Network Working Group                                            C. Wood
Internet-Draft                                                  T. Pauly
Intended status: Informational                               D. Schinazi
Expires: October 14, 2018                                     Apple Inc.
                                                          April 12, 2018

                          TLS Ticket Requests


   TLS session tickets enable stateless connection resumption for
   clients without server-side per-client state.  Servers vend session
   tickets to clients, at their discretion, upon connection
   establishment.  Clients store and use tickets when resuming future
   connections.  Moreover, clients should use tickets at most once for
   session resumption, especially if such keying material protects early
   application data.  Single-use tickets bound the number of parallel
   connections a client may initiate by the number of tickets received
   from a given server.  To address this limitation, this document
   describes a mechanism by which clients may request tickets as needed
   during a connection.

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

   This Internet-Draft will expire on October 14, 2018.

Copyright Notice

   Copyright (c) 2018 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

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   ( 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
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   2
   2.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Ticket Requests . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Negotiation . . . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   5
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .   5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   As per [RFC5077], and as described in [I-D.ietf-tls-tls13], TLS
   servers send clients session tickets at their own discretion in
   NewSessionTicket messages.  Clients are in complete control of how
   many tickets they may use when establishing future and subsequent
   connections.  For example, clients may open multiple TLS connections
   to the same server for HTTP, or may race TLS connections across
   different network interfaces.  The latter is especially useful in
   transport systems that implement Happy Eyeballs [RFC8305].  Since
   connection concurrency and resumption is controlled by clients, a
   mechanism to request tickets on demand is desirable.

   This document specifies a new TLS post-handshake message -
   TicketRequest - that may be used to request tickets via
   NewSessionTicket messages in TLS 1.3.  Ticket requests may carry
   optional application-specific contexts to define the ways in which
   tickets may be used.  NewSessionTicket responses reciprocate this
   application context in an extension.

1.1.  Requirements Language

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

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2.  Use Cases

   The ability to request one or more tickets is useful for a variety of

   o  Parallel HTTP connections: To minimize ticket reuse while still
      improving performance, it may be useful to use multiple, distinct
      tickets when opening parallel connections.  Clients must therefore
      bound the number of parallel connections they initiate by the
      number of tickets in their possession, or risk ticket re-use.

   o  Connection racing: Happy Eyeballs V2 [RFC8305] describes
      techniques for performing connection racing.  The Transport
      Services Architecture implementation from
      [I-D.brunstrom-taps-impl] also describes how connections may race
      across interfaces and address families.  In cases where clients
      have early data to send and want to minimize or avoid ticket re-
      use, unique tickets for each unique connection attempt are useful.
      Moreover, as some servers may implement single-use tickets (and
      even session ticket encryption keys), distinct tickets will be
      needed to prevent premature ticket invalidation by racing.

   o  Connection priming: In some systems, connections may be primed or
      bootstrapped by a centralized service or daemon for faster
      connection establishment.  Requesting tickets on demand allows
      such services to vend tickets to clients to use for accelerated
      handshakes with early data.  (Note that if early data is not
      needed by these connections, this method SHOULD NOT be used.
      Fresh handshakes SHOULD be performed instead.)

   o  Less ticket waste: Currently, TLS servers use application-
      specific, and often implementation-specific, logic to determine
      how many tickets to issue.  By moving the burden of ticket count
      to clients, servers do not generate wasteful tickets for clients.

3.  Ticket Requests

   TLS tickets may be requested via a TicketRequest post-handshake
   message, ticket_request(TBD).  Its structure is shown below.

   struct {
       opaque identifier<0..255>;
       opaque context<0..2^16-1>;
   } TicketRequest;

   o  identifier: A unique value for this ticket request.  Clients
      SHOULD fill this in with a monotonically increasing counter.

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   o  context: An opaque context to be used when generating the ticket
      request.  Clients and servers may use this context to implement or
      exchange data to be included in the ticket computation.  Clients
      SHOULD make this field empty if it is not needed.

   Upon receipt of a TicketRequest message, servers MAY reply with a
   NewSessionTicket message, as defined in [I-D.ietf-tls-tls13].  The
   latter message MUST carry two extensions, ticket_identifer and
   ticket_context, defined below.

   enum {
   } ExtensionType;

   The value of ticket_identifier MUST match that of the corresponding
   TicketRequest identifier field.  The value of ticket_context MAY be
   used by servers to convey ticket context to clients.  Its value MUST
   be empty if the corresponding TicketRequest context field is empty.

   Servers SHOULD place a limit on the number of tickets they are
   willing to vend to clients.  Servers MUST NOT send more than 255
   tickets to clients, as this is the limit imposed by the request and
   response identifier size.  Servers SHOULD NOT send unsolicited
   NewSessionTickets to clients that express support for TicketRequests.

   When operated over an unreliable transport, e.g., DTLS, servers may
   not always be able to identify retransmissions of ticket requests.
   In this case, when a server S receives a TicketRequest with new
   identifier N it MUST generate a new ticket and SHOULD cache it
   locally for some period of time T.  If S receives a TicketRequest
   with identifier N within time period T, S SHOULD reply with the same
   ticket previously generated (and cached).  If S receives a
   TicketRequest with identifier N outside time period T, S SHOULD reply
   with an empty NewSessionTicket, i.e., a NewSessionTicket with
   extension ticket_identifier carrying N, appropriate ticket_context
   extension, and empty ticket field.

4.  Negotiation

   Clients negotiate use of ticket requests via a new ExtensionType,
   ticket_request(TBD).  The extension_data for this extension MUST be
   empty, i.e., have length of 0.  Servers that support ticket requests
   MAY echo this extension in the EncryptedExtensions.  Clients MUST NOT
   send ticket requests to servers that do not signal support for this

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   message.  If absent from a ClientHello, servers MUST NOT generate
   responses to TicketRequests issued by the client.

5.  IANA Considerations

   ((TODO: codepoint for post-handshake message type and extensions))

6.  Security Considerations

   Ticket re-use is a security and privacy concern.  Moreover, pre-
   fetching as a means of avoiding or amortizing handshake costs must be
   used carefully.  If servers do not rotate session ticket encryption
   keys frequently, clients may be encouraged to obtain and use tickets
   beyond common lifetime windows of, e.g., 24 hours.  Despite ticket
   lifetime hints provided by servers, clients SHOULD dispose of pre-
   fetched tickets after some reasonable amount of time that mimics the
   ticket rotation period.

7.  Acknowledgments

   The authors would like to thank Eric Rescorla, Martin Thomson, and
   Nick Sullivan for discussions on earlier versions of this draft.

8.  Normative References

              Brunstrom, A., Pauly, T., Enghardt, T., Grinnemo, K.,
              Jones, T., Tiesel, P., Perkins, C., and M. Welzl,
              "Implementing Interfaces to Transport Services", draft-
              brunstrom-taps-impl-00 (work in progress), March 2018.

              Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", draft-ietf-tls-tls13-28 (work in progress),
              March 2018.

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

   [RFC5077]  Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
              "Transport Layer Security (TLS) Session Resumption without
              Server-Side State", RFC 5077, DOI 10.17487/RFC5077,
              January 2008, <>.

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   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [RFC8305]  Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2:
              Better Connectivity Using Concurrency", RFC 8305,
              DOI 10.17487/RFC8305, December 2017, <https://www.rfc-

Authors' Addresses

   Christopher A. Wood
   Apple Inc.
   One Apple Park Way
   Cupertino, California 95014
   United States of America


   Tommy Pauly
   Apple Inc.
   One Apple Park Way
   Cupertino, California 95014
   United States of America


   David Schinazi
   Apple Inc.
   One Apple Park Way
   Cupertino, California 95014
   United States of America


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