Network Working Group                                          R. Barnes
Internet-Draft                                          BBN Technologies
Intended status: Informational                                 A. Cooper
Expires: April 25, 2013                                              CDT
                                                              O. Kolkman
                                                              NLnet Labs
                                                        October 22, 2012

        Technical Considerations for Internet Service Filtering


   The Internet is structured to be an open communications medium.  This
   openness is one of the key underpinnings of Internet innovation, but
   it can also allow communications that may be viewed as undesirable by
   certain parties.  Thus, as the Internet has grown, so have mechanisms
   to limit the extent and impact of abusive or allegedly illegal
   communications.  Recently, there has been an increasing emphasis on
   "blocking" or "filtering," the active prevention of abusive or
   allegedly illegal communications.  This document examines several
   technical approaches to Internet content blocking in terms of their
   alignment with the overall Internet architecture.  In general, the
   approach to content filtering that is most coherent with the Internet
   architecture is to inform endpoints about potentially undesirable
   services, so that the communicants can avoid engaging in abusive or
   illegal communications.

Status of this Memo

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   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 25, 2013.

Copyright Notice

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   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   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
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Architectural Principles . . . . . . . . . . . . . . . . . . .  4
     2.1.  End-to-End Connectivity and "Transparency" . . . . . . . .  4
     2.2.  Layering . . . . . . . . . . . . . . . . . . . . . . . . .  5
     2.3.  Distribution and Mobility  . . . . . . . . . . . . . . . .  5
     2.4.  Locality and Autonomy  . . . . . . . . . . . . . . . . . .  6
   3.  Examples of Blocking . . . . . . . . . . . . . . . . . . . . .  7
   4.  Blocking Design Patterns . . . . . . . . . . . . . . . . . . .  8
     4.1.  Intermediary-Based Blocking  . . . . . . . . . . . . . . .  9
     4.2.  Server-Based Blocking  . . . . . . . . . . . . . . . . . . 12
     4.3.  Endpoint-Based Blocking  . . . . . . . . . . . . . . . . . 15
   5.  Summary of Trade-offs  . . . . . . . . . . . . . . . . . . . . 16
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 18
   7.  Informative References . . . . . . . . . . . . . . . . . . . . 19
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21

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1.  Introduction

   The original design goal of the Internet was to enable communications
   between hosts.  As this goal was met and people started using the
   Internet to communicate, however, it became apparent that some hosts
   were engaging in arguably undesirable communications.  The most
   famous early example of undesirable communications was the Morris
   worm, which used the Internet to infect many hosts in 1988.  As the
   Internet has evolved into a rich communications medium, so have
   mechanisms to restrict undesirable communications.

   Efforts to restrict or deny access to Internet resources have evolved
   over time.  As noted in [RFC4084], some Internet service providers
   impose restrictions on which applications their customers may use and
   which traffic they allow on their networks.  These restrictions are
   often imposed with customer consent, where customers may be
   enterprises or individuals.  Increasingly, however, both governmental
   and private sector entities are seeking to block access to certain
   content, traffic, or communications without the knowledge or
   agreement of affected users.  Where these entities do not directly
   control networks, they aim to make use of intermediary systems to
   effectuate the blocking.

   Entities may seek to block Internet content for a diversity of
   reasons, including defending against security threats, restricting
   access to content thought to be objectionable, and preventing illegal
   activity.  While blocking remains highly contentious in many cases,
   the desire to restrict access to content will likely continue to

   This document aims to clarify the technical implications and trade-
   offs of various blocking strategies and to identify the potential for
   different strategies to come into conflict with the Internet's
   architecture or cause harmful side effects ("collateral damage").
   The strategies broadly fall into three categories:

   1.  Control by intermediaries (intermediary-based filtering)

   2.  Manipulation of authoritative data (server-based filtering)

   3.  Reputation and authentication systems (endpoint-based filtering)

   Examples of blocking or attempted blocking using the DNS, HTTP
   proxies, domain name seizures, spam filters, and RPKI manipulation
   are used to illustrate each category's properties.

   Whether particular forms of blocking are lawful in particular
   jurisdictions raises complicated legal questions that are outside the

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   scope of this document.

2.  Architectural Principles

   To understand the implications of different blocking strategies, it
   is important to understand the key principles that have informed the
   design of the Internet.  While much of this ground has been well trod
   before, this section highlights four architectural principles that
   have a direct impact on the viability of content blocking: end-to-end
   connectivity and "transparency", layering, distribution and mobility,
   and locality and autonomy.

2.1.  End-to-End Connectivity and "Transparency"

   The end-to-end principle is "the core architectural guideline of the
   Internet" [RFC3724].  Adherence to the principle of vesting endpoints
   with the functionality to accomplish end-to-end tasks results in a
   "transparent" network in which packets are not filtered or
   transformed en route [RFC2775].  This transparency in turn is a key
   requirement for providing end-to-end security features on the
   network.  Modern security mechanisms that rely on trusted hosts
   communicating via a secure channel without intermediary interference
   enable the network to support e-commerce, confidential communication,
   and other similar uses.

   The end-to-end principle is fundamental for Internet security, and
   the foundation on which Internet security protocols are built.
   Protocols such as TLS and IPsec [RFC5246][RFC4301] are designed to
   ensure that each endpoint of the communication knows the identity of
   the other endpoint, and that only the endpoints of the communication
   can access the secured contents of the communication.  For example,
   when a user connects to a bank's web site, TLS ensures that the
   user's banking information is communicated to the bank and nobody

   Some blocking strategies require intermediaries to insert themselves
   within the end-to-end communications path, potentially breaking
   security properties of Internet protocols.  In these cases it can be
   difficult or impossible for endpoints to distinguish between
   attackers and the entities conducting blocking.

   A similar notion to the end-to-end principle is the notion of
   "transparency," that is, the idea that the network should provide a
   generic connectivity service between endpoints, with minimal
   interaction by intermediaries aside from routing packets from source
   to destination.  In "Reflections on Internet Transparency" [RFC4924],
   the IAB assessed the relevance of this principle and concluded that

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   "far from having lessened in relevance, technical implications of
   intentionally or inadvertently impeding network transparency play a
   critical role in the Internet's ability to support innovation and
   global communication."

2.2.  Layering

   Internet applications are built out of a collection of loosely-
   coupled components or "layers."  Different layers serve different
   purposes, such as routing, transport, and naming (see [RFC1122],
   especially Section 1.1.3).  The functions at these layers are
   developed autonomously and almost always operated by different
   entities.  For example, in many networks, physical and link-layer
   connectivity is provided by an "access provider", while IP routing is
   performed by an "Internet service provider" -- and application-layer
   services are provided by a completely separate entity (e.g., a web
   server).  Upper-layer protocols and applications rely on combinations
   of lower-layer functions in order to work.  As a consequence of the
   end-to-end principle, functionality at higher layers tends to be more
   specialized, so that many different specialized applications can make
   use of the same generic underlying network functions.

   As a result of this structure, actions taken at one layer can affect
   functionality or applications at higher layers.  For example,
   manipulating routing or naming functions to restrict access to a
   narrow set of resources via specific applications will likely affect
   all applications that depend on those functions.

   In a similar manner, the physical distance between a host and a
   service provider at a particular layer grows as one moves up the
   stack.  A host must be physically connected to a link-layer access
   provider network, and its distance from its ISP is limited by the
   length of the link that connects it, but Internet applications can be
   delivered to the host from anywhere in the world.

   Thus, as one considers changes at each layer of the stack, changes at
   higher layers become more specific in terms of application, but more
   broad in terms of the size and geography of hosts impacted.  Changes
   to an access network will only affect a relatively small, well-
   defined set of users (namely, those connected to the access network),
   but can affect all applications for those users.  Changes to an
   application service can affect users across the entire Internet, but
   only for that specific application.

2.3.  Distribution and Mobility

   The Internet is designed as a distributed system both geographically
   and topologically.  Resources can be made globally accessible

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   regardless of their physical location or connectivity providers used.
   Resources are also highly mobile -- moving content from one physical
   or logical address to another can often be easily accomplished.

   This distribution and mobility underlies a large part of the
   resiliency of the Internet.  Internet routing can survive major
   outages such as cuts in undersea fibers because the distributed
   routing system of the Internet allows individual networks to
   collaborate to route traffic.  Application services are commonly
   protected using distributed servers.  For example, even though the
   2010 earthquake in Haiti destroyed almost all of the Internet
   infrastructure in the country, the Haitian top-level domain name
   (.ht) had no interruption in service because it was also accessible
   via servers in the United States, Canada, and France.

   Undesirable communications also benefit from this resiliency --
   resources that are blocked or restricted in one part of the Internet
   can be reconstituted in another part of the Internet.  If a web site
   is prevented from using a domain name or set of IP addresses, the web
   site can simply move to another domain name or network.

2.4.  Locality and Autonomy

   The basic unit of Internet routing is an "Autonomous System" -- a
   network that manages its own routing internally.  The concept of
   autonomy is present in many aspects of the Internet, as is the
   related concept of locality, the idea that local changes should not
   have a broader impact on the network.

   These concepts are critical to the stability and scalability of the
   Internet.  With millions of individual actors engineering different
   parts of the network, there would be chaos if every change had impact
   across the entire Internet.

   Locality implies that the impact of technical changes made to realize
   blocking will only be within a defined scope.  As discussed above,
   this scope might be narrow in one dimension (set of users or set of
   applications affected) but broad in another.  Changes made to
   effectuate blocking are often targeted at a particular locality, but
   result in blocking outside of the intended scope.  For example, web
   filtering systems in India and China have been shown to cause
   "collateral damage" by blocking users in Oman and the US from
   accessing web sites in Germany and Korea

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3.  Examples of Blocking

   As noted above, systems to restrict or block Internet communications
   have evolved alongside the Internet technologies they seek to
   restrict.  Looking back at the history of the Internet, there have
   been several such systems deployed, with varying degrees of

   o  Firewalls: Firewalls are a very common form of service blocking,
      employed at many points in today's Internet.  Typically, firewalls
      block according to content-neutral rules, e.g., blocking all
      inbound connections or outbound connections on certain ports.
      Firewalls can be deployed either on end hosts (under user
      control), or at network boundaries.

   o  Web Filtering: HTTP and HTTPS are common targets for blocking and
      filtering, typically targeted at specific URLs.  Some enterprises
      use HTTP blocking to block non-work-appropriate web sites, and
      several nations require HTTP and HTTPS filtering by their ISPs in
      order to block illegal content.  HTTPS is a challenge for these
      systems, because the URL in an HTTPS request is carried inside the
      secure channel.  To block access to content made accessible via
      HTTPS, filtering systems thus must either block based only on IP
      address, or else obtain a trust anchor certificate that is trusted
      by endpoints (and thus act as a man in the middle).  These
      filtering systems often take the form of "portals" or "enterprise
      proxies."  These portals present their own HTTPS certificates that
      are invalid for any given domain according to normal validation
      rules, but may still be trusted if the user install a security

   o  Spam Filtering: Spam filtering is one of the oldest forms of
      service blocking, in the sense that it denies spammers access to
      recipients' mailboxes.  Spam filters evaulate messages based on a
      variety of criteria and information sources to decide whether a
      given message is spam.  For example, DNS Reverse Black Lists use
      the reverse DNS to flag whether an IP address is a known spam
      source [RFC5782].  Spam filters are typically either installed on
      user devices (e.g., in a mail client) or operated by a mail domain
      on behalf of users.

   o  Domain name seizure: In recent years, US law enforcement
      authorities have been issuing legal orders to domain name
      registries to seize domain names associated with the distribution
      of counterfeit goods and other allegedly illegal activity
      [US-ICE].  When domain names are seized, DNS queries for the
      seized names are typically redirected to resolve to U.S.
      government IP addresses that host information about the seizure,

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      either by an authoritative server or by an intermediate resolver.
      The effectiveness of domain seizures is limited by the mobility
      principle, since the application using the seized name can simply
      use another name.  Seizures can come into conflict with the
      locality principle, since content is blocked not only within the
      jurisdiction of the seizure, but globally, even when it may be
      affirmatively legal elsewhere [RojaDirecta].  When domain
      redirection is effected via redirections at intermediate resolvers
      rather than at authoritative servers, it directly contradicts the
      DNS security architecture [RFC4033].

   o  Safe Browsing: Modern web browsers provide some measures to
      prevent users from accessing malicious web sites.  For instance,
      before loading a URL, current versions of Google Chrome and
      Firefox web browsers use the Google Safe Browsing service to
      determine whether or not a given URL is safe to load
      [SafeBrowsing].  The DNS can also be used to mark domains as safe
      or unsafe [RFC5782].

   o  Manipulation of routing and addressing data: Governments have
      recently intervened in the management of IP addressing and routing
      information in order to maintain control over a specific set of
      DNS servers.  As part of an internationally coordinated response
      to the DNSChanger malware, a Dutch court ordered the RIPE NCC to
      freeze the accounts of several resource holders as a means to
      limit the resource holders' ability to use certain address blocks
      [GhostClickRIPE].  These actions have led to concerns that the
      resource certification system and related secure routing
      technologies developed by the IETF SIDR working group might be
      subject to government manipulation as well [RFC6480], potentially
      for the purpose of denying targeted networks access to the

4.  Blocking Design Patterns

   Considering a typical end-to-end Internet communcation, there are
   three logical points at which blocking mechanisms can be put in
   place: the middle and either end.  Mechanisms based in the middle
   usually involve an intermediary device in the network that observes
   Internet traffic and decides which communications to block.  At the
   service end of a communication, authoritative databases (such as the
   DNS) and servers can be manipulated to deny or alter service
   delivery.  At the user end of a communication, authentication and
   reputation systems enable user devices (and users) to make decisions
   about which communications should be blocked.

   In this section, we discuss these three "blocking design patterns"

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   and how they align with the Internet architectural principles
   outlined above.  In general, the third pattern -- informing user
   devices of which services should be blocked -- is the most consistent
   with the Internet architecture.

4.1.  Intermediary-Based Blocking

   A common goal for blocking systems is for the system to be able to
   block communications without the consent or cooperation of either
   endpoint to the communication.  Such systems are thus implemented
   using intermediary devices in the network, such as firewalls or
   filtering systems.  These systems inspect user traffic as it passes
   through the network, decide based on the content of a given
   communication whether it should be blocked, and then block or allow
   the communication as desired.

   Common examples of intermediary-based filtering are firewalls and
   network-based web-filtering systems.  For example, web filtering
   devices usually inspect HTTP requests to determine the URL being
   requested, compare that URL to a list of black-listed or white-listed
   URLs, and allow the request to proceed only if it is permitted by
   policy (or at least not forbidden).  Firewalls perform a similar
   function for other classes of traffic in addition to HTTP.

   It should be noted that these "intermediaries" are often not far from
   the edge of the network.  For example, many enterprise networks
   operate firewalls that block certain web sites, as do some
   residential ISPs.  In some cases, this filtering is done with the
   consent or cooperation of the affected users.  PCs within an
   enterprise, for example, might be configured to trust an enterprise
   proxy, a residential ISP might offer a "safe browsing" service, or
   mail clients might authorize mail servers on the local network to
   filter spam on their behalf.  These cases are effectively equivalent
   to the "Endpoint-Based Blocking" scenarios discussed below, since the
   endpoint has authorized the intermediary to block on its behalf.  The
   challenges discussed in this section arise mostly for scenarios where
   endpoints are not assumed to cooperate with filtering (i.e., they
   might have incentives to circumvent filtering).

   Accomplishing blocking by using intermediaries conflicts with the
   end-to-end and transparency principles noted above.  The very goal of
   blocking in this way is to impede transparency for particular content
   or communications.  For this reason, intermediary-based approaches to
   blocking run into several technical issues that limit their viability
   in practice.  In particular, many issues arise from the fact that an
   intermediary needs to have access to a sufficient amount of traffic
   to make its blocking determinations.

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   The first challenge to obtaining this traffic is simply gaining
   access to the constituent packets.  The Internet is designed to
   deliver packets from source to destination -- not to any particular
   point along the way.  In practice, inter-network routing is often
   asymmetric, and for sufficiently complex local networks, intra-
   network traffic flows can be asymmetric as well.

   This asymmetry means that an intermediary will often see only one
   half of a given communication (if it sees any of it at all), limiting
   its ability to make decisions based on the content of the
   communication.  For example, a URL-based filter cannot make blocking
   decisions if it only has access to HTTP responses (not requests).
   Routing can sometimes be forced to be symmetric within a given
   network using routing configuration, NAT, or layer-2 mechanisms
   (e.g., MPLS), but these mechanisms are frequently brittle, complex,
   and costly -- and often reduce network performance relative to
   asymmetric routing.

   Once an intermediary has access to traffic, it must identify which
   packets must be filtered.  This decision is usually based on some
   combination of information at the network layer (e.g., IP addresses),
   transport layer (ports), or application layer (URLs).  The
   communicating endpoints can deny the intermediary access to these by
   using encryption (see below), but IP addresses must be visible, even
   if packets are protected with IPsec.  However, blocking based on IP
   addresses is the simplest form of filtering to circumvent, because a
   filtered site need only change a single DNS record to move all of its
   services to a new IP address.  Indeed, in the face of IP-based
   blocking in some networks, services such as The Pirate Bay are now
   using cloud hosting services so that their IP addresses are difficult
   for intermediaries to predict [BT-TPB][TPB-cloud].

   If application content is encrypted with a security protocol such as
   IPsec or TLS, then the intermediary will require the ability to
   decrypt the packets to examine application content.  Since security
   protocols are designed to provide end-to-end security (i.e., to
   prevent intermediaries from examining content), the intermediary
   would need to masquerade as one of the endpoints, breaking the
   authentication in the security protocol, reducing the security of the
   users and services affected, and interfering with legitimate private

   If the intermediary is unable to decrypt the security protocol, then
   its blocking determinations for secure sessions can only be based on
   unprotected attributes, such as IP addresses and port numbers.  Some
   blocking systems today still attempt to block based on these
   attributes, for example by blocking TLS traffic to known proxies that
   could be used to tunnel through the blocking system.

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   However, as the Telex project recently demonstrated, if an endpoint
   cooperates with a server, it can create a TLS tunnel that is
   indistinguishable from legitimate traffic [Telex].  For example, if a
   banking website operated a Telex server, then a blocking system would
   be unable to distinguish legitimate encrypted banking traffic from
   Telex-tunneled traffic to that server (potentially carrying content
   that the blocking system would have blocked).

   Thus, in principle it is impossible to block tunneled traffic through
   an intermediary device without blocking all secure traffic.  (The
   only limitation in practice is the requirement for special software
   on the client.)  In most cases, blocking all secure traffic is an
   unacceptable consequence of blocking, since security is often
   required for services such as online commerce, enterprise VPNs, and
   management of critical infrastructure.  If governments or network
   operators were to force these services to use insecure protocols so
   as to effectuate blocking, they would expose their users to the
   various attacks that the security protocols were put in place to

   Some network operators may assume that only blocking access to
   resources available via unsecure channels is sufficient for their
   purposes -- i.e., that the size of the user base that will be willing
   to use secure tunnels and/or special software to circumvent the
   blocking is low enough to make blocking via intermediaries
   worthwhile.  Under that assumption, one might decide that there is no
   need to control secure traffic, and thus that intermediary-based
   blocking is an attractive option.

   However, the longer such blocking systems are in place, the more
   likely it is that efficient and easy-to-use tunnelling tools will
   become available.  The proliferation of the Tor network, for example,
   and its increasingly sophisticated blocking-avoidance techniques
   demonstrate that there is energy behind this trend [Tor].  Thus,
   intermediary-based blocking becomes less effective over time.

   Intermediary-based blocking is a key contributor to the arms race
   that has led to the development of these kinds of tools, the result
   of which is to create unecessary layers of complexity in the
   Internet.  Before content-based blocking became common, the next best
   option for intermediaries was port blocking, the widespread use of
   which has driven more applications and services to use ports (80 most
   commonly) that are unlikely to be blocked.  In turn, intermediaries
   shifted to finer-grained content blocking over port 80, content
   providers shifted to encrypted channels, and intermediaries began
   seeking to identify those channels.  Because the premise of
   intermediary-based blocking is that endpoints have incentives to
   circumvent it, this cat-and-mouse game is an invetiable by-product of

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   this form of blocking.

   In sum, blocking via intermediaries is only effective in a fairly
   constrained set of circumstances.  First, the routing structure of
   the network needs to be such that the intermediary has access to any
   communications it intends to block.  Second, the blocking system
   needs an out-of-band mechanism to mitigate the risk of secure
   protocols being used to avoid blocking (e.g., human analysts
   identifying IP addresses of tunnel endpoints), which may be resource-
   prohibitive, especially if tunnel endpoints begin to change
   frequently.  If the network is sufficiently complex, or the risk of
   tunneling too high, then intermediary-based blocking is unlikely to
   be effective, and in any case this type of blocking drives the
   development of increasingly complex layers of circumvention.

4.2.  Server-Based Blocking

   [TO DO: Likely to distinguish server-based filtering from
   infrastructure-based (DNS, RPKI) filtering in a future version.]

   Internet services are driven by physical devices such as web servers,
   DNS servers, certificate authorities, WHOIS databases, and Internet
   Route Registries.  These devices control the structure and
   availability of Internet applications by providing data elements that
   are used by application code.  For example, changing an A or AAAA
   record on a DNS server will change the IP address that is bound to a
   given domain name; applications trying to communicate with the host
   at that name will then communicate with the host at the new address.

   As physical objects, the servers that underlie Internet applications
   exist within the jurisdiction of governments, and their operators are
   thus subject to certain local laws.  It is thus possible for laws to
   be structured to facilitate blocking of Internet services operated
   within a jurisdiction, either via direct government action or by
   allowing private actors to demand blocking (e.g., through lawsuits).

   The "seizure" of domain names discussed above is an example of this
   type of blocking.  Even though some of the affected domain names
   belonged to non-US entities (e.g., RojaDirecta is Spanish), they were
   subject to blocking by the US government because certain servers were
   operated in the US.  Government officials required the operators of
   the parent zones of a target name (e.g., "com" for "") to
   direct queries for that name to a set of government-operated name
   servers.  Users of services under a target name would thus be unable
   to locate the servers providing services for that name, denying them
   the ability to access these services.  The action of the Dutch police
   against the RIPE NCC is of a similar character, limiting the ability
   of certain ISPs to manage their Internet services by controlling

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   their WHOIS information.

   Blocking services by disabling or manipulating servers does respect
   the end-to-end principle, since the affected server is one end of the
   blocked communication.  However, its interactions with layering,
   resource mobility, and autonomy can limit its effectiveness and cause
   undesirable consequences.

   The layered architecture of the Internet means that there are several
   points at which access to a service can be blocked.  The service can
   be denied Internet access (via control of routers), DNS services (DNS
   servers), or application-layer services (application servers, e.g.,
   web servers).  Blocking via these channels, however, can be both
   amplified and limited by the global nature of the Internet.

   On the one hand, the global nature of Internet resources amplifies
   blocking actions, in the sense that it increases the risk of
   overblocking -- collateral damage to legitimate use of a resource.  A
   given network or domain name might host both legitimate services and
   services that governments desire to block.  A service hosted under a
   domain name and operated in a jurisdiction where it is considered
   undesirable might be considered legitimate in another jurisdiction; a
   blocking action in the host jurisdiction would deny legitimate
   services in the other.

   On the other hand, the distributed and mobile nature of Internet
   resources limits the effectiveness of blocking actions.  Because an
   Internet service can be reached from anywhere on the Internet, a
   service that is blocked in one jurisdiction can often be moved or re-
   instantiated in another jurisdiction.  Likewise, services that rely
   on blocked resources can often be rapidly re-configured to use non-
   blocked resources.  For example, in a process known as "snowshoe
   spamming," a spam originator uses addresses in many different
   networks as sources for spam.  This technique is already widely used
   to spread spam generation across a variety of resources and
   jursidictions to prevent spam blocking from being effective.

   The efficacy of server-based blocking is further limited by the
   autonomy principle discussed above.  If the Internet community
   realizes that a blocking decision has been made and wishes to counter
   it, then local networks can "patch" the authoritative data to avoid
   the blocking.  For example, in 2008, Pakistan Telecom attempted to
   deny access to YouTube within Pakistan by announcing bogus routes for
   YouTube address space to peers in Pakistan.  YouTube was temporarily
   denied service on a global basis due to a route leak, but service was
   restored in approximately two hours because network operators around
   the world re-configured their routers to ignore the blocking routes
   [RenesysPK].  In the context of SIDR and secure routing, a similar

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   re-configuration could be done if a resource certificate were to be
   revoked in order to block routing to a given network.

   In the DNS context, similar work-arounds are available.  If a domain
   name were blocked by changing authoritative records, network
   operators can restore service simply by extending TTLs on cached pre-
   blocking records in recursive resolvers, or by statically configuring
   resolvers to return un-blocked results for the affected name.  Indeed
   these techniques are commonly used in practice to provide service to
   domains that have been disrupted, such as the .ht domain during the
   2010 earthquake in Haiti [EarthquakeHT].  While the point of these
   measures was to counter the effects of a natural disaster rather than
   to counter filtering, the same technical means can also counter the
   effects of filtering based on modifications to the authoritative
   server for a domain.

   Resources such as the DNS, the RPKI, and the Internet Route
   Registries are generic technical databases intended to record certain
   facts about the network.  The DNS, for example, stores information
   about which servers provide services for a given name; the RPKI about
   which entities have been allocated IP addresses.  To offer
   specialized Internet services and applications, different entities
   rely on these generic records in different ways.  Thus the effects of
   changes to the databases can be much more difficult to predict than,
   for example, the effect of shutting down a web server (which fulfills
   the specific purpose of serving web content).

   Server-based blocking also has a variety of other implications that
   may reduce the stability, accessibility, and usability of the global
   Internet.  Server-side blocking may encourage the development of
   parallel or "underground" server-side infrastructure, for example.
   These considerations are further discussed in ISOC's whitepaper on
   DNS filtering [ISOCFiltering], but they also apply to other global
   Internet resources.

   In summary, server-based blocking can sometimes be used to
   immediately block a target service by removing some of the resources
   it depends on.  However, such blocking actions often have harmful
   side effects due to the global nature of Internet resources.  The
   global mobility of Internet resources, together with the autonomy of
   the networks that comprise the Internet, can mean that the effects of
   server-based blocking can be quickly be negated.  To adapt a quote by
   John Gilmore, "The Internet treats blocking as damage and routes
   around it".

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4.3.  Endpoint-Based Blocking

   Internet users and their devices make thousands of decisions every
   day as to whether to engage in particular Internet communications.
   Users decide whether to click on links in suspect email messages;
   browsers advise users on sites that have suspicious characteristics;
   spam filters evaluate the validity of senders and messages.  If the
   hardware and software making these decisions can be instructed not to
   engage in certain communications, then the communications are
   effectively blocked because they never happen.

   There are several systems in place today that advise user systems
   about which communications they should engage in.  As discussed
   above, several modern browsers consult with "Safe Browsing" services
   before loading a web site in order to determine whether the site
   could potentially be harmful.  Spam filtering is one of the oldest
   blocking systems in the Internet; modern blocking systems typically
   make use of one or more "reputation" or "blacklist" databases in
   order to make decisions about whether a given message or sender
   should be blocked.  These systems typically have the property that
   many blocking systems (browsers, MTAs) share a single reputation

   This approach to blocking is consistent with the Internet
   architectural principles discussed above, dealing well with the end-
   to-end principle, layering, mobility, and locality/autonomy.

   Much like server-based blocking, endpoint-based blocking is performed
   at one end of an Internet communication, and thus avoids the problems
   related to end-to-end security mechanisms that intermediary-based
   blocking runs into.  Endpoint-based blocking also lacks some of the
   limitations of server-based blocking: While server-based blocking can
   only see and affect the portion of an application that happens at a
   given server (e.g., DNS name resolution), endpoint-based blocking has
   visibility into the entire application, across all layers and
   transactions.  This visibility provides endpoint-based blocking
   systems with a much richer set of information on which to make
   blocking decisions.

   In particular, endpoint-based blocking deals well with adversary
   mobility.  If a blocked service relocates resources or uses different
   resources, a server-based blocking approach may not be able to affect
   the new resources.  An intermediary-based blocking system may not
   even be able to tell whether the new resources are being used, if the
   blocked service uses secure protocols.  By contrast, endpoint-based
   blocking systems can detect when a blocked service's resources have
   changed (because of their full visibility into transactions) and
   adjust blocking as quickly as new blocking data can be sent out

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   through a reputation system.

   Finally, in an endpoint-based blocking system, blocking actions are
   performed autonomously, by individual endpoints or their delegates.
   The effects of blocking are thus local in scope, minimizing the
   effects on other users or other, legitimate services.

   The primary challenge to endpoint-based blocking is that it requires
   the cooperation of endpoints.  Where this cooperation is willing,
   this is a fairly low barrier, requiring only reconfiguration or
   software update.  Where cooperation is unwilling, it can be
   challenging to enforce cooperation for large numbers of endpoints.
   If cooperation can be achieved, endpoint-based blocking can be much
   more effective than other approaches because it is so coherent with
   the Internet's architectural principles.

5.  Summary of Trade-offs

   Intermediary-based blocking is a relatively low-cost blocking
   solution in some cases, but a poor fit with the Internet
   architecture, especially the end-to-end principle.  It thus suffers
   from several limitations.

   o  Examples: Firewalls, web filtering systems.

   o  A single intermediary device can be used to block access by many
      users to many services.

   o  Intermediary blocking can be done without the cooperation of
      either endpoint to a communication (although having that
      cooperation makes it more likely to be effective).

   o  Intermediaries often lack sufficient information to make blocking
      decisions, due to routing asymmetry or encryption.

   o  Intermediary blocking sometimes involves breaking end-to-end
      security assurances.

   o  Tunneling through blocking is difficult to prevent without
      preventing legitimate secure services.

   Server-based blocking can provide rapid effects for resources under
   the control of the blocking entity, but its ultimate effectiveness is
   limited by the global, autonomous nature of Internet resources and
   networks, and it may create undesirable collateral damage to Internet

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   o  Examples: Domain name seizures, WHOIS account freezing, RPKI
      certificate revocation.

   o  Internet services that depend on specific resources can be blocked
      by disabling those resources.

   o  Blocked resources can often be easily relocated or reinstantiated
      in a location where they will not be blocked.

   o  Resources used by undesirable services are often also used by
      legitimate services, resulting in collateral damage.

   o  Autonomy of Internet networks and users allows them to "route
      around" blocking.

   Endpoint-based blocking matches well with the overall design of the

   o  Examples: Safe browsing, spam filtering, enterprise HTTPS proxies
      (explicitly trusted by clients).

   o  Endpoints block services by deciding whether or not to engage in a
      given communication.

   o  Blocking system has full visibility into all layers involved in a

   o  Adversary mobility can be quickly observed so that blocking
      systems can be updated to account for it.

   o  Requires cooperation of endpoints.

   Because it agrees so well with Internet architectural principles,
   endpoint-based blocking is the form of Internet service blocking that
   is least harmful to the Internet.  It is likely to be the most
   effective long-term technical filtering mechanism in many cases.

   While this document has focused on technical mechanisms used to
   filter Internet content, a variety of non-technical mechanisms may
   also be available depending on the particular context and goals of
   the public or private entity seeking to restrict access to content.
   For example, purveyors of illegal online content can be pursued
   through international cooperation, by using the criminal justice
   system, and by targeting the funding that supports their activities
   through collaboration with financial services companies
   [click-trajectories].  Thus even in cases where endpoint-based
   filtering is not viewed as a viable means of restricting access to
   content, entities seeking to filter may find other strategies for

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   achieving their goals that do not involve interfering with the
   architecture or operation of the Internet.

   In reality, the various approaches discussed above are all applied
   for different reasons.  Often, the choice of a filtering solution is
   constrained by practical limitations on which parts of the network
   are under the control of the entity implementing filtering, and which
   parts of the network are trusted to cooperate.  For example, an ISP
   that is subject to filtering requirements might implement an
   intermediary-based filtering approach because it cannot be sure that
   endpoints will cooperate in filtering.  As discussed above,
   government agencies tasked with disabling certain foreign web sites
   have done so by manipulating servers that are within their own
   jurisdictions, since those are the servers they can access.  An
   enterprise with filtering requirements might require install a
   certain filtering software package on enterprise-owned PCs.

   It is therefore realistic to expect that certain entities will
   continue to attempt to conduct intermediary- or server-based
   filtering since they may not have control over the endpoints they
   wish to affect or because the endpoints do not have incentives to
   consent to the filtering.  In some cases, an approach that combines
   one of these with endpoint-based filtering can help strike a better
   balance.  For example, a filtering system might make it possible for
   some endpoints to cooperate or "opt in" to filtering, rather than
   deploying a purely intermediary-based solution.

   Those with a desire to filter should take into account the
   limitations discussed in this document and wholistically assess the
   space of technical and non-technical solutions at their disposal and
   the likely effectiveness of each combination of approaches.

6.  Security Considerations

   The primary security concern related to Internet service blocking is
   the effect that it has on the end-to-end security model of many
   Internet security protocols.  When blocking is enforced by an
   intermediary with respect to a given communication, the blocking
   system may need to obtain access to confidentiality-protected data to
   make blocking decisions.  Mechanisms for obtaining such access
   typically require the blocking system to defeat the authentication
   mechanisms built into security protocols.

   For example, some enterprise firewalls will dynamically create TLS
   certificates under a trust anchor recognized by endpoints subject to
   blocking.  These certificates allow the firewall to authenticate as
   any website, so that it can act as a man-in-the-middle on TLS

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   connections passing through the firewall.

   Modifications such as these obviously make the firewall itself a
   point of weakness.  If an attacker can gain control of the firewall
   or compromise the key pair used by the firewall to sign certificates,
   he will have access to the plaintext of all TLS sessions for all
   users behind that firewall, in a way that is undetectable to users.

   When blocking systems are unable to inspect and block secure
   protocols, it is tempting to simply block those protocols.  For
   example, a web blocking system that is unable to hijack HTTPS
   connections might simply block any attempted HTTPS connection.
   However, since Internet security protocols are commonly used for
   critical services such as online commerce and banking, blocking these
   protocols would block access to these services as well, or worse,
   force them to be conducted over insecure protocols.

   Security protocols can, of course, also be used a mechanism for
   blocking services.  For example, if a blocking system can insert
   invalid credentials for one party in an authentication protocol, then
   the other end will typically terminate the connection based on the
   authentication failure.  However, it is typically much simpler to
   simply block secure protocols than to exploit those protocols for
   service blocking.

7.  Informative References

   [BT-TPB]   Meyer, D., "BT blocks The Pirate Bay", June 2012, <http://

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

              Raj Upadhaya, G., ".ht: Recovering DNS from the Quake",
              March 2010, <

              RIPE NCC, "RIPE NCC Blocks Registration in RIPE Registry
              Following Order from Dutch Police", 2012, <http://

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              Citizen Lab, "Routing Gone Wild", July 2012,

              Internet Society, "DNS: Finding Solutions to Illegal On-
              line Activities", 2012, <

   [RFC1122]  Braden, R., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122, October 1989.

   [RFC2775]  Carpenter, B., "Internet Transparency", RFC 2775,
              February 2000.

   [RFC3724]  Kempf, J., Austein, R., and IAB, "The Rise of the Middle
              and the Future of End-to-End: Reflections on the Evolution
              of the Internet Architecture", RFC 3724, March 2004.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, March 2005.

   [RFC4084]  Klensin, J., "Terminology for Describing Internet
              Connectivity", BCP 104, RFC 4084, May 2005.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.

   [RFC4924]  Aboba, B. and E. Davies, "Reflections on Internet
              Transparency", RFC 4924, July 2007.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC5782]  Levine, J., "DNS Blacklists and Whitelists", RFC 5782,
              February 2010.

   [RFC6480]  Lepinski, M. and S. Kent, "An Infrastructure to Support
              Secure Internet Routing", RFC 6480, February 2012.

              Brown, M., "Pakistan hijacks YouTube", February 2008, <htt

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              Masnick, M., "Homeland Security Seizes Spanish Domain Name
              That Had Already Been Declared Legal", 2011, <http://

              Google, "Safe Browsing API", 2012,

              "The Pirate Cloud", October 2012,

   [Telex]    Wustrow, E., Wolchok, S., Goldberg, I., and J. Halderman,
              "Telex: Anticensorship in the Network Infrastructure",
              August 2011, <>.

   [Tor]      "Tor Project: Anonymity Online", 2012,

   [US-ICE]   U.S. Immigration and Customs Enforcement, "Operation in
              Our Sites", 2011, <

              Levchenko, K., Pitsillidis, A., Chacra, N., Enright, B.,
              Felegyhazi, M., Grier, C., Halvorson, T., Kreibich, C.,
              Liu, H., McCoy, D., Weaver, N., Paxson, V., Voelker, G.,
              and S. Savage, "Click Trajectories: End-to-End Analysis of
              the Spam Value Chain", 2011,

Authors' Addresses

   Richard Barnes
   BBN Technologies
   1300 N. 17th St
   Arlington, VA  22209

   Phone: +1 703 284 1340

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   Alissa Cooper
   1634 Eye St. NW, Suite 1100
   Washington, DC  20006


   Olaf Kolkman
   NLnet Labs


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