1. Network Simulation NET441
Queuing Disciplines
Network Simulation
NET441

2. Flow control vs Congestion control
Flow control involves preventing senders from overrunning the capacity of the receivers
Congestion control involves preventing too much data from being injected into the network, thereby causing switches or links to become overloaded

3. Congestion Control and Resource Allocation
Resources
Bandwidth of the links
Buffers at the routers and switches
Packets contend at a router for the use of a link, with each contending packet placed in a queue waiting for its turn to be transmitted over the link

4. Congestion Control and Resource Allocation
When too many packets are contending for the same link
The queue overflows
Packets get dropped
Network is congested!
Network should provide a congestion control mechanism to deal with such a situation

5. Congestion Control and Resource Allocation
Congestion control involves both hosts and routers
In network elements
Various queuing disciplines can be used to control the order in which packets get transmitted and which packets get dropped
At the hosts’ end
The congestion control mechanism paces how fast sources are allowed to send packets

6. Network Control Issues
Resources are limited.
Identify the resources:
Buffer space
Bandwidth allocation
One could simply “route around” congested links. Put a large edge weight on a congested link to route around it. That doesn't solve the inherent problem though.

7. Flows
We talk about “flows” in the context of queuing because of the ease in which they can be viewed at different levels:
Process to process
Host to host
Institution to institution
Region to region
In general, a flow refers to a sequence of packets sent between a source/destination pair, following the same route through the network.

8. Queuing Disciplines
Routers must implement some queuing discipline that governs how packets are buffered or prioritized.
One can think of queuing disciplines as rules for the allocating bandwidth or rules for the allocation of buffer space within the router.
The book discusses two common disciplines:
FIFO
Fair Queuing

9. FIFO Queuing
FIFO queuing is called first-come-first- served (FCFS) queuing
First packet that arrives at a router is first packet to be transmitted
Amount of buffer space at each router is finite
Tail drop - If a packet arrives and the queue (buffer space) is full, then the router discards that packet

10. (a) FIFO queuing; (b) tail drop at a FIFO queue.

11. FIFO Queuing – Priority Queuing
A simple variation on basic FIFO queuing is priority queuing
Each packet marked with a priority
The routers then implement multiple FIFO queues, one for each priority class
Router always transmits packets out of the highest-priority queue if that queue is nonempty before moving on to the next priority queue.
Within each priority, packets are still managed in a FIFO manner.

12. Priority Queuing !
The problem with priority queuing, of course, is that the high-priority queue can starve out all the other queues.
That is, as long as there is at least one high- priority packet in the high-priority queue, lower-priority queues do not get served
For this to be viable, there need to be hard limits on how much high-priority traffic is inserted in the queue

13. Fair Queuing
The main problem with FIFO queuing is that it does not discriminate between different traffic sources, or it does not separate packets according to the flow to which they belong.
Fair queuing (FQ) maintains a separate queue for each flow currently being handled by the router.
The router then services these queues in round- robin algorithm

14. Fair Queuing - Round-robin service
Round-robin service of four flows at a router

15. Fair Queuing - Round-robin service
The router then services these queues in a sort of round- robin ,as illustrated in Figure 6.6.
When a flow sends packets too quickly, then its queue fills up.
When a queue reaches a particular length, additional packets belonging to that flow’s queue are discarded.
In this way, a given source cannot arbitrarily increase its share of the network’s capacity at the expense of other flows.
Note that FQ does not involve the router telling the traffic sources anything about the state of the router or in any way limiting how quickly a given source sends packets.
In other words, FQ is still designed to be used in conjunction with an end-to-end congestion-control mechanism. It simply segregates traffic so that ill-behaved traffic sources do not interfere with those that are faithfully implementing the end-to-end algorithm. FQ also enforces fairness among a collection of flows managed by a wellbehaved congestion-controlalgorithm.

16. Fair Queuing
The main complication with Fair Queuing is that the packets being processed at a router are not necessarily the same length.
To truly allocate the bandwidth of the outgoing link in a fair manner, it is necessary to take packet length into consideration.
For example, if a router is managing two flows, one with byte packets and the other with 500-byte packets (perhaps because of fragmentation upstream from this router), then a simple round-robin servicing of packets from each flow’s queue will give the first flow two thirds of the link’s bandwidth and the second flow only one-third of its bandwidth.

17. Queuing Disciplines
What we really want is bit-by-bit round-robin; that is, the router transmits a bit from flow 1, then a bit from flow 2, and so on.
However, it is not feasible to interleave the bits from different packets.
Simulates this behavior instead
Determine when a given packet would finish being transmitted if it were being sent using bit-by-bit round- robin
Use this finishing time to sequence the packets for transmission.

18. Queuing Disciplines Fair Queuing
To understand the algorithm for approximating bit-by- bit round robin, consider the behavior of a single flow
For this flow, let
Pi : denote the length of packet i
Si: time when the router starts to transmit packet i
Fi: time when router finishes transmitting packet i
Fi = Si + Pi

19. Queuing Disciplines Fair Queuing
When do we start transmitting packet i?
Depends on whether packet i arrived before or after the router finishes transmitting packet i-1 for the flow
Let Ai denote the time that packet i arrives at the router
Then Si = max(Fi-1, Ai)
Fi = max(Fi-1, Ai) + Pi

20. Queuing Disciplines Fair Queuing
Now for every flow, we calculate Fi for each packet that arrives using our formula
We then treat all the Fi as timestamps
Next packet to transmit is always the packet that has the lowest timestamp
The packet that should finish transmission before all others

21. Queuing Disciplines Fair Queuing Example of fair queuing in action:
packets with earlier finishing times are sent first;
sending of a packet already in progress is completed

22. Bandwidth Sharing
Because FQ is work-conserving, any bandwidth that is not used by one flow is automatically available to other flows.
Thus we can think of FQ as providing a guaranteed minimum share of bandwidth to each flow
For example:
if we have 4 flows passing through a router, and all of them are sending packets,
then each one will receive 1/4 of the bandwidth.
if one of them is idle long enough that all its packets drain out of the router’s queue
then the available bandwidth will be shared among the remaining 3 flows, which will each now receive 1/3 of the bandwidth

23. Weighted Fair Queuing (WFQ)
allows a weight to be assigned to each flow (queue).
specifies how many bits to send (BW) each time the router services that queue.
Example:
a router has 3 flows (queues), one queue has a weight of 2, the second queue has a weight of 3, and the third queue has a weight of 1. Assuming that each flow always contains a packet waiting to be sent, what is the percentage of BW that is assigned to each flow?
Source: Peterson & Davie, 2007 p 473

24. WFQ, cont.
Solution
The first flow will get 1/3 of the available BW.
The second flow will get ½ of the available BW.
The third flow will get 1/6 of the available BW.
Simple FQ gives each queue a weight of 1, which means that logically only 1 bit is transmitted from each queue each time around. This results in each flow getting 1/nth of the bandwidth when there are n flows.
PQ1 , PQ2 and PQ3 would be allocated as: BW1 = w1/(w1+w2+w3), BW2 = w2/(w1+w2+w3), BW3 = w3/(w1+w2+w3)
Source: Peterson & Davie, 2007 p 473

25. Example
Suppose a router has 3 input flows and one output. It receives the packets listed in Table 1 all at about the same time, in the order listed, during a period in which the output port is busy but all queues are otherwise empty. Give the order in which the packets are transmitted, assuming:
Fair queuing.
Weighted fair queuing with:
flow 1 having a weight of 2, 
flow 2 having twice as much share as flow 1, 4
and flow 3 having 1.5 times as much share as flow 1. 3
Source: Peterson & Davie, 2007 p 529

26. Example, cont. Packet Size Flow 1 200 2 3 160 4 120 5 6 210 7 150 8 90
Table 1
Source: Peterson & Davie, 2007 p 530

27. Solution
(a) Fi is the cumulative per-flow size. Consider Ai = 0 as all packets are received at about the same time so there is no waiting.
Packet
Size
Flow Fi 1
200 2
400 3
160 4
120 5 6
210 7
150 8 90
Source: Peterson & Davie, 2007 p 737

28. Solution, cont. Packet Size Flow Fi 1 200 2 400 3 160 4 120 280 5 440
210 7
150
360 8 90
450

29. Solution, cont.
So, packets are sent in increasing order of Fi: Packet 3, Packet 1, Packet 6, Packet 4, Packet 7, Packet 2, Packet 5, Packet 8.
Packet
Size
Flow Fi 1
200 2
400 3
160 4
120
280 5
440 6
210 7
150
360 8 90
450

30. Solution, cont.
(b) Flow 1 has a weight of 2, so Flow 2 has a weight of 4, so Flow 3 has a weight of 3, so
Packet
Size
Flow
Weighted Fi 1
200
100 2 3
160 40 4
120 5 6
210 7
150 8 90
Source: Peterson & Davie, 2007 p 737

31. Solution, cont. Packet Size Flow Weighted Fi 1 200 100 2 3 160 40 4
120 70 5
110 6
210 7
150 8 90

32. Solution, cont.
So, packets are sent in increasing order of weighted Fi:
Packet 3, Packet 4, Packet 6, Packet 1, Packet 5, Packet 7, Packet 8, Packet 2.
Packet
Size
Flow
Weighted Fi 1
200
100 2 3
160 40 4
120 70 5
110 6
210 7
150 8 90

33. Quality of Service Approaches to QoS Support
fine-grained approaches, which provide QoS to individual applications or flows
coarse-grained approaches, which provide QoS to large classes of data or aggregated traffic
In the first category we find “Integrated Services,” a QoS architecture developed in the IETF and often associated with RSVP (Resource Reservation Protocol).
In the second category lies “Differentiated Services,” which is probably the most widely deployed QoS mechanism.

34. Reference
Computer Networks: A systems approach by Larry Peterson and Bruce Davie, published by Morgan Kaufmann (Fourth edition ISBN: ).