RFC 8987

DHCPv6 Prefix Delegating Relay Requirements

📅 published February 2021, from ietf

RFC 8987: DHCPv6 Prefix Delegating Relay Requirements 🎀

Okay bestie, let's do the vibe check first 💅 This is RFC 8987, DHCPv6 Prefix Delegating Relay Requirements, and it's an Internet Standards Track document, so it's an actual official Internet standard 👑 The IETF published it in February 2021 with ISSN 2070-1721. The authors are I. Farrer from Deutsche Telekom AG, N. Kottapalli from Benu Networks, M. Hunek from the Technical University of Liberec, and R. Patterson from Sky UK Ltd. ✨ The header doesn't list anything it obsoletes or updates, so it stands on its own 🌸

Abstract ✨

So this doc is all about the operational problems that show up when you use DHCPv6 relays with prefix delegation 🧚 Those problems can stop delegation from working and cause routing failures, which is so not it 💀 To fix that, the doc lays out the functional requirements you need for running DHCPv6 relays with prefix delegation 🔥

It is recommended that any network operator using DHCPv6 prefix delegation with relays ensure that these requirements are followed on their networks 🫶

Status of This Memo 📜

This one is an Internet Standards Track document, so it's the real deal 👑

It's a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community, it got public review, and the Internet Engineering Steering Group (IESG) approved it for publication 🎀 If you want more on Internet Standards, check Section 2 of RFC 7841.

For the current status, any errata, and how to send feedback, head to https://www.rfc-editor.org/info/rfc8987 🦋

Copyright Notice ⚖️

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) as they stand on the publication date. Please read these carefully, because they spell out your rights and restrictions for this document 📖 Code Components pulled out of this document must include the Simplified BSD License text described in Section 4.e of the Trust Legal Provisions, and they come without warranty, as the Simplified BSD License describes 💜

Table of Contents 🗂️

   1.  Introduction
   2.  Terminology
     2.1.  General
     2.2.  Topology
     2.3.  Requirements Language
   3.  Problems Observed with Existing Delegating Relay
           Implementations
     3.1.  DHCP Messages Not Being Forwarded by the Delegating Relay
     3.2.  Delegating Relay Loss of State on Reboot
     3.3.  Multiple Delegated Prefixes for a Single Client
     3.4.  Dropping Messages from Devices with Duplicate MAC Addresses
           and DUIDs
     3.5.  Forwarding Loops between Client and Relay
   4.  Requirements for Delegating Relays
     4.1.  General Requirements
     4.2.  Routing Requirements
     4.3.  Service Continuity Requirements
     4.4.  Operational Requirements
   5.  IANA Considerations
   6.  Security Considerations
   7.  References
     7.1.  Normative References
     7.2.  Informative References
   Acknowledgements
   Authors' Addresses

1. Introduction 🌈

Okay, so for ISPs offering native IPv6 access with prefix delegation to their customers, the usual setup is to put a DHCPv6 relay agent function in the ISP's Layer 3 customer edge device, and then run a separate, centralized DHCPv6 server infrastructure 🏠 [RFC8415] describes what a DHCPv6 relay does, and Section 19.1.3 of [RFC8415] mentions this exact deployment scenario. It just doesn't go into detail on all the functional requirements a relay needs in order to behave predictably in this setup 🤔

A DHCPv6 relay agent for prefix delegation is a function that routers commonly implement, but the implementations vary in what they can do and how they work with clients and servers 🫠 That can cause operational problems, like messages not getting forwarded by the relay, or delegated prefixes becoming unreachable 💔 So this document gives a set of requirements for devices that implement a relay function for prefix delegation 🎀

Not in scope: the mechanisms a relay uses to inject routes (including aggregated ones) on its network-facing interface, based on prefixes learned from a server via DHCP prefix delegation (DHCP-PD) 🙅‍♀️

Multi-hop DHCPv6 relaying is not affected 🦋 The requirements here apply only to the DHCP relay agent co-located with the first-hop router that the DHCPv6 client requesting the prefix is connected to. That means no changes are needed for any subsequent relays along the path 🧚

2. Terminology 📚

2.1. General

This document uses the terminology defined in [RFC8415]. However, when it defines the functional elements for prefix delegation, [RFC8415], Section 4.2 defines the term "delegating router" like this:

The router that acts as a DHCP server and responds to requests for delegated prefixes.

This document is about deployments where the DHCPv6 relay and DHCPv6 server functions are separated, so the term "delegating router" doesn't get used here 🙃 Instead, the doc introduces a new term for the relaying function:

Delegating relay: A delegating relay acts as an intermediate device, forwarding DHCPv6 messages containing IA_PD and IAPREFIX options between the client and server. It does not implement a DHCPv6 server function. It is also responsible for routing traffic for the delegated prefixes 💅

When the term "relay" shows up alone in this document, it should be understood to mean a delegating relay, unless it's specifically stated otherwise ✨

In CableLabs DOCSIS environments, the Cable Modem Termination System (CMTS) would be considered a delegating relay with respect to Customer Premises Devices (CPEs) ([DOCSIS_3.1], Section 5.2.7.2). A Broadband Network Gateway (BNG) in a DSL-based access network may be a delegating relay if it does not implement a local DHCPv6 server function ([TR-092], Section 4.10) 📡

[RFC8415] defines the "DHCP server" (or "server") as:

A node that responds to requests from clients. It may or may not be on the same link as the client(s). Depending on its capabilities, if it supports prefix delegation it may also feature the functionality of a delegating router.

This document covers the cases where a DHCPv6 server is not on the same link as the client, which is exactly why the delegating relay is needed 🔗 The server supports prefix delegation and can lease prefixes to clients, but it isn't responsible for the other jobs a delegating router handles, like managing routes for the delegated prefixes 🛣️

The term "requesting router" has been used before to describe the DHCP client that requests prefixes for use. This document adopts the terminology of [RFC8415] and uses "DHCP client" or "client" interchangeably for this element 🫶

2.2. Topology

Here's the deployment topology this document cares about 🗺️

    +
    |             ------- uplink ------>
    |                                       _    ,--,_
    |   +--------+       +------------+   _(  `'      )_    +--------+
    +---+   PD   |-------| Delegating |--(   Operator   )---| DHCPv6 |
    |   | Client |       |    relay   |   `(_ Network_)'    | server |
    |   +--------+       +----------- +      `--'`---'      +--------+
    |
    |             <----- downlink ------
    +                 (client facing)
    Client
    Network

                        Figure 1: Topology Overview

The client requests prefixes through the downlink interface of the delegating relay 📥 The resulting prefixes are used to address the client network 🏡 The delegating relay is responsible for forwarding DHCP messages, including prefix delegation requests and responses, between the client and the server. Messages go from the delegating relay to the server using multicast or unicast over the operator uplink interface 📤

The delegating relay is the operator's Layer 3 edge facing the client. It's also responsible for routing traffic to and from clients on the client network, using addresses from the delegated prefixes 🦋

2.3. Requirements Language

Here's the big one. 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 🎀 Anything in lowercase is just regular English, so no hidden requirements 🙈

3. Problems Observed with Existing Delegating Relay Implementations 🙀

The sections below describe problems that have been seen with delegating relay implementations in commercially available devices 🛠️

3.1. DHCP Messages Not Being Forwarded by the Delegating Relay

Some delegating relay implementations have been observed not forwarding messages between the client and the server 😭 This usually happens when a client sends a message the delegating relay doesn't expect. For example, the delegating relay already has an active PD lease entry for an existing client on a port. A new client then connects to that port and sends a Solicit message. The delegating relay drops the Solicit messages until it either receives a DHCP Release message from the original client or the existing lease times out. That's a real problem when a client device needs replacing because it failed 🥲

Sometimes, on top of dropping messages, the delegating relay also generates error messages and sends them to the client. One example is NoBinding messages, sent when the delegating relay doesn't have an active delegated prefix lease 🚫

3.2. Delegating Relay Loss of State on Reboot

For client traffic to route properly, the delegating relay needs a matching routing table entry for each active prefix delegated to a connected client 🗺️ If a delegating relay doesn't store this state persistently across reboots, it won't be able to forward traffic to the client's delegated leases until the state is rebuilt through new DHCP messages 🔄

3.3. Multiple Delegated Prefixes for a Single Client

DHCPv6 [RFC8415] lets a client include more than one instance of OPTION_IA_PD in its messages, so it can ask the server for multiple prefix delegations 🎁 If the server is set up for this, it supplies one (or more) OPTION_IAPREFIX for each received OPTION_IA_PD, and each one carries information about a different delegated prefix ✨

Some delegating relay implementations only support a single delegated prefix per DHCP Unique Identifier (DUID) 😬 In those cases, only one IPv6 route for one of the delegated prefixes gets installed. That means the other prefixes delegated to the client are unreachable 💀

3.4. Dropping Messages from Devices with Duplicate MAC Addresses and DUIDs

okay bestie, real talk 😭 client devices with duplicate MAC addresses and/or DUIDs are a whole thing, and they've already been deployed out there 🫠. In some networks, tracking down and swapping out those devices costs way too much, so they just stay put 💀.

Delegating relays have been spotted restricting forwarding of client messages from one client DUID to a single interface. So if that same client DUID shows up from a second client on another interface while there's already an active lease, the messages from that second client get dropped 🙅‍♀️. The result is that the second client can't get a prefix delegation at all 💔.

heads up 👀 in some access networks, the MAC address and/or DUID are used as part of device identification and authentication. In those networks, enforcing uniqueness of the MAC address and/or DUID is a necessary function and is not considered a problem 🔐.

3.5. Forwarding Loops between Client and Relay

the loop situation is giving chaos 🔁 If the client loses info about an active prefix lease it was delegated while the lease entry and associated route are still active in the delegating relay, the relay keeps forwarding traffic to the client. The client sends that traffic right back to the relay, which is the client's default gateway (learned via a Router Advertisement (RA)).

The loop keeps going until either the client gets reprovisioned via DHCP or the lease ages out in the relay. Not me going in circles 😵‍💫.

4. Requirements for Delegating Relays ✨

okay so this is the part where we get into the actual rules 🎀 To fix the problems from Section 3 and to head off other undesirable behavior, this section lays out a set of functional requirements for the delegating relay.

Also, relay implementers, a friendly reminder 💜: [RFC8415] makes it clear that relays MUST forward packets that either contain message codes it may not understand (Section 19 of [RFC8415]) or options that it does not understand (Section 16 of [RFC8415]). Forward the unknown stuff, no cap 🫶.

4.1. General Requirements

G-1: The delegating relay MUST forward messages bidirectionally between the client and server without changing the contents of the message. Main character energy: untouched messages only 💅.

G-2: The relay MUST allow for multiple prefixes to be delegated for the same client IA_PD. These delegations may have different lifetimes ⏳.

G-3: The relay MUST allow for multiple prefixes (with or without separate IA_PDs) to be delegated to a single client connected to a single interface, identified by its DHCPv6 Client Identifier (DUID) 🧚.

G-4: A delegating relay may have one or more interfaces on which it acts as a relay, as well as one or more interfaces on which it does not (for example, in an ISP, it might act as a relay on all southbound interfaces but not on the northbound interfaces). The relay SHOULD allow the same client identifier (DUID) to have active delegated prefix leases on more than one interface simultaneously unless client DUID uniqueness is necessary for the functioning or security of the network. This is to allow client devices with duplicate DUIDs to function on separate broadcast domains 🌈.

G-5: The maximum number of simultaneous prefixes delegated to a single client MUST be configurable 🎛️.

G-6: The relay MUST implement a mechanism to limit the maximum number of active prefix delegations on a single port for all client identifiers and IA_PDs. This value MUST be configurable 🚧.

G-7: It is RECOMMENDED that delegating relays support at least 8 active delegated leases per client device and use this as the default limit 🍓.

G-8: The delegating relay MUST update the lease lifetimes based on the client's reply messages it forwards to the client and only expire the delegated prefixes when the valid lifetime has elapsed ⏰.

G-9: On receipt of a Release message from the client, the delegating relay MUST expire the active leases for each of the IA_PDs in the message 👋.

4.2. Routing Requirements

R-1: The relay MUST maintain a local routing table that is dynamically updated with leases and the associated next hops as they are delegated to clients. When a delegated prefix is released or expires, the associated route MUST be removed from the relay's routing table 🗺️.

R-2: The delegating relay's routing entry MUST use the same prefix length for the delegated prefix as given in the IA_PD 📏.

R-3: The relay MUST provide a mechanism to dynamically update ingress filters permitting ingress traffic sourced from client delegated leases and blocking packets from invalid source prefixes. This is to implement anti-spoofing as described in [BCP38]. The delegating relay's ingress filter entry MUST use the same prefix length for the delegated prefix as given in the IA_PD 🛡️.

R-4: The relay MAY provide a mechanism to dynamically advertise delegated leases into a routing protocol as they are learned. If such a mechanism is implemented, when a delegated lease is released or expires, the delegated route MUST be withdrawn from the routing protocol. The mechanism by which the routes are inserted and deleted is out of the scope of this document 📣.

R-5: To prevent routing loops, the relay SHOULD implement a configurable policy to drop potential looping packets received on any DHCP-PD client-facing interfaces.

The policy SHOULD be configurable on a per-client or per-destination basis.

Looping packets are those with a destination address in a prefix delegated to a client connected to that interface, as follows:

For point-to-point links, when the packet's ingress and egress interfaces match.
For multi-access links, when the packet's ingress and egress interface match, and the source link-layer and next-hop link-layer addresses match.

An ICMPv6 Type 1, Code 6 (Destination Unreachable, reject route to destination) error message MAY be sent as per [RFC4443], Section 3.1. The ICMP policy SHOULD be configurable 🔄.

4.3. Service Continuity Requirements

S-1: To preserve active client prefix delegations across relay restarts, the relay SHOULD implement at least one of the following:

Implement DHCPv6 Bulk Leasequery as defined in [RFC5460].
Store active prefix delegations in persistent storage so they can be reread after the reboot.

S-2: If a client's next-hop link-local address becomes unreachable (e.g., due to a link-down event on the relevant physical interface), routes for the client's delegated prefixes MUST be retained by the delegating relay unless they are released or removed due to expiring DHCP timers. This is to reestablish routing for the delegated prefix if the client next hop becomes reachable without the delegated prefixes needing to be relearned 🔌.

S-3: The relay SHOULD implement DHCPv6 Active Leasequery as defined in [RFC7653] to keep the local lease database in sync with the DHCPv6 server 🔁✨.

4.4. Operational Requirements

O-1: The relay SHOULD implement an interface allowing the operator to view the active delegated prefixes. This SHOULD provide information about the delegated lease and client details such as the client identifier, next-hop address, connected interface, and remaining lifetimes 👀.

O-2: The relay SHOULD provide a method for the operator to clear active bindings for an individual lease, client, or all bindings on a port 🧹.

O-3: To facilitate troubleshooting of operational problems between the delegating relay and other elements, it is RECOMMENDED that a time synchronization protocol be used by the delegating relays and DHCP servers ⏱️.

5. IANA Considerations 🌸

This document has no IANA actions. Nothing to register, nothing to fight over 💅.

6. Security Considerations 🚨

okay this section is SERIOUS, so pay attention bestie 😤

This document does not add any new security considerations beyond those mentioned in Section 4 of [RFC8213] and Section 22 of [RFC8415].

If the delegating relay implements [BCP38] filtering, then the filtering rules will need to be dynamically updated as delegated prefixes are leased.

[RFC8213] describes a method for securing traffic between the relay agent and server by sending DHCP messages over an IPsec tunnel. It is RECOMMENDED that this be implemented by the delegating relay 🔐.

Failure to implement requirement G-6 may have specific security implications, such as a resource depletion attack on the relay. Yikes, that's giving villain arc 💀.

The operational requirements in Section 4.4 may introduce additional security considerations. It is RECOMMENDED that the operational security practices described in [RFC4778] be implemented 🛡️✨.

7. References 📚

7.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>.

[RFC4443] Conta, A., Deering, S., and M. Gupta, Ed., "Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification", STD 89, RFC 4443, DOI 10.17487/RFC4443, March 2006, <https://www.rfc-editor.org/info/rfc4443>.

[RFC4778] Kaeo, M., "Operational Security Current Practices in Internet Service Provider Environments", RFC 4778, DOI 10.17487/RFC4778, January 2007, <https://www.rfc-editor.org/info/rfc4778>.

[RFC5460] Stapp, M., "DHCPv6 Bulk Leasequery", RFC 5460, DOI 10.17487/RFC5460, February 2009, <https://www.rfc-editor.org/info/rfc5460>.

[RFC7653] Raghuvanshi, D., Kinnear, K., and D. Kukrety, "DHCPv6 Active Leasequery", RFC 7653, DOI 10.17487/RFC7653, October 2015, <https://www.rfc-editor.org/info/rfc7653>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC8213] Volz, B. and Y. Pal, "Security of Messages Exchanged between Servers and Relay Agents", RFC 8213, DOI 10.17487/RFC8213, August 2017, <https://www.rfc-editor.org/info/rfc8213>.

[RFC8415] Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A., Richardson, M., Jiang, S., Lemon, T., and T. Winters, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 8415, DOI 10.17487/RFC8415, November 2018, <https://www.rfc-editor.org/info/rfc8415>.

7.2. Informative References

[BCP38] Ferguson, P. and D. Senie, "Network Ingress Filtering: Defeating Denial of Service Attacks which employ IP Source Address Spoofing", BCP 38, RFC 2827, May 2000.

<https://www.rfc-editor.org/info/bcp38>

[DOCSIS_3.1] CableLabs, "MAC and Upper Layer Protocols Interface Specification", Version 10, DOCSIS 3.1, January 2017, <https://www.cablelabs.com/specification/CM-SP-MULPIv3.1>.

[TR-092] Broadband Forum, "Broadband Remote Access Server (BRAS) Requirements Document", Technical Report TR-092, August 2004, <https://www.broadband-forum.org/download/TR-092.pdf>.

Acknowledgements ✨

okay bestie, the authors wanna give a shoutout to Bernie Volz, Ted Lemon, and Michael Richardson 🫶 they dropped some really valuable comments and the doc is better because of them 💖

Authors' Addresses 💌

Ian Farrer 🌸

Ian Farrer
Deutsche Telekom AG
Landgrabenweg 151
53227 Bonn
Germany

Email: ian.farrer@telekom.de

Naveen Kottapalli 🦋

Naveen Kottapalli
Benu Networks
WeWork Galaxy, 43 Residency Road
Bangalore 560025
Karnataka
India

Email: nkottapalli@benunets.com

Martin Hunek 🧚

Martin Hunek
Technical University of Liberec
Studentska 1402/2
46017 Liberec
Czech Republic

Email: martin.hunek@tul.cz

Richard Patterson 💅

Richard Patterson
Sky UK Ltd.
1 Brick Lane
London
E1 6PU
United Kingdom

Email: richard.patterson@sky.uk

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