Network Working Group W. Dec, Ed.
Internet-Draft Cisco Systems
Intended status: Standards Track T. Mrugalski
Expires: March 14, 2012 ISC
T. Sun
China Mobile
B. Sarikaya
Huawei USA
September 11, 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 impractical.

Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 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 14, 2012.

Copyright Notice

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

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 [RFC4191].

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 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-hop.

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.

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 routers.

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 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 available.

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 expire.

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 changes.

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]).

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.

 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                       |
.                                                               .
.                                                               .

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.

 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                     .
.                                                               .

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 infinity.
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.
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.
Fixed length 16 octet field containing an IPv6 prefix.
RT_PREFIX options:
Options specific to this particular prefix.

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 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 reachable.

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 document: OPTION_NEXT_HOP and OPTION_RT_PREFIX.

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 addresses.

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 suggestions.

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

[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.
[I-D.ietf-v6ops-ipv6-multihoming-without-ipv6nat] Troan, O, Miles, D, Matsushima, S, Okimoto, T and D Wing, "IPv6 Multihoming without Network Address Translation", Internet-Draft draft-ietf-v6ops-ipv6-multihoming-without-ipv6nat-03, November 2011.
[I-D.ietf-dhc-secure-dhcpv6] Jiang, S and S Shen, "Secure DHCPv6 Using CGAs", Internet-Draft draft-ietf-dhc-secure-dhcpv6-03, June 2011.

Authors' Addresses

Wojciech Dec editor Cisco Systems Haarlerbergweg 13-19 1101 CH Amsterdam, The Netherlands EMail:
Tomasz Mrugalski Internet Systems Consortium, Inc. 950 Charter Street Redwood City, CA 94063 USA Phone: +1 650 423 1345 EMail:
Tao Sun China Mobile Unit2, 28 Xuanwumenxi Ave Beijing, Xuanwu District 100053 China EMail:
Behcet Sarikaya Huawei USA 1700 Alma Dr. Suite 500 Plano, TX 75075 United States Phone: +1 972-509-5599 EMail: