Using Early Data in HTTP
draft-ietf-httpbis-replay-00

httpbis Working Group                                         M. Thomson
Internet-Draft                                                   Mozilla
Intended status: Standards Track                           M. Nottingham
Expires: March 10, 2018                                             true
                                                              W. Tarreau
                                                    HAProxy Technologies
                                                      September 06, 2017

                        Using Early Data in HTTP
                      draft-ietf-httpbis-replay-00

Abstract

   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
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   This Internet-Draft will expire on March 10, 2018.

Copyright Notice

   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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   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 4NN (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
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Appendix A.  Acknowledgments  . . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

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
   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 words "MUST", "MUST NOT", "SHOULD", and "MAY" are used in this
   document.  It's not shouting; when they are capitalized, they have
   the special meaning defined in [RFC2119].

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.  Assuming that a replayed ClientHello
       will not result in additional connections being made by the
       client, 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 4NN (Too Early) status code
       (Section 5.2), in cases where the risk of replay is judged too
       great.

   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
   significant.

   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
   above.

   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 a
   server might initiate processing prior to completion of the handshake
   it needs 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
   mechanism.

   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,
   or none of it.  Once a server has decided to accept early data, it

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   MUST process all requests in early data, even if the server rejects
   the request by sending a 4NN (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.  For HTTP/2,
   this means re-sending the 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 the 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 TLS).

   Clients that use early data MUST retry requests upon receipt of a 4NN
   (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

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   arrived in early data or arrived with the "Early-Data" header field
   set to "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 are
      received in early data.

   o  The 4NN (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 4NN (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

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   An intermediary that forwards a request received in TLS early data
   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 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 4NN (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 4NN (Too Early) status
   code.

5.2.  The 4NN (Too Early) Status Code

   A 4NN (Too Early) status code indicates that the server is unwilling
   to risk processing a request that might be replayed.

   Clients (user-agents and intermediaries) that sent the request in
   early data MUST automatically retry the request when receiving a 4NN
   (Too Early) response status code.  Such retries MUST NOT be sent in
   early data, and SHOULD NOT be sent if the TLS handshake on the
   original connection does not successfully complete.

   Intermediaries that receive a 4NN (Too Early) status code MAY
   automatically retry requests after allowing the handshake to complete
   unless the original request contained the "Early-Data" header field
   when it was received.  Otherwise, an intermediary MUST forward the
   4NN (Too Early) status code.

   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 4NN status
   code unless one of these conditions is met.

   The 4NN (Too Early) status code is not cacheable by default.  Its
   payload is not the representation of any identified resource.

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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 server that receives those
   requests understands the "Early-Data" header field and will correctly
   generate a 4NN (Too Early) status code.  A gateway that isn't certain
   about server support SHOULD either delay forwarding the request until
   the TLS handshake completes, or send a 4NN (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 need to
   agree and either reject the request or delay processing.

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 4NN (Too Early)
   status code often leads to clients retrying requests, which could
   result in increased load.

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

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   Author/Change controller:  IETF

   Specification document(s):  This document

   Related information:  (empty)

   This document registers the 4NN (Too Early) status code in the
   "Hypertext Transfer Protocol (HTTP) Status Code" registry established
   in [RFC7231].

   Value:  4NN

   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>.

   [HEADERS]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, RFC 3864,
              DOI 10.17487/RFC3864, September 2004, <https://www.rfc-
              editor.org/info/rfc3864>.

   [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>.

   [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>.

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

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8.2.  Informative References

   [HQ]       Bishop, M., "Hypertext Transfer Protocol (HTTP) over
              QUIC", draft-ietf-quic-http-05 (work in progress), August
              2017.

   [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>.

Appendix A.  Acknowledgments

   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, and Victor Vasiliev.

Authors' Addresses

   Martin Thomson
   Mozilla

   Email: martin.thomson@gmail.com

   Mark Nottingham
   true

   Email: mnot@mnot.net

   Willy Tarreau
   HAProxy Technologies

   Email: willy@haproxy.org

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