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

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.

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:

• Reject the optimization, in which case it ignores the attribute.
• 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 reasonably likely that when ICE is in use the Answer will arrive first, for two reasons:

• 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.
• 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.

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

8.2. Informative References

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.

    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