S6C10 - Queuing

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					     S6C10 - Queuing

Which Packet Gets Processed First
• the process the router uses to schedule packets for
  transmission during periods of congestion.
   – mission-critical and delay-sensitive traffic can be sent
• four methods of queuing:
   – first in,first out (FIFO) queuing; priority queuing (PQ);
     custom queuing (CQ); and weighted fair queuing
      • Only one queuing method per interface
            FIFO Quequing
• Simplest algorithm for packet transmission.
  – transmission occurs in the same order as
    messages are received
  – Until recently, FIFO queuing was the default
    for all router interfaces
• Mission critical packet can be stuck in line
  behind massive ftp download
• Most effective on WAN links with combination
  of bursty traffic and relatively lower data rates
• Most effective when applied to links at T1/E1
  bandwidth speeds or lower.
   – If congestion on the WAN link does not exist, there is
     no reason to implement traffic prioritization.
   – Effective on WAN links that experience temporary
   – Add bandwidth if congesting is constant
                Queuing Policy
• provides an appropriate level of service for all
• control expensive WAN costs
   – Determines relative importance of different traffic types
   – Determines which queuing scheme is best
   – Provides ability to prioritize, reserve, and manage
     network resources,
   – ensures the seamless integration and migration of
     disparate technologies without unnecessary costs.
 Choosing a Cisco IOS Queuing
• Determine whether the WAN is congested
• Decide whether strict control over traffic
  prioritization is necessary and whether automatic
  configuration is acceptable
   – study the types of traffic using the interface
   – decide relative priority
• Establish a queuing policy
• Determine whether any of the traffic types you
  identified in your traffic pattern analysis can
  tolerate a delay.
          Weighted Fair Queuing
• Dynamic queuing strategy
   – used by default on serial interfaces at E1 speeds (2.048
      Mbps) and below.
   – Disabled on serial interfaces that use X.25,
      Synchronous Data Link Control (SDLC), or
      compressed Point-to-Point Protocol (PPP).
• Uses a complex algorithm to sort the packets that make up
  the different conversations on an interface
• Automatically allocates bandwidth to all types of network
   – prioritizes delay-sensitive packets so that high-volume
      conversations don’t consume all of the available
   – Low-volume traffic streams (which are the majority of
      traffic) receive preferential service, transmitting their
      entire loads in a timely fashion.
          WQF Discriminators
• Discrimination of traffic into conversations is
  based on packet-header addressing.
• Common conversation discriminators include:
   –    Source/destination network address
   –    Source/destination MAC address
   –    Source/destination port or socket numbers
   –    Frame Relay Data Link Connection Identifier
     (DLCI) value
   –    Quality of service/type of service (QoS/ToS) value
     Configuration Commands
• Router(config-if)#fair-queue
  – congestive-discard-threshold is the number of
    messages to queue for high-volume traffic
  – Fair-queue command appears in the output only
    if the congestive discard threshold is modified
    to a value other than 64
  – Use no fair-queue command to disable WFQ
  – and enable FIFO queuing on an interface
                Priority Queuing
• highest-priority traffic always gets dispatched
  before any other packets
   – Assign traffic to one of four output queues: high,
     medium, normal, or low priority
       • Once the high queue is empty, the router checks the medium
       • the lower-priority queue might not be serviced within an
         acceptable time frame, or even at all.
       • used on low-speed WAN links
   – Packet is classified; if the appropriate queue is full, the
     packet is dropped
   Configuring Priority Queuing
• Priority list is a set of rules that describe the
  waypackets should be assigned to PQs
   – Router(config)#priority-list list-number
     protocol protocol-name {high | medium |
     normal | low} queue-keyword keyword-value
      • Router(config)#priority-list 1 protocol ip high tcp 23
      • Router(config)#access-list 10 permit
      • Router(config)#priority-list 1 protocol ip high list 10
     Priority Queuing Examples
• Establish queuing priorities on packets entering
  from a given interface:
   – Router(config)#priority-list list-number interface
     interface-type interface-number {high | medium |
     normal | low}
• Place traffic from E0 in medium priority
   – Router(config)#priority-list 2 interface ethernet 0
• Change the number of packets
   – Router(config)#priority-list list-number queue-limit
     high-limit medium-limit normal-limit low-limit
      • Router(config)#priority-list 4 queue-limit 10 40 60 80
                Custom Queuing
• Reserves a minimum amount of bandwidth for every kind
  of traffic
   – delay-sensitive and mission-critical traffic can be
      assigned a large percentage of available bandwidth,
   – low-priority traffic receives a smaller portion.
       • can configure up to 16 queues
       • Each queue is serviced sequentially
           – until the number of bytes sent exceeds the configurable byte
           – or until the queue is empty.
• Important for time-sensitive protocols, such as voice,
  video, or IBMs SNA
   – require predictable response time.
   – Queue 0 is a system queue that handles system packets
     such as keepalives.
       • emptied before the other custom queues.
                Custom Queuing
• Traffic filtering - The forwarding application-
  such as IP, IPX, or AppleTalk-
   – applies a set of filters or access-list entries to each
     message that it forwards.
       • messages are placed in queues, based on the filtering.
• Queued message forwarding –
   – CQ uses a round-robin dispatching algorithm to
     forward traffic.
       • Each queue continues to transmit packets until the configured
         byte limit is reached.
       • When the threshold of this queue is reached or the queue is
         empty, the queuing software services the next queue in
  Configuring Custom Queuing
• Send all traffic from Ethernet interface 0 to
  custom queue 1.
• Send all IP traffic to custom queue 2.
• Send all IPX traffic to custom queue 3.
• Send all AppleTalk traffic to custom queue
    Custom Queuing Examples
• Router(config)#queue-list list-number
  protocol protocol-name queue-number
  queue-keyword keyword-value
• Router(config)#queue-list list-number
  interface interface-type interface-number
      Queuing Show Commands
•   show queueing
•   show interfaces
•   show queueing custom
•   show queueing priority
•   show queueing fair
           Data Compression
• identifies patterns in a stream of data, and
  chooses a more efficient method of
  representing the same information
  – algorithm is applied to the data to remove as
    much redundancy as possible
     • Shannon's Limit
        – how much a given source of data can be compressed
   Types of Data Compression
• Link compression (also known as per-
  interface compression)
• Payload compression (also known as
  per-virtual-circuit compression)
• TCP header compression
  – By default data is sent uncompressed
             Link Compression
• uses either the Predictor or STAC algorithm to
  compress the traffic
• To ensure error correction and packet sequencing
  (Cisco High-Level Data Link Control [HDLC]
  uses STAC compression only):
   – Predictor - Predicts the next sequence of characters in
     the data stream by using an index to look up a sequence
     in a compression dictionary
   – STAC - Developed by STAC Electronics, STAC is a
     Lempel-Ziv (LZ)-based compression algorithm. It
     searches the input data stream for redundant strings and
     replaces them with what is called a token,
          Payload Compression
• (also known as per-virtual-circuit compression)
   – compresses only the data portion (including the Layer 3
     and Layer 4 headers) of the data stream
   – frame header is left untouched
• appropriate for virtual network services such as
  Switched Multimegabit Data Service (SMDS),
  Frame Relay, and Asynchronous Transfer Mode
• Use the frame-relay payload-compress command
  to enable STAC compression on a specified Frame
  Relay point-to-point interface or subinterface:
   –    Router(config-if)#frame-relay payload-compr
   TCP/IP Header Compression
• subscribes to the Van Jacobson Algorithm, which
  is defined in RFC 1144.
  – protocol specific and compresses only the TCP/IP
    header, which leaves the Layer 2 header intact to allow
    a packet with a compressed TCP/IP header to travel
    across a WAN link
  – Don’t implement both Layer 2 payload compression
    and TCP/IP header compression
  – header compression is generally used at lower speeds,
    such as 64-kbps links.
     • Router(config-if)#ip tcp header-compression [passive]
    Compression Considerations
• Modem compression - In dial environ-ments,
  compression can occur in the modem.
• Encrypted data - Compression is a Layer 2
  function. When a data stream is encrypted by the
  client application, it is then passed onto the router
  for routing or compression service
• CPU cycles versus memory - The amount of
  memory that a router must have varies according
  to the protocol being compressed

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