Given that the receiver-pull model grants more control to receivers in terms of traffic delivery, and only receivers know what they want to receive, the receiver-pull model has clear advantages in restraining unwanted traffic compared to the sender-push model. Moreover, the above discussion also makes it clear that the sender is accountable to a greater degree in the receiver-pull model than in the sender-push model. This brings us to the following key idea which underlies the theme of this paper: When designing any communication protocol, it is advantageous to first consider using a receiver-pull model which inherently provides greater protection against unwanted traffic.
The receiver-pull based model is a relatively low-cost design choice that can be considered early during any communication system design. Even if the receiver-pull model results in slightly greater protocol complexity, it can greatly help to simplify accountability and authentication issues by placing the overheads where they truly belong - at the sender of the unwanted traffic.
A legitimate concern with a receiver-pull model is that it may end up increasing the cost of sending messages for malicious as well as legitimate senders. We will show in the next section through an example of a receiver-pull based email architecture that, using simple design optimizations, one can easily lower the sending cost for legitimate senders while still holding senders of unwanted content accountable.
We do not claim that a receiver-pull based model may be universally suitable for all forms of communications. For example, soldiers in the middle of a desert war may not want to rely on remote senders being reachable when trying to retrieve their messages. However, in many important applications, such as civilian use of email, mobile text messages, and asynchronous voice messages, the receiver-pull architecture appears to offer strong advantages in fight against unwanted traffic.