DHC Working Group D. Miles, Ed.
Internet-Draft S. Ooghe
Intended status: Informational Alcatel-Lucent
Expires: January 3, 2010 W. Dec
Cisco Systems
S. Krishnan
A. Kavanagh
Ericsson
July 2, 2009
Lightweight DHCPv6 Relay Agent (LDRA)
draft-ietf-dhc-dhcpv6-ldra-00
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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 DSLAMs and Ethernet switches) that do
not support IPv6 control or routing functions.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Message Format . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. Relay-Forward Message . . . . . . . . . . . . . . . . . . 5
4.2. Relay-Reply Message . . . . . . . . . . . . . . . . . . . 5
4.3. Mandatory DHCP Options . . . . . . . . . . . . . . . . . . 5
4.3.1. Relay-Message Option . . . . . . . . . . . . . . . . . 5
4.3.2. Interface-ID Option . . . . . . . . . . . . . . . . . 6
5. Agent Behaviour . . . . . . . . . . . . . . . . . . . . . . . 6
5.1. Relaying a Client Message . . . . . . . . . . . . . . . . 6
5.1.1. Client Message Validation . . . . . . . . . . . . . . 7
5.1.2. Trusted and Untrusted Interfaces . . . . . . . . . . . 7
5.2. Relaying a Relay-Reply message from the network . . . . . 8
6. Network Topology . . . . . . . . . . . . . . . . . . . . . . . 9
6.1. Client and Server on Same Link . . . . . . . . . . . . . . 9
6.2. Client and Server behind Relay Agent . . . . . . . . . . . 10
6.3. Relay Agent in Front of LDRA . . . . . . . . . . . . . . . 11
7. Server Considerations . . . . . . . . . . . . . . . . . . . . 12
8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 12
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
10. Security Considerations . . . . . . . . . . . . . . . . . . . 13
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
11.1. Normative References . . . . . . . . . . . . . . . . . . . 13
11.2. Informative References . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
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1. Introduction
DHCPv6 Relay-Agents [RFC3315] 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. A 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 DHCP Relay draft[L2RA]
proposed for DHCPv4 operation.
Unlike a DHCPv6 Relay-Agent specified in [RFC3315], a 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 [RFC2119].
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 [RFC3315], 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 may be as an equivalent
to the Broadband Forum TR-101 Layer 2 DHCP Relay Agent[TR-101]
described in [L2RA]
3. Terminology
Address An IP layer identifier for an interface or
set of interfaces
Host A non-routing IPv6 node that is
participating in DHCPv6 message exchange
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IP Internet Protocol Version 6 (IPv6)
LDRA Lightweight DHCPv6 Relay Agent
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.
Node A device that implements IPv6
Router A node that forwards packets not directly
addressed to itself
Access Node A device that combines many interfaces onto
one link. An access node is not IP-aware
in the data path
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
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.
Unspecified address An IP address that is comprised entirely of
zeros
4. Message Format
The Lightweight DHCPv6 Relay Agent (LDRA) exchanges DHCP messages
between clients and servers using the message formats established in
[RFC3315].
To maintain interoperability with existing DHCP relays and servers
the message format is unchanged from [RFC3315]. The LDRA implements
the same message types as a normal DHCPv6 Relay Agent. They are:
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o Relay-Forward Messages
o Relay-Reply Messages
4.1. 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
[RFC3315]
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 Interface-ID Relay Agent Option MUST be
included in the Relay-Forward message. The LDRA MAY insert
additional relay agent options.
4.2. 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, Interface-ID and 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 de-
capsulated message should be forwarded.
4.3. Mandatory DHCP Options
Parameters are exchanged between DHCP client, relay-agent and server
through the use of DHCP options. There are a set of mandatory DHCP
options that MUST be included by the LDRA in all Relay-Forward and
Relay-Reply messages. These are the:
o Relay-Message Option
o Interface-ID Option
4.3.1. 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,
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excluding UDP and IP headers) is encapsulated in a Relay Message
option [RFC3315].
As an LDRA does not implement ICMPv6, fragmentation of Relay-Messages
is not supported. If a Relay-Message would exceed the MTU of the
outgoing interface it MUST be discarded and an error condition SHOULD
be logged.
4.3.2. Interface-ID Option
The LDRA MUST include the Interface-ID option [RFC3315] in all Relay-
Forward messages. When a 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
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.
5. Agent Behaviour
The LDRA MUST have each of its interfaces configured as either
client-facing or network (DHCPv6 server) facing. The LDRA uses the
notion of client-facing and network-facing interfaces to process the
DHCPv6 messages.
5.1. Relaying a Client Message
When a DHCPv6 message (defined in [RFC3315]) is received on any
client-facing interface, the LDRA MUST intercept and process the
message. 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
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created Relay-Forward message to the value of the hop-count field in
the received message incremented by 1 as specified in [RFC3315].
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.
5.1.1. Client Message Validation
On receipt of a DHCP message on the client facing interface, the LDRA
MUST discard a message if it is of one of following message types:
o ADVERTISE (2)
o REPLY (7)
o RECONFIGURE (10)
o RELAY-REPLY (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.
5.1.2. Trusted and Untrusted Interfaces
In [RFC3046] DHCPv4 relay-agents had their client-facing interfaces
set to trusted and untrusted. An LDRA MUST implement a trusted/
untrusted definition for all client-facing interfaces that SHOULD be
configurable per interface. When a client-facing interface is deemed
untrusted the LDRA MUST discard any message received from the client-
facing interface of type RELAY-FORWARD (12).
In DHCPv4 it was not possible for a DHCP server to determine whether
the client or an intermediate relay agent had added relay agent
options and thus trusted interfaces (relay-agent interfaces that
would allow any DHCP options to be present on incoming messages) and
untrusted interfaces (relay-agent interfaces that would ensure there
are no relay-agent options on incoming messages) were defined. In
DHCPv6, relay agents encapsulate the received message into the Relay-
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Message Option in addition to adding any relay-agent options. This
nested message behaviour allows a server to identify the options each
relay-agent has inserted along the path, whenever the data path
between LDRA and server falls within a protected or operator
controlled environment.
When an LDRA is deployed, DHCPv6 servers MAY be configured with the
Relay-Forward hop-count of the LDRA to instruct at which level of
nesting the relay-agent options should be parsed. This removes the
need for an interface to be configured as trusted or untrusted by
providing the DHCPv6 server with an awareness of the LDRA logical
location in the DHCP relay path. This behaviour is dependent on the
interception of all DHCP messages by the LDRA and the incrementing of
the Relay-Forward hop-count if a Relay-Forward message is received
from the client-facing LDRA interface.
5.2. Relaying a Relay-Reply message from the network
When a valid Relay-Reply is received on any network-facing access
node interface, it MUST be intercepted by the LDRA. The LDRA MUST
listen to all IP traffic that has a link-local scoped source address,
link-local scoped destination address, protocol type UDP and
destination port 547. The LDRA SHOULD ignore any message that does
not meet this criteria and MUST allow it to be forwarded like any
other packet. The LDRA MAY be configured to listen only to a
specific destination address if it has been configured as a node
(implementing a full IP stack).
The LDRA MUST intercept and process all DHCP Relay-Reply messages and
MUST silently discard all other DHCP message types.
In addition to the validity checks performed by a relay agent in
[RFC3315] , the Relay-Reply message is considered valid by the LDRA
if:
- 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 MUST be
identical and MUST be 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
The LDRA copies the peer-address into the destination IP address
field, and MAY use the destination link-layer address (MAC address)
or Interface-ID to determine which interface to send the message to
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the client. The contents of the Relay Message Option is 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.
6. Network Topology
The LDRA intercepts any DHCPv6 message received on client-facing
interfaces with a destination IP address of
All_DHCP_Relay_Agents_and_Servers (FF02::1:2). 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.
6.1. 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 was
not relayed.
+--------+
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 and then to the server, the server would receive the
following Relay-Forward message from the LDRA:
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src-ip: client link-local address
dst-ip: All_DHCP_Relay_Agents_and_Servers
msg-type: RELAY-FORWARD
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>
6.2. 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:
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src-ip: relayB
dst-ip: server
msg-type: RELAY-FORWARD
hop-count: 1
link-address: relayB address from link A
peer-address: client link-local address
Relay-Message Option, which contains:
msg-type: RELAY-FORWARD
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>
6.3. 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 [RFC3315] 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.
+--------+
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 the LDRA to another relay agent and then to the server, the server
would receive the following Relay-Forward message from the LDRA:
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src-ip: relayB
dst-ip: server
msg-type: RELAY-FORWARD
hop-count: 2
link-address: relayB address from link A
peer-address: relayC
Relay-Message Option, which contains:
msg-type: RELAY-FORWARD
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-FORWARD
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>
7. Server Considerations
Although permitted in [RFC3315] the LDRA makes specific use of Relay-
Forward link-address fields with a zero value.
o A DHCPv6 server MUST ignore any Relay-Forward link-addresses field
with a zero value in Relay-Forward messages when searching for the
inner-most link-address field. This allows the DHCPv6 server to
select an address appropriate to the L3 link and supports a
combination of L3 DHCPv6 relay agents and LDRA.
o If no non-zero Relay-Forward link-address is found, the DHCPv6
server should act as though the message were directly received.
This is the case where no LDRA is present.
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 and
Alfred Hoenes.
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Comments are solicited and should be addressed to the DHC WG mailing
list (dhcwg@ietf.org) and/or the author.
9. IANA Considerations
This document does not introduce any new namespaces for the IANA to
manage.
10. Security Considerations
Although the LDRA only listens to client-originated IPv6 traffic sent
to the All_DHCPv6_Servers_and_Relay_Agents address on UDP port 547,
the LDRA SHOULD implement some form of rate-limiting on received
messages to prevent excessive process utilisation. 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.
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[RFC3315] Droms, R., "Dynamic Host Configuration Protocol for IPv6
(DHCPv6)", RFC 3315, July 2003.
11.2. Informative References
[L2RA] Joshi, B., "Layer 2 Relay Agent Information", IETF
Draft draft-ietf-dhc-l2ra-02, May 2008.
[RFC3046] 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.
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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
Phone:
Email: sven.ooghe@alcatel-lucent.com
Wojciech Dec
Cisco Systems
Haarlerberdweg 13-19
1101 CH Amsterdam,
The Netherlands
Phone:
Email: wdec@cisco.com
Suresh Krishnan
Ericsson
8400 Blvd Decarie
Town of Mount Royal, Quebec
Canada
Email: suresh.krishnan@ericsson.com
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Alan Kavanagh]
Ericsson
8400 Blvd Decarie
Town of Mount Royal, Quebec
Canada
Email: alan.kavanagh@ericsson.com
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