Network Working Group                                            J. Hall
Internet-Draft                                                       CDT
Intended status: Informational                                  M. Aaron
Expires: February 24, 2020                                    CU Boulder
                                                                S. Adams
                                                                B. Jones
                                                             N. Feamster
                                                         August 23, 2019

              A Survey of Worldwide Censorship Techniques


   This document describes the technical mechanisms used by censorship
   regimes around the world to block or impair Internet traffic.  It
   aims to make designers, implementers, and users of Internet protocols
   aware of the properties being exploited and mechanisms used to censor
   end-user access to information.  This document makes no suggestions
   on individual protocol considerations, and is purely informational,
   intended to be a reference.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Drafts is at

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   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on February 24, 2020.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Technical Prescription  . . . . . . . . . . . . . . . . . . .   3
   3.  Technical Identification  . . . . . . . . . . . . . . . . . .   4
     3.1.  Points of Control . . . . . . . . . . . . . . . . . . . .   4
     3.2.  Application Layer . . . . . . . . . . . . . . . . . . . .   5
       3.2.1.  HTTP Request Header Identification  . . . . . . . . .   5
       3.2.2.  HTTP Response Header Identification . . . . . . . . .   6
       3.2.3.  Instrumenting Content Providers . . . . . . . . . . .   7
       3.2.4.  Deep Packet Inspection (DPI) Identification . . . . .   8
     3.3.  Transport Layer . . . . . . . . . . . . . . . . . . . . .  10
       3.3.1.  Shallow Packet Inspection and TCP/IP Header
               Identification  . . . . . . . . . . . . . . . . . . .  10
       3.3.2.  Protocol Identification . . . . . . . . . . . . . . .  11
   4.  Technical Interference  . . . . . . . . . . . . . . . . . . .  12
     4.1.  Application Layer . . . . . . . . . . . . . . . . . . . .  12
       4.1.1.  DNS Interference  . . . . . . . . . . . . . . . . . .  12
     4.2.  Transport Layer . . . . . . . . . . . . . . . . . . . . .  14
       4.2.1.  Performance Degradation . . . . . . . . . . . . . . .  14
       4.2.2.  Packet Dropping . . . . . . . . . . . . . . . . . . .  15
       4.2.3.  RST Packet Injection  . . . . . . . . . . . . . . . .  15
     4.3.  Multi-layer and Non-layer . . . . . . . . . . . . . . . .  16
       4.3.1.  Distributed Denial of Service (DDoS)  . . . . . . . .  16
       4.3.2.  Network Disconnection or Adversarial Route
               Announcement  . . . . . . . . . . . . . . . . . . . .  17
   5.  Non-Technical Prescription  . . . . . . . . . . . . . . . . .  18
   6.  Non-Technical Interference  . . . . . . . . . . . . . . . . .  18
     6.1.  Self-Censorship . . . . . . . . . . . . . . . . . . . . .  18
     6.2.  Domain Name Reallocation  . . . . . . . . . . . . . . . .  19
     6.3.  Server Takedown . . . . . . . . . . . . . . . . . . . . .  19
     6.4.  Notice and Takedown . . . . . . . . . . . . . . . . . . .  19
   7.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  19
   8.  Informative References  . . . . . . . . . . . . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  29

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1.  Introduction

   Censorship is where an entity in a position of power - such as a
   government, organization, or individual - suppresses communication
   that it considers objectionable, harmful, sensitive, politically
   incorrect or inconvenient.  (Although censors that engage in
   censorship must do so through legal, military, or other means, this
   document focuses largely on technical mechanisms used to achieve
   network censorship.)

   This document describes the technical mechanisms that censorship
   regimes around the world use to block or degrade Internet traffic
   (see [RFC7754] for a discussion of Internet blocking and filtering in
   terms of implications for Internet architecture, rather than end-user
   access to content and services).

   We describe three elements of Internet censorship: prescription,
   identification, and interference.  Prescription is the process by
   which censors determine what types of material they should block,
   i.e. they decide to block a list of pornographic websites.
   Identification is the process by which censors classify specific
   traffic to be blocked or impaired, i.e. the censor blocks or impairs
   all webpages containing "sex" in the title or traffic to  Interference is the process by which the censor
   intercedes in communication and prevents access to censored materials
   by blocking access or impairing the connection.

2.  Technical Prescription

   Prescription is the process of figuring out what censors would like
   to block [Glanville-2008].  Generally, censors aggregate information
   "to block" in blacklists or using real-time heuristic assessment of
   content [Ding-1999].  There are indications that online censors are
   starting to use machine learning techniques as well [Tang-2016].

   There are typically three types of blacklists: Keyword, domain name,
   or Internet Protocol (IP) address.  Keyword and domain name blocking
   take place at the application level (e.g.  HTTP), whereas IP blocking
   tends to take place using routing data in TCP/IP headers.  The
   mechanisms for building up these blacklists are varied.  Censors can
   purchase from private industry "content control" software, such as
   SmartFilter, which allows filtering from broad categories that they
   would like to block, such as gambling or pornography.  In these
   cases, these private services attempt to categorize every semi-
   questionable website as to allow for meta-tag blocking (similarly,
   they tune real-time content heuristic systems to map their
   assessments onto categories of objectionable content).

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   Countries that are more interested in retaining specific political
   control, a desire which requires swift and decisive action, often
   have ministries or organizations, such as the Ministry of Industry
   and Information Technology in China or the Ministry of Culture and
   Islamic Guidance in Iran, which maintain their own blacklists.

3.  Technical Identification

3.1.  Points of Control

   Internet censorship, necessarily, takes place over a network.
   Network design gives censors a number of different points-of-control
   where they can identify the content they are interested in filtering.
   An important aspect of pervasive technical interception is the
   necessity to rely on software or hardware to intercept the content
   the censor is interested in.  This requirement, the need to have the
   interception mechanism located somewhere, logically or physically,
   implicates various general points-of-control:

   o  *Internet Backbone:* If a censor controls the gateways into a
      region, they can filter undesirable traffic that is traveling into
      and out of the region by packet sniffing and port mirroring at the
      relevant exchange points.  Censorship at this point of control is
      most effective at controlling the flow of information between a
      region and the rest of the Internet, but is ineffective at
      identifying content traveling between the users within a region.

   o  *Internet Service Providers:* Internet Service Providers are
      perhaps the most natural point of control.  They have a benefit of
      being easily enumerable by a censor paired with the ability to
      identify the regional and international traffic of all their
      users.  The censor's filtration mechanisms can be placed on an ISP
      via governmental mandates, ownership, or voluntary/coercive

   o  *Institutions:* Private institutions such as corporations,
      schools, and cyber cafes can put filtration mechanisms in place.
      These mechanisms are occasionally at the request of a censor, but
      are more often implemented to help achieve institutional goals,
      such as to prevent the viewing of pornography on school computers.

   o  *Personal Devices:* Censors can mandate censorship software be
      installed on the device level.  This has many disadvantages in
      terms of scalability, ease-of-circumvention, and operating system
      requirements.  The emergence of mobile devices exacerbate these
      feasibility problems.

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   o  *Services:* Application service providers can be pressured,
      coerced, or legally required to censor specific content or flows
      of data.  Service providers naturally face incentives to maximize
      their potential customer base and potential service shutdowns or
      legal liability due to censorship efforts may seem much less
      attractive than potentially excluding content, users, or uses of
      their service.

   o  *Certificate Authorities:* Authorities that issue
      cryptographically secured resources can be a significant point of
      control.  Certificate Authorities that issue certificates to
      domain holders for TLS/HTTPS or Regional/Local Internet Registries
      that issue Route Origination Authorizations to BGP operators can
      be forced to issue rogue certificates that may allow compromises
      in confidentiality guarantees - allowing censorship software to
      engage in identification and interference where not possible
      before - or integrity guarantees - allowing, for example,
      adversarial routing of traffic.

   o  *Content Distribution Networks (CDNs):* CDNs seek to collapse
      network topology in order to better locate content closer to the
      service's users in order to improve quality of service.  These can
      be powerful points of control for censors, especially if the
      location of a CDN results in easier interference.

   At all levels of the network hierarchy, the filtration mechanisms
   used to detect undesirable traffic are essentially the same: a censor
   sniffs transmitting packets and identifies undesirable content, and
   then uses a blocking or shaping mechanism to prevent or impair
   access.  Identification of undesirable traffic can occur at the
   application, transport, or network layer of the IP stack.  Censors
   are almost always concerned with web traffic, so the relevant
   protocols tend to be filtered in predictable ways.  For example, a
   subversive image would always make it past a keyword filter, but the
   IP address of the site serving the image may be blacklisted when
   identified as a provider of undesirable content.

3.2.  Application Layer

3.2.1.  HTTP Request Header Identification

   An HTTP header contains a lot of useful information for traffic
   identification; although "host" is the only required field in an HTTP
   request header (for HTTP/1.1 and later), an HTTP method field is
   necessary to do anything useful.  As such, "method" and "host" are
   the two fields used most often for ubiquitous censorship.  A censor
   can sniff traffic and identify a specific domain name (host) and
   usually a page name (GET /page) as well.  This identification

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   technique is usually paired with TCP/IP header identification (see
   Section 3.3.1) for a more robust method.

   *Tradeoffs:* Request Identification is a technically straight-forward
   identification method that can be easily implemented at the Backbone
   or ISP level.  The hardware needed for this sort of identification is
   cheap and easy-to-acquire, making it desirable when budget and scope
   are a concern.  HTTPS will encrypt the relevant request and response
   fields, so pairing with TCP/IP identification (see Section 3.3.1) is
   necessary for filtering of HTTPS.  However, some countermeasures such
   as URL obfuscation [RSF-2005] can trivially defeat simple forms of
   HTTP Request Header Identification.

   *Empirical Examples:* Studies exploring censorship mechanisms have
   found evidence of HTTP header/ URL filtering in many countries,
   including Bangladesh, Bahrain, China, India, Iran, Malaysia,
   Pakistan, Russia, Saudi Arabia, South Korea, Thailand, and Turkey
   [Verkamp-2012] [Nabi-2013] [Aryan-2012].  Commercial technologies
   such as the McAfee SmartFilter and NetSweeper are often purchased by
   censors [Dalek-2013].  These commercial technologies use a
   combination of HTTP Request Identification and TCP/IP Header
   Identification to filter specific URLs.  Dalek et al. and Jones et
   al. identified the use of these products in the wild [Dalek-2013]

3.2.2.  HTTP Response Header Identification

   While HTTP Request Header Identification relies on the information
   contained in the HTTP request from client to server, response
   identification uses information sent in response by the server to
   client to identify undesirable content.

   *Tradeoffs:* As with HTTP Request Header Identification, the
   techniques used to identify HTTP traffic are well-known, cheap, and
   relatively easy to implement, but is made useless by HTTPS, because
   the response in HTTPS is encrypted, including headers.

   The response fields are also less helpful for identifying content
   than request fields, as "Server" could easily be identified using
   HTTP Request Header identification, and "Via" is rarely relevant.
   HTTP Response censorship mechanisms normally let the first n packets
   through while the mirrored traffic is being processed; this may allow
   some content through and the user may be able to detect that the
   censor is actively interfering with undesirable content.

   *Empirical Examples:* In 2009, Jong Park et al. at the University of
   New Mexico demonstrated that the Great Firewall of China (GFW) has
   used this technique [Crandall-2010].  However, Jong Park et al. found

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   that the GFW discontinued this practice during the course of the
   study.  Due to the overlap in HTTP response filtering and keyword
   filtering (see Section 3.2.3), it is likely that most censors rely on
   keyword filtering over TCP streams instead of HTTP response

3.2.3.  Instrumenting Content Providers

   In addition to censorship by the state, many governments pressure
   content providers to censor themselves.  Due to the extensive reach
   of government censorship, we need to define content provider as any
   service that provides utility to users, including everything from web
   sites to locally installed programs.  The defining factor of keyword
   identification by content providers is the choice of content
   providers to detect restricted terms on their platform.  The terms to
   look for may be provided by the government or the content provider
   may be expected to come up with their own list.

   *Tradeoffs:* By instrumenting content providers to identify
   restricted content, the censor can gain new information at the cost
   of political capital with the companies it forces or encourages to
   participate in censorship.  For example, the censor can gain insight
   about the content of encrypted traffic by coercing web sites to
   identify restricted content, but this may drive away potential
   investment.  Coercing content providers may encourage self-
   censorship, an additional advantage for censors.  The tradeoffs for
   instrumenting content providers are highly dependent on the content
   provider and the requested assistance.

   *Empirical Examples:* Researchers have discovered keyword
   identification by content providers on platforms ranging from instant
   messaging applications [Senft-2013] to search engines [Rushe-2015]
   [Cheng-2010] [Whittaker-2013] [BBC-2013] [Condliffe-2013].  To
   demonstrate the prevalence of this type of keyword identification, we
   look to search engine censorship.

   Search engine censorship demonstrates keyword identification by
   content providers and can be regional or worldwide.  Implementation
   is occasionally voluntary, but normally is based on laws and
   regulations of the country a search engine is operating in.  The
   keyword blacklists are most likely maintained by the search engine
   provider.  China is known to require search engine providers to
   "voluntarily" maintain search term blacklists to acquire/keep an
   Internet content provider (ICP) license [Cheng-2010].  It is clear
   these blacklists are maintained by each search engine provider based
   on the slight variations in the intercepted searches [Zhu-2011]
   [Whittaker-2013].  The United Kingdom has been pushing search engines
   to self-censor with the threat of litigation if they don't do it

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   themselves: Google and Microsoft have agreed to block more than
   100,000 queries in U.K. to help combat abuse [BBC-2013]

   Depending on the output, search engine keyword identification may be
   difficult or easy to detect.  In some cases specialized or blank
   results provide a trivial enumeration mechanism, but more subtle
   censorship can be difficult to detect.  In February 2015, Microsoft's
   search engine, Bing, was accused of censoring Chinese content outside
   of China [Rushe-2015] because Bing returned different results for
   censored terms in Chinese and English.  However, it is possible that
   censorship of the largest base of Chinese search users, China, biased
   Bing's results so that the more popular results in China (the
   uncensored results) were also more popular for Chinese speakers
   outside of China.

3.2.4.  Deep Packet Inspection (DPI) Identification

   Deep Packet Inspection has become computationally feasible as a
   censorship mechanism in recent years [Wagner-2009].  Unlike other
   techniques, DPI reassembles network flows to examine the application
   "data" section, as opposed to only the header, and is therefore often
   used for keyword identification.  DPI also differs from other
   identification technologies because it can leverage additional packet
   and flow characteristics, i.e. packet sizes and timings, to identify
   content.  To prevent substantial quality of service (QoS) impacts,
   DPI normally analyzes a copy of data while the original packets
   continue to be routed.  Typically, the traffic is split using either
   a mirror switch or fiber splitter, and analyzed on a cluster of
   machines running Intrusion Detection Systems (IDS) configured for

   *Tradeoffs:* DPI is one of the most expensive identification
   mechanisms and can have a large QoS impact [Porter-2010].  When used
   as a keyword filter for TCP flows, DPI systems can cause also major
   overblocking problems.  Like other techniques, DPI is less useful
   against encrypted data, though DPI can leverage unencrypted elements
   of an encrypted data flow (e.g., the Server Name Indicator (SNI) sent
   in the clear for TLS) or statistical information about an encrypted
   flow (e.g., video takes more bandwidth than audio or textual forms of
   communication) to identify traffic.

   Other kinds of information can be inferred by comparing certain
   unencrypted elements exchanged during TLS handshakes to similar data
   points from known sources.  This practice, called TLS fingerprinting,
   allows a probabilistic identification of a party's operating system,
   browser, or application based on a comparison of the specific
   combinations of TLS version, ciphersuites, compression options, etc.

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   sent in the ClientHello message to similar signatures found in
   unencrypted traffic [Husak-2016].

   Despite these problems, DPI is the most powerful identification
   method and is widely used in practice.  The Great Firewall of China
   (GFW), the largest censorship system in the world, has used DPI to
   identify restricted content over HTTP and DNS and inject TCP RSTs and
   bad DNS responses, respectively, into connections [Crandall-2010]
   [Clayton-2006] [Anonymous-2014].

   *Empirical Examples:* Several studies have found evidence of DPI
   being used to censor content and tools.  Clayton et al.  Crandal et
   al., Anonymous, and Khattak et al., all explored the GFW and Khattak
   et al. even probed the firewall to discover implementation details
   like how much state it stores [Crandall-2010] [Clayton-2006]
   [Anonymous-2014] [Khattak-2013].  The Tor project claims that China,
   Iran, Ethiopia, and others must have used DPI to block the obsf2
   protocol [Wilde-2012].  Malaysia has been accused of using targeted
   DPI, paired with DDoS, to identify and subsequently knockout pro-
   opposition material [Wagstaff-2013].  It also seems likely that
   organizations not so worried about blocking content in real-time
   could use DPI to sort and categorically search gathered traffic using
   technologies such as NarusInsight [Hepting-2011].  Server Name Indication

   In encrypted connections using Transport Layer Security (TLS), there
   may be servers that host multiple "virtual servers" at a give network
   address, and the client will need to specify in the (unencrypted)
   Client Hello message which domain name it seeks to connect to (so
   that the server can respond with the appropriate TLS certificate)
   using the Server Name Indication (SNI) TLS extension [RFC6066].
   Since SNI is sent in the clear, censors and filtering software can
   use it as a basis for blocking, filtering, or impairment by dropping
   connections to domains that match prohibited content (e.g., may be censored while is not)

   Domain fronting has been one popular way to avoid identification by
   censors [Fifield-2015].  To avoid identification by censors,
   applications using domain fronting put a different domain name in the
   SNI extension than the one encrypted by HTTPS.  The visible SNI would
   indicate an unblocked domain, while the blocked domain remains hidden
   in the encrypted application header.  Some encrypted messaging
   services relied on domain fronting to enable their provision in
   countries employing SNI-based filtering.  These services used the
   cover provided by domains for which blocking at the domain level
   would be undesirable to hide their true domain names.  However, the

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   companies holding the most popular domains have since reconfigured
   their software to prevent this practice.  It may be possible to
   achieve similar results using potential future options to encrypt SNI
   in TLS 1.3.

   *Tradeoffs:* Some clients do not send the SNI extension (e.g.,
   clients that only support versions of SSL and not TLS) or will fall
   back to SSL if a TLS connection fails, rendering this method
   ineffective.  In addition, this technique requires deep packet
   inspection techniques that can be computationally and
   infrastructurally expensive and improper configuration of an SNI-
   based block can result in significant overblocking, e.g., when a
   second-level domain like populardomain.example is inadvertently
   blocked.  In the case of encrypted SNI, pressure to censor may
   transfer to other points of intervention, such as content and
   application providers.

   *Empirical Examples:* While there are many examples of security firms
   that offer SNI-based filtering [Trustwave-2015] [Sophos-2015]
   [Shbair-2015], the government of South Korea was recently observed
   using SNI-based filtering.  Cite to Gatlan

3.3.  Transport Layer

3.3.1.  Shallow Packet Inspection and TCP/IP Header Identification

   Of the various shallow packet inspection methods, TCP/IP Header
   Identification is the most pervasive, reliable, and predictable type
   of identification.  TCP/IP headers contain a few invaluable pieces of
   information that must be transparent for traffic to be successfully
   routed: destination and source IP address and port.  Destination and
   Source IP are doubly useful, as not only does it allow a censor to
   block undesirable content via IP blacklisting, but also allows a
   censor to identify the IP of the user making the request.  Port is
   useful for whitelisting certain applications.

   *Trade-offs:* TCP/IP identification is popular due to its simplicity,
   availability, and robustness.

   TCP/IP identification is trivial to implement, but is difficult to
   implement in backbone or ISP routers at scale, and is therefore
   typically implemented with DPI.  Blacklisting an IP is equivalent to
   installing a /32 route on a router and due to limited flow table
   space, this cannot scale beyond a few thousand IPs at most.  IP
   blocking is also relatively crude, leading to overblocking, and
   cannot deal with some services like Content Distribution Networks

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   (CDN), that host content at hundreds or thousands of IP addresses.
   Despite these limitations, IP blocking is extremely effective because
   the user needs to proxy their traffic through another destination to
   circumvent this type of identification.

   Port-blocking is generally not useful because many types of content
   share the same port and it is possible for censored applications to
   change their port.  For example, most HTTP traffic goes over port 80,
   so the censor cannot differentiate between restricted and allowed
   content solely on the basis of port.  Port whitelisting is
   occasionally used, where a censor limits communication to approved
   ports, such as 80 for HTTP traffic and is most effective when used in
   conjunction with other identification mechanisms.  For example, a
   censor could block the default HTTPS port, port 443, thereby forcing
   most users to fall back to HTTP.

3.3.2.  Protocol Identification

   Censors sometimes identify entire protocols to be blocked using a
   variety of traffic characteristics.  For example, Iran impairs the
   performance of HTTPS traffic, a protocol that prevents further
   analysis, to encourage users to switch to HTTP, a protocol that they
   can analyze [Aryan-2012].  A simple protocol identification would be
   to recognize all TCP traffic over port 443 as HTTPS, but more
   sophisticated analysis of the statistical properties of payload data
   and flow behavior, would be more effective, even when port 443 is not
   used [Hjelmvik-2010] [Sandvine-2014].

   If censors can detect circumvention tools, they can block them, so
   censors like China are extremely interested in identifying the
   protocols for censorship circumvention tools.  In recent years, this
   has devolved into an arms race between censors and circumvention tool
   developers.  As part of this arms race, China developed an extremely
   effective protocol identification technique that researchers call
   active probing or active scanning.

   In active probing, the censor determines whether hosts are running a
   circumvention protocol by trying to initiate communication using the
   circumvention protocol.  If the host and the censor successfully
   negotiate a connection, then the censor conclusively knows that host
   is running a circumvention tool.  China has used active scanning to
   great effect to block Tor [Winter-2012].

   *Trade-offs:* Protocol Identification necessarily only provides
   insight into the way information is traveling, and not the
   information itself.

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   Protocol identification is useful for detecting and blocking
   circumvention tools, like Tor, or traffic that is difficult to
   analyze, like VoIP or SSL, because the censor can assume that this
   traffic should be blocked.  However, this can lead to over-blocking
   problems when used with popular protocols.  These methods are
   expensive, both computationally and financially, due to the use of
   statistical analysis, and can be ineffective due to its imprecise

   *Empirical Examples:* Protocol identification can be easy to detect
   if it is conducted in real time and only a particular protocol is
   blocked, but some types of protocol identification, like active
   scanning, are much more difficult to detect.  Protocol identification
   has been used by Iran to identify and throttle SSH traffic to make it
   unusable [Anonymous-2007] and by China to identify and block Tor
   relays [Winter-2012].  Protocol Identification has also been used for
   traffic management, such as the 2007 case where Comcast in the United
   States used RST injection to interrupt BitTorrent Traffic

4.  Technical Interference

4.1.  Application Layer

4.1.1.  DNS Interference

   There are a variety of mechanisms that censors can use to block or
   filter access to content by altering responses from the DNS
   [AFNIC-2013] [ICANN-SSAC-2012], including blocking the response,
   replying with an error message, or responding with an incorrect

   "DNS mangling" is a network-level technique where an incorrect IP
   address is returned in response to a DNS query to a censored
   destination.  An example of this is what some Chinese networks do (we
   are not aware of any other wide-scale uses of mangling).  On those
   Chinese networks, every DNS request in transit is examined
   (presumably by network inspection technologies such as DPI) and, if
   it matches a censored domain, a false response is injected.  End
   users can see this technique in action by simply sending DNS requests
   to any unused IP address in China (see example below).  If it is not
   a censored name, there will be no response.  If it is censored, an
   erroneous response will be returned.  For example, using the command-
   line dig utility to query an unused IP address in China of
   for the name "www.uncensored.example" compared with
   "www.censored.example" (censored at the time of writing), we get an
   erroneous IP address "" as a response:

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   % dig +short +nodnssec @ A www.uncensored.example
   ;; connection timed out; no servers could be reached

   % dig +short +nodnssec @ A www.censored.example

   There are also cases of what is colloquially called "DNS lying",
   where a censor mandates that the DNS responses provided - by an
   operator of a recursive resolver such as an Internet access provider
   - be different than what authoritative resolvers would provide

   DNS cache poisoning refers to a mechanism where a censor interferes
   with the response sent by an authoritative DNS resolver to a
   recursive resolver by responding more quickly than the authoritative
   resolver can respond with an alternative IP address [Halley-2008].
   Cache poisoning occurs after the requested site's name servers
   resolve the request and attempt to forward the true IP back to the
   requesting device; on the return route the resolved IP is recursively
   cached by each DNS server that initially forwarded the request.
   During this caching process if an undesirable keyword is recognized,
   the resolved IP is "poisoned" and an alternative IP (or NXDOMAIN
   error) is returned more quickly than the upstream resolver can
   respond, causing an erroneous IP address to be cached (and
   potentially recursively so).  The alternative IPs usually direct to a
   nonsense domain or a warning page.  Alternatively, Iranian censorship
   appears to prevent the communication en-route, preventing a response
   from ever being sent [Aryan-2012].

   *Trade-offs:* These forms of DNS interference require the censor to
   force a user to traverse a controlled DNS hierarchy (or intervening
   network on which the censor serves as a Active Pervasive Attacker
   [RFC7624] to rewrite DNS responses) for the mechanism to be
   effective.  It can be circumvented by a technical savvy user that
   opts to use alternative DNS resolvers (such as the public DNS
   resolvers provided by Google, OpenDNS, Telcomix, or FDN) or Virtual
   Private Network technology.  DNS mangling and cache poisoning also
   imply returning an incorrect IP to those attempting to resolve a
   domain name, but in some cases the destination may be technically
   accessible; over HTTP, for example, the user may have another method
   of obtaining the IP address of the desired site and may be able to
   access it if the site is configured to be the default server
   listening at this IP address.  Target blocking has also been a
   problem, as occasionally users outside of the censors region will be
   directed through DNS servers or DNS-rewriting network equipment
   controlled by a censor, causing the request to fail.  The ease of
   circumvention paired with the large risk of content blocking and
   target blocking make DNS interference a partial, difficult, and less

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   than ideal censorship mechanism.  Additionally, the above mechanisms
   rely on DNSSEC not being deployed or DNSSEC validation not being
   active on the client or recursive resolver.

   *Empirical Examples:* DNS interference, when properly implemented, is
   easy to identify based on the shortcomings identified above.  Turkey
   relied on DNS interference for its country-wide block of websites
   such Twitter and YouTube for almost week in March of 2014 but the
   ease of circumvention resulted in an increase in the popularity of
   Twitter until Turkish ISPs implementing an IP blacklist to achieve
   the governmental mandate [Zmijewki-2014].  Ultimately, Turkish ISPs
   started hijacking all requests to Google and Level 3's international
   DNS resolvers [Zmijewki-2014].  DNS interference, when incorrectly
   implemented, has resulted in some of the largest "censorship
   disasters".  In January 2014, China started directing all requests
   passing through the Great Fire Wall to a single domain,, due to an improperly configured DNS poisoning
   attempt; this incident is thought to be the largest Internet-service
   outage in history [AFP-2014] [Anon-SIGCOMM12].  Countries such as
   China, Iran, Turkey, and the United States have discussed blocking
   entire TLDs as well, but only Iran has acted by blocking all Israeli
   (.il) domains [Albert-2011].

4.2.  Transport Layer

4.2.1.  Performance Degradation

   While other interference techniques outlined in this section mostly
   focus on blocking or preventing access to content, it can be an
   effective censorship strategy in some cases to not entirely block
   access to a given destination, or service but instead degrade the
   performance of the relevant network connection.  The resulting user
   experience for a site or service under performance degradation can be
   so bad that users opt to use a different site, service, or method of
   communication, or may not engage in communication at all if there are
   no alternatives.  Traffic shaping techniques that rate-limit the
   bandwidth available to certain types of traffic is one example of a
   performance degradation.

   *Trade offs:* While implementing a performance degradation will not
   always eliminate the ability of people to access a desire resource,
   it may force them to use other means of communication where
   censorship (or surveillance) is more easily accomplished.

   *Empirical Examples:* Iran has been known to shape the bandwidth
   available to HTTPS traffic to encourage unencrypted HTTP traffic

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4.2.2.  Packet Dropping

   Packet dropping is a simple mechanism to prevent undesirable traffic.
   The censor identifies undesirable traffic and chooses to not properly
   forward any packets it sees associated with the traversing
   undesirable traffic instead of following a normal routing protocol.
   This can be paired with any of the previously described mechanisms so
   long as the censor knows the user must route traffic through a
   controlled router.

   *Trade offs:* Packet Dropping is most successful when every
   traversing packet has transparent information linked to undesirable
   content, such as a Destination IP.  One downside Packet Dropping
   suffers from is the necessity of blocking all content from otherwise
   allowable IPs based on a single subversive sub-domain; blogging
   services and github repositories are good examples.  China famously
   dropped all github packets for three days based on a single
   repository hosting undesirable content [Anonymous-2013].  The need to
   inspect every traversing packet in close to real time also makes
   Packet Dropping somewhat challenging from a QoS perspective.

   *Empirical Examples:* Packet Dropping is a very common form of
   technical interference and lends itself to accurate detection given
   the unique nature of the time-out requests it leaves in its wake.
   The Great Firewall of China has been observed using packet dropping
   as one of its primary mechanisms of technical censorship
   [Ensafi-2013].  Iran has also used Packet Dropping as the mechanisms
   for throttling SSH [Aryan-2012].  These are but two examples of a
   ubiquitous censorship practice.

4.2.3.  RST Packet Injection

   Packet injection, generally, refers to a man-in-the-middle (MITM)
   network interference technique that spoofs packets in an established
   traffic stream.  RST packets are normally used to let one side of TCP
   connection know the other side has stopped sending information, and
   thus the receiver should close the connection.  RST Packet Injection
   is a specific type of packet injection attack that is used to
   interrupt an established stream by sending RST packets to both sides
   of a TCP connection; as each receiver thinks the other has dropped
   the connection, the session is terminated.

   *Trade-offs:* RST Packet Injection has a few advantages that make it
   extremely popular as a censorship technique.  RST Packet Injection is
   an out-of-band interference mechanism, allowing the avoidance of the
   the QoS bottleneck one can encounter with inline techniques such as
   Packet Dropping.  This out-of-band property allows a censor to
   inspect a copy of the information, usually mirrored by an optical

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   splitter, making it an ideal pairing for DPI and Protocol
   Identification [Weaver-2009] (this asynchronous version of a MITM is
   often called a Man-on-the-Side (MOTS)).  RST Packet Injection also
   has the advantage of only requiring one of the two endpoints to
   accept the spoofed packet for the connection to be interrupted.

   The difficult part of RST Packet Injection is spoofing "enough"
   correct information to ensure one end-point accepts a RST packet as
   legitimate; this generally implies a correct IP, port, and (TCP)
   sequence number.  Sequence number is the hardest to get correct, as
   [RFC0793] specifies an RST Packet should be in-sequence to be
   accepted, although the RFC also recommends allowing in-window packets
   as "good enough".  This in-window recommendation is important, as if
   it is implemented it allows for successful Blind RST Injection
   attacks [Netsec-2011].  When in-window sequencing is allowed, It is
   trivial to conduct a Blind RST Injection, a blind injection implies
   the censor doesn't know any sensitive (encrypted) sequencing
   information about the TCP stream they are injecting into, they can
   simply enumerate the ~70000 possible windows; this is particularly
   useful for interrupting encrypted/obfuscated protocols such as SSH or
   Tor. RST Packet Injection relies on a stateful network, making it
   useless against UDP connections.  RST Packet Injection is among the
   most popular censorship techniques used today given its versatile
   nature and effectiveness against all types of TCP traffic.

   *Empirical Examples:* RST Packet Injection, as mentioned above, is
   most often paired with identification techniques that require
   splitting, such as DPI or Protocol Identification.  In 2007, Comcast
   was accused of using RST Packet Injection to interrupt traffic it
   identified as BitTorrent [Schoen-2007], this later led to a US
   Federal Communications Commission ruling against Comcast
   [VonLohmann-2008].  China has also been known to use RST Packet
   Injection for censorship purposes.  This interference is especially
   evident in the interruption of encrypted/obfuscated protocols, such
   as those used by Tor [Winter-2012].

4.3.  Multi-layer and Non-layer

4.3.1.  Distributed Denial of Service (DDoS)

   Distributed Denial of Service attacks are a common attack mechanism
   used by "hacktivists" and malicious hackers, but censors have used
   DDoS in the past for a variety of reasons.  There is a huge variety
   of DDoS attacks [Wikip-DoS], but on a high level two possible impacts
   tend to occur; a flood attack results in the service being unusable
   while resources are being spent to flood the service, a crash attack
   aims to crash the service so resources can be reallocated elsewhere
   without "releasing" the service.

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   *Trade-offs:* DDoS is an appealing mechanism when a censor would like
   to prevent all access to undesirable content, instead of only access
   in their region for a limited period of time, but this is really the
   only uniquely beneficial feature for DDoS as a censorship technique.
   The resources required to carry out a successful DDoS against major
   targets are computationally expensive, usually requiring renting or
   owning a malicious distributed platform such as a botnet, and
   imprecise.  DDoS is an incredibly crude censorship technique, and
   appears to largely be used as a timely, easy-to-access mechanism for
   blocking undesirable content for a limited period of time.

   *Empirical Examples:* In 2012 the U.K.'s GCHQ used DDoS to
   temporarily shutdown IRC chat rooms frequented by members of
   Anonymous using the Syn Flood DDoS method; Syn Flood exploits the
   handshake used by TCP to overload the victim server with so many
   requests that legitimate traffic becomes slow or impossible
   [Schone-2014] [CERT-2000].  Dissenting opinion websites are
   frequently victims of DDoS around politically sensitive events in
   Burma [Villeneuve-2011].  Controlling parties in Russia
   [Kravtsova-2012], Zimbabwe [Orion-2013], and Malaysia
   [Muncaster-2013] have been accused of using DDoS to interrupt
   opposition support and access during elections.  In 2015, China
   launched a DDoS attack using a true MITM system collocated with the
   Great Firewall, dubbed "Great Cannon", that was able to inject
   JavaScript code into web visits to a Chinese search engine that
   commandeered those user agents to send DDoS traffic to various sites

4.3.2.  Network Disconnection or Adversarial Route Announcement

   While it is perhaps the crudest of all censorship techniques, there
   is no more effective way of making sure undesirable information isn't
   allowed to propagate on the web than by shutting off the network.
   The network can be logically cut off in a region when a censoring
   body withdraws all of the Boarder Gateway Protocol (BGP) prefixes
   routing through the censor's country.

   *Trade-offs:* The impact to a network disconnection in a region is
   huge and absolute; the censor pays for absolute control over digital
   information with all the benefits the Internet brings; this is never
   a long-term solution for any rational censor and is normally only
   used as a last resort in times of substantial unrest.

   *Empirical Examples:* Network Disconnections tend to only happen in
   times of substantial unrest, largely due to the huge social,
   political, and economic impact such a move has.  One of the first,
   highly covered occurrences was with the Junta in Myanmar employing
   Network Disconnection to help Junta forces quash a rebellion in 2007

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   [Dobie-2007].  China disconnected the network in the Xinjiang region
   during unrest in 2009 in an effort to prevent the protests from
   spreading to other regions [Heacock-2009].  The Arab Spring saw the
   the most frequent usage of Network Disconnection, with events in
   Egypt and Libya in 2011 [Cowie-2011] [Cowie-2011b], and Syria in 2012
   [Thomson-2012].  Russia has indicated that it will attempt to
   disconnect all Russian networks from the global internet in April
   2019 as part of a test of the nation's network independence.  Reports
   also indicate that, as part of the test disconnect, Russian telecom
   firms must route all traffic to state-operated monitoring points.
   cite ZD Net

5.  Non-Technical Prescription

   As the name implies, sometimes manpower is the easiest way to figure
   out which content to block.  Manual Filtering differs from the common
   tactic of building up blacklists in that it doesn't necessarily
   target a specific IP or DNS, but instead removes or flags content.
   Given the imprecise nature of automatic filtering, manually sorting
   through content and flagging dissenting websites, blogs, articles and
   other media for filtration can be an effective technique.  This
   filtration can occur on the Backbone/ISP level - China's army of
   monitors is a good example [BBC-2013b] - but more commonly manual
   filtering occurs on an institutional level.  Internet Content
   Providers such as Google or Weibo, require a business license to
   operate in China.  One of the prerequisites for a business license is
   an agreement to sign a "voluntary pledge" known as the "Public Pledge
   on Self-discipline for the Chinese Internet Industry".  The failure
   to "energetically uphold" the pledged values can lead to the ICPs
   being held liable for the offending content by the Chinese government

6.  Non-Technical Interference

6.1.  Self-Censorship

   Self-censorship is one of the most interesting and effective types of
   censorship; a mix of Bentham's Panopticon, cultural manipulation,
   intelligence gathering, and meatspace enforcement.  Simply put, self-
   censorship is when a censor creates an atmosphere where users censor
   themselves.  This can be achieved through controlling information,
   intimidating would-be dissidents, swaying public thought, and
   creating apathy.  Self-censorship is difficult to document, as when
   it is implemented effectively the only noticeable tracing is a lack
   of undesirable content; instead one must look at the tools and
   techniques used by censors to encourage self-censorship.  Controlling
   Information relies on traditional censorship techniques, or by

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   forcing all users to connect through an intranet, such as in North
   Korea.  Intimidation is often achieved through allowing Internet
   users to post "whatever they want," but arresting those who post
   about dissenting views, this technique is incredibly common
   [Calamur-2013] [AP-2012] [Hopkins-2011] [Guardian-2014]
   [Johnson-2010].  A good example of swaying public thought is China's
   "50-Cent Party," reported to be composed of somewhere between 20,000
   [Bristow-2013] and 300,000 [Fareed-2008] contributors who are paid to
   "guide public thought" on local and regional issues as directed by
   the Ministry of Culture.  Creating apathy can be a side-effect of
   successfully controlling information over time and is ideal for a
   censorship regime [Gao-2014].

6.2.  Domain Name Reallocation

   Because domain names are resolved recursively, if a root name server
   reassigns or delists a domain, all other DNS servers will be unable
   to properly forward and cache the site.  Domain name registration is
   only really a risk where undesirable content is hosted on TLD
   controlled by the censoring country, such as .cn or .ru
   [Anderson-2011] or where legal processes in countries like the United
   States result in domain name seizures and/or DNS redirection by the
   government [Kopel-2013].

6.3.  Server Takedown

   Servers must have a physical location somewhere in the world.  If
   undesirable content is hosted in the censoring country the servers
   can be physically seized or the hosting provider can be required to
   prevent access [Anderson-2011].

6.4.  Notice and Takedown

   In some countries, legal mechanisms exist where an individual can
   issue a legal request to a content host that requires the host to
   take down content.  Examples include the voluntary systems employed
   by companies like Google to comply with "Right to be Forgotten"
   policies in the European Union [Google-RTBF] and the copyright-
   oriented notice and takedown regime of the United States Digital
   Millennium Copyright Act (DMCA) Section 512 [DMLP-512].

7.  Contributors

   This document benefited from discussions with Stephane Bortzmeyer,
   Nick Feamster, and Martin Nilsson.

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8.  Informative References

              AFNIC, "Report of the AFNIC Scientific Council:
              Consequences of DNS-based Internet filtering", 2013,

              AFP, "China Has Massive Internet Breakdown Reportedly
              Caused By Their Own Censoring Tools", 2014,

              Albert, K., "DNS Tampering and the new ICANN gTLD Rules",
              2011, <

              Anderson, R. and S. Murdoch, "Access Denied: Tools and
              Technology of Internet Filtering", 2011,

              Anonymous, "The Collateral Damage of Internet Censorship
              by DNS Injection", 2012,

              Anonymous, "How to Bypass Comcast's Bittorrent
              Throttling", 2012, <

              Anonymous, "GitHub blocked in China - how it happened, how
              to get around it, and where it will take us", 2013,

              Anonymous, "Towards a Comprehensive Picture of the Great
              Firewall's DNS Censorship", 2014,

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   [AP-2012]  Associated Press, "Sattar Beheshit, Iranian Blogger, Was
              Beaten In Prison According To Prosecutor", 2012,

              Aryan, S., Aryan, H., and J. Halderman, "Internet
              Censorship in Iran: A First Look", 2012,

              BBC News, "Google and Microsoft agree steps to block abuse
              images", 2013, <>.

              BBC, "China employs two million microblog monitors state
              media say", 2013,

              Bortzmayer, S., "DNS Censorship (DNS Lies) As Seen By RIPE
              Atlas", 2015,

              Bristow, M., "China's internet 'spin doctors'", 2013,

              Calamur, K., "Prominent Egyptian Blogger Arrested", 2013,

              CERT, "TCP SYN Flooding and IP Spoofing Attacks", 2000,

              Cheng, J., "Google stops Hong Kong auto-redirect as China
              plays hardball", 2010, <

              Clayton, R., "Ignoring the Great Firewall of China", 2006,

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              Condliffe, J., "Google Announces Massive New Restrictions
              on Child Abuse Search Terms", 2013, <

              Cowie, J., "Egypt Leaves the Internet", 2011,

              Cowie, J., "Libyan Disconnect", 2011,

              Crandall, J., "Empirical Study of a National-Scale
              Distributed Intrusion Detection System: Backbone-Level
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              Dalek, J., "A Method for Identifying and Confirming the
              Use of URL Filtering Products for Censorship", 2013,

              Ding, C., Chi, C., Deng, J., and C. Dong, "Centralized
              Content-Based Web Filtering and Blocking: How Far Can It
              Go?", 1999, <

              Digital Media Law Project, "Protecting Yourself Against
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              Dobie, M., "Junta tightens media screw", 2007,

              Ensafi, R., "Detecting Intentional Packet Drops on the
              Internet via TCP/IP Side Channels", 2013,

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              Fareed, M., "China joins a turf war", 2008,

              Fifield, D., Lan, C., Hynes, R., Wegmann, P., and V.
              Paxson, "Blocking-resistant communication through domain
              fronting", 2015,

              Gao, H., "Tiananmen, Forgotten", 2014,

              Glanville, J., "The Big Business of Net Censorship", 2008,

              Google, Inc., "Search removal request under data
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              The Gaurdian, "Chinese blogger jailed under crackdown on
              'internet rumours'", 2014,

              Halley, B., "How DNS cache poisoning works", 2014,

              Heacock, R., "China Shuts Down Internet in Xinjiang Region
              After Riots", 2009, <

              Electronic Frontier Foundation, "Hepting vs. AT&T", 2011,

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              Hjelmvik, E., "Breaking and Improving Protocol
              Obfuscation", 2010,

              Hopkins, C., "Communications Blocked in Libya, Qatari
              Blogger Arrested: This Week in Online Tyranny", 2011,

              Husak, M., Cermak, M., Jirsik, T., and P. Celeda, "HTTPS
              traffic analysis and client identification using passive
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              ICANN Security and Stability Advisory Committee (SSAC),
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              the Domain Name System", 2012,

              Johnson, L., "Torture feared in arrest of Iraqi blogger",
              2011, <

              Jones, B., "Automated Detection and Fingerprinting of
              Censorship Block Pages", 2014,

              Khattak, S., "Towards Illuminating a Censorship Monitor's
              Model to Facilitate Evasion", 2013, <http://0b4af6cdc2f0c5

              Kopel, K., "Operation Seizing Our Sites: How the Federal
              Government is Taking Domain Names Without Prior Notice",
              2013, <>.

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              Kravtsova, Y., "Cyberattacks Disrupt Opposition's
              Election", 2012,

              Marczak, B., Weaver, N., Dalek, J., Ensafi, R., Fifield,
              D., McKune, S., Rey, A., Scott-Railton, J., Deibert, R.,
              and V. Paxson, "An Analysis of China's "Great Cannon"",

              Muncaster, P., "Malaysian election sparks web blocking/
              DDoS claims", 2013,

              Nabi, Z., "The Anatomy of Web Censorship in Pakistan",
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              n3t2.3c, "TCP-RST Injection", 2011,

              Orion, E., "Zimbabwe election hit by hacking and DDoS
              attacks", 2013,

              Porter, T., "The Perils of Deep Packet Inspection", 2010,

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,

   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066,
              DOI 10.17487/RFC6066, January 2011,

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   [RFC7624]  Barnes, R., Schneier, B., Jennings, C., Hardie, T.,
              Trammell, B., Huitema, C., and D. Borkmann,
              "Confidentiality in the Face of Pervasive Surveillance: A
              Threat Model and Problem Statement", RFC 7624,
              DOI 10.17487/RFC7624, August 2015,

   [RFC7754]  Barnes, R., Cooper, A., Kolkman, O., Thaler, D., and E.
              Nordmark, "Technical Considerations for Internet Service
              Blocking and Filtering", RFC 7754, DOI 10.17487/RFC7754,
              March 2016, <>.

              Reporters Sans Frontieres, "Technical ways to get around
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              for users in the US", 2013,

              Sandvine, "Technology Showcase on Traffic Classification:
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              Schoen, S., "EFF tests agree with AP: Comcast is forging
              packets to interfere with user traffic", 2007,

              Schone, M., Esposito, R., Cole, M., and G. Greenwald,
              "Snowden Docs Show UK Spies Attacked Anonymous, Hackers",
              2014, <

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              Senft, A., "Asia Chats: Analyzing Information Controls and
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              Shbair, W., Cholez, T., Goichot, A., and I. Chrisment,
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              Sophos, "Understanding Sophos Web Filtering", 2015,

              Tang, C., "In-depth analysis of the Great Firewall of
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              Thomson, I., "Syria Cuts off Internet and Mobile
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              Trustwave, "Filter: SNI extension feature and HTTPS
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              Verkamp, J. and M. Gupta, "Inferring Mechanics of Web
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              Villeneuve, N., "Open Access: Chapter 8, Control and
              Resistance, Attacks on Burmese Opposition Media", 2011,

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              VonLohmann, F., "FCC Rules Against Comcast for BitTorrent
              Blocking", 2008, <

              Wagner, B., "Deep Packet Inspection and Internet
              Censorship: International Convergence on an 'Integrated
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              Wagstaff, J., "In Malaysia, online election battles take a
              nasty turn", 2013,

              Weaver, N., Sommer, R., and V. Paxson, "Detecting Forged
              TCP Packets", 2009, <

              Whittaker, Z., "1,168 keywords Skype uses to censor,
              monitor its Chinese users", 2013,

              Wikipedia, "Denial of Service Attacks", 2016,

              Wilde, T., "Knock Knock Knockin' on Bridges Doors", 2012,

              Winter, P., "How China is Blocking Tor", 2012,

              Zhu, T., "An Analysis of Chinese Search Engine Filtering",

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              Zmijewki, E., "Turkish Internet Censorship Takes a New
              Turn", 2014, <

Authors' Addresses

   Joseph Lorenzo Hall


   Michael D. Aaron
   CU Boulder


   Stan Adams


   Ben Jones


   Nick Feamster


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