Network Working Group                                           T. Pauly
Internet-Draft                                                Apple Inc.
Intended status: Standards Track                             D. Schinazi
Expires: 23 May 2021                                          Google LLC
                                                               C.A. Wood
                                                        19 November 2020

                          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.

Discussion Venues

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

   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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   This Internet-Draft will expire on 23 May 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 (
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
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   provided without warranty as 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 . . . . . . . . . . . . . . . . . . . . .   6
   5.  Performance Considerations  . . . . . . . . . . . . . . . . .   6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   As as described in [RFC8446], TLS servers vend clients an arbitrary
   number of session tickets at their own discretion in NewSessionTicket
   messages.  There are at least three limitations with this design.

   First, servers vend some (often hard-coded) number of tickets per
   connection.  Some server implementations return a different default
   number of tickets for session resumption than for the initial
   connection that created the session.  No static choice, whether
   fixed, or resumption-dependent is ideal for all situations.

   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 parallel 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

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   requesting more than one ticket is desirable for avoiding ticket
   reuse.  See [RFC8446], Appendix C.4 for discussion of ticket reuse

   Third, all tickets in the client's possession ultimately derive from
   some initial connection.  Especially when the client was initially
   authenticated with a client certificate, that session may need to be
   refreshed from time to time.  Consequently, a server may periodically
   force a new connection even when the client presents a valid ticket.
   When that happens, it is possible that any other tickets derived from
   the same original session are equally invalid.  A client avoids a
   full handshake on subsequent connections if it replaces all stored
   tickets with new ones obtained from the just performed full
   handshake.  The number of tickets the server should vend for a new
   connection may therefore need to be larger than the number for
   routine resumption.

   This document specifies a new TLS extension - "ticket_request" - that
   clients can use to express their desired number of session tickets.
   Servers can use this extension as a hint for 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 of (D)TLS.

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

   *  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.

   *  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 such cases, clients may use more than one ticket while racing

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      connection attempts in order to establish one successful
      connection.  Having multiple tickets equips clients with enough
      tickets to initiate connection racing while avoiding ticket re-use
      and ensuring that their cache of tickets does not empty during
      such races.  Moreover, as some servers may implement single-use
      tickets, distinct tickets prevent premature ticket invalidation by

   *  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.

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

3.  Ticket Requests

   As discussed in Section 1, clients may want different numbers of
   tickets for new or resumed connections.  Clients may indicate to
   servers their desired number of tickets to receive on a single
   connection, in the case of a new or resumed 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 new_session_count;
       uint8 resumption_count;
   } ClientTicketRequest;

   new_session_count  The number of tickets desired by the client when
      the server chooses to negotiate a new connection.

   resumption_count  The number of tickets desired by the client when
      the server is willing to resume using a ticket presented in this

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   A client starting a new connection SHOULD set new_session_count to
   the desired number of session tickets and resumption_count to 0.
   Once a client's ticket cache is primed, a resumption_count of 1 is a
   good choice that allows the server to replace each ticket with a new
   ticket, without over-provisioning the client with excess tickets.
   However, clients which race multiple connections and place a separate
   ticket in each will ultimately end up with just the tickets from a
   single resumed session.  In that case, clients can send a
   resumption_count equal to the number of sessions they are attempting
   in parallel.  (Clients which send a resumption_count less than the
   number of parallel connection attempts might end up with zero

   When a client presenting a previously obtained ticket finds that the
   server nevertheless negotiates a new connection, the client SHOULD
   assume that any other tickets associated with the same session as the
   presented ticket are also no longer valid for resumption.  This
   includes tickets obtained during the initial (new) connection and all
   tickets subsequently obtained as part of subsequent resumptions.
   Requesting more than one ticket in cases when servers complete a new
   connection helps keep the session cache primed.

   Servers SHOULD NOT send more tickets than requested for the
   connection type selected by the server (new or resumed connection).
   Moreover, servers SHOULD place a limit on the number of tickets they
   are willing to send, whether for new or resumed connections, to save
   resources.  Therefore, the number of NewSessionTicket messages sent
   will typically be the minimum of the server's self-imposed limit and
   the number requested.  Servers MAY send additional tickets, typically
   using the same limit, if the tickets that are originally sent are
   somehow invalidated.

   A server which supports and uses a client "ticket_request" extension
   MUST also send the "ticket_request" extension in the
   EncryptedExtensions message.  It contains the following structure:

   struct {
       uint8 expected_count;
   } ServerTicketRequestHint;

   expected_count  The number of tickets the server expects to send in
      this connection.

   Servers MUST NOT send the "ticket_request" extension in any handshake
   message, including ServerHello or HelloRetryRequest messages.  A
   client MUST abort the connection with an "illegal_parameter" alert if
   the "ticket_request" extension is present in any server handshake

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   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 ClientTicketRequest 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, EE", and "Recommended" column
   being set to "Y".

5.  Performance Considerations

   Servers can send tickets in NewSessionTicket messages any time after
   the server Finished message (see [RFC8446]; Section 4.6.1).  A server
   which chooses to send a large number of tickets to a client can
   potentially harm application performance if the tickets are sent
   before application data.  For example, if the transport connection
   has a constrained congestion window, ticket messages could delay
   sending application data.  To avoid this, servers should prioritize
   sending application data over tickets when possible.

6.  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 cached tickets after some reasonable amount of time that
   mimics the session ticket encryption key rotation period.
   Specifically, as specified in Section 4.6.1 of [RFC8446], clients
   MUST NOT cache tickets for longer than 7 days.

   In some cases, a server may send NewSessionTicket messages
   immediately upon sending the server Finished message rather than
   waiting for the client Finished.  If the server has not verified the
   client's ownership of its IP address, e.g., with the TLS Cookie
   extension (see [RFC8446]; Section 4.2.2), an attacker may take
   advantage of this behavior to create an amplification attack
   proportional to the count value toward a target by performing a
   (DTLS) key exchange over UDP with spoofed packets.  Servers SHOULD
   limit the number of NewSessionTicket messages they send until they
   have verified the client's ownership of its IP address.

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   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

7.  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.
   Viktor Dukhovni contributed text allowing clients to send multiple
   counts in a ticket request.

8.  References

8.1.  Normative References

              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-
              dtls13-39, 2 November 2020, <

   [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, <>.

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

8.2.  Informative References

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

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   [TAPS]     Brunstrom, A., Pauly, T., Enghardt, T., Grinnemo, K.,
              Jones, T., Tiesel, P., Perkins, C., and M. Welzl,
              "Implementing Interfaces to Transport Services", Work in
              Progress, Internet-Draft, draft-ietf-taps-impl-08, 2
              November 2020, <

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
   101 Townsend St
   San Francisco,
   United States of America


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