TCP Sack processing

>>SACKs are advisory, not guarantees
Could you be a little more elaborate on that. or point me to some rfc
or other doc ?

From RFC 2018:

   The SACK option is advisory, in that, while it notifies the data
   sender that the data receiver has received the indicated segments,
   the data receiver is permitted to later discard data which have been
   reported in a SACK option.

- Vern