Transport Layer Security (TLS) False Start
RFC 7918

Document Type RFC - Informational (August 2016; No errata)
Last updated 2016-08-10
Replaces draft-bmoeller-tls-falsestart
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Internet Engineering Task Force (IETF)                        A. Langley
Request for Comments: 7918                                   N. Modadugu
Category: Informational                                       B. Moeller
ISSN: 2070-1721                                                   Google
                                                             August 2016

               Transport Layer Security (TLS) False Start

Abstract

   This document specifies an optional behavior of Transport Layer
   Security (TLS) client implementations, dubbed "False Start".  It
   affects only protocol timing, not on-the-wire protocol data, and can
   be implemented unilaterally.  A TLS False Start reduces handshake
   latency to one round trip.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   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).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see 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
   http://www.rfc-editor.org/info/rfc7918.

Copyright Notice

   Copyright (c) 2016 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
   (http://trustee.ietf.org/license-info) 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
   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.

Langley, et al.               Informational                     [Page 1]
RFC 7918                     TLS False Start                 August 2016

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Notation . . . . . . . . . . . . . . . . . . . .   4
   3.  False Start Compatibility . . . . . . . . . . . . . . . . . .   4
   4.  Client-Side False Start . . . . . . . . . . . . . . . . . . .   4
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
     5.1.  Symmetric Cipher  . . . . . . . . . . . . . . . . . . . .   6
     5.2.  Protocol Version  . . . . . . . . . . . . . . . . . . . .   7
     5.3.  Key Exchange and Client Certificate Type  . . . . . . . .   7
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Appendix A.  Implementation Notes . . . . . . . . . . . . . . . .  10
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   A full handshake in TLS protocol versions up to TLS 1.2 [RFC5246]
   requires two full protocol rounds (four flights) before the handshake
   is complete and the protocol parties may begin to send application
   data.  Thus, using TLS can add a latency penalty of two network
   round-trip times for application protocols in which the client sends
   data first, such as HTTP [RFC7230].  Figure 1 (copied from [RFC5246])
   shows the message flow for a full handshake.

      Client                                               Server

      ClientHello                  -------->
                                                      ServerHello
                                                     Certificate*
                                               ServerKeyExchange*
                                              CertificateRequest*
                                   <--------      ServerHelloDone
      Certificate*
      ClientKeyExchange
      CertificateVerify*
      [ChangeCipherSpec]
      Finished                     -------->
                                               [ChangeCipherSpec]
                                   <--------             Finished
      Application Data             <------->     Application Data

                Figure 1: Message Flow for a Full Handshake

Langley, et al.               Informational                     [Page 2]
RFC 7918                     TLS False Start                 August 2016

   This document describes a technique that alleviates the latency
   burden imposed by TLS: the client-side TLS False Start.  If certain
   conditions are met, the client can start to send application data
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