End-to-end QoS behavior

The QoS settings on each network device must be aligned to achieve the desired end-to-end QoS behavior for a network. Three service types can be used to categorize and prioritize network traffic:

  • Best Effort Service
  • Ethernet Class of Service (CoS)
  • Internet Differentiated Services (DiffServ)

For a network as a whole, it is best to select one service type to use as the primary end-to-end behavior, and then use the other two service types as needed.

Best effort service

This is the simplest service type. All traffic is treated equally in a first-come, first-served manner. If the traffic load is low in relation to the capacity of the network links, then there is no need for the administrative complexity and costs of maintaining a more complex end-to-end policy. This is sometimes called over-provisioning, as all link speeds are much higher than peak loads on the network.

Class of Service

Class of Service (CoS) is a method for classifying network traffic at layer 2 by marking 802.1Q VLAN Ethernet frames with one of eight service classes.

CoS

Traffic type

Example protocols

7

Network Control

STP, PVST

6

Internetwork Control

BGP, OSPF, PIM

5

Voice (<10ms latency)

VoIP(UDP)

4

Video (<100ms latency)

RTP

3

Critical Applications

SQL RPC, SNMP

2

Excellent Effort

NFS, SMB

0

Best Effort

HTTP, TELNET

1

Background

SMTP, IMAP

CoS 1 is deliberately set as the lowest CoS. This enables a traffic service level below the default (best effort) traffic level to be specified.

The 3-bit Priority Code Point (PCP) field within the 16-bit Ethernet VLAN tag is used to mark the CoS.

+--------+--------+--------+----------+-----------+-------- | mac-da | mac-sa | 0x8100 | VLAN tag | ethertype | data... +--------+--------+--------+----------+-----------+-------- / \ / \ / \ +-----+-----+---------+ | pcp | dei | vlan_id | +-----+-----+---------+

Differentiated services

Differentiated services (DiffServ) is a method for classifying network traffic at layer 3 by marking packets with one of 64 different service classes. Services classes are identified by the Differentiated services Code Point (DSCP) value. Some common DSCP values are:

DSCP

Name

Service class

RFC

56

CS6

Network Control

2474

46

EF

Telephony

3246

40

CS5

Signaling

2474

34, 36, 38

AF41, AF42, AF43

Multimedia Conferencing

2597

32

CS4

Real-Time Interactive

2474

26, 28, 30

AF31, AF32, AF33

Multimedia Streaming

2597

24

CS3

Broadcast Video

2474

18, 20, 22

AF21, AF22, AF23

Low-Latency Data

2597

16

CS2

OAM

2474

00

CS0,BE,DF

Best Effort

2474

10, 12, 14

AF11, AF12, AF13

Bulk Data

2597

08

CS1

Low-Priority Data

3662

DSCP CS1 (08) CoS 1 is deliberately set as the lowest priority. This enables a traffic service level below the standard (best effort or default forwarding) level to be specified.

The DSCP value is carried within the IPv4 DSCP field or the upper 6-bits of the 8-bit IPv6 Traffic Class (TC) field.

IPv4

+----+-----+----+----+---+-------+----+------+-------+------+------+-------

|ver |dscp |ecn |len |id |offset |ttl |proto |chksum |ip-sa |ip-da | data..

+----+-----+----+----+---+-------+----+------+-------+------+------+-------

+------+-----+

| dscp | ecn |

+------+-----+

\ /

\ /

IPv6 \ /

+-----+-----+-----+-------+-------------+-----------+-------+-------+--------

| ver | tc | len | label | next_header | hop_limit | ip-sa | ip-da | data...

+-----+-----+-----+-------+-------------+-----------+-------+-------+--------