DICE Working Group S. Keoh
Internet-Draft University of Glasgow Singapore
Intended status: Standards Track S.S. Kumar, Ed.
Expires: August 18, 2014 O. Garcia-Morchon
E. Dijk
Philips Research
A. Rahman
February 14, 2014

DTLS-based Multicast Security for Low-Power and Lossy Networks (LLNs)


The CoAP and 6LoWPAN standards are fast emerging as key protocols in the area of resource-constrained devices. Such IP-based systems are foreseen to be used for building and lighting control systems where wireless devices interconnect with each other, forming low-power and lossy networks (LLNs). Both multicast and its security are key needs in these networks. This draft presents a method for securing IPv6 multicast communication in LLNs based on the DTLS which is already supported for unicast communication for CoAP devices. This draft deals with the adaptation of the DTLS record layer to protect multicast group communication, assuming that all group members already have the group security association parameters in their possession. The adapted DTLS record layer provides message confidentiality, integrity and replay protection to group messages using the group keying material before sending the message via IPv6 multicast to the group.

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 August 18, 2014.

Copyright Notice

Copyright (c) 2014 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.

Table of Contents

1. Introduction

There is an increased use of wireless control networks in environmental monitoring, industrial automation, lighting controls and building management systems. This is mainly driven by the fact that the independence from physical control wires allows for freedom of placement, portability and for reducing the cost of installation as less cable placement and drilling are required. Consequently, there is an ever growing number of electronic devices, sensors and actuators that have become Internet connected, thus creating a trend towards the Internet-of-Things (IoT). These connected devices are equipped with communication capability that enables them to interact with each other as well as with the wider Internet services. However, the devices in such wireless control networks are characterized by power constraints (as these are usually battery-operated), have limited computational resources (low CPU clock, small RAM and flash storage) and often, the communication bandwidth is limited and unreliable (e.g., IEEE 802.15.4 radio). Hence, such wireless control networks are also known as Low-power and Lossy Networks (LLNs).

In addition to the usual device-to-device unicast communication that allow devices to directly interact with each other, group communication is an important feature in LLNs. It is more effective in LLNs to convey messages to a group of devices without requiring the sender to perform multiple time and energy consuming unicast transmissions to reach each individual group member. For example, in a building and lighting control system, the heating, ventilation, air-conditioning and lighting devices are often grouped according to the layout of the building, and control commands are issued simultaneously to a group of devices. Group communication for LLNs is based on the Constrained Application Protocol (CoAP) [I-D.ietf-core-coap] sent over IP- multicast [I-D.ietf-core-groupcomm].

Currently, CoAP messages are protected using Datagram Transport Layer Security (DTLS) [RFC6347]. However, DTLS is currently used to secure a connection between two endpoints and it cannot be used to protect multicast group communication. Group communication in LLNs is equally important and should be secured as it is also vulnerable to the usual attacks over the air (eavesdropping, tampering, message forgery, replay, etc). There have been a lot of previous efforts in IETF to standardize mechanisms to secure multicast communication such as [RFC3830], [RFC4082], [RFC3740], [RFC4046], and [RFC4535]. However, these approaches are not necessarily suitable for LLNs which have much more limited bandwidth and resources. For example, the MIKEY Architecture [RFC3830] is mainly designed to facilitate multimedia distribution, while TESLA [RFC4082] is proposed as a protocol for broadcast authentication of the source and not for protecting the confidentiality of multicast messages. [RFC3740] and [RFC4046] provide reference architectures for multicast security. [RFC4535] describes Group Secure Association Key Management Protocol (GSAKMP), a security framework for creating and managing cryptographic groups on a network which can be reused for key management in our context with any needed adaptation for LLNs.

This draft describes an approach to use DTLS as mandated in CoAP unicast to also support multicast security. We will assume that all devices in the group already have a group security association parameters based on a key management mechanism which is outside the scope of this draft. This draft focuses primarily on the adaptation of the DTLS record layer to protect multicast messages to be sent to the group, and thus providing confidentiality, integrity and replay protection to the CoAP group messages.

Lastly, even though this draft is written from the perspective of securing CoAP based group communication, it is important to note that DTLS is a powerful and flexible security protocol. Thus use of DTLS-based multicast for application layer protocols other than CoAP are possible as long as they follow the approach outlined in this draft.

1.1. 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].

This specification uses the following terminology:

1.2. Outline

This draft is structured as follows: Section 2 motivates the proposed solution with group communication use cases in LLNs and derives a set of requirements. Section 3 provides an overview of the proposed DTLS-based multicast security assuming that all devices in the group already have a group security association parameters in their possession. In Section 4, we describe the details of the adaptation of DTLS record layer for confidentiality and integrity protection of the multicast messages. Section 6 presents the security considerations.

2. Use Cases and Requirements

This section defines the use cases for group communication in LLNs and specifies a set of security requirements for these use cases.

2.1. Group Communication Use Cases

The "Group Communication for CoAP" draft [I-D.ietf-core-groupcomm] provides the necessary background for multicast based CoAP communication in LLNs and the interested reader is encouraged to first read this document to understand the non-security related details. This document also lists a few multicast group communication uses cases with detailed descriptions and some are listed here briefly:

  1. Lighting control: enabling synchronous operation of a group of 6LoWPAN [RFC4944] [RFC6282] connected lights in a room/floor/building. This ensures that the light preset like on/off/dim-level of a large group of luminaries are changed at the same time, hence providing a visual synchronicity of light effects to the user.
  2. Parameter update: configuration settings of a group of similar devices are updated simultaneously and efficiently.
  3. Device and Service discovery: information about the devices in the local network and their capabilities can be queried and requested using multicast, e.g. by a commissioning device. The responses are sent back in unicast.

Elaborating on one of the main use cases that this document addresses, Lighting control, consider a building equipped with 6LoWPAN IP-connected lighting devices, switches, and 6LoWPAN border routers; the devices are organized in groups according to their physical location in the building, e.g., lighting devices and switches in a room/floor can be configured as a single multicast group. The switches are then used to control the lighting devices in the group by sending on/off/dimming commands to all lighting devices in the group. 6LoWPAN border routers that are connected to an IPv6 network backbone (which is also multicast enabled) are used to interconnect 6LoWPANs in the building. Consequently, this would also enable multicast groups to be formed across different physical subnets (which may be individually protected with L2 security). In such a multicast group, group messages can traverse from one physical subnet to another physical subnet through a IPv6 backbone which may not be protected. Additionally, other non-lighting devices (like window blind controls) may share the physcial subnet for networking.

2.2. Security Requirements

The "Miscellaneous CoAP Group Communication Topics" draft [I-D.dijk-core-groupcomm-misc] already defines a set of security requirements for group communication in LLNs. We re-iterate and further describe those security requirements in this section with respect to the use cases:

  1. Multicast communication topology: We consider both 1-to-N (one sender with multiple listeners) and M-to-N (multiple senders with multiple listeners) communication topologies. The 1-to-N communication topology is the simplest group communication scenario that would serve the needs of a typical LLN. For example, in the simple lighting control use case, the switch is the only entity that is responsible for sending control commands to a group of lighting devices. In more advanced lighting control use cases, a N-to-M communication topology would be required, for example if multiple sensors (presence or day-light) are responsible to trigger events to a group of lighting devices.
  2. Multicast group size: The security solutions should support the typical group sizes that "Group Communication for CoAP" draft [I-D.ietf-core-groupcomm] intends to support. Group size is the combination of the number of Senders and Listeners in a group with possible overlap (a Sender can also be a Listener but need not be always). In LLN use cases mentioned in the document, the number of Senders (normally the controlling devices) is much smaller than the number of Listeners (the controlled devices). A security solution that supports 1 to 50 Senders would cover the group sizes required for most use cases that are relevant for this document. The total number of group devices must be in the range of 2 to 100 devices. Groups larger than these should be divided into smaller independent multicast groups such as grouping lights of a building per floor.
  3. Establishment of a GSA: A secure mechanism must be used to distribute keying materials, multicast security policies and security parameters to members of a multicast group. A GSA must be established by the group controller (which manages the multicast group) among the group members. The 6LoWPAN border router, a device in the 6LoWPAN, or a remote server outside the 6LoWPAN could play the role of the group controller. However, GSA establishment is outside the scope of this draft, and it is anticipated that an activity in IETF dedicated to the design of a generic key management scheme for the LLN will include this feature preferably based on [RFC3740], [RFC4046] and [RFC4535].
  4. Multicast data confidentiality: Multicast message should be encrypted, as some control commands when sent in the clear could pose unforeseen privacy risks to the users of the system.
  5. Multicast data replay protection: It must not be possible to replay a multicast message as this would disrupt the operation of the group communication.
  6. Multicast data group authentication and integrity: It is essential to ensure that a multicast message originated from a member of the group and that messages have not been tampered with by attackers who are not members. The multicast group key which is known to all group members is used to provide authenticity to the multicast messages (e.g., using a Message Authentication Code, MAC). This assumes that all other group members are trusted not to tamper with the multicast message.
  7. Multicast data security ciphersuite: All group members must use the same ciphersuite to protect the authenticity, integrity and confidentiality of multicast messages. The ciphersuite is part of the GSA. Typically authenticity is more important than confidentiality in LLNs. Therefore the proposed multicast data security protocol must support at least ciphersuites with MAC only (NULL encryption) and AEAD [RFC5116] ciphersuites. Other ciphersuites that are defined for data record security in DTLS should also be preferably supported.
  8. Multicast data source authentication: Source authenticity is required if the group members are assumed to be untrusted and can tamper with the multicast messages. This can happen if nodes of the group can be easily compromised. Source authenticity helps to minimize the risk of any node compromise leading to the compromise of the whole multicast group. Source authenticity can be typically provided using public-key cryptography in which every multicast message is signed by the sender. Alternatively, a lightweight broadcast authentication, i.e., TESLA [RFC4082] can be deployed, however it requires devices in the multicast group to have a trusted clock and have the ability to loosely synchronize their clocks with the sender. Source authenticity mechanisms should be preferably defined at the application layer. The transport layer group level security can provide an additional layer of security for the source authenticity mechanism against DoS attacks. However, even with source authenticity the risk still remains that compromise of a sender can still compromise the whole group.
  9. Forward security: Devices that leave the group should not have access to any future GSAs. This ensures that a past member device cannot continue to decrypt confidential data that is sent in the group. It also ensures that this device cannot send encrypted and/or integrity protected data after it leaves the group. The GSA update mechanism has to be defined as part of the key management scheme.
  10. Backward confidentiality: A new device joining the group should not have access to any old GSAs. This ensures that a new member device cannot decrypt data sent before it joins the group. The key management scheme should ensure that the GSA is updated to ensure backward confidentiality.

3. Overview of DTLS-based Secure Multicast

The goal of this draft is to secure CoAP Group communication over 6LoWPAN networks, by extending the use of the DTLS security protocol to allow for the use of DTLS record layer with minimal adaptation. The IETF CoRE WG has selected DTLS [RFC6347] as the default must-implement security protocol for securing CoAP, therefore it is desirable that DTLS be extended to facilitate CoAP-based group communication. Reusing DTLS for different purposes while guaranteeing the required security properties can avoid the need to implement multiple security protocols and this is especially beneficial when the target deployment consists of resource-constrained embedded devices. This section first describes group communication based on IP multicast, and subsequently sketches a solution for securing group communication using DTLS.

3.1. IP Multicast

Devices in the LLN are categorized into two roles, (1) sender and (2) listener. Any node in the LLN may have one of these roles, or both roles. The application(s) running on a device basically determine these roles by the function calls they execute on the IP stack of the device.

In principle, a sender or listener does not require any prior access procedures or authentication to send or listen to a multicast message [RFC5374]. A sender to an IPv6 multicast group sets the destination of the packet to an IPv6 address that has been allocated for IPv6 multicast. A device becomes a listener by "joining" to the specific IPv6 multicast group by registering with a network routing device, signaling its intent to receive packets sent to that particular IPv6 multicast group. Figure 1 depicts a 1-to-N multicast communication and the roles of the nodes. Any device can in principle decide to listen to any IPv6 multicast address. This also means applications on the other devices do not know, or do not get notified, when new listeners join the LLN. More details on the IPv6 multicast and CoAP group communication can be found in [I-D.ietf-core-groupcomm]. This draft does not intend to modify any of the underlying group communication or multicast routing protocols.


               |. |
             --| ++++
    ++++    /  ++|. |
    |A |---------| ++++
    |  |    \    ++|B |
    ++++     \-----|  |
   Sender          ++++

Figure 1: The roles of nodes in a 1-to-N multicast communication topology

3.2. Securing Multicast in LLNs

A group controller in an LLN creates a multicast group. The group controller may be hosted by a remote server, or a border router that creates a new group over the network. In some cases, devices may be configured using a commissioning tool that mediates the communication between the devices and the group controller. The controller in the network can be discovered by the devices using various methods defined in [I-D.vanderstok-core-dna] such as DNS-SD [RFC6763] and Resource Directory [I-D.ietf-core-resource-directory]. The group controller communicates with individual device to add them to the new group. Additionally it distributes the GSA consisting of keying material, security policies security parameters and ciphersuites using a standardized key management for LLN which is outside the scope of this draft. Additional ciphersuites may need to be defined to convey the bulk cipher algorithm, MAC algorithm and key lengths within the key management protocol. We provide two examples of ciphersuites (based on the security requirements) that could be defined as part of a future key management mechanism:

	Ciphersuite MTS_WITH_AES_128_CCM_8 = {TBD1, TBD2}
	Ciphersuite MTS_WITH_NULL_SHA256   = {TBD3, TBD4}

Ciphersuite MTS_WITH_AES_128_CCM_8 is used to provide confidentiality, integrity and authenticity to the multicast messages where the encryption algorithm is AES [FIPS.197.2001], key length is 128-bit, and the authentication function is CCM [RFC6655] with a Message Authentication Code (MAC) length of 8 octets. Similar to [RFC4785], the ciphersuite MTS_WITH_NULL_SHA is used when confidentiality of multicast messages is not required, it only provides integrity and authenticity protection to the multicast message. When this ciphersuite is used, the message is not encrypted but the MAC must be included in which it is computed using a HMAC [RFC2104] that is based on Secure Hash Function SHA256 [FIPS.180-2.2002]. Depending on the future needs, other ciphersuites with different cipher algorithms and MAC length may be supported.

Senders in the group can encrypt and authenticate the CoAP group messages from the application using the keying material into the DTLS record. The authenticated encrypted message is passed down to the lower layer of the IPv6 protocol stack for transmission to the multicast address as depicted in Figure 2. The listeners when receiving the message, use the multicast IPv6 destination address and port (i.e., Multicast identifier) to look up the GSA needed for that group connection. The received message is then decrypted and the authenticity is verified using the keying material for that connection.

|        | +--------+------------------------------------+ |
|        | |        | +-------------+------------------+ | |
|        | |        | |             | +--------------+ | | |
|   IP   | |   UDP  | | DTLS Record | |   multicast  | | | |
| header | | header | |    Header   | |    message   | | | |
|        | |        | |             | +--------------+ | | |
|        | |        | +-------------+------------------+ | |
|        | +--------+------------------------------------+ |

Figure 2: Sending a multicast message protected using DTLS Record Layer

4. Multicast Data Security

This section describes in detail the use of DTLS record layer to secure multicast messages. This assumes that group membership has been configured by the group controller, and all member devices in the group have the GSA.

4.1. SecurityParameter derivation

The GSA is used to derive the same "SecurityParameters" structure as defined in [RFC5246] for all devices.

The SecurityParameters.ConnectionEnd should be set to "server" for senders and "client" for listeners. The current read and write states can be derived from SecurityParameters by generating the six keying materials:

	client write MAC key
	server write MAC key
	client write encryption key
	server write encryption key
	client write IV
	server write IV

This requires that the client_random and server_random within the SecurityParameters are also set to the same value for all devices as part of the GSA to derive the same keying material for all devices in the group with the PRF function defined in Section 6.3 of [RFC5246] . Alternatively, the GSA could directly include the above six keying material when being configured in all group devices.

The current read and write states are instantiated for all group members based on the keying material and according to their roles: senders use "server write" parameters for the write state and listeners use "server write" parameters for the read state. Additionally each connection state contains the sequence number which is incremented for each record sent; the first record sent has the sequence number 0.

4.2. Record layer adaptation

In this section, we describe in detail the adaptation of the DTLS Record layer to enable multiple senders in the group to securely send information using a common group key, while preserving the confidentiality, integrity and freshness of the messages.

The following Figure 3 illustrates the structure of the DTLS record layer header, the epoch and seq_number are used to ensure message freshness and to detect message replays.

| 1 Byte  | 2 Byte  | 2 Byte | 6 Byte | 2 Byte |            |       |
| Content | Version | epoch  |  seq_  | Length | Ciphertext |  MAC  |
|   Type  | Ma | Mi |        | number |        |   (Enc)    | (Enc) |

Figure 3: The DTLS record layer header and optionally encrypted payload and MAC

The epoch is fixed by the DTLS handshake and the seq_number is initialized to 0. The seq_number is increased by one whenever a sender sends a new record message. This is the mechanism of DTLS to detect message replay. Finally, the message is protected (encrypted and authenticated with a MAC) using the session keys in the "server write" parameters.

One of the problems with supporting multiple senders is that, the seq_number used by senders need to be synchronized to avoid their reuse, otherwise packets sent by different senders may get discarded as replayed packets. Further, the bigger problem is using a single key in a multiple sender scenario leads to nonce reuse in AEAD cipher suites like AES-CCM [RFC6655] and AES-GCM [RFC5288] as defined in DTLS. Nonce reuse can completely break the security of these cipher suites.

According to the AES-CCM for TLS, Section 3 [RFC6655], the CCMNonce is a combination of a salt value and the sequence number.

	struct {
   		opaque salt[4];
   		opaque nonce_explicit[8];
   	} CCMNonce;

The salt is the "client write IV" (when the client is sending) or the "server write IV" (when the server is sending) as defined in the "SecurityParameters". Further [RFC6655] requires that the value of the nonce_explicit MUST be distinct for each distinct invocation of the CCM encrypt function for any fixed key. When the nonce_explicit is equal to the sequence number of the TLS packets, the CCMNonce has the structure as below:

	struct {
   		uint32 client_write_IV; // low order 32-bits
   		uint64 seq_num;         // TLS sequence number
   	} CCMClientNonce.

   	struct {
   		uint32 server_write_IV; // low order 32-bits
   		uint64 seq_num; 		// TLS sequence number
   	} CCMServerNonce.

In DTLS, the 64-bit sequence number is the 16-bit epoch concatenated with the 48-bit seq_number. Therefore to prevent that the CCMNonce is reused, either all senders need to synchronize or separate non-overlapping sequence number spaces need to be created for each sender. Synchronization between senders is especially hard in LLN and therefore we go for the second approach of separating the sequence number spaces by embedding a unique sender identifier in the sequence number as suggested in [RFC5288].

Thus in addition to configuring each device in the group with the GSA, the controller needs to assign a unique SenderID to each device which has the sender role in the group. The size of the SenderID is 1-octet based on the requirement for the supported group size mentioned in Section 2.2. The list of SenderIDs are then distributed to all the group members by the controller.

The existing DTLS record layer header is adapted such that the 6-octet seq_number field is split into a 1-octet SenderID field and a 5-octet "truncated" trunc_seq_number field. Figure 4 illustrates the adapted DTLS record layer header.

   | 1 Byte  | 2 Byte  | 2 Byte | 1 Byte | 5 Byte    | 2 Byte |
   | Content | Version | Epoch  | Sender | trunc_seq_| Length |
   |   Type  | Ma | Mi |        |   ID   | number    |        |

Figure 4: The adapted DTLS record layer header

4.3. Sending Secure Multicast Messages

Senders in the multicast group when sending a CoAP group message from the application, create the adapted DTLS record payload based on the "server write" parameters. Each sender in the group uses its own unique SenderID in the DTLS record layer header. It also manages its own epoch and trunc_seq_number in the "server write" connection state; the first record sent has the trunc_seq_number 0. After creating the DTLS record, the trun_seq_number is incremented in the "server write" connection state. The adapted DTLS record is then passed down to UDP and IPv6 layer for transmission on the multicast IPv6 destination address and port.

4.4. Receiving Secure Multicast Messages

When a listeners receives a protected multicast message from the sender, it looks up the corresponding "client read" connection state based on the multicast IP destination and port of the packet. This is fundamentally different from standard DTLS logic in that the current "client read" connection state is bound to the source IP address and port.

Listener devices in a multiple senders multicast group, need to store multiple "client read" connection states for the different senders linked to the SenderIDs. The keying material is same for all senders however the epoch and the trunc_seq_number of the last received packets needs to be kept different for different senders.

The listeners first perform a "server write" keys lookup by using the multicast IPv6 destination address and port of the packet. By knowing the keys, the listeners decrypt and check the MAC of the message. This guarantees that no one outside the group has spoofed the SenderID, as it is protected by the MAC. Subsequently, by authenticating the SenderID field, the listeners retrieve the "client read" connection state which contains the last stored epoch and trunc_seq_number of the sender, which is used to check the freshness of the message received. The listeners must ensure that the epoch is the same and trunc_seq_number in the message received is higher than the stored value, otherwise the message is discarded. Alternatively a windowing mechanism can be used to accept genuine out-of-order packets. Once the authenticity and freshness of the message have been checked, the listeners can pass the message to the higher layer protocols. The epoch and the trunc_seq_number in the corresponding "client read" connection state are updated as well.

4.5. Proxy Operation

CoAP allows a client to designate a (forward) proxy to process its CoAP request for both unicast and multicast scenarios as described in Section 2.10 of [I-D.ietf-core-groupcomm]. In this case, the proxy (and not the client) appears as the originating point to the destination server for the CoAP request.

As mentioned in Section 11.2 of [I-D.ietf-core-coap], proxies are by their nature men-in-the-middle and break DTLS protection of CoAP message exchanges. Therefore, in a DTLS-based multicast scenario involving a proxy, a two-step approach is required. First, the client will send a unicast DTLS request to the proxy. The proxy will then receive and decrypt the unicast message. The proxy will then take the contents of the received message and create a new multicast message and secure it using DTLS-based multicast before sending it out to the group. For this approach to work properly, the client needs to be able to designate the proxy as an authorized sender. The mechanism for this authorization is outside the scope of this draft.

5. IANA Considerations

This memo includes no request to IANA.

6. Security Considerations

Some of the security issues that should be taken into consideration are discussed below.

6.1. Group level security

This proposal uses a single group key to protect communication within the group. This requires that all group members are trusted, for e.g. they do not forge messages as a different sender in the group. In many usecase, the devices in a group belong to a common authority and are configured by a commissioner. In a professional lighting scenario, the roles of the senders and listeners are configured by the lighting commissioner and devices follow those roles.

The use of the protocol should take into consideration the risk of compromise of a group device in a deployment scenario. Therefore the group size should be limited to 100 devices unless additional source authenticity mechanisms are implemented at the application layer. Further, the damage due to a compromised key can be limited by increasing the frequency of rekeying based on the unique unicast key-pair shared by each device with the controller. Additionally the risk of compromise is reduced when deployments are in physcially secured locations, like lighting inside office buildings.

6.2. Late joiners

Listeners who are late joiners to a multicast group, do not know the current epoch and trun_seq_number being used by different senders. When they receive a packet from a sender with a random trunc_seq_number in it, it is impossible for the listener to verify if the packet is fresh and has not been replayed by an attacker. To overcome this late joiner security issue, we can use the techniques similar to AERO [I-D.mcgrew-aero] where the late joining listener on receiving the first packet from a particular sender, initialize its last seen epoch and trunc_seq_number in the "client read" state for that sender, however does not pass this packet to the application layer and instead drops it. This provides a reference point to identify if future packets are fresher than the last seen packet. Alternatively, the group controller which can act as a listener in the multicast group can maintain the epoch and trunc_seq_number of each sender. When late joiners send a request to the group controller to join the multicast group, the group controller can send the list of epoch and trunc_seq_numbers as part of the GSA.

6.3. Uniqueness of SenderIDs

It is important that SenderIDs are unique to maintain the security properties of the DTLS record layer messages. However in the event that two or more senders are configured with the same SenderID, a mechanism needs to be present to avoid a security weakness and recover from the situation. One such mechanism is that all senders of the multicast group are also listeners. This allows a sender which receives a packet from a different device with its own SenderID in the DTLS header to become aware of a clash. Once aware, the sender can inform the controller on a secure channel about the clash along with the source IP address. The controller can then provide a different SenderID to either device or both.

6.4. Reduced sequence number space

The DTLS record layer seq_number is truncated from 6 octets to 5 octets. This reduction of the seq_number space should be taken into account to ensure that epoch is incremented before the trunc_seq_number wraps over. The sender or the controller can increase the epoch number by sending a ChangeCipherSpec message whenever the trunc_seq_number has been exhausted. This should be done as part of the key management mechanism which is not defined in this draft.

7. Acknowledgements

The authors greatly acknowledge discussion, comments and feedback from Dee Denteneer, Peter van der Stok, Zach Shelby and Michael StJohns. Additionally thank David McGrew for suggesting options for recovering from a SenderID clash, and John Foley for the extensive review and pointing us to the AERO draft. We also appreciate prototyping and implementation efforts by Pedro Moreno Sanchez who worked as an intern at Philips Research.

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.
[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated Encryption", RFC 5116, January 2008.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5288] Salowey, J., Choudhury, A. and D. McGrew, "AES Galois Counter Mode (GCM) Cipher Suites for TLS", RFC 5288, August 2008.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security Version 1.2", RFC 6347, January 2012.
[RFC6655] McGrew, D. and D. Bailey, "AES-CCM Cipher Suites for Transport Layer Security (TLS)", RFC 6655, July 2012.
[I-D.ietf-core-coap] Shelby, Z., Hartke, K. and C. Bormann, "Constrained Application Protocol (CoAP)", Internet-Draft draft-ietf-core-coap-14, March 2013.
[I-D.ietf-core-groupcomm] Rahman, A. and E. Dijk, "Group Communication for CoAP", Internet-Draft draft-ietf-core-groupcomm-05, February 2013.

8.2. Informative References

[RFC2104] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing for Message Authentication", RFC 2104, February 1997.
[RFC3740] Hardjono, T. and B. Weis, "The Multicast Group Security Architecture", RFC 3740, March 2004.
[RFC3830] Arkko, J., Carrara, E., Lindholm, F., Naslund, M. and K. Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830, August 2004.
[RFC4046] Baugher, M., Canetti, R., Dondeti, L. and F. Lindholm, "Multicast Security (MSEC) Group Key Management Architecture", RFC 4046, April 2005.
[RFC4082] Perrig, A., Song, D., Canetti, R., Tygar, J.D. and B. Briscoe, "Timed Efficient Stream Loss-Tolerant Authentication (TESLA): Multicast Source Authentication Transform Introduction", RFC 4082, June 2005.
[RFC4535] Harney, H., Meth, U., Colegrove, A. and G. Gross, "GSAKMP: Group Secure Association Key Management Protocol", RFC 4535, June 2006.
[RFC4785] Blumenthal, U. and P. Goel, "Pre-Shared Key (PSK) Ciphersuites with NULL Encryption for Transport Layer Security (TLS)", RFC 4785, January 2007.
[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J. and D. Culler, "Transmission of IPv6 Packets over IEEE 802.15.4 Networks", RFC 4944, September 2007.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2", RFC 4949, August 2007.
[RFC5374] Weis, B., Gross, G. and D. Ignjatic, "Multicast Extensions to the Security Architecture for the Internet Protocol", RFC 5374, November 2008.
[RFC6282] Hui, J. and P. Thubert, "Compression Format for IPv6 Datagrams over IEEE 802.15.4-Based Networks", RFC 6282, September 2011.
[RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service Discovery", RFC 6763, February 2013.
[FIPS.197.2001] National Institute of Standards and Technology, "Advanced Encryption Standard (AES)", FIPS PUB 197, November 2001.
[FIPS.180-2.2002] National Institute of Standards and Technology, "Secure Hash Standard", FIPS PUB 180-2, August 2002.
[I-D.ietf-core-resource-directory] Shelby, Z., Krco, S. and C. Bormann, "CoRE Resource Directory", Internet-Draft draft-ietf-core-resource-directory-00, June 2013.
[I-D.vanderstok-core-dna] Stok, P., Lynn, K. and A. Brandt, "CoRE Discovery, Naming, and Addressing", Internet-Draft draft-vanderstok-core-dna-02, July 2012.
[I-D.dijk-core-groupcomm-misc] Dijk, E. and A. Rahman, "Miscellaneous CoAP Group Communication Topics", Internet-Draft draft-dijk-core-groupcomm-misc-05, December 2013.
[I-D.mcgrew-aero] McGrew, D. and J. Foley, "Authenticated Encryption with Replay prOtection (AERO)", Internet-Draft draft-mcgrew-aero-00, October 2013.

Appendix A. Change Log

(To be removed by RFC editor before publication.)

Changes from keoh-03 to keoh-04:

  • Added description of Proxy operation in a DTLS-based multicast scenario in Section 4.5 (Proxy Operation).
  • Corrected text in Section 2.2 (Security Requirements), item "h", to indicate that multicast source authentication is not specified in this version of the draft.
  • Clarified that draft is written primarily for securing of CoAP based group communication, but that other protocols may also be supported if they have similar characteristics. See Section 1 (Introduction).
  • Ran IETF spell checker and ID-Nits tools and corrected various issues throughout the document.
  • Various editorial updates.

Changes from keoh-04 to keoh-05:

  • In section 2.1, removed the firmware upgrade usecase and clarified the commissioning use case. The lighting use-case expanded with shared and multiple subnets issues.
  • In Section 2.2, (b) reduced the group size to 100; (h) clarified data source authenticity
  • Added new Section 6.1 (Group level security) in security considerations to make clear the risks of the single group key.

Authors' Addresses

Sye Loong Keoh University of Glasgow Singapore Republic PolyTechnic, 9 Woodlands Ave 9 Singapore, 838964 SG EMail: SyeLoong.Keoh@glasgow.ac.uk
Sandeep S. Kumar (editor) Philips Research High Tech Campus 34 Eindhoven, 5656 AE NL EMail: sandeep.kumar@philips.com
Oscar Garcia-Morchon Philips Research High Tech Campus 34 Eindhoven, 5656 AE NL EMail: oscar.garcia@philips.com
Esko Dijk Philips Research High Tech Campus 34 Eindhoven, 5656 AE NL EMail: esko.dijk@philips.com
Akbar Rahman InterDigital 1000 Sherbrooke Street West Montreal, H3A 3G4 CA EMail: akbar.rahman@interdigital.com