Department of Computer Sciences

The University of Texas at Austin

Zmail : Zero-Sum Free Market Control of Spam

Benjamin J. Kuipers, Alex X. Liu,

Aashin Gautam, Mohamed G. Gouda

The University of Texas at Austin 2

Spam Problem

Volume of Spam─ 2001: 8% of all email traffic was spam

─ 2004: 60% of all email traffic was spam

─ # emails per day: 31 billion (60% spam = 18.6 billion)

─ # spam emails per person per day = 6

Cost of Spam─ 2003: $10 billion/year in U.S. (Ferris Research)

$20.5 worldwide (Radicati Group)

─ Just cost for adding spam processing servers

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Anti-Spam: Legal Approaches

Punish spammers by anti-spam laws

Problems─Hard to define spam

● First Amendment Right to Free Speech

─Hard to enforce anti-spam laws● Spammers move to another country

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Anti-Spam: Filtering Approaches

Filter out spam based on text content

Problems─False Positive

● What if a job offer email gets filtered?

─Can be foiled by spammers● For example, imbedding spam text in images

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Anti-Spam: Economic Approaches

Require senders to pay something

Human Effort: Sender pays human effort─ Inconvenient─ Inefficient─ Challenge can be perceived as rude

Computational Cost: Sender pays CPU time─ Inefficient

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Anti-Spam: Zmail

Current cost for spamming is almost zero User’s attention is the important resource

consumed Basic idea of Zmail:

─ The sender of an email pays money directly to the receiver.

─ Zero sum (unlike USPS): The amount of money charged to the sender

=The amount of money rewarded to the receiver

─ The money earned can be used to send email.

─ The cost of sending one email is a unit called an e-penny.

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Impact of Market Forces

Spammers─ Cost of spamming significantly increases─ Bulk email advertising become more targeted

Normal Users─ Most users receive more email than they send─ If not, pay money or subscribing commercial email

ESPs (Email Service Providers)─ Less spam, less bandwidth, less computational cost

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Zmail Architecture

ESP ESP

ESP

User

User

User

User

User

User

User

Bank

User

User

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The Zmail Protocol: two parties Bank:

─ Each compliant ESP has an account with bank─ Manage accounting for compliant ESPs─ ESP buy and sell e-pennies with bank─ Exchange e-pennies between real money for ESPs

ESPs:─ Manage accounting for each of its users─ Users buy and sell e-pennies with their ESPs

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The Zmail Protocol: Email Transfer

An email: user s of ESP A user r of ESP B

Sending:─ A reduces s’s balance by one─ A records that A owes B one more e-penny

Receiving:─ B increments r’s balance by one─ B records that A owes B one more e-penny

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The Zmail Protocol: Misbehavior Detection

What if an ESP lies?

Periodically, every ESP reports to the bank how much they owe to every other ESPs

Misbehavior Detection

─ A says that A owes B x e-pennies

─ B says that A owes B y e-pennies

─ If x != y, then Bank further investigates who lies.

─ Most ESPs are honest.

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Mailing Lists

List server: email distributor + subscriber DB A subscriber sends an email to distributor, distributor forwards

the email to everyone Solution: for each email from distributor, receiver’s ESP sends

back an ack email.

Extra benefit: subscriber DB is always updated

Distributor Sender

1

2

100

-1+1+1

+1

+1

-100-1

-1

-1

+100

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Zombies and Email Viruses

Zombie: sends out spam at user’s expense

Virus: sends email to everyone in address book

Solution: each user specify a limit on the number of email that he can send a day.

Extra benefit: detecting virus and zombies

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Incremental Deployment

Zmail runs on top of SMTP

No change to SMTP

Deployment: starting from two compliant ESPs

A user may decide to segregate, filter, or discard email from non-compliant ESPs.

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Bank Setup

Internet already has some central authorities such as IANA that controls IP assignment

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Conclusions

Zmail gives no free ride to spammers

Zmail requires no definition of spam

Zmail works with other anti-spam techniques

Zmail can be incrementally deployed