title: "TLS Application-Layer Protocol Settings Extension" abbrev: "TLS ALPS" docname: draft-vvv-tls-alps-latest category: std
ipr: trust200902 area: General workgroup: TLS Working Group keyword: Internet-Draft
stand_alone: yes pi: [toc, sortrefs, symrefs]
ins: D. Benjamin
name: David Benjamin
organization: Google
email: davidben@google.com
- ins: V. Vasiliev name: Victor Vasiliev organization: Google email: vasilvv@google.com
- ins: V. Tan name: Victor Tan organization: Google LLC email: victortan@google.com
normative: RFC2119:
informative:
--- 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.
--- middle
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:
- 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.
- 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.
- 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.
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.
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 {{?RFC9000}} or HTTP/2 SETTINGS frame {{?RFC7540}}, but puts it in contrast to similar mechanisms in TLS.
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.
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:
- The client sends an extension in ClientHello that enumerates all ALPN values for which ALPS is supported.
- The server sends an encrypted extension containing the server settings.
- The client sends an encrypted extension containing the client settings.
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.
{: #alps-full title="ALPS exchange in a full TLS handshake"}
A TLS client can enable ALPS by specifying an application_settings extension
in the ClientHello message. An early experimental deployment of this protocol
used the value 17513 for application_settings. It was then replaced with an
experimental deployment which used the value 17613. 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.
If the server chooses an ALPN value for which the client has offered ALPS
support, the server MAY additionally negotiate ALPS in this connection and
determine a server settings value. This is an opaque blob as specified by the
ALPN protocol. If not accepting early data (see {{early-data}}), the server then
sends an application_settings extension in its EncryptedExtensions message.
The value of the extension_data field in that case SHALL the server settings
value.
If the client receives an EncryptedExtensions message containing an
application_settings extension from the server, it first checks that ALPN was
negotiated and that the selected ALPN protocol was one of the ones advertised in
ApplicationSettingsSupport. If not, it MUST abort the connection with an
"illegal_parameter" alert. Otherwise, it MUST send an EncryptedExtensions
message (see {{encrypted-extensions}}) containing an application_settings
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.
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 in {{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.
[[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.]]
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.
When sending a ClientHello, clients MUST NOT offer early data with a PSK that has application settings if the PSK's client application settings are different from those the client would send for the PSK's ALPN protocol. If the PSK does not have application settings, the client MAY offer early data with the PSK independent of its ALPS configuration.
When processing a ClientHello, in addition to the checks specified by {{Section 4.2.10 of RFC8446}}, the server MUST verify the following before accepting early data:
-
If the server did not negotiate ALPS for the connection, the PSK does not have application settings.
-
If the server did negotiate ALPS for the connection, the PSK has application settings and the PSK's server settings value matches the value selected by the server.
If either check fails, the server MUST NOT accept early data. It MAY continue to negotiate the PSK.
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.
The checks in this section are analogous to the requirement that ALPN is preserved across early data. They ensure that early data does not change the result of the ALPN or ALPS negotiation.
Protocols using ALPS MUST define an application profile describing how the negotiated settings values are used in the protocol. Absent such a profile, client and server applications MUST NOT configure their TLS implementations to negotiate ALPS with the corresponding ALPN value. The profile MUST define the syntaxes of the client and server protocol settings values and how they are processed.
Protocols MAY mandate the use of ALPS when negotiated over TLS. This may be appropriate for new protocols that can depend on ALPS. Applications implementing the protocol would then check that ALPS was negotiated and, if not, terminate the connection.
Alternatively, protocols MAY make the use of ALPS optional. This may be appropriate for existing protocols, where there is already a deployment of non-ALPS clients or servers. ALPS would then be an extension to the existing protocol, and the application profile would define the protocol changes when negotiated. Note that, for any given connection, ALPS will be consistently negotiated or not negotiated on both the client and server. This means adjusting protocol behavior based on ALPS is unambiguous.
As described in {{early-data}}, early data does not change the result of the ALPS negotiation, so protocols that use TLS early data MAY change client and server ALPS preferences across connections. However, when either value changes, early data is rejected. Thus protocols using both ALPS and early data SHOULD ensure the client and server preferences change infrequently. For example, the set of HTTP/2 extensions {{?RFC7540}} implemented by a server only changes when new features are deployed. However, including different reserved setting identifiers (see {{Section 7.2.4.1 of ?I-D.ietf-quic-http}}) in each connection would not perform well.
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.
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.
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.
--- back
{:numbered="false"}
This document has benefited from contributions and suggestions from Nick Harper, David Schinazi, Renjie Tang and many others.