A comparison of Mechanisms for Improving tcp performance over Wireless Links By: Hari B., Venkata P. et al

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