Transcript PowerPoint

Quality of Service Support
in Packet Networks
Tiziana Ferrari
[email protected]
Italian National Institute for Nuclear Physics
INFN - CNAF
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Overview
• Problem statement
• technical solutions:
– ATM
– RSVP and RSVP to ATM SVC mapping
– differentiated services (diffserv)
• Diffeserv in detail
• Diffserv: a case study
• Diffserv test activities (TF-TANT)
• comments
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Quality of Service
• Heterogeneous networks
– capacity
– transmission technology
– bottlenecks
• congestion and lack of transmission guarantees
• heterogeneous application requirements
–
–
–
–
–
interactive: telnet, remote X sessions, web browsing
non-interactive and packet loss tolerant: ftp, mailing
delay sensitive: real time applications (e.g. remote control)
delay variation sensitive and packet loss: voice over IP, videoconferencing
...
Quality of Service
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Quality of Service: basic concepts
• Service: a pre-defined type of packet treatment during transmission
across the network
– qualitative
– quantitative
> delay
> instantaneous delay variation
> packet loss probability
> throughput
> MTU (Maximum Transfer Unit)
> priority (e.g. for congestion treatment)
• Class: set of packets to which a given service applies.
Classification is based on traffic filters. A filter defines a set of packet
matching rules. Matching is based on the content of packet fields.
E.g. filter := (pack(src) = SRC) && (pack(dest) = DEST) &&
(pack(pro) = TCP)
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Quality of Service: benefits
• Benefits:
– network. congestion management, congestion independent QoS
parameters, traffic engineering
> differentiated allocation of expensive network resources e.g.
over intercontinental connections
> multiple services
– application:
> within a single application: differentiated treatment of streams
according to their requirements and priority
> multiple applications: applications hierarchy according to
priorities
• today: single service, best-effort
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Solutions
• Cell based networks:
– ATM (permanent and/or switched connections)
• IP based networks:
– RSVP (resource ReSerVation Protocol) and integrated
services
• heterogeneous networks:
– RSVP to ATM SVC mapping (IP and ATM)
– differentiated services
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ATM (Asynchronous Transfer Mode)
• Cell based
• pre-defined classes of services:
– Constant Bit Rate (peak cell rate - PCR-)
– Variable Bit Rate (sustainable cell rate, PCR, maximum burst size MBS-)
> real time
> non real time
– Available Bit Rate (minimum cell rate -MCR-, PCR, rate
increase/decrease factors)
– Unspecified bit rate (PCR)
• today: deployed as backbone technology (GARR, European national
research networks, TEN-155, ESnet), not common as LAN technology.
Only permanent connections are commonly deployed to support traffic
engineering.
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ATM (cont)
•
•
•
•
•
Permanent and/or dynamic connections (PVC, SVC)
1-to-1 or 1-to-many
end-to-end signalling protocol for ATM connection set-up
traffic engineering
Disadvantages:
– not widely deployed as LAN technology (no end-to-end
connectivity)
– no native ATM applications
– IP over ATM: overhead
– few applications supporting traffic profile definition
– addressing scheme not compatible with IP
– signalling only in few backbones -> lack of interoperability
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RSVP
• RSVP: signalling protocol for IP based applications
–
–
–
–
–
–
–
traffic profile definition - source reservation profile specification - destination 3 classes of service: best-effort, controlled load, guaranteed
reservation: (soft state)
QoS support in heterogeneous network
multicast is supported
QoS support to the application
• Advantage: IP compatible
• Disadvantages:
– requires RSVP support on each router on the path from tx to rx
– lack of scalability
– admission control and policy management
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RSVP to ATM SVC mapping
•
•
•
•
ATM signalling in the core
RSVP signalling at the edge (access networks)
reservation parameter mapping at the boundary
Advantages:
– deploys ATM features in the backbones
– QoS support in heterogeneous environments
– QoS to the application
– interim solution -> intserv - diffserv (scalability)
• Disadvantages:
– application: RSVP capable, traffic profile specification
– RSVP support at both edges
– connection set-up latency: not convenient for delay sensitive
applications transmitting small chunks of data
– still relays on ATM signalling in the core
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RSVP to ATM mapping: features
• Translation of intserv classes of services and parameters
into ATM classes of services and parameters
• deployment of best-effort connections (UBR in the ATM
core) for initial transmission of RSVP messages (PATH,
RESV)
• combination of RSVP and ATM admission control
• ATM: tx initiated signalling vs RSVP: rx initiated
reservation request
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Scenarios
• Mapping in the end-system
LIS 2
LIS 1
ATM
ATM
RSVP -> ATM
LIS 3
ATM
NRN - TEN-155 - NRN
• mapping in the router
RSVP
RSVP
1. PATH
ATM
4. SVC
3. RSVP -> ATM
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2. RESV
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Differentiated Services:
Architecture
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Differentiated services
cont.
• Why diffserv?
– new technology
– independent of layer 2 technologies
– interoperability between independent national research networks
(different requirements, infrastructures, policies and management)
– traffic aggregation
– scalability: no reservation state maintained in the routers
– no signalling
– QoS for networks not ATM based
•
•
•
•
RFC 2474: Definition of the Differentiated Services Field (DS Field) in the
IPv4 and IPv6 headers
RFC 2475: An Architecture for Differentiated Services
RFC 2598: An Expedited Forwarding PHB
RFC 2597: Assured Forwarding PHB Group
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Diffserv architecture: building blocks
• Label: DS field (1 byte), DS Code Point (6 bits)
0
6 7
DSCP
CU
• packet classification
• packet scheduling
• traffic conditioning:
– metering
– marking
– policing
– shaping
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DS building blocks: logical view
meter
classifier
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marker
Shaper/
policer
Quality of Service Support in Packet Networks
scheduler
16
Diffserv: traffic aggregation and (re)marking
aggregation
and re-marking
aggregation
marking
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aggregation
re-marking
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Terminology
• Per Hop Behaviour (PHB): the externally observable forwarding
behaviour applied to a DS-compliant node to packets with same label
(DS codepoint)
• PHB Group: set of PHBs which can only meaningfully specified and
implemented simultaneously (e.g. with common constraints on queue
servicing and queue management). E.g. 4 PHB each associated to a
different drop priority. A single PHB is a special case of PHB Group.
• Service: quantitative or statistical definition of significant
characteristics of packet transmission in one direction across the
network in terms of throughput, delay, jitter, loss, priority in access to
network resources. Services are implemented through PHBs. The
service describes the overall treatment end-to-end.
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Terminology (cont)
• DS codepoint: specific value of the DSCP field
• DS behaviour aggregate: packets with same code point
• DS domain: contiguous set of nodes with same service provisioning
policies and same code point numbering scheme
• DS region: set of contiguous DS domains
• DS ingress/egress node: DS node handling packets entering/leaving
the DS domain it belongs to
• classifier: entity selecting packets according to the content of packet
headers according to a defined rule
• BA classifier: a classifier which only takes into account the DS field
content
Interior/Ingress/Egress Node
DS Region
DS Domain
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Terminology (cont)
• Conditioning: metering, marking
• Policing: packet discard according to the state of a corresponding
meter enforcing a traffic profile
• Metering: the process of measuring the temporal properties of a traffic
stream selected by a classifier
• Marking: the process of setting the DS codepoint in a packet based on
defined rules
• Service Level Agreement: traffic contract between a customer and
service provider specifying the forwarding service the customer’s
traffic should receive
• Service Provisioning Policy: specification of
– microflow mapping into a DS Behaviour Aggregate
– conditioning configuration
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Diffserv architecture: network model for TEN-155
DS domain
NRN
DS domain
MPLS
Non DS capable
domain
DS domain
NRN
Marking
policing
scheduling
TEN-155
marking
shaping
DS domain
DS domain
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Diffserv architecture: PHBs
• Standardised PHBs:
– Expedited Forwarding (low delay, low delay variation,
guaranteed bandwidth)
– Assured Forwarding (Behaviour Aggregate, 4 classes,
3 drop priorities per class)
• Experimental PHBs
• PHB class selectors
3
0
7
precedence
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PHB: Expedited Forwarding
• For the implementation of services requiring a reservation profile like:
low loss, low latency, low jitter, assured bandwidth
• loss, latency and jitter  queue management (small queues)
in order to prevent a queue from building up, in the EF queue of
each transit node, the aggregate maximum arrival rate < departure rate
• EF implementation based on
– scheduling (for traffic isolation and support of bandwidth
guarantees)
– policing
– shaping
• EF traffic can preempt other classes, for this reason the maximum EF
rate has to be limited through policing
• EF codepoint: 101|110
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PHB: Assured Forwarding
• 4 independent AF forwarding classes
• 3 drop priorities in each AF class
• given two packets in node with drop precedence p and q
respectively, with p < q, pack(p) is always transmitted
BEFORE pack(q)
• at the boundary between two AF domains, traffic
conditioning can apply: shaping, per class discarding, drop
precedence remarking and AF class reassignment. Traffic
conditioning has to avoid REORDERING -> performance
gain on the rx side
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PHB: Assured Forwarding (cont)
• In case of long term congestion AF packets are dropped
• drop algorithm:
WRED (Weighted Random Early Discard) for the implementation of
a gradual discard mechanism based on congestion levels and
proportional to the drop precedence of a given microflow
• Codepoints:
class 1 class2 class3 class4
low drop prec
001|010 010|010 011|010 100|010
medium drop prec
001|100 010|100 011|100 100|010
high drop prec
001|110 010|110 110|110 100|110
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PHB: Class Selectors and Experimental PHBs
• Class selectors:
– XXX000 where x = 1 or 0
precedence
0
TOS byte
3
7
– for backward compatibility with precedence field of the old so
called TOS (Type Of Service) byte. TOS was replaced by the DS
field. Precedence = [0, 7]
– if pred(pack1) < pred(pack2) then
p_drop(pack1) > p_drop(pack2)
• Experimental PHB:
– not standardised codepoint and packet forwarding behaviour
– definition up to the ISP
– requires PHB mapping at the boundary
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Diffserv architecture:
scheduling
policing and classification
traffic metering
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Scheduling
• Scheduling: queue service policy for differentiated treatment of
packets among queues
• Examples of packet scheduling algorithms:
– simple priority queue (high priority queues have the highest
priority, arrival rate < departure rate, low priority traffic starvation
is possible)
– weighted round robin queuing (queues serviced in round robin
fashion, service time proportional to the weight)
– weighted fair queuing (minimum rate guaranteed per class, service
time of each packet in each queue is a function of the packet size
and of the queue weight. Current service time is updated every
time a packet is sent)
– class based queuing (maximum rate per class is configured)
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Congestion management and service enforcement:
policing
Policing: traffic which exceeds a given rate threshold is treated
differently from conforming traffic. E.g. Exceeding packets can
be dropped, re-marked, transmitted as best-effort etc.
Policing is deployed for service level agreement enforcement:
- to limit the input rate at the edge
- at the boundary between domains to guarantee a fair deployment of the
service among different domains
Customer
Premise
Network Edge Packet
Classifier and Policer
Policy
Specification
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Policing: token bucket
• Policing is based on traffic metering. A typical algorithm is
called token bucket.
Input pack stream
drop probability = 1
Exceed burst
0 < drop probability < 1
Normal burst (number of tokens available),
drop probability = 0
output pack stream
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R: departure rate at which tokens
are replenished
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Token bucket: algorithm
• tokens replenished at regular intervals
• Normal burst: max number of tokens which can be in the bucket (in
bytes)
• Exceed burst: to avoid tail drop in favour of gradual drop
• Actual debt AD =  ADi where is the number of borrowed tokens,
– ADi decreases of R tokens per time unit
• Compounded debt
CD =  ADi where ADi
CD = 0 after a packet drop
packi is dropped if CB > exceed burst
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Token bucket: algorithm (cont)
Example (by BoB Olsen)
token rate = 1 data_unit/time_unit
normal burst size = 2 data_units (DUs)
extended burst = 4 DUs.
rate = 2 DUs arrive per time unit.
After 2 time units, the stream has used up its normal burst and must begin
borrowing one DU per time unit, beginning at time unit 3.
Time tocken available DU arrivals
0
2
1
2-2+1=1
2
2
1-2+1=0
2
3
0-2+1=-1
2
4
-1-2+1=-2
2
5
-2-2+1=-3
2
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Actual Debt
Compounded Debt
0
0
1
2
3 (temp)
Quality of Service Support in Packet Networks
0
0
1
3
6 (temp) > 4
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Token bucket: algorithm (cont)
•
•
At this time a packet is dropped because the new compounded debt (6) would
exceed the extended burst limit (4). This causes CD to effectively become 0,
and lowers AD back down to 2. The values 3 and 6 were only temporary and
do not remain valid in the case where a packet is dropped. The final values for
time unit 5 are given below.
Time tocken available DU arrivals Actual Debt Compounded Debt
5
-2-2+1=-3
2
3 (temp)
6 (temp) > 4
drop
5
-2
2
2 (*)
0
6
-2-2+1=-3
2
3
3
7
-3-2+1=-4
2
4 (temp)
7 (temp)
drop
7
-3
2
3
0
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Congestion Avoidance:
Weighted Random Early Discard (WRED)
• Goal: to avoid congestion before it occurs by dropping
isolated packets among different streams. Packets are dropped
according to the current amount of data into a buffer. If
buffer_level < thr1
p(drop) = 0
case 1
thr1 < buffer_level < thr2 0 < p(drop) < 1
case 2
otherwise
p(drop) = 1
case 3
• (W)RED is a congestion avoidance algorithm for TCP traffic based on the
TCP flow control features (TCP reduces the output rate when a single
packet is dropped before real congestion occurs)
• WRED: like RED but p(drop) in case 2 depends on the DS codepoint of
the packet. Packets with low priority experience packet drop before
packets with higher priority
• with both RED and WRED packet drop is randomly distributed among
several flows
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Weighted Random Early Discard (WRED) - cont -
Standard
Bronze
Silver
Gold
Queue Length
40%
60%
70%
Packet Drop
Probability
Packet Drop
Probability
Class 6 gets 50% minimum
Class 4 gets 30% minimum
Class 2 gets 20% minimum
The remaining traffic gets 10%
90%
Max
•
Gold Class 6 will never get drop unless extreme congestion : 90%of queue depth
•
Silver Class 4 will not get dropped unless severe congestion :70%of queue depth
•
Bronze Class 2 will start drop at 60%queue occupancy
•
Standard The remaining traffic will start drop at 40% of queue occupancy
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Differentiated Services:
Case Study
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Characterisation of the application
Characterisation needed to identify the requirements of the
application: service definition
• 1. Hardware of the trigger: remote control - ROBIN – few transactions (low bandwidth), TCP traffic on a limited well-known set
of TCP ports, IP address of the server known
– client - server, one connection to a server at a time
 low paket loss, delay sensitive application, reliability, burst
tolerance
• 2. Monitoring of quality and correctness of the results of
the analysis - ROOT – exchange of analysis results (root object)
– low bandwidth consumption
– client - server, IP address of the server is known
 bandwidth guarantee, more tolerance to packet loss
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Models of network deployment
1. Trigger hardware control
Power PC
VME
Client 1
Server 1
Client 2
VME
Server 2
...
Client 3
...
VME
Client n
Server m
1. Monitoring of analysis
bottlenecks
Browser 1
server
Browser 2
Browser 3
...
Browser n
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Service for trigger control packets
• SERVICE 1:
– low drop probability
– delay bound (minimum queue size)
– precedence: highest precedence, higher than monitoring
packets precedence
– Capacity: a minimum network capacity guaranteed, in
case of spare capacity, more bandwidth can be
allocated.
– No upper limit in rate for maximum burst tolerance
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Service 1: implementation
pack(src/dest) = (s1 || s2 || … || s m) && pack(TCP_port) in [x, y] then
pack(label) = max precedence
policing: always transmit
delay: buffer size of 2*MTU
minimum service rate = m * r * Nclient or rate = m * R * Nclient
m: overbooking factor
r: estimated rate consumed by 1 client, R: estimated rate consumed by 1 server
Nclient : number of clients downstream
If
VME
Server 1
VME
R
2*m*r
Server 2
...
R
2*m*r
r
Client 2
r
Client 3
r
Client 4
Server 8
Marking client -> server
Marking server -> client
Scheduling client -> server
Scheduling server -> client
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Client 1
4*m*r
R
VME
r
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Service for monitoring traffic
• SERVICE 2:
– precedence: higher than best effort, lower than service 1
packets
– drop probability: packets can be dropped in case of
congestion.
– Label: lower precedence
– capacity: minimum guaranteed bandwidth, more
bandwidth can be allocated if available
– maximum upper rate threshold: for fair bandwidth
allocation between several clients
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Service 2: implementation
If
pack(src/dest) = s1 then
if rate < max, pack(label) = medium prcedence
else drop
(marking)
(policing)
shaping: buffer size > server or client burst size
minimum service rate = r * Nclient (client), or rate = R * Nclient (server)
r: estimated rate needed for 1 client,
R: estimated rate needed by the server to support m clients
Nclient : number of clients downstream
precedence: < precedence(service 1)
2*r
R
2*r
Policing server -> client
Marking client -> server
Marking server -> client
Scheduling client -> server
Scheduling server -> client
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Client 1
r
Client 2
r
Client 3
4*r
Server
...
Policing client -> server
r
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r
Client 4
42
Diffserv testing and QoS measurement
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Test network
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LAN layout (example)
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Equipment
•
•
•
•
Test workstations
traffic generators (3 SmartBits, Netcom System loan)
ATM switches in the test sites
cabletron ethernet switch
• DS capable platforms:
– CISCO: C7200 or C7500 (partial CISCO loan)
– IBM: IBM 2212 and IBM 2216 (2 routers in 5 sites,
IBM donation)
– Linux
– Cabletron (LAN switch)
– (Nortel, Torrent)
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CISCO: traffic policing (CAR)
•
•
•
•
•
CAR: Committed Access Rate
Multi-field classification: OK
packet marking (precedence setting): OK
exceed action testing: OK
policing (at a configurable rate): two parameters token
bucket for TCP performance optimisation
– normal burst
– exceed burst
--> parameter tuning
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CAR: test of exceed actions
Throughput:
SWITCH: 1.20 Mbps
DANTE: 0.38 Mbps
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CAR: TCP burst tolerance
• Normal and exceed burst tuning with TCP traffic
• single and multiple TCP streams
optimum values are functions of the rate R at which
traffic is policed, in particular:
normal burst = 0.5 sec * R
exceed burst = 2 * normal burst
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CAR: TCP burst tolerance (cont)
Throughput of 1 TCP connection (Mbps)
Exceed (bytes)
32000
48000
64000
96000
0.98
1.23
1.23
1.25
1.09
1.21
1.25
1.18
1.24
1.24
Normal
(bytes)
128000
32000
1.25
48000
125
64000
1.25
96000
1.25
128000
1.25
Table 4: throughput of 1 TCP connection for increasing values of the normal and exceed burst size
Aggregate throughput of 5 concurrent TCP connection (Mbps)
Normal
Exceed (bytes)
(bytes)
32000
48000
64000
96000
128000
32000
1.26
1.26
1.25
1.26
1.25
48000
1.25
1.26
1.25
126
64000
1.25
1.27
1.25
96000
1.26
1.26
128000
1.25
Table 5: throughput of 5 TCP connections for increasing values of the normal and exceed burst size
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CISCO: scheduling (CB-WFQ)
Scheduling mechanism to provide minimum bandwidth
guarantees to classes
• class definition: precedence or MF classification --> OK
• bandwidth allocation: no starvation, no bandwidth
consumption --> OK
• traffic isolation: scenarios
– UDP high priority + UDP best-effort
– TCP high priority + UDP best-effort
– TCP high priority + TCP best-effort
UDP --> OK (always)
TCP: inconsistent results with 1 TCP stream due to cell shaping problems
in the ATM part of the network, good results with several TCP strams
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CB-WFQ: set-up
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IBM: scheduling (Self Clocked Fair Queuing)
• Policy = (traffic profile, validity period, diffserv action)
• diffserv action = (type of marking, queue type, bandwidth)
• optimum traffic isolation (tests only with UDP)
Premium: 163.8 Kbps guaranteed (8% PPP link bw)
Assured: 819.2 Kbps (40% PPP bw)
Test
number
1
2
3
4
5
6
7
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Premium TCP traffic throughput, target rate: 163 Kbps
Streams
BE
Assured
Premium
throughput
Throughput
throughput
(Kbps)
(Kbps)
(Kbps)
BE
1967.7
/
/
A
/
1968.0
/
P
/
/
159.8
BE + A
649.8
1367.0
/
BE + P
1852.5
/
159.8
A+P
/
1852.0
159.8
BE + A + P
617.8
1236.9
159.8
Quality of Service Support in Packet Networks
Total throughput
(Kbps)
1967.7
1968.0
159.8
2016.8
2012.3
2011.8
2014.6
53
IBM: EF policing
Small TCP burst tolerance in a policer can completely starve
a TCP stream. Token bucket depth is key parameter
--> tuning need according to the rate at which traffic is policed
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IBM: EF policing (cont)
EF with TCP traffic, target rate = 163 Kbps
Bucket size
(bytes)
2200
4400
6600
8800
11000
13200
15400
17600
64000
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Test length
(sec)
Connection stalled
60
60
120
180
240
300
360
420
480
300
300
300
300
300
300
TCP Throughput
(Kbps)
~0
0.97
35.2
74.7
89.8
88.5
95.6
98.2
99.3
100.6
118.9
124.4
124.8
126.0
125.3
125.0
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QoS measurement
• Resource allocation monitoring
– for resource allocation and network dimensioning
• performance measurement
– passive
– active (invasive traffic)
– for service validation
– to understand the effect on end-to-end performance of
buffering in one router or in a chain
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Performance measurement
• parameters of interest:
– one-way delay, requirements: clock synchronisation
> GPS based synchronisation
• SmartBits (Netcom Systems)
> NTP (Network Time Protocol)
• NTP client / server hierarchy
– one-way delay variation
– packet loss
– throughput
– RTT
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Services: examples
• Virtual leased line:
– point to point
– one to many
• capacity allocation on congested links (e.g. US links)
• better-than-best-effort (qualitative definition)
• delay bound and delay variation sensitive traffic classes
• rate limiting of invasive traffic
• ...
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More information at
• diffserv:
– http://www.cnaf.infn.it/~ferrari/tfng/ds
• QoS measurement:
– http://www.cnaf.infn.it/~ferrari/tfng/qosmon
• QBONE: US initiative for testing, validation and
deployment of services based on the expedited forwarding
PHB of the pr
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Comments
• Diffserv building blocks: already supported by several
vendors
• diffserv: no changes in applications required
• diffserv goal: simplified approach to QoS for its support in
backbones from today
• good interim test results
• diffserv in the future:
– Packet Over Sonet (POS) vs ATM
– VLL vs ATM permanent connections
Tiziana Ferrari
Quality of Service Support in Packet Networks
60
Comments (cont)
• diffserv still requires QoS support end-to-end (but diffserv
can be implemented in some domains, provided that the
end-to-end service is homogeneous)
Dedicated connection
diffserv
diffserv
ATM/POS
Production network
diffserv
diffserv
diffserv
Production network
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Quality of Service Support in Packet Networks
61
Comments (cont)
• Issues
– effects of high degree aggregation? More testing
needed
– interoperability between different platforms: effect on
end-to-end services?
– performance of marking, classification and scheduling
at high speed?
– Tools for service monitoring….
– Diffserv in production? 1 year?
Tiziana Ferrari
Quality of Service Support in Packet Networks
62
Discussion
• Deployment of diffserv in HEP
• Issues in diffserv deployment in HEP
• recommendations
Tiziana Ferrari
Quality of Service Support in Packet Networks
63