Internet DRAFT - draft-hartke-core-codtls
draft-hartke-core-codtls
CoRE Working Group K. Hartke
Internet-Draft O. Bergmann
Intended status: Informational Universitaet Bremen TZI
Expires: January 17, 2013 July 16, 2012
Datagram Transport Layer Security in Constrained Environments
draft-hartke-core-codtls-02
Abstract
This draft considers some obstacles in implementing Datagram
Transport Layer Security (DTLS) in constrained environments, and
presents some ideas for a constrained version of DTLS that is
friendly to constrained nodes and networks.
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 January 17, 2013.
Copyright Notice
Copyright (c) 2012 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.
Hartke & Bergmann Expires January 17, 2013 [Page 1]
Internet-Draft Constrained DTLS July 2012
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Background . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Potential Problems and Possible Solutions . . . . . . . . . . 4
2.1. Handshake Message Fragmentation . . . . . . . . . . . . . 4
2.2. Timer Values . . . . . . . . . . . . . . . . . . . . . . . 6
2.3. Connection Initiation . . . . . . . . . . . . . . . . . . 7
2.4. Connection Closure . . . . . . . . . . . . . . . . . . . . 9
2.5. Application Data Fragmentation . . . . . . . . . . . . . . 9
2.6. Data size . . . . . . . . . . . . . . . . . . . . . . . . 10
2.7. Code size . . . . . . . . . . . . . . . . . . . . . . . . 11
3. Stateless Header Compression . . . . . . . . . . . . . . . . . 12
3.1. Records . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.2. Handshake Messages . . . . . . . . . . . . . . . . . . . . 13
4. RESTful DTLS Handshake . . . . . . . . . . . . . . . . . . . . 15
5. Security Considerations . . . . . . . . . . . . . . . . . . . 17
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
8.1. Normative References . . . . . . . . . . . . . . . . . . . 17
8.2. Informative References . . . . . . . . . . . . . . . . . . 17
Appendix A. Templates . . . . . . . . . . . . . . . . . . . . . . 20
A.1. secp256r1 . . . . . . . . . . . . . . . . . . . . . . . . 20
A.2. secp384r1 . . . . . . . . . . . . . . . . . . . . . . . . 21
A.3. secp521r1 . . . . . . . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23
Hartke & Bergmann Expires January 17, 2013 [Page 2]
Internet-Draft Constrained DTLS July 2012
1. Introduction
1.1. Background
Nodes that take part in the "Internet of Things" often have strict
limitations regarding their computational power, memory size (both
ROM and RAM), and power management [I-D.ietf-lwig-guidance]. Network
communication, especially wireless, also imposes constraints that
need to be considered during protocol design, e.g. low bitrate,
variable delay and possibly high packet loss. Moreover, frames at
the link layer might be much smaller than the IPv6 minimum MTU of
1280 bytes and therefore require additional mapping mechanisms such
as 6LoWPAN [RFC4944] for IEEE 802.15.4 wireless networks
[IEEE.802-15-4], which in turn may exacerbate the limitations of the
network: E.g., as high loss rates are anticipated by design,
application protocols usually try to avoid fragmentation at the
network layer.
However, application protocols often delegate security mechanisms to
transport layer security protocols. More often than not, the
protocol overhead from securing the communication is highly relevant
to the overall performance of the systems.
One protocol that has received significant attention recently for
constrained node/network applications is Datagram Transport Layer
Security (DTLS) [RFC6347]. DTLS is derived from and inherits some
characteristics from TLS [RFC5246]. Although DTLS has not been
designed with constrained nodes/networks in mind, it is thought to be
usable in such environments [SOS12]. Still, there are a few
challenges when it comes to implement DTLS.
1.2. Overview
The present document considers some obstacles in implementing DTLS in
constrained environments, and presents a few ideas to make DTLS more
friendly to constrained nodes and networks.
The ideas generally fall into one of the following categories:
Implementation guidance: Implementation techniques for achieving
light-weight implementations of DTLS, without affecting
conformance to the relevant specifications or interoperability
with other implementations. This includes techniques for reducing
complexity, memory footprint, or power usage. The result may
eventually be incorporated into [I-D.ietf-lwig-guidance].
Hartke & Bergmann Expires January 17, 2013 [Page 3]
Internet-Draft Constrained DTLS July 2012
Protocol profile: Use of DTLS in a particular way, for example, by
changing MAYs into MUSTs or MUST NOTs, or by requiring or
precluding certain extensions or cipher suites. Existing DTLS
implementations ought to continue to be used without change if
they can be configured accordingly.
Stateless header compression: Compression of DTLS records without
explicitly building any compression context state. This is done
by using shorter forms to represent the same bits of information
or relying on information that is already shared by the client and
server. Existing DTLS implementations can continue to be used if
a thin layer is added that handles compression/decompression.
Breaking changes: New implementations are required that do not
interoperate with implementations of DTLS.
1.3. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
(Note that this document itself is informational, but it is
discussing normative statements.)
The term "byte" is used in its now customary sense as a synonym for
"octet".
2. Potential Problems and Possible Solutions
2.1. Handshake Message Fragmentation
DTLS records can be large in size for a single 6LoWPAN [RFC4944]
payload: [IEEE.802-15-4] specifies a physical layer MTU of only 127
bytes, which yields about 60-80 bytes of payload after adding MAC
layer and adaptation layer headers. Although 6LoWPAN supports the
fragmentation of IPv6 packets into small link-layer frames, this
doesn't really work well for constrained applications and networks.
DTLS offers fragmentation at the handshake layer and hence can get
around IP fragmentation. However, this can add a significant
overhead on the number of datagrams and bytes transferred (see
Table 1). Packet loss is also still a big problem for the
constrained nodes; buffers must be large enough to hold all messages
after reassembly and losing a single fragment will cause all
fragments of a message flight to be retransmitted. This is very
likely especially during key and certificate exchange as these will
not fit within a packet without fragmentation in most 6LoWPANs.
Hartke & Bergmann Expires January 17, 2013 [Page 4]
Internet-Draft Constrained DTLS July 2012
+--------------+-----------------+------------------+---------------+
| UDP data | Number of | Total number of | Proportion of |
| size limit | datagrams | bytes | header data |
| (bytes) | transferred | transferred | |
+--------------+-----------------+------------------+---------------+
| 50 | 27 | 1,182 | 55 % |
| 55 | 21 | 1,037 | 49 % |
| 60 | 20 | 1,081 | 51 % |
| 65 | 18 | 1,003 | 47 % |
| 70 | 15 | 912 | 42 % |
| 75 | 14 | 875 | 39 % |
| 80 | 13 | 874 | 39 % |
| 85 | 12 | 849 | 37 % |
| 90 | 12 | 849 | 37 % |
| 1,152 | 6 | 802 | 34 % |
+--------------+-----------------+------------------+---------------+
Table 1: Number of datagrams and bytes transferred using different
limits for DTLS fragmentation in an example DTLS handshake
(TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 with raw public key certificate)
Possible Solutions include:
o Use IP fragmentation instead of DTLS fragmentation. If no X.509
certificates are involved, the handshake messages of one flight
typically require less than 400 bytes combined. Since all
messages of a flight are retransmitted anyway when a single
fragment is lost, the difference between performing the
fragmentation at the DTLS layer and at the IP layer is probably
not huge. A recipient must still be prepared to receive
arbitrarily fragmented handshake messages at the DTLS layer,
though.
o Reduce the number of bytes to be transferred, so the overhead of
header data becomes smaller when fragmenting for small packet
sizes, and fewer packets need to be transmitted that could
potentially be lost:
* Use an out-of-band mechanism to exchange large blobs. For
example, the TLS Cached Information Extension
[I-D.ietf-tls-cached-info] allows to omit the exchange of
fairly static information, such as the server certificate, if
this information is already available.
* Use 6LoWPAN General Header Compression
[I-D.bormann-6lowpan-ghc] to compress DTLS messages, as
proposed in [DCOSS12].
Hartke & Bergmann Expires January 17, 2013 [Page 5]
Internet-Draft Constrained DTLS July 2012
* Use some DTLS-specific kind of Stateless Header Compression, as
shown in Section 3. This can significantly reduce the number
of datagrams and bytes transferred, and in particular also the
proportion of header data in the number of bytes transferred
(see Table 2).
* Use compressed point formats for elliptic curve points.
* Use self-delimiting numeric values [RFC6256] instead of fixed-
size numeric values.
* Use a bit field instead of multiple type fields to indicate
which handshake messages are present in a datagram.
o Perform the DTLS handshake over another protocol, for example,
CoAP [I-D.ietf-core-coap] with its support for block-wise
transfers [I-D.ietf-core-block], as shown in Section 4.
+--------------+-----------------+------------------+---------------+
| UDP data | Number of | Total number of | Proportion of |
| size limit | datagrams | bytes | header data |
| (bytes) | transferred | transferred | |
+--------------+-----------------+------------------+---------------+
| 50 | 15 (56 %) | 592 (50 %) | 10 % |
| 55 | 13 (62 %) | 585 (56 %) | 9 % |
| 60 | 13 (65 %) | 621 (57 %) | 14 % |
| 65 | 11 (61 %) | 588 (59 %) | 10 % |
| 70 | 11 (73 %) | 573 (63 %) | 7 % |
| 75 | 11 (79 %) | 573 (65 %) | 7 % |
| 80 | 10 (77 %) | 567 (65 %) | 6 % |
| 85 | 10 (83 %) | 567 (67 %) | 6 % |
| 90 | 10 (83 %) | 567 (67 %) | 6 % |
| 1,152 | 6 (100 %) | 617 (77 %) | 14 % |
+--------------+-----------------+------------------+---------------+
Table 2: Number of datagrams and bytes transferred in the same
example DTLS handshake but using Stateless Header Compression
(Section 4)
2.2. Timer Values
DTLS leaves the choice of timer values to the implementation, but
makes the following recommendation:
"Implementations SHOULD use an initial timer value of 1 second
(the minimum defined in RFC 6298 [RFC6298]) and double the value
at each retransmission, up to no less than the RFC 6298 maximum of
60 seconds." [RFC6347]
Hartke & Bergmann Expires January 17, 2013 [Page 6]
Internet-Draft Constrained DTLS July 2012
Given the time required by some algorithms when executed on a
constrained devices (see Table 3), an initial value of 1 second can
easily lead to spurious retransmissions.
+-------------+--------------+-----------+------------+-------------+
| Algorithm | Library | Memory | Execution | Comparable |
| | | footprint | time | RSA key |
| | | (bytes) | (seconds) | length |
+-------------+--------------+-----------+------------+-------------+
| RSA 1024 | AvrCryptolib | 640 | 199.7 | |
| RSA 2048 | AvrCryptolib | 1,280 | 1,587.6 | |
| ECDSA 160r1 | TinyECC | 892 | 2.3 | 1024 |
| ECDSA 192r1 | TinyECC | 1,008 | 3.6 | 1536 |
| ECDSA 160r1 | Wiselib | 842 | 20.2 | 1024 |
| ECDSA 192r1 | Wiselib | 952 | 34.6 | 1536 |
| ECDSA 163k1 | Relic | 2,804 | 0.3 | 1024 |
| ECDSA 233k1 | Relic | 3,675 | 1.8 | 2048 |
+-------------+--------------+-----------+------------+-------------+
Table 3: RSA private key operation and ECDSA signature performance
(from [I-D.aks-crypto-sensors])
Possible Solutions include:
o Adjust the timer value to meet the conditions of constrained nodes
and low-power, lossy networks.
o Add some kind of acknowledgment message to DTLS that allows an
implementation to confirm the receipt of a message before
preparing the next message flight.
2.3. Connection Initiation
Nodes with very constrained main memory also suffer from the
complexity of the DTLS handshake protocol. We envision that the
acceptance of DTLS as security protocol for embedded devices would
significantly increase if a less complex connection initiation
procedure with a smaller number of handshake messages was defined.
Compared to TLS, DTLS exacerbates the connection initiation: A DTLS
handshake has an additional roundtrip that results from the addition
of a stateless cookie exchange. This exchange is designed to prevent
certain denial-of-service attacks: consumption of excessive server
resources caused by the transmission of a series of handshake
initiation requests, and use of the server as an amplifier by sending
connection initiation messages with a forged source of the victim.
Hartke & Bergmann Expires January 17, 2013 [Page 7]
Internet-Draft Constrained DTLS July 2012
Possible Solutions include:
o Create the DTLS connection before it is needed, so it doesn't take
a long time to set it up when it's actually needed. This works if
a server has do deal with a relatively small overall number of
clients that wish to interact with the server. Care must be taken
such that not all clients perform their handshake at the same
time, as a handshake requires considerably more memory than
keeping a connection open. (See also Section 2.4 below.)
o Shorten the handshake to four flights. This may be possible
without losing the denial-of-service roundtrip if the cipher suite
permits that the server remains stateless after sending the
ServerHello and if the flight fits in one datagram (see Figure 1).
Client Server
------ ------
ClientHello --------> Flight 1
HelloVerifyRequest \
ServerHello Flight 2
<-------- ServerHelloDone /
(remain stateless)
ClientHello \
"ServerHello" \
ClientKeyExchange Flight 3
[ChangeCipherSpec] /
Finished --------> /
[ChangeCipherSpec] \ Flight 4
<-------- Finished /
Figure 1: Artist's impression of a four-flight DTLS handshake with
Pre-Shared Key
o As an alternative, client puzzles could be used as a mechanism for
mitigating denial-of-service attacks, resulting in a four-flight
exchange similar to the one in HIP DEX [I-D.moskowitz-hip-rg-dex].
The application of client puzzles to TLS has been shown
[USENIX01]. However, a puzzle would be needed that ideally takes
less effort for a constrained device and more effort for a less
constrained device.
Hartke & Bergmann Expires January 17, 2013 [Page 8]
Internet-Draft Constrained DTLS July 2012
2.4. Connection Closure
Although a connection needs considerably less memory after a
handshake has finished, it still requires, e.g., around 80 bytes with
AES-128-CCM [I-D.mcgrew-tls-aes-ccm] for the keys, sequence numbers
and anti-replay window. More memory is needed if session resumption
is supported, to keep the 48-byte master secret and negotiated
connection parameters. This limits how many connections a
constrained device can maintain at a given time. Often, constrained
devices will have a fixed number of "slots" for connections rather
than allocating memory dynamically for each connection.
DTLS provides a facility for secure connection closure. When a valid
closure alert is received, an implementation can be assured that no
further data will be received on that connection. It is noteworthy,
though, that the closure alert is not a handshake message and thus is
not retransmitted when packet loss occurs.
Possible Solutions include:
o Maintain the session for as long as possible. When the server
runs out of resources, it can close connections, e.g., using a
Least Frequently Used (LFU) eviction policy. The client simply
assumes that the connection is active until the server rejects its
application data, in which case the client initiates a new
connection.
o Use the DTLS Heartbeat Extension [RFC6520] to figure out from time
to time if the connection is still active.
2.5. Application Data Fragmentation
Messages larger than an IP fragment result in undesired packet
fragmentation. DTLS does not support fragmentation of application
data. If an implementation of an application layer protocol such as
CoAP [I-D.ietf-core-coap] wants to avoid IP fragmentation, it must
fit the application data (e.g., a CoAP message) and all headers
within a single IP packet.
DTLS has a per-record overhead of 13 bytes for the record header.
AEAD ciphers such as AES-CCM [I-D.mcgrew-tls-aes-ccm] eat up
additional space to carry the explicit nonce and the authentication
tag. Thus, cipher suites like TLS_PSK_WITH_AES_128_CCM_8 or
TLS_ECDHE_ECDSA_AES_128_CCM_8 requires 16 additional bytes, leading
to an overall overhead of 29 bytes for the header of each encrypted
DTLS packet. With packet sizes of 60-80 bytes, this takes a
considerable portion of the available packet size away (see Table 4).
Hartke & Bergmann Expires January 17, 2013 [Page 9]
Internet-Draft Constrained DTLS July 2012
+------------------+------------------------+-----------------------+
| UDP data size | Number of bytes left | ... with Stateless |
| limit (bytes) | for application data | Header Compression |
+------------------+------------------------+-----------------------+
| 50 | 21 (42 %) | 39 (78 %) |
| 55 | 26 (47 %) | 44 (80 %) |
| 60 | 31 (52 %) | 49 (82 %) |
| 65 | 36 (55 %) | 54 (83 %) |
| 70 | 41 (59 %) | 59 (84 %) |
| 75 | 46 (61 %) | 64 (85 %) |
| 80 | 51 (64 %) | 69 (86 %) |
| 85 | 56 (66 %) | 74 (87 %) |
| 90 | 61 (68 %) | 79 (88 %) |
| 1,152 | 1,123 (97 %) | 1,141 (99 %) |
+------------------+------------------------+-----------------------+
Table 4: Number of bytes left for data in an ApplicationData record
using DTLS and DTLS with Stateless Header Compression (Section 4)
Possible Solutions include:
o Elide the GenericAEADCipher.nonce_explicit field when AES-CCM is
used. The GenericAEADCipher.nonce_explicit field is set to the
16-bit epoch concatenated with the 48-bit sequence number, which
means that the epoch and sequence number are unnecessarily
included twice in each record.
o Elide the DTLS version field where it is implicitly clear. Since
the DTLS version is negotiated in the handshake, there should not
be a need to specify the DTLS version in each and every record.
o Elide the length field of the last record in a datagram. DTLS
records specify their length so multiple records can be
transmitted in a single datagram. When DTLS is used with UDP
(which preserves the boundaries of all message sent), the length
field of the last record in a datagram can be calculated from the
UDP payload length.
For example, when using the Stateless Header Compression presented in
Section 3 and eliminating the redundant epoch and sequence number
information, the number of bytes left in an ApplicationData record
for application data can be significantly increased (see Table 4).
2.6. Data size
As fragmented handshake messages can arrive at a constrained node in
any order, the receiver must provide a message buffer that is large
enough to hold multiple fragments. When several handshake messages
Hartke & Bergmann Expires January 17, 2013 [Page 10]
Internet-Draft Constrained DTLS July 2012
forming a single flight are sent out in parallel, it is likely that
the receiver's resources are too limited to order fragments from
distinct handshake messages. Avoiding this might require additional
resources on the server side to ensure serialization of a flight's
messages.
Furthermore, since handshake messages can be fragmented arbitrarily
and with overlaps, the receiver must, in addition to the message
buffer, keep track of the fragments received so far.
Possible retransmissions require even more buffer space as replay-
protection requires encryption of every single packet that is to be
transmitted. In particular, this renders destructive in-place
encryption impossible as the source data must be preserved.
Possible Solutions include:
o Use the same sequence number when retransmitting a message, so the
plaintext could be encrypted in-place without the need for a
second buffer. The security implications of this change need to
be carefully analyzed.
o Favour cryptographic algorithms that use less memory, possibly
resulting in a slower performance.
o Add some kind of acknowledgment message to DTLS that allows an
receiver to confirm the receipt of a message, and let the sender
wait for the acknowledgment before it sends the next part of the
flight.
2.7. Code size
Although probably not as severe as data size limits, the code size of
a DTLS implementation also can play a role, in particular for
constrained devices at the lower bound of Class 1 devices.
Possible Solutions include:
o Avoid static tables for cryptographic functions where possible, as
typical embedded platforms are more restricted in RAM than in non-
volatile memory such as flash ROM. Instead, their procedural
equivalent is to be used, although less efficient during run-time.
o Use using pre-composed messages instead of writing code, e.g., for
encoding or decoding ASN.1 structures, as shown in Appendix A.
Hartke & Bergmann Expires January 17, 2013 [Page 11]
Internet-Draft Constrained DTLS July 2012
3. Stateless Header Compression
Stateless Header Compression compresses the headers of records and
handshake messages. The compression is lossless, does not increase
the record length and is done without explicitly building any
compression context state.
The Finished MAC is computed as if each handshake message had been
sent uncompressed.
3.1. Records
Records are compressed by specifying the type, version, epoch,
sequence_number and length fields using a variable number of bytes.
A prefix is added in front of the structure to indicate the length of
each field or to specify the value of the field directly. If the
value is specified directly, the field itself is elided. The format
of the prefix is as follows:
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| T | V | E |1 1 0| S | L |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The fields in the prefix are defined as follows:
T: Describes the type field.
0 - Content Type 20 (ChangeCipherSpec)
1 - 8-bit type field
2 - Content Type 22 (Handshake)
3 - Content Type 23 (Application Data)
V: Describes the version field.
0 - Version 254.255 (DTLS 1.0)
1 - 16-bit version field
2 - Version 254.253 (DTLS 1.2)
3 - Reserved for future use
E: Describes the epoch field.
0 - Epoch 0
1 - Epoch 1
2 - Epoch 2
3 - Epoch 3
4 - Epoch 4
Hartke & Bergmann Expires January 17, 2013 [Page 12]
Internet-Draft Constrained DTLS July 2012
5 - 8-bit epoch field
6 - 16-bit epoch field
7 - Implicit -- same as previous record in the datagram
S: Describes the sequence_number field.
0 - Sequence number 0
1 - 8-bit sequence_number field
2 - 16-bit sequence_number field
3 - 24-bit sequence_number field
4 - 32-bit sequence_number field
5 - 40-bit sequence_number field
6 - 48-bit sequence_number field
7 - Implicit -- number of previous record in the datagram + 1
L: Describes the length field.
0 - Length 0
1 - 8-bit length field
2 - 16-bit length field
3 - Implicit -- last record in the datagram
3.2. Handshake Messages
Handshake messages are compressed in a similar way. A prefix is
added in front of the structure to indicate the length of each field
or to specify the value of the field directly. If the value is
specified directly, the field itself is elided. The format of the
prefix is as follows:
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0| T | L | S | O | C |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The fields in the prefix are defined as follows:
T: Describes the msg_type field.
0 - 8-bit msg_type field
1 - Handshake Type 1 (Client Hello)
2 - Handshake Type 2 (Server Hello)
3 - Handshake Type 3 (Hello Verify Request)
4 - Reserved for future use
5 - Reserved for future use
6 - Reserved for future use
7 - Handshake Type 11 (Certificate)
Hartke & Bergmann Expires January 17, 2013 [Page 13]
Internet-Draft Constrained DTLS July 2012
8 - Handshake Type 12 (Server Key Exchange)
9 - Handshake Type 13 (Certificate Request)
10 - Handshake Type 14 (Server Hello Done)
11 - Handshake Type 15 (Certificate Verify)
12 - Handshake Type 16 (Client Key Exchange)
13 - Reserved for future use
14 - Reserved for future use
15 - Handshake Type 20 (Finished)
L: Describes the length field.
0 - Implicit -- last message in the record
1 - 8-bit length field
2 - 16-bit length field
3 - 24-bit length field
S: Describes the message_seq field.
0 - Message sequence number 0
1 - Message sequence number 1
2 - Message sequence number 2
3 - Message sequence number 3
4 - Message sequence number 4
5 - Message sequence number 5
6 - Message sequence number 6
7 - Message sequence number 7
8 - Message sequence number 8
9 - Message sequence number 9
10 - Message sequence number 10
11 - Message sequence number 11
12 - Message sequence number 12
13 - 8-bit message_seq field
14 - 16-bit message_seq field
15 - Implicit -- number of previous message in the record + 1
O: Describes the fragment_offset field.
0 - Offset 0
1 - 8-bit fragment_offset field
2 - 16-bit fragment_offset field
3 - 24-bit fragment_offset field
C: Describes the fragment_length field.
0 - Implicit -- last message in the record
1 - 8-bit fragment_length field
2 - 16-bit fragment_length field
3 - 24-bit fragment_length field
Hartke & Bergmann Expires January 17, 2013 [Page 14]
Internet-Draft Constrained DTLS July 2012
4. RESTful DTLS Handshake
Where DTLS is used in conjunct with the Constrained Application
Protocol (CoAP) [I-D.ietf-core-coap], it might be beneficial to use
CoAP with its support for block-wise transfers [I-D.ietf-core-block]
instead of DTLS's convoluted handshake protocol to transport DTLS
handshake messages.
CoAP, like HTTP, is designed for applications following the REST
architectural style [REST]. So the DTLS connection is modeled as a
CoAP resource which gets created when a client wants to initiate a
connection, and gets updated to modify the state and parameters of
the connection. A well-known URI path [RFC5785] is used to identify
a collection resource that models the set of active connections and
allows new connections to be created.
Client Server
------ ------
POST /.well-known/dtls
ClientHello ----->
1.xx Verify
<----- HelloVerifyRequest
POST /.well-known/dtls
ClientHello ----->
2.01 Created /session/4ad6bc29
ServerHello
Certificate*
ServerKeyExchange*
CertificateRequest*
<----- ServerHelloDone
PATCH /session/4ad6bc29
Certificate*
ClientKeyExchange
CertificateVerify*
[ChangeCipherSpec]
Finished ----->
2.04 Changed
[ChangeCipherSpec]
<----- Finished
Figure 2: Message Flights for Full Handshake
Hartke & Bergmann Expires January 17, 2013 [Page 15]
Internet-Draft Constrained DTLS July 2012
Client Server
------ ------
PATCH /session/4ad6bc29
ClientHello ----->
2.04 Changed
ServerHello
[ChangeCipherSpec]
<----- Finished
PATCH /session/4ad6bc29
[ChangeCipherSpec]
Finished ----->
<----- 2.04 Changed
Figure 3: Message Flights for Session-Resuming Handshake
There are the following possible operations:
o POST to well-known URI: requests the server to create a new
session resource.
o PATCH session resource: requests the server to change session
parameters, or to resume a session.
o GET session resource: returns a representation of the session.
o DELETE session resource: requests the server to delete the session
resource and free all resources related to the session.
The following protocols and URI schemes are used:
o CoAP [I-D.ietf-core-coap]
o coap+codtls://
The following well-known URIs are used:
o /.well-known/dtls
The following media types are used:
o application/dtls
(The exact definition of these items is TBD.)
Hartke & Bergmann Expires January 17, 2013 [Page 16]
Internet-Draft Constrained DTLS July 2012
5. Security Considerations
Beyond stateless header compression and profiling, changes to the
TLS/DTLS protocol need to be performed extremely carefully. No
analysis has been done in the present version of this draft.
6. IANA Considerations
This draft includes no request to IANA.
7. Acknowledgements
Thanks to Angelo P. Castellani, Stefan Jucker and Shahid Raza for
helpful comments and discussions that have shaped the document.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, January 2012.
8.2. Informative References
[DCOSS12] Raza, S., Trabalza, D., and T. Voigt, "6LoWPAN Compressed
DTLS for CoAP", 8th IEEE International Conference on
Distributed Computing in Sensor Systems, May 2012.
[I-D.aks-crypto-sensors]
Sethi, M., Arkko, J., Keranen, A., and H. Rissanen,
"Practical Considerations and Implementation Experiences
in Securing Smart Object Networks",
draft-aks-crypto-sensors-02 (work in progress),
March 2012.
[I-D.bormann-6lowpan-ghc]
Bormann, C., "6LoWPAN Generic Compression of Headers and
Header-like Payloads", draft-bormann-6lowpan-ghc-04 (work
in progress), March 2012.
Hartke & Bergmann Expires January 17, 2013 [Page 17]
Internet-Draft Constrained DTLS July 2012
[I-D.ietf-core-block]
Bormann, C. and Z. Shelby, "Blockwise transfers in CoAP",
draft-ietf-core-block-08 (work in progress),
February 2012.
[I-D.ietf-core-coap]
Shelby, Z., Hartke, K., Bormann, C., and B. Frank,
"Constrained Application Protocol (CoAP)",
draft-ietf-core-coap-10 (work in progress), June 2012.
[I-D.ietf-lwig-guidance]
Bormann, C., "Guidance for Light-Weight Implementations of
the Internet Protocol Suite", draft-ietf-lwig-guidance-01
(work in progress), July 2012.
[I-D.ietf-tls-cached-info]
Santesson, S. and H. Tschofenig, "Transport Layer Security
(TLS) Cached Information Extension",
draft-ietf-tls-cached-info-11 (work in progress),
December 2011.
[I-D.ietf-tls-oob-pubkey]
Wouters, P., Gilmore, J., Weiler, S., Kivinen, T., and H.
Tschofenig, "TLS Out-of-Band Public Key Validation",
draft-ietf-tls-oob-pubkey-03 (work in progress),
April 2012.
[I-D.mcgrew-tls-aes-ccm]
McGrew, D. and D. Bailey, "AES-CCM Cipher Suites for TLS",
draft-mcgrew-tls-aes-ccm-03 (work in progress),
February 2012.
[I-D.mcgrew-tls-aes-ccm-ecc]
McGrew, D., Bailey, D., Campagna, M., and R. Dugal, "AES-
CCM ECC Cipher Suites for TLS",
draft-mcgrew-tls-aes-ccm-ecc-02 (work in progress),
October 2011.
[I-D.moskowitz-hip-rg-dex]
Moskowitz, R., "HIP Diet EXchange (DEX)",
draft-moskowitz-hip-rg-dex-06 (work in progress),
May 2012.
[IEEE.802-15-4]
"Information technology - Telecommunications and
information exchange between systems - Local and
metropolitan area networks - Specific requirements - Part
15.4: Wireless Medium Access Control (MAC) and Physical
Hartke & Bergmann Expires January 17, 2013 [Page 18]
Internet-Draft Constrained DTLS July 2012
Layer (PHY) Specifications for Low-Rate Wireless Personal
Area Networks (WPANs)", IEEE Standard 802.15.4,
September 2006, <http://standards.ieee.org/getieee802/
download/802.15.4-2006.pdf>.
[REST] Fielding, R., "Architectural Styles and the Design of
Network-based Software Architectures", Ph.D. Dissertation,
University of California, Irvine, 2000, <http://
www.ics.uci.edu/~fielding/pubs/dissertation/
fielding_dissertation.pdf>.
[RFC4492] Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B.
Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites
for Transport Layer Security (TLS)", RFC 4492, May 2006.
[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4
Networks", RFC 4944, September 2007.
[RFC5785] Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
Uniform Resource Identifiers (URIs)", RFC 5785,
April 2010.
[RFC6256] Eddy, W. and E. Davies, "Using Self-Delimiting Numeric
Values in Protocols", RFC 6256, May 2011.
[RFC6298] Paxson, V., Allman, M., Chu, J., and M. Sargent,
"Computing TCP's Retransmission Timer", RFC 6298,
June 2011.
[RFC6520] Seggelmann, R., Tuexen, M., and M. Williams, "Transport
Layer Security (TLS) and Datagram Transport Layer Security
(DTLS) Heartbeat Extension", RFC 6520, February 2012.
[SOS12] Arkko, J. and H. Tschofenig, "Conclusions from the
Workshop on Smart Object Security", Workshop on Smart
Object Security, March 2012, <http://
www.lix.polytechnique.fr/hipercom/SmartObjectSecurity/
slides/sos-conclusions.ppt>.
[USENIX01]
Dean, D. and A. Stubblefield, "Using Client Puzzles to
Protect TLS", 10th USENIX Security Symposium, August 2001,
<http://static.usenix.org/events/sec01/full_papers/dean/
dean.pdf>.
Hartke & Bergmann Expires January 17, 2013 [Page 19]
Internet-Draft Constrained DTLS July 2012
Appendix A. Templates
When elliptic curve cryptography is used, building and parsing the
bodies of Certificate, ServerKeyExchange and ClientKeyExchange
messages mainly involves the encoding and decoding of elliptic curve
points. The points are encapsulated in a mix of DTLS structures and
ASN.1 sequences. For a given elliptic curve, some parts of a message
body are static, which allows using pre-composed messages instead of
writing lots of memory consuming code pertaining to DTLS and ASN.1.
This appendix provides templates for the bodies of the Certificate,
ServerKeyExchange and ClientKeyExchange messages used in a DTLS
handshake with raw public key certificates [I-D.ietf-tls-oob-pubkey]
and the ECDHE_ECDSA key exchange [RFC4492].
The templates are given for the named curves secp256r1, secp384r1 and
secp521r1; these curves are equivalent to the NIST P-256, P-384, and
P-521 curves. They are required in [I-D.mcgrew-tls-aes-ccm-ecc].
The same curve is used in each case for both the raw public key
certificate and the ephemeral keys. Points are represented in
uncompressed point format.
Note: The templates have not been independently verified yet.
A.1. secp256r1
Raw Public Key Certificate:
30 59 30 13 06 07 2a 86 48 ce 3d 02 01 06 08 2a
86 48 ce 3d 03 01 07 03 42 00 04 __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __
ECDSA-capable public key (x, y)
Hartke & Bergmann Expires January 17, 2013 [Page 20]
Internet-Draft Constrained DTLS July 2012
Server Key Exchange:
03 00 17 41 04 __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ 00 46 30 44 02 20 __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ 02 20 __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __
ephemeral ECDH public key (x, y) and ECDSA signature (r, s)
Client Key Exchange:
41 04 __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __
ephemeral ECDH public key (x, y)
A.2. secp384r1
Raw Public Key Certificate:
30 76 30 10 06 07 2a 86 48 ce 3d 02 01 06 05 2b
81 04 00 22 03 62 00 04 __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __
ECDSA-capable public key (x, y)
Hartke & Bergmann Expires January 17, 2013 [Page 21]
Internet-Draft Constrained DTLS July 2012
Server Key Exchange:
03 00 18 61 04 __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ 00 66 30 64 02 30 __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ 02 30 __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __
ephemeral ECDH public key (x, y) and ECDSA signature (r, s)
Client Key Exchange:
61 04 __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __
ephemeral ECDH public key (x, y)
A.3. secp521r1
Raw Public Key Certificate:
30 81 9b 30 10 06 07 2a 86 48 ce 3d 02 01 06 05
2b 81 04 00 23 03 81 86 00 04 __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __
ECDSA-capable public key (x, y)
Hartke & Bergmann Expires January 17, 2013 [Page 22]
Internet-Draft Constrained DTLS July 2012
Server Key Exchange:
03 00 19 85 04 __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ 00 8b 30 81 88 02 42
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ 02 42 __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __
ephemeral ECDH public key (x, y) and ECDSA signature (r, s)
Client Key Exchange:
85 04 __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __
__ __ __ __ __ __
ephemeral ECDH public key (x, y)
Hartke & Bergmann Expires January 17, 2013 [Page 23]
Internet-Draft Constrained DTLS July 2012
Authors' Addresses
Klaus Hartke
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63905
Email: hartke@tzi.org
Olaf Bergmann
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63904
Email: bergmann@tzi.org
Hartke & Bergmann Expires January 17, 2013 [Page 24]
