TCP over Wireless Networks
CS 653, Fall 2010
UNIVERSITY OF MASSACHUSETTS, AMHERST • 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
UNIVERSITY OF MASSACHUSETTS, AMHERST • 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
UNIVERSITY OF MASSACHUSETTS, AMHERST • 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
UNIVERSITY OF MASSACHUSETTS, AMHERST • 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
FH
BS
MH
Link-Layer
ARQ
Internet
Packet 1
Link-Ack 1
Packet 2
Link-Ack 2
Packet 3
Timeout
Packet 3
Link-Ack 3
UNIVERSITY OF MASSACHUSETTS, AMHERST • 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
UNIVERSITY OF MASSACHUSETTS, AMHERST • 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
UNIVERSITY OF MASSACHUSETTS, AMHERST • 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
TCP
TCP
Buffer
FH
BS
Internet
UNIVERSITY OF MASSACHUSETTS, AMHERST • Department of Computer Science
MH
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
UNIVERSITY OF MASSACHUSETTS, AMHERST • 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
TCP
SRP/UDP
Buffer
FH
BS
Internet
UNIVERSITY OF MASSACHUSETTS, AMHERST • Department of Computer Science
MH
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
UNIVERSITY OF MASSACHUSETTS, AMHERST • 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
UNIVERSITY OF MASSACHUSETTS, AMHERST • 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
FH
BS
MH
Internet
Packet 1
Packet 2
Packet 3
Packet 4
Packet 1
Packet 2
Ack 1
Ack 2
Ack 1
Ack 2
Packet 3
Packet 4
Ack 2
Blocks Dup-Ack
Packet 3
UNIVERSITY OF MASSACHUSETTS, AMHERST • Department of Computer Science
TCP-aware Link Layers: Snoop
Pros
No modification to FH and MH
BS only keeps soft state—BS failure does not break TCP
Cons
Does not work with encrypted packets
Does not work if data packets and acks traverse different
paths
Increases RTT—high timeout
UNIVERSITY OF MASSACHUSETTS, AMHERST • 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
FH
BS
MH
Internet
Packet 3
Packet 4
Log Arrival Time
Delay
Packet 3
Packet 4
Ack 2
Packet 3
Ack 3
Arrival Time+=Delay
Ack’ 3
UNIVERSITY OF MASSACHUSETTS, AMHERST • 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 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
UNIVERSITY OF MASSACHUSETTS, AMHERST • 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
UNIVERSITY OF MASSACHUSETTS, AMHERST • 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
UNIVERSITY OF MASSACHUSETTS, AMHERST • 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-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
FH
BS
MH
Internet
Log Hole(Pkt 2)
Packet 1
Ack 1
Ack 1
Ack 1
Packet 3
Packet 4
Ack 1
Ack 1(ELN)
Ack 1(ELN)
UNIVERSITY OF MASSACHUSETTS, AMHERST • Department of Computer Science
Packet 1
Packet 2
Packet 3
Packet 4
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
FH
BS
Internet
UNIVERSITY OF MASSACHUSETTS, AMHERST • Department of Computer Science
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
Summary
Hop: a Reliable Hop-by-hop Block Transfer Protocol
UNIVERSITY OF MASSACHUSETTS, AMHERST • Department of Computer Science
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
UNIVERSITY OF MASSACHUSETTS, AMHERST • 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
UNIVERSITY OF MASSACHUSETTS, AMHERST • Department of Computer Science
Review slides
UNIVERSITY OF MASSACHUSETTS, AMHERST • 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
UNIVERSITY OF MASSACHUSETTS, AMHERST • 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
Window=4
Packet 2
Ack 1
Packet 3
Ack 2
Packet 4
Ack 3
Ack 4
Packet 5
Packet 6
Packet 7
Ack 4
Packet 8
Ack 4
Duplicate Acks
Ack 4
Timeout
Packet 5
Window=2
Packet 6
UNIVERSITY OF MASSACHUSETTS, AMHERST • Department of Computer Science
TCP Review: Flow Control
Selective Ack
Receiver can indicate missing segments
Handles “holes” efficiently
Co-exists with cumulative acks
TCP SACK
0-99
100-199
200-299
300-399
400-499
UNIVERSITY OF MASSACHUSETTS, AMHERST • 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
UNIVERSITY OF MASSACHUSETTS, AMHERST • Department of Computer Science
TCP Review: Congestion Control
Congestion Avoidance
Incrementally increases window after threshold reached
Congestion
Window
Congestion
Avoidance
Slow
Start
Time
UNIVERSITY OF MASSACHUSETTS, AMHERST • 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
Congestion
Avoidance
Timeout
Congestion
Avoidance
Slow
Start
Slow
Start
Time
UNIVERSITY OF MASSACHUSETTS, AMHERST • Department of Computer Science
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.
Congestion
Window
Three Dup-Acks
Congestion
Avoidance
Congestion
Avoidance
Slow
Start
Time
UNIVERSITY OF MASSACHUSETTS, AMHERST • Department of Computer Science