·
Several DiffServ RFCs suggest a set of values to
use in the DSCP field.
·
RFC 2598 defines a DSCP of decimal 46, with a
name Expedited Forwarding (EF).
o
According to this RFC,
o
during periods When thigh-priority traffic
reaches or exceeds the interface bandwidth,
o
packet marked as EF should be given queuing
preference
o
so that
they experience minimal latency,
o
But the packets should be policed to prevent
them from taking over a link
o
& preventing any other types of traffic from
exiting an interface
·
These suggested settings, and the associated QoS
behaviour recommended when using each setting, are called Per-Hop Behaviours
(PHBs) by DiffServ.
·
Above example is called Expedited Forwarding PHB
·
Class Selector PHB and DSCP values
o
IPP overlaps with the 1st 3bits of
the DSCP field
§
Because the DS field is simply a redefinition of
the original ToS byte in the IP header.
o
Because of this overlap, RFC 2475 defines a set
of DSCP values and PHBs, called Class Selector(CS) PHBs,
§
That provides backward compatibility with IPP.
o
A C&M feature can set a CS DSCP value, and
if another router or switch just looks at the IPP field,
§
The value will make sense from an IPP perspective.
o
Below table lists the CS DSCP names &
values, and the corresponding IPP values and names.
|
DSCP Class Selector Name
|
Binary DSCP Values
|
IPP Binary Values
|
IPP Names
|
|
Default/CS0
|
000000
|
000
|
Routine
|
|
CS1
|
001000
|
001
|
Priority
|
|
CS2
|
010000
|
010
|
Immediate
|
|
CS3
|
011000
|
011
|
Flash
|
|
CS4
|
100000
|
100
|
Flash Override
|
|
CS5
|
101000
|
101
|
Critic/Critical
|
|
CS6
|
110000
|
110
|
Internetwork Control
|
|
CS7
|
111000
|
111
|
Network Control
|
o
CS PHB define above eight DSCP values their text
names ,
o
It also suggests a simple set of QoS action that
should be taken based on the CS values.
o
The CS PHB simply states that packets with
larger CS DSCPs should be given better queuing preference than packets with
lower CS DSCPs.
·
Assured Forwarding PHB & DSCP Values
o
The Assured Forwarding (AF) PHB (RFC 2597)
defines four classes for queuing purposes,
o
Along with three levels of drop probability
inside each queue.
o
To mark packets and distinguish into which of
four queues a packet should be placed,
o
along with one of three drop priorities inside
each queue,
o
the AF PHB defines 12 DSCP values their
meanings.
o
The name of the AF DSCPs confirm to the
following format:
o
AFxy
§
Where x implies one of four queues(values 1
through 4),
§
y implies one of three drop priorities(values 1
through 3)
·
The AF PHB suggested that
o
The higher the value of x in the DSCP name AFxy,
§
The better the queuing treatment a packet should
get.
o
The higher the value of y in the DSCP name AFxy,
§
The worse the drop treatment for those packets.
§
(treating a packet worse for drop purposes means
that the packet has a higher probability of being dropped)
·
Assured Forwarding DSCP values-Names,
Binary Values, and Decimal Values
|
Queue Class
|
Low Drop Probability
|
Medium Drop Probability
|
High Drop Probability
|
|
|
Name/Decimal/Binary
|
Name/Decimal/Binary
|
Name/Decimal/Binary
|
|
1
|
AF11/10/001010
|
AF12/12/0011001
|
AF13/14/001110
|
|
2
|
AF21/18/010010
|
AF22/20/010100
|
AF23/22/010110
|
|
3
|
AF31/26/011010
|
AF32/28/011100
|
AF33/30/011110
|
|
4
|
AF41/34/100010
|
AF42/36/100100
|
AF43/38/100110
|
·
A text AF PHB names do not follow the
“bigger-is-better” logic in all cases
o
AF11 is better than AF13 if both are in same queuing class,
§
AF11 has lower probability of being dropped than
AF13
·
The binary version of the AF DSCP values shows
the patterns of values.
o
1st 3 bits of Binary DSCP values
designate queue class(bits 0-2, left to right)
o
Next two bits (bits 3 & 4) designate the
drop preference
o
As a result, queuing tools that operate only on
IPP can still react to the AF DSCP values,
§
Essentially making the AF DSCPs backward
compatible with non-DiffServ nodes for queuing process.
o
Convert from the AF name to the decimal
equivalent use simple following formulae
§
AF31
§
8x + 2y = decimal value
§
8*3 + 2*1 = 26
·
Expedited
(speed & efficient)Forwarding
PHB & DSCP Values
§
RFC 2598 defines the Expedited Forwarding (EF)
PHB,
§
This RFC defines a very simple pair of PHB
actions
o
Queue EF packets so that they get scheduled
quickly, to give them low latency
o
Police (regularly watch & guard) the EF packets
so that they do not consume all bandwidth on the link or starve other queues.
§
The DSCP value defined for EF is named EF, with
decimal value 46, binary value 101110.
