Congestion control important notes the retransmission

Congestion Control
Important notes:
– the retransmission of the Lost or delayed Packets only
treats one symptom of the network congestion , however:
– the retransmission does not treat the actual cause of the
network congestion
– Therefore:
» we have to find a mechanism to control the congestion in
the network
» Also, Fairness should be applied to all links in the
network to enhance the congestion control


When a connection slows down, probably due to congestion -
how it will affect other connections?

If n
k
connections share a congested link k with capacity R
k
, then
to ensure fairness each connection should receive r = R
k
/ n
k
of the
total bandwidth –
split the total Bandwidth fairly among all inputs

The Causes & Effects
of Congestion
Three Scenarios
-
Scenario 1: Two equal-rate senders share a single link
-
Scenario 2: Throughput analysis
-
Scenario 3: Multihop paths

B
1
B
2
T
3
B
3
CPU
The Causes and Effects of Congestion
Scenario 1: Two equal-rate senders share a single link

Two sources send data at an average rate of 
in
to two receivers
across a shared link with capacity R
 Data is delivered to the application at the receiver at rate 
out

Packets are queued at the router
 Assume the router has infinite storage capacity - ( Therefore , no packets will be
lost and there is no need for retransmissions)
Router

in

in 
out
R
2
x

in
R
1

The first cost of congestion is:
the Large queuing delays

Causes/costs of congestion: scenario 1
Assumptions :

two senders and
two receivers

one router in the
middle - with an
infinite buffer

output link
capacity: R

no retransmission

maximum per-connection
throughput: R/2
unlimited shared
output link buffers
Host A
original data:

in
Host B
throughput:

out
R/2
R/2

out

in
R/2
delay

in

large delays as arrival rate, 
in
,
approaches capacity

2


By definition, in this portion of the curve , 
out
=  ´
in
= 
in

Retransmission scenarios:
 “ Perfect-Known loss " — Retransmissions occur only when there is an actual loss
 Early — Delayed packets are retransmitted
R /2
R /2

out
Throughput
R /4
 ´
in
Ideal throughput ( 
in
=  ´
in
)
Known loss retransmissions ( 
out
<  ´
in
)
Early retransmissions (  ´
in
= 2 
in
)
(Each segment transmitted twice)
Early retransmissions plus loss
The Causes and Effects of Congestion
Scenario 2: Throughput analysis

Assumptions :

two senders and two receivers

one router in the middle with finite buffer - as a result, some packets
can be lost

sender retransmits the timed-out packet
finite shared output
link buffers
Host A

in
: original data
Host B

out
 '
in
:
original data, plus
retransmitted data
Causes/costs of congestion: scenario 2

idealization : Ideal knowledge

sender sends only when space is
available in the router buffers
finite shared output
link buffers

in
: original data

out
 '
in
:
original data, plus
retransmitted data
copy
free buffer space!