Understanding IP Precedence, TOS & DSCP

People using NetFlow Analyzer
wonder, what these reports DSCP, TOS actually means ? Here is the
blog which explain more detail about these fields.

The NetFlow packets exported from the
device originally contains ToS value on each flow. From the ToS
value, the analyzing software derives the DSCP.

Type of Service (TOS):-

The Type of Service field is present in
IP Header and it was originally defined in RFC 791.

The Type of Service octet consists of
three fields. The last 3 bits ( 7,6,5) are for the first field,
labeled “Precedence” , intended to denote the importance
or priority of the datagram. The second field, labeled “TOS”
, denotes how the network should make tradeoffs between throughput,
delay, reliability, and cost.The first field, labeled “MBZ”
( for “must be zero” ) above, is currently unused. The
originator of a datagram sets this field to zero (unless
participating in an Internet protocol experiment which makes use of
that bit). Routers and recipients of datagrams ignore the value of
this field. This field is copied on fragmentation.

DSCP(Differentiated Service Code
Point)

The definition to ToS was entirely
changed in the RFC 2474 and it is now been called as Differentiated
Service (DS). On the 8 fields, the upper 6 bit contains value called
Differentiated Service Code Point(DSCP). The last 2 bits are used for
Explicit Congestion Notification and it is defined in RFC 3168.

Given below are the interpretation
of ToS and DSCP fields:

DSCP interpretation (RFC2474) :

RFC791/1349:


In DSCP the most
significant three bits are directly proportional to IP Precedence,
The below table explains on DSCP to IP precedence Conversion table.

DSCP Name DS Field Value IP Precedence
Binary Decimal
CS0 000 000 0 0
CS1 001 000 8 1
AF11 001 010 10 1
AF12 001 100 12 1
AF13 001 110 14 1
CS2 010 000 16 2
AF21 010 010 18 2
AF22 010 100 20 2
AF23 010 110 22 2
CS3 011 000 24 3
AF31 011 010 26 3
AF32 011 100 28 3
AF33 011 110 30 3
CS4 100 000 32 4
AF41 100 010 34 4
AF42 100 100 36 4
AF43 100 110 38 4
CS5 101 000 40 5
EF 101 110 46 5
CS6 110 000 48 6
CS7 111 000 56 7

NetFlow Analyzer
DSCP and TOS Reports:-

NetFlow Analyzer
reports on both DSCP and TOS, the DSCP report generated from NetFlow
Analyzer will show each DSCP marking traffic utilization and
application and conversation marked for DSCP.

The TOS report
will show corresponding ToS values for each DSCP.

Click here to know about Cisco recommended QoS base lines. 

For 30 Day Trial, click Download now.

Thanks and Regards

Praveen Kumar

You Can Learn More About the ManageEngine Product Line By Going to www.ManageEngine.ca

The original article/video can be found at Understanding IP Precedence, TOS & DSCP

About the Author: Shannon Lewis

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