Internet DRAFT - draft-thomson-rtcweb-ice-dtls
draft-thomson-rtcweb-ice-dtls
rtcweb M. Thomson
Internet-Draft Skype
Intended status: Standards Track March 28, 2012
Expires: September 29, 2012
Using Datagram Transport Layer Security (DTLS) For Interactivity
Connectivity Establishment (ICE) Connectivity Checking: ICE-DTLS
draft-thomson-rtcweb-ice-dtls-00
Abstract
Interactivity Connectivity Establishment (ICE) connectivity checking
using the Datagram Transport Layer Security (DTLS) handshake is
described. The DTLS handshake provides sufficient information to
identify valid candidates and establish consent.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. ICE using DTLS . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Without Optimization . . . . . . . . . . . . . . . . . . . 4
3.2. With Optimization . . . . . . . . . . . . . . . . . . . . 5
3.3. Connectivity Check and Nomination . . . . . . . . . . . . 5
3.4. ICE Controller Selection . . . . . . . . . . . . . . . . . 5
3.5. DTLS Cookie Handling . . . . . . . . . . . . . . . . . . . 6
4. Indicating Continuing Consent to Receive . . . . . . . . . . . 7
5. Parallel STUN . . . . . . . . . . . . . . . . . . . . . . . . 7
6. Signaling in SDP . . . . . . . . . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
10.1. Normative References . . . . . . . . . . . . . . . . . . . 9
10.2. Informative References . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10
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1. Introduction
User experience with real time applications depends greatly on low
latency. At session setup time, optimizing the total number of
network round trips between a user action and the receipt of media is
key to improving user experience.
Interactivity Connectivity Establishment (ICE) [RFC5245] performs a
crucial function in establishing a functional transport flow between
peers in the presence of network address translation (NAT)
middleboxes. Performing the complete ICE procedure can add
significant additional latency to session setup.
A complete ICE connectivity check and candidate nomination requires -
at best - one additional round trip. This assumes aggressive
nomination and all the associated drawbacks. Without aggressive
nomination, two round trips are added.
A transport session that is secured with Datagram Transport Layer
Security (DTLS) [RFC6347] requires at least two additional round
trips to establish once ICE negotiation completes.
A method is described that removes the need for blocking ICE
connectivity checks and nominations with only minimal additional
state overhead at each peer. This allows a secured session setup
without additional latency. In addition, the negative consequences
of aggressive nomination are avoided.
Peers identify candidates using information encoded in the DTLS
cookie. This avoids the need for a DTLS HelloVerifyRequest and
corresponding round trip.
Continuing consent to receive media in the session is verified using
the DTLS heartbeat extension [RFC6520].
An extension to the Session Description Protocol (SDP) [RFC4566]
identifies candidates that support this method.
2. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
described in BCP 14, RFC 2119 [RFC2119] and indicate requirement
levels for compliant implementations.
The term "server" and "client" are used in this document to refer to
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the peers that act in the DTLS server and client roles. This is
distinct from the caller and callee who are participants in the media
session.
3. ICE using DTLS
The DTLS ClientHello and ServerHello messages can be used as a
replacement connectivity check request.
3.1. Without Optimization
A complete session setup using ICE and DTLS optimistically requires
between 5 and 7 round trips to complete. From the instant that the
caller initiates a call:
1. Caller gathers server reflexive and relay candidates.
2. Caller signals call creation. Callee signals response, callee
sends first connectivity check - a Session Traversal Utilities
for NAT (STUN) [RFC5389] Binding request.
3. Caller responds to connectivity check, creates own connectivity
check. Callee responds to connectivity check.
4. Caller creates an ICE nomination (a STUN Binding request with
USE-CANDIDATE set). Callee responds to nomination. Callee sends
DTLS ClientHello.
5. Caller sends a DTLS HelloVerifyRequest with a cookie. Callee re-
sends the ClientHello with the cookie.
6. Caller sends a DTLS ServerHello and associated messages. Callee
sends a DTLS Finished and ChangeCipherSpec.
7. Caller validates the Finished message. Caller can begin media
transmission. Caller sends a Finished message. Callee validates
the Finished message. Callee can begin media transmission.
This sequence assumes that no packets are lost or blocked during
setup. It also assumes that the callee creates the DTLS ClientHello
in order to save half a round trip, which probably doesn't correspond
with established practice.
The delay imposed by acquiring consent for the call from the callee
potentially dwarfs any delays from session setup. Low latency setup
is most applicable to pre-authorized sessions and situations where an
automated system is able to rapidly accept calls.
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3.2. With Optimization
Using the DTLS handshake messages for connectivity checking takes far
fewer round trips in the best case:
1. Caller gathers server reflexive and relay candidates.
2. Caller signals call creation, indicating support for ICE-DTLS.
Callee signals response. Callee sends DTLS ClientHello including
a specially formatted cookie.
3. Caller sends a DTLS ServerHello and associated messages. Callee
sends a DTLS Finished and ChangeCipherSpec.
4. Caller validates the Finished message. Caller can begin media
transmission. Caller sends a Finished message. Callee validates
the Finished message. Callee can begin media transmission.
3.3. Connectivity Check and Nomination
Validating the DTLS handshake requires that peers retain copies of
the handshake messages. A (DTLS) client that sends multiple
ClientHello messages in place of connectivity checks MUST retain the
contents of each of those messages in order to later successfully
complete the DTLS handshake.
This requires additional state retention - especially at the (DTLS)
server - when multiple connectivity checks are sent and received.
This information does not add significantly to the state burden
imposed by ICE. Assuming that the client does not alter the
configuration for each session, the Random and Cookie fields will
vary in every ClientHello.
Continuing the handshake past the *Hello messages indicates that the
candidate pair in use has been nominated. Once a Finished message
has been received for any session, any state retained for other
candidates within the same ICE negotiation MAY be discarded and any
sessions abandoned.
3.4. ICE Controller Selection
This process implies that the peer in the DTLS client role is acting
as the ICE controller. Since both peers need to send packets in
order to create the session, a mechanism for determining which of the
two clients is the controller is necessary.
A DTLS peer that receives a ClientHello when it has one of its own
ClientHello messages outstanding continues to send. A peer that
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receives a message is more likely to be able to respond to those
messages on the same transport flow than it is to successfully send
its own messages. Based on the receipt of a message alone, there is
no way to tell if the other peer has successfully received any other
packets. Therefore, the peer creates the ServerHello and associated
messages in response.
A peer that receives a ServerHello when its own ServerHello is
outstanding must choose whether to end the handshake, or whether it
is the controller. Only the ICE controller is able to continue the
handshake.
Unless the ICE controller is selected by other means, the ICE
controller is the peer that has the largest numeric public key value,
taken from the DTLS Certificate message.
3.5. DTLS Cookie Handling
The HelloVerifyRequest message in DTLS is used to determine that a
client is genuine and is able to receive packets. This allows a
server to avoid allocating state for a client based on the receipt of
an arbitrary packet.
Where a signaling relationship already exists between client and
server, the cookie exchange provides less utility. However, the
cookie still provides protection against receipt of spoofed
ClientHello packets.
Populating the DTLS cookie with information from the signaling allows
a server to determine that a packet is genuine without requiring a
round trip for confirmation. Using the ICE username fragment and
password ensures that no additional state is required to use this
method.
The DTLS cookie includes information from the signaling, chosen by
the DTLS server, plus a HMAC [RFC2104]. The HMAC that ensures that
access to packets does not enable the sending of spoofed ClientHello
messages.
The value of the Cookie is formed using a method similar to the ICE
short term credential mechanism. The ICE user is formed by
concatenating the username fragment from the peer, a colon (':') and
the local username fragment. The HMAC is calculated using the ICE
password as the key, with the text being a concatenation of the
Random value from the DTLS ClientHello and the ICE user. No other
information from the ClientHello is authenticated.
The Cookie is calculated as follows:
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ice_user = ice_ufrag_peer | ':' | ice_ufrag_local
Cookie = HMAC[hash](ice_pwd_peer, Random | ice_user) | ice_user
... where '|' implies concatenation. This method is roughly
equivalent to the one used by ICE when forming STUN packets.
Hash agility is achieved by signaling the hash to use.
Implementations MUST support SHA-1 [RFC3174]. See Section 6 for an
example.
Multiple hash algorithms MAY be signaled to indicate that multiple
different cookies are accepted.
In order to support ICE usernames longer than 12 octets or hash
algorithms other than SHA-1, DTLS 1.2 [RFC6347] MUST be supported.
DTLS 1.2 expands the size of the cookie field to 255 octets.
4. Indicating Continuing Consent to Receive
An important security concern for the web context is that a media
sender has a means of checking that a media receiver consents to
receive that media. Since consent can be revoked, a regular check is
necessary to ensure that media is not unwanted.
The DTLS heartbeat extension [RFC6520] provides a means of signaling
liveness of a DTLS session. A successful response also indicates
that the receiver consents to the continuing receipt of data.
In order to support this feature, peers MUST use the heartbeat
extension and MUST NOT send peer_not_allowed_to_send in the
handshake.
5. Parallel STUN
The main drawback of this optimization is that it is not possible to
gather peer reflexive addresses using the connectivity check.
Performing a STUN Binding request in parallel to the DTLS handshake
allows a client to gather a peer reflexive address without additional
latency.
6. Signaling in SDP
The Session Description Protocol (SDP) [RFC4566] can be used to
signal support for this feature.
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The "ice-dtls" attribute is set to the textual name of the hash
function used in the HMAC. Valid values are taken from the IANA
registry of Hash Function Textual Names [IANAHashText], with multiple
values separated by spaces.
7. Security Considerations
The Random field in the DTLS handshake provides more entropy than the
corresponding field in STUN (224 bits vs. 96 bits). Both use an
equivalent HMAC method. This makes guessing the correct ClientHello
considerably harder than guessing the correct STUN Binding request.
The state maintained by peers using the DTLS handshake is increased
marginally over what is required to perform ICE. Each peer is
required to retain all the messages in the DTLS handshake in order to
correctly form the Finished message. Since each peer also
authenticates every ClientHello, only hosts with access to signaling
are able to create state in this fashion.
State accumulation can be limited using the same methods recommended
for the STUN Amplification Attack (Section 18.5.2 in [RFC5245]).
8. IANA Considerations
[[Note to IANA/RFC Editor: Please replace instance of RFCXXXX with
the number of the published RFC and remove this note.]]
This specification defines a new SDP 'ice-dtls' attribute following
the procedures of Section 8.2.4 of [RFC4566].
Contact Name: Martin Thomson, martin.thomson@gmail.com
Attribute Name: ice-dtls
Long Form: ice-dtls
Type of Attribute: session- or media-level
Charset Considerations: The attribute is not subject to the charset
attribute.
Purpose: This attribute indicates that DTLS is supported as an
alternative mechanism for ICE connectivity checking and consent.
The values of the attribute indicate the hash functions that can
be used to calculate the value of the DTLS cookie.
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Appropriate Values: A whitespace-separated list of values taken from
the IANA registry of Hash Function Textual Names [IANAHashText].
9. Acknowledgments
Cullen Jennings provided the initial idea for this optimization.
10. References
10.1. Normative References
[IANAHashText]
Internet Asigned Numbers Authority, "IANA registry of Hash
Function Textual Names".
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
February 1997.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3174] Eastlake, D. and P. Jones, "US Secure Hash Algorithm 1
(SHA1)", RFC 3174, September 2001.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245,
April 2010.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, January 2012.
[RFC6520] Seggelmann, R., Tuexen, M., and M. Williams, "Transport
Layer Security (TLS) and Datagram Transport Layer Security
(DTLS) Heartbeat Extension", RFC 6520, February 2012.
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10.2. Informative References
[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
October 2008.
Author's Address
Martin Thomson
Skype
3210 Porter Drive
Palo Alto, CA 94304
US
Phone: +1 650-353-1925
Email: martin.thomson@gmail.com
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