An Investigation into Transport Protocols and Data Transport ...

4.6. Summary 74
D-SACK
As an extension of the SACK mechanism, if a duplicate segment was received,
then the receiver should send an ack, containing as its first SACK block, a
reference to the duplicate data. Using SACK information and the D-SACK
extensions [FMMP00] it is possible to infer the amount of re-ordering on a
path and it is possible to make TCP more robust to re-ordering by using this
information to set the fast retransmit threshold depending on the reordering
detected on a path [BA02, ZKFP02].
4.6 Summary
The functions and implementation of the Transmission Control Protocol
(TCP) were presented and some of the key changes to its algorithms in order
to better utilise network resources upon congestion were given.
As the Internet is a shared resource of many end-nodes sending traffic
into a ‘black-box’ network, it is important that users do not send data into
the network such that congestion collapse occurs. Should the transport algorithms
not support a mechanism that is aware of the need for appropriate
use of these resources, then congestion collapse will invariably occur. An important
aspect of TCP is that it introduces the idea of ‘congestion control’ in
order to regulate the rate at which data is sent (and retransmitted) into the
Internet. Because of the success of the Slow Start, RT O calculation, Fast
Retransmit and Fast Recovery mechanisms, it is widely believed that TCP
in all of its variants has prevented the occurrence of Internet collapse.
TCP implements the notion of a congestion window, cwnd, which is the
protocols’ estimation of the rate at which the network can handle network