UDT
Bo Liu
11/1/2012
Inspired by Yunhong GU
OUTLINE
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Goal of UDT
Three conditions
Congestion control of UDT
UDT Format
Composable UDT
What is UDT
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UDP-based Data Transfer Protocol
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Implemented in Application Layer
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Built on top of UDP with reliability control
and congestion control
UDT overview
Applications
UDT Socket
Applications
UDT
Socket API
UDP
Goal of UDT
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BDP-Bandwidth Delay Product
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Product of a data link's capacity and its endto-end delay
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Equivalent to the maximum amount of data
on the network circuit at any given time
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Commonly known as a long fat network
Goal of UDT
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Effectively utilize a high BDP network
 But
why do we need UDT?
Three Conditions
 Physical
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condition
networkbandwidth today has been expanded
to 10Gb/s with 100Gb/s emerging
Three conditions
 Application
condition
 High-energy
physics, astronomy,
earth science, and other high
performance computing areas
Three conditions
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TCP is not suitable for a high BDP network
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TCP has been found inefficient as the
network bandwidth-delay product (BDP)
increases
TCP's drawback
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Congestion control algorithm
Congestion control of UDT
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Rate control algorithm
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Rate contrl interval
Congestion control of UDT
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Rate control algorithm
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Slow start is controlled by FLow Control
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Similar to TCP, but only occurs at the session beginning.
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Window based flow control (Flow Control)
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FC limits the number of unacknowledged packets.
FC is triggered on each received ACK.
Congestion control of UDT
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Rate control algorithm
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(1) Positive ack
x←x+α(x)
α(x) is non-increasing and it approaches 0 as
x increases, i.e., limx→+∞α(x) = 0
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 ( x)  10
log( B  x ) 
c
Congestion control of UDT
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B(Mb/s)
B<=0.1
0.1<B<=1
1<B<=10
10<B<=100
a(x)(packets/interval)
0.00067
0.001
0.01
0.1
Congestion control of UDT
Congestion control of UDT
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Rate Control Algorithm
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(2) Negative ACK
x←(1-β)·x (0 < β < 1)
Decrease
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Randomized decrease factor
  = 1 – (8/9)n
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UDT Format
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Data packet
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Control packet
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Distinguished by the 1st bit (flag bit) of the
packet header
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UDT Format
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Data packet PCI
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01234567890123456789012345678901
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+|0|
Packet Sequence Number
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+|FF |O|
Message Number
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
Time Stamp
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+Destination Socket ID
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
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UDT Format
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Control Packet PCI
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01234567890123456789012345678901
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+|1| Type | Reserved
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+Additional Info
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+Time Stamp
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Destination Socket ID
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+Control Information Field ˜
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
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UDT Format
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Control Packet PCI
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8 types of control packets
UDT Format
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TYPE 0x0: Protocol Connection Handshake
Additional Info: Undefined
Control Info:
1) 32 bits: UDT version
2) 32 bits: initial packet sequence number
3) 32 bits: maximum packet size
4) 32 bits: socket ID
UDT Format
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TYPE 0x2: Acknowledgement (ACK)
Control Info:
1) 32 bits: The packet sequence number to which all the
previous packets have been received (excluding)
2) 32 bits: RTT (in microseconds)
3) 32 bits: RTT variance
4) 32 bits: Available buffer size (in bytes)
5) 32 bits: Estimated link capacity (in number of packets
per second)
UDT Format
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TYPE 0x3: Negative Acknowledgement
(NAK)
Additional Info: Undefined
Control Info:
1) 32 bits integer array of compressed loss
information
UDT Format
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TYPE 0x5: Shutdown
Additional Info: Undefined
Control Info: None
UDT Format
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TYPE 0x7: Message Drop Request:
Additional Info: Message ID
Control Info:
1) 32 bits: First sequence number in the
message
2) 32 bits: Last sequence number in the
message
Composable UDT
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TCP-based Protocol
TCP variants
UDP-based Protocol
UDT
Composable UDT
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Easy implementation and deployment of
new control algorithms
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Easy evaluation of new control algorithms
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Application awareness support and dynamic
configuration
Reference
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http://tools.ietf.org/pdf/draft-gg-udt-03.pdf
Thank you for attending!
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Bo Liu
11/1/2012