的是使用的IP标志的不分段[DF]位在哪里。由于碎片是不可见的高层，他们不关心了。

I am curious to know where the "Don't Fragment" [DF] Bit of the IP Flags is used. As fragmentation is invisible to higher layers and they don't care too.

我也找了一个例子。

非常感谢在前进。

推荐答案

碎片并不总是看不到所有的上层。一些早期（甚至可能电流）微控制器的TCP / IP协议栈没有实现全部功能，如破碎处理。在这种情况下该标志的使用将确保数据包的原始形式，而不是大量的另一端无法处理的片段来了。

Fragmentation is not always invisible to all upper layers. Some early (and probably even current) micro-controller TCP/IP stacks did not implement the full capabilities such as fragmentation handling. Use of the flag in that situation would ensure that the packet arrived in its original form instead of a lot of fragments which the other end couldn't handle.

另外，使用UDP时，它没有必要对所有的片段到达目的地这样，preventing碎片意味着消息要么到达或不到达 - 没有可能性，只有一个位的UDP数据报将到达目的地。我不记得保持多久，TCP / IP协议栈上等待失踪片段组装的IP数据包，但使用的DF标志，意味着有在这段时间绑起来没有不必要的资源。

In addition, when using UDP, it's not necessary for all the fragments to arrive at the destination so, preventing fragmentation means the message either arrives or doesn't arrive - there is no possibility that only a bit of the UDP datagram will reach the destination. I can't recall how long the TCP/IP stack held on to unassembled IP packets waiting for missing fragments, but use of the DF flag meant there were no unnecessary resources tied up during that time.

最后，你可以将它用于网络基础设施的测试行为，比如当你得到一个数据包，它比最大传输单元更大发生了什么（DF将prevent从被零碎信息包通过挤压的孔）。

Finally, you can use it for testing behavior of network infrastructure, such as what happens when you get a packet that's bigger than the maximum transmission unit (DF will prevent that packet from being fragmented to 'squeeze through' the hole).

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