QoS

Terms

FIFO — First in, First out

• Default behavior

Differentiated Services

• Giving packet flows different levels of network service
• Based on classification
• AKA, DiffServ

Integrated Services

• Packet flows explicitly reserve bandwidth along a path
• Uses admission control
• AKA, IntServ

RSVP — Resource Reservation Protocol

• An IntServ Implementation

Marking

• Changing the DSCP bits in the IP header field

DSCP — Differentiated Services Code Point

• The markings of an IP packet that allows DiffServ

PHB — Per Hop Behavior

• What a node should or shouldn’t do with marked traffic.

Queuing

• Hold a packet in memory
• Delays transmission
• Expensive, because memory is expensive

LLQ — Low latency queuing

• Describes queue behavior for the EF PHB:
  • Never drop
  • Never delay
  • Send immediately
  • Police aggressively

EF — Expedited Forwarding

• The highest PHB for network data

Control Traffic

• Routing Traffic
  • OSPF, EIGRP, IS-IS, BGP, etc

CAR — Committed Access Rate

• Contractual data rate a traffic source will flow at
• SLA oriented

SLA — Service Level Agreement

• A Business agreements about data servicing requirements

WFQ — Weighted Fair Queuing

• Legacy default on serial interfaces at E1 speeds and below ~2.048 Mbps
• Sorts traffic into high bw and low bw classes.

CBWFQ — Class Based Weighted Fair Queuing

• AKA, Modular QoS
• Multiple queues
• Bandwidth limits
• Different kinds of queues, like LLQ

MQC

• Modular QoS CLI

PQ — Priority Queue

• A queue that is served first, even if other queues have been waiting longer
• An LLQ is an example of a PQ

Type of Service

How these 8 bits get used has changed over the years.

                   0 1 2 3 4 5 6 7
                  ┌─────┬─────┬─┬─┐
   RFC 791 (1981) │IP Pr│ ToS │0│0│
                  └─────┴─────┴─┴─┘

                   0 1 2 3 4 5 6 7
                  ┌─────┬───────┬─┐
  RFC 1349 (1992) │IP Pr│  TOS  │0│
                  └─────┴───────┴─┘

                   0 1 2 3 4 5 6 7
                  ┌───────────┬─┬─┐
  RFC 2474 (1998) │    DSCP   │0│0│
                  └───────────┴─┴─┘

                   0 1 2 3 4 5 6 7
                  ┌───────────┬───┐
  RFC 3168 (2001) │    DSCP   │ECN│
                  └───────────┴───┘

PHB - Per Hop Behaviors

Assured Forwarding

Assured Forwarding PHB Group

AF uses the first 6 bits to create 4 traffic classes, 4 is best.

Within those classes, there is a drop precedence, or … at what point of queue congestion should this traffic be dropped.

Used for RED, or WRED.

Four AF classes, each should get it’s own resources.

Drop

 Precedence      Class 1        Class 2        Class 3       Class 4

            ┌───────────────┬───────────────┬───────────────┬──────────────┐
   Low    │ │ AF11  001 010 │ AF21  010 010 │ AF31  011 010 │ AF41 100 010 │
   Medium │ │ AF12  001 100 │ AF22  010 100 │ AF32  011 100 │ AF42 100 100 │
   High   ▼ │ AF13  001 110 │ AF23  010 110 │ AF33  011 110 │ AF43 100 110 │
            └───────────────┴───────────────┴───────────────┴──────────────┘
             ────────────►  Importance  to Business/Net work ───────────►

Again, with DSCP

Drop

 Precedence      Class 1        Class 2        Class 3       Class 4

            ┌───────────────┬───────────────┬───────────────┬──────────────┐
   Low    │ │ AF11  DSCP 10 │ AF21  DSCP 18 │ AF31  DSCP 26 │ AF41 DSCP 34 │
   Medium │ │ AF12  DSCP 12 │ AF22  DSCP 20 │ AF32  DSCP 28 │ AF42 DSCP 36 │
   High   ▼ │ AF13  DSCP 14 │ AF23  DSCP 22 │ AF33  DSCP 30 │ AF43 DSCP 38 │
            └───────────────┴───────────────┴───────────────┴──────────────┘
             ────────────►  Importance  to Business/Network ───────────►

Yields the following formula.

DSCP = 8 (class) + 2 (drop)

QoS Consequences

LAN QoS with voice (buffer management)

• One voice packet, no voice, but modem will retrain
• Two voice packets, audio clip, fax call disconnection.
• VoIP QoS cannot be fixed by adding bandwidth. You simply cannot drop these
• packets.

QoS Commands

RFC 4594 — DiffServ Service Classes

Source: RFC 4594 (Aug 2006), updated by RFC 5865 and RFC 8622. AF drop precedence: x1=low, x2=medium, x3=high drop probability.

References

An Architecture for Differentiated Services