A comparison of Mechanisms for Improving tcp performance over Wireless Links By: Hari B., Venkata P. et al
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#70905
A Comparison of Mechanisms for Improving TCP Performance
over Wireless Links
By: Hari B., Venkata P. et. al.
Presented by: Nitin Bahadur
How I plan to keep you
Awake
Review of TCP Congestion Control and Wireless issues
Discussion of techniques presented in the paper
Evaluation of some techniques
What is a Handoff ???
TCP Congestion Control
Fast Retransmit
if
three
duplicate acks before timeout
, retransmit
Fast Recovery
no slow start after retransmit
go directly to half the last successful congestion win. ( Cwin = Cwin/2 )
Coarse grained Timeouts
Implications to Wireless Networks
Wireless losses are different from congestion losses
weak signal,
corruption
, incomplete packet, lost bits
TCP treats both losses similarly
reduces congestion window size
degrades performance for wireless
Coarse grained timeouts are bad for lossy wireless networks
slower retransmissions
consistent
small window size
reduced bandwidth !!!!
Solutions
Approaches presented in the paper
Split Connection
End 2 End
Link Layer TCP aware
Other recent ones
Split Connection Schemes
Divide TCP connection into 2 connections…..isolate wired network from wireless network
Use SACK or SMART
for performance enhancement
End -End Schemes w/ SMART or SACK
Using SMART/SACK, sender can detect multiple losses faster
Faster and efficient retransmit scheme
No need for 3 duplicate acks or coarse timeout
End -End model is maintained
E2E w/ Explicit Loss Notification
Pkt. Loss on wireless link -> Ack. w/
ELN bit set
Sender retransmits on receiving
first
(not third) duplicate ack w/ ELN bit set
Power and time saving !!!!!
Sender does not invoke congestion control in such cases large congestion window……even at high rate wireless losses
LL-SMART-TCP-Aware Scheme
Maintain cache of
un-acked
packets at Home Agent
Use a LL retransmission scheme with finer granularity timeout
Use SMART
for efficient retransmissions
Suppress duplicate ack from reaching sender
Effectiveness of LL and E2E schemes
Handoff Issues
Mobile hosts (MH) and cell
Handoff takes place when MH changes Base Station
Conclusion
The paper presented a taxonomy and comparison of various approaches
But all approaches have drawbacks…….so none have become a standard today.
The results presented do not consider losses arising from
congestion
…..so are not practical.
Explicit Bad State Notification (EBSN)
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
Multiple Acks Proposal
Base Station sends a
Partial Ack
to sender
Base station reliably sends packets to mobile client
Sender does not retransmit/invoke congestion control on
timeout
, just discards the
Partial Ack
Receiver sends
Complete Ack
to sender
Similar to ELN……but results in
excess traffic towards sender
Delayed Duplicate Acks (Dupacks)
TCP - unaware technique, good for encrypted data
Base Station uses a LL retransmission scheme
This scheme uses LL acks…not TCP duplicate acks !!
TCP receiver delays 3rd & other Dupacks
High Priority to LL acks & retransmitted pkts
Other Proposed Schemes
Explicit Loss Notification to Receiver (ELNR)
Explicit Delayed Dupack Activation Notification (EDDAN)
Wireless Explicit Congestion Notification (WECN)
Forward Explicit Congestion Notification (FECN)
Extended Link Failure Notification (ELFN)
Appropriate
Byte Counting
Loss Predictors
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