Transcript Uma Arquitetura para Simulação de Protocolos para - PUC-Rio

General Approaches for
Implementing Seamless
Handover
Markus Endler
PUC-Rio - Brazil
[email protected]
Vera Nagamuta
IME/USP - Brazil
[email protected]
Outline
 Motivation
 Main Concepts
 Bird Eye’s View
 General Approaches
 Case Studies
 Architectural Framework
 Conclusions & Future Research
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Motivation
 In mobile networks, migration is a central issue, with
impact on service performance, QoS, reliability, security,..
 Handover is the process “behind the scene” that handles
all the necessary changes when switching (access)
domains.
 Types of migration:
 micro-migration: between access points within a wireless network
 macro-migration: between distinct networks
 Common tasks in mobile/cellular networks:
 message re-routing
 state transfer and context re-establishment (e.g. discovery of
servers)
 The goal of Seamless Handover is to make migration
transparent to the communicating parties.
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Motivation
Seamless Handover has been investigated in several contexts:
 Medium Access Layer of cellular networks
 Goal: Switch channels with smallest possible traffic disruption
 IETF Mobile IP Route Optimization, IPv6
 Goal: minimum loss of IP packets
 Wireless ATM
 Goal: Fast re-establishment of a virtual circuit
 Mobile Objects/Agents
 Goal: Reliable interaction among objects (no message loss)
 Heterogeneous wireless Networks
 Goal: Provide continuous service across multiple administrative
domains and wireless technologies
 Seamlesness is also an issue in other fields: e.g. virtual
reality, human-computer interaction, etc.
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Meanings of Seamless
The meaning of “seamless” depends on:
 QoS requirements of the upper network/application
layers
 type of service (e.g. virtual circuit vs. packet switching)
 properties of the wireless network (e.g. degree of cell
overlay, network structure, etc.)
 Examples:
 reliable message delivery in causal order
 delivery of data frames with max delay 
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Motivation
Thesis: Despite the differences in the meaning of
“Seamless” and the tasks of the Handover for a specific
service/network, there exist common patterns of
solutions, which could be described in general terms.
Main Goals:
 Investigate Handover protocols for micro-mobility
 they are a fundamental component of distributed algorithms for
Mobile Networks
 Identify and classify the canonic solution approaches
 may be re-used for different networks/applications
 Define an unifying Framework and identify the common
used techniques
 gives support for developing distributed services for mobile
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networks
Main Concepts
 A Migrating Element (ME) moves between Domains (e.g.
from Domo to Domn), while it is interacting with one or
more Correspondent Elements (CE)
 In each Domain, a Domain Representative (DomRep) is
responsible for serving ME when it is within its Domain
 DomRep is a network access point which delivers
Downstream Events from the network to ME and
Upstream Events from ME to network (e.g. signals,
packets, messages, etc.)
Handover is a distributed protocol between DomRepo,
DomRepn (and propably other network nodes) that
 creates alternative/replicated routes for data,
 establishes the execution context at DomRepn, and
 updates ME´s network-resident state
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Bird Eye’s View:
Major Events
hp
network
hi
hc
NDy
ODx
ME
mp
mi
How to provide the service (event
delivery) in the migration interval
[mi,mc] like before and after
migration?
hi-ack
mc
(serviced by either DomRepo or DomRepn)
{m,h}p : preparation for a migration/handover
{m,h}i : initiation of a migration/handover
{m,h}c: completion of a migration/handover
ODx: old Domain exit
NDy: new Domain entry
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General Approaches
Approaches:
 Non-Coordinated Redundant Service
 Coordinated Redundant Service
 New Domain Service
 Old Domain Service
 Interrupted Service with Exceptions
 Main Differences:
if DomRepo or DomRepn (or both) will serve ME and
if (and how) they synchronize the switching of
service responsibility
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General Approach #1
 Non-Coordinated Redundant Service
 both DomRepo or DomRepn deliver/receive events
 requires access to either DomRepo OR DomRepn during the
migration interval
 redundant events must be identified & filtered out
 non-optimal use of resources
CE
DomRepo
DomRepn
ME
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General Approach #2
 Coordinated Redundant Service
 both DomRepo or DomRepn could serve ME, but agree on a
precise moment to switch service (“switching point”)
 switching point must be synchronized with update of ME´s
network-resident state
 saves resources and avoids filtering of redundant events
CE
DomRepo
synch
DomRepn
ME
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General Approach #3
 New Domain Service
 only DomRepn serves ME during migration interval
 requires NDy  mi
( = happens before)
 DomRepo must forward events to DomRepn
 DomRepn must merge direct and forwarded events
CE
DomRepo
DomRepn
ME
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General Approach #4
 Old Domain Service
 only DomRepn serves ME during migration interval
 requires hc  ODx
 if DomRepn receives Downstream events directly it either buffers
them or redirects them to DomRepn
 may be required if context establishment at DomRepn is timeconsuming, or if DomRepo  DomRepn forwarding is not costeffective
CE
DomRepo
DomRepn
ME
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General Approach #5
 Interrupted Service with Exceptions
 ME is not served by any DomRep during migration interval (i.e.
delivery is interrupted)
 Correspondent Element (or proxy) gets an exception and is
requested to re-send events
 Feasible only if:
 There are no upstream Events (e.g. ME is quiescent during
migration) or proxy at ME buffer events, and
 There is no requirement about timely delivery
 Major advantage: does not require overlapping access areas
CE
Not found
DomRepo
DomRepn
ME
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Case Studies
 Wireless LANs
 Non-Coordinated Reduntant
 DomReps are AccessPoints (hubs)
 all frames are received by all DomReps, which transcode them to
the wireless link
 ME must filter out redundant frames
 Multicast-based Handover  Non-Coordinated Reduntant
 ME has a IP multicast address, and registers with a multicast router
 DomRepo and DomRepn are included in the multicast tree
 multicast router forwards any IP packets to both DomReps, which
relay them to ME
 Mobile IP Seamless Handover  New Domain
 ME informs new Care-of Address to DomRepo, which forwards IP
packets to DomRepn
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 forwarding pointer expires after some time
Case Studies
 Mobile Database Transaction
 Old Domain
 a ME requests read locks at the current DomRep, and even after
migration locks are maintained at DomRepo
 before granting any new lock DomRepn checks for confliciting locks
at previous DomReps (i.e. DomRepo)
 from concurrency control point of view, ME is still at DomRepo
 Wireless ATM
 Coordinated Redundant
 handover is responsible for switching connection between CE and
DomRepo to DomRepn (path redirection at a ATM crossover switch)
 switching point = when new path is set at ATM crossover switch
 Mobile Agent Systems  Interrupted Service & Exceptions
 proxies hold the bindings to the mobile objects
 if proxy attempts to communicate through a stale binding, this
raises an exception, and proxy transparently, gets new location,
updates binding, and re-submits request
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Elements involved
Buffer/Merge
Not Found
DomRepN
Correspondent/
proxy
Router/
Switch
Context Forward
ME
Filter
setNewPath
DomRepO
Lookup
Update
Buffer/Merge
Address
Registry
Interactions and Alternative Paths in a Downstream
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Core Tasks and Mechanisms
 Context Establishment
 prepare/update network for interaction at new Domain
e.g. update router, address registry, discover services
 Stream Uniformness
 mechanisms to guarantee less disruption of event streams
e.g. replication, redirection, retransmission
 Delivery Control
 mechanisms to control/synchronize delivery of events by DomReps
e.g. bufferingOn/Off, Enable/Disable delivery, etc.
 Stream Unification
 mechanisms to merge streams (over different paths) to single
stream
e.g. Event forwarding, Stream merge, filtering, etc.
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Architectural Framework
The five General Approaches differ in how the mechanisms &
tasks are assigned to the several elements and how they
are synchronized.
Hence, we are building an unifying architectural framework,
where the mechanisms are the hot spots.
Framework instantiation will depend on:
 the application requirements
 specifics of the mobile network/system
 specific meaning of “seamless” (e.g. set of properties to be
satisfied in the migration interval)
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Conclusion
Handover is a fundamental part of distributed protocols in
any network with mobility.
For micro-mobility fast handover is desirable, but usually
provided only as “best effort”.
Seamless/Transparent Handover is required for applications
which have strict requirements of Quality of Service.
Although there are several implementations of “seamless
handover” for different networks/services/applications
most solutions employ similar, general approaches.
Our framework shall be useful for designing new protocols for
seamless handover for a wide range of networks and
services.
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Future Research
But there are still many open questions…
 Are there other general approaches? Can the approaches
be further detailed?
 Is it possible to describe each approach as a composition
of sub-approaches for each of the core tasks?
 Is it possible to derive new protocols for seamless
handover by combining known protocols/mechanisms?
 How does location management affect handover protocols?
 How to formalize some of the intuitive meanings of
seamless handover?
 Are there other meanings of seamless from other
application areas?
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General Approaches for
Implementing Seamless
Handover
Questions?
Cellular IP (CIP)
 Routers have cache with soft-state entries (cleared by timeouts)
 Entries are updated or refreshed by data (or Route-update) packets
sent by the ME
 In the overlapping area, packets from both DomReps are received
Route-Update
Packet
CE
DomRepN
CIP Gateway
Route-Update
Packet
ME
Filter
DomRepO
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