SRTP-DTLS Multiplexing: Multiplexing Scheme Updates for Secure Real-time Transport Protocol (SRTP) Extension for Datagram Transport Layer Security (DTLS)
How one UDP port can carry STUN, ZRTP, DTLS, TURN channels and RTP by looking at the first byte.
Petit-Huguenin, M. and G. Salgueiro, "Multiplexing Scheme Updates for Secure Real-time Transport Protocol (SRTP) Extension for Datagram Transport Layer Security (DTLS)", RFC 7983, DOI 10.17487/RFC7983, September 2016, <https://www.rfc-editor.org/info/rfc7983>.
@misc{rfc7983,
series = {Request for Comments},
number = 7983,
howpublished = {RFC 7983},
publisher = {RFC Editor},
doi = {10.17487/RFC7983},
url = {https://www.rfc-editor.org/info/rfc7983},
author = {M. Petit-Huguenin and G. Salgueiro},
title = {{Multiplexing Scheme Updates for Secure Real-time Transport Protocol (SRTP) Extension for Datagram Transport Layer Security (DTLS)}},
pagetotal = 13,
year = 2016,
month = sep,
} RFC text
Contents
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.
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
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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
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.
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.
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.
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)
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.
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
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].
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
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>.
[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.
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
ISO Plain Language
On this page
§1Why the Original Scheme Had to Be Fixed
Section 5.1.2 of RFC 5764 defines a scheme for a Real-time Transport Protocol (RTP) receiver to demultiplex DTLS, STUN and SRTP/SRTCP packets that arrive on the RTP port, so that one port can carry the encrypted media and the key exchange that sets it up. The scheme works by looking at the first byte of each packet. Unfortunately, it created four problems.
First, it implicitly allocated codepoints for new STUN methods without an IANA registry reflecting those allocations. Second, it did not take into account the fact that ZRTP also needs to be demultiplexed with the other packet types explicitly mentioned in Section 5.1.2 of RFC 5764. Third, it implicitly allocated codepoints for new TLS ContentTypes without an IANA registry reflecting these allocations. Fourth, it did not take into account the fact that TURN's use of STUN can create TURN channels that also need to be demultiplexed.
Overlapping ranges between different IANA registries become an issue when a new codepoint is allocated in one of these registries without carefully analysing 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, it creates a requirement for coordination between codepoint assignments where none should exist, and the RFC calls that organizationally and socially undesirable. Because RFC 5764 has been widely deployed, there has to be awareness of this issue and of how it must be dealt with. So even if the feature related to a codepoint is not initially thought to be useful in the context of demultiplexing, the relevant IANA registry expert should at least raise a flag when the allocated codepoint irrevocably prevents multiplexing.
The RFC states two goals. The first is to make sure that future allocations in any of the affected protocols are done with the full knowledge of their impact on multiplexing, achieved by updating RFC 5764 and modifying the IANA registries with instructions for coordination between the protocols at risk. The second is to permit the addition of new protocols to the list of multiplexed protocols in a manner that does not break existing implementations. At the time of writing, the flaws in the demultiplexing scheme had been unavoidably inherited by other documents, such as RFC 7345 and SDP-BUNDLE, so those and any other affected documents need to be corrected with the updates this document provides.
The original rules took values out of the numbering systems of other protocols without saying so, so two unrelated protocols could end up with the same number and be confused on the wire. This RFC sorts out the ranges and tells the registries to watch out for each other from now on.
§3STUN Method Codepoints
The demultiplexing scheme in RFC 5764 says the receiver can identify a STUN packet by looking at the first byte: if the value of this byte is 0 or 1, the packet is STUN. The problem with this implicit allocation is that it restricts the codepoints for STUN methods to values between 0x000 and 0x07F. That in turn reduces the number of possible STUN method codepoints assigned by IETF Review, the range 0x000 to 0x7FF, from 2048 to only 128, and eliminates the possibility of having STUN method codepoints assigned by Designated Expert, the range 0x800 to 0xFFF.
To preserve the Designated Expert range, the RFC allocates the values 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. Those codepoints can still be allocated, but they require IETF Review with a document that will properly evaluate the risk of an assignment overlapping with other registries.
The registry itself is rearranged from two ranges into three. It used to be 0x000-0x7FF by IETF Review and 0x800-0xFFF by Designated Expert. It becomes 0x000-0x07F by IETF Review, 0x080-0x0FF by Designated Expert, and 0x100-0xFFF Reserved.
STUN messages were quietly borrowing the first two numbers of the method numbering space, which left only a fraction of it for real methods. The RFC gives STUN four numbers of its own and reserves the rest for the multiplexing scheme.
| Range | Old policy | New policy |
|---|---|---|
| 0x000-0x07F | IETF Review | IETF Review |
| 0x080-0x0FF | IETF Review | Designated Expert |
| 0x100-0x7FF | IETF Review | Reserved |
| 0x800-0xFFF | Designated Expert | Reserved |
§4Adding ZRTP
ZRTP is a protocol for a media path Diffie-Hellman exchange to agree on a session key and parameters for establishing unicast SRTP sessions for Voice over IP applications. ZRTP is media path keying, because it is multiplexed on the same port as RTP and does not require support in the signalling protocol.
In order to prevent future documents from assigning values from the unused range to a new protocol, the RFC modifies the RFC 5764 demultiplexing algorithm to properly account for ZRTP by allocating the values from 16 to 19 for this purpose.
ZRTP is a second way to agree on the keys for encrypted media, done on the media path itself rather than in the signalling. It needed a range of its own, and this RFC gives it four numbers.
§5TLS ContentTypes
The RFC 5764 scheme dictates that if the value of the first byte is between 20 and 63 inclusive, the packet is DTLS. For DTLS 1.0 and DTLS 1.2, 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 that functionality is desirable.
The RFC notes that RFC 5764 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 RFC 5764 describes.
The registry gains an explicit warning. The codepoints 0 to 19 and 64 to 255 are marked as unassigned and requiring coordination, and the middle of the old range is split out: 25 to 63 becomes plain unassigned, while what used to be 25 to 255 becomes 64 to 255 requiring coordination.
For encrypted media, the first byte of a packet is the type of the TLS record, and the values from 20 to 63 already belong to that meaning. The RFC marks the values on either side of that range as needing a conversation with the multiplexing rules before anyone uses them.
§6TURN Channels
When used with Interactive Connectivity Establishment (ICE), an implementation of RFC 5764 can receive packets on the same socket from three different paths: directly from the source, through a NAT, and relayed by a TURN server. Even if the ICE algorithm succeeded in selecting a non-relayed path, it is still possible to receive data from the TURN server. When ICE is used with aggressive nomination, the media path can quickly change until it stabilizes, and 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. The problem is that the RFC 5764 demultiplexing scheme does not define what to do with packets received over a TURN channel, because these packets start with a first byte whose value is between 64 and 127 inclusive. If the TURN server was instructed to send data over a TURN channel, the demultiplexing scheme specified in RFC 5764 will reject these packets. Current implementations violate RFC 5764 for values 64 to 127 and instead parse packets with such values as TURN.
To prevent future documents from assigning values from the unused range to a new protocol, the RFC modifies the demultiplexing algorithm 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 RFC 5764.
When traffic is relayed, the relay can wrap each packet in a short label instead of the usual heavy STUN header. Those labelled packets had no home in the old rules, so the RFC gives them a range of sixteen numbers.
§7The New Demultiplexing Algorithm
Section 7 replaces the text in Section 5.1.2 of RFC 5764. The receiver looks at the first byte of the packet. If the value of this byte is between 0 and 3 inclusive, the packet is STUN. If the value is between 16 and 19 inclusive, the packet is ZRTP. If the value is between 20 and 63 inclusive, the packet is DTLS. If the value is between 64 and 79 inclusive, the packet is a TURN Channel. If the value is between 128 and 191 inclusive, 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, the packet MUST be dropped and an alert MAY be logged.
The old text had only three ranges: 0 or 1 for STUN, 20 to 63 for DTLS, and 128 to 191 for RTP. Everything else was undefined, and the new text also says what to do with a packet that matches nothing.
The rule is now a single lookup table on the first byte of the packet, with a clear answer for every value, including an instruction to throw away anything unrecognised.
| First byte | Protocol | Note |
|---|---|---|
| 0 to 3 | STUN | Values 2 and 3 added by this RFC |
| 16 to 19 | ZRTP | Allocated by this RFC |
| 20 to 63 | DTLS | Unchanged from RFC 5764 |
| 64 to 79 | TURN Channel | Allocated by this RFC |
| 128 to 191 THIS RFC | RTP or RTCP | Unchanged from RFC 5764 |
| Anything else | Dropped | An alert may be logged |
+----------------+
| [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
+----------------+§8Security Considerations
The RFC is short here. The document updates existing IANA registries and adds a new range for TURN channels in the demultiplexing algorithm, and these modifications do not introduce any specific security considerations beyond those detailed in RFC 5764.
Changing which numbers mean what does not add any new risk here. The security rules of the document it updates still apply.
§9IANA Considerations
Three registries are touched. The STUN Methods registry gains a registration for the reserved codepoints 0x100 to 0xFFF, named as reserved for DTLS-SRTP multiplexing collision avoidance, which cannot be made available for assignment without IETF Review, and its ranges are reassigned as described earlier: 0x000-0x07F by IETF Review and 0x080-0x0FF by Designated Expert.
The TLS ContentType registry gains two reserved entries: the values 0 to 19 and the values 64 to 255, each described as unassigned and requiring coordination, with the DTLS-OK field recorded as N/A for both.
The TURN Channel Numbers registry gains a reservation of 0x5000 to 0xFFFF, named as reserved for DTLS-SRTP multiplexing collision avoidance.
IANA keeps three lists of numbers that this scheme depends on, and each one now carries a note saying which parts are off limits and why.