Aaron's Worthless Words

Here are some more notes from my studies.  Of course, no one cares about them but me, but it’s my blog.  I’m sure someone will find it useful.  Please help to correct dumbass mistakes.

• 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

Aaron Conaway

I like to lean my head to the left, hit it with the palm of my right hand, and document what knowledge falls out.

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Here’s another set of notes from my ONT studies.  I’m sure someone will find it useful.  Please help to correct dumbass mistakes.

• 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

Aaron Conaway

I like to lean my head to the left, hit it with the palm of my right hand, and document what knowledge falls out.

More Posts - Website

I’ll try to keep it a little shorter this time.

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

Aaron Conaway

I like to lean my head to the left, hit it with the palm of my right hand, and document what knowledge falls out.

More Posts - Website