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.

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
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   publication of this document.  Please review these documents
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   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

   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).

   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

   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.

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: 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