Quality of Service in IP Networks

Queue Management
5
Principles for QOS Guarantees (more)
o Applications misbehave (audio sends packets at a rate higher than 1Mbps
assumed above);
o PRINCIPLE 2: provide protection (isolation) for one class from other
classes
o Require Policing Mechanisms to ensure sources adhere to bandwidth
requirements; Marking and Policing need to be done at the edges:
Queue Management
6
Principles for QOS Guarantees (more)
o Alternative to Marking and Policing: allocate a set portion of bandwidth
to each application flow; can lead to inefficient use of bandwidth if one
of the flows does not use its allocation
o PRINCIPLE 3: While providing isolation, it is desirable to use
resources as efficiently as possible
Queue Management
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Principles for QOS Guarantees (more)
o Cannot support traffic beyond link capacity
o PRINCIPLE 4: Need a Call Admission Process; application flow
declares its needs, network may block call if it cannot satisfy the
needs
Queue Management
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Policing Mechanisms
o Three criteria:
l (Long term) Average Rate (100 packets per sec or 6000 packets per min??), crucial
aspect is the interval length
l Peak Rate: e.g., 6000 p p minute Avg and 1500 p p sec Peak
l (Max.) Burst Size: Max. number of packets sent consecutively, ie over a short
period of time
Queue Management
9
Traffic shaping
o How to prevent congestion?
l it may result from burstiness
l arrivals more deterministic, better performance
– example : nbr of customers in D/D/1 vs. G/D/1
l control the rate and burst size
– traffic description
o Service contract
l if the network knows the type of the traffic, it can reserve resources to support
the traffic
l contract between the source and the network
– source: traffic description
– network: QoS guarantee if the traffic conformsto the description
– if the traffic is not conformant, penalty: reject a packet, no guarantees of the QoS
(traffic policing)
o More details in Network Calculus course
Queue Management
10
Leaky bucket
o Limited size buffer with constant departurerate
l r if buffer not empty
l 0 if buffer empty
o Equivalent to the queue G/D/1/N
o Fixed size packets
l one packet per clock tick
o Variable size packets
l number of bytes per clock tick
o Packet loss if buffer filled
r
B
Queue Management
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Token bucket
arrival of tokens :
rate r
B
test
packet buffer
Queue Management
12
Token bucket
o Tokens generated with rate r
l 1 token : 1 packet or k bytes
o Packetmust wait for a token before transmission
l no losses
l allows limited bursts (a little bit more than B)
o When packets are not generated, tokens accumulate
l n tokens - burst of n packets
l if bucket filled, tokens are lost
o Mean departure rate : r
Queue Management
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Burst duration
o Burst duration - S sec
o Size of the bucket - B bytes
o Maximal departure rate - p bytes/s
o Token arrival rate - r bytes /s
l burst of B + r S bytes
l burst of pS
l B + rS = pS → S = B/(r - p)
o Example
l B = 250 Kb, p = 25 Mb/s, r = 2 Mb/s
l S = 11 ms
Queue Management
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Traffic description
o Flow A : r = 1 MB/s, B = 1 byte
o Flow B : r = 1 MB/s, B = 1 Mbyte
l during2 s, the flow saves 2 s × 0.5 MB/s = 1 MB
rate
1 MB/s
2 MB/s
3 MB/s
time
1 s 2 s 3 s 4 s
Flow A
Flow B

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