IP over 802.16: IP over IEEE 802.16 Problem Statement and Goals

Network Working Group                                        J. Jee, Ed.
Request for Comments: 5154                                          ETRI
Category: Informational                                   S. Madanapalli
                                                      Ordyn Technologies
                                                               J. Mandin
                                                                  Runcom
                                                              April 2008


            IP over IEEE 802.16 Problem Statement and Goals

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Abstract

   This document specifies problems in running IP over IEEE 802.16
   networks by identifying specific gaps in the IEEE 802.16 Media Access
   Control (MAC) for IPv4 and IPv6 support.  This document also provides
   an overview of IEEE 802.16 network characteristics and convergence
   sublayers.  Common terminology used for the base guideline while
   defining the solution framework is also presented.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Overview of the IEEE 802.16 MAC Layer  . . . . . . . . . . . .  4
     3.1.  Transport Connections  . . . . . . . . . . . . . . . . . .  4
     3.2.  IEEE 802.16 PDU Format . . . . . . . . . . . . . . . . . .  5
     3.3.  IEEE 802.16 Convergence Sublayer . . . . . . . . . . . . .  5
   4.  IP over IEEE 802.16 Problem Statement and Goals  . . . . . . .  6
     4.1.  Root Problem . . . . . . . . . . . . . . . . . . . . . . .  6
     4.2.  Point-to-Point Link Model for IP CS: Problems  . . . . . .  8
     4.3.  Ethernet-Like Link Model for Ethernet CS: Problems . . . .  9
     4.4.  IP over IEEE 802.16 Goals  . . . . . . . . . . . . . . . . 10
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   6.  Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 11
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 11
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 12
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 12

1.  Introduction

   Broadband Wireless Access networks address the inadequacies of low
   bandwidth wireless communication for user requirements such as high
   quality data/voice service, fast mobility, wide coverage, etc.  The
   IEEE 802.16 Working Group on Broadband Wireless Access Standards
   develops standards and recommended practices to support the
   development and deployment of broadband Wireless Metropolitan Area
   Networks [IEEE802.16].

   Recently the WiMAX Forum, and in particular, its NWG (Network Working
   Group) is defining the IEEE 802.16 network architecture (e.g., IPv4,
   IPv6, Mobility, Interworking with different networks, AAA, etc.).
   The NWG is thus taking on work at layers above those defined by the
   IEEE 802 standards (typically limited to the physical and link-layers
   only).  Similarly, WiBro (Wireless Broadband), a Korean effort, which
   focuses on the 2.3 GHz spectrum band, is also based on the IEEE
   802.16 specification [IEEE802.16].

   IEEE 802.16 [IEEE802.16] is point-to-point and connection-oriented at
   the MAC, physically arranged in a point-to-multipoint structure with
   the Base Station (BS) terminating one end of each connection and an
   individual Subscriber Station (SS) terminating the other end of each
   connection.  The IEEE 802.16 convergence sublayer (CS) is at the
   uppermost part of the MAC that is responsible for assigning transmit-
   direction Service Data Units (originating from a higher layer
   application, e.g., IP or Ethernet at the BS or SS) to a specific
   outbound transport connection.  IEEE 802.16 defines two convergence
   sublayer types, the ATM Convergence Sublayer (CS) and the Packet CS.
   The IP Specific Subpart (IP CS) and the 802.3 Ethernet Specific
   Subpart (Ethernet CS) of Packet CS are within the current scope of
   IETF efforts.

   There is complexity in configuring the IP Subnet over IEEE 802.16
   network because of its point-to-point connection-oriented feature and
   the existence of IP CS and Ethernet CS, which assume different
   higher-layer functionality.  An IP Subnet is a topological area that
   uses the same IP address prefix where that prefix is not further
   subdivided except into individual addresses as specified in
   [RFC4903].  The IP Subnet configuration is dependent on the
   underlying link-layer's characteristic and decides the overall IP
   operation on the network.  The IP CS and Ethernet CS of IEEE 802.16
   assume different higher layer capabilities: IP routing functionality
   in the case of IP CS and bridging functionality in the case of
   Ethernet CS.  This means that the link-layer's characteristics
   beneath IP can change according to the adopted convergence sublayers.

   This document provides the feasible IP Subnet model for each IP CS
   and Ethernet CS and specifies the problems in running IP for each
   case.  This document also presents an overview of IEEE 802.16 network
   characteristics specifically focusing on the convergence sublayers
   and the common terminology to be used for the base guideline while
   defining solution frameworks.

2.  Terminology

   Subscriber Station (SS): An end-user equipment that provides
   connectivity to the IEEE 802.16 networks.  It can be either fixed/
   nomadic or mobile equipment.  In mobile environment, SS represents
   the Mobile Subscriber Station (MS) introduced in [IEEE802.16e].

   Base Station (BS): A generalized equipment set that provides
   connectivity, management, and control between the subscriber station
   and the IEEE 802.16 networks.

   Access Router (AR): An entity that performs an IP routing function to
   provide IP connectivity for the subscriber station (SS or MS).

   Protocol Data Unit (PDU): This refers to the data format passed from
   the lower edge of the MAC to the PHY, which typically contains SDU
   data after fragmentation/packing, encryption, etc.

   Service Data Unit (SDU): This refers to the data format passed to the
   upper edge of the MAC

   IP Subnet: Topological area that uses the same IP address prefix
   where that prefix is not further subdivided except into individual
   addresses as specified from [RFC4903].

   Link: Topological area bounded by routers, which decrement the IPv4
   TTL or IPv6 Hop Limit when forwarding the packet as specified from
   [RFC4903].

   Transport Connection: The MAC layer connection in IEEE 802.16 between
   an SS (MS) and BS with a specific Quality of Service (QoS)
   attributes.  Several types of connections are defined and these
   include broadcast, unicast, and multicast.  Each transport connection
   is uniquely identified by a 16-bit connection identifier (CID).  A
   transport connection is a unique connection intended for user
   traffic.  The scope of the transport connection is between the SS
   (MS) and the BS.

   Connection Identifier (CID): A 16-bit value that identifies a
   connection to equivalent peers in the IEEE 802.16 MAC of the SS (MS)
   and BS.

   Ethernet CS: The 802.3/Ethernet CS specific part of the Packet CS
   defined in [IEEE802.16].

   802.1Q CS: The 802.1Q (VLAN) specific part of the Packet CS defined
   in [IEEE802.16].

   IP CS: The IP specific subpart of the Packet CS defined in
   [IEEE802.16].

   IPv4 CS: The IP specific subpart of the Packet CS, Classifier 1
   (Packet, IPv4)

   IPv6 CS: The IP specific subpart of the Packet CS, Classifier 2
   (Packet, IPv6).

3.  Overview of the IEEE 802.16 MAC Layer

   IEEE 802.16 [IEEE802.16] is point-to-point and connection-oriented at
   the MAC, physically arranged in a point-to-multipoint structure with
   the BS terminating one end of each connection and an individual SS
   terminating the other end of each connection.  Each SS in the network
   possesses a 48-bit MAC address.  The BS possesses a 48-bit unique
   identifier called "BSId".  The BS and SS learn each others' MAC
   Address/BSId during the SS's entry into the network.  Additionally,
   the BS may possess a 48-bit MAC address, but this is only known to
   the SS if using the Ethernet CS.

3.1.  Transport Connections

   User data traffic in both the BS-bound (uplink) and SS-bound
   (downlink) directions is carried on unidirectional "transport
   connections".  Each transport connection has a particular set of
   associated parameters indicating characteristics such as
   cryptographic suite and quality of service.

   After successful entry of an SS to the IEEE 802.16 network, no data
   traffic is possible as there are no transport connections between the
   BS and the SS yet.  Transport connections are established by a
   3-message signaling sequence within the MAC layer (usually initiated
   by the BS).

   A downlink-direction transport connection is regarded as "multicast"
   if it has been made available (via MAC signaling) to more than one
   SS.  Uplink-direction connections are always unicast.

3.2.  IEEE 802.16 PDU Format

   An IEEE 802.16 PDU (i.e., the format that is transmitted over the
   airlink) consists of a Generic MAC header, various optional
   subheaders, and a data payload.

   The IEEE 802.16 Generic MAC header carries the Connection Identifier
   (CID) of the connection with which the PDU is associated.  We should
   observe that there is no source or destination address present in the
   raw IEEE 802.16 MAC header.

3.3.  IEEE 802.16 Convergence Sublayer

   The IEEE 802.16 convergence sublayer (CS) is the component of the MAC
   that is responsible for mapping between the MAC service and the
   internal connection oriented service of the MAC CPS (Common Part
   Sublayer), through classification and encapsulation.  The
   classification process assigns transmit-direction Service Data Units
   (originating from a higher layer application, e.g., an IP stack at
   the BS or SS) to a specific outbound transport connection.  The
   convergence sublayer maintains an ordered "classifier table".  Each
   entry in the classifier table includes a classifier and a target CID.
   A classifier, in turn, consists of a conjunction of one or more
   subclassifiers -- where each subclassifier specifies a packet field
   (e.g., the destination MAC address in an Ethernet frame, or the Type
   of Service (TOS) field of an IP datagram contained in an Ethernet
   frame) together with a particular value or range of values for the
   field.  To perform classification on an outbound Service Data Unit,
   the convergence sublayer proceeds from the first entry of the
   classifier table to the last, and evaluates the fields of the Service
   Data Unit for a match with the table entry's classifier.  When a
   match is found, the convergence sublayer associates the Service Data
   Unit with the target CID (for eventual transmission), and the
   remainder of the IEEE 802.16 MAC and PHY processing can take place.

   IEEE 802.16 defines two convergence sublayer types, the ATM CS and
   the Packet CS.  The ATM CS supports ATM directly.  The Packet CS is
   subdivided into three specific subparts.

   o  "The IP Specific Subpart" carries IP packets over a point-to-point
      connection.

   o  "The 802.3 Ethernet Specific Subpart" carries packets encoded in
      the 802.3/Ethernet packet format with 802.3 style headers.

   o  "The 802.1Q VLAN Specific Subpart" carries 802 style packets that
      contain 802.1Q VLAN Tags.

   Classifiers applied to connections at the time of connection
   establishment further classify and subdivide the nature of the
   traffic over a connection.

   The classifications that apply to the Ethernet CS include packet over
   the 802.3/Ethernet CS, IPv4 over the 802.3/Ethernet CS, IPv6 over the
   802.3/Ethernet CS, 802.3/Ethernet CS with RObust Header Compression
   (ROHC) header compression and 802.3/Ethernet with Enhanced Compressed
   Real-Time Protocol (ECRTP) header compression.

   The classifications that apply to the 802.1Q/VLAN CS include IPv4
   over 802.1Q/VLAN and IPv6 over 802.1Q/VLAN.

   It should be noted that while the 802.3/Ethernet CS has a packet
   classification that does not restrict the IP version (packet over the
   802.3/Ethernet CS), the IP CS and 802.1Q/VLAN CS do.  All the IP
   classifiers for those CSs are either IPv4 or IPv6.

   The classifiers enable the MAC to be sure of the presence of fields
   in the headers and so to be able to apply the payload header
   suppression (PHS) feature of IEEE 802.16 to those headers.

   For the sake of brevity in this document, the following naming
   conventions will be used for particular classifications of particular
   subparts of particular CSs.

   o  IPv4 CS: Packet CS, IP Specific Subpart, Classifier 1 (Packet,
      IPv4)

   o  IPv6 CS: Packet CS, IP Specific Subpart, Classifier 2 (Packet,
      IPv6)

   o  Ethernet CS: Packet CS, 802.3/Ethernet Subpart, Classifier 3
      (Packet, 802.3/Ethernet)

   An implementation of IEEE 802.16 can support multiple CS types.

   We can observe that the CS type, subpart, and classification actually
   defines the type of data interface (e.g., IPv4/IPv6 or 802.3) that is
   presented by IEEE 802.16 to the higher layer application.

4.  IP over IEEE 802.16 Problem Statement and Goals

4.1.  Root Problem

   The key issue when deploying IP over IEEE 802.16 networks is how to
   configure an IP Subnet over that link, which is connection-oriented
   and point-to-point in the MAC level.  IP Subnet is a topological area
   that uses the same IP address prefix where that prefix is not further
   subdivided except into individual addresses.  [RFC4903] There are
   three different IP Subnet models [RFC4968] that are possible for IEEE
   802.16 network:

   1) Point-to-point Link Model

   2) Ethernet-like Link Model

   3) Shared IPv6 Prefix Link Model

   The specific problems and issues when adopting the above IP Subnet
   models to the IEEE 802.16 network are as below:

   In the point-to-point link model, each SS under a BS resides on a
   different IP Subnet.  Therefore, only a certain SS and an AR exist
   under an IP Subnet, and IP packets with destination address of link
   local scope are delivered only within the point-to-point link between
   a SS and an AR.  PPP [RFC1661] has been widely used for this kind of
   point-to-point link.  However, the direct use of PPP is not possible
   on the IEEE 802.16 network because IEEE 802.16 does not define a
   convergence sublayer, which can encapsulate and decapsulate PPP
   frames.  Therefore, there needs to be a mechanism to provide a point-
   to-point link between an SS and an AR in case of IP CS.  The other
   alternative is to utilize PPP over Ethernet by using the Ethernet CS.
   However, Ethernet CS assumes the upper layer's bridging functionality
   to realize the Ethernet-like link model.

   In the Ethernet-like link model, all SSs under an AR reside on the
   same IP Subnet.  This also applies when SSs are connected with
   different BSs.  This Ethernet-like link model assumes that underlying
   link-layer provides the equivalent functionality like Ethernet, for
   example, native broadcast and multicast.  It seems feasible to apply
   IEEE 802.16's Ethernet CS to configure this link model.  However,
   IEEE 802.16's MAC feature is still connection-oriented, and does not
   provide multicast and broadcast connection for IP packet transfer.
   Therefore, we need a mechanism like IEEE 802.1D to realize multicast
   and broadcast.  Moreover, frequent IP multicast and broadcast
   signaling should be avoided so as not to wake up the SSs that are in
   sleep/idle mode [IEEE802.16e].

   The shared IPv6 prefix link model eventually results in multi-link
   subnet problems [RFC4903].  In IEEE 802.16, the BS assigns separate
   IEEE 802.16 connections for SSs.  Therefore, SSs are placed on
   different links.  In this situation, distributing shared IPv6 prefix
   for SSs, which are placed on different links causes multi-link subnet
   problems.  This applies to IP CS and even to Ethernet CS if no
   bridging functionality is implemented on top of the BS or between the
   BS and the AR.

   We identified the feasible IP Subnet models for IEEE 802.16 networks
   depending on the convergence sublayers.  At the current stage, only
   the IP CS and Ethernet CS of IEEE 802.16 are within the scope of
   ongoing IETF work.  Following are the feasible IP Subnet models for
   each convergence sublayer used.

   1.  Point-to-Point Link model for IP CS.

   2.  Ethernet-like Link Model for Ethernet CS.

   According to the point-to-point feature of the IEEE 802.16 MAC, the
   Point-to-Point link model is the feasible IP Subnet model in the case
   of IP CS.  For the Ethernet CS, the Ethernet-like link model is the
   feasible IP Subnet model.  However, in this model unnecessary
   multicast and broadcast packets within an IP Subnet should be
   minimized.

4.2.  Point-to-Point Link Model for IP CS: Problems

   - Address Resolution:

   Address Resolution is the process by which IP nodes determine the
   link-layer address of a destination node on the same IP Subnet given
   only the destination's IP address.  In the case of IP CS, the IEEE
   802.16 MAC address is not used as part of the IEEE 802.16 frame so
   typical usage of the Address Resolution Protocol (ARP) or Neighbor
   cache does not apply.  Thus, performing the address resolution may be
   redundant in the case of IP CS.  For IPv4, ARP cannot be carried by
   the IP CS, so is not used either by the SS or by the BS.  For IPv6,
   address resolution is the function of IP layer, and IP reachability
   state is maintained through neighbor discovery packets.  Therefore,
   blocking neighbor discovery packets would break the neighbor
   unreachability detection model.

   - Router Discovery:

   The BS needs to send the Router Advertisement (RA) with separate IP
   prefix in unicast manner for each SS explicitly to send periodic
   router advertisements in IEEE 802.16 Networks.

   - Prefix Assignment:

   Separate IP prefix should be distributed for each SS to locate them
   on different IP Subnets.  When an SS moves between BSs under the same
   AR, the AR needs to redistribute the same IP Subnet prefix, which the
   SS used at the previous BS.

   - Next-Hop:

   SS's next-hop always needs to be the AR that provides the IP
   connectivity at that access network.

   - Neighbor Unreachability Detection (NUD):

   Because the SS always sees an AR as the next hop, the NUD is required
   only for that AR.  Also the requirement of NUD may depend on the
   existence of a connection to the BS for that particular destination.

   - Address Autoconfiguration:

   Because a unique prefix is assigned to each SS, the IP Subnet
   consists of only one SS and an AR.  Therefore, duplicate address
   detection (DAD) is trivial.

4.3.  Ethernet-Like Link Model for Ethernet CS: Problems

   - Address Resolution:

   For Ethernet CS, the sender needs to perform an address resolution to
   fill the destination Ethernet address field even though that address
   is not used for transmitting an IEEE 802.16 frame on the air.  That
   Ethernet destination address is used for a BS or bridge to decide
   where to forward that Ethernet frame after decapsulating the IEEE
   802.16 frame.  When the destination's IP address has the same address
   prefix with its own, the sender should set the Ethernet frame's
   destination address as the destination itself.  To acquire that
   address, the address resolution should be performed throughout
   conventional broadcast- and multicast-based ARP or Neighbor Discovery
   Protocol (NDP).  However, if not filtered (e.g., [RFC4541]), these
   multicast and broadcast packets result in the problem of waking up
   the SSs that are in sleep/idle mode [IEEE802.16e].

   - Router Discovery:

   All SSs under the AR are located in the same broadcast domain in the
   Ethernet-like link model.  In this environment, sending periodic
   Router Advertisements with the destination of all-nodes multicast
   address results in the problem of waking up the SSs that are in
   sleep/idle mode [IEEE802.16e].

   - Prefix Assignment:

   Because the same IP prefix is shared with multiple SSs, an IP Subnet
   consists of multiple SSs and an AR.  The SS assumes that there exist
   on-link neighbors and tries to resolve the L2 address for the on-link
   prefixes.  However, direct communication using link-layer address
   between two SSs is not possible with Ethernet CS alone; bridging
   functionality must be added on top of the BS or between the BS and
   AR.

   - Next-Hop:

   When Ethernet CS is used and the accompanying Ethernet capability
   emulation is implemented, the next-hop for the destination IP with
   the same global prefix with the sender or link local address type
   should be the destination itself not an AR.

   - Neighbor Unreachability Detection (NUD):

   All SSs under the same AR are all the neighbors.  Therefore, the NUD
   is required for all the SSs and AR.

   - Address Autoconfiguration:

   Duplicate Address Detection (DAD) should be performed among multiple
   SSs and an AR, which use the same IP prefix.  The previous multicast-
   based DAD causes the problem of waking up the SSs that are in sleep/
   idle mode [IEEE802.16e].

4.4.  IP over IEEE 802.16 Goals

   The following are the goals in no particular order that point at
   relevant work to be done in IETF.

   Goal #1.  Define the way to provide the point-to-point link model for
             IP CS.

   Goal #2.  Reduce the power consumption caused by waking up sleep/idle
             [IEEE802.16e] terminals for Ethernet-like link model.

   Goal #3.  Avoid multi-link subnet problems.

   Goal #4.  Allow applicability of security schemes such as SEcure
             Neighbor Discovery (SEND) [RFC3971].

   Goal #5.  Do not introduce any new security threats.

   Goal #6.  Review management requirements and specifically the
             interfaces and specific management model (objects) for IP
             over IEEE 802.16 in collaboration with IEEE 802.16 working
             group.

5.  Security Considerations

   This documents describes the problem statement and goals for IP over
   IEEE 802.16 networks and does not introduce any new security threats.
   The IEEE 802.16 link-layer employs cryptographic security mechanisms
   as specified in [IEEE802.16][IEEE802.16e].

6.  Contributors

   This document is a joint effort of the problem statement team of the
   IETF 16ng Working Group.  The team members include Junghoon Jee, Syam
   Madanapalli, Jeff Mandin, Gabriel Montenegro, Soohong Daniel Park,
   and Maximilian Riegel.

   The problem statement team members can be reached at:

      Junghoon Jee, jhjee@etri.re.kr

      Syam Madanapalli, smadanapalli@gmail.com

      Jeff Mandin, j_mandin@yahoo.com

      Gabriel Montenegro, g_e_montenegro@yahoo.com

      Soohong Daniel Park, soohong.park@samsung.com

      Maximilian Riegel, maximilian.riegel@nsn.com

7.  Acknowledgments

   The authors would like to express special thank to David Johnston for
   his help with Section 3, "Overview of the IEEE 802.16 MAC Layer", and
   for carefully reviewing the entire document, and also to Phil Roberts
   for suggesting the reorganization of the document depending on the
   baseline IP subnet models.

   The authors also would like to thank Jari Arkko, HeeYoung Jung,
   Myung-Ki Shin, Eun-Kyoung Paik, Jaesun Cha, and the KWISF (Korea
   Wireless Internet Standardization Forum) for their comments and
   contributions.

8.  References

8.1.  Normative References

   [RFC1661]      Simpson, W., "The Point-to-Point Protocol (PPP)",
                  STD 51, RFC 1661, July 1994.

   [RFC3971]      Arkko, J., Kempf, J., Zill, B., and P. Nikander,
                  "SEcure Neighbor Discovery (SEND)", RFC 3971,
                  March 2005.

8.2.  Informative References

   [IEEE802.16]   IEEE Std 802.16-2004, "IEEE Standard for Local and
                  metropolitan area networks, Part 16: Air Interface for
                  Fixed Broadband Wireless Access Systems",
                  October 2004.

   [IEEE802.16e]  IEEE Std 802.16e, "IEEE standard for Local and
                  metropolitan area networks, Part 16:Air Interface for
                  fixed and Mobile broadband wireless access systems",
                  October 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.

   [RFC4903]      Thaler, D., "Multi-Link Subnet Issues", RFC 4903,
                  June 2007.

   [RFC4968]      Madanapalli, S., "Analysis of IPv6 Link Models for
                  802.16 Based Networks", RFC 4968, August 2007.

Authors' Addresses

   Junghoon Jee (editor)
   ETRI
   161 Gajeong-dong Yuseong-gu
   Daejeon  305-700
   Korea

   Phone: +82 42 860 5126
   EMail: jhjee@etri.re.kr


   Syam Madanapalli
   Ordyn Technologies
   1st Floor, Creator Building, ITPL
   Bangalore - 560066
   India

   EMail: smadanapalli@gmail.com


   Jeff Mandin
   Runcom

   EMail: j_mandin@yahoo.com

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