u niversity of m assachusetts, a mherst department of computer science tcp over wireless networks cs...

UUNIVERSITY OF NIVERSITY OF MMASSACHUSETTSASSACHUSETTS, A, AMHERST • MHERST • Department of Computer Science Department of Computer Science

TCP over Wireless Networks

CS 653, Fall 2010


Outline

TCP over Wireless: ProblemsTCP over Wireless: Solutions

Link-layer basedSplit TCPTCP-aware link layerExplicit notificationNetwork-assisted congestion control

SummaryHop: a Reliable Hop-by-hop Block Transfer Protocol


TCP over Wireless: Problems

Disambiguating wireless bit-errors from congestionFrequent window reduction due to errorsFrequent timeout due to burst losses

High variabilityContention + channel errors high lossContention + link-layer retransmission high RTT variance

Need end-to-end connection all the timeWireless links have intermittent connectivity


Outline





Link-layer based Schemes: ARQ

Automatic Repeat reQuest (ARQ)Widely used in wireless link-layer protocolsUses retransmissions and acks to cover wireless errors

Packet 1Link-Ack 1

Packet 2Link-Ack 2

Packet 3

Packet 3

Timeout

Link-Ack 3

Internet Internet

FH BS MH

Link-Layer ARQ


Link-layer based Schemes: ARQPros

No modification to upper layers

ConsFast 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


Outline





Split TCP: Indirect TCP

I-TCP splits end-to-end TCP connection into two connectionsFixed host to BSBS to mobile host

Two TCP connections with independent flow/congestion control contextsPackets buffered at BS

Internet Internet

FH BS MH

TCP TCPBuffer


Split TCP: Indirect TCP

ProsSeparates flow and congestion control of wireless and wired

--higher throughput at sender

ConsBreaks TCP end-to-end semantics

Ack at FH does not mean MH has received the packet

BS failure causes loss of dataNeither FH nor MH can recover the data

On path change, data has to be forwarded to new BS

Wireless part is the bottleneck


Split TCP: Selective Repeat Protocol

Similar to I-TCP but uses SRP/UDP over wireless linkRaj Yavatkar, Namrata Bhagawat, Improving End-to-End Performance of TCP over Mobile Internetworks, 1994

ProsBetter performance over wireless links

ConsAll cons of I-TCP except last one

Internet Internet

FH BS MH

TCP SRP/UDPBuffer


Split-TCP: Mobile TCP

Similar to I-TCP but tries to keep TCP end-to-end semanticsBS 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

ProsData will be recovered eventually after BS failureBS buffer does not overflow

ConsWorse performanceStill not exactly the TCP semantics


Outline





TCP-aware Link Layers: Snoop

Link layer is aware of TCP trafficBS 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 Internet

FH BS MH

Packet 1Ack 1Packet 2

Ack 2

Packet 3

Packet 4Ack 2

Packet 1

Ack 1

Packet 2

Ack 2Packet 3

Packet 4

Packet 3Blocks Dup-Ack


TCP-aware Link Layers: Snoop

ProsNo modification to FH and MHBS only keeps soft state—BS failure does not break TCP

ConsDoes not work with encrypted packetsDoes not work if data packets and acks traverse different pathsIncreases RTT—high timeout


TCP-aware Link Layers: WTCP

Similar to SnoopWTCP 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 Internet

FH BS MH

Packet 3Packet 4

Ack 2

Packet 3

Ack’ 3

Packet 4

Packet 3

Ack 3

Log Arrival Time

Arrival Time+=Delay

Delay


TCP-aware Link Layers: Delayed DupAcks Protocol

Similar to Snoop, BS does link-layer retrans but does not drop dup-acksMH delays the third duplicate ack—will 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


Outline


Link-layer based Split TCP TCP-aware link layer Explicit notification Backpressure-based approaches

Summary Hop: a Reliable Hop-by-hop Block Transfer Protocol


Outline TCP over Wireless: Problems TCP over Wireless: Solutions

Link-layer based Split TCP TCP-aware link layer Explicit notification Network-assisted congestion control



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-ack’s 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 Internet

FH BS MH

Packet 1

Ack 1

Packet 2Packet 3Packet 4

Packet 1

Packet 3Packet 4

Ack 1Ack 1

Log Hole(Pkt 2)

Ack 1(ELN)Ack 1(ELN)

Ack 1


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-ack’s 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 Internet

FH BS MH


Outline TCP over Wireless: Problems TCP over Wireless: Solutions

Link-layer based Split TCP TCP-aware link layer Explicit notification Network-assisted congestion control



Backpressure Approaches: RAIN RAIN—Reliable 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-network—prompt 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


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


Review slides


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


TCP Review: Flow Control Cumulative Ack

Receiver acks back expecting sequence number Sends duplicate acks when “holes” are detected

Sender ReceiverPacket 1

Ack 1Packet 2

Packet 3 Ack 2

Ack 3Packet 4

Packet 5

Packet 6

Ack 4

Ack 4

Ack 4Packet 7

Packet 8

Ack 4

Packet 5

Packet 6

Timeout

Duplicate Acks

Window=4

Window=2


TCP Review: Flow Control Selective Ack

Receiver can indicate missing segments Handles “holes” efficiently Co-exists with cumulative acks

0-99 100-199 200-299 300-399 400-499

TCP SACK


TCP Review: Congestion Control Slow Start

Actually grows exponentially Starts from small window size(2KB) Multiplicatively increases window until threshold reached or

lossCongestionWindow

Slow Start

Time


TCP Review: Congestion Control Congestion Avoidance

Incrementally increases window after threshold reached

CongestionWindow

Slow Start

CongestionAvoidance

Time


TCP Review: Congestion Control Timeout

Decreases window size to 1 Halves threshold Timeout value = f(Mean_RTT, Dev_RTT)

CongestionWindow

Slow Start

CongestionAvoidance

Timeout

Slow Start

CongestionAvoidance

Time


TCP Review: Congestion Control Fast Retransmits (TCP-Reno)

Don’t 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: Can’t handle bursty (3+) acks; Dup-acks are also sent if pkts out of order.

CongestionWindow

Slow Start

CongestionAvoidance

Three Dup-Acks

Time

CongestionAvoidance

u niversity of m assachusetts, a mherst department of computer science tcp over wireless networks cs...

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