Piggybacked DTLS Handshakes in SDP
draft-rescorla-dtls-in-sdp-01

Versions: 00 01                                                         
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.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.




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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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