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QoS (Quality of Service) - part1

This is part 1 of 2. My shortish summarized version of Quality of Service (QoS), Basic concepts, Marking and Congestion Management.




Quality Of Service (QoS) Basics:

Main Traffic-type Characteristics:

  • Voice traffic flow is smooth in nature, subject to packet drops, delay and has predictable bandwidth usage.
  • Video traffic is bursty and has greedy traffic usage.
  • Data traffic varies per application.

Main Traffic Challenges:

  • Lack of bandwith
  • Packet loss
  • Delay
  • Jitter (Variation in delay)

Latency/Delay Types:

  • Propagation delay: time taken for head of signal to travel from sender to receiver.
  • Serialization delay: time needed to place data on the wire/medium.
  • Processing delay: time spend to take from input interface and move to output interface.
  • Packetization delay: time taken to turn data into packets.
  • Queueing delay: time spend in the queues of output interface.

QoS Methods:

  • Classification – identifying and groupiong different traffic types.
  • Marking – tags/colors packets so it can be identified elsewhare in a network.
  • Policing – drops or marks packets when specified limit is reached.
  • Shaping – queues packets when a specified limit is reached.
  • Congestion Avoidence
    • FIFO (First-in First-out) – Tail-drop
    • RED (Random Early Detection) – Drops packets randomly
    • WRED (Weighted Random Early Detection) – Drops packets with specified markings
  • Congestion Management – Ordering packets in most efficient way
  • Link Efficiency
    • Compression
    • Link Fragmentation and Interleaving (LFI)

Modular QoS CLI (MQC):

  1. Configure class-map
  2. Configure policy-map
  3. Apply service-policy (ONLY 1 per interface per direction)

Classification:

  • Inspecting one or more aspects of a packet to see what it’s carrying.

Network Based Application Recognition (NBAR):

  • Deep packet inspection to identify traffic
  • OSI Layers 5 – 7
  • Identification “signatures” (PDLM files / PDLM packs) can be extended (ip nbar pdlm flash://whatever.pdlm)

Example:

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class-map MYMAP
  match protocol <SOME_PROTOCOL_NAME>

NBAR Stats Collection:

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interface Gi0/0
  ip nbar protocol-discovery
  load-interval 60

show ip nbar protocol-discovery stats bit-rate top-n 10
show ip nbar unclassified-port-stats (Unknown traffic)

Marking:

  • Writing information to a packet to identify the classification decision.
  • Mark packets as close to source (trust boundry) as possible.

Types Of Marking:

  • Layer 2 – Typically stripped at Layer 3 hops
    • example: COS, MPLS (Experimental bits), Frame Relay (DE bit), ATM (CLD bit)
  • Layer 3 – Passes through Layer 3 hops, excluding NAT
    • example: IP Precedence, DSCP

Example:

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class-map MATCH_HTTP
  match protocol http

policy-map MARKING_HTTP
  class MATCH_HTTP
    set ip precedence <PRECEDENCE_VALUE> (0 - 7)

interface Gi0/0
  service-policy <INPUT_OR_OUTPUT> MARKING_HTTP

IP Precedence and DSCP:


  • 7 – Network – Reserved
  • 6 – Internet – Reserved
  • 5 – Critical – Voice bearer (RTP)
  • 4 – Flash Override – Video
  • 3 – Flash – Voice signaling (RTCP) or Video
  • 2 – Immediate – High priority data
  • 1 – Priority – Medium priority data
  • 0 – Routine – Best effort data

Congestion Management:

  • FIFO (First-in First-out):
    • Best effort
    • ONLY 1 queue (1 send and 1 receive)
    • method: FIFO
    • NO delay guarentee
    • NO bandwidth guarentee
  • Priority Queueing:
    • Can cause bandwith starvation for “NOT high” trafffic.
    • 4 queues (High, Medium, Normal, Low)
    • method: strict priority
    • ONLY High priority delay guarentee
    • NO bandwidth guarentee
  • Custom Queueing:
    • Assigns bytes per protocol per queue.
    • 16 queues
    • method: Round-robin
    • NO delay guarentee
    • Bandwidth GUARENTEE
  • Weighted Fair Queueing (WFQ):
    • Cisco default for links less than 2048 bit/s
    • Number of queues per flow
    • method: Weighted Fair (least top talker gets priority)
    • NO delay guarentee
    • NO bandwidth guarentee
  • Class-based Weighted Fair Queueing (CBWFQ):
    • Up to 256 classes (class-maps)
    • Use % of bandwidth per class-map
    • method: N/A
    • NO delay guarentee
    • Bandwidth GUARENTEE
  • Low Latency Queueing (LLQ):
    • 1 Priority queue plus CBWFQ
    • method: Policed priority (Uses combination of other queueing methods)
    • Delay GUARENTEE for priority queue traffic
    • Bandwidth GUARENTEE

Marking and LLQ Implementation:

NOTE: Will ONLY see effect when there is congestion!


Router 2 Configuration:

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class-map MATCH_HTTP
  match protocol http

class-map MATCH_FTP
  match protocol ftp

policy-map MARK_TRAFFIC
  class MATCH_HTTP
    set dscp af21
  class MATCH_FTP
    set dscp af11

interface Gi0/1
  service-policy output MARK_TRAFFIC

show class-map
show policy-map
show policy-map interface Gi0/1

Router 1 Configuration:

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class-map MATCH_AF11
  match dscp af11

class-map MATCH_AF21
  match dscp af21

policy-map LLQ
  class MATCH_AF11
    bandwidth percent 15 (WFQ)
  class MATCH_AF21
    priority percent 15 (LLQ)
  class class-default
    fair-queue (CBWFQ)

interface Gi0/1
  service-policy output LLQ

show class-map  
show policy-map
show policy-map interface Gi0/1

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