draft-tuexen-tsvwg-rfc6083-bis.xml

<?xml version='1.0' encoding='utf-8'?>

<rfc xmlns:xi="http://www.w3.org/2001/XInclude"
     xml:lang="en"
     ipr="pre5378Trust200902"
     submissionType="IETF"
     consensus="true"
     category="std"
     docName="draft-tuexen-tsvwg-rfc6083-bis-07-to-be"
     obsoletes="6083"
     version="3"
     tocDepth="2">

<front>
<title abbrev="DTLS 1.3 for SCTP">
       Datagram Transport Layer Security (DTLS) 1.3
       for Stream Control Transmission Protocol (SCTP)
</title>
<seriesInfo name="Internet-Draft" value="draft-tuexen-tsvwg-rfc6083-bis-07-to-be"/>

<!-- ************** MICHAEL TUEXEN *************** -->
<author initials="M." surname="Tüxen" fullname="Michael Tüxen">
<organization abbrev="Münster Univ. of Applied Sciences">
              Münster University of Applied Sciences</organization>
<address>
    <postal>
        <street>Stegerwaldstr. 39</street>
        <city>48565 Steinfurt</city>
        <country>Germany</country>
    </postal>
    <email>tuexen@fh-muenster.de</email>
</address>
</author>

<!-- ************** HANNES TSCHOFENIG *************** -->
<author initials="H." surname="Tschofenig" fullname="Hannes Tschofenig">
<address>
    <email>hannes.tschofenig@gmx.net</email>
</address>
</author>

<!-- ************** TIRUMALESWAR REDDY *************** -->
<author initials="T." surname="Reddy" fullname="Tirumaleswar Reddy">
<organization>Nokia</organization>
<address>
    <email>k.tirumaleswar_reddy@nokia.com</email>
</address>
</author>

<date/>

<abstract>
<t>This document describes the usage of the Datagram Transport Layer
Security (DTLS) 1.3 protocol over the Stream Control Transmission
Protocol (SCTP) and obsoletes RFC 6083.</t>
<t>DTLS 1.3 over SCTP provides communications privacy for applications that
use SCTP as their transport protocol and allows client/server applications to
communicate in a way that is designed to prevent eavesdropping and
detect tampering or message forgery.</t>
<t>Applications using DTLS 1.3 over SCTP can use almost all transport
features provided by SCTP and its extensions.</t>
</abstract>

</front>

<middle>
<section>
<name>Introduction</name>
<t>This document describes the usage of the Datagram Transport Layer
Security (DTLS) 1.3 protocol, as defined in
<xref target='RFC9147'/>, over the
Stream Control Transmission Protocol (SCTP), as defined in
<xref target='RFC9260'/>.</t>

<t>Prior versions of DTLS are out of scope for this document.
The use of DTLS 1.0 is described in <xref target='RFC6083'/>.
For the rest of the document we assume version 1.3 when referring to DTLS
unless the context requires it to refer to a dedicated version.</t>

<t>DTLS over SCTP provides communications privacy for applications that
use SCTP as their transport protocol and allows client/server applications to
communicate in a way that is designed to prevent eavesdropping and
detect tampering or message forgery.</t>
<t>Applications using DTLS over SCTP can use almost all transport
features provided by SCTP and its extensions.</t>

<t>TLS is designed to run on top of a
byte-stream-oriented transport protocol providing a reliable,
in-sequence delivery.</t>

<t>TLS over SCTP, as described in <xref target='RFC3436'/>, has limitations:</t>
<ul>
<li><t>It does not support the unordered delivery of SCTP user
 messages.</t></li>
<li><t>It does not support partial reliability, as defined in
<xref target='RFC3758'/>.</t></li>
<li><t>It only supports the usage of the same number of streams in both
directions.</t></li>
<li><t>It uses a TLS connection for every bidirectional stream, which requires
a substantial amount of resources and message exchanges if a large number
of streams is used.</t></li>
</ul>

<t>DTLS over SCTP, as described in this document, overcomes these limitations
of TLS over SCTP.
This specification supports:</t>
<ul>
<li><t>preservation of message boundaries.</t></li>
<li><t>a large number of unidirectional and bidirectional streams.</t></li>
<li><t>ordered and unordered delivery of SCTP user messages.</t></li>
<li><t>the partial reliability extension, as defined in
<xref target='RFC3758'/>.</t></li>
<li><t>the dynamic address reconfiguration extension, as defined in
<xref target='RFC5061'/>.</t></li>
</ul>
<t>The list above matches the design of DTLS over SCTP based on
<xref target='RFC6083'/>.</t>
<t>This specification supports two ways of relaxing the message size
limitation, which is imposed by the maximum plaintext record size
of 2<sup>14</sup> bytes:</t>
<ul>
<li><t>Using DTLS extensions allows to bump this limit to
2<sup>32</sup> - 256 bytes.</t></li>
<li><t>If only ordered and reliable messages are relevant, remove the limit at
all by using a fragmentation and reassembly method making use of the Payload
Protocol Identifier (PPID).</t></li>
</ul>
<t>However, the following limitation still apply:</t>
<ul>
<li><t>The DTLS user cannot perform the SCTP-AUTH key management
because this is done by the DTLS layer.</t></li>
</ul>
<t>DTLS establishes session keys for SCTP-AUTH in the same style as
<xref target='RFC5763'/> defines how DTLS establishes keys for SRTP.
This specification utilizes a new version of SCTP-AUTH, described in
<xref target='I-D.ietf-tsvwg-rfc4895-bis'/> utilizing modern cryptographic
algorithms.</t>

<t>The method described in this document requires that the SCTP implementation
supports the optional feature of fragmentation of SCTP user messages as
defined in <xref target='RFC9260'/> and the SCTP authentication extension
defined in <xref target='I-D.ietf-tsvwg-rfc4895-bis'/>.</t>
<t>The design of this specification is based on the following principles:</t>
<ul>
<li><t>Re-use RFC 6083 as much as possible.  Large parts of RFC 6083 are
re-used.</t></li>
<li><t>Focus of DTLS 1.3 and use its features and extensions.</t></li>
<li><t>Minimize the implementation effort.</t></li>
<li><t>Re-use the SCTP-AUTH extension but utilize a modernized design.</t></li>
</ul>
</section>

<section>
<name>Conventions and Terminology</name>
<t>The key words "<bcp14>MUST</bcp14>", "<bcp14>MUST NOT</bcp14>",
"<bcp14>REQUIRED</bcp14>", "<bcp14>SHALL</bcp14>", "<bcp14>SHALL NOT</bcp14>",
"<bcp14>SHOULD</bcp14>", "<bcp14>SHOULD NOT</bcp14>",
"<bcp14>RECOMMENDED</bcp14>", "<bcp14>NOT RECOMMENDED</bcp14>",
"<bcp14>MAY</bcp14>", and "<bcp14>OPTIONAL</bcp14>" in this document are to
be interpreted as described in
BCP 14 <xref target="RFC2119"/> <xref target="RFC8174"/> when, and only when,
they appear in all capitals, as shown here.</t>

<t>This document uses the following terms:</t>
<dl>
<dt>Association:</dt>
<dd><t>An SCTP association.</t></dd>
<dt>Stream:</dt>
<dd><t>A unidirectional stream of an SCTP association.
It is uniquely identified by a stream identifier.</t></dd>
</dl>

<t>This document uses the following terms:</t>
<dl>
<dt>DTLS:</dt>
<dd><t>Datagram Transport Layer Security</t></dd>
<dt>MTU:</dt>
<dd><t>Maximum Transmission Unit</t></dd>
<dt>PPID:</dt>
<dd><t>Payload Protocol Identifier</t></dd>
<dt>SCTP:</dt>
<dd><t>Stream Control Transmission Protocol</t></dd>
<dt>TCP:</dt>
<dd><t>Transmission Control Protocol</t></dd>
<dt>TLS:</dt>
<dd><t>Transport Layer Security</t></dd>
</dl>
</section>

<section>
<name>DTLS 1.3 Considerations</name>

<section>
<name>Version of DTLS</name>
<t>This document is based on <xref target='RFC9147'/>.</t>
</section>

<section anchor="size">
<name>Message Sizes</name>
<t>DTLS, as specified in <xref target='RFC9147'/>, limits the maximum plaintext
record size to 2<sup>14</sup> bytes.</t>
<t>If this limit is too restrictive, there are at least two options:</t>
<ol>
<li><t>To bump the maximum plaintext record size limit to
2<sup>32</sup> - 256 bytes, the "record_size_limit" extension, which is defined
in <xref target='RFC8449'/>, <bcp14>MAY</bcp14> be used in combination with
<xref target='I-D.ietf-tls-tlsflags'/> and
<xref target='I-D.mattsson-tls-super-jumbo-record-limit'/>.</t></li>
<li><t>If only ordered and reliable messages are relevant, the PPID based
fragmentation and reassembly method specified in
<xref target='I-D.tuexen-tsvwg-sctp-ppid-frag'/> <bcp14>MAY</bcp14> be used
to remove the limit at all.</t></li>
</ol>
</section>

<section>
<name>Replay Detection</name>
<t>The DTLS protocol allows two forms of replay protection:
replay detection of handshake messages and
replay detection of application data payloads.
Handshake messages must be reliably transmitted and replay detection is
essential.</t>
<t>Contrary to handshake messages, detecting replays of application data
messages is optional.
If enabled, this replay detection may result in the DTLS layer dropping
messages.
Since DTLS/SCTP provides a reliable service, if requested by the application,
replay detection cannot be used.
Therefore, replay detection for application data payloads of DTLS
<bcp14>MUST NOT</bcp14> be used.</t>
</section>

<section>
<name>Path MTU Discovery</name>
<t>SCTP provides Path MTU discovery and fragmentation/reassembly for user
messages.
Since DTLS can send maximum sized messages Path MTU discovery of DTLS
<bcp14>MUST NOT</bcp14> be used.</t>
<t>Still, DTLS cannot completely ignore the PMTU for reasons mentioned
in Section 4.4 of <xref target='RFC9147'/>.
The DTLS record framing expands the datagram size, thus lowering the
effective PMTU.</t> <!-- Fix me -->
<t>As recommended in Section 4.4 of <xref target='RFC9147'/>, the DTLS record
layer <bcp14>SHOULD</bcp14> also allow the upper layer protocol to discover
the amount of record expansion expected by the DTLS processing.</t>
</section>

<section>
<name>Retransmission of Messages</name>
<t>SCTP provides a reliable and in-sequence transport service for DTLS messages
that require it.
Therefore, DTLS procedures for retransmissions of handshake messages
<bcp14>MUST NOT</bcp14> be used.
For the DTLS stack the appearance is that there is no message loss and
consequently no retransmission needed.</t>
<!-- Dont fire timers! SCTP can take a longer time than needed -->
</section>

<section>
<name>Exporter</name>
<t>TLS defines an exporter interface, which is inherited by DTLS.
The exporter interface allows the application using DTLS to obtain
keying material from the DTLS stack.
In <xref target='RFC8446'/> the exporter values are computed as:</t>
<sourcecode>
TLS-Exporter(label, context_value, key_length) =
    HKDF-Expand-Label(Derive-Secret(Secret, label, ""),
                      "exporter", Hash(context_value), key_length)
</sourcecode>
<t>For use with this specification the label <bcp14>MUST</bcp14> be set to
"EXPORTER_DTLS13_OVER_SCTP", an empty context_value field and a key length of
64 bytes.</t>
</section>

<section>
<name>Application Message Length</name>
<t>The size of a DTLS record header depends on several factors, namely</t>
<ul>
<li><t>the use of the DTLS Connection ID (CID) feature,</t></li>
<li><t>the size of the sequence number,</t></li>
<li><t>the presence of the length field, and</t></li>
<li><t>the use of padding to conceal the true message length.</t></li>
</ul>
<t>While the CID needs to be negotiated with the peer, the other parameters can
be configured in DTLS stacks.
The size of the records can, however, be influenced with the record size limit
extension and the flags extension.</t>
<t>Hence, an SCTP user application has a number of configuration options
to adjust the use of DTLS to best fit a given deployment environment.</t>
</section>
</section>

<section>
<name>SCTP Considerations</name>

<section>
<name>Mapping of DTLS Records</name>
<t>The supported maximum length of SCTP user messages <bcp14>MUST</bcp14> be at
least the size of the maximum size of a DTLSCiphertext.
In particular, the SCTP implementation <bcp14>MUST</bcp14> support fragmentation
of user messages.</t>
<t>Every SCTP user message <bcp14>MUST</bcp14> consist of exactly one DTLS
record.</t>
</section>

<section>
<name>DTLS Connection Handling</name>
<t>Each DTLS connection <bcp14>MUST</bcp14> be established and terminated
within the same SCTP association.
A DTLS connection <bcp14>MUST NOT</bcp14> span multiple SCTP associations.</t>
</section>

<section>
<name>Payload Protocol Identifier Usage</name>
<t>Application protocols using DTLS over SCTP <bcp14>SHOULD</bcp14>
register and use a separate payload protocol identifier (PPID)
and <bcp14>SHOULD NOT</bcp14> reuse the PPID that they registered for running
directly over SCTP.</t>
<t>Using the same PPID does not harm as long as the application can determine
whether or not DTLS is used.
However, for protocol analyzers, for example, it is much easier if a separate
PPID is used.</t>
<t>This means, in particular, that there is no specific PPID for DTLS.</t>
</section>

<section anchor="stream">
<name>Stream Usage</name>
<t>All DTLS messages except ApplicationData protocol messages
<bcp14>MUST</bcp14> be transported on stream 0 with unlimited reliability and
with the ordered delivery feature.</t>
<t>DTLS messages of the ApplicationData protocol <bcp14>SHOULD</bcp14> use
multiple streams other than stream 0;
they <bcp14>MAY</bcp14> use stream 0 for everything if they do not care about
minimizing head of line blocking.</t>
</section>

<section>
<name>Chunk Handling</name>
<t>DATA chunks of SCTP <bcp14>MUST</bcp14> be sent in an authenticated way,
as described in <xref target='I-D.ietf-tsvwg-rfc4895-bis'/>.
Other chunks <bcp14>MAY</bcp14> be sent in an authenticated way.
This makes sure that an attacker cannot modify the stream in which a message
is sent or affect the ordered/unordered delivery of the message.</t>
<t>If PR-SCTP, as defined in <xref target='RFC3758'/>, is used,
FORWARD-TSN chunks <bcp14>MUST</bcp14> also be sent in an authenticated way,
as described in <xref target='I-D.ietf-tsvwg-rfc4895-bis'/>.
Hence, it is not possible for an attacker to drop messages and
use forged FORWARD-TSN, SACK, and/or SHUTDOWN chunks to hide this dropping.</t>
</section>

<section>
<name>Post-Handshake Authentication</name>
<t>If the SCTP peers require periodic re-authentication after DTLS handshake is
complete to provide proof of ownership of an updated identity (e.g.,
X.509 certificate), the mechanism defined in <xref target='RFC9261'/> MUST be
leveraged by the peers for mutual re-authentication.
The application-layer protocol would have to be used to send the authenticator
request, either by the SCTP client or the SCTP server using the established
DTLS connection.</t>
<t>A specific PPID is used to multiplex/de-multiplex user messages used for
performing the procedures defined in <xref target='RFC9261'/> with other
user messages.</t> 

</section>

<section>
<name>Rekeying</name>
<t>The Extended Key Update mechanism for DTLS 1.3, defined in
<xref target='I-D.ietf-tls-extended-key-update'/>, specifies the
ExtendedKeyUpdate message, which indicates that the sender wishes to update its
cryptographic keys to ensure forward secrecy.
In DTLS, updating traffic keys requires the epoch value to  be incremented.
Since epoch values cannot wrap, the maximum number of epoch updates is
2<sup>64</sup> (this includes epoch updates during the handshake itself).</t>
<t>When the sender of the ExtendedKeyUpdate message receives an
ExtendedKeyUpdateResponse, it knows that it can safely switch from the old
epoch to the new epoch once the NewKeyUpdate message has been successfully
transmitted.
This process is described in Section 8 of <xref target='RFC9147'/> and
Section 8 of <xref target='I-D.ietf-tls-extended-key-update'/>.
While DTLS 1.3 will automatically perform this switch, the SCTP application
using DTLS 1.3 for updating keys with SCTP-AUTH requires updating of keying
material.
Although it is not mandatory to synchronize SCTP-AUTH key updates with DTLS key
changes, it is RECOMMENDED to switch keys for use with SCTP-AUTH once a
NewKeyUpdate message has been successfully transmitted.</t>
</section>

<section>
<name>Handshake</name>
<t>A DTLS implementation discards DTLS messages from older epochs
after some time, as described in <xref target='RFC9147'/>.
This is not acceptable when a reliable data transfer is performed.</t>
<!-- FIXME MT: This is missing details. -->
</section>

<section>
<name>Handling of Endpoint-Pair Shared Secrets</name>
<t>The endpoint-pair shared secret for Shared Key Identifier 0
is empty and <bcp14>MUST</bcp14> be used when establishing a DTLS connection.
A new endpoint-pair shared secret MUST be established using the exporter
interface defined in <xref target='RFC8446'/>, as described in Section 3.6.
The new Shared Key Identifier <bcp14>MUST</bcp14> be the old Shared Key
Identifier incremented by 1. If the old one is 65535, the new one
<bcp14>MUST</bcp14> be 1.</t>
<t>Before sending the Finished message, the active SCTP-AUTH key
<bcp14>MUST</bcp14> be switched to the new one.</t>
<t>Once the corresponding Finished message from the peer has
been received, the old SCTP-AUTH key <bcp14>SHOULD</bcp14> be removed.</t>
<t>Since ExtendedKeyUpdate messages are used to change the application traffic
keys it is necessary to update the keying material initially exported
via the TLS 1.3 exporter interface.</t>
<t>The key derivation function MUST be used once the NewKeyUpdate has been
transmitted successfully. The function follows the design of the exporter
interface and deriving the updated Exported Keying Material is discussed in
Section 11 of <xref target='I-D.ietf-tls-extended-key-update'/>.</t>
<t>For use with this specification the label MUST be set to
"DERIVE_DTLS13_OVER_SCTP", a context_value field containing a 64 bit
sequence number and a key length of 64 bytes.
The 64 bit number logically corresponds to the epoch value but there is no
requirement for the DTLS stack to expose the epoch value to this key derivation
function interfacing the SCTP stack for setting the SCTP-AUTH keying
material.
The sequence number MUST be increased with every DTLS key update.</t>
</section>

<section>
<name>Shutdown</name>
<t>To prevent DTLS from discarding DTLS user messages while it is
shutting down, a close_notify alert <bcp14>MUST</bcp14> only be sent after
all outstanding SCTP user messages have been acknowledged by the
SCTP peer and <bcp14>MUST NOT</bcp14> still be revoked by the SCTP peer.</t>
<t>Prior to processing a received close_notify, all other
received SCTP user messages that are buffered in the SCTP layer
<bcp14>MUST</bcp14> be read and processed by DTLS.</t>
</section>
</section>

<section anchor="iana">
<name>IANA Considerations</name>
<t>IANA is asked to update in the the TLS Exporter Label registry,
which was established in <xref target='RFC5705'/> and updated by
<xref target='RFC8447'/>, the Reference of the label "EXPORTER_DTLS_OVER_SCTP"
to this document.</t>
</section>

<section>
<name>Security Considerations</name>
<t>The security considerations given in
<xref target='RFC4347'/>,
<xref target='RFC4895'/>, and
<xref target='RFC9260'/>
also apply to this document.</t>
<t>It is possible to authenticate DTLS endpoints based on IP addresses
in certificates.
SCTP associations can use multiple addresses per SCTP endpoint.
Therefore, it is possible that DTLS records will be sent from a different
IP address than that originally authenticated.
This is not a problem provided that no security decisions are made
based on that IP address.
This is a special case of a general rule: all decisions should be based on
the peer's authenticated identity, not on its transport layer identity.</t>
<t>For each message, the SCTP user also provides a stream
identifier, a flag to indicate whether the message is sent
ordered or unordered, and a payload protocol identifier.
Although DTLS can be used to provide privacy for the actual user message,
none of these three are protected by DTLS.
They are sent as clear text, because they are part of the SCTP DATA chunk
header.</t>
<t>While TLS 1.3 reduced the number of supported cipher suites and removed a
number of cipher suites, including all NULL cipher algorithm.
However, <xref target='RFC9150'/> later re-introduced support for cipher suites
that do not support confidentiality protection.
Negotiating such cipher suites will not provide communications privacy for
SCTP applications and will not provide privacy for user messages and
<bcp14>MUST NOT</bcp14> be used with this specification.</t>
</section>

<section anchor="acks">
<name>Acknowledgments</name>
<t>The authors wish to thank Eric Rescorla and Robin Seggelmann for being
coauthors of <xref target='RFC6083'/>, which is the basis of this document.</t>
<t>The authors wish to thank
Anna Brunstrom,
Lars Eggert,
Gorry Fairhurst,
Ian Goldberg,
Alfred Hoenes,
Carsten Hohendorf,
Stefan Lindskog,
Daniel Mentz,
and Sean Turner
for their invaluable comments on <xref target='RFC6083'/>.</t>
<t>Finally, the authors wish to thank
Eric Rescorla and
Martin Thomson
for their invaluable comments on this document.</t>
</section>

</middle>

<back>
<references>
<name>References</name>
<references>
<name>Normative References</name>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.2119.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.3758.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.4347.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.4895.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.5705.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.5763.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.8449.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.8174.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.8446.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.8447.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.9147.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.9260.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.9261.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml3/reference.I-D.ietf-tsvwg-rfc4895-bis.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml3/reference.I-D.ietf-tls-tlsflags.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml3/reference.I-D.mattsson-tls-super-jumbo-record-limit"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml3/reference.I-D.tuexen-tsvwg-sctp-ppid-frag"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml3/reference.I-D.draft-ietf-tls-extended-key-update-02"/>
</references>
<references>
<name>Informative References</name>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.3436.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.5061.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.6083.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.9150.xml"/>
</references>
</references>
</back>
</rfc>