| Internet-Draft | DCHP 4o6 Relay Agent | September 2024 |
| Porfiri, et al. | Expires 16 March 2025 | [Page] |
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.¶
This note is to be removed before publishing as an RFC.¶
Status information for this document may be found at https://datatracker.ietf.org/doc/draft-porfiri-dhc-dhcpv4-over-dhcpv6-ra/.¶
Source for this draft and an issue tracker can be found at https://github.com/mirjak/dhc-dhcpv4-over-dhcpv6-ra.¶
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[RFC7341] 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 [RFC7341] 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 [RFC6221] or L3RA [RFC8415] do not foresee the possibility to handle legacy DHCP, other than implementing 4o6 in client.¶
This document specifies an [RFC7341]-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 [RFC7341] that allows a Relay Agent to perform the 4o6 DHCP en- and decapsultion instead of the client.¶
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 [RFC7341].¶
DHCP Relay Agent (or RA): This is a concept in all of the protocols, BOOTP [RFC0951] [RFC1542], DHCPv4 [RFC2131] [RFC2132], and DHCPv6 [RFC8415], 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 [RFC6221].¶
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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
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 Figure 1.¶
This document specifies the encapsulation and decapsulation described in [RFC7341] 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 Section 5 of [RFC7341] apply shall be applied to the 4o6RA instead.¶
To maintain interoperability with existing DHCP relays and servers, the message format is unchanged from [RFC8415]. The 4o6RA implements the same message types as a normal DHCPv6 Relay Agent Section 6 of [RFC7341].¶
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 Section 6 of [RFC6221].¶
The DHCPv6 server must be compliant with 4o6 according to [RFC7341]. No additional requirements on DHCPv6 server are set by this specification.¶
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 [RFC2131] and DHCPv6 [RFC3315] 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 [RFC6221] 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 [RFC7969], 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 [RFC7341] 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.¶
As shown in Figure 2, 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 [RFC6221] 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 Section 5.3.2 of [RFC6221]. 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.¶
The assumed architecture is shown in Figure 3 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 Figure 4.¶
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.¶
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 [RFC7341] still apply.¶
The mechanism described differs from [RFC7341] as the DHCP client actually sends and receveis DHCP messages, whereas in [RFC7341] 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.¶
This document has no IANA actions.¶
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.¶
Figure 5 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.¶
The authors would also like to acknowledge interesting discussions in this problem space with Sarah Gannon, Ines Ramadza and Siddharth Sharma.¶