Using Early Data in HTTP
RFC 8470

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
   Provisions Relating to IETF Documents
   (https://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
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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.

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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.

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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",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "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 data
   to form a single stream.  This can mean that requests are entirely
   contained within early data or that 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.

   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.

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

   1.  The server can reject early data at the TLS layer.  A server
       cannot selectively reject early data, so this results in all
       requests sent in early data being discarded.

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   2.  The server can choose to delay processing of early data until
       after 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.  If the server
       receives multiple requests in early data, it can determine
       whether to defer HTTP processing on a per-request basis.

   3.  The server can cause a client to retry individual requests 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 great.

   All of these techniques are equally effective; a server can use the
   method that best suits it.

   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).  The primary risk associated with using early
   data is in the actions a server takes when processing a request;
   processing a duplicated request might result in duplicated effects
   and side effects.  Appendix E.5 of [TLS13] also describes other
   effects produced by processing duplicated requests.

   The request method's safety ([RFC7231], Section 4.2.1) is one way to
   determine this.  However, some resources produce 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, origin servers MUST either reject early data or
   implement the techniques described in this document for ensuring that
   requests are not processed prior to TLS handshake completion.

   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 account for the possibility
   of replay of early data and how that 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.

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   Intermediary servers do not have sufficient information to decide
   whether early data can be processed, 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 mechanism.

   Note that a server cannot choose to selectively reject early data at
   the TLS layer.  TLS only permits a server to either accept all early
   data 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
   requests that it might defer until the handshake completes.

4.  Using Early Data in HTTP Clients

   A client that wishes to use early data commences by 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 ([RFC7231], Section 4.2.1) in early data when it is
   available and MUST 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 were new.  This could
   entail using a different negotiated protocol [ALPN] than the one
   optimistically used for the early data.  Any requests sent in early
   data will need to be sent again, unless the client decides to abandon
   those requests.

   Automatic retry creates the potential for a replay attack.  An
   attacker intercepts a connection that uses early data and copies the
   early data to another server instance.  The second server instance
   accepts and processes the early data, even though it will not
   complete the TLS handshake.  The attacker then allows the original
   connection to complete.  Even if the early data is detected as a
   duplicate and rejected, the first server instance might allow the
   connection to complete.  If the client then retries requests that
   were sent in early data, the request will be processed twice.

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   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 the latter 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
   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 hop.  Likewise, it is necessary to have some means of
   explicitly triggering a retry when early data is not desired.
   Finally, it is necessary to know whether the client will actually
   perform such a retry.

   To meet these needs, two signaling mechanisms are defined:

   o  The Early-Data header field is included in requests that might
      have been forwarded by an intermediary prior to the completion of
      the TLS handshake with its client.

   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.

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5.1.  The Early-Data Header Field

   The Early-Data request header field indicates that the request has
   been conveyed in early data and 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:

   GET /resource HTTP/1.0
   Host: example.com
   Early-Data: 1

   An intermediary that forwards a request prior to the completion of
   the TLS handshake with its client 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
   the request might have been subject to a replay and might already
   have been forwarded by it or another instance (see Section 6.2).

   An intermediary MUST NOT remove this header field if it is present in
   a request.  Early-Data MUST NOT appear in a Connection header field.

   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 header field and
   cannot be safely processed MUST be rejected using the 425 (Too Early)
   status code.

   The Early-Data header field carries a single bit of information, and
   clients MUST include at most one instance.  Multiple or invalid
   instances of the header field MUST be treated as equivalent to a
   single instance with a value of 1 by a server.

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   An Early-Data header field MUST NOT be included in responses or
   request trailers.

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 are expected to retry
   the request when receiving a 425 (Too Early) response status code.  A
   user agent SHOULD retry automatically, but any 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 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.  In
   particular, a request that is replayed might result in a different
   response, which might be observable from the length of protected data
   even if the content remains confidential.

6.1.  Gateways and Early Data

   A gateway MUST NOT forward requests that were received in early data
   unless it knows that the origin server it will forward to understands
   the Early-Data header field and will correctly generate a 425 (Too
   Early) status code.  A gateway that is uncertain about origin server
   support for a given request SHOULD either delay forwarding the

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   request until the TLS handshake with its client completes or send a
   425 (Too Early) status code in response.

   A gateway without at least one potential origin server that supports
   the Early-Data header field expends significant effort for what can
   at best be a modest performance benefit from enabling early data.  If
   no origin server supports early data, it is more efficient to disable
   early data entirely.

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
   processing.

   Disabling early data, delaying requests, or rejecting requests with
   the 425 (Too Early) status code are all equally good measures for
   mitigating replay attacks on requests that might be vulnerable to
   replay.  Server instances can implement any of these measures and be
   considered consistent, even if different instances use different
   methods.  Critically, this means that it is possible to employ
   different mitigations in reaction to other conditions, such as server
   load.

   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.

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.  A server MAY
   process requests received in early data after handshake completion
   only if it can rely on other instances correctly handling replays of
   the same requests.

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7.  IANA Considerations

   This document registers the Early-Data header field in the "Permanent
   Message Header Field Names" registry located at
   <https://www.iana.org/assignments/message-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 "HTTP
   Status Codes" registry located at <https://www.iana.org/assignments/
   http-status-codes>.

   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,
              <https://www.rfc-editor.org/info/rfc5234>.

   [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,
              <https://www.rfc-editor.org/info/rfc7230>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

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   [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,
              <https://www.rfc-editor.org/info/rfc7231>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [TLS13]    Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

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, <https://www.rfc-editor.org/info/rfc7301>.

   [HQ]       Bishop, M., "Hypertext Transfer Protocol (HTTP) over
              QUIC", Work in Progress, draft-ietf-quic-http-14, August
              2018.

   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,
              <https://www.rfc-editor.org/info/rfc7540>.

Acknowledgments

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

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Authors' Addresses

   Martin Thomson
   Mozilla

   Email: martin.thomson@gmail.com

   Mark Nottingham
   Fastly

   Email: mnot@mnot.net

   Willy Tarreau
   HAProxy Technologies

   Email: willy@haproxy.org

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