Personal T. Melsen Internet-Draft Ericsson Expires: March 1, 2005 S. Blake Modular Networks August 31, 2004 MAC-Forced Forwarding: A Method for Traffic Separation on an Ethernet Access Network draft-melsen-mac-forced-fwd-03.txt Status of this Memo This document is an Internet-Draft and is subject to all provisions of section 3 of RFC 3667. By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she become aware will be disclosed, in accordance with RFC 3668. 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 March 1, 2005. Copyright Notice Copyright (C) The Internet Society (2004). 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 Melsen & Blake Expires March 1, 2005 [Page 1] Internet-Draft MAC-Forced Forwarding August 2004 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. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1 Access Network Requirements . . . . . . . . . . . . . . . 4 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 Router . 6 3.2 Responding to ARP Requests . . . . . . . . . . . . . . . . 6 3.3 Filtering Upstream Traffic . . . . . . . . . . . . . . . . 7 4. Access Router Considerations . . . . . . . . . . . . . . . . . 7 5. Resiliency Considerations . . . . . . . . . . . . . . . . . . 7 6. Multicast Considerations . . . . . . . . . . . . . . . . . . . 8 7. IPv6 Considerations . . . . . . . . . . . . . . . . . . . . . 8 8. Security Considerations . . . . . . . . . . . . . . . . . . . 8 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 9 10.1 Normative References . . . . . . . . . . . . . . . . . . . . 9 10.2 Informative References . . . . . . . . . . . . . . . . . . . 9 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 10 Intellectual Property and Copyright Statements . . . . . . . . 11 Melsen & Blake Expires March 1, 2005 [Page 2] Internet-Draft MAC-Forced Forwarding August 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 +-----------| +----+ +----+ | Melsen & Blake Expires March 1, 2005 [Page 3] Internet-Draft MAC-Forced Forwarding August 2004 1.1 Access Network Requirements There are two basic requirements that a remote access network 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 Melsen & Blake Expires March 1, 2005 [Page 4] Internet-Draft MAC-Forced Forwarding August 2004 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 [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. The solution proposed in this document avoids these problems. 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 Melsen & Blake Expires March 1, 2005 [Page 5] Internet-Draft MAC-Forced Forwarding August 2004 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 [RFC2131]. 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. 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 Melsen & Blake Expires March 1, 2005 [Page 6] Internet-Draft MAC-Forced Forwarding August 2004 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. 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 Melsen & Blake Expires March 1, 2005 [Page 7] Internet-Draft MAC-Forced Forwarding August 2004 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-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. Melsen & Blake Expires March 1, 2005 [Page 8] Internet-Draft MAC-Forced Forwarding August 2004 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. 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. 10.2 Informative References [RFC1009] Braden, R. and J. Postel, "Requirements for Internet gateways", RFC 1009, June 1987. [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 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 Melsen & Blake Expires March 1, 2005 [Page 9] Internet-Draft MAC-Forced Forwarding August 2004 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 Modular Networks 2 Davis Drive PO Box 12076 Research Triangle Park, NC 27709 USA Phone: +1 919 765-0027 x2016 EMail: slblake@modularnet.com Melsen & Blake Expires March 1, 2005 [Page 10] Internet-Draft MAC-Forced Forwarding August 2004 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights 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; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat 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 implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Disclaimer of Validity This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Copyright Statement Copyright (C) The Internet Society (2004). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Melsen & Blake Expires March 1, 2005 [Page 11]