第一章 概述——1.TCP/IP设计遵循的两个原则

1.端到端原则（end-to-end principle）

当我们设计一个大的系统（如操作系统或协议族）时，随之而来的一个问题通常是在什么位置实现某个功能。影响TCP/IP协议族设计的一个重要原则是端到端原则（end-to-end principle）：只有在通信系统端点的应用知识的帮助下，才能完全和正确地实现问题中提到的功能。这种观点表明底层功能不应以完美为目标，这是因为对应用程序需求做出完美推测是不可能的。

2.命运共享原则（Fate-sharing)

什么是命运共享原则，以下是维基的解释:

Fate-sharing is an engineering design philosophy where related parts of a system are yoked together, so that they either fail together or not at all. Fate-sharing is an example of the end-to-end principle. The term "fate-sharing" was defined by David D. Clark in his 1988 paper "The Design Philosophy of the DARPA Internet Protocols" as follows:
The fate-sharing model suggests that it is acceptable to lose the state information associated with an entity if, at the same time, the entity itself is lost. Specifically, information about transport level synchronization is stored in the host which is attached to the net and using its communication service.
A good example of fate-sharing is the transmission of routing messages in routing protocols such as BGP, where the failure of a link or link interface automatically has the effect of terminating routing announcements through that interface, ultimately resulting in the tearing down of the state for that route at each end of the link. Similar considerations apply to TCP.

它说命运共享是端到端的一种范例，我的理解是：命运共享就是把所有维护一个活动的通信关联（如虚拟连接）所必须的状态都放在通信端点，通过端点间的状态信息交换来维持连接，而不是通过各自检测实际的整个通信连接中的中间实体（路由器，交换机，线路等）是否有效。这样当某个中间实体或端点失效时，整个连接的所有实体（包括端点实体）都将失效，也就是整个通信的失效。
想一想命运共享的替代方法：如果中间实体必须维护有关通信的状态，那么您将面临复制该状态的难题，即该连接通过的所有中间实体都得保持一份通信状体的拷贝。
参考资料：https://www.niem.es/2008/04/term-fate-sharing.html