QoS / CoS in the LAN

1
QoS / CoS in the LAN

• Byron D. Early
• Chad D. Burnham
• University of Denver
• UTS - Network Services
• WestNet January 15, 2004
• ASU Tempe, AZ

2
QoS / CoS Definition

• Techniques to enhance network performance for
  traffic types deemed essential to your
  institutions business model
• Bandwidth
• Delay
• Jitter
• Packet Loss

3
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
• Institutions business model (mission-critical)

4
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

5
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

6
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

7
QoS / CoS Parameters (cont.)

• Acceptable Delays (typical)
• Telephony lt 150 ms
• Video Conferencing (VC) lt 500 ms
• Encoding / Decoding 125-250 ms (each)
• WAN Transit 50-100 ms
• LAN Transit lt 1-5 ms (per node)
• Jitter lt 20 on one-way delay
• H.323 Pt-to-Pt 300 ms

8
Application Requirements
experpt from Cisco IP QoS, 2002 by Zdravko
Nikolov
9
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

10
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.

11
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)

12
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

13
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

14
Types of QoS / CoS (cont.)

• IntServ DiffServ models can also be used in
  combination to achieve end-to-end QoS
• True end-to-end QoS requires by all devices along
  forwarding path

15
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 2748-2753)
  offloads admission control to central policy
  server

16
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)

17
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

18
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)

19
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)

20
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

21
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

22
DiffServ - Per Hop Behavior (PHB)

• RFC 2475 Foundation of DiffServ
• Forwarding Behavior applied _at_ each DS-complaint
  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

23
DiffServ DSCPCode Points

• RFC 2474 Field Format
• Obsoletes RFC 791
• ToS IP Precedence
• Code Points are backward compatible
• Default configs recommended mappings

24
Diffserv Assured Forwarding (AF) PHB Type

• RFC 2597
• 12 recommended Code Points
• 4 independent classes each having 3 Levels of
  drop precedence

25
Diffserv DS Field Format

• IP Header Comparison IP Precedence/ToS DS Code
  points

• In IPv6 Traffic Class Octet

26
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

27
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

28
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 Switch-to-IP
  Phone
• Usually treated as trusted by network
  administrators CoS value is unchanged

29
Layer 2 CoS Marking

• Layer 2 ISL Frame

• ISL CoS uses 3 least significant bits of user
  field in ISL header

30
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

31
QoS / CoS Summary Table
32
References

• Cisco Catalyst QoS Quality of Service in Campus
  Networks
• Michael Flannagan, Richard Froom Kevin Turek
• ISBN1-58705-120-6
• IP QoS (Cisco, 2002)
• Zdravko Nikolov (znikolov_at_cisco.com)
• Polycomm User Group Presentation
• http//www.pug.com/conference/2003_Conference/Pres
  entations/A1-QoS-and_CoS.pdf
• Kris Acharya, Optimal Systems, Inc.(on
  assignment at Pfizer, Inc.)
• September 15th, 2003
• Eva Heinold - CCCSC München - eva.heinold_at_hp.com
• http//www.decus.de/slides/sy2003/08_04/1g02.pdf
• Jeff Caruso Network World
• http//www.nwfusion.com/newsletters/lans/2003/1215
  lan1.html