6'033 Lecture 13 Congestion Control

1
6.033 Lecture 13Congestion Control

• Sam Madden
• March 18, 2009

• Key Ideas
• In-order delivery
• Congestion collapse
• Congestion control via exponential backoff

2
Reordering
head sno of last delivered packet size length
of reordering queue slot position to insert new
packet

• recv(p)
• slot p.sno head
• if (slot gt head size)
• drop
• else
• new isempty(slot)
• if (new) add p to slot
• ack p
• if (slot head and new)
• deliver prefix to app
• head head len(prefix)

head 1
head size
P2
P4
P5
size 5
Reordering Buffer
3
Reordering
head sno of last delivered packet size length
of reordering queue slot position to insert new
packet

• recv(p)
• slot p.sno head
• if (slot gt head size)
• drop
• else
• new isempty(slot)
• if (new) add p to slot
• ack p
• if (slot head and new)
• deliver prefix to app
• head head len(prefix)

head 3
head size
P4
P5
size 5
Reordering Buffer
4
T0-1
Router Queue
Sender
Head
5
T1-2
Router Queue
Sender
6
T1-2
Router Queue
Sender
7
T1-2
Router Queue
Sender
8
T1-2
Router Queue
Sender
9
T2-3
Router Queue
Sender
10
T2-3
Router Queue
Sender
11
T3-4
Router Queue
Sender
12
T3-4
Router Queue
Sender
13
T3-4
Sender is spending all of its time
retransmitting Queue is filling with duplicate
packets No new transmissions are getting
through! Congestion collapse
Router Queue
Sender
14
T4-5
Router Queue
Sender
15
T5-6
Router Queue
Sender
16
T6-7
Router Queue
Sender