TLS E. Rescorla, Ed.
Internet-Draft RTFM, Inc.
Obsoletes: 6347 (if approved) H. Tschofenig, Ed.
Intended status: Standards Track ARM Limited
Expires: April 15, 2018 October 12, 2017
The Datagram Transport Layer Security (DTLS) Connection Identifier
draft-rescorla-tls-dtls-connection-id-00
Abstract
This document specifies the "Connection ID" concept for the Datagram
Transport Layer Security (DTLS) protocol, version 1.2 and version
1.3.
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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Terminology . . . . . . . . . . . . . . . . . 3
3. The "connection_id" Extension . . . . . . . . . . . . . . . . 3
4. Post-Handshake Messages . . . . . . . . . . . . . . . . . . . 4
5. Record Layer Extensions . . . . . . . . . . . . . . . . . . . 5
6. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 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
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 to amortize the cost of the initial
handshake to subsequent application data protection. Ideally, the
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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
is 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 the device needs to
enter extended sleep periods to increase the battery lifetime and is
therefore subject to rebinding. This leads to connection failure,
with the resulting cost of a new handshake.
This document defines an extension to DTLS to add a connection ID to
each DTLS record. The presence of the connection ID is negotiated
via a DTLS extension. It also defines a DTLS 1.3 post-handshake
message to change connection ids.
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 the DTLS specifications
since this document defines an extension to DTLS 1.2 and DTLS 1.3.
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 CID structure, which contains 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).
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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 which 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 (DTLS 1.2
language) or to a PSK-based handshake using the ticket-based
approach.
In DTLS 1.2, 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 1.2 in general does not allow post-handshake messages
that do not themselves begin other handshakes. In DTLS 1.3, which
does allow such messages, we use post-handshake message to update the
connection ID Section 4 and to request new IDs.
DTLS 1.2 peers switch to the new record layer format when encryption
is enabled. The same is true for DTLS 1.3 but since the DTLS 1.3
enables encryption early in the handshake phase the connection ID
will be enabled earlier. For this reason, the connection ID needs to
go in the DTLS 1.3 ServerHello.
4. Post-Handshake Messages
In DTLS 1.3, if the client and server have negotiated the
"connection_id" extension, either side can send a new connection ID
which it wishes the other side to use in a NewConnectionId message:
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enum {
cid_immediate(0), cid_spare(1), (255)
} ConnectionIdUsage;
struct {
opaque cid<0..2^8-1>;
ConnectionIdUsage usage;
} NewConnectionId;
cid Indicates the CID which the sender wishes the peer to use.
usage Indicates whether the new CID should be used immediately or is
a spare. If usage is set to "cid_immediate", then the new CID
MUST be used immediately for all future records. If it is set to
"cid_spare", then either CID MAY be used, as described in
Section 7.
If the client and server have negotiated the "connection_id"
extension, either side can request a new CID using the
RequestConnectionId message.
struct {
} RequestConnectionId;
Endpoints SHOULD respond to RequestConnectionId by sending a
NewConnectionId with usage "cid_spare" as soon as possible. Note
that an endpoint MAY ignore requests which it considers excessive
(though they MUST be ACKed as usual).
5. Record Layer Extensions
This extension is applicable for use with DTLS 1.2 and DTLS 1.3.
This extension can be used with the optimized DTLS 1.3 record layer
format.
Figure 1 and Figure 2 illustrate the record formats of DTLS 1.2 and
DTLS 1.3, respectively.
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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
struct {
opaque content[DTLSPlaintext.length];
ContentType type;
uint8 zeros[length_of_padding];
} DTLSInnerPlaintext;
struct {
ContentType opaque_type = 23; /* application_data */
ProtocolVersion legacy_record_version = {254,253); // DTLSv1.2
uint16 epoch; // DTLS-related field
uint48 sequence_number; // DTLS-related field
opaque cid[cid_length]; // New field
uint16 length;
opaque encrypted_record[length];
} DTLSCiphertext;
Figure 2: DTLS 1.3 Record Format with Connection ID
Besides the "cid" field, all other fields are defined in the DTLS 1.2
and DTLS 1.3 specifications.
Note that for both record formats, it is not possible to parse the
records without knowing if the connection ID is in use and how long
it is.
6. Example
Below is an example exchange for DTLS 1.3 using a single connection
id in each direction.
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Client Server
------ ------
ClientHello
(connection_id=5)
-------->
<-------- HelloRetryRequest
(cookie)
ClientHello -------->
(connection_id=5)
+cookie
<-------- ServerHello
EncryptedExtensions
(connection_id=100)
Certificate
CertificateVerify
Finished
Certificate -------->
CertificateVerify
Finished
<-------- Ack
Application Data ========>
(cid=100)
<======== Application Data
(cid=5)
Figure 3: Example DTLS 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].
An on-path adversary, who is able to observe the DTLS 1.2 protocol
exchanges between the DTLS client and the DTLS server, is able to
link the initial handshake to all subsequent payloads carrying the
same connection id pair (for bi-directional communication). In DTLS
1.3, it is possible to provide new encrypted connection IDs, though
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of course those IDs are immediately used on the wire. Without multi-
homing and 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.
In order to prevent this, implementations SHOULD attempt to use fresh
connection IDs whenever they change local addresses or ports (though
this is not always possible to detect). In DTLS 1.3, The
RequestConnectionId message can be used to ask for new IDs in order
to ensure that you have a pool of suitable IDs.
This document does not change the security properties of DTLS 1.2
[RFC6347] and DTLS 1.3 [I-D.ietf-tls-dtls13]. 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 two values in the "TLS Handshake Type"
registry, defined in [RFC5246], for request_connection_id (TBD), and
new_connection_id (TBD), as defined in this document.
9. References
9.1. Normative 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-dtls13-01 (work in progress), July
2017.
[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>.
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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>.
9.2. Informative References
[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
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Appendix A. History
RFC EDITOR: PLEASE REMOVE THE THIS SECTION
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
Archives of the list can be found at: https://www.ietf.org/mail-
archive/web/tls/current/index.html
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:
* Thomas Fossati
Nokia
thomas.fossati@nokia.com
* 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.)
- Jana Iyengar (Google)
- Daniel Kahn Gillmor (ACLU)
- Patrick McManus (Sole Proprietor)
- Ian Swett (Google)
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- 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
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