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 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 July 14, 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 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. Melsen Expires July 14, 2004 [Page 1] Internet-Draft MAC Forced Forwarding January 2004 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 Melsen Expires July 14, 2004 [Page 2] Internet-Draft MAC Forced Forwarding January 2004 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 Melsen Expires July 14, 2004 [Page 3] Internet-Draft MAC Forced Forwarding January 2004 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. Melsen Expires July 14, 2004 [Page 4] Internet-Draft MAC Forced Forwarding January 2004 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. Melsen Expires July 14, 2004 [Page 5] Internet-Draft MAC Forced Forwarding January 2004 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 Melsen Expires July 14, 2004 [Page 6] Internet-Draft MAC Forced Forwarding January 2004 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 Melsen Expires July 14, 2004 [Page 7] Internet-Draft MAC Forced Forwarding January 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. 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