Traversal Using Relays around NAT (TURN) Uniform Resource Identifiers
draft-ietf-behave-turn-uri-02

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Abstract

This document defines two URI schemes and the resolution mechanism to generate a list of server transport addresses that can be tried to create a Traversal Using Relays around NAT (TURN) allocation.

Table of Contents

1.  Introduction
2.  Terminology
3.  Syntax of a TURN or TURNS URI
4.  TURN or TURNS URI Resolution
5.  Example
6.  Security Considerations
7.  IANA Considerations
    7.1.  TURN URI Registration
    7.2.  TURNS URI Registration
    7.3.  RELAY Application Service Tag Registration
    7.4.  turn.udp Application Protocol Tag Registration
    7.5.  turn.tcp Application Protocol Tag Registration
    7.6.  turn.tls Application Protocol Tag Registration
8.  Running Code Considerations
9.  Acknowledgements
10.  References
    10.1.  Normative References
    10.2.  Informative References
Appendix A.  Release notes
    A.1.  Modifications between -02 and -01
    A.2.  Modifications between -01 and -00
    A.3.  Design Notes
    A.4.  TODO List
§  Author's Address

1.  Introduction

The TURN specification (Rosenberg, J., Mahy, R., and P. Matthews, “Traversal Using Relays around NAT (TURN): Relay Extensions to Session Traversal Utilities for NAT (STUN),” April 2009.) [I‑D.ietf‑behave‑turn] defines a process for a TURN client to find TURN servers by using DNS SRV resource records, but this process does not let the TURN server administrators provision the preferred TURN transport protocol between the client and the server and for the TURN client to discover this preference. This document defines a S-NAPTR application (Daigle, L. and A. Newton, “Domain-Based Application Service Location Using SRV RRs and the Dynamic Delegation Discovery Service (DDDS),” January 2005.) [RFC3958] for this purpose. This application defines "RELAY" as application service tag and "turn.udp", "turn.tcp", and "turn.tls" as application protocol tags.

To simplify the provisioning of TURN clients, this document also defines a TURN and a TURNS URI scheme and a resolution mechanism to convert these URIs into a list of IP addresses, ports and TURN transport protocols.

Another usage of the resolution mechanism described in this document would be Remote Hosting as described in [RFC3958] (Daigle, L. and A. Newton, “Domain-Based Application Service Location Using SRV RRs and the Dynamic Delegation Discovery Service (DDDS),” January 2005.) section 4.4. For example a VoIP provider who does not want to deploy TURN servers could use the servers deployed by another company but could still want to provide configuration parameters to its customers without explicitly showing this relationship. The mechanism permits one to implement this indirection, without preventing the company hosting the TURN servers from managing them as it see fit.

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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

3.  Syntax of a TURN or TURNS URI

A TURN/TURNS URI has the following ABNF syntax [RFC5234] (Crocker, D. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” January 2008.):

turnURI   = scheme ":" host [ ":" port ] [ "?transport=" transport ]
scheme    = "turn" / "turns"
transport = "udp" / "tcp" / transport-ext
transport-ext = 1*unreserved

<host>, <port> and <unreserved> are specified in [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.).

4.  TURN or TURNS URI Resolution

The URI resolution mechanism is used only to create an allocation. All other transactions use the IP address, transport and port used for a successful allocation creation.

The URI resolution algorithm uses <scheme>, <host>, <port> and <transport> as input. It also uses a list ordered by preference of TURN transports (UDP, TCP, TLS) supported by the application using the TURN client. The output of the algorithm is a list of {IP address, transport, port} tuples that a TURN client can try in order to create an allocation on a TURN server.

An Allocate error response as specified in section 6.4 of [I‑D.ietf‑behave‑turn] (Rosenberg, J., Mahy, R., and P. Matthews, “Traversal Using Relays around NAT (TURN): Relay Extensions to Session Traversal Utilities for NAT (STUN),” April 2009.) is processed as a failure as specified by [RFC3958] (Daigle, L. and A. Newton, “Domain-Based Application Service Location Using SRV RRs and the Dynamic Delegation Discovery Service (DDDS),” January 2005.) section 2.2.4. The resolution stops when a TURN client gets a successful Allocate response from a TURN server. After an allocation succeeds or all the allocations fail the resolution context MUST be discarded and the URI resolution algorithm MUST be restarted from the beginning for any subsequent allocation. Servers blacklisted as described in section 6.4 of [I‑D.ietf‑behave‑turn] (Rosenberg, J., Mahy, R., and P. Matthews, “Traversal Using Relays around NAT (TURN): Relay Extensions to Session Traversal Utilities for NAT (STUN),” April 2009.) should not be used for the specified duration even if returned by a subsequent resolution.

In some steps <transport> and <scheme> have to be converted to a TURN transport. If <scheme> is defined as "turn" and <transport> is defined as "udp" then the TURN UDP transport is used. If <scheme> is defined as "turn" and <transport> is defined as "tcp" then the TURN TCP transport is used. If <scheme> is defined as "turns" and <transport> is defined as "tcp" then the TURN TLS transport is used.

First the resolution algorithm checks that the URI can be resolved with the list of TURN transports supported:

• If <scheme> is defined as "turn" and <transport> is defined as "udp" but the list of TURN transports does not contain UDP then the resolution MUST stop with an error.
• If <scheme> is defined as "turn" and <transport> is defined as "tcp" but the list of TURN transports does not contain TCP or TLS then the resolution MUST stop with an error.
• If <scheme> is defined as "turns" and <transport> is defined as "udp" then the algorithm MUST stop with an error.
• If <scheme> is defined as "turns" and <transport> is defined as "tcp" but the list of TURN transports does not contain TLS then the resolution MUST stop with an error.
• If <scheme> is defined as "turns" and <transport> is not defined but the list of TURN transports does not contain TLS then the resolution MUST stop with an error.
• If <transport> is defined but unknown then the resolution MUST stop with an error.

Then the algorithm applies the following steps:

1. If <host> is an IP address then it indicates the specific IP address to be used. If <port> is not defined, the default port declared in [I‑D.ietf‑behave‑turn] (Rosenberg, J., Mahy, R., and P. Matthews, “Traversal Using Relays around NAT (TURN): Relay Extensions to Session Traversal Utilities for NAT (STUN),” April 2009.) for the SRV service name defined in <scheme> is used. If <transport> is defined then <scheme> and <transport> are converted to a TURN transport as specified above. If <transport> is not defined, the TURN transports supported by the application are tried by preference order. If the TURN client cannot contact a TURN server with this IP address and port on any of the transports then the resolution MUST stop with an error.
   
   
2. If <host> is a domain name and <port> is defined, then <host> is resolved to a list of IP addresses via DNS A and AAAA queries. If <transport> is defined then <scheme> and <transport> are converted to a TURN transport as specified above. If <transport> is not defined, the TURN transports supported by the application are tried by preference order. If the TURN client cannot contact a TURN server with this port and any combination of transports and resolved IP addresses then the resolution MUST stop with an error.
   
   
3. If <host> is a domain name and <port> is not defined but <transport> is defined then <host> is converted to a list of IP address and port tuples via a DNS SRV query as defined in [I‑D.ietf‑behave‑turn] (Rosenberg, J., Mahy, R., and P. Matthews, “Traversal Using Relays around NAT (TURN): Relay Extensions to Session Traversal Utilities for NAT (STUN),” April 2009.) section 6.1. <scheme> is used for the service name and <transport> is used for the protocol name in the SRV algorithm (Gulbrandsen, A., Vixie, P., and L. Esibov, “A DNS RR for specifying the location of services (DNS SRV),” February 2000.) [RFC2782]. If the TURN client cannot contact a TURN server at any of the IP address, port and transport tuples returned by the SRV algorithm then the resolution MUST stop with an error. The SRV algorithm recommends doing an A query if the SRV query returns an error or no SRV RR. In this case the default port declared in [I‑D.ietf‑behave‑turn] (Rosenberg, J., Mahy, R., and P. Matthews, “Traversal Using Relays around NAT (TURN): Relay Extensions to Session Traversal Utilities for NAT (STUN),” April 2009.) for the SRV service name defined in <scheme> must be used for contacting the TURN server. Also in this case, this specification modifies the SRV algorithm by recommending an A or AAAA query.
   
   
4. If <host> is a domain name and <port> and <transport> are not defined, then <host> is converted to an ordered list of IP address, port and transport tuples via the S-NAPTR algorithm defined in [RFC3958] (Daigle, L. and A. Newton, “Domain-Based Application Service Location Using SRV RRs and the Dynamic Delegation Discovery Service (DDDS),” January 2005.) with a "RELAY" Application Service Tag. The TURN transports supported by the application are converted in Application Protocol Tags by using "turn.udp" if the TURN transport is UDP, "turn.tcp" if the TURN transport is TCP and "turn.tls" if the TURN transport is TLS. The order to try the protocol tags is provided by the ranking of the first set of NAPTR records. If multiple protocol tags have the same ranking, the preferred order set by the application is used. If the TURN client cannot contact a TURN server with any of the IP address, port and transport tuples returned by the S-NAPTR algorithm then the resolution MUST stop with an error. If the first NAPTR SRV query does not return any result then <host> is converted to a list of IP address and port tuples by using the algorithm specified in step 3 for each of the TURN transports supported by the application by order of preference.

5.  Example

With the DNS RRs in Figure 1 and a preferred protocol list of {TLS, TCP, UDP}, the resolution algorithm will convert the "turn:example.com" URI to the list of IP addresses, port and protocol tuples in Table 1.

example.com.
IN NAPTR 100 10 "" "RELAY:turn.udp" "" datagram.example.com.
IN NAPTR 200 10 "" "RELAY:turn.tcp:turn.tls" "" stream.example.com.

datagram.example.com.
IN NAPTR 100 10 "S" "RELAY:turn.udp" "" _udp._turn.example.com.

stream.example.com.
IN NAPTR 100 10 "S" "RELAY:turn.tcp" "" _turn._tcp.example.com.
IN NAPTR 200 10 "A" "RELAY:turn.tls" "" a.example.com.

_turn._udp.example.com.
IN SRV   0   0  3478 a.example.com.

_turn._tcp.example.com.
IN SRV   0   0  5000 a.example.com.

a.example.com.
IN A     192.0.2.1

6.  Security Considerations

Security considerations for TURN are discussed in [I‑D.ietf‑behave‑turn] (Rosenberg, J., Mahy, R., and P. Matthews, “Traversal Using Relays around NAT (TURN): Relay Extensions to Session Traversal Utilities for NAT (STUN),” April 2009.).

The Application Service Tag and Application Protocol Tags defined in this document do not introduce any specific security issues beyond the security considerations discussed in [RFC3958] (Daigle, L. and A. Newton, “Domain-Based Application Service Location Using SRV RRs and the Dynamic Delegation Discovery Service (DDDS),” January 2005.).

The "turn" and "turns" URI schemes do not introduce any specific security issues beyond the security considerations discussed in [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.).

7.  IANA Considerations

This section contains the registration information for the "turn" and "turns" URI Schemes (in accordance with [RFC4395] (Hansen, T., Hardie, T., and L. Masinter, “Guidelines and Registration Procedures for New URI Schemes,” February 2006.)), one S-NAPTR Application Service Tag, and three S-NAPTR Application Protocol Tags (in accordance with [RFC3958] (Daigle, L. and A. Newton, “Domain-Based Application Service Location Using SRV RRs and the Dynamic Delegation Discovery Service (DDDS),” January 2005.)).

7.1.  TURN URI Registration

URI scheme name: turn

Status: permanent

URI scheme syntax: See Section 3 (Syntax of a TURN or TURNS URI).

URI scheme semantics: See Section 4 (TURN or TURNS URI Resolution).

Encoding considerations: There are no encoding considerations beyond those in [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.).

Applications/protocols that use this URI scheme name:

The "turn" URI scheme is intended to be used by applications that might need access to a TURN server.

Interoperability considerations: N/A

Security considerations: See Section 6 (Security Considerations).

Contact: Marc Petit-Huguenin <petithug@acm.org>

Author/Change controller: The IESG

References: This document.

7.2.  TURNS URI Registration

URI scheme name: turns

Status: permanent

URI scheme syntax: See Section 3 (Syntax of a TURN or TURNS URI).

URI scheme semantics: See Section 4 (TURN or TURNS URI Resolution).

Encoding considerations: There are no encoding considerations beyond those in [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.).

Applications/protocols that use this URI scheme name:

The "turns" URI scheme is intended to be used by applications that might need access to a TURN server.

Interoperability considerations: N/A

Security considerations: See Section 6 (Security Considerations).

Contact: Marc Petit-Huguenin <petithug@acm.org>

Author/Change controller: The IESG

References: This document.

7.3.  RELAY Application Service Tag Registration

Application Protocol Tag: RELAY

Intended usage: See Section 4 (TURN or TURNS URI Resolution).

Interoperability considerations: N/A

Security considerations: See Section 6 (Security Considerations).

Relevant publications: This document.

Contact information: Marc Petit-Huguenin <petithug@acm.org>

Author/Change controller: The IESG

7.4.  turn.udp Application Protocol Tag Registration

Application Protocol Tag: turn.udp

Intended usage: See Section 4 (TURN or TURNS URI Resolution).

Interoperability considerations: N/A

Security considerations: See Section 6 (Security Considerations).

Relevant publications: This document.

Contact information: Marc Petit-Huguenin <petithug@acm.org>

Author/Change controller: The IESG

7.5.  turn.tcp Application Protocol Tag Registration

Application Protocol Tag: turn.tcp

Intended usage: See Section 4 (TURN or TURNS URI Resolution).

Interoperability considerations:

Security considerations: See Section 6 (Security Considerations).

Relevant publications: This document.

Contact information: Marc Petit-Huguenin <petithug@acm.org>

Author/Change controller: The IESG

7.6.  turn.tls Application Protocol Tag Registration

Application Protocol Tag: turn.tls

Intended usage: See Section 4 (TURN or TURNS URI Resolution).

Interoperability considerations: N/A

Security considerations: See Section 6 (Security Considerations).

Relevant publications: This document.

Contact information: Marc Petit-Huguenin <petithug@acm.org>

Author/Change controller: The IESG

8.  Running Code Considerations

• Zap. Eilon Yardeni, 8x8 Inc. Implements version -00
• Reference Implementation of TURN URI parser and resolver.. Marc Petit-Huguenin. Implements version -00 to -02

9.  Acknowledgements

Thanks to Alfred E. Heggestad, Eilon Yardeni, Dan Wing, Alfred Hoenes and Jim Kleck for their comments, suggestions and questions that helped to improve this document.

This document was written with the xml2rfc tool described in [RFC2629] (Rose, M., “Writing I-Ds and RFCs using XML,” June 1999.).

10.  References

10.1. Normative References

10.2. Informative References

Appendix A.  Release notes

This section must be removed before publication as an RFC.

A.1.  Modifications between -02 and -01

• Shorten the abstract so it does not overflow on the second page.
• Added text to explicitly say that the resolution is only to create an allocation.
• Fixed the default port for TLS in the example.
• Changed some priority in the example for RFC3958 section 2.2.5.
• Fixed the service/protocol order for the SRV RR in the example.
• Removed reference to draft-wood-tae-specifying-uri-transports as it has an experimental status.

A.2.  Modifications between -01 and -00

• Fixed the contact email.
• Changed the IPR to trust200902.
• Added case for transport defined but unknown.
• Moved RFC 3958 to Normative References.
• Added study of draft-wood-tae-specifying-uri-transports in TODO list.

A.3.  Design Notes

• The Application Service Tag is "RELAY" so other relaying mechanisms than TURN (e.g., TWIST) can be registered as Application Protocol Tags.
• S-NAPTR was preferred to U-NAPTR because there is no use case for U-NAPTR.
• <password> is not used in the URIs because it is deprecated. <username> is not used in the URIs because it is not used to guide the resolution mechanism.
• As discussed in Dublin, there is no generic parameters in the URI to prevent compatibity issues.
• Adding optional capabilities (IPv6 allocation, preserve bit, etc...) in the resolution process was rejected at the Dublin meeting.

A.4.  TODO List

(Empty)

Author's Address