Transport Layer Security (TLS) Application Layer Protocol Negotiation Extension
draft-ietf-tls-applayerprotoneg-01

Abstract

This document describes a Transport Layer Security (TLS) extension for application layer protocol negotiation within the TLS handshake. For instances in which the TLS connection is established over a well known TCP/IP port not associated with the desired application layer protocol, this extension allows the application layer to negotiate which protocol will be used within the TLS session.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on October 27, 2013.

Copyright Notice

Copyright (c) 2013 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Introduction
• 2. Requirements Language
• 3. Application Layer Protocol Negotiation
• 3.1. The Application Layer Protocol Negotiation Extension
• 3.2. Protocol Selection
• 4. Design Considerations
• 5. Security Considerations
• 6. IANA Considerations
• 7. Acknowledgements
• 8. References
• 8.1. Normative References
• 8.2. Informative References
• Authors' Addresses

1. Introduction

Increasingly, application layer protocols are encapsulated in the TLS security 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 which 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 version ([RFC2616], [I-D.ietf-httpbis-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. Implementations MUST ensure that an empty string is not included and that no byte strings are truncated.

Experimental protocol names, which are not registered by IANA, will start with the following sequence of bytes: 0x65, 0x78, 0x70 ("exp").

Servers that receive a client hello 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 SHALL be 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
            

Client                                              Server

ClientHello                     -------->       ServerHello
  (ALPN extension &                               (ALPN extension &
   list of protocols)                              selected protocol)
                                                [ChangeCipherSpec]
                                <--------       Finished
[ChangeCipherSpec]
Finished                        -------->
Application Data                <------->       Application Data
            

enum {
	no_application_protocol(120),
	(255)
} AlertDescription;