Transcript Mobile IP: Introduction

Cellular IP: Introduction
Reference: “Design, implementation, and evaluation of cellular IP”;
Campbell, A.T.; Gomez, J.; Kim, S.; Valko, A.G.; Chieh-Yih
Wan; Turanyi, Z.R.; IEEE Personal Communications,
Volume: 7 Issue: 4, Aug. 2000; Page(s): 42 –49
(CellularIPIntro-3.pdf)
Introduction
• Trend
– Large numbers of mobile users equipped with
wireless IP-enabled communicators will have
access to a wide array of Web-based mobile
multimedia services
– High-speed access can be achieved by using
smaller and smaller cell sizes
 Resulting in coverage area with a larger number of
base stations
 Picocellular environments: simple, low-cost wireless
infrastructure
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Introduction (cont)
• Seamless mobility
– The ability of the network to support fast handoff
between base stations with low delay and minimum or
zero packet loss
• Paging
– In essence, Mobile hosts will be in an idle state but
passively connected to the network infrastructure
– It will be sufficient for the wireless Internet only to know
the approximate location of its population of idle users
– The exact location of idle mobile hosts only becomes
important when data needs to be forwarded to them, in
which case the network needs to be able to efficiently
search and find these users in a scalable and timely
manner
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Introduction (cont)
• Passive connectivity
– Efficient location tracking in support of idle users and
paging in support of active communications
– Handle the location tracking of active and idle mobile
hosts independently
– E.g. keeping the approximate location information of
idle users requires less signaling and thus reduced the
load of the network (air and wired)
• Mobile IP doest not support the notion of
seamless mobility, passive connectivity, or
paging  Cellular IP (micro mobility)
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Related Work
• Hawaii
– Support seamless mobility, passive connectivity,
and paging
– Hawaii nodes are IP routers
 Assume intra-domain routing protocol is operating in the
access network, allowing each node to have routes to
other nodes
 The routing information is used to exchange explicit
signaling messages, and forward packets between old
and new access points during handoff
 For large number of mobile hosts (e.g. tens of thousands),
the use of an all-IP-based router solution for picocellular
networks may become prohibitively expensive
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Related Work (cont)
• About Cellular IP
– Layer 3 routing protocol
 Replaces IP routing but without modifying the IP packet
format and forwarding mechanism
 Location management is integrated with routing
 Per-host location information stored in Cellular IP nodes
is the next hop route for a given mobile host
– Adopting the learning feature of Ethernet switches
 Layer 2 solution (low cost to support large number of MH)
 Cellular IP uses data packets to refresh location
management state and can operate at layer two or three
 The use of explicit signaling messages is limited by using
the IP data packets to convey location and paging
information
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Protocol Overview
• Network model
Mapping (X, BS3)
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Protocol Overview (cont)
• Base station
– Wireless access point and router of IP packets
while performing all mobility-related functions
– Built on a regular IP forwarding engine with the
exception that IP routing is replaced by
Cellular IP routing
• Gateway router
– Connects Cellular IP networks to the Internet
– Mobile hosts attached to an access network
use the IP address of the gateway as their
Mobile IP care-of-address
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Protocol Overview (cont)
• General principles
– To minimize control messaging, regular data
packets transmitted by mobile hosts are used
to refresh host location information
– Uplink packets are routed from a mobile host
to the gateway on a hop-by-hop basis
– The path taken by these packets is cached by
all intermediate base stations
– To route downlink packets addressed to a
mobile host, the path used by recently
transmitted packets from the mobile host is
reversed
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Protocol Overview (cont)
– When the mobile host has no data to transmit,
it sends a small, special IP packets toward the
gateway to maintain its downlink routing state
– Passive connectivity: mobile hosts that have
not received packets for some period of time
allow their downlink routes to be cleared from
the cache (soft state timer)
– Paging is used to route packets to idle mobile
hosts
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CIP Routing
• Cellular IP gateway
– Periodically broadcasts a beacon packet that is
flooded in the access network
• Base stations
– Record the neighbor they last received this
beacon from and use it to route packets toward
the gateway
• Packet transmission
– All packets transmitted by mobile hosts,
regardless their destination address, are routed
toward the gateway using these routes
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CIP Routing (cont)
– As these packets pass each node en route to the
gateway, their route information is recorded in
the routing cache of each base station en route
– For a mobile host that is not regularly
transmitting data packets, to keep its routing
cache mappings valid, the host transmits route
update packets in the uplink at regular intervals
called route-update time
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CIP Handoff
• Hard handoff
– Mobile hosts listen to beacons transmitted by
base stations and initiate handoff based on
signal strength measurement
– To perform a handoff, a MH tunes its radio to a
new BS and sends a route-update packet
– The route update message creates routing
cache mappings en route to the gateway
configuring the downlink route cache to point
toward the new base station
– Define handoff latency = the time between
handoff initiation and the arrival of the first
packet along the new route
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CIP Handoff (cont)
– For hard handoff, handoff latency = round-trip
time between the MH and the crossover base
station. In the worst case, the crossover point
is the gateway
– During the interval of handoff latency,
downlink packets may be lost
– Although packets may get lost during a hard
handoff, the time taken to redirect packets to
the new point of attachment is shorter than
that in Mobile IP
– Method to reduced packet loss during handoff:
relying on interaction between the old and new
base stations (the idea of HAWAII)
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Hard Handoff
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Semi-soft handoff
– Reduce packet loss during handoff
– 1. In order to reduce handoff latency, the
routing cache mappings associated with the
new base station must be created before the
actual handoff takes place
– Before a mobile host hands off to a new base
station, it sends a semi-soft packet to the new
base station and immediately returns to
listening to the old base station
– The semi-soft packet is used to establish new
routing cache mappings between the
crossover and new base stations
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Semi-soft handoff (cont)
– 2. After a semi-soft delay, the mobile host
performs a regular handoff
– The semi-soft delay can be an arbitrary value
that is proportional to the mobile-to-gateway
round-trip time
– The delay ensures that by the time the MH
finally tunes its radio to the new BS, its
downlink packets are being delivery through
both the old and the new BS
– i.e. the downlink packets consumes twice the
amount of resources during the period of
semi-soft delay
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Semi-soft handoff (cont)
– Q: does semi-soft handoff ensures smooth
handoff (no packet loss during handoff) ?
– Consider the time to transmit packets from the
crossover point to the new base station
– Case 1. New BS is behind the old one
 Duplicate packets (which does not disrupt many APs)
– Case 2. New BS is ahead
 Packets will be missing
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Semi-soft handoff (cont)
Crossover
Crossover
Pkt i
Pkt i
Pkt i
Old BS
MH
New BS
MH
New BS
MH
Case 1
Pkt i
Old BS
MH
Case 2
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Semi-soft handoff (cont)
– Solution for Case 2: by temporarily introducing
a constant delay along the new path between
the crossover and new base stations using a
simple delay device mechanism (delay buffers)
– Optimally, the device delay should be located
at the crossover base station
– After handoff is complete, the mobile host
sends a data or route-update packet along the
new path, which disables the delaying
mechanism.
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CIP Paging
• Base stations are geographically grouped
into paging areas
• When there is no call ongoing, mobile
hosts only need to report their position to
the network if they move between paging
areas
• This makes location update and handoff
support for idle hosts unnecessary
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Paging Areas
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CIP Paging (cont)
• Idle mobile host
– That has not transmitted packets for a systemspecific active-state-timeout
– Transmits paging-update packets at regular
intervals defined by a paging-update-time
• Base stations
– May optionally maintain paging cache
– Paging cache mappings have a longer timeout
period called paging-timeout, hence a longer
interval exists between consecutive pagingupdate packets
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CIP Paging (cont)
• Paging action
– When a packet is addressed to an idle mobile host, and
the gateway or base stations find no valid routing cache
mapping for the destination
– If the base station has no paging cache, it will forward the
packet to all of its interfaces except the one the packet
came through
– Base stations that have paging cache will only forward a
paging packet (the first data packet) to the mapped
interface if the destination has a valid paging cache
mapping
– If there is no paging cache in an access network, the first
packet addressed to an idle mobile will be broadcast
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CIP Security
• Only authenticated (control) packets can
establish or change cache mappings
• Data packets can only refresh existing
mappings
• Session keys used by mobile hosts to
perform authentication must be promptly
available at the new base station during
handoff
• The session key is calculated using an
MD5 hashing function
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CIP Security (cont)
• A special session key is used in CIP access
networks, and each BS can independently
calculate session keys
• The session key is a secure hash, which combines:
– The IP address of a MH (IPMH)
– A random # (RMH) assigned to a MH when it first registers
with an access network
– A network secret (Knetwork) known by all BS within in an
access network
• Session key Ksession = MD5 (IPMH, RMH, Knetwork)
• A session key is first calculated and transmitted to
a MH when it first contacts the CIP network during
global mobility authentication and authorization
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CIP Security (cont)
AAA
Ksession ,RMH
MH
Route-Update
BS
IPMH RMH Authentication Info.
Encryption with Ksession
1. BS can quickly calculate the session
key by combining the IP address and
the random value found in the control
packet with the network secret.
2. BS can validate authentication easily
with the session key.
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Performance Evaluation
• Testbed
100Mbps full duplex
2Mbps WaveLAN
MH can dynamically change frequency to perform handoff
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UDP Performance
MH Handoff every 5sec
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UDP Performance (cont)
• Input
– The mobile host receives 100 byte UDP
packets at rates of 25 and 50 packets/s while
making periodic handoffs between B2 and B3
every 5s
• Discussion
– Hard handoff causes packet losses
proportional to the round-trip time and to the
downlink packet rate
– Semi-soft handoff eliminates packet loss
 Buffering a single packet in the delay device is
sufficient to eliminate loss even in the case of a large
round-trip time where hard handoff results in the
loss of up to 4 packets
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TCP Performance
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TCP Performance (cont)
• Input
– TCP Reno, downloading 16 Mbytes of data
from a correspondent host to a mobile host
• Discussion
– Hard handoff
 The performance of TCP degrades as the handoff
frequency increases due to packet loss
– Semi-soft handoff
 Reduced packet loss and significantly improved the
transport throughput
 For 8-packet delay buffer, the packet loss is
eliminated at the higher handoff rates
 The results look promising
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TCP Performance (cont)
• The throughput measured at zero handoff rate
is marginally lower than 1.6Mbps achieved
using standard IP routing
– 1. IP is implemented in the kernel and CIP in user
space
– 2. CIP uses PCAP (Berkeley Packet Filter’s Packet
Capture Library) to forward packets which is not
optimized for IP forwarding.
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Scalability
• High throughput with large # of mobile hosts
– Efficient routing cache search
– Routing cache misses  search paging cache
• Input
– Permanent, random cache mappings
– Multi-homed 300MHz PC base station
– Radio interface  100Mbps Ethernet
• Discussion
– Operation of CIP routing cache is very similar to the
self-learning operation of Ethernet switches
– CIP software base stations are capable of supporting
large # of mobile hosts and high aggregate
throughput
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Scalability (cont)
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Future Work
• Quality of service provisioning
• Multiple gateways in Cellular IP Networks
– Mobile host should be capable of changing
gateways (care-of-address) during normal
operations
• Specification, source code for Cellular IP
– Web page: comet.columbia.edu/cellularip
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