RTCWEB M. Perumal
Internet-Draft D. Wing
Intended status: Standards Track R. Ravindranath
Expires: November 20, 2014 T. Reddy
Cisco Systems
M. Thomson
Mozilla
May 19, 2014
STUN Usage for Consent Freshness
draft-ietf-rtcweb-stun-consent-freshness-03
Abstract
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.
Status of This Memo
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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 . . . . . . . . . . . . . . . . . . . . . 5
6. DiffServ Treatment for Consent packets . . . . . . . . . . . 5
7. W3C API Implications . . . . . . . . . . . . . . . . . . . . 5
8. Security Considerations . . . . . . . . . . . . . . . . . . . 6
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
10. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 6
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
11.1. Normative References . . . . . . . . . . . . . . . . . . 6
11.2. Informative References . . . . . . . . . . . . . . . . . 7
Appendix A. Example Implementation . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 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
consent.
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
interface).
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.
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2. 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: It is the mechanism of obtaining permission to send traffic
to a certain transport address. This is the initial consent to
send traffic, which is obtained 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
[RFC5245].
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 (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
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obtained following the procedure described in this document. The
consent expires after a fixed amount of time.
Explicit consent to send is indicated 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 requests and
responses are authenticated using the same short-term credentials as
the initial ICE exchange, but using a new (fresh) transaction-id each
time consent needs to be refreshed. Implementations MUST obtain
fresh consent before their existing consent expires. 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 new
(fresh) transaction-id (so that round-trip time can be calculated),
and 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-INTEGRITY
is considered a consent response.
The initial Consent to send traffic is obtained by ICE. Consent
expires after 30 seconds. That is, if 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, the endpoint MUST
cease transmission on that 5-tuple.
To meet the security needs of consent, an untrusted application
(e.g., JavaScript) MUST NOT be able to obtain or control the ICE
transaction-id, because that enables spoofing STUN responses,
falsifying consent
An endpoint that is only receiving application traffic (recvonly)
does not need to obtain consent which can slightly conserve its
resources. However, the endpoint needs to ensure its NAT or firewall
mappings persist which can be done using keepalive or other
techniques (see Section 10 of [RFC5245] and see [RFC6263]). If the
endpoint wants send application traffic, it needs to first obtain
consent if its consent expired.
4.2. Immediate Revocation of 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.
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Consent for sending traffic on the media or data channel is revoked
by receipt of a an authenticated message that closes the connection
(for instance, a TLS fatal alert).
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.
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 7.1.2.4 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.
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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
sufficient.
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
2010.
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[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
[I-D.ietf-avtcore-srtp-ekt]
McGrew, D. and D. Wing, "Encrypted Key Transport for
Secure RTP", draft-ietf-avtcore-srtp-ekt-02 (work in
progress), February 2014.
[I-D.ietf-tsvwg-rtcweb-qos]
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
only.
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
less.
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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
India
Email: mperumal@cisco.com
Dan Wing
Cisco Systems
821 Alder Drive
Milpitas, California 95035
USA
Email: dwing@cisco.com
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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
Suite 300
650 Castro Street
Mountain View, California 94041
US
Email: martin.thomson@gmail.com
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