WirelessAware Protocol Stacks

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Wireless-Aware Protocol Stacks

• Carey Williamson
• Professor and iCORE Chair
• Department of Computer Science
• University of Calgary

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Introduction

• It is an exciting time to be an Internet
  researcher (or even a user!)
• The last 10-15 years of Internet evolution have
  brought us
• World Wide Web (WWW)
• Media streaming applications
• Peer-to-peer (P2P) applications
• Wi-Fi wireless LANs and hotspots
• Mobile/pervasive/ubiquitous computing

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Main Theme of this Talk

• Existing layered Internet protocol stack does not
  lend itself well to providing optimal performance
  for diversity of service demands and environments
• Subtle multi-layer protocol interactions
• Challenge performance transparency
• Who should bend users or protocols?
• Explore the role of awareness in (wireless)
  Internet protocol performance

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Network Protocol Stack

• Application supporting network applications and
  end-user services
• FTP, SMTP, HTTP, DNS, NTP, ...
• Transport end to end data transfer
• TCP, UDP, RTP, SCTP, XTP
• Network routing of datagrams from source to
  destination
• IPv4, IPv6, BGP, RIP, routing protocols
• Data Link frames, channel access, flow/error
  control
• PPP, Ethernet, IEEE 802.11a/b/g
• Physical raw transmission of bits

001101011...
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Viewpoint

• Layered design is good
  layered implementation is bad - AST??
• Good
• unifying framework for describing protocols
• modularity, black-boxes, plug and play
  functionality, well-defined interfaces (SE)
• Bad
• increases overhead (interface boundaries)
• compromises performance (ignorance)

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Main Example TCP

• The Transmission Control Protocol (TCP) is the
  protocol that sends your data reliably
• Used for email, Web, ftp, telnet,
• Makes sure that data is received correctly right
  data, right order, exactly once
• Detects and recovers from any problems that occur
  at the IP network layer
• Mechanisms for reliable data transfer
• sequence numbers, ACKs, flow control, timers,
  retransmissions, congestion control...

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In Praise of TCP

• TCP is the 4 wheel drive of transport layer
  protocols
• general purpose, robust, go anywhere
• The TCP protocol has undergone only minor changes
  in 30 years of existence
• original version circa 1974
• congestion control mechanism 1988
• TCP has witnessed dramatic changes in network
  technology and in computing technology over that
  same time period!

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Criticisms of TCP

• TCP is a high overhead protocol
• hefty headers (20-byte TCP, 20-byte IP)
• extra network packets for handshaking
• extra RTTs for handshaking
• Performance problems aplenty
• high delay-bandwidth product networks
• multiple packet losses in same window
• fairness problems (RTT bias, phasing, losses)
• World Wide Web (small transfers vs large ones)
• wireless networks (losses, mobility, etc.)

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The Problem Restated
TCP or not TCP? That is the question!
- Hamlet, Act 3, Scene 1
William Shakespeare (1608)
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TCP Performance Problems

• Examples
• TCP over ATM Networks
• TCP and the Web
• TCP over Wireless Networks
• TCP over Wireless Ad Hoc Networks
• Wireless Media Streaming Performance

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Example 1a TCP over ATM

• TCP on 10 Mbps Ethernet LAN Boggs88
• throughput approximately 9.5 Mbps
• TCP on 140 Mbps ATM LAN Moldeklev94
• throughput approximately 0.2 Mbps!!!
• Why? TCP deadlock problem
• large ATM MTU size, relatively small TCP
  send/receive socket buffer sizes
• interaction between TCP delayed ACKs and Nagles
  Algorithm and socket copy rules
• Solution proper config gt 70 Mbps

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Example 1b TCP over ATM

• TCP throughput and efficiency suffer over ATM
  networks Romanow94
• Why? The jigsaw-puzzle problem
• TCP packets large ATM cell size small
• one TCP packet many ATM cells (N)
• under overload, ATM switch discards cells
• ATM cells do not have sequence numbers!!!
• retransmission at the TCP packet layer only
• lose 1 cell, resend N cells for each packet loss
• Solution
• mark packet boundaries use PPD/EPD

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Example 2 TCP and the Web
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Example 2 TCP and the Web
The classic approach in HTTP/1.0 is to use
one HTTP request per TCP connection, serially.
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Example 2 TCP and the Web
The persistent HTTP approach can re-use
the same TCP connection for multiple HTTP
transfers, one after another, serially. Amortizes
TCP overhead, but maintains TCP state longer at
server. Mogul 1995
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Example 3 TCP over Wireless

• Wireless TCP Performance Problems

Low capacity, high error rate
Wired Internet
High capacity, low error rate
Wireless Access
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Example 3 TCP over Wireless

• Solution wireless-aware TCP (I-TCP, ProxyTCP,
  Snoop-TCP, split connections...)

The assumption Loss Congestion no longer
applies
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Example 4a TCP over Ad Hoc

• Multi-hop ad hoc networking

Steve
Carey
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Example 4a TCP over Ad Hoc

• Multi-hop ad hoc networking

Steve
Carey
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Example 4a TCP over Ad Hoc

• Multi-hop ad hoc networking

Steve
Carey
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Example 4a TCP over Ad Hoc

• Multi-hop ad hoc networking

Steve
Carey
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Example 4a TCP over Ad Hoc

• Problem TCP vs Dynamic ad hoc routing
• Node movement can disrupt the IP routing path at
  any time, disrupting TCP connection
• Yet another way to lose packets!!!
• Route discovery delays are unpredictable
• can vary from 10 ms to 5 s or more Gupta 2004
• Possible solutions
• Explicit Loss Notification (ELN) Balakrishnan et
  al.1998
• Wireless loss inference techniques Liu et al.
  2003
• Fast TCP handoffs Tan et al. 1999

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Example 4b TCP over Ad Hoc
(Assume ideal world)
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Example 4b TCP over Ad Hoc
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Example 4b TCP over Ad Hoc
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Example 4b TCP over Ad Hoc
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Example 4b TCP over Ad Hoc
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Example 4b TCP over Ad Hoc
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Example 4b TCP over Ad Hoc
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Example 4b TCP over Ad Hoc
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Example 4b TCP over Ad Hoc
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Example 4b TCP over Ad Hoc
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Example 4b TCP over Ad Hoc
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Example 4b TCP over Ad Hoc
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Example 4b TCP over Ad Hoc
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Example 4b TCP over Ad Hoc
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Example 4b TCP over Ad Hoc
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Example 4b TCP over Ad Hoc
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Example 4b TCP over Ad Hoc
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Example 4b TCP over Ad Hoc
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Example 4b TCP over Ad Hoc
(Reality check)
Issue 1 node A cant use both of these links at
the same time - shared wireless
channel - transmit or receive, but not
both
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Example 4b TCP over Ad Hoc
Issue 2 cant use both of these links at same
time - range overlap at A -
hidden node problem - exposed node
problem
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Example 4b TCP over Ad Hoc
Issue 3 LOTS of contention for the channel - in
steady state, all stations want to send - need
RTS/CTS to resolve contention
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RTS Request-To-Send CTS Clear-To-Send
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Example 4b TCP over Ad Hoc
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RTS
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Example 4b TCP over Ad Hoc
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Example 4b TCP over Ad Hoc
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Example 4b TCP over Ad Hoc
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Example 4b TCP over Ad Hoc
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Example 4b TCP over Ad Hoc
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Example 4b TCP over Ad Hoc
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Example 4b TCP over Ad Hoc
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Example 4b TCP over Ad Hoc
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Example 4b TCP over Ad Hoc
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Example 4b TCP over Ad Hoc
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Example 4b TCP over Ad Hoc
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Example 4b TCP over Ad Hoc
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Example 4b TCP over Ad Hoc
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Example 4b TCP over Ad Hoc
Issue 4 TCP uses ACKS to indicate reliable data
delivery - bidirectional traffic (DATA, ACKS) -
even more contention!!!
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Example 4b TCP over Ad Hoc
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Example 4b TCP over Ad Hoc
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Example 4b TCP over Ad Hoc
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Example 4b TCP over Ad Hoc
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Example 4b TCP over Ad Hoc
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Example 4b TCP over Ad Hoc
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Example 4b TCP over Ad Hoc
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Example 4b TCP over Ad Hoc
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Example 4b TCP over Ad Hoc
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Example 4b TCP over Ad Hoc
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Example 4b TCP over Ad Hoc
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Example 4b TCP over Ad Hoc
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Example 4b TCP over Ad Hoc

• Problem TCP flow control vs. wireless ad hoc
• The bursty nature of TCP packet transmissions
  creates contention for the shared wireless
  channel among forwarding nodes
• Causes intra-flow and inter-flow contention
• Possible solutions
• Bound TCP window size Fu et al. 2003
• Link-layer RED Fu et al. 2003
• Rate-based flow control Gupta et al. 2004
• Multi-channel MAC protocols Kuang et al. 2004
• Careful channel scheduling Wormsbecker et al.
  2006

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Example 5 Wireless Streaming
Wireless Sniffer
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Example 5 Contd
Wireless Sniffer
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(No Transcript)
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The Bad Apple Phenomenon

• What? One user with poor or transient wireless
  connectivity in the WLAN disrupts performance for
  everyone!!!
• Why? Shared broadcast WLAN lots of MAC-layer
  retransmissions FIFO server queue Head of Line
  (HOL) blocking
• Solutions?
• Disable MAC-layer retransmissions (yuck!)
• Multiple queues and packet scheduling
• Station-based adaptation Cao et al. 2006

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Some Examples of Our Work

• Context-Aware (CATNIP) TCP
• Multi-Channel MAC Protocols
• Bi-directional MAC protocols (Bi-MCMAC)
• Multi-Rate Multi-Channel (MRMC) WLANs
• Fairness Issues in IEEE 802.11b WLANs
• Wireless Cross-Layer Design
• Scalable Wireless Media Streaming

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Example 1 CATNIP TCP

• Context-Aware Transport/Network Internet
  Protocol (CATNIP)
• Motivation Like kittens, TCP connections are
  born with their eyes shut - CLW
• Question How much better could TCP perform if it
  knew what it was trying to do (e.g., 14 KB Web
  document transfer)?

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Motivation for CATNIP TCP

• Main observation
• Not all packet losses are created equal
• Losses early in the transfer have a huge adverse
  impact on the transfer latency
• Losses near the end of the transfer always cost a
  retransmit timeout
• Losses in the middle may or may not hurt,
  depending on congestion window size at the time
  of the loss (because of the TCP fast retransmit
  mechanism)

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Web/TCP Pain Profile
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Design of CATNIP

• Make TCP smarter by conveying application-layer
  context to TCP/IP
• modifies socket API

Application
Transport
Network
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CATNIP TCP Sources

• What could sources do differently?
• Rate-Based Pacing of Last Window (RBPLW)
• Early Congestion Avoidance (ECA)
• Selective Packet Marking (SPM) use a 1-bit field
  in the reserved portion of the TCP/IP header to
  convey packet priority information
• 0 low priority 1 high priority (crucial
  pkts)
• SPM is the most effective Wu et al. 2002
• Implementable using DiffServ codepoints Wu et
  al. 2006

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CATNIP in the Internet

• What could IP router do differently?
• If it knew which packets mattered most
• CATNIP-Good avoid discarding them when
  congested (if possible)
• CATNIP-Bad throw them away!

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Simulation/Emulation Results

• Sources have relatively little control
• IP routers have all the power
• Adding context-awareness at IP routers improves
  both mean and standard deviation of Web page
  transfer times (e.g., by 20-60 for 1-5 packet
  loss)
• SPM and CATNIP-Good provide most of the benefit

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Example 2 Bi-MCMAC Protocol
Issue 2 cant use both of these links at same
time - range overlap at A -
hidden node problem - exposed node
problem
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Multi-Channel MAC Protocols
- Sender transmits RTS on control channel,
indicating candidate set of channels to use - CTS
reply indicates channel to use (if any) - Sender
and receiver tune to that channel
for the duration of that packet transmission
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A big win! - Confines contention to control
channel only - Reduces hidden node/exposed node
problems - Shortens the contention distance (3
to 2) - Significantly improves throughput
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Bidirectional Reservations
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Old way RTS CTS DATA 3 (ack) RTS CTS ACK 1
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Our new way RTS CTS CRN DATA 3 ACK 1 (ack)
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DATA/ACK
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Performance Evaluation

• Simulation using ns-2 network simulator
• Three protocols
• 802.11 MAC, MCMAC, Bi-MCMAC
• Static ad hoc networks
• Performance metrics
• Fairness
• Throughput (FTP-like bulk data transfer
  applications)
• Response Time (Web-like applications)

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Fairness Example
MCMAC
802.11 MAC
Bi-MCMAC
Total 736 kbps
Total 1406 kbps
Total 1528 kbps
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Summary and Conclusions

• There are performance advantages to bending the
  rules in the layered Internet protocol stack
  (esp. wireless!)
• The general notion of awareness is worth
  exploring in many contexts
• wireless networks, ad hoc routing, TCP/IP, mobile
  computing, adaptive applications
• wireless mesh networks, sensor networks
• Many interesting issues to explore!!

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The End

• For more information
• Email carey_at_cpsc.ucalgary.ca
• Web URL www.cpsc.ucalgary/carey
• Credits
• Guangwei Bai, Jean Cao, Mingwei Gong, Abhinav
  Gupta, Tianbo Kuang, Hongxia Sun, Ian
  Wormsbecker, Qian Wu
• The rest of my iCORE research team
• Questions?