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

Nitin Bahadur Advanced Computer Networks Spring 2000

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

Nitin Bahadur Advanced Computer Networks Spring 2000

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

Nitin Bahadur Advanced Computer Networks Spring 2000

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 !!!!

Nitin Bahadur Advanced Computer Networks Spring 2000

Solutions

• Approaches presented in the paper– Split Connection– End 2 End – Link Layer TCP aware

• Other recent ones

Nitin Bahadur Advanced Computer Networks Spring 2000

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

Nitin Bahadur Advanced Computer Networks Spring 2000

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

Nitin Bahadur Advanced Computer Networks Spring 2000

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

Nitin Bahadur Advanced Computer Networks Spring 2000

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

Nitin Bahadur Advanced Computer Networks Spring 2000

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

Nitin Bahadur Advanced Computer Networks Spring 2000

Effectiveness of LL and E2E schemes

Nitin Bahadur Advanced Computer Networks Spring 2000

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

Nitin Bahadur Advanced Computer Networks Spring 2000

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 !!

Nitin Bahadur Advanced Computer Networks Spring 2000

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

Nitin Bahadur Advanced Computer Networks Spring 2000

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

Nitin Bahadur Advanced Computer Networks Spring 2000

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

Nitin Bahadur Advanced Computer Networks Spring 2000

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