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explicitly the sender of the reception and the sender could retransmit this segment although it has already been received. A possible solution to improve the performance of selective repeat is to provide additional information about the received segments in the cknowledgements that are returned by the receiver. For example,the receiver could add in he returned acknowledgement the list of the sequence numbers of all segments that have already been received. Such acknowledgements are sometimes called selective cknowledgements. This is illustrated in the figure below. In the figure above, when the sender receives C(OK,0,[2]), it knows that all segments up to and including D have been correctly received. It also knows that segment D(2,...) has been received and can cancel the retransmission timer associated to this segment. However, this segment should not be removed from the sending buffer before the reception of a cumulative acknowledgement (C(OK,2) in the figure above) that covers this segment. Note: Maximum window size with go-back-n and selective repeat A transport protocol that uses n bits to encode its sequence number can send up to 2n different segments. However, to ensure a reliable delivery of the segments, go-back-n and selective repeat cannot use a sending window of 2n segments. Consider first go-back-n and assume that a sender sends 2n segments. These segments are received in-sequence by the destination, but all the returned acknowledgements are lost. The sender will retransmit all segments and they will all be accepted by the receiver and delivered a second time to the user. It is easy to see thatthis problem can be avoided if the maximum size of the sending window is 2n 1 segments. A similar problem occurs with selective repeat. However, as the receiver accepts out-of-sequence segments, a sending window of 2n 1 segments is not sufficient to ensure a reliable delivery of all segments. It can be easily shown that to avoid this problem, a selective repeat sender cannot use a window that is larger than 2n 2 segments. Go-back-n or selective repeat are used by transport protocols to provide a reliable data transfer above an unreliable network layer service. Until now, we have assumed that the size of the sliding window was fixed for the entire lifetime of the connection. In practice a transport layer entity is usually implemented in the operating system and shares memory with other parts of the system. Furthermore, a transport layer entity must support several (possibly hundreds or thousands) of transport connections at the same time. This implies that the memory which can be used to support the sending or the receiving buffer of a transport connection may change during the lifetime of the connection 4 . Thus, a transport protocol must allow the sender and the receiver to adjust their window sizes. To deal with this issue, transport protocols allow the receiver to advertise the current size of its receiving window in all the acknowledgements that it sends. The receiving window advertised by the receiver bounds the size of the sending buffer used by the sender. In practice, the sender maintains two state variables : swin, the size of its sending window (that may be adjusted by the system) and rwin, the size of the receiving window advertised by the receiver. At any time, the number of unacknowledged segments cannot be larger than min(swin,rwin) 5 . The utilisation of dynamic windows is illustrated in the figure below. The receiver may adjust its advertised receive window based on its current memory consumption, but also to limit the bandwidth used by the sender. In practice, the receive buffer can also shrink as the application may not able to process the received data quickly enough. In this case, the receive buffer may be completely full and the advertised receive window may shrink to 0. When the sender receives an acknowledgement with a receive window set to 0, it is blocked until it receives an acknowledgement with a positive receive window. Unfortunately, as shown in the figure below, the loss of this acknowledgement could cause a deadlock as the sender waits for an acknowledgement while the receiver is waiting for a data segment. To solve this problem, transport protocols rely on a special timer : the persistence timer. This timer is started
by the sender whenever it receives an acknowledgement advertising a receive window set to 0. When the timer expires, the sender retransmits an old segment in order to force the receiver to send a new acknowledgement, and hence send the current receive window size. To conclude our description of the basic mechanisms found in transport protocols, we still need to discuss the impact of segments arriving in the wrong order. If two consecutive segments are reordered, the receiver relies on their sequence numbers to reorder them in its receive buffer. Unfortunately, as transport protocols reuse the same sequence number for different segments, if a segment is delayed for a prolonged period of time, it might still be accepted by the receiver. This is illustrated in the figure below where segment D is delayed.
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