RTCWEB                                                        M. Perumal
Internet-Draft                                                   D. Wing
Intended status: Standards Track                         R. Ravindranath
Expires: October 13, November 20, 2014                                      T. Reddy
                                                           Cisco Systems
                                                              M. Thomson
                                                          April 11,
                                                            May 19, 2014

                    STUN Usage for Consent Freshness


   To prevent sending excessive traffic to an endpoint, periodic consent
   needs to be obtained from that remote endpoint.

   This document describes a consent mechanism using a new STUN usage.
   This same mechanism can also determine connection loss ("liveness")
   with a remote peer.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Design Considerations . . . . . . . . . . . . . . . . . . . .   3
   4.  Solution Overview . . . . . . . . . . . . . . . . . . . . . .   3
     4.1.  Expiration of Consent . . . . . . . . . . . . . . . . . .   3
     4.2.  Immediate Revocation of Consent . . . . . . . . . . . . .   4
   5.  Connection Liveness . . . . . . . . . . . . . . . . . . . . .   4   5
   6.  DiffServ Treatment for Consent packets  . . . . . . . . . . .   5
   7.  W3C API Implications  . . . . . . . . . . . . . . . . . . . .   5
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   5   6
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   10. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . .   6
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     11.1.  Normative References . . . . . . . . . . . . . . . . . .   6
     11.2.  Informative References . . . . . . . . . . . . . . . . .   6   7
   Appendix A.  Example Implementation . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7   8

1.  Introduction

   To prevent attacks on peers, RTP endpoints have to ensure the remote
   peer wants to receive traffic.  This is performed both when the
   session is first established to the remote peer using ICE
   connectivity checks, and periodically for the duration of the session
   using the procedures defined in this document.

   When a session is first established, WebRTC implementations are
   required to perform STUN connectivity checks as part of ICE
   [RFC5245].  That initial consent is not described further in this
   document and it is assumed that ICE is being used for that initial

   Related to consent is loss of connectivity ("liveness").  Many
   applications want notification of connection loss to take appropriate
   actions (e.g., alert the user, try switching to a different

   This document describes a new STUN usage with a request and response
   messages which verifies the remote peer's consent to receive traffic,
   and can also detect loss of liveness.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   Consent:  It is the mechanism of obtaining permission to send traffic
      to a certain transport address.  This is usually the initial consent to
      send traffic, which is obtained via ICE. by ICE or a TCP handshake.

   Consent Freshness:  Permission to continue sending traffic to a
      certain transport address.  This is performed by the procedure
      described in this document.

   Session Liveness:  Detecting loss of connectivity to a certain
      transport address.  This is performed by the procedure described
      in this document.

   Transport Address:  The remote peer's IP address and (UDP or TCP)
      port number.

3.  Design Considerations

   Although ICE requires periodic keepalive traffic to keep NAT bindings
   alive (Section 10 of [RFC5245], [RFC6263]), those keepalives are sent
   as STUN Indications which are send-and-forget, and do not evoke a
   response.  A response is necessary both for consent to continue
   sending traffic, as well as to verify session liveness.  Thus, we
   need a request/response mechanism for consent freshness.  ICE can be
   used for that mechanism because ICE already requires ICE agents
   continue listening for ICE messages, as described in section 10 of

4.  Solution Overview

   There are two ways consent to send traffic is revoked: expiration of
   consent and immediate revocation of consent, which are discussed in
   the following sections.

4.1.  Expiration of Consent

   A WebRTC browser performs a combined consent freshness and session
   liveness test using STUN request/response as described below:

   An endpoint MUST NOT send application data (in WebRTC this means RTP
   or SCTP data) (e.g., RTP, RTCP, SCTP,
   DTLS) on an ICE-initiated connection unless the receiving endpoint
   consents to receive the data.  After a successful ICE connectivity
   check on a particular transport address, subsequent consent MUST be
   obtained following the procedure described in this document.  The
   consent expires after a fixed amount of time.

   Explicit consent to send is indicated by:

   1.  Sending by sending an ICE binding
   request to the remote peer's Transport Address and receiving a
   matching and authenticated ICE binding response from the inverted
   remote peer's Transport Address.  These ICE binding request/response requests and
   responses are authenticated using the same short- term short-term credentials as
   the initial ICE exchange, but using a new (fresh) transaction-id each
   time consent needs to be
       refresh. refreshed.  Implementations MUST obtain
   fresh consent before their existing consent expires.  When obtaining fresh consent  If an ICE
   binding response is not received within 1.5 times the previous round
   trip time, another ICE binding request is sent using the a STUN
       connectivity check (or response) could new
   (fresh) transaction-id (so that round-trip time can be lost, and re-
       transmissions MUST use the same STUN transaction-id, calculated),
   and re-
       transmissions re-transmissions MUST NOT be sent more frequently than every
   500ms or the smoothed round-trip time (from previous consent
   freshness checks or RTP round-trip time), whichever is less.  For the
   purposes of this document, receipt of an ICE response with the
   matching transaction-id of its request with a valid MESSAGE-
   is considered an authenticated packet. a consent response.

   The initial Consent to send traffic is obtained by ICE.  Consent
   expires after 15 30 seconds.  That is, if an authenticated
   packet (e.g., DTLS, SRTP, ICE) a valid STUN binding response
   corresponding to one of the STUN requests sent in the last 30 seconds
   has not been received from the inverted 5-tuple after 15 seconds, 5-tuple, the application endpoint MUST
   cease transmission on that 5-tuple.

   Consent is ended immediately by receipt of a an authenticated message
   that closes

   To meet the connection (for instance, a TLS fatal alert).

   Receipt security needs of consent, an unauthenticated end-of-session message untrusted application
   (e.g., TCP FIN)
   does not indicate loss of consent.  Thus, an endpoint receiving an
   unauthenticated end-of-session message SHOULD continue sending media
   (over connectionless transport) or attempt JavaScript) MUST NOT be able to re-establish obtain or control the
   connection (over connection-oriented transport) until ICE
   transaction-id, because that enables spoofing STUN responses,
   falsifying consent expires
   or it receives an authenticated message revoking consent.

   Although receiving authenticated packets is sufficient for consent,

   An endpoint that is still RECOMMENDED only receiving application traffic (recvonly)
   does not need to send messages obtain consent which can slightly conserve its
   resources.  However, the endpoint needs to keep ensure its NAT or firewall
   bindings alive
   mappings persist which can be done using keepalive or other
   techniques (see Section 10 of [RFC5245] and see [RFC6263]).

   To meet  If the security
   endpoint wants send application traffic, it needs to first obtain
   consent if its consent expired.

4.2.  Immediate Revocation of consent, Consent

   The previous section explained how consent expires due to a timeout.
   In some cases it is useful to signal a connection is terminated,
   rather than relying on a timeout.  This is done by immediately
   revoking consent.

   Consent for sending traffic on the media or data channel is revoked
   by receipt of a an implementation MUST ensure authenticated message that closes the connection
   (for instance, a TLS fatal alert).

   Receipt of an application unauthenticated message that closes a connection (e.g., Javascript application) is
   TCP FIN) does not able to
   obtain indicate revocation of consent.  Thus, an endpoint
   receiving an unauthenticated end-of-session message SHOULD continue
   sending media (over connectionless transport) or control STUN information relevant attempt to consent, specifically re-
   establish the ICE transaction-id MUST NOT be accessible to upper-level
   applications. connection (over connection-oriented transport) until
   consent expires or it receives an authenticated message revoking

5.  Connection Liveness

   A connection is considered "live" if packets are received from a
   remote endpoint within an application-dependent period.  An
   application can request a notification when there are no packets
   received for a certain period (configurable).

   Similarly, if packets haven't been received within a certain period,
   an application can request a consent check (heartbeat) be generated.
   These two time intervals might be controlled by the same
   configuration item.

   Sending consent checks (heartbeats) at a high rate could allow a
   malicious application to generate congestion, so applications MUST
   NOT be able to send heartbeats faster than 1 per second.

6.  DiffServ Treatment for Consent packets

   It is RECOMMENDED that STUN consent checks use the same Diffserv
   Codepoint markings as the media packets sent on that transport
   address.  This follows the recommendation of ICE connectivity check
   described in section of [RFC5245].

   Note: It is possible that different Diffserv Codepoints are used by
   different media over the same transport address
   [I-D.ietf-tsvwg-rtcweb-qos].  In that case, what should this document
   recommend as the Codepoint for STUN consent packets ?

7.  W3C API Implications

   For the consent freshness and liveness test the W3C specification
   should provide APIs as described below:

   1.  Ability for the browser to notify the JavaScript that a consent
       freshness transaction has failed for a media stream and the
       browser has stopped transmitting for that stream.

   2.  Ability for the JavaScript to start and stop liveness test and
       set the liveness test interval.

   3.  Ability for the browser to notify the JavaScript that a liveness
       test has failed for a media stream.

8.  Security Considerations

   This document describes a security mechanism.

   The security considerations discussed in [RFC5245] should also be
   taken into account.

   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 others
   keys, any of those parties could have sent (or spoofed) the packet.
   Such shared key distributions are possible with some MIKEY [RFC3830]
   modes, Security Descriptions [RFC4568], and EKT
   [I-D.ietf-avtcore-srtp-ekt].  Thus, in such shared keying
   distributions, receipt of an authenticated SRTP packet is not

9.  IANA Considerations

   This document does not require any action from IANA.

10.  Acknowledgement

   Thanks to Eric Rescorla, Harald Alvestrand, Bernard Aboba, Magnus
   Westerland, Cullen Jennings and Simon Perreault for their valuable
   inputs and comments.

11.  References

11.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", RFC 5245, April

   [RFC6263]  Marjou, X. and A. Sollaud, "Application Mechanism for
              Keeping Alive the NAT Mappings Associated with RTP / RTP
              Control Protocol (RTCP) Flows", RFC 6263, June 2011.

11.2.  Informative References

              McGrew, D. and D. Wing, "Encrypted Key Transport for
              Secure RTP", draft-ietf-avtcore-srtp-ekt-02 (work in
              progress), February 2014.

              Dhesikan, S., Druta, D., Jones, P., and J. Polk, "DSCP and
              other packet markings for RTCWeb QoS", draft-ietf-tsvwg-
              rtcweb-qos-00 (work in progress), April 2014.

   [RFC3830]  Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K.
              Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830,
              August 2004.

   [RFC4568]  Andreasen, F., Baugher, M., and D. Wing, "Session
              Description Protocol (SDP) Security Descriptions for Media
              Streams", RFC 4568, July 2006.

Appendix A.  Example Implementation

   This section describes one possible implementation algorithm of
   consent.  This section is non-normative and provided for reference

   The solution uses three values:

   1.  A consent timer, Tc, whose value is set to 30 seconds.

   2.  A packet receipt timer, Tr, whose value is determined by the
       application.  Tr can be greater than 1 but less than 30 seconds
       and has a default value of 5 seconds.

   3.  A consent timeout, Tf, which is how many seconds elapse without a
       consent response before the browser ceases transmission of media.
       Its value is be 30 seconds or less.

   4.  A retransmission Timer, Tret, whose value is determined by the
       RTT of a given path.  The duration of this timer is set to 1.5
       times of (500 ms or the smoothened round-trip time (from previous
       consent freshness checks or RTP round-trip time)), whichever is

   A WebRTC browser performs a combined consent freshness and session
   liveness test using STUN request/response as described below:

   Every Tc seconds, the WebRTC browser sends a STUN Binding Request to
   the peer.  The difference from ICE connectivity check is that there
   is no exponential back off for retransmissions.

   If a valid STUN Binding Response is received, the consent timer is
   reset to the time of receiving the response and fires again Tc
   seconds later.

   If a valid STUN Binding Response is not received after Tret
   milliseconds, the STUN Binding Request is retransmitted (with a new
   Transaction ID).  As long as a valid STUN Binding Response is not
   received, this retransmission is repeated every Tret milliseconds
   until Tf seconds have elapsed or a valid response is received.  If no
   valid response is received after Tf seconds, the WebRTC browser quits
   transmitting traffic to this remote peer.  The streams that are being
   sent on a flow(5-tuple) for which a consent has failed will be
   stopped.  If the default value of Tf is 30 seconds then media
   transmission will stop Consent (Tf) expires.

Authors' Addresses

   Muthu Arul Mozhi Perumal
   Cisco Systems
   Cessna Business Park
   Sarjapur-Marathahalli Outer Ring Road
   Bangalore, Karnataka  560103

   Email: mperumal@cisco.com

   Dan Wing
   Cisco Systems
   821 Alder Drive
   Milpitas, California  95035

   Email: dwing@cisco.com
   Ram Mohan Ravindranath
   Cisco Systems
   Cessna Business Park
   Sarjapur-Marathahalli Outer Ring Road
   Bangalore, Karnataka  560103

   Email: rmohanr@cisco.com

   Tirumaleswar Reddy
   Cisco Systems
   Cessna Business Park, Varthur Hobli
   Sarjapur Marathalli Outer Ring Road
   Bangalore, Karnataka  560103

   Email: tireddy@cisco.com

   Martin Thomson
   Suite 300
   650 Castro Street
   Mountain View, California  94041

   Email: martin.thomson@gmail.com