Technical Considerations for Internet Service Blocking and Filtering

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 too have mechanisms to restrict undesirable communications. Efforts to restrict or deny access to Internet resources have evolved over time. As noted in , 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 communications 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. Entities may seek to block or filter Internet content for a diversity of reasons, including defending against security threats, restricting access to content thought to be objectionable, and preventing nefarious or illegal activity. While blocking and filtering remain highly contentious in many cases, the desire to restrict 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 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. 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 potentially cause harmful side effects ("collateral damage"). Blocking is principally taken up (whether voluntarily or not) by three types of entities: Operators of networks (including enterprises) Operators of infrastructure services (for naming, routing, and other core Internet functions) Operators of endpoints (including end users and application providers) Examples of blocking or attempted blocking using the DNS, HTTP proxies, spam filters, and RPKI manipulation are used to illustrate each category's properties. 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 filtering or are considering 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. In , ICANN's Security and Stability Advisory Committee (SSAC) assessed the aspects of blocking using the DNS. This document attempts to take a broader perspective on blocking and filtering and genaralizes from some of SSAC's findings.

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.

The end-to-end principle is "the core architectural guideline of the Internet" . 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 . 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 an array of 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 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 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 (network-based web filtering systems, for example), potentially breaking security properties of Internet protocols. In these cases it can be difficult or impossible for endpoints to distinguish between attackers and "authorized" entities conducting blocking.

"Different Players on Different Layers." 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 , 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. 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.

The Internet is designed as a distributed system both geographically and topologically. Resources can be made globally accessible regardless of their physical location or connectivity providers used. Resources are also commonly 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 and for alternative paths to reach a given destination to be rapidly computed. Application services are commonly protected using distributed servers. 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 (see, for example, ). 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. 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. Alternatively, users may choose to use different sets of protocols to establish desired service. If voice communication based on SIP is blocked, users are likely to use propriety protocols that allow them to talk to each other. Thus distribution and mobility can hamper efforts to block communications in a number of ways.

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 (where "local" may be denote a particular service provider or a particular geography). These concepts are critical to the stability, scalability, and ability to innovate 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. For example, 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 a particular application service 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/or infrastructure services are so intertwined with each other that filtering a single service or in a single location can have broad effects in multiple directions. 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 .

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 effectiveness. 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, protocols and network layer addresses. More advanced configurations perform deep packet inspection or traffic flow analysis and filter or block based on rich (content-specific) rules and policies. Many firewalls include web filtering capabilities (see below). Firewalls can be deployed either on end hosts (under user control), or at network boundaries. 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 content deemed illegal. HTTPS is a challenge for these systems, because the URL in an HTTPS request is carried inside the secure (encrypted) 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 installs a security exception. (See further discussion in .) 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 evaluate 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 . 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, 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 . 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 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 . 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 , 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 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 . The DNS can also be used to store third party information that mark domains as safe or unsafe . 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 (also see ). These actions have led to concerns that the number resource certification system and related secure routing technologies developed by the IETF SIDR working group might be subject to government manipulation as well , potentially for the purpose of denying targeted networks access to the Internet.

Broadly speaking, completing a typical end-to-end Internet communication requires the involvement of three classes of entities, each of which has different means available to it for blocking communications. An end user originates the communication, which may be destined for another end user or application provider on the other end. At these endpoints, authentication and reputation systems enable devices and their users to make decisions about which communications should be blocked. The endpoints are each connected to a series of networks, the operators of which may also be capable of blocking. Network-based mechanisms usually involve an intermediary device in the network that observes Internet traffic and decides which communications to block. Finally, most communications depend on common "infrastructure" services (for lack of a better term) that globally support many different kinds of applications. These include the Domain Name System, the routing infrastructure, and their supporting information services such as the WHOIS databases. The operators of these services can alter the information they store in their associated databases or provide to endpoints in order to block communications from occuring. In this section, we discuss these three "blocking design patterns" and how they align with the Internet architectural principles outlined above. In general, endpoint-based blocking is the most consistent with the Internet architecture.

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. Common examples of network-based blocking 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. Some blocking systems focus on specific application-layer traffic, while others filter traffic based on lower layer criteria (transport protocol and source or destination addresses or ports). In addition to blocking communications based on their ultimate destination or source, network-based blocking may target discovery, rendezvous or store-and-forward components that are critical to the establishment of an application service but are not operated at either end-point of the communications. 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. Network operators can block the use of SIP-based services by blocking access to SIP servers. [Q: Does this actually happen?] It should be noted that "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 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 share some of the properties of the "Endpoint-Based Blocking" scenarios discussed in 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.

Blocking that uses intermediaries in the network 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. The first challenge 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. This asymmetry means that an intermediary will often see only one half of a given communication (if it sees any of it at all), which may limit its ability to effectively filter. For example, a filter aimed at requests destined for particular URLs cannot make accurate blocking decisions if it is only in the data path for HTTP responses and 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 or other content). 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 occupants of the same address (in violation of the locality principle.)

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 is the simplest form of filtering 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 . 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) 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 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 . 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 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 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 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 . 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 most commonly) that are unlikely to be blocked. In turn, network operators shifted to finer-grained content blocking over port 80, content providers shifted to encrypted channels, and operators began seeking to identify those channels. 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. In sum, blocking via intermediaries within networks 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), 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 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.

Internet applications often require or rely on support from common, global infrastructure 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. These infrastructure 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 shutting down a web server (which fulfills the specific purpose of serving web content). As physical objects, the servers that are used to provide infrastructure services exist within the jurisdiction of governments, 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 infrastructure services within a jurisdiction, either via direct government action or by allowing private actors to demand blocking (e.g., through lawsuits). Blocking by infrastructure operators is at odds with the locality principle. On the one hand, the global nature of Internet services and resources amplifies blocking actions, in the sense that it increases the risk of overblocking -- collateral damage to legitimate use of a resource. 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. The efficacy of infrastructure-based blocking is further limited by the autonomy principle. 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 infrastructure 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. Below we provide a few specific examples for routing, DNS, and WHOIS services. These examples demonstrate that for these types of infrastructure services (services that are often considered a global commons), jusrisdiction-specific legal and ethical motivations impinge on and are constrained by the principles of locality and autonomy. 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 route leak beyond the Pakistan ISP's scope (violation of locality), but service was restored in approximately two hours because network operators around the world re-configured their routers to ignore the bogus routes (which demonstrates autonomy). 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 US 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 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 "example.com") to direct queries for that name to a set of US-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 (violation of locality). Similar work-arounds 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, violating locality, and in that reduced trust in the global system may encourage networks to develop their own autonomous solutions. Blocking of infrastructure 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 ISOC's whitepaper on DNS filtering , but they also apply to other global Internet resources. In summary, infrastructure-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 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 infrastructure-based blocking can be negated on short order in some cases. To adapt a quote by John Gilmore, "The Internet treats blocking as damage and routes around it".

[TO DO: Should this section only address "user"-based blocking or should it also discuss server-based blocking (e.g., a web server that prevents abusive users from accessing content?)] 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 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 service. 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. 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 infrastructure-based blocking: while infrastructure-based blocking can only see and affect the infrastructure service at hand (e.g., DNS name resolution), endpoint-based blocking (depending on how it is designed) can have visibility into the entire application, across all layers and transactions. This visibility can provide 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, an infrastructure- 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. Finally, 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. 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.

Network-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. Examples: Firewalls, web filtering systems. A single intermediary device can be used to block the access of many users to many services. Network-based blocking can be done without the cooperation of either endpoint to a communication (although having that cooperation makes it more likely to be effective). Network intermediaries often lack sufficient information to make blocking decisions, due to routing asymmetry or encryption. Network-based blocking sometimes involves breaking end-to-end security assurances. Tunneling through blocking is difficult to prevent without preventing legitimate secure services. Infrastructure-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 services. Examples: Domain name seizures, WHOIS account freezing, RPKI certificate revocation. Internet services that depend on specific resources can be blocked by disabling those resources. Blocked resources can often be easily relocated or reinstantiated in a location where they will not be blocked. Resources used by undesirable services are often also used by legitimate services, resulting in collateral damage. Autonomy of Internet networks and users allows them to "route around" blocking. [OK Perhaps: Autonomy principle allows users to innovatively find methods to route around the blocking.] Endpoint-based blocking matches well with the overall design of the Internet. Examples: Safe browsing, spam filtering Endpoints block services by deciding whether or not to engage in a given communication. Blocking system has full visibility into all layers involved in a communication. Adversary mobility can be quickly observed so that blocking systems can be updated to account for it. 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. From a technical perspective, it is the most preferred option because it maintains transparency of the network, vests functionality at the endpoints in accordance with the end-to-end principle, can be applied granularly so as to avoid collateral damage, and accommodates mobile adversaries. Entities seeking to filter and for whom endpoint-based blocking is a potential choice should view its technical benefits as distinct advtanges compared to the other approaches. In reality, the various approaches discussed above are all applied for different reasons, and particular entities may not consider endpoint-based filtering to be viable. 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 infrastructure servers that are within their own jurisdictions, based on legal claims to obtain access to those servers. An enterprise with filtering requirements might require employees to 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 network- or infrastructure-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 network-based solution. 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 . 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 achieving their goals that do not involve interfering with the architecture or operation of the Internet. Those with a desire to filter should take into account the limitations discussed in this document and holistically assess the space of technical and non-technical solutions at their disposal and the likely effectiveness of each combination of approaches.

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