Lecture 14: Transport Layer

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Lecture 14 Transport Layer

• Prev. summary
• TCP flow control
• TCP congestion control

Application
Transport
Network
Link

• Todays lecture
• TCP latencies

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Overview
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How does actual TCP latencies look like?

• Assumptions
• Assume fixed congestion window, W
• throughput of R bps
• S MSS (bits)
• O object size (bits)
• no retransmissions (no loss, no corruption)

• Q latency until receipt of the object?
• TCP connection establishment
• data transfer delay at the rate arranged by the
  window size

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Fixed Congestion Window
Two cases
1) ACK arrives before windows worth of data
sent 2) Wait for ACK after sending windows worth
of data sent

• fixed congestion window, W
• throughput of R bps
• S MSS (bits)
• O object size (bits)
• no retransmissions

• Case I
• WS/R gt RTT S/R

Case 2 latency 2RTT O/R (K-1)S/R RTT -
WS/R
Case 1 latency 2RTT O/R
latency 2RTT O/R max0, (K-1)S/R RTT -
WS/R
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Dynamic Congestion Window
K windows cover the object
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Dynamic Congestion Window
Out of infinite windows, will stall Q times Q
log (1RTT/(S/R)) 1
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Out of K windows, will stall P times P min
Q, K-1
p
latency 2RTT O/R P(RTTS/R) - (2 -1)S/R
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Example Scenarios
O 100 Kbytes S 536 bytes K 8 RTT100 ms
a) Large Object Size
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Large Object
O 100 Kbytes S 536 bytes K 8 RTT100 ms
a) Large Object Size
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Large vs Small Object
O 5 Kbytes S 536 bytes K 4 RTT100 ms
O 100 Kbytes S 536 bytes K 8 RTT100 ms
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Example Scenarios
O 5 Kbytes S 536 bytes K 4 RTT1 sec
c) Larger RTT
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Larger RTT
O 5 Kbytes S 536 bytes K 4 RTT1 sec
O 5 Kbytes S 536 bytes K 4 RTT100 ms
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In Practice, can be complicated...
Host A
Host B
initialize Congwin 1 for (each segment ACKed)
Congwin until (loss event OR
CongWin gt threshold)
one segment
two segments
two segments

• In practice, slow start can result in a slower
  increase than exponential due to the position of
  lost packets.