An Investigation into Transport Protocols and Data Transport ...

4.5. TCP Variants 72
drops within a window. To avoid this TCP NewReno was proposed (see
Section 4.5.4).
TCP-SACK has been shown to perform well even at a high level of packet
losses in the network [MM96].
SACK Blocks
SACK attaches detailed information called SACK Blocks in TCP Options
[Pos81b] on each and every ack. Each SACK block specifies a contiguous and
isolated block of data not covered by the TCP Cumulative Acknowledgment
field. Therefore cumulative SACK Blocks reports in the ack segments help
the sender to identify ‘holes’ in the sequence numbers at the receiver; on the
receipt of an ack containing the SACK Block, the sender can infer which
segments are lost and hence retransmit only those segments. Therefore,
SACKs enables a TCP sender to maintain an image of the receiver’s queue
and enables recovery from multiple losses per window within roughly one
RTT.
Each SACK block is described using absolute sequence numbers, with
two sequence numbers describing each block, each taking 4B:
1st Left edge of the block, i.e. the first byte of the block successfully received.
2nd Right edge of the block; i.e. the sequence number of the byte immediately
following the last byte in the block.
Given that the TCP options field is limited to 40B, the maximum number
of SACK blocks per ack is limited 8 to four. However, as it is expected that
SACK will often be used in conjunction with other TCP Options such as
8 Each TCP option has a necessary associated overhead.