An Investigation into Transport Protocols and Data Transport ...

4.5. TCP Variants 70
partial acks do not take NewReno out of Fast Recovery and it retransmits
one packet per RTT until all the lost packets are retransmitted thus avoiding
the multiple fast retransmits from a single window of data [Flo94b].
The downside of this is that it may take many RTT’s to recover from
a loss episode, and enough new data must be present in order to keep the
ack clock running. Otherwise, re-initiation of the ack clock through a RT O
timeout is still necessary.
The modifications to Reno to enable NewReno are as follows:
Multiple Packet Loss
When first entering Fast Retransmit, the highest sequence number sent so far
is saved in the variable recover. Normal retransmission of data and the Fast
Recovery algorithm are run as normal. However, when a new ack arrives,
an additional check is performed to ensure that this ack covers the value of
recover. If the sequence number of the ack is less than that of recover,
then this ack is a partial ack and signals that another segment was lost from
the same window of data.
As such, TCP can retransmit the segment reported as expected by the
partial ack. There are two versions of NewReno algorithm which differ in
the way that the RT O is updated under this scenario [FHG04]. Under the
‘slow-and-steady’ variant, the RT O is reset on every partial ack. Whilst
with the ‘impatient’ variant the RT O is only set after the first partial ack.
Due to this RT O update, the impatient variant is more likely to be able
to recover quickly under very lossy conditions (or large cwnds) by resorting
to timeouts, whilst the ‘slow-but-steady’ variant will take approximately n
RTTs to recover (where n is the number of lost packets in the window)