WRED "dscp-based" uses AF Drop Prob bits?

[PAUL TRIVINO]

I am trying to configure WRED for traffic classes, and am using DSCP for my marking (as opposed to ip precedence). The QOS-Plus class notes describe bits 4-5 (from the left) of the 6-bit DSCP value as "drop probability". If I use DSCP values with different drop probabilities, will this automatically be used by WRED to control drops?

For example, if I use AF41 for voice, AF32 for business apps and AF23 for default/best effort, are these automatically going to be treated as less-to-more drop-likely?

Thanx.

[ariela]

Hi,

logically, I think yes ... but I don't know how, "automagically"? :)

Hope a guru could help us

Regards

Andrea

[PAUL TRIVINO]

Thanx, Andrea. I am thinkiing of "random-detect dscp-based" in an output policy - is it really DSCP based?

[AhsanMohiuddin]

hi guys,

you should use the "set ip dscp" policy-map configuration command to set the DSCP to AF41, AF32 and so on.

There are three steps to making WRED work on an interface.

1) Set WRED thresholds: On a multilayer switch (such as 3550), use the

" wrr-queue random-detect max-threshold [..] "

interface configuration command to set WRED Thresholds for each queue.

2) Associate Class of Service (CoS) value with egress interface queues / WRED thresholds:

" mls qos cos-map "

3) Map internal DSCP value to CoS:

" mls qos map dscp-cos "

Once you have completed above steps, this is how things will happen.

A) Policy map will classify traffic streams, and markdown their DSCP to your requirement: voice traffic as AF41, Business Apps as AF32 and so on. Note that this new value overwrites the original DSCP value in the IP Header (now called Internal DSCP).

B) The queues on egress interface have WRED enabled and thresholds configured (done in step 1).

C) When a packet arrives at egress interface, it needs to be encapsulated inside a frame, so the CoS bits need to be set. The CoS value will be re-generated using the (internal) DSCP in the IP Header.

D) Once CoS value for each frame is available, the egress interface will put frames into different queues and thresholds as configured in step 3.

Once these steps are completed, WRED should start using the Drop Probability for conforming traffic to its thresholds values. For example, AF41 may translate to CoS 5, which may in turn be queued on queue 4, threshold 2 (thereby getting max priority)

The default config may work well for you. Change anything only if you need to.

Hope this post helps. (Please rate all helpful posts.)

Regards,

Ahsan Mohiuddin

Field Service Engineer

Societe Intl de Telecom Aeronautics

+92.333.206.3831

GMT+5

[ariela]

Thanks Ahsan,

but our question is general. What exactly the AF "drop probability", and when it works.

Any advice will be appreciated.

Regards

Andrea

[AhsanMohiuddin]

Hi Andrea,

ok, this time I will try to be brief, and to the point =)

The IP Header has a DSCP Byte. This is how its bits look like.

aaa.bbb.cc

cc are unused (always set to 0)

aaa are DSCP CLASS SELECTOR bits. Its values are interpreted as follows:

aaa Class

000 Default

001 AF1

010 AF2

011 AF3

100 AF4

101 EF

110 -

111 -

The bits bbb are called DSCP DROP PRECEDENCE bits. The least significant 'b' is always set to zero. Each

DSCP class has 3 possible Drop Precdence values.

bbb Drop Precedence

010 Low

100 Medium

110 High

Combined with Class Selector (aaa) bits, the Drop Precedence bits would look like this: for example, if aaa= 011 (AF3) and drop precedence is 010 (Low), the aaa.bbb pair is = 011 010. The corresponding DSCP code will be AF31.

If Drop precedence for same class (AF3) was set to 100 (Medium), the resultant DSCP code will be AF32.

Now there are two ways a QoS-capable device treats these DSCP bits in an IP header, 1) based on its class 2) within the class, the packet's drop precedence.

For example, a class AF4 packet will receive better Quality of service as compared to class AF3 packet.

HOWEVER, in case of contention WITHIN class AF4, a packet with DSCP code AF41 (drop precedence = Low) is LESS likely to be dropped than packet with DSCP code AF42 (drop precedence = Med)

Hope this helps.

Regards,

Ahsan Mohiuddin

Field Service Engineer

Societe Intl de Telecom Aeronautics

+92.333.206.3831

GMT+5

[ariela]

Hi Ahsan,

thanks for your answer.

you said:

"HOWEVER, in case of contention WITHIN class AF4, a packet with DSCP code AF41 (drop precedence = Low) is LESS likely to be dropped than packet with DSCP code AF42 (drop precedence = Med)"

but WRED on AF4, for example, drops more likely a packet with DSCP code AF42 than AF41 automatically, or we have to do something?

Hope my question will be clear

Regards

Andrea

[PAUL TRIVINO]

Ahsan/Andrea: thanx for your input(s). It sounds from Ahsan's answers (and I fully understand DSCP encoding etc.) that the DSCP simply allows more granular classification of traffic. I say this becasue, IF the Drop Probability bits automatically controled WRED (et. al.), then we would not have to manipulate queue thresholds etc.

Thanx!

[ariela]

ptrivino,

I agree with you.

I've lost the right way :)

Tnx

Andrea

[AhsanMohiuddin]

bah! I think I am very good at explaining things really badly =P

Consider this example:

policy-map Germany2006

class France

set ip dscp AF41

class Portugal

set ip dscp AF42

class Germany

set ip dscp AF31

exit

mls qos map dscp-cos 41 to 5

mls qos map dscp-cos 31 to 5

mls qos map dscp-cos 42 to 3

wrr-queue random-detect max-threshold 2 50 75

wrr-queue cos-maps 2 2 4-5

wrr-queue cos-maps 2 1 1-3

AS YOU CAN SEE, its all in your control how you want the DSCP bits to be used. Here, class France (DSCP= 41) and Portugal (DSCP=42) have better DSCP classes (class 4) than Germany (DSCP=31).

HOWEVER, later in the config, we are translating DSCP 41,31 to CoS=5, and DSCP 42 to CoS=3 (which is to say that, for the course of this config, Drop Precedence bits are more important to us than DSCP class bits) (NOTE: DSCP 41,31 have LEAST Drop Precedence within their respective classes)

As a result, Portugal will get assigned to Queue 2, threshold 1, while Germany/France get Queue 2, Threshold 2 (which means higher priority for GER/FRA compared to POR)

As you can see, nothing happens automatically really. Everything is configurable. And which packets are dropped and which are given priority, depends on what configuration you have.

Please ask if you dont understand any thing

HTH,

Ahsan Mohiuddin

Field Service Engineer

Societe Intl de Telecom Aeronautics

+92.333.206.3831

GMT+5