RFC9146: Connection Identifier for DTLS 1.2
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(NAT rebinding)
Internet Engineering Task Force (IETF) E. Rescorla, Ed.
Request for Comments: 9146 Mozilla
Updates: 6347 H. Tschofenig, Ed.
Category: Standards Track T. Fossati
ISSN: 2070-1721 Arm Limited
A. Kraus
Bosch.IO GmbH
March 2022
Connection Identifier for DTLS 1.2
Abstract
This document specifies the Connection ID (CID) construct for the
Datagram Transport Layer Security (DTLS) protocol version 1.2.
A CID is an identifier carried in the record layer header that gives
the recipient additional information for selecting the appropriate
security association. In "classical" DTLS, selecting a security
association of an incoming DTLS record is accomplished with the help
of the 5-tuple. If the source IP address and/or source port changes
during the lifetime of an ongoing DTLS session, then the receiver
will be unable to locate the correct security context.
The new ciphertext record format with the CID also provides content
type encryption and record layer padding.
This document updates RFC 6347.
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
https://www.rfc-editor.org/info/rfc9146.
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Table of Contents
1. Introduction
2. Conventions and Terminology
3. The "connection_id" Extension
4. Record Layer Extensions
5. Record Payload Protection
5.1. Block Ciphers
5.2. Block Ciphers with Encrypt-then-MAC Processing
5.3. AEAD Ciphers
6. Peer Address Update
7. Example
8. Privacy Considerations
9. Security Considerations
10. IANA Considerations
10.1. Extra Column Added to the TLS ExtensionType Values
Registry
10.2. New Entry in the TLS ExtensionType Values Registry
10.3. New Entry in the TLS ContentType Registry
11. References
11.1. Normative References
11.2. Informative References
Acknowledgements
Contributors
Authors' Addresses
1. Introduction
The Datagram Transport Layer Security (DTLS) protocol [RFC6347] was
designed for securing data sent over datagram transports (e.g., UDP).
DTLS, like TLS, starts with a handshake, which can be computationally
demanding (particularly when public key cryptography is used). After
a successful handshake, symmetric key cryptography is used to apply
data origin authentication, integrity, and confidentiality
protection. This two-step approach allows endpoints to amortize the
cost of the initial handshake across subsequent application data
protection. Ideally, the second phase where application data is
protected lasts over a long period of time, since the established
keys will only need to be updated once the key lifetime expires.
In DTLS as specified in RFC 6347, the IP address and port of the peer
are used to identify the DTLS association. Unfortunately, in some
cases, such as NAT rebinding, these values are insufficient. This is
a particular issue in the Internet of Things when devices enter
extended sleep periods to increase their battery lifetime. The NAT
rebinding leads to connection failure, with the resulting cost of a
new handshake.
This document defines an extension to DTLS 1.2 to add a Connection ID
(CID) to the DTLS record layer. The presence of the CID is
negotiated via a DTLS extension.
Adding a CID to the ciphertext record format presents an opportunity
to make other changes to the record format. In keeping with the best
practices established by TLS 1.3, the type of the record is
encrypted, and a mechanism is provided for adding padding to
obfuscate the plaintext length.
2. Conventions and Terminology
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.
This document assumes familiarity with DTLS 1.2 [RFC6347]. The
presentation language used in this document is described in Section 3
of [RFC8446].
3. The "connection_id" Extension
This document defines the "connection_id" extension, which is used in
ClientHello and ServerHello messages.
The extension type is specified as follows.
enum {
connection_id(54), (65535)
} ExtensionType;
The extension_data field of this extension, when included in the
ClientHello, MUST contain the ConnectionId structure. This structure
contains the CID value the client wishes the server to use when
sending messages to the client. A zero-length CID value indicates
that the client is prepared to send using a CID but does not wish the
server to use one when sending.
struct {
opaque cid<0..2^8-1>;
} ConnectionId;
A server willing to use CIDs will respond with a "connection_id"
extension in the ServerHello, containing the CID it wishes the client
to use when sending messages towards it. A zero-length value
indicates that the server will send using the client's CID but does
not wish the client to include a CID when sending.
Because each party sends the value in the "connection_id" extension
it wants to receive as a CID in encrypted records, it is possible for
an endpoint to use a deployment-specific constant length for such
connection identifiers. This can in turn ease parsing and connection
lookup -- for example, by having the length in question be a compile-
time constant. Such implementations MUST still be able to send CIDs
of different lengths to other parties. Since the CID length
information is not included in the record itself, implementations
that want to use variable-length CIDs are responsible for
constructing the CID in such a way that its length can be determined
on reception.
In DTLS 1.2, CIDs are exchanged at the beginning of the DTLS session
only. There is no dedicated "CID update" message that allows new
CIDs to be established mid-session, because DTLS 1.2 in general does
not allow TLS 1.3-style post-handshake messages that do not
themselves begin other handshakes. When a DTLS session is resumed or
renegotiated, the "connection_id" extension is negotiated afresh.
If DTLS peers have not negotiated the use of CIDs, or a zero-length
CID has been advertised for a given direction, then the record format
and content type defined in RFC 6347 MUST be used to send in the
indicated direction(s).
If DTLS peers have negotiated the use of a non-zero-length CID for a
given direction, then once encryption is enabled, they MUST send with
the record format defined in Figure 3 (see Section 4) with the new
Message Authentication Code (MAC) computation defined in Section 5
and the content type tls12_cid. Plaintext payloads never use the new
record format or the CID content type.
When receiving, if the tls12_cid content type is set, then the CID is
used to look up the connection and the security association. If the
tls12_cid content type is not set, then the connection and the
security association are looked up by the 5-tuple and a check MUST be
made to determine whether a non-zero-length CID is expected. If a
non-zero-length CID is expected for the retrieved association, then
the datagram MUST be treated as invalid, as described in
Section 4.1.2.1 of [RFC6347].
When receiving a datagram with the tls12_cid content type, the new
MAC computation defined in Section 5 MUST be used. When receiving a
datagram with the record format defined in RFC 6347, the MAC
calculation defined in Section 4.1.2 of [RFC6347] MUST be used.
4. Record Layer Extensions
This specification defines the CID-enhanced record layer format for
DTLS 1.2, and [DTLS13] specifies how to carry the CID in DTLS 1.3.
To allow a receiver to determine whether a record has a CID or not,
connections that have negotiated this extension use a distinguished
record type tls12_cid(25). The use of this content type has the
following three implications:
* The CID field is present and contains one or more bytes.
* The MAC calculation follows the process described in Section 5.
* The real content type is inside the encryption envelope, as
described below.
Plaintext records are not impacted by this extension. Hence, the
format of the DTLSPlaintext structure is left unchanged, as shown in
Figure 1.
struct {
ContentType type;
ProtocolVersion version;
uint16 epoch;
uint48 sequence_number;
uint16 length;
opaque fragment[DTLSPlaintext.length];
} DTLSPlaintext;
Figure 1: DTLS 1.2 Plaintext Record Payload
When CIDs are being used, the content to be sent is first wrapped
along with its content type and optional padding into a
DTLSInnerPlaintext structure. This newly introduced structure is
shown in Figure 2.
struct {
opaque content[length];
ContentType real_type;
uint8 zeros[length_of_padding];
} DTLSInnerPlaintext;
Figure 2: New DTLSInnerPlaintext Payload Structure
content: Corresponds to the fragment of a given length.
real_type: The content type describing the cleartext payload.
zeros: An arbitrary-length run of zero-valued bytes may appear in
the cleartext after the type field. This provides an opportunity
for senders to pad any DTLS record by a chosen amount as long as
the total stays within record size limits. See Section 5.4 of
[RFC8446] for more details. (Note that the term TLSInnerPlaintext
in RFC 8446 refers to DTLSInnerPlaintext in this specification.)
The DTLSInnerPlaintext byte sequence is then encrypted. To create
the DTLSCiphertext structure shown in Figure 3, the CID is added.
struct {
ContentType outer_type = tls12_cid;
ProtocolVersion version;
uint16 epoch;
uint48 sequence_number;
opaque cid[cid_length]; // New field
uint16 length;
opaque enc_content[DTLSCiphertext.length];
} DTLSCiphertext;
Figure 3: DTLS 1.2 CID-Enhanced Ciphertext Record
outer_type: The outer content type of a DTLSCiphertext record
carrying a CID is always set to tls12_cid(25). The real content
type of the record is found in DTLSInnerPlaintext.real_type after
decryption.
cid: The CID value, cid_length bytes long, as agreed at the time the
extension has been negotiated. Recall that each peer chooses the
CID value it will receive and use to identify the connection, so
an implementation can choose to always receive CIDs of a fixed
length. If, however, an implementation chooses to receive CIDs of
different lengths, the assigned CID values must be self-
delineating, since there is no other mechanism available to
determine what connection (and thus, what CID length) is in use.
enc_content: The encrypted form of the serialized DTLSInnerPlaintext
structure.
All other fields are as defined in RFC 6347.
5. Record Payload Protection
Several types of ciphers have been defined for use with TLS and DTLS,
and the MAC calculations for those ciphers differ slightly.
This specification modifies the MAC calculation as defined in
[RFC6347] and [RFC7366], as well as the definition of the additional
data used with Authenticated Encryption with Associated Data (AEAD)
ciphers provided in [RFC6347], for records with content type
tls12_cid. The modified algorithm MUST NOT be applied to records
that do not carry a CID, i.e., records with content type other than
tls12_cid.
The following fields are defined in this document; all other fields
are as defined in the cited documents.
cid: Value of the negotiated CID (variable length).
cid_length: The length (in bytes) of the negotiated CID (one-byte
integer).
length_of_DTLSInnerPlaintext: The length (in bytes) of the
serialized DTLSInnerPlaintext (two-byte integer). The length MUST
NOT exceed 2^14.
seq_num_placeholder: 8 bytes of 0xff.
Note that "+" denotes concatenation.
5.1. Block Ciphers
The following MAC algorithm applies to block ciphers that do not use
the Encrypt-then-MAC processing described in [RFC7366].
MAC(MAC_write_key,
seq_num_placeholder +
tls12_cid +
cid_length +
tls12_cid +
DTLSCiphertext.version +
epoch +
sequence_number +
cid +
length_of_DTLSInnerPlaintext +
DTLSInnerPlaintext.content +
DTLSInnerPlaintext.real_type +
DTLSInnerPlaintext.zeros
);
The rationale behind this construction is to separate the MAC input
for DTLS without the connection ID from the MAC input with the
connection ID. The former always consists of a sequence number
followed by some content type other than tls12_cid; the latter always
consists of the seq_num_placeholder followed by tls12_cid. Although
2^64-1 is potentially a valid sequence number, tls12_cid will never
be a valid content type when the connection ID is not in use. In
addition, the epoch and sequence_number are now fed into the MAC in
the same order as they appear on the wire.
5.2. Block Ciphers with Encrypt-then-MAC Processing
The following MAC algorithm applies to block ciphers that use the
Encrypt-then-MAC processing described in [RFC7366].
MAC(MAC_write_key,
seq_num_placeholder +
tls12_cid +
cid_length +
tls12_cid +
DTLSCiphertext.version +
epoch +
sequence_number +
cid +
DTLSCiphertext.length +
IV +
ENC(content + padding + padding_length)
);
5.3. AEAD Ciphers
For ciphers utilizing AEAD, the following modification is made to the
additional data calculation.
additional_data = seq_num_placeholder +
tls12_cid +
cid_length +
tls12_cid +
DTLSCiphertext.version +
epoch +
sequence_number +
cid +
length_of_DTLSInnerPlaintext;
6. Peer Address Update
When a record with a CID is received that has a source address
different from the one currently associated with the DTLS connection,
the receiver MUST NOT replace the address it uses for sending records
to its peer with the source address specified in the received
datagram, unless the following three conditions are met:
* The received datagram has been cryptographically verified using
the DTLS record layer processing procedures.
* The received datagram is "newer" (in terms of both epoch and
sequence number) than the newest datagram received. Reordered
datagrams that are sent prior to a change in a peer address might
otherwise cause a valid address change to be reverted. This also
limits the ability of an attacker to use replayed datagrams to
force a spurious address change, which could result in denial of
service. An attacker might be able to succeed in changing a peer
address if they are able to rewrite source addresses and if
replayed packets are able to arrive before any original.
* There is a strategy for ensuring that the new peer address is able
to receive and process DTLS records. No strategy is mandated by
this specification, but see note (*) below.
The conditions above are necessary to protect against attacks that
use datagrams with spoofed addresses or replayed datagrams to trigger
attacks. Note that there is no requirement for the use of the anti-
replay window mechanism defined in Section 4.1.2.6 of [RFC6347].
Both solutions, the "anti-replay window" or "newer" algorithm, will
prevent address updates from replay attacks while the latter will
only apply to peer address updates and the former applies to any
application layer traffic.
Note that datagrams that pass the DTLS cryptographic verification
procedures but do not trigger a change of peer address are still
valid DTLS records and are still to be passed to the application.
(*) Note: Application protocols that implement protection against
spoofed addresses depend on being aware of changes in peer
addresses so that they can engage the necessary mechanisms. When
delivered such an event, an address validation mechanism specific
to the application layer can be triggered -- for example, one that
is based on successful exchange of a minimal amount of ping-pong
traffic with the peer. Alternatively, a DTLS-specific mechanism
may be used, as described in [DTLS-RRC].
DTLS implementations MUST silently discard records with bad MACs or
that are otherwise invalid.
7. Example
Figure 4 shows an example exchange where a CID is used
unidirectionally from the client to the server. To indicate that a
zero-length CID is present in the "connection_id" extension, we use
the notation 'connection_id=empty'.
Client Server
------ ------
ClientHello -------->
(connection_id=empty)
<-------- HelloVerifyRequest
(cookie)
ClientHello -------->
(connection_id=empty)
(cookie)
ServerHello
(connection_id=100)
Certificate
ServerKeyExchange
CertificateRequest
<-------- ServerHelloDone
Certificate
ClientKeyExchange
CertificateVerify
[ChangeCipherSpec]
Finished -------->
<CID=100>
[ChangeCipherSpec]
<-------- Finished
Application Data ========>
<CID=100>
<======== Application Data
Legend:
<...> indicates that a connection ID is used in the record layer
(...) indicates an extension
[...] indicates a payload other than a handshake message
Figure 4: Example DTLS 1.2 Exchange with CID
Note: In the example exchange, the CID is included in the record
layer once encryption is enabled. In DTLS 1.2, only one handshake
message is encrypted, namely the Finished message. Since the
example shows how to use the CID for payloads sent from the client
to the server, only the record layer payloads containing the
Finished message or application data include a CID.
8. Privacy Considerations
The CID replaces the previously used 5-tuple and, as such, introduces
an identifier that remains persistent during the lifetime of a DTLS
connection. Every identifier introduces the risk of linkability, as
explained in [RFC6973].
An on-path adversary observing the DTLS protocol exchanges between
the DTLS client and the DTLS server is able to link the observed
payloads to all subsequent payloads carrying the same ID pair (for
bidirectional communication). Without multihoming or mobility, the
use of the CID exposes the same information as the 5-tuple.
With multihoming, a passive attacker is able to correlate the
communication interaction over the two paths. The lack of a CID
update mechanism in DTLS 1.2 makes this extension unsuitable for
mobility scenarios where correlation must be considered. Deployments
that use DTLS in multihoming environments and are concerned about
these aspects SHOULD refuse to use CIDs in DTLS 1.2 and switch to
DTLS 1.3 where a CID update mechanism is provided and sequence number
encryption is available.
This specification introduces record padding for the CID-enhanced
record layer, which is a privacy feature not available with the
original DTLS 1.2 specification. Padding allows the size of the
ciphertext to be inflated, making traffic analysis more difficult.
More details about record padding can be found in Section 5.4 and
Appendix E.3 of [RFC8446].
Finally, endpoints can use the CID to attach arbitrary per-connection
metadata to each record they receive on a given connection. This may
be used as a mechanism to communicate per-connection information to
on-path observers. There is no straightforward way to address this
concern with CIDs that contain arbitrary values. Implementations
concerned about this aspect SHOULD refuse to use CIDs.
9. Security Considerations
An on-path adversary can create reflection attacks against third
parties because a DTLS peer has no means to distinguish a genuine
address update event (for example, due to a NAT rebinding) from one
that is malicious. This attack is of particular concern when the
request is small and the response large. See Section 6 for more on
address updates.
Additionally, an attacker able to observe the data traffic exchanged
between two DTLS peers is able to replay datagrams with modified IP
addresses / port numbers.
The topic of peer address updates is discussed in Section 6.
10. IANA Considerations
This document implements three IANA updates.
10.1. Extra Column Added to the TLS ExtensionType Values Registry
IANA has added an extra column named "DTLS-Only" to the "TLS
ExtensionType Values" registry to indicate whether an extension is
only applicable to DTLS and to include this document as an additional
reference for the registry.
10.2. New Entry in the TLS ExtensionType Values Registry
IANA has allocated an entry in the existing "TLS ExtensionType
Values" registry for connection_id(54), as described in the table
below. Although the value 53 had been allocated by early allocation
for a previous version of this document, it is incompatible with this
document. Therefore, the early allocation has been deprecated in
favor of this assignment.
+=======+===============+=====+===========+=============+===========+
| Value |Extension Name | TLS | DTLS-Only | Recommended | Reference |
| | | 1.3 | | | |
+=======+===============+=====+===========+=============+===========+
| 54 |connection_id | CH, | Y | N | RFC 9146 |
| | | SH | | | |
+-------+---------------+-----+-----------+-------------+-----------+
Table 1
A new column, "DTLS-Only", has been added to the registry. The valid
entries are "Y" if the extension is only applicable to DTLS, "N"
otherwise. All the pre-existing entries are given the value "N".
Note: The value "N" in the "Recommended" column is set because
this extension is intended only for specific use cases. This
document describes the behavior of this extension for DTLS 1.2
only; it is not applicable to TLS, and its usage for DTLS 1.3 is
described in [DTLS13].
10.3. New Entry in the TLS ContentType Registry
IANA has allocated tls12_cid(25) in the "TLS ContentType" registry.
The tls12_cid content type is only applicable to DTLS 1.2.
11. References
11.1. 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>.
[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>.
[RFC7366] Gutmann, P., "Encrypt-then-MAC for Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", RFC 7366, DOI 10.17487/RFC7366, September 2014,
<https://www.rfc-editor.org/info/rfc7366>.
[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>.
[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>.
11.2. Informative References
[DTLS-RRC] Tschofenig, H., Ed. and T. Fossati, "Return Routability
Check for DTLS 1.2 and DTLS 1.3", Work in Progress,
Internet-Draft, draft-ietf-tls-dtls-rrc-05, 7 March 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-tls-
dtls-rrc-05>.
[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-
dtls13-43, 30 April 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-tls-
dtls13-43>.
[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>.
Acknowledgements
We would like to thank Hanno Becker, Martin Duke, Lars Eggert, Ben
Kaduk, Warren Kumari, Francesca Palombini, Tom Petch, John Scudder,
Sean Turner, Éric Vyncke, and Robert Wilton for their review
comments.
Finally, we want to thank the IETF TLS Working Group chairs, Chris
Wood, Joseph Salowey, and Sean Turner, for their patience, support,
and feedback.
Contributors
Many people have contributed to this specification, and we would like
to thank the following individuals for their contributions:
Yin Xinxing
Huawei
Email: yinxinxing@huawei.com
Nikos Mavrogiannopoulos
RedHat
Email: nmav@redhat.com
Tobias Gondrom
Email: tobias.gondrom@gondrom.org
Additionally, we would like to thank the Connection ID task force
team members:
* Martin Thomson (Mozilla)
* Christian Huitema (Private Octopus Inc.)
* Jana Iyengar (Google)
* Daniel Kahn Gillmor (ACLU)
* Patrick McManus (Mozilla)
* Ian Swett (Google)
* Mark Nottingham (Fastly)
The task force team discussed various design ideas, including
cryptographically generated session IDs using hash chains and public
key encryption, but dismissed them due to their inefficiency. The
approach described in this specification is the simplest possible
design that works, given the limitations of DTLS 1.2. DTLS 1.3
provides better privacy features, and developers are encouraged to
switch to the new version of DTLS.
Authors' Addresses
Eric Rescorla (editor)
Mozilla
Email: ekr@rtfm.com
Hannes Tschofenig (editor)
Arm Limited
Email: hannes.tschofenig@arm.com
Thomas Fossati
Arm Limited
Email: thomas.fossati@arm.com
Achim Kraus
Bosch.IO GmbH
Email: achim.kraus@bosch.io