networking - SR & GBN : Out-of-window ACKs

Question

我目前正在研究相当基础的网络，我目前正在研究可靠传输的主题。我正在使用 Kurrose & Ross 的 Computer Networking 一书，其中两个复习问题如下:

With the selective-repeat/go-back-n protocol, it is possible for the sender to receive an ACK for a packet that falls outside of its current window?

对于 SR 版本，我对这个问题的回答如下:

Yes, if the window size is too big for the sequence number space. For example, a receiver gets a number of packets equal to the space of the sequence numbers. Its receive window has thus moved so that it is expecting a new set of packets with the same sequence numbers as the last one. The receiver now sends an ACK for each of the packets, but all of them are lost along the way. This eventually causes the sender to timeout for each of the previous set of packets, and retransmits each of them. The receiver think that this duplicate set of packets are really the new ones that it is expecting, and it sends ACKs for each of them that successfully reaches the sender. The sender now experiences a similar kind of confusion, where it thinks that the ACKs are confirmations that each of the old packets have been received, when they are really ACKs meant for the new, yet-to-be-sent packets.

我很确定这是正确的(否则，请告诉我!)，因为这种情况似乎是为什么窗口大小应该小于或等于序列号空间大小的一半的经典理由。 SR协议(protocol)，但是GBN呢？

是否会出现相同的环绕问题，使答案几乎相同？如果没有，是否还有其他情况会导致典型的 GBN 发送方在其窗口之外接收到 ACK？

关于后者，我能想到的唯一例子如下:

A GBN sender sends packets A & B in order. The receiver receives both in order, and sends one cumulative ACK covering every packet before and up to A, and then another one covering every packet before and up to B (including A). The first one is so heavily delayed that the second one arrives first to the sender, causing its window to slide beyond A & B. When the first one finally arrives, it needlessly acknowledges that everything up to A has been correctly received, when A is already outside of the sender's window.

与前一个相比，这个例子似乎相当无害且不太可能，所以我怀疑它的正确性(但如果我错了，请再次纠正我!)。

Answer 1

在实际世界中， 怎么样？重复 ACK 延迟了足够长的时间以从窗口掉出？

该协议(protocol)位于发送方和接收方之间，但它无法控制媒体(网络路径)的行为方式。

根据设计，协议(protocol)仍然是可靠的，但实现应该能够处理这种窗口外的重复 ACK。