EE 281 Lecture Notes - Lecture 12: Tcp Congestion Control, Network Congestion, Sliding Window Protocol
Lost ACK
If an acknowledgement is lost and the sender times out (meaning that there were no additional segments sent), the sender will
retransmit that segment. The receiver, upon getting it, will see that it does not need that sequence number (it is a duplicate
segment) and will discard the segment but will send an acknowledgement back to the sender.
Another case is when the sender sent two segments but the acknowledgement to the first one was lost. Because
acknowledgements are cumulative and the sender gets the second, cumulative acknowledgement, it knows that the sender has
received all the bytes and a retransmission is not necessary.
Delayed ACKs
If the network round-trip time suddenly became much longer than expected, acknowledgements to a sequence of segments might
arrive after a timeout. When the timeout occurs, the sender will retransmit only the earliest unacknowledged segment and restart
the timer. Suppose now that acknowledgements for the previously transmitted segments arrive. TCP will process them and adjust
the base of its send buffer if those bytes are no longer needed. When the receiver gets the duplicate packet, it will discard it but
send an acknowledgement. Some time later, the sender will receive this acknowledgement but see that it is a duplicate and hence
discard it.
TCP Fast Retransmit
As we have seen, TCP uses pipelining and can send multiple segments before waiting for acknowledgements. If a receiver
detects a missing sequence number, it means one of two things: a segment was lost (either discarded due to queue overflows or
due to data corruption) or that a segment is delivered out of sequence. TCP does not use negative acknowledgements but sends an
acknowledgement for every received out-of-sequence segment with the sequence number of the next byte it needs. In the case
where a segment has indeed been lost, every segment after that will be acknowledged with the same sequence number. Hence,
the receiver will see a stream of duplicate acknowledgements.
If TCP receives three segments with duplicate acknowledgement numbers, it assumes a segment was lost (the assumption is that
it is unlikely that one segment was routed or delayed such that three others arrived first). Instead of waiting for an RTO to occur,
TCP will transmit that missing segment (the one with the sequence number of the ACK). This technique is called a fast
retransmit because the protocol retransmits the segment without waiting for the RTO to occur.
Selective Acknowledgements
TCP behaves in a similar manner, but not quite the same, as the Go-Back-N protocol. The sender only keeps track of the earliest
sequence number that has been transmitted but not acknowledged. A distinction is that, in the case of a timeout, Go-Back-N will
retransmit all segments in its window while TCP will only retransmit the lowest (earliest) segment.
With this behavior, having the receiver store out-of-order segments is optional. The receiver may store them. If it does, and the
receipt of a segment fills in a hole (missing segment), then TCP will send back a cumulative ACK, ensuring that the sender will
not have to retransmit those segments. However, the sender does not know if this will happen and has to hold on to all
unacknowledged segments in its send buffer just in case any or all of those segments have to be resent.
An optional enhancement to the TCP protocol uses the options field in the header to allow the receiver to send over a list of {start
byte, end byte} values to identify the specific range of bytes that have been received. These are called Selective
Acknowledgements and make the protocol behave like a Selective Repeat protocol. The acknowledgement number in the TCP
header is unchanged but the list of received byte ranges allows the sender to remove those enumerated segments from its send
buffer since it knows that they will never have to be retransmitted.
Receiver-regulated flow control
It would be pointless to keep sending data to the receiver if the receiver’s application isn’t keeping up with reading it. In addition
to slowing down transmission due to congestion (which we will look at next), TCP allows the receiver to tell the sender to tell it
how much space it has in the receive window (the free space in the receive buffer). It does this by placing the size of the receive
window into the receive window field in the TCP header in each segment that is sent to the sender.
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Document Summary
If an acknowledgement is lost and the sender times out (meaning that there were no additional segments sent), the sender will retransmit that segment. The receiver, upon getting it, will see that it does not need that sequence number (it is a duplicate segment) and will discard the segment but will send an acknowledgement back to the sender. Another case is when the sender sent two segments but the acknowledgement to the first one was lost. Because acknowledgements are cumulative and the sender gets the second, cumulative acknowledgement, it knows that the sender has received all the bytes and a retransmission is not necessary. If the network round-trip time suddenly became much longer than expected, acknowledgements to a sequence of segments might arrive after a timeout. When the timeout occurs, the sender will retransmit only the earliest unacknowledged segment and restart the timer. Suppose now that acknowledgements for the previously transmitted segments arrive.
