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
draft-ietf-httpbis-replay-02
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
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
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
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.
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
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
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
Thomson, et al. Expires May 28, 2018 [Page 1]
Internet-Draft HTTP Early Data November 2017
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
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. Expires May 28, 2018 [Page 2]
Internet-Draft HTTP Early Data November 2017
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 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.
Thomson, et al. Expires May 28, 2018 [Page 3]
Internet-Draft HTTP Early Data November 2017
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
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 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
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,
Thomson, et al. Expires May 28, 2018 [Page 4]
Internet-Draft HTTP Early Data November 2017
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
Thomson, et al. Expires May 28, 2018 [Page 5]
Internet-Draft HTTP Early Data November 2017
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
completing.
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:
Thomson, et al. Expires May 28, 2018 [Page 6]
Internet-Draft HTTP Early Data November 2017
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 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
code.
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"
Thomson, et al. Expires May 28, 2018 [Page 7]
Internet-Draft HTTP Early Data November 2017
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
processing.
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.
Thomson, et al. Expires May 28, 2018 [Page 8]
Internet-Draft HTTP Early Data November 2017
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,
<https://www.rfc-editor.org/info/rfc5234>.
Thomson, et al. Expires May 28, 2018 [Page 9]
Internet-Draft HTTP Early Data November 2017
[] 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>.
[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", 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, <https://www.rfc-editor.org/info/rfc7301>.
[HQ] Bishop, M., "Hypertext Transfer Protocol (HTTP) over
QUIC", draft-ietf-quic-http-07 (work in progress), October
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>.
Thomson, et al. Expires May 28, 2018 [Page 10]
Internet-Draft HTTP Early Data November 2017
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, Eric Rescorla, Kyle Rose, and Victor Vasiliev.
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
Thomson, et al. Expires May 28, 2018 [Page 11]
