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Versions: (draft-rescorla-tls-dtls-connection-id)
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In: Publication_Requested
TLS E. Rescorla, Ed.
Internet-Draft RTFM, Inc.
Updates: 6347 (if approved) H. Tschofenig, Ed.
Intended status: Standards Track T. Fossati
Expires: September 12, 2019 Arm Limited
March 11, 2019
Connection Identifiers for DTLS 1.2
draft-ietf-tls-dtls-connection-id-04
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.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
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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 September 12, 2019.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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than English.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Terminology . . . . . . . . . . . . . . . . . 3
3. The "connection_id" Extension . . . . . . . . . . . . . . . . 3
4. Record Layer Extensions . . . . . . . . . . . . . . . . . . . 5
5. Record Payload Protection . . . . . . . . . . . . . . . . . . 7
6. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 8
7. Security and Privacy Considerations . . . . . . . . . . . . . 10
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
9.1. Normative References . . . . . . . . . . . . . . . . . . 11
9.2. Informative References . . . . . . . . . . . . . . . . . 11
Appendix A. History . . . . . . . . . . . . . . . . . . . . . . 13
Appendix B. Working Group Information . . . . . . . . . . . . . 13
Appendix C. Contributors . . . . . . . . . . . . . . . . . . . . 14
Appendix D. Acknowledgements . . . . . . . . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15
1. Introduction
The Datagram Transport Layer Security (DTLS) protocol was designed
for securing connection-less transports, like 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
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initial handshake across subsequent application data protection.
Ideally, the second phase where application data is protected lasts
over a longer period of time since the established keys will only
need to be updated once the key lifetime expires.
In the current version of DTLS, 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 CID to the
DTLS record layer. The presence of the CID is negotiated via a DTLS
extension.
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 RFC
2119 [RFC2119].
This document assumes familiarity with DTLS 1.2 [RFC6347].
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(TBD), (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 with a CID but does not wish the
server to use one when sending. Alternatively, this can be
interpreted as the client wishes the server to use a zero-length CID;
the result is the same.
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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 with the client's CID but does
not wish the client to include a CID (or again, alternately, to use a
zero-length CID).
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 globally 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. Implementations, which want to use variable-length CIDs,
are responsible for constructing the CID in such a way that its
length can be determined on reception. Such implementations must
still be able to send CIDs of different length to other parties.
Note that there is no CID length information included in the record
itself.
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 then the RFC
6347-defined record format and content type MUST be used.
If DTLS peers have negotiated the use of a CIDs using the ClientHello
and the ServerHello messages then the peers need to take the
following steps.
The DTLS peers determine whether incoming and outgoing messages need
to use the new record format, i.e., the record format containing the
CID. The new record format with the the tls12_cid content type is
only used once encryption is enabled. Plaintext payloads never use
the new record type and the CID content type.
For sending, if a zero-length CID has been negotiated then the RFC
6347-defined record format and content type MUST be used (see
Section 4.1 of [RFC6347]) else the new record layer format with the
tls12_cid content type defined in Figure 1 MUST be used.
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When transmitting a datagram with the tls12_cid content type, the new
MAC computation defined in Section 5 MUST be used.
For 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 security
association is looked up by the 5-tuple and a check MUST be made to
determine whether the expected CID value is indeed zero length. If
the check fails, then the datagram MUST be dropped.
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 RFC 6347-defined record format the MAC calculation
defined in Section 4.1.2 of [RFC6347] MUST be used.
4. Record Layer Extensions
This specification defines the DTLS 1.2 record layer format and
[I-D.ietf-tls-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 which have negotiated this extension use a distinguished
record type tls12_cid(25). 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 true content type is inside the encryption envelope, as
described below.
When CIDs are being used, the content to be sent is first wrapped
along with its content type and optional padding into a
DTLSInnerPlaintext:
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struct {
ContentType type;
ProtocolVersion version;
uint16 epoch;
uint48 sequence_number;
uint16 length;
opaque fragment[DTLSPlaintext.length];
} DTLSPlaintext;
struct {
opaque content[DTLSPlaintext.length];
ContentType real_type;
uint8 zeros[length_of_padding];
} DTLSInnerPlaintext;
content A copy of DTLSPlaintext.fragment
real_type A copy of DTLSPlaintext.type
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 value is then encrypted and the CID added to
produce the final DTLSCiphertext.
struct {
ContentType special_type = tls12_cid; /* 25 */
ProtocolVersion version;
uint16 epoch;
uint48 sequence_number;
opaque cid[cid_length]; // New field
uint16 length;
opaque enc_content[DTLSCiphertext.length];
} DTLSCiphertext;
Figure 1: DTLSCiphertext with CID
special_type The outer content type of a DTLSCiphertext record
carrying a CID is always set to the value 25 (tls12_cid). The
actual 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.
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enc_content The encrypted form of the serialized DTLSInnerPlaintext
structure.
All other fields are as defined in RFC 6347.
5. Record Payload Protection
This specification modifies the MAC calculation defined in [RFC6347]
and [RFC7366] as well as the definition of the additional data used
with 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.
- Block Ciphers:
MAC(MAC_write_key, seq_num +
tls12_cid + // New input
DTLSPlaintext.version +
cid + // New input
cid_length + // New input
length_of_DTLSInnerPlaintext + // New input
DTLSInnerPlaintext.content + // New input
DTLSInnerPlaintext.real_type + // New input
DTLSInnerPlaintext.zeros // New input
)
- Block Ciphers with Encrypt-then-MAC processing:
MAC(MAC_write_key, seq_num +
DTLSCipherText.type +
DTLSCipherText.version +
DTLSPlaintext.version +
cid + // New input
cid_length + // New input
length of (IV + DTLSCiphertext.enc_content) +
IV +
DTLSCiphertext.enc_content);
- AEAD Ciphers:
additional_data = seq_num + DTLSPlaintext.type +
DTLSPlaintext.version +
cid + // New input
cid_length + // New input
length_of_DTLSInnerPlaintext;
Where:
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cid Value of the negotiated CID.
cid_length 1 byte field indicating the length of the negotiated CID.
All other fields are as defined in the cited documents.
length_of_DTLSInnerPlaintext The length (in bytes) of the serialised
DTLSInnerPlaintext. The length MUST NOT exceed 2^14.
6. Examples
Figure 2 shows an example exchange where a CID is used uni-
directionally from the client to the server. To indicate that a
zero-length CID we use the term 'connection_id=empty'.
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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 2: 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
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how to use the CID for payloads sent from the client to the server
only the record layer payload containing the Finished messagen
contains a CID. Application data payloads sent from the client to
the server contain a CID in this example as well.
7. Security and 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].
In addition, endpoints can use the CID to attach arbitrary metadata
to each record they receive. This may be used as a mechanism to
communicate per-connection information to on-path observers. There
is no straightforward way to address this with CIDs that contain
arbitrary values; implementations concerned about this SHOULD refuse
to use connection ids.
An on-path adversary, who is able to observe 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 connection id pair (for bi-directional communication). Without
multi-homing or mobility, the use of the CID is not different to the
use of the 5-tuple.
With multi-homing, an adversary is able to correlate the
communication interaction over the two paths, which adds further
privacy concerns.
Importantly, the sequence number makes it possible for a passive
attacker to correlate packets across CID changes. Thus, even if a
client/server pair do a rehandshake to change CID, that does not
provide much privacy benefit.
The CID-enhanced record layer introduces record padding; a privacy
feature not available with the original DTLS 1.2 RFC. Padding allows
to inflate the size of the ciphertext making traffic analysis more
difficult. More details about the padding can be found in
Section 5.4 and Appendix E.3 of RFC 8446.
8. IANA Considerations
IANA is requested to allocate an entry to the existing TLS
"ExtensionType Values" registry, defined in [RFC5246], for
connection_id(TBD) defined in this document.
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IANA is requested to allocate tls12_cid(25) in the "TLS ContentType
Registry".
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>.
[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>.
[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>.
[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>.
9.2. Informative References
[I-D.ietf-tls-dtls13]
Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", draft-ietf-tls-dtls13-30 (work in progress),
November 2018.
[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>.
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9.3. URIs
[1] mailto:tls@ietf.org
[2] https://www1.ietf.org/mailman/listinfo/tls
[3] https://www.ietf.org/mail-archive/web/tls/current/index.html
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Appendix A. History
RFC EDITOR: PLEASE REMOVE THE THIS SECTION
draft-ietf-tls-dtls-connection-id-03
- Updated list of contributors
- Updated list of contributors and acknowledgements
- Updated example
- Changed record layer design
- Changed record payload protection
- Updated introduction and security consideration section
- Author- and affiliation changes
draft-ietf-tls-dtls-connection-id-02
- Move to internal content types a la DTLS 1.3.
draft-ietf-tls-dtls-connection-id-01
- Remove 1.3 based on the WG consensus at IETF 101
draft-ietf-tls-dtls-connection-id-00
- Initial working group version (containing a solution for DTLS 1.2
and 1.3)
draft-rescorla-tls-dtls-connection-id-00
- Initial version
Appendix B. Working Group Information
RFC EDITOR: PLEASE REMOVE THE THIS SECTION
The discussion list for the IETF TLS working group is located at the
e-mail address tls@ietf.org [1]. Information on the group and
information on how to subscribe to the list is at
https://www1.ietf.org/mailman/listinfo/tls [2]
Archives of the list can be found at: https://www.ietf.org/mail-
archive/web/tls/current/index.html [3]
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Appendix C. Contributors
Many people have contributed to this specification and we would like
to thank the following individuals for their contributions:
* Yin Xinxing
Huawei
yinxinxing@huawei.com
* Nikos Mavrogiannopoulos
RedHat
nmav@redhat.com
* Tobias Gondrom
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,
if these privacy properties are important in a given deployment.
Finally, we want to thank the IETF TLS working group chairs, Chris
Wood, Joseph Salowey, and Sean Turner, for their patience, support
and feedback.
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Appendix D. Acknowledgements
We would like to thank Achim Kraus for his review feedback.
Authors' Addresses
Eric Rescorla (editor)
RTFM, Inc.
EMail: ekr@rtfm.com
Hannes Tschofenig (editor)
Arm Limited
EMail: hannes.tschofenig@arm.com
Thomas Fossati
Arm Limited
EMail: thomas.fossati@arm.com
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