]> Ericsson

claudio.porfiri@ericsson.com

Cisco

suresh.krishnan@gmail.com

Ericsson

jari.arkko@ericsson.com

Ericsson

mirja.kuhlewind@ericsson.com

Internet Dynamic Host Configuration dhcp This document describes a mechanism for networks with legacy IPv4-only clients to use services provided by DHCPv6 using DHCPv4-over-DHCPv6 (DHCP 4o6) in a Relay Agent. RFC7341 specifies use of DHCPv4-over-DHCPv6 in the client only. This document specifies a RFC7341-based approach that allows DHCP 4o6 to be deployed as a Relay Agent (4o6RA) that implements the 4o6 DHCP en- and decapsulation if this is not possible at the client. About This Document Status information for this document may be found at . Source for this draft and an issue tracker can be found at .

Introduction describes a transport mechanism for carrying DHCPv4 messages using the DHCPv6 protocol for the dynamic provisioning of IPv4 addresses and other DHCPv4 specific configuration parameters across IPv6-only networks. The deployment of requires implementation in all DHCP clients and at the DHCPv6 server. However, if the client only supports IPv4 and cannot easily be replaced or updated due to a number of technical or business reasons, this approach does not work. Similarly, the specifications for DHCPv6 Relay Agents such as LDRA or L3RA do not foresee the possibility to handle legacy DHCP, other than implementing 4o6 in client. This document specifies an -based solution that can be implemented in intermediate nodes such as L2 switches or routers, without putting any requirements on clients. No new protocols or extensions are needed, instead this document specifies an amendment to that allows a Relay Agent to perform the 4o6 DHCP en- and decapsultion instead of the client.

Conventions and Definitions The following terms and acronyms are used in this document:

• DHCPv4 over DHCPv6 (or 4o6): The architecture, the procedures and the protocols described in the DHCPv4-over-DHCPv6 document .
• DHCP Relay Agent (or RA): This is a concept in all of the protocols, BOOTP , DHCPv4 , and DHCPv6 , although the details differ between the protocols.
• Lightweight DHCPv6 Relay Agent (or LDRA): This is an extension of the original DHCPv6 Relay Agent mechanism, to support also Layer 2 devices performing a Relay Agent function .
• DHCPv4 over DHCPv6 Relay Agent (or 4o6RA): The 4o6 Relay Agent (as specified in this document) is the part of an RA implementing 4o6.

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.

DHCPv4 over DHCPv6 Relay Agent (4o6RA) This document assume an network, where IPv4-only clients are connected to an uplink network that supports IPv6 only and limited IPv4 services. To address such a network setup, this document proposes to extend DHCPv6 Relay Agents with DHCPv4 over DHCPv6, as shown in .

Architecture Overview with legacy DHCP client

This document specifies the encapsulation and decapsulation described in to be performed in the Relay Agent whereas the DHCP Client does not require any change. In this case it is up to the Relay Agent to provide the full 4o6 DHCP set of functionality whereas the legacy client is not aware of being served via a 4o6 DHCP service. All prerequisites and configuration that to the DHCP client in apply shall be applied to the 4o6RA instead. To maintain interoperability with existing DHCP relays and servers, the message format is unchanged from . The 4o6RA implements the same message types as a normal DHCPv6 Relay Agent . In this specification, the 4o6RA creates the DHCPV4-QUERY Message and encapsulates the DHCP request message received from the legacy DHCPv4 client. When DHCPV4-RESPONSE Message is received by the 4o6 Relay Agent, it looks for the DHCPv4 Message option within this message. If this option is not found, the DHCPv4-response message MUST be discarded. If the DHCPv4 Message option is present, the 4o6RA MUST extract the DHCPv4 message and forward the encapsulated DHCPv4-response to the legacy DHCPv4 client. Any Layer 2 Relay Agent receiving DHCPV4-QUERY or DHCPV4-RESPONSE messages will handle them as specified in . The DHCPv6 server must be compliant with 4o6 according to . No additional requirements on DHCPv6 server are set by this specification.

Using 4o6RA for Topology Discovery In some networks the configuration of a client host may depend on the topology. However, when a new client host gets connected to the network, it may be unaware of the topology and respectively how it has to be configured. The DHCPv4 and DHCPv6 protocol specifications describe how addresses can be allocated to clients based on network topology information provided by the DHCP relay infrastructure. In IPv6 networks, Topology discover can be realized using DHCPv6 Relay Agents that insert relay agent options in DHCPv6 message exchanges in order to identify the client-facing interfaces, e.g. using the Serial Number or other hardcoded information. Then, a reference host that is responsible for providing configuration to the client host can obtain topology information from the DHCP server. Address allocation decisions are integral to the allocation of addresses and prefixes in DHCP. The argument is described in details in , here we want to guarantee that 4o6RA does not break any legacy capability when related to the use of topology. In the scenario described in the DHCPv6 Relay Agent knows the interface where the encapsulated DHCP request is received. Moving 4o6 in the intermediate node rather than at the client breaks the topology propagation as 4o6RA-only does not provide any interface information in the encapsulated message.

Topology broken path

As shown in , the introduction of 4o6 at the edge of the IPv6 network hides the L2 network from the DHCPv6 RA. In order to preserve the topology information, it is recommended that the implementation of 4o6RA is combined with the implementation of LDRA and that the implementation has a mechanism for LDRA to get interface information that can be used for the Interface-ID option, as specified in . The internal mechanisms to exchange interface information, their format and whether the interface information contains an indication that a 4o6RA is involved are out of the scope for this document.

Topology path preserved with LDRA

The assumed architecture is shown in where the whole RA is built up with cooperating 4o6RA and LDRA, and an internal interface to propagated topology information from 4o6RA to LDRA. In a simple case, where the same node hosts teh 4o6RA and the DHCP4o6 server, it might be enough to only use 4o6RA, as shown in .

Topology path preserved 4o6 RA in DHCP server

Deployment Considerations As the client is not aware of the 4o6RA, the network deployment needs to ensure that all DHCPv4 broad- and unicast messages from the client are routed over the 4o6RA. This can e.g. be achieved by placing the 4o6RA in a cetral position that can observe all traffic from the clients or use of address translation with the 4o6RA address for unicast messages.

Security Considerations This documents applies 4o6 DHCP in a scenario where legacy IPv4 clients are connected to 4o6 DHCP Relay Agent that performs the en- and decapsulation. This document does not change anything else in the 4o6 DHCP specification and therefore the security consideration of still apply. The mechanism described differs from as the DHCP client actually sends and receveis DHCP messages, whereas in it only sends DHCPv6 messages. This makes it possible that DHCP messages could reach a DHCP server without using the 4o6RA. While this can cause errornous states in both the client and DHCP server and potentially even lead to misconfigrations that impact reachability, this is not seen as a security concern rather than a deloyment error. More generally, the legacy IPv4 client is not aware of this mechanism, however, even when 4o6 DHCP is used, the client does not have any control about the information provided by the Relay agent. As such this change does not raise any additional security concerns.

IANA Considerations This document has no IANA actions.

References Normative References Newer high-speed public Internet access technologies call for a high- speed modem to have a local area network (LAN) attachment to one or more customer premise hosts. It is advantageous to use the Dynamic Host Configuration Protocol (DHCP) as defined in RFC 2131 to assign customer premise host IP addresses in this environment. However, a number of security and scaling problems arise with such "public" DHCP use. This document describes a new DHCP option to address these issues. This option extends the set of DHCP options as defined in RFC 2132. [STANDARDS-TRACK] This document proposes a Lightweight DHCPv6 Relay Agent (LDRA) that is used to insert relay agent options in DHCPv6 message exchanges identifying client-facing interfaces. The LDRA can be implemented in existing access nodes (such as Digital Subscriber Link Access Multiplexers (DSLAMs) and Ethernet switches) that do not support IPv6 control or routing functions. [STANDARDS-TRACK] This document defines a new Relay Agent Identifier sub-option for the Dynamic Host Configuration Protocol (DHCP) Relay Agent Information option. The sub-option carries a value that uniquely identifies the relay agent device within the administrative domain. The value is normally administratively configured in the relay agent. The sub-option allows a DHCP relay agent to include the identifier in the DHCP messages it sends. IPv4 connectivity is still needed as networks migrate towards IPv6. Users require IPv4 configuration even if the uplink to their service provider supports IPv6 only. This document describes a mechanism for obtaining IPv4 configuration information dynamically in IPv6 networks by carrying DHCPv4 messages over DHCPv6 transport. Two new DHCPv6 messages and two new DHCPv6 options are defined for this purpose. DHCP servers have evolved over the years to provide significant functionality beyond that described in the DHCP base specifications. One aspect of this functionality is support for context-specific configuration information. This memo describes some such features and explains their operation. 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. 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. 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. Informative References This RFC describes an IP/UDP bootstrap protocol (BOOTP) which allows a diskless client machine to discover its own IP address, the address of a server host, and the name of a file to be loaded into memory and executed. The bootstrap operation can be thought of as consisting of TWO PHASES. This RFC describes the first phase, which could be labeled `address determination and bootfile selection'. After this address and filename information is obtained, control passes to the second phase of the bootstrap where a file transfer occurs. The file transfer will typically use the TFTP protocol, since it is intended that both phases reside in PROM on the client. However BOOTP could also work with other protocols such as SFTP or FTP. This RFC suggests a proposed protocol for the ARPA-Internet community, and requests discussion and suggestions for improvements. Some aspects of the BOOTP protocol were rather loosely defined in its original specification. In particular, only a general description was provided for the behavior of "BOOTP relay agents" (originally called "BOOTP forwarding agents"). The client behavior description also suffered in certain ways. This memo attempts to clarify and strengthen the specification in these areas. [STANDARDS-TRACK] This document specifies the current set of DHCP options. Future options will be specified in separate RFCs. The current list of valid options is also available in ftp://ftp.isi.edu/in-notes/iana/assignments. [STANDARDS-TRACK] 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]

Example Use Case: Topology Discovery for IPv4-only Radio Unit in the RAN Switched Fronthaul In Radio Access Networks (RANs) the Fronthaul is the network segment that connects Radio Units, the distributed radio elements in a mobile network, to other network elements. The aggregation of Radio Unit devices (also known as Switched Fronthaul) hides the relationship between the Radio Units themselves and the physical ports where they are connected. The Radio Units are the client hosts in the switched Fronthaul network and need to be configured based on their Topology.

Layer 2 Switched Fronthaul Example

shows multiple Radio Units that are connected to one Baseband Unit by means of a Layer 2 switched network. The Baseband Unit is the central processing unit that handles baseband information. A Baseband Unit is often placed rather centrally, while the Radio Units need to be distributed to be co-located with or near the antennas. Traffic between Radio Units and Baseband Units is both IP-based and Layer-2-based and may pass a hierarchy of L2 switches. In order to properly address the Radio Unit, the Baseband Unit needs to associate the Radio Unit's MAC address to the L2 switch and respective port where the Radio Unit is connected. To realize this device configuration in the Switched Fronthaul network, DHCPv6 can be used to discover the network Topology. With the L2 switched network between the clients and the server, one of the clients is responsible for the configuration of the other clients based on their topology. Updating of the software on the clients is not possible often not possible and clients may be IPv4-only.

Acknowledgments The authors would also like to acknowledge interesting discussions in this problem space with Sarah Gannon, Ines Ramadza and Siddharth Sharma.