- Buffer:
- Buffer =
computer memory used to
store data (until
the data is
no longer needed)
- Computers have a finite amount of buffers (memory)
- Buffer =
computer memory used to
store data (until
the data is
no longer needed)
- Sender/Receiver buffer:
- Send buffer =
buffer used by the sender to
store
data frames
- Receive buffer = buffer used by the receiver to store data frames
- Send buffer =
buffer used by the sender to
store
data frames
- Note:
- There are
no buffers
used for
ACK frames !!!
- ACK frames are
processed
immediately when
they are
received
- An ACK frame will update some status information (= variables) at the sender
- ACK frames are
processed
immediately when
they are
received
- There are
no buffers
used for
ACK frames !!!
- Outstanding frame:
- Outstanding frame = a data frame transmitted by the sender that has not been acknowledged
Consequently:
- The sender does not know
whether an
outstanding frame:
- will be (correctly) received or
- will be lost (corrupted)
- The
buffering rule
that a
sender must
obey:
- The sender must keep all outstanding frames in its send buffer
Reason:
- An outstanding frame
can
be lost
Therefore:
- The sender must keep all outstanding frames so it will be able to retransmit it when the frame is lost !!!
Graphically illustrated:
- Constraint
on sender to
ensure
reliable communication:
# outstanding frames ≤ send buffer size (# frames that send buffer can hold)Reason to impose this constraint:
- Suppose
the sender has
k buffers
and has
k+1
outstanding data frames:
Then: one of the outstanding data frames has not been buffered !!!
- In the example: frame #5 has not been buffered
- If
that unbuffered
outstanding frame is
lost:
- The sender cannot retransmit the lost data frame !!!
- Suppose
the sender has
k buffers
and has
k+1
outstanding data frames:
- To achieve the
best performance, use:
# outstanding data frames = send buffer size