Application Layer Protocol Negotiation (ALPN) for Session Traversal Utilities for NAT (STUN) and Traversal Using Relays around NAT (TURN)
draft-ietf-tram-alpn-01

Abstract

Application Layer Protocol Negotiation (ALPN) labels for the Session Traversal Utilities for NAT (STUN) and Traversal Using Relays around NAT (TURN) are defined in this document to allow the application layer to negotiate STUN, TURN within the Transport Layer Security (TLS) connection. The STUN ALPN protocol identifier and TURN ALPN 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.ietf-tram-stun-dtls].

ALPN [RFC7301] enables an endpoint to positively identify STUN protocol and TURN 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.

TURN ALPN is useful in the following scenarios:

1. Consider an Enterprise network that deploys a TURN server in a DeMilitarized Zone (DMZ) to audit all media sessions from inside the Enterprise premises to any external peer. In this deployment, an Enterprise firewall could use the TURN ALPN identifer to detect, and act accordingly, the use of a TURN server outisde the Enterprise domain (i.e., a TURN server provided by an application server, access network etc).
2. If a firewall is configured to 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. However, despite the restrictions imposed by the firewall, the TURN server can still be reached on the allowed HTTPS port if the TURN ALPN identifier is used to establish usage of TURN as part of the TLS handshake. In this case, the TURN 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.
3. If a TURN server is in a resource exhausted state then it could use the TURN ALPN identifier sent by the client to identify that the connection will be used to allocate resouces, which the server cannot accommodate, and hence reject the (D)TLS handshake with a fatal error.

This document defines entries ("stun") and ("turn") in the "Application Layer Protocol Negotiation (ALPN) Protocol IDs" registry established by [RFC7301] to identify the STUN protocol and usage of TURN.

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 [RFC7301].

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)

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 [RFC7301]. It also does not impact security of TLS/DTLS session establishment nor 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, Brandon Williams 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