CLASSIFICATION / MARKING

• The purpose of QoS is to give certain kinds of NETWORK TRAFFIC priority over other during congestion
• CLASSIFICATION organizes network TRAFFIC (PACKETS) into TRAFFIC CLASSES (CATEGORIES)
• CLASSIFICATION is fundamental to QoS.
  • To give PRIORITY to certain types of TRAFFIC, you have to IDENTIFY which types of TRAFFIC to give PRIORITY to.
• There are MANY methods of CLASSIFYING TRAFFIC
  • An ACL : TRAFFIC which is permitted by the ACL will be given certain TREATMENT, other TRAFFIC will not
  • NBAR (Network Based Application Recognition) performs a DEEP PACKET INSPECTION, looking beyond the LAYER 3 and LAYER 4 information up to LAYER 7 to identify the specific kinds of TRAFFIC
  • In the LAYER 2 and LAYER 3 HEADERS there are specific FIELDS used for this purpose
• The PCP (PRIORITY CODE POINT) FIELD of the 802.1Q Tag (in the ETHERNET HEADER) can be used to identify HIGH / LOW PRIORITY TRAFFIC
  • ** ONLY when there is a dot1q tag!
• The DSCP (DIFFERENTIATED SERVICES CODE POINT) FIELD of the IP HEADER can also be used to identify HIGH / LOW PRIORITY TRAFFIC

PCP / CoS

• PCP is also known as CoS (CLASS OF SERVICE)
• It’s use is defined by IEEE 802.1p
• 3 bits = 8 possible values (2^3 = 8)

• PCP VALUE 0:

  • “BEST EFFORT” DELIVERY means there is no guarantee that data is delivered or that it meets ANY QoS Standard. This is REGULAR TRAFFIC - NOT HIGH PRIORITY

• PCP VALUE 3 and 5:

  • IP PHONES MARK call signaling TRAFFIC (used to establish calls) as PCP3
    • They MARK the actual VOICE TRAFFIC as PCP5

• Because PCP is found in the dot1q header, it can only be used over the following connections:

  • TRUNK LINKS
  • ACCESS LINKS with a VOICE VLAN

• In the diagram below, TRAFFIC between R1 and R2, or between R2 and EXTERNAL DESTINATIONS will not have a dot1q tag. So, traffic over those links PCP cannot be marked with a PCP value.

THE IP ToS BYTE

(6 bits for DSCP and 2 bits for ECN)

IP PRECEDENCE (OLD)

• Standard IPP markings are similar to PCP:
  • 6 and 7 are reserved to ‘network control traffic’ (ie: OSPF Messages between ROUTERS)
  • 5 = VOICE
  • 4 = VIDEO
  • 3 = VOICE SIGNALLING
  • 0 = BEST EFFORT
• With 6 and 7 reserved, 6 possible values remain
• Although 6 values is sufficient for many NETWORKS, the QoS REQUIREMENTS of some NETWORKS demand more flexibility

DSCP (CURRENT)

• RFC 2474 (1998) defines the DSCP field, and other ‘DiffServ’ RFCs elaborate on its use

• With IPP updated to DSCP, new STANDARD MARKINGS had to be decided on

  • By having generally agreed upon STANDARD MARKINGS for DIFFERENT KINDS of TRAFFIC:
    • QoS DESIGN and IMPLEMENTATION is simplified.
    • QoS works better between ISPs and ENTERPRISES
    • etc.

• You should be AWARE of the FOLLOWING STANDARD MARKINGS:

  • DEFAULT FORWARDING (DF) - Best Effort TRAFFIC
  • EXPEDITED FORWARDING (EF) - Low Loss / Latency / Jitter TRAFFIC (usually voice)
  • ASSURED FORWARDING (AF) - A set of 12 STANDARD VALUES
  • CLASS SELECTOR (CS) - A set of 8 STANDARD VALUES, provides backward compatibility with IPP

DF / EF

DEFAULT FORWARDING (DF)

• Used for BEST EFFORT TRAFFIC
• The DSCP marking for DF is 0

EXPEDITED FORWARDING (EF)

• EF is used for TRAFFIC that requires Low Loss / Latency / Jitter
• The DSCP marking for EF is 46

ASSURED FORWARDING (AF)

• Defines FOUR TRAFFIC CLASSES
• ALL PACKETS in a CLASS have the same PRIORITY
• Within each CLASS, there are THREE LEVELS of DROP PRECEDENCE
  • HIGHER DROP PRECEDENCE = More likely to DROP the PACKET during CONGESTION

EXAMPLES:

• AF41 gets the BEST TREATMENT (Highest Priority / Lowest Drop)
• AF13 gets the WORST TREATMENT (Lowest Priority / Highest Drop)

CLASS SELECTOR (CS)

• Defines EIGHT DSCP values for backward compatibility with IPP
• The THREE BITS that were added for DSCP are set to 0, and the original IPP bits are used to make 8 values

RFC 4954

• RFC 4954 was developed with help of Cisco to bring ALL of these VALUES together and STANDARDIZE their use

• The RFC offers MANY specific recommendations, but here are a few KEY ones:

  • VOICE TRAFFIC : EF
  • INTERACTIVE VIDEO : AF4x
  • STREAMING VIDEO : AF3x
  • HIGH PRIORITY DATA : AF2x
  • BEST EFFORT : DF

TRUST BOUNDARIES

• The TRUST BOUNDARY of a NETWORK defines where the DEVICE TRUST / DON’T TRUST the QoS MARKINGS of received messages
• If the MARKINGS are TRUSTED:
  • DEVICE will forward the message without changing the MARKINGS
• If the MARKINGS are NOT TRUSTED:
  • DEVICE will change the MARKINGS according to configured POLICY

• If an IP PHONE is connected to the SWITCH PORT, it is RECOMMENDED to move the TRUST BOUNDARY to the IP PHONES
• This is done via CONFIGURATION on the SWITCH PORT connected to the IP PHONE
• If a user MARKS their PC’s TRAFFIC with a HIGH PRIORITY, the MARKING will be CHANGED (not trusted)

QUEUING / CONGESTION MANAGEMENT

• When a NETWORK DEVICE receives TRAFFIC at a FASTER PACE than it can FORWARD out of the appropriate INTERACE, PACKETS are placed in that INTERFACE’S QUEUE as they wait to be FORWARDED
• When a QUEUE becomes FULL, PACKETS that don’t FIT in the QUEUE are dropped (Tail Drop)
• RED and WRED DROP PACKETS early to avoid TAIL DROP

• An essential part of QoS is the use of MULTIPLE QUEUES

  • This is where CLASSIFICATION plays a role.
  • DEVICE can match TRAFFIC based on various factors (like DSCP MARKINGS in the IP HEADER) and then place it in the appropriate QUEUE

• HOWEVER, the DEVICE is only able to forward one FRAME out of an INTERFACE at once SO a SCHEDULER, is used to decide which QUEUE TRAFFIC is FORWARDED from the next

  • PRIORITZATION allows the SCHEDULER to give certain QUEUES more PRIORITY than others

• A COMMON scheduling method is WEIGHTED ROUND-ROBIN
  • ROUND-ROBIN:
    • PACKETS taken from each QUEUE in order, cyclically
  • WEIGHTED:
    • More DATA taken from HIGH PRORITY QUEUES each time the SCHEDULER reaches that QUEUE

• CBWFQ (CLASS BASED WEIGHED FAIR QUEUING)
  • Popular method of SCHEDULING
  • Uses WEIGHTED ROUND-ROBIN SCHEDULER while guaranteeing each QUEUE a certain PERCENTAGE of the INTERFACE’S bandwidth during CONGESTION

• ROUND-ROBIN SCHEDULING is NOT IDEAL for VOICE / VIDEO TRAFFIC
  • Even if VOICE / VIDEO TRAFFIC receives a guaranteed MINIMUM amount of BANDWIDTH, ROUND-ROBIN can add DELAY and JITTER because even the HIGH PRIORITY QUEUES have to wait their turn in the SCHEDULER

• LLQ (LOW LATENCY QUEUING)
  • Designates ONE (or more) QUEUES as strict priority queues
  • This means that if there is TRAFFIC in the QUEUE, the SCHEDULER will ALWAYS take the next PACKET from that QUEUE until it is EMPTY
  • This is VERY EFFECTIVE for reducing the DELAY and JITTER of VOICE / VIDEO TRAFFIC
  • HOWEVER, LLQ has a DOWNSIDE of potentially starving other QUEUES if there is always TRAFFIC in the DESIGNATED STRICT PRIORITY QUEUE
    • POLICING can control the AMOUNT of TRAFFIC allowed in the STRICT PRIORITY QUEUE so that it can’t take all of the link’s BANDWIDTH

SHAPING / POLICING

• TRAFFIC SHAPING and POLICING are both used to control the RATE of TRAFFIC

• SHAPING

  • Buffers TRAFFIC in a QUEUE if the TRAFFIC RATE goes over the CONFIGURED RATE

• POLICING

  • DROPS TRAFFIC if the TRAFFIC RATE goes over the CONFIGURED RATE
    • POLICING also has the option of RE-MARKING the TRAFFIC, instead of DROPPING
  • “BURST” TRAFFIC over the CONFIGURED RATE is allowed for a short period of time
  • This accommodates DATA APPLICATIONS which typically are “bursty” in nature (ie: not constant stream)
  • The amount of BURST TRAFFIC allowed is configurable

• In BOTH cases, CLASSIFICATION can be used to ALLOW for different RATES for different KINDS of TRAFFIC

• WHY would you want to LIMIT the RATE that TRAFFIC is SENT / RECEIVED ?