16ng Working Group S. Madanapalli
Internet-Draft Ordyn Technologies
Intended status: Standards Track Soohong D. Park
Expires: December 16, 2009 Samsung Electronics
S. Chakrabarti
IP Infusion
G. Montenegro
Microsoft Corporation
June 14, 2009
Transmission of IPv4 packets over IEEE 802.16's IP Convergence Sublayer
draft-ietf-16ng-ipv4-over-802-dot-16-ipcs-06
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Copyright (c) 2009 IETF Trust and the persons identified as the
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Abstract
IEEE 802.16 is an air interface specification for wireless broadband
access. IEEE 802.16 has specified multiple service specific
Convergence Sublayers for transmitting upper layer protocols. The
packet CS (Packet Convergence Sublayer) is used for the transport of
all packet-based protocols such as Internet Protocol (IP) and IEEE
802.3 (Ethernet). The IP-specific part of the Packet CS enables the
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 the IP-specific part of the Packet Convergence Sublayer
of IEEE 802.16.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Typical Network Architecture for IPv4 over IEEE 802.16 . . . . 4
3.1. IEEE 802.16 IPv4 Convergence Sublayer Support . . . . . . 5
4. IPv4 CS link in 802.16 Networks . . . . . . . . . . . . . . . 5
4.1. IPv4 CS link establishment . . . . . . . . . . . . . . . . 5
4.2. Frame Format for IPv4 Packets . . . . . . . . . . . . . . 5
4.3. Maximum Transmission Unit . . . . . . . . . . . . . . . . 6
5. Subnet Model and IPv4 Address Assignment . . . . . . . . . . . 8
5.1. IPv4 Unicast Address Assignment and Router Discovery . . . 8
5.2. Address Resolution Protocol . . . . . . . . . . . . . . . 9
5.3. IP Multicast Address Mapping . . . . . . . . . . . . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
9.1. Normative References . . . . . . . . . . . . . . . . . . . 10
9.2. Informative References . . . . . . . . . . . . . . . . . . 10
Appendix A. Multiple Convergence Layers - Impact on Subnet
Model . . . . . . . . . . . . . . . . . . . . . . . . 11
Appendix B. Sending and Receiving IPv4 Packets . . . . . . . . . 11
Appendix C. WiMAX IPCS MTU size . . . . . . . . . . . . . . . . . 12
Appendix D. Thoughts on handling multicast-broadcast IP
packets . . . . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
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1. Introduction
IEEE 802.16 [IEEE802_16] is a connection oriented access technology
for the last mile. The IEEE 802.16 specification includes the PHY
and MAC layers. The MAC includes various Convergence Sublayers (CS)
for transmitting higher layer packets including IPv4 packets
[IEEE802_16].
The scope of this specification is limited to the operation of IPv4
over the IP-specific part of the packet CS (referred to as "IPv4 CS")
for hosts served by a network that utilizes the IEEE Std 802.16 air
interface.
This document specifies a method for encapsulating and transmitting
IPv4 [RFC0791] packets over the IPv4 CS of IEEE 802.16. This
document also specifies the MTU and address assignment method for
hosts using IPv4 CS.
This document also discusses ARP (Address Resolution Protocol) and
Multicast Address Mapping.
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 [RFC2119].
2. Terminology
o Subscriber station (SS), Mobile Station (MS), Mobile Node (MN) -
The terms subscriber station, mobile station, and mobile node are
used interchangeably in this document and mean the same, i.e., an
IP host. Notice that this usage is more informal than that in
IEEE 802.16, in which SS and MS refer to the interface
implementing the IEEE 802.16 MAC and PHY layers and not to the
entire host.
Other terminology in this document is based on the definitions in
[RFC5154].
3. Typical Network Architecture for IPv4 over IEEE 802.16
The network architecture follows what is described in [RFC5154] and
[RFC5121]. In a nutshell, each MS is attached to an Access Router
(AR) through a Base Station (BS), a layer 2 entity (from the
perspective of the IPv4 link between the MS and access router (AR)).
For further information on the typical network architecture, see
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[RFC5121] section 5.
3.1. IEEE 802.16 IPv4 Convergence Sublayer Support
As described in [IEEE802_16], the IP-specific part of the packet CS
allows the transmission of either IPv4 or IPv6 payloads. In this
document, we are focusing on the IPv4 over Packet Convergence
Sublayer.
For further information on the IEEE 802.16 Convergence Sublayer and
encapsulation of IP packets, see [RFC5121] section 4 and
[IEEE802_16].
4. IPv4 CS link in 802.16 Networks
In 802.16, the transport connection between an MS and a BS is used to
transport user data, i.e., IPv4 packets in this case. A transport
connection is represented by a service flow, and multiple transport
connections can exist between an MS and a BS.
When an AR and a BS are colocated, the collection of transport
connections to an MS is defined as a single IPv4 link. When an AR
and a BS are separated, it is recommended that a tunnel be
established between the AR and a BS whose granularity is no greater
than 'per MS' or 'per service flow' (An MS can have multiple service
flows which are identified by a service flow ID). Then the tunnel(s)
for an MS, in combination with the MS's transport connections, forms
a single point-to-point IPv4 link.
Each host belongs to a different IPv4 link and is assigned an unique
IPv4 address per recommendations in [RFC4968].
4.1. IPv4 CS link establishment
In order to enable the sending and receiving of IPv4 packets between
the MS and the AR, the link between the MS and the AR via the BS
needs to be established. This section explains the link
establishment procedures following section 6.2 of [RFC5121]. Steps
1-4 are same as indicated in 6.2 of [RFC5121]. In step 5, support
for IPv4 is indicated. In step 6, a service flow is created that can
be used for exchanging IP layer signaling messages, e.g. address
assignment procedures using DHCP.
4.2. Frame Format for IPv4 Packets
IPv4 packets are transmitted in Generic IEEE 802.16 MAC frames in the
data payloads of the 802.16 PDU ( see section 3.2 of [RFC5154] ).
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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 1: IEEE 802.16 MAC Frame Format for IPv4 Packets
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
4.3. Maximum Transmission Unit
The MTU value for IPv4 packets on an IEEE 802.16 link is configurable
(e.g., see the bottom of this section for some possible mechanisms).
The default MTU for IPv4 packets over an IEEE 802.16 link
SHOULD be 1500 octets. Given the possibility for "in-the-network"
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tunneling, supporting this MTU at the endhosts has implications on
the underlying network, for example, as discussed in [RFC4459].
Per [RFC5121] section 6.3, the IP MTU can vary to be larger or
smaller than 1500 octets.
if an MS transmits 1500-octet packets in a deployment with a smaller
MTU, packets from the MS may be dropped at the link-layer silently.
Unlike IPv6, in which departures from the default MTU are readily
advertised via the MTU option in Neighbor Discovery (via router
advertisement), there is no similarly reliable mechanism in IPv4, as
the legacy IPv4 client implementations do not determine the link MTU
by default before sending packets. Even though there is a DHCP
option to accomplish this, DHCP servers are required to provide the
MTU information only when requested.
Discovery and configuration of the proper link MTU value ensures
adequate usage of the network bandwidth and resources. Accordingly,
deployments should avoid packet loss due to a mismatch between the
default MTU and the configured link MTUs.
Some of the mechanisms available for the IPv4 CS host to find out
the link's MTU value and mitigate MTU-related issues are:
o The IEEE recently revised 802.16 (see IEEE 802.16-2009
[IEEE802_16]) to (among other things) allow providing the Service
Data Unit or MAC MTU in the IEEE 802.16 SBC-REQ/SBC-RSP phase,
such that IEEE 802.16 compliant clients can infer and configure
the negotiated MTU size for the IPv4 CS link. However, the
implementation must communicate the negotiated MTU value to the IP
layer to adjust the IP Maximum payload size for proper handling of
fragmentation. Note that this method is useful only when MS is
directly connected to the BS.
o Configuration and negotiation of MTU size at the network layer by
using the DHCP interface MTU option [RFC2132].
This document recommends that implementations of IPv4 and IPv4 CS
clients SHOULD implement the DHCP interface MTU option [RFC2132] in
order to configure its interface MTU accordingly.
In the absence of DHCP MTU configuration, the client node (MS) has
two alternatives: 1) use the default MTU (1500 bytes) or 2) determine
the MTU by the methods described in IEEE 802.16-2009[IEEE802_16].
Additionally, the clients are encouraged to run PMTU [RFC1191] or
PPMTUD [RFC4821]. However, the PMTU mechanism has inherent problems
of packet loss due to ICMP messages not reaching the sender and IPv4
routers not fragmenting the packets due to the DF bit being set in
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the IP packet. The above mentioned path MTU mechanisms will take
care of the MTU size between the MS and its correspondent node across
different flavors of convergence layers in the access networks.
5. Subnet Model and IPv4 Address Assignment
The Subnet Model recommended for IPv4 over IEEE 802.16 using IPv4 CS
is based on the point-to-point link between MS and AR [RFC4968],
hence each MS shall be assigned an address with 32bit prefix-length
or subnet-mask. The point-to-point link between MS and AR is
achieved using a set of IEEE 802.16 MAC connections (identified by
service flows) and an L2 tunnel (e.g., a 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.
5.1. IPv4 Unicast Address Assignment and Router Discovery
DHCP [RFC2131] SHOULD be used for assigning IPv4 address for the MS.
DHCP messages are transported over the IEEE 802.16 MAC connection to
and from the BS and relayed to the AR. In case the DHCP server does
not reside in the AR, the AR SHOULD implement a DHCP relay Agent
[RFC1542].
Router discovery messages [RFC1256] contain router solicitation and
router advertisements. The Router solicitation messages (multicast
or broadcast) from the MS are delivered to the AR via the BS through
the point-to-point link. The BS SHOULD map the all-routers multicast
nodes or broadcast nodes for router discovery to the AR's IP address
and deliver directly to the AR. Similarly a router advertisement to
the all-nodes multicast nodes will be either unicast to each MS by
the BS separately or put onto a multicast connection to which all MSs
are listening to. If no multicast connection exists, and the BS does
not have the capability to aggregate and disaggregate the messages to
and from the MS hosts, then the AR implementation must ensure that
unicast messages are sent to the corresponding individual MS hosts
within the set of broadcast or multicast recipients. This
specification simply assumes that the multicast service is provided.
How the multicast service is implemented in an IEEE 802.16 Packet CS
deployment is out of scope of this document.
The 'Next hop' IP address of the IPv4 CS MS is always the IP address
of the AR, because MS and AR are attached via a point-to-point link.
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5.2. Address Resolution Protocol
The IPv4 CS does not allow for transmission of ARP [RFC0826] packets.
Furthermore, in a point-to-point link model, address resolution is
not needed.
5.3. IP Multicast Address Mapping
IPv4 multicast packets are carried over the point-to-point link
between the AR and the MS (via the BS). The IPv4 multicast packets
are classified normally at the IPv4 CS if the IEEE 802.16 MAC
connection has been set up with a multicast IP address as a
classification parameter for the destination IP address. The IPv4
multicast address may be mapped into a multicast CID as defined in
the IEEE 802.16 specification. The mapping mechanism at the BS or
the relative efficiency of using a multicast CID as opposed to
simulating multicast by generating multiple unicast messages are out
of scope of this document. For further considerations on the use of
multicast CIDs see [I-D.ietf-16ng-ip-over-ethernet-over-802-dot-16].
6. 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
[IEEE802_16]. 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 [WMF].
7. IANA Considerations
This document has no actions for IANA.
8. Acknowledgements
The authors would like to acknowledge the contributions of Bernard
Aboba, Dave Thaler, Jari Arkko, Bachet Sarikaya, Basavaraj Patil,
Paolo Narvaez, and Bruno Sousa for their review and comments. The
working group members Burcak Beser, Wesley George, Max Riegel and DJ
Johnston helped shape the MTU discussion for IPv4 CS link. Thanks to
many other members of the 16ng working group who commented on this
document to make it better.
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9. References
9.1. Normative References
[IEEE802_16]
"IEEE Std 802.16-2009, Draft Standard for Local and
Metropolitan area networks, Part 16: Air Interface for
Broadband Wireless Access Systems", May 2009.
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
[RFC0826] 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.
[RFC1542] Wimer, W., "Clarifications and Extensions for the
Bootstrap Protocol", RFC 1542, October 1993.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, March 1997.
9.2. Informative References
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990.
[RFC1256] Deering, S., "ICMP Router Discovery Messages", RFC 1256,
September 1991.
[RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, March 1997.
[RFC4459] Savola, P., "MTU and Fragmentation Issues with In-the-
Network Tunneling", RFC 4459, April 2006.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, March 2007.
[RFC4840] Aboba, B., Davies, E., and D. Thaler, "Multiple
Encapsulation Methods Considered Harmful", RFC 4840,
April 2007.
[RFC4968] Madanapalli, S., "Analysis of IPv6 Link Models for 802.16
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Based Networks", RFC 4968, August 2007.
[RFC5121] Patil, B., Xia, F., Sarikaya, B., Choi, JH., and S.
Madanapalli, "Transmission of IPv6 via the IPv6
Convergence Sublayer over IEEE 802.16 Networks", RFC 5121,
February 2008.
[RFC5154] Jee, J., Madanapalli, S., and J. Mandin, "IP over IEEE
802.16 Problem Statement and Goals", RFC 5154, April 2008.
[I-D.ietf-16ng-ip-over-ethernet-over-802-dot-16]
Riegel, M., Jeong, S., and H. Jeon, "Transmission of IP
over Ethernet over IEEE 802.16 Networks",
draft-ietf-16ng-ip-over-ethernet-over-802-dot-16-08 (work
in progress), January 2009.
[WMF] "WiMAX End-to-End Network Systems Architecture Stage 2-3
Release 1.2,
http://www.wimaxforum.org/technology/documents",
January 2008.
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 IPv4 CS only) cannot
communicate at data link layer and requires interworking at IP layer.
For this reason, these two nodes must be configured to be on two
different subnets. For more information refer to [RFC4840].
Appendix B. 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 are transmitted, a 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 (if these two are not co-located). This IEEE
802.16 transport connection provides a point-to-point link between
the MS and AR. All the packets originated at the MS always reach the
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
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on upper layer (IP and transport layers) header fields to place 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 optionally reconstructs the payload header if PHS is in use.
It then tunnels the packet that has been received on a particular MAC
connection to the AR. Similarly the packets received on a tunnel
interface from the AR, would be mapped to a particular CID using the
IPv4 classification mechanism.
AR performs normal routing for the packets that it receives,
processing them per 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. The particular method is out of scope of this
specification as it need not depend on any particularities of IEEE
802.16.
Appendix C. WiMAX IPCS MTU size
WiMAX (Worldwide Interoperability for Microwave Access) forum has
defined a network architecture[WMF]. Furthermore, WiMAX has
specified IPv4 CS support for transmission of IPv4 packets between MS
and BS over the IEEE 802.16 link. The WiMAX IPv4 CS and this
specification are similar. One significant difference, however, is
that the WiMAX Forum [WMF] has specified the IP MTU as 1400 octets
[WMF] as opposed to 1500 in this specification.
Hence if an IPv4 CS MS configured with an MTU of 1500 octet enters a
WiMAX network, some of the issues mentioned in this specification may
arise. As mentioned in section 4.3, the possible mechanisms are not
guaranteed to work. Furthermore, an IPv4 CS client is not capable of
doing ARP probing to find out the link MTU. On the other hand, it is
imperative for an MS to know the link MTU size. In practice, MS
should be able to sense or deduce the fact that they are operating
within a WiMAX network (e.g., given the WiMAX-specific
particularities of the authentication and network entry procedures),
and adjust their MTU size accordingly. This document makes no
further assumptions in this respect.
Appendix D. Thoughts on handling multicast-broadcast IP packets
Although this document does not directly specify details of multicast
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or broadcast packet handling, here are some suggestions:
While uplink connections from the MSs to the BS provide only unicast
transmission capabilities, downlink connections can be used for
multicast transmission to a group of MSs as well as unicast
transmission from the BS to a single MS. For all-node IP addresses,
the AR or BS should have special mapping and the packets should be
distributed to all active point-to-point connections by the AR or by
the BS. All-router multicast packets and any broadcast packets from
a MS will be forwarded to the AR by the BS. If BS and MS are co-
located, then the first approach is more useful. If the AR and BS
are located separately then the second approach should be
implemented. An initial capability exchange message should be
performed between BS and AR (if they are not co-located) to determine
who would perform the distribution of multicast/broadcast packets.
Such mechansim should be part of L2 exchange during the connection
setup and is out of scope of this document. In order to save energy
of the wireless end devices in the IEEE 802.16 wireless network, it
is recommened that the multicast and broadcast from network side to
device side should be reduced. Only DHCP, IGMP, Router advertisemnet
packets are allowed on the downlink for multicast and broadcast IP
addresses. Other protocols using multicast and broadcast IP
addresses should be permitted through local AR/BS configuration.
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
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Samita Chakrabarti
IP Infusion
1188 Arques Avenue
Sunnyvale, CA
USA
Email: samitac@ipinfusion.com
Gabriel Montenegro
Microsoft Corporation
Redmond, Washington
USA
Email: gabriel.montenegro@microsoft.com
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