- At some later time:
- The other TCP module will close its TCP connection
- Caviat:
- There is a subtle problem that make the close operation a little bit of a headache
- The error-free
protocol for
closing an
half-open
TCP connection is:
- (Recall we
have assumed that
the client closes
its TCP connection
first)
The server sends a FIN (disconnect) request to the client:
- When the client
receives the FIN message,
it deletes (invalidates) the
ACK no for the
TCP connection:
The client must acknowledges the FINISH message:
- When the server receives the
ACK message, it
deletes (invalidates) the
send seq. no:
- (Recall we
have assumed that
the client closes
its TCP connection
first)
- Consider the following
error scenario:
- (Recall we
have assumed that
the client closes
its TCP connection
first)
The server sends a FIN (disconnect) request to the client:
- When the client
receives the FIN message,
it deletes (invalidates) the
ACK no for the
TCP connection:
The client must acknowledges the FINISH message:
- The ACK message is
lost:
- The server will
time out and
retransmits the
FIN request:
Problem:
- The client has an
invalid ACK no
- The client is unable to acknowledge the re-transmitted FIN request !!!!
- The client has an
invalid ACK no
- (Recall we
have assumed that
the client closes
its TCP connection
first)
- Note: in the
above scenario
- The client will
send a
RESET back to the
server
Graphically:
- The client will
send a
RESET back to the
server
- Solution:
- When a TCP module receives
a FIN request:
- The TCP module does
not
delete/invalidate the
ACK no
immediately
- The TCP module set an
alarm for:
2 × MSL (= Maximum Segment Livetime)
- The TCP module will delete/invalidate the ACK no after 2 × MSL
- The TCP module does
not
delete/invalidate the
ACK no
immediately
- When a TCP module receives
a FIN request:
- Example:
- The ACK message is
lost:
Now:
- The client will retain the ACK no !!!
- The server
wil time out and
retransmit
the FIN request:
The client can now verify and retransmits the ACK
Note:
- The retransmitted FIN message
will reset the
timer to
2 × MSL !!!
(In case the retransmitted ACK is lost !!!)
- The retransmitted FIN message
will reset the
timer to
2 × MSL !!!
- The ACK message is
lost:
- MSL:
- Maximum segment lifetime =
the time a
TCP segment can
exist in the
Internet
It is arbitrarily defined to be 2 minutes long
Wikipedia: click here
- Maximum segment lifetime =
the time a
TCP segment can
exist in the
Internet