An Investigation into Transport Protocols and Data Transport ...

4.5. TCP Variants 71
[Hoe96]. In both cases, the TCP connection only exits Fast Recovery if the
new ack’s sequence number is larger than recover.
False Retransmits
In order to prevent the retransmission of segments that have already been
received by the receiver 6 , the same recover variable as per the previous
discussion is used 7 . With the initial call into Fast Retransmit after a loss is
detected, the RT O is reset. Upon consecutive RT O’s, the highest sequence
number transmitted so far is also recorded into recover. By checking the
received acks against recover, the TCP sender is able to prevent having to
retransmit unnecessary data.
The sender comes out of Fast Recovery and Fast Retransmits only after
all outstanding packets (at the time of first loss) have been acked.
4.5.5 Selective Acknowledgments (SACKs)
With Selective Acknowledgment, SACK [MMFR96], the data receiver can
inform the sender about all segments that have arrived successfully, so the
sender need retransmit only the segments that have actually been lost. However,
TCP-SACK requires that both TCP senders and receivers be modified
to support SACKs.
In order to be backwards compatible with existing non-SACK TCP receivers,
SACK capability is negotiated during the TCP handshake. If neither
of the TCP hosts implement the TCP SACK option, then TCP-SACK behaves
the same as TCP Reno - with its associated problem with multiple
6 As retransmitted packets may also be dropped.
7 Note that the original ‘experimental’ draft of NewReno does not implement the check
against recover.