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Keywords: Ethernet, Access Network, ARP, DHCP
Network Working Group T. Melsen
Request for Comments: 4562 S. Blake
Category: Informational Ericsson
June 2006
MAC-Forced Forwarding:
A Method for Subscriber Separation on an Ethernet Access Network
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document describes a mechanism to ensure layer-2 separation of
Local Area Network (LAN) stations accessing an IPv4 gateway over a
bridged Ethernet segment.
The mechanism - called "MAC-Forced Forwarding" - implements an
Address Resolution Protocol (ARP) proxy function that prohibits
Ethernet Media Access Control (MAC) address resolution between hosts
located within the same IPv4 subnet but at different customer
premises, and in effect directs all upstream traffic to an IPv4
gateway. The IPv4 gateway provides IP-layer connectivity between
these same hosts.
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Table of Contents
1. Introduction ....................................................2
1.1. Access Network Requirements ................................3
1.2. Using Ethernet as an Access Network Technology .............4
2. Terminology .....................................................5
3. Solution Aspects ................................................6
3.1. Obtaining the IP and MAC Addresses of the Access Routers ...6
3.2. Responding to ARP Requests .................................7
3.3. Filtering Upstream Traffic .................................8
3.4. Restricted Access to Application Servers ...................8
4. Access Router Considerations ....................................8
5. Resiliency Considerations .......................................9
6. Multicast Considerations ........................................9
7. IPv6 Considerations ............................................10
8. Security Considerations ........................................10
9. Acknowledgements ...............................................11
10. References ....................................................11
10.1. Normative References .....................................11
10.2. Informative References ...................................12
1. Introduction
The main purpose of an access network is to provide connectivity
between customer hosts and service provider access routers (ARs),
typically offering reachability to the Internet and other IP networks
and/or IP-based applications.
An access network may be decomposed into a subscriber line part and
an aggregation network part. The subscriber line - often referred to
as "the first mile" - is characterized by an individual physical (or
logical, in the case of some wireless technologies) connection to
each customer premises. The aggregation network - "the second mile"
- performs aggregation and concentration of customer traffic.
The subscriber line and the aggregation network are interconnected by
an Access Node (AN). Thus, the AN constitutes the border between
individual subscriber lines and the common aggregation network. This
is illustrated in the following figure.
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Access Aggregation Access Subscriber Customer
Routers Network Nodes Lines Premises
Networks
+----+ |
--+ AR +-----------| +----+
+----+ | | +----------------[]--------
|--------+ AN |
| | +----------------[]--------
| +----+
|
| +----+
| | +----------------[]--------
|--------+ AN |
| | +----------------[]--------
| +----+
|
| +----+
| | +----------------[]--------
|--------+ AN |
+----+ | | +----------------[]--------
--+ AR +-----------| +----+
+----+ |
1.1. Access Network Requirements
There are two basic requirements that an access network solution must
satisfy:
1. Layer-2 separation between customer premises.
2. High IPv4 address assignment efficiency.
It is required that all traffic to and from customer hosts located at
different premises (i.e., accessed via different subscriber lines or
via different access networks) be forwarded via an AR, and not
bridged or switched at layer-2 (Requirement 1; see also requirement
R-40 in [TR101]). This enables the access network service provider
to use the AR(s) to perform security filtering, policing, and
accounting of all customer traffic. This implies that within the
access network, layer-2 traffic paths should not exist that
circumvent an AR (with some exceptions; see Section 3.4).
In ATM-based access networks, the separation of individual customer
hosts' traffic is an intrinsic feature achieved by the use of ATM
permanent virtual connections (PVCs) between the customers' access
device (e.g., DSL modem) and the AR (typically co-located/integrated
with access control functionality in a Broadband Remote Access Server
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(BRAS)). In this case, the AN is an ATM-based Digital Subscriber
Line Access Multiplexer (DSLAM).
This document, however, targets Ethernet-based access networks.
Techniques other than ATM PVCs must be employed to ensure the desired
separation of traffic to and from individual customer hosts.
Efficient address assignment is necessary to minimize consumption of
the scarce IPv4 address space (Requirement 2). See [RFC3069] for
further discussion. Address assignment efficiency is improved if
host addresses are assigned out of one or more large pools, rather
than by being assigned out of separate, smaller subnet blocks
allocated to each customer premises. IPv6 address assignment
efficiency is of much less concern, and it is anticipated that IPv6
deployments will allocate separate IPv6 subnet blocks to each
customer premises [v6BB].
1.2. Using Ethernet as an Access Network Technology
A major aspect of using Ethernet as an access technology is that
traffic pertaining to different customer hosts is conveyed over a
shared broadcast network. Layer-2 isolation between customer
premises networks could be provided by implementing access router
functionality in each EAN, treating each subscriber line as a
separate IP interface. However, there are a variety of reasons why
it is often desirable to avoid IP routing in the access network,
including the need to satisfy regulatory requirements for direct
layer-2 accessibility to multiple IP service providers. In addition,
this solution would not solve Requirement 2.
To avoid IP routing within the access network, the Ethernet
aggregation network is bridged via EANs to individual Ethernet
networks at the customers' premises. If the EANs were standard
Ethernet bridges, then there would be direct layer-2 visibility
between Ethernet stations (hosts) located at different customers'
premises. Specifically, hosts located within the same IP subnet
would have this visibility. This violates Requirement 1 (Section
1.1) and introduces security issues, as malicious end-users thereby
can attack hosts at other customers' premises directly at the
Ethernet layer.
Existing standardized solutions may be deployed to prevent layer-2
visibility between stations:
o PPP over Ethernet [RFC2516]. The use of PPPoE creates individual
PPP sessions between hosts and one or more BRASes over a bridged
Ethernet topology. Traffic always flows between a BRAS and hosts,
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never directly between hosts. The AN can force upstream traffic
to flow only to the BRAS initially selected by the host.
o VLAN per-customer premises network [RFC3069]. Traffic to/from
each customer premises network can be separated into different
VLANs across the aggregation network between the AN and the AR.
Both solutions provide layer-2 isolation between customer hosts, but
they are not considered optimal for broadband access networks,
because:
o PPPoE does not support efficient multicast: packets must be
replicated on each PPPoE session to hosts listening on a specific
multicast group. This negates one of the major advantages of
using Ethernet (instead of ATM) as an access technology. This is
an especially problematic limitation for services such as IPTV,
which require high bandwidth per-multicast group (channel), and
which may often have hundreds or thousands of listening customer
hosts per group.
o Using VLANs to isolate individual customer premises networks also
forces multicast packets to be replicated to each VLAN with a
listening host. Furthermore, the basic limit of a maximum of 4096
VLANs per-Ethernet network limits the scalability of the solution.
This scalability limit can be removed by deploying VLAN stacking
techniques within the access network, but this approach increases
provisioning complexity.
The solution proposed in this document avoids these problems.
2. Terminology
Access Node (AN)
The entity interconnecting individual subscriber lines to the
shared aggregation network.
Access Router (AR)
The entity interconnecting the access network to the Internet or
other IP-based networks. The AR provides connectivity between
hosts on the access network at different customer premises. It is
also used to provide security filtering, policing, and accounting
of customer traffic.
Application Server (AS)
A server, usually owned by a service provider, that attaches
directly to the aggregation network and is directly reachable at
layer-2 by customer hosts.
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Ethernet Access Node (EAN)
An Access Node supporting Ethernet-based subscriber lines and
uplinks to an Ethernet-based aggregation network and MAC-Forced
Forwarding. For example, for xDSL access, the EAN is an
Ethernet-centric DSLAM. The EAN is a special type of filtering
bridge that does not forward Ethernet broadcast and multicast
frames originating on a subscriber line to other subscriber lines,
but either discards them or forwards them upstream (towards the
aggregation network). The EAN also discards unicast Ethernet
frames that originate on a subscriber line and are not addressed
to an AR.
3. Solution Aspects
The basic property of the solution is that the EAN ensures that
upstream traffic is always sent to a designated AR, even if the IP
traffic should ultimately flow between customer hosts located within
the same IP subnet.
The solution has three major aspects:
1. Initially, the EAN obtains the IP and MAC addresses of the allowed
target ARs for each customer host.
2. The EAN replies to any upstream ARP request [RFC0826] from
customer hosts with the MAC address of an allowed target AR.
3. The EAN discards any upstream unicast traffic to MAC addresses
other than the allowed target ARs. The EAN also discards all
non-essential broadcast and multicast packets received on
subscriber lines.
These aspects are discussed in the following sections.
3.1. Obtaining the IP and MAC Addresses of the Access Routers
An access network may contain multiple ARs, and different hosts may
be assigned to different (groups of) ARs. This implies that the EAN
must register the assigned AR addresses on a per-customer host basis.
For each customer host, one of the ARs is acting as the default
gateway. If a customer has simultaneous access to multiple ARs, the
other ARs typically will provide access to other IP networks.
The EAN learns the IPv4 address of the allowed target ARs in one of
two ways, depending on the host IPv4 address assignment method. For
each host using Dynamic Host Configuration Protocol (DHCP), the EAN
learns the AR IPv4 addresses dynamically by snooping the DHCPACK
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reply to a host [RFC2131]. If a host using DHCP shall have
simultaneous access to multiple ARs, DHCP option 121 [RFC3442] or
DHCP option 33 [RFC2132] must be used to specify them for that host.
If static address assignment is used instead of DHCP, then AR IPv4
addresses must be pre-provisioned in the EAN by the network operator.
In both cases, the EAN will ARP to determine the ARs' corresponding
MAC addresses. This can be done immediately after the IPv4 addresses
are learned or when the MAC addresses are first required.
The DHCP server can associate customer hosts with subscriber lines if
the EAN uses the DHCP Relay Agent Information Option (82) to convey a
subscriber line identifier to the DHCP server in DHCP messages
flowing upstream from the customer host [RFC3046].
3.2. Responding to ARP Requests
If all customer networks were assigned individual IP subnet blocks
(and if routing protocols were blocked inside the access network),
then all upstream traffic would normally go to an AR (typically the
default gateway), and the EAN could validate all upstream traffic by
checking that the destination MAC address matched that of an AR.
However, to comply with Requirement 2 of Section 1.1, residential
customer networks are not (usually) assigned individual IPv4 subnet
blocks. In other words, several hosts located at different premises
are within the same IPv4 subnet. Consequently, if a host wishes to
communicate with a host at another premises, an ARP request is issued
to obtain that host's corresponding MAC address. This request is
intercepted by the EAN's ARP proxy, and an ARP reply is sent,
specifying an allowed AR MAC address (typically the default
gateway's) as the requested layer-2 destination address, in a manner
similar to the "proxy ARP" mechanism described in [RFC1812]. In this
way, the ARP table of the requesting host will register an AR MAC
address as the layer-2 destination for any host within that IPv4
subnet (except those at the same customer premises; see below).
ARP requests for an IPv4 address of an allowed target AR are replied
to by the EAN's ARP proxy with that AR's MAC address, rather than the
MAC address of the default gateway AR.
An exception is made when a host is ARPing for another host located
within the same premises network. If this ARP request reaches the
EAN, it should be discarded, because it is assumed to be answered
directly by the target host within the premises network. The EAN
must keep track of all assigned IPv4 addresses on a subscriber line
so that it can detect these ARP requests and discard them.
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3.3. Filtering Upstream Traffic
Since the EAN's ARP proxy will always reply with the MAC address of
an AR, the requesting host will never learn MAC addresses of hosts
located at other premises. However, malicious customers or
malfunctioning hosts may still try to send traffic using other
unicast destination MAC addresses. The EAN must discard all unicast
frames received on a subscriber line that are not addressed to a
destination MAC address for an allowed AR (with some exceptions; see
Section 3.4.
Similarly, broadcast or multicast packets received on a subscriber
line must never be forwarded on other subscriber lines, but only on
EAN uplinks to the aggregation network. An EAN must discard all
non-ARP broadcast packets received on subscriber lines, except when
DHCP is in use, in which case, the EAN must forward client-to-server
DHCP broadcast messages (DHCPDISCOVER, DHCPREQUEST, DHCPDECLINE,
DHCPINFORM) [RFC2131] upstream. An EAN should rate limit upstream
broadcast packets.
Broadcast packets forwarded on an EAN uplink may be forwarded to
other EANs by the aggregation network. EANs should discard all
broadcast packets received from the aggregation network, except ARPs
from ARs for subscriber hosts and server-to-client DHCP messages
(DHCPOFFER, DHCPACK, DHCPNAK) [RFC2131], when DHCP is in use.
Filtering of multicast packets to and from an EAN uplink is discussed
in Section 6.
3.4. Restricted Access to Application Servers
The previous discussion (Section 3.1) describes how customer hosts
are allowed direct layer-2 connectivity only to one or more ARs.
Similarly, a customer host could be allowed direct layer-2 access to
one or more Application Servers (ASes) which are directly connected
to the aggregation network. There is no functional difference in the
way MAC-Forced Forwarding treats access to ARs and ASes.
4. Access Router Considerations
Traffic between customer hosts that belong to the same IPv4 subnet
but are located at different customer premises will always be
forwarded via an AR. In this case, the AR will forward the traffic
to the originating network, i.e., on the same interface from where it
was received. This normally results in an ICMP redirect message
[RFC0792] being sent to the originating host. To prevent this
behavior, the ICMP redirect function for aggregation network
interfaces must be disabled in the AR.
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5. Resiliency Considerations
The operation of MAC-Forced Forwarding does not interfere with or
delay IP connectivity recovery in the event of a sustained AR
failure. Use of DHCP to configure hosts with information on
multiple, redundant ARs, or use of Virtual Router Redundancy Protocol
(VRRP) [RFC3768] to implement AR redundancy, allows IP connectivity
to be maintained.
MAC-Forced Forwarding is a stateful protocol. If static IPv4 address
assignment is used in the access network, then the EAN must be pre-
provisioned with state information for the customer hosts which may
be reached via a subscriber line, and the ARs associated with those
hosts. In the event of a transient EAN failure, the EAN's state
database can be quickly recovered from its configuration storage.
If DHCP is used to assign IPv4 addresses in the access network, then
MAC-Forced Forwarding operates as a soft-state protocol. Since the
DHCP and ARP messages that are snooped to construct the EAN state
database are usually sent infrequently, a transient failure may not
be detected by either the AR(s) or the customer hosts. Therefore, a
transient failure of an EAN could lead to an extended loss of
connectivity. To minimize connectivity loss, an EAN should maintain
its dynamic state database in resilient storage to permit timely
database and connectivity restoration.
The EAN is a single point of attachment between a subscriber line and
the aggregation network; hence, the EAN is a single point of
connectivity failure. Customers seeking more resilient connectivity
should multi-home.
6. Multicast Considerations
Multicast traffic delivery for streams originating within the
aggregation network or further upstream and delivered to one or more
customer hosts in an access network is supported in a scalable manner
by virtue of Ethernet's native multicast capability. Bandwidth
efficiency can be enhanced if the EAN behaves as an IGMP snooping
bridge; e.g., if it snoops on IGMP Membership Report and Leave Group
messages originating on subscriber lines to prune the set of
subscriber lines on which to forward particular multicast groups
[RFC3376].
An EAN must discard all IPv4 multicast packets received on a
subscriber line other than IGMP Membership Report and Leave Group
messages [RFC3376]. If a customer host wishes to source multicast
packets to a group, the host must tunnel them upstream to a multicast
router; e.g., an AR acting as a Protocol Independent Multicast -
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Sparse Mode (PIM-SM) Designated Router [RFC2362]. An AR will forward
them back into the access network if there are any listening customer
hosts.
EAN processing of IPv6 multicast packets is discussed in the next
section.
7. IPv6 Considerations
MAC-Forced Forwarding is not directly applicable for IPv6 access
networks for the following reasons:
1. IPv6 access networks do not require the same efficiency of address
allocation as IPv4 access networks. It is expected that customer
premises networks will be allocated unique network prefixes (e.g.,
/48) accommodating large numbers of customer subnets and hosts
[v6BB].
2. IPv6 nodes do not use ARP, but instead use the Neighbor Discovery
Protocol [RFC2461] for layer-2 address resolution.
To simultaneously support both IPv6 and MAC-Forced Forwarding for
IPv4, an EAN can implement the unicast, broadcast, and multicast
filtering rules described in Section 3.3. To correctly perform
unicast filtering, the EAN must learn the IPv6 and MAC addresses of
the allowed ARs for a particular subscriber line. It can learn these
addresses either through static configuration or by snooping Router
Discovery messages exchanged between the customer premises router and
one or more ARs [RFC2461].
Multicast is an intrinsic part of the IPv6 protocol suite.
Therefore, an EAN must not indiscriminately filter IPv6 multicast
packets flowing upstream, although it may rate limit them. Detailed
IPv6 multicast filtering rules are not discussed in this document.
8. Security Considerations
MAC-Forced Forwarding is, by its nature, a security function,
ensuring layer-2 isolation of customer hosts sharing a broadcast
access medium. In that sense, it provides security equivalent to
alternative PVC-based solutions. Security procedures appropriate for
any shared access medium are equally appropriate when MAC-Forced
Forwarding is employed. It does not introduce any additional
vulnerabilities over those of standard Ethernet bridging.
In addition to layer-2 isolation, an EAN implementing MAC-Forced
Forwarding must discard all upstream broadcast packets, except for
valid DHCP messages, and ARP requests (which are proxied by the EAN).
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In particular, the EAN must discard any DHCP server replies
originating on a subscriber line. Further, an EAN may rate limit
upstream broadcast DHCP messages.
An EAN implementing MAC-Forced Forwarding must keep track of IPv4
addresses allocated on subscriber lines. Therefore, the EAN has
sufficient information to discard upstream traffic with spoofed IPv4
source addresses.
9. Acknowledgements
The authors would like to thank Ulf Jonsson, Thomas Narten, James
Carlson, Rolf Engstrand, Tomas Thyni, and Johan Kolhi for their
helpful comments.
10. References
10.1. Normative References
[RFC0792] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, September 1981.
[RFC0826] Plummer, D., "Ethernet Address Resolution Protocol: Or
converting network protocol addresses to 48.bit Ethernet
address for transmission on Ethernet hardware", STD 37,
RFC 826, November 1982.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC
2131, March 1997.
[RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, March 1997.
[RFC2362] Estrin, D., Farinacci, D., Helmy, A., Thaler, D., Deering,
S., Handley, M., Jacobson, V., Liu, C., Sharma, P., and L.
Wei, "Protocol Independent Multicast-Sparse Mode (PIM-SM):
Protocol Specification", RFC 2362, June 1998.
[RFC3046] Patrick, M., "DHCP Relay Agent Information Option", RFC
3046, January 2001.
[RFC3376] Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
Thyagarajan, "Internet Group Management Protocol, Version
3", RFC 3376, October 2002.
[RFC3442] Lemon, T., Cheshire, S., and B. Volz, "The Classless
Static Route Option for Dynamic Host Configuration
Protocol (DHCP) version 4", RFC 3442, December 2002.
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10.2. Informative References
[RFC1812] Baker, F., "Requirements for IP Version 4 Routers", RFC
1812, June 1995.
[RFC3768] Hinden, R., "Virtual Router Redundancy Protocol (VRRP)",
RFC 3768, April 2004.
[RFC2461] Narten, T., Nordmark, E., and W. Simpson, "Neighbor
Discovery for IP Version 6 (IPv6)", RFC 2461, December
1998.
[RFC2516] Mamakos, L., Lidl, K., Evarts, J., Carrel, D., Simone, D.,
and R. Wheeler, "A Method for Transmitting PPP Over
Ethernet (PPPoE)", RFC 2516, February 1999.
[RFC3069] McPherson, D. and B. Dykes, "VLAN Aggregation for
Efficient IP Address Allocation", RFC 3069, February 2001.
[TR101] DSL Forum, "Migration to Ethernet-Based DSL Aggregation",
Technical Report TR-101, April 2006.
[v6BB] Asadullah, S., Ahmed, A., Popoviciu, C., Savola, P., and
J. Palet, "ISP IPv6 Deployment Scenarios in Broadband
Access Networks", Work in Progress.
Authors' Addresses
Torben Melsen
Ericsson
Faelledvej
Struer DK-7600
Denmark
EMail: Torben.Melsen@ericsson.com
Steven Blake
Ericsson
920 Main Campus Drive
Suite 500
Raleigh, NC 27606
USA
Phone: +1 919 472 9913
EMail: steven.blake@ericsson.com
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