16ng Working Group S. Madanapalli
Internet-Draft Ordyn Technologies
Intended status: Standards Track Soohong D. Park
Expires: November 19, 2007 Samsung Electronics
S. Chakrabarti
Azaire Networks
May 18, 2007
Transmission of IPv4 packets over IEEE 802.16's IP Convergence Sublayer
draft-madanapalli-16ng-ipv4-over-802-dot-16-ipcs-01.txt
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Copyright (C) The IETF Trust (2007).
Abstract
IEEE 802.16 is an air interface specification for wireless broadband
access. IEEE has specified the service specific convergence
sublayers (CS) in the IEEE 802.16 MAC to be used by upper layer
protocols. Asynchronous Transfer Mode Convergence Sublayer (ATM CS)
and Packet Convergence Sublayer (Packet CS) represent the two main
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service specific convergence sublayers for the IEEE 802.16. The
packet CS is used for transport for all packet-based protocols such
as Internet Protocol (IP), IEEE 802.3 (Ethernet) and IEEE 802.1Q
(VLAN). The IP specific part of the Packet CS enables transport of
IPv4 packets directly over the IEEE 802.16 MAC.
This document specifies the frame format, the Maximum Transmission
Unit (MTU) and address assignment procedures for transmitting IPv4
packets over IP Convergence Sublayer (IPCS) of the IEEE 802.16. This
document also provides the details of why the ARP cannot be sent over
the IEEE 802.16 links using IPCS and a recommendation for this.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Typical Network Architecture for IPv4 over IEEE 802.16 . . . . 3
4. Frame Format for IPv4 Packets . . . . . . . . . . . . . . . . 4
5. Maximum Transmission Unit . . . . . . . . . . . . . . . . . . 5
6. Subnet Model and IPv4 Address Assignment . . . . . . . . . . . 5
7. Address Resolution Protocol . . . . . . . . . . . . . . . . . 6
8. IP Multicast Address Mapping . . . . . . . . . . . . . . . . . 6
9. Security Considerations . . . . . . . . . . . . . . . . . . . 6
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 6
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
12.1. Normative References . . . . . . . . . . . . . . . . . . 7
12.2. Informative References . . . . . . . . . . . . . . . . . 7
Appendix A. Multiple Convergence Layers - Impact on Subnet
Model . . . . . . . . . . . . . . . . . . . . . . . . 7
Appendix B. Consideration for ARP Implementation . . . . . . . . 8
Appendix C. Sending and Receiving IPv4 Packets . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
Intellectual Property and Copyright Statements . . . . . . . . . . 10
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1. Introduction
IEEE 802.16 [7] is a connection oriented access technology for the
last mile without bi-directional native multicast support. IEEE
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. And also IEEE 802.16
MAC does not use the Source and Destination MAC addresses, instead it
uses the Connection Identifiers (CIDs), which are assigned
dynamically while setting up the MAC connections, for transmitting
the IEEE 802.16 frames between a Mobile Station (MS) and a Base
Station (BS).
This document specifies a method for encapsulating and transmitting
IPv4 [2] and Address Resolution Protocol (ARP) packets over IP CS of
IEEE 802.16. This document also specifies the MTU and address
assignment method for the IEEE 802.16 based networks using IPCS. As
the IEEE 802.16 MAC does not use the source and destination MAC
addresses for the frame transmission, this document recommends
avoiding ARP and Mapping of multicast IP address to MAC address.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [1].
2. Terminology
The terminology in this document is based on the definitions in [10],
in addition to the ones specified in this section.
Access Router (AR): An entity that performs an IP routing function to
provide IP connectivity for Mobile Stations.
3. Typical Network Architecture for IPv4 over IEEE 802.16
In a network that utilizes the IEEE 802.16 air interface, each MS is
attached to an Access Router (AR) through a Base Station (BS), a
layer 2 entity. The AR can be an integral part of the BS or the AR
could be an entity beyond the BS within the access network. IPv4
packets between the MS and BS are carried over a point-to-point MAC
transport connection which has a unique connection identifier (CID).
The packets between BS and AR are carried using L2 tunnel (typically
GRE tunnel) so that MS and AR are seen as layer 3 peer entities. At
least one L2 tunnel is required for each MS, so that IP packets can
be sent to MSs before they acquire IP addresses. The figure below
illustrates the network architecture.
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+-----+ CID1 +------+ +-----------+
| MS1 |----------+| BS |----------| AR |-----Internet
+-----+ / +------+ +-----------+
. / ____________
. CIDn / ()__________()
+-----+ / L2 Tunnel
| MSn |-----/
+-----+
Figure 1: Typical Network Architecture for IPv4 over IEEE 802.16
The above network model serves as an example and is shown to
illustrate the point to point link between the MS and the AR. The L2
tunnel is not required if BS and AR are integrated into a single box.
4. Frame Format for IPv4 Packets
IPv4 packets are transmitted in Generic IEEE 802.16 MAC frames as
shown in the following figure.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|H|E| TYPE |R|C|EKS|R|LEN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LEN LSB | CID MSB |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| CID LSB | HCS |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 |
+- -+
| header |
+- -+
| and |
+- -+
/ payload /
+- -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|CRC (optional) |
+-+-+-+-+-+-+-+-+
Figure 2: IEEE 802.16 MAC Frame Format for IPv4 Packets
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H: Header Type (1 bit). Shall be set to zero indicating that it
is a Generic MAC PDU.
E: Encryption Control. 0 = Payload is not encrypted; 1 = Payload
is encrypted.
R: Reserved. Shall be set to zero.
C: CRC Indicator. 1 = CRC is included, 0 = 1 No CRC is included
EKS: Encryption Key Sequence
LEN: The Length in bytes of the MAC PDU including the MAC header
and the CRC if present (11 bits)
CID: Connection Identifier (16 bits)
HCS: Header Check Sequence (8 bits)
CRC: An optional 8-bit field. CRC appended to the PDU after
encryption.
TYPE: This field indicates the subheaders (Mesh subheader,
Fragmentation Subheader, Packing subheader etc and special payload
types (ARQ) present in the message payload
5. Maximum Transmission Unit
The Length parameter of IEEE 802.16 MAC frame has a size of 11 bits.
Hence the total PDU size is 2048 bytes. The IPv4 payload can be a
maximum value of 2038 bytes ( Total PDU size (2048) - (MAC Header (6)
+ CRC (4)), which is the maximum possible MTU. The minimum MTU
required for IPv4 is 576 bytes [4]. The default MTU value of 1400
bytes SHOULD be used so that it is most likely that the packets are
not fragmented between BS and AR when Ethernet is used for transport.
The actual MTU value can be set by the Path MTU Discovery [9] or by
manual configuration of each MS.
6. Subnet Model and IPv4 Address Assignment
The Subnet Model recommended for IPv4 over IEEE 802.16 using IP CS is
based on point-to-point link between MS and AR, hence each MS will be
on different IP subnet. The point-to-point link between MS and AR is
achieved using a set of IEEE 802.16 MAC connections (identified by
CIDs) and at least an L2 tunnel (usually GRE tunnel) per MS between
BS and AR. If the AR is co-located with the BS then the set of IEEE
802.16 MAC connections between the MS and BS/AR represent the
point-to- point connection.
DHCP [5] SHOULD be used for assigning IPv4 address for the MSs. DHCP
messages are transported over IEEE 802.16 MAC transported connection
to and from the AR. In case DHCP server does not reside in AR, the
AR SHOULD implement DHCP relay Agent [6].
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7. Address Resolution Protocol
The IEEE 802.16 frame header does not contain the source and
destination MAC addresses, instead it uses the Connection Identifier
(CID) for the delivery of MAC frames. This makes classical Address
Resolution Protocol (ARP) [3] trivial and unnecessary. > Also, IEEE
802.16 IPCS cannot classify the ARP packets as ARP runs directly over
Ethernet and does not contain IP header. Thus ARP packets are not
transmitted over IEEE 802.16 air interface when using IPCS.
8. IP Multicast Address Mapping
In IEEE 802.16, MAC address is not user for delivering the frames as
well as there is no concept of multicast MAC address. Hence, the
Mapping of multicast IP address to an IEEE 802.16 MAC address is not
required. The IPv4 multicast packets are classified normally at the
IPCS if the IEEE 802.16 MAC connection has been setup with a
multicast IP address as a classification parameter for the
destination IP address.
9. Security Considerations
This document specifies transmission of IPv4 packets over IEEE 802.16
networks with IPv4 Convergence Sublayer and does not introduce any
new vulnerabilities to IPv4 specifications or operation. The
security of the IEEE 802.16 air interface is the subject of [7]. In
addition, the security issues of the network architecture spanning
beyond the IEEE 802.16 base stations is the subject of the documents
defining such architectures, such as WiMAX Network Architecture [8].
10. IANA Considerations
This document has no actions for IANA.
11. Acknowledgements
The authors would like to acknowledge the contributions of Bachet
Sarikaya, Basavaraj Patil, Paolo Narvaez, Bruno Sousa and Bernard
Aboba for their review and comments.
12. References
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12.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
[3] Plummer, D., "Ethernet Address Resolution Protocol: Or
converting network protocol addresses to 48.bit Ethernet
address for transmission on Ethernet hardware", STD 37,
RFC 826, November 1982.
[4] Braden, R., "Requirements for Internet Hosts - Communication
Layers", STD 3, RFC 1122, October 1989.
[5] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.
[6] Wimer, W., "Clarifications and Extensions for the Bootstrap
Protocol", RFC 1542, October 1993.
12.2. Informative References
[7] "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.
[8] "WiMAX End-to-End Network Systems Architecture Stage 2-3
Release 1.0.0, http://www.wimaxforum.org/technology/documents",
March 2007.
[9] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990.
[10] Jee, J., "IP over 802.16 Problem Statement and Goals",
February 2007, <http://www.ietf.org/internet-drafts/
draft-ietf-16ng-ps-goals-01.txt>.
[11] Aboba, B., Davies, E., and D. Thaler, "Multiple Encapsulation
Methods Considered Harmful", RFC 4840, April 2007.
Appendix A. Multiple Convergence Layers - Impact on Subnet Model
Two different MSs using two different convergence sublayers (e.g. an
MS using Ethernet CS only and another MS using IP CS only) cannot
communicate at data link layer and requires interworking at IP layer.
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For this reason, these two nodes must be configured to be on two
different subnets. For more information refer [11].
Appendix B. Consideration for ARP Implementation
A node may trigger ARP for address resolution when there is no
corresponding entry in ARP cache, in such cases, the node should
synthesize the ARP response locally for smooth operation of IP layer.
Appendix C. Sending and Receiving IPv4 Packets
IEEE 802.16 MAC is a point-to-multipoint connection oriented air-
interface, and the process of sending and receiving of IPv4 packets
is different from multicast capable shared medium technologies like
Ethernet.
Before any packets being transmitted, IEEE 802.16 transport
connection must be established. This connection consists of IEEE
802.16 MAC transport connection between MS and BS and an L2 tunnel
between BS and AR. This IEEE 802.16 transport connection provides a
point-to-point link between MS and AR. all the packets originated at
the MS always reach AR before being transmitted to the final
destination.
IPv4 packets are carried directly in the payload of IEEE 802.16
frames when the IPv4 CS is used. IPv4 CS classifies the packet based
on upper layer (IP and transport layers)header fields to put the
packet on one of the available connections identified by the CID.
The classifiers for the IPv4 CS are source and destination IPv4
addresses, source and destinations ports, Type-of-Service and IP
protocol field. The CS may employ Packet Header Suppression (PHS)
after the classification.
The BS tunnels the packet that has been received on a particular MAC
connection to the AR. BS reconstructs the payload header if the PHS
is in use before the packet is tunneled to the AR. Similarly the
packets received on a tunnel interface from the AR, would be mapped
to a particular CID using IPv4 classification mechanism.
AR performs normal routing for the packets that it receives and
forwards the packet based on its forwarding table. However the DHCP
relay agent in the AR, MUST maintain the tunnel interface on which it
receives DHCP requests, so that it can relay the DHCP responses to
the correct MS. One way of doing this is to have a mapping between
MAC address and Tunnel Identifier.
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Authors' Addresses
Syam Madanapalli
Ordyn Technologies
1st Floor, Creator Building, ITPL
Bangalore - 560066
India
Email: smadanapalli@gmail.com
Soohong Daniel Park
Samsung Electronics
416 Maetan-3dong, Yeongtong-gu
Suwon 442-742
Korea
Email: soohong.park@samsung.com
Samita Chakrabarti
Azaire Networks
3121 Jay Street
Santa Clara, CA
USA
Email: samitac2@gmail.com
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