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: Push vs. Pull: Implications : Push vs. Pull: Implications : Abstract


In recent years the Internet has been increasingly plagued by the seemingly-never-ending unwanted traffic, manifesting itself in large volumes of unsolicited bulk emails (spam), frequent outbreaks of virus/worm attacks, and large scale Distributed Denial of Services (DDoS) attacks. For example, it was estimated that 32 billion spam messages were sent daily on the Internet as of November 2003 [10]. Worse, spammers and virus/worm attackers are increasingly joining force to automate spamming by hijacking (home) user machines through virus/worm attacks. A recent study reported that as high as 80% of spam messages were sent from compromised user machines (zombies) [14]. In this paper, we focus our attention on spam-like unwanted Internet traffic, which plagues critical Internet applications and services such as emails, mobile text messages, and asynchronous voice messages (where a recorded voice message is sent to a list of receivers). We refer to such applications collectively as message services. In this paper, we are especially interested in the implications of the protocol design on controlling unwanted traffic on the Internet.

Given the importance of controlling spam for preserving the value of the messaging systems, this issue has attracted a great amount of attention in both networking research and industrial communities. Many different spam control schemes (in the context of Internet emails) have been proposed, and some of them have been deployed on the Internet [2,7,8,11,12,13]. On the other hand, despite these anti-spam research and development efforts, the proportion of spam seen on the Internet has been continuously on the rise. It is estimated that nowadays spam messages constitute 79% of all business emails, up from 68% since the US federal Can-Spam Act of 2003 took effect in January 2004 [1]. It was also reported that 80% of mobile phone text messages were unsolicited in Japan [15], where SMS (Short Message Services) is popular, and is therefore attractive to spammers.

In this paper we argue that the difficulties in restraining spam can be attributed to the lack of receiver control over how messages should be delivered on the Internet. For example, in the current SMTP-based email delivery architecture [9], any user can send an email to another at will, regardless of whether or not the receiver is willing to accept the message. In the early days of the Internet development, this was not a big problem as people on the network largely trusted each other. However, since the commercialization of the Internet in mid-1990, the nature of the Internet community has changed. It has become less trustworthy, and email spam is possibly one of the most notable examples of the untrustworthy nature of the Internet.

In order to effectively address the issue of spam in the untrustworthy Internet, we argue that receivers must gain greater control over if and when a message should be delivered to them. Asynchronous messages on the Internet are delivered primarily using two different models: sender-push and receiver-pull (or a combination of the two). They differ in who initiates the message delivery process. In the sender-push model, senders control the delivery of traffic, and receivers passively accept whatever the senders push to them. The current SMTP-based email delivery system is a typical example of this model. In contrast, the receiver-pull model grants receivers the control over if and when they want to retrieve data from the senders. In this model, senders can only prepare the data but they cannot push the data to receivers. Examples of the receiver-pull model include the HTTP-based web access services and the FTP-based file transfers.

As we will discuss in the next section, the receiver-pull model comes with several appealing advantages because it grants receivers greater control over the message delivery mechanism. It takes advantage of the fact that receivers have more reliable knowledge of what traffic they want to receive. Moreover, the receiver-pull model may also simplify the challenging issues related to the resource usage accountability and sender authentication. For example, because spammers need to store and manage email messages on their own mail servers (waiting for receivers to pull), it becomes relatively easier to hold spammers responsible for the resources they consume. As a proof of concept, in this paper we present examples of three asynchronous messaging applications - emails, mobile text messages, and asynchronous voice messages.

The objective of the paper is two-fold. First, through the example designs of the message applications, we would like to demonstrate the feasibility and advantages of using receiver-pull model to design protocols for asynchronous messaging applications. Second, and more importantly, we want to raise the explicit awareness of the difference between the sender-push and receiver-pull models, and argue that, the receiver-pull model should be the strongly favored design choice, whenever appropriate.

The rest of the paper is structured as follows. In Section 2 we elaborate on the two different traffic models on the Internet. We outline the example design to support emails, mobile text messages, and asynchronous voice messages using the receiver-pull model in Section 3. We summarize the paper in Section 4.

: Push vs. Pull: Implications : Push vs. Pull: Implications : Abstract