]> Google, LLC

warren@kumari.net

Cisco Systems, Inc.

suresh.krishnan@gmail.com

Independent

rajiv.asati@gmail.com

Google, LLC

Shibuya 3-21-3 Japan lorenzo@google.com

Google, LLC

1 Darling Island Rd Pyrmont 2009 Australia furry13@gmail.com

Beijing University of Posts and Telecommunications

No. 10 Xitucheng Road Beijing Haidian District 100083 China shengjiang@bupt.edu.cn

Internet Dynamic Host Configuration Internet-Draft This document defines a method to inform a DHCPv6 server that a device has one or more self-generated or statically configured addresses. About This Document The latest revision of this draft can be found at . Status information for this document may be found at . Discussion of this document takes place on the Dynamic Host Configuration Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

Introduction It is very common operational practice, especially in enterprise networks, to use IPv4 DHCP logs for troubleshooting or forensics purposes. Examples of this include a help desk dealing with a ticket such as "The CEO's laptop cannot connect to the printer"; if the MAC address of the printer is known (for example from an inventory system), the printer's IPv4 address can be retrieved from the DHCP log or lease table and the printer pinged to determine if it is reachable. Another common example is a Security Operations team discovering suspicious events in outbound firewall logs and then consulting DHCP logs to determine which employee's laptop had that IPv4 address at that time so that they can quarantine it and remove the malware. This operational practice relies on the DHCP server knowing the IP address assignments. This works quite well for IPv4 addresses, as most addresses are either assigned by DHCP or statically configured by the network operator. For IPv6, however, this practice is much harder to implement, as devices often self-configure IPv6 addresses via SLAAC . This document provides a mechanism for a device to inform the DHCPv6 server that the device has a self-configured IPv6 address (or has a statically configured address), and thus provides parity with IPv4, by making DHCPv6 infrastructure aware of self-assigned IPv6 addresses.

Conventions and Definitions 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 BCP 14 when, and only when, they appear in all capitals, as shown here.

Registration Mechanism Overview The DHCPv6 protocol is used as the address registration protocol when a DHCPv6 server performs the role of an address registration server. This document introduces a new Address Registration (OPTION_ADDR_REG_ENABLE) option which indicates that the server supports the registration mechanism. Before registering any addresses, the client MUST determine whether the network supports address registration. It can do this by including the Address Registration option code in the Option Request option (see Section 21.7 of ) of the Information-Request, Solicit, Request, Renew, or Rebind messages it sends to the server as part of the regular stateless or stateful DHCPv6 configuration process. If the server supports address registration, it includes an Address Registration option in its Advertise or Reply messages. To avoid undesired multicast traffic, if the DHCPv6 infrastructure does not support (or is not willing to receive) any address registration information, the client MUST NOT register any addresses using the mechanism in this specification. Otherwise, the client registers addresses as described below. After successfully assigning a self-generated or statically configured Valid () IPv6 address on one of its interfaces, a client implementing this specification multicasts an ADDR-REG-INFORM message (see Section 4.2) in order to inform the DHCPv6 server that this self-generated address is in use. Each ADDR-REG-INFORM message contains a DHCPv6 IA Address option to specify the address being registered. The address registration mechanism overview is shown in Fig.1. | | | | | | | | src: link-local address | | --------------------------------------------> | | INFORMATION-REQUEST or SOLICIT/... | | - OPTION REQUEST OPTION | | -- OPTION_ADDR_REG_ENABLE | | | | ... | | | | | |<--------------------------------------------- | | REPLY or ADVERTISE MESSAGE | | - OPTION_ADDR_REG_ENABLE | | | | | | src: address being registered | | --------------------------------------------> | | ADDR-REG-INFORM MESSAGE |Register/ | |log addresses | | | | | <-------------------------------------------- | | ADDR-REG-REPLY MESSAGE | | | ]]> Figure 1: Address Registration Procedure Overview

DHCPv6 Address Registration Procedure

DHCPv6 Address Registration Option The Address Registration option (OPTION_ADDR_REG_ENABLE) indicates that the server supports the mechanism described in this document. The format of the Address Registration option is described as follows: Figure 2: DHCPv6 Address Registration option If a client has the address registration mechanism enabled, it MUST include this option in all Option Request options that it sends. A server which is configured to support the address registration mechanism MUST include this option in Advertise and Reply messages if the client message it is replying to contained this option in the Option Request option.

DHCPv6 Address Registration Request Message The DHCPv6 client sends an ADDR-REG-INFORM message to inform that an IPv6 address is assigned to the client's interface. The format of the ADDR-REG-INFORM message is described as follows: Figure 3: DHCPv6 ADDR-REG-INFORM message The client MUST generate a transaction ID as described in and insert this value in the "transaction-id" field. The client MUST include the Client Identifier option in the ADDR-REG-INFORM message. The ADDR-REG-INFORM message MUST NOT contain the Server Identifier option and MUST contain exactly one IA Address option containing the address being registered. The valid-lifetime and preferred-lifetime fields in the option MUST match the current Valid Lifetime and Preferred Lifetime of the address being registered. The ADDR-REG-INFORM message is dedicated for clients to initiate an address registration request toward an address registration server. Consequently, clients MUST NOT put any Option Request Option(s) in the ADDR-REG-INFORM message. Clients MAY include other options, such as the Client FQDN Option . The client sends the DHCPv6 ADDR-REG-INFORM message to the All_DHCP_Relay_Agents_and_Servers multicast address (ff02::1:2). The client MUST send separate messages for each address being registered. Unlike other types of messages, which are sent from the link-local address of the client, the ADDR-REG-INFORM message MUST be sent from the address being registered. This is primarily for "fate sharing" purposes - for example, if the network implements some form of layer-2 security to prevent a client from spoofing other clients' MAC addresses, this prevents an attacker from spoofing ADDR-REG-INFORM messages. On clients with multiple interfaces, the client MUST only send the packet on the network interface that has the address being registered, even if it has multiple interfaces with different addresses. If the same address is configured on multiple interfaces, then the client MUST send ADDR-REG-INFORM each time the address is configured on an interface that did not previously have it, and refresh each registration independently from the others. The client MUST only send the ADDR-REG-INFORM message for valid () addresses of global scope (). This includes ULA addresses, which are defined in to have global scope. This also includes statically assigned addresses of global scope (such addresses are considered to be valid indefinitely). The client MUST NOT send the ADDR-REG-INFORM message for addresses configured by DHCPv6. The client SHOULD NOT send the ADDR-REG-INFORM message unless it has received a Router Advertisement message with either M or O flags set to 1. Clients MUST discard any received ADDR-REG-INFORM messages.

Server message processing Servers MUST discard any ADDR-REG-INFORM messages that meet any of the following conditions:

• the message does not include a Client Identifier option;
• the message includes a Server Identifier option;
• the message does not include the IA Address option, or the IP address in the IA Address option does not match the source address of the original ADDR-REG-INFORM message sent by the client. The source address of the original message is the source IP address of the packet if it is not relayed, or the Peer-Address field of the innermost Relay-Forward message if it is relayed.
• the message includes an Option Request Option.

If the message is not discarded, the address registration server SHOULD verify that the address being registered is "appropriate to the link" as defined by or within a prefix delegated to the client via DHCPv6-PD (see Section 6.3 of ). If the address being registered fails this verification, the server MUST drop the message, and SHOULD log this fact. If the message passes the verification, the server:

• MUST log the address registration information (as is done normally for clients to which it has assigned an address), unless configured not to do so. The server SHOULD log the client DUID and the link-layer address, if available. The server MAY log any other information.
• SHOULD register a binding between the provided Client Identifier and IPv6 address in its database, if no binding exists. The lifetime of the binding is equal to the Valid Lifetime of the address reported by the client. If there is already a binding between the registered address and the same client, the server MUST update its lifetime. If there is already a binding between the registered address and another client, the server SHOULD log the fact and update the binding.
• SHOULD mark the address as unavailable for use and not include it in future ADVERTISE messages.
• MUST send back an ADDR-REG-REPLY message to ensure the client does not retransmit.

If a client is multihomed (connected to multiple administrative domains, each operating its own DHCPv6 infrastructure), the requirement to verify that the registered address is appropriate for the link or belongs to a delegated prefix ensures that each DHCPv6 server only registers bindings for addresses from the given administrative domain. Although a client "MUST NOT send the ADDR-REG-INFORM message for addresses configured by DHCPv6", if a server does receive such a message, it SHOULD log and discard it. DHCPv6 relay agents and switches that relay address registration messages directly from clients MUST include the client's link-layer address in the relayed message using the Client Link-Layer Address option () if they would do so for other DHCPv6 client messages such as SOLICIT, REQUEST, and REBIND.

DHCPv6 Address Registration Acknowledgement The server MUST acknowledge receipt of a valid ADDR-REG-INFORM message by sending back an ADDR-REG-REPLY message. The format of the ADDR-REG-REPLY message is described as follows: Figure 4: DHCPv6 ADDR-REG-REPLY message If the ADDR-REG-INFORM message that the server is replying to was not relayed, then the IPv6 destination address of the message MUST be the address being registered. If the ADDR-REG-INFORM message was relayed, then the server MUST construct the Relay-reply message as specified in section 19.3. The server MUST copy the transaction-id from the ADDR-REG-INFORM message to the transaction-id field of the ADDR-REG-REPLY. The ADDR-REG-REPLY message MUST contain an IA Address option for the address being registered. The option MUST be identical to the one in the ADDR-REG-INFORM message that the server is replying to. Servers MUST ignore any received ADDR-REG-REPLY messages. Clients MUST discard any ADDR-REG-REPLY messages that meet any of the following conditions:

• The IPv6 destination address does not match the address being registered.
• The IA Address option does not match the address being registered.
• The address being registered is not assigned to the interface receiving the message.
• The transaction-id does not match the transaction-id the client used in the corresponding ADDR-REG-INFORM message.

The ADDR-REG-REPLY message only indicates that the ADDR-REG-INFORM message has been received and that the client should not retransmit it. The ADDR-REG-REPLY message MUST NOT be considered as any indication of the address validity and MUST NOT be required for the address to be usable. DHCPv6 relays, or other devices that snoop ADDR-REG-REPLY messages, MUST NOT add or alter any forwarding or security state based on the ADDR-REG-REPLY message.

Signaling Address Registration Support To avoid undesired multicast traffic, the client MUST NOT register addresses using this mechanism unless the DHCPv6 infrastructure supports address registration. The client can discover this by including using the OPTION_ADDR_REG_ENABLE option in the Option Request options that it sends. If the client receives and processes an Advertise or Reply message with the OPTION_ADDR_REG_ENABLE option, it concludes that the DHCPv6 infrastructure supports address registration. When the client detects address registration support, it MUST start the registration process (unless configured not to do so) and MUST immediately register any addresses that are already in use. Once the client starts the registration process, it MUST NOT stop registering addresses until it disconnects from the link, even if subsequent Advertise or Reply messages do not contain the OPTION_ADDR_REG_ENABLE option. The client MUST discover whether the DHCPv6 infrastructure supports address registration every time it connects to a network or when it detects it has moved to a new link, without utilizing any prior knowledge about address registration support on that network or link. This client behavior allows networks to progressively roll out support for the address registration option across the DHCPv6 infrastructure without causing clients to frequently stop and restart address registration if some of the network's DHCPv6 servers support it and some of them do not. A client with multiple interfaces MUST discover address registration support for each interface independently. The client MUST NOT send address registration messsages on a given interface unless the client has discovered that the interface is connected to a network which supports address registration.

Retransmission To reduce the effects of packet loss on registration, the client MUST retransmit the registration message. Retransmissions SHOULD follow the standard retransmission logic specified by section 15 of with the following default parameters:

• IRT 1 sec
• MRC 3

The client SHOULD allow these parameters to be configured by the administrator. To comply with section 16.1 of , the client MUST leave the transaction ID unchanged in retransmissions of an ADDR-REG-INFORM message. When the client retransmits the registration message, the lifetimes in the packet MUST be updated so that they match the current lifetimes of the address. If an ADDR-REG-REPLY message is received for the address being registered, the client MUST stop retransmission.

Registration Expiry and Refresh The client MUST refresh registrations to ensure that the server is always aware of which addresses are still valid. The client SHOULD perform refreshes as described below.

SLAAC Addresses For an address configured using SLAAC, a function AddrRegRefreshInterval(address) is defined as 80% of the address's current Valid Lifetime. When calculating this value, the client applies a multiplier of AddrRegDesyncMultiplier to avoid synchronization causing a large number of registration messages from different clients at the same time. AddrRegDesyncMultiplier is a random value uniformly distributed between 0.9 and 1.1 (inclusive) and is chosen by the client when it starts the registration process, to ensure that refreshes for addresses with the same lifetime are coalesced (see below). Whenever the client registers or refreshes an address, it calculates a NextAddrRegRefreshTime for that address as AddrRegRefreshInterval seconds in the future but does not schedule any refreshes. Whenever the network changes the Valid Lifetime of an existing address by more than 1%, for example, by sending a Prefix Information option (PIO, ) with a new Valid Lifetime, the client calculates a new AddrRegRefreshInterval. The client schedules a refresh for min(now + AddrRegRefreshInterval, NextAddrRegRefreshTime). If the refresh would be scheduled in the past, then the refresh occurs immediately. Justification: this algorithm ensures that refreshes are not sent too frequently, while ensuring that the server never believes that the address has expired when it has not. Specifically, after every registration:

• If the network never changes the lifetime of an address (e.g., if no further PIOs are received, or if all PIO lifetimes decrease in step with the passage of time), then no refreshes occur. Refreshes are not necessary, because the address expires at the time the server expects it to expire.
• Any time the network changes the lifetime of an address (i.e., changes the time at which the address will expire) the client ensures that a refresh is scheduled, so that server will be informed of the new expiry.
• Because AddrRegDesyncMultiplier is at most 1.1, the refresh never occurs later than a point 88% between the time when the address was registered and the time when the address will expire. This allows the client to retransmit the registration for up to 12% of the original interval before it expires. This may not be possible if the network sends a Router Advertisement (RA, ) very close to the time when the address would have expired. In this case, the client refreshes immediately, which is the best it can do.
• The 1% tolerance ensures that the client will not refresh or reschedule refreshes if the Valid Lifetime experiences minor changes due to transmission delays or clock skew between the client and the router(s) sending the Router Advertisement.
• AddrRegRefreshCoalesce (Section 4.6.3) allows battery-powered clients to wake up less often. In particular, it allows the client to coalesce refreshes for multiple addresses formed from the same prefix, such as the stable and privacy addresses. Higher values will result in fewer wakeups, but may result in more network traffic, because if a refresh is sent early, then the next RA received will cause the client to immediately send a refresh message.
• In typical networks, the lifetimes in periodic Router Advertisements either contain constant values, or values that decrease over time to match another lifetime, such as the lifetime of a prefix delegated to the network. In both these cases, this algorithm will refresh on the order of once per address lifetime, which is similar to the number of refreshes that are necessary using stateful DHCPv6.
• Because refreshes occur at least once per address lifetime, the network administrator can control the address refresh frequency by appropriately setting the Valid Lifetime in the Prefix Information Option.

Statically Assigned Addresses A statically assigned address has an infinite valid lifetime which is not affected by Router Advertisements. Therefore whenever the client registers or refreshes a statically assigned address, the next refresh is scheduled for StaticAddrRegRefreshInterval seconds in the future. The default value of StaticAddrRegRefreshInterval is 4 hours. This ensures static addresses are still refreshed periodically, but refreshes for static addresses do not cause excessive multicast traffic. The StaticAddrRegRefreshInterval interval SHOULD be configurable.

Transmitting Refreshes When a refresh is performed, the client MAY refresh all addresses assigned to the interface that are scheduled to be refreshed within the next AddrRegRefreshCoalesce seconds. The value of AddrRegRefreshCoalesce is implementation-dependent, and a suggested default is 60 seconds. Registration refresh packets MUST be retransmitted using the same logic as used for initial registrations (see the 'Retransmission' section above). The client MUST generate a new transaction ID when refreshing the registration. When a Client-Identifier-to-IPv6-address binding expires, the server MUST remove it and consider the address as available for use. The client MAY choose to notify the server when an address is no longer being used (e.g., if the client is disconnecting from the network, the address lifetime expired, or the address is being removed from the interface). To indicate that the address is not being used anymore the client MUST set the preferred-lifetime and valid-lifetime fields of the IA Address option in the ADDR-REG-INFORM message to zero. If the server receives a message with a valid-lifetime of zero, it MUST act as if the address has expired.

Client configuration DHCP clients SHOULD allow the administrator to disable sending ADDR-REG-INFORM messages. This could be used, for example, to reduce network traffic on networks where the servers are known not to support the message type. Sending the messages SHOULD be enabled by default.

Security Considerations An attacker may attempt to register a large number of addresses in quick succession in order to overwhelm the address registration server and / or fill up log files. Similar attack vectors exist today, e.g., an attacker can DoS the server with messages containing spoofed DHCP Unique Identifiers (DUIDs) . If a network is using FCFS SAVI , then the DHCPv6 server can trust that the ADDR-REG-INFORM message was sent by the legitimate holder of the address. This prevents a client from registering an address configured on another client. One of the use cases for the mechanism described in this document is to identify sources of malicious traffic after the fact. Note, however, that as the device itself is responsible for informing the DHCPv6 server that it is using an address, a malicious or compromised device can simply not send the ADDR-REG-INFORM message. This is an informational, optional mechanism, and is designed to aid in troubleshooting and forensics. On its own, it is not intended to be a strong security access mechanism. In particular, the ADDR-REG-INFORM message MUST NOT be used for authentication and authorization purposes, because in addition to the reasons above, the packets containing the message may be dropped.

Privacy Considerations If the network doesn't have MLD snooping enabled, then IPv6 link-local multicast traffic is effectively transmitted as broadcast. In such networks, an on-link attacker listening to DHCPv6 messages might obtain information about IPv6 addresses assigned to the client. As ADDR-REG-INFORM messages contain unique identifiers such as the client's DUID, the attacker may be able to track addresses being registered and map them to the same client, even if the client uses randomized MAC addresses. This privacy consideration is not specific to the proposed mechanism. Section 4.3 of discusses using the DUID for device tracking in DHCPv6 environments and provides mitigation recommendations. In general, hiding information about the specific IPv6 address from on-link observers should not be considered a security measure, as such information is usually disclosed via Duplicate Address Detection to all nodes anyway, if MLD snooping is not enabled. If MLD snooping is enabled, an attacker might be able to join the All_DHCP_Relay_Agents_and_Servers multicast address (ff02::1:2) group to listen for address registration messages. However, the same result can be achieved by joining the All Routers Address (ff02::2) group and listen to Gratuitous Neighbor Advertisement messages . It should be noted that this particular scenario shares the fate with DHCPv6 address assignment: if an attacker can join the All_DHCP_Relay_Agents_and_Servers multicast group, they would be able to monitor all DHCPv6 messages sent from the client to DHCPv6 servers and relays, and therefore obtain the information about addresses being assigned via DHCPv6. Layer 2 isolation allows mitigating this threat by blocking onlink peer-to-peer communication between nodes.

IANA Considerations This document introduces the following new entities which require an allocation out of the Dynamic Host Configuration Protocol for IPv6 (DHCPv6) registry group defined at http://www.iana.org/assignments/dhcpv6-parameters/:

• one new DHCPv6 option, described in Section 4.1 which requires an allocation out of the Option Codes registry:
  • Value: TBA0
  • Description: OPTION_ADDR_REG_ENABLE
  • Client ORO: Yes
  • Singleton Option: Yes
  • Reference: This document (draft-ietf-dhc-addr-notification)
• two new DHCPv6 messages which require an allocation out of the Message Types registry:
  • ADDR-REG-INFORM message (TBA1) described in Section 4.2
  • ADDR-REG-REPLY (TBA2) described in Section 4.3.
  • Reference: This document (draft-ietf-dhc-addr-notification)

References Normative References In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. This document specifies the architectural characteristics, expected behavior, textual representation, and usage of IPv6 addresses of different scopes. According to a decision in the IPv6 working group, this document intentionally avoids the syntax and usage of unicast site-local addresses. [STANDARDS-TRACK] This document defines an IPv6 unicast address format that is globally unique and is intended for local communications, usually inside of a site. These addresses are not expected to be routable on the global Internet. [STANDARDS-TRACK] This document specifies the steps a host takes in deciding how to autoconfigure its interfaces in IP version 6. The autoconfiguration process includes generating a link-local address, generating global addresses via stateless address autoconfiguration, and the Duplicate Address Detection procedure to verify the uniqueness of the addresses on a link. [STANDARDS-TRACK] This document specifies the format and mechanism that is to be used for encoding the client link-layer address in DHCPv6 Relay-Forward messages by defining a new DHCPv6 Client Link-Layer Address option. This document describes the Dynamic Host Configuration Protocol for IPv6 (DHCPv6): an extensible mechanism for configuring nodes with network configuration parameters, IP addresses, and prefixes. Parameters can be provided statelessly, or in combination with stateful assignment of one or more IPv6 addresses and/or IPv6 prefixes. DHCPv6 can operate either in place of or in addition to stateless address autoconfiguration (SLAAC). This document updates the text from RFC 3315 (the original DHCPv6 specification) and incorporates prefix delegation (RFC 3633), stateless DHCPv6 (RFC 3736), an option to specify an upper bound for how long a client should wait before refreshing information (RFC 4242), a mechanism for throttling DHCPv6 clients when DHCPv6 service is not available (RFC 7083), and relay agent handling of unknown messages (RFC 7283). In addition, this document clarifies the interactions between models of operation (RFC 7550). As such, this document obsoletes RFC 3315, RFC 3633, RFC 3736, RFC 4242, RFC 7083, RFC 7283, and RFC 7550. The Dynamic Host Configuration Protocol (DHCP) provides a framework for passing configuration information to hosts on a TCPIP network. DHCP is based on the Bootstrap Protocol (BOOTP), adding the capability of automatic allocation of reusable network addresses and additional configuration options. [STANDARDS-TRACK] RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. This document specifies a new Dynamic Host Configuration Protocol for IPv6 (DHCPv6) option that can be used to exchange information about a DHCPv6 client's Fully Qualified Domain Name (FQDN) and about responsibility for updating DNS resource records (RRs) related to the client's address assignments. [STANDARDS-TRACK] Some DHCP options carry unique identifiers. These identifiers can enable device tracking even if the device administrator takes care of randomizing other potential identifications like link-layer addresses or IPv6 addresses. The anonymity profiles are designed for clients that wish to remain anonymous to the visited network. The profiles provide guidelines on the composition of DHCP or DHCPv6 messages, designed to minimize disclosure of identifying information. Neighbor Discovery (RFC 4861) is used by IPv6 nodes to determine the link-layer addresses of neighboring nodes as well as to discover and maintain reachability information. This document updates RFC 4861 to allow routers to proactively create a Neighbor Cache entry when a new IPv6 address is assigned to a node. It also updates RFC 4861 and recommends that nodes send unsolicited Neighbor Advertisements upon assigning a new IPv6 address. These changes will minimize the delay and packet loss when a node initiates connections to an off-link destination from a new IPv6 address. Informative References This memo describes First-Come, First-Served Source Address Validation Improvement (FCFS SAVI), a mechanism that provides source address validation for IPv6 networks using the FCFS principle. The proposed mechanism is intended to complement ingress filtering techniques to help detect and prevent source address spoofing. [STANDARDS-TRACK] This document specifies the Neighbor Discovery protocol for IP Version 6. IPv6 nodes on the same link use Neighbor Discovery to discover each other's presence, to determine each other's link-layer addresses, to find routers, and to maintain reachability information about the paths to active neighbors. [STANDARDS-TRACK]

Acknowledgments Many thanks to Bernie Volz for significant review and feedback, as well as Hermin Anggawijaya, Carlos Jesus Bernardos, Brian Carpenter, Stuart Cheshire, Roman Danyliw, Alan DeKok, James Guichard, James Guichard, Erik Kline, Mallory Knodel, Murray Kucherawy, David Lamparter, Ted Lemon, Eric Levy-Abegnoli, Aditi Patange, Jim Reid, Michael Richardson, Patrick Rohr, John Scudder, Mark Smith, Gunter Van de Velde, Eric Vyncke, Timothy Winters, Peter Yee for their feedback, comments and guidance. We apologize if we inadvertently forgot to acknowledge anyone's contributions.

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