1. Wireless TCP February 22, 2002 © 2002 Yongguang Zhang CS 395T - Mobile Computing and Wireless Networks Department of Computer SciencesTHE UNIVERSITY OF TEXAS AT AUSTIN

2. Spring 2002© 2002 Yongguang Zhang2 What Will We Learn? The “Mobile/Wireless” Effects on TCP –What can affect TCP? –Why should TCP worry about these effects? To Hide or Not to Hide, That is the Question. –Will layering principle work? General Solutions –Ways to adjust TCP –Ways to design a better link layer

3. Spring 2002© 2002 Yongguang Zhang3 Effects of Mobile/Wireless High Error Rate –An abnormal event in wired communication (fiber- optic or copper) –Fact-of-life in wireless communication Interference Obstruction Intermittent Connectivity –Back-to-back losses (due to radio shadow) –Hand-off latency (due to mobility) –Disconnection (due to mobility)

4. Spring 2002© 2002 Yongguang Zhang4 Effects of Mobile/Wireless (II) Bandwidth –Mismatch between wired and wireless segment Wireless network is usually the bottleneck –Asymmetric links Latency –Wireless network: longer latency Due to media access, like TDMA Due to MAC, like DAMA –Some networks: much longer latency GEO Satellite: minimum 250ms (prorogation delay)

5. Spring 2002© 2002 Yongguang Zhang5 Effects of Mobile/Wireless (III) Variation of Link Characteristics –In almost every aspect –Due to media access, MAC –Due to attempts by link layer to compensate some effects of “wireless/mobile” Variation of End-to-end Path Characteristics –Vertical handoff –User pattern Drastic and Sudden Changes of the Above

6. Spring 2002© 2002 Yongguang Zhang6 To Hide or Not to Hide? Some Effects Can Be Hidden –Link layer mechanism to hide the effects of wireless ARQ, FEC, MAC with QoS, … –Network layer hides the effect of mobility Mobile-IP, … Some Effects Cannot Be Easily Hidden –Latency Hiding Can Generate New Effects –ARQ introduces jitter and bandwidth overhead

7. Spring 2002© 2002 Yongguang Zhang7 Learn The Trade-Offs Link-Layer Mechanisms to Hide the Effects Indirect/Proxy Approach to Isolate the Effects Deal with the Effects in Transport Layer

8. Spring 2002© 2002 Yongguang Zhang8 Dealing with Error Losses Fundamental Problem –High loss rate in wireless networks –TCP interprets loss as congestion Timeout/retransmission Congestion avoidance Improvement Strategy –Dealing with multiple losses in a window –Keeping congestion window from shrinking –Avoiding unnecessary retransmissions

9. Spring 2002© 2002 Yongguang Zhang9 Classification Shield TCP Sender from Wireless Losses (so it won’t react poorly) –Link-layer protocols –Link-assist protocols –Split connection protocols Make TCP Sender Aware of Wireless Losses (so it can do something about it) –Explicit loss notification

10. Spring 2002© 2002 Yongguang Zhang10 Link-Layer Protocols Approaches: –Local loss detection through ARQ (with sack) –Local retransmission (with faster timeout) Pros: –Layering approach -- invisible to higher-level –Does not break TCP end-to-end semantics Cons: –May adversely interact with higher-level mechanisms –Introduce variable delay

11. Spring 2002© 2002 Yongguang Zhang11 Link-Assist Protocols Approach –TCP-awareness: snooping TCP packets –Loss detection through observing dup-ack –Local retransmission –Suppress evidence of losses (duplicated ack) Pros: –Better results than pure link-layer protocols –Does not break TCP end-to-end semantics Cons: –Violate the layering principle

12. Spring 2002© 2002 Yongguang Zhang12 Split TCP Approach –Two separated connections (c.f. proxy architecture) Fixed node (TCP sender)  base-station  mobile node (TCP receiver) Pros: –Isolate the effort of wireless loss to local TCP segment So that a wireless link looks like robust but slower wired link –No change to TCP or link layer Cons: –Breaks TCP end-to-end semantics –TCP may not be effective for “one-hop” network

13. Spring 2002© 2002 Yongguang Zhang13 Explicit Loss Notification Approach –Base-station marks ACK to show non-congestion loss –Sender responds by retransmission, but not congestion control procedures Pros: –Sender can distinguish congestion loss from error loss –Does not break end-to-end semantics –No need to buffer or cache packets at base-station

14. Spring 2002© 2002 Yongguang Zhang14 More on ELN How does each end of a wireless link detect an error loss? Send-side detection –Relay on Snooping Duplicated ACKs –May have broken the layering principle –What if the wireless link is a transmit link? Receive-side detection –Interaction between transport layer and link layer?

15. Spring 2002© 2002 Yongguang Zhang15 Overall Results Must Prevent Sender from Entering Congestion Control Procedures –Snooping seems the best approach so far local retransmission Suppress duplicated ACKs Should Use Selective ACKs (for bursty errors)

16. Spring 2002© 2002 Yongguang Zhang16 Dealing with Asymmetry Problem with Asymmetry –TCP’s forward progress depends on timely receipt of acks Types of asymmetry –Bandwidth –Latency –Media-access –Packet error rate –Others? (cost, etc.)

17. Spring 2002© 2002 Yongguang Zhang17 Illustration Assuming MSS=512, –512+20+20-byte data packets vs 20+20-byte acks: the ratio is 15 Assuming 10 Mbps forward channel and 100 Kbps back link: ratio of bandwidths is 100 Implies there cannot be more than 1 ack for every 6 packets before back link is saturated

18. Spring 2002© 2002 Yongguang Zhang18 Approaches ACK Congestion Control ACK Filtering Periodic ACK