- Consider the
following scenario:
- A client and a
server has
already established
a TCP connection :
- Then
a connection request
with the
same connection identifiers
(srcIP, srcPort#, destIP, destPort#)
arrives:
Furthermore:
- The SYN request contains a different sequence number
- A client and a
server has
already established
a TCP connection :
- $64,000 question:
- Is this connection request
always
a bogus request ?
(If so, we can safely ignore it....)
Answer:
- No !!!!...
The connection request can be a legitimate request !!!
I will show this fact with the following 2 sceanrios...
- Is this connection request
always
a bogus request ?
- Possibility 1:
the connection request
was a
legitimate request
- The client and
server had
a TCP connection:
- But the
client has
crashed:
The TCP connection state values are lossed !!!
- The client
has recovered and
sends
a new TCP connection
(using the same
TCP connection identifiers)
- The client and
server had
a TCP connection:
- Possibility 2:
the connection request
was a
bogus request
- The client picks the
random number 7625 and
sends
a connection request
to the server
However:
- The SYN message is delayed (due to congestion)
- The client then
times out
and retransmits
the
TCP connection request to the
server:
Furthermore:
- The retransmitted request was received first
- The client and
server
completes
the TCP connection protocol:
(In the mean time, the delayed connection request has not yet arrived)
- The client
then sends 30 bytes of data
(which will move the window):
- The server
sends the
ACK for the data:
- When the client
receives + processes the
ACK message,
the receiver will
update its
send sequence number :
We are now in the same state as givenm in the staring scenario:
- At this moment, the
delayed connection request
is received:
Clearly, in this scenario, the connection request is bogus (invalid)
- The client picks the
random number 7625 and
sends
a connection request
to the server
- Fact 1:
- The bogus connection request
is
indistinguishable
from a
legitimate connection request
In fact:
- If the client has crashed, the connection request would have been a legitimate connection request (and must be processed) !!!
- The bogus connection request
is
indistinguishable
from a
legitimate connection request
- Fact 2:
- The server does
not have
sufficient information that:
- Allow the server to decide whether the connection request is bogus or legitimate.
Caveat:
- The server
currently has
a TCP connection that
may still be
valid !!!
- The server
cannot
discard the
existing
TCP connection !!!
(Because the TCP connection may be alive (valid) !!!
- The server
cannot
discard the
existing
TCP connection !!!
- The server does
not have
sufficient information that:
- Fact 3:
- The
client
does have the
necessay
information
that:
- Allow the client to decide whether the connection request is bogus or legitimate.
- For a
legitimate connection request:
- The client's
TCP state is:
- The client's
TCP state is:
- For a
bogus connection request:
- The client has a
different
TCP state value
:
- The client has a
different
TCP state value
:
Conclussion from fact 3:
- The client is able to process a bogus connection request differently from a legitimate connection request !!!!!!
- The
client
does have the
necessay
information
that: