sliding window. sliding window - sender side cumulative acknowledgments not sentsent, no...

Sliding Window

Sliding window - Sender side

Cumulative AcknowledgmentsNot sent Sent, no ACK ACK:ed Free

Sending buffer at the sender:

Old data sent that has already been ACK:ed(Could as well be marked as free space)

New data sent to transport layerby application, but not yet sent

Free buffer space where application canwrite new data to be sent




Data that has been sent, but not ACK:edAlso called the Sending window

This is the sliding window (yes, it slides!)

This data can not be sent yet, as the sliding window in this example has a maximum size of 10




ACTION: An ACK of the oldest sent packet arrives

• The window slides so that the left border is in line with the oldest outstanding ACK

• The unsent segments that fit within the window are sent




• The data is placed in free buffer slots

ACTION: The application has more data to send




• Older sent but un-ACK:ed segments are now considered to be ACK:ed

ACTION: An ACK arrives in the middle of the window

• The window slides and unsent segments within the window are sent

• The window shrinks by one segment as there is no more than 9 segments outstanding




• The data is placed in free buffer slots

ACTION: The application has more data to send

• As the window is currently 9 segments wide, it can grow by one segment

• The new data that fits within the window is sent




• The ACK is silently ignored

ACTION: An ACK of already ACK:ed segments arrives

• Must keep track of outstanding data– Data sent, but not ACK:ed

• Must not exceed maximum window size– Configuration parameter– Affects memory consumption

• Must adjust window size when – An ACK inside the window arrives– New data that can fit within window arrives

from application


Cumulative Acknowledgments

Sliding window – Receiver side


Y X

Not received Received Read Free

Read buffer at the receiver:

N/A

Data that was previously received andthat has been delivered to application

Data that was previously received,but not yet delivered to theapplication

Data not yet received



Y X



N/A

Free buffer space wherenew segments that arereceived can be stored

Space unavailableto new segments

The sliding window holdsdata received but not yetread. Must also be able tokeep ”holes” like segmentY in the segments

This sliding window hassize 12, max size 14



Y X



• Store in read buffer, register as received

ACTION: Segment X arrives

• Send cumulative ACK Y to indicate that receiver is waiting for Y Y

N/A



Y X



• Can not fit into the buffer, must be discarded

ACTION: Segment X+2 arrives

• Send cumulative ACK Y to indicate that receiver is waiting for Y Y

N/A



Y X



• Only two segments are returned, still waiting for Y

ACTION: Applications try to read 5 segments

• Application is informed of how much data was read

• The unavailable segment at the end of the buffer becomes available

N/A



Y X



• Store in read buffer, register as received

ACTION: Segment Y arrives

• Send cumulative ACK (X+1) to indicate that receiver is waiting for (X+1) X+1

N/A



Role of sliding window is different at receiver– Represents the maximum buffer size for

segments received but still not read– If a segment that does not fit inside the window

arrives (either too new or too old), it is discarded.• However, an ACK is sent

– To avoid running out of buffer space, receiver can inform sender about available buffer space in each ACK

sliding window. sliding window - sender side cumulative acknowledgments not sentsent, no...

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