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Using Early Data in HTTP

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Document Type This is an older version of an Internet-Draft that was ultimately published as an RFC.
Authors Martin Thomson , Mark Nottingham , Willy Tarreau
Last updated 2018-05-01 (Latest revision 2017-11-23)
Replaces draft-thomson-http-replay
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httpbis Working Group                                         M. Thomson
Internet-Draft                                                   Mozilla
Intended status: Standards Track                           M. Nottingham
Expires: May 28, 2018                                             Fastly
                                                              W. Tarreau
                                                    HAProxy Technologies
                                                       November 24, 2017

                        Using Early Data in HTTP


   This document explains the risks of using early data for HTTP and
   describes techniques for reducing them.  In particular, it defines a
   mechanism that enables clients to communicate with servers about
   early data, to assure correct operation.

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
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on May 28, 2018.

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   Copyright (c) 2017 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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   ( in effect on the date of
   publication of this document.  Please review these documents
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   include Simplified BSD License text as described in Section 4.e of

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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 . . . . . . . . . . . . . . .   6
     5.2.  The 425 (Too Early) Status Code . . . . . . . . . . . . .   7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
     6.1.  Gateways and Early Data . . . . . . . . . . . . . . . . .   8
     6.2.  Consistent Handling of Early Data . . . . . . . . . . . .   8
     6.3.  Denial of Service . . . . . . . . . . . . . . . . . . . .   8
     6.4.  Out of Order Delivery . . . . . . . . . . . . . . . . . .   9
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   TLS 1.3 [TLS13] introduces the concept of early data (also known as
   zero round trip data or 0-RTT data).  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 if the client has
   spoken to the same server recently.

   When used with HTTP [HTTP], early data allows clients to send
   requests immediately, avoiding the one or two round trip delay 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

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

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   To help mitigate the risk of replays in HTTP, this document gives an
   overview of techniques for controlling these risks in servers, and
   defines requirements for clients when sending requests in early data.

   The advice in this document also applies to use of 0-RTT in HTTP over
   QUIC [HQ].

1.1.  Conventions and Definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Early Data in HTTP

   Conceptually, early data is concatenated with other application to
   form a single stream.  This can mean that requests are entirely
   contained within early data, or only part of a request is early.  In
   a multiplexed protocol, like HTTP/2 [RFC7540] or HTTP/QUIC [HQ],
   multiple requests might be partially delivered in early data.

   The model that this document assumes is that once the TLS handshake
   completes, the early data received on that TLS connection is known to
   not be a replayed copy of that data.  However, it is important to
   note that this does not mean that early data will not be or has not
   been replayed on another connection.

3.  Supporting Early Data in HTTP Servers

   A server decides whether or not to offer a client the ability to send
   early data on future connections when sending the TLS session ticket.

   When a server enables early data, there are a number of techniques it
   can use to mitigate the risks of replay:

   1.  TLS [TLS13] mandates the use of replay detection strategies that
       reduce the ability of an attacker to successfully replay early
       data.  These anti-replay techniques reduce but don't completely
       eliminate the chance of data being replayed and ensure a fixed
       upper limit to the number of replays.

   2.  The server can choose whether it will process early data before
       the TLS handshake completes.  By deferring processing, it can
       ensure that only a successfully completed connection is used for
       the request(s) therein.  This provides the server with some
       assurance that the early data was not replayed.

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   3.  If the server receives multiple requests in early data, it can
       determine whether to defer HTTP processing on a per-request
       basis.  This may require buffering the responses to preserve
       ordering in HTTP/1.1.

   4.  The server can cause a client to retry a request and not use
       early data by responding with the 425 (Too Early) status code
       (Section 5.2), in cases where the risk of replay is judged too

   For a given request, the level of tolerance to replay risk is
   specific to the resource it operates upon (and therefore only known
   to the origin server).  In general, if processing a request does not
   have state-changing side effects, the consequences of replay are not

   The request method's safety ([RFC7231], Section 4.2.1) is one way to
   determine this.  However, some resources do elect to associate side
   effects with safe methods, so this cannot be universally relied upon.

   It is RECOMMENDED that origin servers allow resources to explicitly
   configure whether early data is appropriate in requests.  Absent such
   explicit information, they SHOULD mitigate against early data in
   requests that have unsafe methods, using the techniques outlined

   A request might be sent partially in early data with the remainder of
   the request being sent after the handshake completes.  This does not
   necessarily affect handling of that request; what matters is when the
   server starts acting upon the contents of a request.  Any time any
   server instance might initiate processing prior to completion of the
   handshake, all server instances need to consider how a possible
   replay of early data could affect that processing (see also
   Section 6.2).

   A server can partially process requests that are incomplete.  Parsing
   header fields - without acting on the values - and determining
   request routing is likely to be safe from side-effects, but other
   actions might not be.

   Intermediary servers do not have sufficient information to make this
   determination, so Section 5.2 describes a way for the origin to
   signal to them that a particular request isn't appropriate for early
   data.  Intermediaries that accept early data MUST implement that

   Note that a server cannot choose to selectively reject early data at
   the TLS layer.  TLS only permits a server to accept all early data,

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   or none of it.  Once a server has decided to accept early data, it
   MUST process all requests in early data, even if the server rejects
   the request by sending a 425 (Too Early) response.

   A server can limit the amount of early data with the
   "max_early_data_size" field of the "early_data" TLS extension.  This
   can be used to avoid committing an arbitrary amount of memory for
   deferred requests.  A server SHOULD ensure that when it accepts early
   data, it can defer processing of requests until after the TLS
   handshake completes.

4.  Using Early Data in HTTP Clients

   A client that wishes to use early data commences sending HTTP
   requests immediately after sending the TLS ClientHello.

   By their nature, clients have control over whether a given request is
   sent in early data - thereby giving the client control over risk of
   replay.  Absent other information, clients MAY send requests with
   safe HTTP methods (see [RFC7231], Section 4.2.1) in early data when
   it is available, and SHOULD NOT send unsafe methods (or methods whose
   safety is not known) in early data.

   If the server rejects early data at the TLS layer, a client MUST
   start sending again as though the connection was new.  This could
   entail using a different negotiated protocol [ALPN] than the one
   optimistically used for the early data.  If HTTP/2 is selected after
   early data is rejected, a client sends another connection preface.
   Any requests sent in early data MUST be sent again, unless the client
   decides to abandon those requests.

   This automatic retry exposes the request to a potential replay
   attack.  An attacker sends early data to one server instance that
   accepts and processes the early data, but allows that connection to
   proceed no further.  The attacker then forwards the same messages
   from the client to another server instance that will reject early
   data.  The client then retries the request, resulting in the request
   being processed twice.  Replays are also possible if there are
   multiple server instances that will accept early data, or if the same
   server accepts early data multiple times (though this would be in
   violation of requirements in Section 8 of [TLS13]).

   Clients that use early data MUST retry requests upon receipt of a 425
   (Too Early) status code; see Section 5.2.

   An intermediary MUST NOT use early data when forwarding a request
   unless early data was used on a previous hop, or it knows that the
   request can be retried safely without consequences (typically, using

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   out-of-band configuration).  Absent better information, that means
   that an intermediary can only use early data if the request either
   arrived in early data or arrived with the "Early-Data" header field
   set to "1" (see Section 5.1).

5.  Extensions for Early Data in HTTP

   Because HTTP requests can span multiple "hops", it is necessary to
   explicitly communicate whether a request has been sent in early data
   on a previous connection.  Likewise, some means of explicitly
   triggering a retry when early data is not desirable is necessary.
   Finally, it is necessary to know whether the client will actually
   perform such a retry.

   To meet these needs, two signalling mechanisms are defined:

   o  The "Early-Data" header field is included in requests that might
      have been forwarded by an intermediary prior to the TLS handshake

   o  The 425 (Too Early) status code is defined for a server to
      indicate that a request could not be processed due to the
      consequences of a possible replay attack.

   They are designed to enable better coordination of the use of early
   data between the user agent and origin server, and also when a
   gateway (also "reverse proxy", "Content Delivery Network", or
   "surrogate") is present.

   Gateways typically don't have specific information about whether a
   given request can be processed safely when it is sent in early data.
   In many cases, only the origin server has the necessary information
   to decide whether the risk of replay is acceptable.  These extensions
   allow coordination between a gateway and its origin server.

5.1.  The Early-Data Header Field

   The "Early-Data" request header field indicates that the request has
   been conveyed in early data, and additionally indicates that a client
   understands the 425 (Too Early) status code.

   It has just one valid value: "1".  Its syntax is defined by the
   following ABNF [ABNF]:

   Early-Data = "1"

   For example:

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   GET /resource HTTP/1.0
   Early-Data: 1

   An intermediary that forwards a request prior to the completion of
   the TLS handshake MUST send it with the "Early-Data" header field set
   to "1" (i.e., it adds it if not present in the request).  An
   intermediary MUST use the "Early-Data" header field if it might have
   forwarded the request prior to handshake completion (see Section 6.2
   for details).

   An intermediary MUST NOT remove this header field if it is present in
   a request.

   The "Early-Data" header field is not intended for use by user agents
   (that is, the original initiator of a request).  Sending a request in
   early data implies that the client understands this specification and
   is willing to retry a request in response to a 425 (Too Early) status
   code.  A user agent that sends a request in early data does not need
   to include the "Early-Data" header field.

   A server cannot make a request that contains the Early-Data header
   field safe for processing by waiting for the handshake to complete.
   A request that is marked with Early-Data was sent in early data on a
   previous hop.  Requests that contain the Early-Data field and cannot
   be safely processed MUST be rejected using the 425 (Too Early) status

5.2.  The 425 (Too Early) Status Code

   A 425 (Too Early) status code indicates that the server is unwilling
   to risk processing a request that might be replayed.

   User agents that send a request in early data MUST automatically
   retry the request when receiving a 425 (Too Early) response status
   code.  Such retries MUST NOT be sent in early data.

   In all cases, an intermediary can forward a 425 (Too Early) status
   code.  Intermediaries MUST forward a 425 (Too Early) status code if
   the request that it received and forwarded contained an "Early-Data"
   header field.  Otherwise, an intermediary that receives a request in
   early data MAY automatically retry that request in response to a 425
   (Too Early) status code, but it MUST wait for the TLS handshake to
   complete on the connection where it received the request.

   The server cannot assume that a client is able to retry a request
   unless the request is received in early data or the "Early-Data"

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   header field is set to "1".  A server SHOULD NOT emit the 425 status
   code unless one of these conditions is met.

   The 425 (Too Early) status code is not cacheable by default.  Its
   payload is not the representation of any identified resource.

6.  Security Considerations

   Using early data exposes a client to the risk that their request is
   replayed.  A retried or replayed request can produce different side
   effects on the server.  In addition to those side effects, replays
   and retries might be used for traffic analysis to recover information
   about requests or the resources those requests target.

6.1.  Gateways and Early Data

   A gateway that forwards requests that were received in early data
   MUST only do so if it knows that the origin server that receives
   those requests understands the "Early-Data" header field and will
   correctly generate a 425 (Too Early) status code.  A gateway that
   isn't certain about origin server support SHOULD either delay
   forwarding the request until the TLS handshake with its client
   completes, or send a 425 (Too Early) status code in response.  A
   gateway that is uncertain about whether an origin server supports the
   "Early-Data" header field SHOULD disable early data.

6.2.  Consistent Handling of Early Data

   Consistent treatment of a request that arrives in - or partially in -
   early data is critical to avoiding inappropriate processing of
   replayed requests.  If a request is not safe to process before the
   TLS handshake completes, then all instances of the server (including
   gateways) need to agree and either reject the request or delay

   A server MUST NOT act on early data before the handshake completes if
   it and any other server instance could make a different decision
   about how to handle the same data.

6.3.  Denial of Service

   Accepting early data exposes a server to potential denial of service
   through the replay of requests that are expensive to handle.  A
   server that is under load SHOULD prefer rejecting TLS early data as a
   whole rather than accepting early data and selectively processing
   requests.  Generating a 503 (Service Unavailable) or 425 (Too Early)
   status code often leads to clients retrying requests, which could
   result in increased load.

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6.4.  Out of Order Delivery

   In protocols that deliver data out of order (such as QUIC [HQ]) early
   data can arrive after the handshake completes.  This leads to
   potential ambiguity about the status of requests and could lead to
   inconsistent treatment (see Section 6.2).  Implementations MUST
   either ensure that any early data that is delivered late is either
   discarded or consistently identified and processed.

7.  IANA Considerations

   This document registers the "Early-Data" header field in the "Message
   Headers" registry [HEADERS].

   Header field name:  Early-Data

   Applicable protocol:  http

   Status:  standard

   Author/Change controller:  IETF

   Specification document(s):  This document

   Related information:  (empty)

   This document registers the 425 (Too Early) status code in the
   "Hypertext Transfer Protocol (HTTP) Status Code" registry established
   in [RFC7231].

   Value:  425

   Description:  Too Early

   Reference:  This document

8.  References

8.1.  Normative References

   [ABNF]     Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,

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   [HEADERS]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, RFC 3864,
              DOI 10.17487/RFC3864, September 2004,

   [HTTP]     Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,

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

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,

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

   [TLS13]    Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", draft-ietf-tls-tls13-21 (work in progress),
              July 2017.

8.2.  Informative References

   [ALPN]     Friedl, S., Popov, A., Langley, A., and E. Stephan,
              "Transport Layer Security (TLS) Application-Layer Protocol
              Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
              July 2014, <>.

   [HQ]       Bishop, M., "Hypertext Transfer Protocol (HTTP) over
              QUIC", draft-ietf-quic-http-07 (work in progress), October

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

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   This document was not easy to produce.  The following people made
   substantial contributions to the quality and completeness of the
   document: Subodh Iyengar, Benjamin Kaduk, Ilari Liusavaara, Kazuho
   Oku, Eric Rescorla, Kyle Rose, and Victor Vasiliev.

Authors' Addresses

   Martin Thomson


   Mark Nottingham


   Willy Tarreau
   HAProxy Technologies


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