Application Layer Protocol Negotiation (ALPN) for Session Traversal Utilities for NAT (STUN)
draft-ietf-tram-alpn-00

Abstract

An Application Layer Protocol Negotiation (ALPN) label for the Session Traversal Utilities for NAT (STUN) protocol is defined in this document to allow the application layer to negotiate STUN within the Transport Layer Security (TLS) connection. The STUN ALPN protocol identifier applies to both TLS and Datagram Transport Layer Security (DTLS).

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Table of Contents

• 1. Introduction
• 2. Terminology
• 3. IANA Considerations
• 4. Security Considerations
• 5. Acknowledgements
• 6. References
• 6.1. Normative References
• 6.2. Informative References
• Authors' Addresses

1. Introduction

STUN can be securely transported using TLS-over-TCP (referred to as TLS [RFC5246]), as specified in [RFC5389], or TLS-over-UDP (referred to as DTLS [RFC6347]), as specified in [I-D.petithuguenin-tram-turn-dtls].

ALPN [I-D.ietf-tls-applayerprotoneg] enables an endpoint to positively identify STUN protocol uses in TLS/DTLS and distinguish them from other TLS/DTLS protocols. With ALPN, the client sends the list of supported application protocols as part of the TLS/DTLS ClientHello message. The server chooses a protocol and sends the selected protocol as part of the TLS/DTLS ServerHello message. The application protocol negotiation can thus be accomplished within the TLS/DTLS handshake, without adding network round-trips, and allows the server to associate a different certificate with each application protocol, if desired.

For example, a firewall could block all outgoing traffic except for TCP traffic to specific ports (e.g., 443 for HTTPS). A TURN server listening on its default ports (3478 for TCP/UDP, 5349 for TLS) would not be reachable in this case. However, despite the restrictions imposed by the firewall, the TURN server can still be reached on the allowed HTTPS port if an ALPN STUN protocol identifier is used to establish the STUN application layer protocol as part of the TLS handshake. In this case, the STUN ALPN identifier sent by the client will be used by the server to identify that the client intends to make a TURN request and it must act as a TURN server to relay the traffic to and from the remote peer. Similarly, with Quick UDP Internet Connections (QUIC) [QUIC], a UDP-based transport protocol that operates under SPDY [I-D.mbelshe-httpbis-spdy], a TURN server could be operated on the same ports as that of a SPDY server.

This document defines an entry ("stun") in the "Application Layer Protocol Negotiation (ALPN) Protocol IDs" registry established by [I-D.ietf-tls-applayerprotoneg] to identify the STUN protocol.

[[TODO: In various offline discussions some have expressed a desire to add an additional ALPN protocol identifier for TURN (see IANA Considerations below for example registration). ALPN can be used more granularly to externally identify more of the protocol variants and their different properties (i.e., STUN and TURN over TLS/DTLS). The advantage in dividing it this way is that these different forms can be externally identified (obviously, there isn't any inherent value in the different identifiers from within the TLS handshake).There are two main disadvantages. the first is that this two application protocol approach may make implementations more complicated/confusing. The second is that there may be difficulty in differentiating the two with ALPN when TURN was specifically designed to be able to run on the same port as STUN usage (in section 13 of RFC 5389). Section 4.1.1.2 of RFC 5245 explicitly says that "If the Allocate request is rejected because the server lacks resources to fulfill it, the agent SHOULD instead send a Binding request to obtain a server reflexive candidate." Does that prove there is no need to differentiate TURN and STUN request on UDP/TCP or TLS and now DTLS? Are there sufficiently meaningful differences between the usages to warrant separate STUN and TURN ALPN identifiers?]]

2. Terminology

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. IANA Considerations

The following entry is to be added to the "Application Layer Protocol Negotiation (ALPN) Protocol IDs" registry established by [I-D.ietf-tls-applayerprotoneg].

The "stun" label identifies STUN over TLS/DTLS:

• Protocol: Session Traversal Utilities for NAT (STUN)
• Identification Sequence: 0x73 0x74 0x75 0x6E ("stun")
• Specification: This document (RFCXXXX)

[[TODO: Shown only as an example. Remove the below registry entry if open issue above dictates a single STUN ALPN identifier is sufficient.]]

The "turn" label identifies TURN over TLS/DTLS:

• Protocol: Traversal Using Relays around NAT (TURN)
• Identification Sequence: 0x74 0x75 0x72 0x6E ("turn")
• Specification: This document (RFCXXXX)

4. Security Considerations

The ALPN STUN protocol identifier does not introduce any specific security considerations beyond those detailed in the TLS ALPN Extension specification [I-D.ietf-tls-applayerprotoneg]. It also does not impact the security of TLS/DTLS session establishment nor the application data exchange.

5. Acknowledgements

This work benefited from the discussions and invaluable input by the various members of the TRAM working group. These include Simon Perrault, Paul Kyzivat, and Andrew Hutton. Special thanks to Martin Thomson and Oleg Moskalenko for their constructive comments, suggestions, and early reviews that were critical to the formulation and refinement of this document.

6. References

6.1. Normative References

6.2. Informative References

Authors' Addresses