Transcript QoS / CoS in the LAN - Denver University

QoS / CoS in the LAN
Byron D. Early
Chad D. Burnham
University of Denver
UTS - Network Services
WestNet – January 15, 2004
ASU – Tempe, AZ
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QoS / CoS Definition
• Techniques to enhance network
performance for traffic types deemed
essential to your institution’s business
model:
– Bandwidth
– Delay
– Jitter
– Packet Loss
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“Managed Unfairness”
• Goal: predictable end-to-end service
levels for selected (“preferred”) traffic
– Prioritizing: “preferential packet forwarding”
given to selected network traffic types at the
expense of lower priority traffic
– Preferential Treatment Based On:
• Traffic type
• Institution’s business model (“mission-critical”)
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QoS / CoS Parameters
• Bandwidth:
–Bandwidth Management:
•Does not create additional bandwidth
•“Reallocate” existing bandwidth to
satisfy requirements of applications
•Weakest link determines maximum
available bandwidth
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QoS / CoS Parameters
• Delay (3 Major Types):
–Processing: encode/decode; queuing
–Serialization: transmission onto circuit
–End-to-End: total packet/frame delay
from source-to-destination
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QoS/CoS Parameters (cont.)
• Jitter: “delay variations” from one
frame/packet to another for a given flow
• Packet Loss: packets/frames lost in
“forwarding path”
– Buffer overflows
– Transmissions errors
– QoS: Traffic policing
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QoS / CoS Parameters (cont.)
• Acceptable Delays (typical):
– Telephony: < 150 ms
– Video Conferencing (VC): < 500 ms
• Encoding / Decoding: 125-250 ms (each)
• WAN Transit: 50-100+ ms
• LAN Transit: < 1-5 ms (per node)
• Jitter: < 20% on one-way delay
– H.323 Pt-to-Pt: ~300 ms
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Application Requirements
experpt from Cisco “IP QoS”, 2002 by Zdravko Nikolov
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Congestion & Performance
• Network Traffic: unpredictable &
“bursty” nature fundamentally drives
need for QoS/CoS
• Transmission Queues:
– Limited size transmit buffers need overfill
protection
• “Tail Drop”: full transmit queue drops all
incoming packets (inefficient TCP windowing)
• Interface Queues use QoS to intelligently
manage which packets are dropped
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Interface Queues
• “Intelligently” protect transmit queues from
being overwhelmed
• QoS/CoS Techniques: should impact traffic
only under CONGESTED conditions
–
–
–
–
IP Precedence (ToS)
Class-based Weighted Fair Queuing (CBWFQ)
Low Latency Queuing (LLQ)
Etc.
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Why QoS in a Switched
Environment?
• Increasing Bandwidth is not a panacea:
– High Cost: prohibitive for higher-speed
links
– Does not solve “TCP windowing” issue of
taking as much bandwidth as possible
– Interactive traffic: requires low delay &
jitter (VoIP, VC)
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Initial QoS Planning
• Identify “congestion points” in campus LAN
hierarchy
– Switch “uplink speeds”
– LAN-to-LAN speed mismatches
• Classify critical applications requiring
preferential forwarding in your environment
• Implement QoS techniques at congestion
points to match traffic requirements
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Types of QoS / CoS
• Best Effort (BE): no QoS applied to
packet/frames along forwarding path
– default behavior
• Integrated Services Model (IntServ):
end-station or network node signals
network neighbors with QoS request
• Differentiated Services Model (Diffserv):
network recognizes traffic classes
requiring QoS
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Types of QoS / CoS (cont.)
• IntServ & DiffServ models can also be
used in combination to achieve endto-end QoS
• True end-to-end QoS requires by all
devices along forwarding path
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IntServ: RSVP
• RFC 1633 / 2205-2215 (RSVP)
• Resource Reservation Protocol (RSVP):
– Identifies application (flow)
– Signaling determines if required network
resources are available
– Admission Control determines if application
(flow) will be granted resources
• Common Open Policy Service (COPS; RFC 27482753) offloads admission control to “central
policy server”
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IntServ: RSVP (cont.)
• RSVP Process:
– Sender sends path message to receiver
about QoS capabilities of intermediate nodes
– Receiver processes and generates
“upstream” request to reserve resources
– UNI-Directional Process (requires each end
point to reserve resources)
– Uses existing mechanisms (WFQ, etc)
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Differentiated Services
• RFC 2475 (DiffServ)
• Most Generally Accepted QoS Model
• Different Services to Different Traffic
types - that can scale!
• Uses Packet Classification and
Marking [DSFIELD]
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Differentiated Services - (cont.)
• Packet Classification
– Layer 2 & Layer 3
– ACL,URL,MIME Type, NBAR – to identify traffic
– Perform as close as possible to source
• Packet Marking
– Based on Classification (used to distinguish)
– Marking is carried throughout network
– Scalable: Deployed on 1st Layer-3-capable
device (Limiting burden on core devices)
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Differentiated Services - (cont.)
• Congestion Management
– Isolates and prioritizes various classes
of traffic
– Re-ordering of packet transmissions
– Impacts delay and jitter
– Egress function (CBWFQ & LLQ)
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Differentiated Services - (cont.)
• Congestion Avoidance
– TCP Based – cause a smaller TCP Window
– Weighted Random Early Detection (WRED)
– Random dropping to prevent exhaustion of
queue
• “Tail-drop” Condition
– Uses DiffServ Code point (DSCP) or IP
Precedence
• Traffic Conditioning
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Differentiated Services - (cont.)
• Traffic Conditioning
Policers
•
•
•
•
Drop packets exceeding specified rate
UDP does not re-transmit dropped packets
Better for VoIP
Cisco: CAR
Shapers
• Limits rate of packets using buffers
• Adds delay which is not good for VoIP & VC
• Cisco: GTS, FRTS, Class-based etc
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DiffServ - Per Hop Behavior
**(PHB)**
• RFC 2475 – Foundation of DiffServ
• Forwarding Behavior applied @ each DScomplaint node to a DS “behavior
aggregate” (BA)
– BA: Collection of packets with the same
DiffServ Code Point traversing a node in a given
direction
• Based on single or multiple criteria
• MF Classifier (MF): Source/Destination address, DS
field, Protocol ID, Ports
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DiffServ – DSCP
“Code Points”
• RFC 2474 – Field Format
• Obsoletes RFC 791
– ToS – IP Precedence
• Code Points are backward compatible
• Default configs = recommended
mappings
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Diffserv
Assured Forwarding (AF) –
PHB Type
• RFC 2597
• 12 recommended Code Points
– 4 independent classes each having 3 Levels
of “drop precedence”
Class
Low Drop
Medium Drop
High Drop
AF1
001010 (AF11)
001100 (AF12)
001110 (AF13)
AF2
010010 (AF21)
010100 (AF22)
010110 (AF23)
AF3
011010 (AF31)
011100 (AF32)
011110 (AF33)
AF4
100010 (AF41)
100100 (AF42)
100110 (AF43)
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Diffserv
DS Field Format
• IP Header Comparison: IP Precedence/ToS
& DS Code points
BIT
IPv4 (RFC 791)
DS Field
0
1
IP Precedence
2
3
4
5
"ToS Bits"
DSCP
6
7
Unused
ECN
• In IPv6 = “Traffic Class” Octet
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DiffServ: Expedited Forwarding (EF)
• RFC 2598
• Node forwards packet ASAP
– DSCP 46 (101110)
• Real-time traffic requiring low delay & jitter
• Marking Mechanisms:
– CAR, policy-based Routing, Dial Peers, Class-based
marking, Class-based Policer
• Cisco: LLQ
– single strict priority queue extends CBWFQ
• Risk: Too much EF traffic can lead to
“starvation” of non EF traffic!
– Police EF traffic rate
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Classification, Marking & Mapping
• Layer 2 CoS frames are classified and
marked in the “ISL” or “802.1Q” header
• Frames passing from L2 to L3 lose
header information
• Mapping Problem between L2 & L3:
– 64 DSCP Values (0-63)
– 8 CoS Value (0-7)
– Groups of DSCP values must be mapped to
single CoS values
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QoS / CoS “Trust Concepts”
• How ingress packets are handled on interfaces
• End-User-Ports:
– Generally treated as “untrusted” by network
administrators because OS allow users to set CoS
values
– Switch changes CoS to Best Effort (0) when frame
is forwarded
• Switch-to-Switch, Switch-to-Router & Switchto-IP Phone:
– Usually treated as “trusted” by network
administrators & CoS value is unchanged
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Layer 2 CoS Marking
• Layer 2 ISL Frame
•ISL CoS: uses 3 least significant bits of “user field”
in ISL header
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Layer 2 CoS Marking (cont.)
• Layer 2 802.1q/p Frame
•802.1q/p CoS: uses 3 bits of “user priority” portion
of “tag field”
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QoS / CoS Summary Table
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References
• Cisco Catalyst QoS: Quality of Service in Campus
Networks
– Michael Flannagan, Richard Froom & Kevin Turek
– ISBN#1-58705-120-6
• IP QoS (Cisco, 2002)
– Zdravko Nikolov ([email protected])
• Polycomm User Group Presentation:
– http://www.pug.com/conference/2003_Conference/Presentations
/A1-QoS-and_CoS.pdf
– Kris Acharya, Optimal Systems, Inc.
(on assignment at Pfizer, Inc.)
– September 15th, 2003
• Eva Heinold - CCCSC München - [email protected]
– http://www.decus.de/slides/sy2003/08_04/1g02.pdf
• Jeff Caruso: Network World
– http://www.nwfusion.com/newsletters/lans/2003/1215lan1.html
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