Internet Engineering Task Force (IETF) M. Perumal Request for Comments: 7675 Ericsson Category: Standards Track D. Wing ISSN: 2070-1721 Cisco Systems, Inc. R. Ravindranath T. Reddy Cisco Systems M. Thomson Mozilla October 2015 Session Traversal Utilities for NAT (STUN) Usage for Consent Freshness Abstract To prevent WebRTC applications, such as browsers, from launching attacks by sending traffic to unwilling victims, periodic consent to send needs to be obtained from remote endpoints. This document describes a consent mechanism using a new Session Traversal Utilities for NAT (STUN) usage. Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7675.
response messages that verifies the remote peer's ongoing consent to receive traffic. This consent expires after a period of time and needs to be continually renewed, which ensures that consent can be terminated. This document defines what it takes to obtain, maintain, and lose consent to send. Consent to send applies to a single 5-tuple. How applications react to changes in consent is not described in this document. The consent mechanism does not update the ICE procedures defined in [RFC5245]. Consent is obtained only by full ICE implementations. An ICE-lite agent (as defined in Section 2.7 of [RFC5245]) does not generate connectivity checks or run the ICE state machine. Hence, an ICE-lite agent does not generate consent checks and will only respond to any checks that it receives. No changes are required to ICE-lite implementations in order to respond to consent checks, as they are processed as normal ICE connectivity checks. 2. Applicability This document defines what it takes to obtain, maintain, and lose consent to send using ICE. Sections 4.4 and 5.3 of [WebRTC-SA] further explain the value of obtaining and maintaining consent. Other applications that have similar security requirements to verify peer consent before sending non-ICE packets can use the consent mechanism described in this document. The mechanism of how applications are made aware of consent expiration is outside the scope of the document. 3. 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]. Consent: The mechanism of obtaining permission from the remote endpoint to send non-ICE traffic to a remote transport address. Consent is obtained using ICE. Note that this is an application- level consent; no human intervention is involved. Consent Freshness: Maintaining and renewing consent over time. Transport Address: The remote peer's IP address and UDP or TCP port number.
4. Design Considerations Although ICE requires periodic keepalive traffic to keep NAT bindings alive (see Section 10 of [RFC5245] and also [RFC6263]), those keepalives are sent as STUN Indications that are send-and-forget, and do not evoke a response. A response is necessary for consent to continue sending traffic. Thus, we need a request/response mechanism for consent freshness. ICE can be used for that mechanism because ICE implementations are already required to continue listening for ICE messages, as described in Section 10 of [RFC5245]. STUN binding requests sent for consent freshness also serve the keepalive purpose (i.e., to keep NAT bindings alive). Because of that, dedicated keepalives (e.g., STUN Binding Indications) are not sent on candidate pairs where consent requests are sent, in accordance with Section 20.2.3 of [RFC5245]. When Secure Real-time Transport Protocol (SRTP) is used, the following considerations are applicable. SRTP is encrypted and authenticated with symmetric keys; that is, both sender and receiver know the keys. With two party sessions, receipt of an authenticated packet from the single remote party is a strong assurance the packet came from that party. However, when a session involves more than two parties, all of whom know each other's keys, any of those parties could have sent (or spoofed) the packet. Such shared key distributions are possible with some Multimedia Internet KEYing (MIKEY) [RFC3830] modes, Security Descriptions [RFC4568], and Encrypted Key Transport (EKT) [EKT]. Thus, in such shared keying distributions, receipt of an authenticated SRTP packet is not sufficient to verify consent. The mechanism proposed in the document is an optional extension to the ICE protocol; it can be deployed at one end of the two-party communication session without impact on the other party. 5. Solution Initial consent to send traffic is obtained using ICE [RFC5245]. An endpoint gains consent to send on a candidate pair when the pair enters the Succeeded ICE state. This document establishes a 30-second expiry time on consent. 30 seconds was chosen to balance the need to minimize the time taken to respond to a loss of consent with the desire to reduce the occurrence of spurious failures. ICE does not identify when consent to send traffic ends. This document describes two ways in which consent to send ends: expiration of consent and immediate revocation of consent, which are discussed in the following sections.
5.1. Expiration of Consent A full ICE implementation obtains consent to send using ICE. After ICE concludes on a particular candidate pair and whenever the endpoint sends application data on that pair consent is maintained following the procedure described in this document. An endpoint MUST NOT send data other than the messages used to establish consent unless the receiving endpoint has consented to receive data. Connectivity checks that are paced as described in Section 16 of [RFC5245], and responses to connectivity checks are permitted. That is, no application data (e.g., RTP or Datagram Transport Layer Security (DTLS)), can be sent until consent is obtained. Explicit consent to send is obtained and maintained by sending a STUN binding request to the remote peer's transport address and receiving a matching, authenticated, non-error STUN binding response from the remote peer's transport address. These STUN binding requests and responses are authenticated using the same short-term credentials as the initial ICE exchange. Note: Although TCP has its own consent mechanism (TCP acknowledgements), consent is necessary over a TCP connection because it could be translated to a UDP connection (e.g., [RFC6062]). Consent expires after 30 seconds. That is, if a valid STUN binding response has not been received from the remote peer's transport address in 30 seconds, the endpoint MUST cease transmission on that 5-tuple. STUN consent responses received after consent expiry do not re-establish consent and may be discarded or cause an ICMP error. To prevent expiry of consent, a STUN binding request can be sent periodically. To prevent synchronization of consent checks, each interval MUST be randomized from between 0.8 and 1.2 times the basic period. Implementations SHOULD set a default interval of 5 seconds, resulting in a period between checks of 4 to 6 seconds. Implementations MUST NOT set the period between checks to less than 4 seconds. This timer is independent of the consent expiry timeout. Each STUN binding request for consent MUST use a new STUN transaction identifier, as described in Section 6 of [RFC5389]. Each STUN binding request for consent is transmitted once only. A sender therefore cannot assume that it will receive a response for every consent request, and a response might be for a previous request (rather than for the most recently sent request).
and authenticated STUN response with error code Forbidden (403). Note however that consent revocation messages can be lost on the network, so an endpoint could resend these messages, or wait for consent to expire. Receipt of an unauthenticated message that closes a connection (e.g., TCP FIN) does not indicate revocation of consent. Thus, an endpoint receiving an unauthenticated end-of-session message SHOULD continue sending media (over connectionless transport) or attempt to re-establish the connection (over connection-oriented transport) until consent expires or it receives an authenticated message revoking consent. Note that an authenticated Secure Real-time Transport Control Protocol (SRTCP) BYE does not terminate consent; it only indicates the associated SRTP source has quit. 6. DiffServ Treatment for Consent It is RECOMMENDED that STUN consent checks use the same Diffserv Codepoint markings as the ICE connectivity checks described in Section 188.8.131.52 of [RFC5245] for a given 5-tuple. Note: It is possible that different Diffserv Codepoints are used by different media over the same transport address [WebRTC-QoS]. Such a case is outside the scope of this document. 7. DTLS Applicability The DTLS applicability is identical to what is described in Section 4.2 of [RFC7350]. 8. Security Considerations This document describes a security mechanism, details of which are mentioned in Sections 4.1 and 4.2 of [RFC7350]. Consent requires 96 bits transaction ID defined in Section 6 of [RFC5389] to be uniformly and randomly chosen from the interval 0 .. 2**96-1, and be cryptographically strong. This is good enough security against an off-path attacker replaying old STUN consent responses. Consent Verification to avoid attacks using a browser as an attack platform against machines is discussed in Sections 3.3 and 4.2 of [WebRTC-SEC]. The security considerations discussed in [RFC5245] should also be taken into account.
9. References 9.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <http://www.rfc-editor.org/info/rfc2119>. [RFC5245] Rosenberg, J., "Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal for Offer/Answer Protocols", RFC 5245, DOI 10.17487/RFC5245, April 2010, <http://www.rfc-editor.org/info/rfc5245>. [RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing, "Session Traversal Utilities for NAT (STUN)", RFC 5389, DOI 10.17487/RFC5389, October 2008, <http://www.rfc-editor.org/info/rfc5389>. 9.2. Informative References [EKT] Mattsson, J., McGrew, D., and D. Wing, "Encrypted Key Transport for Secure RTP", Work in Progress, draft-ietf-avtcore-srtp-ekt-03, October 2014. [RFC3830] Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K. Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830, DOI 10.17487/RFC3830, August 2004, <http://www.rfc-editor.org/info/rfc3830>. [RFC4568] Andreasen, F., Baugher, M., and D. Wing, "Session Description Protocol (SDP) Security Descriptions for Media Streams", RFC 4568, DOI 10.17487/RFC4568, July 2006, <http://www.rfc-editor.org/info/rfc4568>. [RFC4953] Touch, J., "Defending TCP Against Spoofing Attacks", RFC 4953, DOI 10.17487/RFC4953, July 2007, <http://www.rfc-editor.org/info/rfc4953>. [RFC5961] Ramaiah, A., Stewart, R., and M. Dalal, "Improving TCP's Robustness to Blind In-Window Attacks", RFC 5961, DOI 10.17487/RFC5961, August 2010, <http://www.rfc-editor.org/info/rfc5961>.
[RFC6062] Perreault, S., Ed. and J. Rosenberg, "Traversal Using Relays around NAT (TURN) Extensions for TCP Allocations", RFC 6062, DOI 10.17487/RFC6062, November 2010, <http://www.rfc-editor.org/info/rfc6062>. [RFC6263] Marjou, X. and A. Sollaud, "Application Mechanism for Keeping Alive the NAT Mappings Associated with RTP / RTP Control Protocol (RTCP) Flows", RFC 6263, DOI 10.17487/RFC6263, June 2011, <http://www.rfc-editor.org/info/rfc6263>. [RFC7350] Petit-Huguenin, M. and G. Salgueiro, "Datagram Transport Layer Security (DTLS) as Transport for Session Traversal Utilities for NAT (STUN)", RFC 7350, DOI 10.17487/RFC7350, August 2014, <http://www.rfc-editor.org/info/rfc7350>. [WebRTC-QoS] Dhesikan, S., Jennings, C., Druta, D., Jones, P., and J. Polk, "DSCP and other packet markings for RTCWeb QoS", Work in Progress, draft-ietf-tsvwg-rtcweb-qos-04, July 2015. [WebRTC-SA] Rescorla, E., "WebRTC Security Architecture", Work in Progress, draft-ietf-rtcweb-security-arch-11, March 2015. [WebRTC-SEC] Rescorla, E., "Security Considerations for WebRTC", Work in Progress, draft-ietf-rtcweb-security-08, February 2015. Acknowledgements Thanks to Eric Rescorla, Harald Alvestrand, Bernard Aboba, Magnus Westerlund, Cullen Jennings, Christer Holmberg, Simon Perreault, Paul Kyzivat, Emil Ivov, Jonathan Lennox, Inaki Baz Castillo, Rajmohan Banavi, Christian Groves, Meral Shirazipour, David Black, Barry Leiba, Ben Campbell, and Stephen Farrell for their valuable inputs and comments. Thanks to Christer Holmberg for doing a thorough review.
Authors' Addresses Muthu Arul Mozhi Perumal Ericsson Ferns Icon Doddanekundi, Mahadevapura Bangalore, Karnataka 560037 India Email: firstname.lastname@example.org Dan Wing Cisco Systems, Inc. 170 West Tasman Drive San Jose, California 95134 United States Email: email@example.com Ram Mohan Ravindranath Cisco Systems Cessna Business Park Sarjapur-Marathahalli Outer Ring Road Bangalore, Karnataka 560103 India Email: firstname.lastname@example.org Tirumaleswar Reddy Cisco Systems Cessna Business Park, Varthur Hobli Sarjapur Marathalli Outer Ring Road Bangalore, Karnataka 560103 India Email: email@example.com Martin Thomson Mozilla 650 Castro Street, Suite 300 Mountain View, California 94041 United States Email: firstname.lastname@example.org