Download Congestion Control

Congestion Control
• Reasons:
- too many packets in the network and not
enough buffer space
S = rate at which packets are generated
R = rate at which receivers take packets
S – R == access packet rate
• Some nodes are slow
- links used by too many sources
- example: two high speed links feeding
into a low speed link
Flow control vs congestion control
• Flow control may be required even if
congestion is not present and vice versa
Classification
• Router-centric and host-centric
• Router-centric
- controlled by the routers
(inside the network)
- each router decides which packets to
forward and which to drop to ensure the
service model
• Host centric:
- control is done at the end points
- source adjusts the rate and the receiver
provides feedback
• Reservation-based vs feedback based
- reservation based:
* each source requests resources
(bandwidth, buffers) when flow is
established.
* flow is rejected if not enough resources
are available
• Feedback based:
- no resources are reserved
- discard packets if buffers are full
-explicit or implicit feedback to adjust the
sending rate
• Window-based vs Rate based
• Both mechanism control the speed of the
sender;
• Although any combination of the above
strategies are possible, only two are popular
• best effort model: feedback, host-centric
and window based
• QoS model: reservation based, router
centric and rate based
Evaluation of congestion control
• Throughput
• Delay must also be taken into account:
- all links are totally packed;
high-throughput but long delays
Throughput/delay
* fairness
Queuing Disciplines
• Each router must maintain queues for
packets to be transmitted
• Packet forwarding rule
• Packet dropping rule
FIFO disciplines
• One queue at each router
• The first packet to arrive is the first to be
transmitted
• If the queue is full, discard the packet
• Most IP routers are FIFO
Priority queues
• One queue for each priority level
• Transmit from highest priority queue first
• Packet dropping policy:
drop low priority packets
Fair queuing
• FIFO does not distinguish packets of
different flows
• Problem: all responsibility is pushed to
sources (no network control)
• Some sources could flood the network
Fair queuing
• Separate queue for each flow
• Router serves the queues in round-robin
fashion
• When a queue is full, its packets are
discarded according to some discipline
• In this scheme, a source cannot hog the
network
• Enforces fairness but no priority
• Simple fair queuing is not entirely fair as
packets may be of different lengths
• Solution: bit-by-bit round robin
Fair queuing
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Use clocks for each flow
Assume clock ticks for each bit transmitted
Pi = length of packet numbered i
Si = time at which packet i transmission is started
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Fi = finish time for packet i
Ai = arrival time for packet i
Fi = Si + Pi
Si = max(Fi-1, Ai), Fi = max(Fi-1, Ai) + Pi
• All flows: treat Fi as timestamps
• Transmit the packet with the lowest
timestamp
• Variation: weighted fair queuing
• Weight is assigned to each flow
• Weight specifies how many bits to transmit
each time the router services that queue
• Source must communicate the weight
information: QoS
Feedback based control
• If congestion is detected then send a control
packet to source
• Problem: too many control packets
• Congestion bit in message:
- this bit is sent to 1 if congestion is
detected and a choke packet is sent
- subsequent routers do not send choke
packet if congestion bit is already set.
Implicit feedback
• Randomly drop packets
• Increased timeout
TCP congestion control
• Each source determines how much capacity
is available for a given flow in the
networks.
• Acks are used to reach an operating point:
self-clocking
Additive increase/multiplicative
decrease
• Window = min(Congestion Window,
advertised window)
• Linear increase
• Multiplicative decrease
Slow start
• Linear additive increase takes too long
Fast retransmit
• Timeouts may not be accurate; results in
slow response
• Duplicate acks
• Fast recovery: reduce congestion window