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TLS Working Group                                            V. Vasiliev
Internet-Draft                                                    Google
Intended status: Standards Track                        15 December 2020
Expires: 18 June 2021

     Transport Layer Security (TLS) Resumption across Server Names


   This document specifies a way for the parties in the Transport Layer
   Security (TLS) protocol to indicate that an individual session ticket
   can be used to perform resumption even if the Server Name of the new
   connection does not match the Server Name of the original.

Discussion Venues

   This note is to be removed before publishing as an RFC.

   Discussion of this document takes place on the TLS Working Group
   mailing list (tls@ietf.org), which is archived at

   Source for this draft and an issue tracker can be found at

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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 18 June 2021.

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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
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   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   3
   3.  The Extension . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   3
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   4
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   Transport Layer Security protocol [RFC8446] allows the clients to use
   an abbreviated handshake in cases where the client has previously
   established a secure session with the same server.  This mechanism is
   known as "session resumption", and its positive impact on performance
   makes it desirable to be able to use it as frequently as possible.

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   Modern application-level protocols, HTTP in particular, often require
   accessing multiple servers within a single workflow.  Since the
   identity of the server is established through its certificate, in the
   ideal case, the resumption would be possible to all of the domains
   for which the certificate is valid (see [PERF] for a survey of
   potential practical impact of such approach).  TLS, starting with
   version 1.3, defines the SNI value to be a property of an individual
   connection that is not retained across sessions ([RFC8446],
   Section 4.2.11).  However, in the absence of additional signals, it
   discourages using a session ticket when the SNI value does not match
   ([RFC8446], Section 4.6.1), as there is normally no reason to assume
   that all servers sharing the same certificate would also share the
   same session keys.  The extension defined in this document allows the
   server to provide such a signal in-band.

2.  Conventions and Definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  The Extension

   The server MAY send a resumption_across_names(TBD) extension in a
   NewSessionTicket message.  That extension SHALL have an empty body.
   If the extension is sent, it indicates that the client MAY use the
   ticket for any SNI value for which the certificate presented by the
   server is valid.  The server MUST handle the ticket correctly by
   either resuming and using a new SNI provided by the client, or by
   ignoring the ticket.

   The server MAY send the extension if it reasonably believes that any
   server for any identity presented in its certificate would be capable
   of accepting that ticket.  The server SHOULD NOT send the extension
   otherwise, since, if the client follows the single-use ticket policy
   recommended by [RFC8446], sending the ticket results in it being no
   longer usable regardless of whether resumption has succeeded.

4.  Security Considerations

   This document does not alter any of the security requirements of
   [RFC8446], but merely lifts a performance-motivated "SHOULD NOT"
   recommendation from Section 4.6.1.  Notably, it still relies on the
   server certificate being re-validated against the new SNI at the
   session resumption time.

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   If a client certificate has been associated with the session, the
   client MUST use the same policy on whether to present said
   certificate to the server as if it were a new TLS session.  For
   instance, if the client would show a certificate choice prompt for
   every individual domain it connects to, it MUST show that prompt for
   the new host when performing cross-domain resumption.

   Cross-domain resumption, like other similar mechanisms (e.g. cross-
   domain HTTP connection reuse), can incentivize the server deployments
   to create server certificates valid for a wider range of domains than
   they would otherwise.  However, any increase in the scope of a
   certificate comes at a cost: the wider is the scope of the
   certificate, the wider is the impact of the key compromise for that
   certificate.  In addition, creating a certificate that is valid for
   multiple hostnames can lead to complications if some of those
   hostnames change ownership, or otherwise require a different
   operational domain.

   Session tickets can contain arbitrary information, and thus could be
   potentially used to re-identify a user from a previous connection.
   Cross-domain resumption expands the potential list of servers to
   which an individual ticket could be presented.  Client applications
   should partition the session cache between connections that are meant
   to be uncorrelated.  For example, the Web use case uses network
   partition keys to separate cache lookups [FETCH].

5.  IANA Considerations

   IANA (will add/has added) the following entry to the "TLS
   ExtensionType Values" table of the "Transport Layer Security (TLS)
   Extensions" registry:

   Value  TBD

   Extension Name  resumption_across_names

   TLS 1.3  NST

   Recommended  N

   Reference  This document

6.  References

6.1.  Normative References

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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,

6.2.  Informative References

   [FETCH]    WHATWG, "Fetch Standard", December 2020,

   [PERF]     Sy, E., Moennich, M., Mueller, T., Federrath, H., and M.
              Fischer, "Enhanced Performance for the encrypted Web
              through TLS Resumption across Hostnames", 7 February 2019,


   Cross-name resumption has been previously implemented in the QUIC
   Crypto protocol as a preloaded list of hostnames.

   Erik Sy has previously proposed a similar mechanism for TLS, draft-
   sy-tls-resumption-group (https://datatracker.ietf.org/doc/draft-sy-
   tls-resumption-group/).  This document incorporates ideas from that

   This document has benefited from contributions and suggestions from
   David Benjamin, Nick Harper, David Schinazi, Ryan Sleevi, Ian Swett
   and many others.

Author's Address

   Victor Vasiliev

   Email: vasilvv@google.com

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