draft-ietf-dhc-topo-conf-05.txt   draft-ietf-dhc-topo-conf-06.txt 
Network Working Group T. Lemon Network Working Group T. Lemon
Internet-Draft Nominum, Inc. Internet-Draft Nominum, Inc.
Intended status: Informational T. Mrugalski Intended status: Informational T. Mrugalski
Expires: January 7, 2016 ISC Expires: April 21, 2016 ISC
July 6, 2015 October 19, 2015
Customizing DHCP Configuration on the Basis of Network Topology Customizing DHCP Configuration on the Basis of Network Topology
draft-ietf-dhc-topo-conf-05 draft-ietf-dhc-topo-conf-06
Abstract Abstract
DHCP servers have evolved over the years to provide significant DHCP servers have evolved over the years to provide significant
functionality beyond that which is described in the DHCP base functionality beyond that which is described in the DHCP base
specifications. One aspect of this functionality is support for specifications. One aspect of this functionality is support for
context-specific configuration information. This memo describes some context-specific configuration information. This memo describes some
such features and makes recommendations as to how they can be used. such features and makes recommendations as to how they can be used.
Status of This Memo Status of This Memo
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 7, 2016. This Internet-Draft will expire on April 21, 2016.
Copyright Notice Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Identifying Client's Location by DHCP Servers . . . . . . . . 3 3. Identifying Client's Location by DHCP Servers . . . . . . . . 3
3.1. DHCPv4 Specific Behavior . . . . . . . . . . . . . . . . 7 3.1. DHCPv4 Specific Behavior . . . . . . . . . . . . . . . . 7
3.2. DHCPv6 Specific Behavior . . . . . . . . . . . . . . . . 7 3.2. DHCPv6 Specific Behavior . . . . . . . . . . . . . . . . 7
4. Simple Subnetted Network . . . . . . . . . . . . . . . . . . 9 4. Simple Subnetted Network . . . . . . . . . . . . . . . . . . 9
5. Relay agent running on a host . . . . . . . . . . . . . . . . 11 5. Relay Agent Running on a Host . . . . . . . . . . . . . . . . 11
6. Cascade relays . . . . . . . . . . . . . . . . . . . . . . . 11 6. Cascaded Relays . . . . . . . . . . . . . . . . . . . . . . . 11
7. Regional Configuration Example . . . . . . . . . . . . . . . 12 7. Regional Configuration Example . . . . . . . . . . . . . . . 12
8. Dynamic Lookup . . . . . . . . . . . . . . . . . . . . . . . 14 8. Multiple subnets on the same link . . . . . . . . . . . . . . 14
9. Multiple subnets on the same link . . . . . . . . . . . . . . 15 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 15
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 15 10. Security Considerations . . . . . . . . . . . . . . . . . . . 15
11. Security Considerations . . . . . . . . . . . . . . . . . . . 16 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 16 12.1. Normative References . . . . . . . . . . . . . . . . . . 16
13.1. Normative References . . . . . . . . . . . . . . . . . . 16 12.2. Informative References . . . . . . . . . . . . . . . . . 16
13.2. Informative References . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17
1. Introduction 1. Introduction
The DHCPv4 [RFC2131] and DHCPv6 [RFC3315] protocol specifications The DHCPv4 [RFC2131] and DHCPv6 [RFC3315] protocol specifications
describe how addresses can be allocated to clients based on network describe how addresses can be allocated to clients based on network
topology information provided by the DHCP relay infrastructure. topology information provided by the DHCP relay infrastructure.
Address allocation decisions are integral to the allocation of Address allocation decisions are integral to the allocation of
addresses and prefixes in DHCP. addresses and prefixes in DHCP.
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Existing DHCP server implementations do in fact provide such Existing DHCP server implementations do in fact provide such
capabilities; the goal of this document is to describe those capabilities; the goal of this document is to describe those
capabilities for the benefit both of operators and of protocol capabilities for the benefit both of operators and of protocol
designers who may wish to use DHCP as a means for configuring their 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 own services, but may not be aware of the capabilities provided by
most modern DHCP servers. most modern DHCP servers.
2. Terminology 2. Terminology
o Routable IP address: an IP address with a scope of use wider than o CPE device: Customer Premise Equipment device. Typically a router
the local link. belonging to the customer that connects directly to the provider
link.
o PE router: provider edge router. The provider router closest to o DHCP, DHCPv4, and DHCPv6. DHCP refers to the Dynamic Host
Configuration Protocol in general and applies to both DHCPv4 and
DHCPv6. The terms DHCPv4 and DHCPv6 are used in contexts where it
is necessary to avoid ambiguity and explain differences.
o PE router: Provider Edge router. The provider router closest to
the customer. the customer.
o CPE device: customer premise equipment device. Typically a router o Routable IP address: an IP address with a scope of use wider than
belonging to the customer that connects directly to the provider the local link.
link.
o Shared subnet: a case where two or more subnets of the same o Shared subnet: a case where two or more subnets of the same
protocol family are available on the same link. 'Shared subnet' protocol family are available on the same link. 'Shared subnet'
terminology is typically used in Unix environments. It is terminology is typically used in Unix environments. It is
typically called 'multinet' in Windows environment. The typically called 'multinet' in Windows environment. The
administrative configuration inside a Microsoft DHCP server is administrative configuration inside a Microsoft DHCP server is
called 'DHCP Superscope'. called 'DHCP Superscope'.
3. Identifying Client's Location by DHCP Servers 3. Identifying Client's Location by DHCP Servers
Figure 1 illustrates a small hierarchy of network links with Link D Figure 1 illustrates a small hierarchy of network links with Link D
serving as a backbone to which the DHCP server is attached. serving as a backbone to which the DHCP server is attached.
Figure 2 illustrates a more complex case. Although some of its Figure 2 illustrates a more complex case. Although some of its
aspects are unlikely to be seen in an actual production networks, aspects are unlikely to be seen in actual production networks, they
they are beneficial for explaining finer aspects of the DHCP are beneficial for explaining finer aspects of the DHCP protocols.
protocols. Note that some nodes act as routers (which forward all Note that some nodes act as routers (which forward all IP traffic)
IPv6 traffic) and some are relay agents (i.e. run DHCPv6 specific and some are relay agents (i.e. run DHCP specific software that
software that forwards only DHCPv6 traffic). forwards only DHCP traffic).
Link A Link B Link A Link B
|===+===========| |===========+======| |===+===========| |===========+======|
| | | |
| | | |
+---+---+ +---+---+ +---+---+ +---+---+
| relay | | relay | | relay | | relay |
| A | | B | | A | | B |
+---+---+ +---+---+ +---+---+ +---+---+
| | | |
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route packets to an off-link DHCP server, therefore some kind of route packets to an off-link DHCP server, therefore some kind of
relay mechanism is required. relay mechanism is required.
The details of how packet delivery between clients and servers works The details of how packet delivery between clients and servers works
are different between DHCPv4 and DHCPv6, but the essence is the same: are different between DHCPv4 and DHCPv6, but the essence is the same:
whether or not the client actually has an IP configuration, it whether or not the client actually has an IP configuration, it
generally communicates with the DHCP server by sending its requests generally communicates with the DHCP server by sending its requests
to a DHCP relay agent on the local link; this relay agent, which has 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 a routable IP address, then forwards the DHCP requests to the DHCP
server (directly or via other relays). In later stages of the server (directly or via other relays). In later stages of the
configuration when the client has aquired an address and certain configuration when the client has acquired an address and certain
conditions are met, it is possible for the client to send packets conditions are met, it is possible for the client to send packets
directly to the server, thus bypassing the relays. The conditions directly to the server, thus bypassing the relays. The conditions
for such behavior are different for DHCPv4 and DHCPv6 and are for such behavior are different for DHCPv4 and DHCPv6 and are
discussed in sections Section 3.1 and Section 3.2. discussed in sections Section 3.1 and Section 3.2.
The DHCP server uses an IP address from the client's message which is To determine the client's point of attachment and link specific
on the same link as the client to perform address assignment configuration, the server typically uses the client facing IP address
decisions or to select subnet-specific configuration for the client. of the relay agent. In some cases the server may use the routable IP
The address that the server uses is the DHCP client's routable IP address of the client, if the client has the routable IP address
address or the client facing address of the relay agent. The server assigned to its interface and it is transmitted in the DHCP message.
is then able to determine the client's point of attachment and select The server is then able to determine the client's point of attachment
appropriate subnet- or link-specific configuration. and select appropriate subnet- or link-specific configuration.
Sometimes it is useful for the relay agents to provide additional Sometimes it is useful for the relay agents to provide additional
about the topology. A number of extensions have been defined for information about the topology. A number of extensions have been
this purpose. The specifics are different, but the core principle defined for this purpose. The specifics are different, but the core
remains the same: the relay agent knows exactly where the original principle remains the same: the relay agent knows exactly where the
request came from, so it provides an indentifier that will help the original request came from, so it provides an identifier that will
server to choose appropriate address pool and configuration help the server to choose appropriate address pool and configuration
parameters. Examples of such options are mentioned in the following parameters. Examples of such options are mentioned in the following
sections. sections.
Finally, clients may be connected to the same link as the server, so Finally, clients may be connected to the same link as the server, so
no relay agents are required. In such cases, the DHCPv4 server no relay agents are required. In such cases, the DHCPv4 server
typically uses the IPv4 address assigned to the network interface typically uses the IPv4 address assigned to the network interface
over which the transmission was received to select appropriate over which the transmission was received to select an appropriate
subnet. This is more complicated for DHCPv6, as the DHCPv6 server is subnet. This is more complicated for DHCPv6, as the DHCPv6 server is
not required to have any globally unique addresses. In such cases, not required to have any globally unique addresses. In such cases,
an additional configuration information may be required. Some additional configuration information may need to be required. Some
servers allow indicating that a given subnet is directly reachable servers allow indicating that a given subnet is directly reachable
over specific local network interface. over a specific local network interface.
3.1. DHCPv4 Specific Behavior 3.1. DHCPv4 Specific Behavior
In some cases in DHCPv4, when a DHCPv4 client has a routable IPv4 In some cases in DHCPv4, when a DHCPv4 client has a routable IPv4
address, the message is unicast to the DHCPv4 server rather than address, the message is unicast to the DHCPv4 server rather than
going through a relay agent. Examples of such transmissions are going through a relay agent. Examples of such transmissions are
renewal (DHCPREQUEST) and address release (DHCPRELEASE). renewal (DHCPREQUEST) and address release (DHCPRELEASE).
The relay agent that receives client's message sets GIADDR field to The relay agent that receives client's message sets giaddr field to
the address of the network interface the message was received on. the address of the network interface the message was received on.
The relay agent may insert a relay agent option [RFC3046]. The relay agent may insert a relay agent option [RFC3046].
There are several options defined that are useful for subnet There are several options defined that are useful for subnet
selection in DHCPv4. [RFC3527] defines Link Selection sub-option selection in DHCPv4. [RFC3527] defines the Link Selection sub-option
that is iserted by a relay agent. This option is particularly useful that is inserted by a relay agent. This option is particularly
when the relay agent needs to specify the subnet/link on which a DHCP useful when the relay agent needs to specify the subnet/link on which
client resides, which is different from an IP address that can be a DHCPv4 client resides, which is different from an IP address that
used to communicate with the relay agent. Virtual Subnet Selection can be used to communicate with the relay agent. The Virtual Subnet
Option, specified in [RFC6607] is used for the same purpose (i.e. Selection sub-option, specified in [RFC6607], can also be added by a
relay agents insert that information), but it also covers additional relay agent to specify information in a VPN environment. In certain
use cases in VPN environment. In certain cases it is useful for the cases, it is useful for the client itself to specify the Virtual
client itself to specify this option, e.g. when there are no relay Subnet Selection option, e.g. when there are no relay agents involved
agents involved during VPN set up process. during the VPN set up process.
Another option that may influence the subnet selection is IPv4 Subnet Another option that may influence the subnet selection is the IPv4
Selection Option, defined in [RFC3011], which allows the client to Subnet Selection Option, defined in [RFC3011], which allows the
explicitly request allocation from a given subnet. client to explicitly request allocation from a given subnet.
3.2. DHCPv6 Specific Behavior 3.2. DHCPv6 Specific Behavior
In DHCPv6 unicast communication is possible in case where the server In DHCPv6 unicast communication is possible in case where the server
is configured with a Server Unicast option (see Section 22.12 in is configured with a Server Unicast option (see Section 22.12 in
[RFC3315]) and clients are able to take advantage of it. In such [RFC3315]) and clients are able to take advantage of it. In such
cases, once a client is assigned a, presumably global, address, it is cases, once a client is assigned a, presumably global, address, it is
able to contact the server directly, bypassing any relays. It should able to contact the server directly, bypassing any relays. It should
be noted that such a mode is completely controllable by be noted that such a mode is completely controllable by
administrators in DHCPv6. (They may simply choose to not configure administrators in DHCPv6. (They may simply choose to not configure
server unicast option, thus forcing clients to send their messages server unicast option, thus forcing clients to send their messages
always via relay agents in every case). always via relay agents in every case).
In the DHCPv6 protocol, there are two core mechanisms defined in In the DHCPv6 protocol, there are two core mechanisms defined in
[RFC3315] that allow server to distinguish which link the relay agent [RFC3315] that allow a server to distinguish which link the relay
is connected to. The first mechanism is a link-address field in the agent is connected to. The first mechanism is the link-address field
Relay-forward and Relay-reply messages. Somewhat contrary to its in the Relay-forward and Relay-reply messages. Somewhat contrary to
name, relay agents insert in the link-address field an address that its name, relay agents insert in the link-address field an address
is typically global and can be used to uniquely identify the link on that is typically global and can be used to uniquely identify the
which the client is located. In normal circumstances this is the link on which the client is located. In normal circumstances this is
solution that is easiest to maintain, as existing address assignments the solution that is easiest to maintain, as existing address
can be used and no additional administrative actions (like assigning assignments can be used and no additional administrative actions
dedicated identifers for each relay agent, making sure they are (like assigning dedicated identifiers for each relay agent, making
unique and maintaining a list of such identifiers) are needed. It sure they are unique and maintaining a list of such identifiers) are
requires, however, for the relay agent to have an address with a needed. It requires, however, for the relay agent to have an address
scope larger than link-local configured on its client-facing with a scope larger than link-local configured on its client-facing
interface. interface.
If for whatever reason that is not feasible (e.g. because the relay The second mechanism uses Interface-Id option (see Section 22.18 of
agent does not have a global address or ULA [RFC4193] configured on [RFC3315]) inserted by the relay agent, which identifies the link
the client-facing interface), the relay agent includes an Interface- that the client is connected to. This mechanism may be used for
Id option (see Section 22.18 of [RFC3315]) that identifies the link example, when the relay agent does not have a globally unique address
clients are connected to. If the interface-id is unique within an or ULA [RFC4193] configured on the client-facing interface) on the
administrative domain, the interface-id value may be used to select client facing interface, thus making the first mechanism not
the appropriate subnet. As there is no guarantee for the uniqueness feasible. If the interface-id is unique within an administrative
([RFC3315] only mandates the interface-id to be unique within a domain, the interface-id value may be used to select the appropriate
single relay agent context), it is up to the administrator to check subnet. As there is no guarantee for the uniqueness ([RFC3315] only
whether the relay agents deployed use unique interface-id values. If mandates the interface-id to be unique within a single relay agent
they aren't, Interface-id cannot be used to determine client's point context), it is up to the administrator to check whether the relay
of attachment. agents deployed use unique interface-id values. If the interface-id
values are not unique, the Interface-id option can not be used to
determine the client's point of attachment.
It should be noted that Relay-forward and Relay-reply messages are It should be noted that Relay-forward and Relay-reply messages are
exchanged between relays and servers only. Clients are never exposed exchanged between relays and servers only. Clients are never exposed
to those messages. Also, servers never receive Relay-reply messages. to those messages. Also, servers never receive Relay-reply messages.
Relay agents must be able to process both Relay-forward (sending Relay agents must be able to process both Relay-forward (sending
already relayed message further towards the server, when there is already relayed message further towards the server, when there is
more than one relay agent in a chain) and Relay-reply (when sending more than one relay agent in a chain) and Relay-reply (when sending
back the response towards the client, when there is more than one back the response towards the client, when there is more than one
relay agent in a chain). relay agent in a chain).
For completeness, we also mention an uncommon, but valid case, where For completeness, we also mention an uncommon, but valid case, where
relay agents set link-local address in the link-address field in relay agents use a link-local address in the link-address field in
relayed Relay-forward messages. This may happen if the relay agent relayed Relay-forward messages. This may happen if the relay agent
doesn't have any address with a larger scope. Even though link local doesn't have any address with a larger scope on the interface
addresses cannot be automatically used to associate relay agent with connected to that specific link. Even though link-local addresses
a given link, with sufficient information provided the server is cannot be automatically used to associate relay agent with a given
still able to correctly select the proper link. That requires the link, with additional configuration information the server may still
DHCP server software to be able to specify relay agent link-address able to select the proper link. That requires the DHCP server
or a feature similar to 'shared subnets' (see Section 9). Network software to be able to specify relay agent link-address associated
administrator has to manually configure additional information that a with each link or a feature similar to 'shared subnets' (see
given subnet uses a relay agent with link-address X. Alternatively, Section 8). Both may or may not be supported by the server software.
if the relay agent uses link address X and relays messages from a Network administrator has to manually configure additional
subnet A, an administrator can configure that subnet A is a shared information that a given subnet uses a relay agent with link-address
subnet with a very small X/128 subnet. That is not a recommended X. Alternatively, if the relay agent uses link address X and relays
configuration, but in cases where it is impossible for relay agents messages from a subnet A, an administrator can configure that subnet
to get an address from the subnet they are relaying from, it may be a A is a shared subnet with a very small X/128 subnet. That is not a
viable solution. recommended configuration, but in cases where it is impossible for
relay agents to get an address from the subnet they are relaying
from, it may be a viable solution.
DHCPv6 also has support for more finely grained link identification, DHCPv6 also has support for more finely grained link identification,
using Lightweight DHCPv6 Relay Agents [RFC6221] (LDRA). In this using Lightweight DHCPv6 Relay Agents [RFC6221] (LDRA). In this
case, the link-address field is set to Unspecified_address (::), but case, the link-address field is set to Unspecified_address (::), but
the DHCPv6 server also receives an Interface-Id option from the relay the DHCPv6 server also receives an Interface-Id option from the relay
agent that can be used to more precisely identify the client's agent that can be used to more precisely identify the client's
location on the network. location on the network.
What this means in practice is that the DHCP server in all cases has 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 sufficient information to pinpoint, at the very least, the layer 3
link to which the client is connected, and in some cases which layer 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 2 link the client is connected to, when the layer 3 link is
aggregated out of multiple layer 2 links. aggregated out of multiple layer 2 links.
In all cases, then, the DHCP server will have a link-identifying IP In all cases, then, the DHCPv6 server will have a link-identifying IP
address, and in some cases it may also have a link-specific address, and in some cases it may also have a link-specific
identifier (e.g. Interface-Id Option or Link Address Option defined identifier (e.g. Interface-Id Option or Link Address Option defined
in Section 5 of [RFC6977]). It should be noted that there the link- in Section 5 of [RFC6977]). It should be noted that there the link-
specific identifier is unique only within the scope of the link- specific identifier is unique only within the scope of the link-
identifying IP address. For example, link-specific indentifier of identifying IP address. For example, link-specific identifier of
"eth0" for a relay agent with IPv4 address 192.0.2.1 means something "eth0" for a relay agent with IPv4 address 2001:db8::1 means
different than "eth0" for a relay agent with address 192.0.2.123. something different than "eth0" for a relay agent with address
2001:db8::2.
It is also possible for link-specific identifiers to be nested, so It is also possible for link-specific identifiers to be nested, so
that the actual identifier that identifies the link is an aggregate 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 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 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 were received from a single LDRA, so we do not treat it specially in
the discussion below, but sites that use chained LDRA configurations the discussion below, but sites that use chained LDRA configurations
will need to be aware of this when configuring their DHCP servers. will need to be aware of this when configuring their DHCPv6 servers.
The Virtual Subnet Selection Options, present in DHCPv4, are also The Virtual Subnet Selection Options, present in DHCPv4, are also
defined for DHCPv6. The use case is the same as in DHCPv4: the relay defined for DHCPv6. The use case is the same as in DHCPv4: the relay
agent inserts VSS options that can help the server to select the agent inserts VSS options that can help the server to select the
appropriate subnet with its address pool and associated configuration appropriate subnet with its address pool and associated configuration
options. See [RFC6607] for details. options. See [RFC6607] for details.
4. Simple Subnetted Network 4. Simple Subnetted Network
Consider Figure 1 in the context of a simple subnetted network. In Consider Figure 1 in the context of a simple subnetted network. In
skipping to change at page 10, line 43 skipping to change at page 10, line 49
}, },
"192.0.2.192/26": { "192.0.2.192/26": {
"options": { "options": {
"routers": ["192.0.2.193"] "routers": ["192.0.2.193"]
}, },
"on-link": ["G"] "on-link": ["G"]
} }
} }
} }
Figure 3: Configuration example Figure 3: Configuration Example
In Figure 3, we see a configuration example for this scenario: a set 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 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: 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 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 only has one value, and that value is the IP address of the router
specific to the prefix. specific to the prefix.
When the DHCP server receives a request, it searches the list of When the DHCP server receives a request, it searches the list of
prefixes for one that encloses the link-identifying IP address prefixes for one that encloses the link-identifying IP address
skipping to change at page 11, line 19 skipping to change at page 11, line 26
options to the client. options to the client.
So for example a client connected to link A in the example would have So for example a client connected to link A in the example would have
a link-identifying IP address within the 192.0.2.0/26 prefix, so the a link-identifying IP address within the 192.0.2.0/26 prefix, so the
DHCP server would match it to that prefix. Based on the DHCP server would match it to that prefix. Based on the
configuration, the DHCP server would then return a routers option configuration, the DHCP server would then return a routers option
containing a single IP address: 192.0.2.1. A client on link F would containing a single IP address: 192.0.2.1. A client on link F would
have a link-identifying address in the 192.0.2.128/26 prefix, and have a link-identifying address in the 192.0.2.128/26 prefix, and
would receive a routers option containing the IP address 192.0.2.129. would receive a routers option containing the IP address 192.0.2.129.
5. Relay agent running on a host 5. Relay Agent Running on a Host
A relay agent is a DHCP software that may be run on any IP node. A relay agent is DHCP software that may be run on any IP node.
Although it is typically run on a router, this is by no means Although it is typically run on a router, this is by no means
required by the DHCP protocol. The relay agent is simply a service required by the DHCP protocol. The relay agent is simply a service
that operates on a link, receiving link-local multicasts (IPv6) or that operates on a link, receiving link-local multicasts (IPv6) or
broadcasts (IPv4) and relaying them, using IP routing, to a DHCP broadcasts (IPv4) and relaying them, using IP routing, to a DHCP
server. As long as the relay has an IP address on the link, and a 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 default route or more specific route through which it can reach a
DHCP server, it need not be a router, or even have multiple DHCP server, it need not be a router, or even have multiple
interfaces. interfaces.
A relay agent can be run on a host connected to two links. That case A 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 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 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 connected to the same links. The relay agent software is running on
that host. That is uncommon, but a valid configuration. that host. That is uncommon, but a valid configuration.
6. Cascade relays 6. Cascaded Relays
Let's observe another case, shown in Figure 2. Note that in this Let's observe another case, shown in Figure 2. Note that in this
configuration, the clients connected to link G will send their configuration, the clients connected to link G will send their
requests to relay D which will forward its packets directly to the requests to relay D which will forward its packets directly to the
DHCP server. That is typical, but not the only possible DHCP server. That is typical, but not the only possible
configuration. It is possible to configure relay agent D to forward 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 client messages to relay E which in turn will send it to the DHCP
server. This configuration is sometimes referred to as cascade relay server. This configuration is sometimes referred to as cascade relay
agents. agents.
Note that the relaying mechanism works differently in DHCPv4 and in Note that the relaying mechanism works differently in DHCPv4 and in
DHCPv6. In DHCPv4 only the first relay is able to set the GIADDR DHCPv6. In DHCPv4 only the first relay is able to set the giaddr
field in the DHCPv4 packet. Any following relays that receive that field in the DHCPv4 packet. Any following relays that receive that
packet will not change it as the server needs GIADDR information from packet will not change it as the server needs giaddr information from
the first relay (i.e. the closest to the client). The server will the first relay (i.e. the closest to the client). The server will
send the response back to the GIADDR address, which is the address of send the response back to the giaddr address, which is the address of
the first relay agent that saw the client's message. That means that the first relay agent that saw the client's message. That means that
the client messages travel on a different path than the server's the client messages travel on a different path than the server's
responses. A message from client connected to link G will travel via 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 relay D, relay E and to the server. A response message will be sent
from the server to relay D via router B, and relay D will send it to from the server to relay D via router B, and relay D will send it to
the client on link G. the client on link G.
Relaying in DHCPv6 is more structured. Each relay agent encapsulates Relaying in DHCPv6 is more structured. Each relay agent encapsulates
a packet that is destined to the server and sends it towards the a packet that is destined to the server and sends it towards the
server. Depending on the configuration, that can be a server's server. Depending on the configuration, that can be a server's
skipping to change at page 13, line 7 skipping to change at page 13, line 7
PE routers connected to each regional backbone; we have kept the PE routers connected to each regional backbone; we have kept the
number small for simplicity. number small for simplicity.
In the example presented in Figure 4, the goal is to configure all In the example presented in Figure 4, the goal is to configure all
the devices within a region with server addresses local to that the devices within a region with server addresses local to that
region, so that service traffic does not have to be routed between region, so that service traffic does not have to be routed between
regions unnecessarily. regions unnecessarily.
{ {
"prefixes": { "prefixes": {
"2001:db8:0:0::/40": { "2001:db8::/40": {
"on-link": ["A"] "on-link": ["A"]
}, },
"2001:db8:100:0::/40": { "2001:db8:100::/40": {
"on-link": ["B"] "on-link": ["B"]
}, },
"2001:db8:200:0::/40": { "2001:db8:200::/40": {
"on-link": ["F"] "on-link": ["F"]
}, },
"2001:db8:300:0::/40": { "2001:db8:300::/40": {
"on-link": ["G"] "on-link": ["G"]
} }
}, },
"links": { "links": {
"A": {"region": "omashu"}, "A": {"region": "omashu"},
"B": {"region": "omashu"}, "B": {"region": "omashu"},
"F": {"region": "gaoling"}, "F": {"region": "gaoling"},
"G": {"region": "gaoling"} "G": {"region": "gaoling"}
}, },
"regions": { "regions": {
skipping to change at page 13, line 44 skipping to change at page 13, line 44
"gaoling": { "gaoling": {
"options": { "options": {
"sip-servers": ["sip.gaoling.example.org"], "sip-servers": ["sip.gaoling.example.org"],
"dns-servers": ["dns1.gaoling.example.org", "dns-servers": ["dns1.gaoling.example.org",
"dns2.gaoling.example.org"] "dns2.gaoling.example.org"]
} }
} }
} }
} }
Figure 4: An example regions configuration Figure 4: Regional Configuration Example
In this example, when a request comes in to the DHCP server with a 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 link-identifying IP address in the 2001:DB8::/40 prefix, it is
identified as being on link A. The DHCP server then looks on the identified as being on link A. The DHCPv6 server then looks on the
list of links to see what region the client is in. Link A is 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 identified as being in omashu. The DHCPv6 server then looks up
in the set of regions, and discovers a list of region-specific omashu in the set of regions, and discovers a list of region-specific
options. options.
The DHCP server then resolves the domain names listed in the options The DHCP server then resolves the domain names listed in the options
and sends a sip-server option containing the IP addresses that the and sends a sip-server option containing the IP addresses that the
resolver returned for sip.omashu.example.org, and a dns-server option resolver returned for sip.omashu.example.org, and a dns-server option
containing the IP addresses returned by the resolver for containing the IP addresses returned by the resolver for
dns1.omashu.example.org and dns2.omashu.example.org. Depending on dns1.omashu.example.org and dns2.omashu.example.org. Depending on
the server capability and configuration, it may cache resolved the server capability and configuration, it may cache resolved
responses for specific period of time, repeat queries every time or responses for specific period of time, repeat queries every time or
even keep the response until reconfiguration or shutdown. even keep the response until reconfiguration or shutdown. For more
detailed discussion see Section 7 of [RFC7227].
Similarly, if the DHCP server receives a request from a DHCP client Similarly, if the DHCP server receives a request from a DHCP client
where the link-identifying IP address is contained by the prefix where the link-identifying IP address is contained by the prefix
2001:DB8:300:0::/40, then the DHCP server identifies the client as 2001:DB8:300::/40, then the DHCP server identifies the client as
being connected to link G. The DHCP server then identifies link G 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- being in the gaoling region, and returns the sip-servers and dns-
servers options specific to that region. servers options specific to that region.
As with the previous example, the exact configuration syntax and As with the previous example, the exact configuration syntax and
structure shown above does not precisely match what existing DHCP structure shown above does not precisely match what existing DHCP
servers do, but the behavior illustrated in this example can be servers do, but the behavior illustrated in this example can be
accomplished with most existing modern DHCP servers. accomplished with most existing modern DHCP servers.
8. Dynamic Lookup 8. Multiple subnets on the same link
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, most modern DHCP servers handle name resolution by querying There are scenarios where there is more than one subnet from the same
the resolver each time a DHCP packet comes in. This means that if protocol family (i.e. two or more IPv4 subnets or two or more IPv6
DHCP servers and DNS servers are managed by different administrative subnets) configured on the same link. Such a configuration is often
entities, there is no need for the administrators of the DHCP servers referred to as 'shared subnets' in Unix environments or 'multinet' in
and DNS servers to communicate when changes are made. When changes Microsoft terminology.
are made to the DNS server, these changes are promptly and
automatically adopted by the DHCP server, as long as the DNS server
is managed appropriately (see the next paragraph). Similarly, when
DHCP server configurations change, DNS server administrators need not
be aware of this.
It should be noted that even though the DHCP server may be configured The most frequently mentioned use case is a network renumbering where
to query the DNS resolver every time it uses configured names, the some services are migrated to the new addressing scheme, but some
changes made in the DNS zone may not be visible to the server until aren't yet.
the DNS cache expires. In general, it is the responsibility of the
DNS zone's administrator to ensure that existing cache does not cause
a trouble when a change is made to the zone; it should be usually
reasonable for the DHCP server to rely on it. However, if this is
not desired or if the management of the DNS zone is not very
reliable, the DHCP server can be configured to query the
authoritative DNS server directly, bypassing any caching DNS servers.
It is worth noting that DNS is not the only way to resolve names, and Second example is expanding the allocation space. In DHCPv4 and for
not all DHCP servers support other techniques (e.g., NIS+ or WINS). DHCPv6 Prefix Delegation, there could be cases where multiple subnets
However, since these protocols have all but vanished from common use, are needed, because a single subnet may be too small to accommodate
this won't be an issue in new deployments. the client population.
9. Multiple subnets on the same link The third use case covers allocating addresses (or delegation
prefixes) that are not the same as topological information. For
example, the link-address is on prefix X and the addresses to be
delegated is on prefix Y. This could be based on differentiating
information (i.e., whether device is CPE or CM in DOCSIS) or just
because the link-address/giaddr is different from the actual
allocation space.
There are scenarios where there is more than one subnet from the same The fourth use case is a cable network, where cable modems and the
protocol family (i.e. two or more IPv4 subnets or two or more IPv6 devices connected behind them are connected to the same layer 2 link.
subnets) configured on the same layer 3 link. One example is a slow However, operators want the cable modems and user devices to get
network renumbering where some services are migrated to the new addresses from distinct address spaces, so users couldn't easily
addressing scheme, but some aren't yet. Second example is a cable access their modems management interfaces.
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
subnets' in Unix environments or 'multinet' in Microsoft terminology.
To support such a configuration, additional differentiating To support such a configuration, additional differentiating
information is required. Many DHCP server implementations offer a information is required. Many DHCP server implementations offer a
feature that is typically called client classification. The server feature that is typically called client classification. The server
segregates incoming packets into one or more classes based on certain segregates incoming packets into one or more classes based on certain
packet characteristics, e.g. presence or value of certain options or packet characteristics, e.g. presence or value of certain options or
even a match between existing options. Servers require additional even a match between existing options. Servers require additional
information to handle such configuration, as they cannot use the information to handle such configuration, as they cannot use the
topographical property of the relay addresses alone to properly topographical property of the relay addresses alone to properly
choose a subnet. Exact details of such operation is not part of the choose a subnet. Exact details of such operation is not part of the
DHCPv4 or DHCPv6 protocols and is implementation dependent. DHCPv4 or DHCPv6 protocols and is implementation dependent.
10. Acknowledgments 9. Acknowledgments
Thanks to Dave Thaler for suggesting that even though "everybody Thanks to Dave Thaler for suggesting that even though "everybody
knows" how DHCP servers are deployed in the real world, it might be knows" how DHCP servers are deployed in the real world, it might be
worthwhile to have an IETF document that explains what everybody worthwhile to have an IETF document that explains what everybody
knows, because in reality not everybody is an expert in how DHCP knows, because in reality not everybody is an expert in how DHCP
servers are administered. Thanks to Andre Kostur, Carsten Strotmann, servers are administered. Thanks to Andre Kostur, Carsten Strotmann,
Simon Perreault, Jinmei Tatuya, Suresh Krishnan, Qi Sun, Jean- Simon Perreault, Jinmei Tatuya, Suresh Krishnan, Qi Sun, Jean-
Francois Tremblay, Marcin Siodelski and Bernie Volz for their Francois Tremblay, Marcin Siodelski and Bernie Volz for their
reviews, comments and feedback. reviews, comments and feedback.
11. Security Considerations 10. Security Considerations
This document explains existing practice with respect to the use of This document explains existing practice with respect to the use of
Dynamic Host Configuration Protocol [RFC2131] and Dynamic Host Dynamic Host Configuration Protocol [RFC2131] and Dynamic Host
Configuration Protocol Version 6 [RFC3315]. The security Configuration Protocol Version 6 [RFC3315]. The security
considerations for these protocols are described in their considerations for these protocols are described in their
specifications and in related documents that extend these protocols. specifications and in related documents that extend these protocols.
This document introduces no new functionality, and hence no new This document introduces no new functionality, and hence no new
security considerations. security considerations.
12. IANA Considerations 11. IANA Considerations
The IANA is hereby absolved of any requirement to take any action in The IANA is hereby absolved of any requirement to take any action in
relation to this document. relation to this document.
13. References 12. References
12.1. Normative References
13.1. Normative References
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC [RFC2131] Droms, R., "Dynamic Host Configuration Protocol",
2131, March 1997. RFC 2131, DOI 10.17487/RFC2131, March 1997,
<http://www.rfc-editor.org/info/rfc2131>.
[RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., [RFC3315] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
and M. Carney, "Dynamic Host Configuration Protocol for C., and M. Carney, "Dynamic Host Configuration Protocol
IPv6 (DHCPv6)", RFC 3315, July 2003. for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July
2003, <http://www.rfc-editor.org/info/rfc3315>.
13.2. Informative References 12.2. Informative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities", [RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987. STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<http://www.rfc-editor.org/info/rfc1034>.
[RFC3011] Waters, G., "The IPv4 Subnet Selection Option for DHCP", [RFC3011] Waters, G., "The IPv4 Subnet Selection Option for DHCP",
RFC 3011, November 2000. RFC 3011, DOI 10.17487/RFC3011, November 2000,
<http://www.rfc-editor.org/info/rfc3011>.
[RFC3046] Patrick, M., "DHCP Relay Agent Information Option", RFC [RFC3046] Patrick, M., "DHCP Relay Agent Information Option",
3046, January 2001. RFC 3046, DOI 10.17487/RFC3046, January 2001,
<http://www.rfc-editor.org/info/rfc3046>.
[RFC3527] Kinnear, K., Stapp, M., Johnson, R., and J. Kumarasamy, [RFC3527] Kinnear, K., Stapp, M., Johnson, R., and J. Kumarasamy,
"Link Selection sub-option for the Relay Agent Information "Link Selection sub-option for the Relay Agent Information
Option for DHCPv4", RFC 3527, April 2003. Option for DHCPv4", RFC 3527, DOI 10.17487/RFC3527, April
2003, <http://www.rfc-editor.org/info/rfc3527>.
[RFC4193] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast [RFC4193] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
Addresses", RFC 4193, October 2005. Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005,
<http://www.rfc-editor.org/info/rfc4193>.
[RFC6221] Miles, D., Ooghe, S., Dec, W., Krishnan, S., and A. [RFC6221] Miles, D., Ed., Ooghe, S., Dec, W., Krishnan, S., and A.
Kavanagh, "Lightweight DHCPv6 Relay Agent", RFC 6221, May Kavanagh, "Lightweight DHCPv6 Relay Agent", RFC 6221,
2011. DOI 10.17487/RFC6221, May 2011,
<http://www.rfc-editor.org/info/rfc6221>.
[RFC6607] Kinnear, K., Johnson, R., and M. Stapp, "Virtual Subnet [RFC6607] Kinnear, K., Johnson, R., and M. Stapp, "Virtual Subnet
Selection Options for DHCPv4 and DHCPv6", RFC 6607, April Selection Options for DHCPv4 and DHCPv6", RFC 6607,
2012. DOI 10.17487/RFC6607, April 2012,
<http://www.rfc-editor.org/info/rfc6607>.
[RFC6977] Boucadair, M. and X. Pougnard, "Triggering DHCPv6 [RFC6977] Boucadair, M. and X. Pougnard, "Triggering DHCPv6
Reconfiguration from Relay Agents", RFC 6977, July 2013. Reconfiguration from Relay Agents", RFC 6977,
DOI 10.17487/RFC6977, July 2013,
<http://www.rfc-editor.org/info/rfc6977>.
[RFC7159] Bray, T., "The JavaScript Object Notation (JSON) Data [RFC7159] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", RFC 7159, March 2014. Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
2014, <http://www.rfc-editor.org/info/rfc7159>.
[RFC7227] Hankins, D., Mrugalski, T., Siodelski, M., Jiang, S., and
S. Krishnan, "Guidelines for Creating New DHCPv6 Options",
BCP 187, RFC 7227, DOI 10.17487/RFC7227, May 2014,
<http://www.rfc-editor.org/info/rfc7227>.
Authors' Addresses Authors' Addresses
Ted Lemon Ted Lemon
Nominum, Inc. Nominum, Inc.
2000 Seaport Blvd 2000 Seaport Blvd
Redwood City, CA 94063 Redwood City, CA 94063
USA USA
Phone: +1-650-381-6000 Phone: +1-650-381-6000
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