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TCP Flow Control (nagle’s algorithm). 오 남 호 분산 처리 실험실 ( nhooh@cs.chonbuk.ac.kr). TCP Flow Control. TCP uses a specialized sliding window mechanism to solve important problems efficient transmission and flow control Efficient transmission

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tcp flow control nagle s algorithm

TCP Flow Control (nagle’s algorithm)

오 남 호

분산 처리 실험실

(nhooh@cs.chonbuk.ac.kr)

tcp flow control
TCP Flow Control
  • TCP uses a specialized sliding window mechanism to solve important problems
    • efficient transmission and flow control
  • Efficient transmission
    • TCP window mechanism make it possible to send multiple segments before an ack arrives
      • increasing total throughput
  • Flow control
    • allowing the receiver to restrict transmission until it has sufficient buffer space to accommodate more data

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tcp flow control i

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TCP Flow Control (I)
  • TCP Sliding window
    • first pointer
      • separating octets that have been sent and acknowledged from octets yet to be acknowledged
    • second pointer
      • define the highest octet in the sequence that can be sent before more acknowledgements are received
    • third pointer
      • Separate those octet that have already been sent from those octets that have not been sent

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tcp flow control ii
TCP Flow Control (II)
  • Difference between the TCP sliding window protocol and the simplified sliding window protocol
    • TCP allows the window size to vary over time
    • each ack contain window advertisement
      • specifying the receiver’s current buffer size
  • Increased window advertisement
    • sender increases the size of its sliding window and proceeds to send octets that have not been acknowledged
  • Decreased window advertisement
    • sender decreases the size of its window and stops sending octets beyond the boundary

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tcp flow control iv
TCP Flow Control (IV)
  • The advantage of using a variable size window is that it provides flow control as well as reliable transfer
    • if the receiver’s buffer begin to become full
      • it cannot tolerate more packets, so it sends a smaller window advertisement
      • in the extreme case, the receiver advertises a window size of zero to stop all transmissions
  • Having a mechanism for flow control is essential in an internet environment
    • where machines of various speeds and sizes communicate through networks and routers of various speeds and capacities

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tcp flow control problems i
TCP Flow Control Problems (I)
  • The small packet problem
    • occurs when the source sends many small packets
  • The silly window syndrome
    • occurs when the destination reads a small number of bytes at a time from its buffer

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tcp flow control problems ii
TCP Flow Control Problems (II)
  • Consider an interactive application where the source host sends each keystroke one at a time to the destination host
    • each keystroke is 1 byte, after adding TCP/IP overhead, a 41 byte packet is generated
    • when the destination receives the packet, it returns a 40 byte ack. packet
    • when the destination removes the byte from its buffer, a 40 byte window update packet is sent
    • some applications echo the types character back to the source, creating another 41-byte packet
  • The small packet problem seriously degrades throughputs

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avoiding silly window syndrome i
Avoiding silly window Syndrome (I)
  • Sending machine avoids transmitting a small amount of data in each segment
  • Receiving machine avoids sending small increments in window advertisement that can trigger small data packet
  • Receive-side silly window avoidance
    • before sending an updated window advertisement after advertising a zero window
      • wait for space to become available that is either at least 50% of the total buffer size or equal to a maximum sized segment

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avoiding silly window syndrome ii
Avoiding silly window Syndrome (II)
  • Two approaches have been taken to implement silly window avoidance on the receive side
    • in the first approach
      • TCP acknowledges each segment that arrives,
        • but does not advertise an increase in its window until the window reaches the limits specified by the silly window avoidance
    • in the second approach
      • TCP delay sending an acknowledgement when silly window avoidance specifies that the window is not sufficiently large to advertise

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avoiding silly window syndrome iii
Avoiding silly window Syndrome (III)
  • Send-side silly window avoidance
    • when a sending application generates additional data to be sent over a connection for which previous data has been transmitted but not acknowledge
      • place the new data in the output buffer as usual, but don’t send additional segments until there is sufficient data to fill a maximum-sized segment
    • if still waiting to send when an acknowledgement arrives,
      • send all data that has accumulated in the buffer

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how tcp solves the spp
How TCP solves the SPP
  • Nagle’s algorithm
    • when data is sent one byte at a time, send only the first byte
    • buffer all remaining bytes until the first one is acknowledged
    • after receiving the ack., send all the buffered bytes in one packet
  • This algorithm reduces the amount of bandwidth required to support interactive applications

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