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Versions: (draft-barnes-blocking-considerations) 00 01 02 03 04 05 06 07 08 09 RFC 7754

Internet Architecture Board                                    R. Barnes
Internet-Draft                                                   Mozilla
Intended status: Informational                                 A. Cooper
Expires: January 7, 2016                                           Cisco
                                                              O. Kolkman
                                                              NLnet Labs
                                                          D. Thaler, Ed.
                                                               Microsoft
                                                            July 6, 2015


  Technical Considerations for Internet Service Blocking and Filtering
               draft-iab-filtering-considerations-07.txt

Abstract

   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 objectionable
   communications.  Recently, there has been an increasing emphasis on
   "blocking" and "filtering," the active prevention of such
   communications.  This document examines several technical approaches
   to Internet blocking and filtering in terms of their alignment with
   the overall Internet architecture.  In general, the approach to
   blocking and 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
   objectionable communications.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 7, 2016.




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Copyright Notice

   Copyright (c) 2015 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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Filtering Examples  . . . . . . . . . . . . . . . . . . . . .   4
   3.  Characteristics of Blocking Systems . . . . . . . . . . . . .   6
     3.1.  Entities that set blocking policies . . . . . . . . . . .   6
     3.2.  Purposes of blocking  . . . . . . . . . . . . . . . . . .   7
     3.3.  Blacklist vs. Whitelist Model . . . . . . . . . . . . . .   7
     3.4.  Intended targets of blocking  . . . . . . . . . . . . . .   7
     3.5.  Components used for blocking  . . . . . . . . . . . . . .   8
   4.  Evaluation of Blocking Design Patterns  . . . . . . . . . . .  10
     4.1.  Criteria for evaluation . . . . . . . . . . . . . . . . .  10
       4.1.1.  Scope: What content or services can be blocked for
               what set of users?  . . . . . . . . . . . . . . . . .  10
       4.1.2.  Granularity: How specific is the blocking?  Will
               blocking one service also block others? . . . . . . .  10
       4.1.3.  Efficacy: How easy is it for a resource or service to
               avoid being blocked?  . . . . . . . . . . . . . . . .  11
       4.1.4.  Security: How does the blocking impact existing trust
               infrastructures?  . . . . . . . . . . . . . . . . . .  12
     4.2.  Network-Based Blocking  . . . . . . . . . . . . . . . . .  13
       4.2.1.  Scope . . . . . . . . . . . . . . . . . . . . . . . .  13
       4.2.2.  Granularity . . . . . . . . . . . . . . . . . . . . .  14
       4.2.3.  Efficacy and security . . . . . . . . . . . . . . . .  15
       4.2.4.  Summary . . . . . . . . . . . . . . . . . . . . . . .  17
     4.3.  Rendezvous-Based Blocking . . . . . . . . . . . . . . . .  17
       4.3.1.  Scope . . . . . . . . . . . . . . . . . . . . . . . .  18
       4.3.2.  Granularity . . . . . . . . . . . . . . . . . . . . .  18
       4.3.3.  Efficacy  . . . . . . . . . . . . . . . . . . . . . .  18
       4.3.4.  Security and other implications . . . . . . . . . . .  19
       4.3.5.  Examples  . . . . . . . . . . . . . . . . . . . . . .  19
       4.3.6.  Summary . . . . . . . . . . . . . . . . . . . . . . .  20
     4.4.  Endpoint-Based Blocking . . . . . . . . . . . . . . . . .  21



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       4.4.1.  Scope and granularity . . . . . . . . . . . . . . . .  21
       4.4.2.  Efficacy  . . . . . . . . . . . . . . . . . . . . . .  22
       4.4.3.  Security  . . . . . . . . . . . . . . . . . . . . . .  22
       4.4.4.  Server Endpoints  . . . . . . . . . . . . . . . . . .  22
       4.4.5.  Summary . . . . . . . . . . . . . . . . . . . . . . .  23
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  23
   6.  Conclusion  . . . . . . . . . . . . . . . . . . . . . . . . .  24
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  25
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  25
   9.  IAB Members at the Time of This Writing . . . . . . . . . . .  25
   10. Informative References  . . . . . . . . . . . . . . . . . . .  25
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  29

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 communications that were viewed as undesirable by
   certain parties.  The most famous early example of undesirable
   communications was the Morris worm [Morris], which used the Internet
   to infect many hosts in 1988.  As the Internet has evolved into a
   rich communications medium, so too have mechanisms to restrict
   communications viewed as undesirable, ranging from acceptable use
   policies enforced through informal channels to technical blocking
   mechanisms.

   Efforts to restrict or deny access to Internet resources and services
   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 or
   filter access to certain content, traffic, or services without the
   knowledge or agreement of affected users.  Where these entities do
   not directly control networks themselves, they commonly aim to make
   use of intermediary systems to effectuate the blocking or filtering.

   While blocking and filtering remain highly contentious in many cases,
   the desire to restrict communications or access to content will
   likely continue to exist.

   The difference between "blocking" and "filtering" is a matter of
   scale and perspective.  "Blocking" often refers to preventing access
   to resources in the aggregate, while "filtering" refers to preventing
   access to specific resources within an aggregate.  Both blocking and
   filtering can be effectuated at the level of "services" (web hosting



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   or video streaming, for example) or at the level of particular
   "content."  For the analysis presented in this document, the
   distinction between blocking and filtering does not create
   meaningfully different conclusions.  Hence, in the remainder of this
   document, we will treat the terms as being generally equivalent and
   applicable to restrictions on both content and services.

   This document aims to clarify the technical implications and trade-
   offs of various blocking strategies and to identify the potential for
   different strategies to potentially cause harmful side effects
   ("collateral damage") for Internet users and the overall Internet
   architecture.  This analysis is limited to technical blocking
   mechanisms.  Enforcement of blocking via contractual terms or legal
   action is out of scope.

   Filtering may be considered legal, illegal, ethical, or unethical in
   different places, at different times, and by different parties.  This
   document is intended for an audience of entities that are conducting
   filtering or are considering conducting filtering and who want to
   understand the implications of their decisions with respect to the
   Internet architecture and the trade-offs that come with each type of
   filtering strategy.  This document does not present formulas on how
   to make those trade-offs; it is likely that filtering decisions
   require knowledge of context-specific details.  Whether particular
   forms of filtering are lawful in particular jurisdictions raises
   complicated legal questions that are outside the scope of this
   document.  For similar reasons, questions about the ethics of
   particular forms of filtering are also out of scope.

2.  Filtering Examples

   Blocking systems 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 by different entities
   and for different purposes.

   Firewalls: Firewalls of various sorts are very commonly employed at
   many points in today's Internet [RFC2979].  They can be deployed
   either on end hosts (under user or administrator control), or in the
   network, typically at network boundaries.  While the Internet
   Security Glossary [RFC4949] contains an extended definition of a
   firewall, informally, most people would tend to think of a firewall
   as simply "something that blocks unwanted traffic" (see [RFC4948] for
   a discussion on many types of unwanted traffic).  While there are
   many sorts of firewalls, there are several specific types of firewall
   functionality worth noting.





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   o  Stateless Packet Filtering: Stateless packet filters block
      according to content-neutral rules, e.g., blocking all inbound
      connections or outbound connections on certain ports, protocols,
      or network-layer addresses.

   o  Stateful Packet Filtering: More advanced configurations require
      keeping state used to enforce flow-based policies, e.g., blocking
      inbound traffic for flows that have not been established, or
      blocking packets or flows based on the content carried by other
      packets.

   o  Deep Packet Inspection: Yet more advanced configurations perform
      deep packet inspection and filter or block based on the content
      carried.  Many firewalls include web filtering capabilities (see
      below).

   Web Filtering: HTTP and HTTPS are common targets for blocking and
   filtering, typically targeted at specific URIs.  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 content deemed illegal.  HTTPS is a challenge for these
   systems, because the URI in an HTTPS request is carried inside the
   encrypted channel.  To block access to content made accessible via
   HTTPS, filtering systems thus must either block based on network- and
   transport-layer headers (IP address and/or port), 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
   administrator installs a security exception.  (See further discussion
   in Section 5.)

   Spam Filtering: Spam filtering is one of the oldest forms of content
   filtering.  Spam filters evaluate messages based on a variety of
   criteria and information sources to decide whether a given message is
   spam.  For example, DNS 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.

   Domain Name Seizure: In recent years, U.S. 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 alleged 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.  The effectiveness of domain seizures is limited



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   by application mobility -- applications using the seized name can
   switch to using another name.  Seizures can also have overbroad
   effects, since access to 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 end-to-end assumptions
   in the DNS security architecture [RFC4033], potentially causing
   validation failures by validating end-nodes.

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

   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](also
   see Section 4.3).  These actions have led to concerns that the number
   resource certification system and related secure routing technologies
   developed by the IETF's SIDR working group might be subject to
   government manipulation as well [RFC6480], potentially for the
   purpose of denying targeted networks access to the Internet.

3.  Characteristics of Blocking Systems

   At a generic level, blocking systems can be characterized by four
   attributes: the entity that sets the blocking policy, the purpose of
   the blocking, the intended target of the blocking, and the Internet
   component(s) used as the basis of the blocking system.

3.1.  Entities that set blocking policies

   Parties that institute blocking policies include governments, courts,
   enterprises, network operators, application providers, and individual
   end users.  In some cases, these parties use their own technical
   assets to conduct blocking; for example, a network operator might
   install a firewall in its own networking equipment, or a web
   application provider might block responses between its web server and
   certain clients.  In other cases, particularly in the case of
   blocking initiated by governments, the entity that institutes the



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   blocking policy works with other entities to effectuate blocking
   using technical assets that it does not control.

3.2.  Purposes of blocking

   Entities may be motivated to filter for a variety of purposes:

   o  Preventing or responding to security threats.  Network operators,
      enterprises, application providers, and end users often block
      communications that are believed to be associated with security
      threats or network attacks.

   o  Restricting objectionable content or services.  Certain
      communications may be viewed as undesirable, harmful, or illegal
      by particular governments, enterprises, or users (e.g., parents).
      Governments may seek to block communications that are deemed to be
      defamation, hate speech, obscenity, intellectual property
      infringement, or otherwise objectionable.  Enterprises may seek to
      restrict employees from accessing content that is not deemed to be
      work appropriate.  Parents may restrict their children from
      accessing content or services targeted for adults.

   o  Restricting access based on business arrangements.  Some networks
      are designed so as to only provide access to certain content or
      services ("walled gardens"), or to only provide limited access
      until end users pay for full Internet services (captive portals
      provided by hotspot operators, for example).

3.3.  Blacklist vs. Whitelist Model

   Note that the purpose for which blocking occurs often dictates
   whether the blocking system operates on a blacklist model, where
   communications are allowed by default but a subset are blocked, or a
   whitelist model, where communications are blocked by default with
   only a subset allowed.  Captive portals, walled gardens, and
   sandboxes used for security or network endpoint assessment usually
   require a whitelist model since the scope of communications allowed
   is narrow.  Blocking for other purposes often uses a blacklist model
   since only individual content or traffic is intended to be blocked.

3.4.  Intended targets of blocking

   Entities institute blocking systems so as to target particular
   content, services, endpoints, or some combination of these.  For
   example, a "content" filtering system used by an enterprise might
   block access to specific URIs whose content is deemed by the
   enterprise to be inappropriate for the work place.  This is distinct
   from a "service" filtering system that blocks all web traffic



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   (perhaps as part of a parental control system on an end user device),
   and also distinct from an "endpoint" filtering system in which a web
   application blocks traffic from specific endpoints that are suspected
   of malicious activity.

   As discussed in Section 4, the design of a blocking system may affect
   content, services, or endpoints other than those that are the
   intended targets.  For example, the domain name seizures described
   above target particular web pages associated with illegal activity,
   but by removing the domains from use, they affect all services made
   available by the hosts associated with those names, including mail
   services and web services unrelated to the illegal activity.

3.5.  Components used for blocking

   Broadly speaking, the process of delivering an Internet service
   involves three different components:

   1.  Endpoints: The actual content of the service is typically an
       application layer protocol between two or more Internet hosts.
       In many protocols, there are two endpoints, a client and a
       server.

   2.  Network services: The endpoints communicate by way of a
       collection of IP networks that use routing protocols to determine
       how to deliver packets between the endpoints.

   3.  Rendezvous services: Service endpoints are typically identified
       by identifiers than are more "human-friendly" than IP addresses.
       Rendezvous services allow one endpoint to figure out how to
       contact another endpoint based on an identifier.

   Consider, for example, an HTTP transaction fetching the content of
   the URI <http://example.com/index.html>.  The client endpoint is an
   end host running a browser.  The client uses the DNS as a rendezvous
   service when it performs a AAAA query to obtain the IP address for
   the server name "example.com".  The client then establishes a
   connection to the server, and sends the actual HTTP request.  The
   server then responds to the HTTP request.

   As another example, in the SIP protocol, the client and server are IP
   phones, and the rendezvous service is provided by an application-
   layer SIP proxy as well as the DNS.

   Blocking access to Internet content, services, or endpoints is done
   by controlling one or more of the components involved in the
   provision of the communications involved in accessing the content,
   services or endpoints.  In the HTTP example above, the successful



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   completion of the HTTP request could have been prevented in several
   ways:

   o  [Endpoint] Preventing the client from making the request

   o  [Endpoint] Preventing the server from responding to the request

   o  [Endpoint] Preventing the client from making a DNS request for
      example.com

   o  [Network] Preventing the request from reaching the server

   o  [Network] Preventing the response from reaching the client

   o  [Network] Preventing the client from reaching the DNS server

   o  [Network] Preventing the DNS response from reaching the client

   o  [Rendezvous] Preventing the DNS server from providing the client
      the correct IP address of the server

   Most entities that desire to block communications will have access to
   only one or two components, and therefore their choices for how to
   effectuate blocking will be limited.  End users and application
   providers can usually only control their own software and hardware,
   which means that they are limited to endpoint-based filtering.  Some
   network operators offer filtering services that their customers can
   activate individually, in which case end users might have network-
   based filtering systems available to them.  Network operators can
   control their own networks and the rendezvous services for which they
   provide infrastructure support (e.g., DNS resolvers) or to which they
   may have access (e.g., SIP proxies), but not usually endpoints.
   Enterprises usually have access to their own networks and endpoints
   for filtering purposes.  Governments might make arrangements with the
   operators or owners of any of the three components that exist within
   their jurisdictions to effectuate filtering.

   In the next section, blocking systems designed according to each of
   the three patterns -- network services, rendezvous services, and
   endpoints -- are evaluated for their technical and architectural
   implications.  The analysis is as agnostic as possible as to which
   kind of entity sets the blocking policy (government, end user,
   network operator, application provider, or enterprise), but in some
   cases the way in which a particular blocking design pattern is used
   might differ depending on the entity that desires to block.  For
   example, a network-based firewall provided by an ISP that parents can
   elect to use for parental control purposes will likely function
   differently from one that all ISPs in a particular jurisdiction are



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   required to use by the local government, even though in both cases
   the same component (network) forms the basis of the blocking system.

4.  Evaluation of Blocking Design Patterns

4.1.  Criteria for evaluation

   To evaluate the technical implications of each of the blocking design
   patterns, we compare them based on four criteria: scope, granularity,
   efficacy, and security.

4.1.1.  Scope: What content or services can be blocked for what set of
        users?

   The Internet is comprised of many distinct autonomous networks and
   applications, which means that the impact of a blocking system will
   only be within a defined scope.  For example, blocking within 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.  Blocking effectuated by an
   application provider can affect users across the entire Internet, but
   only for that specific application.  Thus the scope of the impact
   might be narrow in one dimension (set of users or set of applications
   affected) but broad in another.  In some cases, applications and
   rendezvous services are so intertwined with each other that filtering
   a single service or in a single network location can have broad
   effects in multiple directions.  Blocking systems are generally
   viewed as less objectionable if the scope of their impact is as
   narrow as possible while still being effective.

4.1.2.  Granularity: How specific is the blocking?  Will blocking one
        service also block others?

   Internet applications are built out of a collection of loosely-
   coupled components or "layers."  Different layers serve different
   purposes, and rely on or offer different functions 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", IP routing is performed by an "Internet service
   provider," and application-layer services are provided by completely
   separate entities (e.g., web servers).  Upper-layer protocols and
   applications rely on combinations of lower-layer functions in order
   to work.  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.




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   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.  As with the scope
   criteria, blocking systems are generally viewed as less objectionable
   when they are highly granular and do not cause collateral damage to
   content or services unrelated to the target of the blocking
   [RFC4924].

   Even within the application layer, the granularity of blocking can
   vary depending on how targeted the blocking system is designed to be.
   Blocking all traffic associated with a particular application
   protocol is less granular than blocking only traffic associated with
   a subset of application instances that make use of that protocol.
   Sophisticated heuristics that make use of information about the
   application protocol, lower-layer protocols, payload signatures,
   source and destination addresses, inter-packet timing, packet sizes,
   and other characteristics are sometimes used to narrow the subset of
   traffic to be blocked.

   Design flaws in blocking systems may also cause the effects of
   blocking to be overbroad.  For example, web filtering systems in
   India and China have been shown to cause "collateral damage" by
   unwittingly blocking users in Oman and the U.S. from accessing web
   sites in Germany and Korea
   [IN-OM-filtering][CCS-GFC-collateral-damage].

4.1.3.  Efficacy: How easy is it for a resource or service to avoid
        being blocked?

   Although blocking a resource or service might have some immediate
   effect, efficacy must be evaluated in terms of whether it is easy to
   circumvent.  Simply doing a one-time policy is often unlikely to have
   lasting efficacy (e.g., see [CleanFeed]).

   For blacklist-style blocking, the distributed and mobile nature of
   Internet resources limits the effectiveness of blocking actions.  A
   service that is blocked in one jurisdiction can often be moved or re-
   instantiated in another jurisdiction (see, for example,
   [Malicious-Resolution]).  Likewise, services that rely on blocked
   resources can often be rapidly re-configured to use non-blocked
   resources.  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, or simply uses alternate syntaxes to express the
   same resource name such as with internationalized names (see the
   discussion of false negatives in [RFC6943]).




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   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 jurisdictions to prevent spam blocking from
   being effective.

   In the presence of either blacklist or whitelist systems, users may
   choose to use different sets of protocols or otherwise alter their
   traffic characteristics to circumvent filters.  As discussed in
   [I-D.blanchet-iab-internetoverport443], many applications shift their
   traffic to port 80 or 443 when other ports are blocked.  This sort of
   circumvention based on shifting ports can succeed because port
   selection is designed to only be meaningful to endpoints, not to the
   network.  If voice communication based on SIP [RFC3261] is blocked,
   users are likely to use proprietary protocols that allow them to talk
   to each other.  Some filtering systems are only capable of
   identifying IPv4 traffic and therefore by shifting to IPv6 users may
   be able to evade filtering.  Using IPv6 with header options, using
   multiple layers of tunnels, or using encrypted tunnels can also make
   it more challenging for blocking systems to find transport ports
   within packets, making port-based blocking more difficult.  Thus
   distribution and mobility can hamper efforts to block communications
   in a number of ways.

4.1.4.  Security: How does the blocking impact existing trust
        infrastructures?

   Modern security mechanisms rely on trusted hosts communicating via a
   secure channel without intermediary interference.  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(s), 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 securely communicated to the bank and nobody
   else, ensuring the data remains confidential while in transit.

   Some blocking strategies require intermediaries to insert themselves
   within the end-to-end communications path, potentially breaking
   security properties of Internet protocols [RFC4924].  In these cases
   it can be difficult or impossible for endpoints to distinguish
   between attackers and "authorized" entities conducting blocking.

   Finally, one needs to evaluate whether a blocking mechanism can be
   used by an end-user to efficiently locate blocked resources that can
   then be accessed via other mechanisms that circumvent the blocking
   mechanism.  For example, Clayton [CleanFeed] showed how special
   treatment in one blocking system could be detected by end users in



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   order to efficiently locate illegal websites, which was thus counter-
   productive to the policy objective of the blocking mechanism.

4.2.  Network-Based Blocking

   Being able to block access to resources without the consent or
   cooperation of either endpoint to a communication is viewed as a
   desirable feature by some entities that deploy blocking systems.
   Systems that have this property are often implemented using
   intermediary devices in the network, such as firewalls or filtering
   systems.  These systems inspect traffic as it passes through the
   network, decide based on the characteristics or content of a given
   communication whether it should be blocked, and then block or allow
   the communication as desired.  For example, web filtering devices
   usually inspect HTTP requests to determine the URI being requested,
   compare that URI to a list of blacklisted or whitelisted URIs, 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.  Some blocking systems focus
   on specific application-layer traffic, while others, such as router
   ACLs, filter traffic based on lower layer criteria (transport
   protocol and source or destination addresses or ports).

   Intermediary systems used for blocking 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 endpoints.  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 share some of the properties of the "Endpoint-
   Based Blocking" scenarios discussed in Section 4.4 below, since the
   endpoint has made an informed decision to authorize the intermediary
   to block on its behalf and is therefore unlikely to attempt to
   circumvent the blocking.  From an architectural perspective, however,
   they may create many of the same problems as network-based filtering
   conducted without consent.

4.2.1.  Scope

   In the case of government-initiated blocking, a network exists within
   specific jurisdictions, and the operators are thus subject to
   jurisdictional laws.  It is thus possible for laws to be structured
   to effectuate blocking by imposing obligations on the operators of
   networks within a jurisdiction, either via direct government action
   or by allowing private actors to demand blocking (e.g., through
   lawsuits).



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   Regardless of who is responsible for a blocking policy, enforcement
   can be done using Stateless Packet Filtering, Stateful Packet
   Filtering, or Deep Packet Inspection as defined in Section 2.  While
   network-based Stateless Packet Filtering has granularity issues
   discussed in Section 4.2.2, network-based Stateful Packet Filtering
   and Deep Packet Inspection approaches often run into several
   technical issues that limit their viability in practice.  For
   example, many issues arise from the fact that an intermediary needs
   to have access to a sufficient amount of traffic to make its blocking
   determinations.

   For residential or consumer networks with many egress points, the
   first step to obtaining this traffic is simply gaining access to the
   constituent packets.  The Internet is designed to deliver packets
   hop-by-hop 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 [asymmetry].

   This asymmetry means that an intermediary in a network with many
   egress points may, depending on topology and configuration, see only
   one half of a given communication (if it sees any of it at all),
   which may limit the scope of the communications that it can filter.
   For example, a filter aimed at requests destined for particular URIs
   cannot make accurate blocking decisions if it is only in the data
   path for HTTP responses and not requests.  Asymmetry may be
   surmountable given a filtering system with enough distributed,
   interconnected filtering nodes that can coordinate information about
   flows belonging to the same communication or transaction, but
   depending on the size of the network this may imply significant
   complexity in the filtering system.  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 can sometimes result
   in reduced network performance relative to asymmetric routing.
   Enterprise networks may also be less susceptible to these problems if
   they route all traffic through a small number of egress points.

4.2.2.  Granularity

   Once an intermediary in a network 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 (URIs or
   other content).  Deep Packet Inspection type blocking based on
   application-layer attributes can be potentially more granular and
   less likely to cause collateral damage than blocking all traffic
   associated with a particular address, which can impact unrelated



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   occupants of the same address.  However, more narrowly focused
   targeting may be more complex, less efficient, or easier to
   circumvent than filtering that sweeps more broadly, and entities that
   seek to block may balance these attributes against each other when
   choosing a blocking system.

4.2.3.  Efficacy and security

   Regardless of the layer at which blocking occurs, it may be open to
   circumvention, particularly in cases where network endpoints have not
   authorized the blocking.  The communicating endpoints can deny the
   intermediary access to attributes at any layer by using encryption
   (see below).  IP addresses must be visible, even if packets are
   protected with IPsec, but blocking based on IP addresses can be
   trivial to circumvent.  A filtered site may be able to quickly change
   its IP address using only a few simple steps: changing a single DNS
   record and provisioning the new address on its server or moving its
   services to the new 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].

   Indeed, Poort, et al.  [Poort] found that "any behavioural change in
   response to blocking access to TPB has had no lasting net impact on
   the overall number of downloaders from illegal sources, as new
   consumers have started downloading from illegal sources and people
   learn to circumvent the blocking while new illegal sources may be
   launched, causing file sharing to increase again", and that these
   results "are in line with a tendency found in the literature that any
   effects of legal action against file sharing often fade out after a
   period of typically six months."

   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
   communication.  Besides, various techniques that use public databases
   with whitelisted keys (e.g., DANE [RFC6698]) enable users to detect
   these sort of intermediaries.  Those users are then likely to act as
   if the service is blocked.

   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, protocol IDs and port



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   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.

   However, as the Telex project recently demonstrated, if an endpoint
   cooperates with a relay in the network (e.g., a Telex station), it
   can create a TLS tunnel that is indistinguishable from legitimate
   traffic [Telex].  For example, if an ISP used by a banking website
   were to operate a Telex station at one of its routers, then a
   blocking system would be unable to distinguish legitimate encrypted
   banking traffic from Telex-tunneled traffic (potentially carrying
   content that would have been filtered).

   Thus, in principle in a blacklist system 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 prevent.

   Some 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 network-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 tunneling 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,
   network-based blocking becomes less effective over time.

   Network-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 unnecessary layers of complexity in the Internet.
   Before content-based blocking became common, the next best option for
   network operators was port blocking, the widespread use of which has
   driven more applications and services to use ports (80 and 443 most
   commonly) that are unlikely to be blocked.  In turn, network



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   operators shifted to finer-grained content blocking over port 80,
   content providers shifted to encrypted channels, and operators began
   seeking to identify those channels (although doing so can be
   resource-prohibitive, especially if tunnel endpoints begin to change
   frequently).  Because the premise of network-based blocking is that
   endpoints have incentives to circumvent it, this cat-and-mouse game
   is an inevitable by-product of this form of blocking.

4.2.4.  Summary

   In sum, network-based blocking is only effective in a fairly
   constrained set of circumstances.  First, the traffic needs to flow
   through the network in such a way that the intermediary device 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).  If the
   network is sufficiently complex, or the risk of tunneling too high,
   then network-based blocking is unlikely to be effective, and in any
   case this type of blocking drives the development of increasingly
   complex layers of circumvention.  Network-based blocking can be done
   without the cooperation of either endpoint to a communication, but it
   has the serious drawback of breaking end-to-end security assurances
   in some cases.  The fact that network-based blocking is premised on
   this lack of cooperation results in arms races that increase the
   complexity of both application design and network design.

4.3.  Rendezvous-Based Blocking

   Internet applications often require or rely on support from common,
   global rendezvous services, including the DNS, certificate
   authorities, WHOIS databases, and Internet Route Registries.  These
   services control or register the structure and availability of
   Internet applications by providing data elements that are used by
   application code.  Some applications also have their own specialized
   rendezvous services.  For example, to establish an end-to-end SIP
   call the end-nodes (terminals) will rely on presence and session
   information supplied by SIP servers.

   Global rendezvous services are comprised of 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




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   shutting down a web server (which fulfills the specific purpose of
   serving web content).

   Although rendezvous services are discussed as a single category, the
   precise characteristics and implications of blocking each kind of
   rendezvous service are slightly different.  This section provides
   examples to highlight these differences.

4.3.1.  Scope

   In the case of government-initiated blocking, the servers that are
   used to provide rendezvous services exist within specific
   jurisdictions, and their operators are thus subject to jurisdictional
   laws.  It is thus possible for laws to be structured to effectuate
   blocking by imposing obligations on the operators of rendezvous
   services within a jurisdiction, either via direct government action
   or by allowing private actors to demand blocking (e.g., through
   lawsuits).

   The scope of blocking conducted by other entities will depend on
   which servers those entities can access.  For example, network
   operators and enterprises may be capable of conducting blocking using
   their own DNS resolvers or application proxies within their networks,
   but not authoritative servers controlled by others.

4.3.2.  Granularity

   Blocking based on global rendezvous services tends to be overbroad
   because the resources blocked often support multiple services.  This
   can cause collateral damage to legitimate uses of a resource.  For
   example, a given address 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.

4.3.3.  Efficacy

   The distributed nature of the Internet limits the efficacy of
   blocking based on rendezvous services.  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 that the
   rendezvous service provides to avoid the blocking (although the
   development of DNSSEC and the RPKI are causing this to change by
   requiring updates to be authorized).  In the DNS case, registrants
   whose names get blocked can relocate their resources to different
   names.



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   Endpoints can also choose not to use a particular rendezvous service.
   They might switch to a competitor or use an alternate mechanism (for
   example, IP literals in URIs to circumvent DNS filtering).

4.3.4.  Security and other implications

   Blocking of global rendezvous services also has a variety of other
   implications that may reduce the stability, accessibility, and
   usability of the global Internet.  Infrastructure-based blocking may
   erode the trust in the general Internet and encourage the development
   of parallel or "underground" infrastructures causing forms of
   Internet balkanisation, for example.  This risk may become more acute
   as the introduction of security infrastructures and mechanisms such
   as DNSSEC and RPKI "hardens" the authoritative data -- including
   blocked names or routes -- that the existing infrastructure services
   provide.  Those seeking to circumvent the blocks may opt to use less-
   secure but unblocked parallel services.  As applied to the DNS, these
   considerations are further discussed in the advisory [SAC-056] from
   ICANN's Security and Stability Advisory Committee (SSAC), and in
   ISOC's whitepaper on DNS filtering [ISOCFiltering], but they also
   apply to other global Internet resources.

4.3.5.  Examples

   Below we provide a few specific examples for routing, DNS, and WHOIS
   services.  These examples demonstrate that for these types of
   rendezvous services (services that are often considered a global
   commons), jusrisdiction-specific legal and ethical motivations for
   blocking can both have collateral effects in other jurisdictions and
   be circumvented because of the distributed nature of the Internet.

   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 as a result of a route leak beyond the Pakistan ISP's
   scope, but service was restored in approximately two hours because
   network operators around the world re-configured their routers to
   ignore the bogus routes [RenesysPK].  In the context of SIDR and
   secure routing, a similar re-configuration could theoretically be
   done if a resource certificate were to be revoked in order to block
   routing to a given network.

   In the DNS realm, one of the recent cases of U.S. law enforcement
   seizing domain names involved RojaDirecta, a Spanish web site.  Even
   though several of the affected domain names belonged to Spanish
   entities, they were subject to blocking by the U.S. government
   because certain servers were operated in the U.S.  Government
   officials required the operators of the parent zones of a target name



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   (e.g., "com" for "example.com") to direct queries for that name to a
   set of U.S.-government-operated name servers.  Users of other
   services under a target name (e.g., e-mail) would thus be unable to
   locate the servers providing services for that name, denying them the
   ability to access these services.

   Similar workarounds as those that were used in the Pakistan Telecom
   case are also available in the DNS case.  If a domain name is 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.  However, depending on
   availability of valid signature data, these types of workarounds will
   not work with DNSSEC-signed data.

   The action of the Dutch authorities against the RIPE NCC, where RIPE
   was ordered to freeze the accounts Internet resource holders, is of a
   similar character.  By controlling the account holders' WHOIS
   information, this type of action limited the ability of the ISPs in
   question to manage their Internet resources.  This example is
   slightly different from the others because it does not immediately
   impact the ability of ISPs to provide connectivity.  While ISPs use
   (and trust) the WHOIS databases to build route filters or use the
   databases for trouble-shooting information, the use of the WHOIS
   databases for those purposes is voluntary.  Thus, seizure of this
   sort may not have any immediate effect on network connectivity, but
   it may impact overall trust in the common infrastructure.  It is
   similar to the other examples in that action in one jurisdiction can
   have broader effects, and in that the global system may encourage
   networks to develop their own autonomous solutions.

4.3.6.  Summary

   In summary, rendezvous-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 can have harmful side
   effects due to the global nature of Internet resources and the fact
   that many different application-layer services rely on generic,
   global databases for rendezvous purposes.  The fact that Internet
   resources can quickly shift between network locations, names, and
   addresses, together with the autonomy of the networks that comprise
   the Internet, can mean that the effects of rendezvous-based blocking
   can be negated on short order in some cases.  For some applications,
   rendezvous services are optional to use, not mandatory.  Hence they
   are only effective when the endpoint or the endpoint's network
   chooses to use them; they can be routed around by choosing not to use
   the rendezvous service or migrating to an alternative one.  To adapt




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   a quote by John Gilmore, "The Internet treats blocking as damage and
   routes around it".

4.4.  Endpoint-Based Blocking

   Internet users and their devices constantly make decisions 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
   types of filtering in the Internet; modern filtering 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 filtering systems (browsers, MTAs) share a single
   reputation service.  Even the absence of provisioned PTR records for
   an IP address may result in email messages not being accepted.

4.4.1.  Scope and granularity

   Endpoint-based blocking lacks some of the limitations of rendezvous-
   based blocking: while rendezvous-based blocking can only see and
   affect the rendezvous service at hand (e.g., DNS name resolution),
   endpoint-based blocking can see into the entire application, across
   all layers and transactions.  This visibility can provide endpoint-
   based blocking systems with a much richer set of information for
   making narrow blocking decisions.  Support for narrow granularity
   depends on how the application protocol client and server are
   designed, however.  A typical endpoint-based firewall application may
   have less ability to make fine-grained decisions than an application
   that does its own blocking (see [RFC7288] for further discussion).

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






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4.4.2.  Efficacy

   Endpoint-based blocking deals well with mobile adversaries.  If a
   blocked service relocates resources or uses different resources, a
   rendezvous- or network-based blocking approach may not be able to
   affect the new resources (at least not immediately).  A network-based
   blocking system may not even be able to tell whether the new
   resources are being used, if the previously 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 through a reputation
   system.

   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.
   That challenge is exacerbated when the endpoints are a diverse set of
   static, mobile or visiting 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.

4.4.3.  Security

   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 network-based blocking runs into and the
   challenges to global trust infrastructures that rendezvous-based
   blocking creates.

4.4.4.  Server Endpoints

   In this discussion of endpoint-based blocking, the focus has been on
   the consuming side of the end-to-end communication, mostly the client
   side of a client-server type connection.  However, similar
   considerations apply to the content-producing side of end-to-end
   communications, regardless of whether that endpoint is a server in a
   client-server connection or a peer in a peer-to-peer type of
   connection.

   For instance, for blocking of web content, narrow targeting can be
   achieved through whitelisting methods like password authentication
   whereby passwords are available only to authorized clients.  For
   example, a web site might only make adult content available to users




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   who provide credit card information, which is assumed to be a proxy
   for age.

   The fact that content-producing endpoints do not take it upon
   themselves to block particular forms of content in response to
   requests from governments or other parties can sometimes motivate
   those latter parties to engage in blocking elsewhere within the
   Internet.

4.4.5.  Summary

   Out of the three design patterns, endpoint-based blocking is the
   least likely to cause collateral damage to Internet services or the
   overall Internet architecture.  Endpoint-based blocking systems can
   see into all layers involved in a communication, allowing blocking to
   be narrowly targeted.  Adversary mobility can be accounted for as
   soon as reputation systems are updated with new adversary
   information.  One potential drawback of endpoint-based blocking is
   that it requires the endpoint's cooperation; effectuating blocking at
   an endpoint when it is not in the endpoint's interest is therefore
   difficult to accomplish because the endpoint's user can disable the
   blocking or switch to a different endpoint.

5.  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 often
   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
   connections passing through the firewall.  This is not unlike an
   external attacker using compromised certificates to intercept TLS
   connections.

   Modifications such as these obviously make the firewall itself an
   attack surface.  If an attacker can gain control of the firewall or
   compromise the key pair used by the firewall to sign certificates,
   the attacker will have access to the unencrypted data of all current
   and recorded TLS sessions for all users behind that firewall, in a
   way that is undetectable to users.  Besides, if the compromised key-



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   pairs can be extracted from the firewall, all users, not only those
   behind the firewall, that rely on that public key are vulnerable.

   When blocking systems are unable to inspect and surgically block
   secure protocols, it is tempting to completely block those protocols.
   For example, a web blocking system that is unable to inspect 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 communication.

   Security protocols can, of course, also be used as mechanisms 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.

6.  Conclusion

   Filtering will continue to occur on the Internet.  We conclude that,
   whenever possible, filtering should be done on the end point.
   Cooperative end points are most likely to have sufficient contextual
   knowledge to effectively target blocking.  It is realistic to expect
   that at times filtering will not be done on the end points.  In these
   cases, promptly informing the endpoint that blocking has occurred
   provides necessary transparency to redress any errors, particularly
   as they relate to any collatoral damage introduced by errant filters.

   Black list approaches are often a game of "Cat and Mouse", where
   those with the content move it around to avoid blocking.  At the same
   time, white lists provide similar risks because sites that had
   "acceptable" content may become targets for "unacceptable content",
   and similarly, access to perfectly inoffensive and perhaps useful or
   productive content is unnecessarily blocked.

   From a technical perspective, there are no perfect or even good
   solutions.  There is only least bad.  On that front, we posit that a
   hybrid approach that combines endpoint-based filtering with network
   filtering may prove least damaging.  An endpoint may choose to
   participate in a filtering regime in exchange for the network
   providing broader unfiltered access.

   Finally, we note that where filtering is occurring to address content
   that is generally agreed to be inappropriate or illegal, strong
   cooperation among service providers and governments may provide



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   additional means to identify both the victims and the perpetrators
   through non-filtering mechanisms, such as partnerships with the
   finance industry to identify and limit illegal transactions.

7.  IANA Considerations

   This document requires no actions by the IANA.

8.  Acknowledgments

   Thanks to the many reviewers who provided helpful comments,
   especially Bill Herrin, Eliot Lear, Patrik Falstrom, Pekka Savola,
   and Russ White.

9.  IAB Members at the Time of This Writing

   Jari Arkko
   Mary Barnes
   Marc Blanchet
   Ralph Droms
   Ted Hardie
   Joe Hildebrand
   Russ Housley
   Erik Nordmark
   Robert Sparks
   Andrew Sullivan
   Dave Thaler
   Brian Trammell
   Suzanne Woolf

10.  Informative References

   [asymmetry]
              John, W., Dusi, M., and K. Claffy, "Estimating routing
              symmetry on single links by passive flow measurements",
              2010, <http://dl.acm.org/citation.cfm?id=1815506>.

   [BT-TPB]   Meyer, D., "BT blocks The Pirate Bay", June 2012,
              <http://www.zdnet.com/
              bt-blocks-the-pirate-bay-4010026434/>.

   [CCS-GFC-collateral-damage]
              "The Collateral Damage of Internet Censorship by DNS
              Injection", July 2012,
              <http://conferences.sigcomm.org/sigcomm/2012/paper/
              ccr-paper266.pdf>.





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   [CleanFeed]
              Clayton, R., "Failures in a Hybrid Content Blocking
              System", Fifth Privacy Enhancing Technologies Workshop,
              PET 2005, 2005,
              <http://www.cl.cam.ac.uk/~rnc1/cleanfeed.pdf>.

   [click-trajectories]
              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,
              <http://cseweb.ucsd.edu/~savage/papers/Oakland11.pdf>.

   [EarthquakeHT]
              Raj Upadhaya, G., ".ht: Recovering DNS from the Quake",
              March 2010,
              <http://www.apricot.net/apricot2010/__data/assets/
              pdf_file/0019/19018/Lightning-Talk_03_Gaurab-Upadhaya-
              dotht-apricot-lightning.pdf>.

   [GhostClickRIPE]
              RIPE NCC, "RIPE NCC Blocks Registration in RIPE Registry
              Following Order from Dutch Police", 2012,
              <http://www.ripe.net/internet-coordination/news/
              about-ripe-ncc-and-ripe/ripe-ncc-blocks-registration-in-
              ripe-registry-following-order-from-dutch-police>.

   [I-D.blanchet-iab-internetoverport443]
              Blanchet, M., "Implications of Blocking Outgoing Ports
              Except Ports 80 and 443", draft-blanchet-iab-
              internetoverport443-02 (work in progress), July 2013.

   [IN-OM-filtering]
              Citizen Lab, , "Routing Gone Wild", July 2012,
              <https://citizenlab.org/2012/07/routing-gone-wild/>.

   [ISOCFiltering]
              Internet Society, "DNS: Finding Solutions to Illegal On-
              line Activities", 2012, <http://www.internetsociety.org/
              what-we-do/issues/dns/
              finding-solutions-illegal-line-activities>.









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   [Malicious-Resolution]
              Dagon, D., Provos, N., Lee, C., and W. Lee, "Corrupted DNS
              Resolution Paths: The Rise of a Malicious Resolution
              Authority", 2008,
              <http://www.citi.umich.edu/u/provos/papers/
              ndss08_dns.pdf>.

   [Morris]   Kehoe, B., "The Robert Morris Internet Worm", 1992,
              <http://groups.csail.mit.edu/mac/classes/6.805/articles/
              morris-worm.html>.

   [Poort]    Poort, J., Leenheer, J., van der Ham, J., and C. Dumitru,
              "Baywatch: Two approaches to measure the effects of
              blocking access to The Pirate Bay", Telecommunications
              Policy 38:383-392, 2014,
              <http://staff.science.uva.nl/~vdham/research/
              publications/1401-Baywatch.pdf>.

   [RenesysPK]
              Brown, M., "Pakistan hijacks YouTube", February 2008,
              <http://www.renesys.com/blog/2008/02/
              pakistan_hijacks_youtube_1.shtml>.

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

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

   [RFC2979]  Freed, N., "Behavior of and Requirements for Internet
              Firewalls", RFC 2979, October 2000.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

   [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.




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   [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.

   [RFC4948]  Andersson, L., Davies, E., and L. Zhang, "Report from the
              IAB workshop on Unwanted Traffic March 9-10, 2006", RFC
              4948, August 2007.

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2", RFC
              4949, August 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.

   [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS)
              Protocol: TLSA", RFC 6698, August 2012.

   [RFC6943]  Thaler, D., "Issues in Identifier Comparison for Security
              Purposes", RFC 6943, May 2013.

   [RFC7288]  Thaler, D., "Reflections on Host Firewalls", RFC 7288,
              June 2014.

   [RojaDirecta]
              Masnick, M., "Homeland Security Seizes Spanish Domain Name
              That Had Already Been Declared Legal", 2011,
              <http://www.techdirt.com/articles/20110201/10252412910/
              homeland-security-seizes-spanish-domain-name-that-had-
              already-been-declared-legal.shtml>.

   [SAC-056]  "SSAC Advisory on Impacts of Content Blocking via the
              Domain Name System", October 2012,
              <http://www.icann.org/en/groups/ssac/documents/
              sac-056-en.pdf>.

   [SafeBrowsing]
              Google, "Safe Browsing API", 2012,
              <https://developers.google.com/safe-browsing/>.




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

   [Tor]      "Tor Project: Anonymity Online", 2012,
              <https://www.torproject.org/>.

   [TPB-cloud]
              "The Pirate Cloud", October 2012,
              <http://thepiratebay.se/blog/224>.

   [US-ICE]   U.S. Immigration and Customs Enforcement, "Operation in
              Our Sites", 2011,
              <http://www.ice.gov/doclib/news/library/factsheets/pdf/
              operation-in-our-sites.pdf>.

Authors' Addresses

   Richard Barnes
   Mozilla
   Suite 300
   650 Castro Street
   Mountain View, CA  94041
   US

   Email: rlb@ipv.sx


   Alissa Cooper
   Cisco
   707 Tasman Drive
   Milpitas, CA  95035
   USA

   Email: alcoop@cisco.com


   Olaf Kolkman
   NLnet Labs
   Science Park 400
   Amsterdam  1098 XH
   Netherlands

   Email: olaf@nlnetlabs.nl







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   Dave Thaler (editor)
   Microsoft
   One Microsoft Way
   Redmond, WA  98052
   US

   Email: dthaler@microsoft.com












































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