RTCWEB WG E. Rescorla
Internet-Draft RTFM, Inc.
Intended status: Informational October 31, 2016
Expires: May 4, 2017
Piggybacked DTLS Handshakes in SDP
draft-rescorla-dtls-in-sdp-01
Abstract
This document describes a mechanism for embedding DTLS handshake
messages in SDP descriptions. This technique allows implementations
to shave a full round-trip off of DTLS-SRTP session establishment,
while retaining compatibility with ordinary DTLS-SRTP endpoints.
Status of This Memo
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This Internet-Draft will expire on May 4, 2017.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 4
2.1. DTLS 1.2 . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. DTLS 1.3 . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Attribute Definition . . . . . . . . . . . . . . . . . . . . 7
4. Interactions . . . . . . . . . . . . . . . . . . . . . . . . 7
4.1. ICE . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.2. Forking . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.3. RTCWEB Identity . . . . . . . . . . . . . . . . . . . . . 8
5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Normative References . . . . . . . . . . . . . . . . . . 9
8.2. Informative References . . . . . . . . . . . . . . . . . 10
8.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Appendix A. Speculative: Server False-Start . . . . . . . . . . 11
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 13
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
DTLS-SRTP [RFC5763][RFC5763] uses a DTLS [RFC6347] handshake to
establish keys which are then used to key SRTP [RFC3711]. The DTLS
negotiation is tied to the offer/answer [RFC3264] transaction via an
"a=fingerprint" attribute [RFC4572] in the SDP [RFC4566]. The common
message flow is shown below for DTLS 1.2.
This figure and the rest of this document adopt the following
assumptions about network behavior:
o ICE [RFC5245] is in use but that both endpoints implement
endpoint-independent filtering [RFC5389] so that STUN checks
succeed immediately.
o Signaling messages take the same time to be delivered as direct
messages [this is generally false.]
Links to detailed diagrams with a more accurate vertical scale can be
found below each diagram.
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Alice Signaling Service Bob
-----------------------------------------------------------
^ Offer + fingerprint --------->
| Offer + fingerprint --------> ^
| |
| <------ Answer + fingerprint |
| <-------- Answer + fingerprint |
| <------------------------------------------------- STUN-REQ |
| STUN-REQ -------------------------------------------------> |
| STUN-RESP-------------------------------------------------> |
| <--------------------------------------------- ClientHello |
| <------------------------------------------------ STUN-RESP |
4 |
R ServerHello |
T ServerKeyExchange |
T Certificate 3
| CertificateRequest R
| ServerHelloDone -----------------------------------------> T
| T
| ClientKeyExchange |
| Certificate |
| CertificateVerify |
| [ChangeCipherSpec] |
| <------------------------------------------------ Finished |
| |
| [ChangeCipherSpec] |
| Finished -------------------------------------------------> |
| Media ----------------------------------------------------> v
v <------------------------ Media----------------------------
Figure 1: Standard DTLS-SRTP Negotiation
Better picture [1]
In this flow, the earliest that Alice can start sending media is
after receiving Bob's Finished and the earliest Bob can start sending
media is upon receiving Alice's Finished, and neither side can send
any DTLS messages until they have had a successful STUN check. The
result is that in the best case, Alice receives media four round
trips after sending the offer and Bob receives media three round
trips after receiving Alice's offer.
This document describes a technique for improving call setup time by
piggybacking the first round of DTLS messages on the signaling
messages. This reduces latency by a full round trip for both DTLS
1.2 and DTLS 1.3 handshakes, and for DTLS 1.3 [I-D.ietf-tls-tls13]
allows the answerer to start sending media immediately upon receiving
the offer, or, if ICE is used, upon ICE completion.
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2. Protocol Overview
The basic concept, as shown in Figure 2, is for Alice to send her
ClientHello in her Offer and Bob to send the server's first flight
(ServerHello...ServerHelloDone for DTLS 1.2) in his Answer.
2.1. DTLS 1.2
Alice Signaling Service Bob
-----------------------------------------------------------
^ Offer
| + fingerprint
| + ClientHello --------->
| Offer
| + fingerprint
| + ClientHello ------------> ^
| |
| Answer |
| + fingerprint |
| + ServerHello |
| + ServerKeyExchange |
| + Certificate |
| + CertificateRequest |
| <------------ ServerHelloDone |
3 |
T Answer 2
T + fingerprint R
T + ServerHello T
| + ServerKeyExchange T
| + Certificate |
| + CertificateRequest |
| <------------ ServerHelloDone |
| <------------------------------------------------ STUN-REQ |
| STUN-REQ -------------------------------------------------> |
| STUN-RESP-------------------------------------------------> |
| <------------------------------------------------ STUN-RESP |
| ClientKeyExchange |
| Certificate |
| CertificateVerify |
| [ChangeCipherSpec] |
| Finished -------------------------------------------------> v
| Media ---------------------------------------------------->
| [ChangeCipherSpec]
| <------------------------------------------------ Finished
v <--------------------------------------------------- Media
Figure 2: Piggybacked DTLS-SRTP Negotiation (TLS 1.2)
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Better picture [2]
Note that in this flow, the active/passive (DTLS client/server) roles
are reversed and Alice becomes the client. Because this is a
basically symmetrical transaction, this is not an issue.
It should be immediately apparent that this exchange shaves off a
full round trip from Bob's perspective (despite actually only shaving
a half a round trip from the number of messages). The reason is that
Bob does not need to wait for Alice's Finished to send but can
piggyback his data on his Finished.
This change also shaves off a round trip from Alice's perspective
because Alice can now safely perform TLS False Start
[I-D.ietf-tls-falsestart] and send traffic prior to receiving Bob's
Finished message. When only fingerprints are carried in the
handshake, then extensions such as [RFC7301] indicators and DTLS-SRTP
negotiation are not protected. However, in this case because those
indicators are carried in the hello messages which are now tied to
the signaling channel, they are authenticated via the same mechanisms
that authenticate the fingerprint.
Note: One could argue that under some conditions Bob could do False
Start in the ordinary handshake, but it's much harder to analyze and
even then it leaves Alice one round trip slower than she would be
with this optimization.
2.2. DTLS 1.3
Figure Figure 3 shows the impact of this optimization on DTLS 1.3.
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Alice Signaling Service Bob
-----------------------------------------------------------
^ Offer
| + fingerprint
| + ClientHello --------->
| Offer
| + fingerprint
| + ClientHello ------------> ^
| |
| Answer |
| + fingerprint |
| + ServerHello |
| + CertificateRequest |
| + Certificate |
| + CertificateVerify |
| <------------------- Finished |
| Answer |
3 + fingerprint |
R + ServerHello 2
T + CertificateRequest R
T + Certificate T
| + CertificateVerify T
| <------------------- Finished |
| <------------------------------------------------ STUN-REQ |
| STUN-REQ -------------------------------------------------> |
| STUN-RESP-------------------------------------------------> |
| <------------------------------------------------ STUN-RESP |
| |
| ClientKeyExchange |
| Certificate |
| CertificateVerify |
| [ChangeCipherSpec] |
| Finished -------------------------------------------------> |
| Media ----------------------------------------------------> v
| [ChangeCipherSpec]
| <------------------------------------------------ Finished
v <--------------------------------------------------- Media
Figure 3: Piggybacked DTLS-SRTP Negotiation (TLS 1.3)
Better picture [3]
Alice cannot send any sooner than with DTLS 1.2 because sending at
the point when she receives Bob's first message is already optimal.
It may be possible for Bob to shave off yet another round trip,
however. As described in Appendix A.
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3. Attribute Definition
This document defines a new media-level SDP attribute, "a=dtls-
message". This message is used to contain DTLS messages. The syntax
of this attribute is:
attribute =/ dtls-message-attribute
dtls-message-attribute = "dtls-message" ":" role SP value
role = "client" / "server"
value = 1*(ALPHA / DIGIT / "+" / "/" / "=" )
; base64 encoded message
An offeror which wishes to use the optimization defined in this
document shall send his ClientHello in the "a=dtls-message" attribute
of its initial offer with the role "client" and MUST use
"a=setup:actpass". This allows the peer to either:
o Reject the optimization, in which case it ignores the attribute.
o Accept the optimization, in which case it MUST use
"a=setup:passive" and send its first flight (starting with
ServerHello) and using the role "server" in its response. These
messages are simply serialized end-to-end as they would be on the
wire. It MAY also choose to send its first flight separately in
the media channel; DTLS implementations already handle retransmits
properly.
The offerer MUST be able to detect whether an incoming DTLS message
is a ClientHello or a ServerHello and adapt accordingly.
In subsequent negotiations, implementations MUST maintain these
roles.
4. Interactions
This optimization has a number of interactions with existing pieces
of protocol machinery.
4.1. ICE
When ICE is in use, there is a race condition between the answerer's
ICE checks (at which point it will be able to send the first flight
on the media channel) and the answerer's Answer, which contains the
first flight. For this reason, we allow implementations to send the
first flight on both channels. However, as a practical matter it is
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reasonably likely that when ICE is in use the Answer will arrive
first, for two reasons:
o The answerer consumes a full RTT doing a STUN check to verify the
path to the offerer (even in the best case where the first STUN
check succeeds). Thus, even if the path through the signaling
server is twice as expensive as the direct path, there is a
reasonable chance that the answer will arrive first.
o If the offerer is behind a NAT without endpoint-independent
filtering, the answerer's ICE checks will be discarded until the
offerer sends its own ICE checks, which it can only do upon
receiving the answer.
In this case, although a comparison of Figure 1 and Figure 2 would
show the ClientHello (in ordinary DTLS) and the ServerHello (when
piggybacked) as arriving at the same time, in fact the ServerHello
may arrive up to a full RTT first, but the offerer can SEND its
second flight immediately upon its STUN check succeeding, which
happens first, thus increasing the advantage of this technique.
4.2. Forking
This technique does not interact very well with forking. Because
each ClientHello is only usable for one server, the system must
somehow ensure that only one of the forks takes up the piggybacked
offers. The easiest approach is for any intermediary which does a
fork to strip out the "a=dtls-message" attribute. An alternative
would be to add another attribute which could be stripped out (this
might interact better with RTCWEB Identity). Note that [RFC4474]
protects against any SDP modifications, but I think at this point
it's clear that that's not practical.
4.3. RTCWEB Identity
RTCWEB Identity assertions need to cover these DTLS messages.
5. Examples
[we need examples.]
6. Security Considerations
The security implications of this technique are described throughout
this document.
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7. IANA Considerations
This specification defines the "dtls-message" SDP attribute per the
procedures of Section 8.2.4 of [RFC4566]. The required information
for the registration is included here:
Contact Name: Eric Rescorla (ekr@rftm.com)
Attribute Name: dtls-message
Long Form: dtls-message
Type of Attribute: session-level
Charset Considerations: This attribute is not subject to the charset
attribute.
Purpose: This attribute carries piggybacked DTLS message.
Appropriate Values: This document
8. References
8.1. Normative References
[I-D.ietf-tls-falsestart]
Langley, A., Modadugu, N., and B. Moeller, "Transport
Layer Security (TLS) False Start", draft-ietf-tls-
falsestart-02 (work in progress), May 2016.
[I-D.ietf-tls-tls13]
Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", draft-ietf-tls-tls13-14 (work in progress),
July 2016.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
DOI 10.17487/RFC3264, June 2002,
<http://www.rfc-editor.org/info/rfc3264>.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, DOI 10.17487/RFC3711, March 2004,
<http://www.rfc-editor.org/info/rfc3711>.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
July 2006, <http://www.rfc-editor.org/info/rfc4566>.
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[RFC4572] Lennox, J., "Connection-Oriented Media Transport over the
Transport Layer Security (TLS) Protocol in the Session
Description Protocol (SDP)", RFC 4572,
DOI 10.17487/RFC4572, July 2006,
<http://www.rfc-editor.org/info/rfc4572>.
[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>.
[RFC5763] Fischl, J., Tschofenig, H., and E. Rescorla, "Framework
for Establishing a Secure Real-time Transport Protocol
(SRTP) Security Context Using Datagram Transport Layer
Security (DTLS)", RFC 5763, DOI 10.17487/RFC5763, May
2010, <http://www.rfc-editor.org/info/rfc5763>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <http://www.rfc-editor.org/info/rfc6347>.
[RFC7301] Friedl, S., Popov, A., Langley, A., and E. Stephan,
"Transport Layer Security (TLS) Application-Layer Protocol
Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
July 2014, <http://www.rfc-editor.org/info/rfc7301>.
8.2. Informative References
[I-D.thomson-avtcore-sdp-uks]
Thomson, M. and E. Rescorla, "Unknown Key Share Attacks on
uses of Transport Layer Security with the Session
Description Protocol (SDP)", draft-thomson-avtcore-sdp-
uks-00 (work in progress), October 2016.
[RFC4474] Peterson, J. and C. Jennings, "Enhancements for
Authenticated Identity Management in the Session
Initiation Protocol (SIP)", RFC 4474,
DOI 10.17487/RFC4474, August 2006,
<http://www.rfc-editor.org/info/rfc4474>.
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8.3. URIs
[1] https://raw.githubusercontent.com/ekr/dtls-in-sdp/master/normal-
12.png
[2] https://raw.githubusercontent.com/ekr/dtls-in-sdp/master/
piggybacked-12.png
[3] https://raw.githubusercontent.com/ekr/dtls-in-sdp/master/
piggybacked-13.png
[4] https://raw.githubusercontent.com/ekr/dtls-in-sdp/master/
piggybacked-13-falsestart.png
Appendix A. Speculative: Server False-Start
WARNING: THE FOLLOWING SECTION HAS NOT RECEIVED ANY REAL SECURITY
REVIEW AND MAY BE A REALLY BAD IDEA.
It has been observed that as if Alice uses a fresh DH ephemeral, then
Bob knows (because he can trust the signaling service) that Alice's
DH ephemeral corresponds to Alice and can therefore encrypt under the
joint DH shared secret without waiting for Alice's CertificateVerify,
as shown in Figure 4.
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Alice Signaling Service Bob
-----------------------------------------------------------
^ Offer
| + fingerprint
| + ClientHello --------->
| Offer
| + fingerprint
| + ClientHello ------------> ^
| |
| Answer |
| + fingerprint |
| + ServerHello |
2 + CertificateRequest |
R + Certificate |
T + CertificateVerify |
T <------------------- Finished |
| Answer |
| + fingerprint |
| + ServerHello 2
| + CertificateRequest R
| + Certificate T
| + CertificateVerify T
| <------------------- Finished |
| <------------------------------------------------ STUN-REQ |
| STUN-REQ -------------------------------------------------> |
| STUN-RESP-------------------------------------------------> |
v <--------------------------------------------------- Media |
<------------------------------------------------ STUN-RESP |
|
ClientKeyExchange |
Certificate |
CertificateVerify |
[ChangeCipherSpec] |
Finished -------------------------------------------------> |
Media ----------------------------------------------------> v
[ChangeCipherSpec]
<------------------------------------------------ Finished
Figure 4: Piggybacked DTLS-SRTP Negotiation (TLS 1.3 with false
start)
Better picture [4]
This has demonstrably inferior security properties if Alice is using
a long-term key (for key continuity or fingerprint validation),
because Bob has not yet verified that Alice controls that key and
does not even know if Alice is using a fresh DH ephemeral, if
implementations decide to adopt this optimization, they must do
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something hacky like Send data immediately but generate an error if
the handshake, including a signature, does not complete within some
reasonable period (a small number of measured round trips) [Just one
reason why this is a questionable technique.]. This technique may
also complicate dealing with the issues raised in
[I-D.thomson-avtcore-sdp-uks].
Appendix B. Acknowledgements
Thanks to Cullen Jennings, Martin Thomson, and Justin Uberti for
helpful suggestions.
Author's Address
Eric Rescorla
RTFM, Inc.
Email: ekr@rtfm.com
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