网络有中间结点和端结点，中间结点提供传输服务，端结点接受传输服务。对于Internet 来说，路由器就是中间结点，电脑就是端结点。
下面为了叙述方便，我们把中间结点组成的网络称为网络。那么，网络提供传输服务，端结点接受服务。
端结点希望传输是可靠的：不丢数据、数据不错序等等。为了做到这一点，端结点有两个方案可选择：第一要求网络做到传输可靠，第二端结点自己解决这个问题。
做第二种选择——自己解决——就是端到端方案。Internet 就是做的这种选择，所以TCP是端到端传输协议。
TCP怎么自己做到可靠传输的呢？可以参考计算机网络有关书籍。下面以寄信做个比喻。
邮局的服务很差，经常丢信。用户可以找邮局要求提高服务质量，用户也可以自己解决这个问题——采取端到端方案：
用户寄信时给每封信编上号，要求亲友收到后寄一个回执确实。如果在回执该到的时间（足够长）没到，就重复寄信一直到收到回执为止，也可以寄n次后放弃——通信失败。

因为这是英语写作博客，所以下面给出英语译文：

A network involves intermediate nodes and end ones. The former provides transport services for the latter. In Internet, intermediate nods are routers while end nods are PCs or other devices. 

For simplicity, we suppose a network merely involves intermediate nodes. So the network provides transport services for end nodes.

End nodes expect that delivery will be reliable, namely no data loss or no order errors. To achieve this, end nodes have two alternatives: to ask the network for reliable transport services, or to address reliable transport themselves. 

The second alternative is end to end approach. Internet does this way, where TCP is an end to end protocol. 

How does TCP accomplish reliable delivery? Please refer to books on computer networks.

In the following, we take letters as an example to explain what end to end is.

A post office fails to provide satisfactory service. Letters are lost sometimes. Customers don’t take the trouble to ask the post office to improve service quality. Instead, they deal with the problem themselves. They use acknowledgments. When a customer sends a letter, he will wait for a certain period of time for an acknowledgment. If the acknowledgment fails to arrive on time, the customer sends the letter again till he gets an acknowledgment.

（注：大家常提到端到端，本文解释其真正含义。本博客今后会不局限于英语写作。）