RFC 7301 PROPOSED STANDARD TLS · NEGOTIATION · WEB
TLS ALPN: Transport Layer Security (TLS) Application-Layer Protocol Negotiation Extension
Negotiates the application protocol during the TLS handshake without adding network round-trips.
RFC EDITOR
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>.
BIBTEX
@misc{rfc7301,
series = {Request for Comments},
number = 7301,
howpublished = {RFC 7301},
publisher = {RFC Editor},
doi = {10.17487/RFC7301},
url = {https://www.rfc-editor.org/info/rfc7301},
author = {S. Friedl and A. Popov and A. Langley and E. Stephan},
title = {{Transport Layer Security (TLS) Application-Layer Protocol Negotiation Extension}},
pagetotal = 9,
year = 2014,
month = jul,
} RFC text
Contents
Internet Engineering Task Force (IETF) S. Friedl
Request for Comments: 7301 Cisco Systems, Inc.
Category: Standards Track A. Popov
ISSN: 2070-1721 Microsoft Corp.
A. Langley
Google Inc.
E. Stephan
Orange
July 2014
Transport Layer Security (TLS)
Application-Layer Protocol Negotiation Extension
Abstract
This document describes a Transport Layer Security (TLS) extension
for application-layer protocol negotiation within the TLS handshake.
For instances in which multiple application protocols are supported
on the same TCP or UDP port, this extension allows the application
layer to negotiate which protocol will be used within the TLS
connection.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7301.
Copyright Notice
Copyright (c) 2014 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
(http://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
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. Application-Layer Protocol Negotiation . . . . . . . . . . . 3
3.1. The Application-Layer Protocol Negotiation Extension . . 3
3.2. Protocol Selection . . . . . . . . . . . . . . . . . . . 5
4. Design Considerations . . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1. Normative References . . . . . . . . . . . . . . . . . . 8
8.2. Informative References . . . . . . . . . . . . . . . . . 8
1. Introduction
Increasingly, application-layer protocols are encapsulated in the TLS
protocol [RFC5246]. This encapsulation enables applications to use
the existing, secure communications links already present on port 443
across virtually the entire global IP infrastructure.
When multiple application protocols are supported on a single server-
side port number, such as port 443, the client and the server need to
negotiate an application protocol for use with each connection. It
is desirable to accomplish this negotiation without adding network
round-trips between the client and the server, as each round-trip
will degrade an end-user's experience. Further, it would be
advantageous to allow certificate selection based on the negotiated
application protocol.
This document specifies a TLS extension that permits the application
layer to negotiate protocol selection within the TLS handshake. This
work was requested by the HTTPbis WG to address the negotiation of
HTTP/2 ([HTTP2]) over TLS; however, ALPN facilitates negotiation of
arbitrary application-layer protocols.
With ALPN, the client sends the list of supported application
protocols as part of the TLS ClientHello message. The server chooses
a protocol and sends the selected protocol as part of the TLS
ServerHello message. The application protocol negotiation can thus
be accomplished within the TLS handshake, without adding network
round-trips, and allows the server to associate a different
certificate with each application protocol, if desired.
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Application-Layer Protocol Negotiation
3.1. The Application-Layer Protocol Negotiation Extension
A new extension type ("application_layer_protocol_negotiation(16)")
is defined and MAY be included by the client in its "ClientHello"
message.
enum {
application_layer_protocol_negotiation(16), (65535)
} ExtensionType;
The "extension_data" field of the
("application_layer_protocol_negotiation(16)") extension SHALL
contain a "ProtocolNameList" value.
opaque ProtocolName<1..2^8-1>;
struct {
ProtocolName protocol_name_list<2..2^16-1>
} ProtocolNameList;
"ProtocolNameList" contains the list of protocols advertised by the
client, in descending order of preference. Protocols are named by
IANA-registered, opaque, non-empty byte strings, as described further
in Section 6 ("IANA Considerations") of this document. Empty strings
MUST NOT be included and byte strings MUST NOT be truncated.
Servers that receive a ClientHello containing the
"application_layer_protocol_negotiation" extension MAY return a
suitable protocol selection response to the client. The server will
ignore any protocol name that it does not recognize. A new
ServerHello extension type
("application_layer_protocol_negotiation(16)") MAY be returned to the
client within the extended ServerHello message. The "extension_data"
field of the ("application_layer_protocol_negotiation(16)") extension
is structured the same as described above for the client
"extension_data", except that the "ProtocolNameList" MUST contain
exactly one "ProtocolName".
Therefore, a full handshake with the
"application_layer_protocol_negotiation" extension in the ClientHello
and ServerHello messages has the following flow (contrast with
Section 7.3 of [RFC5246]):
Client Server
ClientHello --------> ServerHello
(ALPN extension & (ALPN extension &
list of protocols) selected protocol)
Certificate*
ServerKeyExchange*
CertificateRequest*
<-------- ServerHelloDone
Certificate*
ClientKeyExchange
CertificateVerify*
[ChangeCipherSpec]
Finished -------->
[ChangeCipherSpec]
<-------- Finished
Application Data <-------> Application Data
Figure 1
* Indicates optional or situation-dependent messages that are not
always sent.
An abbreviated handshake with the
"application_layer_protocol_negotiation" extension has the following
flow:
Client Server
ClientHello --------> ServerHello
(ALPN extension & (ALPN extension &
list of protocols) selected protocol)
[ChangeCipherSpec]
<-------- Finished
[ChangeCipherSpec]
Finished -------->
Application Data <-------> Application Data
Figure 2
Unlike many other TLS extensions, this extension does not establish
properties of the session, only of the connection. When session
resumption or session tickets [RFC5077] are used, the previous
contents of this extension are irrelevant, and only the values in the
new handshake messages are considered.
3.2. Protocol Selection
It is expected that a server will have a list of protocols that it
supports, in preference order, and will only select a protocol if the
client supports it. In that case, the server SHOULD select the most
highly preferred protocol that it supports and that is also
advertised by the client. In the event that the server supports no
protocols that the client advertises, then the server SHALL respond
with a fatal "no_application_protocol" alert.
enum {
no_application_protocol(120),
(255)
} AlertDescription;
The protocol identified in the
"application_layer_protocol_negotiation" extension type in the
ServerHello SHALL be definitive for the connection, until
renegotiated. The server SHALL NOT respond with a selected protocol
and subsequently use a different protocol for application data
exchange.
4. Design Considerations
The ALPN extension is intended to follow the typical design of TLS
protocol extensions. Specifically, the negotiation is performed
entirely within the client/server hello exchange in accordance with
the established TLS architecture. The
"application_layer_protocol_negotiation" ServerHello extension is
intended to be definitive for the connection (until the connection is
renegotiated) and is sent in plaintext to permit network elements to
provide differentiated service for the connection when the TCP or UDP
port number is not definitive for the application-layer protocol to
be used in the connection. By placing ownership of protocol
selection on the server, ALPN facilitates scenarios in which
certificate selection or connection rerouting may be based on the
negotiated protocol.
Finally, by managing protocol selection in the clear as part of the
handshake, ALPN avoids introducing false confidence with respect to
the ability to hide the negotiated protocol in advance of
establishing the connection. If hiding the protocol is required,
then renegotiation after connection establishment, which would
provide true TLS security guarantees, would be a preferred
methodology.
5. Security Considerations
The ALPN extension does not impact the security of TLS session
establishment or application data exchange. ALPN serves to provide
an externally visible marker for the application-layer protocol
associated with the TLS connection. Historically, the application-
layer protocol associated with a connection could be ascertained from
the TCP or UDP port number in use.
Implementers and document editors who intend to extend the protocol
identifier registry by adding new protocol identifiers should
consider that in TLS versions 1.2 and below the client sends these
identifiers in the clear. They should also consider that, for at
least the next decade, it is expected that browsers would normally
use these earlier versions of TLS in the initial ClientHello.
Care must be taken when such identifiers may leak personally
identifiable information, or when such leakage may lead to profiling
or to leaking of sensitive information. If any of these apply to
this new protocol identifier, the identifier SHOULD NOT be used in
TLS configurations where it would be visible in the clear, and
documents specifying such protocol identifiers SHOULD recommend
against such unsafe use.
6. IANA Considerations
The IANA has updated its "ExtensionType Values" registry to include
the following entry:
16 application_layer_protocol_negotiation
This document establishes a registry for protocol identifiers
entitled "Application-Layer Protocol Negotiation (ALPN) Protocol IDs"
under the existing "Transport Layer Security (TLS) Extensions"
heading.
Entries in this registry require the following fields:
o Protocol: The name of the protocol.
o Identification Sequence: The precise set of octet values that
identifies the protocol. This could be the UTF-8 encoding
[RFC3629] of the protocol name.
o Reference: A reference to a specification that defines the
protocol.
This registry operates under the "Expert Review" policy as defined in
[RFC5226]. The designated expert is advised to encourage the
inclusion of a reference to a permanent and readily available
specification that enables the creation of interoperable
implementations of the identified protocol.
The initial set of registrations for this registry is as follows:
Protocol: HTTP/1.1
Identification Sequence:
0x68 0x74 0x74 0x70 0x2f 0x31 0x2e 0x31 ("http/1.1")
Reference: [RFC7230]
Protocol: SPDY/1
Identification Sequence:
0x73 0x70 0x64 0x79 0x2f 0x31 ("spdy/1")
Reference:
http://dev.chromium.org/spdy/spdy-protocol/spdy-protocol-draft1
Protocol: SPDY/2
Identification Sequence:
0x73 0x70 0x64 0x79 0x2f 0x32 ("spdy/2")
Reference:
http://dev.chromium.org/spdy/spdy-protocol/spdy-protocol-draft2
Protocol: SPDY/3
Identification Sequence:
0x73 0x70 0x64 0x79 0x2f 0x33 ("spdy/3")
Reference:
http://dev.chromium.org/spdy/spdy-protocol/spdy-protocol-draft3
7. Acknowledgements
This document benefitted specifically from the Next Protocol
Negotiation (NPN) extension document authored by Adam Langley and
from discussions with Tom Wesselman and Cullen Jennings, both of
Cisco.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC7230] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
(HTTP/1.1): Message Syntax and Routing", RFC 7230, June
2014.
8.2. Informative References
[HTTP2] Belshe, M., Peon, R., and M. Thomson, "Hypertext Transfer
Protocol version 2", Work in Progress, June 2014.
[RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption without
Server-Side State", RFC 5077, January 2008.
Authors' Addresses
Stephan Friedl
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
USA
Phone: (720)562-6785
EMail: sfriedl@cisco.com
Andrei Popov
Microsoft Corp.
One Microsoft Way
Redmond, WA 98052
USA
EMail: andreipo@microsoft.com
Adam Langley
Google Inc.
USA
EMail: agl@google.com
Emile Stephan
Orange
2 avenue Pierre Marzin
Lannion F-22307
France
EMail: emile.stephan@orange.com
ISO Plain Language
On this page
PROBLEMA server can support several application protocols on the same port. The client and server need to agree which protocol to use for a connection.
MECHANISMThe client lists supported protocols in an extension to ClientHello. The server selects one and returns it in an extension to ServerHello.
WHY IT MATTERSALPN places protocol selection in the TLS handshake. RFC 7301 is a Proposed Standard and has been updated by RFC 8447.
§1Why Negotiate a Protocol in TLS
Application protocols can share a server-side port, so a client and server need to select one for each connection. RFC 7301 defines Application-Layer Protocol Negotiation (ALPN) as a TLS extension for that choice.
The client and server exchange the choice within the TLS handshake, avoiding extra network round-trips. The RFC says a server can also use the negotiated protocol when selecting a certificate.
When one port supports several application protocols, the two sides need to choose one. ALPN carries that choice in the TLS handshake.
§3.1The ALPN Extension Data
The client can include extension type application_layer_protocol_negotiation(16) in ClientHello. Its ProtocolNameList contains non-empty protocol identifiers as byte strings, ordered from most to least preferred. Each name is 1 to 255 bytes, and the list is 2 to 65535 bytes.
The server's response uses the same structure, but its list must contain exactly one protocol name. The RFC requires that protocol names not be truncated.
The client sends a preference-ordered list of protocol identifiers. The server returns one identifier that it selected.
§3.1Where ALPN Appears in the Handshake
The client places the ALPN extension in ClientHello, and a server that supports a suitable choice can return the extension in ServerHello. The RFC describes this for both full and abbreviated handshakes; ALPN does not add new handshake messages.
ALPN establishes a property of the connection, not the session. When a connection resumes, the previous ALPN values are irrelevant; the parties use the values in the new handshake.
ALPN information travels in the ClientHello and ServerHello messages. A resumed connection negotiates the protocol again in its new handshake.
ClientServer
- ClientHello with protocol list
- ServerHello with selected protocol
§3.2How the Server Selects a Protocol
The server is expected to keep its own preference-ordered list. It should select its most preferred protocol that also appears in the client's list.
If the server supports none of the protocols that the client offered, it sends the fatal no_application_protocol alert, whose description value is 120. Once selected, the protocol is definitive for the connection until renegotiation; the server must not use a different protocol for application data.
The server chooses a protocol that both sides support. If there is no shared choice, the server sends an error alert and ends the handshake.
§4Visible Protocol Names and Security
The server's selected ALPN protocol is sent in plaintext. The RFC says ALPN does not change the security of TLS session establishment or application data, but the selected protocol is visible to observers.
In TLS 1.2 and earlier, the client also sends its offered identifiers in the clear. The RFC warns that identifiers could reveal personal information or enable profiling, and says such identifiers should not be used where they are visible in plaintext.
Protocol identifiers can be visible to observers during the handshake. Identifiers that reveal personal or sensitive information should not be sent in the clear.
§6Protocol Identifier Registry
The RFC assigns extension type 16 to ALPN and creates the ALPN Protocol IDs registry. Each registration includes the protocol name, the exact sequence of identifying bytes, and a reference to its specification.
The registry uses Expert Review. Initial entries include http/1.1 and spdy/1, spdy/2, and spdy/3.
An official registry records each protocol name and the bytes that identify it. Expert review is required for new entries.
§7References
[RFC5246]RFC 5246Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, August 2008.August 2008[RFC5077]RFC 5077Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig, "Transport Layer Security (TLS) Session Resumption without Server-Side State", RFC 5077, January 2008.January 2008[RFC2119]RFC 2119Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.March 1997