Mobile Cloud Networking and Mobility Control 133
In CMT, there is no distinction between the primary and secondary
paths. All paths are equivalent, and are used simultaneously in data
transmission. CMT employs the same system of sequencing numbers
as SCTP. CMT uses the round-robin algorithm to send the data along
the available paths. The sending of data to the transmitter is monitored
by the size of the congestion window (CWND) for each path. CMT
sends the data along a path as soon as the congestion window for that
path becomes available. When several paths can be used for the
transmission, the path is selected using the round-robin model. CMT
fills the congestion window of each path before moving on to the next.
Despite the benefits of using multipath transmission, CMT brings
with it the phenomenon of de-sequencing, which degrades
performances, because of the fact that the paths in CMT may have
different characteristics in terms of bandwidth and delay.
CMT identifies three causes of de-sequencing of data at the
receiver’s end:
– needless fast retransmissions. In a multipath transmission, paths
may have different bandwidths and delays. It may be that an
acknowledgement from a fast path will indicate the loss of a packet
while that packet is still in transit on a slower path, and only arrives at
its destination later on. CMT uses the same congestion control
mechanism as TCP. When the transmitter receives three duplicate
acknowledgements indicating the loss of a packet, it deems that packet
to be lost in the network and triggers the fast retransmission of the
packet. However, this retransmission is needless, because it is the desequencing which is the cause of the duplicated acknowledgements;
– inaccurate updating of the congestion window. The mechanism
by which the congestion window is updated only allows it to be
increased if a new cumulative acknowledgement (CUM ACK) number
is received by the transmitter. When the acknowledgements with the
same CUM ACK have arrived, even if they contain new data gaps, the
transmitter does not modify its congestion window. As the
phenomenon of de-sequencing occurs regularly in CMT, several
acknowledgements with the same acknowledgement are sent to the
transmitter. When the data gaps are covered by a new
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In CMT, there is no distinction between the primary and secondary
paths. All paths are equivalent, and are used simultaneously in data
transmission. CMT employs the same system of sequencing numbers
as SCTP. CMT uses the round-robin algorithm to send the data along
the available paths. The sending of data to the transmitter is monitored
by the size of the congestion window (CWND) for each path. CMT
sends the data along a path as soon as the congestion window for that
path becomes available. When several paths can be used for the
transmission, the path is selected using the round-robin model. CMT
fills the congestion window of each path before moving on to the next.
Despite the benefits of using multipath transmission, CMT brings
with it the phenomenon of de-sequencing, which degrades
performances, because of the fact that the paths in CMT may have
different characteristics in terms of bandwidth and delay.
CMT identifies three causes of de-sequencing of data at the
receiver’s end:
– needless fast retransmissions. In a multipath transmission, paths
may have different bandwidths and delays. It may be that an
acknowledgement from a fast path will indicate the loss of a packet
while that packet is still in transit on a slower path, and only arrives at
its destination later on. CMT uses the same congestion control
mechanism as TCP. When the transmitter receives three duplicate
acknowledgements indicating the loss of a packet, it deems that packet
to be lost in the network and triggers the fast retransmission of the
packet. However, this retransmission is needless, because it is the desequencing which is the cause of the duplicated acknowledgements;
– inaccurate updating of the congestion window. The mechanism
by which the congestion window is updated only allows it to be
increased if a new cumulative acknowledgement (CUM ACK) number
is received by the transmitter. When the acknowledgements with the
same CUM ACK have arrived, even if they contain new data gaps, the
transmitter does not modify its congestion window. As the
phenomenon of de-sequencing occurs regularly in CMT, several
acknowledgements with the same acknowledgement are sent to the
transmitter. When the data gaps are covered by a new
www.it-ebooks.info