1. Providing QoS in IP Networks
Future: next generation Internet with QoS guarantees
Integrated Services: firm guarantees
Differentiated Services: differential guarantees
simple model for sharing and congestion studies:

2. Principles for QoS Guarantees
Example: 1Mbps IP phone, FTP share 1.5 Mbps link.
bursts of FTP can congest router, cause audio packets to be excessively delayed or lost
want to give priority to audio over FTP
Principle 1
packet marking/classification needed for router to distinguish among packets belonging to different classes of traffic; new router policy needed to treat packets accordingly

3. Principles for QoS Guarantees
what if applications misbehave (e.g. audio sends higher than declared rate)?
policing: force the flow to adhere to certain criteria
Token bucket
Packet classification/marking and policing done at network edge (in the host or at an edge router)
Principle 2
provide protection (isolation) for one class from others

4. Principles for QoS Guarantees
Another way to provide flow isolation: allocating fixed (non-sharable) bandwidth to each flowinefficient use of bandwidth if flow doesn’t use its allocation
Principle 3
While providing isolation among flows, it is desirable
to use resources as efficiently as possible

5. Principles for QoS Guarantees
Basic fact of life: can not support traffic demands beyond link capacity
Principle 4
Call Admission: flow declares its QoS requirement,
network either accepts the flow or blocks the flow

6. Scheduling Mechanisms
scheduling: choose next packet to send on link
FIFO (first in first out) scheduling: send in order of arrival to queue
Drawbacks of FIFO scheduling
No special treatment is given to packets from flows that are of higher priority or are more delay sensitive
A greedy TCP connection can crowd out other well-behaved connections
Buffer full: drop or preempt.

7. Scheduling Mechanisms
Priority scheduling:
Multiple priority classes, each has its own queue
A packet’s priority class may depend on an explicit marking or other header info, e.g. source/dest IP address, source/dest port number, protocol ID.
Transmit a packet from the highest priority class that has a nonempty queue

8. Scheduling Mechanisms
Round Robin scheduling:
one queue for each flow
cyclically scan the queues, serving one packet from each queue (if available)
No advantage in being greedy
Work-conserving queuing discipline: never allow the link to remain idle whenever there are packets queued for transmission
Drawback: flow with shorter average packet size is penalized

9. Scheduling Mechanisms
Byte-by-byte round robin:
One queue for each flow
Scan the queues repeatedly, byte-for-byte, until find the tick on which each packet will be finished
Packets sorted in order of finishing time and sent in that order
Drawback: all flows are given the same bandwidth

10. Scheduling Mechanisms
Weighted Fair Queuing (WFQ): approximate fluid fair queuing (FFQ)
FFQ: allows different flows to have different service shares.
A separate FIFO queue for each flow sharing the link.
When there are N nonempty queues, the server serves the N packets at the head of the queues simultaneously
At any time t, the service rate for a nonempty queue i is
where wi is the weight associated with queue i, B(t) is the set of nonempty queues, and C is the link speed.

11. WFQ FFQ is impractical because
Only one queue can receive service at a time
An entire packet must be served before another packet can be served
WFQ: When the server is ready to transmit the next packet at time t, it picks the first packet that would complete service in the corresponding FFQ system if no additional packets were to arrive after time t

12. Policing Mechanisms
Goal: regulate the rate at which a flow is allowed to inject packets into the network
Three policing criteria:
(Long term) Average Rate: how many packets can be sent per time interval
crucial question: what is the interval length?
Peak Rate: max. number of packets that can be sent over a short period of time.
Burst Size: max. number of packets that can be sent consecutively (with no intervening idle)

13. Policing Mechanisms
Token Bucket: limit input to specified Burst Size and Average Rate.
bucket can hold b tokens
tokens generated at rate r token/sec
Token added to bucket if bucket not full, ignored otherwise
A packet must remove a token from the token bucket before it is transmitted into the network

14. Policing Mechanisms For a token-bucket-policed flow:
The max. burst size is b packets
Over interval of length t: max. number of packets admitted is (r t + b)  r limits the long term average rate
Byte-count token bucket:
Each token represents the right to send k bytes
A packet can be transmitted if enough tokens are available to cover its length in bytes
If a flow’s guaranteed rate is greater than or equal to the flow’s average rate, then
Token bucket + WFQ = guaranteed upper bound on end-to-end delay and delay jitter, i.e., QoS guarantee!