1 Improving TCP Performance over Mobile Networks HALA ELAARAG Stetson University Speaker : Aron ACM Computing Surveys 2002

2 Overview Regular TCP Regular TCP The Problems ? The Problems ? Why regular TCP is not suitable? Why regular TCP is not suitable? The solutions to improve the performance The solutions to improve the performance Link layer Link layer End to end End to end Split connection Split connection Conclusions Conclusions

3 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. 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. Objective: to improve the performance of TCP over mobile wireless networks.

4 The behavior of regular TCP Congestion control Congestion control Slow-start Slow-start Congestion avoidance Congestion avoidance Fast Retransmit Fast Retransmit

5 Problems with wireless and mobile networks High bit error rates High bit error rates Disconnections Disconnections Limited and variable bandwidth Limited and variable bandwidth Cell size Cell size Power scarcity Power scarcity Dynamic network topology Dynamic network topology

6 Why regular TCP is not suitable? TCPs main problem is the delay caused by packet losses due to congestion. TCPs 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. 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. If regular TCP is used on a mobile network, it can severely degrade performance.

7 Problems of mobile TCP implement Non-congestion delay Non-congestion delay Serial timeouts Serial timeouts Packet size variation Packet size variation

8 The solutions to improve the performance Link layer protocols Link layer protocols RLP RLP AIRMAIL AIRMAIL Snoop Snoop End-to-end protocols End-to-end protocols Reno Reno New-Reno New-Reno SACK SACK FR FR EBSN EBSN Split-connection Split-connection MTCP MTCP I-TCP I-TCP M-TCP M-TCP WAP WAP

9 Link layer protocols Objective – Objective – Increase the quality of the lossy wireless link Increase the quality of the lossy wireless link Solve the problem at the link layer Solve the problem at the link layer Transport layer protocol is too slow to recover from losses Transport layer protocol is too slow to recover from losses Congestion control mechanisms of transport layer are unnecessarily triggered, throughput is decreased Congestion control mechanisms of transport layer are unnecessarily triggered, throughput is decreased

10 Link layer protocols – RLP Radio Link Protocol (RLP) Radio Link Protocol (RLP) Automatic Repeat reQuest (ARQ) for radio channels. Automatic Repeat reQuest (ARQ) for radio channels. Retransmit a packet when transmitter make sure it was not received. Retransmit a packet when transmitter make sure it was not received. May Solve High bit error rates at link layer May Solve High bit error rates at link layer

11 Link layer protocols –AIRMAIL Approaches to improve link layer protocol performance Approaches to improve link layer protocol performance Automatic Repeat reQuest (ARQ) Automatic Repeat reQuest (ARQ) Forward Error Correction (FEC) Forward Error Correction (FEC) Mobility and handoff processing by window management Mobility and handoff processing by window management and state transfer

12 Link layer protocols – Snoop Snoop protocol (TCP-aware link-layer schemes) Snoop protocol (TCP-aware link-layer schemes) Introduce a module--snoop agent at the base station Introduce a module--snoop agent at the base station monitors every packet that passes through the TCP connection in both directions monitors every packet that passes through the TCP connection in both directions Maintains a cache of TCP packets sent from the sender that havent yet been acknowledged by the receiver. Maintains a cache of TCP packets sent from the sender that havent 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 If detect packet loss (use duplicate ACKs or local timeout), retransmit the packet if it is in the cache and suppress the duplicate ACKs

13 End-to-end protocols – Reno New-Reno SACK TCP Change Fast Retransmit to include Fast Recovery Change Fast Retransmit to include Fast Recovery New-Reno SACK TCP improve the performance when multiple packets lost in the same window 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 Improvement of regular TCP in wireless is expected to be limited

14 End-to-end protocols – FR Fast retransmission scheme Fast retransmission scheme Providing smooth hand-offs on networks that lose packet during handoff. Providing smooth hand-offs on networks that lose packet during handoff. When mobile IP software signal hand-off complete, When mobile IP software signal hand-off complete, mobile host signals fixed host to invoke retransmission scheme. Focus on hand-off Focus on hand-off

15 End-to-end protocols – EBSN Explicit Bad State Notification Explicit Bad State Notification Base Station sends EBSN message to sender if packets cannot be transmitted successfully Base Station sends EBSN message to sender if packets cannot be transmitted successfully Sender changes Timeout based on current RTT Sender changes Timeout based on current RTT Timeout is reset to original on receipt of new ack. Timeout is reset to original on receipt of new ack. Eliminates unnecessary timeouts Eliminates unnecessary timeouts EBSN

16 Split Connection Schemes Divide TCP connection into 2 connections Divide TCP connection into 2 connections Isolate wired network from wireless network Isolate wired network from wireless network TCP IITCP I Wired Link Wireless Link

17 Split-connection – MTCP Protect the wired connection from the impact of the erratic behavior of wireless connection Protect the wired connection from the impact of the erratic behavior of wireless connection Use session layer protocol at BS and MH Use session layer protocol at BS and MH Selective Repeat Protocol (SRP) – recover quickly packet loss Selective Repeat Protocol (SRP) – recover quickly packet loss TCP IITCP I Wired Link Wireless Link

18 Split-connection – I-TCP I-TCP (Indirect TCP) I-TCP (Indirect TCP) The idea is the same with MTCP The idea is the same with MTCP

19 Split-connection – M-TCP Three-level hierarchy architecture Three-level hierarchy architecture High-Speed Network SH Mobile Support Station(MSS) Cell Mobile Host(MH) Supervisor Host

20 M-TCP cont. End-to-end TCP connection End-to-end TCP connection SH Fixed Host (Sender) Mobile Host (Receiver) SH-TCPM-TCP TCP TCP connection is split at the SH TCP connection is split at the SH The SH does not send an ack to FH unless SH has received an ack from MH The SH does not send an ack to FH unless SH has received an ack from MH Maintains end-to-end semantics Maintains end-to-end semantics

21 M-TCP cont. TCP Persist Mode TCP Persist Mode When a new ack is received with receiver s advertised window = 0, the sender enters 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 Sender does not send any data in persist mode When a positive window advertisement is received, sender exits 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 On exiting persist mode, RTO and congestion window are same as before the persist mode

22 M-TCP cont. Advantages Advantages Maintains the TCP end-to-end semantics Maintains the TCP end-to-end semantics In case disconnection, avoids useless retransmission and slow start In case disconnection, avoids useless retransmission and slow start Need not buffer at SH Need not buffer at SH Efficient handoff Efficient handoff Adapt to dynamically changing bandwidth over starved link Adapt to dynamically changing bandwidth over starved link Disadvantages Disadvantages SH does not act as proxy SH does not act as proxy Packet loss on wireless link is propagated to the sender 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 Requires modifications to MH protocol software and new network elements like the bandwidth management module

23 Split-connection – WAP

24 Split-connection – WAP cont.

25 Comparison of categories RLPAirmailSnoopFREBSNMTCPI-TCPM-TCP High BER Bursty error Handoff Long Disconnections Frequent Disconnections Bandwidth Cell size Power scarcity Serial timeouts Packet size variation End-to-end TCP semantics Compatability

26 Conclusion Avoid erroneously triggering congestion control mechanisms on the fixed host. Avoid erroneously triggering congestion control mechanisms on the fixed host. Avoid the serial timeout problem 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. Be reliable, by solving the problems arising from the lossy wireless links and their bursty high BER.

27 Conclusions cont. Can efficiently deal with handoff. Can efficiently deal with handoff. Can handle frequent and long disconnections of the mobile host. Can handle frequent and long disconnections of the mobile host. Take into consideration the limited bandwidth and power scarcity of mobile hosts. Take into consideration the limited bandwidth and power scarcity of mobile hosts.

28 Conclusions cont. Use a dynamic packet size depending on the dynamic bandwidth available for mobile hosts. 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. Preferably provide compatibility; that is, do not require any software on the fixed hosts.