



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
   Provisions Relating to IETF Documents
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   publication of this document.  Please review these documents
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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: ian.farrer@telekom.de


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

   Email: nkottapalli@benunets.com


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

   Email: martin.hunek@tul.cz


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

   Email: richard.patterson@sky.uk
