Transcript Improving TCP Performance over Mobile Networks ACM Computing

ACM Computing Surveys 2002
Improving TCP Performance
over Mobile Networks
HALA ELAARAG
Stetson University
Speaker : Aron
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Overview
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Regular TCP
The Problems ?
Why regular TCP is not suitable?
The solutions to improve the performance
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Link layer
End to end
Split connection
 Conclusions
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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.
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The behavior of regular TCP
 Congestion control
 Slow-start
 Congestion avoidance
 Fast Retransmit
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Problems with wireless and mobile networks
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High bit error rates
Disconnections
Limited and variable bandwidth
Cell size
Power scarcity
Dynamic network topology
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Why regular TCP is not suitable?
TCP’s 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.
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Problems of mobile TCP implement
 Non-congestion delay
 Serial timeouts
 Packet size variation
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The solutions to improve the performance
 Link layer protocols
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RLP
AIRMAIL
Snoop
 End-to-end protocols
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Reno
New-Reno
SACK
FR
EBSN
 Split-connection
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MTCP
I-TCP
M-TCP
WAP
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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
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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
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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 haven’t 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
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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
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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
EBSN
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Split Connection Schemes
Divide TCP connection into 2 connections
 Isolate wired network from wireless network
TCP II
Wireless Link
TCP I
Wired 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
Wireless Link
TCP I
Wired 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
M-TCP
TCP
SH
Fixed Host
Mobile Host
(Sender)
(Receiver)
SH-TCP
M-TCP
 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 receiver’s 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
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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
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Split-connection – WAP
Web Server
WAP Gateway
WML
WML Encoder
WMLScript
WSP/WTP
WMLScript
Compiler
HTTP
CGI
Scripts
etc.
WTAI
Protocol Adapters
Content
WML Decks
with WML-Script
Client
Etc.
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Split-connection – WAP
cont.
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Comparison of categories
RLP
Airmail
Snoop
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Bursty error
Handoff
High BER
FR
EBSN
MTCP
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M-TCP
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Long Disconnections
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Frequent Disconnections
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Bandwidth
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I-TCP
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Cell size
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Power scarcity
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Serial timeouts
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Packet size variation
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End-to-end TCP semantics
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Compatability
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√
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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.
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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.
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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.
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