RFC 7675

Session Traversal Utilities for NAT (STUN) Usage for Consent Freshness

📅 published October 2015, from ietf

okay bestie, this is RFC 7675 🎀 and it's all about making sure nobody sends traffic to someone who didn't say yes 🫶 Here's the title block tea ☕

  • Number and title: RFC 7675, Session Traversal Utilities for NAT (STUN) Usage for Consent Freshness
  • Published by: the Internet Engineering Task Force (IETF) 🌸
  • Date: October 2015
  • Category: Standards Track, so it's an Internet Standards Track document 🦋 (ISSN: 2070-1721)
  • Authors: M. Perumal (Ericsson), D. Wing (Cisco Systems, Inc.), R. Ravindranath and T. Reddy (both Cisco Systems), and M. Thomson (Mozilla) 👑
  • Obsoletes / Updates: the header doesn't list anything this one obsoletes or updates, so no drama there ✨

Abstract

okay so the big problem is this: WebRTC apps, like browsers, could start launching attacks by sending traffic to unwilling victims 😭 To stop that, the app has to keep asking remote endpoints for periodic consent to send 🙈

This document describes a consent mechanism that uses a brand 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 (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 ✨

okay so endpoints have to make sure the remote peer actually wants to receive traffic, otherwise they're basically helping attackers 😭 That check needs to happen before sending anything. In deployments like WebRTC, it's how we stop a sketchy JavaScript from using your browser as a launchpad for attacks 💀 The check happens twice: once when the session first gets set up, through Interactive Connectivity Establishment (ICE) [RFC5245] connectivity checks, and then again regularly for the whole session using the procedures in this document 🫶

When a session first starts, ICE implementations get an initial consent to send by doing STUN connectivity checks 🌸 This document describes a new STUN usage with request and response messages that check whether the remote peer still consents to receive traffic. That consent expires after a while, so it has to be renewed over and over, which is what makes it possible to terminate consent 🦋

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 💅

okay this part is so good 😭 ICE needs periodic keepalive traffic to keep NAT bindings alive (see Section 10 of [RFC5245] and also [RFC6263]). But those keepalives are sent as STUN Indications that are send-and-forget, and they do not evoke a response. A response is necessary for consent to continue sending traffic 🌸 So we need a request/response mechanism for consent freshness. ICE can be used for that, because ICE implementations are already required to keep 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. 🌸

okay so a full ICE implementation gets consent to send using ICE 💖. After ICE wraps up on a particular candidate pair, and every single time the endpoint sends application data on that pair, consent gets maintained using the procedure 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 allowed 🎀. So basically, no application data (like RTP or Datagram Transport Layer Security (DTLS)) can go out 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 then receiving a matching, authenticated, non-error STUN binding response from that same 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 still necessary over a TCP connection because it could be translated to a UDP connection (e.g., [RFC6062]) 🧚.

Consent expires after 30 seconds ⏰. That means if a valid STUN binding response hasn't been received from the remote peer's transport address in 30 seconds, the endpoint MUST cease transmission on that 5-tuple 💀. STUN consent responses that show up after consent expiry do not re-establish consent, and they may be discarded or cause an ICMP error 🙅‍♀️.

To keep consent from expiring, a STUN binding request can be sent periodically 🌸. To stop consent checks from syncing up with each other, 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, which gives a period between checks of 4 to 6 seconds ⭐. Implementations MUST NOT set the period between checks to less than 4 seconds 🚫. This timer runs independently 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 1️⃣. So a sender can't assume it'll get a response for every consent request, and a response might be for a previous request instead of the most recently sent one 🙃.

An endpoint SHOULD wait for a binding response to 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 would enable spoofing of STUN responses and falsify consent 🚨. Not okay, not me.

To prevent attacks on the peer during an ICE restart, an endpoint that keeps sending traffic on the previously validated candidate pair during the restart MUST continue to perform consent freshness on that candidate pair, as described earlier 🛡️.

TCP does give some protection from off-path attackers ([RFC5961], [RFC4953]), but there's still a risk that an attacker could make a TCP sender send forever by spoofing ACKs 😱. To prevent that attack, consent checks MUST be performed over all transport connections, including TCP 🔒. That way, an off-path attacker spoofing TCP segments can't make a TCP sender keep sending once the consent timer expires (30 seconds) ✋.

An endpoint doesn't need to maintain consent if it isn't sending application data 😌. However, an endpoint MUST regain consent before it resumes sending application data 🔁. Without any packets, bindings in middleboxes for the flow might expire 📦. Also, having one peer unable to send is bad for lots of protocols 😬. Unless there's better information about the network, if an endpoint needs to make sure its NAT or firewall mappings don't expire, it can use 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 a new session, or an ICE restart, is needed to get consent to send on the affected candidate pair 🔄.

Sometimes it's useful to signal that consent is terminated instead of just waiting for a timeout 💁‍♀️.

Consent for sending application data is immediately revoked when an authenticated message that closes the connection is received (e.g., a Transport Layer Security (TLS) fatal alert), or when a valid and authenticated STUN response with error code Forbidden (403) is received 💥. But heads up: consent revocation messages can get lost on the network, so an endpoint could resend them, or just wait for consent to expire 🙃.

Receiving an unauthenticated message that closes a connection (e.g., TCP FIN) does not mean consent was revoked 🙅‍♀️. So an endpoint that gets an unauthenticated end-of-session message SHOULD keep sending media (over connectionless transport) or try to re-establish the connection (over connection-oriented transport) until consent expires, or until it receives an authenticated message revoking consent 🫶.

Also, an authenticated Secure Real-time Transport Control Protocol (SRTCP) BYE does not terminate consent 🙌. It only tells you the associated SRTP source has quit 👋.

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's possible that different Diffserv Codepoints get used by different media over the same transport address [WebRTC-QoS]. That case is outside the scope of this document 🤷‍♀️.

7. DTLS Applicability 🔐

The DTLS applicability is identical to what's described in Section 4.2 of [RFC7350] 💖.

8. Security Considerations 🚨

This is the serious part, bestie, so pay attention 🫡. This document describes a security mechanism, and the details are in Sections 4.1 and 4.2 of [RFC7350]. Consent requires the 96-bit transaction ID defined in Section 6 of [RFC5389] to be uniformly and randomly chosen from the interval 0 .. 2**96-1, and to be cryptographically strong 🔥. That's good enough security against an off-path attacker replaying old STUN consent responses 🛡️. Consent verification, which is about avoiding attacks that use 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 🫶

Huge 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 💜. Extra big 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
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