TLS E. Rescorla, Ed.
Internet-Draft RTFM, Inc.
Obsoletes: 6347 (if approved) H. Tschofenig, Ed.
Intended status: Standards Track ARM Limited
Expires: May 18, 2018 T. Fossati
Nokia
T. Gondrom
Huawei
November 14, 2017
The Datagram Transport Layer Security (DTLS) Connection Identifier
draft-rescorla-tls-dtls-connection-id-02
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
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-
Drafts is at http://datatracker.ietf.org/drafts/current/.
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 May 18, 2018.
Rescorla, et al. Expires May 18, 2018 [Page 1]
Internet-Draft DTLS Connection ID November 2017
Copyright Notice
Copyright (c) 2017 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
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Terminology . . . . . . . . . . . . . . . . . 3
3. The "connection_id" Extension . . . . . . . . . . . . . . . . 3
4. Post-Handshake Messages . . . . . . . . . . . . . . . . . . . 5
5. Record Layer Extensions . . . . . . . . . . . . . . . . . . . 5
6. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 6
7. Security and Privacy Considerations . . . . . . . . . . . . . 8
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
9.1. Normative References . . . . . . . . . . . . . . . . . . 9
9.2. Informative References . . . . . . . . . . . . . . . . . 10
Appendix A. History . . . . . . . . . . . . . . . . . . . . . . 11
Appendix B. Working Group Information . . . . . . . . . . . . . 11
Appendix C. Contributors . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
Rescorla, et al. Expires May 18, 2018 [Page 2]
Internet-Draft DTLS Connection ID November 2017
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
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;
Rescorla, et al. Expires May 18, 2018 [Page 3]
Internet-Draft DTLS Connection ID November 2017
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).
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.
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. Note that such implementations must still be able to
send other length connection identifiers to other parties.
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.
Rescorla, et al. Expires May 18, 2018 [Page 4]
Internet-Draft DTLS Connection ID November 2017
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:
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.
Rescorla, et al. Expires May 18, 2018 [Page 5]
Internet-Draft DTLS Connection ID November 2017
Figure 1 and Figure 2 illustrate the record formats of DTLS 1.2 and
DTLS 1.3, respectively.
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. Examples
Below is an example exchange for DTLS 1.3 using a single connection
id in each direction.
Rescorla, et al. Expires May 18, 2018 [Page 6]
Internet-Draft DTLS Connection ID November 2017
Client Server
------ ------
ClientHello
(connection_id=5)
-------->
<-------- HelloRetryRequest
(cookie)
ClientHello -------->
(connection_id=5)
+cookie
<-------- ServerHello
(connection_id=100)
EncryptedExtensions
(cid=5)
Certificate
(cid=5)
CertificateVerify
(cid=5)
Finished
(cid=5)
Certificate -------->
(cid=100)
CertificateVerify
(cid=100)
Finished
(cid=100)
<-------- Ack
(cid=5)
Application Data ========>
(cid=100)
<======== Application Data
(cid=5)
Figure 3: Example DTLS 1.3 Exchange with Connection IDs
Below is an example exchange for DTLS 1.2 using a connection id used
uni-directionally from the client to the server.
Rescorla, et al. Expires May 18, 2018 [Page 7]
Internet-Draft DTLS Connection ID November 2017
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 4: 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
Rescorla, et al. Expires May 18, 2018 [Page 8]
Internet-Draft DTLS Connection ID November 2017
contain arbitrary values; implementations concerned about this SHOULD
refuse to use connection ids.
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 observed payloads 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
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.
[[OPEN ISSUE: Sequence numbers leak connection IDs. We need to
update the document to address this. One possibility would be the
technique documented in https://quicwg.github.io/base-drafts/draft-
ietf-quic-transport.html#packet-number-gap.]]
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
Rescorla, et al. Expires May 18, 2018 [Page 9]
Internet-Draft DTLS Connection ID November 2017
[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-dtls13-02 (work in progress), October
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>.
[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
Rescorla, et al. Expires May 18, 2018 [Page 10]
Internet-Draft DTLS Connection ID November 2017
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:
* 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)
- 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.
Rescorla, et al. Expires May 18, 2018 [Page 11]
Internet-Draft DTLS Connection ID November 2017
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
Rescorla, et al. Expires May 18, 2018 [Page 12]