Improving TCP Performance over Mobile Networks

1
Improving TCP Performance over Mobile Networks
ACM Computing Surveys 2002

• HALA ELAARAG
• Stetson University
• Speaker Aron

2
Overview

• Regular TCP
• The Problems ?
• Why regular TCP is not suitable?
• The solutions to improve the performance
• Link layer
• End to end
• Split connection
• Conclusions

3
Introduction

• Mobile users would like to use the same
  applications over the wireless link and with the
  same quality of service (QoS) they are getting
  over a wired link.
• Objective to improve the performance of TCP over
  mobile wireless networks.

4
The behavior of regular TCP

• Congestion control
• Slow-start
• Congestion avoidance
• Fast Retransmit

5
Problems with wireless and mobile networks

• High bit error rates
• Disconnections
• Limited and variable bandwidth
• Cell size
• Power scarcity
• Dynamic network topology

6
Why regular TCP is not suitable?

• TCPs main problem is the delay caused by packet
  losses due to congestion.
• Wired links have low bit error rates (BER), as
  opposed to wireless links that suffer from high
  bit error rates.
• If regular TCP is used on a mobile network, it
  can severely degrade performance.

7
Problems of mobile TCP implement

• Non-congestion delay
• Serial timeouts
• Packet size variation

8
The solutions to improve the performance

• Link layer protocols
• RLP
• AIRMAIL
• Snoop
• End-to-end protocols
• Reno
• New-Reno
• SACK
• FR
• EBSN
• Split-connection
• MTCP
• I-TCP
• M-TCP
• WAP

9
Link layer protocols

• Objective
• Increase the quality of the lossy wireless link
• Solve the problem at the link layer
• Transport layer protocol is too slow to recover
  from losses
• Congestion control mechanisms of transport layer
  are unnecessarily triggered, throughput is
  decreased

10
Link layer protocols RLP

• Radio Link Protocol (RLP)
• Automatic Repeat reQuest (ARQ) for radio
  channels.
• Retransmit a packet when transmitter make sure it
  was not received.
• May Solve High bit error rates at link layer

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Link layer protocols AIRMAIL

• Approaches to improve link layer protocol
  performance
• Automatic Repeat reQuest (ARQ)
• Forward Error Correction (FEC)
• Mobility and handoff processing by window
  management
• and state transfer

12
Link layer protocols Snoop

• Snoop protocol (TCP-aware link-layer schemes)
• Introduce a module--snoop agent at the base
  station
• monitors every packet that passes through the TCP
  connection in both directions
• Maintains a cache of TCP packets sent from the
  sender that havent yet been acknowledged by the
  receiver.
• If detect packet loss (use duplicate ACKs or
  local timeout), retransmit the packet if it is in
  the cache and suppress the duplicate ACKs

13
End-to-end protocols
Reno? New-Reno?SACK TCP

• Change Fast Retransmit to include Fast Recovery
• New-Reno?SACK TCP improve the performance when
  multiple packets lost in the same window
• Improvement of regular TCP in wireless is
  expected to be limited

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End-to-end protocols FR

• Fast retransmission scheme
• Providing smooth hand-offs on networks that lose
  packet during handoff.
• When mobile IP software signal hand-off
  complete,
• mobile host signals fixed host to invoke
  retransmission scheme.
• Focus on hand-off

15
End-to-end protocols EBSN

• Explicit Bad State Notification
• Base Station sends EBSN message to sender if
  packets cannot be transmitted successfully
• Sender changes Timeout based on current RTT
• Timeout is reset to original on receipt of new
  ack.
• Eliminates unnecessary timeouts

16
Split Connection Schemes

• Divide TCP connection into 2 connections
• Isolate wired network from wireless network

TCP II
TCP I
Wired Link
Wireless Link
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Split-connection MTCP

• Protect the wired connection from the impact of
  the erratic behavior of wireless connection
• Use session layer protocol at BS and MH
• Selective Repeat Protocol (SRP) recover quickly
  packet loss

TCP II
TCP I
Wired Link
Wireless Link
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Split-connection I-TCP

• I-TCP (Indirect TCP)
• The idea is the same with MTCP

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Split-connection M-TCP

• Three-level hierarchy architecture

High-Speed Network
Supervisor Host
SH
SH
Cell
Mobile Support Station(MSS)
Mobile Host(MH)
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M-TCP cont.

• End-to-end TCP connection

• TCP connection is split at the SH
• The SH does not send an ack to FH unless SH has
  received an ack from MH
• Maintains end-to-end semantics

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M-TCP cont.

• TCP Persist Mode
• When a new ack is received with receivers
  advertised window 0, the sender enters persist
  mode
• Sender does not send any data in persist mode
• When a positive window advertisement is received,
  sender exits persist mode
• On exiting persist mode, RTO and congestion
  window are same as before the persist mode

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M-TCP cont.

• Advantages
• Maintains the TCP end-to-end semantics
• In case disconnection, avoids useless
  retransmission and slow start
• Need not buffer at SH
• Efficient handoff
• Adapt to dynamically changing bandwidth over
  starved link
• Disadvantages
• SH does not act as proxy
• Packet loss on wireless link is propagated to the
  sender
• Requires modifications to MH protocol software
  and new network elements like the bandwidth
  management module

23
Split-connection WAP
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Split-connection WAP cont.
25
Comparison of categories
26
Conclusion

• Avoid erroneously triggering congestion control
  mechanisms on the fixed host.
• Avoid the serial timeout problem on the fixed
  host.
• Be reliable, by solving the problems arising from
  the lossy wireless links and their bursty high
  BER.

27
Conclusions cont.

• Can efficiently deal with handoff.
• Can handle frequent and long disconnections of
  the mobile host.
• Take into consideration the limited bandwidth and
  power scarcity of mobile hosts.

28
Conclusions cont.

• Use a dynamic packet size depending on the
  dynamic bandwidth available for mobile hosts.
• Preferably provide compatibility that is, do not
  require any software on the fixed hosts.