Draft: draft-irtf-pearg-censorship

Commentary: This PR has a ton of line edits and typos fixed courtesy of Jane Coffin .
We describe three elements of Internet censorship: prescription,
identification, and interference.  The document contains three major
sections, each corresponding to one of these elements.  Prescription
is the process by which censors determine what types of material they
should censor, e.g., classifying pornographic websites as
undesirable.  Identification is the process by which censors classify
specific traffic or traffic identifiers to be blocked or impaired,
e.g., deciding that webpages containing "sex" in an HTTP Header or
that accept traffic through the URL www.sex.example are likely to be
undesirable.  Interference is the process by which censors intercede
in communication and prevents access to censored materials by
blocking access or impairing the connection, e.g., implementing a
technical solution capable of identifying HTTP headers or URLs and
ensuring they are rendered wholly or partially inaccessible.
We describe three elements of Internet censorship: prescription,
identification, and interference.  The document contains three major
sections, each corresponding to one of these elements.  Prescription
is the process by which censors determine what types of material they
should censor, e.g., classifying pornographic websites as
undesirable.  Identification is the process by which censors classify
specific traffic or traffic identifiers to be blocked or impaired,
e.g., deciding that webpages containing "sex" in an HTTP (Hypertext
Transport Protocol) Header or that accept traffic through the URL
www.sex.example are likely to be undesirable.  Interference is the
process by which censors intercede in communication and prevent
access to censored materials by blocking access or impairing the
connection, e.g., implementing a technical solution capable of
identifying HTTP headers or URLs and ensuring they are rendered
wholly or partially inaccessible.
Prescription is the process of figuring out what censors would like
to block Glanville-2008.  Generally, censors aggregate information
"to block" in blocklists, databases of image hashes ekr-2021, or use
real-time heuristic assessment of content Ding-1999.  Some national
networks are designed to more naturally serve as points of control
Leyba-2019.  There are also indications that online censors use
probabilistic machine learning techniques Tang-2016.  Indeed, web
crawling and machine learning techniques are an active research idea
in the effort to identify content deemed as morally or commercially
harmful to companies or consumers in some jurisdictions SIDN2020.
Prescription is the process of figuring out what censors would like
to block Glanville-2008.  Generally, censors aggregate information
"to block" in blocklists, databases of image hashes ekr-2021, or use
real-time heuristic assessment of content Ding-1999.  Some national
networks are designed to more naturally serve as points of control
Leyba-2019.  There are also indications that online censors use
probabilistic machine learning techniques Tang-2016.  Indeed, web
crawling and machine learning techniques are an active research area
in the effort to identify content deemed as morally or commercially
harmful to companies or consumers in some jurisdictions SIDN2020.
The mechanisms for building up these blocklists vary.  Censors can
purchase from private industry "content control" software, which lets
censors filter traffic from broad categories they would like to
block, such as gambling or pornography Knight-2005.  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.
The mechanisms for building up these blocklists vary.  Censors can
purchase from private industry "content control" software, which lets
censors filter traffic from broad categories they would like to
block, such as gambling or pornography Knight-2005.  In these cases,
these private services attempt to categorize every semi-questionable
website to allow for meta-tag blocking.  Similarly, they tune real-
time content heuristic systems to map their assessments onto
categories of objectionable content.
Content-layer filtering of images and video require institutions or
organizations to store hashes of images or videos to be blocked in
databases, which can then be compared, with some degree of tolerance,
to content that is sent, received or stored using centralized,
content applications and services ekr-2021.
Content-layer filtering of images and video requires institutions or
organizations to store hashes of images or videos to be blocked in
databases, which can then be compared, with some degree of tolerance,
to content that is sent, received or stored using centralized,
content applications and services ekr-2021.
Internet censorship takes place in all parts of the network topology.
It may be implemented in the network itself (e.g. local loop or
backhaul), on the services side of communication (e.g. web hosts,
cloud providers or content delivery networks), in the ancillary
services eco-system (e.g. domain name system or certificate
authorities) or on the end-client side (e.g. in an end-user device
such as a smartphone, laptop or desktop or software executed on such
devices).  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.  There are various logical and
physical points-of-control censors may use for interception
mechanisms, including, though not limited to, the following.
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.
Some national network designs naturally serve as more effective
chokepoints and points of control Leyba-2019.
Internet Backbone: If a censor controls elements of Internet
network infrastructure, such as the international gateways into a
region or Internet exchange points, those chokepoints can be used
to filter undesirable traffic that is traveling into and out of
the region by packet sniffing and port mirroring.  Censorship at
gateways 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, which would have to be accomplished at exchange points or
other network aggregation points.  Some national network designs
naturally serve as more effective chokepoints and points of
control Leyba-2019.
Certificate Authorities (CAs) for Public-Key Infrastructures
(PKIs): Authorities that issue cryptographically secured resources
can be a significant point of control.  CAs that issue
certificates to domain holders for TLS/HTTPS (the Web PKI) or
Regional/Local Internet Registries (RIRs) that issue Route
Origination Authorizations (ROAs) to BGP operators can be forced
to issue rogue certificates that may allow compromise, i.e., by
allowing censorship software to engage in identification and
interference where not possible before.  CAs may also be forced to
revoke certificates.  This may lead to adversarial traffic routing
or TLS interception being allowed, or an otherwise rightful origin
or destination point of traffic flows being unable to communicate
in a secure way.
Services: Application service providers can be pressured, coerced,
or legally required to censor specific content or data flows.
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.  Services have increasingly become focal points of
censorship discussions, as well as the focus of discussions of
moral imperatives to use censorship tools.
Certificate Authorities (CAs) for Public-Key Infrastructures
(PKIs): Authorities that issue cryptographically secured resources
can be a significant point of control.  CAs that issue
certificates to domain holders for TLS/HTTPS (the Web PKI) or
Regional/Local Internet Registries (RIRs) that issue Route
Origination Authorizations (ROAs) to BGP operators can be forced
to issue rogue certificates that may allow compromise, i.e., by
allowing censorship software to engage in identification and
interference where it may not have been possible before.  CAs may
also be forced to revoke certificates.  This may lead to
adversarial traffic routing or TLS interception being allowed, or
an otherwise rightful origin or destination point of traffic flows
being unable to communicate in a secure way.
Services: Application service providers can be pressured, coerced,
or legally required to censor specific content or data flows.
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.  Services have increasingly become focal points of
censorship discussions, as well as the focus of discussions of
moral imperatives to use censorship tools.
Content sites: On the service side of communications lie many
platforms that publish user-generated content require terms of
service compliance with all content and user accounts in order to
avoid intermediary liability for the web hosts.  In aggregate
these policies, actions and remedies are known as content
moderation.  Content moderation happens above the services or
application layer, but these mechanisms are built to filter, sort
and block content and users thus making them available to censors
through direct pressure on the private entity.
Content sites: On the service side of communications lie many
platforms that publish user-generated content and require terms of
service compliance with all content and user accounts in order to
avoid intermediary liability for the web hosts.  In aggregate,
these policies, actions and remedies are known as content
moderation.  Content moderation happens above the services or
application layer, but these mechanisms are built to filter, sort
and block content and users thus making them available to censors
through direct pressure on the private entity.
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.  (Of course, if a personal device is treated with
censorship software before sale and this software is difficult to
reconfigure, this may work in favor of those seeking to control
information, say for children, students, customers, or employees.)
The emergence of mobile devices exacerbate these feasibility
problems.  This software can also be mandated by institutional
actors acting on non-governmentally mandated moral imperatives.
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.  (Of course, if a personal device is treated with
censorship software before sale and this software is difficult to
reconfigure, this may work in favor of those seeking to control
information, say for children, students, customers, or employees.)
The emergence of mobile devices has exacerbate these feasibility
problems.  This software can also be mandated by institutional
actors acting on non-governmentally mandated moral imperatives.
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 transport identification (see sec_thid) is
necessary for HTTPS filtering.  However, some countermeasures can
trivially defeat simple forms of HTTP Request Header Identification.
For example, two cooperating endpoints - an instrumented web server
and client - could encrypt or otherwise obfuscate the "host" header
in a request, potentially thwarting techniques that match against
"host" header values.
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 (Hyptertext Transport Protocol Secure) will
encrypt the relevant request and response fields, so pairing with
transport identification (see sec_thid) is necessary for HTTPS
filtering.  However, some countermeasures can trivially defeat simple
forms of HTTP Request Header Identification.  For example, two
cooperating endpoints - an instrumented web server and client - could
encrypt or otherwise obfuscate the "host" header in a request,
potentially thwarting techniques that match against "host" header
values.
good.foo.example is not) Shbair-2015.  There are undergoing
standardization efforts in the TLS Working Group to encrypt SNI I-
D.ietf-tls-sni-encryption I-D.ietf-tls-esni and recent research shows
promising results in the use of encrypted SNI in the face of SNI-
based filtering Chai-2019 in some countries.
good.foo.example is not) Shbair-2015.  There are ongoing
standardization efforts in the TLS Working Group to encrypt SNI I-
D.ietf-tls-sni-encryption I-D.ietf-tls-esni and recent research shows
promising results in the use of encrypted SNI in the face of SNI-
based filtering Chai-2019 in some countries.
Empirical Examples: There are many examples of security firms that
offer SNI-based filtering products Trustwave-2015 Sophos-2015 Shbair-
2015, and the governments of China, Egypt, Iran, Qatar, South Korea,
Turkey, Turkmenistan, and the UAE all do widespread SNI filtering or
blocking OONI-2018 OONI-2019 NA-SK-2019 CitizenLab-2018 Gatlan-2019
Chai-2019 Grover-2019 Singh-2019.  SNI blocking against QUIC traffic
has been first observed in Russia in March 2022 Elmenhorst-2022.
Empirical Examples: There are many examples of security firms that
offer SNI-based filtering products Trustwave-2015 Sophos-2015 Shbair-
2015, and the governments of China, Egypt, Iran, Qatar, South Korea,
Turkey, Turkmenistan, and the UAE all do widespread SNI filtering or
blocking OONI-2018 OONI-2019 NA-SK-2019 CitizenLab-2018 Gatlan-2019
Chai-2019 Grover-2019 Singh-2019.  SNI blocking against QUIC traffic
was first observed in Russia in March 2022 Elmenhorst-2022.
Tradeoffs: Censoring based on the server certificate requires DPI
techniques that can be more computationally expensive compared to
other methods.  Additionally, the certificate is sent later in the
TLS Handshake compared to the SNI field, forcing the censor to track
the connection for longer.
Tradeoffs: Censoring based on the server certificate requires DPI
techniques that can be more computationally expensive compared to
other methods.  Additionally, the certificate is sent later in the
TLS Handshake compared to the SNI field, forcing the censor to track
the connection longer.
A commonly used method of instrumenting content distributors consists
of keyword identification to detect restricted terms on their
platform.  Governments may provide the terms on such keyword lists.
Alternatively, the content provider may be expected to come up with
their own list.
An increasingly common method of instrumeting content distribution
consists of hash matching to detect and action images and videos
known to be restricted either by governments, institutions,
organizations or the distributor themselves ekr-2021.
A commonly used method of instrumenting content distributors consists
of keyword identification to detect restricted terms on their
platforms.  Governments may provide the terms on such keyword lists.
Alternatively, the content provider may be expected to come up with
their own list.
An increasingly common method of instrumenting content distribution
consists of hash matching to detect and take action on images and
videos known to be restricted either by governments, institutions,
organizations or the distributor themselves ekr-2021.
100,000 queries in U.K. to help combat abuse BBC-2013 Condliffe-2013.
European Union law, as well as US law, requires modification of
search engine results in response to either copyright, trademark,
data protection or defamation concerns EC-2012.
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.
100,000 queries in the U.K. to help combat abuse BBC-2013 Condliffe-
2013.  European Union law, as well as US law, requires modification
of search engine results in response to either copyright, trademark,
data protection or defamation concerns EC-2012.
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.
Disassociation by content distributors from certain categories of
users has happened for instance in Spain, as a result of the conflict
between the Catalunyan independence movement and the Spanish legal
presumption of a unitary state Lomas-2019.  E-sport event organizers
have also disassociated themselves from top players who expressed
political opinions in relation to the 2019 Hong Kong protests Victor-
2019.  See also discon.
Disassociation by content distributors from certain categories of
users has happened for instance in Spain, as a result of the conflict
between the Catalan independence movement and the Spanish legal
presumption of a unitary state Lomas-2019.  E-sport event organizers
have also disassociated themselves from top players who expressed
political opinions in relation to the 2019 Hong Kong protests Victor-
2019.  See also discon.
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.
sent in the ClientHello message to similar signatures found in
unencrypted traffic Husak-2016.
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.
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, uses 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.
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, uses DPI to
identify restricted content over HTTP and DNS and to inject TCP RSTs
and bad DNS responses, respectively, into connections Crandall-2010
Clayton-2006 Anonymous-2014.
Trade-offs: header identification is popular due to its simplicity,
availability, and robustness.
Trade offs: header identification is popular due to its simplicity,
availability, and robustness.
protocols above IP, e.g.  TCP and QUIC.
protocols above IP, e.g., TCP and QUIC.
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 web
content solely on the basis of port.  HTTPS goes over port 443, with
similar consequences for the censor except only partial metadata may
now be available to the censor.  Port allowlisting 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.  A counter-example is that port 25 (SMTP) has long
been blocked on residential ISPs' networks to reduce the risk for
email spam, but in doing so also prohibits residential ISP customers
from running their own email servers.
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 web
content solely on the basis of port.  HTTPS goes over port 443, with
similar consequences for the censor except only partial metadata may
now be available to the censor.  Port allowlisting 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.  A counter-example is that port 25 (SMTP) has long
been blocked on residential ISP networks to reduce the risk of email
spam, but doing this also prohibits residential ISP customers from
running their own email servers.
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.
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 competition between censors and circumvention
tool developers.  As part of this competition, 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 the
host is running a circumvention tool.  China has used active scanning
to great effect to block Tor Winter-2012.
Trade offs: Protocol identification only provides insight into the
way information is traveling, and not the information itself.
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 Winter-2012.  In
2020, Iran deployed an allowlist protocol filter, which only allowed
three protocols to be used (DNS, TLS, and HTTP) on specific ports and
censored any connection it could not identify Bock-2020.  In 2022,
Russia seemed to have used protocol identification to block most
HTTP/3 connections Elmenhorst-2022.
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 (injection of a TCP RST packet into the stream) to
interrupt BitTorrent Traffic Winter-2012.  In 2020, Iran deployed an
allowlist protocol filter, which only allowed three protocols to be
used (DNS, TLS, and HTTP) on specific ports and censored any
connection it could not identify Bock-2020.  In 2022, Russia seemed
to have used protocol identification to block most HTTP/3 connections
Elmenhorst-2022.
Trade-offs: The impact of residual censorship is to provide users
with further discouragement from trying to access forbidden content,
though it is not clear how successful it is at accomplishing this.
Trade offs: The impact of residual censorship is to provide users
with further discouragement from trying to access forbidden content,
though it is not clear how successful it is at accomplishing this.
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 name server would provide
Bortzmeyer-2015.
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 an authoritative name server would provide
Bortzmeyer-2015.
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
than ideal censorship mechanism.
problem, as occasionally users outside of the censor's 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
than ideal censorship mechanism.
Previously, techniques were used for e.g. censorship that relied on
DNS requests being passed in cleartext over port 53 SSAC-109-2020.
With the deployment of encrypted DNS (e.g., DNS-over-HTTPS RFC8484)
these requests are now increasingly passed on port 443 with other
HTTPS traffic, or in the case of DNS-over-TLS RFC7858 no longer
passed in the clear (see also sec_thid).
Previously, techniques were used for censorship that relied on DNS
requests being passed in cleartext over port 53 SSAC-109-2020.  With
the deployment of encrypted DNS (e.g., DNS-over-HTTPS RFC8484) these
requests are now increasingly passed on port 443 with other HTTPS
traffic, or in the case of DNS-over-TLS RFC7858 no longer passed in
the clear (see also sec_thid).
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.
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 to 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.
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.  Notably, packet dropping during the handshake
or working connection is the only interference technique observed for
QUIC traffic so far, e.g. in India, Iran, Russia and Uganda
Elmenhorst-2022.
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: Although ineffective against non-TCP protocols (QUIC,
IPSec), RST Packet Injection has a few advantages that make it
extremely popular as a technique employed for censorship.  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 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.
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 technical censorship mechanisms Ensafi-2013.
Iran has also used Packet Dropping as the mechanism for throttling
SSH Aryan-2012.  These are but two examples of a ubiquitous
censorship practice.  Notably, packet dropping during the handshake
or working connection is the only interference technique observed for
QUIC traffic to date (e.g., in India, Iran, Russia and Uganda
Elmenhorst-2022).
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 a
TCP connection know the other side has stopped sending information,
and that 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: Although ineffective against non-TCP protocols (QUIC,
IPSec), RST Packet Injection has a few advantages that make it
extremely popular as a technique employed for censorship.  RST Packet
Injection is an out-of-band interference mechanism, allowing the
avoidance of 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 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.
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.
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, subsequently leading 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.
While it is perhaps the crudest of all techniques employed for
censorship, 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 Border 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 by losing the benefits a globally-accessible Internet
brings.  Network disconnections are also politically expensive -
citizens used to being able to access Internet platforms and services
see such disconnections as a loss of civil liberty.  Network
disconnection is rarely a long-term solution for any censor and is
normally only used as a last resort in times of substantial unrest.
While it is perhaps the crudest of all techniques employed for
censorship, 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 entity withdraws all of the Border Gateway Protocol
(BGP) prefixes routing through the censor's country.
Trade offs: The impact of a network disconnection in a region is huge
and absolute; the censor pays for absolute control over digital
information by losing the benefits a globally-accessible Internet
brings.  Network disconnections are also politically expensive as
citizens accustomed to accessing Internet platforms and services see
such disconnections as a loss of civil liberty.  Network
disconnection is rarely a long-term solution for any censor and is
normally only used as a last resort in times of substantial civil
unrest in a country.
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 telecommunications firms must now
route all traffic to state-operated monitoring points Cimpanu-2019.
India was the country that saw the largest number of internet
shutdowns per year in 2016 and 2017 Dada-2017.
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 telecommunications firms must now
route all traffic to state-operated monitoring points Cimpanu-2019.
India saw the largest number of Internet shutdowns per year in 2016
and 2017 Dada-2017.
Trade-offs: A global leak of a misrouted website can overwhelm an ISP
if the website gets a lot of traffic.  It is not a permanent solution
because incorrect BGP routes that leak globally can be fixed, though
within a jurisdiction only the ISP/IXP is in a position to correct
them for local users.
Empirical examples: In 2008 Pakistan Telecom censored Youtube at the
request of the Pakistan government by changing its BGP routes for the
website.  The new routes were announced to the ISP's upstream
providers and beyond.  The entire Internet began directing Youtube
routes to Pakistan Telecom and continued doing so for many hours.  In
2018 nearly all Google services and Google cloud customers like
Spotify all lost more than one hour of service after it lost control
of several million of its IP addresses.  Those IP prefixes were being
misdirected to China Telecom, a Chinese government-owned ISP Google-
2018}, in a manner similar to the BGP hijacking of US government and
military websites by China Telecom in 2010.  ISPs in both Russia
(2022) and Myanmar (2021) have tried to hijack the same Twitter
prefix more than once MANRS.
Trade offs: A global leak of a misrouted website can overwhelm an ISP
if the website gets a lot of traffic.  It is not a permanent solution
because incorrect BGP routes that leak globally can be fixed, but
leaks within a jurisdiction can only be corrected by an ISP/IXP for
local users.
Empirical examples: In 2008, Pakistan Telecom censored Youtube at the
request of the Pakistan government by changing its BGP routes for the
website.  The new routes were announced to the ISP's upstream
providers and beyond.  The entire Internet began directing Youtube
routes to Pakistan Telecom and continued doing so for many hours.  In
2018 nearly all Google services and Google cloud customers, like
Spotify, all lost more than one hour of service after it lost control
of several million of its IP addresses.  Those IP prefixes were being
misdirected to China Telecom, a Chinese government-owned ISP Google-
2018}, in a manner similar to the BGP hijacking of US government and
military websites by China Telecom in 2010.  ISPs in both Russia
(2022) and Myanmar (2021) have tried to hijack the same Twitter
prefix more than once MANRS.
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 at 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.
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 technique employed for
censorship.  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
Marczak-2015.
Distributed Denial of Service attacks are a common attack mechanism
used by "hacktivists" and malicious hackers.  Censors have also used
DDoS in the past for a variety of reasons.  There is a wide variety
of DDoS attacks Wikip-DoS, but at a high level two possible impacts
from the attack 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.
Trade offs: DDoS is an appealing mechanism when a censor would like
to prevent all access to undesirable content, instead of only
preventing access in their region for a limited period of time.  The
latter is really the only uniquely beneficial feature for DDoS as a
technique employed for censorship.  The resources required to carry
out a successful DDoS against major targets are computationally
expensive, usually requiring rental or ownership of a malicious
distributed platform such as a botnet, and they are 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 signals intelligence
organization, the Government Communications Headquarters (GCHQ), used
DDoS to temporarily shutdown Internet Relay Chat (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 like
the DDoS 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 Marczak-2015.
Trade-offs: Censorship in depth can be attractive for censors to
deploy, as it offers additional guarantees about censorship: even if
someone evades one type of censorship, they may still be blocked by
another.  The main drawback to this approach is the cost to initial
deployment, as it requires the system to deploy multiple censorship
systems in tandem.
Trade offs: Censorship in depth can be attractive for censors to
deploy, as it offers additional guarantees about censorship: even if
someone evades one type of censorship, they may still be blocked by
another.  The main drawback to this approach is the cost to initial
deployment, as it requires the system to deploy multiple censorship
systems in tandem.
Empirical Examples: Censorship in depth is present in many large
censoring nation states today.  Researchers have observed China has
deployed significant censorship in depth, often censoring the same
resource across multiple protocols Chai-2019, Bock-2020b or deploying
additional censorship systems to censor the same content and protocol
Bock-2021b.  Iran also has deployed a complimentary protocol filter
to limit which protocols can be used on certain ports, forcing users
to rely on protocols their censorship system can filter Bock-2020.
Empirical Examples: Censorship in depth is present in many large
censoring nation states today.  Researchers have observed that China
has deployed significant censorship in depth, often censoring the
same resource across multiple protocols Chai-2019 Bock-2020b, or
deploying additional censorship systems to censor the same content
and protocol Bock-2021b.  Iran also has deployed a complimentary
protocol filter to limit which protocols can be used on certain
ports, forcing users to rely on protocols their censorship system can
filter Bock-2020.
As mentioned in passing by Murdoch-2011, 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 - in
cases where a server is virtualized in a cloud infrastructure where
it may not necessarily have a fixed physical location - the hosting
provider can be required to prevent access.
As mentioned in passing by Murdoch-2011, 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 - in
cases where a server is virtualized in a cloud infrastructure where
it may not necessarily have a fixed physical location - the hosting
provider can be required to prevent access.
Domain names are catalogued in so-called name-servers operated by
legal entities called registries.  These registries can be made to
cede control over a domain name to someone other than the entity
which registered the domain name through a legal procedure grounded
in either private contracts or public law.  Domain name seizures is
increasingly used by both public authorities and private entities to
deal with undesired content dissemination ICANN2012 EFF2017.
In addition to establishing a thorough resource for describing
censorship techniques this document implicates critical areas for
future work.
Taken as a whole the apparent costs of implementation of censorship
techniques indicate a need for better classification of censorship
regimes as they evolve and mature and specifying censorship
circumvention techniques themselves.  Censors maturity refers to the
technical maturity required of the censor to perform the specific
censorship technique.  Future work might classify techniques by
essentially how hard a censor must work, including what
infrastructure is required, in order to successfully censor content,
users or services.
Domain names are catalogued in name-servers operated by legal
entities called registries.  These registries can be made to cede
control over a domain name to someone other than the entity which
registered the domain name through a legal procedure grounded in
either private contracts or public law.  Domain name seizures is
increasingly used by both public authorities and private entities to
deal with undesired content dissemination ICANN2012 EFF2017.
In addition to establishing a thorough resource for describing
censorship techniques, this document implicates critical areas for
future work.
Taken as a whole the apparent costs of implementation of censorship
techniques indicate a need for better classification of censorship
regimes as they evolve and mature, and specifying censorship
circumvention techniques themselves.  Censors maturity refers to the
technical maturity required of the censor to perform the specific
censorship technique.  Future work might classify techniques by
essentially how hard a censor must work, including what
infrastructure is required, in order to successfully censor content,
users or services.