Internet Engineering Task Force (IETF) M. Perumal

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.

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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

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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.

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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.

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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).

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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

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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.

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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>.

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[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.

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Authors' Addresses

Muthu Arul Mozhi Perumal
Ericsson
Ferns Icon
Doddanekundi, Mahadevapura
Bangalore, Karnataka 560037
India
Email: [email protected]

Dan Wing
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, California 95134
United States
Email: [email protected]

Ram Mohan Ravindranath
Cisco Systems
Cessna Business Park
Sarjapur-Marathahalli Outer Ring Road
Bangalore, Karnataka 560103
India
Email: [email protected]

Tirumaleswar Reddy
Cisco Systems
Cessna Business Park, Varthur Hobli
Sarjapur Marathalli Outer Ring Road
Bangalore, Karnataka 560103
India
Email: [email protected]

Martin Thomson
Mozilla
650 Castro Street, Suite 300
Mountain View, California 94041
United States
Email: [email protected]

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