DefenestraTor: Throwing out windows in Tor

Mashael AlSabah, Kevin Bauer, Ian Goldberg, Dirk Grunwald, Damon McCoy, Stefan Savage, Geoffrey M. Voelker

    Research output: Chapter in Book/Report/Conference proceedingConference contribution


    Tor is one of the most widely used privacy enhancing technologies for achieving online anonymity and resisting censorship. While conventional wisdom dictates that the level of anonymity offered by Tor increases as its user base grows, the most significant obstacle to Tor adoption continues to be its slow performance. We seek to enhance Tor's performance by offering techniques to control congestion and improve flow control, thereby reducing unnecessary delays. To reduce congestion, we first evaluate small fixed-size circuit windows and a dynamic circuit window that adaptively re-sizes in response to perceived congestion. While these solutions improve web page response times and require modification only to exit routers, they generally offer poor flow control and slower downloads relative to Tor's current design. To improve flow control while reducing congestion, we implement N23, an ATM-style per-link algorithm that allows Tor routers to explicitly cap their queue lengths and signal congestion via back-pressure. Our results show that N23 offers better congestion and flow control, resulting in improved web page response times and faster page loads compared to Tor's current design and other window-based approaches. We also argue that our proposals do not enable any new attacks on Tor users' privacy.

    Original languageEnglish (US)
    Title of host publicationPrivacy Enhancing Technologies - 11th International Symposium, PETS 2011, Proceedings
    PublisherSpringer Verlag
    Number of pages21
    ISBN (Print)9783642222627
    StatePublished - 2011

    Publication series

    NameLecture Notes in Computer Science (including subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics)
    Volume6794 LNCS
    ISSN (Print)0302-9743
    ISSN (Electronic)1611-3349

    ASJC Scopus subject areas

    • Theoretical Computer Science
    • General Computer Science


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