134 Software Networks
acknowledgement, the congestion window is only increased with the
new data acknowledged in the most recent acknowledgement. The
previously-acknowledged data in the gaps do not contribute to the
growth of the congestion window. In other words, the congestion
window update does not exactly repeat the volume of data transmitted;
– increased acknowledgement traffic. The principle of delayed
acknowledgements in TCP is also used in SCTP. Instead of sending an
acknowledgement for each and every packet received, the use of a
group
acknowledgement
for
several
packets
reduces
acknowledgement traffic. SCTP uses this mechanism if the packets
reach the receiver in the correct order. De-sequenced packets must be
acknowledged immediately. However, as it is quite common for desequencing to occur in CMT, if the receiver cannot delay the sending
of the acknowledgements, then acknowledgement traffic is greatly
increased, which may impact the network’s performance. CMT
includes the following solutions for these problems:
- in order to prevent needless retransmissions, CMT offers the
algorithm SFR (Split Fast Retransmit), which enables the transmitter
to correctly interpret duplicate acknowledgements. SFR defines a
virtual buffer for each destination within the transmitter’s
retransmission buffer. With the additional information of each
destination, such as the highest acknowledged TSN for each
destination, the transmitter can distinguish between de-sequencing and
actual data loss, with a view to correctly triggering fast retransmission.
A chunk with the TSN T for destination M is deemed lost if and only
if T is lower than the highest acknowledged TSN for destination M;
- the algorithm CUC (Cwnd Update for CMT) is proposed to
correctly update the congestion windows for the paths. At the level of
the transmitter, each destination has a variable known as the PSEUDOCUMACK, which represents the smallest anticipated TSN. Upon receipt
of a SACK acknowledgement, the transmitter checks whether there is
a change of PSEUDO-CUMACK for each destination. An increase of a
PSEUDO-CUMACK triggers the updating of the congestion winder of
the corresponding destination, even if the CUM ACK does not advance.
Thus, the congestion window for each destination increases in parallel
to the acknowledged data, without having to wait for the new CUM
ACK;
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acknowledgement, the congestion window is only increased with the
new data acknowledged in the most recent acknowledgement. The
previously-acknowledged data in the gaps do not contribute to the
growth of the congestion window. In other words, the congestion
window update does not exactly repeat the volume of data transmitted;
– increased acknowledgement traffic. The principle of delayed
acknowledgements in TCP is also used in SCTP. Instead of sending an
acknowledgement for each and every packet received, the use of a
group
acknowledgement
for
several
packets
reduces
acknowledgement traffic. SCTP uses this mechanism if the packets
reach the receiver in the correct order. De-sequenced packets must be
acknowledged immediately. However, as it is quite common for desequencing to occur in CMT, if the receiver cannot delay the sending
of the acknowledgements, then acknowledgement traffic is greatly
increased, which may impact the network’s performance. CMT
includes the following solutions for these problems:
- in order to prevent needless retransmissions, CMT offers the
algorithm SFR (Split Fast Retransmit), which enables the transmitter
to correctly interpret duplicate acknowledgements. SFR defines a
virtual buffer for each destination within the transmitter’s
retransmission buffer. With the additional information of each
destination, such as the highest acknowledged TSN for each
destination, the transmitter can distinguish between de-sequencing and
actual data loss, with a view to correctly triggering fast retransmission.
A chunk with the TSN T for destination M is deemed lost if and only
if T is lower than the highest acknowledged TSN for destination M;
- the algorithm CUC (Cwnd Update for CMT) is proposed to
correctly update the congestion windows for the paths. At the level of
the transmitter, each destination has a variable known as the PSEUDOCUMACK, which represents the smallest anticipated TSN. Upon receipt
of a SACK acknowledgement, the transmitter checks whether there is
a change of PSEUDO-CUMACK for each destination. An increase of a
PSEUDO-CUMACK triggers the updating of the congestion winder of
the corresponding destination, even if the CUM ACK does not advance.
Thus, the congestion window for each destination increases in parallel
to the acknowledged data, without having to wait for the new CUM
ACK;
www.it-ebooks.info