Network Working Group                                        W. Dec, Ed.
Internet-Draft                                             Cisco Systems
Intended status: Standards Track                            T. Mrugalski
Expires: March 13, 2012                                              ISC
                                                                  T. Sun
                                                            China Mobile
                                                             B. Sarikaya
                                                              Huawei USA
                                                      September 10, 2011

                          DHCPv6 Route Options


   This document describes DHCPv6 Route Options for provisioning IPv6
   routes on DHCPv6 client nodes.  This is expected to improve the
   ability of an operator to configure and influence a nodes' ability to
   pick an appropriate route to a destination when this node is multi-
   homed and where other means of route configuration may be

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on March 13, 2012.

Copyright Notice

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   Copyright (c) 2011 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
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   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 . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Problem overview . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  DHCPv6 Based Solution  . . . . . . . . . . . . . . . . . . . .  4
     3.1.  Default route configuration  . . . . . . . . . . . . . . .  4
     3.2.  Configuring on-link routes . . . . . . . . . . . . . . . .  5
     3.3.  Deleting obsolete route  . . . . . . . . . . . . . . . . .  5
     3.4.  Applicability to routers . . . . . . . . . . . . . . . . .  5
     3.5.  Updating Routing Information . . . . . . . . . . . . . . .  5
     3.6.  Limitations  . . . . . . . . . . . . . . . . . . . . . . .  6
   4.  DHCPv6 Route Options . . . . . . . . . . . . . . . . . . . . .  7
     4.1.  Next Hop Option Format . . . . . . . . . . . . . . . . . .  7
     4.2.  Route Prefix Option Format . . . . . . . . . . . . . . . .  8
   5.  DHCPv6 Server Behavior . . . . . . . . . . . . . . . . . . . . 10
   6.  DHCPv6 Client Behavior . . . . . . . . . . . . . . . . . . . . 10
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   9.  Contributors and Acknowledgements  . . . . . . . . . . . . . . 12
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 12
     10.2. Informative References . . . . . . . . . . . . . . . . . . 12
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13

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1.  Introduction

   The Neighbor Discovery (ND) protocol [RFC4861] provides a mechanism
   for hosts to discover one or more default routers on a directly
   connected network segment.  Extensions to the Router Advertisement
   (RA) protocol defined in [RFC4191] allow hosts to discover the
   preferences for multiple default routers on a given link, as well as
   any specific routes advertised by these routers.  This allows network
   administrators to better handle multi-homed host topologies and
   influence the route selection by the host.  This ND based mechanism
   however is sub optimal or impractical in some multi-homing scenarios,
   where DHCPv6 [RFC3315] is seen to be more viable.

   This draft defines the DHCPv6 Route Options for provisioning IPv6
   routes on DHCPv6 clients.  The proposed option is primarily envisaged
   for use by DHCPv6 client nodes that are capable of making basic IP
   routing decisions and maintaining an IPv6 routing table, broadly in
   line with the capabilities of a generic host as described in

   Throughout the document the words node and client are used as a
   reference to the device with such routing capabilities, hosting the
   DHCPv6 client software.  The route information is taken to be
   equivalent to static routing, and limited in the number of required
   routes to a handful.

2.  Problem overview

   The solution described in this document applies to multi-homed
   scenarios including ones where the client is simultaneously connected
   to multiple access network (e.g.  WiFi and 3G).  The following
   scenario is used to illustrate the problem as found in typical multi-
   homed residential access networks.  It is duly noted that the problem
   is not specific to IPv6, occurring also with IPv4, where it is today
   solved by means of DHCPv4 classless route information option
   [RFC3442], or alternative configuration mechanisms.

   In multi-homed networks, a given user's node may be connected to more
   than one gateway.  Such connectivity may be realized by means of
   dedicated physical or logical links that may also be shared with
   other users nodes.  In such multi-homed networks it is quite common
   for the network operator to offer the delivery of a particular type
   of IP service via a particular gateway, where the service can be
   characterised by means of specific destination IP network prefixes.
   Thus, from an IP routing perspective in order for the user node to
   select the appropriate gateway for a given destination IP prefix,
   recourse needs to be made to classic longest destination match IP

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   routing, with the node acquiring such prefixes into its routing
   table.  This is typically the remit of dynamic Internal Gateway
   Protocols (IGPs), which however are rarely used by operators in
   residential access networks.  This is primarily due to operational
   costs and a desire to contain the complexity of user nodes and IP
   Edge devices to a minimum.  While, IP Route configuration may be
   achieved using the ICMPv6 extensions defined in [RFC4191], this
   mechanism does not lend itself to other operational constraints such
   as the desire to control the route information on a per node basis,
   the ability to determine whether a given node is actually capable of
   receiveing/processing such route information.  A preferred mechanism,
   and one that additionally also lends itself to centralized management
   independent of the management of the gateways, is that of using the
   DHCP protocol for conveying route information to the nodes.

3.  DHCPv6 Based Solution

   A DHCPv6 based solution allows an operator an on demand and node
   specific means of configuring static routing information.  Such a
   solution also fits into network environments where the operator
   prefers to manage Residential Gateway (RG) configuration information
   from a centralized DHCP server.
   [I-D.ietf-v6ops-ipv6-multihoming-without-ipv6nat] provides additional
   background to the need for a DHCPv6 solution to the problem.

   In terms of the high level operation of the solution defined in this
   draft, a DHCPv6 client interested in obtaining routing information
   request the route options using the DHCPv6 Option Request Option
   (ORO) sent to a server.  A Server, when configured to do so, provides
   the requested route information as part of a nested options structure
   covering; the next-hop address; the destination prefix; the route
   metric; any additional options applicable to the destination or next-

3.1.  Default route configuration

   Defined mechanism may be used to configure default route.  Default
   route may be specified in two ways.

   In bandwidth constrained networks, server MAY send NEXT_HOP option
   without any RT_PREFIX options.  NEXT_HOP option that does not contain
   any RT_PREFIX options designate default router.  Second way of
   defining default route is to convey RT_PREFIX option that specifies
   ::/0 route, included as suboption in NEXT_HOP.  First approach has
   the benefit of consuming less bandwidth, while the second one allows
   definition of default route lifetime and metric.

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   Server MUST NOT define more than one default prefix (i.e. both
   defined configuration methods are mutually exclusive).  Unless there
   are significant bandwidth restrictions, mechanism that uses ::/0
   RT_PREFIX option SHOULD be used.

3.2.  Configuring on-link routes

   Server may also configure on-link routes, i.e. routes that are
   available directly over the link, not via routers.  To specify on-
   link routes, server MAY include RTPREFIX option directly in Advertise
   and Reply messages.

3.3.  Deleting obsolete route

   There are two mechanisms that allow removing a route.  Each defined
   route has a route lifetime.  If specific route is not refreshed and
   its timer reaches 0, client MUST remove corresponding entry from
   routing table.

   In cases, where faster route removal is needed, server SHOULD return
   RT_PREFIX option with route lifetime set to 0.  Client that receives
   RT_PREFIX with route lifetime set to 0 MUST remove specified route
   immediately, even if its previous lifetime did not expire yet.

3.4.  Applicability to routers

   Contrary to Router Adverisement mechanism, defined in [RFC4861] that
   explicitly limits configuration to hosts, routing configuration over
   DHCPv6 defined in this document may be used by both hosts and

   One of the envisaged usages for this solution are residential
   gateways (RG) or Customer Premises Equipment (CPE).  Those devices
   very often perform routing.  It may be useful to configure routing on
   such devices over DHCPv6.  One example of such use may be a class of
   premium users that are allowed to use dedicated router that is not
   available to regular users.

3.5.  Updating Routing Information

   Network configuration occassionally changes, due to failure of
   existing hardware, migration to newer equipment or many other
   reasons.  Therefore there a way to inform clients that routing
   information have changed is required.

   There are several ways to inform clients about new routing
   information.  Every client SHOULD periodically refresh its
   configuration, according to Information Refresh Time Option, so

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   server may send updated information the next time client refreshes
   its information.  New routes may be configured at that time.  As
   every route has associated lifetime, client is required to remove its
   routes when this timer expires.  This method is particularly useful,
   when migrating to new router is undergoing, but old router is still

   Server MAY also announce routes via soon to be removed router with
   lifetimes set to 0.  This will cause the client to remove its routes,
   despite the fact that previously received lifetime may not yet

   Aforementioned methods are useful, when there is no urgent need to
   update routing information.  Bound by timer set by value of
   Information Refresh Time Option, clients may use outdated routing
   information until next scheduled renewal.  Depending on configured
   value this delay may be not acceptable in some cases.  In such
   scenarios, administrators are advised to use RECONFIGURE mechanism,
   defined in [RFC3315].  Server transmits RECONFIRGURE message to each
   client, thus forcing it to immediately start renewal process.

   See also Section 3.6 about limitations regarding dynamic routing.

3.6.  Limitations

   Defined mechanism is not intended to be used as a dynamic routing
   protocol.  It should be noted that proposed mechanism cannot
   automatically detect routing changes.  In networks that use dynamic
   routing and also employ this mechanism, clients may attempt using
   routes configured over DHCPv6 even though routers or specific routes
   ceased to be available.  This may cause black hole routing problem.
   Therefore it is not recommended to use this mechanism in networks
   that use dynamic routing protocols.  This mechanism SHOULD NOT be
   used in such networks, unless network operator can provide a way to
   update DHCP server information in case of router availability

   Discussion: It should be noted that DHCPv6 server is not able to
   monitor health of existing routers.  As there are currently more than
   60 options defined for DHCPv6, it is infeasible to implement
   mechanism that would monitor huge set of services and stop announcing
   its availability in case of service outage.  Therefore in case of
   prolonged unavailability human interverntion is required to change
   DHCPv6 server configuration.  If that is considered a problem,
   network administrators should consider using other alternatives, like
   RA and ND mechanisms (see [RFC4861]).

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4.  DHCPv6 Route Options

   A DHCPv6 client interested in obtaining routing information includes
   the NEXT_HOP and RT_PREFIX options as part of its Option Request
   Option (ORO) in messages directed to a server (as allowed by
   [RFC3315], i.e.  Solicit, Request, Renew, Rebind or Information-
   request messages).  A Server, when configured to do so, provides the
   requested route information using zero, one or more NEXT_HOP options
   in messages sent in response (Advertise, and Reply).  So as to allow
   the route options to be both extensible, as well as conveying
   detailed info for routes, use is made of a nested options structure.
   Server sends one or more NEXT_HOP options that specify the IPv6 next
   hop addresses.  Each NEXT_HOP option conveys in turn zero, one or
   more RT_PREFIX options that represents the IPv6 destination prefixes
   reachable via the given next hop.  Server includes RT_PREFIX directly
   in message to indicate that given prefix is available directly on-
   link.  Server MAY send a single NEXT_HOP without any RT_PREFIX
   suboptions or with RT_PREFIX that contains ::/0 to indicate available
   default route.  The Formats of the NEXT_HOP and RT_PREFIX options are
   defined in the following sub-sections.

   The DHCPv6 Route Options format borrows from the principles of the
   Route Information Option defined in [RFC4191].

4.1.  Next Hop Option Format

   Each IPv6 route consists of an IPv6 next hop address, an IPv6
   destination prefix (a.k.a. the destination subnet), and a host
   preference value for the route.  Elements of such route (e.g.  Next
   hops and prefixes associated with them) are conveyed in NEXT_HOP
   option that contains RT_PREFIX suboptions.

   The Next Hop Option defines the IPv6 address of the next hop, usually
   corresponding to a specific next-hop router.  For each next hop
   address there can be zero, one or more prefixes reachable via that
   next hop.

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      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |        OPTION_NEXT_HOP        |          option-len           |
     |                                                               |
     |                    IPv6 Next Hop Address                      |
     |                       (16 octets)                             |
     |                                                               |
     |                                                               |
     |                        NEXT_HOP options                       |
     .                                                               .
     .                                                               .

                   Figure 1: IPv6 Next Hop Option Format

   option-code:  OPTION_NEXT_HOP (TBD).

   option-len:  16 + Length of NEXT_HOP options field.

   IPv6 Next Hop Address:  16 octet long field that specified IPv6
             address of the next hop.

   NEXT_HOP options:  Options associated with this Next Hop. This
             includes, but is not limited to, zero, one or more
             RT_PREFIX options that specify prefixes reachable through
             the given next hop.

4.2.  Route Prefix Option Format

   The Route Prefix Option is used to convey information about a single
   prefix that represents the destination network.  The Route Prefix
   Option is used as a sub-option in the previously defined Next Hop
   Option.  It may also be sent directly in message to indicate that
   route is available directly on-link.

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      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |       OPTION_RT_PREFIX        |          option-len           |
     |                         Route lifetime                        |
     | Prefix-Length |     Metric    |                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
     |                            Prefix                             |
     |                          (16 octets)                          |
     |                                                               |
     |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                               |                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
     .                                                               .
     .                         RT_PREFIX options                     .
     .                                                               .

                   Figure 2: Route Prefix Option Format

   option-code:  OPTION_RT_PREFIX (TBD).

   option-len:  18 + length of RT_PREFIX options.

   Route lifetime  32-bit unsigned integer.  Specifies lifetime of the
             route information, expressed in seconds.  There are 2
             special values defined. 0 means that route is no longer
             valid and must be removed by clients. 0xffffffff means

   Prefix Length:  8-bit unsigned integer.  The length in bits of the IP
             Prefix.  The value ranges from 0 to 128.  This field
             represents the number of valid leading bits in the prefix.

   Metric:   Route Metric. 8-bit signed integer.  The Route Metric
             indicates whether to prefer the next hop associated with
             this prefix over others, when multiple identical prefixes
             (for different next hops) have been received.

   Prefix:   Fixed length 16 octet field containing an IPv6 prefix.

   RT_PREFIX options:  Options specific to this particular prefix.

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5.  DHCPv6 Server Behavior

   When configured to do so, a DHCPv6 server shall provide the Next Hop
   and Route Prefix Options in ADVERTISE and REPLY messages sent to a
   client that requested the route option.  Each Next Hop Option sent by
   the server must convey at least one Route Prefix Option.

   Server includes NEXT_HOP option with possible RT_PREFIX suboptions to
   designate that specific routes are available via routers.  Server
   includes RT_PREFIX options directly in Advertise and Reply messages
   to inform that specific routes are available directly on-link.

   If there is more than one route available via specific next hop,
   server MUST send only one NEXT_HOP for that next hop, which contains
   multiple RT_PREFIX options.  Server MUST NOT send more than one
   identical (i.e. with equal next hop address field) NEXT_HOP option.

   Servers SHOULD NOT send Route Option to clients that did not
   explicitly requested it, using the ORO.

   Servers MUST NOT send Route Option in messages other than ADVERTISE
   or REPLY.

   Servers MAY also include Status Code Option, defined in Section 22.13
   of the [RFC3315] to indicate the status of the operation.

   Servers MUST include the Status Code Option, if the requested routing
   configuration was not successful and SHOULD use status codes as
   defined in [RFC3315] and [RFC3633].

   The maximum number of routing information in one DHCPv6 message
   depend on the maximum DHCPv6 message size defined in [RFC3315]

6.  DHCPv6 Client Behavior

   A DHCPv6 client compliant with this specification MUST request the
   NEXT_HOP and RT_PREFIX Options in an Option Request Option (ORO) in
   the following messages: Solicit, Request, Renew, Rebind, and
   Information-Request.  The messages are to be sent as and when
   specified by [RFC3315].

   When processing a received Route Options a client MUST substitute a
   received 0::0 value in the Next Hop Option with the source IPv6
   address of the received DHCPv6 message.  It MUST also associate a
   received Link Local next hop addresses with the interface on which
   the client received the DHCPv6 message containing the route option.
   Such a substitution and/or association is useful in cases where the

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   DHCPv6 server operator does not directly know the IPv6 next-hop
   address, other than knowing it is that of a DHCPv6 relay agent on the
   client LAN segment.  DHCPv6 Packets relayed to the client are sourced
   by the relay using this relay's IPv6 address, which could be a link
   local address.

   The Client SHOULD refresh assigned route information periodically.
   The generic DHCPv6 Information Refresh Time Option, as specified in
   [RFC4242], can be used when it is desired for the client to
   periodically refresh of route information.

   The routes conveyed by the Route Option should be considered as
   complimentary to any other static route learning and maintenance
   mechanism used by, or on the client with one modification: The client
   MUST flush DHCPv6 installed routes following a link flap event on the
   DHCPv6 client interface over which the routes were installed.  This
   requirement is necessary to automate the flushing of routes for
   clients that may move to a different network.

   Client MUST confirm that routers announced over DHCPv6 are reachable,
   using one of methods suitable for specific network type.  The most
   common mechanism is Neighbor Unreachability Detection (NUD),
   specified in [RFC4861].  Client SHOULD use NUD to verify that
   received routers are reachable before adjusting its routing tables.
   Client MAY use other reachibality verification mechanisms specific to
   used network technology.  To avoid potential long-lived routing black
   holes, client MAY periodically confirm that router is still

7.  IANA Considerations

   A DHCPv6 option number of TBD for the introduced Route Option.  IANA
   is requested to allocate three DHCPv6 option codes referencing this

8.  Security Considerations

   The overall security considerations discussed in [RFC3315] apply also
   to this document.  The Route option could be used by malicious
   parties to misdirect traffic sent by the client either as part of a
   denial of service or man-in-the-middle attack.  An alternative denial
   of service attack could also be realized by means of using the route
   option to overflowing any known memory limitations of the client, or
   to exceed the client's ability to handle the number of next hop

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   Neither of the above considerations are new and specific to the
   proposed route option.  The mechanisms identified for securing DHCPv6
   as well as reasonable checks performed by client implementations are
   deemed sufficient in addressing these problems.

   It is essential that clients verify that announced routers are indeed
   reachable, as specified in Section 6.  Failing to do so may create
   black hole routing problem.

   This mechanism may introduce severe problems if deployed in networks
   that use dynamic routing protocols.  See Section 3.6 for details.

   Reader is also encouraged to read DHCPv6 security considerations
   document [I-D.ietf-dhc-secure-dhcpv6].

9.  Contributors and Acknowledgements

   This document would not have been possible without the significant
   contribution provided by: Arifumi Matsumoto, Hui Deng, Richard
   Johnson, and Zhen Cao.

   The authors would also like to thank Alfred Hines, Ralph Droms, Ted
   Lemon, Ole Troan, Dave Oran, Dave Ward, Joel Halpern, Marcin
   Siodelski and Alexandru Petrescu for their comments and useful

10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3315]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
              and M. Carney, "Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6)", RFC 3315, July 2003.

   [RFC3633]  Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
              Host Configuration Protocol (DHCP) version 6", RFC 3633,
              December 2003.

10.2.  Informative References

              Jiang, S. and S. Shen, "Secure DHCPv6 Using CGAs",
              draft-ietf-dhc-secure-dhcpv6-03 (work in progress),

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              June 2011.

              Troan, O., Miles, D., Matsushima, S., Okimoto, T., and D.
              Wing, "IPv6 Multihoming without Network Address
              draft-ietf-v6ops-ipv6-multihoming-without-ipv6nat-01 (work
              in progress), August 2011.

   [RFC3442]  Lemon, T., Cheshire, S., and B. Volz, "The Classless
              Static Route Option for Dynamic Host Configuration
              Protocol (DHCP) version 4", RFC 3442, December 2002.

   [RFC4191]  Draves, R. and D. Thaler, "Default Router Preferences and
              More-Specific Routes", RFC 4191, November 2005.

   [RFC4242]  Venaas, S., Chown, T., and B. Volz, "Information Refresh
              Time Option for Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6)", RFC 4242, November 2005.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              September 2007.

Authors' Addresses

   Wojciech Dec (editor)
   Cisco Systems
   Haarlerbergweg 13-19
   1101 CH Amsterdam
   The Netherlands


   Tomasz Mrugalski
   Internet Systems Consortium, Inc.
   950 Charter Street
   Redwood City, CA  94063

   Phone: +1 650 423 1345

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   Tao Sun
   China Mobile
   Unit2, 28 Xuanwumenxi Ave
   Beijing, Xuanwu District  100053


   Behcet Sarikaya
   Huawei USA
   1700 Alma Dr. Suite 500
   Plano, TX  75075
   United States

   Phone: +1 972-509-5599

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