CS 268: EndHost Mobility and AdHoc Routing

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CS 268 End-Host Mobility and Ad-Hoc Routing

• Ion Stoica
• Feb 18, 2004

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Overview

• Wireless
• End-host mobility
• Ad-hoc routing

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Wireless

• Wireless connectivity proliferating
• Satellite, line-of-sight microwave, line-of-sight
  laser, cellular data (CDMA, GPRS, 3G), wireless
  LAN (802.11a/b), Bluetooth
• More cell phones than currently allocated IP
  addresses
• Wireless ? non-congestion related loss
• Signal fading distance, buildings, rain,
  lightning, microwave ovens, etc.
• Non-congestion related loss ?
• Reduced efficiency for transport protocols that
  depend on loss as implicit congestion signal
  (e.g. TCP)

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Problem
Best possible TCP with no errors (1.30 Mbps)
TCP Reno (280 Kbps)
Sequence number (bytes)
Time (s)
2 MB wide-area TCP transfer over 2 Mbps Lucent
WaveLAN (from Hari Balakrishnan)
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Solutions

• Modify transport layer
• Modify link layer protocol
• Hybrid

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Modify Transport Protocol

• Explicit Loss Signal
• Distinguish non-congestion losses
• Explicit Loss Notification (ELN) BK98
• If packet lost due to interference, set header
  bit
• Only needs to be deployed at wireless router
• Need to modify end hosts
• How to determine loss cause?
• What if ELN gets lost?

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Modify Link Layer

• Advantages
• Limited changes only link-layer affected
• Preserve end-to-end (TCP) semantics
• Three types of losses
• Total packet loss
• Partial packet loss
• Packet corrupted by bit errors
• Three methods to reduce packet loss
• Packet retransmission
• Forward error correction
• Packet shrinking

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Retransmission

• Advantages
• Optimal overhead only lost packets are
  retransmitted
• Disadvantages nasty interactions between TCP
  control look and link-level retransmission
• Both TCP and link-layer can retransmit same
  packets
• Can introduce packet reordering
• Can introduce highly variable delays

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FEC

• Forward Error Correction (FEC) codes
• k data blocks, use code to generate ngtk coded
  blocks
• Can recover original k blocks from any k of the n
  blocks
• n-k blocks of overhead
• Trade bandwidth for loss
• Can recover from loss in time independent of link
  RTT
• Useful for links that have long RTT (e.g.
  satellite)
• Pay n-k overhead whether loss or not
• Need to adapt n, k depending on current channel
  conditions

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FEC Packet Shrinking

• Advantages
• No changes at end hosts or base-stations above
  link-layer
• Decrease packet loss
• Do not introduce variability
• Disadvantages
• Overhead can be quite high, e.g., packet
  segmentation/reassembly, encoding/decoding

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Flex Eckhardt Steenkiste 98

• Combine the three types of error control ? seven
  policies (three fixed and four adaptive)
• Most sophisticated Flex
• When two or more packets in a window of ten are
  truncated ? reduces safe packet size by 85
• When three consecutive packets do not experience
  truncation ? linearly increase packet size
• When two or more packets in a window of ten
  cannot be decoded ? decrease user data by 15
  (more conservative coding)
• When three consecutive packets can be decoded ?
  increase user data linearly
• Note adaptation exhibits a linear-increase
  multiplicative-decrease behavior

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Hybrid Indirect-TCP Bakre Badrinath 94

• Split TCP connection into 2 TCPs
• Advantages
• Optimize performance for wireless TCP
• No changes to protocol for fixed hosts
  (transparent to fixed hosts)
• Disadvantages
• Violate end-to-end TCP semantics (why?)
• High overhead, because dual stack at BS
• Might introduce high delays because packet
  buffering

wireless TCP
regular TCP
Internet
Mobile Host (MH)
Fixed Host (FH)
Base Station (BS)
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Hybrid Snoop-TCP Balakrishnan et al. 95

• Insert a snoop agent between fixed host (FH)
  and mobile host (MH)
• Monitor traffic, retransmit packets and discard
  acknowledgements
• Notes
• Avoid violating end-to-end semantics
• What about layering?

TCP
Internet
Mobile Host (MH)
packet retransmissions
Fixed Host (FH)
Base Station (BS)
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Overview

• Wireless
• End-host mobility
• Ad-hoc routing

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Motivation and Problem

• Network Layer mobility
• Movement IP address change
• Problem
• Location
• I take my cell phone to London
• How do people reach me?
• Migration
• I walk between base stations while talking on my
  cell phone
• I download or web surf while riding in car or
  public transit
• How to maintain flow?

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Solutions

• Mobile IP (v4 and v6)
• TCP Migrate
• Multicast

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Mobile IP

• Use indirection to deal with location and
  migration
• Point of indirection Home Agent (HA)
• Resides in Mobile Hosts (MH) home network
• Uses MHs home IP address
• As MH moves, it sends its current IP address to
  HA
• Correspondent Host (CH) contacts MH through HA
• HA tunnels packets to MH using encapsulation
• MH sends packets back to CH
• Tunnels packets back to HA (bi-directional
  tunneling)
• Sends directly to CH (triangle routing)

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Mobile IP Properties

• Advantages
• Preserves location privacy
• CH does not have to be modified
• Disadvantages
• Triangle routing and especially bidirectional
  tunneling increase latency and consume bandwidth
• HA is single point of failure

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Mobile IP Route Optimization

• CH uses HA to contact MH initially
• MH sends its location directly back to CH
• CH and MH communicate directly
• Lose location privacy
• CH must be modified

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TCP Migrate SB00

• Location uses dynamic DNS updates
• When MH moves to new IP address, it updates its
  home DNS server with new hostname to IP address
  mapping
• Migration
• When MH moves, it sends update to CH
• Advantage
• No new infrastructure
• Incremental deployable
• Efficient routing
• Disadvantages
• Only works for TCP
• Both CH and MH need new TCP implementation
• No location privacy

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i3 Based Mobility (Z03)

• Receiver R maintains a trigger (id, R) in the i3
  infrastructure sender sends packets to id
• Advantages
• Support simultaneous mobility
• Efficient routing receiver can chose id to map
  on a close i3 server
• Ensure privacy
• Disadvantage
• Require a new infrastructure

Sender
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Other solutions

• Network specific mobility schemes
• Cellular phones, 802.11b
• Cannot handle mobility across networks (e.g. move
  laptop from cell phone to 802.11b) or between
  same network type in different domains (e.g.
  laptop from Soda Hall 802.11b to campus 802.11b)
• Other mobility models
• Terminal/personal mobility
• e.g.accessing email through IMAP from different
  computers
• Session mobility
• e.g. talking on cell phone, transfer call in
  progress to office phone

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Summary

• Not that important today
• Few portable, wireless IP telephony devices
• Cell phones have their own network-specific
  mobility schemes
• IP-based wireless networks are not ubiquitous
  enough to be seamless
• PDA (e.g. palm pilot) are too weak to do handle
  long-lived flows
• Future
• Cellular networks will become IP-based, need IP
  mobility scheme
• PDA are becoming more powerful

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Overview

• Wireless
• End-host mobility
• Ad-hoc routing

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Motivation

• Internet goal decentralized control
• Someone still has to deploy routers and set
  routes
• Ad Hoc routing
• Every node is a router
• Better wireless coverage
• Better fault tolerance (e.g. node bombed, stepped
  on, exhausted power)
• No configuration (e.g. temporary association)
• Dedicated router costs money

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Routing

• DSDV destination-sequenced distance vector
• TORA Temporally-Ordered Routing Algorithm
• DSR Dynamic Source Routing
• AODV Ad hoc On-demand Distance Vector
• TORA, DSR, and AODV are all on-demand routing
  protocols

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DSDV

• Hop-by-hop distance vector
• Routing table contains entries for every other
  reachable node
• Nodes pass their routing tables to neighbors
  periodically
• Routing tables are updates using standard
  distance vector algorithm
• Old routes are ignored using sequence numbers
• O(n) routing state / node, O(nk) communication
  size / node / period
• k average node degree

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TORA

• Temporally-Ordered Routing Algorithm
• Interested in finding multiple routes from S?D
• Find routes on demand
• Flood query to find destination
• Flood query response to form multiple routes
• O(m) routing state / node, O(nk) communication /
  node / route update
• m nodes to communicate with worst case mn

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DSR

• Dynamic Source Routing
• Packet headers contain entire route
• Flood query to find destination
• Intermediate nodes dont have to maintain routing
  state
• Nodes listen for and cache queries, responses as
  optimization
• Nodes gratuitously sends response packets to
  shorten paths when they hear packets with
  sub-optimal routes
• O(m) routing state / nodes, O(nk) communication
  / node / route update
• Smaller constant than other protocols
• O(n1/k) space required in header

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AODV

• Ad Hoc On-Demand Distance Vector
• Flood query to find destination
• Reply is sent back to source along the reverse
  path
• Intermediate nodes listen for reply to set up
  routing state
• State is refreshed periodically
• O(m) routing state / node, O(nk) communication /
  node / route update

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Results

• Avoid synchronization in timers
• TORA does not scale to 50 node
• Suffers control traffic congestion collapse
• DSDV fails to deliver packets when movement is
  frequent
• Only maintains one route/destination
• AODV has high routing overhead when movement is
  frequent
• Combination of DSDV maintenance of state
  flooding of DSR
• DSR does well compared to others
• Designed by authors ? not surprising!
• LJC00 shows congestion collapse beyond 300
  nodes

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Related Work

• Greedy Perimeter Stateless Routing (GPSR) Karp
  and Kung, Mobicom 2000
• Separate addressing from naming
• Assume everyone has GPS
• Do Cartesian routing
• Separate scalable, efficient, fault tolerant
  service to map from names to addresses
• How to deal with selfish users? MGL00
• Listen to neighbors to make sure they are
  forwarding
• Convey black list information back to source
• Route around selfish nodes

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Conclusions

• Proliferation of wireless network interfaces
  provide ready market
• Ad hoc provides less configuration, more fault
  tolerance, better coverage, lower cost
• Many interesting and unsolved problems