Using Early Data in HTTP
RFC 8470

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Internet Engineering Task Force (IETF)                        M. Thomson
Request for Comments: 8470                                       Mozilla
Category: Standards Track                                  M. Nottingham
ISSN: 2070-1721                                                   Fastly
                                                              W. Tarreau
                                                    HAProxy Technologies
                                                          September 2018

                        Using Early Data in HTTP

Abstract

   Using TLS early data creates an exposure to the possibility of a
   replay attack.  This document defines mechanisms that allow clients
   to communicate with servers about HTTP requests that are sent in
   early data.  Techniques are described that use these mechanisms to
   mitigate the risk of replay.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8470.

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
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   (https://trustee.ietf.org/license-info) in effect on the date of
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Thomson, et al.              Standards Track                    [Page 1]
RFC 8470                     HTTP Early Data              September 2018

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Conventions and Definitions . . . . . . . . . . . . . . .   3
   2.  Early Data in HTTP  . . . . . . . . . . . . . . . . . . . . .   3
   3.  Supporting Early Data in HTTP Servers . . . . . . . . . . . .   3
   4.  Using Early Data in HTTP Clients  . . . . . . . . . . . . . .   5
   5.  Extensions for Early Data in HTTP . . . . . . . . . . . . . .   6
     5.1.  The Early-Data Header Field . . . . . . . . . . . . . . .   7
     5.2.  The 425 (Too Early) Status Code . . . . . . . . . . . . .   8
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
     6.1.  Gateways and Early Data . . . . . . . . . . . . . . . . .   8
     6.2.  Consistent Handling of Early Data . . . . . . . . . . . .   9
     6.3.  Denial of Service . . . . . . . . . . . . . . . . . . . .   9
     6.4.  Out-of-Order Delivery . . . . . . . . . . . . . . . . . .   9
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   TLS 1.3 [TLS13] introduces the concept of early data (also known as
   zero round-trip time (0-RTT) data).  If the client has spoken to the
   same server recently, early data allows a client to send data to a
   server in the first round trip of a connection, without waiting for
   the TLS handshake to complete.

   When used with HTTP [HTTP], early data allows clients to send
   requests immediately, thus avoiding the one or two round-trip delays
   needed for the TLS handshake.  This is a significant performance
   enhancement; however, it has significant limitations.

   The primary risk of using early data is that an attacker might
   capture and replay the request(s) it contains.  TLS [TLS13] describes
   techniques that can be used to reduce the likelihood that an attacker
   can successfully replay a request, but these techniques can be
   difficult to deploy and still leave some possibility of a successful
   attack.

   Note that this is different from automated or user-initiated retries;
   replays are initiated by an attacker without the awareness of the
   client.

Thomson, et al.              Standards Track                    [Page 2]
RFC 8470                     HTTP Early Data              September 2018

   To help mitigate the risk of replays in HTTP, this document gives an