16ng Working Group                                   S. Madanapalli, Ed.
Internet-Draft                                                 LogicaCMG
Intended status: Informational                          October 10, 2006
Expires: April 13, 2007

         Analysis of IPv6 Link Models for 802.16 based Networks

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Copyright Notice

   Copyright (C) The Internet Society (2006).


   This document provides different IPv6 link models that are suitable
   for 802.16 based networks and provides analysis of various
   considerations for each link model and the applicability of each link
   model under different deployment scenarios.  This document is result
   of a Design Team that was formed to analyze the IPv6 link models for
   802.16 based networks.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  IPv6 Link Models for 802.16 based Networks . . . . . . . . . .  4
     3.1.  Shared IPv6 Prefix Link Model  . . . . . . . . . . . . . .  4
       3.1.1.  Prefix Assignment  . . . . . . . . . . . . . . . . . .  5
       3.1.2.  Address Autoconfiguration  . . . . . . . . . . . . . .  5
       3.1.3.  Duplicate Address Detection  . . . . . . . . . . . . .  5
       3.1.4.  Considerations . . . . . . . . . . . . . . . . . . . .  6
       3.1.5.  Applicability  . . . . . . . . . . . . . . . . . . . .  7
     3.2.  Point-to-point Link Model  . . . . . . . . . . . . . . . .  7
       3.2.1.  Prefix Assignment  . . . . . . . . . . . . . . . . . .  8
       3.2.2.  Address Autoconfiguration  . . . . . . . . . . . . . .  8
       3.2.3.  Considerations . . . . . . . . . . . . . . . . . . . .  8
       3.2.4.  Applicability  . . . . . . . . . . . . . . . . . . . . 10
     3.3.  Ethernet Like Link Model . . . . . . . . . . . . . . . . . 10
       3.3.1.  Prefix Assignment  . . . . . . . . . . . . . . . . . . 11
       3.3.2.  Address Autoconfiguration  . . . . . . . . . . . . . . 11
       3.3.3.  Duplicate Address Detection  . . . . . . . . . . . . . 11
       3.3.4.  Considerations . . . . . . . . . . . . . . . . . . . . 11
       3.3.5.  Applicability  . . . . . . . . . . . . . . . . . . . . 12
   4.  Renumbering  . . . . . . . . . . . . . . . . . . . . . . . . . 12
   5.  Effect on Dormant Mode . . . . . . . . . . . . . . . . . . . . 13
   6.  SeND Support . . . . . . . . . . . . . . . . . . . . . . . . . 13
   7.  Conclusions and Relevant Link Models . . . . . . . . . . . . . 13
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
   11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 14
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 16
   Intellectual Property and Copyright Statements . . . . . . . . . . 17

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1.  Introduction

   802.16 [1] [2] is a connection oriented access technology for the
   last mile without bi-directional native multicast support. 802.16 has
   only downlink multicast support and there is no mechanisms defined
   for mobile stations to be able to send multicast packets that can be
   mapped to downlink multicast connection.  This could be a problem for
   IP protocols (e.g.  ARP, IPv6 ND) that traditionally assume the
   availability of multicast at the link layer.  This is further
   complicated by the definition of commercial network models like
   WiMAX, which defines the WiMAX transport connection that extend the
   802.16b MAC transport connection all the way to an access router by
   using a tunnel between the base station and the access router.  This
   leads to multiple ways of deploying IP over 802.16 based networks.

   This document looks at various considerations in selecting a link
   model for 802.16 based networks and provides an analysis of the
   various possible link models.

2.  Terminology

   Access Router (AR): An entity that performs an IP routing function to
   provide IP connectivity for Mobile Stations.  In WiMAX Networks, the
   AR is an Access Service Network Gateway.

   Access Service Network (ASN): A complete set of network functions
   needed to provide radio access to a Mobile Station.

   Base Station (BS): A generalized equipment sets providing
   connectivity, management, and control of the subscriber station (SS).

   Dormant Mode: A state in which a mobile station restricts its ability
   to receive normal IP traffic by reducing monitoring of radio
   channels.  This allows the mobile station to save power and reduces
   signaling load on the network.  In the dormant mode, the MS is only
   listening at scheduled intervals to the paging channel.  The network
   (e.g. the AR) maintains state about an MS which has transitioned to
   dormant mode and can page it when needed.

   IP Link: This has been defined in [3] as below.
      - a communication facility or medium over which nodes can
      communicate at the link layer, i.e., the layer immediately below
      IP.  Examples are Ethernets (simple or bridged), PPP links, X.25,
      Frame Relay, or ATM networks as well as Internet (or higher) layer
      "tunnels", such as tunnels over IPv4 or IPv6 itself.

   Mobile Station (MS): An end-user equipment that provides connectivity

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   to 802.16 based networks.

3.  IPv6 Link Models for 802.16 based Networks

   This section discusses various IPv6 link models for 802.16 based
   networks and provides their operational considerations in practical
   deployment scenarios.

3.1.  Shared IPv6 Prefix Link Model

   The following figures illustrates high level view of the link model
   using a shared prefix for an IPv6 link wherein one more prefixes
   advertised on the link would be used by all nodes attached to the
   IPv6 link.

        | MS1 |-----+
        +-----+     |
        +-----+     |     +-----+          +--------+
        | MS2 |-----+-----| BS1 |----------|   AR   |-------[Internet]
        +-----+     |     +-----+          +--------+
           .        |           ____________
           .        |          ()__________()
        +-----+     |             L2 Tunnel
        | MSn |-----+

               Figure 1. Shared IPv6 Prefix Link Model

   The above figure shows the case where the BS and AR exist as separate
   entities.  In this case a tunnel exists between the BS and AR per MS

   In this link model, the link between the MS and the AR at the IPv6
   layer is viewed as a shared link and the lower layer link between the
   MS and BS is a point-to-point link.  This point-to-point link between
   the MS and BS is extended all the way to the AR when the granularity
   of the tunnel between the BS and AR is on per MS basis.  This is
   illustrated in the following figure below.

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        +----+                                     +----+
        |    |      IPv6 (Shared link)             |    |
        | L3 |=====================================|    |
        |    |                                     |    |
        |----|   PTP conn. +----+   L2 Tunnel      | AR |---[Internet]
        | L2 |-------------| BS |==================|    |
        |    |             |    |                  |    |
        +----+             +----+                  |    |
                                                   |    |
                           +----+   L2 Tunnel      |    |
                           | BS |==================|    |
                           |    |                  |    |
                           +----+                  +----+

         Figure 2. Shared IPv6 Prefix Link Model - Layered View

   In this link model, an AR can serve one or more BSs.  All MSs
   connected to BSs that are served by an AR are on the same IPv6 link.
   This model is different from Ethernet Like Link model wherein the
   later model provides Ethernet link abstraction and multicast
   capability to IPv6 layer, whereas the Shared IPv6 Prefix Link Model
   defined here does not provide native link layer multicast and
   broadcast capabilities.

3.1.1.  Prefix Assignment

   One or more IPv6 prefixes are assigned to the link and hence shared
   by all the nodes that are attached to the link.  The prefixes are
   advertised with autonomous flag (A-Flag) set and the On-link flag
   (L-flag) reset for address autoconfiguration so that the nodes may
   not make an on-link assumption for the addresses in those prefixes.

3.1.2.  Address Autoconfiguration

   The standard IPv6 address autoconfiguration mechanisms, which are
   specified in [4] [6] are used.

3.1.3.  Duplicate Address Detection

   The DAD procedure as specified in [4] does not adapt well to the
   802.16 air interface as there is no native multicast support.  The
   DAD can be performed with MLD snooping and the AR relaying the DAD
   probe to the address owners in case if the address is duplicate,
   called Relay DAD.  In this method, the MS behavior is same as

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   specified in [4] and the optimization is achieved with the support of
   AR, which maintains MLD table for a list of multicast addresses and
   the nodes that joined the multicast address.  The relay DAD works as
   1.  An MS constructs a Link Local Address as specified in [4].
   2.  The MS constructs a solicited node multicast address for the
       corresponding Link Local Address and sends an MLD Join request
       for the solicited node multicast address.
   3.  The MS starts verifying address uniqueness by sending a DAD NS on
       the initial MAC transport connection.
   4.  The AR consults the MLD table for who joined the multicast
       address.  If the AR does not find any entry in the MLD table, the
       AR silently discards the DAD NS.  If the AR founds a match, the
       AR relays the DAD NS to the address owner.
   5.  The address owner defends the address by sending DAD NA, which is
       relayed to the DAD originating MS via the AR.
   6.  If the DAD originating MS does not receive any response (DAD NA)
       to its DAD NS, the MS assigns the address to its interface.  If
       the MS receives the DAD NA, the MS discards the tentative address
       and behaves as specified in [4].

3.1.4.  Considerations  Reuse of existing standards

   The shared IPv6 prefix model uses the existing specification and does
   not require any protocol changes or any new protocols.  However this
   model requires implementation changes for DAD optimization on the AR.  On-link Multicast Support

   No native on-link multicast is possible with this method.  However
   the multicast can be supported with support of AR performing MLD
   snooping(?).  Consistency in IP Link Definition

   The definition of IPv6 link is consistent for all procedures and
   functionalities except for the support of native on-link multicast
   support.  Packet Forwarding

   All the packets travel to the AR before being delivered to the final
   destination as the layer 2 transport connection exists between the MS
   and AR.  The AR handles the packets with external IPv6 addresses
   normally.  However the packets with link local destination addresses
   are relayed by the AR to destination without decrementing the hop-

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   limit.  Changes to Host Implementation

   This link model does not require any implementation changes to the
   host side.  However the hosts are required to perform duplicate
   address detection for all addresses even if the host is reusing the
   interface identifier.  Changes to Router Implementation

   This link model requires MLD snooping in the AR for supporting Relay

3.1.5.  Applicability

   This model is applicable to service provider public access networks
   like cellular networks in conjunction with IPv6 CS.

   This link model is also under consideration of the WiMAX Forum
   Network Working Group for using with IPv6 CS access.

3.2.  Point-to-point Link Model

   In this model, a set of MAC transport connections between an MS and
   the AR are treated as a single link.  The point-to-point link model
   follows the recommendations of [8].  In this model, each link between
   an MS and the AR is allocated a separate, unique prefix or a set of
   unique prefixes by the AR.  No other node under the AR has the same
   prefixes on the link between it and the AR.  The following diagram
   illustrates this model.

                            +----+                   +----+
        +-----+             |    |      Tunnel       |    |
        | MS1 |-------------|....|===================|    |
        +-----+             |    |                   |    |
                            |    |                   |    |
        +-----+             |    |      Tunnel       |    |
        | MS2 |-------------|....|===================|    |---[Internet]
        +-----+             |    |                   | AR |
                            | BS |                   |    |
        +-----+             |    |      Tunnel       |    |
        | MS3 |-------------|....|===================|    |
        +-----+             |    |                   |    |
                            +----+                   +----+

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               Figure 3. Point-to-point Link Model

   There are multiple possible ways that the point-to-point link between
   the AR and the MS can be implemented.
   1.  One way to accomplish this is to run PPP on the link [8].
       Running PPP requires that the 802.16 link use the Ethernet CS and
       PPP over Ethernet [10].  Since the IPv6 CS does not support PPP,
       whether PPP can be run depends on the network architecture.
   2.  If the actual physical medium is shared, like Ethernet, but PPP
       is not run, the link can be made point to point between the MS
       and AR by having each MS on a separate VLAN [12].
   3.  If neither PPP nor VLAN is used, the set of 802.16 connections
       can be viewed as a virtual point-to-point link for the purpose of
       neighbor discovery and address configuration.  For IPv6 CS, this
       may be used to implement the point-to-point link.

3.2.1.  Prefix Assignment

   Prefixes are assigned to the link using the standard [3] Router
   Advertisement mechanism.  The AR assigns a unique prefix or set of
   unique prefixes for each MS.  In the prefix information options, both
   the A-flag and L-flag are set to 1, as they can be used for address
   autoconfiguration and the prefixes are on link.

3.2.2.  Address Autoconfiguration

   MSs perform link local as well as global address autoconfiguration
   exactly as specified in [4], including duplicate address detection.
   Because there is only one other node on the link, the AR, there is
   only a possibility of an address conflict with the AR, so collisions
   are statistically very unlikely, and easy to fix if they should

   If DHCP is used for address configuration ('M=1' in the Router
   Advertisement), the DHCP server must provide addresses with a
   separate prefix per MS.  The prefix must of course match a prefix the
   ASN Gateway has advertised to the MS (if any).

3.2.3.  Considerations  Reuse of existing standards

   This solution reuses RFC 2461, 2462, and if PPP is used, RFC 2472 and
   RFC 2516.  No changes in these protocols are required, the protocols
   must only be configured properly.

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   If PPP is not used, any VLAN solution, such as IEEE 802.1Q [9], or
   any L2 tunnel can be used.  On-link Multicast Support

   Since the link between the MS and the AR is point to point, any
   multicast can only be sent by one or the other node.  Link local
   multicast between other nodes and the AR will not be seen.  Consistency in IP Link Definition

   The IP link is fully consistent with a standard IP point-to-point
   link, without exception.  Packet Forwarding

   The MS always sends all packets to the AR, because it is the only
   other node on the link.  Link local unicast and multicast packets are
   also forwarded only between the two.

   When the p2p link model is used, the BS acts as a bridge.  For each
   MS, the BS bridges the unique prefix or set of prefixes assigned by
   the AR to the link between itself and the MS.  This means, in
   particular, that the per MS prefix or set of prefixes are routed on
   both sides (wireless and wired) of the BS, and that the BS needs to
   participate in all 802 standard bridging protocols.  Changes to Host Implementation

   Host implementations follow standard IPv6 stack procedures.  No
   changes needed.  Changes to Router Implementation

   If PPP is used, no changes in router implementations are needed.  If
   PPP is not used, the AR must be capable of doing the following:
   1.  Each MS is assigned a separate VLAN when 802.1X [13] or each MS
       must have an L2 tunnel to the AR to aggregate all the connections
       to the MS and present these set of connections as an interface to
       the IPv6 layer.
   2.  The AR must be configured to include a unique prefix or set of
       prefixes for each MS.  This unique prefix or set of prefixes must
       be included in Router Advertisements every time they are sent,
       and if DHCP is used, the addresses leased to the MS must include
       only the uniquely advertised prefixes.

   Note that, depending on the router implementation, these functions
   may or may not be possible with simple configuration.  No protocol

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   changes are required, however.

3.2.4.  Applicability

   In enterprise networks, shared services including printers, fax
   machines, and other such on-line services are often available in on
   the local link.  These services are typically discovered using some
   kind of link local service discovery protocol.  The unique prefix per
   MS model is not appropriate for these kinds of deployments, since it
   is not possible to have shared link services in the ASN.

   The p2p link model is applicable to deployments where there are no
   shared services in the ASN.  Such deployments are typical of service
   provider networks like cellular network which provide public access
   to wireless network.

3.3.  Ethernet Like Link Model

   This model describes a scheme for configuration and provisioning of
   an IEEE 802.16 networks so that it emulates a broadcast link in a
   manner similar to Ethernet.  Figure 4 illustrates an example of the
   Ethernet model.  This model essentially functions like an Ethernet
   link, which means the model works as described in [3], [4].

   One way to construct an Ethernet like link is to implement bridging
   between BSs and AR like switched Ethernet.  In the Figure 4, bridging
   performs link aggregation between BSs and AR.  Bridging also supports
   multicast packet filtering.  Another way to implement this model is
   by using VLAN function [12].

   In this model, an IPv6 prefix is shared by multiple MSs on top of
   IEEE 802.16 point-to-multipoint links.  Also this model supports
   multiple access routers and multiple hosts behind an MS as shown in
   Figure 4.

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              +-----+             +---+      +----+   +-----+    ISP 1
              | MS1 |-+           |   |  +---|AR1 |---| ER1 |===>Network
              +-----+ |           |  S|  |   +----+   +-----+
              +-----+ |  +-----+  |E w|  |
              | MS2 |-+--| BS1 |--|t i|  |
              +-----+    +-----+  |h t|--+
                                  |  c|  |   +----+   +-----+    ISP 2
     +-----+  +-----+    +-----+  |  h|  +---|AR2 |---| ER2 |===>Network
     |Hosts|--|MS/GW|----| BS2 |--|   |      +----+   +-----+
     +-----+  +-----+    +-----+  +---+
     A network
     may exists behind

                  Figure 4: Ethernet Like Link Model

3.3.1.  Prefix Assignment

   Prefixes are assigned as specified in [3], [4].

3.3.2.  Address Autoconfiguration

   It is the same as described in [4].

3.3.3.  Duplicate Address Detection

   It is the same as described in [4].

3.3.4.  Considerations  Reuse of existing standards

   All the IPv6 standards can be preserved or reused in this model.  On-link Multicast Support

   On-link multicast can be emulated in unicast manner by efficiently
   bridging between all BSs with IEEE 802.16 providing the links between
   the MSs and the bridge on top of the BS.  Nevertheless, in case of
   bridging, direct inter-MSs communication may not be not allowed due
   to restrictions from the service providers.  Another way to emulate
   on-link multicast is to use VLAN.

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   This model is consistent with the IP link definition.  Packet Forwarding

   When properly configured and assisted by simple bridging or VLAN
   functions, IEEE 802.16 can emulate a simple broadcast network like
   Ethernet.  Changes to Host Implementation

   No special impact on host implementation.  Changes to Router Implementation

   No special impact on router implementation under a separated AR-BS
   model, if the bridging is implemented in BS.  Some networks e.g.
   WiMAX networks may require bridging or VLAN to be implemented in the
   AR (ASN Gateway).

3.3.5.  Applicability

   This model works with the Ethernet CS and is chosen for fixed/nomadic
   WiMAX networks by the WiMAX Forum Network Working Group.

4.  Renumbering

   If the downstream prefixes managed by the AR are involved in
   renumbering, it may be necessary to renumber each link under the AR.
   [11] discusses recommended procedures for renumbering.

   If the prefixes are advertised in RAs, the AR must withdraw the
   existing prefixes and advertise the new ones.  Since each MS
   irrespective of the link model is on a separate point-to-point link
   at the MAC level because of the 802.16 connection oriented
   architecture, the AR must send an RA withdrawing the old prefix and
   advertising the new one to each link.  In point-to-point link model,
   the number of RAs sent is equal to the number of nodes the AR serves,
   whereas in the other two models, the AR sends a single RA to BS that
   is sent to all the MSs as separate RAs.

   If DHCP is used to assign addresses, either the DHCP address lease
   lifetime may be reduced prior to the renumbering event to encourage
   MSs to renew their addresses quickly or a DHCP Reconfigure message
   may be sent to each of the MSs by the server to cause them to renew
   their addresses.

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   In conclusion, the amount of traffic on the air-interface is same for
   all link models.  However the number of RAs sent by the AR to BS can
   be more compared to the other two models.

5.  Effect on Dormant Mode

   If the network needs to deliver packets to an MS, which is in dormant
   mode, the AR pages the MS.  The MS that is monitoring the paging
   channel receives the page and transitions out of the dormant mode to
   active mode.  It establishes connectivity with the network by
   requesting and obtaining the radio resources.  The network is then
   able to deliver the packets to the MS.  In many networks, packets
   destined to an MS in dormant mode are buffered at the AR in the
   network until connectivity is established.

   Support for dormant MSs is critical in mobile networks and hence it
   is a necessary feature.  Paging capability and optimizations possible
   for paging an MS are not either enhanced or handicapped by the link
   model itself.  However the multicast capability within a link may
   cause for an MS to wake up for unwanted packet.  This can be avoided
   by filtering the multicast packets and delivering the packets to only
   for MSs that are listening for particular multicast packets.  As the
   Shared IPv6 Prefix model does not have the multicast capability and
   point-to-point link model has only one node on the link, they do not
   have any effect on the dormant mode.  The Ethernet like link model
   may have the multicast capability, which requires filtering at the BS
   to support the dormant mode for the MSs.

6.  SeND Support

   SEND is fully supported by all models described in this document.
   And SeND should be used to secure the neighbor discovery procedures.

7.  Conclusions and Relevant Link Models

   Ethernet Like Link model would be used when the deployment requires
   the use of Ethernet CS as this is the only model being proposed for
   the Ethernet CS and running IPv6 over Ethernet is well understood.

   For IPv6 CS, point-to-point link model appears to the choice because
   of its simplicity for performing the DAD and does not break any
   existing applications or require defining any new protocol.  However
   the IPv6 shared prefix model would be defined if there is any
   interest from service provider community.

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8.  Security Considerations

   This document provides the analysis of various IPv6 link models for
   802.16 based networks and this document as such does not introduce
   any new security threats.  Nonetheless, this document encourages
   usage of SeND for securing the neighbor discovery messages on any
   IPv6 link.

9.  IANA Considerations

   This document has no actions for IANA.

10.  Acknowledgements

   This document is a result of discussions in v6subnet design team for
   IPv6 Prefix Model Analysis.  The members of this design team in
   alphabetical order were: Dave Thaler, David Johnston, Junghoon Jee,
   Max Riegel, Myungki Shin and Syam Madanapalli.  The discussion in the
   DT was benefited from the active participation of James Kempf, Behcet
   Sarikaya, Basavaraj Patil and JinHyeock Choi in the DT mailing list.
   The DT thanks the chairs (Gabriel Montenegro Soohong Daniel Park) and
   Shepherding AD (Jari Arkko) for their active participation and

11.  Contributors

   The members who provided the text based on the DT discussion are:

      Myung-Ki Shin

      James Kempf
      DoCoMo Communications Labs USA

      Soohong Daniel Park
      Samsung Electronics

      Dave Thaler

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      JinHyeock Choi
      Samsung Advanced Institute of Technology

      Behcet Sarikaya
      Huawei USA

12.  References

   [1]   "IEEE 802.16-2004, IEEE standard for Local and metropolitan
         area networks, Part 16:Air Interface for fixed broadband
         wireless access systems", October 2004.

   [2]   "IEEE 802.16e, IEEE standard for Local and metropolitan area
         networks, Part 16:Air Interface for fixed and Mobile broadband
         wireless access systems", October 2005.

   [3]   Narten, T., Nordmark, E., and W. Simpson, "Neighbor Discovery
         for IP Version 6 (IPv6)", RFC 2461, December 1998.

   [4]   Thomson, S. and T. Narten, "IPv6 Stateless Address
         Autoconfiguration", RFC 2462, December 1998.

   [5]   Narten, T. and R. Draves, "Privacy Extensions for Stateless
         Address Autoconfiguration in IPv6", RFC 3041, January 2001.

   [6]   Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M.
         Carney, "Dynamic Host Configuration Protocol for IPv6
         (DHCPv6)", RFC 3315, July 2003.

   [7]   Hinden, R. and S. Deering, "IP Version 6 Addressing
         Architecture", RFC 4291, February 2006.

   [8]   Wasserman, M., "Recommendations for IPv6 in Third Generation
         Partnership Project (3GPP) Standards", RFC 3314,
         September 2002.

   [9]   Haskin, D. and E. Allen, "IP Version 6 over PPP", RFC 2472,
         December 1998.

   [10]  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.

   [11]  Baker, F., Lear, E., and R. Droms, "Procedures for Renumbering
         an IPv6 Network without a Flag Day", RFC 4192, September 2005.

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   [12]  "IEEE, Virtual Bridged Local Area Networks, IEEE 802.1Q",
         May 2003.

   [13]  "IEEE, Port-based Network Access Control, IEEE 802.1X",
         December 2004.

Author's Address

   Syam Madanapalli (editor)
   125 Yemlur P.O.
   Off Airport Road
   Bangalore  560037

   Email: smadanapalli@gmail.com

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Internet-Draft         IPv6 Link Models for 802.16          October 2006

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