TCP/IP Tutorial and Technical Overview - IBM Redbooks

SACK option Selective Acknowledgment (SACK) allows the receiver
to inform the sender about all the segments that are
received successfully. Therefore, the sender will only
send the segments that got lost. If the number of the
segments that have been lost since the last SACK is
too large, the SACK option will be too large. As a
result, the number of blocks that can be reported by
the SACK option is limited to four. To reduce this, the
SACK option should be used for the most recent
received data. See Figure 4-16 for more details.
Figure 4-16 TCP: SACK option
Timestamps option The timestamps option sends a time stamp value that
indicates the current value of the time stamp clock of
the TCP sending the option. The Timestamp Echo
Value can only be used if the ACK bit is set in the TCP
header. See Figure 4-17 for more details.
Figure 4-17 TCP: Timestamps option
Padding All zero bytes are used to fill up the TCP header to a total
length that is a multiple of 32 bits.
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