Internet DRAFT - draft-vvv-tls-alps
draft-vvv-tls-alps
TLS Working Group D. Benjamin
Internet-Draft V. Vasiliev
Intended status: Standards Track Google
Expires: 25 March 2021 21 September 2020
TLS Application-Layer Protocol Settings Extension
draft-vvv-tls-alps-01
Abstract
This document describes a Transport Layer Security (TLS) extension
for negotiating application-layer protocol settings (ALPS) within the
TLS handshake. Any application-layer protocol operating over TLS can
use this mechanism to indicate its settings to the peer in parallel
with the TLS handshake completion.
Discussion Venues
This note is to be removed before publishing as an RFC.
Discussion of this document takes place on the TLS Working Group
mailing list (tls@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/tls/
(https://mailarchive.ietf.org/arch/browse/tls/).
Source for this draft and an issue tracker can be found at
https://github.com/vasilvv/tls-alps (https://github.com/vasilvv/tls-
alps).
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on 25 March 2021.
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Copyright Notice
Copyright (c) 2020 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/
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
3. Semantics . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Wire Protocol . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. Client Encrypted Extensions . . . . . . . . . . . . . . . 6
4.2. 0-RTT Handshakes . . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1. Normative References . . . . . . . . . . . . . . . . . . 8
7.2. Informative References . . . . . . . . . . . . . . . . . 8
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
An application-layer protocol often starts with both parties
negotiating parameters under which the protocol operates; for
instance, HTTP/2 [RFC7540] uses a SETTINGS frame to exchange the list
of protocol parameters supported by each endpoint. This is usually
achieved by waiting for TLS handshake [RFC8446] to complete and then
performing the application-layer handshake within the application
protocol itself. This approach, despite its apparent simplicity at
first, has multiple drawbacks:
1. While the server is technically capable of sending configuration
to the peer as soon as it sends its Finished message, most TLS
implementations do not allow any application data to be sent
until the Finished message is received from the client. This
adds an extra round-trip to the time of when the server settings
are available to the client.
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2. In QUIC, any settings delivered within the application layer can
arrive after other application data; thus, the application has to
operate under the assumption that peer's settings are not always
available.
3. If the application needs to be aware of the server settings in
order to send 0-RTT data, the application has to manually
integrate with the TLS stack to associate the settings with TLS
session tickets.
This document introduces a new TLS extension, "application_settings",
that allows applications to exchange settings within the TLS
handshake. Through doing that, the settings can be made available to
the application as soon as the handshake completes, and can be
associated with TLS session tickets automatically at the TLS layer.
This approach allows the application protocol to be designed with the
assumption that it has access to the peer's settings whenever it is
able to send data.
2. 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.
3. Semantics
Settings are defined to be an opaque blob that is specified by the
application when initiating a TLS connection. The settings are meant
to be a _declaration_ of the protocol parameters supported by the
sender. While in this version of the extension the server settings
are always sent first, this may change in future versions; thus, the
application MUST NOT vary client settings based on the ones received
from the server.
ALPS is _not_ a negotiation mechanism: there is no notion of
rejecting peer's settings, and the settings are not responses to one
another. Nevertheless, it is possible for parties to coordinate
behavior by, for instance, requiring a certain parameter to be
present in both client and server settings. This makes ALPS
mechanism similar to QUIC transport parameters
[I-D.ietf-quic-transport] or HTTP/2 SETTINGS frame [RFC7540], but
puts it in contrast to similar mechanisms in TLS.
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Settings are exchanged as a part of the TLS handshake that is
encrypted with the handshake keys. When the server settings are
sent, the identity of the client has not been yet established;
therefore, an application MUST NOT use ALPS if it requires the
settings to be available only to the authenticated clients.
The ALPS model provides applications with a guarantee that the
settings are available before any application data can be written.
Note that this implies that when the full handshake is performed, the
server can no longer send data immediately after sending its Finished
message; it has to wait for the client to respond with its settings.
This may negatively impact the latency of the protocols where the
server sends the first message, however it should be noted that
sending application data before receiving has not been widely
supported by TLS implementations, nor has it been allowed in
situations when establishing client identity through TLS is required.
ALPS can only be used in conjunction with Application-Layer Protocol
Negotiation: the client MUST offer ALPN [RFC7301] if advertising ALPS
support, and the server MUST NOT reply with ALPS unless it is also
negotiating ALPN. The ALPS payload is protocol-dependent, and as
such it MUST be specified with respect to a selected ALPN.
4. Wire Protocol
ALPS is only supported in TLS version 1.3 or later, as the earlier
versions do not provide any confidentiality protections for the
handshake data. The exchange is performed in three steps:
1. The client sends an extension in ClientHello that enumerates all
ALPN values for which ALPS is supported.
2. The server sends an encrypted extension containing the server
settings.
3. The client sends an encrypted extension containing the client
settings.
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Client Server
ClientHello
+ alpn
+ alps -------->
ServerHello
{EncryptedExtensions}
+ alpn
+ alps
...
<-------- {Finished}
{EncryptedExtensions}
+ alps
{Certificate*}
{CertificateVerify*}
{Finished} -------->
+ Indicates extensions sent in the
previously noted message.
{} Indicates messages protected using
the handshake keys.
* Indicates optional messages that are
not related to ALPS.
Figure 1: ALPS exchange in a full TLS handshake
A TLS client can enable ALPS by specifying an "application_settings"
extension in the ClientHello message. The value of the
"extension_data" field for this extension SHALL be a
ApplicationSettingsSupport struct:
struct {
ProtocolName supported_protocols<2..2^16-1>;
} ApplicationSettingsSupport;
Here, the "supported_protocols" field indicates the names of the
protocols (as defined in [RFC7301]) for which ALPS exchange is
supported; this is necessary for the situations when the client
offers multiple ALPN values but only supports ALPS in some of them.
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If the server chooses an ALPN value for which the client has offered
ALPS support, the server MAY negotiate ALPS by sending an
"application_settings" extension in its EncryptedExtensions message.
The value of the "extension_data" field in that case SHALL be an
opaque blob containing the server settings as specified by the
application protocol.
If the client receives an EncryptedExtensions message containing an
"application_settings" extension from the server, it MUST send an
EncryptedExtensions message (see Section 4.1) containing an
"application_extensions" extension. The value of the
"extension_data" in this extension SHALL be an opaque blob containing
the client settings as specified by the application protocol. A
server which negotiates ALPS MUST abort the handshake with a
"missing_extension" alert if the client's EncryptedExtensions is
missing this extension.
4.1. Client Encrypted Extensions
This specification introduces the client EncryptedExtensions message.
The format and HandshakeType code point match the server
EncryptedExtensions message. When sent, it is encrypted with
handshake traffic keys and sent by the client after receiving the
server Finished message and before the client sends the Certificate,
CertificateVerify (if any), and Finished messages. It SHALL be
appended to the Client Handshake Context, as defined Section 4.4 of
[RFC8446]. It additionally SHALL be inserted after the server
Finished in the Post-Handshake Handshake Context.
The client MUST send the EncryptedExtensions message if any extension
sent in the server EncryptedExtension message contains the CEE token
in the TLS 1.3 column of the TLS ExtensionType Values registry.
Otherwise, the client MUST NOT send the message. The server MUST
abort the handshake with a "unexpected_message" alert if the message
was sent or omitted incorrectly.
The client MAY send an extension in the client EncryptedExtension
message if that extension's entry in the registry contains a CEE
token and the server EncryptedExtensions message included the
extension. Otherwise, the client MUST NOT send the extension. If a
server receives an extension which does not meet this criteria, it
MUST abort the handshake with an "unsupported_extension" alert.
Future extensions MAY use the client EncryptedExtensions message by
including the CEE token in the TLS 1.3 registry. The above rules
ensure clients will not send EncryptedExtensions messages to older
servers, but will send EncryptedExtensions when some negotiated
extension uses it.
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[[TODO: Section 4.6.1 of RFC8446 allows the server to predict the
client Finished flight and send a ticket early. This is still
possible with 0-RTT handshakes here because we omit rather than
repeat the redudant ALPS information, but, in the general extension
case, client EncryptedExtensions breaks this. Extension order is
unpredictable. We should resolve this conflict, either by dropping
that feature or removing flexibility here.]]
4.2. 0-RTT Handshakes
ALPS ensures settings are available before reading and writing
application data, so handshakes which negotiate early data instead
use application settings from the PSK. To use early data with a PSK,
the TLS implementation MUST associate both client and server
application settings, if any, with the PSK. For a resumption PSK,
these values are determined from the original connection. For an
external PSK, this values should be configured with it. Existing
PSKs are considered to not have application settings.
If the server accepts early data, the server SHALL NOT send an
"application_settings" extension, and thus the client SHALL NOT send
a "application_settings" extension in its EncryptedExtensions
message. Unless the server has sent some other extension which uses
client EncryptedExtensions, the client SHALL NOT send an
EncryptedExtensions message. Instead, the connection implicitly uses
the PSK's application settings, if any.
If the server rejects early data, application settings are negotiated
independently of the PSK, as if early data were not offered.
If the client wishes to send different client settings for the
connection, it MUST NOT offer 0-RTT. Conversely, if the server
wishes to use send different server settings, it MUST reject 0-RTT.
Note that the ALPN itself is similarly required to match the one in
the original connection, thus the settings only need to be remembered
or checked for a single application protocol. Implementations are
RECOMMENDED to first determine the desired application protocol and
settings independent of early data, and then decline to offer or
accept early data if the values do not match the PSK. This preserves
any ALPN and ALPS configuration specified by the calling application.
5. Security Considerations
ALPS is protected using the handshake keys, which are the secret keys
derived as a result of (EC)DHE between the client and the server.
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In order to ensure that the ALPS values are authenticated, the TLS
implementation MUST NOT reveal the contents of peer's ALPS until
peer's Finished message is received, with exception of cases where
the ALPS has been carried over from the previous connection.
6. IANA Considerations
IANA will update the "TLS ExtensionType Values" registry to include
"application_settings" with the value of TBD; the list of messages in
which this extension may appear is "CH, EE, CEE".
7. References
7.1. Normative References
[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>.
[RFC7301] 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>.
[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>.
[RFC8446] 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>.
7.2. Informative References
[I-D.ietf-quic-transport]
Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
and Secure Transport", Work in Progress, Internet-Draft,
draft-ietf-quic-transport-30, 9 September 2020,
<http://www.ietf.org/internet-drafts/draft-ietf-quic-
transport-30.txt>.
[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>.
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Acknowledgments
This document has benefited from contributions and suggestions from
Nick Harper, David Schinazi, Renjie Tang and many others.
Authors' Addresses
David Benjamin
Google
Email: davidben@google.com
Victor Vasiliev
Google
Email: vasilvv@google.com
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