Personal T. Melsen
Internet-Draft S. Blake
Expires: September 27, 2004 Ericsson
March 29, 2004
MAC Forced Forwarding: An ARP proxy method for ensuring traffic
separation between hosts sharing an Ethernet access network
draft-melsen-mac-forced-fwd-02.txt
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document describes a mechanism to ensure layer-2 separation of
LAN stations accessing an IPv4 gateway over a shared Ethernet
segment.
The mechanism - called "MAC Forced Forwarding" - implements an ARP
proxy function that prohibits MAC address resolution between hosts
located within the same IP subnet but at different customer premises,
and in effect directs all upstream traffic to the IP gateway. The IP
gateway provides IP-layer connectivity between these same hosts.
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Conventions used in this document
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
described in BCP 14, [RFC2119] and indicate requirement levels for
compliant implementations.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Access Network Requirements . . . . . . . . . . . . . . . . . 3
1.2 Using Ethernet as an Access Network Technology . . . . . . . . 4
1.3 Solution Characteristics . . . . . . . . . . . . . . . . . . . 5
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Solution Aspects . . . . . . . . . . . . . . . . . . . . . . . 6
3.1 Obtaining the IP and MAC addresses of the Access Router . . . 6
3.2 Responding to ARP Requests . . . . . . . . . . . . . . . . . . 7
3.3 Filtering Upstream Traffic . . . . . . . . . . . . . . . . . . 7
4. Access Router Considerations . . . . . . . . . . . . . . . . . 7
5. Resiliency Considerations . . . . . . . . . . . . . . . . . . 8
6. Multicast Considerations . . . . . . . . . . . . . . . . . . . 8
7. IPv6 Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. Security Considerations . . . . . . . . . . . . . . . . . . . 9
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 10
Intellectual Property and Copyright Statements . . . . . . . . 11
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1. Introduction
The main purpose of a remote access network is to provide
connectivity between customer hosts and service provider access
routers (AR), typically offering access to the Internet and other IP
networks and/or IP-based applications.
A remote 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
connection to each customer premise. 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.
Access Aggregation Access Subscriber Customer
Routers Network Nodes Lines Premise
Networks
+----+ |
--+ AR +-----------| +----+
+----+ | | +----------------[]--------
|--------+ AN |
| | +----------------[]--------
| +----+
|
| +----+
| | +----------------[]--------
|--------+ AN |
| | +----------------[]--------
| +----+
|
| +----+
| | +----------------[]--------
|--------+ AN |
+----+ | | +----------------[]--------
--+ AR +-----------| +----+
+----+ |
1.1 Access Network Requirements
There are two basic requirements that a remote access network
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solution must satisfy:
1. Layer-2 traffic separation MUST be provided between customer
premises.
2. High IPv4 address assignment efficiency MUST be supported.
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 at layer-2 (Requirement 1). 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.
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 access server
(BRAS)). In this case, the Access Node 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 deployed to ensure the desired
separation of traffic to and from individual customer hosts.
Efficient address assignment is necessary to minimize consumption of
scarce IPv4 address space (Requirement 2). See [RFC3069] for a
discussion on why this requirement is relevant. 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
subnet blocks allocated to each customer premise.
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. To avoid IP routing in the access network,
the Ethernet aggregation network is bridged via Access Nodes to
individual Ethernet networks at the customers' premises. If the
Access Node were a standard bridge then there would be direct
visibility between Ethernet stations (hosts) located at different
customers' premises due to the nature of Ethernet. 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 can attack hosts at other customers'
premises directly at the Ethernet layer.
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Existing standardized solutions may be deployed to prevent layer-2
visibility between stations:
o PPP over Ethernet [RFC2516]. The use of PPPoE creates individual
tunnels between hosts and one or more Access Concentrators (AC)
over a bridged Ethernet topology. Traffic always flows between an
AC and hosts, never between hosts. The Access Node can enforce
that upstream traffic will only go to the AC initially selected by
the host.
o VLAN per customer [RFC3069]. Individual traffic streams can be
separated in different VLANs between the AN and the AR.
Both solutions provide layer-2 isolation between customer hosts, but
they are not considered optimal for broadband remote access networks,
because:
o PPPoE does not support efficient multicast, one of the major
advantages of using Ethernet (instead of ATM) as an access
technology.
o Using VLANs to separate individual customer hosts is not
appealing, since that is regarded as requiring cumbersome
provisioning. Furthermore, the basic limit of a maximum of 4096
VLANs per-Ethernet network limits the scalability of the solution.
This limit can be removed by deploying VLAN stacking techniques
within the access network, but this approach only adds to the
provisioning complexity.
1.3 Solution Characteristics
The solution proposed in this document has the following main
characteristics:
1. Traffic between individual customer hosts is isolated over the
Ethernet access network. Traffic always flows between customer
hosts and the AR, and never directly between customer hosts at
different premises.
2. IP addresses are assigned to customer hosts in an efficient
manner. Specifically, allocating individual IP subnets to each
customer network is NOT a requirement for this solution to
function.
3. The solution works for both dynamically assigned IP addresses
(via DHCP) and statically assigned IP addresses.
4. IP over Ethernet is used as the access protocol to ensure
efficient multicast support (for multicast streams originating
upstream from the access network; see Section 6).
5. VLANs are NOT used to separate traffic pertaining to individual
customer hosts, due to scalability and provisioning issues.
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2. Terminology
Ethernet Access Node (EAN)
The entity interconnecting individual subscriber lines and the
shared aggregation network, e.g., 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 to an AR. The EAN also
discards unicast Ethernet frames originating on a subscriber line
and not addressed to an AR.
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.
3. Solution Aspects
The basic property of the solution is that the EAN ensures that
upstream traffic is always sent to the designated AR, even if the IP
traffic goes between customer hosts located in the same IP subnet.
The solution has three major aspects:
1. Initially, the EAN obtains the IP and MAC address of the target
AR.
2. The EAN replies with this MAC address to any upstream ARP
[RFC0826] request from customer hosts.
3. The EAN filters out any upstream traffic to MAC addresses other
than the target AR.
These aspects are discussed in the following sections.
3.1 Obtaining the IP and MAC addresses of the Access Router
The AR is typically the default gateway of the host.
The EAN may learn the IP address of the AR in one of two ways,
depending on the host IP address assignment method. If each host uses
DHCP, the AR IP address is dynamically learned by snooping the DHCP
reply to a host. Otherwise, the AR IP address is pre-provisioned by
the network operator. In both cases, the EAN will need to determine
the corresponding MAC address, using ARP. This can be done
immediately after the IP address is learned, or when the MAC address
is first required.
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An access network may contain multiple ARs, and different hosts may
be assigned different ARs. This implies that the EAN MUST register
the assigned AR address on a per-host basis.
3.2 Responding to ARP Requests
If all customer networks were assigned individual IP subnets, 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 the AR.
However, to comply with Requirement 2 of Section 1.1, residential
customer networks are not (usually) assigned individual IP subnets.
In other words, several hosts located at different premises are
within the same IP subnet. Consequently, if a host wishes to
communicate with a host at another premise, an ARP is issued to
obtain that host's corresponding MAC address. This ARP request is
intercepted by the EAN's ARP proxy, and responded to with an ARP
reply, indicating the AR MAC address as the requested layer-2
destination, in a manner similar to the "proxy ARP" mechanism
described in [RFC1009]. In this way, the ARP table of the requesting
host will register the AR MAC address as the layer-2 destination for
any host within that IP subnet.
An exception is made when a host is ARPing for another host located
within the same premise. If this ARP request reaches the EAN, it is
discarded, because it is assumed to be answered directly by a host
locally within the premise.
3.3 Filtering Upstream Traffic
Since the EAN's ARP proxy will always reply with the MAC address of
the 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
destination MAC addresses. This traffic MUST be discarded by the EAN.
4. Access Router Considerations
Traffic between hosts that belong to the same IP subnet but are
located at different 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
when DHCP is used as the IP address allocation mechanism, or when two
or more ARs implement VRRP [RFC2338].
MAC Forced Forwarding is a stateful protocol. If static IP address
assignment is used in the access network, then the EAN state database
can be quickly recovered in the event of a transient EAN failure.
Otherwise, transient failure of an EAN can lead to temporary loss of
connectivity, since the DHCP and ARP messages that are snooped to
construct the EAN state database are usually infrequent, and a
transient failure may not be detected by either the AR(s) or the
customer premise hosts. For high availability applications, EANs used
in access networks using dynamic IP address assignment SHOULD employ
resilient storage of their state database to permit timely
restoration of connectivity in the event of a transient EAN failure.
6. Multicast Considerations
Multicast traffic delivery for streams originating upstream from the
access network and delivered to one or more customer premise hosts in
an access network is supported in a scalable manner by virtue of
Ethernet's native multicast capability. 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 multicast packets.
Support for multicast streams originating from a customer premise
host is not discussed in this document.
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 premise networks will be allocated unique network
prefixes (e.g., /48) accommodating large numbers of customer
subnets and hosts.
2. IPv6 nodes do not use ARP, but instead use the Neighbor Discovery
protocol [RFC2461] for layer-2 address resolution.
3. IPv6 nodes do not necessarily use DHCP to obtain IP addresses and
IP gateway information, but may instead use the Stateless Address
Autoconfiguration protocol [RFC2462].
Furthermore, there is no practical deployment experience using a MAC
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Forced Forwarding-type approach in an IPv6 access network.
Some principles of MAC Address Forwarding may be applicable in an
IPv6 access network design and merit further study.
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.
A MAC Forced Forwarding implementation MUST ensure that only
authentic DHCP replies are used in the dynamic discovery of AR
addresses. One way to accomplish this is to reject any upstream DHCP
replies, i.e., replies originated on a subscriber line.
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.
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.
[RFC1009] Braden, R. and J. Postel, "Requirements for Internet
gateways", RFC 1009, June 1987.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2338] Knight, S., Weaver, D., Whipple, D., Hinden, R., Mitzel,
D., Hunt, P., Higginson, P., Shand, M. and A. Lindem,
"Virtual Router Redundancy Protocol", RFC 2338, April
1998.
[RFC2461] Narten, T., Nordmark, E. and W. Simpson, "Neighbor
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Discovery for IP Version 6 (IPv6)", RFC 2461, December
1998.
[RFC2462] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, 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.
Authors' Addresses
Torben Melsen
Ericsson
Faelledvej
Struer DK-7600
Denmark
EMail: Torben.Melsen@ericsson.com
Steven Blake
Ericsson
IP Infrastructure
920 Main Campus Drive
Suite 500
Raleigh, NC 27606
USA
Phone: +1 919 472-9913
EMail: steven.blake@ericsson.com
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