Network Working Group J. Uberti Internet-Draft Google Intended status: Standards Track July 15, 2013 Expires: January 16, 2014 A REST API For Access To TURN Services draft-uberti-behave-turn-rest-00 Abstract This document describes a proposed standard REST API for obtaining access to TURN services via ephemeral (i.e. time-limited) credentials. These credentials are vended by a web service over HTTP, and then supplied to and checked by a TURN server using the standard TURN protocol. The usage of ephemeral credentials ensures that access to the TURN server can be controlled even if the credentials can be discovered by the user, as is the case in WebRTC where TURN credentials must be specified in Javascript. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on January 16, 2014. Copyright Notice Copyright (c) 2013 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must Uberti Expires January 16, 2014 [Page 1] Internet-Draft TURN Server REST API July 2013 include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. HTTP Interactions . . . . . . . . . . . . . . . . . . . . . . 3 2.1. Request . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.2. Response . . . . . . . . . . . . . . . . . . . . . . . . 4 3. WebRTC Interactions . . . . . . . . . . . . . . . . . . . . . 5 4. TURN Interactions . . . . . . . . . . . . . . . . . . . . . . 5 4.1. Client . . . . . . . . . . . . . . . . . . . . . . . . . 5 4.2. Server . . . . . . . . . . . . . . . . . . . . . . . . . 5 5. Implementation Notes . . . . . . . . . . . . . . . . . . . . 6 5.1. Revocation . . . . . . . . . . . . . . . . . . . . . . . 6 5.2. Key Rotation . . . . . . . . . . . . . . . . . . . . . . 6 6. Security Considerations . . . . . . . . . . . . . . . . . . . 6 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 9.1. Normative References . . . . . . . . . . . . . . . . . . 7 9.2. Informative References . . . . . . . . . . . . . . . . . 7 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8 1. Introduction TURN [RFC5766] is a protocol that is often used to improve the connectivity of P2P applications. By providing a cloud-based relay service, TURN ensures that a connection can be established even when one or both sides is incapable of a direct P2P connection. However, as a relay service, it imposes a nontrivial cost on the service provider. Therefore, access to a TURN service is almost always access-controlled. TURN provides a mechanism to control access via [RFC5389] long-term credentials that are provided as part of the TURN protocol. It is expected that these credentials will be kept secret; if the credentials are discovered, the TURN server could be used by unauthorized users or applications. However, in web applications, ensuring this secrecy is typically impossible. Uberti Expires January 16, 2014 [Page 2] Internet-Draft TURN Server REST API July 2013 To address this problem, this document proposes an API that can be used to retrieve ephemeral TURN credentials from a web service. These credentials can then be used as long-term credentials with a standard TURN server with a custom authentication module. For simplicity, the design has been kept intentionally stateless; the only interaction needed between the web service and the TURN service is to share a secret key. 2. HTTP Interactions To retrieve a new set of credentials, the client makes a HTTP GET request, specifying TURN as the service to allocate credentials for, and optionally specifying a user id parameter. The purpose of the user id parameter is to simplify debugging on the TURN server, as well as provide the ability to control the number of credentials handed out for a specific user, if desired. The TURN credentials and their lifetime are returned as JSON, along with URIs that indicate how to connect to the server using the TURN protocol. To avoid the need for state passing between the web service and TURN server, the returned credentials consist of a TURN username that encodes all the necessary state (expiry time and application user id), and a TURN password that is a digest of this state, signed with the shared secret key. Since the returned credentials are ephemeral, they will eventually expire. This does not affect existing TURN allocations, as they are tied to a specific 5-tuple, but requests to allocate new TURN ports will fail after the expiry time. This is significant in the case of an ICE restart, where the client will need to allocate a new set of candidates, including TURN candidates. To get a new set of ephemeral credentials, the client can simply re-issue the original HTTP request with the same parameters, which will return the new credentials in its JSON response. To prevent unauthorized use, the HTTP requests can be ACLed by various means, e.g. IP address (if coming from a server), Origin header, User-Agent header, login cookie, API key, etc. 2.1. Request The request includes the following parameters, specified in the URL: o service: specifies the desired service (turn) o username: an optional user id to be associated with the credentials Uberti Expires January 16, 2014 [Page 3] Internet-Draft TURN Server REST API July 2013 o key: if an API key is used for authentication, the API key Example: GET /?service=turn&username=mbzrxpgjys 2.2. Response The response is returned with content-type "application/json", and consists of a JSON object with the following parameters: o username: the TURN username to use, which is a colon-delimited combination of the expiration timestamp and the username parameter from the request (if specified). The timestamp is intended to be opaque to the web application, so its format is arbitrary, but for simplicity, use of UNIX timestamps is recommended. o password: the TURN password to use; this value is computed from the a secret key shared with the TURN server and the returned username value, by performing base64(hmac(secret key, returned username)). HMAC-SHA1 is one HMAC algorithm that can be used, but any algorithm that incorporates a shared secret is acceptable, as long as both the web server and TURN server use the same algorithm and secret. o ttl: the duration for which the username and password are valid, in seconds. A value of one day (86400 seconds) is recommended. o uris: an array of TURN URIs, in the form specified in [I-D.petithuguenin-behave-turn-uris]. This is used to indicate the different addresses and/or protocols that can be used to reach the TURN server. These URIs SHOULD specify a hostname, IPv4, or IPv6 address for the TURN server, as well as the port and transport to use; this avoids the need for a DNS SRV or S-NAPTR lookup as specified in [RFC5928]. Example: { "username" : "12334939:mbzrxpgjys", "password" : "adfsaflsjfldssia", "ttl" : 86400, "uris" : [ "turn:1.2.3.4:9991?transport=udp", "turn:1.2.3.4:9992?transport=tcp", "turns:1.2.3.4:443?transport=tcp" ] Uberti Expires January 16, 2014 [Page 4] Internet-Draft TURN Server REST API July 2013 } 3. WebRTC Interactions The returned JSON is parsed into an RTCIceServer object, and supplied as part of the RTCConfiguration object that is used when creating a RTCPeerConnection. Example: var iceServer = { "username": response.username, "credential": response.password, "uris": response.uris }; var config = {"iceServers": [iceServer]}; var pc = new RTCPeerConnection(config); When the credentials are updated (e.g. because they are about to expire), a new RTCConfiguration with the updated credentials can be supplied to the existing RTCPeerConnection via the updateIce method. This update must not affect existing TURN allocations, because TURN requires that the username stay constant for an allocation, but the new credentials will be used for any new allocations. [TODO: make sure this behavior is specified in the W3C API spec] 4. TURN Interactions 4.1. Client The WebRTC client will perform a standard TURN allocation sequence using the long-term credentials mechanism specified in [RFC5389], Section 10.2, using the "username" value from the returned JSON for its USERNAME attribute, and the "password" value for the password input to the MESSAGE-INTEGRITY hash. 4.2. Server The TURN server will process the request using the long-term credentials mechanism specifed in [RFC5389]. Note that the REALM value supplied by the server is not meaningful in this context, and can be set to any valid value. When processing ALLOCATE requests, the TURN server MUST split the USERNAME attribute into its timestamp and user id components, and Uberti Expires January 16, 2014 [Page 5] Internet-Draft TURN Server REST API July 2013 verify that the timestamp, which indicates when the credentials expire, has not yet been reached. If this verification fails, it SHOULD reject the request with a 401 (Unauthorized) error. If desired, the TURN server can optionally verify that the parsed user id value corresponds to a currently valid user of an external service (e.g. is currently logged in to the web app that is making use of TURN). This requires proprietary communication between the TURN server and external service on each ALLOCATE request, and is not necessary for typical applications. If this external verification fails, it SHOULD reject the request with a 401 (Unauthorized) error. For non-ALLOCATE requests, the TURN server merely verifies that the USERNAME matches the USERNAME that was used in the ALLOCATE (since it must remain constant). As in RFC 5766, the TURN server MUST verify the MESSAGE-INTEGRITY using the password associated with the supplied USERNAME. For the usage outlined in this document, the password will always be constructed using the supplied username and the shared secret as indicated in the "HTTP Interactions" section above. Because the password is derived from the USERNAME, successful verification of the MESSAGE-INTEGRITY ensures that the USERNAME (and the expiration time contained within) is trustworthy. 5. Implementation Notes 5.1. Revocation In the system as described here, revoking specific credentials is not possible. The assumption is that TURN services are of low enough value that waiting for the timeout to expire is a valid approach for dealing with possibly-compromised credentials. In extreme abuse cases, TURN server blacklists of timestamp+username values can be supplied by an administrator to stop abuse of specific credential sets. 5.2. Key Rotation As indicated in [RFC2104], periodic rotation of the shared secret to protect against key compromise is RECOMMENDED. To facilitate the rollover, the TURN server SHOULD be able to validate incoming MESSAGE-INTEGRITY tokens based on at least 2 shared secrets at any time. 6. Security Considerations Uberti Expires January 16, 2014 [Page 6] Internet-Draft TURN Server REST API July 2013 Because the USERNAME values in a TURN ALLOCATE request are typically visible to eavesdroppers, inclusion of an externally identifying user id, such as a login name, may allow a passive attacker to determine the identities of the parties in a conversation. To prevent this problem, use of opaque user id values is recommended. This mechanism assumes that the clocks of the web server and TURN server are roughly in sync. Given the expected large TTLs for the vended credentials, clock skew on the order of seconds to minutes should not cause an issue. However, if the TURN server's clock was mistakenly set to a date signficantly in the past, credentials could be accepted for far longer than their intended lifetime. 7. IANA Considerations None. 8. Acknowledgements Harald Alvestrand, Alfred Godoy, and Philipp Hancke provided key input on the initial design. Dave Cridland, Cullen Jennings, Oleg Moskalenko, and Matthew Robertson pointed out several errors and omissions. 9. References 9.1. Normative References [I-D.petithuguenin-behave-turn-uris] Petit-Huguenin, M., Nandakumar, S., Salgueiro, G., and P. Jones, "Traversal Using Relays around NAT (TURN) Uniform Resource Identifiers", draft-petithuguenin-behave-turn- uris-03 (work in progress), January 2013. [RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing, "Session Traversal Utilities for NAT (STUN)", RFC 5389, October 2008. [RFC5766] Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using Relays around NAT (TURN): Relay Extensions to Session Traversal Utilities for NAT (STUN)", RFC 5766, April 2010. 9.2. Informative References [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- Hashing for Message Authentication", RFC 2104, February 1997. Uberti Expires January 16, 2014 [Page 7] Internet-Draft TURN Server REST API July 2013 [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. [RFC5928] Petit-Huguenin, M., "Traversal Using Relays around NAT (TURN) Resolution Mechanism", RFC 5928, August 2010. Author's Address Justin Uberti Google 747 6th St S Kirkland, WA 98033 USA Email: justin@uberti.name Uberti Expires January 16, 2014 [Page 8]