Personal T. Melsen
Internet-Draft S. Blake
Expires: August 2004 Ericsson
February 2004
MAC Forced Forwarding: An ARP proxy method for ensuring traffic
separation between hosts sharing an Ethernet access network
draft-melsen-mac-forced-fwd-01.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that other
groups may also distribute working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at http://
www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on August 13, 2004.
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.
Melsen & Blake Expires August 13, 2004 [Page 1]
Internet-Draft MAC Forced Forwarding February 2004
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Using Ethernet as an Access Network Technology . . . . . . . . 4
1.2 Solution Characteristics . . . . . . . . . . . . . . . . . . . 5
2. Conventions used in this document . . . . . . . . . . . . . . 5
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Solution Aspects . . . . . . . . . . . . . . . . . . . . . . . 6
4.1 Obtaining the IP and MAC addresses of the Access Router . . . 6
4.2 Responding to ARP Requests . . . . . . . . . . . . . . . . . . 6
4.3 Filtering Upstream Traffic . . . . . . . . . . . . . . . . . . 7
5. Access Router Considerations . . . . . . . . . . . . . . . . . 7
6. Resiliency Considerations . . . . . . . . . . . . . . . . . . 7
7. Multicast Considerations . . . . . . . . . . . . . . . . . . . 8
8. IPv6 Considerations . . . . . . . . . . . . . . . . . . . . . 8
9. Security Considerations . . . . . . . . . . . . . . . . . . . 9
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 10
Intellectual Property and Copyright Statements . . . . . . . . 11
Melsen & Blake Expires August 13, 2004 [Page 2]
Internet-Draft MAC Forced Forwarding February 2004
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 +-----------| +----+
+----+ |
It is often strongly desired 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. This enables the access network service provider to use the
Melsen & Blake Expires August 13, 2004 [Page 3]
Internet-Draft MAC Forced Forwarding February 2004
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 DSLAM.
This document, however, targets traffic separation for Ethernet-based
access networks, i.e., where techniques other than ATM PVCs are
deployed to ensure the desired separation of traffic belonging to
individual customer hosts.
1.1 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. In this
architecture there is 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 will
have this functionality. This not only violates the requirement to
send all traffic via the AR, it also introduces security issues, as
malicious end-users 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. 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. 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
still 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 as an access technology (instead of
ATM).
Melsen & Blake Expires August 13, 2004 [Page 4]
Internet-Draft MAC Forced Forwarding February 2004
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 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 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. See RFC 3069 [3] for a discussion on why this
requirement is relevant.
3. IP over Ethernet is used as the access protocol to ensure
efficient multicast support (see Section 7).
4. VLANs are NOT used to separate traffic pertaining to individual
customer hosts, due to scalability and provisioning issues.
5. The solution works for both dynamically assigned IP addresses
(via DHCP) and statically assigned IP addresses.
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 interconnecting individual subscriber lines and the
shared aggregation network. e.g., for xDSL access, the EAN is an
Ethernet-centric DSLAM (Digital Subscriber Line Access
Multiplexer). 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.
Melsen & Blake Expires August 13, 2004 [Page 5]
Internet-Draft MAC Forced Forwarding February 2004
4. 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 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.
4.1 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.
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.
4.2 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 1.2, residential
customer networks are not 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 this way, the
Melsen & Blake Expires August 13, 2004 [Page 6]
Internet-Draft MAC Forced Forwarding February 2004
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 customers 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 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, RFC 792 [4], being sent
to the originating host. To prevent this behavior, the ICMP redirect
function for aggregation network interfaces MUST be disabled in the
AR.
6. 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 [5].
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 sustained 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. EANs used in access networks using dynamic
IP address assignment MUST employ resilient storage of their state
database to permit timely restoration of connectivity in the event of
a transient EAN failure.
Melsen & Blake Expires August 13, 2004 [Page 7]
Internet-Draft MAC Forced Forwarding February 2004
7. 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 customer-originated multicast streams is more
complicated. The access network must avoid reflecting a multicast
packet received on a subscriber line back out onto that line (to
suppress duplicate packet reception on hosts at that customer
premise). The EAN could bridge multicast frames onto (listening)
subscriber lines excluding the originating one, but this would
violate the requirement that all customer-originated traffic be
forwarded via an AR. Alternatively, the multicast packets could be
sent upstream to the appropriate AR, which would forward them back
out onto the same subnet. This would require non-standard behavior
on the part of the AR, and would require the EAN to filter downstream
multicast traffic based on source IP address (not delivering
multicast packets to the subscriber line serving the originating
host).
8. 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 [6] 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 [7].
Furthermore, there is no practical deployment experience using a MAC
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.
Melsen & Blake Expires August 13, 2004 [Page 8]
Internet-Draft MAC Forced Forwarding February 2004
9. 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.
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.
10. Acknowledgements
The authors would like to thank Ulf Jonsson, Thomas Narten, James
Carlson, Rolf Engstrand, and Johan Kolhi for their helpful comments.
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.
[5] 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.
[6] Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery for
IP Version 6 (IPv6)", RFC 2461, December 1998.
[7] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
Melsen & Blake Expires August 13, 2004 [Page 9]
Internet-Draft MAC Forced Forwarding February 2004
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
Melsen & Blake Expires August 13, 2004 [Page 10]
Internet-Draft MAC Forced Forwarding February 2004
Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
Full Copyright Statement
Copyright (C) The Internet Society (2004). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assignees.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
Melsen & Blake Expires August 13, 2004 [Page 11]
Internet-Draft MAC Forced Forwarding February 2004
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
Melsen & Blake Expires August 13, 2004 [Page 12]