SOLUTION QUIZ 5 CMPT 371 Consider flow control in TCP. Explain ...

SOLUTION QUIZ 5 CMPT 371
Consider flow control in TCP. Explain two of the following mechanisms in detail
a) Fast retransmit
b) Use of sequence numbers and acknowledgement numbers
c) Cumulative acknowledgements
SOLUTION
a)
Fast retransmit is a mechanism used by TCP to improve TCP’s efficiency. TCP does not use
NACKs so the only obvious way to know if a packet needs to be retransmitted is to wait for the
retransmission timer for the packet to expire. The duration of the waiting period for the ACK
(the duration of the retransmission timer) may be large. In fact it may be large enough that the
contents of the entire sliding window (or congestion window if it is smaller) has been transmitted
before the waiting period expires. Then, the time between the end of the transmission of the data
in the sliding window (or congestions window) and the expiration of the timeout timer is not
used for transmission. Not using this time for transmission reduces the efficiency of TCP
transport. To improve the efficiency of TCP transport an alternate method of instigating
retransmission of a lost packet is used. This alternate method is known as fast retransmission.
Fast retransmission relies upon the acknowledgements that are received as a result of the
receiver’s receipt of packets after the lost packet. Each time the receiver receives a packet it will
send an acknowledgement to the source. This acknowledgement will indicate the next octet of
data the receiver expects (the first octet in the lost packet). If a packet has been lost the packets
following it will send the same ACK as the packet that preceded it because the receiver is still
waiting for the same next packet of data. The duplicate ACKs received also implicitly tell the
source that a packet that is not the next expected packet has been received. If three duplicate
ACKs are received (the original, then three identical copies of the same ACK) the source will
take this to mean that a packet has been lost, and will automatically retransmit the next packet
the receiver is expecting (as indicated in the duplicate ACKs. This alternate mechanism to cause
retransmission is called fast retransmit.)

) TCP uses cumulate acknowledgements (ACKs). Each time a packet is received, the receiver
sends an ACK is to the source. That ACK indicates the next octet of data (from the stream going
from the source to the destination) the receiver expects to receive from the source. If a packet is
received out of order the next expected octet (the same one as before) is still given in the ACK.
Rather than acknowledge the particular octet that has just arrived the receiver will acknowledge
the next octet of data expected. When that packet arrives the system will again acknowledge the
(new) next octet of data required.

c) Each TCP packet contains two fields the sequence number and the ACK number. These two
fields indicate which data is in the packet and/or which data is expected next respectively. TCP is
a full duplex protocol, this means that data can flow in both directions simultaneously. Consider
data flowing from Host A to Host B above as a continuous stream broken into octets (8 bit
chunks). The first octet sent will have a randomly chosen octet number.
The octet number indicates the location of the octet in the stream. If the first octet is 42, the
second will be 43, the third octet 44, and so on. The sequence number of a packet going from
Host A to Host B is the number of the first octet of data from the data stream flowing from Host
A to Host B that is in that packet. Next consider data flowing from Host B to Host A above as a
continuous stream broken into octets (8 bit chunks). The first octet sent will have a randomly
chosen octet number (in the diagram above octet 79). The sequence number of a packet going
from Host B to Host A is the number of the first octet of data from the data stream flowing from
Host B to Host A that is carried in that packet. Next, again consider the stream of data from A to
B. Each time A sends a packet to B it will expect to receive an ACK from B indicating that B has
received the packet. The ACK that B sends to A will have the ACK bit set in the TCP header (to
indicate it is an ACK). When the ACK bit in a packet going from B to A is set then the ACK
number in the TCP header indicates the next octet of data (from the data stream going from A to
B) that the receiver expects to receive. Finally, consider again the stream of data from B to A.
Each time B sends a packet to A it will expect to receive an ACK from A indicating that A has
received the packet. The ACK that A sends to B will have the ACK bit set in the TCP header (to
indicate it is an ACK). When the ACK bit in a packet going from A to B is set then the ACK
number in the TCP header indicates the next octet of data (from the data stream going from B to
A) that the receiver expects to receive.