Wireless TCP Performance Issues

Transcription

1 Wireless TCP Performance Issues

2 Issues, transport layer protocols Set up and maintain end-to-end connections Reliable end-to-end delivery of data Flow control Congestion control Udp? Assume TCP for the rest of these slides

3 Tutorial: TCP 101 The Transmission Control Protocol (TCP) is the protocol that sends your data reliably Used for , Web, ftp, telnet, p2p, Makes sure that data is received correctly: right data, right order, exactly once Detects and recovers from any problems that occur at the IP network layer Mechanisms for reliable data transfer: sequence numbers, acknowledgements, timers, retransmissions, flow control...

4 TCP 101 (Cont d) TCP is a connection-oriented protocol SYN SYN/ACK GET URL ACK Web Client YOUR DATA HERE Web Server FIN ACK FIN/ACK

5 TCP 101 (Cont d) TCP slow-start and congestion avoidance ACK

6 TCP 101 (Cont d) TCP slow-start and congestion avoidance ACK

7 TCP 101 (Cont d) TCP slow-start and congestion avoidance ACK

8 TCP 101 (Cont d) This (exponential growth) slow start process continues until either: packet loss: after a brief recovery phase, you enter a (linear growth) congestion avoidance phase based on slow-start threshold found limit reached: slow-start threshold, or maximum advertised receive window size all done: terminate connection and go home

9 CA: Additive Increase, Multiplicative Decrease We have additive increase in the absence of loss events After loss event, decrease congestion window by half multiplicative decrease ssthresh = W/2 9

10 TCP Reno Window Note how there is Fast Recovery after cutting Window in half Reached initial ssthresh value; switch to CA mode Slow Start Time 10

11 Detecting Packet Loss Assumption: loss indicates congestion Option 1: time-out Waiting for a time-out can be long! Option 2: duplicate ACKs How many? At least Sender Receiver 11

12 TCP Reno Window Note how there is Fast Recovery after cutting Window in half Reached initial ssthresh value; switch to CA mode Slow Start Time 12

13 How do losses occur? Congestion control assumes: due to congestion packets queue in router buffers if queue is full, arriving packets dropped (Drop-Tail) packet being transmitted (delay) A B packets queueing (delay) free (available) buffers: arriving packets dropped (loss) if no free buffers

14 How do losses occur? In wireless (and mobile) environment... We find many other reasons A Packet losses due to - wireless characteristics (e.g., interference, bit errors, etc.) - mobility (including handover issues)

15 Wireless, mobility: impact on higher layer protocols logically, impact should be minimal best effort service model remains unchanged TCP and UDP can (and do) run over wireless, mobile but performance-wise: packet loss/delay due to bit-errors (discarded packets, delays for link-layer retransmissions), and handoff TCP interprets loss as congestion, will decrease congestion window un-necessarily delay impairments for real-time traffic limited bandwidth of wireless links

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17 Also, not all packet losses the same What happens when a packet loss occurs? Quiz Time... Consider a 14-packet Web document For simplicity, consider only a single packet loss 17

18 SeqNum Key: Data Packet Ack Packet Time 18

19 SeqNum Key: Data Packet Ack Packet? Time 19

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22 SeqNum Key: Data Packet Ack Packet? Time 22

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25 SeqNum Key: Data Packet Ack Packet? Time 25

26 SeqNum Key: Data Packet Ack Packet Time 26

27 SeqNum Key: Data Packet Ack Packet Time 27

28 SeqNum Key: Data Packet Ack Packet? Time 28

29 SeqNum Key: Data Packet Ack Packet Time 29

30 TCP 301 (Cont d) Main observation: Not all packet losses are created equal Losses early in the transfer have a huge adverse impact on the transfer latency Losses near the end of the transfer always cost at least a retransmit timeout Losses in the middle may or may not hurt, depending on congestion window size at the time of the loss 30

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32 TCP Reno Window Note how there is Fast Recovery after cutting Window in half Reached initial ssthresh value; switch to CA mode Slow Start Time 32

33 Let s reason about TCP throughput Wired: What s the average throughout of TCP as a function of window size and RTT? Ignore slow start Let W be the window size when loss occurs. When window is W, throughput is W/RTT Just after loss, window drops to W/2, throughput to W/2RTT. Average throughout:.75 W/RTT Loss rate proportional to 1/W 2 33

34 TCP under lots of losses Throughput in terms of loss rate: 1.22 MSS RTT Wireless TCP versions or handling losses where they occur L 34

35 Example #1 Wireless TCP Performance Problems Low capacity, high error rate Wireless Access Wired Internet Hard to distinguish losses here from losses here High capacity, low error rate

36 Example #1 (Cont d) Solution: wireless-aware TCP (I-TCP, ProxyTCP, Snoop-TCP, split connections...)

37 Example trends and issues... Middle boxes [e2e arguments, equation] Customized wireless TCP solutions Multi-path TCP 37

38 Example #2 Wireless TCP Fairness Problems D DATA Wireless Bottleneck U ACK AP DATA ACK Wired Internet Loss of ACK = Loss of DATA

39 Example #3 Multi-hop ad hoc networking Kelly Carey

40 Example #3 (Cont d) Multi-hop ad hoc networking Kelly Carey

41 Example #3 (Cont d) Multi-hop ad hoc networking Kelly Carey

42 Example #3 (Cont d) Multi-hop ad hoc networking Kelly Carey

43 Example #3 (Cont d) Multi-hop ad hoc networking Kelly Carey

44 Example #3 (Cont d) Multi-hop ad hoc networking Kelly Carey

45 Example #3 (Cont d) Multi-hop ad hoc networking Kelly Carey

46 Example #3 (Cont d) Multi-hop ad hoc networking (e.g., TCP-F, TCP-ELFN, etc.) Kelly Carey

47 Example #3 (Cont d) Multi-hop ad hoc networking (e.g., TCP-F, TCP-ELFN, etc.) Kelly Carey

48 Example #3 (Cont d) Multi-hop ad hoc networking (e.g., TCP-F, TCP-ELFN, etc.) Kelly Carey

49 Example #3 (Cont d) Multi-hop ad hoc networking (e.g., TCP-F, TCP-ELFN, etc.) Kelly Carey

50 Example #3 (Cont d) Multi-hop ad hoc networking (e.g., TCP-F, TCP-ELFN, etc.) Kelly Carey

51 TCP performance issues in adhoc networks Route failure TCP may lose a congestion window of packets Large RTT variation Degrades TCP throughput Route re-computations Incur extra delay Inflated re-transmission time-out

52 TCP performance issues in adhoc networks (cont d) Network partition TCP may reset the connection if the partition lasts for more than 100 sec. TCP enters slow-start phase after cutting down its window size to 1 Degrades TCP throughput

53 Summary of Wireless TCP TCP is the four wheel drive of TP s Wireless is a newly emerging technology with rapidly growing deployment popularity TCP and Wireless don t fit together all that well Making TCP smarter about wireless helps!

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55 More slides

56 TCP performance issues in Ad-hoc networks Misinterpretation of packet loss E.g., packet loss/delay due to bit-errors (discarded packets, delays for link-layer retransmissions), and handoff Frequent path breaks Network partitioning and remerging Path length effects Misinterpretation of congestion window Asymmetric link behavior Uni-directional paths Multi-path routing The use of sliding window

57 More interesting problems Two interesting subproblems: Dynamic ad hoc routing: node movement can disrupt the IP routing path at any time, disrupting TCP connection; yet another way to lose packets!!!; possible solution: Explicit Loss Notification (ELN)? Handoff? Route prediction? TCP flow control: the bursty nature of TCP packet transmissions can create contention for the shared wireless channel among forwarding nodes; collisions between DATA and ACKs possible solution: rate-based flow control? Burst mode? Spatial reuse of channels?

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