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Internet Engineering Task Force (IETF)                        M. PerumalRequest for Comments: 7675                                      EricssonCategory: Standards Track                                        D. WingISSN: 2070-1721                                      Cisco Systems, Inc.                                                         R. Ravindranath                                                                T. Reddy                                                           Cisco Systems                                                              M. Thomson                                                                 Mozilla                                                            October 2015Session Traversal Utilities for NAT (STUN) Usage for Consent FreshnessAbstract   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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc7675.Perumal, et al.              Standards Track                    [Page 1]

RFC 7675            STUN Usage for Consent Freshness        October 2015Copyright Notice   Copyright (c) 2015 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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   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 Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .22.  Applicability . . . . . . . . . . . . . . . . . . . . . . . .33.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .34.  Design Considerations . . . . . . . . . . . . . . . . . . . .45.  Solution  . . . . . . . . . . . . . . . . . . . . . . . . . .45.1.  Expiration of Consent . . . . . . . . . . . . . . . . . .55.2.  Immediate Revocation of Consent . . . . . . . . . . . . .66.  DiffServ Treatment for Consent  . . . . . . . . . . . . . . .77.  DTLS Applicability  . . . . . . . . . . . . . . . . . . . . .78.  Security Considerations . . . . . . . . . . . . . . . . . . .79.  References  . . . . . . . . . . . . . . . . . . . . . . . . .89.1.  Normative References  . . . . . . . . . . . . . . . . . .89.2.  Informative References  . . . . . . . . . . . . . . . . .8   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .9   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .101.  Introduction   To prevent attacks on peers, endpoints have to ensure the remote peer   is willing to receive traffic.  Verification of peer consent before   sending traffic is necessary in deployments like WebRTC to ensure   that a malicious JavaScript cannot use the browser as a platform for   launching attacks.  This is performed both when the session is first   established to the remote peer using Interactive Connectivity   Establishment (ICE) [RFC5245] connectivity checks, and periodically   for the duration of the session using the procedures defined in this   document.   When a session is first established, ICE implementations obtain an   initial consent to send by performing STUN connectivity checks.  This   document describes a new STUN usage with exchange of request andPerumal, et al.              Standards Track                    [Page 2]

RFC 7675            STUN Usage for Consent Freshness        October 2015   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 inSection 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.  Sections4.4 and5.3 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.Perumal, et al.              Standards Track                    [Page 3]

RFC 7675            STUN Usage for Consent Freshness        October 20154.  Design Considerations   Although ICE requires periodic keepalive traffic to keep NAT bindings   alive (seeSection 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 inSection 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 withSection 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.Perumal, et al.              Standards Track                    [Page 4]

RFC 7675            STUN Usage for Consent Freshness        October 20155.1.  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 inSection 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 inSection 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).Perumal, et al.              Standards Track                    [Page 5]

RFC 7675            STUN Usage for Consent Freshness        October 2015   An endpoint SHOULD await a binding response for each request it sends   for a time based on the estimated round-trip time (RTT) (seeSection 7.2.1 of [RFC5389]) with an allowance for variation in   network delay.  The RTT value can be updated as described in   [RFC5389].  All outstanding STUN consent transactions for a candidate   pair MUST be discarded when consent expires.   To meet the security needs of consent, an untrusted application   (e.g., JavaScript or signaling servers) MUST NOT be able to obtain or   control the STUN transaction identifier, because that enables   spoofing of STUN responses, falsifying consent.   To prevent attacks on the peer during ICE restart, an endpoint that   continues to send traffic on the previously validated candidate pair   during ICE restart MUST continue to perform consent freshness on that   candidate pair as described earlier.   While TCP affords some protection from off-path attackers ([RFC5961],   [RFC4953]), there is still a risk an attacker could cause a TCP   sender to send forever by spoofing ACKs.  To prevent such an attack,   consent checks MUST be performed over all transport connections,   including TCP.  In this way, an off-path attacker spoofing TCP   segments cannot cause a TCP sender to send once the consent timer   expires (30 seconds).   An endpoint does not need to maintain consent if it does not send   application data.  However, an endpoint MUST regain consent before it   resumes sending application data.  In the absence of any packets, any   bindings in middleboxes for the flow might expire.  Furthermore,   having one peer unable to send is detrimental to many protocols.   Absent better information about the network, if an endpoint needs to   ensure its NAT or firewall mappings do not expire, this can be done   using keepalive or other techniques (seeSection 10 of [RFC5245] and   see [RFC6263]).   After consent is lost, the same ICE credentials MUST NOT be used on   the affected 5-tuple again.  That means that a new session, or an ICE   restart, is needed to obtain consent to send on the affected   candidate pair.5.2.  Immediate Revocation of Consent   In some cases, it is useful to signal that consent is terminated   rather than relying on a timeout.   Consent for sending application data is immediately revoked by   receipt of an authenticated message that closes the connection (e.g.,   a Transport Layer Security (TLS) fatal alert) or receipt of a validPerumal, et al.              Standards Track                    [Page 6]

RFC 7675            STUN Usage for Consent Freshness        October 2015   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 inSection 7.1.2.4 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 inSection 4.2 of [RFC7350].8.  Security Considerations   This document describes a security mechanism, details of which are   mentioned in Sections4.1 and4.2 of [RFC7350].  Consent requires 96   bits transaction ID defined inSection 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 Sections3.3 and4.2 of   [WebRTC-SEC].   The security considerations discussed in [RFC5245] should also be   taken into account.Perumal, et al.              Standards Track                    [Page 7]

RFC 7675            STUN Usage for Consent Freshness        October 20159.  References9.1.  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",RFC4953, 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>.Perumal, et al.              Standards Track                    [Page 8]

RFC 7675            STUN Usage for Consent Freshness        October 2015   [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.Perumal, et al.              Standards Track                    [Page 9]

RFC 7675            STUN Usage for Consent Freshness        October 2015Authors' Addresses   Muthu Arul Mozhi Perumal   Ericsson   Ferns Icon   Doddanekundi, Mahadevapura   Bangalore, Karnataka  560037   India   Email: muthu.arul@gmail.com   Dan Wing   Cisco Systems, Inc.   170 West Tasman Drive   San Jose, California  95134   United States   Email: dwing@cisco.com   Ram Mohan Ravindranath   Cisco Systems   Cessna Business Park   Sarjapur-Marathahalli Outer Ring Road   Bangalore, Karnataka  560103   India   Email: rmohanr@cisco.com   Tirumaleswar Reddy   Cisco Systems   Cessna Business Park, Varthur Hobli   Sarjapur Marathalli Outer Ring Road   Bangalore, Karnataka  560103   India   Email: tireddy@cisco.com   Martin Thomson   Mozilla   650 Castro Street, Suite 300   Mountain View, California  94041   United States   Email: martin.thomson@gmail.comPerumal, et al.              Standards Track                   [Page 10]