Network Working Group                                         M. Thomson
Internet-Draft                                                   Mozilla
Updates: 7540, 8441 (if approved)                       16 December 2020
Intended status: Standards Track
Expires: 19 June 2021

            Optimizations for Using TLS Early Data in HTTP/2


   This proposes an extension to HTTP/2 that enables the use of server
   settings by clients that send requests in TLS early data.  In
   particular, this allows extensions to the protocol to be used.

   This amends the definition of settings defined in RFC 7540 and RFC
   8441 and introduces new registration requirements for HTTP/2

Discussion Venues

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

   Discussion of this document takes place on the HTTP Working Group
   mailing list (, which is archived at

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   This Internet-Draft will expire on 19 June 2021.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   3
   3.  EARLY_DATA_SETTINGS Setting . . . . . . . . . . . . . . . . .   3
     3.1.  Server Handling . . . . . . . . . . . . . . . . . . . . .   3
     3.2.  Client Handling . . . . . . . . . . . . . . . . . . . . .   4
     3.3.  Use for Resumption  . . . . . . . . . . . . . . . . . . .   4
   4.  Settings in Early Data  . . . . . . . . . . . . . . . . . . .   5
     4.1.  Existing HTTP/2 Settings  . . . . . . . . . . . . . . . .   5
     4.2.  CONNECT Protocol  . . . . . . . . . . . . . . . . . . . .   6
     4.3.  Replay Attack Risk  . . . . . . . . . . . . . . . . . . .   7
     4.4.  Advertising Less-Permissive Values  . . . . . . . . . . .   7
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   HTTP/2 [HTTP2] does not include any special provisions for the use of
   TLS early data as it was published prior to the introduction the
   feature in TLS 1.3 [TLS].  As a result, when using HTTP/2 with TLS
   early data, clients are forced to assume defaults for the server

   Using the initial value of settings can adversely affect performance
   as it can take an additional round trip or two to receive the
   connection preface from the server.  This is especially noticeable
   for new features that are added using extensions.  Clients that wish
   to use extensions therefore have to deal with extended delays before
   they can confirm server support for the extension.

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   In contrast, HTTP/3 [HTTP3] was defined for use with QUIC [QUIC],
   which includes early data (or 0-RTT) as a core features.  The use in
   HTTP/3 demonstrates the value of access to non-default values of
   server configuration, especially for performance.

   This document defines a new setting for servers and clients to
   indicate a willingness to remember settings from a previous
   connection when attempting TLS early data.  This allows clients to
   rely on capabilities established in a previous connection.  This also
   offers servers the ability to place tighter restrictions on use of
   early data than the initial values of settings otherwise allows.

2.  Conventions and Definitions

   This document relies on concepts from [HTTP2] and [TLS].


   The EARLY_DATA_SETTINGS setting (0xTBD) is sent to indicate support
   for remembering the value of settings in TLS early data.

   A server that advertises a value for EARLY_DATA_SETTINGS of 1 MUST
   remember all settings defined as being applicable to early data; see
   Section 4.  A client that advertises a value for EARLY_DATA_SETTINGS
   of 1 and has received a value of 1 from a server MUST respect these
   settings when attempting early data.

3.1.  Server Handling

   An EARLY_DATA_SETTINGS value of 1 indicates that the server will
   respect any settings that can apply to early data if it accepts the
   early data; see Section 4.  A value of 0, the initial value,
   indicates that settings assume their initial values for resumed
   connections (that is, the default behavior in HTTP/2).

   Any session tickets that are sent by the server subsequent to a
   SETTINGS frame containing EARLY_DATA_SETTINGS set to 1 are affected
   by this feature.  The value of all applicable settings apply to each
   session ticket as TLS NewSessionTicket messages are received.

   In addition, setting a value of 1 in the SETTINGS frame that is part
   of the connection preface has the effect of applying to all session
   tickets sent prior to that point; the settings that are used for
   those session tickets is taken from the connection preface.

   Initial values for settings are used if those settings are not
   explicitly sent in a SETTINGS frame.

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   A server does not need to wait for a SETTINGS acknowledgment before
   it sends a TLS NewSessionTicket message.  Values from SETTINGS frames
   apply immediately to any subsequent TLS NewSessionTicket messages.

   Note:  As the arrival of SETTINGS frames is strictly ordered with
      respect to TLS NewSessionTicket messages, this ensures that the
      value of settings that apply to each session ticket is

   Once set to a value of 1, a server can set this value to 0 in
   subsequent SETTINGS frames to indicate that updated settings values
   do not apply to early data.  This could be used by a server to set
   values that are more permissive than it might be willing to accept
   for early data.

   A server that might have set EARLY_DATA_SETTINGS to 1 and does not
   remember the value of settings MUST reject early data.  Similarly, a
   server that cannot respect the values that it previously set MUST
   reject early data.

   A server that advertises a value of 1 MUST remember settings even if
   the client does not indicate support for EARLY_DATA_SETTINGS.

3.2.  Client Handling

   A client advertises a value of 1 for EARLY_DATA_SETTINGS to indicate
   that it will respect the settings that a server sets when attempting
   to use early data if the server also advertises a value of 1; see
   Section 4.4.

   A client that advertises a value of 1 for EARLY_DATA_SETTINGS MUST
   remember the value of all applicable server settings at the time that
   a TLS NewSessionTicket was received if the server settings include a
   a value of 1 for EARLY_DATA_SETTINGS.  These settings values are then
   used for server settings in place of initial values if early data is
   accepted by the server.

   A client MUST NOT set a value of 0 for EARLY_DATA_SETTINGS after it
   advertises a value of 1.  A server can treat a change in the value of
   EARLY_DATA_SETTINGS from 1 to 0 as a connection error (see
   Section 5.4.1 of [HTTP2]) of type PROTOCOL_ERROR.

3.3.  Use for Resumption

   It might have been possible to define a similar setting to indicate
   that a server would respect settings for TLS session resumption more
   generally.  This would have the benefit of providing starting values
   for clients that differ from the protocol-defined initial values.

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   However, resumption does not come with a clear rejection signal in
   the same way as early data.  Servers would not have any way to
   invalidate previous settings short of rejecting resumption, which
   could have undesirable performance consequences.  Furthermore, a
   setting of that type would be difficult for clients to adapt to as
   many clients do not currently condition their behavior on whether the
   underlying TLS connection is resumed or full.

   There are potential advantages from the mechanism in this draft as it
   provides a way for clients to use non-initial values for settings
   even where 0.5-RTT data is not sent by the server.  Clients that want
   the performance gains provided by the EARLY_DATA_SETTINGS setting,
   but do not want any exposure to replay attack can use early data and
   limit their use of that to sending the connection preface, which
   carries no risk from replay.

4.  Settings in Early Data

   Some settings cannot apply during TLS early data.  Other settings
   might represent too much of a risk of replay attack.  For a setting
   to be usable in early data, a definition MUST be provided for how the
   value is handled.  This definition MUST include either an analysis
   showing that use of the setting in early data is safe, or rules for
   managing the risk of replay attack arising from its use; see
   Section 4.3 for details.

   Exposure to replay attacks does not automatically disqualify settings
   from use with EARLY_DATA_SETTINGS.  As noted in Section 3.3, there is
   value in being able to use remembered values of settings in place of
   initial values, even if the functions enabled by the setting cannot
   be used in early data.

   Table 1 in Section 6 contains a summary of existing settings and
   whether they are remembered when EARLY_DATA_SETTINGS is enabled.

4.1.  Existing HTTP/2 Settings

   This document amends the definition of extensions defined in [HTTP2]
   to permit their use with early data.

   The ENABLE_PUSH setting only applies to clients.  Though [HTTP2] does
   not prohibit servers from advertising a value, there is no value in
   doing so.  ENABLE_PUSH is marked as not remembered for early data.

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   The other settings defined in [HTTP2] all represent resource limits
   that could apply to early data.  These values can all be remembered
   and applied to early data.  As resource limits, their use does not
   carry actionable information and so none of these settings cannot
   contribute to the risk of replay attacks.

   A server that advertises a value for EARLY_DATA_SETTINGS of 1 MUST
   remember all settings defined in [HTTP2], aside from ENABLE_PUSH.  A
   client that advertises a value for EARLY_DATA_SETTINGS of 1 and
   receives a value of 1 MUST respect these settings when attempting
   early data.

4.2.  CONNECT Protocol

   The setting defined in [HTTP-WS] governs CONNECT requests.  This
   document establishes this setting as applicable to early data.

   Using CONNECT to establish a TCP connection is observable behavior
   that might in itself comprise a risk of replay as it would allow an
   attacker to use replay attacks to learn about any CONNECT requests
   were included in early data.  A server could tentatively allocate a
   connection that was pre-emptively made to a CONNECT request that
   arrives in early data without significant risk of leaking significant
   information, but establishing a connection in reaction to the request
   would leak information.

   In addition, any actions that are taken based on any early data sent
   in the CONNECT tunnel presents a potential risk in the event of a
   replay attack.  Even connection establishment might result in side-
   effects that can be exploited in the event of a replay attack.

   For this reason, a client that sends a CONNECT request in early data
   cannot expect the request to be processed until the handshake is
   complete.  A server MUST delay processing of any CONNECT request
   until the handshake is complete, or reject any attempt with a 425
   (Too Early) status code.

   Though this limits the applicability of the capability,
   SETTINGS_ENABLE_CONNECT_PROTOCOL is marked as requiring servers to
   remember the value when accepting early data.  This allows clients to
   send requests in early data, or before receiving the connection
   preface from the server.

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4.3.  Replay Attack Risk

   Use of TLS early data requires careful consideration of the potential
   for replay attack.  [HTTP-REPLAY] provides a discussion about what
   this means for HTTP requests.  That advice applies to settings that
   might affect the generation or handling of HTTP requests.

   Extensions to HTTP/2 that are used by a client before the handshake
   completes might not be limited to those that affect requests.

   Extensions that are limited in effect to the state of the HTTP/2
   connection have limited exposure to replay attacks.  Replayed
   connection attempts cannot be completed successfully, so any effect
   is discarded.

   Extensions that might affect requests or result in other activity not
   limited to connection state MUST define rules for how the risk of
   replay attack is managed.  Techniques similar to those in
   [HTTP-REPLAY], such as deferral of processing and rejection could be
   used.  Extensions that do not include describe any analysis of or
   mitigations for the risk of replay attack MUST indicate in their
   definition that they cannot be used in 0-RTT.

4.4.  Advertising Less-Permissive Values

   One potential value of advertising the EARLY_DATA_SETTINGS setting is
   that a server is able to restrict the resources that a client can
   consume with early data.  Though TLS provides the max_early_data_size
   field in the early_data extension, which limits the total data that
   the server can accept, there might be other resources that a server
   does not wish to commit.

   If a client does not support EARLY_DATA_SETTINGS, it could consume
   resources up to the limits implied by initial values of settings.
   This includes a number of request streams that is only bounded by the
   value of max_early_data_size.

   A server might choose to condition support for early data on client
   support for EARLY_DATA_SETTINGS, only sending session tickets that
   permit use of early data after receiving a value of 1.  In this way,
   a server can rely on clients respecting any stricter limits to
   resource usage that are advertised.

   A server cannot rely on being able to limit resource usage in this
   way beyond early data.  The server might be forced to reject early
   data, at which point the client uses the initial values for settings.

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5.  Security Considerations

   The potential for replay attacks on early data is significant and
   needs consideration; see Section 4.3 for details.

   An endpoint that offers this setting requires a larger amount of
   state associated with sessions that might be resumed with early data.
   This state is bounded in size and can be offloaded using session
   tickets, so this is expected to be manageable.

6.  IANA Considerations

   The "HTTP/2 Settings" registry established in HTTP/2 [HTTP2] is
   modified to include a new field for each entry, titled "Early Data".
   This field has one of two values:

   *  A value of "Y" indicates that the value of this setting advertised
      by a server is remembered by that if it advertises the
      EARLY_DATA_SETTINGS setting.  In so doing, clients can rely on the
      value of the setting when attempting to use TLS early data.
      Clients MUST remember settings values and respect any values it
      has remembered when attempting to use early data.

   *  A value of "N" indicates that the setting does not need to be
      remembered by a server or respected by a client when accepting or
      attempting early data.  The client needs to observe initial values
      for settings until the server sends its first SETTINGS frame.

   New registrations to this registry MUST specify a value for this

   Initial values for existing values are listed in Table 1.

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         | Code | Name                             | Early Data |
         |  0x1 | HEADER_TABLE_SIZE                |     Y      |
         |  0x2 | ENABLE_PUSH                      |     N      |
         |  0x3 | MAX_CONCURRENT_STREAMS           |     Y      |
         |  0x4 | INITIAL_WINDOW_SIZE              |     Y      |
         |  0x5 | MAX_FRAME_SIZE                   |     Y      |
         |  0x6 | MAX_HEADER_LIST_SIZE             |     Y      |
         |  0x8 | SETTINGS_ENABLE_CONNECT_PROTOCOL |     Y      |
         | 0x10 | TLS_RENEG_PERMITTED              |     N      |

                 Table 1: Early Data Values for Settings

7.  References

7.1.  Normative References

   [HTTP-WS]  McManus, P., "Bootstrapping WebSockets with HTTP/2",
              RFC 8441, DOI 10.17487/RFC8441, September 2018,

   [HTTP2]    Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,

   [QUIC]     Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", Work in Progress, Internet-Draft,
              draft-ietf-quic-transport-33, 13 December 2020,

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

7.2.  Informative References

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              Thomson, M., Nottingham, M., and W. Tarreau, "Using Early
              Data in HTTP", RFC 8470, DOI 10.17487/RFC8470, September
              2018, <>.

   [HTTP3]    Bishop, M., "Hypertext Transfer Protocol Version 3
              (HTTP/3)", Work in Progress, Internet-Draft, draft-ietf-
              quic-http-32, 20 October 2020, <

Author's Address

   Martin Thomson


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