15-441 Computer Networking
TCP Enhancements
Hui Zhang, Fall 2012
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Overview
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TCP is a general purpose transport protocol
Design decisions are made based on a set of
assumptions
TCP performance suffers when one or more
assumptions do not hold in some application
scenario’s
We want to understand
- Common scenario’s when standard TCP does not
perform well
- What assumptions are broken in each scenario?
- What different design decisions can be made to
improve TCP performance in each scenario?
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Hui Zhang, Fall 2012
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Scenario 1: Network With Large
Delay Bandwidth Product
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Example: a supercomputer in UIUC
communicating with another supercomputer in
CERN of Switzerland over a 10Gbps network
Assume round trip time (RTT) is 200 ms
What is the maximum throughput with standard
TCP?
Hui Zhang, Fall 2012
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The Key Formula and The Answer
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MaxThroughput <= MaxWindowSize/RTT
Reasons:
- MaxWindowSize is the maximum of outstanding bytes a
sender can send before receiving the first acknowledgement
packet
- RTT is the minimum period between the time the first byte
sent by the sender and the first ack (with respect to the first
bye) is received by the sender
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What is the maximum window size in TCP?
- 64 KB as limited by the 16 bits window size in TCP header
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Therefore, the answer is 2.56 Mbps (64 KB/200ms)
for one TCP connection
Hui Zhang, Fall 2012
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TCP Large Window Option
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TCP option allows additional 14 bits window size
The total window size up to 30 bits (1GB)
Question: what is the minimum additional number of bits
for the window that the supercomputers need to use to
fully utilize the 10Gbps link?
Answer
- Window size in number of bytes: 250 GB (10Gbps * 200ms/8)
- # of bits needed to encode the window: 28 bits
- # of additional bits: 12
Hui Zhang, Fall 2012
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Large Delay Bandwidth Network
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In the previous example, when the link is fully utilized, how
many packets (TCP segments) are in transit before the 1st
ack is received by the sender?
For simplicity assume each segment has 1KB payload
The answer is 250,000 (250MB/1KB)
Hui Zhang, Fall 2012
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Flow Control vs. Congestion Control
in Large Delay Bandwidth Networks
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TCP’s sender window is limited by both awnd (for flow
control purpose) and cwnd (for congestion purpose)
The previous example considers only the limitation
imposed by flow control
Now let’s consider the effect of congestion control
- Assuming NO packet loss, how much time does it take for the
sender cwnd to grow to 1GB?
You should try to work this work yourself
- What happens if there are multiple packet losses in one window?
Hui Zhang, Fall 2012
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Implication of Decision of Using
Cumulative Ack in TCP
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Standard TCP uses cumulative ack
- Plus: robust with respect to lost acks
- Minus: not robust with respect to lost packets
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The drawback of cumulative ack is more serious in
networks with networks with large delay bandwidth
product
- Large number of packets per RTT
- The sender learns only one packet loss per RTT
- A small number of packet losses can result in Retransmission
Timeout (RTO)
- With a RTO, the sender will retransmit all packets starting from the
packet that causes the timeout
- many of these packets may have been received by the receiver,
however, the sender won’t be able to know --- the cumulative ack
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Hui
Zhang, Fall
2012 provide this information!
does
not
TCP Selective Ack Option
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Receivers informs the sender about each of packets that
has been received successfully
Sender transmits only the packet that have not been
acked
Hui Zhang, Fall 2012
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Scenario 2: Performance
Degradation in Wireless Networks
Sequence number (bytes)
2.0E+06
Expected
TCP performance
(1.30 Mbps)
1.5E+06
TCP Reno
(280 Kbps)
1.0E+06
5.0E+05
0.0E+00
0
10
20
30
40
50
60
Time (s)
2 MB wide-area TCP transfer over 2 Mbps Lucent WaveLAN
Hui Zhang, Fall 2012
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Wireless Bit-Errors
Router
Computer 1
Computer 2
Loss  Congestion
3
2
2
1
2
0
Loss  Congestion
Burst losses lead to coarse-grained timeouts
Result: Low throughput
Hui Zhang, Fall 2012
Wireless
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TCP Problems Over Noisy Links
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Wireless links are inherently error-prone
- Fades, interference, attenuation
- Errors often happen in bursts
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TCP cannot distinguish between corruption and
congestion
- TCP unnecessarily reduces window, resulting in low
throughput and high latency
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Burst losses often result in timeouts
Sender retransmission is the only option
- Inefficient use of bandwidth
Hui Zhang, Fall 2012
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Proposed Solutions
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Incremental deployment
- Solution should not require modifications to fixed hosts
- If possible, avoid modifying mobile hosts
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End-to-end protocols
- Selective ACKs, Explicit loss notification
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Split-connection protocols
- Separate connections for wired path and wireless hop
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Reliable link-layer protocols
- Error-correcting codes
- Local retransmission
Hui Zhang, Fall 2012
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Approach Styles (End-to-End)
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Improve TCP implementations
- Not incrementally deployable
- Improve loss recovery (SACK, NewReno)
- Help it identify congestion (ELN, ECN)
• ACKs include flag indicating wireless loss
- Trick TCP into doing right thing  E.g. send extra
dupacks
Wired link
Hui Zhang, Fall 2012
Wireless link
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Approach Styles (Link Layer)
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More aggressive local rexmit than TCP
- Bandwidth not wasted on wired links
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Possible adverse interactions with transport layer
- Interactions with TCP retransmission
- Large end-to-end round-trip time variation
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FEC does not work well with burst losses
Wired link
Wireless link
ARQ/FEC
Hui Zhang, Fall 2012
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Scenario 3: Asymmetric TCP
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Example of Asymmetric networks
- Cable modems: 10 Mbps down, 512 kbps up
- ADSL: 8 Mbps down, 1 Mbps up
- May also due to congested condition on reverse path
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Forward path throughout may be smaller than
forward path link capacity, why?
Slower bandwidth on reverse path stretches out
ACKs (ACK dilation)
Hui Zhang, Fall 2012
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Scenario 4: Small File and Transaction
Applications
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Most files are smaller than 1 KB
- One TCP segment
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No chance for fast recovery/fast retransmission (why?)
Packet loss results in RTO
Hui Zhang, Fall 2012
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Summary: TCP Key Design
Decisions & Assumptions
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Application: large number of bytes & average
throughput
- What about small file and transaction applications?
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Packet drop is congestion signal
- Wireless network?
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Cumulative ack
- Single packet loss with large RTT*BW environment
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16 bits win size
- Large RTT*BW environment
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Ack clocking
- Asymmetric path
Hui Zhang, Fall 2012
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Tradeoffs Between General-Purpose
Protocol vs. Special Purpose Protocol
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TCP is general purpose protocol
- One size hard to fit for all
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Why not developing special purpose protocol?
- Metcalf’s law of network utility
- There is huge value just to be able to communicate with
anyone
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TCP option strikes the balance
- Backward compatible with hosts not implementing the
options (still be able to communicate, may not be high
performance)
- Two hosts both implementing the same option can take
advantage of the benefit of the option
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TCP option does incur additional complexity and
overhead
Hui Zhang, Fall 2012
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