Transmission of IPv4 packets over IEEE 802.16's IP Convergence Sublayer
draft-ietf-16ng-ipv4-over-802-dot-16-ipcs-05

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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), IEEE 802.3 (Ethernet) and IEEE 802.1Q (VLAN). 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.

Table of Contents

1.  Introduction
2.  Terminology
3.  Typical Network Architecture for IPv4 over IEEE 802.16
    3.1.  IEEE 802.16 IPv4 Convergence Sublayer Support
4.  IPv4 CS link in 802.16 Networks
    4.1.  IPv4 CS link establishment
    4.2.  Frame Format for IPv4 Packets
    4.3.  Maximum Transmission Unit
5.  Subnet Model and IPv4 Address Assignment
    5.1.  IPv4 Unicast Address Assignment and Router Discovery
    5.2.  Address Resolution Protocol
    5.3.  IP Multicast Address Mapping
6.  Security Considerations
7.  IANA Considerations
8.  Acknowledgements
9.  References
    9.1.  Normative References
    9.2.  Informative References
Appendix A.  Multiple Convergence Layers - Impact on Subnet Model
Appendix B.  Sending and Receiving IPv4 Packets
Appendix C.  WiMAX IPCS MTU size
Appendix D.  Thoughts on handling multicast-broadcast IP packets
§  Authors' Addresses

1.  Introduction

IEEE 802.16 [IEEE802_16] (, “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.) 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] (, “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.).

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" or simply "IP CS" in this document).

This document specifies a method for encapsulating and transmitting IPv4 [RFC0791] (Postel, J., “Internet Protocol,” September 1981.) packets over the IP CS of IEEE 802.16. This document also specifies the MTU and address assignment method for the IEEE 802.16 based networks using IP CS.

This document also discusses ARP (Address Resolution Protocol) and Multicast Address Mapping whose operations are similar to any other point-to-point link model.

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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

2.  Terminology

The terminology in this document is based on the definitions in [RFC5154] (Jee, J., Madanapalli, S., and J. Mandin, “IP over IEEE 802.16 Problem Statement and Goals,” April 2008.).

3.  Typical Network Architecture for IPv4 over IEEE 802.16

The network architecture follows what is described in [RFC5154] (Jee, J., Madanapalli, S., and J. Mandin, “IP over IEEE 802.16 Problem Statement and Goals,” April 2008.) and [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,” February 2008.). 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 IPv6 link between the MS and access router (AR)).

For further information on the typical network architecture, see [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,” February 2008.) section 5.

3.1.  IEEE 802.16 IPv4 Convergence Sublayer Support

As described in [IEEE802_16] (, “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.), 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] (Patil, B., Xia, F., Sarikaya, B., Choi, JH., and S. Madanapalli, “Transmission of IPv6 via the IPv6 Convergence Sublayer over IEEE 802.16 Networks,” February 2008.) section 4 and [RFC5154] (Jee, J., Madanapalli, S., and J. Mandin, “IP over IEEE 802.16 Problem Statement and Goals,” April 2008.) section 3.3.

4.  IPv4 CS link in 802.16 Networks

This document defines the IPv4 CS link as a point-to-point link between the MS and the AR using a set of service flows consisting of MAC transport connections between a MS and BS, and L2 tunnel(s) between a BS and AR. It is recommended that a tunnel be established between the AR and a BS based on 'per MS' or 'per service flow' (An MS can have multiple service flows each of which are identified by a unique service flow ID). Then the tunnel(s) for an MS, in combination with the MS's MAC transport connections, forms a single point-to-point link. Each MS belongs to a different link and is assigned an unique IPv4 address per recommendations in [RFC4968] (Madanapalli, S., “Analysis of IPv6 Link Models for 802.16 Based Networks,” August 2007.).

To summarize:

• IPv4 CS uses the IPv4 header fields to classify the packets and map to the appropriate CID.
• A point-to-point link between MS and AR is established.

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] (Patil, B., Xia, F., Sarikaya, B., Choi, JH., and S. Madanapalli, “Transmission of IPv6 via the IPv6 Convergence Sublayer over IEEE 802.16 Networks,” February 2008.). Steps 1-4 are same as indicated in 6.2 of [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,” February 2008.). In step 5, support for IPv4 is indicated. In step 6, an initial service flow is created that can be used for exchanging IP layer signaling messages, e.g. address assignment procedures using DHCP.

The address assignment procedure depends on the MS mode - i,e. whether it is acting as a Mobile IPv4 client or a Proxy Mobile IP client or a Simple IP client. In the most common case, the MS requests an IP address 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] (Jee, J., Madanapalli, S., and J. Mandin, “IP over IEEE 802.16 Problem Statement and Goals,” April 2008.) ).

                     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) |
                    +-+-+-+-+-+-+-+-+

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.  The default MTU for IPv4 packets over an IEEE 802.16    link SHOULD be 1500 octets.

Per [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,” February 2008.) 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, 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: 

• The IEEE is currently revising 802.16 (see 802.16Rev2 [802_16REV2] (Johnston, D., “SDU MTU Capability Declaration,” March 2008.)) 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 future 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.
• Configuration and negotiation of MTU size at the network layer by using the DHCP interface MTU option [RFC2132] (Alexander, S. and R. Droms, “DHCP Options and BOOTP Vendor Extensions,” March 1997.).

This document recommends that all future 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 [802_16REV2] (Johnston, D., “SDU MTU Capability Declaration,” March 2008.).

Additionally, the clients are encouraged to run PMTU [RFC1191] (Mogul, J. and S. Deering, “Path MTU discovery,” November 1990.) or PPMTUD [RFC4821] (Mathis, M. and J. Heffner, “Packetization Layer Path MTU Discovery,” March 2007.). 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 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 IP CS is based on the point-to-point link between MS and AR [RFC4968] (Madanapalli, S., “Analysis of IPv6 Link Models for 802.16 Based Networks,” August 2007.), 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 CIDs) 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] (Droms, R., “Dynamic Host Configuration Protocol,” March 1997.) 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 DHCP relay Agent [RFC1542] (Wimer, W., “Clarifications and Extensions for the Bootstrap Protocol,” October 1993.).

Although DHCP is the recommended method of address assignment, it is possible that the MS could be a pure Mobile IPv4 [RFC3344] (Perkins, C., “IP Mobility Support for IPv4,” August 2002.) device which will be offered an IP address from its home network after successful Mobile IP [RFC3344] (Perkins, C., “IP Mobility Support for IPv4,” August 2002.) registration. In such situations, the mobile host SHOULD use the default link MTU in order to avoid any link-layer packet loss due to larger than supported packet size in the IP CS link.

Router discovery messages [RFC1256] (Deering, S., “ICMP Router Discovery Messages,” September 1991.) 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 IP CS MS is always the IP address of the AR, because MS and AR are attached via a point-to-point link.

5.2.  Address Resolution Protocol

The IP CS does not allow for transmission of ARP [RFC0826] (Plummer, D., “Ethernet Address Resolution Protocol: Or converting network protocol addresses to 48.bit Ethernet address for transmission on Ethernet hardware,” November 1982.) 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 IP 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] (Jeon, H., Riegel, M., and S. Jeong, “Transmission of IP over Ethernet over IEEE 802.16 Networks,” September 2009.).

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] (, “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.). 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] (, “WiMAX End-to-End Network Systems Architecture Stage 2-3 Release 1.2, http://www.wimaxforum.org/technology/documents,” January 2008.).

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.

9.  References

9.1. Normative References

9.2. Informative References

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. For this reason, these two nodes must be configured to be on two different subnets. For more information refer to [RFC4840] (Aboba, B., Davies, E., and D. Thaler, “Multiple Encapsulation Methods Considered Harmful,” April 2007.).

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 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. One way of doing this is to have a mapping between MAC address and Tunnel Identifier.

Appendix C.  WiMAX IPCS MTU size

WiMAX (Worldwide Interoperability for Microwave Access) forum has defined a network architecture[WMF] (, “WiMAX End-to-End Network Systems Architecture Stage 2-3 Release 1.2, http://www.wimaxforum.org/technology/documents,” January 2008.). 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] (, “WiMAX End-to-End Network Systems Architecture Stage 2-3 Release 1.2, http://www.wimaxforum.org/technology/documents,” January 2008.) has specified the IP MTU as 1400 octets [WMF] (, “WiMAX End-to-End Network Systems Architecture Stage 2-3 Release 1.2, http://www.wimaxforum.org/technology/documents,” January 2008.) 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 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