RTCWEB M. Perumal
Internet-Draft Ericsson
Intended status: Standards Track D. Wing
Expires: June 20, 2015 R. Ravindranath
T. Reddy
Cisco Systems
M. Thomson
Mozilla
December 17, 2014
STUN Usage for Consent Freshness
draft-ietf-rtcweb-stun-consent-freshness-11
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 Session
Traversal Utilities for NAT (STUN) usage.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Design Considerations . . . . . . . . . . . . . . . . . . . . 3
4. Solution . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4.1. Expiration of Consent . . . . . . . . . . . . . . . . . . 3
4.2. Immediate Revocation of Consent . . . . . . . . . . . . . 5
5. DiffServ Treatment for Consent . . . . . . . . . . . . . . . 6
6. DTLS applicability . . . . . . . . . . . . . . . . . . . . . 6
7. API Recommendations . . . . . . . . . . . . . . . . . . . . . 6
8. Security Considerations . . . . . . . . . . . . . . . . . . . 6
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
10. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 7
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
11.1. Normative References . . . . . . . . . . . . . . . . . . 7
11.2. Informative References . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
To prevent attacks on peers, endpoints have to ensure the remote peer
is willing to receive traffic. 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.
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Consent is obtained only by full ICE implementations. An ICE-lite
implementation will not generate consent checks, but will just
respond to consent checks 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. 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 to send to a remote
transport address. Initial consent is obtained using ICE.
Consent Freshness: Maintaining and renewing consent over time.
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 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]. If consent is
performed then there is no need to send keepalive messages.
4. Solution
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 full ICE implementation performs consent freshness test using STUN
request/response as described below:
An endpoint MUST NOT send data other than paced STUN connectivity
checks or responses toward any transport address unless the receiving
endpoint consents to receive data. That is, no application data
(e.g., RTP or DTLS) can be sent until consent is obtained. After a
successful ICE connectivity check on a particular transport address,
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consent MUST be maintained following the procedure described in this
document.
Explicit consent to send is obtained and maintained by sending an
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]).
Initial consent to send traffic is obtained using ICE. Consent
expires after 30 seconds. That is, if a valid STUN binding response
corresponding to any STUN request sent in the last 30 seconds has not
been received from the remote peer's transport address, 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.
Each STUN binding request for consent MUST use a new
cryptographically strong [RFC4086] STUN transaction ID. Each STUN
binding requests for consent is transmitted once only. Hence, the
sender cannot assume that it will receive a response for each consent
request, and a response might be for a previous request (rather than
for the most recently sent request). Consent expiration causes
immediate termination of all outstanding STUN consent transactions.
Each STUN transaction is maintained until one of the following
criteria is fulfilled:
o A STUN response associated with the transaction is received; or
o A STUN response associated to a newer transaction is received.
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 ID, because that enables spoofing of
STUN responses, falsifying consent.
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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 can not cause a TCP sender to send once the consent timer
expires (30 seconds).
An endpoint that is not sending any application data does not need to
maintain consent. However, failure to send could cause any NAT or
firewall mappings for the flow to expire. Furthermore, having one
peer unable to send is detrimental to many protocols.
After consent is lost for any reason, 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.
4.2. 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 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 SRTCP BYE does not terminate consent; it
only indicates the associated SRTP source has quit.
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5. 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
[I-D.ietf-tsvwg-rtcweb-qos]. Such a case is outside the scope of
this document.
6. DTLS applicability
The DTLS applicability is identical to what is described in
Section 4.2 of [RFC7350].
7. API Recommendations
The W3C specification MAY provide the following API points to provide
feedback and control over consent:
1. Generate an event when consent has expired for a given 5-tuple,
meaning that transmission of data has ceased. This could
indicate what application data is affected, such as media or data
channels.
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 to verify consent.
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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, Christer Holmberg, Simon Perreault, Paul
Kyzivat, Emil Ivov, Jonathan Lennox, Inaki Baz Castillo, Rajmohan
Banavi and Christian Groves for their valuable inputs and comments.
Thanks to Christer Holmberg for doing a through review.
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.
[RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Requirements for Security", BCP 106, RFC 4086, June 2005.
[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245, April
2010.
[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]
Mattsson, J., McGrew, D., and D. Wing, "Encrypted Key
Transport for Secure RTP", draft-ietf-avtcore-srtp-ekt-03
(work in progress), October 2014.
[I-D.ietf-rtcweb-overview]
Alvestrand, H., "Overview: Real Time Protocols for
Browser-based Applications", draft-ietf-rtcweb-overview-13
(work in progress), November 2014.
[I-D.ietf-tsvwg-rtcweb-qos]
Dhesikan, S., Jennings, C., Druta, D., Jones, P., and J.
Polk, "DSCP and other packet markings for RTCWeb QoS",
draft-ietf-tsvwg-rtcweb-qos-03 (work in progress),
November 2014.
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[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.
[RFC4953] Touch, J., "Defending TCP Against Spoofing Attacks", RFC
4953, July 2007.
[RFC5961] Ramaiah, A., Stewart, R., and M. Dalal, "Improving TCP's
Robustness to Blind In-Window Attacks", RFC 5961, August
2010.
[RFC6062] Perreault, S. and J. Rosenberg, "Traversal Using Relays
around NAT (TURN) Extensions for TCP Allocations", RFC
6062, November 2010.
[RFC7350] Petit-Huguenin, M. and G. Salgueiro, "Datagram Transport
Layer Security (DTLS) as Transport for Session Traversal
Utilities for NAT (STUN)", RFC 7350, August 2014.
Authors' Addresses
Muthu Arul Mozhi Perumal
Ericsson
Ferns Icon
Doddanekundi, Mahadevapura
Bangalore, Karnataka 560037
India
Email: muthu.arul@gmail.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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