Internet DRAFT - draft-westerlund-tsvwg-dtls-over-sctp-bis
draft-westerlund-tsvwg-dtls-over-sctp-bis
TSVWG M. Westerlund
Internet-Draft J. Preuß Mattsson
Obsoletes: 6083 (if approved) C. Porfiri
Intended status: Standards Track Ericsson
Expires: 26 August 2021 M. Tüxen
Münster Univ. of Appl. Sciences
22 February 2021
Datagram Transport Layer Security (DTLS) over Stream Control
Transmission Protocol (SCTP)
draft-westerlund-tsvwg-dtls-over-sctp-bis-01
Abstract
This document describes a proposed update for the usage of the
Datagram Transport Layer Security (DTLS) protocol to protect user
messages sent over the Stream Control Transmission Protocol (SCTP).
DTLS over SCTP provides mutual authentication, confidentiality,
integrity protection, and replay protection for applications that use
SCTP as their transport protocol and allows client/server
applications to communicate in a way that is designed to give
communications privacy and to prevent eavesdropping and detect
tampering or message forgery.
Applications using DTLS over SCTP can use almost all transport
features provided by SCTP and its extensions. This document intends
to obsolete RFC 6083 and removes the 16 kB limitation on user message
size by defining a secure user message fragmentation so that multiple
DTLS records can be used to protect a single user message. It
further updates the DTLS versions to use, as well as the HMAC
algorithms for SCTP-AUTH, and simplifies the implementation by some
stricter requirements on the establishment procedures.
Discussion Venues
This note is to be removed before publishing as an RFC.
Discussion of this document takes place on the TSVWG Working Group
mailing list (tsvwg@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/tsvwg/.
Source for this draft and an issue tracker can be found at
https://github.com/gloinul/draft-westerlund-tsvwg-dtls-over-sctp-bis.
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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
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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 26 August 2021.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1.1. Comparison with TLS for SCTP . . . . . . . . . . . . 4
1.1.2. Changes from RFC 6083 . . . . . . . . . . . . . . . . 5
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
1.3. Abbreviations . . . . . . . . . . . . . . . . . . . . . . 5
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. DTLS Considerations . . . . . . . . . . . . . . . . . . . . . 6
3.1. Version of DTLS . . . . . . . . . . . . . . . . . . . . . 6
3.2. Cipher Suites . . . . . . . . . . . . . . . . . . . . . . 6
3.3. Message Sizes . . . . . . . . . . . . . . . . . . . . . . 6
3.4. Replay Protection . . . . . . . . . . . . . . . . . . . . 8
3.5. Path MTU Discovery . . . . . . . . . . . . . . . . . . . 8
3.6. Retransmission of Messages . . . . . . . . . . . . . . . 8
4. SCTP Considerations . . . . . . . . . . . . . . . . . . . . . 8
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4.1. Mapping of DTLS Records . . . . . . . . . . . . . . . . . 8
4.2. DTLS Connection Handling . . . . . . . . . . . . . . . . 9
4.3. Payload Protocol Identifier Usage . . . . . . . . . . . . 9
4.4. Stream Usage . . . . . . . . . . . . . . . . . . . . . . 10
4.5. Chunk Handling . . . . . . . . . . . . . . . . . . . . . 10
4.6. SCTP-AUTH Hash Function . . . . . . . . . . . . . . . . . 11
4.7. Renegotiation . . . . . . . . . . . . . . . . . . . . . . 11
4.8. DTLS Epochs . . . . . . . . . . . . . . . . . . . . . . . 11
4.9. Handling of Endpoint-Pair Shared Secrets . . . . . . . . 11
4.10. Shutdown . . . . . . . . . . . . . . . . . . . . . . . . 12
5. DTLS over SCTP Service . . . . . . . . . . . . . . . . . . . 12
5.1. Adaptation Layer Indication in INIT/INIT-ACK . . . . . . 12
5.2. DTLS/SCTP "dtls_over_sctp_maximum_message_size"
Extension . . . . . . . . . . . . . . . . . . . . . . . . 12
5.3. DTLS over SCTP Initialization . . . . . . . . . . . . . . 13
5.4. Client Use Case . . . . . . . . . . . . . . . . . . . . . 14
5.5. Server Use Case . . . . . . . . . . . . . . . . . . . . . 14
5.6. RFC 6083 Fallback . . . . . . . . . . . . . . . . . . . . 14
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
6.1. TLS Exporter Label . . . . . . . . . . . . . . . . . . . 15
6.2. DTLS "dtls_over_sctp_buffer_size_limit" Extension . . . . 15
6.3. SCTP Parameter . . . . . . . . . . . . . . . . . . . . . 15
7. Security Considerations . . . . . . . . . . . . . . . . . . . 16
7.1. Cryptographic Considerations . . . . . . . . . . . . . . 16
7.2. Downgrade Attacks . . . . . . . . . . . . . . . . . . . . 17
7.3. DTLS/SCTP Message Sizes . . . . . . . . . . . . . . . . . 17
7.4. Authentication and Policy Decisions . . . . . . . . . . . 17
7.5. Privacy Considerations . . . . . . . . . . . . . . . . . 18
7.6. Pervasive Monitoring . . . . . . . . . . . . . . . . . . 18
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 19
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
9.1. Normative References . . . . . . . . . . . . . . . . . . 19
9.2. Informative References . . . . . . . . . . . . . . . . . 20
Appendix A. Motivation for Changes . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
1. Introduction
1.1. Overview
This document describes the usage of the Datagram Transport Layer
Security (DTLS) protocol, as defined in [I-D.ietf-tls-dtls13], over
the Stream Control Transmission Protocol (SCTP), as defined in
[RFC4960] with Authenticated Chunks for SCTP (SCTP-AUTH) [RFC4895].
This specification provides mutual authentication of endpoints,
confidentiality, integrity protection, and replay protection of user
messages for applications that use SCTP as their transport protocol.
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Thus it allows client/server applications to communicate in a way
that is designed to give communications privacy and to prevent
eavesdropping and detect tampering or message forgery. DTLS/SCTP
uses DTLS for mutual authentication, key exchange with perfect
forward secrecy for SCTP-AUTH, and confidentiality of user messages.
DTLS/SCTP use SCTP and SCTP-AUTH for integrity protection and replay
protection of user messages.
Applications using DTLS over SCTP can use almost all transport
features provided by SCTP and its extensions. DTLS/SCTP supports:
* preservation of message boundaries.
* a large number of unidirectional and bidirectional streams.
* ordered and unordered delivery of SCTP user messages.
* the partial reliability extension as defined in [RFC3758].
* the dynamic address reconfiguration extension as defined in
[RFC5061].
* large user messages.
The method described in this document requires that the SCTP
implementation supports the optional feature of fragmentation of SCTP
user messages as defined in [RFC4960]. To efficiently implement and
support larger user messages it is also recommended that I-DATA
chunks as defined in [RFC8260] as well as an SCTP API that supports
partial user message delivery as discussed in [RFC6458].
1.1.1. Comparison with TLS for SCTP
TLS, from which DTLS was derived, is designed to run on top of a
byte-stream-oriented transport protocol providing a reliable, in-
sequence delivery. TLS over SCTP as described in [RFC3436] has some
serious limitations:
* It does not support the unordered delivery of SCTP user messages.
* It does not support partial reliability as defined in [RFC3758].
* It only supports the usage of the same number of streams in both
directions.
* 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.
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1.1.2. Changes from RFC 6083
The DTLS over SCTP solution defined in RFC 6083 had the following
limitation:
* The maximum user message size is 2^14 bytes, which is a single
DTLS record limit.
This update that replaces RFC6083 defines the following changes:
* Removes the limitations on user messages sizes by defining a
secure fragmentation mechanism.
* Defines a DTLS extension for the endpoints to declare the user
message size supported to be received.
* Mandates that more modern DTLS version are required (DTLS 1.2 or
1.3)
* Mandates use of modern HMAC algorithm (SHA-256) in the SCTP
authentication extension [RFC4895].
* Recommends support of [RFC8260] to enable interleaving of large
SCTP user messages to avoid scheduling issues.
* Recommends support of partial message delivery API, see [RFC6458]
if larger usage messages are intended to be used.
* Applies stricter requirements on always using DTLS for all user
messages in the SCTP association.
* Requires that SCTP-AUTH is applied to all SCTP Chunks that can be
authenticated.
1.2. Terminology
This document uses the following terms:
Association: An SCTP association.
Stream: A unidirectional stream of an SCTP association. It is
uniquely identified by a stream identifier.
1.3. Abbreviations
DTLS: Datagram Transport Layer Security
MTU: Maximum Transmission Unit
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PPID: Payload Protocol Identifier
SCTP: Stream Control Transmission Protocol
TCP: Transmission Control Protocol
TLS: Transport Layer Security
2. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. DTLS Considerations
3.1. Version of DTLS
This document is based on DTLS 1.3 [I-D.ietf-tls-dtls13], but works
also for DTLS 1.2 [RFC6347]. Earlier versions of DTLS MUST NOT be
used. It is expected that DTLS/SCTP as described in this document
will work with future versions of DTLS.
3.2. Cipher Suites
For DTLS 1.2, the cipher suites forbidden by [RFC7540] MUST NOT be
used. Cipher suites without encryption MUST NOT be used.
3.3. Message Sizes
DTLS/SCTP, automatically fragments and reassembles user messages.
This specification defines how to fragment the user messages into
DTLS records, where each DTLS 1.3 record allows a maximum of 2^14
protected bytes. Each DTLS record adds some overhead, thus using
records of maximum possible size are recommended to minimize the
overhead.
The sequence of DTLS records is then fragmented into DATA or I-DATA
Chunks to fit the path MTU by SCTP. The largest possible user
messages using the mechanism defined in this specification is 2^64-1
bytes.
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The security operations and reassembly process requires that the
protected user message, i.e. with DTLS record overhead, is buffered
in the receiver. This buffer space will thus put a limit on the
largest size of plain text user message that can be transferred
securely.
A receiver that doesn't support partial delivery of user messages
from SCTP [RFC6458] will advertise its largest supported protected
message using SCTP's mechanism for Advertised Receiver Window Credit
(a_rwnd) as specified in Section 3.3.2 of [RFC4960]. Note that the
a_rwnd value is across all user messages being delivered.
For a receiver supporting partial delivery of user messages a_rwnd
will not limit the maximum size of the DTLS protected user message
because the receiver can move parts of the DTLS protected user
message from the SCTP receiver buffer into a buffer for DTLS
processing. When each complete DTLS record have been received from
SCTP, it can be processed and the plain text fragment can, in its
turn, be partially delivered to the user application.
Thus, the limit of the largest user message is dependent on buffering
allocated for DTLS processing as well as the DTLS/SCTP API to the
application. To ensure that the sender have some understanding of
the maximum receiver size a TLS extension
"dtls_over_sctp_maximum_message_size" Section 5.2 is used to signal
the endpoints receiver capability when it comes to user message size.
All implementors of this specification MUST support user messages of
at least 16383 bytes. Where 16383 bytes is the supported message
size in RFC 6083. By requiring this message size in this document,
we ensure compatibility with existing usage of RFC 6083, not
requiring the upper layer protocol to implement additional features
or requirements.
Due to SCTP's capability to transmit concurrent user messages the
total memory consumption in the receiver is not bounded. In cases
where one or more user messages are affected by packet loss, the DATA
chunks may require more data in the receiver's buffer.
The necessary buffering space for a single user message of
dtls_over_sctp_maximum_message_size (MMS) is dependent on the
implementation.
When no partial data delivery is supported, the message size is
limited by the a_rwnd as this is the largest protected user message
that can be received and then processed by DTLS and where the plain
text user message is expected to be no more than the signalled MMS.
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With partial processing it is possible to have a receiver
implementation that is bound to use no more buffer space than MMS
(for the plaintext) plus one maximum size DTLS record. The later
assumes that one can realign the start of the buffer after each DTLS
record has been consumed. A more realistic implementation is two
maximum DTLS record sizes.
If an implementation supports partial delivery in both the SCTP API
and the ULP API, and also partial processing in the DTLS/SCTP
implementation, then the buffering space in the DTLS/SCTP layer ought
to be no more than two DTLS records. In which case the MMS to set is
dependent on the ULP and the endpoints capabilities.
3.4. Replay Protection
SCTP-AUTH [RFC4895] does not have explicit replay protection.
However, the combination of SCTP-AUTH's protection of DATA or I-DATA
chunks and SCTP user message handling will prevent third party
attempts to inject or replay SCTP packets resulting in impact on the
received protected user message. In fact this document's solution is
dependent on SCTP-AUTH and SCTP to prevent reordering of the DTLS
records within each protected user message.
DTLS optionally supports record replay detection. Such replay
detection could result in the DTLS layer dropping valid messages
received outside of the DTLS replay window. As DTLS/SCTP provides
replay protection even without DTLS replay protection, the replay
detection of DTLS MUST NOT be used.
3.5. Path MTU Discovery
DTLS Path MTU Discovery MUST NOT be used. Since SCTP provides own
Path MTU discovery and fragmentation/reassembly for user messages,
and according to Section 3.3, DTLS can send maximum sized DTLS
Records.
3.6. Retransmission of Messages
SCTP provides a reliable and in-sequence transport service for DTLS
messages that require it. See Section 4.4. Therefore, DTLS
procedures for retransmissions MUST NOT be used.
4. SCTP Considerations
4.1. Mapping of DTLS Records
The SCTP implementation MUST support fragmentation of user messages
using DATA [RFC4960], and optionally I-DATA [RFC8260] chunks.
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DTLS/SCTP works as a shim layer between the user message API and
SCTP. The fragmentation works similar as the DTLS fragmentation of
handshake messages. On the sender side a user message fragmented
into fragments m0, m1, m2, each no larger than 2^14 - 1 = 16383
bytes.
m0 | m1 | m2 | ... = user_message
The resulting fragments are protected with DTLS and the records are
concatenated
user_message' = DTLS( m0 ) | DTLS( m1 ) | DTLS( m2 ) ...
The new user_message', i.e the protected user message, is the input
to SCTP.
On the receiving side DTLS is used to decrypt the records. If a DTLS
decryption fails, the DTLS connection and the SCTP association are
terminated. Due to SCTP-AUTH preventing delivery of corrupt
fragments of the protected user message this should only occur in
case of implementation errors or internal hardware failures.
The DTLS Connection ID SHOULD NOT be negotiated (Section 9 of
[I-D.ietf-tls-dtls13]). If DTLS 1.3 is used, the length field MUST
NOT be omitted and a 16 bit sequence number SHOULD be used.
4.2. DTLS Connection Handling
The DTLS connection MUST be established at the beginning of the SCTP
association and be terminated when the SCTP association is
terminated, (i.e. there's only one DTLS connection within one
association). A DTLS connection MUST NOT span multiple SCTP
associations.
As it is required to establish the DTLS connection at the beginning
of the SCTP association, either of the peers should never send any
SCTP user messages that are not protected by DTLS. So the case that
an endpoint receives data that is not either DTLS messages on Strea 0
or protecetd user messages in the form of a sequence of DTLS Records
on any stream is a protocol violation. The receiver MAY terminate
the SCTP association due to this protocol violation.
4.3. Payload Protocol Identifier Usage
SCTP Payload Protocol Identifiers are assigned by IANA. Application
protocols using DTLS over SCTP SHOULD register and use a separate
Payload Protocol Identifier (PPID) and SHOULD NOT reuse the PPID that
they registered for running directly over SCTP.
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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.
This means, in particular, that there is no specific PPID for DTLS.
4.4. Stream Usage
All DTLS Handshake, Alert, or ChangeCipherSpec (DTLS 1.2 only)
messages MUST be transported on stream 0 with unlimited reliability
and with the ordered delivery feature.
DTLS messages of the record protocol, which carries the protected
user messages, SHOULD use multiple streams other than stream 0; they
MAY use stream 0 as long as the ordered message semantics is
acceptable. On stream 0 protected user messages as well as any DTLS
messages that isn't record protocol will be mixed, thus the
additional head of line blocking can occur.
4.5. Chunk Handling
DATA chunks of SCTP MUST be sent in an authenticated way as described
in [RFC4895]. All other chunks that can be authenticated, i.e. all
chunk types that can be listed in the Chunk List Parameter [RFC4895],
MUST also 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.
If PR-SCTP as defined in [RFC3758] is used, FORWARD-TSN chunks MUST
also be sent in an authenticated way as described in [RFC4895]. This
makes sure that it is not possible for an attacker to drop messages
and use forged FORWARD-TSN, SACK, and/or SHUTDOWN chunks to hide this
dropping.
I-DATA chunk type as defined in [RFC8260] is RECOMMENDED to be
supported to avoid some of the down sides that large user messages
have on blocking transmission of later arriving high priority user
messages. However, the support is not mandated and negotiated
independently from DTLS/SCTP. If I-DATA chunks are used then they
MUST be sent in an authenticated way as described in [RFC4895].
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4.6. SCTP-AUTH Hash Function
When using DTLS/SCTP, the SHA-256 Message Digest Algorithm MUST be
supported in the SCTP-AUTH [RFC4895] implementation. SHA-1 MUST NOT
be used when using DTLS/SCTP. [RFC4895] requires support and
inclusion of of SHA-1 in the HMAC-ALGO parameter, thus, to meet both
requirements the HMAC-ALGO parameter will include both SHA-256 and
SHA-1 with SHA-256 listed prior to SHA-1 to indicate the preference.
4.7. Renegotiation
Renegotiation enables rekeying and reauthentication inside an DTLS
1.2 connection. It is up to the upper layer to use/allow it or not.
Application writers should be aware that allowing renegotiations may
result in changes of security parameters. Renegotiation has been
removed from DTLS 1.3 and partly replaced with Post-Handshake
messages such as KeyUpdate. See Section 7 for security
considerations regarding rekeying.
4.8. DTLS Epochs
In general, DTLS implementations SHOULD discard records from earlier
epochs, as described in Section 4.2.1 of [I-D.ietf-tls-dtls13]. To
avoid discarding messages, the processing guidelines in Section 4.2.1
of DTLS 1.3 [I-D.ietf-tls-dtls13] or Section 4.1 or DTLS 1.2
[RFC6347] should be followed.
4.9. Handling of Endpoint-Pair Shared Secrets
SCTP-AUTH [RFC4895] is keyed using Endpoint-Pair Shared Secrets. In
SCTP associations where DTLS is used, DTLS is used to establish these
secrets. The endpoints MUST NOT use another mechanism for
establishing shared secrets for SCTP-AUTH.
The endpoint-pair shared secret for Shared Key Identifier 0 is empty
and MUST be used when establishing a DTLS connection. In DTLS 1.2,
whenever the main secret changes, a 64-byte shared secret is derived
from every main secret and provided as a new endpoint-pair shared
secret by using the TLS-Exporter. In DTLS 1.3, the exporter_secret
never change. For DTLS 1.3, the exporter is described in [RFC8446].
For DTLS 1.2, the exporter is described in [RFC5705]. The exporter
MUST use the label given in Section Section 6 and no context. The
new Shared Key Identifier MUST be the old Shared Key Identifier
incremented by 1. If the old one is 65535, the new one MUST be 1.
Before sending the DTLS Finished message, the active SCTP-AUTH key
MUST be switched to the new one.
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Once the corresponding Finished message from the peer has been
received, the old SCTP-AUTH key SHOULD be removed.
4.10. Shutdown
To prevent DTLS from discarding DTLS user messages while it is
shutting down, a CloseNotify message MUST only be sent after all
outstanding SCTP user messages have been acknowledged by the SCTP
peer and MUST NOT be revoked by the SCTP peer.
Prior to processing a received CloseNotify, all other received SCTP
user messages that are buffered in the SCTP layer MUST be read and
processed by DTLS.
5. DTLS over SCTP Service
The adoption of DTLS over SCTP according to the current description
is meant to add to SCTP the option for transferring encrypted data.
When DTLS over SCTP is used, all data being transferred MUST be
protected by chunk authentication and DTLS encrypted. Chunks that
need to be received in an authenticated way will be specified in the
CHUNK list parameter according to [RFC4895]. Error handling for
authenticated chunks is according to [RFC4895].
5.1. Adaptation Layer Indication in INIT/INIT-ACK
At the initialization of the association, a sender of the INIT or
INIT ACK chunk that intends to use DTLS/SCTP as specified in this
specification MUST include an Adaptation Layer Indication Parameter
with the IANA assigned value TBD to inform its peer that it is able
to support DTLS over SCTP per this specification.
5.2. DTLS/SCTP "dtls_over_sctp_maximum_message_size" Extension
The endpoint's DTLS/SCTP maximum message size is declared in the
"dtls_over_sctp_maximum_message_size" TLS extension. The
ExtensionData of the extension is MessageSizeLimit:
uint64 MessageSizeLimit;
The value of MessageSizeLimit is the maximum plaintext user message
size in octets that the endpoint is willing to receive. When the
"dtls_over_sctp_maximum_message_size" extension is negotiated, an
endpoint MUST NOT send a user message larger than the
MessageSizeLimit value it receives from its peer.
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This value is the length of the user message before DTLS
fragmentation and protection. The value does not account for the
expansion due to record protection, record padding, or the DTLS
header.
The "dtls_over_sctp_maximum_message_size" MUST be used to negotiate
maximum message size for DTLS/SCTP. A DTLS/SCTP endpoint MUST treat
the omission of "dtls_over_sctp_maximum_message_size" as a fatal
error unless supporting RFC 6083 fallback Section 5.6, and it SHOULD
generate an "illegal_parameter" alert. Endpoints MUST NOT send a
"dtls_over_sctp_maximum_message_size" extension with a value smaller
than 16383. An endpoint MUST treat receipt of a smaller value as a
fatal error and generate an "illegal_parameter" alert.
The "dtls_over_sctp_maximum_message_size" MUST NOT be send in TLS or
in DTLS versions earlier than 1.2. In DTLS 1.3, the server sends the
"dtls_over_sctp_maximum_message_size" extension in the
EncryptedExtensions message.
During resumption, the maximum message size is renegotiated.
5.3. DTLS over SCTP Initialization
Initialization of DTLS/SCTP requires all the following options to be
part of the INIT/INIT-ACK handshake:
RANDOM: defined in [RFC4895]
CHUNKS: list of permitted chunks, defined in [RFC4895]
HMAC-ALGO: defined in [RFC4895]
ADAPTATION-LAYER-INDICATION: defined in [RFC5061]
When all the above options are present, the Association will start
with support of DTLS/SCTP. The set of options indicated are the
DTLS/SCTP Mandatory Options. No data transfer is permitted before
DTLS handshake is complete. Chunk bundling is permitted according to
[RFC4960]. The DTLS handshake will enable authentication of both the
peers and also have the declare their support message size.
The extension described in this document is given by the following
message exchange.
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--- INIT[RANDOM; CHUNKS; HMAC-ALGO; ADAPTATION-LAYER-IND] --->
<- INIT-ACK[RANDOM; CHUNKS; HMAC-ALGO; ADAPTATION-LAYER-IND] -
------------------------ COOKIE-ECHO ------------------------>
<------------------------ COOKIE-ACK -------------------------
---------------- AUTH; DATA[DTLS Handshake] ----------------->
...
...
<--------------- AUTH; DATA[DTLS Handshake] ------------------
5.4. Client Use Case
When a SCTP Client initiates an Association with DTLS/SCTP Mandatory
Options, it can receive an INIT-ACK also containing DTLS/SCTP
Mandatory Options, in that case the Association will proceed as
specified in the previous Section 5.3 section. If the peer replies
with an INIT-ACK not containing all DTLS/SCTP Mandatory Options, the
Client can decide to keep on working with RFC 6083 fallback, plain
data only, or to ABORT the association.
5.5. Server Use Case
If a SCTP Server supports DTLS/SCTP, when receiving an INIT chunk
with all DTLS/SCTP Mandatory Options it must reply with INIT-ACK also
containing the all DTLS/SCTP Mandatory Options, then it must follow
the sequence for DTLS initialization Section 5.3 and the related
traffic case. If a SCTP Server supports DTLS, when receiving an INIT
chunk with not all DTLS/SCTP Mandatory Options, it can decide to
continue by creating an Association with RFC 6083 fallback, plain
data only or to ABORT it.
5.6. RFC 6083 Fallback
This section discusses how an endpoint supporting this specification
can fallback to follow the DTLS/SCTP behavior in RFC 6083. It is
recommended to define a setting that represents the policy to allow
fallback or not. However, the possibility to use fallback is based
on the ULP can operate using user messages that are no longer than
16383 bytes and where the security issues can be mitigated or
considerd acceptable. Fallback is NOT RECOMMEND to be enabled as it
enables downgrade to weaker algorithms and versions of DTLS.
A SCTP client that receives an INIT-ACK that is not compliant
according this specification may in certain cases potentially perform
an fallback to RFC 6083 behavior. The first case is when the SCTP
client receives an INIT-ACK doesn't contain the SCTP-Adaptation-
Indication parameter with the DTLS/SCTP adaptation layer codepoint
but do include the SCTP-AUTH parameters on a server that are expected
to provide services using DTLS. The second case is when the INIT-ACK
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do contain the SCTP-Adaptation-Indication parameter with the correct
code point, however the HMAC-ALGO or the Chunks parameters values are
such that do not fullfil the requirement of this specification but do
meet the requirements of RFC 6083. In either of these cases the
client could attempt DTLS per RFC 6083 as fallback. However, the
fallback attempt should only be performed if policy says that is
acceptable.
If fallback is allowed it is possible that the client will send plain
text user messages prior to DTLS handshake as it is allowed per RFC
6083. So that needs to be part of the consideration for a policy
allowing fallback. When performing the the DTLS handshake, the
server is required accepting that lack of the TLS extension
"dtls_over_sctp_maximum_message_size" and can't treat it as fatal
error. In case the "dtls_over_sctp_maximum_message_size" TLS
extension is present in the handshake the server SHALL continue the
handshake including the extension with its value also, and from that
point follow this specification. In case the TLS option is missing
RFC 6083 applies.
6. IANA Considerations
6.1. TLS Exporter Label
RFC 6083 defined a TLS Exporter Label registry as described in
[RFC5705]. IANA is requested to update the reference for the label
"EXPORTER_DTLS_OVER_SCTP" to this specification.
6.2. DTLS "dtls_over_sctp_buffer_size_limit" Extension
This document registers the "dtls_over_sctp_maximum_message_size"
extension in the TLS "ExtensionType Values" registry established in
[RFC5246]. The "dtls_over_sctp_maximum_message_size" extension has
been assigned a code point of TBD. This entry [[will be|is]] marked
as recommended ([RFC8447] and marked as "Encrypted" in (D)TLS 1.3
[I-D.ietf-tls-dtls13]. The IANA registry [RFC8447] [[will
list|lists]] this extension as "Recommended" (i.e., "Y") and
indicates that it may appear in the ClientHello (CH) or
EncryptedExtensions (EE) messages in (D)TLS 1.3
[I-D.ietf-tls-dtls13].
6.3. SCTP Parameter
IANA is requested to assign a Adaptation Code Point for DTLS/SCTP.
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7. Security Considerations
The security considerations given in [I-D.ietf-tls-dtls13],
[RFC4895], and [RFC4960] also apply to this document.
7.1. Cryptographic Considerations
Over the years, there have been several serious attacks on earlier
versions of Transport Layer Security (TLS), including attacks on its
most commonly used ciphers and modes of operation. [RFC7457]
summarizes the attacks that were known at the time of publishing and
BCP 195 [RFC7525] provides recommendations for improving the security
of deployed services that use TLS.
When DTLS/SCTP is used with DTLS 1.2 [RFC6347], DTLS 1.2 MUST be
configured to disable options known to provide insufficient security.
HTTP/2 [RFC7540] gives good minimum requirements based on the attacks
that where publicly known in 2015. DTLS 1.3 [I-D.ietf-tls-dtls13]
only define strong algorithms without major weaknesses at the time of
publication. Many of the TLS registries have a "Recommended" column.
Parameters not marked as "Y" are NOT RECOMMENDED to support.
DTLS 1.3 requires rekeying before algorithm specific AEAD limits have
been reached. The AEAD limits equations are equally valid for DTLS
1.2 and SHOULD be followed for DTLS/SCTP, but are not mandated by the
DTLS 1.2 specification. HMAC-SHA-256 as used in SCTP-AUTH has a very
large tag length and very good integrity properties. The SCTP-AUTH
key can be used until the DTLS handshake is re-run at which point a
new SCTP-AUTH key is derived using the TLS-Exporter.
DTLS/SCTP is in many deployments replacing IPsec. For IPsec, NIST
(US), BSI (Germany), and ANSSI (France) recommends very frequent re-
run of Diffie-Hellman to provide Perfect Forward Secrecy. ANSSI
writes "It is recommended to force the periodic renewal of the keys,
e.g. every hour and every 100 GB of data, in order to limit the
impact of a key compromise." [ANSSI-DAT-NT-003].
For many DTLS/SCTP deployments the DTLS connections are expected to
have very long lifetimes of months or even years. For connections
with such long lifetimes there is a need to frequently re-
authenticate both client and server.
When using DTLS 1.2 [RFC6347], AEAD limits, frequant re-
authentication and frequent re-run of Diffie-Hellman can be achieved
with frequent renegotiation, see TLS 1.2 [RFC5246]. When
renegotiation is used both clients and servers MUST use the
renegotiation_info extension [RFC5746] and MUST follow the
renegotiation guidelines in BCP 195 [RFC7525].
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In DTLS 1.3 renegotiation has been removed from DTLS 1.3 and partly
replaced with Post-Handshake KeyUpdate. When using DTLS 1.3
[I-D.ietf-tls-dtls13], AEAD limits and frequent rekeying can be
achieved by sending frequent Post-Handshake KeyUpdate messages.
Symmetric rekeying gives less protection against key leakage than re-
running Diffie-Hellman. After leakage of
application_traffic_secret_N, a passive attacker can passively
eavesdrop on all future application data sent on the connection
including application data encrypted with
application_traffic_secret_N+1, application_traffic_secret_N+2, etc.
The is no way to do Post-Handshake server authentication or Ephemeral
Diffie-Hellman inside a DTLS 1.3 connection. Note that KeyUpdate
does not update the exporter_secret.
7.2. Downgrade Attacks
A peer supporting DTLS/SCTP according to this specification, DTLS/
SCTP according to [RFC6083] and/or SCTP without DTLS may be
vulnerable to downgrade attacks where on on-path attacker interferes
with the protocol setup to lower or disable security. If possible,
it is RECOMMENDED that the peers have a policy only allowing DTLS/
SCTP according to this specification.
7.3. DTLS/SCTP Message Sizes
The DTLS/SCTP maximum message size extension enables secure negation
of a message sizes that fit in the DTLS/SCTP buffer, which improves
security and availability. Very small plain text user fragment sizes
might generate additional work for senders and receivers, limiting
throughput and increasing exposure to denial of service.
The maximum message size extension does not protect against peer
nodes intending to negatively affect the peer node through flooding
attacks. The attacking node can both send larger messages than the
expressed capability as well as initiating a large number of
concurrent user message transmissions that never are concluded. For
the target of the attack it is more straight forward to determine
that a peer is ignoring the node's stated limitation.
7.4. Authentication and Policy Decisions
DTLS/SCTP MUST be mutually authenticated. It is RECOMMENDED that
DTLS/SCTP is used with certificate based authentication. All
security decisions MUST be based on the peer's authenticated
identity, not on its transport layer identity.
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It is possible to authenticate DTLS endpoints based on IP addresses
in certificates. SCTP associations can use multiple IP addresses per
SCTP endpoint. Therefore, it is possible that DTLS records will be
sent from a different source IP address or to a different destination
IP address than that originally authenticated. This is not a problem
provided that no security decisions are made based on the source or
destination IP addresses.
7.5. Privacy Considerations
[RFC6973] suggests that the privacy considerations of IETF protocols
be documented.
For each SCTP user message, the 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/SCTP
provides privacy for the actual user message, the other three
information fields are not confidentiality protected. They are sent
as clear text, because they are part of the SCTP DATA chunk header.
It is RECOMMENDED that DTLS/SCTP is used with certificate based
authentication in DTLS 1.3 [I-D.ietf-tls-dtls13] to provide identity
protection. DTLS/SCTP MUST be used with a key exchange method
providing Perfect Forward Secrecy. Perfect Forward Secrecy
significantly limits the amount of data that can be compromised due
to key compromise.
7.6. Pervasive Monitoring
As required by [RFC7258], work on IETF protocols needs to consider
the effects of pervasive monitoring and mitigate them when possible.
Pervasive Monitoring is widespread surveillance of users. By
encrypting more information including user identities, DTLS 1.3
offers much better protection against pervasive monitoring.
Massive pervasive monitoring attacks relying on key exchange without
forward secrecy has been reported. By mandating perfect forward
secrecy, DTLS/SCTP effectively mitigate many forms of passive
pervasive monitoring and limits the amount of compromised data due to
key compromise.
In addition to the privacy attacks discussed above, surveillance on a
large scale may enable tracking of a user over a wider geographical
area and across different access networks. Using information from
DTLS/SCTP together with information gathered from other protocols
increases the risk of identifying individual users.
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8. Acknowledgments
The authors of RFC 6083 which this document is based on are Michael
Tuexen, Eric Rescorla, and Robin Seggelmann.
The RFC 6083 authors thanked Anna Brunstrom, Lars Eggert, Gorry
Fairhurst, Ian Goldberg, Alfred Hoenes, Carsten Hohendorf, Stefan
Lindskog, Daniel Mentz, and Sean Turner for their invaluable
comments.
9. References
9.1. 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,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3758] Stewart, R., Ramalho, M., Xie, Q., Tuexen, M., and P.
Conrad, "Stream Control Transmission Protocol (SCTP)
Partial Reliability Extension", RFC 3758,
DOI 10.17487/RFC3758, May 2004,
<https://www.rfc-editor.org/info/rfc3758>.
[RFC4895] Tuexen, M., Stewart, R., Lei, P., and E. Rescorla,
"Authenticated Chunks for the Stream Control Transmission
Protocol (SCTP)", RFC 4895, DOI 10.17487/RFC4895, August
2007, <https://www.rfc-editor.org/info/rfc4895>.
[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
RFC 4960, DOI 10.17487/RFC4960, September 2007,
<https://www.rfc-editor.org/info/rfc4960>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<https://www.rfc-editor.org/info/rfc5246>.
[RFC5705] Rescorla, E., "Keying Material Exporters for Transport
Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705,
March 2010, <https://www.rfc-editor.org/info/rfc5705>.
[RFC5746] Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
"Transport Layer Security (TLS) Renegotiation Indication
Extension", RFC 5746, DOI 10.17487/RFC5746, February 2010,
<https://www.rfc-editor.org/info/rfc5746>.
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[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <https://www.rfc-editor.org/info/rfc6347>.
[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015,
<https://www.rfc-editor.org/info/rfc7540>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8260] Stewart, R., Tuexen, M., Loreto, S., and R. Seggelmann,
"Stream Schedulers and User Message Interleaving for the
Stream Control Transmission Protocol", RFC 8260,
DOI 10.17487/RFC8260, November 2017,
<https://www.rfc-editor.org/info/rfc8260>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[RFC8447] Salowey, J. and S. Turner, "IANA Registry Updates for TLS
and DTLS", RFC 8447, DOI 10.17487/RFC8447, August 2018,
<https://www.rfc-editor.org/info/rfc8447>.
[I-D.ietf-tls-dtls13]
Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", Work in Progress, Internet-Draft, draft-ietf-tls-
dtls13-40, 20 January 2021, <http://www.ietf.org/internet-
drafts/draft-ietf-tls-dtls13-40.txt>.
9.2. Informative References
[RFC3436] Jungmaier, A., Rescorla, E., and M. Tuexen, "Transport
Layer Security over Stream Control Transmission Protocol",
RFC 3436, DOI 10.17487/RFC3436, December 2002,
<https://www.rfc-editor.org/info/rfc3436>.
[RFC5061] Stewart, R., Xie, Q., Tuexen, M., Maruyama, S., and M.
Kozuka, "Stream Control Transmission Protocol (SCTP)
Dynamic Address Reconfiguration", RFC 5061,
DOI 10.17487/RFC5061, September 2007,
<https://www.rfc-editor.org/info/rfc5061>.
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[RFC6083] Tuexen, M., Seggelmann, R., and E. Rescorla, "Datagram
Transport Layer Security (DTLS) for Stream Control
Transmission Protocol (SCTP)", RFC 6083,
DOI 10.17487/RFC6083, January 2011,
<https://www.rfc-editor.org/info/rfc6083>.
[RFC6458] Stewart, R., Tuexen, M., Poon, K., Lei, P., and V.
Yasevich, "Sockets API Extensions for the Stream Control
Transmission Protocol (SCTP)", RFC 6458,
DOI 10.17487/RFC6458, December 2011,
<https://www.rfc-editor.org/info/rfc6458>.
[RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
Morris, J., Hansen, M., and R. Smith, "Privacy
Considerations for Internet Protocols", RFC 6973,
DOI 10.17487/RFC6973, July 2013,
<https://www.rfc-editor.org/info/rfc6973>.
[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
2014, <https://www.rfc-editor.org/info/rfc7258>.
[RFC7457] Sheffer, Y., Holz, R., and P. Saint-Andre, "Summarizing
Known Attacks on Transport Layer Security (TLS) and
Datagram TLS (DTLS)", RFC 7457, DOI 10.17487/RFC7457,
February 2015, <https://www.rfc-editor.org/info/rfc7457>.
[RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
2015, <https://www.rfc-editor.org/info/rfc7525>.
[ANSSI-DAT-NT-003]
Agence nationale de la sécurité des systèmes
d'information, ., "Recommendations for securing networks
with IPsec", ANSSI Technical Report DAT-NT-003 , August
2015,
<https://www.ssi.gouv.fr/uploads/2015/09/NT_IPsec_EN.pdf>.
Appendix A. Motivation for Changes
This document proposes a number of changes to RFC 6083 that have
various different motivations:
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Supporting Large User Messages: RFC 6083 allowed only user messages
that could fit within a single DTLS record. 3GPP has run into this
limitation where they have at least four SCTP using protocols (F1,
E1, Xn, NG-C) that can potentially generate messages over the size of
16384 bytes.
New Versions: Almost 10 years has passed since RFC 6083 was written,
and significant evolution has happened in the area of DTLS and
security algorithms. Thus DTLS 1.3 is the newest version of DTLS and
also the SHA-1 HMAC algorithm of RFC 4895 is getting towards the end
of usefulness. Thus, this document mandates usage of relevant
versions and algorithms.
Clarifications: Some implementation experiences has been gained that
motivates additional clarifications on the specification.
* Avoid unsecured messages prior to DTLS handshake have completed.
* Make clear that all messages are encrypted after DTLS handshake.
Authors' Addresses
Magnus Westerlund
Ericsson
Email: magnus.westerlund@ericsson.com
John Preuß Mattsson
Ericsson
Email: john.mattsson@ericsson.com
Claudio Porfiri
Ericsson
Email: claudio.porfiri@ericsson.com
Michael Tüxen
Münster University of Applied Sciences
Stegerwaldstrasse 39
48565 Steinfurt
Germany
Email: tuexen@fh-muenster.de
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