spring 2000nitin bahaduradvanced computer networks a comparison of mechanisms for improving tcp...

Nitin Bahadur Advanced Computer Networks Spring 2000

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 ???

• New approaches proposed in recent years


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

Assumes packet losses are due to congestion

Assumes an underlying wired network


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


Cumulative Ack. + Seq. no. of packet causing the ack.

1, 3

021 3

Sender can determine which packets were lost before

timeout occurs or 3 duplicate acks are received

0 3

Smart assumes no packet reordering on wired link

Receiver sends ack for up to 3 sets of non-contiguous data received

Base Station

0 - 9

0 1 2 3 4 5 6 7 8 9

0-2 4-6 8-9


Split Connection Schemes• Divide TCP connection into 2 connections…..isolate wired

network from wireless network

• Use SACK or SMART for performance enhancement

• Loss of end-end TCP model• Limited buffering available at base station • Timeouts on wired TCP due to retransmissions on wireless TCP• Problems in handoff as it now involves 2 TCP connections

TCP II TCP I

Wired N/wWireless Link


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

• Still considers wireless losses as due to congestion

• Does invoke congestion control….small congestion window


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

How to distinguish b/w congestion and wireless losses ?

Scheme does not detect multiple losses….add SACK/SMART


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

0 1 2 3

Base Station

0 3

Layer Violation !!!!!

Bursty losses/slow wireless links lead to TCP sender timeouts while agent is trying to retransmit

1,3


Effectiveness of LL and E2E schemes


Handoff Issues• Mobile hosts (MH) and cell

• Handoff takes place when MH changes Base Station

• Losses due to handoff….During establishing of new route/new cell registration• Rerouting through BS or direct routing to MH ???

Sender

X


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.

How I wish the figures were animated for better understanding !!


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

0 1 20

EBSN


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

Receiver cannot distinguish between congestion and transmission losses…..performance degradation


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

spring 2000nitin bahaduradvanced computer networks a comparison of mechanisms for improving tcp...

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