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rfc:rfc7675

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.

Perumal, et al. Standards Track [Page 1] RFC 7675 STUN Usage for Consent Freshness October 2015

Copyright Notice

 Copyright (c) 2015 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
 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.  Applicability . . . . . . . . . . . . . . . . . . . . . . . .   3
 3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
 4.  Design Considerations . . . . . . . . . . . . . . . . . . . .   4
 5.  Solution  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   5.1.  Expiration of Consent . . . . . . . . . . . . . . . . . .   5
   5.2.  Immediate Revocation of Consent . . . . . . . . . . . . .   6
 6.  DiffServ Treatment for Consent  . . . . . . . . . . . . . . .   7
 7.  DTLS Applicability  . . . . . . . . . . . . . . . . . . . . .   7
 8.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
 9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
   9.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
   9.2.  Informative References  . . . . . . . . . . . . . . . . .   8
 Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   9
 Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1. 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 and

Perumal, 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 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.

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

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.

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

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).

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) (see
 Section 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 (see Section 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 valid

Perumal, 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 in
 Section 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 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.

Perumal, et al. Standards Track [Page 7] RFC 7675 STUN Usage for Consent Freshness October 2015

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>.

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 2015

Authors' 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.com

Perumal, et al. Standards Track [Page 10]

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