Session Description Protocol (SDP) Offer/Answer Considerations for Datagram Transport Layer Security (DTLS) and Transport Layer Security (TLS)

Summary: Has a DISCUSS. Needs one more YES or NO OBJECTION position to pass.

Eric Rescorla Discuss

Discuss (2017-08-15 for -28)
1. Assuming I understand this document correctly, it conflicts with
the guidance in JSEP. Specifically, S 4 says:

   No default value is defined for the SDP 'tls-id' attribute.
   Implementations that wish to use the attribute MUST explicitly
   include it in SDP offers and answers.  If an offer or answer does not
   contain a 'tls-id' attribute (this could happen if the offerer or
   answerer represents an existing implementation that has not been
   updated to support the 'tls-id' attribute), unless there is another
   mechanism to explicitly indicate that a new DTLS association is to be
   established, a modification of one or more of the following
   characteristics MUST be treated as an indication that an endpoint
   wants to establish a new DTLS association:

   o  DTLS setup role; or

   o  fingerprint set; or

   o  local transport parameters; or

   o  ICE ufrag value

This seems to say that if there is no tls-id attribute, then an ICE restart
(which necessitates a ufrag change) requires a DTLS restart. JSEP isn't
incredibly clear on this point, but 5.7.3 seems to say that tls-id
neeed not be present:

      *  tls-id value, which MUST be set according to
         [I-D.ietf-mmusic-dtls-sdp], Section 5.  If this is a re-offer
         and the tls-id value is different from that presently in use,
         the DTLS connection is not being continued and the remote
         description MUST be part of an ICE restart, together with new
         ufrag and password values.  If this is an answer, the tls-id
         value, if present, MUST be the same as in the offer.

I believe that the first sentence is in error, as we clearly
can't have JSEP implementations requiring that tls-id be present.

   o  If the remote DTLS fingerprint has been changed or the tls-id has
      changed, tear down the DTLS connection.  This includes the case
      when the PeerConnection state is "have-remote-pranswer".  If a
      DTLS connection needs to be torn down but the answer does not
      indicate an ICE restart or, in the case of "have-remote-pranswer",
      new ICE credentials, an error MUST be generated.  If an ICE
      restart is performed without a change in tls-id or fingerprint,
      then the same DTLS connection is continued over the new ICE
I think the best interpretation of this is that if tls-id is not present
(and hence unchanged) then ICE restart does not cause DTLS restart.
This is also my memory of the consensus in RTCWEB. In any case, these
two documents clearly must match.

2. S 4 says:

   The mux category [I-D.ietf-mmusic-sdp-mux-attributes] for the 'tls-
   id' attribute is 'IDENTICAL', which means that the attribute value
   must be identical across all media descriptions being multiplexed

This is not actually what JSEP requires:

   different categories.  To avoid unnecessary duplication when
   bundling, attributes of category IDENTICAL or TRANSPORT MUST NOT be
   repeated in bundled m= sections, repeating the guidance from
   [I-D.ietf-mmusic-sdp-bundle-negotiation], Section 8.1.  This includes

I suspect this is old text.

3. S 7.1 says:
   If DTLS is transported on top of a connection-oriented transport
   protocol (e.g., TCP or SCTP), where all IP packets are acknowledged,

This is incorrect, because none of these protocols ack all IP packets.

   all DTLS packets associated with a previous DTLS association MUST be
   acknowledged (or timed out) before a new DTLS association can be
   established on the same instance of that transport (5-tuple).

More generally, I'm not sure that this is useful, because the
required semantic isn't *acknowledged* but rather that the receiver
can appropriately demux. So, say you just stop sending DTLS on
connection A and start sending on B, what's the delimiter, given
that you don't require close_notify here? IIRC, we just decided to
punt on this whole thing. Does anyone try to have successive
connections over the same transport, even when it's connection oriented?

4. The demux instructions seem to have gotten lost from 6.7.1. At minimum
these need a reference to RFC 7983.
Comment (2017-08-15 for -28)
S 5.1.
   media session immediately (see [RFC8122]).  Note that it is
   permissible to wait until the other side's fingerprint(s) has been
   received before establishing the connection; however, this may have
   undesirable latency effects.

I agree that it's permissible, but why would you do this? This does
not seem like helpful guidance.

S 10.
Please do something about the "NEW" constructions. I literally had to
pull these into ediff to know what had changed. That's not useful to
people. I'm not a fan of this construction in general, but at minimum
you need to explain what has changed.

S 9.
   Regardless of the
   previous existence of a DTLS association, the SDP 'setup' attribute
   MUST be included according to the rules defined in [RFC4145] and if
   ICE is used, ICE restart MUST be initiated.

What is the rationale for this rule?

Ben Campbell Yes

Deborah Brungard No Objection

Suresh Krishnan No Objection

Warren Kumari No Objection

Comment (2017-08-15 for -28)
This falls well into the "This is outside my area of expertise" part of:
This ballot position may be interpreted as "This is outside my area of expertise or have no cycles", in that you exercise the ability to move a document forward on the basis of trust towards the other ADs.

Mirja K├╝hlewind (was Discuss) No Objection

Comment (2017-10-05 for -30)
Thanks for addressing my discuss by simply removing section 7.1!

Old comments:

A couple mostly editorial comments:
- Probably a nit: In section 3.2 'must' is used while in section 3.3 'MUST' is used. I would assume that both sections should probably use the same.

- in sec 5.1: "Because of
   this, if an unordered transport is used for the DTLS association, a
   new transport (3-tuple) must be allocated by at least one of the
   endpoints so that DTLS packets can be de-multiplexed."
Why is this a 3-tuple (instead of a 5-tuple)? I guess you talk about the source address, source port, transport 3-tuple? May say this more explicitly.  Also the word of the use transport is confusing to me here because it's used for the transport protocol as well as for the transport 'connection' (if a connection-oriented transport protocol is used). Maybe s/new transport/new flow/? Moreover, there should probably be a 'MUST' here instead of 'must'!

- sec 5.2:"In addition, the offerer MUST insert in the
   offer an SDP 'tls-id' attribute with a unique attribute value."
Is that a MUST or rather a SHOULD? The rest of the text reads like this should be a SHOULD.

- Shouldn't this document cite RFC6347 normatively, e.g. here (sec 5.3):
"... the answerer MUST initiate a DTLS handshake by sending a
   DTLS ClientHello message towards the offerer."

- I would like to see more discussion about linkability based on the introduction of the "tls-id" in the security considerations section.

Alexey Melnikov (was Discuss) No Objection

Comment (2017-08-31 for -29)
Thank you for addressing my DISCUSS/comments.

Kathleen Moriarty No Objection

Comment (2017-08-16 for -28)
I agree with Alexey's discuss, thanks for addressing it.

Alvaro Retana No Objection

Comment (2017-08-11 for -28)
Mostly a nit...

RFC4572 has been Obsoleted by RFC8122.  I think that this change in status implicitly means that all references to RFC4572 should now really refer to RFC8122.  I then don't think there's a need to explicitly Update the references from RFC4572 to RFC8122.

In the text Updating RFC5763 (10.2.1), the document says: "The reference to [RFC4572] is replaced with a reference to [RFC8122]."  I don't think that is necessary.

Also, the Update to RFC7345 (10.3.3) adds a Normative bibliographical entry to RFC8122, but no text is updated to point at that RFC.  I don't think this is necessary either.

Adam Roach (was Discuss) No Objection

Comment (2017-08-17 for -28)
Thanks for the quick answer to my DISCUSS.

I agree with the core assertion of EKR's DISCUSS: this document needs to be aligned with JSEP. I think we're going to need a little additional work figuring out which document needs to change where they disagree. In addition to those areas he highlights in his DISCUSS, the following text is also in conflict:

DTLS-SDP: "the offerer and answerer generate their own local 'tls-id' attribute values, and the combination of both values identify the DTLS association."

JSEP: "If this is an answer, the tls-id value, if present, MUST be the same as in the offer."

[Note: this does appear to be an issue in JSEP rather than this document]

I would think the long-form title of this document should include "TLS," to reflect that it also contains TLS-related procedures.

Section 1: "...but currently there is no way..." will not age well once this is an RFC. Suggest "...previously, there was no way..." or somesuch.

Section 2 uses RFC 2119 boilerplate, and then the very next sentence uses a non-normative "must." I would strongly recommend moving to RFC 8174 boilerplate.

The conventional name for DTLS-SRTP is "DTLS-SRTP" -- please change replace "SRTP-DTLS" with "DTLS-SRTP" everywhere it appears.

The last paragraph in section 5.4 starts with "NOTE" (which implementors frequently read as non-normative) and then contains a normative statement. Suggest removing "NOTE:"

Please expand the following acronyms upon first use and in the title;
see for guidance.

 - SDP - Session Description Protocol
 - DTLS - Datagram Transport Layer Security
 - TLS - Transport Layer Security
 - ICE - Interactive Connectivity Establishment
 - SCTP - Stream Control Transmission Protocol
 - SRTP - Secure Realtime Transport Protocol
 - UDPTL - UDP Transport Layer

Alia Atlas No Record

Benoit Claise No Record

Alissa Cooper No Record

Spencer Dawkins No Record

Terry Manderson No Record