Overview
Motivation
Fast Handoff
Paging
Fast
Security/AAA
Micro-Mobile QOS
Charateristics
Hierarchical
Mobility
Hierarchical Tunneling
Mobile-Specific Routing
Columbia IP Micro-mobility Suite (CIMS)
Cellular IP
Hawaii
Hierarchical Mobile IP
CIMS Papers
Wireless access to the Internet may outstrip all other forms of access in the
near future. It is likely that mobile users will expect similar levels of
service quality as wireline users. Such a vision presents a number of technical
challenges for Mobile IP in terms of performance and scalability.
Recently, a number of micro-mobility protocols have been discussed in
the IETF Mobile IP Working Group that address some of these performance and
scalability issues. Micro-mobility protocols are designed for environments where
mobile hosts change their point of attachment to the network so frequently that
the basic Mobile IP protocol tunneling mechanism introduces network overhead in
terms of increased delay, packet loss and signaling. For example, many real-time
wireless applications (e.g., voice-over-IP) would experience noticeable
degradation of service with frequent handoff. Establishment of new tunnels can
introduce additional delays in the handoff process causing packet loss and
delayed delivery of data to applications. This delay is inherent in the
round-trip incurred by Mobile IP as the registration request is sent to the home
agent and the response sent back to the foreign agent. Micro-mobility protocols
aim to handle local movement (e.g., within a domain) of mobile hosts without
interaction with the Mobile IP enabled Internet. This has the benefit of
reducing delay and packet loss during handoff and eliminating registration
between mobile hosts and possibly distant home agents when mobile hosts remain
inside their local coverage areas. Eliminating registration in this manner
reduces the signaling load experienced by the core network in support of
mobility.
As the numbers of wireless users grow so will the signaling overhead
associated with mobility management. In cellular networks, registration and
paging techniques are used to minimize the signaling overhead and optimize
mobility management performance. Currently, Mobile IP supports registration but
not paging. An important characteristic of micro-mobility protocols is their
ability to reduce the signaling overhead related to frequent mobile migrations
and power consumption taking into account a mobile host's operational mode (i.e., active or idle).
When wireless access to Internet becomes the norm then Mobile IP will have to
provide efficient and scalable location tracking in support of idle users, and
paging in support of active communications. Support for "passive
connectivity'' to the wireless Internet balances a number of important design
considerations. For example, only keeping the approximate location information
of idle users requires significantly less signaling and thus reduces the load
over the air interface and in the core network. Reducing signaling over the air
interfaces in this manner also has the benefit of preserving the power reserves
of mobile hosts. Saving power is an important issue for wireless Internet.
In the what follows, we discuss the motivation behind the development of IP
micro-mobility protocols. We outline some of charactertics that these protocols
possess and provide an overview of the Columbia IP Micro-mobility Suite (CIMS),
which include ns code for Cellular
IP, Hawaii and Hierarchical Mobile IP.
We conclude by pointing to a couple of recent papers that are related to the
CIMS code release. The first paper provides an overview of IP micro-mobility
protocols and the second provides a performance evaluation of CIMS
itself.
Micro-mobility protocols aim to support fast handoff control with minimum or
zero packet loss, and to minimize signaling through the introduction of paging
techniques thereby reducing registration to a minimum. These enhancements are
necessary for the Internet to scale to support very large volumes of wireless
subscribers. In this section, we discuss a number of issues that motivate the
design of micro-mobility protocols; these are, fast handoff,
IP paging, fast security/AAA services and quality of service (QOS)
support.
Fast Handoff. Support for fast handoff, which reduces delay and packet
loss during handoff, is an important attributed of micro-mobility protocols. A
number of design choices influence handoff performance including handoff
control, buffering and forwarding techniques, radio behavior, movement detection
and prediction, and coupling and synchronization between the IP and radio
layers. Tighter coupling between layers, for example, may minimize handoff
latency but may impact the general applicability of the solution. The Mobile IP working
group has considered a number of contributions that cover a wide set of design
choices. Many proposals discuss seamless handoff (i.e., zero or near zero loss)
where data is forwarded between the old and new access points during handoff.
Many of these approaches support fairly complex signaling, buffering and
synchronization procedures. Layer three movement detection (e.g., eager cell
switching) plays an important roll in handoff performance. The delay associated
with recognizing and registering at a new access point can have a significant
impact on mobility and data delivery. Signal strength based handoff schemes may
provide better solutions. In this case layer three handoff control is triggered
by a layer two event. Given the wide diversity of wireless devices it is
difficult to define the operation and interaction of these radios in a global
mobility aware network, without falling into link specific definitions. There is
a need to define an open radio API that captures the essence of each wireless
technology without exposing complex link specific details. This may help
facilitate layer two ``triggered'' handoff across a variety of radio
technologies. Support for hard handoff and variants of soft handoff are under
discussion in the working group. Many proposals support mobile-controlled
handoff schemes, while others, consider network-controlled handoff. Other
important design issues relate to assumptions governing the detection/prediction
of host movement between different access points, the level of coupling between
layer two and three, and the degree of synchronization between radio handoff and
Mobile IP registration process.
Paging. Typically, fixed hosts connected to the Internet (e.g.,
desktop computers connected to LANs) remain on-line for extended periods of time
even though most of the time they do not communicate. Being ``always connected''
in this manner results in being reachable around the clock with instant access
to Internet resources. Mobile subscribers connected to the wireless Internet
will expect similar service. In the case of mobile hosts maintaining location
information in support of being continuously reachable would require frequent
location updates which would consume precious bandwidth and battery power. This
signaling overhead and mobile host power consumption can be reduced through the introduction of paging. Mobile
hosts are expected to typically operate on batteries with limited lifetime. This
makes it important to save idle mobile hosts from having to transmit frequent
location update messages. This requires explicit support from networking
protocols, such as the ability to track location approximately and the ability
to page idle mobile hosts. Idle mobile hosts do not have to register if they
move within the same paging area. Rather, they only register if they change
paging area. Paging has been implemented by a number of micro-mobility protocols
including Cellular IP and Hawaii, and recently proposed as extensions to
Hierarchical Mobile IP.
Fast Security/AAA. One of the goals of micro-mobility protocols is to
support fast handoff control for mobile hosts that frequently handoff. The
performance of network services that contribute to handoff latency should be
optimized in support of this goal. Therefore, networking functions (e.g.,
security, billing, etc.) invoked during handoff should be designed to assist
this real-time operation. While authenticating location update messages seems
necessary in most cases, data encryption over the air interface or in the fixed
network may be not always needed. User authentication for authorization or
accounting may be required in some cases, while anonymous free access is
sufficient in others. The extent to which various micro-mobility protocols
support security and Authentication, Authorization and Account (AAA) functions has a large impact on the practical applicability of the protocol. The
security model adopted by micro-mobility protocols impacts network and device
performance, quality of service, manageability and the interoperation with other
(possibly global) AAA systems. Because mobile hosts need to be authenticated
during handoff, the security mechanisms used need to be responsive to the
handoff time-scale found in micro-mobility environments. In particular the
traditional AAA model where security-aware servers are potentially located at
far away locations may be not responsive enough to accommodate fast handoff.
Session keys for example that may be used by mobile hosts to perform
authentication must be promptly available at the new base station during
handoff. Timeliness of the authentication process is critical in the case of
micro-mobility due to the real-time nature of handoff. In contrast, global
mobility solutions may have broader requirements such as user identification,
bilateral billing and service provisioning agreements. These boarder
requirements may out weigh the need to support fast handoff control where the
scalability of the global AAA system is of more importance than handoff. One can
envision, however, micro-mobility protocols that build on global AAA preferences
by offering enhanced services (e.g., fast session key management) to aid fast
handoff.
Micro-Mobile QOS. Micro-mobility protocols will have to support the
delivery of a variety of traffic including best effort and real-time traffic.
There has been very little work on a suitable QOS model for micro-mobility.
Extending the differentiated services model to micro-mobility seems like a
logical starting point. However, the differentiated services concepts such as
aggregation, per-hop behavior, service level agreement and slow time scale
resource management may be impractical in wireless IP networks. For example, it
may be impractical to allocate resources at every base station in a wireless
access network in support of a service level agreement that offers assured
service, or to use traffic engineering techniques that promote under utilization
of wireless links in support of some per-hop behavior characteristic. In Mobile
IP a host acquires a new address each time it hands off to a new base station. A
new reservation between the mobile host and its home agent would be triggered in
this case. This would be rather disruptive in support of micro-mobility because
most of the path between the home agent and mobile host would remain unchanged.
Work on QOS support for micro-mobility is predicated on differentiated services
first being resolved in the wired network.
Micro-mobility proposals can be characterized into a number of categories.
Hierarchical Mobility. Hierarchical mobility management reduces the
performance impact of mobility by handling local migrations locally and hiding
them from home agents. In this case, the Internet address known by a home agent
no longer reflects a mobile host's exact point of attachment. Rather, it
represents the address of a gateway that is common to a potentially large
numbers of network access points. When a mobile host moves from one access point
to another one (which is reachable through the same gateway) then the home agent
need not be informed. The role of micro-mobility protocols is to ensure that
packets arriving at the gateway are forwarded to the appropriate access point.
In order to route packets to the mobile host's actual point of attachment,
micro-mobility protocols maintain a ``location data base'' that maps host
identifiers to location information. Most micro-mobility protocols require hosts
that participate in mobile routing to maintain a list of host entries and search
this list for each downlink packet. List entries in these protocols are assigned
timers and are removed after a pre-specified time unless refreshed. Each entry
contains a pointer to the next node toward the mobile host's actual point of
attachment. To forward a downlink packet, nodes must read the original
destination address, find the corresponding entry and forward the packet to the
next node. Two styles of hierarchical mobility are supported by micro-mobility,
these are, ``hierarchical tunneling'' and ``mobile-specific routing''
techniques, as discussed in the next two sections, respectively.
Hierarchical Tunneling. In hierarchical tunneling approaches the
location data base is maintained in a distributed form by a set of foreign
agents in the access network. Each foreign agent reads the incoming packet's
original destination address and searches its visitor list for a corresponding
entry. If the entry exists then it contains the address of next lower level
foreign agent. The sequence of visitor list entries corresponding to a
particular mobile host constitutes the host's location information and
determines the route taken by its downlink packets. Entries are created and
maintained by registration messages transmitted by mobile hosts. These proposals
rely on a tree-like structure of foreign agents. Encapsulated traffic from the
home agent is delivered to the root foreign agent. Each foreign agent on the
tree decapsulates and then reencapsulates data packets as they are forwarded
down the tree of foreign agents toward the mobile host's point of attachment. As
a mobile host moves between different access points, location updates are made
at the optimal point on the tree, tunneling traffic to the new access point.
These protocols sometimes require the mobile host to send new types of messages
or to be aware that a hierarchical tunneling protocol is in use. Examples of
micro-mobility protocols that use hierarchical tunneling include regional
tunneling management used by a number of Hierarchical Mobile IP proposals.
Mobile-Specific Routing. Mobile-specific routing approaches avoid the
overhead introduced by decapsulation and reencapsulation schemes, as is the case
with hierarchical tunneling approaches. These proposals use routing to forward
packets toward a mobile host's point of attachment using mobile specific routes.
These schemes typically introduce implicit (e.g., based on snooping data) or
explicit signaling to update mobile-specific routes or they are aware that a
routing protocol is in use. In the case of Cellular IP mobile hosts attached to
an access network use the IP address of the gateway as their Mobile IP care-of
address. The gateway decapsulates packets and forwards them toward a base
station. Inside the access network, mobile hosts are identified by their home
address and data packets are routed using mobile-specific routing without
tunneling or address conversion. The routing protocol ensures that packets are
delivered to the host's actual location. Examples of micro-mobility protocols
that use mobile-specific routing include Cellular IP and Hawaii.
The CMIS v1.0 release includes ns implementations of Cellular IP, Hawaii, and
Hierarchical Mobile IP. The Cellular IP implementation supports hard and
semi-soft handoff, and IP paging. The Hawaii implementation supports Unicast
Non-Forwarding (UNF) and Multiple Stream Forwarding (MSF) schemes. Hawaii's IP
paging capability is currently not supported in CIMS. In addition, the CIMS
implementation of Hierarchical Mobile IP currently does not support IP
paging. These and other features will be added in due course.
In what follows, we provide an overview of the Cellular IP, Hawaii, and
Hierarchical Mobile IP proposals. Each protocol is identified as having one or more of the following
protocol design attributes: (h) fast handoff, (p) paging, (s)
fast security, (m) hierarchical mobility,(t) hierarchical
tunneling and (r) mobile-specific routing. We use these design attribute
to present a simple taxonomy in the table below.
Cellular IP (h,p,s,m,r).The Cellular IP (CIP) proposal from
Columbia University and Ericsson supports fast handoff and paging techniques.
Location management and handoff support are integrated with routing in Cellular
IP access networks. To minimize control messaging, regular data packets
transmitted by mobile hosts are used to refresh host location information.
Cellular IP uses mobile originated data packets to maintain reverse path routes.
Nodes in a Cellular IP access network monitor (i.e., ``snoop'') mobile
originated packets and maintain a distributed, hop-by-hop location data base
that is used to route packets to mobile hosts. Cellular IP uses IP addresses to
identify mobile hosts. The loss of downlink packets when a mobile host moves
between access points is reduced by customized handoff procedures. Cellular IP
supports two types of handoff scheme. Cellular IP hard handoff is based on
simple approach that trades off some packet loss in exchange for minimizing
handoff signaling rather than trying to guarantee zero packet loss. Cellular IP
semisoft handoff exploits the notion that some mobile hosts can simultaneously
receive packets from the new and old base stations during handoff. Semisoft
handoff minimizes packet loss providing improved TCP and UDP performance over
hard handoff. Distinguishing idle and active mobile hosts reduces power
consumption at the terminal side. The location of idle hosts is tracked only
approximately by Cellular IP. Therefore, mobile hosts do not have to update
their location after each handoff. This extends battery life and reduces air
interface traffic. When packets need to be sent to an idle mobile host, the host
is paged using a limited scope broadcast. A mobile host becomes active upon
reception of a paging packet and starts updating its location until it moves to
an idle state again. Cellular IP also supports a fast security model that is
suitable for micro-mobility environments based on fast session key management.
Rather than defining new signaling, Cellular IP access networks use special
session keys where base stations independently calculate session keys. This
eliminates the need for signaling in support of session key management, which
would inevitably add additional delay to the handoff process.
Hawaii (h,p,m,r). The Hawaii protocol from Lucent
Technologies proposes a separate routing protocol to handle intra-domain
mobility. Hawaii relies on Mobile IP to provide wide-area inter-domain mobility.
A mobile host entering a new foreign agent domain is assigned a collocated
care-of address. The mobile node retains its care-off address unchanged while
moving within the foreign domain, thus the home agent does not need to be
involved unless the mobile node moves to a new domain. Nodes in a Hawaii network
execute a generic IP routing protocol and maintain mobility specific routing
information as per host routes added to legacy routing tables. In this sense
Hawaii nodes can be considered as enhanced IP routers, where the existing packet
forwarding function is reused. Location information (i.e., mobile-specific
routing entries) is created, updated and modified by explicit signalling
messages sent by mobile hosts. Hawaii defines four alternative path setup
schemes that control handoff between access points. An appropriate path setup
scheme is selected depending on the operator's priorities between eliminating
packet loss, minimizing handoff latency and maintaining packet ordering. Hawaii
uses IP multicasting to page mobile hosts when incoming data packets arrive at
an access network and no recent routing information is available. As mentioned
above, the Hawaii implementation supports Unicast
Non-Forwarding (UNF) and Multiple Stream Forwarding (MSF) schemes. Hawaii's IP
paging capability is currently not supported in CIMS v1.0.
Hierarchical Mobile IP (h,p,s,m,t).The Hierarchical
Mobile IP (HMIP) proposal from Ericsson and Nokia employs a hierarchy of foreign
agents to locally handle Mobile IP registration. In this protocol mobile hosts
send mobile IP registration messages (with appropriate extensions) to update
their respective location information. Registration messages establish tunnels
between neighboring foreign agents along the path from the mobile host to a
gateway foreign agent. Packets addressed to mobile hosts travel in this network
of tunnels, which can be viewed as a separate routing network overlay on top of
IP. The use of tunnels makes it possible to employ the protocol in an IP network
that carries non-mobile traffic as well. Typically one level of hierarchy is
considered where all foreign agents are connected to the gateway foreign agent. In this case, direct tunnels connect the gateway foreign agent to foreign agents
that are located at access points. CIMS v1.0 is configured to support one
level but this can be modified to multiple levels. Paging extensions for Hierarchical Mobile IP
allows idle mobile nodes to operate in a power saving mode while
located within a paging area. The location of mobile hosts is known to home
agents and is represented by paging areas. After receiving a packet addressed to
a mobile host located in a foreign network, the home agent tunnels that packet
to the paging foreign agent, which then pages the mobile host to re-establishes
a path toward the current point of attachment. Paging a mobile node can take
place using a specific communication time-slot in the paging area similar to the
paging channel in second generation cellular systems. Paging schemes increase
the amount of time a mobile host can remain in a power saving mode. In this
case, the mobile host only needs to wakeup at predefined time intervals to check
for incoming paging requests. Table 1 shows a simple comparison of CIP, Hawaii
and HMIP.
