Personal T. Melsen
Internet-Draft Ericsson
Expires: July 14, 2004 January 14, 2004
MAC Forced Forwarding: An ARP proxy method for ensuring traffic
separation between hosts sharing an Ethernet access network
draft-melsen-mac-forced-fwd-00.txt
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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 IP gateway over a shared Ethernet segment.
The mechanism - called "MAC forced forwarding" - implements an ARP
proxy function that in effect directs all upstream traffic to the IP
gateway, also if a station is trying to obtain direct Ethernet
connectivity to another station within the same IP subnet but located
at another end-user premise.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Using Ethernet as Access Network Technology . . . . . . . . . 3
1.2 Solution Characteristics . . . . . . . . . . . . . . . . . . . 4
2. Conventions used in this document . . . . . . . . . . . . . . 5
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Solution Aspects . . . . . . . . . . . . . . . . . . . . . . . 5
4.1 Obtaining the IP and MAC address of the Access Router . . . . 5
4.2 Responding to ARP Requests . . . . . . . . . . . . . . . . . . 6
4.3 Filtering Upstream Traffic . . . . . . . . . . . . . . . . . . 6
5. Access Router Considerations . . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 6
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 7
Intellectual Property and Copyright Statements . . . . . . . . 8
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1. Introduction
The main task of a remote access network is to provide connectivity
between end-user devices and access routers (AR), typically offering
access to IP networks and/or IP-centric applications.
A remote access network scenario 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 end-user premise. The
aggregation network - "the second mile" - performs aggregation and
concentration of end-user traffic.
The subscriber line and the aggregation network are separated by an
Access Node (AN). Thus, the AN constitutes the border between
individual subscriber lines and the common aggregation network.
Traffic to and from end-user devices located at different premises
(i.e. going via different subscriber lines) always go via an AR. This
implies that within the aggregation network, no local traffic paths
exist that circumvents the AR. This aspect enables the network
service provider to let his AR(s) perform security filtering,
policing and charging of all end-user traffic.
In ATM-centric aggregation networks based on DSL technology, the
separation of individual end-users' traffic is an intrinsic feature
caused by the use of ATM permanent virtual circuits (PVCs) between
the end-users' DSL modems and the AR (typically co-located/integrated
with access control functionality in a broadband access server, BAS).
In this case, the Access Node is an ATM-centric DSLAM.
This document, however, is targeting traffic separation for Ethernet-
centric aggregation networks, i.e. where other techniques than ATM
PVCs are deployed to ensure the desired separation of traffic
belonging to individual end-users.
1.1 Using Ethernet as Access Network Technology
A major aspect of using Ethernet as aggregation network technology is
that traffic pertaining to different end-users is conveyed over a
shared broadcast network. To avoid IP routing in the access network,
this Ethernet aggregation network is bridged via Access Nodes to
individual Ethernet networks at the end-users' premises. This
architecture, however, creates by nature the ability to have direct
layer-2 visibility between Ethernet stations (hosts) located at
different end-users' premises. Specifically, hosts located within the
same IP subnet will have this functionality. This not only violates
the requirement of sending all traffic via the IP gateway, it also
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introduces security issues, as a malicious host can attack another
host directly on layer-2.
Existing standardized solutions may be deployed to prevent layer-2
visibility between stations:
o PPP over Ethernet. 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 end-user. Individual traffic streams can be separated in
different VLANs between the AN and the AR.
Both solutions provide layer-2 isolation between end-users, but still
they are not considered optimal for broadband remote access networks.
o PPPoE does not support efficient multicast, one of the major
advantages of using Ethernet as access technology instead of ATM.
Furthermore, the use of PPP seriously limits IP and Ethernet
related QoS mechanisms.
o Using VLANs to separate individual end-users is not appealing,
since that is regarded as requiring cumbersome provisioning.
Furthermore, the basic limit of 4096 VLANs in an Ethernet network
does not present a scalable solution. This limit is removed by
deploying VLAN stacking techniques within the access network, but
this solution only adds to the provisioning complexity.
1.2 Solution Characteristics
The solution proposed in this document has the following main
characteristics:
1. Traffic between individual end-users is isolated over the
Ethernet access network. Traffic always goes between end-user
device and AR, never directly between end-user devices on
different premises.
2. IP addresses are assigned to end-users on an efficient basis.
Specifically, individual IP subnets per end-user network is NOT a
requirement for the solution to function. See RFC 3069 [3] for a
discussion on why this requirement is relevant.
3. IP over Ethernet is used as access protocol, to ensure an
efficient multicast architecture and support for adequate QoS
mechanisms.
4. VLANs are NOT used to separate traffic pertaining to individual
end-users, due to scalability and provisioning issues.
5. The solution works for both dynamically assigned IP addresses
(via DHCP) and statically assigned IP addresses.
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2. 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, RFC 2119 [2] and indicate requirement levels for
compliant implementations.
3. Terminology
Ethernet Access Node (EAN)
The entity interfacing between individual subscriber lines and the
shared access network. E.g. for xDSL access, the EAN is an
Ethernet-centric DSLAM (Digital Subscriber Line Access
Multiplexer).
4. Solution Aspects
The basic property of the solution is to have the EAN ensure that
upstream traffic is always sent to the designated AR, even if the IP
traffic goes between end-users located in the same IP subnet.
The solution may be described as having three major aspects:
1. Initially, the EAN obtains corresponding IP and MAC address of
target AR.
2. The EAN replies with this MAC address to any upstream ARP request
from end-user devices.
3. The EAN filters out any upstream traffic to other MAC addresses
than the target AR.
These aspects are discussed in the following sections.
4.1 Obtaining the IP and MAC address 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 the host uses
DHCP, the AR IP address is dynamically learned by snooping the DHCP
reply towards the host. Otherwise, the AR IP address is
pre-provisioned by the network operator. In both cases, the EAN will
need to resolve 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.
An access network may contain multiple ARs, and different hosts may
be assigned different ARs. This implies that the EAN MUST register
the allowed AR address on a per-user basis.
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4.2 Responding to ARP Requests
If all end-users 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 matches the AR.
However, to comply with requirement 2 of section 1.2, residential
end-users are not assigned individual IP subnets. In other words,
several hosts located at different premises share an IP subnet.
Consequently, if a host wishes to communicate with a host on another
premise, an ARP is issued to obtain the 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 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.
4.3 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 end-users or
malfunctioning hosts may still try to send traffic using other
destination MAC addresses. This traffic MUST be discarded by the EAN.
5. Access Router Considerations
Traffic between hosts within same IP subnet but located at different
premises will always go via an IP Gateway. In this case, the AR will
forward the traffic to the originating network, i.e. down the same
interface from where it was received. This normally results in an
ICMP redirect message, RFC 0792 [4], being sent to the originating
host. To prevent this behavior, the ICMP redirect function MUST be
disabled in the AR.
6. Security Considerations
MAC Forced Forwarding is by nature a security function, ensuring
layer-2 isolation between end-users sharing a broadcast access media.
The solution MUST ensure that only authentic DHCP replies are used in
the dynamic learning of AR addresses. One way is to reject any
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upstream DHCP replies, i.e. replies from end-users.
7. Acknowledgements
Add any acknowledgements.
References
[1] Bradner, S., "The Internet Standards Process -- Revision 3", BCP
9, RFC 2026, October 1996.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] McPherson, D. and B. Dykes, "VLAN Aggregation for Efficient IP
Address Allocation", RFC 3069, February 2001.
[4] Postel, J., "Internet Control Message Protocol", STD 5, RFC 792,
September 1981.
Author's Address
Torben Melsen
Ericsson
Faelledvej
Struer DK-7600
Denmark
EMail: Torben.Melsen@ericsson.com
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