Transmission of IPv4 Packets over the IP Convergence Sublayer of IEEE 802.16
draft-ietf-16ng-ipv4-over-802-dot-16-ipcs-07

16ng Working Group                                        S. Madanapalli
Internet-Draft                                        Ordyn Technologies
Intended status: Standards Track                         Soohong D. Park
Expires: December 4, 2010                            Samsung Electronics
                                                          S. Chakrabarti
                                                             IP Infusion
                                                           G. Montenegro
                                                   Microsoft Corporation
                                                            June 2, 2010

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

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on December 4, 2010.

Copyright Notice

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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  . . . . . .  4
   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  . . . . . . . . . . . . .  9
     5.2.  Address Resolution Protocol  . . . . . . . . . . . . . . .  9
     5.3.  IP Broadcast and Multicast . . . . . . . . . . . . . . . .  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
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12

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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.

   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  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].

3.  Typical Network Architecture for IPv4 over IEEE 802.16

   The network architecture follows what is described in [RFC5154] and
   [RFC5121].  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] 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

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   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

      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).

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   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].

   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.

   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

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   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
   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.

   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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5.1.  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.

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

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   document to make it better.

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.

   [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.

   [WMF]      "WiMAX End-to-End Network Systems Architecture Stage 2-3
              Release 1.2, http://www.wimaxforum.org/", 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
   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.

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   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, 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.

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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
   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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