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rfc:rfc7983

Internet Engineering Task Force (IETF) M. Petit-Huguenin Request for Comments: 7983 Impedance Mismatch Updates: 5764 G. Salgueiro Category: Standards Track Cisco Systems ISSN: 2070-1721 September 2016

                    Multiplexing Scheme Updates
      for Secure Real-time Transport Protocol (SRTP) Extension
            for Datagram Transport Layer Security (DTLS)

Abstract

 This document defines how Datagram Transport Layer Security (DTLS),
 Real-time Transport Protocol (RTP), RTP Control Protocol (RTCP),
 Session Traversal Utilities for NAT (STUN), Traversal Using Relays
 around NAT (TURN), and ZRTP packets are multiplexed on a single
 receiving socket.  It overrides the guidance from RFC 5764 ("SRTP
 Extension for DTLS"), which suffered from four issues described and
 fixed in this document.
 This document updates RFC 5764.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 7841.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7983.

Petit-Huguenin & Salgueiro Standards Track [Page 1] RFC 7983 Multiplexing Scheme Updates for RFC 5764 September 2016

Copyright Notice

 Copyright (c) 2016 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.
 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
 2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
 3.  Implicit Allocation of Codepoints for New STUN Methods  . . .   4
 4.  Multiplexing of ZRTP  . . . . . . . . . . . . . . . . . . . .   5
 5.  Implicit Allocation of New Codepoints for TLS ContentTypes  .   5
 6.  Multiplexing of TURN Channels . . . . . . . . . . . . . . . .   7
 7.  Updates to RFC 5764 . . . . . . . . . . . . . . . . . . . . .   8
 8.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
 9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   9.1.  STUN Methods  . . . . . . . . . . . . . . . . . . . . . .  10
   9.2.  TLS ContentType . . . . . . . . . . . . . . . . . . . . .  10
   9.3.  Traversal Using Relays around NAT (TURN) Channel Numbers   11
 10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
   10.1.  Normative References . . . . . . . . . . . . . . . . . .  11
   10.2.  Informative References . . . . . . . . . . . . . . . . .  12
 Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  13
 Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

Petit-Huguenin & Salgueiro Standards Track [Page 2] RFC 7983 Multiplexing Scheme Updates for RFC 5764 September 2016

1. Introduction

 Section 5.1.2 of "Datagram Transport Layer Security (DTLS) Extension
 to Establish Keys for the Secure Real-time Transport Protocol (SRTP)"
 [RFC5764] defines a scheme for a Real-time Transport Protocol (RTP)
 [RFC3550] receiver to demultiplex DTLS [RFC6347], Session Traversal
 Utilities for NAT (STUN) [RFC5389], and Secure Real-time Transport
 Protocol (SRTP) / Secure Real-time Transport Control Protocol (SRTCP)
 [RFC3711] packets that are arriving on the RTP port.  Unfortunately,
 this demultiplexing scheme has created problematic issues:
 1.  It implicitly allocated codepoints for new STUN methods without
     an IANA registry reflecting these new allocations.
 2.  It did not take into account the fact that ZRTP [RFC6189] also
     needs to be demultiplexed with the other packet types explicitly
     mentioned in Section 5.1.2 of RFC 5764.
 3.  It implicitly allocated codepoints for new Transport Layer
     Security (TLS) ContentTypes without an IANA registry reflecting
     these new allocations.
 4.  It did not take into account the fact that the Traversal Using
     Relays around NAT (TURN) usage of STUN can create TURN channels
     that also need to be demultiplexed with the other packet types
     explicitly mentioned in Section 5.1.2 of RFC 5764.
 Having overlapping ranges between different IANA registries becomes
 an issue when a new codepoint is allocated in one of these registries
 without carefully analyzing the impact it could have on the other
 registries when that codepoint is demultiplexed.  Among other
 downsides of the bad design of the demultiplexing algorithm detailed
 in [RFC5764], it creates a requirement for coordination between
 codepoint assignments where none should exist, and that is
 organizationally and socially undesirable.  However, RFC 5764 has
 been widely deployed, so there must be an awareness of this issue and
 how it must be dealt with.  Thus, even if the feature related to a
 codepoint is not initially thought to be useful in the context of
 demultiplexing, the respective IANA registry expert should at least
 raise a flag when the allocated codepoint irrevocably prevents
 multiplexing.

Petit-Huguenin & Salgueiro Standards Track [Page 3] RFC 7983 Multiplexing Scheme Updates for RFC 5764 September 2016

 The first goal of this document is to make sure that future
 allocations in any of the affected protocols are done with the full
 knowledge of their impact on multiplexing.  This is achieved by
 updating [RFC5764], which includes modifying the IANA registries with
 instructions for coordination between the protocols at risk.
 A second goal is to permit the addition of new protocols to the list
 of existing multiplexed protocols in a manner that does not break
 existing implementations.
 At the time of this writing, the flaws in the demultiplexing scheme
 were unavoidably inherited by other documents, such as [RFC7345] and
 [SDP-BUNDLE].  So in addition, these and any other affected documents
 will need to be corrected with the updates this document provides.

2. Terminology

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in [RFC2119].

3. Implicit Allocation of Codepoints for New STUN Methods

 The demultiplexing scheme in [RFC5764] states that the receiver can
 identify the packet type by looking at the first byte.  If the value
 of this first byte is 0 or 1, the packet is identified to be STUN.
 The problem with this implicit allocation is that it restricts the
 codepoints for STUN methods (as described in Section 18.1 of
 [RFC5389]) to values between 0x000 and 0x07F, which in turn reduces
 the number of possible STUN method codepoints assigned by IETF Review
 (i.e., the range 0x000 - 0x7FF) from 2048 to only 128 and eliminates
 the possibility of having STUN method codepoints assigned by
 Designated Expert (i.e., the range 0x800 - 0xFFF).
 To preserve the Designated Expert range, this document allocates the
 values 2 and 3 to also identify STUN methods.
 The IANA Registry for STUN methods has been modified to mark the
 codepoints from 0x100 to 0xFFF as Reserved.  These codepoints can
 still be allocated, but require IETF Review with a document that will
 properly evaluate the risk of an assignment overlapping with other
 registries.

Petit-Huguenin & Salgueiro Standards Track [Page 4] RFC 7983 Multiplexing Scheme Updates for RFC 5764 September 2016

 In addition, this document also updates the IANA registry such that
 the STUN method codepoints assigned in the 0x080-0x0FF range are also
 assigned via Designated Expert.  The "STUN Methods" registry has been
 changed as follows:
 OLD:
 0x000-0x7FF     IETF Review
 0x800-0xFFF     Designated Expert
 NEW:
 0x000-0x07F     IETF Review
 0x080-0x0FF     Designated Expert
 0x100-0xFFF     Reserved

4. Multiplexing of ZRTP

 ZRTP [RFC6189] is a protocol for media path Diffie-Hellman exchange
 to agree on a session key and parameters for establishing unicast
 SRTP sessions for Voice over IP (VoIP) applications.  The ZRTP
 protocol is media path keying because it is multiplexed on the same
 port as RTP and does not require support in the signaling protocol.
 In order to prevent future documents from assigning values from the
 unused range to a new protocol, this document modifies the [RFC5764]
 demultiplexing algorithm to properly account for ZRTP [RFC6189] by
 allocating the values from 16 to 19 for this purpose.

5. Implicit Allocation of New Codepoints for TLS ContentTypes

 The demultiplexing scheme in [RFC5764] dictates that if the value of
 the first byte is between 20 and 63 (inclusive), then the packet is
 identified to be DTLS.  For DTLS 1.0 [RFC4347] and DTLS 1.2
 [RFC6347], that first byte corresponds to the TLS ContentType field.
 Considerations must be taken into account when assigning additional
 ContentTypes in the codepoint ranges 0 to 19 and 64 to 255, so this
 does not prevent demultiplexing when this functionality is desirable.
 Note that [RFC5764] describes a narrow use of DTLS that works as long
 as the specific DTLS version used abides by the restrictions on the
 demultiplexing byte (the ones that this document imposes on the "TLS
 ContentType Registry").  Any extension or revision to DTLS that
 causes it to no longer meet these constraints should consider what
 values may occur in the first byte of the DTLS message and what
 impact it would have on the multiplexing that [RFC5764] describes.

Petit-Huguenin & Salgueiro Standards Track [Page 5] RFC 7983 Multiplexing Scheme Updates for RFC 5764 September 2016

 With respect to TLS packet identification, this document explicitly
 adds a warning to the codepoints from 0 to 19 and from 64 to 255
 indicating that allocations in these ranges require coordination, as
 described in this document.  The "TLS ContentType Registry" has been
 changed as follows:
 OLD:
 0-19    Unassigned
 20      change_cipher_spec
 21      alert
 22      handshake
 23      application_data
 24      heartbeat
 25-255  Unassigned
 NEW:
 0-19    Unassigned (Requires coordination; see RFC 7983)
 20      change_cipher_spec
 21      alert
 22      handshake
 23      application_data
 24      heartbeat
 25-63   Unassigned
 64-255  Unassigned (Requires coordination; see RFC 7983)

Petit-Huguenin & Salgueiro Standards Track [Page 6] RFC 7983 Multiplexing Scheme Updates for RFC 5764 September 2016

6. Multiplexing of TURN Channels

 When used with Interactive Connectivity Establishment (ICE)
 [RFC5245], an implementation of RFC 5764 can receive packets on the
 same socket from three different paths, as shown in Figure 1:
 1.  Directly from the source
 2.  Through a NAT
 3.  Relayed by a TURN server
     +------+
     | TURN |<------------------------+
     +------+                         |
        |                             |
        | +-------------------------+ |
        | |                         | |
        v v                         | |
 NAT -----------                    | |
        | | +---------------------+ | |
        | | |                     | | |
        v v v                     | | |
    +----------+              +----------+
    | RFC 5764 |              | RFC 5764 |
    +----------+              +----------+
      Figure 1: Packet Reception by an Implementation of RFC 5764
 Even if the ICE algorithm succeeded in selecting a non-relayed path,
 it is still possible to receive data from the TURN server.  For
 instance, when ICE is used with aggressive nomination, the media path
 can quickly change until it stabilizes.  Also, freeing ICE candidates
 is optional, so the TURN server can restart forwarding STUN
 connectivity checks during an ICE restart.
 TURN channels are an optimization where data packets are exchanged
 with a 4-byte prefix instead of the standard 36-byte STUN overhead
 (see Section 2.5 of [RFC5766]).  The problem is that the RFC 5764
 demultiplexing scheme does not define what to do with packets
 received over a TURN channel since these packets will start with a
 first byte whose value will be between 64 and 127 (inclusive).  If
 the TURN server was instructed to send data over a TURN channel, then
 the demultiplexing scheme specified in RFC 5764 will reject these
 packets.  Current implementations violate RFC 5764 for values 64 to
 127 (inclusive) and they instead parse packets with such values as
 TURN.

Petit-Huguenin & Salgueiro Standards Track [Page 7] RFC 7983 Multiplexing Scheme Updates for RFC 5764 September 2016

 In order to prevent future documents from assigning values from the
 unused range to a new protocol, this document modifies the
 demultiplexing algorithm in RFC 5764 to properly account for TURN
 channels by allocating the values from 64 to 79 for this purpose.
 This modification restricts the TURN channel space to a more limited
 set of possible channels when the TURN client does the channel
 binding request in combination with the demultiplexing scheme
 described in [RFC5764].

7. Updates to RFC 5764

 This document updates the text in Section 5.1.2 of [RFC5764] as
 follows:
 OLD TEXT
 The process for demultiplexing a packet is as follows.  The receiver
 looks at the first byte of the packet.  If the value of this byte is
 0 or 1, then the packet is STUN.  If the value is in between 128 and
 191 (inclusive), then the packet is RTP (or RTCP, if both RTCP and
 RTP are being multiplexed over the same destination port).  If the
 value is between 20 and 63 (inclusive), the packet is DTLS.  This
 process is summarized in Figure 3.
                  +----------------+
                  | 127 < B < 192 -+--> forward to RTP
                  |                |
      packet -->  |  19 < B < 64  -+--> forward to DTLS
                  |                |
                  |       B < 2   -+--> forward to STUN
                  +----------------+
   Figure 3: The DTLS-SRTP receiver's packet demultiplexing algorithm.
        Here the field B denotes the leading byte of the packet.
 END OLD TEXT

Petit-Huguenin & Salgueiro Standards Track [Page 8] RFC 7983 Multiplexing Scheme Updates for RFC 5764 September 2016

 NEW TEXT
 The process for demultiplexing a packet is as follows.  The receiver
 looks at the first byte of the packet.  If the value of this byte is
 in between 0 and 3 (inclusive), then the packet is STUN.  If the
 value is between 16 and 19 (inclusive), then the packet is ZRTP.  If
 the value is between 20 and 63 (inclusive), then the packet is DTLS.
 If the value is between 64 and 79 (inclusive), then the packet is
 TURN Channel.  If the value is in between 128 and 191 (inclusive),
 then the packet is RTP (or RTCP, if both RTCP and RTP are being
 multiplexed over the same destination port).  If the value does not
 match any known range, then the packet MUST be dropped and an alert
 MAY be logged.  This process is summarized in Figure 3.
                  +----------------+
                  |        [0..3] -+--> forward to STUN
                  |                |
                  |      [16..19] -+--> forward to ZRTP
                  |                |
      packet -->  |      [20..63] -+--> forward to DTLS
                  |                |
                  |      [64..79] -+--> forward to TURN Channel
                  |                |
                  |    [128..191] -+--> forward to RTP/RTCP
                  +----------------+
   Figure 3: The DTLS-SRTP receiver's packet demultiplexing algorithm.
 END NEW TEXT

8. Security Considerations

 This document updates existing IANA registries and adds a new range
 for TURN channels in the demultiplexing algorithm.
 These modifications do not introduce any specific security
 considerations beyond those detailed in [RFC5764].

Petit-Huguenin & Salgueiro Standards Track [Page 9] RFC 7983 Multiplexing Scheme Updates for RFC 5764 September 2016

9. IANA Considerations

9.1. STUN Methods

 This specification contains the registration information for reserved
 STUN Methods codepoints, as explained in Section 3 and in accordance
 with the procedures defined in Section 18.1 of [RFC5389].
 Value:   0x100-0xFFF
 Name:   Reserved (For DTLS-SRTP multiplexing collision avoidance, see
    RFC 7983.  Cannot be made available for assignment without IETF
    Review.)
 Reference:   RFC 5764, RFC 7983
 This specification also reassigns the ranges in the STUN Methods
 Registry as follows:
 Range:   0x000-0x07F
 Registration Procedures:   IETF Review
 Range:   0x080-0x0FF
 Registration Procedures:   Designated Expert

9.2. TLS ContentType

 This specification contains the registration information for reserved
 TLS ContentType codepoints, as explained in Section 5 and in
 accordance with the procedures defined in Section 12 of [RFC5246].
 Value:   0-19
 Description:   Unassigned (Requires coordination; see RFC 7983)
 DTLS-OK:   N/A
 Reference:   RFC 5764, RFC 7983
 Value:   64-255
 Description:   Unassigned (Requires coordination; see RFC 7983)
 DTLS-OK:   N/A
 Reference:   RFC 5764, RFC 7983

Petit-Huguenin & Salgueiro Standards Track [Page 10] RFC 7983 Multiplexing Scheme Updates for RFC 5764 September 2016

9.3. Traversal Using Relays around NAT (TURN) Channel Numbers

 This specification contains the registration information for reserved
 codepoints in the "Traversal Using Relays around NAT (TURN) Channel
 Numbers" registry, as explained in Section 6 and in accordance with
 the procedures defined in Section 18 of [RFC5766].
 Value:   0x5000-0xFFFF
 Name:   Reserved (For DTLS-SRTP multiplexing collision avoidance, see
    RFC 7983.)
 Reference:   RFC 7983

10. References

10.1. Normative References

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119,
            DOI 10.17487/RFC2119, March 1997,
            <http://www.rfc-editor.org/info/rfc2119>.
 [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
            Jacobson, "RTP: A Transport Protocol for Real-Time
            Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
            July 2003, <http://www.rfc-editor.org/info/rfc3550>.
 [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>.
 [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>.
 [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
            (TLS) Protocol Version 1.2", RFC 5246,
            DOI 10.17487/RFC5246, August 2008,
            <http://www.rfc-editor.org/info/rfc5246>.
 [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>.

Petit-Huguenin & Salgueiro Standards Track [Page 11] RFC 7983 Multiplexing Scheme Updates for RFC 5764 September 2016

 [RFC5764]  McGrew, D. and E. Rescorla, "Datagram Transport Layer
            Security (DTLS) Extension to Establish Keys for the Secure
            Real-time Transport Protocol (SRTP)", RFC 5764,
            DOI 10.17487/RFC5764, May 2010,
            <http://www.rfc-editor.org/info/rfc5764>.
 [RFC5766]  Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
            Relays around NAT (TURN): Relay Extensions to Session
            Traversal Utilities for NAT (STUN)", RFC 5766,
            DOI 10.17487/RFC5766, April 2010,
            <http://www.rfc-editor.org/info/rfc5766>.
 [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>.

10.2. Informative References

 [RFC4347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
            Security", RFC 4347, DOI 10.17487/RFC4347, April 2006,
            <http://www.rfc-editor.org/info/rfc4347>.
 [RFC6189]  Zimmermann, P., Johnston, A., Ed., and J. Callas, "ZRTP:
            Media Path Key Agreement for Unicast Secure RTP",
            RFC 6189, DOI 10.17487/RFC6189, April 2011,
            <http://www.rfc-editor.org/info/rfc6189>.
 [RFC7345]  Holmberg, C., Sedlacek, I., and G. Salgueiro, "UDP
            Transport Layer (UDPTL) over Datagram Transport Layer
            Security (DTLS)", RFC 7345, DOI 10.17487/RFC7345, August
            2014, <http://www.rfc-editor.org/info/rfc7345>.
 [SDP-BUNDLE]
            Holmberg, C., Alvestrand, H., and C. Jennings,
            "Negotiating Media Multiplexing Using the Session
            Description Protocol (SDP)", Work in Progress,
            draft-ietf-mmusic-sdp-bundle-negotiation-32, August 2016.

Petit-Huguenin & Salgueiro Standards Track [Page 12] RFC 7983 Multiplexing Scheme Updates for RFC 5764 September 2016

Acknowledgements

 The implicit STUN Method codepoint allocations problem was first
 reported by Martin Thomson in the RTCWEB mailing list and discussed
 further with Magnus Westerlund.
 Thanks to Simon Perreault, Colton Shields, Cullen Jennings, Colin
 Perkins, Magnus Westerlund, Paul Jones, Jonathan Lennox, Varun Singh,
 Justin Uberti, Joseph Salowey, Martin Thomson, Ben Campbell, Stephen
 Farrell, Alan Johnston, Mehmet Ersue, Matt Miller, Spencer Dawkins,
 Joel Halpern, and Paul Kyzivat for the comments, suggestions, and
 questions that helped improve this document.

Authors' Addresses

 Marc Petit-Huguenin
 Impedance Mismatch
 Email: marc@petit-huguenin.org
 Gonzalo Salgueiro
 Cisco Systems
 7200-12 Kit Creek Road
 Research Triangle Park, NC  27709
 United States of America
 Email: gsalguei@cisco.com

Petit-Huguenin & Salgueiro Standards Track [Page 13]

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