Reliable Communication Layers
In this lesson, we discuss the basics of a reliable communication protocol such as TCP.
We'll cover the following...
To build a reliable communication layer, we need some new mechanisms and techniques to handle packet loss. Let us consider a simple example in which a client is sending a message to a server over an unreliable connection. The first question we must answer: how does the sender know that the receiver has actually received the message?
Acknowledgment (ack)
The technique that we will use is known as an acknowledgment, or ack for short. The idea is simple: the sender sends a message to the receiver; the receiver then sends a short message back to acknowledge its receipt. The figure below depicts the process.
When the sender receives an acknowledgment of the message, it can then rest assured that the receiver did indeed receive the original message. However, what should the sender do if it does not receive an acknowledgment?
Timeout
To handle this case, we need an additional mechanism, known as a timeout. When the sender sends a message, the sender now sets a timer to go off after some period of time. If in that time, no acknowledgment has been received, the sender concludes that the message has been lost. The sender then simply performs a retry of the send, sending the same message again with hopes that this time, it will get through. For this approach to work, the sender must keep a copy of the message around, in case it needs to send it again. The combination of the timeout and then retry have led some to call the approach timeout/retry; pretty clever crowd, those networking types, no? The figure below shows an example.
Unfortunately, timeout/retry in this form is not quite enough. The figure below shows an example of packet loss which could lead to trouble.
In this example, it is not the original message that gets lost, but the acknowledgment. From the perspective of the sender, the situation seems the same: no ack was received, and thus a timeout and retry are in order. But from the perspective of the receiver, it is quite different: now the same message has been received twice! While there may be cases where this is ...