·
One header in all types of Packets
·
The IP header, LAN trunking header, Frame Relay
header, and ATM cell header all have at least one field that can be used to
perform some form of QoS marketing.
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Main topics covered are IP header IP Precedence
(IPP) and Differentiated Services Code Point (DSCP) fields.
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IPP Precedence & DSCP Compared
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IP header is defined in RFC 791
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It includes 1-byte field called the Type of
Service (ToS) byte
·
ToS byte used as field to mark a packet for
treatment with QoS tools
·
ToS byte further subdivided, with the high-order
3bits defined as the IP Precedence (IPP) field.
·
The complete list of values from the ToS byte’s
original IPP 3-bit field, and the corresponding names
|
Name
|
Decimal Value
|
Binary Value
|
|
Routine
|
Precedence 0
|
000
|
|
Priority
|
Precedence 1
|
001
|
|
Immediate
|
Precedence 2
|
010
|
|
Flash
|
Precedence 3
|
011
|
|
Flash Override
|
Precedence 4
|
100
|
|
Critic/Critical
|
Precedence 5
|
101
|
|
Internetwork Control
|
Precedence 6
|
110
|
|
Network Control
|
Precedence 7
|
111
|
·
ToS byte include the flag fields (from
bits 3 to 6)
o
They are toggled on or off to imply a particular
QoS service.
o
Flags were not used very often,
o
So the ToS byte’s main purpose was to hold the
3-bit IPP field
·
A series of RFCs collectively called Differentiated
Services (DiffServ) came along later.
o
Diffserv needed more than 3bits to mark packets,
o
So DiffServ standardized a redefinition of the
ToS byte
o
The ToS byte itself was renamed the Differentiated
Services (DS) field.
o
And IPP was replaced with a 6-bit field
(high-order bits 0-5) called Differentiated Services Code Point(DSCP)
field.
o
Later, RFC 3168 defined the lower-order 2 bits
of the DS field for use with the QoS Explicit Congestion Notification (ECN)
feature.
·
·
Above figure shows ToS byte’s format with the
pre-DiffServ and Post-DiffServ definition of the field.
·
C&M tools often mark DSCP or IPP because the
IP packet remains intact as it is forwarded throughout an ip network.
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