Aaditeshwar Seth Patrick Darragh Srinivasan Keshav School of

Aaditeshwar Seth Patrick Darragh Srinivasan Keshav School of
A Generalized Architecture for Tetherless Computing in Disconnected Networks
Aaditeshwar Seth
Patrick Darragh
Srinivasan Keshav
School of Computer Science
University of Waterloo
Waterloo, Canada, N2L 3G1.
Abstract: In the emerging paradigm of tetherless computing,
client applications running on small, inexpensive, and smart
mobile devices maintain opportunistic wireless connectivity with
back-end services running on centralized computers, enabling
novel classes of applications. These applications require an
infrastructure that is mobility-aware, disconnection-resilient and
integrates support for identity management. We propose an
architecture that provides this functionality essentially by adding
mobility management to Delay Tolerant Networks [1]. We show
that our architecture supports tetherless computing even in
highly partitioned networks.1
Index Terms: Network Architecture, Mobility, Disconnection
Tolerant Networks, Opportunistic Connectivity
I.
INTRODUCTION
In the emerging paradigm of tetherless computing, client
applications running on small, inexpensive, and smart mobile
devices maintain opportunistic wireless connectivity with
back-end services running on centralized computers, enabling
novel classes of applications that address problems ranging
from rural development, to environmental monitoring,
healthcare and education. For instance:
• A bus carrying an 802.11 access point can wirelessly pick
up email as it drives past rural areas that are far from Internet
connectivity [1, 6, 18]. Later, the bus can forward the email to
an email server on the Internet when it passes an Internetconnected wireless hot spot.
• A health care provider can record ‘vitals’ and test results
of home-care patients in a mobile device. When driving past
an access point, this information can be relayed to a central
database for archiving and also sent to the attending physician
for follow-up diagnosis.
In these examples, an edge application client opportunistically
communicates with a centralized server over one or more
wireless links. Such tetherless applications require us to
address three key problems. First, to allow scaleable address
aggregation, IP addresses implicitly signify location. So, when
a host moves, its IP address may change. How can a sender
locate a receiver whose IP address periodically changes?
Second, TCP connections may restart each time a mobile
disconnects and reconnects. How can a client and server
maintain communication progress despite disconnections and
partitioned networks? Third, a mobile client may attempt to
1
The research reported in this paper extends the ideas
presented in [13]. A detailed comparison can be found in
Section II. E.
communicate with a server from an access point that is
managed by a third party. How can the third party authorize
the mobile and how can the mobile authenticate the third
party? Based on these considerations, we enumerate some
essential goals for a tetherless computing architecture:
1. Mobility transparency: It should be possible to locate a
mobile even as its (potentially private) IP address changes.
2. Disconnection transparency: Communication state should
persist across disconnection periods. Moreover, simultaneous
presence of both ends of a (transport-layer) end-to-end link
should not be necessary because networks might be
partitioned based on scheduled or unscheduled disconnections.
3. Identity management: A mobile user’s identity should be
verified during opportunistic connections. Moreover, mobile
users should be protected from eavesdropping rogue access
points. Although we recognize the importance of achieving
this goal, we do not consider issues of identity management
further in this paper, deferring that to future work.
4. Low control overhead: The architecture should maximize
the usage of communication opportunities by minimizing
control overheads. This is challenging: statistics in [6] show
that 30% of the total connection time of a fast moving mobile
host is taken up by connection establishment.
We have proposed an architecture in [13] that addresses these
goals in the special case where mobile users are either
disconnected or, when connected, can directly access the
Internet (i.e they are present at one end of a transport layer
connection that terminates inside the Internet). Here, we
present a general solution that provides opportunistic
connectivity to mobiles that may never have direct access to
the Internet, relying instead on the services of a proxy to carry
their data to and from the Internet. This is necessary, for
instance, to support PDAs that access the Internet by means of
a bus-based data mule. This apparently minor change has
major repercussions on the architecture. We compare our
solutions in Section II.E
II. RELATED WORK
We present a detailed survey of related work in [13]. In the
interests of space, we present only a summary of related work
here.
A. Cellular networks
Cellular telephone networks certainly seem to have solved the
problem of tetherless communication, providing identity
management and nearly seamless voice communication
despite mobility and transient disconnections. However,
cellular telephony (and data over cellular links) is low-
bandwidth—on the order of 100kbps, even with 3G—and
expensive. Moreover, widespread penetration of cellular
networks is limited to urban areas; most rural areas, especially
in developing countries, cannot afford ubiquitous cellular
coverage. Furthermore, 802.11-based wireless networks
experience more handoffs and roaming than cellular networks,
due to the fact that they have a much smaller coverage area.
For these reasons, existing cellular network solutions cannot
be trivially retargeted for tetherless computing
The real challenge here is to mimic the considerable
capabilities of cellular networks by composing a large number
of heterogeneously administered wireless LANs and
potentially using a motorized backhaul. This would result in a
network that would not support interactive communication,
but would be far cheaper and would have up to 500 times the
capacity of cellular networks.
B. Network layer mobility solutions
Although schemes such as mobile IP [7], HIP [19] and I3 [2]
provide mobility transparency (and with HIP, identity
management), they cannot function effectively in partitioned
networks. In particular, they do not address the problem of
updating a location register when a mobile is able to access the
Internet only through a proxy.
C. Transport layer disconnection tolerance solutions
This includes protocols like TCP Migrate [5] and Rocks-andRacks [4], which essentially provide OSI session layer
functionality to resume a TCP connection on network
reconnection. However, these protocols only support TCP, and
also only on an end-to-end basis. Therefore they cannot
function in a highly partitioned network.
D. Delay Tolerant Networks (DTN)
Unmodified TCP cannot be used over challenged networks
where there are frequent disconnections, or when the network
regions to be traversed are heterogeneous and connectivity is
not always available. DTN [1] instead proposes the notion of
bundle transfer, where the data is wrapped into bundles
(similar to email messages) and these bundles are transferred
on an overlay network. DTN routers have a large persistent
bundle store, where bundles await transfer to the next DTN
router whenever connections become available. Custody
transfer occurs as the bundles are transmitted across the
network, and the responsibility of reliable delivery of each
bundle is passed from one custodian DTN router to the next.
DTN addresses the problem of disconnections in a novel
manner, even though it constrains applications to be noninteractive in nature. End-to-end connectivity is not necessary
with DTN. Senders and receivers can inject and retrieve
bundles from the DTN overlay according to their individual
connectivity schedules. A side benefit of DTN is that high
throughput rates can be obtained for opportunistic delivery if
bundles are routed to DTN routers physically close to potential
receivers [13]. However, the existing DTN architecture has no
support for mobility, which is an essential requirement for
tetherless computing.
E. DTN/I3 Architecture for Tetherless Computing
In [13], we proposed an architecture that handled transport
layer disconnections using DTN, and mobility using I3 [3].
However, this architecture does not handle the case where a
mobile never has direct access to the Internet, always relying
on a proxy to ferry its data to and from the Internet. In such a
situation:
1. I3 triggers in the Internet cannot be easily updated,
because there is never a transport level connection
between the mobile and the I3 server. Trigger updates
c o u l d be tunneled through to I3 servers, but this
complicates the architecture.
2. All data, even that bound for the local region, must travel
first to the Internet region’s I3 servers, which is inefficient
3. The mobile does not have access to a I3-DNS server to
translate from a destination name to its trigger.
In this paper, we present an alternative architecture that avoids
these problems.
III. TETHERLESS COMPUTING ARCHITECTURE (TCA)
A. Overview and definitions
Based on the goals outlined in Section I, we believe that the
following features must be included in any architecture for
tetherless computing.
1. Intermediate persistent storage: End to end connections
will not be always possible for disconnected mobiles. Hence,
intermediate infrastructure nodes should have a persistent
storage capability where senders can inject data and receivers
can pick up data according to their individual connectivity
schedules.
2. Lookup on a globally unique identifier: Each mobile host
should possess a GUID that maps to the current location of the
mobile. The GUID should not change as the mobile moves,
and its mapping should be updated periodically with the new
location.
3. Forwarding to changing mobile locations: The data
forwarding function must incorporate a lookup step.
In our architecture, we use a distributed hash table (DHT)
infrastructure such as OpenHash [11] for looking up the
location of a mobile based on its GUID. A DTN overlay
network is used for intermediate persistent storage, and data
forwarding within the DTN network is handled by DTN
routing protocols.
We first present some definitions.
1. Region: We follow the DTN definition of a region i.e a
collection of mutually reachable DTN routers, determined by
administrative policies, communication protocols, naming
conventions, or connection types [17]. Regions may also
represent physical boundaries.
2. Gateways: These are DTN routers with interfaces on
more than one region.
3. Custodians: These DTN routers act as always-available
proxies for intermittently connected mobile hosts. Custodians
store data on behalf of disconnected mobile hosts and deliver
them whenever the hosts reconnect to the network. Note that
because custodian DTN routers must always be directly
available on the Internet we impose the simplifying constraint
that custodians must not be mobile.
4. Near area: This is defined as the set of wireless access
points (APs) that are ‘closer’ to a particular custodian DTN
router than any other DTN router. We do not define closeness
precisely; one candidate would be the long-term mean RTT
delay between an AP and the custodian DTN router.
5. Mobility within a near area is near mobility. Preauthentication can used during near mobility to help reduce
reconnection delays [8, 9].
6. Mobility between near areas is far mobility. We assume
that near mobility is far more common than far mobility, and
we optimize our architecture for this case.
7. Local DTN router: This is the DTN router that
communicates directly with a mobile. A local DTN router may
or may not also be a custodian. A local DTN router that is not
a custodian may itself be mobile. For instance this models a
bus that travels between villages with a DTN router on board.
B. Location management
We identify all mobile hosts using an opaque globally unique
identifier (GUID). The Internet region, which has special
status in our architecture, maintains a DHT that maps from a
mobile’s GUID (I) to its current region (R). Following cellular
telephony terminology [15], we call this lookup table the
Home Location Register (HLR). Unlike cellular networks (and
unlike Mobile IP), our HLR uses a DHT to gain scalability
and fault-resilience. If a mobile is simultaneously present in
multiple regions—for instance, when the mobile host is
reachable on two different bus routes—the HLR will resolve
the GUID to multiple regions.
Each region maintains at least one Visitor Location Register
(VLR) that is either stored at, or accessible to, all of its
gateways. The VLRs store a mapping from the GUIDs (I) of
all mobile hosts currently in the region to the custodian DTN
router (C) of the mobile. Multiple mappings for different
custodians can also be stored if the mobile can pick up bundles
from more than one custodian.
Finally, each custodian maintains a Local Location Register
(LLR) that maps from the GUID to the best last-hop fixed or
mobile local DTN router (M) for each mobile. If the mobile
picks up data directly from the custodian, without an
intermediate local DTN router, this is reflected in the LLR.
This three-stage lookup hierarchy is shown in Fig. 1. When a
mobile device moves, its location information is updated, if
necessary, in zero or more location registers. If a mobile
moves such that its local DTN router doesn’t change, then
none of the location registers are updated after the move. If a
mobile moves such that its custodian doesn’t change, but its
local DTN router changes (i.e. a near move), then only the
LLR at the custodian is updated to reflect the new best local
DTN router for the mobile. If a mobile changes custodians
within a region, the LLR at the new custodian has to be set up,
and all VLRs in the region have to be updated to point to the
new custodian. Finally, on a far move, the HLR, VLR, and
LLR must all be updated both in the new and old regions. It is
important the HLR only be modified after all the VLRs in the
new region are aware of the mobile, as described in Section
III.E.
There are many ways of maintaining the location registers. If
the region is large (like the Internet region) then the location
register can be maintained in a DHT like OpenHash, or in a
central database. If the region is small then it can be
maintained in a lookup table at each gateway and custodian.
C. Discovering local and custodian DTN routers
When a mobile moves, its location must be updated in the
location registers. Therefore, the mobile device needs to
determine its new region, custodian, and local DTN router. A
local DTN router can always be queried for a list of ‘nearby’
custodians and regions accessible through it. So, the problem
reduces to that of a mobile finding a local DTN router after it
has associated with a new wireless access point.
Note that when a mobile associates with a wireless access
point, one of two cases must hold.
1. If the wireless access point is not itself connected to the
Internet, then, in order for data to be ferried to the
Internet, it must be co-resident with a mobile DTN router
(as on a bus). This must therefore also be the local DTN
router.
2. If the wireless access point is connected to the Internet,
then the mobile can access a centralized location service
with location information, such as the SSID of the access
point, the current zip code or its GPS location, to find a
‘close’ local DTN router. Alternatively, when the access
point is initially set up, this information can be handconfigured in the same way that it is configured with the
address of a DNS server.
A local DTN router, when queried, may return more than one
choice of custodian. Indeed, since the local DTN router may
be one overlay hop away from all custodians on the Internet,
the choice of custodian may be non trivial. We believe that a
scheme similar to I3 trigger sampling [3] can be used to
choose a ‘close’ custodian. We are looking into algorithms for
optimal choice of custodian in ongoing research. In any case,
our scheme will work correctly with any custodian – choosing
a ‘close’ custodian is simply a performance optimization.
D. Late binding of regions
The current DTN architecture [17] supports the notion of late
binding, where the administrative ID portion of the DTN
L LR
Internet
HLR
(I, R)
Region (R)
VLR
Local DTN
router (M)
Custodian (C)
(I, C)
Fig. 1: Three tiered hierarchy of lookups in TCA
TCP
Mobile (I)
TCP
address is bound to an actual next hop only at the destination
region. We extend this notion to allow even a node’s region to
be late bound. More precisely, we assume that every DTN
router has a default route that is its next hop to get to the
Internet region. We also assume the existence of a special
region name called ‘unknown’. When a DTN router gets a
bundle addressed to the ‘unknown’ region, it either forwards
the bundle on its default route to the Internet, or, if it has an
interface on the Internet, looks up the destination’s GUID in
the HLR to rewrite the ‘unknown’ region with the mobile’s
current region. Subsequently, the bundle is forwarded using
normal DTN routing. This optimization allows a disconnected
node to send a bundle to a destination knowing only its
GUID—it does not have to query the HLR for the mobile’s
current region.
E. Avoiding race conditions during location updates
Mobility intrinsically introduces race conditions. Bundles may
be sent to a mobile’s old region, or they may arrive to a
gateway or custodian in the new region before it has heard of
the mobile. To avoid race conditions, the location registers
must be updated with care. The basic principle is to always
have new location information reliably set up, using a group
communication protocol [20], before old information is
deleted (‘make then break’). We now outline an algorithm for
location update management in case of an inter-region far
move (the case of a near move is a straightforward subset):
1. The mobile associates with a wireless AP and is given an
IP address, for instance using DHCP. It discovers its local
DTN router using one of the techniques in III.C.
2. The mobile tells its local DTN router that it has moved to
its new region from its old region.
3. The local DTN informs the mobile of its choice of
‘nearby’ custodians.
4. The mobile chooses one or more custodians and informs
the local DTN router.
5. The local DTN router participates in a group
communication protocol to update all the custodians’
LLRs to make itself as their next hop to get to the mobile.
6. When Step 5 terminates, one of the chosen custodians
participates in a group communication protocol with all
the gateways in the region to update their VLRs so that
the GUID of the mobile maps to its set of chosen
custodians.
7. When Step 6 terminates, one of the gateways updates the
HLR to point the mobile’s GUID to its region.
8. Next this gateway participates in a group communication
protocol with the set of gateways in the mobile’s old
region to update their VLRs so that these VLRs ‘unmap’
the GUID by mapping the GUID to ‘undefined’.
9. When Step 8 terminates, the gateway reliably multicasts
an update to all the custodians in the old region. The
custodians send any stored bundles to the mobile in the
new region by rewriting the bundles’ destination region as
‘unknown’ and forwarding them on its default route.
Since this is the last step, group communication is not
necessary.
10. Any bundles addressed to the old region whose GUID is
unmapped are rewritten with an ‘unknown’ destination
region and sent back into the Internet region.
This ‘make-then-break’ approach has the interlocks necessary
to prevent race conditions and provides ‘eventually always
consistent’ semantics. For instance, if a VLR points to an old
custodian, bundles reaching the old custodian will either be
stored and eventually forwarded (when the location update
reaches the custodian), or the custodian will find the GUID to
be unmapped, in which case the bundles will automatically be
forwarded to the new custodian. A detailed algorithm can be
found in [14].
F. Multiple home regions
Our architecture treats the Internet region as a special region
that maintains the HLR. In fact, multiple regions can be
designated as home-regions by embedding the Id of the homeregion within the GUID of the mobile hosts. Thus, each
mobile host can belong to its own home-region that maintains
an HLR for it. All lookup or data delivery requests for a
mobile host automatically get routed to its home-region. This
allows administrative regions to manage their own HLR and
GUID assignments.
G. Example
Fig. 2 illustrates the scheme in operation. The shaded region
represents the Internet region. Region R1 is directly connected
to the Internet, while Region R2 is connected through Region
R1 to the Internet. (Note that R2 may have a scheduled link to
R1 and so may never be able to directly access the Internet
region.) The network contains custodians DTN-1 through
DTN-3, and M-DTN represents a mobile DTN router, such as a
bus.
Data transfer is illustrated from a fixed sender located in the
Internet, through a custodian in the Internet, to a mobile
receiver that changes its location from L-1 to L-5. In Fig. 2a,
the mobile host is present in Region R2, where it does a near
move within the vicinity of DTN-1. This is followed by intraregion move to the DTN-2 area in Fig. 2b. An inter-region far
move occurs in Fig. 2c to the DTN-3 area in the Internet
region, where the mobile host moves to a new location L-4.
Finally, the mobile host in location L-5 gains connectivity
through a mobile local DTN node, also in the Internet region.
IV. DISCUSSION
A. Globally unique Ids and routing
Address aggregation and mobility are mutually antagonistic
concepts. Address aggregation is possible only when nodes
with similar addresses are topologically close, so that an
address range can be assigned a common next hop; mobility
means that this is precisely not the case—even if nodes with
similar addresses were close by to begin with, over time they
would move apart. Therefore, any scheme for mobility must
support location-independent GUIDs that are mapped either to
location-specific addresses, or are directly incorporated into
routing tables. By the former, we mean that a node has a
location-specific (aggregable) address that changes as it
moves, and a lookup table maps from the GUID to the current
location. By the latter, we mean that each router, for each
GUID, keeps track of the next hop. The latter solution is not
scaleable: it would mean that the routing table size at every
router would be size of at least the number of mobiles in the
system. Consequently, every mobility solution must use some
form of translation from a GUID to a location-specific
address, with aggregate (address-range) based routing and
forwarding tables.
The current DTN architecture uses globally unique node
identifiers of the form (region, admin id), where the admin id
is opaque outside the region. This allows one level of
aggregation, but suffers from the problem that the identifier
changes as a mobile changes regions. Consequently, we need
to have different a GUID to identify mobile nodes, and we
have introduced this into the DTN architecture.
We could have mapped from a GUID directly to the locationspecific address of a mobile in the Internet DHT. However,
this would have required a deluge of DHT updates even for
moves within a region. Our approach--to use the DHT to map
from a GUID to a region--allows us to avoid DHT updates for
the common case of near mobility. On the other hand, it forces
us to maintain at least one other lookup table to actually
determine a node’s location-specific identifier. In fact, we
further partition lookup in two: the VLR maps to the
custodian’s address, and at a custodian the LLR maps to the
best next hop. This is because the only way to reach the
mobile is through a custodian. So, it makes sense for the VLR
to point to the custodian rather than directly to the mobile.
Note that unlike the HLR, the VLR only needs to track
mobiles actually in the region, so it can be much smaller.
Vanilla DTN routing ought to be enough for a custodian to
choose the best next hop to a mobile’s location-specific
address. Since this routing has yet to be defined, at the time of
this writing, we use a simple GUID-based lookup table to map
from a custodian to the next-hop local DTN router. Because of
default routes, this table can be small.
Given that we need GUIDs anyway, the question is where to
put them. One option is to extend a mobile node’s DTN
address with a GUID. Our solution, a minor optimization, is to
have all regions use the GUID as the admin ID.
Note also that non-mobile nodes can be assigned aggregable
(location-specific) addresses. We are looking into assigning
Host mobility
Data transmission
Mobile DTN route
HLR
VLR
LLR
Gateway DTN router
Mobile DTN router
Custodian DTN router
Fig. 2: Mobility in TCA
Sender/Receiver
fixed nodes an aggregable address, and allocating GUIDs only
for mobile nodes.
B. Cellular telephony architectures
TCA is modeled on cellular telephony architecture [15]. In
cellular networks, multiple Base Station Systems (BSS) are
grouped under a single MSC (Mobile Switching Center). A
mobile’s GUID is mapped in a VLR (Visitor Location
Register) to an MSC. The VLR and MSC’s lookup tables
locate a mobile’s BSS. A global HLR (Home Location
Register) tracks the current location of the mobile host. TCA
works similarly, with the HLR pointing to the current region,
and each gateway hosting a VLR for that region.
Differences in the two architectures arise because of the
partitioned network structure in TCA. Cellular telephony
assumes always-available connectivity of the mobile host with
the central management system. For example, although SMS
(Short Message Service) can be viewed as a form of delay
tolerant data transfer in cellular telephony, SMS data is always
stored on a central SMS Center (SMSC). In contrast, the
partitioned network in TCA requires a distributed network of
custodians that are used to relay the data from one mobile host
to another. Other macroscopic differences between the
architectures are reviewed in Section II.A.
Detecting movement is not always simple. Consider a mobile
host reachable on two bus routes that lead to different regions.
How does the mobile host determine whether it is present in
the same physical location but on two different bus routes, or
has moved physically to a new location on another bus route?
The answer might be obtained using GPS or by the buses
carrying information about the immediate postal code of the
area that they are driving through, or possibly by other means.
We are looking into some solutions to this problem.
As mentioned in Section IV, construction of a hierarchical
address based routing scheme for fixed DTN nodes is an open
area. We are exploring this further by looking at IPv6 to
allocate addresses on hierarchical topologies for a fixed DTN
network. Augmenting routing with optimization on deliverytime based metrics is another enhancement [16].
Finally, identity management is a substantial open issue in
tetherless computing. We are currently addressing this issue
using well-known techniques in identity-based cryptography.
VI. REFERENCES
[1]
[2]
V.
CONCLUSIONS AND FUTURE WORK
We believe that the emerging paradigm of tetherless
computing requires an infrastructure that deals well with
mobility, disconnection, and identity management. The
architecture proposed in this paper (TCA) achieves the goals
enumerated in Section I. Unlike the solution in [13], our
architecture seamlessly supports mobility and disconnection
even in networks where end points may never have a direct
connection to the Internet, relying instead on a proxy to ferry
data to and from it. We showed how to avoid race conditions
in such networks and presented a novel technique of latebinding region names to avoid expensive lookups from
disconnected nodes. We also discussed the role of GUIDs in
mobility, and their impact on DTN architecture. We now
consider some avenues for future work.
Should all addresses be late bound? In some cases it might be
more efficient to first do a lookup and then dispatch the data
addressed directly to the current region of the receiver.
Furthermore, in the case of pre-scheduled transmissions, the
receiver itself can update the sender with its current location
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questions: Which nodes should be looked up instead of
delayed-bound? What should be the degree of caching or
aggressiveness in the lookups? We have attempted to answer
these questions to some extent in [14] by using service
discovery protocols like Jini and SDP.
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