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
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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.
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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.
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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.
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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.
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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
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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
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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.
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- 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
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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].
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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.
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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.
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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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RFC, FYI, BCP