IPv4 over 802.16: Transmission of IPv4 Packets over the IP Convergence Sublayer of IEEE 802.16

Internet Engineering Task Force (IETF)                    S. Madanapalli
Request for Comments: 5948                             iRam Technologies
Category: Standards Track                                        S. Park
ISSN: 2070-1721                                      Samsung Electronics
                                                          S. Chakrabarti
                                                             IP Infusion
                                                           G. Montenegro
                                                   Microsoft Corporation
                                                             August 2010


     Transmission of IPv4 Packets over the IP Convergence Sublayer
                             of IEEE 802.16

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 the 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) layer.

   This document specifies the frame format, the Maximum Transmission
   Unit (MTU), and the 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 is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc5948.

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Table of Contents

   1. Introduction ....................................................3
   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 .................................4
      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 ...........................8
      5.2.  Address Resolution Protocol ...............................8
      5.3.  IP Broadcast and Multicast ................................8
   6. Security Considerations .........................................8
   7. Acknowledgements ................................................9
   8. References ......................................................9
      8.1. Normative References .......................................9
      8.2. Informative References .....................................9
   Appendix A.  Multiple Convergence Layers -- Impact on Subnet
                Model ................................................11
   Appendix B.  Sending and Receiving IPv4 Packets ...................11
   Appendix C.  WiMAX IPv4 CS MTU Size ...............................12

1.  Introduction

   IEEE 802.16 [IEEE802_16] is a connection-oriented access technology
   for the last mile.  The IEEE 802.16 specification includes the
   Physical (PHY) and Media Access Control (MAC) layers.  The MAC layer
   includes various Convergence Sublayers (CSs) 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 refer to 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.

   o  Last mile -- The term "last mile" is used to refer to the final
      leg of delivering connectivity from a communications provider to a
      customer.

   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 (L2) 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
   document, we are focusing on IPv4 over the Packet Convergence
   Sublayer.

   For further information on the IEEE 802.16 Convergence Sublayer and
   encapsulation of IP packets, see Section 4 of [RFC5121] 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 co-located, 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 a unique
   IPv4 address, similar to the recommendations discussed in "Analysis
   of IPv6 Link Models for IEEE 802.16 Based Networks" ([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 procedure, as described in Section 6.2 of [RFC5121].
   Steps 1-4 are the same as those indicated in Section 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]).

                        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: Cyclic Redundancy Check (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.  The CRC is appended to the PDU
      after encryption.

      TYPE: This field indicates the subheaders (Mesh subheader,
      Fragmentation subheader, Packing subheader, etc.) and special
      payload types (e.g., Automatic Repeat reQuest (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 end 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 end hosts 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  Recent revision of 802.16 by the IEEE (see IEEE 802.16-2009
      [IEEE802_16]) to (among other things) allow the provision of the
      Service Data Unit or MAC MTU in the IEEE 802.16 SBC-REQ/SBC-RSP
      phase, such that clients that are compliant with IEEE 802.16 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 the 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 use 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 Path MTU (PMTU)
   Discovery [RFC1191] or Packetization Layer Path MTU Discovery
   (PLPMTUD) [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 Don't
   Fragment (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 the MS and the AR
   [RFC4968]; hence, each MS shall be assigned an address with a 32-bit
   prefix length or subnet mask.  The point-to-point link between the MS
   and the AR is achieved using a set of IEEE 802.16 MAC connections
   (identified by service flows) and an L2 tunnel (e.g., a Generic
   Routing Encapsulation (GRE) tunnel) for each MS between the BS and
   the AR.  If the AR is co-located with the BS, then the set of IEEE
   802.16 MAC connections between the MS and the 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 the MS and the AR are attached via a point-to-
   point link.

5.1.  IPv4 Unicast Address Assignment

   DHCP [RFC2131] SHOULD be used for assigning an 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 Address Resolution
   Protocol (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 the 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 the
   Worldwide Interoperability for Microwave Access (WiMAX) network
   architecture [WMF].

7.  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 the IPv4 CS link.
   Thanks to many other members of the 16ng Working Group who commented
   on this document to make it better.

8.  References

8.1.  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.

8.2.  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 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 the data link layer and require interworking at the 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 IPv4 packets is
   different from multicast-capable shared-medium technologies like
   Ethernet.

   Before any packets are transmitted, an IEEE 802.16 transport
   connection must be established.  This connection consists of an
   IEEE 802.16 MAC transport connection between the MS and the BS and an
   L2 tunnel between the BS and the AR (if these two are not
   co-located).  This IEEE 802.16 transport connection provides a point-
   to-point link between the MS and the AR.  All the packets originating
   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 destination 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.

   The 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 IPv4 CS MTU Size

   The 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
   the MS and the 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 octets 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 IEEE 802.16 from the
   point of view of the IETF, in spite of the WiMAX Forum's deployment.

Authors' Addresses

   Syam Madanapalli
   iRam Technologies
   #H304, Shriram Samruddhi, Thubarahalli
   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, WA
   USA

   EMail: gabriel.montenegro@microsoft.com