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Versions: (draft-jee-16ng-ps-goals) 00 01 02
03 04 RFC 5154
Network Working Group J. Jee, Editor
Internet-Draft ETRI
Intended status: Informational S. Madanapalli
Expires: June 21, 2008 Ordyn Technologies
J. Mandin
Runcom
December 19, 2007
IP over 802.16 Problem Statement and Goals
draft-ietf-16ng-ps-goals-04.txt
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Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
This document specifies problems in running the IETF IP protocols
over IEEE 802.16 networks by identifying specific gaps in the 802.16
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
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defining the solution framework is also presented.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Overview of the IEEE 802.16 MAC layer . . . . . . . . . . . . 5
3.1. Transport Connections . . . . . . . . . . . . . . . . . . 5
3.2. 802.16 PDU format . . . . . . . . . . . . . . . . . . . . 5
3.3. 802.16 Convergence Sublayer . . . . . . . . . . . . . . . 6
4. IP over IEEE 802.16 Problem Statement and Goals . . . . . . . 7
4.1. Root Problem . . . . . . . . . . . . . . . . . . . . . . . 7
4.2. Point-to-Point Link model for IP CS: Problems . . . . . . 9
4.3. Ethernet-like Link model for Ethernet CS: Problems . . . . 10
4.4. IP over IEEE 802.16 Goals . . . . . . . . . . . . . . . . 11
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
6. Security Considerations . . . . . . . . . . . . . . . . . . . 11
7. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 12
8. Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . 12
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
9.1. Normative References . . . . . . . . . . . . . . . . . . . 12
9.2. Informative References . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
Intellectual Property and Copyright Statements . . . . . . . . . . 14
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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].
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. The 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. 802.16 defines two
convergence sublayer types, the ATM 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 16ng WG scope.
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 protocols
for each case. This document also presents an overview of 802.16
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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 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 sets that provides
connectivity, management, and control between the subscriber station
and the 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 802.16 MAC to the 802.16 PHY, which typically
contains Service Data Unit data after fragmentation, encryption, etc.
Service Data Unit (SDU): This refers to the data format passed to the
upper edge of the 802.16 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 802.16 between a SS
(MS) and BS with a specific 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 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].
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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
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 a SS to the 802.16 network, no data traffic
is possible as there are yet no transport connections between the BS
and the SS. 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. 802.16 PDU format
An 802.16 PDU (ie. the format that is transmitted over the airlink)
consists of a Generic MAC header, various optional subheaders, and a
data payload.
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The 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 802.16 MAC header.
3.3. 802.16 Convergence Sublayer
The 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
(eg. the destination MAC address in an Ethernet frame, or the 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 802.16 MAC
and PHY processing can take place.
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 classifie 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
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802.3/Ethernet CS, 802.3/Ethernet CS with ROHC header compression and
802.3/Ethernet with 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 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 802.16 can support multiple CS types.
We can observe that the CS type, subpart and classification actually
defines the type of data interface (eg. IPv4/IPv6 or 802.3) that is
presented by 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 network 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
802.16 network:
1) Point-to-point Link Model
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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 802.16 network because 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 a 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
802.16's Ethernet CS to configure this link model. However, 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 not to wake up sleep/idle [IEEE802.16e]
SSs.
The shared IPv6 prefix link model eventually results in multi-link
subnet problems [RFC4903]. In 802.16, the BS assigns separate 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
16ng. Following are the feasible IP Subnet models for each
convergence sublayer used.
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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 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 IPCS, the 802.16
MAC address is not used as part of the 802.16 frame so typical usage
of the 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 RA with separate IP prefix in unicast manner
for each SS explicitly to send periodic router advertisements in
802.16 Networks.
- Prefix Assignment:
Separate IP prefix should be distributed for each SS to locate them
on different IP Subnets. When a 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 which 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
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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 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 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 NDP. However, if not filtered
(e.g., [RFC4541]), these multicast and broadcast packets result in
the problem of waking up the sleep/idle [IEEE802.16e] SSs.
- 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 sleep/idle
[IEEE802.16e] SSs.
- 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 only with Ethernet CS without adding
bridging functionality 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
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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 are using the same IP prefix. The previous
multicast-based DAD causes the problem of waking up the sleep/idle
[IEEE802.16e] SSs.
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 multilink subnet problems.
Goal #4. Allow applicability of security schemes such as SeND
[RFC3971].
Goal #5. Do not introduce any new security threats.
5. IANA Considerations
This document does not require any actions from IANA.
6. Security Considerations
This documents describes the problem statement and goals for IP over
802.16 networks and does not introduce any new security threats. The
802.16 link-layer employs cryptographic security mechanisms as
specified in [IEEE802.16][IEEE802.16e].
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7. Contributors
This document is a joint effort of the problem statement team of the
16ng WG. The team members include Junghoon Jee, Syam Madanapalli,
Jeff Mandin, Gabriel Montenegro, Soohong Daniel Park and Maximilian
Riegel.
The problem statment team members can be reached at:
Junghoon Jee, jhjee@etri.re.kr
Syam Madanapalli, smadanapalli@gmail.com
Jeff Mandin, jeff@streetwaves-networks.com
Gabriel Montenegro, g_e_montenegro@yahoo.com
Soohong Daniel Park, soohong.park@samsung.com
Maximilian Riegel, maximilian.riegel@nsn.com
8. Acknowledgment
The authors would like to express special thank to David Johnston for
his help with section 4, "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 KWISF (Korea Wireless
Internet Standardization Forum) for their comments and contributions.
9. References
9.1. 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.
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9.2. 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
ETRI
161 Gajeong-dong Yuseong-gu
Daejeon 305-350
Korea
Email: jhjee@etri.re.kr
Syam Madanapalli
Ordyn Technologies
Email: smadanapalli@gmail.com
Jeff Mandin
Runcom
Email: jeff@streetwaves-networks.com
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