@ujued #11
1 、联系上下文来看，楼主说的 『 100%避免』指的应该是：
精心挑选的 ISN 无法 100% 的避免『误收旧包』和『恶意伪造』这两种情况。

2 、这个全看个人喜好，我赞同『三次』更容易带来误解

3 、其实我感觉楼主这里应该没有将它描述为充分条件的意思，如果描述为这样可能会更严谨：
TCP 设计了若干基于序列号的机制，用于保证通信的可靠性。


4 、联系上下文来看，楼主所说的『不使用 0 作为 ISN 』应该指的是：
『不固定使用 0 』 作为 ISN 。而不是说 ISN 『始终不应为 0 』。


另外，『不固定使用 0 』最初应该只是为了避免 『误收旧包』。
后续 rfc1948 提出的『不使用容易被猜到的数字』才是为了安全原因。

这一点可以在 rfc793 中查证：
To avoid confusion we must prevent segments from one incarnation of a connection from being used while the same sequence numbers may still be present in the network from an earlier incarnation. We want to assure this, even if a TCP crashes and loses all knowledge of the sequence numbers it has been using. When new connections are created, an initial sequence number (ISN) generator is employed which selects a new 32 bit ISN. The generator is bound to a (possibly fictitious) 32 bit clock whose low order bit is incremented roughly every 4 microseconds. Thus, the ISN cycles approximately every 4.55 hours. Since we assume that segments will stay in the network no more than the Maximum Segment Lifetime (MSL) and that the MSL is less than 4.55 hours we can reasonably assume that ISN's will be unique.