Transport Layer Security (TLS) Next Protocol Negotiation Extension
draft-agl-tls-nextprotoneg-00

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Abstract

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.

Table of Contents

1.  Introduction
2.  Requirements Notation
3.  Next Protocol Negotiation Extension
4.  Discussion
5.  Security considerations
6.  IANA Considerations
7.  Acknowledgements
8.  References
    8.1.  Normative References
    8.2.  Informative References
Appendix A.  Changes
§  Author's Address
§  Intellectual Property and Copyright Statements

1.  Introduction

The use of TLS (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2,” August 2008.) [RFC5246] over TCP (Postel, J., “Transmission Control Protocol,” September 1981.) [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 (Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” June 1999.) [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 (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.) [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 (Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig, “Transport Layer Security (TLS) Session Resumption without Server-Side State,” January 2008.) [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.  Discussion

It is expected that the server will include a list of supported protocols in the ServerHello message and that the client will select one and include that in the NextProtocol message.

If the client were to include a list of supported protocols in the ClientHello message and the server were to select one, the additional handshake message could be avoided. However, as the client would be announcing its intentions in the clear, such a design invites interference from the network. By using an encrypted handshake message, the network is unable to discriminate between TLS connections on the grounds of application layer protocol.

5.  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 (Dingledine, R., Matthewson, N., and P. Syverson, “Tor: The Second-Generation Onion Router,” August 2004.) [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.

6.  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.

7.  Acknowledgements

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

8.  References

8.1. Normative References

8.2. Informative References

Appendix A.  Changes

To be removed by RFC Editor before publication

Author's Address

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