• Congestion
  • Speed mismatch – traffic leaves a lower-bandwidth interface than the one it came in on
  • Aggregation problem – lots of links with one egress of equal bandwidth
  • Confluence problem – a bunch of traffic needs to egress out of the same interface
• Queuing
  • Transmit queue (TxQ) – hardware queue; there’s only one you can’t touch
  • Software queue – where packets wait to be sent; there are many queue-types that you modified to police traffic
• FIFO
  • If I beat you to the router, I leave the router first.
  • Possible long delays, jitter, and starvation
• Priority queuing (PQ)
  • Four queues
    • High-priority
    • Medium-priority
    • Normal-priority
    • Low-priority
  • Scheduler starts from high and work to low
  • When the high queue is empty, it processes a packet from medium, then starts all over
  • Can you say starvation?
• Round robin queuing (RR)
  • One packet from this queue, one from the next, etc., then start over again
• Custom queuing (CQ)
  • Weighted round robin
  • Queues are given weights (bandwidth guarantees)
• Weighted Fair Queuing (WFQ)
  • Default queuing on slow links ( < E1 )
  • Divides traffic into flows
  • Equal bandwidth is given to each flow
  • Provides faster scheduling to low-volume flows
  • Provides more bandwidth to higher-priority flows
  • Flows identified by a hash
    • Source IP
    • Destination IP
    • Protocol number
    • ToS
    • Source port
    • Destination port
  • Each unique has is a new flow
  • No way to allocate bandwidth among the flows
  • By default, up to 256 queues are made, but that is changeable to a power of 2 between 16 and 4096
  • If the max number of flows is reached, queues are reused for other flows
  • If a queue is full, a packet may be dropped.
  • WFQ early dropping drops packets when the queue reaches the congestive discard threshold (CDT)
  • Advantages
    • Simple configuration
    • No starvation
    • Guarantee processing of all flows
    • Drops packets from big-hitter flows
    • Faster service no low-hitters (interactive) flows
    • Standard on (nearly) all IOS devices
  • Disadvantages
    • Classification and scheduling are not configurable
    • Only on slow links
    • No guarantee of bandwidth or delay
• Class-based Weighted Fair Queuing (CBWFQ)
  • User-defined queues for flexibility
  • Configured with class-maps via MQC
  • Weights are calculated based on values give in class-map
    • Bandwidth – guarantee this much bandwidth
    • Bandwidth percent – give me this much of the available bandwidth
    • Bandwidth remaining percent
  • Advantages
    • User-defined traffic classes
    • Each queue gets its own bandwidth
    • Scalability
  • Disadvantages
    • No delay guarantee (not good for real-time application like voice)
  • Configuring

    class-map TESTCM1
     match access-group 100
    !
    class-map TESTCM2
     match access-group 200
    !
    policy-map TESTPM
     class TESTCM1
      bandwidth 64
     class TESTCM2
      bandwidth 128

• Low-latency Queuing
  • Includes strict priority queue for delay-sensitive data
  • Strict priority queue is policed to avoid starvation of other queues
  • Configured the same way as normal CBWFQ, but with the priority keyword
  • This configuration makes TESTCM2 a priority queue

  class-map TESTCM1
   match access-group 100
  !
  class-map TESTCM2
   match access-group 200
  !
  policy-map TESTPM
   class TESTCM1
    bandwidth 64
   class TESTCM2
    priority bandwidth 128

• Classification is done with traffic desriptors
  • Ingress interface
  • CoS value on ISL or 802.1P frames
  • Source/destination IP address
  • IP Precedence or DSCP value
  • MPLS EXP
  • Application type
• Layer 3 QoS
  • Type of Service (ToS) is 8-bit field.
  • First 3 bits of ToS are the IP precedence.
  • First 6 bits of ToS are the DSCP value.
  • Last 2 bits of ToS are explicit congestion notification (ECN).
• Layer 2 QoS
  • Ethernet
    • Class of Service (CoS)
    • On 802.1P frame
    • 3-bit priority (PRI) field
      • 000 – Routine – Best-effort
      • 001 – Priority – Medium priority
      • 010 – Immediate – High priority
      • 011 – Flash – Call signaling
      • 100 – Flash-Override – Video conferencing
      • 101 – Critical – Voice bearer
      • 110 – Internet – Reserved
      • 111 – Network – Reserved
  • Frame Relay
    • 1-bit discard eligible (DE) field
  • ATM
    • 1-bit cell loss priority (CLP) field
  • MPLS (layer 2 1/2)
    • 3-bit experimental (EXP) field
    • By default, the 3 most significant ToS bits (IP Precedence bits) are copied to EXP
• Per-hop Behavior (PHB)
  • “an externally observable fowarding behavior of a network node toward a group of IP packets that have the same DSCP value”
  • In other words, treat packets with the same DSCP value in the same manner – scheduling, queuing, policing, etc.
  • Behavior aggregate (BA) is a group of packets with the same DSCP value
• DSCP
  • DSCP is chopped up into 4 PHBs
    • Class selector PHB – (000) old IP precedence compatibility
    • Default PHB – (000) best effort
    • Assured forwarding (AF) PHB – (001, 010, 011, 100) guarantee bandwidth
      • Provides 4 queues for 4 classes of traffic (AF1-4)
      • Also specifies drop preference (ex., AF41, A13) where second number is preference (higher is more probable to be dropped)
      • Each queue must have (W)RED to avoid drops
      • No queue is any better than the other
      • Backward compatible with IP precedence
  • • Expedited forwarding (EF) PHB – (101) low delay
      • Minimum delay
      • Bandwidth guarantee
      • Policing
• Trust boundaries
  • Establish DSCP values as close to the source as possible
    • On the device (IP phone), access switch, or distribution switch
    • The core should never assign DSCP values
  • Only trust DSCP values from devices you trust
  • Examine and rewrite values from untrust sources
• Network-based Application Recognition (NBAR)
  • Protocol discovery – discovers what protocols you’re running on your network
  • Traffic statistics collection – keeps tracks of stats on each protocol
  • Traffic classification – NBAR protocols can be used in class-maps to define traffic to be services
  • Packet description language models (PDLMs) – table of what protocols NBAR recognizes
  • Limitations
    • Doesn’t work on EtherChannel interfaces
    • Only handles 24 URLs, hosts, or MIME types
    • Only analyzes first 400 bytes of the packets
    • Requires CEF
    • Doesn’t work on HTTPS, multicasts, or fragments
    • Ignored traffic destined for the router itself
  • NBAR commands
    • Router(config)# ip nbar pdlm pdlm-name : Update the PDLM table
    • Router(config)# ip nbar port-map protocol-name [tcp|udp] port-number : Adds an entry to the PDLM table
    • Router# show ip nbar port-map protocol-name : Shows what’s in the PDLM table
    • Router# show ip nbar protocol-discovery : Shows what’s been discovered
    • Router(config-cmap)# match protocol name : a class-map match for an NBAR-discovered protocol
  • Special protocol matching
    • Can match beyond the port number with deep packet inspection
    • Matches HTTP hostname, URL, or MIME type
    • Matches fast-track P2P
    • Matches RTP content

Major issues for converged enterprise networks

• Available bandwidth: competition among applications
  • Fixes
    • Increase bandwidth: More power!
    • Properly queue based on classification and marking: QoS
    • Compress: cRTP, TCP header compression, etc.
• Delay: Lead time to get a packet to the destination
  • Types of delay
    • Processing delay: routing, switch delay
    • Queuing delay: how long a frame stays in an output queue
    • Serialization delay:  how long to put the frame on the wire
    • Propagation delay: the time to cross the physical medium
• Jitter (delay variation): Variation is the delay
  • Different delays mean different arrival times
  • De-jitter buffers save up packets to reduce jitter (like the old CD writers)
  • Fixes
    • More bandwidth
    • Prioritize sensitive data and forward first
    • Remark (reclassify) packets based on sensitivity
    • Enable L2 payload compression: make sure compression delay isn’t worse than the jitter
    • Use header compression
• Packet loss: Packets are lost in the network somewhere
  • Fixes
    • More bandwidth
    • Increase buffers space: more room for the queue on the interface
    • Provide guaranteed bandwidth: Queuing and QoS
    • Congestion avoidance
      • Random Early Detection (RED) and weighted RED (WRED) drop packets before the queue is full
      • Selective dropping is better than FIFO or LIFO dropping

QoS History

• Priority queuing: gives certain data the right-of-way for transmission
• Weighted Fair Queuing (WFQ): prevents small packets from waiting too long for big packets
• RTP priority queuing: Gives voice packets the right-of-way
• CAC: Makes sure we don’t fill up the queue or pipe with voice traffic

Implementing QoS

• Step 1: Identify traffic types and requirements
  • Network audit
  • Business audit
  • Define bandwidth requirements for each class found
• Step 2: Classify the traffic
  • Common classes
    • VOIP
    • Mission-critical
    • Signal traffic: for VOIP
    • Transactional application: SAP, ERP
    • Best-effort: Everything else
    • Scavenger: Crap you don’t care about like P2P and your boss’s email
• Step 3: Define policies for each class
  • Tasks for each class
    • Set max bandwidth
    • Set min bandwidth
    • Assign relative priorities
    • Apply congestion avoidance, congestion management, etc.

QoS Models

• Best-effort: no QoS
  • Scalable
  • Easy
  • No service guarantee: doesn’t care what you’re trying to do
  • No service differentiation: all traffic is equal
• Integrated Service (IntServ)
  • Hard-QoS
  • Uses RSVP to guarantee bandwidth through the entire path
  • Requires
    • Admission control
    • Classification
    • Polices the traffic (ceiling)
    • Queuing
    • Scheduling
  • Advantages
    • End-to-end resource admission control
    • Per-request policy admission control
    • Signaling of dynamic ports
  • Disadvantages
    • Continuous signaling
    • Not scalable
• Differentiated Services (DiffServ)
  • Soft-QoS
  • Configured on each hop
  • Traffic is classified
  • Enforces different treatment on different classes
  • Defined based on business requirements
  • Benefits
    • Scalable
    • Supports lots of service levels
  • Drawbacks
    • No absolute guarantee of service
    • Complex configuration throughout network

QoS Implementation Methods

• CLI
  • Old school
  • Not used any more
• Modules QoS CLI (MQC)
  • Step 1: class-map
  • Step 2: policy-map
  • Step 3: service-policy
• AutoQoS
  • Automatically generates classes and policies based on traffic it sees
  • Super-simple
  • Requires CEF, NBAR, and correct bandwidth statements
• SDM QoS Wizard
  • Next, next, next
  • Can be used to implement, monitor, or troubleshoot QoS