Network Working Group A. Langley
Internet-Draft Google Inc
Intended status: Standards Track January 2010
Expires: July 03, 2010

Transport Layer Security (TLS) Next Protocol Negotiation Extension


This document describes a Transport Layer Security (TLS) extension for application layer protocol negotiation. This allows the application layer to negotiate which protocol should be performed over the secure connection in a manner which avoids additional round trips and which is independent of the application layer protocols.

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Table of Contents

1. Introduction

The use of TLS [RFC5246] over TCP [RFC0793] unavoidably adds at least a round trip to the time taken to establish a connection. By allowing the application layer to perform negotiation during that round trip one gains several advantages:

Firstly, for various reasons, different application layer protocols are increasingly being carried over TLS using a small set of TCP port numbers, most often port 443. Rather than forcing systems administrators to use different IP addresses for every service, negotiation allows multiple services to exist with the same IP address.

Secondly, newer versions of the same application layer protocol can perform discovery and avoid additional round trips after the TLS handshake.

To illustrate the second point, consider HTTP [RFC2616] pipelining. Although it has seen little adoption because of interference from middleware, there is no reason not to use it over TLS where the transport is known to be pristine.

However, even over TLS, the client cannot make pipelined requests until the first reply indicates that the remote peer supports the feature. If the client could negotiate HTTP/1.1 support before the first request, it could start sending pipelined requests immediately.

2. Requirements Notation

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 RFC 2119 [RFC2119].

3. Next Protocol Negotiation Extension

A new extension type (next_protocol_negotiation(TBD)) is defined and MAY be included by the client in its ClientHello message. If, and only if, the server sees this extension in the ClientHello, it MAY choose to include the extension in its ServerHello.

enum {
  next_protocol_negotiation(TBD), (65535)
} ExtensionType;

A new handshake message type (next_protocol(TBD)) is defined. If, and only if, the server included a next_protocol_negotiation extension in its ServerHello message, the client MUST send a NextProtocol message after its ChangeCipherSpec and before its Finished message.

struct {
  opaque selected_protocol<0..255>;
  opaque padding<0..255>;
} NextProtocol;

The extension_data field of a next_protocol_negotiation in a ClientHello MUST be empty.

The extension_data field in a ServerHello and the NextProtocol message contain opaque bytes to be used by the application layer to negotiate the application layer protocol. The format of this data is not specified in this draft.

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.

For the same reasons, after a handshake has been performed for a given connection, renegotiations on the same connection MUST NOT include the next_protocol_negotiation extension.

4. Security considerations

The server's list of supported protocols is still advertised in the clear with this extension. This may be undesirable for certain protocols (such as Tor [tor]) where one could imagine that hostile networks would terminate any TLS connection with a server that advertised such a capability. In this case, clients may wish to opportunistically select a protocol that wasn't advertised by the server. However, the workings of such a scheme are outside the scope of this document.

5. IANA Considerations

This document requires IANA to update its registry of TLS extensions to assign an entry, referred herein as next_protocol_negotiation.

This document also requires IANA to update its registry of TLS handshake types to assign an entry, referred herein as next_protocol.

6. Acknowledgements

This document benefited specifically from discussions with Wan-Teh Chang and Nagendra Modadugu.

7. References

7.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, August 2008.

7.2. Informative References

[RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, September 1981.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC5077] Salowey, J., Zhou, H., Eronen, P. and H. Tschofenig, "Transport Layer Security (TLS) Session Resumption without Server-Side State", RFC 5077, January 2008.
[tor] Dingledine, R., Matthewson, N. and P. Syverson, "Tor: The Second-Generation Onion Router", August 2004.

Appendix A. Changes

To be removed by RFC Editor before publication

Author's Address

Adam Langley Google Inc EMail: