Network Working Group S. Madanapalli, Ed.
Request for Comments: 4968 Ordyn Technologies
Category: Informational August 2007 Analysis of IPv6 Link Models for IEEE 802.16 Based Networks
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright (C) The IETF Trust (2007).
This document provides different IPv6 link models that are suitable
for IEEE 802.16 based networks and provides analysis of various
considerations for each link model and the applicability of each link
model under different deployment scenarios. This document is the
result of a design team (DT) that was formed to analyze the IPv6 link
models for IEEE 802.16 based networks.
This is further complicated by the definition of commercial network
models like WiMAX, which defines the WiMAX transport connection that
extends the IEEE 802.16 MAC transport connection all the way to an
access router by using a tunnel between the base station and the
access router . This leads to multiple ways of deploying IP over
IEEE 802.16 based networks.
This document looks at various considerations in selecting a link
model for IEEE 802.16 based networks and provides an analysis of the
various possible link models. And finally, this document provides a
recommendation for choosing one link model that is best suitable for
The terminology in this document is based on the definitions in ,
in addition to the ones specified in this section.
Access Router (AR): An entity that performs an IP routing function to
provide IP connectivity for Mobile Stations. In WiMAX Networks, the
AR is an Access Service Network Gateway.
Access Service Network (ASN) - The ASN is defined as a complete set
of network functions needed to provide radio access to a WiMAX
subscriber. The ASN is the access network to which the MS attaches.
The IPv6 access router is an entity within the ASN. The term ASN is
specific to the WiMAX network architecture.
Dormant Mode: A state in which a mobile station restricts its ability
to receive normal IP traffic by reducing monitoring of radio
channels. This allows the mobile station to save power and reduces
signaling load on the network. In the dormant mode, the MS is only
listening at scheduled intervals to the paging channel. The network
(e.g., the AR) maintains state about an MS that has transitioned to
dormant mode and can page it when needed.
3. IPv6 Link Models for IEEE 802.16 Based Networks
This section discusses various IPv6 link models for IEEE 802.16 based
networks and provides their operational considerations in practical
3.1. Shared IPv6 Prefix Link Model
In this model, all MSs attached to an AR share one or more prefixes
for constructing their global IPv6 addresses, however this model does
not provide any multicast capability. The following figures
illustrates a high-level view of this link model wherein one or more
prefixes advertised on the link would be used by all the MSs attached
to the IPv6 link.
| MS1 |-----+
+-----+ | +-----+ +--------+
| MS2 |-----+-----| BS1 |----------| AR |-------Internet
+-----+ | +-----+ +--------+
. | ____________
. | ()__________()
+-----+ | L2 Tunnel
| MSn |-----+
Figure 1. Shared IPv6 Prefix Link Model
The above figure shows the case where the BS and AR exist as separate
entities. In this case, a tunnel exists between the BS and AR per MS
In this link model, the link between the MS and the AR at the IPv6
layer is viewed as a shared link, and the lower layer link between
the MS and BS is a point-to-point link. This point-to-point link
between the MS and BS is extended all the way to the AR when the
granularity of the tunnel between the BS and AR is on a per MS basis.
This is illustrated in the following figure below.
| | IPv6 (Shared link) | |
| L3 |=====================================| |
| | | |
|----| PTP conn. +----+ L2 Tunnel | AR |---Internet
| L2 |-------------| BS |==================| |
| | | | | |
+----+ +----+ | |
+----+ L2 Tunnel | |
| BS |==================| |
| | | |
Figure 2. Shared IPv6 Prefix Link Model - Layered View
In this link model, an AR can serve one or more BSs. All MSs
connected to BSs that are served by an AR are on the same IPv6 link.
This model is different from an Ethernet Like Link model wherein the
later model provides an Ethernet link abstraction and multicast
capability to the IPv6 layer, whereas the Shared IPv6 Prefix Link
Model defined here does not provide native link-layer multicast and
3.1.1. Prefix Assignment
One or more IPv6 prefixes are assigned to the link and hence shared
by all the nodes that are attached to the link. The prefixes are
advertised with the autonomous flag (A-Flag) set and the On-link flag
(L-flag) reset for address autoconfiguration so that the nodes may
not make an on-link assumption for the addresses in those prefixes.
3.1.2. Address Autoconfiguration
The standard IPv6 address autoconfiguration mechanisms, which are
specified in  , are used.
3.1.3. Duplicate Address Detection
The DAD procedure, as specified in , does not adapt well to the
IEEE 802.16 air interface as there is no native multicast support.
The DAD can be performed with MLD (Multicast Listener Discovery)
snooping  and the AR relaying the DAD probe to the address owners
in case the address is a duplicate, called Relay DAD. In this
method, the MS behavior is the same as specified in  and the
optimization is achieved with the support of AR, which maintains the
MLD table for a list of multicast addresses and the nodes that joined
the multicast address. The relay DAD works as below:
1. An MS constructs a Link Local Address as specified in .
2. The MS constructs a solicited node multicast address for the
corresponding Link Local Address and sends an MLD Join request
for the solicited node multicast address.
3. The MS starts verifying address uniqueness by sending a DAD NS on
the initial MAC transport connection.
4. The AR consults the MLD table for who joined the multicast
address. If the AR does not find any entry in the MLD table, the
AR silently discards the DAD NS. If the AR finds a match, the AR
relays the DAD NS to the address owner.
5. The address owner defends the address by sending DAD NA, which is
relayed to the DAD originating MS via the AR.
6. If the DAD originating MS does not receive any response (DAD NA)
to its DAD NS, the MS assigns the address to its interface. If
the MS receives the DAD NA, the MS discards the tentative address
and behaves as specified in .
184.108.40.206. Reuse of Existing Specifications
The shared IPv6 prefix model uses the existing specification and does
not require any protocol changes or any new protocols. However, this
model requires implementation changes for DAD optimization on the AR.
220.127.116.11. On-link Multicast Support
No native on-link multicast is possible with this method. However,
the multicast can be supported with using a backend process in AR
that maintains the multicast members list and forwards the multicast
packets to the MSs belonging to a particular multicast group in a
unicast manner. MLD snooping  should be used for maintaining the
multicast members list.
18.104.22.168. Consistency in IP Link Definition
The definition of an IPv6 link is consistent for all procedures and
functionalities except for the support of native on-link multicast
22.214.171.124. Packet Forwarding
All the packets travel to the AR before being delivered to the final
destination as the layer 2 transport connection exists between the MS
and AR. The AR normally handles the packets with external IPv6
addresses. However, the packets with link local destination
addresses are relayed by the AR to the destination without
decrementing the hop-limit.
126.96.36.199. Changes to Host Implementation
This link model does not require any implementation changes for the
188.8.131.52. Changes to Router Implementation
This link model requires MLD snooping in the AR for supporting Relay
This model is good for providing shared on-link services in
conjunction with the IP convergence sublayer with IPv6 classifiers.
However, in public access networks like cellular networks, this model
cannot be used for the end users to share any of their personal
devices/services with the public.
This link model was also under consideration of the WiMAX Forum
Network Working Group for use with IPv6 CS (Convergence Sublayer)
3.2. Point-to-Point Link Model
In this model, a set of MAC transport connections between an MS and
an AR are treated as a single link. The point-to-point link model
follows the recommendations of . In this model, each link between
an MS and an AR is allocated a separate, unique prefix or a set of
unique prefixes by the AR. No other node under the AR has the same
prefixes on the link between it and the AR. The following diagram
illustrates this model.
+-----+ | | Tunnel | |
| MS1 |-------------|....|===================| |
+-----+ | | | |
| | | |
+-----+ | | Tunnel | |
| MS2 |-------------|....|===================| |---Internet
+-----+ | | | AR |
| BS | | |
+-----+ | | Tunnel | |
| MS3 |-------------|....|===================| |
+-----+ | | | |
Figure 3. Point-to-Point Link Model
There are multiple possible ways that the point-to-point link between
the AR and the MS can be implemented.
1. One way to accomplish this is to run PPP on the link .
Running PPP requires that the IEEE 802.16 link use the Ethernet
CS and PPP over Ethernet . Since the IPv6 CS does not support
PPP, whether PPP can be run depends on the network architecture.
2. If the actual physical medium is shared, like Ethernet, but PPP
is not run, the link can be made point to point between the MS
and AR by having each MS on a separate VLAN .
3. If neither PPP nor VLAN is used, the set of IEEE 802.16
connections can be viewed as a virtual point-to-point link.
3.2.1. Prefix Assignment
Prefixes are assigned to the link using the standard  Router
Advertisement mechanism. The AR assigns a unique prefix or a set of
unique prefixes for each MS. In the prefix information options, both
the A-flag and L-flag are set to 1, as they can be used for address
autoconfiguration and the prefixes are on the link.
3.2.2. Address Autoconfiguration
MSs perform link local as well as global address autoconfiguration
exactly as specified in , including duplicate address detection.
Because there is only one other node on the link, the AR, there is
only a possibility of an address conflict with the AR, so collisions
are statistically very unlikely, and easy to fix if they should
If DHCP is used for address configuration ('M=1' in the Router
Advertisement), the DHCP server must provide addresses with a
separate prefix per MS. The prefix must of course match a prefix
that the ASN Gateway has advertised to the MS (if any).
184.108.40.206. Reuse of Existing Specifications
This solution reuses RFC 2461, 2462, and, if PPP is used, RFC 2472
and RFC 2516. No changes in these protocols are required; the
protocols must only be configured properly.
If PPP is not used, any VLAN solution, such as IEEE 802.1Q  or any
L2 tunnel, can be used.
220.127.116.11. On-link Multicast Support
Since the link between the MS and the AR is point to point, any
multicast can only be sent by one or the other node. Link local
multicast between other nodes and the AR will not be seen.
18.104.22.168. Consistency in IP Link Definition
The IP link is fully consistent with a standard IP point-to-point
link, without exception.
22.214.171.124. Packet Forwarding
The MS always sends all packets to the AR because it is the only
other node on the link. Link local unicast and multicast packets are
also forwarded only between the two.
126.96.36.199. Changes to Host Implementation
Host implementations follow standard IPv6 stack procedures. No
changes are needed.
188.8.131.52. Changes to Router Implementation
If PPP is used, no changes in router implementations are needed. If
PPP is not used, the AR must be capable of doing the following:
1. Each MS is assigned a separate VLAN when IEEE 802.1X  or each
MS must have an L2 tunnel to the AR to aggregate all the
connections to the MS and present these set of connections as an
interface to the IPv6 layer.
2. The AR must be configured to include a unique prefix or a set of
prefixes for each MS. This unique prefix or set of prefixes must
be included in Router Advertisements every time they are sent,
and if DHCP is used, the addresses leased to the MS must include
only the uniquely advertised prefixes.
Note that, depending on the router implementation, these functions
may or may not be possible with simple configuration. No protocol
changes are required, however.
In enterprise networks, shared services including printers, fax
machines, and other such online services are often available on the
local link. These services are typically discovered using some kind
of link local service discovery protocol. The unique prefix per MS
model is not appropriate for these kinds of deployments, since it is
not possible to have shared link services in the ASN.
The p2p link model is applicable to deployments where there are no
shared services in the ASN. Such deployments are typical of service
provider networks like cellular networks, which provide public access
to wireless networks.
3.3. Ethernet-Like Link Model
This model describes a scheme for configuration and provisioning of
an IEEE 802.16 network so that it emulates a broadcast link in a
manner similar to Ethernet. Figure 4 illustrates an example of the
Ethernet model. This model essentially functions like an Ethernet
link, which means the model works as described in , .
One way to construct an Ethernet-like link is to implement bridging
 between BSs and an AR, like a switched Ethernet. In Figure 4,
bridging performs link aggregation between BSs and an AR. Bridging
also supports multicast packet filtering.
+-----+ +---+ +----+
| MS1 |---+ | | +---|AR1 |---Internet
+-----+ | | S| | +----+
+-----+ | +-----+ |E w| |
| MS2 |---+---| BS1 |---|t i| |
+-----+ +-----+ |h t|---+
| c| | +----+
+-----+ +-----+ +-----+ | h| +---|AR2 |---Internet
|Hosts|--|MS/GW|-------| BS2 |---| | +----+
+-----+ +-----+ +-----+ +---+
may exist behind
Figure 4: Ethernet Like Link Model3.3.1. Prefix Assignment
Prefixes are assigned as specified in , .
3.3.2. Address Autoconfiguration
It is the same as described in .
3.3.3. Duplicate Address Detection
It is the same as described in .
184.108.40.206. Reuse of Existing Specifications
All the IPv6 standards can be preserved or reused in this model.
220.127.116.11. On-link Multicast Support
On-link multicast can be emulated in a unicast manner by efficiently
bridging between all BSs with IEEE 802.16 providing the links between
the MSs and the bridge on top of the BS. MLD snooping should be used
for efficient forwarding of multicast packets as specified in .
Nevertheless, in case of bridging, direct inter-MSs communication may
not be not allowed due to restrictions from the service providers.
18.104.22.168. Consistency in IP Link Definition
This model is consistent with the IP link definition.
22.214.171.124. Packet Forwarding
When properly configured and assisted by simple bridging, IEEE 802.16
can emulate a simple broadcast network like Ethernet.
126.96.36.199. Changes to Host Implementation
No special impact on host implementation.
188.8.131.52. Changes to Router Implementation
No special impact on router implementation under a separated AR-BS
model, if the bridging is implemented in BS. Some networks, e.g.,
WiMAX networks, may require bridging to be implemented in the AR (ASN
This model works with the Ethernet CS and is chosen for fixed/nomadic
WiMAX networks by the WiMAX Forum Network Working Group.
If the downstream prefixes managed by the AR are involved in
renumbering, it may be necessary to renumber each link under the AR.
 discusses recommended procedures for renumbering.
If the prefixes are advertised in RAs, the AR must withdraw the
existing prefixes and advertise the new ones. Since each MS,
irrespective of the link model, is on a separate point-to-point link
at the MAC level because of the IEEE 802.16 connection oriented
architecture, the AR must send an RA withdrawing the old prefix and
advertising the new one to each link. In a point-to-point link
model, the number of RAs sent is equal to the number of nodes the AR
serves, whereas in the other two models, the AR sends a single RA to
BS that is sent to all the MSs as separate RAs.
If DHCP is used to assign addresses, either the DHCP address lease
lifetime may be reduced prior to the renumbering event to encourage
MSs to renew their addresses quickly, or a DHCP Reconfigure message
may be sent to each of the MSs by the server to cause them to renew
In conclusion, the amount of traffic on the air-interface is the same
for all link models. However, the number of RAs sent by the AR to BS
can be better compared to the other two models.
5. Effect on Dormant Mode
If the network needs to deliver packets to an MS, which is in dormant
mode, the AR pages the MS. The MS that is monitoring the paging
channel receives the page and transitions out of the dormant mode to
active mode. It establishes connectivity with the network by
requesting and obtaining the radio resources. The network is then
able to deliver the packets to the MS. In many networks, packets
destined to an MS in dormant mode are buffered at the AR in the
network until connectivity is established.
Support for dormant MSs is critical in mobile networks, hence it is a
necessary feature. Paging capability and optimizations possible for
paging an MS are neither enhanced nor handicapped by the link model
itself. However, the multicast capability within a link may cause
for an MS to wake up for an unwanted packet. This can be avoided by
filtering the multicast packets and delivering the packets to only
for MSs that are listening for particular multicast packets. As the
Shared IPv6 Prefix model does not have the multicast capability and
the point-to-point link model has only one node on the link, neither
has any effect on the dormant mode. The Ethernet-like link model may
have the multicast capability, which requires filtering at the BS to
support the dormant mode for the MSs.
6. Effect on Routing
The model used in an IEEE 802.16 network may have a significant
impact on how routing protocols are run over such a network. The
deployment model presented in this document discusses the least
impacting model on routing as connectivity on the provider edge is
intentionally limited to point-to-point connectivity from one BS to
any one of multiple MSs. Any other deployment model may cause a
significant impact on routing protocols, however, they are outside
the scope of this document.
7. Conclusions and Relevant Link Models
Ethernet-Like Link models would be used when the deployment requires
the use of Ethernet CS, as this is the only model being proposed for
the Ethernet CS and running IPv6 over Ethernet is well understood.
For IP CS with IPv6 classifiers, a point-to-point link model appears
to be the choice because of its simplicity for performing the DAD and
because it does not break any existing applications nor requires
defining any new protocol. However, the IPv6 shared prefix model
would be defined if there is any interest from the service provider
8. Security Considerations
This document provides the analysis of various IPv6 link models for
IEEE 802.16 based networks, and as such does not introduce any new
security threats. No matter what the link model is, the networks
employ the same link-layer security mechanisms defined in .
However, the chosen link model affects the scope of link local
communication, and this may have security implications for protocols
that are designed to work within the link scope. This is the concern
for a shared link model compared with other models wherein private
resources e.g., personal printer, cannot be put onto a public WiMAX
network. This may restrict the usage of a shared prefix model to
enterprise environments. The Neighbor Discovery related security
issues are document in   and these are applicable for all the
models described in this document. The model specific security
considerations are documented in their respective protocol
This document is a result of discussions in the v6subnet design team
for IPv6 Prefix Model Analysis. The members of this design team are
(in alphabetical order): Dave Thaler, David Johnston, Junghoon Jee,
Max Riegel, Myungki Shin and Syam Madanapalli. The discussion in the
DT was benefited from the active participation of James Kempf, Behcet
Sarikaya, Basavaraj Patil and JinHyeock Choi in the DT mailing list.
The DT thanks the chairs (Gabriel Montenegro and Soohong Daniel Park)
and Shepherding AD (Jari Arkko) for their active participation and
The members who provided the text based on the DT discussion are:
DoCoMo Communications Labs USA
Soohong Daniel Park
Samsung Advanced Institute of Technology
11.1. Normative References
 Narten, T., Nordmark, E., and W. Simpson, "Neighbor Discovery
for IP Version 6 (IPv6)", RFC 2461, December 1998.
 Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
 Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M.
Carney, "Dynamic Host Configuration Protocol for IPv6
(DHCPv6)", RFC 3315, July 2003.
11.2. Informative References
 "IEEE 802.16-2004, IEEE standard for Local and metropolitan
area networks, Part 16:Air Interface for fixed broadband
wireless access systems", October 2004.
 "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.
 Jee, J., "IP over IEEE 802.16 Problem Statement and Goals",
Work in Progress, October 2006.
 Christensen, M., Kimball, K., and F. Solensky, "Considerations
for Internet Group Management Protocol (IGMP) and Multicast
Listener Discovery (MLD) Snooping Switches", RFC 4541,
 Wasserman, M., "Recommendations for IPv6 in Third Generation
Partnership Project (3GPP) Standards", RFC 3314,
 Mamakos, L., Lidl, K., Evarts, J., Carrel, D., Simone, D., and
R. Wheeler, "A Method for Transmitting PPP Over Ethernet
(PPPoE)", RFC 2516, February 1999.
 Baker, F., Lear, E., and R. Droms, "Procedures for Renumbering
an IPv6 Network without a Flag Day", RFC 4192, September 2005.
 "IEEE, Virtual Bridged Local Area Networks, IEEE 802.1Q",
 "IEEE, Port-based Network Access Control, IEEE 802.1X",
 "IEEE Std 802.1D-2004, "IEEE Standard for Local and
metropolitan area networks, Media Access Control (MAC)
Bridges"", June 2004.
 "WiMAX End-to-End Network Systems Architecture", March 2007,
Syam Madanapalli (editor)
1st Floor, Creator Building, ITPL
Bangalore - 560066
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