Transmission of IPv4 packets over IEEE 802.16's IP Convergence Sublayer
draft-ietf-16ng-ipv4-over-802-dot-16-ipcs-07

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 Media Access Control (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.

Status of this Memo

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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
    5.2.  Address Resolution Protocol
    5.3.  IP Broadcast and Multicast
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
§  Authors' Addresses

1.  Introduction

IEEE 802.16 [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.) 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 Std 802.16-2009, Draft Standard for Local and Metropolitan area networks, Part 16: Air Interface for Broadband Wireless Access Systems,” May 2009.).

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] (Postel, J., “Internet Protocol,” September 1981.) packets over the IPv4 CS of IEEE 802.16. This document also specifies the MTU and address assignment method for hosts using IPv4 CS.

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

• Mobile Station (MS) - The term MS is used to mean an IP host. This usage is more informal than that in IEEE 802.16, in which "MS" refers 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] (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.). Namely, 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 the 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 Std 802.16-2009, Draft Standard for Local and Metropolitan area networks, Part 16: Air Interface for Broadband Wireless Access Systems,” May 2009.), 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 [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.).

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] (Madanapalli, S., “Analysis of IPv6 Link Models for 802.16 Based Networks,” August 2007.).

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, 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] (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) |
                    +-+-+-+-+-+-+-+-+

Here, "MSB" means "most significant byte", and "LSB" means "least significant byte".

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 = 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" tunneling, supporting this MTU at the endhosts has implications on the underlying network, for example, as discussed in [RFC4459] (Savola, P., “MTU and Fragmentation Issues with In-the-Network Tunneling,” April 2006.).

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 (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.

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. 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. 5.1. IPv4 Unicast Address Assignment 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]. 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 Broadcast and Multicast Multicast or broadcast packets from the MS are delivered to the AR via the BS through the point-to-point link. This specification simply assumes that the broadcast and multicast services are provided. How these services are implemented in an IEEE 802.16 Packet CS deployment is out of scope of this document. 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 recently revised 802.16 (see IEEE 802.16-2009 [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.)) 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.
• 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 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] (, “IEEE Std 802.16-2009, Draft Standard for Local and Metropolitan area networks, Part 16: Air Interface for Broadband Wireless Access Systems,” May 2009.).

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 IPv4 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 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.

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.

5.1.  IPv4 Unicast Address Assignment

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 a DHCP relay Agent [RFC1542] (Wimer, W., “Clarifications and Extensions for the Bootstrap Protocol,” October 1993.).

5.2.  Address Resolution Protocol

The IPv4 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 Broadcast and Multicast

Multicast or broadcast packets from the MS are delivered to the AR via the BS through the point-to-point link. This specification simply assumes that the broadcast and multicast services are provided. How these services are implemented in an IEEE 802.16 Packet CS deployment is out of scope of this document.

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 Std 802.16-2009, Draft Standard for Local and Metropolitan area networks, Part 16: Air Interface for Broadband Wireless Access Systems,” May 2009.). 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/,” 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 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] (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. 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] (, “WiMAX End-to-End Network Systems Architecture Stage 2-3 Release 1.2, http://www.wimaxforum.org/,” 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/,” 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/,” 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, an MS should be able to sense or deduce the fact that it is operating within a WiMAX network (e.g., given the WiMAX-specific particularities of the authentication and network entry procedures), and adjust its MTU size accordingly. Even though this method is not perfect, and the potential for conflict may remain, this document recommends a default MTU of 1500. This represents the WG's consensus (after much debate) to select the best value for IEEE802.16 from the point of view of the IETF, in spite of WiMAX Forum's deployment.

Authors' Addresses