non-piggy-backed ACK with total IP length = 52

[Andrej van der Zee <andrejvanderzee () gmail com>]

Hi,

I am looking at a ACK-packet with total IP length of 52. Normally I
find non-piggy-backed ACKs only 40 bytes. In this case the ACK comes
after a retransmission. The extra 12 bytes are contained in the TCP
options. In Wireshark, it sais:

Options (12 bytes)
   NOP
   NOP
   SACK: 553-1130
      left edge: 533 (relative)
      right edge: 1130 (relative)

It seems like it tries to indicate which of the duplicate packet it
actually acknowledges by explicitly stating the byte range, in case
the retransmitted packet is longer than the original packet because
more send-data became available before the retransmission. Is my
understand correct?

What is actually contained in these extra 12 bytes?

What does NOP mean?

Thank you,
Andrej
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