Network Working Group H. Jeon Request for Comments: 5692 S. Jeong Category: Standards Track ETRI M. Riegel NSN October 2009
Transmission of IP over Ethernet over IEEE 802.16 Networks
Abstract
This document describes the transmission of IPv4 over Ethernet, as well as IPv6 over Ethernet, in an access network deploying the IEEE 802.16 cellular radio transmission technology. The Ethernet on top of IEEE 802.16 is realized by bridging connections that IEEE 802.16 provides between a base station and its associated subscriber stations. Due to the resource constraints of radio transmission systems and the limitations of the IEEE 802.16 Media Access Control (MAC) functionality for the realization of an Ethernet, the transmission of IP over Ethernet over IEEE 802.16 may considerably benefit by adding IP-specific support functions in the Ethernet over IEEE 802.16 while maintaining full compatibility with standard IP over Ethernet behavior.
Status of This Memo
This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the document authors. All rights reserved.
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This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English.
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Table of Contents
1. Introduction ....................................................4 2. Requirements ....................................................4 3. Terminology .....................................................4 4. The IEEE 802.16 Link Model ......................................4 4.1. Connection-Oriented Air Interface ..........................4 4.2. MAC Addressing in IEEE 802.16 ..............................5 4.3. Unidirectional Broadcast and Multicast Support .............6 4.4. IEEE 802.16 Convergence Sublayer for IP over Ethernet ......6 5. Ethernet Network Model for IEEE 802.16 ..........................6 5.1. IEEE 802.16 Ethernet Link Model ............................7 5.2. Ethernet without Native Broadcast and Multicast Support ....8 5.3. Network-Side Bridging Function .............................8 5.4. Segmenting the Ethernet into VLANs .........................9 6. Transmission of IP over Ethernet over IEEE 802.16 Link ..........9 6.1. Generic IP over Ethernet Network Scenario ..................9 6.2. Transmission of IP over Ethernet ..........................10 6.2.1. IPv4-over-Ethernet Packet Transmission .............10 6.2.2. IPv6-over-Ethernet Packet Transmission .............11 6.2.3. Maximum Transmission Unit ..........................11 6.2.4. Prefix Assignment ..................................11 7. Operational Enhancements for IP over Ethernet over IEEE 802.16 .........................................................12 7.1. IP Multicast and Broadcast Packet Processing ..............12 7.1.1. Multicast Transmission Considerations ..............12 7.1.2. Broadcast Transmission Considerations ..............12 7.2. DHCP Considerations .......................................13 7.3. Address Resolution Considerations .........................13 8. Public Access Recommendations ..................................14 9. Security Considerations ........................................15 10. Acknowledgments ...............................................16 11. References ....................................................16 11.1. Normative References .....................................16 11.2. Informative References ...................................17 Appendix A. Multicast CID Deployment Considerations ..............19 Appendix B. Centralized vs. Distributed Bridging ................19
IEEE 802.16 [802.16] specifies a fixed-to-mobile, broadband wireless access system.
The IEEE 802.16 standard defines a packet CS (Convergence Sublayer) for interfacing with specific packet-based protocols as well as a generic packet CS (GPCS) to provide an upper-layer, protocol- independent interface. This document describes transmission of IPv4 and IPv6 over Ethernet via the Ethernet-specific part of the packet CS as well as of the GPCS in the access network based on IEEE 802.16.
Ethernet has been originally architected and designed for a shared medium while the IEEE 802.16 uses a point-to-multipoint architecture like other cellular radio transmission systems. Hence, Ethernet on top of IEEE 802.16 is realized by bridging between IEEE 802.16 radio connections that connect a BS (Base Station) and its associated SSs (Subscriber Stations).
Under the resource constraints of radio transmission systems and the particularities of the IEEE 802.16 for the realization of Ethernet, it makes sense to add IP-specific support functions in the Ethernet layer above IEEE 802.16 while maintaining full compatibility with standard IP over Ethernet behavior.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].
The IEEE 802.16 MAC establishes connections between a BS and its associated SSs for the transfer of user data over the air. Each of these connections realizes an individual service flow, which is identified by a 16-bit Connection Identifier (CID) number and has a defined Quality of Service (QoS) profile.
Multiple connections can be established between a BS and an SS, each with its particular QoS class and direction. Although the BS and all
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the SSs are associated with unique 48-bit MAC addresses, packets going over the air are only identified in the IEEE 802.16 MAC header by the CID number of the particular connection. The connections are established by MAC management messages between the BS and the SS during network entry or also later on demand.
Each SS has a unique 48-bit MAC address; the 48-bit MAC address is used during the initial ranging process for the identification of the SS and may be verified by the succeeding PKM (Privacy Key Management) authentication phase. Out of the successful authentication, the BS establishes and maintains the list of attached SSs based on their MAC addresses, purely for MAC management purposes.
While MAC addresses are assigned to all the BSs as well as the SSs, the forwarding of packets over the air is only based on the CID value of the particular connection in the IEEE 802.16 MAC header. Not relying on the MAC addresses in the payload for reception of a radio frame allows for the transport of arbitrary source and destination MAC addresses in Ethernet frames between an SS and its BS. This is required for bridging Ethernet frames toward an SS that is attached to a bridge connected to another network.
Due to the managed assignment of the service flows and associated CID values to individual SSs, the BS is able to bundle all unicast connections belonging to a particular SS into a single link on the network side, as shown in Figure 1, so that it provides a single layer-2 link between the SS and its associated wired link on the network side.
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4.3. Unidirectional Broadcast and Multicast Support
Current IEEE 802.16 [802.16] does not support bidirectional native broadcast and multicast for IP packets. While downlink connections can be used for multicast transmission to a group of SSs as well as unicast transmission from the BS to a single SS, uplink connections from the SSs to the BS provide only unicast transmission capabilities. Furthermore, the use of multicast CIDs for realizing downlink multicast transmissions is not necessarily preferable due to the reduced transmission efficiency of multicast CIDs for small multicast groups. Appendix A provides more background information about the issues arising with multicast CIDs in IEEE 802.16 systems.
MBS (Multicast and Broadcast Service), as specified in IEEE 802.16, also does not cover IP broadcast or multicast data because MBS is invisible to the IP layer.
4.4. IEEE 802.16 Convergence Sublayer for IP over Ethernet
IEEE 802.16 provides two solutions to transfer Ethernet frames over IEEE 802.16 MAC connections.
The packet CS is defined for handling packet-based protocols by classifying higher-layer packets depending on the values in the packet header fields and assigning the packets to the related service flow. The packet CS comprises multiple protocol-specific parts to enable the transmission of different kinds of packets over IEEE 802.16. The Ethernet-specific part of the packet CS supports the transmission of Ethernet by defining classification rules based on Ethernet header information.
The GPCS (Generic Packet Convergence Sublayer) may be used as an alternative to transfer Ethernet frames over IEEE 802.16. The GPCS does not define classification rules for each kind of payload but relies on higher-layer functionality outside of the scope of IEEE 802.16 to provide the assignment of packets to particular service flows.
Like in today's wired Ethernet networks, bridging is required to implement connectivity between more than two devices. In IEEE 802.16, the point-to-point connections between SSs and the BS can be bridged so that Ethernet is realized over the IEEE 802.16 access network.
To realize Ethernet on top of IEEE 802.16, all the point-to-point connections belonging to an SS MUST be connected to a network-side bridging function, as shown in Figure 2. This is equivalent to today's switched Ethernet with twisted pair wires or fibres connecting the hosts to a bridge ("Switch").
The network-side bridging function can be realized either by a single centralized network-side bridge or by multiple interconnected bridges, preferably arranged in hierarchical order. The single centralized network-side bridge allows best control of the broadcasting and forwarding behavior of the Ethernet over IEEE 802.16. Appendix B explains the issues of a distributed bridging architecture when no assumptions about the location of the access router can be made.
The BS MUST forward all the service flows belonging to one SS to one port of the network-side bridging function. No more than one SS MUST be connected to one port of the network-side bridging function. The separation method for multiple links on the connection between the BS and the network-side bridging function is out of scope for this document. Either layer-2 transport or layer-3 tunneling may be used.
If the Ethernet over IEEE 802.16 is extended to multiple end stations behind the SS (i.e., SS#4 in the figure below), then the SS SHOULD support bridging according to [802.1D] and its amendment [802.16k], a.k.a. subscriber-side bridge, between all its subscriber-side ports and the IEEE 802.16 air link.
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------------------------ IP Link --------------------------
5.2. Ethernet without Native Broadcast and Multicast Support
Current IEEE 802.16 does not define broadcast and multicast of Ethernet frames. Hence, Ethernet frames that are broadcast or multicast SHOULD be replicated and then carried via unicast transport connections on the IEEE 802.16 access link. The network-side bridging function performs the replication and forwarding for Ethernet broadcast and multicast over the IEEE 802.16 radio links.
The network-side bridging function MUST create a new radio-side port whenever a new SS attaches to any of the BSs of the network, or it MUST remove a radio-side port when an associated SS detaches from the BSs. The method for managing the port on the network-side bridging function may depend on the protocol used for establishing multiple links on the connection between the BS and the network-side bridge. The port-managing method is out of scope for this document.
The network-side bridging function MUST be based on [802.1D] and its amendment [802.16k] to interconnect the attached SSs and pass Ethernet frames between the point-to-point connections associated with the attached SSs. However, to enhance the IEEE 802.16 Ethernet link model by avoiding broadcast or multicast packet flooding,
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additional IP-specific functionalities MAY be provided by the network-side bridging function in addition to the mandatory functions, according to Section 5.1 of [802.1D].
It is possible to restrict the size and coverage of the broadcast domain by segmenting the Ethernet over IEEE 802.16 into VLANs and grouping subsets of hosts into particular VLANs with each VLAN representing an IP link. Therefore, the network-side bridging function MAY be enabled to support VLANs according to [802.1Q] by assigning and handling the VLAN-IDs on the virtual bridge ports.
If an SS is directly connected to a subscriber-side bridge supporting VLANs, the port associated with such an SS MAY be enabled as trunk port. On trunk ports, Ethernet frames are forwarded in the [802.1Q] frame format.
6. Transmission of IP over Ethernet over IEEE 802.16 Link
The generic IP over Ethernet network scenario assumes that all hosts are residing on the same link. It enables the hosts to directly communicate with each other without detouring. There can be multiple Access Routers (ARs) on the link, and these may reside both on the subscriber side as well as on the network side, as shown in Figure 3.
[RFC0894] defines the transmission of IPv4 packets over Ethernet networks. It contains the specification of the encapsulation of the IPv4 packets into Ethernet frames as well as rules for mapping IP addresses onto Ethernet MAC addresses. Hosts transmitting IPv4 over Ethernet packets over the IEEE 802.16 MUST follow the operations specified in [RFC0894].
[RFC2464] defines transmission of IPv6 packets over Ethernet networks, which includes an encapsulation of IPv6 packets into Ethernet frames; that document includes rules for mapping IPv6 addresses to Ethernet addresses (i.e., MAC addresses). Hosts transmitting IPv6-over-Ethernet packets over IEEE 802.16 MUST follow the operations specified in [RFC2464].
6.2.2.1. Router Discovery, Prefix Discovery and Parameter Discovery
Router Discovery, Prefix Discovery, and Parameter Discovery procedures are achieved by receiving Router Advertisement messages. However, periodic Router Advertisement messages can waste radio resource and disturb SSs in dormant mode in IEEE 802.16. Therefore, the AdvDefaultLifetime and MaxRtrAdvInterval SHOULD be overridden with high values specified in Section 8.3 in [RFC5121].
When stateful address autoconfiguration is required, the stateful address configuration according to [RFC3315] MUST be performed. In this case, an AR supports a Dynamic Host Configuration Protocol for IPv6 (DHCPv6) server or relay function.
When stateless address autoconfiguration is required, the stateless address configuration according to [RFC4862] and [RFC4861] MUST be performed.
[RFC2460] mandates 1280 bytes as a minimum Maximum Transmission Unit (MTU) size for the link layer and recommends at least 1500 bytes for IPv6 over Ethernet transmission. [RFC0894] also specifies 1500 bytes as a maximum length of IPv4 over Ethernet. Therefore, the default MTU of IPv6 packets and IPv4 packets on an Ethernet over IEEE 802.16 link MUST be 1500 bytes.
As Ethernet over IEEE 802.16 may only build a part of a larger Ethernet of arbitrary structure, any kind of prefix assignment that is feasible for Ethernet is applicable for Ethernet over IEEE 802.16
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as well. The same IPv4 prefix and the same set of IPv6 prefixes MAY be assigned to all hosts attached to the Ethernet over IEEE 802.16 to make best usage of Ethernet behavior. Sharing the prefix means locating all hosts on the same subnetwork.
7. Operational Enhancements for IP over Ethernet over IEEE 802.16
This section presents operational enhancements in order to improve network performance and radio resource efficiency for transmission of IP packets over Ethernet over IEEE 802.16 networks.
All multicast and multicast control messages can be processed in the network-side bridging function, according to [RFC4541]. Broadcasting messages to all radio-side side ports SHOULD be prevented.
Further information on the prevention of multicasting or broadcasting messages to all radio-side ports is given in the following sections.
Usually, bridges replicate the IP multicast packets and forward them into all of its available ports except the incoming port. As a result, the IP multicast packets would be transmitted over the air -- even to hosts that have not joined the corresponding multicast group. To allow bridges to handle IP multicast more efficiently, the IP multicast membership information should be propagated between bridges.
In the IEEE 802.16 Ethernet link model in Section 5.1, the network- side bridging function can process all multicast data and multicast control messages according to [RFC4541] in order to maintain IP multicast membership states and forward IP multicast data to only ports suitable for the multicast group.
The ordinary bridge floods the IP broadcast packets out of all connected ports except the port on which the packet was received. This behavior is not appropriate with scarce resources and dormant- mode hosts in a wireless network such as an access network based on IEEE 802.16.
The network-side bridging function in the IEEE 802.16 Ethernet link model SHOULD flood all IP broadcast packets except ARP-, DHCPv4-, and Internet Group Management Protocol (IGMP)-related traffic.
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IGMP-related broadcast packets can be forwarded according to the [RFC4541]. ARP-related broadcast SHOULD be processed as specified in Section 7.3.
In the IPv4-over-Ethernet case, DHCPv4 clients may send DHCPDISCOVER and DHCPREQUEST messages with the BROADCAST bit set to request the DHCPv4 server to broadcast its DHCPOFFER and DHCPACK messages. The network-side bridging function SHOULD filter these broadcast DHCPOFFER and DHCPACK messages and forward the broadcast messages only to the host defined by the client hardware address in the chaddr information element.
Alternatively, the DHCP Relay Agent Information option (option 82) [RFC3046] MAY be used to avoid DHCPv4 broadcast replies. Option 82 consists of two types of sub-options: Circuit ID and Remote ID. The DHCPv4 Relay Agent is usually located on the network-side bridging function as the Layer 2 DHCPv4 Relay Agent. The port number of the network-side bridging function can be used as Circuit ID, and Remote ID may be left unspecified. Note that using option 82 requires DHCPv4 servers that are aware of option 82.
In the IPv6-over-Ethernet case, DHCPv6 clients use their link-local addresses and the All_DHCP_Relay_Agents_and_Servers multicast address to discover and communicate with DHCPv6 servers or Relay Agents on their link. Hence, DHCPv6-related packets are unicasted or multicasted. The network-side bridging function SHOULD handle the DHCPv6-related unicast packets based on [802.1D] and SHOULD transmit the DHCPv6-related multicast packets as specified in Section 7.1.1.
In the IPv4-over-Ethernet case, ARP Requests are usually broadcasted to all hosts on the same link in order to resolve an Ethernet MAC address, which would disturb all hosts on the same link. Proxy ARP provides the function in which a device on the same link as the hosts answers ARP Requests instead of the remote host. When transmitting IPv4 packets over the IEEE 802.16 Ethernet link, the Proxy ARP mechanism is used by the network-side bridging function to avoid broadcasting ARP Requests over the air.
The network-side bridging function SHOULD maintain an ARP cache large enough to accommodate ARP entries for all its serving SSs. The ARP cache SHOULD be updated by any packets including ARP Requests from SSs in the same way the normal layer-2 bridging device is updating its Filtering Database according to [802.1D].
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Upon receiving an ARP Request from an SS, the network-side bridging function SHOULD unicast an ARP Reply back to the SS with the Ethernet address of the target host, provided that the target address matches an entry in the ARP cache. However, in case of receiving an ARP Request from a host behind a subscriber-side bridge, the network-side bridging function SHOULD discard the request if the target host is also behind the same subscriber-side bridge, i.e., on the same port of the network-side bridge. Otherwise, the ARP Request MAY be flooded. The network-side bridging function SHOULD silently discard any received self-ARP Request.
In the IPv6-over-Ethernet case, Neighbor Solicitation messages are multicasted to the solicited-node multicast address for the address resolution, including a duplicate address detection. The solicited- node multicast address facilitates the efficient querying of hosts without disturbing all hosts on the same link. The network-side bridging function SHOULD transmit the Neighbor Solicitation messages specified in Section 7.1.1.
In the public access scenario, direct communication between nodes is restricted because of security and accounting issues. Figure 4 depicts the public access scenario.
In this scenario, the AR is connected to a network-side bridge. The AR MAY perform security filtering, policing, and accounting of all traffic from hosts, e.g., like an NAS (Network Access Server).
If the AR functions as the NAS, all the traffic from SSs SHOULD be forwarded to the AR, not bridged at the network-side bridging function -- even in the case of traffic between SSs served by the same AR. The bridge SHOULD forward upstream traffic from hosts toward the AR but MUST perform normal bridging operation for downstream traffic from the AR and MUST bridge SEcure Neighbor Discovery (SEND) [RFC3971] messages to allow applicability of security schemes.
In the IPv4-over-Ethernet case, MAC-Forced Forwarding (MAC-FF) [RFC4562] can be used for the public access network to ensure that traffic from all hosts is always directed to the AR. The MAC-FF is performed in the network-side bridging function; thus, the bridge filters broadcast ARP Requests from all the hosts and responds to the ARP Requests with an Ethernet MAC address of the AR.
In the IPv6-over-Ethernet case, unique IPv6 prefixes per SS can be assigned because doing so forces all IPv6 packets from SSs to be transferred to the AR and thus results in layer-3 separation between
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SSs. Alternatively, common IPv6 prefixes can be assigned to all SSs served by the same AR in order to exploit the efficient multicast support of Ethernet link in the network side. In this case, a Prefix Information Option (PIO) [RFC4861] carrying the common IPv6 prefixes SHOULD be advertised with the On-link flag (L-Flag) reset so that it is not assumed that the addresses matching the prefixes are available on-link.
The AR should relay packets between SSs within the same AR.
This recommendation does not introduce new vulnerabilities to IPv4 and IPv6 specifications or operations. The security of the IEEE 802.16 air interface between SSs and BS is the subject of [802.16], which provides the capabilities of admission control and ciphering of the traffic carried over the air interface. A Traffic Encryption Key (TEK) is generated by the SS and BS on completion of successful mutual authentication and is used to secure the air interface.
The IEEE 802.16 Ethernet link model described in Section 5.1 represents a bridged (switched) Ethernet architecture with point-to- point links between the SS and its bridge port. Even though the bridged Ethernet model prevents messaging between SSs on the same link without passing through the bridge, it is still vulnerable, e.g., by malicious reconfiguration of the address table of the bridge
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in the learning process. This recommendation does not cause new security issues beyond those that are already known for the bridged Ethernet architecture. For example, link security mechanisms according to [802.1AE] can be used on top of this recommendation to resolve the security issues of the bridged Ethernet.
As the generic IP over Ethernet network using IEEE 802.16 emulates a standard Ethernet link, existing IPv4 and IPv6 security mechanisms over Ethernet can still be used. The public access network using IEEE 802.16 can secure isolation of each of the upstream links between hosts and AR by adopting SEcure Neighbor Discovery (SEND) [RFC3971] for securing neighbor discovery processes.
[802.16] IEEE Std 802.16-2009, "IEEE Standard for Local and metropolitan area networks, Part 16: Air Interface for Fixed Broadband Wireless Access Systems", May 2009.
[802.16k] IEEE Std 802.16k-2007, "IEEE Standard for Local and metropolitan area networks, Media Access Control (MAC) Bridges, Amendment 5: Bridging of IEEE 802.16", March 2007.
[802.1D] IEEE Std 802.1D-2004, "IEEE Standard for Local and metropolitan area networks, Media Access Control (MAC) Bridges", June 2004.
[802.1Q] IEEE Std 802.1Q-2005, "IEEE Standard for Local and metropolitan area networks, Virtual Bridged Local Area Networks", May 2005.
[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.
[RFC0894] Hornig, C., "Standard for the transmission of IP datagrams over Ethernet networks", STD 41, RFC 894, April 1984.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, March 1997.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998.
[RFC2464] Crawford, M., "Transmission of IPv6 Packets over Ethernet Networks", RFC 2464, December 1998.
[RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M. Carney, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3315, July 2003.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, September 2007.
[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless Address Autoconfiguration", RFC 4862, September 2007.
[RFC5121] Patil, B., Xia, F., Sarikaya, B., Choi, JH., and S. Madanapalli, "Transmission of IPv6 via the IPv6 Convergence Sublayer over IEEE 802.16 Networks", RFC 5121, February 2008.
[802.1AE] IEEE Std 802.1AE-2006, "IEEE Standard for Local and metropolitan area networks Media Access Control (MAC) Security", August 2006.
[RFC3046] Patrick, M., "DHCP Relay Agent Information Option", RFC 3046, January 2001.
[RFC3971] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure Neighbor Discovery (SEND)", RFC 3971, March 2005.
[RFC4541] Christensen, M., Kimball, K., and F. Solensky, "Considerations for Internet Group Management Protocol (IGMP) and Multicast Listener Discovery (MLD) Snooping Switches", RFC 4541, May 2006.
[RFC4562] Melsen, T. and S. Blake, "MAC-Forced Forwarding: A Method for Subscriber Separation on an Ethernet Access Network", RFC 4562, June 2006.
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[RFC5154] Jee, J., Madanapalli, S., and J. Mandin, "IP over IEEE 802.16 Problem Statement and Goals", RFC 5154, April 2008.
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Appendix A. Multicast CID Deployment Considerations
Multicast CIDs are a highly efficient means to distribute the same information concurrently to multiple SSs under the same BS. However, the deployment of multicast CIDs for multicast or broadcast data services suffers from the following drawbacks.
A drawback of multicast CIDs for Ethernet over IEEE 802.16 is the unidirectional nature of multicast CIDs. While it is possible to multicast information downstream to a number of SSs in parallel, there are no upstream multicast connections. In the upstream direction, unicast CIDs have to be used for sending multicast messages over the air to the BS, requiring a special multicast forwarding function for sending the information back to the other SSs on a multicast CID. While similar in nature to a bridging function, there is no appropriate forwarding model available. [802.1D] cannot take advantage of the multicast CIDs because it relies on unicast connections or bidirectional broadcast connections.
A further drawback of deploying multicast CIDs for distributing broadcast control messages, like ARP Requests, is the inability to prevent the waking up of dormant-mode SSs by messages not aimed for them. Whenever a message is sent over a multicast CID, all associated stations have to power up and receive and process the message. While this behavior is desirable for multicast and broadcast traffic, it is harmful for link-layer broadcast control messages aimed for a single SS, like an ARP Request. All other SSs are wasting scarce battery power for receiving, decoding, and discarding the message. Low power consumption is an extremely important aspect in a wireless communication.
Furthermore, it should be kept in mind that multicast CIDs are only efficient for a large number of subscribed SSs in a cell. Due to incompatibility with advanced radio-layer algorithms based on feedback information from the receiver side, multicast connections require much more radio resources for transferring the same information as unicast connections.
This specification introduces a network-side bridging function, which can be realized either by a centralized device or by multiple interconnected bridges in a distributed manner. One common implementation of the distributed model is the scenario where a bridge is directly attached to the BS, such that the interface between BS and bridging function becomes a software interface within the operation system of the BS/bridge device.
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The operational enhancements described in Section 7 of this document are based on the availability of additional information about all the hosts attached to the Ethernet. Flooding all ports of the bridge can be avoided when a priori information is available to determine the port to which an Ethernet frame has to be delivered.
Best performance can be reached by a centralized database containing all information about the hosts attached to the Ethernet. A centralized database can be established by either a centralized bridge device or a hierarchical bridging structure with dedicated uplink and downlink ports like in the public access case, where the uppermost bridge is able to retrieve and maintain all the information.
As the generic case of the IP over Ethernet over IEEE 802.16 link model does not make any assumptions about the location of the AR (an AR may eventually be attached to an SS), a centralized bridging system is recommended for the generic case. In the centralized system, every connection over the air of a link should be attached to a single centralized bridge.
A distributed bridging model is appropriate, in particular, for the public access mode, where Ethernet frames, which do not have entries in the bridge behind the BS, are sent upstream until finally reaching a bridge that has an entry for the destination MAC address.
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Authors' Addresses
Hongseok Jeon Electronics Telecommunications Research Institute 161 Gajeong-dong, Yuseong-gu Daejeon, 305-350 KOREA