Network Working Group J. Arkko Internet-Draft Ericsson Intended status: Informational April 30, 2019 Expires: November 1, 2019 Changes in the Internet Threat Model draft-arkko-arch-internet-threat-model-00 Abstract Communications security has been at the center of many security improvements in the Internet. The goal has been to ensure that communications are protected against outside observers and attackers. This memo suggests that the existing threat model, while important and still valid, is no longer alone sufficient to cater for the pressing security issues in the Internet. For instance, it is also necessary to protect systems against endpoints that are compromised, malicious, or whose interests simply do not align with the interests of the users. While such protection is difficult, there are some measures that can be taken. It is particularly important to ensure that as we continue to develop Internet technology, non-communications security related threats are properly understood. While the consideration of these issues is relatively new in the IETF, this memo provides some initial ideas about potential broader threat models to consider when designing protocols for the Internet or when trying to defend against pervasive monitoring. Further down the road, updated threat models could result in changes in RFC 3552 (guidelines for writing security considerations) and RFC 7258 (pervasive monitoring), to include proper consideration of non-communications security threats. It may also be necessary to have dedicated guidance on how systems design and architecture affects security. 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 https://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 Arkko Expires November 1, 2019 [Page 1] Internet-Draft Internet Threat Model April 2019 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 November 1, 2019. Copyright Notice Copyright (c) 2019 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 (https://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 . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Improvements in Communications Security . . . . . . . . . . . 5 3. Issues in Security Beyond Communications Security . . . . . . 5 4. The Role of End-to-end . . . . . . . . . . . . . . . . . . . 8 4.1. Guidelines . . . . . . . . . . . . . . . . . . . . . . . 10 5. Potential Changes in IETF Analysis of Protocols . . . . . . . 11 5.1. Changes in RFC 3552 . . . . . . . . . . . . . . . . . . . 11 5.2. Changes in RFC 7258 . . . . . . . . . . . . . . . . . . . 12 5.3. System and Architecture Aspects . . . . . . . . . . . . . 12 6. Other Work . . . . . . . . . . . . . . . . . . . . . . . . . 13 7. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 13 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 13 9. Informative References . . . . . . . . . . . . . . . . . . . 13 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 16 1. Introduction Communications security has been at the center of many security improvements in the Internet. The goal has been to ensure that communications are protected against outside observers and attackers. At the IETF, this approach has been formalized in BCP 72 [RFC3552], which defined the Internet threat model in 2003. The purpose of a threat model is to outline what threats exist in order to assist the protocol designer. But RFC 3552 also ruled some Arkko Expires November 1, 2019 [Page 2] Internet-Draft Internet Threat Model April 2019 threats to be in scope and of primary interest, and some threats out of scope [RFC3552]: The Internet environment has a fairly well understood threat model. In general, we assume that the end-systems engaging in a protocol exchange have not themselves been compromised. Protecting against an attack when one of the end-systems has been compromised is extraordinarily difficult. It is, however, possible to design protocols which minimize the extent of the damage done under these circumstances. By contrast, we assume that the attacker has nearly complete control of the communications channel over which the end-systems communicate. This means that the attacker can read any PDU (Protocol Data Unit) on the network and undetectably remove, change, or inject forged packets onto the wire. However, the communications-security -only threat model is becoming outdated. This is due to three factors: o Advances in protecting most of our communications with strong cryptographic means. This has resulted in much improved communications security, but also higlights the need for addressing other, remaining issues. This is not to say that communications security is not important, it still is: improvements are still needed. Not all communications have been protected, and even out of the already protected communications, not all of their aspects have been fully protected. Fortunately, there are ongoing projects working on improvements. o Adversaries have increased their pressure against other avenues of attack, from compromising devices to legal coercion of centralized endpoints in conversations. o New adversaries and risks have arisen, e.g., due to creation of large centralized information sources. In short, attacks are migrating towards the currently easier targets, which no longer necessarily include direct attacks on traffic flows. In addition, trading information about users and ability to influence them has become a common practice for many Internet services, often without consent of the users. This memo suggests that the existing threat model, while important and still valid, is no longer alone sufficient to cater for the pressing security issues in the Internet. For instance, while it continues to be very important to protect Internet communications against outsiders, it is also necessary to protect systems against Arkko Expires November 1, 2019 [Page 3] Internet-Draft Internet Threat Model April 2019 endpoints that are compromised, malicious, or whose interests simply do not align with the interests of the users. Of course, there are many trade-offs in the Internet on who one chooses to interact with and why or how. It is not the role of this memo to dictate those choices. But it is important that we understand the implications of different practices. It is also important that when it comes to basic Internet infrastructure, our chosen technologies lead to minimal exposure with respect to the non- communications threats. It is particularly important to ensure that non-communications security related threats are properly understood for any new Internet technology. While the consideration of these issues is relatively new in the IETF, this memo provides some initial ideas about potential broader threat models to consider when designing protocols for the Internet or when trying to defend against pervasive monitoring. Further down the road, updated threat models could result in changes in BCP 72 [RFC3552] (guidelines for writing security considerations) and BCP 188 [RFC7258] (pervasive monitoring), to include proper consideration of non-communications security threats. It may also be necessary to have dedicated guidance on how systems design and architecture affects security. The sole consideration of communications security aspects in designing Internet protocols may lead to accidental or increased impact of security issues elsewhere. For instance, allowing a participant to unnecessarily collect or receive information may be lead to a similar effect as described in [RFC8546] for protocols: over time, unnecessary information will get used with all the associated downsides, regardless of what deployment expectations there were during protocol design. The rest of this memo is organized as follows. Section 2 and Section 3 outline the situation with respect to communications security and beyond it. Section 4 discusses how the author believes the Internet threat model should evolve, and what types of threats should be seen as critical ones and in-scope. Section 4.1 will also discuss high-level guidance to addressing these threats. Section 5 outlines the author's suggested future changes to RFC 3552 and RFC 7258 and the need for guidance on the impacts of system design and architecture on security. Comments are solicited on these and other aspects of this document. The best place for discussion is on the arch-discuss list (https://www.ietf.org/mailman/listinfo/ Architecture-discuss). This memo acts also as an input for the IAB retreat discussion on threat models, and it is a submission for the Arkko Expires November 1, 2019 [Page 4] Internet-Draft Internet Threat Model April 2019 IAB DEDR workshop (https://www.iab.org/activities/workshops/dedr- workshop/). Finally, Section 6 highlights other discussions in this problem space and Section 7 draws some conclusions for next steps. 2. Improvements in Communications Security The fraction of Internet traffic that is cryptographically protected has grown tremendously in the last few years. Several factors have contributed to this change, from Snowden revelations to business reasons and to better available technology such as HTTP/2 [RFC7540], TLS 1.3 [RFC8446], QUIC [I-D.ietf-quic-transport]. In many networks, the majority of traffic has flipped from being cleartext to being encrypted. Reaching the level of (almost) all traffic being encrypted is no longer something unthinkable but rather a likely outcome in a few years. At the same time, technology developments and policy choices have driven the scope of cryptographic protection from protecting only the pure payload to protecting much of the rest as well, including far more header and meta-data information than was protected before. For instance, efforts are ongoing in the IETF to assist encrypting transport headers [I-D.ietf-quic-transport], server domain name information in TLS [I-D.ietf-tls-esni], and domain name queries [RFC8484]. There has also been improvements to ensure that the security protocols that are in use actually have suitable credentials and that those credentials have not been compromised, see, for instance, Let's Encrypt [RFC8555], HSTS [RFC6797], HPKP [RFC7469], and Expect-CT [I-D.ietf-httpbis-expect-ct]. This is not to say that all problems in communications security have been resolved - far from it. But the situation is definitely different from what it was a few years ago. Remaining issues will be and are worked on; the fight between defense and attack will also continue. Communications security will stay at the top of the agenda in any Internet technology development. 3. Issues in Security Beyond Communications Security There are, however, significant issues beyond communications security in the Internet. To begin with, it is not necessarily clear that one can trust all the endpoints. Arkko Expires November 1, 2019 [Page 5] Internet-Draft Internet Threat Model April 2019 Of course, the endpoints were never trusted, but the pressures against endpoints issues seem to be mounting. For instance, the users may not be in as much control over their own devices as they used to be due to manufacturer-controlled operating system installations and locked device ecosystems. And within those ecosystems, even the applications that are available tend to have features that users by themselves would most likely not desire to have, such as excessive rights to media, location, and peripherals. There are also designated efforts by various authorities to hack end- user devices as a means of intercepting data about the user. The situation is different, but not necessarily better on the side of servers. The pattern of communications in today's Internet is almost always via a third party that has at least as much information than the other parties have. For instance, these third parties are typically endpoints for any transport layer security connections, and able to see any communications or other messaging in cleartextx. There are some exceptions, of course, e.g., messaging applications with end-to-end protection. With the growth of trading users' information by many of these third parties, it becomes necessary to take precautions against endpoints that are compromised, malicious, or whose interests simply do not align with the interests of the users. Specifically, the following issues need attention: o Security of users' devices and the ability of the user to control their own equipment. o Leaks and attacks related to data at rest. o Coercion of some endpoints to reveal information to authorities or surveillance organizations, sometimes even in an extra-territorial fashion. o Application design patterns that result in cleartext information passing through a third party or the application owner. o Involvement of entities that have no direct need for involvement for the sake of providing the service that the user is after. o Network and application architectures that result in a lot of information collected in a (logically) central location. o Leverage and control points outside the hands of the users or end- user device owners. Arkko Expires November 1, 2019 [Page 6] Internet-Draft Internet Threat Model April 2019 For instance, while e-mail transport security [RFC7817] has become much more widely distributed in recent years, progress in securing e-mail messages between users has been much slower. This has lead to a situation where e-mail content is considered a critical resource by mail providers who use it for machine learning, advertisement targeting, and other purposes. The Domain Name System (DNS) shows signs of ageing but due to the legacy of deployed systems, has changed very slowly. Newer technology [RFC8484] developed at the IETF enables DNS queries to be performed confidentially, but its deployment is happening mostly in browsers that use global DNS resolver services, such as Cloudflare's 1.1.1.1 or Google's 8.8.8.8. This results in faster evolution and better security for end users. However, if one steps back and considers the overall security effects of these developments, the resulting effects can be different. While the security of the actual protocol exchanges improves with the introduction of this new technology, at the same time this implies a move from using a worldwide distributed set of DNS resolvers into more centralised global resolvers. While these resolvers are very well maintained (and a great service), they are potential high-value targets for pervasive monitoring and Denial-of-Service (DoS) attacks. In 2016, for example, DoS attacks were launched against Dyn, one of the largest DNS providers, leading to some outages. It is difficult to imagine that DNS resolvers wouldn't be a target in many future attacks or pervasive monitoring projects. Unfortunately, there is little that even large service providers can do to refuse authority-sanctioned pervasive monitoring. As a result it seems that the only reasonable course of defense is to ensure that no such information or control point exists. There are other examples about the perils of centralised solutions in Internet infrastructure. The DNS example involved an interesting combination of information flows (who is asking for what domain names) as well as a potential ability to exert control (what domains will actually resolve to an address). Routing systems are primarily about control. While there are intra-domain centralized routing solutions (such as PCE [RFC4655]), a control within a single administrative domain is usually not the kind of centralization that we would be worried about. Global centralization would be much more concerning. Fortunately, global Internet routing is performed a among peers. However, controls could be introduced even in this global, distributed system. To secure some of the control exchanges, the Resource Public Key Infrastructure (RPKI) system ([RFC6480]) allows selected Certification Authorities (CAs) to help drive decisions about which participants in the routing infrastructure can Arkko Expires November 1, 2019 [Page 7] Internet-Draft Internet Threat Model April 2019 make what claims. If this system were globally centralized, it would be a concern, but again, fortunately, current designs involve at least regional distribution. In general, many recent attacks relate more to information than communications. For instance, personal information leaks typically happen via information stored on a compromised server rather than capturing communications. There is little hope that such attacks can be prevented entirely. Again, the best course of action seems to be avoid the disclosure of information in the first place, or at least to not perform that in a manner that makes it possible that others can readily use the information. 4. The Role of End-to-end [RFC1958] notes that "end-to-end functions can best be realised by end-to-end protocols": The basic argument is that, as a first principle, certain required end-to-end functions can only be performed correctly by the end- systems themselves. A specific case is that any network, however carefully designed, will be subject to failures of transmission at some statistically determined rate. The best way to cope with this is to accept it, and give responsibility for the integrity of communication to the end systems. Another specific case is end- to-end security. The "end-to-end argument" was originally described by Saltzer et al [Saltzer]. They said: The function in question can completely and correctly be implemented only with the knowledge and help of the application standing at the endpoints of the communication system. Therefore, providing that questioned function as a feature of the communication system itself is not possible. These functional arguments align with other, practical arguments about the evolution of the Internet under the end-to-end model. The endpoints evolve quickly, often with simply having one party change the necessary software on both ends. Whereas waiting for network upgrades would involve potentially a large number of parties from application owners to multiple network operators. The end-to-end model supports permissionless innovation where new innovation can flourish in the Internet without excessive wait for other parties to act. Arkko Expires November 1, 2019 [Page 8] Internet-Draft Internet Threat Model April 2019 But the details matter. What is considered an endpoint? What characteristics of Internet are we trying to optimize? This memo makes the argument that, for security purposes, there is a significant distinction between actual endpoints from a user's interaction perspective (e.g., another user) and from a system perspective (e.g., a third party relaying a message). This memo proposes to focus on the distinction between "real ends" and other endpoints to guide the development of protocols. A conversation between one "real end" to another "real end" has necessarily different security needs than a conversation between, say, one of the "real ends" and a component in a larger system. The end-to-end argument is used primarily for the design of one protocol. The security of the system, however, depends on the entire system and potentially multiple storage, compute, and communication protocol aspects. All have to work properly together to obtain security. For instance, a transport connection between two components of a system is not an end-to-end connection even if it encompasses all the protocol layers up to the application layer. It is not end-to-end, if the information or control function it carries actually extends beyond those components. For instance, just because an e-mail server can read the contents of an e-mail message does not make it a legitimate recipient of the e-mail. This memo also proposes to focus on the "need to know" aspect in systems. Information should not be disclosed, stored, or routed in cleartext through parties that do not absolutely need to have that information. The proposed argument about real ends is as follows: Application functions are best realised by the entities directly serving the users, and when more than one entity is involved, by end-to-end protocols. The role and authority of any additional entities necessary to carry out a function should match their part of the function. No information or control roles should be provided to these additional entities unless it is required by the function they provide. For instance, a particular piece of information may be necessary for the other real endpoint, such as message contents for another user. The same piece of information may not be necessary for any additional parties, unless the information had to do with, say, routing information for the message to reach the other user. When information is only needed by the actual other endpoint, it should be protected and be only relayed to the actual other endpoint. Protocol Arkko Expires November 1, 2019 [Page 9] Internet-Draft Internet Threat Model April 2019 design should ensure that the additional parties do not have access to the information. Note that it may well be that the easiest design approach is to send all information to a third party and have majority of actual functionality reside in that third party. But this is a case of a clear tradeoff between ease of change by evolving that third party vs. providing reasonable security against misuse of information. Note that the above "real ends" argument is not limited to communication systems. Even an application that does not communicate with anyone else than its user may be implemented on top of a distributed system where some information about the user is exposed to untrusted parties. The implications of the system security also extend beyond information and control aspects. For instance, poorly design component protocols can become DoS vectors which are then used to attack other parts of the system. Availability is an important aspect to consider in the analysis along other aspects. 4.1. Guidelines As [RFC3935] says: We embrace technical concepts such as decentralized control, edge- user empowerment and sharing of resources, because those concepts resonate with the core values of the IETF community. To be more specific, this memo suggests the following guidelines for protocol designers: 1. Minimizing information passed to others: Information passed to another party in a protocol exchange should be minimized to guard against the potential compromise of that party. 2. End-to-end protection via other parties: Information passed via another party who does not intrinsically need the information to perform its function should be protected end-to-end to its intended recipient. This guideline is general, and holds equally for sending TCP/IP packets, TLS connections, or application-layer interactions. As [I-D.iab-wire-image] notes, it is a useful design rule to avoid "accidental invariance" (the deployment of on-path devices that over-time start to make assumptions about protocols). However, it is also a necessary security design rule to avoid "accidental disclosure" where information originally thought to be benign and untapped over-time becomes a significant information leak. This guideline can also be applied for Arkko Expires November 1, 2019 [Page 10] Internet-Draft Internet Threat Model April 2019 different aspects of security, e.g., confidentiality and integrity protection, depending on what the specific need for information is in the other parties. 3. Minimizing passing of control functions to others: Any passing of control functions to other parties should be minimized to guard against the potential misuse of those control functions. This applies to both technical (e.g., nodes that assign resources) and process control functions (e.g., the ability to allocate number or develop extensions). Control functions can also become a matter of contest and power struggle, even in cases where their function as such is minimal, as we saw with the IANA transition debates. 4. Avoiding centralized resources: While centralized components, resources, and function provide usually a useful function, there are grave issues associated with them. Protocol and network design should balance the benefits of centralized resources or control points against the threats arising from them. The general guideline is to avoid such centralized resources when possible. And if it is not possible, find a way to allow the centralized resources to be selectable, depending on context and user settings. 5. Explicit agreements: When users and their devices provide information to network entities, it would be beneficial to have an opportunity for the users to state their requirements regarding the use of the information provided in this way. While the actual use of such requirements and the willingness of network entities to agree to them remains to be seen, at the moment even the technical means of doing this are limited. For instance, it would be beneficial to be able to embed usage requirements within popular data formats. 5. Potential Changes in IETF Analysis of Protocols 5.1. Changes in RFC 3552 This memo suggests that changes maybe necessary in RFC 3552. One initial, draft proposal for such changes would be this: OLD: In general, we assume that the end-systems engaging in a protocol exchange have not themselves been compromised. Protecting against an attack when one of the end-systems has been compromised is extraordinarily difficult. It is, however, possible to design Arkko Expires November 1, 2019 [Page 11] Internet-Draft Internet Threat Model April 2019 protocols which minimize the extent of the damage done under these circumstances. NEW: In general, we assume that the end-system engaging in a protocol exchange has not itself been compromised. Protecting against an attack of a protocol implementation itself is extraordinarily difficult. It is, however, possible to design protocols which minimize the extent of the damage done when the other parties in a protocol become compromised or do not act in the best interests the end-system implementing a protocol. In addition, the following new section could be added to discuss the capabilities required to mount an attack: NEW: 3.x. Other endpoint compromise In this attack, the other endpoints in the protocol become compromised. As a result, they can, for instance, misuse any information that the end-system implementing a protocol has sent to the compromised endpoint. 5.2. Changes in RFC 7258 This memo also suggests that additional guidelines may be necessary in RFC 7258. An initial, draft suggestion for starting point of those changes could be adding the following paragraph after the 2nd paragraph in Section 2: NEW: PM attacks include those cases where information collected by a legitimate protocol participant is misused for PM purposes. The attacks also include those cases where a protocol or network architecture results in centralized data storage or control functions relating to many users, raising the risk of said misuse. 5.3. System and Architecture Aspects This definitely needs more attention from Internet technology developers and standards organizations. Here is one possible The design of any Internet technology should start from an understanding of the participants in a system, their roles, and Arkko Expires November 1, 2019 [Page 12] Internet-Draft Internet Threat Model April 2019 the extent to which they should have access to information and ability to control other participants. 6. Other Work See, for instance, [I-D.farrell-etm]. 7. Conclusions More work is needed in this area. To start with, Internet technology developers need to be better aware of the issues beyond communications security, and consider them in design. At the IETF it would be beneficial to include some of these considerations in the usual systematic security analysis of technologies under development. In particular, when the IETF develops infrastructure technology for the Internet (such as routing or naming systems), considering the impacts of data generated by those technologies is important. Minimising data collection from users, minimising the parties who get exposed to user data, and protecting data that is relayed or stored in systems should be a priority. A key focus area at the IETF has been the security of transport protocols, and how transport layer security can be best used to provide the right security for various applications. However, more work is needed in equivalently broadly deployed tools for minimising or obfuscating information provided by users to other entities, and the use of end-to-end security through entities that are involved in the protocol exchange but who do not need to know everything that is being passed through them. Comments on the issues discussed in this memo are gladly taken either privately or on the architecture-discuss mailing list. 8. Acknowledgements The author would like to thank John Mattsson, Mirja Kuehlewind, Alissa Cooper, Stephen Farrell, Eric Rescorla, Simone Ferlin, Kathleen Moriarty, Brian Trammell, Mark Nottingham, Christian Huitema, Karl Norrman, Ted Hardie, Mohit Sethi, Phillip Hallam-Baker, Goran Eriksson and the IAB for interesting discussions in this problem space. 9. Informative References [I-D.farrell-etm] Farrell, S., "We're gonna need a bigger threat model", draft-farrell-etm-00 (work in progress), April 2019. Arkko Expires November 1, 2019 [Page 13] Internet-Draft Internet Threat Model April 2019 [I-D.iab-wire-image] Trammell, B. and M. Kuehlewind, "The Wire Image of a Network Protocol", draft-iab-wire-image-01 (work in progress), November 2018. [I-D.ietf-httpbis-expect-ct] estark@google.com, e., "Expect-CT Extension for HTTP", draft-ietf-httpbis-expect-ct-08 (work in progress), December 2018. [I-D.ietf-quic-transport] Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed and Secure Transport", draft-ietf-quic-transport-20 (work in progress), April 2019. [I-D.ietf-tls-esni] Rescorla, E., Oku, K., Sullivan, N., and C. Wood, "Encrypted Server Name Indication for TLS 1.3", draft- ietf-tls-esni-03 (work in progress), March 2019. [I-D.nottingham-for-the-users] Nottingham, M., "The Internet is for End Users", draft- nottingham-for-the-users-07 (work in progress), March 2019. [RFC1958] Carpenter, B., Ed., "Architectural Principles of the Internet", RFC 1958, DOI 10.17487/RFC1958, June 1996, . [RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC Text on Security Considerations", BCP 72, RFC 3552, DOI 10.17487/RFC3552, July 2003, . [RFC3935] Alvestrand, H., "A Mission Statement for the IETF", BCP 95, RFC 3935, DOI 10.17487/RFC3935, October 2004, . [RFC4655] Farrel, A., Vasseur, J., and J. Ash, "A Path Computation Element (PCE)-Based Architecture", RFC 4655, DOI 10.17487/RFC4655, August 2006, . [RFC6480] Lepinski, M. and S. Kent, "An Infrastructure to Support Secure Internet Routing", RFC 6480, DOI 10.17487/RFC6480, February 2012, . Arkko Expires November 1, 2019 [Page 14] Internet-Draft Internet Threat Model April 2019 [RFC6797] Hodges, J., Jackson, C., and A. Barth, "HTTP Strict Transport Security (HSTS)", RFC 6797, DOI 10.17487/RFC6797, November 2012, . [RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J., Morris, J., Hansen, M., and R. Smith, "Privacy Considerations for Internet Protocols", RFC 6973, DOI 10.17487/RFC6973, July 2013, . [RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May 2014, . [RFC7469] Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469, April 2015, . [RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext Transfer Protocol Version 2 (HTTP/2)", RFC 7540, DOI 10.17487/RFC7540, May 2015, . [RFC7817] Melnikov, A., "Updated Transport Layer Security (TLS) Server Identity Check Procedure for Email-Related Protocols", RFC 7817, DOI 10.17487/RFC7817, March 2016, . [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, . [RFC8484] Hoffman, P. and P. McManus, "DNS Queries over HTTPS (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018, . [RFC8546] Trammell, B. and M. Kuehlewind, "The Wire Image of a Network Protocol", RFC 8546, DOI 10.17487/RFC8546, April 2019, . [RFC8555] Barnes, R., Hoffman-Andrews, J., McCarney, D., and J. Kasten, "Automatic Certificate Management Environment (ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019, . Arkko Expires November 1, 2019 [Page 15] Internet-Draft Internet Threat Model April 2019 [Saltzer] Saltzer, J., Reed, D., and D. Clark, "End-To-End Arguments in System Design", ACM TOCS, Vol 2, Number 4, November 1984, pp 277-288. , n.d.. Author's Address Jari Arkko Ericsson Email: jari.arkko@piuha.net Arkko Expires November 1, 2019 [Page 16]