Network Working Group                                           T. Lemon
Internet-Draft                                             Nominum, Inc.
Intended status: Best Current Practice                      T. Mrugalski
Expires: August 18, 2014               Internet Systems Consortium, Inc.
                                                       February 14, 2014

    Customizing DHCP Configuration on the Basis of Network Topology


   DHCP servers have evolved over the years to provide significant
   functionality beyond that which is described in the DHCP base
   specifications.  One aspect of this functionality is support for
   context-specific configuration information.  This memo describes some
   such features and makes recommendations as to how they can be used.

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 August 18, 2014.

Copyright Notice

   Copyright (c) 2014 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

Lemon & Mrugalski        Expires August 18, 2014                [Page 1]

Internet-Draft         DHCP Topology Customization         February 2014

   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Locality  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Simple Subnetted Network  . . . . . . . . . . . . . . . . . .   7
   5.  Relay agent running on a host . . . . . . . . . . . . . . . .   9
   6.  Cascade relays  . . . . . . . . . . . . . . . . . . . . . . .   9
   7.  Regional Configuration Example  . . . . . . . . . . . . . . .   9
   8.  Dynamic Lookup  . . . . . . . . . . . . . . . . . . . . . . .  11
   9.  Relay Agent Configurations  . . . . . . . . . . . . . . . . .  12
   10. Mutliple subnets on the same link . . . . . . . . . . . . . .  12
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  12
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  13
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  13
     14.2.  Informative References . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   The DHCPv4 [RFC2131] and DHCPv6 [RFC3315] protocol specifications
   describe how addresses can be allocated to clients based on network
   topology information provided by the DHCP relay infrastructure.
   Address allocation decisions are integral to the allocation of
   addresses and prefixes in DHCP.

   The DHCP protocol also describes mechanisms for provisioning devices
   with additional configuration information; for example, DNS [RFC1034]
   server addresses, default DNS search domains, and similar

   Although it was the intent of the authors of these specifications
   that DHCP servers would provision devices with configuration
   information appropriate to each device's location on the network,
   this practice was never documented, much less described in detail.

   Existing DHCP server implementations do in fact provide such
   capabilities; the goal of this document is to describe those
   capabilities for the benefit both of operators and of protocol
   designers who may wish to use DHCP as a means for configuring their
   own services, but may not be aware of the capabilities provided by
   modern DHCP servers.

Lemon & Mrugalski        Expires August 18, 2014                [Page 2]

Internet-Draft         DHCP Topology Customization         February 2014

2.  Terminology

   an IP address with a scope of use wider than the local link.

   provider edge router.  The provider router closest to the customer.

   customer premise equipment device.  Typically a router belonging to
   the customer that connects directly to the provider link.

3.  Locality

   Figure 1 illustrates a simple hierarchy of network links with Link D
   serving as a backbone to which the DHCP server is attached.

   Figure 2 illustrates a more complex case.  Although some of its
   aspects are unlikely to be seen in an actual production networks,
   they are beneficial for explaining finer aspects of the DHCP

Lemon & Mrugalski        Expires August 18, 2014                [Page 3]

Internet-Draft         DHCP Topology Customization         February 2014

              Link A                   Link B
           |===+===========|    |===========+======|
               |                            |
               |                            |
           +---+---+                    +---+---+
           | relay |                    | relay |
           |   A   |                    |   B   |
           +---+---+                    +---+---+
               |                            |
               |       Link C               |
                     +----+---+        +--------+
                     | router |        |  DHCP  |
                     |    A   |        | Server |
                     +----+---+        +----+---+
                          |                 |
                          |                 |
                          |   Link D        |
                     | router |
                     |    B   |
               |       Link E               |
               |                            |
           +---+---+                    +---+---+
           | relay |                    | relay |
           |   C   |                    |   D   |
           +---+---+                    +---+---+
               |                            |
               |                            |
           |===+===========|    |===========+======|
              Link F                   Link G

                        Figure 1: A simple network

Lemon & Mrugalski        Expires August 18, 2014                [Page 4]

Internet-Draft         DHCP Topology Customization         February 2014

              Link A                   Link B            Link H
           |===+==========|    |=========+======|  |======+======|
               |                         |                |
               |                         |                |
           +---+---+                 +---+---+        +---+---+
           | relay |                 | relay |        | relay |
           |   A   |                 |   B   |        |   G   |
           +---+---+                 +---+---+        +---+---+
               |                         |                |
               |       Link C            |                | Link J
           |===+==========+==============+======|  |======+======|
                          |                               |
                          |                               |
                     +----+---+        +--------+     +---+---+
                     | router |        |  DHCP  |     | relay |
                     |    A   |        | Server |     |   F   |
                     +----+---+        +----+---+     +---+---+
                          |                 |             |
                          |                 |             |
                          |   Link D        |             |
                          |         |
                          |         |
                     +----+---+ +---+---+
                     | router | | relay |
                     |    B   | |   E   |
                     +----+---+ +---+---+
                          |         |
                          |         |
               |       Link E               |
               |                            |
           +---+---+                    +---+---+
           | relay |                    | relay |
           |   C   |                    |   D   |
           +---+---+                    +---+---+
               |                            |
               |                            |
           |===+===========|    |===========+======|
              Link F                   Link G

                         Figure 2: Complex network

   This diagram allows us to represent a variety of different network
   configurations and illustrate how existing DHCP servers can provide
   configuration information customized to the particular location from
   which a client is making its request.

Lemon & Mrugalski        Expires August 18, 2014                [Page 5]

Internet-Draft         DHCP Topology Customization         February 2014

   It's important to understand the background of how DHCP works when
   considering this diagram.  DHCP clients are assumed not to have
   routable IP addresses when they are attempting to obtain
   configuration information.

   The reason for making this assumption is that one of the functions of
   DHCP is to bootstrap the DHCP client's IP address configuration; if
   the client does not yet have an IP address configuration, it cannot
   route packets to an off-link DHCP server, and so some kind of relay
   mechanism is required.

   The details of how this works are different between DHCPv4 and
   DHCPv6, but the essence is the same: whether or not the client
   actually has an IP configuration, it generally communicates with the
   DHCP server by sending its requests to a DHCP relay agent on the
   local link; this relay agent, which has a routable IP address, then
   forwards the DHCP requests to the DHCP server.  In some cases in
   DHCPv4, when a DHCP client has a routable IPv4 address, the message
   is unicast to the DHCP server rather than going through a relay
   agent.  In DHCPv6 that is also possible in case where server is
   configured with server unicast option and clients are able to take
   advantage of it.  In such case once the clients get their (presumably
   global) addresses, they are able to contact server directly,
   bypassing relays.  It should be noted that such a mode is completely
   controllable by administrators in DHCPv6.  (They may simply choose to
   not configure server unicast option, thus forcing clients to send
   their messages always via relay agents).

   In either case, the DHCP server is able to obtain an IP address that
   it knows is on-link for the link to which the DHCP client is
   connected: either the DHCPv4 client's routable IPv4 address, or the
   relay agent's IP address on the link to which the client is

   In DHCPv6 protocol, there are two mechanisms defined in [RFC3315]
   that allow server to distinguish which link the relay agent is
   connected to.  The first mechanism is a link-address field in the
   RELAY-FORW and RELAY-REPL messages.  Somewhat contrary to its name,
   relay agents insert an address that is typically global and can be
   used to uniquely identify the link on which the client is located.
   In normal circumstances this is the solution that is easiest to
   maintain.  It requires, however, for the relay agent to have an
   address configured on its client-facing interface.  If for whatever
   reason that is not feasible (e.g. because the relay agent does not
   have a global address), the relay agent includes an interface-id
   option that identifies the link clients are connected to.  It is up
   to administrator to make sure that the interface-id is unique within
   his administrative domain.

Lemon & Mrugalski        Expires August 18, 2014                [Page 6]

Internet-Draft         DHCP Topology Customization         February 2014

   DHCPv6 also has support for more finely grained link identification,
   using Lightweight DHCPv6 Relay Agents [RFC6221] (LDRA).  In this
   case, in addition to receiving an IPv6 address that is on-link for
   the link to which the client is connected, the DHCPv6 server also
   receives an Interface Identifier option from the relay agent that can
   be used to more precisely identify the client's location on the

   What this means in practice is that the DHCP server in all cases has
   sufficient information to pinpoint, at the very least, the layer 3
   link to which the client is connected, and in some cases which layer
   2 link the client is connected to, when the layer 3 link is
   aggregated out of multiple layer 2 links.

   In all cases, then, the DHCP server will have a link-identifying IP
   address, and in some cases it may also have a link-specific
   identifier.  It should be noted that there is no guarantee that the
   link-specific identifier will be unique outside the scope of the
   link-identifying IP address.

   It is also possible for link-specific identifiers to be nested, so
   that the actual identifier that identifies the link is an aggregate
   of two or more link-specific identifiers sent by a set of LDRAs in a
   chain; in general this functions exactly as if a single identifier
   were received from a single LDRA, so we do not treat it specially in
   the discussion below, but sites that use chained LDRA configurations
   will need to be aware of this when configuring their DHCP servers.

   So let's examine the implications of this in terms of how a DHCP
   server can deliver targeted supplemental configuration information to
   DHCP clients.

4.  Simple Subnetted Network

   Consider Figure 1 in the context of a simple subnetted network.  In
   this network, there are four leaf subnets: links A, B, F and G, on
   which DHCP clients will be configured.  Relays A, B, C and D in this
   example are represented in the diagram as IP routers with an embedded
   relay function, because this is a very typical configuration, but the
   relay function can also be provided in a separate server on each

   In a simple network like this, there may be no need for link-specific
   configuration in DHCPv6, since local routing information is delivered
   through router advertisements.  However, in IPv4, it is very typical
   to configure the default route using DHCP; in this case, the default
   route will be different on each link.  In order to accomplish this,

Lemon & Mrugalski        Expires August 18, 2014                [Page 7]

Internet-Draft         DHCP Topology Customization         February 2014

   the DHCP server will need link-specific configuration for the default

   To illustrate, we will use an example from a hypothetical DHCP server
   that uses a simple JSON notation for configuration.  Although we know
   of no DHCP server that uses this specific syntax, every commercial
   DHCP server provides similar functionality.

     {"": {"options": {"routers": [""]}
              "on-link": ["a"]}}
      "": {"options": {"routers": [""}}
              "on-link": ["b"]}
      "": {"options": {"routers": [""}}
              "on-link": ["f"]}
      "": {"options": {"routers": [""}}
              "on-link": ["g"]}}

   Figure 2

                                 Figure 3

   In Figure 3, we see a configuration example for this scenario: a set
   of prefixes, each of which has a set of options and a list of links
   for which it is on-link.  We have defined one option for each prefix:
   a routers option.  This option contains a list of values; each list
   only has one value, and that value is the IP address of the router
   specific to the prefix.

   When the DHCP server receives a request, it searches the list of
   prefixes for one that encloses the link-identifying IP address
   provided by the client or relay agent.  The DHCP server then examines
   the options list associated with that prefix and returns those
   options to the client.

   So for example a client connected to link A in the example would have
   a link-identifying IP address within the prefix, so the
   DHCP server would match it to that prefix.  Based on the
   configuration, the DHCP server would then return a routers option
   containing a single IP address:  A client on link F would
   have a link-identifying address in the prefix, and would
   receive a routers option containing the IP address

Lemon & Mrugalski        Expires August 18, 2014                [Page 8]

Internet-Draft         DHCP Topology Customization         February 2014

5.  Relay agent running on a host

   Relay agent is a DHCP software that may be run on any IP node.
   Although it is typically run on a a router, it doesn't have to be
   one.  Relay agent can be run on a host connected to two links.  That
   case is presented in Figure 2.  There is router B that is connected
   to links D and E. At the same time there is also a host that is
   connected to the same links.  The relay agent software is running on
   that host.  That is uncommon, but legal configuration.

6.  Cascade relays

   Let's observe another case shown in Figure 2.  Note that in typical
   configuration, the clients connected to link G will send their
   requests to relay D which will forward its packets directly to the
   DHCP server.  That is typical, but not the only possible
   configuration.  It is possible to configure relay agent D to forward
   client messages to relay E which in turn will send it to the DHCP
   server.  This configuration is sometimes referred to as cascade relay

   Note that the relaying mechanism works differently in DHCPv4 and in
   DHCPv6.  In DHCPv4 only the first relay is able to set the GIADDR
   field in the DHCPv4 packet.  Any following relays that receive that
   packet will not change it as the server needs GIADDR information from
   the first relay (i.e. the closest to the client).  Server will send
   the response back to the GIADDR address, which is the address of the
   first relay agent that seen the client's message.  That means that
   the client messages travel on a different path than the server's
   responses.  A message from client connected to link G will travel via
   relay D, relay E and to the server.  A response message will be sent
   from the server to relay D which will send it to the client on link

   Relaying in DHCPv6 is more structured.  Each relay agent encapsulates
   a packet that is destined to the server and sends it towards the
   server.  Depending on the configuration that can be server's unicast
   address, a multicast address or next relay agent address.  The next
   relay repeats the encapsulation process.  Although the resulting
   packet is more complex (may have up to 32 levels of encapsulation if
   traveled through 32 relays), every relay may insert its own options
   and it is clear which relay agent inserted which option.

7.  Regional Configuration Example

   In this example, link C is a regional backbone for an ISP.  Link E is
   also a regional backbone for that ISP.  Relays A, B, C and D are PE
   routers, and Links A, B, F and G are actually link aggregators with

Lemon & Mrugalski        Expires August 18, 2014                [Page 9]

Internet-Draft         DHCP Topology Customization         February 2014

   individual layer 2 circuits to each customer--for example, the relays
   might be DSLAMs or cable head-end systems.  At each customer site we
   assume there is a single CPE device attached to the link.

   We further assume that links A, B, F and G are each addressed by a
   single prefix, although it would be equally valid for each CPE device
   to be numbered on a separate prefix.

   In a real-world deployment, there would likely be many more than two
   PE routers connected to each regional backbone; we have kept the
   number small for simplicity.

   In this example, the goal is to configure all the devices within a
   region with server addresses local to that region, so that service
   traffic does not have to be routed between regions unnecessarily.

     {"2001:DB8:0:0::/40":   {"on-link": ["A"]}}
      "2001:DB8:100:0::/40": {"on-link": ["B"]}
      "2001:DB8:200:0::/40": {"on-link": ["F"]}
      "2001:DB8:300:0::/40": {"on-link": ["G"]}}

     {"A": {"region": "omashu"},
      "B": {"region": "omashu"},
      "F": {"region": "gaoling"},
      "G": {"region": "gaoling"}}}

     {"omashu": {"options":
                 {"sip-servers": [""],
                  "dns-servers": ["",
      "gaoling": {"options":
                  {"sip-servers": [""],
                   "dns-servers": ["",

   Figure 3

   In this example, when a request comes in to the DHCP server with a
   link-identifying IP address in the 2001:DB8:0:0::/40 prefix, it is
   identified as being on link A.  The DHCP server then looks on the
   list of links to see what region the client is in.  Link A is
   identified as being in omashu.  The DHCP server then looks up omashu
   in the set of regions, and discovers a list of region-specific

Lemon & Mrugalski        Expires August 18, 2014               [Page 10]

Internet-Draft         DHCP Topology Customization         February 2014

   The DHCP server then resolves the domain names listed in the options
   and sends a sip-server option containing the IP addresses that the
   resolver returned for, and a dns-server option
   containing the IP addresses returned by the resolver for and

   Similarly, if the DHCP server receives a request from a DHCP client
   where the link-identifying IP address is contained by the prefix
   2001:DB8:300:0::/40, then the DHCP server identifies the client as
   being connected to link G.  The DHCP server then identifies link G as
   being in the gaoling region, and returns the sip-servers and dns-
   servers options specific to that region.

   As with the previous example, the exact configuration syntax and
   structure shown above does not precisely match what existing DHCP
   servers do, but the behavior illustrated in this example can be
   accomplished with all existing commercial DHCP servers.

8.  Dynamic Lookup

   In the Regional example, the configuration listed several domain
   names as values for the sip-servers and dns-servers options.  The
   wire format of both of these options contains one or more IPv6
   addresses--there is no way to return a domain name to the client.

   This was understood to be an issue when the original DHCP protocol
   was defined, and historical implementations even from the very early
   days would accept domain names and resolve them.  Some early DHCP
   implementations, particularly those based on earlier BOOTP
   implementations, had very limited capacity for reconfiguration.

   However, all modern commercial DHCP servers handle name resolution by
   querying the resolver each time a DHCP packet comes in.  This means
   that if DHCP servers and DNS servers are managed by different
   administrative entities, there is no need for the administrators of
   the DHCP servers and DNS servers to communicate when changes are
   made.  When changes are made to the DNS server, these changes are
   immediately and automatically adopted by the DHCP server.  Similarly,
   when DHCP server configurations change, DNS server administrators
   need not be aware of this.

   It's worth noting that DNS is not the only way to resolve names, and
   not all DHCP servers support other techniques (e.g., NIS+ or WINS).
   However, since these protocols have all but vanished from common use,
   this won't be an issue in new deployments.

Lemon & Mrugalski        Expires August 18, 2014               [Page 11]

Internet-Draft         DHCP Topology Customization         February 2014

9.  Relay Agent Configurations

   It's worth mentioning that although we talk about relay agents and
   routers in this document mostly as if they are the same device, this
   is by no means required by the DHCP protocol.  The relay agent is
   simply a service that operates on a link, receiving link-local
   multicasts or broadcasts and relaying them, using IP routing, to a
   DHCP server.  As long as the relay has an IP address on the link, and
   a default route or more specific route through which it can reach a
   DHCP server, it need not be a router, or even have multiple

10.  Mutliple subnets on the same link

   There are scenarios where there is more than one subnet from the same
   protocol family (i.e. two or more IPv4 subnets or two or more IPv6
   subnets) configured on the same layer 3 link.  One example is a slow
   network renumbering where some services are migrated to the new
   addressing scheme, but some aren't yet.  Second example is a cable
   network, where cable modems and the devices connected behind them are
   connected to the same layer 2 link.  However, operators want the
   cable modems and user devices to get addresses from distinct address
   spaces, so users couldn't easily access their modems management
   interfaces.  Such a configuration is often referred to as shared

   To support such an configuration, additional differentiating
   information is required.  Many DHCP server implementations offer a
   feature that is typically called client classification.  The server
   segregates incoming packets into one or more classes based on certain
   packet characteristics, e.g. presence or value of certains options or
   even a match between existing options.  Servers require additional
   information to handle such configuration, as it can't use the
   topographical property of the relay addresses alone to properly
   choose a subnet.  Such information is always implementation specific.

11.  Acknowledgments

   Thanks to Dave Thaler for suggesting that even though "everybody
   knows" how DHCP servers are deployed in the real world, it might be
   worthwhile to have an IETF document that explains what everybody
   knows, because in reality not everybody is an expert in how DHCP
   servers are administered.

Lemon & Mrugalski        Expires August 18, 2014               [Page 12]

Internet-Draft         DHCP Topology Customization         February 2014

12.  Security Considerations

   This document explains existing practice with respect to the use of
   Dynamic Host Configuration Protocol [RFC2131] and Dynamic Host
   Configuration Protocol Version 6 [RFC3315].  The security
   considerations for these protocols are described in their
   specifications and in related documents that extend these protocols.
   This document introduces no new functionality, and hence no new
   security considerations.

13.  IANA Considerations

   The IANA is hereby absolved of any requirement to take any action in
   relation to this document.

14.  References

14.1.  Normative References

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol", RFC
              2131, 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.

14.2.  Informative References

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, November 1987.

   [RFC6221]  Miles, D., Ooghe, S., Dec, W., Krishnan, S., and A.
              Kavanagh, "Lightweight DHCPv6 Relay Agent", RFC 6221, May

Authors' Addresses

   Ted Lemon
   Nominum, Inc.
   2000 Seaport Blvd
   Redwood City, CA  94063

   Phone: +1-650-381-6000

Lemon & Mrugalski        Expires August 18, 2014               [Page 13]

Internet-Draft         DHCP Topology Customization         February 2014

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

   Phone: +1 650 423 1345

Lemon & Mrugalski        Expires August 18, 2014               [Page 14]