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QoS (Quality of Service) - part2

This is part 2 of 2. My shortish summarized version of Quality of Service (QoS), Congestion Avoidence, Policing and Shaping.




Congestion Avoidence with WRED:

  • ONLY TCP traffic
  • TCP Global Synchronization happens to TCP flows during periods of congestion. Each sender will reduce their TX rate (at same time) – (Window-size to half) when packet loss occurs.
  • WRED designed to avoid numerous and sudden packet drops that can cause TCP Global Synchronization.
  • Randomly drops packets from TCP flows to minimize synchronization.
  • Dropping becomes more aggressive as queues fill up.

Example:

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class-map MATCH_HTTP
  match protocol http

policy-map SET_HTTP
  class MATCH_HTTP
    random-detect dscp af11 <PACKETS_MIN> <PACKETS_MAX> <MPD_1-65535>
               or
    random-detect dscp-based (auto)

interface Gi0/0
  service-policy <INPUT_OR_OUTPUT> SET_HTTP

show class-map
show policy-map
show policy-map interface Gi0/0

ECN (Explicit Congestion Notification):

  • WRED enhancement
  • Sender and receiver MUST support ECN
  • Instead of dropping packets, tells sender to slow down
  • 2x ECN bits:
    • 00 = NOT ECN capable
    • 01 = ECN capable
    • 10 = ECN capable
    • 11 = Congestion experienced
  • When upstream router gets packet with 11 bit set, it returns ECN-echo packet to sender (application / host) and tells it to slow down.
  • Can be enabled under policy-map with random-detect ecn.

Compression:

  • Payload Compression
  • TCP Header Compression (suppress redundent header information)
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class-map MATCH_HTTP
  match protocol http

policy-map COMPRESS_HTTP
  class MATCH_HTTP
    compression header ip ?

interface Gi0/0
  service-policy <INPUT_OR_OUTPUT> COMPRESS_HTTP

show class-map
show policy-map
show policy-map interface Gi0/0

LFI (Link Fragmentation and Interleaving):

  • Cause higher bandwidth usage due to packets being fragmented and every fragment needs it’s own header.
  • Used to reduce serialization delay
  • NOT used on links faster than 768 Kbps
  • ONLY used on Frame-relay and PPP

Traffic Policing:

  • Drops or marks traffic exceeding specified thresholds
  • Use “Token Bucket” concept

Single Token Bucket:

Example:

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class-map match-any MATCH_PROTOCOLS
  match protocol edonkey
  match protocol napster

policy-map POLICE_PROTOCOLS
  class MATCH_PROTOCOLS
    police <BPS_RATE> conform-action transmit exceed-action drop

interface Gi0/0
  service-policy input POLICE_PROTOCOLS

show class-map
show policy-map
show policy-map interface Gi0/0

Dual Token Bucket:

Example:

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class-map match-any MATCH_PROTOCOLS
  match protocol edonkey
  match protocol napster

policy-map POLICE_PROTOCOLS
  class MATCH_PROTOCOLS
    police <BPS_RATE> <BURST_RATE> <PEAK_BURST> conform-action transmit exceed-action set-dscp-transmit <AF23> violate-action drop

interface Gi0/0
  service-policy input POLICE_PROTOCOLS

show class-map
show policy-map
show policy-map interface Gi0/0

Traffic Shaping:

  • Shapes per class and NOT per interface
  • Queues excess traffic and send at desired rate (class-based)
  • Cannot mark traffic with shaping
  • Can shape per average or peak rate
  • BECN (Backwards Explicit Congestion Notification)
  • FECN (Forward Explicit Congestion Notification)

Example Shaping:

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class-map match-any MATCH_PROTOCOLS
  match protocol edonkey
  match protocol napster

policy-map SHAPE_PROTOCOLS
  class MATCH_PROTOCOLS
    shape peak <BPS> (CIR)
             or
    shape average <BPS> (CIR)

interface Gi0/0
  service-policy input SHAPE_PROTOCOLS

show class-map
show policy-map
show policy-map interface Gi0/0

Example Queueing (Nested policy and class-maps):

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policy-map PRIORITY
  class DATA
    bandwidth 50
  class class-default
    fair-queue

class-map ALL_TRAFFIC
  match class DATA
  match class class-default

policy-map SHAPE_IT
  class ALL_TRAFFIC
    shape average 500000
    service-policy PRIORITY

interface Gi0/0
  service-policy input SHAPE_IT

show class-map
show policy-map
show policy-map interface Gi0/0

Catalyst QoS (Class Of Service (COS)):

  • Layer 2
  • Classification, marking, policing, etc. done in hardware ASICs (CEF)
  • Switch QoS capabilities defined in terms of:
    • Queues (priority / standard)
    • Thresholds (ammount of packets possibly held in queue)
  • Queues can be configured as:
    • Priority Queues
    • Weighted Round-robin (WRR) – Custom queue
    • Weighted Round-robin (WRR) – Priority queue
  • Thresholds can be configured with:
    • Tail drop
    • WRED
  • Also uses class-maps and policy-maps
  • IP phones normaly tags voice packets – can be “caught” by switch and manipulated.
  • Supports TX queues (RX queues ONLY on high end switches)

Example switch QoS support tags:

  • 2Q2T = 2x Queue, 2x Thresholds per Queue
  • 1P2Q2T = 1x Priority Queue, 2x Standard Queues, 2x Thresholds per Queue

Example:

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mls qos (turn it on)

interface Gi0/0
  mls qos trust (trust markings received on port)
  wrr-queue cos-map 1 0 1 2  ---|
  wrr-queue cos-map 2 3 4       |> first row, 1 - 4 = queue number
  wrr-queue cos-map 3 6 7       |> the rest, IP Precedence values 0 - 7
  wrr-queue cos-map 4 5      ---|
  wrr-queue bandwidth <WEIGHT_1-65536> 5 6 10 1
  priority-queue out
  • 5 = queue1, 6 = queue2, 10 = queue3, 1 = queue 4 (Last queue becomes priority queue)
  • 5 + 6 + 10 + 1 = 22
  • Queue1 = 5 / 22 * 100 = 22.73% of total bandwidth
  • Queue2 = 6 / 22 * 100 = 27.27% of total bandwidth
  • Queue3 = 10 / 22 * 100 = 45.45% of total bandwidth
  • Queue4 = 1 / 22 * 100 = 4.55% of total bandwidth

Auto-QoS:

  • cef MUST be anabled (ip cef)
  • bandwidth MUST be configured on interface (bandwidth <BPS>)
  • IP address MUST be assigned

Example:

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interface Gi0/0
  bandwidth <BPS>
  auto qos <   > ?

show class-map
show policy-map
show ip access-list
show running-config

PAK Priority:

  • Router auto-tags all “mission-critical” (control-plane) traffic with IPP6/CS6
  • Is an un-changeable internal mechanism

TX-Ring:

  • Hardware queue of a router
  • Automatically tuned by later Cisco IOS versions
  • Typically holds 32 – 64 packets by default
  • Reduce to 3 on slow, smaller then 1.5Mbit/s links
  • show controllers <INTERFACE> | incl tx_limited

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