draft-ietf-httpbis-replay-04.txt   draft-ietf-httpbis-replay-latest.txt 
HTTP Working Group M. Thomson HTTP Working Group M. Thomson
Internet-Draft Mozilla Internet-Draft Mozilla
Intended status: Standards Track M. Nottingham Intended status: Standards Track M. Nottingham
Expires: December 29, 2018 Fastly Expires: March 28, 2019 Fastly
W. Tarreau W. Tarreau
HAProxy Technologies HAProxy Technologies
June 27, 2018 September 24, 2018
Using Early Data in HTTP Using Early Data in HTTP
draft-ietf-httpbis-replay-04 draft-ietf-httpbis-replay-latest
Abstract Abstract
Using TLS early data creates an exposure to the possibility of a Using TLS early data creates an exposure to the possibility of a
replay attack. This document defines mechanisms that allow clients replay attack. This document defines mechanisms that allow clients
to communicate with servers about HTTP requests that are sent in to communicate with servers about HTTP requests that are sent in
early data. Techniques are described that use these mechanisms to early data. Techniques are described that use these mechanisms to
mitigate the risk of replay. mitigate the risk of replay.
Note to Readers Note to Readers
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 29, 2018. This Internet-Draft will expire on March 28, 2019.
Copyright Notice Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of (https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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2. Early Data in HTTP . . . . . . . . . . . . . . . . . . . . . 3 2. Early Data in HTTP . . . . . . . . . . . . . . . . . . . . . 3
3. Supporting Early Data in HTTP Servers . . . . . . . . . . . . 3 3. Supporting Early Data in HTTP Servers . . . . . . . . . . . . 3
4. Using Early Data in HTTP Clients . . . . . . . . . . . . . . 5 4. Using Early Data in HTTP Clients . . . . . . . . . . . . . . 5
5. Extensions for Early Data in HTTP . . . . . . . . . . . . . . 6 5. Extensions for Early Data in HTTP . . . . . . . . . . . . . . 6
5.1. The Early-Data Header Field . . . . . . . . . . . . . . . 7 5.1. The Early-Data Header Field . . . . . . . . . . . . . . . 7
5.2. The 425 (Too Early) Status Code . . . . . . . . . . . . . 8 5.2. The 425 (Too Early) Status Code . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8 6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6.1. Gateways and Early Data . . . . . . . . . . . . . . . . . 9 6.1. Gateways and Early Data . . . . . . . . . . . . . . . . . 9
6.2. Consistent Handling of Early Data . . . . . . . . . . . . 9 6.2. Consistent Handling of Early Data . . . . . . . . . . . . 9
6.3. Denial of Service . . . . . . . . . . . . . . . . . . . . 9 6.3. Denial of Service . . . . . . . . . . . . . . . . . . . . 9
6.4. Out of Order Delivery . . . . . . . . . . . . . . . . . . 10 6.4. Out-of-Order Delivery . . . . . . . . . . . . . . . . . . 10
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 10 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
8.1. Normative References . . . . . . . . . . . . . . . . . . 10 8.1. Normative References . . . . . . . . . . . . . . . . . . 10
8.2. Informative References . . . . . . . . . . . . . . . . . 11 8.2. Informative References . . . . . . . . . . . . . . . . . 11
8.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 11 8.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 12 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction 1. Introduction
TLS 1.3 [TLS13] introduces the concept of early data (also known as 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 zero round-trip time (0-RTT) data). If the client has spoken to the
send data to a server in the first round trip of a connection, same server recently, early data allows a client to send data to a
without waiting for the TLS handshake to complete, if the client has server in the first round trip of a connection, without waiting for
spoken to the same server recently. the TLS handshake to complete.
When used with HTTP [HTTP], early data allows clients to send When used with HTTP [HTTP], early data allows clients to send
requests immediately, avoiding the one or two round trip delay needed requests immediately, thus avoiding the one or two round-trip delays
for the TLS handshake. This is a significant performance needed for the TLS handshake. This is a significant performance
enhancement; however, it has significant limitations. enhancement; however, it has significant limitations.
The primary risk of using early data is that an attacker might The primary risk of using early data is that an attacker might
capture and replay the request(s) it contains. TLS [TLS13] describes capture and replay the request(s) it contains. TLS [TLS13] describes
techniques that can be used to reduce the likelihood that an attacker techniques that can be used to reduce the likelihood that an attacker
can successfully replay a request, but these techniques can be can successfully replay a request, but these techniques can be
difficult to deploy, and still leave some possibility of a successful difficult to deploy and still leave some possibility of a successful
attack. attack.
Note that this is different from automated or user-initiated retries; Note that this is different from automated or user-initiated retries;
replays are initiated by an attacker without the awareness of the replays are initiated by an attacker without the awareness of the
client. client.
To help mitigate the risk of replays in HTTP, this document gives an To help mitigate the risk of replays in HTTP, this document gives an
overview of techniques for controlling these risks in servers, and overview of techniques for controlling these risks in servers and
defines requirements for clients when sending requests in early data. 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 The advice in this document also applies to use of 0-RTT in HTTP over
QUIC [HQ]. QUIC [HQ].
1.1. Conventions and Definitions 1.1. Conventions and Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP "OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
2. Early Data in HTTP 2. Early Data in HTTP
Conceptually, early data is concatenated with other application data Conceptually, early data is concatenated with other application data
to form a single stream. This can mean that requests are entirely 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 contained within early data or that only part of a request is early.
a multiplexed protocol, like HTTP/2 [RFC7540] or HTTP/QUIC [HQ], In a multiplexed protocol, like HTTP/2 [RFC7540] or HTTP/QUIC [HQ],
multiple requests might be partially delivered in early data. multiple requests might be partially delivered in early data.
The model that this document assumes is that once the TLS handshake The model that this document assumes is that once the TLS handshake
completes, the early data received on that TLS connection is known to 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 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 note that this does not mean that early data will not be or has not
been replayed on another connection. been replayed on another connection.
3. Supporting Early Data in HTTP Servers 3. Supporting Early Data in HTTP Servers
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2. The server can choose to delay processing of early data until 2. The server can choose to delay processing of early data until
after the TLS handshake completes. By deferring processing, it after the TLS handshake completes. By deferring processing, it
can ensure that only a successfully completed connection is used can ensure that only a successfully completed connection is used
for the request(s) therein. This provides the server with some for the request(s) therein. This provides the server with some
assurance that the early data was not replayed. If the server assurance that the early data was not replayed. If the server
receives multiple requests in early data, it can determine receives multiple requests in early data, it can determine
whether to defer HTTP processing on a per-request basis. whether to defer HTTP processing on a per-request basis.
3. The server can cause a client to retry individual requests and 3. The server can cause a client to retry individual requests and
not use early data by responding with the 425 (Too Early) status 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 code (Section 5.2) in cases where the risk of replay is judged
too great. too great.
Any of these techniques is equally effective and a server can use the All of these techniques are equally effective; a server can use the
method that best suits it. method that best suits it.
For a given request, the level of tolerance to replay risk is For a given request, the level of tolerance to replay risk is
specific to the resource it operates upon (and therefore only known specific to the resource it operates upon (and therefore only known
to the origin server). The primary risk associated with using early to the origin server). The primary risk associated with using early
data is in the actions a server takes when processing a request; data is in the actions a server takes when processing a request;
processing a duplicated request might result in duplicated effects processing a duplicated request might result in duplicated effects
and side effects. Appendix E.5 of [TLS13] also describes other and side effects. Appendix E.5 of [TLS13] also describes other
effects produced by processing duplicated requests. effects produced by processing duplicated requests.
The request method's safety ([RFC7231], Section 4.2.1) is one way to The request method's safety ([RFC7231], Section 4.2.1) is one way to
determine this. However, some resources do produce side effects with determine this. However, some resources produce side effects with
safe methods, so this cannot be universally relied upon. safe methods, so this cannot be universally relied upon.
It is RECOMMENDED that origin servers allow resources to explicitly It is RECOMMENDED that origin servers allow resources to explicitly
configure whether early data is appropriate in requests. Absent such configure whether early data is appropriate in requests. Absent such
explicit information, origin servers MUST either reject early data or explicit information, origin servers MUST either reject early data or
implement the techniques described in this document for ensuring that implement the techniques described in this document for ensuring that
requests are not processed prior to TLS handshake completion. requests are not processed prior to TLS handshake completion.
A request might be sent partially in early data with the remainder of A request might be sent partially in early data with the remainder of
the request being sent after the handshake completes. This does not the request being sent after the handshake completes. This does not
necessarily affect handling of that request; what matters is when the necessarily affect handling of that request; what matters is when the
server starts acting upon the contents of a request. Any time any server starts acting upon the contents of a request. Any time any
server instance might initiate processing prior to completion of the server instance might initiate processing prior to completion of the
handshake, all server instances need to account for the possibility handshake, all server instances need to account for the possibility
of replay of early data and how that could affect that processing of replay of early data and how that could affect that processing
(see also Section 6.2). (see also Section 6.2).
A server can partially process requests that are incomplete. Parsing A server can partially process requests that are incomplete. Parsing
header fields - without acting on the values - and determining header fields -- without acting on the values -- and determining
request routing is likely to be safe from side-effects, but other request routing is likely to be safe from side effects but other
actions might not be. actions might not be.
Intermediary servers do not have sufficient information to decide Intermediary servers do not have sufficient information to decide
whether early data can be processed, so Section 5.2 describes a way 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 for the origin to signal to them that a particular request isn't
appropriate for early data. Intermediaries that accept early data appropriate for early data. Intermediaries that accept early data
MUST implement that mechanism. MUST implement that mechanism.
Note that a server cannot choose to selectively reject early data at 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, the TLS layer. TLS only permits a server to either accept all early
or none of it. Once a server has decided to accept early data, it data or none of it. Once a server has decided to accept early data,
MUST process all requests in early data, even if the server rejects it MUST process all requests in early data, even if the server
the request by sending a 425 (Too Early) response. rejects the request by sending a 425 (Too Early) response.
A server can limit the amount of early data with the A server can limit the amount of early data with the
"max_early_data_size" field of the "early_data" TLS extension. This "max_early_data_size" field of the "early_data" TLS extension. This
can be used to avoid committing an arbitrary amount of memory for can be used to avoid committing an arbitrary amount of memory for
requests that it might defer until the handshake completes. requests that it might defer until the handshake completes.
4. Using Early Data in HTTP Clients 4. Using Early Data in HTTP Clients
A client that wishes to use early data commences sending HTTP A client that wishes to use early data commences by sending HTTP
requests immediately after sending the TLS ClientHello. requests immediately after sending the TLS ClientHello.
By their nature, clients have control over whether a given request is By their nature, clients have control over whether a given request is
sent in early data - thereby giving the client control over risk of sent in early data, thereby giving the client control over risk of
replay. Absent other information, clients MAY send requests with replay. Absent other information, clients MAY send requests with
safe HTTP methods (see [RFC7231], Section 4.2.1) in early data when safe HTTP methods ([RFC7231], Section 4.2.1) in early data when it is
it is available, and MUST NOT send unsafe methods (or methods whose available and MUST NOT send unsafe methods (or methods whose safety
safety is not known) in early data. is not known) in early data.
If the server rejects early data at the TLS layer, a client MUST If the server rejects early data at the TLS layer, a client MUST
start sending again as though the connection were new. This could start sending again as though the connection were new. This could
entail using a different negotiated protocol [ALPN] than the one entail using a different negotiated protocol [ALPN] than the one
optimistically used for the early data. Any requests sent in early optimistically used for the early data. Any requests sent in early
data will need to be sent again, unless the client decides to abandon data will need to be sent again, unless the client decides to abandon
those requests. those requests.
Automatic retry creates the potential for a replay attack. An Automatic retry creates the potential for a replay attack. An
attacker intercepts a connection that uses early data and copies the attacker intercepts a connection that uses early data and copies the
early data to another server instance. The second server instance early data to another server instance. The second server instance
accepts and processes the early data, even though it will not accepts and processes the early data, even though it will not
complete the TLS handshake. The attacker then allows the original complete the TLS handshake. The attacker then allows the original
connection to complete. Even if the early data is detected as a connection to complete. Even if the early data is detected as a
duplicate and rejected, the first server instance might allow the duplicate and rejected, the first server instance might allow the
connection to complete. If the client then retries requests that connection to complete. If the client then retries requests that
were sent in early data, the request will be processed twice. were sent in early data, the request will be processed twice.
Replays are also possible if there are multiple server instances that Replays are also possible if there are multiple server instances that
will accept early data, or if the same server accepts early data will accept early data or if the same server accepts early data
multiple times (though the latter would be in violation of multiple times (though the latter would be in violation of
requirements in Section 8 of [TLS13]). requirements in Section 8 of [TLS13]).
Clients that use early data MUST retry requests upon receipt of a 425 Clients that use early data MUST retry requests upon receipt of a 425
(Too Early) status code; see Section 5.2. (Too Early) status code; see Section 5.2.
An intermediary MUST NOT use early data when forwarding a request 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 unless early data was used on a previous hop, or it knows that the
request can be retried safely without consequences (typically, using request can be retried safely without consequences (typically, using
out-of-band configuration). Absent better information, that means out-of-band configuration). Absent better information, that means
that an intermediary can only use early data if the request either that an intermediary can only use early data if the request either
arrived in early data or arrived with the "Early-Data" header field arrived in early data or arrived with the Early-Data header field set
set to "1" (see Section 5.1). to "1" (see Section 5.1).
5. Extensions for Early Data in HTTP 5. Extensions for Early Data in HTTP
Because HTTP requests can span multiple "hops", it is necessary to Because HTTP requests can span multiple "hops", it is necessary to
explicitly communicate whether a request has been sent in early data explicitly communicate whether a request has been sent in early data
on a previous hop. Likewise, some means of explicitly triggering a on a previous hop. Likewise, it is necessary to have some means of
retry when early data is not desirable is necessary. Finally, it is explicitly triggering a retry when early data is not desired.
necessary to know whether the client will actually perform such a Finally, it is necessary to know whether the client will actually
retry. perform such a retry.
To meet these needs, two signalling mechanisms are defined: To meet these needs, two signaling mechanisms are defined:
o The "Early-Data" header field is included in requests that might o The Early-Data header field is included in requests that might
have been forwarded by an intermediary prior to the TLS handshake have been forwarded by an intermediary prior to the completion of
with its client completing. the TLS handshake with its client.
o The 425 (Too Early) status code is defined for a server to o The 425 (Too Early) status code is defined for a server to
indicate that a request could not be processed due to the indicate that a request could not be processed due to the
consequences of a possible replay attack. consequences of a possible replay attack.
They are designed to enable better coordination of the use of early They are designed to enable better coordination of the use of early
data between the user agent and origin server, and also when a data between the user agent and origin server, and also when a
gateway (also "reverse proxy", "Content Delivery Network", or gateway (also "reverse proxy", "Content Delivery Network", or
"surrogate") is present. "surrogate") is present.
Gateways typically don't have specific information about whether a Gateways typically don't have specific information about whether a
given request can be processed safely when it is sent in early data. given request can be processed safely when it is sent in early data.
In many cases, only the origin server has the necessary information In many cases, only the origin server has the necessary information
to decide whether the risk of replay is acceptable. These extensions to decide whether the risk of replay is acceptable. These extensions
allow coordination between a gateway and its origin server. allow coordination between a gateway and its origin server.
5.1. The Early-Data Header Field 5.1. The Early-Data Header Field
The "Early-Data" request header field indicates that the request has The Early-Data request header field indicates that the request has
been conveyed in early data, and additionally indicates that a client been conveyed in early data and that a client understands the 425
understands the 425 (Too Early) status code. (Too Early) status code.
It has just one valid value: "1". Its syntax is defined by the It has just one valid value: "1". Its syntax is defined by the
following ABNF [ABNF]: following ABNF [ABNF]:
Early-Data = "1" Early-Data = "1"
For example: For example:
GET /resource HTTP/1.0 GET /resource HTTP/1.0
Host: example.com Host: example.com
Early-Data: 1 Early-Data: 1
An intermediary that forwards a request prior to the completion of An intermediary that forwards a request prior to the completion of
the TLS handshake with its client MUST send it with the "Early-Data" 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 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 request). An intermediary MUST use the Early-Data header field if
it - or another instance (see Section 6.2) - could have forwarded the the request might have been subject to a replay and might already
request prior to handshake completion if circumstances were have been forwarded by it or another instance (see Section 6.2).
different.
An intermediary MUST NOT remove this header field if it is present in An intermediary MUST NOT remove this header field if it is present in
a request. "Early-Data" MUST NOT appear in a "Connection" header a request. Early-Data MUST NOT appear in a Connection header field.
field.
The "Early-Data" header field is not intended for use by user agents 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 (that is, the original initiator of a request). Sending a request in
early data implies that the client understands this specification and early data implies that the client understands this specification and
is willing to retry a request in response to a 425 (Too Early) status 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 code. A user agent that sends a request in early data does not need
to include the "Early-Data" header field. to include the Early-Data header field.
A server cannot make a request that contains the Early-Data header A server cannot make a request that contains the Early-Data header
field safe for processing by waiting for the handshake to complete. 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 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 previous hop. Requests that contain the Early-Data header field and
be safely processed MUST be rejected using the 425 (Too Early) status cannot be safely processed MUST be rejected using the 425 (Too Early)
code. status code.
The "Early-Data" header field carries a single bit of information and The Early-Data header field carries a single bit of information, and
clients MUST include at most one instance. Multiple or invalid clients MUST include at most one instance. Multiple or invalid
instances of the header field MUST be treated as equivalent to a instances of the header field MUST be treated as equivalent to a
single instance with a value of 1 by a server. single instance with a value of 1 by a server.
A "Early-Data" header field MUST NOT be included in responses or An Early-Data header field MUST NOT be included in responses or
request trailers. request trailers.
5.2. 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 A 425 (Too Early) status code indicates that the server is unwilling
to risk processing a request that might be replayed. to risk processing a request that might be replayed.
User agents that send a request in early data are expected to retry 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 the request when receiving a 425 (Too Early) response status code. A
user agent MAY do so automatically, but any retries MUST NOT be sent user agent SHOULD retry automatically, but any retries MUST NOT be
in early data. sent in early data.
In all cases, an intermediary can forward a 425 (Too Early) status In all cases, an intermediary can forward a 425 (Too Early) status
code. Intermediaries MUST forward a 425 (Too Early) status code if code. Intermediaries MUST forward a 425 (Too Early) status code if
the request that it received and forwarded contained an "Early-Data" the request that it received and forwarded contained an Early-Data
header field. Otherwise, an intermediary that receives a request in header field. Otherwise, an intermediary that receives a request in
early data MAY automatically retry that request in response to a 425 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 (Too Early) status code, but it MUST wait for the TLS handshake to
complete on the connection where it received the request. complete on the connection where it received the request.
The server cannot assume that a client is able to retry a 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" unless the request is received in early data or the Early-Data header
header field is set to "1". A server SHOULD NOT emit the 425 status field is set to "1". A server SHOULD NOT emit the 425 status code
code unless one of these conditions is met. unless one of these conditions is met.
The 425 (Too Early) status code is not cacheable by default. Its The 425 (Too Early) status code is not cacheable by default. Its
payload is not the representation of any identified resource. payload is not the representation of any identified resource.
6. Security Considerations 6. Security Considerations
Using early data exposes a client to the risk that their request is Using early data exposes a client to the risk that their request is
replayed. A retried or replayed request can produce different side replayed. A retried or replayed request can produce different side
effects on the server. In addition to those side effects, replays effects on the server. In addition to those side effects, replays
and retries might be used for traffic analysis to recover information and retries might be used for traffic analysis to recover information
about requests or the resources those requests target. In about requests or the resources those requests target. In
particular, a request that is replayed might result in a different particular, a request that is replayed might result in a different
response, which might be observable from the length of protected data response, which might be observable from the length of protected data
even if the content remains confidential. even if the content remains confidential.
6.1. Gateways and Early Data 6.1. Gateways and Early Data
A gateway MUST NOT forward requests that were received in 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 unless it knows that the origin server it will forward to understands
the "Early-Data" header field and will correctly generate a 425 (Too the Early-Data header field and will correctly generate a 425 (Too
Early) status code. A gateway that is uncertain about origin server Early) status code. A gateway that is uncertain about origin server
support for a given request SHOULD either delay forwarding the support for a given request SHOULD either delay forwarding the
request until the TLS handshake with its client completes, or send a request until the TLS handshake with its client completes or send a
425 (Too Early) status code in response. 425 (Too Early) status code in response.
A gateway without at least one potential origin server that supports A gateway without at least one potential origin server that supports
"Early-Data" header field expends significant effort for what can at the Early-Data header field expends significant effort for what can
best be a modest performance benefit from enabling early data. If no at best be a modest performance benefit from enabling early data. If
origin server supports early data, disabling early data entirely is no origin server supports early data, it is more efficient to disable
more efficient. early data entirely.
6.2. Consistent Handling of Early Data 6.2. Consistent Handling of Early Data
Consistent treatment of a request that arrives in - or partially in - Consistent treatment of a request that arrives in, or partially in,
early data is critical to avoiding inappropriate processing of early data is critical to avoiding inappropriate processing of
replayed requests. If a request is not safe to process before the replayed requests. If a request is not safe to process before the
TLS handshake completes, then all instances of the server (including TLS handshake completes, then all instances of the server (including
gateways) need to agree and either reject the request or delay gateways) need to agree and either reject the request or delay
processing. processing.
Disabling early data, delaying requests, or rejecting requests with Disabling early data, delaying requests, or rejecting requests with
the 425 (Too Early) status code are all equally good measures for the 425 (Too Early) status code are all equally good measures for
mitigating replay attacks on requests that might be vulnerable to mitigating replay attacks on requests that might be vulnerable to
replay. Server instances can implement any of these measures and be replay. Server instances can implement any of these measures and be
considered to be consistent, even if different instances use considered consistent, even if different instances use different
different methods. Critically, this means that it is possible to methods. Critically, this means that it is possible to employ
employ different mitigations in reaction to other conditions, such as different mitigations in reaction to other conditions, such as server
server load. load.
A server MUST NOT act on early data before the handshake completes if A server MUST NOT act on early data before the handshake completes if
it and any other server instance could make a different decision it and any other server instance could make a different decision
about how to handle the same data. about how to handle the same data.
6.3. Denial of Service 6.3. Denial of Service
Accepting early data exposes a server to potential 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 through the replay of requests that are expensive to handle. A
server that is under load SHOULD prefer rejecting TLS early data as a server that is under load SHOULD prefer rejecting TLS early data as a
whole rather than accepting early data and selectively processing whole rather than accepting early data and selectively processing
requests. Generating a 503 (Service Unavailable) or 425 (Too Early) requests. Generating a 503 (Service Unavailable) or 425 (Too Early)
status code often leads to clients retrying requests, which could status code often leads to clients retrying requests, which could
result in increased load. result in increased load.
6.4. Out of Order Delivery 6.4. Out-of-Order Delivery
In protocols that deliver data out of order (such as QUIC [HQ]) early In protocols that deliver data out of order (such as QUIC [HQ]),
data can arrive after the handshake completes. A server MAY process early data can arrive after the handshake completes. A server MAY
requests received in early data after handshake completion only if it process requests received in early data after handshake completion
can rely on other instances correctly handling replays of the same only if it can rely on other instances correctly handling replays of
requests. the same requests.
7. IANA Considerations 7. IANA Considerations
This document registers the "Early-Data" header field in the "Message This document registers the Early-Data header field in the "Permanent
Headers" registry located at https://www.iana.org/assignments/ Message Header Field Names" registry located at
message-headers [4]. <https://www.iana.org/assignments/message-headers>.
Header field name: Early-Data Header field name: Early-Data
Applicable protocol: http Applicable protocol: http
Status: standard Status: standard
Author/Change controller: IETF Author/Change controller: IETF
Specification document(s): This document Specification document(s): This document
Related information: (empty) Related information: (empty)
This document registers the 425 (Too Early) status code in the This document registers the 425 (Too Early) status code in the "HTTP
"Hypertext Transfer Protocol (HTTP) Status Code" registry located at Status Codes" registry located at <https://www.iana.org/assignments/
https://www.iana.org/assignments/http-status-codes [5]. http-status-codes>.
Value: 425 Value: 425
Description: Too Early Description: Too Early
Reference: This document Reference: This document
8. References 8. References
8.1. Normative References 8.1. Normative References
skipping to change at page 11, line 25 skipping to change at page 11, line 25
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231, Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014, DOI 10.17487/RFC7231, June 2014,
<https://www.rfc-editor.org/info/rfc7231>. <https://www.rfc-editor.org/info/rfc7231>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[TLS13] Rescorla, E., "The Transport Layer Security (TLS) Protocol [TLS13] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", draft-ietf-tls-tls13-28 (work in progress), Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
March 2018. <https://www.rfc-editor.org/info/rfc8446>.
8.2. Informative References 8.2. Informative References
[ALPN] Friedl, S., Popov, A., Langley, A., and E. Stephan, [ALPN] Friedl, S., Popov, A., Langley, A., and E. Stephan,
"Transport Layer Security (TLS) Application-Layer Protocol "Transport Layer Security (TLS) Application-Layer Protocol
Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301, Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
July 2014, <https://www.rfc-editor.org/info/rfc7301>. July 2014, <https://www.rfc-editor.org/info/rfc7301>.
[HQ] Bishop, M., "Hypertext Transfer Protocol (HTTP) over [HQ] Bishop, M., "Hypertext Transfer Protocol (HTTP) over
QUIC", draft-ietf-quic-http-13 (work in progress), June QUIC", draft-ietf-quic-http-14 (work in progress), August
2018. 2018.
[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext [RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540, Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015, DOI 10.17487/RFC7540, May 2015,
<https://www.rfc-editor.org/info/rfc7540>. <https://www.rfc-editor.org/info/rfc7540>.
8.3. URIs 8.3. URIs
[1] https://lists.w3.org/Archives/Public/ietf-http-wg/ [1] https://lists.w3.org/Archives/Public/ietf-http-wg/
[2] http://httpwg.github.io/ [2] http://httpwg.github.io/
[3] https://github.com/httpwg/http-extensions/labels/replay [3] https://github.com/httpwg/http-extensions/labels/replay
[4] https://www.iana.org/assignments/message-headers
[5] https://www.iana.org/assignments/http-status-codes
Acknowledgments Acknowledgments
This document was not easy to produce. The following people made This document was not easy to produce. The following people made
substantial contributions to the quality and completeness of the substantial contributions to the quality and completeness of the
document: David Benjamin, Subodh Iyengar, Benjamin Kaduk, Ilari document: David Benjamin, Subodh Iyengar, Benjamin Kaduk, Ilari
Liusavaara, Kazuho Oku, Eric Rescorla, Kyle Rose, and Victor Liusavaara, Kazuho Oku, Eric Rescorla, Kyle Rose, and Victor
Vasiliev. Vasiliev.
Authors' Addresses Authors' Addresses
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