Network Working Group                                           T. Pauly
Internet-Draft                                                Apple Inc.
Intended status: Standards Track                             D. Schinazi
Expires: May 7, 2020                                          Google LLC
                                                                 C. Wood
                                                              Apple Inc.
                                                       November 04, 2019

                          TLS Ticket Requests


   TLS session tickets enable stateless connection resumption for
   clients without server-side, per-client state.  Servers vend an
   arbitrary number of session tickets to clients, at their discretion,
   upon connection establishment.  Clients store and use tickets when
   resuming future connections.  This document describes a mechanism by
   which clients can specify the desired number of tickets needed for
   future connections.  This extension aims to provide a means for
   servers to determine the number of tickets to generate in order to
   reduce ticket waste, while simultaneously priming clients for future
   connection attempts.

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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   This Internet-Draft will expire on May 7, 2020.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
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   described in the Simplified BSD License.

Table of Contents

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

1.  Introduction

   As per [RFC5077], and as described in [RFC8446], TLS servers vend
   clients an arbitrary number of session tickets at their own
   discretion in NewSessionTicket messages.  There are two limitations
   with this design.  First, servers choose some (often hard-coded)
   number of tickets vended per connection.  Second, clients do not have
   a way of expressing their desired number of tickets, which can impact
   future connection establishment.  For example, clients can open
   multiple TLS connections to the same server for HTTP, or race TLS
   connections across different network interfaces.  The latter is
   especially useful in transport systems that implement Happy Eyeballs
   [RFC8305].  Since clients control connection concurrency and
   resumption, a standard mechanism for requesting more than one ticket
   is desirable.

   This document specifies a new TLS extension - "ticket_request" - that
   can be used by clients to express their desired number of session
   tickets.  Servers can use this extension as a hint of the number of
   NewSessionTicket messages to vend.  This extension is only applicable
   to TLS 1.3 [RFC8446], DTLS 1.3 [I-D.ietf-tls-dtls13], and future
   versions thereof.

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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.

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 [TAPS] also describes
      how connections can 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 can 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.  As an
      example, clients might only request one ticket during resumption.
      Moreover, as ticket generation might involve expensive
      computation, e.g., public key cryptographic operations, avoiding
      waste is desirable.

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   o  Decline resumption: Clients can indicate they have no intention of
      resuming connections by sending a ticket request with count of

3.  Ticket Requests

   Clients can indicate to servers their desired number of tickets for a
   single connection via the following "ticket_request" extension:

   enum {
       ticket_request(TBD), (65535)
   } ExtensionType;

   Clients MAY send this extension in ClientHello.  It contains the
   following structure:

   struct {
       uint8 count;
   } TicketRequestContents;

   count  The number of tickets desired by the client.

   A supporting server MAY use TicketRequestContents.count when
   determining how many NewSessionTicket messages to send to a
   requesting client, and SHOULD place a limit on the number of tickets
   sent.  The number of NewSessionTicket messages sent SHOULD be the
   minimum of the server's self-imposed limit and

   Servers that support ticket requests MUST NOT echo "ticket_request"
   in the EncryptedExtensions message.  A client MUST abort the
   connection with an "illegal_parameter" alert if the "ticket_request"
   extension is present in the EncryptedExtensions message.

   If a client receives a HelloRetryRequest, the presence (or absence)
   of the "ticket_request" extension MUST be maintained in the second
   ClientHello message.  Moreover, if this extension is present, a
   client MUST NOT change the value of TicketRequestContents.count in
   the second ClientHello message.

4.  IANA Considerations

   IANA is requested to Create an entry, ticket_request(TBD), in the
   existing registry for ExtensionType (defined in [RFC8446]), with "TLS
   1.3" column values being set to "CH", and "Recommended" column being
   set to "Yes".

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5.  Security Considerations

   Ticket re-use is a security and privacy concern.  Moreover, clients
   must take care when pooling tickets as a means of avoiding or
   amortizing handshake costs.  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 pooled tickets after some reasonable amount of time that
   mimics the ticket rotation period.

   Servers that do not enforce a limit on the number of NewSessionTicket
   messages sent in response to a "ticket_request" extension could leave
   themselves open to DoS attacks, especially if ticket creation is

6.  Acknowledgments

   The authors would like to thank David Benjamin, Eric Rescorla, Nick
   Sullivan, Martin Thomson, Hubert Kario, and other members of the TLS
   Working Group for discussions on earlier versions of this draft.

7.  References

7.1.  Normative References

              Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", draft-ietf-tls-dtls13-33 (work in progress), October

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

   [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, <>.

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

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

7.2.  Informative References

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

   [TAPS]     Brunstrom, A., Pauly, T., Enghardt, T., Grinnemo, K.,
              Jones, T., Tiesel, P., Perkins, C., and M. Welzl,
              "Implementing Interfaces to Transport Services", draft-
              ietf-taps-impl-04 (work in progress), July 2019.

Authors' Addresses

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


   David Schinazi
   Google LLC
   1600 Amphitheatre Parkway
   Mountain View, California 94043
   United States of America


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


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