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U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science TCP over Wireless Networks CS 653, Fall 2010.

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Presentation on theme: "U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science TCP over Wireless Networks CS 653, Fall 2010."— Presentation transcript:

1 U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science TCP over Wireless Networks CS 653, Fall 2010

2 U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science Outline TCP over Wireless: Problems TCP over Wireless: Solutions Link-layer based Split TCP TCP-aware link layer Explicit notification Network-assisted congestion control Summary Hop: a Reliable Hop-by-hop Block Transfer Protocol

3 U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science TCP over Wireless: Problems Disambiguating wireless bit-errors from congestion Frequent window reduction due to errors Frequent timeout due to burst losses High variability Contention + channel errors high loss Contention + link-layer retransmission high RTT variance Need end-to-end connection all the time Wireless links have intermittent connectivity

4 U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science Outline TCP over Wireless: Problems TCP over Wireless: Solutions Link-layer based Split TCP TCP-aware link layer Explicit notification Network-assisted congestion control Summary Hop: a Reliable Hop-by-hop Block Transfer Protocol

5 U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science Link-layer based Schemes: ARQ Automatic Repeat reQuest (ARQ) Widely used in wireless link-layer protocols Uses retransmissions and acks to cover wireless errors Packet 1 Link-Ack 1 Packet 2 Link-Ack 2 Packet 3 Timeout Link-Ack 3 Internet FH BSMH Link-Layer ARQ

6 U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science Link-layer based Schemes: ARQ Pros No modification to upper layers Cons Fast retransmission due to message lost and out-of-order delivery redundant retransmission and window reduction Interacts with TCP retransmissions redundant retransmission H. Balakrishnan, et al. A comparison of mechanisms for improving TCP performance over wireless links, Sigcomm96 Large RTT variance long timeout Head-of-line blocking due to large #retransmission slow link blocks fast link

7 U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science Outline TCP over Wireless: Problems TCP over Wireless: Solutions Link-layer based Split TCP TCP-aware link layer Explicit notification Network-assisted congestion control Summary Hop: a Reliable Hop-by-hop Block Transfer Protocol

8 U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science Split TCP: Indirect TCP I-TCP splits end-to-end TCP connection into two connections Fixed host to BS BS to mobile host Two TCP connections with independent flow/congestion control contexts Packets buffered at BS Internet FH BSMH TCP Buffer

9 U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science Split TCP: Indirect TCP Pros Separates flow and congestion control of wireless and wired --higher throughput at sender Cons Breaks TCP end-to-end semantics Ack at FH does not mean MH has received the packet BS failure causes loss of data Neither FH nor MH can recover the data On path change, data has to be forwarded to new BS Wireless part is the bottleneck

10 U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science Split TCP: Selective Repeat Protocol Similar to I-TCP but uses SRP/UDP over wireless link Raj Yavatkar, Namrata Bhagawat, Improving End-to-End Performance of TCP over Mobile Internetworks, 1994 Pros Better performance over wireless links Cons All cons of I-TCP except last one Internet FH BSMH TCPSRP/UDP Buffer

11 U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science Split-TCP: Mobile TCP Similar to I-TCP but tries to keep TCP end-to-end semantics BS only acks the last packet after it is received by MH Kevin Brown and Suresh Singh. A network architecture for mobile computing. In Proc. IEEE INFOCOM'96, March 1996 Pros Data will be recovered eventually after BS failure BS buffer does not overflow Cons Worse performance Still not exactly the TCP semantics

12 U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science Outline TCP over Wireless: Problems TCP over Wireless: Solutions Link-layer based Split TCP TCP-aware link layer Explicit notification Network-assisted congestion control Summary Hop: a Reliable Hop-by-hop Block Transfer Protocol

13 U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science TCP-aware Link Layers: Snoop Link layer is aware of TCP traffic BS caches data and monitors acks. Retransmits on duplicate acks and drops duplicate acks H. Balakrishnan, et al. Improving TCP/IP Performance over Wireless Networks, 1995 Internet FH BSMH Packet 1 Ack 1Packet 2 Ack 2 Packet 3 Packet 4 Ack 2 Packet 1 Ack 1 Packet 2 Ack 2 Packet 3 Packet 4 Packet 3 Blocks Dup-Ack

14 U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science TCP-aware Link Layers: Snoop Pros No modification to FH and MH BS only keeps soft stateBS failure does not break TCP Cons Does not work with encrypted packets Does not work if data packets and acks traverse different paths Increases RTThigh timeout

15 U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science TCP-aware Link Layers: WTCP Similar to Snoop WTCP corrects RTT by modifying the timestamp in return acks K. Ratnam and I. Matta,WTCP: An Efficient Transmission Control Protocol for Networks with Wireless Links, 1998 Internet FH BSMH Packet 3 Packet 4 Ack 2 Packet 3 Ack 3 Packet 4 Packet 3 Ack 3 Log Arrival Time Arrival Time+=Delay Delay

16 U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science TCP-aware Link Layers: Delayed DupAcks Protocol Similar to Snoop, BS does link-layer retrans but does not drop dup-acks MH delays the third duplicate ackwill not trigger fast retransmissions on FH Miten N. Mehta, Nitin H. Vaidva, Delayed Duplicate-Acknowledgements: A proposal to Improve Performance of TCP on Wireless Links, 1997 Pros: Works with encrypted TCP Cons: Unnecessary delay during congestion loss

17 U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science Outline TCP over Wireless: Problems TCP over Wireless: Solutions Link-layer based Split TCP TCP-aware link layer Explicit notification Backpressure-based approaches Summary Hop: a Reliable Hop-by-hop Block Transfer Protocol

18 U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science Middle Layer: WCP Observation: TCP Acks are expensive, so should be avoided as much as possible Inserts a middle layer between TCP and 802.11 Buffer TCP data frames Suppress TCP Acks Long Le, et. al, Improving TCP Goodput in 802.11 Access Networks, 2007

19 U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science Outline TCP over Wireless: Problems TCP over Wireless: Solutions Link-layer based Split TCP TCP-aware link layer Explicit notification Network-assisted congestion control Summary Hop: a Reliable Hop-by-hop Block Transfer Protocol

20 U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science Explicit Notification: Explicit Loss Notification ELN works with MH is the sender BS monitors TCP segments from MH and logs down holes BS tags a dup-acks ELN bit if corresponds to a logged hole No congestion control on MH if an ELN-ack is received Hari Balakrishnan and Randy H. Katz, Explicit Loss Notification and Wireless Web Performance, 1998 Internet FH BSMH Packet 1 Ack 1 Packet 2 Packet 3 Packet 4 Packet 1 Packet 3 Packet 4 Ack 1 Log Hole(Pkt 2) Ack 1(ELN) Ack 1

21 U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science Explicit Notification: ELN2 Similar to ELN but works when FH is the sender BS monitors TCP segments from FH and logs down holes BS tags a dup-acks ELN bit if not corresponds to any hole No congestion control on FH if an ELN-ack is received S. Biaz and N. Vaidya, Discriminating Congestion Losses from Wireless Losses using Inter-Arrival Times at the Receiver, 1998 Internet FH BSMH

22 U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science Outline TCP over Wireless: Problems TCP over Wireless: Solutions Link-layer based Split TCP TCP-aware link layer Explicit notification Network-assisted congestion control Summary Hop: a Reliable Hop-by-hop Block Transfer Protocol

23 U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science Backpressure Approaches: RAIN RAINReliable Wireless Network Architecture Small buffer and Adaptive FrEeze(SAFE) link layer Congestion control: backpressure w/ small buffer size Contention control: off-the-shelf CSMA/CA, link-layer ARQ Simple transport layer Pros: Pushes congestion and contention control in-networkprompt and accurate info Cons: Link-layer ARQ does not completely solve link-layer contention Small buffer does not utilize the bursty-sending feature of wireless links Chaegwon Lim, et.al, RAIN: A Reliable Wireless Network Architecture, 2006

24 U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science Summary Most above protocols focus on the last mile problem single wireless hop Multi-hop wireless mesh networks are blooming, e.g., Roofnet, Wildnet, … None of above protocols works during network partitions

25 U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science Review slides

26 U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science TCP Review: Flow Control Sliding Window Keeps the sender from sending too fast Receiver specifies window size Sender only sends a window before waiting for acks

27 U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science TCP Review: Flow Control Cumulative Ack Receiver acks back expecting sequence number Sends duplicate acks when holes are detected Sender Receiver Packet 1 Ack 1Packet 2 Packet 3Ack 2 Ack 3 Packet 4 Packet 5 Packet 6 Ack 4 Packet 7 Packet 8 Ack 4 Packet 5 Packet 6 Timeout Duplicate Acks Window=4 Window=2

28 U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science TCP Review: Flow Control Selective Ack Receiver can indicate missing segments Handles holes efficiently Co-exists with cumulative acks 0-99100-199200-299300-399400-499 TCP SACK

29 U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science TCP Review: Congestion Control Slow Start Actually grows exponentially Starts from small window size(2KB) Multiplicatively increases window until threshold reached or loss Congestion Window Slow Start Time

30 U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science TCP Review: Congestion Control Congestion Avoidance Incrementally increases window after threshold reached Congestion Window Slow Start Congestion Avoidance Time

31 U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science TCP Review: Congestion Control Timeout Decreases window size to 1 Halves threshold Timeout value = f(Mean_RTT, Dev_RTT) Congestion Window Slow Start Congestion Avoidance Timeout Slow Start Congestion Avoidance Time

32 U NIVERSITY OF M ASSACHUSETTS, A MHERST Department of Computer Science TCP Review: Congestion Control Fast Retransmits (TCP-Reno) Dont timeout or reduce window to one on single loss On three duplicate acks Retransmits the lost segment right away Reduces window size to 0.5xWindow+#dup-acks Enters congestion avoidance phase Cons: Cant handle bursty (3+) acks; Dup-acks are also sent if pkts out of order. Congestion Window Slow Start Congestion Avoidance Three Dup-Acks Time Congestion Avoidance


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