Issues and Requirements for Server Name Identification (SNI) Encryption in TLS
RFC 8744

Document Type RFC - Informational (July 2020; No errata)
Authors Christian Huitema  , Eric Rescorla 
Last updated 2020-07-28
Replaces draft-huitema-tls-sni-encryption
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Internet Engineering Task Force (IETF)                        C. Huitema
Request for Comments: 8744                          Private Octopus Inc.
Category: Informational                                        July 2020
ISSN: 2070-1721

Issues and Requirements for Server Name Identification (SNI) Encryption
                                 in TLS

Abstract

   This document describes the general problem of encrypting the Server
   Name Identification (SNI) TLS parameter.  The proposed solutions hide
   a hidden service behind a fronting service, only disclosing the SNI
   of the fronting service to external observers.  This document lists
   known attacks against SNI encryption, discusses the current "HTTP co-
   tenancy" solution, and presents requirements for future TLS-layer
   solutions.

   In practice, it may well be that no solution can meet every
   requirement and that practical solutions will have to make some
   compromises.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are candidates for any level of Internet
   Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8744.

Copyright Notice

   Copyright (c) 2020 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.  History of the TLS SNI Extension
     2.1.  Unanticipated Usage of SNI Information
     2.2.  SNI Encryption Timeliness
     2.3.  End-to-End Alternatives
   3.  Security and Privacy Requirements for SNI Encryption
     3.1.  Mitigate Cut-and-Paste Attacks
     3.2.  Avoid Widely Shared Secrets
     3.3.  Prevent SNI-Based Denial-of-Service Attacks
     3.4.  Do Not Stick Out
     3.5.  Maintain Forward Secrecy
     3.6.  Enable Multi-party Security Contexts
     3.7.  Support Multiple Protocols
       3.7.1.  Hiding the Application-Layer Protocol Negotiation
       3.7.2.  Supporting Other Transports than TCP
   4.  HTTP Co-tenancy Fronting
     4.1.  HTTPS Tunnels
     4.2.  Delegation Control
     4.3.  Related Work
   5.  Security Considerations
   6.  IANA Considerations
   7.  Informative References
   Acknowledgements
   Author's Address

1.  Introduction

   Historically, adversaries have been able to monitor the use of web
   services through three primary channels: looking at DNS requests,
   looking at IP addresses in packet headers, and looking at the data
   stream between user and services.  These channels are getting
   progressively closed.  A growing fraction of Internet communication
   is encrypted, mostly using Transport Layer Security (TLS) [RFC8446].
   Progressive deployment of solutions like DNS over TLS [RFC7858] and
   DNS over HTTPS [RFC8484] mitigates the disclosure of DNS information.
   More and more services are colocated on multiplexed servers,
   loosening the relation between IP address and web service.  For
   example, in virtual hosting solutions, multiple services can be
   hosted as co-tenants on the same server, and the IP address and port
   do not uniquely identify a service.  In cloud or Content Delivery
   Network (CDN) solutions, a given platform hosts the services or
   servers of a lot of organizations, and looking up what netblock an IP
   address belongs to reveals little.  However, multiplexed servers rely
   on the Server Name Information (SNI) TLS extension [RFC6066] to
   direct connections to the appropriate service implementation.  This
   protocol element is transmitted in cleartext.  As the other methods
   of monitoring get blocked, monitoring focuses on the cleartext SNI.
   The purpose of SNI encryption is to prevent that and aid privacy.

   Replacing cleartext SNI transmission by an encrypted variant will
   improve the privacy and reliability of TLS connections, but the
   design of proper SNI encryption solutions is difficult.  In the past,
   there have been multiple attempts at defining SNI encryption.  These
   attempts have generally floundered, because the simple designs fail
   to mitigate several of the attacks listed in Section 3.  In the
   absence of a TLS-level solution, the most popular approach to SNI
   privacy for web services is HTTP-level fronting, which we discuss in
   Section 4.

   This document does not present the design of a solution but provides
   guidelines for evaluating proposed solutions.  (The review of HTTP-
   level solutions in Section 4 is not an endorsement of these
   solutions.)  The need for related work on the encryption of the
   Application-Layer Protocol Negotiation (ALPN) parameters of TLS is
   discussed in Section 3.7.1.

2.  History of the TLS SNI Extension

   The SNI extension was specified in 2003 in [RFC3546] to facilitate
   management of "colocation servers", in which multiple services shared
   the same IP address.  A typical example would be multiple websites
   served by the same web server.  The SNI extension carries the name of
   a specific server, enabling the TLS connection to be established with
   the desired server context.  The current SNI extension specification
   can be found in [RFC6066].

   The SNI specification allowed for different types of server names,
   though only the "hostname" variant was specified and deployed.  In
   that variant, the SNI extension carries the domain name of the target
   server.  The SNI extension is carried in cleartext in the TLS
   "ClientHello" message.

2.1.  Unanticipated Usage of SNI Information

   The SNI was defined to facilitate management of servers, but the
   developers of middleboxes found out that they could take advantage of
   the information.  Many examples of such usage are reviewed in
   [RFC8404].  Other examples came out during discussions of this
   document.  They include:

   *  Filtering or censoring specific services for a variety of reasons

   *  Content filtering by network operators or ISPs blocking specific
      websites, for example, to implement parental controls or to
      prevent access to phishing or other fraudulent websites

   *  ISP assigning different QoS profiles to target services

   *  Firewalls within enterprise networks blocking websites not deemed
      appropriate for work

   *  Firewalls within enterprise networks exempting specific websites
      from man-in-the-middle (MITM) inspection, such as healthcare or
      financial sites for which inspection would intrude on the privacy
      of employees

   The SNI is probably also included in the general collection of
   metadata by pervasive surveillance actors [RFC7258], for example, to
   identify services used by surveillance targets.

2.2.  SNI Encryption Timeliness

   The cleartext transmission of the SNI was not flagged as a problem in
   the Security Considerations sections of [RFC3546], [RFC4366], or
   [RFC6066].  These specifications did not anticipate the alternative
   usage described in Section 2.1.  One reason may be that, when these
   RFCs were written, the SNI information was available through a
   variety of other means, such as tracking IP addresses, DNS names, or
   server certificates.

   Many deployments still allocate different IP addresses to different
   services, so that different services can be identified by their IP
   addresses.  However, CDNs commonly serve a large number of services
   through a comparatively small number of addresses.

   The SNI carries the domain name of the server, which is also sent as
   part of the DNS queries.  Most of the SNI usage described in
   Section 2.1 could also be implemented by monitoring DNS traffic or
   controlling DNS usage.  But this is changing with the advent of DNS
   resolvers providing services like DNS over TLS [RFC7858] or DNS over
   HTTPS [RFC8484].

   The subjectAltName extension of type dNSName of the server
   certificate (or in its absence, the common name component) exposes
   the same name as the SNI.  In TLS versions 1.0 [RFC2246], 1.1
   [RFC4346], and 1.2 [RFC5246], servers send certificates in cleartext,
   ensuring that there would be limited benefits in hiding the SNI.
   However, in TLS 1.3 [RFC8446], server certificates are encrypted in
   transit.  Note that encryption alone is insufficient to protect
   server certificates; see Section 3.1 for details.

   The decoupling of IP addresses and server names, deployment of DNS
   privacy, and protection of server certificate transmissions all
   contribute to user privacy in the face of an RFC 7258-style adversary
   [RFC7258].  Encrypting the SNI complements this push for privacy and
   makes it harder to censor or otherwise provide differential treatment
   to specific Internet services.

2.3.  End-to-End Alternatives

   Deploying SNI encryption helps thwart most of the unanticipated SNI
   usages, including censorship and pervasive surveillance, but it also
   will break or reduce the efficacy of the operational practices and
   techniques implemented in middleboxes, as described in Section 2.1.
   Most of these functions can, however, be realized by other means.
   For example, some DNS service providers offer customers the provision
   to "opt in" to filtering services for parental control and phishing
   protection.  Per-stream QoS could be provided by a combination of
   packet marking and end-to-end agreements.  As SNI encryption becomes
   common, we can expect more deployment of such "end-to-end" solutions.

   At the time of this writing, enterprises have the option of
   installing a firewall performing SNI filtering to prevent connections
   to certain websites.  With SNI encryption, this becomes ineffective.
   Obviously, managers could block usage of SNI encryption in enterprise
   computers, but this wide-scale blocking would diminish the privacy
   protection of traffic leaving the enterprise, which may not be
   desirable.  Enterprise managers could rely instead on filtering
   software and management software deployed on the enterprise's
   computers.

3.  Security and Privacy Requirements for SNI Encryption

   Over the past years, there have been multiple proposals to add an SNI
   encryption option in TLS.  A review of the TLS mailing list archives
   shows that many of these proposals appeared promising but were
   rejected after security reviews identified plausible attacks.  In
   this section, we collect a list of these known attacks.

3.1.  Mitigate Cut-and-Paste Attacks

   The simplest SNI encryption designs replace the cleartext SNI in the
   initial TLS exchange with an encrypted value, using a key known to
   the multiplexed server.  Regardless of the encryption used, these
   designs can be broken by a simple cut-and-paste attack, which works
   as follows:

   1.  The user starts a TLS connection to the multiplexed server,
       including an encrypted SNI value.

   2.  The adversary observes the exchange and copies the encrypted SNI
       parameter.

   3.  The adversary starts its own connection to the multiplexed
       server, including in its connection parameters the encrypted SNI
       copied from the observed exchange.

   4.  The multiplexed server establishes the connection to the
       protected service, which sends its certificate, thus revealing
       the identity of the service.

   One of the goals of SNI encryption is to prevent adversaries from
   knowing which hidden service the client is using.  Successful cut-
   and-paste attacks break that goal by allowing adversaries to discover
   that service.

3.2.  Avoid Widely Shared Secrets

   It is easy to think of simple schemes in which the SNI is encrypted
   or hashed using a shared secret.  This symmetric key must be known by
   the multiplexed server and by every user of the protected services.
   Such schemes are thus very fragile, since the compromise of a single
   user would compromise the entire set of users and protected services.

3.3.  Prevent SNI-Based Denial-of-Service Attacks

   Encrypting the SNI may create extra load for the multiplexed server.
   Adversaries may mount denial-of-service (DoS) attacks by generating
   random encrypted SNI values and forcing the multiplexed server to
   spend resources in useless decryption attempts.

   It may be argued that this is not an important avenue for DoS
   attacks, as regular TLS connection attempts also require the server
   to perform a number of cryptographic operations.  However, in many
   cases, the SNI decryption will have to be performed by a front-end
   component with limited resources, while the TLS operations are
   performed by the component dedicated to their respective services.
   SNI-based DoS attacks could target the front-end component.

3.4.  Do Not Stick Out

   In some designs, handshakes using SNI encryption can be easily
   differentiated from "regular" handshakes.  For example, some designs
   require specific extensions in the ClientHello packets or specific
   values of the cleartext SNI parameter.  If adversaries can easily
   detect the use of SNI encryption, they could block it, or they could
   flag the users of SNI encryption for special treatment.

   In the future, it might be possible to assume that a large fraction
   of TLS handshakes use SNI encryption.  If that were the case, the
   detection of SNI encryption would be a lesser concern.  However, we
   have to assume that, in the near future, only a small fraction of TLS
   connections will use SNI encryption.

   This requirement to not stick out may be difficult to meet in
   practice, as noted in Section 5.

3.5.  Maintain Forward Secrecy

   TLS 1.3 [RFC8446] is designed to provide forward secrecy, so that
   (for example) keys used in past sessions will not be compromised even
   if the private key of the server is compromised.  The general
   concerns about forward secrecy apply to SNI encryption as well.  For
   example, some proposed designs rely on a public key of the
   multiplexed server to define the SNI encryption key.  If the
   corresponding private key should be compromised, the adversaries
   would be able to process archival records of past connections and
   retrieve the protected SNI used in these connections.  These designs
   fail to maintain forward secrecy of SNI encryption.

3.6.  Enable Multi-party Security Contexts

   We can design solutions in which a fronting service acts as a relay
   to reach the protected service.  Some of those solutions involve just
   one TLS handshake between the client and the fronting service.  The
   master secret is verified by verifying a certificate provided by the
   fronting service but not by the protected service.  These solutions
   expose the client to a MITM attack by the fronting service.  Even if
   the client has some reasonable trust in this service, the possibility
   of a MITM attack is troubling.

   There are other classes of solutions in which the master secret is
   verified by verifying a certificate provided by the protected
   service.  These solutions offer more protection against a MITM attack
   by the fronting service.  The downside is that the client will not
   verify the identity of the fronting service, which enables fronting
   server spoofing attacks, such as the "honeypot" attack discussed
   below.  Overall, end-to-end TLS to the protected service is
   preferable, but it is important to also provide a way to authenticate
   the fronting service.

   The fronting service could be pressured by adversaries.  By design,
   it could be forced to deny access to the protected service or to
   divulge which client accessed it.  But if a MITM attack is possible,
   the adversaries would also be able to pressure the fronting service
   into intercepting or spoofing the communications between client and
   protected service.

   Adversaries could also mount an attack by spoofing the fronting
   service.  A spoofed fronting service could act as a "honeypot" for
   users of hidden services.  At a minimum, the fake server could record
   the IP addresses of these users.  If the SNI encryption solution
   places too much trust on the fronting server, the fake server could
   also serve fake content of its own choosing, including various forms
   of malware.

   There are two main channels by which adversaries can conduct this
   attack.  Adversaries can simply try to mislead users into believing
   that the honeypot is a valid fronting server, especially if that
   information is carried by word of mouth or in unprotected DNS
   records.  Adversaries can also attempt to hijack the traffic to the
   regular fronting server, using, for example, spoofed DNS responses or
   spoofed IP-level routing, combined with a spoofed certificate.

3.7.  Support Multiple Protocols

   The SNI encryption requirement does not stop with HTTP over TLS.
   Multiple other applications currently use TLS, including, for
   example, SMTP [RFC3207], DNS [RFC7858], IMAP [RFC8314], and the
   Extensible Messaging and Presence Protocol (XMPP) [RFC7590].  These
   applications, too, will benefit from SNI encryption.  HTTP-only
   methods, like those described in Section 4.1, would not apply there.
   In fact, even for the HTTPS case, the HTTPS tunneling service
   described in Section 4.1 is compatible with HTTP 1.0 and HTTP 1.1 but
   interacts awkwardly with the multiple streams feature of HTTP/2
   [RFC7540].  This points to the need for an application-agnostic
   solution, which would be implemented fully in the TLS layer.

3.7.1.  Hiding the Application-Layer Protocol Negotiation

   The Application-Layer Protocol Negotiation (ALPN) parameters of TLS
   allow implementations to negotiate the application-layer protocol
   used on a given connection.  TLS provides the ALPN values in
   cleartext during the initial handshake.  While exposing the ALPN does
   not create the same privacy issues as exposing the SNI, there is
   still a risk.  For example, some networks may attempt to block
   applications that they do not understand or that they wish users
   would not use.

   In a sense, ALPN filtering could be very similar to the filtering of
   specific port numbers exposed in some networks.  This filtering by
   ports has given rise to evasion tactics in which various protocols
   are tunneled over HTTP in order to use open ports 80 or 443.
   Filtering by ALPN would probably beget the same responses, in which
   the applications just move over HTTP and only the HTTP ALPN values
   are used.  Applications would not need to do that if the ALPN were
   hidden in the same way as the SNI.

   In addition to hiding the SNI, it is thus desirable to also hide the
   ALPN.  Of course, this implies engineering trade-offs.  Using the
   same technique for hiding the ALPN and encrypting the SNI may result
   in excess complexity.  It might be preferable to encrypt these
   independently.

3.7.2.  Supporting Other Transports than TCP

   The TLS handshake is also used over other transports, such as UDP
   with both DTLS [DTLS-1.3] and QUIC [QUIC].  The requirement to
   encrypt the SNI applies just as well for these transports as for TLS
   over TCP.

   This points to a requirement for SNI encryption mechanisms to also be
   applicable to non-TCP transports such as DTLS or QUIC.

4.  HTTP Co-tenancy Fronting

   In the absence of TLS-level SNI encryption, many sites rely on an
   "HTTP co-tenancy" solution, often referred to as "domain fronting"
   [DOMFRONT].  The TLS connection is established with the fronting
   server, and HTTP requests are then sent over that connection to the
   hidden service.  For example, the TLS SNI could be set to
   "fronting.example.com" (the fronting server), and HTTP requests sent
   over that connection could be directed to "hidden.example.com"
   (accessing the hidden service).  This solution works well in practice
   when the fronting server and the hidden server are "co-tenants" of
   the same multiplexed server.

   The HTTP domain fronting solution can be deployed without
   modification to the TLS protocol and does not require using any
   specific version of TLS.  There are, however, a few issues regarding
   discovery, client implementations, trust, and applicability:

   *  The client has to discover that the hidden service can be accessed
      through the fronting server.

   *  The client's browser has to be directed to access the hidden
      service through the fronting service.

   *  Since the TLS connection is established with the fronting service,
      the client has no cryptographic proof that the content does, in
      fact, come from the hidden service.  Thus, the solution does not
      mitigate the context sharing issues described in Section 3.6.
      Note that this is already the case for co-tenanted sites.

   *  Since this is an HTTP-level solution, it does not protect non-HTTP
      protocols, as discussed in Section 3.7.

   The discovery issue is common to most SNI encryption solutions.  The
   browser issue was solved in [DOMFRONT] by implementing domain
   fronting as a pluggable transport for the Tor browser.  The multi-
   protocol issue can be mitigated by implementing other applications
   over HTTP, for example, DNS over HTTPS [RFC8484].  The trust issue,
   however, requires specific developments.

4.1.  HTTPS Tunnels

   The HTTP domain fronting solution places a lot of trust in the
   fronting server.  This required trust can be reduced by tunneling
   HTTPS in HTTPS, which effectively treats the fronting server as an
   HTTP proxy.  In this solution, the client establishes a TLS
   connection to the fronting server and then issues an HTTP connect
   request to the hidden server.  This will establish an end-to-end
   HTTPS-over-TLS connection between the client and the hidden server,
   mitigating the issues described in Section 3.6.

   The HTTPS-in-HTTPS solution requires double encryption of every
   packet.  It also requires that the fronting server decrypt and relay
   messages to the hidden server.  Both of these requirements make the
   implementation onerous.

4.2.  Delegation Control

   Clients would see their privacy compromised if they contacted the
   wrong fronting server to access the hidden service, since this wrong
   server could disclose their access to adversaries.  This requires a
   controlled way to indicate which fronting server is acceptable by the
   hidden service.

   This problem is similar to the "word of mouth" variant of the
   "fronting server spoofing" attack described in Section 3.6.  The
   spoofing would be performed by distributing fake advice, such as "to
   reach hidden.example.com, use fake.example.com as a fronting server",
   when "fake.example.com" is under the control of an adversary.

   In practice, this attack is well mitigated when the hidden service is
   accessed through a specialized application.  The name of the fronting
   server can then be programmed in the code of the application.  But
   the attack is harder to mitigate when the hidden service has to be
   accessed through general-purpose web browsers.

   There are several proposed solutions to this problem, such as
   creating a special form of certificate to codify the relation between
   the fronting and hidden server or obtaining the relation between the
   hidden and fronting service through the DNS, possibly using DNSSEC,
   to avoid spoofing.  The experiment described in [DOMFRONT] solved the
   issue by integrating with the Lantern Internet circumvention tool.

   We can observe that CDNs have a similar requirement.  They need to
   convince the client that "www.example.com" can be accessed through
   the seemingly unrelated "cdn-node-xyz.example.net".  Most CDNs have
   deployed DNS-based solutions to this problem.  However, the CDN often
   holds the authoritative certificate of the origin.  There is,
   simultaneously, verification of a relationship between the origin and
   the CDN (through the certificate) and a risk that the CDN can spoof
   the content from the origin.

4.3.  Related Work

   The ORIGIN frame defined for HTTP/2 [RFC8336] can be used to flag
   content provided by the hidden server.  Secondary certificate
   authentication [HTTP2-SEC-CERTS] can be used to manage authentication
   of hidden server content or to perform client authentication before
   accessing hidden content.

5.  Security Considerations

   This document lists a number of attacks against SNI encryption in
   Sections 3 and 4.2 and presents a list of requirements to mitigate
   these attacks.  Current HTTP-based solutions described in Section 4
   only meet some of these requirements.  In practice, it may well be
   that no solution can meet every requirement and that practical
   solutions will have to make some compromises.

   In particular, the requirement to not stick out, presented in
   Section 3.4, may have to be lifted, especially for proposed solutions
   that could quickly reach large-scale deployments.

   Replacing cleartext SNI transmission by an encrypted variant will
   break or reduce the efficacy of the operational practices and
   techniques implemented in middleboxes, as described in Section 2.1.
   As explained in Section 2.3, alternative solutions will have to be
   developed.

6.  IANA Considerations

   This document has no IANA actions.

7.  Informative References

   [DOMFRONT] Fifield, D., Lan, C., Hynes, R., Wegmann, P., and V.
              Paxson, "Blocking-resistant communication through domain
              fronting", DOI 10.1515/popets-2015-0009, 2015,
              <https://www.bamsoftware.com/papers/fronting/>.

   [DTLS-1.3] Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", Work in Progress, Internet-Draft, draft-ietf-tls-
              dtls13-38, 29 May 2020,
              <https://tools.ietf.org/html/draft-ietf-tls-dtls13-38>.

   [HTTP2-SEC-CERTS]
              Bishop, M., Sullivan, N., and M. Thomson, "Secondary
              Certificate Authentication in HTTP/2", Work in Progress,
              Internet-Draft, draft-ietf-httpbis-http2-secondary-certs-
              06, 14 May 2020, <https://tools.ietf.org/html/draft-ietf-
              httpbis-http2-secondary-certs-06>.

   [QUIC]     Thomson, M. and S. Turner, "Using TLS to Secure QUIC",
              Work in Progress, Internet-Draft, draft-ietf-quic-tls-29,
              9 June 2020,
              <https://tools.ietf.org/html/draft-ietf-quic-tls-29>.

   [RFC2246]  Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
              RFC 2246, DOI 10.17487/RFC2246, January 1999,
              <https://www.rfc-editor.org/info/rfc2246>.

   [RFC3207]  Hoffman, P., "SMTP Service Extension for Secure SMTP over
              Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207,
              February 2002, <https://www.rfc-editor.org/info/rfc3207>.

   [RFC3546]  Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
              and T. Wright, "Transport Layer Security (TLS)
              Extensions", RFC 3546, DOI 10.17487/RFC3546, June 2003,
              <https://www.rfc-editor.org/info/rfc3546>.

   [RFC4346]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.1", RFC 4346,
              DOI 10.17487/RFC4346, April 2006,
              <https://www.rfc-editor.org/info/rfc4346>.

   [RFC4366]  Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
              and T. Wright, "Transport Layer Security (TLS)
              Extensions", RFC 4366, DOI 10.17487/RFC4366, April 2006,
              <https://www.rfc-editor.org/info/rfc4366>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <https://www.rfc-editor.org/info/rfc5246>.

   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066,
              DOI 10.17487/RFC6066, January 2011,
              <https://www.rfc-editor.org/info/rfc6066>.

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
              2014, <https://www.rfc-editor.org/info/rfc7258>.

   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,
              <https://www.rfc-editor.org/info/rfc7540>.

   [RFC7590]  Saint-Andre, P. and T. Alkemade, "Use of Transport Layer
              Security (TLS) in the Extensible Messaging and Presence
              Protocol (XMPP)", RFC 7590, DOI 10.17487/RFC7590, June
              2015, <https://www.rfc-editor.org/info/rfc7590>.

   [RFC7858]  Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
              and P. Hoffman, "Specification for DNS over Transport
              Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
              2016, <https://www.rfc-editor.org/info/rfc7858>.

   [RFC8314]  Moore, K. and C. Newman, "Cleartext Considered Obsolete:
              Use of Transport Layer Security (TLS) for Email Submission
              and Access", RFC 8314, DOI 10.17487/RFC8314, January 2018,
              <https://www.rfc-editor.org/info/rfc8314>.

   [RFC8336]  Nottingham, M. and E. Nygren, "The ORIGIN HTTP/2 Frame",
              RFC 8336, DOI 10.17487/RFC8336, March 2018,
              <https://www.rfc-editor.org/info/rfc8336>.

   [RFC8404]  Moriarty, K., Ed. and A. Morton, Ed., "Effects of
              Pervasive Encryption on Operators", RFC 8404,
              DOI 10.17487/RFC8404, July 2018,
              <https://www.rfc-editor.org/info/rfc8404>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [RFC8484]  Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
              <https://www.rfc-editor.org/info/rfc8484>.

Acknowledgements

   A large part of this document originated in discussion of SNI
   encryption on the TLS WG mailing list, including comments after the
   tunneling approach was first proposed in a message to that list:
   <https://mailarchive.ietf.org/arch/msg/tls/
   tXvdcqnogZgqmdfCugrV8M90Ftw>.

   Thanks to Eric Rescorla for his multiple suggestions, reviews, and
   edits to the successive draft versions of this document.

   Thanks to Daniel Kahn Gillmor for a pretty detailed review of the
   initial draft of this document.  Thanks to Bernard Aboba, Mike
   Bishop, Alissa Cooper, Roman Danyliw, Stephen Farrell, Warren Kumari,
   Mirja Kuelewind, Barry Leiba, Martin Rex, Adam Roach, Meral
   Shirazipour, Martin Thomson, Eric Vyncke, and employees of the UK
   National Cyber Security Centre for their reviews.  Thanks to Chris
   Wood, Ben Kaduk, and Sean Turner for helping move this document
   toward publication.

Author's Address

   Christian Huitema
   Private Octopus Inc.
   Friday Harbor, WA 98250
   United States of America

   Email: huitema@huitema.net