An Investigation into Transport Protocols and Data Transport ...

4.5. TCP Variants 73
TCP Timestamps [JBB92] (See Section 5.2.1), typically only 3 SACK blocks
are expected in each ack.
The extra redundancy in having more than one SACK Block in each ack is
necessary due to the lossy nature of network paths and enables ‘robustness’
when acks are lost. It is therefore also important that the SACK option
always reports the block containing the most recently received segment, as
this provides the sender with the most up-to-date information about the state
of the network and the data receiver’s queue.
However, the unacknowledged segments can be treated in different ways
when accounting for outstanding data. The approach promoted by the IETF
is to consider all unacknowledged data to be outstanding in the network.
Holes within SACK blocks indicate potentially lost packets. However, they
may also indicate reordered packets. The method by which SACK is implemented
can be conservative or aggressive in the way it treats the holes
within the SACK blocks. The Forward Acknowledgments (FACK) algorithm
[MM96] takes the more aggressive approach and considers the unacknowledged
holes between the SACK blocks as lost packets and therefore retransmits
the data. The more conservative approach suggested by the IETF is
to consider all unacknowledged segments to be outstanding in the network.
Although the FACK approach can result in better TCP performance it may
increase the retransmission of unnecessary data.
How the sender uses the information provided by SACK is implementationdependent.
For example, Linux uses a Forward Acknowledgment (FACK) algorithm
[MM96] Another implementation is sometimes referred to as Reno+SACK
[MMFR96, MM96].