TCP for wireless links

1. TCP for wireless links Dario Maggiorini dario@dico.unimi.it

2. TCP

3. TCP congestion control The algorithm for TCP congestion control is the main reason we can use the Internet successfully today despite largely unpredictable user access patterns and despite resource bottlenecks and limitations. Without TCP congestion control, the Internet could have become history a long time ago.

4. Source 1 10-Mbps Ethernet Router Destination 1.5-Mbps T1 link 100-Mbps FDDI Source 2 Resource Management Solutions • Handling congestion • pre-allocate resources so as to avoid congestion  • control congestion if (and when) is occurs  • Two points of implementation • routers inside the network (queuing discipline)  • hosts at the edges of the network (transport protocol) 

5. Detecting Congestion • Packet drops indicate congestion • Is that really true? • Why does it work? Packet Dst Src Drop Ack Timeout! No Ack = Congestion!

6. Controlling Congestion – The Effect of Window Size • Note that sender’s window is equal to the number of sender packets in flight (in the network). Why? Source Destination Window A Window’s worth of packets X acks X more packets

7. Controlling Congestion • Reduce window  less packets in the network • Increase window  more packets in the network • Idea: Concept of a congestion window– window is smaller when congestion is larger and vice versa

8. Additive Increase, Multiplicative Decrease • Each time a packet drop occurs, slash window size in half (multiplicative decrease) • Multiplicative decrease is necessary to avoid congestion • When no losses are observed, gradually increase window size (additive increase)

9. Source Destination … AIMD (cont) • In practice: increment a little for each ACK Increment = (MSS*MSS)/CongestionWindow CongestionWindow += Increment • Algorithm • increment CongestionWindow by one packet per RTT (linear increase) • divide CongestionWindow by two whenever a timeout occurs (multiplicative decrease)

10. Fast retransmission/fast recovery TCP Reno (Jacobson 1990) window time CA SS SS: Slow Start CA: Congestion Avoidance

11. Problems • What should the window size be • Initially? • Upon packet loss and timeout? • Pessimistic window size? (e.g., 1) • Additive increase is too slow in ramping up window size – short connections will not fully utilize available bandwidth • Optimistic window size? • Large initial burst may cause router queue overflow

12. Source Destination … Slow Start • Objective: determine the available capacity quickly • Idea: • Use CongestionThreshold as an optimistic CongestionWindow estimate • begin with CongestionWindow = 1 packet • double CongestionWindow each RTT (increment by 1 packet for each ACK) • When CongestionThreshold is crossed, use additive increase

13. Sender Receiver Packet 1 Packet 2 ACK 1 Packet 3 ACK 2 Packet 4 ACK 2 Packet 5 Packet 6 ACK 2 ACK 2 Retransmit packet 3 ACK 6 Fast Retransmit • Problem: coarse-grain TCP timeouts lead to idle periods • Fast retransmit: • Send an ack on every packet reception • Send duplicate of last ack when a packet is received out of order • Use duplicate ACKs to trigger retransmission

14. Self Clocking and Slow Start • Each packet’s transmission is “clocked” by an ACK – no bursts develop … W=1 W=2 W=4 W=5 W=6 W=7 Slow Start

15. Self Clocking in Operation • Each packet’s transmission is “clocked” by an ACK – no bursts develop … … W=32

16. Self Clocking Interrupted • During timeouts, ACKs are drained from the network. Self clocking is interrupted. Next transmission causes a burst. Hence, slow start! ACKs Drained!! Lost Ack32 … … 16 packet burst W=32 Retransmission Timeout Cut window in 1/2

17. Self Clocking and Fast Retransmit / Fast Recovery • When fast retransmit is used, the packet is retransmitted before all ACKs are drained. Slow start is not needed Lost … … Fast Retransmission W=32 Cut window in 1/2

18. Problems with TCP Reno • During slow start • Underutilizes and then swamps path • No “right rate”: cwnd traces a sawtooth • Underutilizes path • Increases queuing delay • Causes loss, reducing throughput • Inherently biased against long RTTs

19. TCP Vegas • Uses congestion avoidance instead of congestion control • Vegas: Congestion avoidance: Predict and avoid congestion before it occurs • Tahoe, Reno: Congestion control: React to congestion after it occurs • Question: How to predict congestion?

20. 1100 900 700 Sending KBps 500 300 100 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 10 5 Queue size in router 0.5 1.0 1.5 4.0 4.5 6.5 8.0 Time (seconds) TCP Vegas • Idea: source watches for some sign that router’s queue is building up and congestion will happen too; e.g., • RTT grows • sending rate flattens 70 60 50 40 KB 30 20 10 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 Time (seconds) Time (seconds) 2.0 2.5 3.0 3.5 5.0 5.5 6.0 7.0 7.5 8.5

21. TCP Vegas • In congestion avoidance • cwnd = (actual rate)x(baseRTT) + 2 pkts • Each RTT, tweak cwnd by 1 pkt if needed • During slow start • To reduce overshoot, increase cwnd only every other RTT • Exit slow start when • cwnd > (actual rate)x(baseRTT) + 1 pkt

22. TCP Vegas (Brakmo & Peterson 1994) window • Converges, no retransmission • … provided buffer is large enough time CA SS

23. TCP Westwood • Sender side only modification of TCP Reno Congestion control that exploits end to end bandwidth estimation. • The bandwidth is estimated by low pass filtering the rate of returning acks. • The bandwidth is used to compute congestion window and slow start threshold.

24. TCP Westwood Overview • Slow Start and Congestion window aware of Bandwidth at time of congestion • The increase after congestion is additive but decrease Adaptive (AIAD) as compared to AIMD (Additive Increase Multiplicative decrease) of Reno

25. TCPW implementation • Sender side Bandwidth Estimation by measuring and low pass filtering the rate of returning acks • When 3 DUPACKS are received ssthresh=(Bandwidth*RTT)/seg_size cwnd =ssthresh • When a coarse timeout expires ssthresh =(B*RTT)/seg_size cwnd =1 • When acks are successfully received TCPW increases cwnd according to Reno’s congestion control algorithm

26. TCPW Advantage over Reno • In case of sudden increase in bottleneck load, reduction can be more drastic then a reduction by half and can be less drastic in other cases. This features improves stability and utilization

27. Go Back N Protocol (GBN)

28. Selective Repeat Protocol

29. Wireless networks

30. Hidden node problem • A and C can send to B but can’t hear each other • CSMA will be ineffective here A B C A’s transmit range C’s transmit range

31. Exposed node problem • B and C can hear each other but could safely send simultaneously to A, D • Compare to spatial reuse in cellular A B C D C’s transmit range B’s transmit range

32. StrategyCSMA with Collision Avoidance Fundamentally, a less greedy approach than Ethernet • When medium busy, choose random backoff interval • Wait for that many idle timeslots to pass before sending • Remember p-persistence … a refinement

33. CSMA/CA • When a collision is inferred, retransmit with binary exponential backoff (like Ethernet) • Use CRC and ACK from receiver to infer “no collision” • Again, exponential backoff helps us adapt “p” as needed

34. TCP over wireless

35. The wireless problem In Wireless lossy links, the sporadic losses are not due to congestion thus it leads to unnecessary window and transmission rate reduction

36. Sources of Errors in Wireless Links • Pauses due to handoff between cells • Packet losses due to futile transmissions: mobile host out of reach of other transceivers (little or no overlap between cells); • Packet losses due to transmission errors in wireless links.

37. Improving the Performance of TCP SH MSS 1 MSS 2 MH Cell 2 Cell 1

38. Smooth Handoff • Cellular networks should strive to provide smooth handoffs in order to eliminate packet losses during cell crossings. • No overlaps are also good!!! • High aggregate bandwidth: adjacent cells can use the same portion of the spectrum; • Support for low-powered mobile receivers; • Accurate location information

39. Comparision of Mechanisms • End-to-end protocols • Split-connection protocols • Link-layer protocols • Hybrid protocols

40. End-to-end Protocols • Sender is aware of the existence of wireless hops. • Selective Acknowledgments (SACKs): sender can recover from multiple packet losses without resorting to a coarse timeout. • Explicit Loss Notification (ELFN): the sender can distinguish between congestion and other forms of losses.

41. Split-connection Protocols • Aims to hide any non-congestion-related losses from the TCP sender. • TCP connection is split between a sender and receiver into two separate connections at the base station: • TCP connection over wired link; • Specialized protocol over wireless link.

42. I-TCP: Indirect TCP I-TCP TCP MSR MH FH • MH = Mobile Host • MSR = Mobile Support Router • FH = Fixed Host

43. TCP/IP in Mobile Environment • Main reason for throughput degradation: • Loss of TCP segments during cell crossovers, especially with non-overlapped cells. • Effects: • Lost segments trigger exponential back off and congestion control at the transmitting host. • Congestion recovery phase may last for several seconds.

44. Indirect Protocol • Different flow control and congestion control for wireless and wired links; • Separate transport protocol supports disconnections, moves and other wireless related features; • MSR manages much of the overhead; • Faster reaction to mobility due to proximity between MSR and MH.

45. MH socket MH socket MH MH MSR-1 MSR-1 FH I-TCP Basics move Wireless TCP I-TCP Handoff MSR1 fhsocket MSR2 fhsocket Regular TCP MSR-2 MSR1 mhsocket MSR2 mhsocket FH socket

46. Link-layer Protocols • Two main classes: • Error correction using techniques such as Forward Error Correction; • Retransmission of lost packets in response to automatic repeat request (ARQ) messages. • Tuned to the characteristics of the wireless link.

47. Hybrid Protocols: The Snoop Prootocol • An agent monitors every packet and maintains a cache of TCP segments that have not yet been acknowledged. • Packet loss is detected by the arrival of a small number of duplicate acks or by a local timeout. • The agent retransmits the lost packet and suppresses the duplicate acks.

48. Observations • TCP-aware link-layer protocol with selective acknowledgments performs the best; • Split-connection approaches is not a requirement for good performance. • Selective acknowledgment is very useful in lossy links, especially for burst losses. • Explicit Loss Notification is worth to try.

49. TCP over satellite links

50. Why Satellite? • Affordable Access to Interactive Broadband Communication to all Areas of the Earth • To Serve users who can not get Service economically by other means • Satellite Types • Transmission Quality