AVTCORE M. Petit-Huguenin
Internet-Draft Impedance Mismatch
Updates: 5764 (if approved) G. Salgueiro
Intended status: Standards Track Cisco Systems
Expires: September 25, 2015 March 24, 2015

Multiplexing Scheme Updates for Secure Real-time Transport Protocol (SRTP) Extension for Datagram Transport Layer Security (DTLS)
draft-ietf-avtcore-rfc5764-mux-fixes-02

Abstract

This document defines how Datagram Transport Layer Security (DTLS), Real-time Transport Protocol (RTP), Real-time Transport Control Protocol (RTCP), Session Traversal Utilities for NAT (STUN), and Traversal Using Relays around NAT (TURN) packets are multiplexed on a single receiving socket. It overrides the guidance from SRTP Extension for DTLS [RFC5764], which suffered from three issues described and fixed in this document.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on September 25, 2015.

Copyright Notice

Copyright (c) 2015 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.


Table of Contents

1. Introduction

Section 5.1.2 of Secure Real-time Transport Protocol (SRTP) Extension for DTLS [RFC5764] defines a scheme for a Real-time Transport Protocol (RTP) [RFC3550] receiver to demultiplex Datagram Transport Layer Security (DTLS) [RFC6347], Session Traversal Utilities for NAT (STUN) [I-D.ietf-tram-stunbis] 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 implicitly allocated codepoints for new Transport Layer Security (TLS) ContentTypes without an IANA registry reflecting these new allocations.
  3. 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.
  4. The current ranges are not efficiently allocated making it harder to introduce new protocols that might require multiplexing.

These flaws in the demultiplexing scheme were unavoidably inherited by other documents, such as [RFC7345] and [I-D.ietf-mmusic-sdp-bundle-negotiation]. These will need to be corrected with the updates this document provides.

1.1. 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 that arises as a result of this implicit allocation is that this 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 from (0x000 - 0x7FF) from 2048 to only 128 and entirely obliterating those STUN method codepoints assigned by Designated Expert (i.e., the range 0x800 - 0xFFF).

To preserve the Designated Expert range, this document allocates the value 2 and 3 to also identify STUN methods.

The IANA Registry for STUN methods is 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.

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 proposed changes to the STUN Method Registry are:

OLD:

0x000-0x7FF     IETF Review
0x800-0xFFF     Designated Expert

NEW:

0x000-0x07F     IETF Review
0x080-0x0FF     Designated Expert
0x100-0xFFF     Reserved

1.2. 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. The problem that arises is that this restricts the TLS ContentType codepoints (as defined in Section 12 of [RFC5246]) to this range, and by extension implicitly allocates ContentType codepoints 0 to 19 and 64 to 255. Unlike STUN, TLS is a mature protocol that is already well established and widely implemented and thus we expect only relatively few new codepoints to be assigned in the future. With respect to TLS packet identification, this document simply explicitly reserves the codepoints from 0 to 19 and from 64 to 255. These codepoints can still be allocated, but require Standards Action with a document that will properly evaluate the risk of an assignment overlapping with other registries. The proposed changes to the TLS ContentTypes Registry are:

OLD:

0-19    Unassigned
20      change_cipher_spec
21      alert
22      handshake
23      application_data
24      heartbeat
25-255  Unassigned

NEW:

0-19    Reserved (MUST be allocated with Standards Action)
20      change_cipher_spec
21      alert
22      handshake
23      application_data
24      heartbeat
25-63   Unassigned
64-255  Reserved (MUST be allocated with Standards Action)

1.3. Multiplexing of TURN Channels

    +------+
    | TURN |<------------------------+
    +------+                         |
       |                             |
       | +-------------------------+ |
       | |                         | |
       v v                         | |
NAT -----------                    | |
       | | +---------------------+ | |
       | | |                     | | |
       v v v                     | | |
   +----------+              +----------+
   | RFC 5764 |              | RFC 5764 |
   +----------+              +----------+

Figure 1: Packet Reception by an RFC 5764 Implementation

When used with ICE [RFC5245], an RFC 5764 implementation 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 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 current RFC 5764 demultiplexing scheme 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.

In order to prevent future documents from assigning values from the unused range to a new protocol, this document modifies the RFC 5764 demultiplexing algorithm to properly account for TURN channels by allocating the values from 64 to 79 for this purpose.

An implementation that uses the source IP address and port to identify TURN channel messages MAY not need to restrict the channel numbers to the above range.

1.4. Demultiplexing Algorithm Test Order

This document also changes the demultiplexing algorithm by imposing the order in which the first byte is tested against the list of existing protocol ranges. This is done in order to ensure that all implementations fail identically in the presence of a new range.

2. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119] when they appear in ALL CAPS. When these words are not in ALL CAPS (such as "must" or "Must"), they have their usual English meanings, and are not to be interpreted as RFC 2119 key words.

3. RFC 5764 Updates

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

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. Then if the value is between 20 and 63 (inclusive), the packet is DTLS. Then if the value is between 64 and 79 (inclusive), the packet is TURN Channel. Then 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). Else 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. When new values or ranges are added, they MUST be tested in ascending order.

                   +----------------+
                   |        [0..3] -+--> forward to STUN
                   |                |
       packet -->  |      [20..63] -+--> forward to DTLS
                   |                |
                   |      [64..79] -+--> forward to TURN Channel
                   |                |
                   |    [128..191] -+--> forward to RTP
                   +----------------+

    Figure 3: The DTLS-SRTP receiver's packet demultiplexing algorithm.

END NEW TEXT

4. Implementation Status

[[Note to RFC Editor: Please remove this section and the reference to [RFC6982] before publication.]]

This section records the status of known implementations of the protocol defined by this specification at the time of posting of this Internet-Draft, and is based on a proposal described in [RFC6982]. The description of implementations in this section is intended to assist the IETF in its decision processes in progressing drafts to RFCs. Please note that the listing of any individual implementation here does not imply endorsement by the IETF. Furthermore, no effort has been spent to verify the information presented here that was supplied by IETF contributors. This is not intended as, and must not be construed to be, a catalog of available implementations or their features. Readers are advised to note that other implementations may exist.

According to [RFC6982], "this will allow reviewers and working groups to assign due consideration to documents that have the benefit of running code, which may serve as evidence of valuable experimentation and feedback that have made the implemented protocols more mature. It is up to the individual working groups to use this information as they see fit".

Note that there is currently no implementation declared in this section, but the intent is to add RFC 6982 templates here from implementers that support the modifications in this document.

5. Security Considerations

This document simply updates existing IANA registries and does not introduce any specific security considerations beyond those detailed in [RFC5764].

6. IANA Considerations

6.1. STUN Methods

This specification contains the registration information for reserved STUN Methods codepoints, as explained in Section 1.1 and in accordance with the procedures defined in Section 18.1 of [RFC5389].

Value:
0x100-0xFFF
Name:
Reserved (MUST be allocated with IETF Review)
Reference:
RFC5764, RFCXXXX

This specification also reassigns the ranges in the STUN Methods Registry as follow:

Range:
0x000-0x07F
Registration Procedures:
IETF Review

Range:
0x080-0x0FF
Registration Procedures:
Designated Expert

6.2. TLS ContentType

This specification contains the registration information for reserved TLS ContentType codepoints, as explained in Section 1.2 and in accordance with the procedures defined in Section 12 of [RFC5246].

Value:
0-19
Description:
Reserved (MUST be allocated with Standards Action)
DTLS-OK:
N/A
Reference:
RFC5764, RFCXXXX

Value:
64-255
Description:
Reserved (MUST be allocated with Standards Action)
DTLS-OK:
N/A
Reference:
RFC5764, RFCXXXX

6.3. TURN Channel Numbers

This specification contains the registration information for reserved TURN Channel Numbers codepoints, as explained in Section 1.3 and in accordance with the procedures defined in Section 18 of [RFC5766].

Value:
0x5000-0xFFFF
Name:
Reserved
Reference:
RFCXXXX

[RFC EDITOR NOTE: Please replace RFCXXXX with the RFC number of this document.]

7. 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 and Justin Uberti for the comments, suggestions, and questions that helped improve this document.

8. References

8.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, July 2003.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E. and K. Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC 3711, March 2004.
[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal for Offer/Answer Protocols", RFC 5245, April 2010.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5389] Rosenberg, J., Mahy, R., Matthews, P. and D. Wing, "Session Traversal Utilities for NAT (STUN)", RFC 5389, October 2008.
[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, May 2010.
[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, April 2010.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security Version 1.2", RFC 6347, January 2012.

8.2. Informative References

[RFC6982] Sheffer, Y. and A. Farrel, "Improving Awareness of Running Code: The Implementation Status Section", RFC 6982, July 2013.
[RFC7345] Holmberg, C., Sedlacek, I. and G. Salgueiro, "UDP Transport Layer (UDPTL) over Datagram Transport Layer Security (DTLS)", RFC 7345, August 2014.
[I-D.ietf-mmusic-sdp-bundle-negotiation] Holmberg, C., Alvestrand, H. and C. Jennings, "Negotiating Media Multiplexing Using the Session Description Protocol (SDP)", Internet-Draft draft-ietf-mmusic-sdp-bundle-negotiation-18, March 2015.
[I-D.ietf-tram-stunbis] Petit-Huguenin, M., Salgueiro, G., Rosenberg, J., Wing, D., Mahy, R. and P. Matthews, "Session Traversal Utilities for NAT (STUN)", Internet-Draft draft-ietf-tram-stunbis-02, March 2015.

Appendix A. Release notes

This section must be removed before publication as an RFC.

A.1. Modifications between draft-ietf-avtcore-rfc5764-mux-fixes-01 and draft-ietf-avtcore-rfc5764-mux-fixes-00

A.2. Modifications between draft-ietf-avtcore-rfc5764-mux-fixes-01 and draft-ietf-avtcore-rfc5764-mux-fixes-00

A.3. Modifications between draft-ietf-avtcore-rfc5764-mux-fixes-00 and draft-petithuguenin-avtcore-rfc5764-mux-fixes-02

A.4. Modifications between draft-petithuguenin-avtcore-rfc5764-mux-fixes-00 and draft-petithuguenin-avtcore-rfc5764-mux-fixes-01

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 US EMail: gsalguei@cisco.com