- There are 2 kinds of
acknowledgement schemes
used in reliability protocols:
- Selective acknowledgement:
- ACK n
(n =
receive sequence number)
acknowledges:
- the (correct) reception of the data frame with send sequence number = n
- ACK n
(n =
receive sequence number)
acknowledges:
- Cumulative acknowledgement:
- ACK n
acknowledges:
- the (correct) reception of
all data frames
with
send sequence
number
< n
I.e.: the (correct) reception of all data frames with send sequence number ≤ (n−1)
- the (correct) reception of
all data frames
with
send sequence
number
< n
Example:
Note:
- ACK 3 means:
- The first of the new frames for the receiver has sequence number #3
- ACK n
acknowledges:
- Selective acknowledgement:
- Fact:
-
ACK n
will
also
acknowledge
- All data frames that have been acked by ACK (n−1)
-
ACK (n−1)
will
also
acknowledge
- All data frames that have been acked by ACK (n−2)
- And so on....
In other words:
- Tranmitting
ACK n
will
implicitly:
- re-transmitting all the ACK frames ACK (n−1), ACK (n−2), ...
-
ACK n
will
also
acknowledge
- Strength of
the cumulative ACK scheme:
- The ACK frames in the cumulative ACK scheme are very well protected
In other words:
- If the ACK n frame was
lost, then:
- the lost ACK n frame can be recovered by the ACK (n+1) frame !!!
Example: if ACK 7 is lost, then data frame 6 is also acknowledged if ACK 8 is received
- Weakness of the
cumulative ACK scheme:
- The receiver
can
not acknowledge:
- correctly data frames that are received out-of-order
Example:
- The sender transmits
frames 1,2,3
Frame 1 was lost
Frame 2, 3 were received correctly
Then:
- The receiver
can not
send ACK 3 !!!
(because ACK 3 will also acknowledge the lost frame 1 !!!)
- The receiver
can not
send ACK 3 !!!
- The receiver
can
not acknowledge:
- Consequence:
- The sender may
time out on
the correctly received
frames (2 and 3)
- This can lead to inefficiency because sender will retransmit some data frames unneccassarily
- The sender may
time out on
the correctly received
frames (2 and 3)
- Strength:
- Extremely efficient:
- The sender will only retransmit the unacknowledged (corrupted) frames
Weakness:
- The ACK frames are
not protected
from loss
- An ACK can be
lost (= corrupted)
- Lost ACK frames will cause (unnecessary) retransmission and reduce efficiency)
- An ACK can be
lost (= corrupted)
- Extremely efficient:
- As of 2014:
- The cumulative acknowledgement
method
is preferred for
long distance communication
because:
- The error rate is
relatively high
So you want to protect the acknowledgement messages !!!
- The error rate is
relatively high
Fact:
- The Transport Control Protocol (TCP) of the Internet Protocol (IP) uses cumulative acknowledgement
- The cumulative acknowledgement
method
is preferred for
long distance communication
because:
- Note:
- With the advent of
high speed and
low error rate
fiber optic cables,
the
selective acknowledgement
method
is being
considered
to replace the
cumulative ack scheme.
(But it's very difficult to switch protocol on a global scale...
I don't expect the Internet to use selective ACK before I'm dead...)
- With the advent of
high speed and
low error rate
fiber optic cables,
the
selective acknowledgement
method
is being
considered
to replace the
cumulative ack scheme.