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IETF RFC 6221
Lightweight DHCPv6 Relay Agent
Last modified on Tuesday, May 17th, 2011
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Internet Engineering Task Force (IETF) D. Miles, Ed.
Request for Comments: 6221 S. Ooghe
Updates: 3315 Alcatel-Lucent
Category: Standards Track W. Dec
ISSN: 2070-1721 Cisco Systems
S. Krishnan
A. Kavanagh
Ericsson
May 2011
Lightweight DHCPv6 Relay Agent
 Abstract
This document proposes a Lightweight DHCPv6 Relay Agent (LDRA) that
is used to insert relay agent options in DHCPv6 message exchanges
identifying client-facing interfaces. The LDRA can be implemented in
existing access nodes (such as Digital Subscriber Link Access
Multiplexers (DSLAMs) and Ethernet switches) that do not support IPv6
control or routing functions.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/RFC 6221.
Miles, et al. Standards Track PAGE 1 
RFC 6221 LDRA May 2011
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://trustee.ietf.org/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 ....................................................3
1.1. Requirements Language ......................................3
2. Background ......................................................3
3. Terminology .....................................................3
4. Server Considerations ...........................................5
5. Message Format ..................................................5
5.1. Relay-Forward Message ......................................5
5.2. Relay-Reply Message ........................................6
5.3. Mandatory DHCP Options .....................................6
5.3.1. Relay-Message Option ................................6
5.3.2. Interface-ID Option .................................6
6. Agent Behaviour .................................................7
6.1. Relaying a Client Message ..................................7
6.1.1. Client Message Validation ...........................8
6.1.2. Trusted and Untrusted Interfaces ....................8
6.2. Relaying a Relay-Reply Message from the Network ............8
7. Network Topology ................................................9
7.1. Client and Server on Same Link .............................9
7.2. Client and Server behind Relay Agent ......................11
7.3. Relay Agent in Front of LDRA ..............................12
8. Contributors ...................................................15
9. Security Considerations ........................................15
10. References ....................................................15
10.1. Normative References .....................................15
10.2. Informative References ...................................16
Miles, et al. Standards Track PAGE 2
RFC 6221 LDRA May 2011
1. Introduction
DHCPv6 Relay Agents [RFC 3315] are deployed to forward DHCPv6 messages
between clients and servers when they are not on the same IPv6 link
and are often implemented alongside a routing function in a common
node. A Lightweight DHCPv6 Relay Agent (LDRA) allows Relay Agent
Information to be inserted by an access node that performs a link-
layer bridging (i.e., non-routing) function. An LDRA resides on the
same IPv6 link as the client and a DHCPv6 Relay Agent or server, and
is functionally the equivalent of the Layer 2 Relay Agent proposed
for DHCPv4 operation in [L2RA].
Unlike a DHCPv6 Relay Agent specified in [RFC 3315], an LDRA does not
implement any IPv6 control functions (e.g., ICMPv6) or have any
routing capability in the node.
1.1. 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 [RFC 2119].
2. Background
A variety of different link-layer network topologies exist for the
aggregation of IPv6 nodes into one or more routers. In Layer 2
aggregation networks (IEEE 802.1D bridging or similar) that have many
nodes on a single link, a DHCPv6 server or DHCP Relay Agent would
normally be unaware of how a DHCP client is attached to the network.
The LDRA allows Relay Agent Information, including the Interface-ID
option [RFC 3315], to be inserted by the access node so that it may be
used by the DHCPv6 server for client identification. A typical
application in a broadband service provider could be equivalent to a
Layer 2 DHCP Relay Agent as described in the Broadband Forum TR-101
report [TR-101] and in [L2RA].
3. Terminology
Access Node A device that combines many interfaces onto
one link. An access node is not IP-aware
in the data path.
Address An IP layer identifier for an interface or
set of interfaces.
Client-facing An interface on the access node that
carries traffic towards the DHCPv6 client.
Miles, et al. Standards Track PAGE 3
RFC 6221 LDRA May 2011
Host A non-routing IPv6 node that is
participating in a DHCPv6 message exchange.
IP Internet Protocol Version 6 (IPv6).
LDRA Lightweight DHCPv6 Relay Agent.
Lightweight Relay Agent A function on the access node that
intercepts DHCP messages between clients
and servers. The function exists as a bump
in the wire on the IP link.
Link A communication facility or medium over
which nodes can communicate at the link
layer.
Link-local address An IP address having only local scope,
indicated by having the address prefix
fe80::/10, that can be used to reach
neighbouring nodes attached to the same
link. Every interface has a link-local
address.
Network-facing An interface on the access node that
carries traffic towards the DHCPv6
server(s).
Node A device that implements IPv6.
Router A node that forwards packets not directly
addressed to itself.
Relay Agent A node that acts as an intermediary to
deliver DHCP messages between clients and
servers and being on the same link as the
client.
Unspecified address An IPv6 address that is comprised entirely
of zeros.
Miles, et al. Standards Track PAGE 4
RFC 6221 LDRA May 2011
4. Server Considerations
This document updates the behaviour specified in Section 11 of DHCP
for IPv6 [RFC 3315]. RFC 3315 states, in part:
If the server receives the message from a forwarding relay agent,
then the client is on the same link as the one to which the
interface, identified by the link-address field in the message
from the relay agent, is attached.
DHCP server implementations conforming to this specification MUST,
for the purposes of address selection, ignore any link-address field
whose value is zero. In the above text from RFC 3315, "link-address"
refers to both the link-address field of the Relay-Forward message,
and the link-address fields in any Relay-Forward messages that may be
nested within the Relay-Forward message.
5. Message Format
The Lightweight DHCPv6 Relay Agent (LDRA) exchanges DHCP messages
between clients and servers using the message formats established in
[RFC 3315].
To maintain interoperability with existing DHCP relays and servers,
the message format is unchanged from [RFC 3315]. The LDRA implements
the same message types as a normal DHCPv6 Relay Agent. They are:
o Relay-Forward Messages
o Relay-Reply Messages
5.1. Relay-Forward Message
The Relay-Forward message is created by any DHCPv6 Relay Agent,
including an LDRA, to forward messages between clients and servers or
other relay agents. These messages are built as specified in
[RFC 3315].
The Relay-Forward message contains relay agent parameters that
identify the client-facing interface on which any reply messages
should be forwarded. These parameters are link-address, peer-
address, and Interface-ID. The link-address parameter MUST be set to
the unspecified address. The peer-address parameter MUST be set as
specified in Section 6.1. The Interface-ID Relay Agent option MUST
be included in the Relay-Forward message. The LDRA MAY insert
additional relay agent options.
Miles, et al. Standards Track PAGE 5
RFC 6221 LDRA May 2011
5.2. Relay-Reply Message
The Relay-Reply message is constructed by a DHCPv6 server to send
parameters to a DHCP client when a relay agent is present between the
server and the client. The Relay-Reply message may be sent after an
initial Relay-Forward message as the parameters link-address, peer-
address, and Interface-ID, as well as the relay agent's IP address,
are learnt from the Relay-Forward message.
The server MUST include the Interface-ID option in the Relay-Reply
Message to indicate to the LDRA the interface on which the
decapsulated message should be forwarded.
5.3. Mandatory DHCP Options
Parameters are exchanged between the DHCP client, Relay Agent, and
server through the use of DHCP options. There is a set of mandatory
DHCP options that MUST be included by the LDRA in all Relay-Forward
messages. These are the:
o Relay-Message Option
o Interface-ID Option
5.3.1. Relay-Message Option
A DHCPv6 Relay Agent relays messages between clients and servers or
other relay agents through Relay-Forward and Relay-Reply message
types. The original client DHCP message (i.e., the packet payload,
excluding UDP and IP headers) is encapsulated in a Relay Message
option [RFC 3315].
If a Relay-Message would exceed the MTU of the outgoing interface, it
MUST be discarded, and an error condition SHOULD be logged.
5.3.2. Interface-ID Option
The LDRA MUST include the Interface-ID option [RFC 3315] in all Relay-
Forward messages. When an LDRA receives a Relay-Reply message with
an Interface-ID option present and link-address unspecified, the LDRA
MUST relay the decapsulated message to the client on the interface
identified in the Interface-ID option.
Servers MAY use the Interface-ID for parameter assignment policies.
The format of the Interface-ID is outside the scope of this
contribution. The Interface-ID SHOULD be considered an opaque value;
i.e., the server SHOULD NOT try to parse the contents of the
Interface-ID option. The LDRA SHOULD use the same Interface-ID value
Miles, et al. Standards Track PAGE 6
RFC 6221 LDRA May 2011
for a given interface, and this value SHOULD be retained across
restarts. This is because if the Interface-ID changes, a server will
not be able to use it reliably in parameter assignment policies.
6. Agent Behaviour
The LDRA MUST have each of its interfaces configured as either
client-facing or network-facing. The LDRA uses the notion of client-
facing and network-facing interfaces to process DHCPv6 messages.
6.1. Relaying a Client Message
The LDRA MUST intercept and process all IP traffic received on any
client-facing interface that has:
o destination IP address set to All_DHCP_Relay_Agents_and_Servers
(ff02::1:2);
o protocol type UDP; and
o destination port 547.
The LDRA MUST also prevent the original message from being forwarded
on the network-facing interface.
The lightweight relay agent adds any other options it is configured
or required to include in the Relay-Forward message. The LDRA MUST
set the link-address field of the Relay-Forward message to the
Unspecified Address (::) and MUST include the Interface-ID option in
all DHCP Relay-Forward messages.
If the message received on the client-facing interface is a Relay-
Forward message, the LDRA MUST set the hop-count field in the newly
created Relay-Forward message to the value of the hop-count field in
the received message, incremented by 1 as specified in [RFC 3315].
The LDRA MUST copy the IP destination and link-layer destination
addresses from the client-originated message into the IP destination
address and link-layer destination address of the Relay-Forward
message.
The LDRA MUST copy the IP source address from the client-originated
message into the peer-address field of the Relay-Forward message.
The LDRA MUST copy the link-layer source address from the client-
originated message into the link-layer source address of the Relay-
Forward message.
Miles, et al. Standards Track PAGE 7
RFC 6221 LDRA May 2011
6.1.1. Client Message Validation
On receipt of a DHCP message on a client-facing interface, the LDRA
MUST discard a message if it is of one of the following message
types:
o ADVERTISE (2)
o REPLY (7)
o RECONFIGURE (10)
o RELAY-REPL (13)
Options contained in the DHCPv6 message MUST NOT be validated by the
LDRA, making it the responsibility of the DHCP server to check
message option validity and allow new options to be introduced
without changes on the LDRA.
6.1.2. Trusted and Untrusted Interfaces
In [RFC 3046], DHCPv4 Relay Agents had their client-facing interfaces
set to "trusted" and "untrusted". An LDRA MUST implement a
configuration setting for all client-facing interfaces, marking them
either as trusted or as untrusted. This setting SHOULD be
configurable per interface. When a client-facing interface is deemed
untrusted, the LDRA MUST discard any message of type RELAY-FORW (12)
received from the client-facing interface.
6.2. Relaying a Relay-Reply Message from the Network
The LDRA MUST intercept and process all IP traffic received on the
network-facing interface that has:
o a link-local scoped source address;
o a link-local scoped destination address;
o protocol type UDP; and
o destination port 547
An LDRA MUST inspect the DHCP message type and only forward Relay-
Reply messages. Other DHCP message types MUST be silently discarded.
Miles, et al. Standards Track PAGE 8
RFC 6221 LDRA May 2011
The Relay-Reply message is considered valid by the LDRA if it passes
the validity checks to be performed by a relay agent per [RFC 3315]
and
- the Interface-ID option is present, and the value corresponds to a
valid interface in the access node;
- the Relay-Reply peer-address and the destination IP address are
identical, and it is a link-local scoped address when no IP
address is configured on the LDRA; and
- the link-address is the Unspecified Address when no IP address is
configured on the LDRA.
If the Relay-Reply message is valid, the LDRA copies the peer-address
into the destination IP address field. The LDRA SHOULD forward the
packet to the correct client-facing interface using the destination
link-layer (Media Access Control (MAC)) address or the Interface-ID
in the Relay-Reply. The LDRA SHOULD NOT retransmit the packet on any
other interface. The contents of the Relay Message option are put
into an IP/UDP packet and then forwarded to the client.
The LDRA MUST copy the link-layer and IP source address from the
Relay-Reply message into the IP/UDP packet that is forwarded to the
client.
7. Network Topology
The LDRA intercepts any DHCPv6 message received on client-facing
interfaces with the traffic pattern specified in Section 6.1. The
LDRA MUST NOT forward the original client message to a network-facing
interface; it MUST process the message and add the appropriate Relay-
Forward options as described in previous sections.
7.1. Client and Server on Same Link
The access node acts as a bridge; it has no information about any IP
prefixes that are valid on the link. Thus, a server should consider
address and parameter assignment as if the client DHCP message were
not relayed.
Miles, et al. Standards Track PAGE 9
RFC 6221 LDRA May 2011
+--------+
Client -------| |
| Access |
Client -------| Node |-----+
| (LDRA) | |
Client -------| | |
+--------+ |
| +--------+
|------| Server |
| +--------+
+--------+ |
Client -------| | |
| Access | |
Client -------| Node |-----+
| (LDRA) |
Client -------| |
+--------+
<--------- IPv6 Link -------->
For example, if a client sent a DHCP Solicit message that was relayed
by the LDRA to the server, the server would receive the following
Relay-Forward message from the LDRA:
src-ip: client link-local address
dst-ip: All_DHCP_Relay_Agents_and_Servers
msg-type: RELAY-FORW
hop-count: 0
link-address: Unspecified_Address
peer-address: client link-local address
Interface-ID Option:
interface-id: LDRA-inserted interface-id
Relay-Message Option, which contains:
msg-type: SOLICIT
Solicit Message Options: <from client>
Miles, et al. Standards Track PAGE 10
RFC 6221 LDRA May 2011
7.2. Client and Server behind Relay Agent
The client and server are on different IPv6 links, separated by one
or more relay agents that will typically act as a router. The LDRA
will send Relay-Forward messages upstream towards the second relay
agent, which in turn will process the messages.
+--------+
Client -------| |
| Access |
Client -------| Node |-----+
| (LDRA) | |
Client -------| | |
+--------+ |
| +--------+ +--------+
|------| RelayB |-------| Server |
| +--------+ +--------+
+--------+ |
Client -------| | |
| Access | |
Client -------| Node |-----+
| (LDRA) |
Client -------| |
+--------+
<------- IPv6 Link A -------> <--IPv6 Link B-->
For example, if a client sent a DHCP Solicit message that was relayed
by the LDRA to another relay agent and then to the server, the server
would receive the following Relay-Forward message from the LDRA:
Miles, et al. Standards Track PAGE 11
RFC 6221 LDRA May 2011
src-ip: relayB
dst-ip: server
msg-type: RELAY-FORW
hop-count: 1
link-address: relayB address from link A
peer-address: client link-local address
Relay-Message Option, which contains:
msg-type: RELAY-FORW
hop-count: 0
link-address: Unspecified_Address
peer-address: client link-local address
Interface-ID Option:
interface-id: LDRA-inserted interface-id
Relay-Message Option, which contains:
msg-type: SOLICIT
Solicit Message Options: <from client>
7.3. Relay Agent in Front of LDRA
The client and server are on different IPv6 links, separated by one
or more relay agents that will typically act as a router, and there
is an [RFC 3315] Relay Agent on the client-facing interface of the
LDRA. The LDRA will send Relay-Forward messages upstream towards the
second relay agent, which in turn will process the messages.
Miles, et al. Standards Track PAGE 12
RFC 6221 LDRA May 2011
+--------+
RelayC -------| |
| Access |
RelayC -------| Node |-----+
| (LDRA) | |
RelayC -------| | |
+--------+ |
| +--------+ +--------+
|------| RelayB |-------| Server |
| +--------+ +--------+
+--------+ |
RelayC -------| | |
| Access | |
RelayC -------| Node |-----+
| (LDRA) |
RelayC -------| |
+--------+
<------- IPv6 Link A -------> <--IPv6 Link B-->
For example, if a client sent a DHCP Solicit message that was relayed
by RelayC and the LDRA to another relay agent, RelayB, and then to
the server, the server would receive the following Relay-Forward
message:
Miles, et al. Standards Track PAGE 13
RFC 6221 LDRA May 2011
src-ip: relayB
dst-ip: server
msg-type: RELAY-FORW
hop-count: 2
link-address: relayB address from link A
peer-address: relayC
Relay-Message Option, which contains:
msg-type: RELAY-FORW
hop-count: 1
link-address: Unspecified_Address
peer-address: relayC
Interface-ID Option:
interface-id: LDRA-inserted interface-id
Relay-Message Option, which contains:
msg-type: RELAY-FORW
hop-count: 0
link-address: global or Unspecified_Address
peer-address: end client address
Interface-ID Option: (if required)
interface-id: relayC-inserted Interface-ID
Relay-Message Option, which contains:
msg-type: SOLICIT
Solicit Message Options: <from end client>
Miles, et al. Standards Track PAGE 14
RFC 6221 LDRA May 2011
8. Contributors
The authors would like to thank the following for their support:
Lieven Levrau, Alastair Johnson, Robert Haylock, Mickey Vucic, Ludwig
Pauwels, Fernando Cuervo, John Kaippallimalil, Fredrik Garneij,
Alfred Hoenes, Ted Lemon, Tatuya Jinmei, David Hankins, and Ralph
Droms.
Comments are solicited and should be addressed to the DHC WG mailing
list (dhcwg@ietf.org) and/or the authors.
9. Security Considerations
The security issues pertaining to DHCPv6 Relay Agents as specified in
Section 23 of [RFC 3315] are also applicable to LDRAs. The LDRA
SHOULD implement some form of rate-limiting on client-originated
traffic in order to prevent excessive process utilisation. The
traffic to be rate-limited can be easily identified since the LDRA
listens only to client-originated IPv6 traffic sent to the
All_DHCPv6_Servers_and_Relay_Agents address on UDP port 547 and does
not process any other client-originated traffic. As DHCP is session-
oriented, messages in excess of the rate-limit may be silently
discarded.
The hop-count-based determination of the trustworthiness of the LDRA
can be easily defeated by a rogue relay agent on the network-facing
interface of the LDRA.
10. References
10.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 3315] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
C., and M. Carney, "Dynamic Host Configuration Protocol
for IPv6 (DHCPv6)", RFC 3315, July 2003.
Miles, et al. Standards Track PAGE 15
RFC 6221 LDRA May 2011
10.2. Informative References
[L2RA] Joshi, B. and P. Kurapati, "Layer 2 Relay Agent
Information", Work in Progress, April 2011.
[RFC 3046] Patrick, M., "DHCP Relay Agent Information Option",
RFC 3046, January 2001.
[TR-101] The Broadband Forum, "Migration to Ethernet-Based DSL
Aggregation", Technical Report TR-101, April 2006.
Miles, et al. Standards Track PAGE 16
RFC 6221 LDRA May 2011
Authors' Addresses
David Miles (editor)
Alcatel-Lucent
L3 / 215 Spring St.
Melbourne, Victoria 3000
Australia
Phone: +61 3 9664 3308
EMail: david.miles@alcatel-lucent.com
Sven Ooghe
Alcatel-Lucent
Copernicuslaan 50
2018 Antwerp,
Belgium
EMail: sven.ooghe@alcatel-lucent.com
Wojciech Dec
Cisco Systems
Haarlerberdweg 13-19
1101 CH Amsterdam,
The Netherlands
EMail: wdec@cisco.com
Suresh Krishnan
Ericsson
8400 Blvd. Decarie
Town of Mount Royal, Quebec
Canada
EMail: suresh.krishnan@ericsson.com
Alan Kavanagh
Ericsson
8400 Blvd. Decarie
Town of Mount Royal, Quebec
Canada
EMail: alan.kavanagh@ericsson.com
Miles, et al. Standards Track PAGE 17
Lightweight DHCPv6 Relay Agent
RFC TOTAL SIZE: 28471 bytes
PUBLICATION DATE: Tuesday, May 17th, 2011
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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