TLS E. Rescorla, Ed.
Internet-Draft RTFM, Inc.
Updates: 6347 (if approved) H. Tschofenig, Ed.
Intended status: Standards Track Arm Limited
Expires: April 25, 2019 T. Fossati
Nokia
T. Gondrom
Huawei
October 22, 2018
Connection Identifiers for DTLS 1.2
draft-ietf-tls-dtls-connection-id-02
Abstract
This document specifies the Connection ID construct for the Datagram
Transport Layer Security (DTLS) protocol version 1.2.
A Connection ID 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
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 25, 2019.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Terminology . . . . . . . . . . . . . . . . . 3
3. The "connection_id" Extension . . . . . . . . . . . . . . . . 3
4. Record Layer Extensions . . . . . . . . . . . . . . . . . . . 5
5. Record Payload Protection . . . . . . . . . . . . . . . . . . 5
6. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 6
7. Security and Privacy Considerations . . . . . . . . . . . . . 7
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
9.1. Normative References . . . . . . . . . . . . . . . . . . 8
9.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. History . . . . . . . . . . . . . . . . . . . . . . 10
Appendix B. Working Group Information . . . . . . . . . . . . . 10
Appendix C. Contributors . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
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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
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 to add a connection ID
(CID) to the DTLS record layer. The presence of the connection ID 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].
The reader is assumed to be familiar with DTLS [RFC6347].
3. The "connection_id" Extension
This document defines a new extension type (connection_id(TBD)),
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, which carries
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the CID which the client wishes the server to use when sending
messages towards it. A zero-length value indicates that the client
is prepared to send with a connection ID but does not wish the server
to use one when sending (alternately, this can be interpreted as the
client wishes the server to use a zero-length CID; the result is the
same).
struct {
opaque cid<0..2^8-1>;
} ConnectionId;
A server which is willing to use CIDs will respond with its own
"connection_id" extension, 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 use a CID (or again, alternately, to use a zero-length
CID).
When a session is resumed, the "connection_id" extension is
negotiated afresh, not retained from previous connections in the
session.
This is effectively the simplest possible design that will work.
Previous design ideas for using cryptographically generated session
ids, either using hash chains or public key encryption, were
dismissed due to their inefficient designs. Note that a client
always has the chance to fall back to a full handshake or more
precisely to a handshake that uses session resumption.
Because each party sends in the extension_data the value that it will
receive as a connection identifier 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. Note that such
implementations must still be able to send other length connection
identifiers to other parties.
In DTLS, connection ids are exchanged at the beginning of the DTLS
session only. There is no dedicated "connection id update" message
that allows new connection ids to be established mid-session, because
DTLS in general does not allow TLS 1.3-style post-handshake messages
that do not themselves begin other handshakes. DTLS peers switch to
the new record layer format when encryption is enabled.
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4. Record Layer Extensions
This extension is applicable for use with DTLS 1.2 and below.
Figure 1 illustrates the record format. [I-D.ietf-tls-dtls13]
specifies how to carry the CID in a DTLS 1.3 record.
struct {
ContentType type;
ProtocolVersion version;
uint16 epoch;
uint48 sequence_number;
opaque cid[cid_length]; // New field
uint16 length;
select (CipherSpec.cipher_type) {
case block: GenericBlockCipher;
case aead: GenericAEADCipher;
} fragment;
} DTLSCiphertext;
Figure 1: DTLS 1.2 Record Format with Connection ID
Note that for both record formats, it is not possible to parse the
records without knowing how long the Connection ID is.
In order to allow a receiver to determine whether a record has 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 two implications:
- The CID field is present
- The true content type is inside the encryption envelope, as
described below.
5. Record Payload Protection
When CID is being used, the DTLSCompressed value is first wrapped
along with the true content type and padding into a
DTLSWrappedCompressed value prior to encryption. The
DTLSWrappedCompressed value is then encrypted.
struct {
opaque compressed[TLSCompressed.length];
ContentType type;
uint8 zeros[length_of_padding];
} DTLSWrappedCompressed;
compressed The value of DTLSCompressed.fragment
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type The true content type.
zeroes Padding, as defined in [RFC8446].
In addition, the CID value is included in the MAC calculation for the
DTLS record as shown below. The MAC algorithm described in
Section 4.1.2.1 of [RFC6347] and Section 6.2.3.1 of [RFC5246] is
extended as follows:
MAC(MAC_write_key, DTLSCompressed.epoch +
DTLSCompressed.sequence_number +
tls12_cid +
DTLSCompressed.version +
cid_length + // New input
cid + // New input
DTLSWrappedCompressed.length +
DTLSWrappedCompressed.fragment);
where "+" denotes concatenation.
6. Examples
Figure 2 shows an example exchange where a connection id is used uni-
directionally from the client to the server.
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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
Figure 2: Example DTLS 1.2 Exchange with Connection IDs
7. Security and Privacy Considerations
The connection id 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 connection ID to attach arbitrary
metadata to each record they receive. This may be used as a
mechanism to communicate per-connection to on-path observers. There
is no straightforward way to address this with connection IDs that
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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 connection id 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.
This document does not change the security properties of DTLS
[RFC6347]. It merely provides a more robust mechanism for
associating an incoming packet with a stored security context.
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.
IANA is requested to allocate the following new values in the "TLS
ContentType Registry":
- alert_with_cid(25)
- handshake_with_cid(26)
- application_data_with_cid(27)
- heartbeat_with_cid(28)
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>.
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[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>.
[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-dtls13-28 (work in progress), July
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>.
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-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
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]
Appendix C. Contributors
Many people have contributed to this specification since the
functionality has been highly desired by the IoT community. We would
like to thank the following individuals for their contributions in
earlier specifications:
* Nikos Mavrogiannopoulos
RedHat
nmav@redhat.com
Additionally, we would like to thank Yin Xinxing (Huawei), Tobias
Gondrom (Huawei), and the Connection ID task force team members:
- Martin Thomson (Mozilla)
- Christian Huitema (Private Octopus Inc.)
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- Jana Iyengar (Google)
- Daniel Kahn Gillmor (ACLU)
- Patrick McManus (Mozilla)
- Ian Swett (Google)
- Mark Nottingham (Fastly)
Finally, we want to thank the IETF TLS working group chairs, Joseph
Salowey and Sean Turner, for their patience, support and feedback.
Authors' Addresses
Eric Rescorla (editor)
RTFM, Inc.
EMail: ekr@rtfm.com
Hannes Tschofenig (editor)
Arm Limited
EMail: hannes.tschofenig@arm.com
Thomas Fossati
Nokia
EMail: thomas.fossati@nokia.com
Tobias Gondrom
Huawei
EMail: tobias.gondrom@gondrom.org
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