Internet Engineering Task Force (IETF) I. Farrer

Internet Engineering Task Force (IETF) I. Farrer
Request for Comments: 8987 Deutsche Telekom AG
Category: Standards Track N. Kottapalli
ISSN: 2070-1721 Benu Networks
M. Hunek
Technical University of Liberec
R. Patterson
Sky UK Ltd.
February 2021

DHCPv6 Prefix Delegating Relay Requirements

Abstract

This document describes operational problems that are known to occur
when using DHCPv6 relays with prefix delegation. These problems can
prevent successful delegation and result in routing failures. To
address these problems, this document provides necessary functional
requirements for operating 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 is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8987.

Copyright Notice

Copyright (c) 2021 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
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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

For Internet service providers that offer native IPv6 access with
prefix delegation to their customers, a common deployment
architecture is to have a DHCPv6 relay agent function located in the
ISP's Layer 3 customer edge device and a separate, centralized DHCPv6
server infrastructure. [RFC8415] describes the functionality of a
DHCPv6 relay, and Section 19.1.3 of [RFC8415] mentions this
deployment scenario, but it does not provide details for all of the
functional requirements that the relay needs to fulfill to operate
deterministically in this deployment scenario.

A DHCPv6 relay agent for prefix delegation is a function commonly
implemented in routing devices, but implementations vary in their
functionality and client/server interworking. This can result in
operational problems such as messages not being forwarded by the
relay or unreachability of the delegated prefixes. This document
provides a set of requirements for devices implementing a relay
function for use with prefix delegation.

The mechanisms for a relay to inject routes (including aggregated
ones) on its network-facing interface based on prefixes learned from
a server via DHCP prefix delegation (DHCP-PD) are out of scope of the
document.

Multi-hop DHCPv6 relaying is not affected. The requirements in this
document are solely applicable to the DHCP relay agent co-located
with the first-hop router to which the DHCPv6 client requesting the
prefix is connected, so no changes to any subsequent relays in the
path are needed.

2. Terminology

2.1. General

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

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

This document is concerned with deployment scenarios in which the
DHCPv6 relay and DHCPv6 server functions are separated, so the term
"delegating router" is not used. Instead, a new term is introduced
to describe 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. The delegating relay does not implement a
DHCPv6 server function. The delegating relay is also responsible
for routing traffic for the delegated prefixes.

Where the term "relay" is used on its own within this document, it
should be understood to be a delegating relay unless 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 serves the deployment cases where a DHCPv6 server is
not located on the same link as the client (necessitating the
delegating relay). The server supports prefix delegation and is
capable of leasing prefixes to clients, but it is not responsible for
other functions required of a delegating router, such as managing
routes for the delegated prefixes.

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

2.2. Topology

The following diagram shows the deployment topology relevant to this
document.

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

Figure 1: Topology Overview

The client requests prefixes via the downlink interface of the
delegating relay. The resulting prefixes will be used for addressing
the client network. The delegating relay is responsible for
forwarding DHCP messages, including prefix delegation requests and
responses between the client and server. Messages are forwarded from
the delegating relay to the server using multicast or unicast via the
operator uplink interface.

The delegating relay provides the operator's Layer 3 edge towards the
client and is responsible for routing traffic to and from clients
connected to the client network using addresses from the delegated
prefixes.

2.3. Requirements Language

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.

3. Problems Observed with Existing Delegating Relay Implementations

The following sections of the document describe problems that have
been observed with delegating relay implementations in commercially
available devices.

3.1. DHCP Messages Not Being Forwarded by the Delegating Relay

Delegating relay implementations have been observed not to forward
messages between the client and server. This generally occurs if a
client sends a message that is unexpected by the delegating relay.
For example, the delegating relay already has an active PD lease
entry for an existing client on a port. A new client is connected to
this port and sends a Solicit message. The delegating relay then
drops the Solicit messages until either it receives a DHCP Release
message from the original client or the existing lease times out.
This causes a particular problem when a client device needs to be
replaced due to a failure.

In addition to dropping messages, in some cases, the delegating relay
will generate error messages and send them to the client, e.g.,
"NoBinding" messages being sent in the event that the delegating
relay does not have an active delegated prefix lease.

3.2. Delegating Relay Loss of State on Reboot

For proper routing of client traffic, the delegating relay requires a
corresponding routing table entry for each active prefix delegated to
a connected client. A delegating relay that does not store this
state persistently across reboots will not be able to forward traffic
to the client's delegated leases until the state is reestablished
through new DHCP messages.

3.3. Multiple Delegated Prefixes for a Single Client

DHCPv6 [RFC8415] allows a client to include more than one instance of
OPTION_IA_PD in messages in order to request multiple prefix
delegations by the server. If configured for this, the server
supplies one (or more) instance of OPTION_IAPREFIX for each received
instance of OPTION_IA_PD, each containing information for a different
delegated prefix.

In some delegating relay implementations, only a single delegated
prefix per DHCP Unique Identifier (DUID) is supported. In those
cases, only one IPv6 route for one of the delegated prefixes is
installed, meaning that other prefixes delegated to a client are
unreachable.

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

It is an operational reality that client devices with duplicate Media
Access Control (MAC) addresses and/or DUIDs exist and have been
deployed. In some networks, the operational costs of locating and
swapping out such devices are prohibitive.

Delegating relays have been observed to restrict forwarding client
messages originating from one client DUID to a single interface. In
this case, if the same client DUID appears from a second client on
another interface while there is already an active lease, messages
originating from the second client are dropped, causing the second
client to be unable to obtain a prefix delegation.

It should be noted that in some access networks, the MAC address and/
or DUID are used as part of device identification and authentication.
In such 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

If the client loses information about an active prefix lease it has
been delegated while the lease entry and associated route are still
active in the delegating relay, then the relay will forward traffic
to the client. The client will return this traffic to the relay,
which is the client's default gateway (learned via a Router
Advertisement (RA)). The loop will continue until either the client
is successfully reprovisioned via DHCP or the lease ages out in the
relay.

4. Requirements for Delegating Relays

To resolve the problems described in Section 3 and to preempt other
undesirable behavior, the following section of the document describes
a set of functional requirements for the delegating relay.

In addition, relay implementers are reminded that [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]).

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.

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.

6. Security Considerations

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.

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

The authors of this document would like to thank Bernie Volz, Ted
Lemon, and Michael Richardson for their valuable comments.

Authors' Addresses

Ian Farrer
Deutsche Telekom AG
Landgrabenweg 151
53227 Bonn
Germany

Email: [email protected]

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

Email: [email protected]

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

Email: [email protected]

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

Email: [email protected]