2000 년 11 월 20 일 전북대학교 분산처리실험실 TCP Flow Control (nagle’s algorithm) 오 남 호 분산 처리 실험실

2000 년 11 월 20 일 전북대학교 분산처리실험실 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

2000 년 11 월 20 일 전북대학교 분산처리실험실 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 …

2000 년 11 월 20 일 전북대학교 분산처리실험실 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

2000 년 11 월 20 일 전북대학교 분산처리실험실 TCP Flow Control (III)

2000 년 11 월 20 일 전북대학교 분산처리실험실 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

2000 년 11 월 20 일 전북대학교 분산처리실험실 TCP Flow Control (V)

2000 년 11 월 20 일 전북대학교 분산처리실험실 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

2000 년 11 월 20 일 전북대학교 분산처리실험실 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

2000 년 11 월 20 일 전북대학교 분산처리실험실 TCP Flow Control Problems (III)

2000 년 11 월 20 일 전북대학교 분산처리실험실 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

2000 년 11 월 20 일 전북대학교 분산처리실험실 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

2000 년 11 월 20 일 전북대학교 분산처리실험실 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

2000 년 11 월 20 일 전북대학교 분산처리실험실 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

2000 년 11 월 20 일 전북대학교 분산처리실험실 How TCP solves the SPP (I)