DHC Work Group                                                 I. Farrer
Internet-Draft                                       Deutsche Telekom AG
Intended status: Standards Track                      Naveen. Kottapalli
Expires: June 3, 2021                                      Benu Networks
                                                                M. Hunek
                                         Technical University of Liberec
                                                            R. Patterson
                                                              Sky UK Ltd
                                                       November 30, 2020

              DHCPv6 Prefix Delegating Relay Requirements


   This memo 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 memo provides necessary functional
   requirements for operating DHCPv6 relays with Prefix Delegation.

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

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on June 3, 2021.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  General . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.2.  Topology  . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.3.  Requirements Language . . . . . . . . . . . . . . . . . .   5
   3.  Problems Observed with Existing Delegating Relay
       Implementations . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  DHCP Messages not being Forwarded by the Delegating
           Relay . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Delegating Relay Loss of State on Reboot  . . . . . . . .   5
     3.3.  Multiple Delegated Prefixes for a Single Client . . . . .   6
     3.4.  Dropping Messages from Devices with Duplicate MAC
           addresses and DUIDs . . . . . . . . . . . . . . . . . . .   6
     3.5.  Forwarding Loops between Client and Relay . . . . . . . .   6
   4.  Requirements for Delegating Relays  . . . . . . . . . . . . .   7
     4.1.  General Requirements  . . . . . . . . . . . . . . . . . .   7
     4.2.  Routing Requirements  . . . . . . . . . . . . . . . . . .   8
     4.3.  Service Continuity Requirements . . . . . . . . . . . . .   9
     4.4.  Operational Requirements  . . . . . . . . . . . . . . . .   9
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

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 separate, centralized DHCPv6
   server infrastructure.  [RFC8415] describes the functionality of a
   DHCPv6 relay and Section 19.1.3 mentions this deployment scenario,
   but does not provide detail for all of the functional requirements

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   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 inter-working.  This can result in
   operational problems such as messages not being forwarded by the
   relay or un-reachability 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-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 that the DHCPv6 client requesting the
   prefix is connected to, 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/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.

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   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  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 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
   [RFC8415] terminology and uses 'DHCP client' or 'client'
   interchangeably for this element.

2.2.  Topology

   The following diagram shows the deployment topology relevant to this

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

                        Figure 1: Topology overview

   The client requests prefixes via the downlink interface of the
   delegating relay.  The resulting prefixes will be used for addressing

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   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

2.3.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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 which 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 it receives either 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 which does not store this

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   state persistently across reboots will not be able to forward traffic
   to client's delegated leases until the state is re-established
   through new DHCP messages.

3.3.  Multiple Delegated Prefixes for a Single Client

   [RFC8415] allows for 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-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

   It is an operational reality that client devices with duplicate MAC
   addresses and/or DUIDs exist and have been deployed.  In this
   situation, 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 MAC address/DUID uniqueness is a
   necessary function and not considered a problem.

3.5.  Forwarding Loops between Client and Relay

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

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4.  Requirements for Delegating Relays

   To resolve the problems described in Section 3 and pre-empt 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 (Section 19 of [RFC8415]) it may not understand, or contain
   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

   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.

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   G-8:    The delegating relay MUST update the lease lifetimes based on
           the Client Reply messages it forwards to the client and only
           expire the delegated prefixes when the valid lifetime has

   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.
           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

           *  For point-to-point links, when the packet's ingress and
              egress interfaces match.

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           *  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

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

           *  Implement DHCPv6 bulk lease query as defined in [RFC5460].

           *  Store active prefix delegations in persistent storage so
              they can be re-read 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 re-
           establish routing for the delegated prefix if the client
           next-hop becomes reachable without the delegated prefixes
           needing to be re-learned.

   S-3:    The relay SHOULD implement DHCPv6 active lease query 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 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

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           that a time synchronization protocol is used by the
           delegating relays and DHCP servers.

5.  Acknowledgements

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

6.  IANA Considerations

   This memo includes no request to IANA.

7.  Security Considerations

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

   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.  In
   this case the IPsec tunnel is functionally the server-facing
   interface and DHCPv6 message snooping can be carried out as
   described.  It is RECOMMENDED that this is implemented by the
   delegating relay.

8.  References

8.1.  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,

   [RFC5460]  Stapp, M., "DHCPv6 Bulk Leasequery", RFC 5460,
              DOI 10.17487/RFC5460, February 2009,

   [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>.

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   [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,

8.2.  Informative References

   [BCP38]    IETF, "Network Ingress Filtering: Defeating Denial of
              Service Attacks which employ IP Source Address Spoofing
              https://tools.ietf.org/html/bcp38", RFC 2827, BCP 38.

              CableLabs, "MAC and Upper Layer Protocols Interface
              Specification", DOCSIS 3.1, January, 2017",

   [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,

   [RFC8213]  Volz, B. and Y. Pal, "Security of Messages Exchanged
              between Servers and Relay Agents", RFC 8213,
              DOI 10.17487/RFC8213, August 2017,

   [TR-092]   Broadband Forum, "Broadband Remote Access Server (BRAS)
              Requirements Document, August, 2004",

Authors' Addresses

   Ian Farrer
   Deutsche Telekom AG
   Landgrabenweg 151
   Bonn, NRW  53227

   Email: ian.farrer@telekom.de

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   Naveen Kottapalli
   Benu Networks
   300 Concord Road
   Billerica, MA  01821

   Email: naveen.sarma@gmail.com

   Martin Hunek
   Technical University of Liberec
   Studentska 1402/2
   Liberec, L  46017

   Email: martin.hunek@tul.cz

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

   Email: richard.patterson@sky.uk

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