- Undeliverable data frame:
- A data frame is undeliverable when some prior frame has not been received
- The task of
the receiver:
- Deliver the data frames from the sender in the order that was transmitted
- The purpose of
receive buffers:
- Receive buffers = used to save undeliverable data frames
Example:
- Sender transmits
5 data frames:
Frame #1 is lost
- The received data frames are
undeliverable:
The undeliverable frames are stored in the receive buffer
- Note:
- The buffered but
undeliverable frames will
become
deliverable if:
- The missing earlier frame is received
Example:
When the receiver receives the retransmitted frame 1, the receiver can deliver all received frames
- The buffered but
undeliverable frames will
become
deliverable if:
- Fact:
- If receive buffer size = k,
then:
- The receiver can buffer at most k − 1 undeliverable data frames
- If receive buffer size = k,
then:
- Example:
- Suppose: receiver has
reserved buffers
for 3 frames:
- The sender transmits
5 frames and
the first frame is
lost:
- The receiver
can only buffer 2 frames:
Because:
- Receiver must
reserve
a buffer
for
the missing frame #1
- Receiver can
buffer the
undeliverable data frames 2, 3
- Receiver must discard the undeliverable data frames 4, 5
(In other words: frames 4 and 5 will be lost !!!)
- Receiver must
reserve
a buffer
for
the missing frame #1
- Suppose: receiver has
reserved buffers
for 3 frames:
- Fact:
- A receiver can
operate correctly with
1 receive buffer !!!
I.e.: minimum # receive buffers = 1 buffer !!!
- A receiver can
operate correctly with
1 receive buffer !!!
- Why the
protocol
works correctly with
only
1 receive buffer:
- Suppose: receiver has
reserved buffers
for 1 frame:
- If
the
first data frame is
received correctly:
the receiver can deliver the data frame:
and re-use the buffer for the next data frame
- If the
first frame is
lost:
- The receiver will
discard all
the undeliverable frames (2,3,4,5):
Because:
- Receiver must reserve a buffer for the missing frame
(The effect is the same as if frames 2, 3, 4 and 5 were lost also)
- The sender can recover the data frame losses with retransmissions !!!
- Suppose: receiver has
reserved buffers
for 1 frame:
- $64,000 question:
- What is the best setting for send buffer and receive buffer ???