Network Working Group W. Dec, Ed.
Internet-Draft Cisco Systems
Intended status: Standards Track T. Mrugalski
Expires: January 25, 2012 ISC
T. Sun
China Mobile
B. Sarikaya
Huawei USA
July 26, 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 http:/⁠/⁠⁠drafts/⁠current/⁠.

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 January 25, 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 (http:/⁠/⁠⁠license-⁠info) 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 gateways. 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 RG configuration information from a centralized DHCP server. [I-D.troan-multihoming-without-nat66] 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 option 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. The overall DHCPv6 design follow a similar approach to that used in the design of the IA_NA, IA_TA and IA_PD options in [RFC3633].

4. DHCPv6 Route Options

A DHCPv6 client interested in obtaining routing information includes the OPTION_IA_RT as par of its DHCPv6 Option Request Option (ORO) in messages directed to a server (as allowed by [RFC3315], i.e. Solicit, Request, Renew, Rebind, Confirm or Information-request messages). A Server, when configured to do so, provides the requested route information using the OPTION_IA_RT option in messages sent in response (Advertise, and Reply). So as to allow the route option to be both extensible, as well as conveying detailed info for routes, use is made of a nested options structure. An IA_RT conveys one or more OPTION_NEXT_HOP options that specify the IPv6 next hop addresses. Each OPTION_NEXT_HOP conveys in turn one or more OPTION_RT_PREFIX options that represents the IPv6 destination prefixes reachable via the given next hop. The Formats of the OPTION_IA_RT, OPTION_NEXT_HOP and OPTION_RT_PREFIX are defined in the following sub-sections

The DHCPv6 Route Option format borrows from the principles of the Route Information Option defined in [RFC4191]. One notable exception with respect to [RFC4191] is however that a Route Lifetime element is not defined. The information conveyed by the DHCPv6 Route Option is considered valid until changed or refreshed by general events that trigger DHCPv6 or route table state changes on a node, thus not requiring a specific route lifetime. In the event that it is desired for the client to request a refresh of the route information (and other stateless DHCPv6 options), use of the generic DHCPv6 Information Refresh Time Option, as specified in [RFC4242] is envisaged.

4.1. DHCPv6 Route Option Format

To separate routing information from other options conveyed in a DHCPv6 message, the DHCPv6 Route Option is defined and is used to convey to a client one or more IPv6 routes. 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 IA_RT's options, rather than in the IA_RT option itself.

 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_IA_RT          |          option-len           |
|                                                               |
.                           IA_RT options                       .
.                                                               .

Figure 1: IPv6 Routes Option Format

Length of the IA_RT options field.
IA_RT options:
Options associated with this IA_RT. This includes, but is not limited to, OPTION_NEXT_HOP options that specify next hop addresses.

The Route option MUST NOT appear in the following DHCPv6 messages: Solicit, Request, Renew, Rebind, Information-Request. The Route Option MAY appear in ADVERTISE and REPLY messages.

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

Discussion: Traditionally, grouping options (IA_NA, IA_TA and IA_RD) contain an identifier field (IAID) that must be unique among identifiers generated by one client. It is used to differentiate between several options of the same type (e.g. several IA_NA options) that may be used simultaneously. However, it is assumed that client will never use more than one IA_RT option therefore such an identifier is not needed.

4.2. Next Hop Option Format

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 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                       |
.                                                               .
.                                                               .

Figure 2: IPv6 Route Option Format

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, one or more OPTION_RT_PREFIX options that specify prefixes reachable through the given next hop.

4.3. 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.

 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           |
| Prefix-Length |     Metric    |                               | 
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
|                            Prefix                             |
|                          (16 octets)                          |
|                                                               |
|                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                               |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
.                                                               .
.                         RT_PREFIX options                     .
.                                                               .

Figure 3: Route Prefix Option Format

18 + length of RT_PREFIX options.
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 s DHCPv6 server shall provide the Routes Option 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.

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]

Discussion: How should server indicate that there are no specific routes for this particular client? The reasonable behavior is to return empty IA_RT option, possibly with Status Code indicating Success. Another approach could be to simply not return any IA_RT option.

6. DHCPv6 Client Behavior

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

When processing a received Route Option 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 MAY 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_IA_RT, 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.

9. Contributors and Acknowledgements

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

The authors would also like to thank Alfred Hines, Ralph Droms, Ted Lemon, Ole Troan, Dave Oran, Dave Ward and Joel Halpern 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.troan-multihoming-without-nat66] Troan, O, Miles, D, Matsushima, S, Okimoto, T and D Wing, "IPv6 Multihoming without Network Address Translation", Internet-Draft draft-troan-multihoming-without-nat66-01, July 2010.

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: