Transcript Lecture 15

Lecture 15
Internet resource allocation and QoS
Resource Reservation Protocol
Integrated Services
Differentiated Services
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QoS in a datagram network?
Packet Classification.
Buffer acceptance algorithms.
Explicit Congestion Notification.
Flow measurements
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A QoS Capable Router
Shaper
Dispatcher
and Buffer
Acceptance
Classifier
Input
flows
Output
link
Policer
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Packet classification
Identify the flow the packet belongs to.
The edge routers may be able to do that.
MPLS – multi protocol label switch. Add an
extra header in front of the IP header. Now
a router decides the output link based upon
the input link and the MPLS header.
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Buffer acceptance algorithms
Tail Drop.
RED – Random Early Detection
RIO – Random Early Detection with In and
Out packet dropping strategies.
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maxThr_out
high load
out
in
minThr_out
high load
medium load
medium load
maxThr_in
low load
low load
minThr_in
sampleQueueLength
(a)
dropProb
1.0
maxDropProb
minThr_out
out
in
avgQue
maxThr_out
minThr_in
sampleQueueLength
maxThr_in
(b)
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Explicit Congestion Notification (ECN)
Routers could prevent congestion by
informing the source of the packets when
they become lightly congested, but before
they start dropping packets.
This strategy is called source quench.
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Source quench
A router sets a congestion notification flag in
the IP header to inform the destination that
signs of congestion are visible.
The destination informs the source by setting
a flag in the TCP header of segments
carrying acknowledgments.
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Problems with ECN
(1) TCP must be modified to support the new
flag.
(2) Routers must be modified to distinguish
between ECN-capable flows and those who
do not support ECN.
(3) IP must be modified to support the
congestion notification flag.
(4) TCP should allow the sender to confirm the
congestion notification to the receiver,
because acknowledgments could be lost.
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Flow measurements
How to choose the measurement interval to
accommodate bursty traffic?
Token bucket
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The token bucket filter
Characterized by : (1) A token rate R, and (2) The
depth of the bucket, B
Basic idea the sender is allocated tokens at a
given rate and can accumulate tokens in the
bucket until the bucket is filled. To send a byte the
sender must have a token. The maximum burst
can be of size B because at most B token can be
accumulated.
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Example
Flow A: generates data at a constant rate of 1 Mbps.
Its filter will support a rate of 1 Mbps and a bucket
depth of 1 byte,
Flow B: alternates between 0.5 and 2.0 Mbps. Its filter
will support a rate of 1 Mbps and a bucket depth of 1
Mbps
Note: a single flow can be described by many token
buckets.
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Example
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Token bucket
L = packet length
C = # of tokens in the bucket
--------------------------------------------------if ( L <= C ) {
accept the packet;
C = C - L;
}
else
drop the packet;
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A shaping buffer delays packets that do not
confirm to the traffic shape
if ( L <= C ) {
accept the packet;
C = C - L;}
else { /* the packet arrived early, delay it */
while ( C < L ) {
wait; }
transmit the packet;
C = C - L;}
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Packet Scheduling
PS and GPS – Processor Sharing &
Generalized Processor Sharing
Round Robin, Weighted Round Robin
Priority Scheduling
Weighted Fair Queuing – practical version of
GPS. Transmits packets in the order of their
finishing time.
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Weighted queuing


q4

q3


q2
q1
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RSVP- Resource Reservation Protocol
Used to establish a path for a flow and
reserve resources along the path.
Requirements:
Accommodate faults – soft state.
 Support unicast as well as multicast.

PATH messages  issued by sender
includes TSpec
RESV messages  issued by the receiver
includes RSpec
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RSVP
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RSVP message
0
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8
version
hlen
packet length (bytes)
ToS
fragment identifier
TTL -time to live
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protocol=46
flags
13-bit fragment offset
header checksum
IP
header
32-bit source IP address
32-bit destination IP address
version
=1
flags
Sent_TTL
type: PATH
RESV
checksum
reserved
length of RSVP message
RSVP message
It consists of one or more RSVP objects. Each object
has a class number and type, length, and a value.
RSVP
header
RSVP
message
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RSVP multicast
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Integrated Services
Support fine-grain QoS for individual flows.
Mechanisms:
Specification of flow requirements - Flowspecs
 Admission decisions
 Resource reservation and policing
 Policy enforcement

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Flowspecs
TSpec – specify the traffic characteristics
Rspec – describe services required from
network.
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Admission decisions
Two classes:
Guaranteed Services – based upon token
buckets
 Controlled Load – approximates a best effort
model in a lightly loaded network.

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Integrated Service Router
RSVP
Messages
Admission Control
Routing
RSVP
Routing
Messages
Resource Reservation
Shaper
Dispatcher
and Buffer
Acceptance
Classifier
IP
packets
Policer
IP
packets
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Differentiated Services
Two classes of traffic
Regular
 Premium

Edge routers mark the packets.
Premium packets enjoy
EF – Expedited Forwarding
 AF – Assured Forwarding

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