Wireless TCP
Performance Issues
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
Tutorial: TCP 101
 The Transmission Control Protocol (TCP) is
the protocol that sends your data reliably
 Used for email, 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...
TCP 101 (Cont’d)
 TCP is a connection-oriented protocol
SYN
SYN/ACK
GET URL
ACK
Web Client
Web Server
YOUR DATA HERE
FIN
ACK
FIN/ACK
TCP 101 (Cont’d)
 TCP slow-start and congestion avoidance
ACK
TCP 101 (Cont’d)
 TCP slow-start and congestion avoidance
ACK
TCP 101 (Cont’d)
 TCP slow-start and congestion avoidance
ACK
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

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
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TCP Reno
Note how there is “Fast Recovery” after cutting Window in half
Window
Reached initial
ssthresh value;
switch to CA mode
Slow Start
Time
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Detecting Packet Loss
 Assumption: loss
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indicates congestion
 Option 1: time-out

Waiting for a time-out can
be long!
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X
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 Option 2: duplicate ACKs
 How many? At least 3.
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Sender
Receiver
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TCP Reno
Note how there is “Fast Recovery” after cutting Window in half
Window
Reached initial
ssthresh value;
switch to CA mode
Slow Start
Time
12
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
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)
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

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

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SeqNum
Key: X Data Packet
+ Ack Packet
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?
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Key: X Data Packet
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Key: X Data Packet
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Key: X Data Packet
+ Ack Packet
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Key: X Data Packet
+ Ack Packet
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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
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TCP Reno
Note how there is “Fast Recovery” after cutting Window in half
Window
Reached initial
ssthresh value;
switch to CA mode
Slow Start
Time
32
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/W2
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TCP under lots of losses
 Throughput in terms of loss rate:
1.22  MSS
RTT L
 Wireless TCP versions or handling losses where
they occur …
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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
Example #1 (Cont’d)
 Solution: “wireless-aware TCP” (I-TCP,
ProxyTCP, Snoop-TCP, split connections...)
Example trends and issues ...
 Middle boxes [e2e arguments, equation]
 Customized wireless TCP solutions
 Multi-path TCP
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Example #2
 Wireless TCP Fairness Problems
D
DATA
ACK
Wireless
Bottleneck AP
Wired Internet
U
DATA
ACK
Loss of ACK = Loss of DATA
Example #3
 Multi-hop “ad hoc” networking
Kelly
Carey
Example #3 (Cont’d)
 Multi-hop “ad hoc” networking
Kelly
Carey
Example #3 (Cont’d)
 Multi-hop “ad hoc” networking
Kelly
Carey
Example #3 (Cont’d)
 Multi-hop “ad hoc” networking
Kelly
Carey
Example #3 (Cont’d)
 Multi-hop “ad hoc” networking
Kelly
Carey
Example #3 (Cont’d)
 Multi-hop “ad hoc” networking
Kelly
Carey
Example #3 (Cont’d)
 Multi-hop “ad hoc” networking
Kelly
Carey
Example #3 (Cont’d)
 Multi-hop “ad hoc” networking (e.g., TCP-F,
TCP-ELFN, etc.)
Kelly
Carey
Example #3 (Cont’d)
 Multi-hop “ad hoc” networking (e.g., TCP-F,
TCP-ELFN, etc.)
Kelly
Carey
Example #3 (Cont’d)
 Multi-hop “ad hoc” networking (e.g., TCP-F,
TCP-ELFN, etc.)
Kelly
Carey
Example #3 (Cont’d)
 Multi-hop “ad hoc” networking (e.g., TCP-F,
TCP-ELFN, etc.)
Kelly
Carey
Example #3 (Cont’d)
 Multi-hop “ad hoc” networking (e.g., TCP-F,
TCP-ELFN, etc.)
Kelly
Carey
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
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
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!
More slides
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
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?