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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 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)

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  1. TCP Flow Control (nagle’s algorithm) 오 남 호 분산 처리 실험실 (nhooh@cs.chonbuk.ac.kr)

  2. 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 -2-

  3. 1 2 3 4 5 6 7 8 9 10 11 … 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 -3-

  4. 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 -4-

  5. TCP Flow Control (III) -5-

  6. 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 -6-

  7. TCP Flow Control (V) -7-

  8. 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 -8-

  9. 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 -9-

  10. TCP Flow Control Problems (III) -10-

  11. 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 -11-

  12. 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 -12-

  13. 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 -13-

  14. 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 -14-

  15. How TCP solves the SPP (I) -15-

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