From: roy bustos (roybustos@xxxxxxxxxxx)
Date: Tue Feb 12 2002 - 18:24:48 GMT-3
DSCP (differentiated services code point) is a packet classification
criterion, like IP precedence.
from docs:
IP Precedence and IP DSCP Marking
Marking a packet with an IP precedence or IP DSCP marking allows users to
classify traffic based on an IP precedence or IP DSCP value, depending on
which value is marked. These marking can be used to identify traffic within
the network, and other interfaces can match traffic based on the IP
Precedence or DSCP markings.
IP Precedence and DSCP markings are also used to decide how packets should
be treated in Weighted Random Early Detection (WRED).
The IP DSCP value is the first 6 bits in the ToS byte, while the IP
Precedence value is the first 3 bits in the ToS value. Therefore, the IP
Precedence value is actually part of the IP DSCP value. For this reason,
both values cannot be set simultaneously. If both values are set at the same
time, the packet will be marked with the IP DSCP value.
If you need to mark packets in your network and all of the devices support
IP DSCP marking and matching, use the IP DSCP marking to mark your packets,
since the IP DSCP markings provide more packet marking options (64
individual values can be marked using IP DSCP marking, while only 8
individual values can be marked using IP precedence marking). If marking by
IP DSCP is somehow undesirable, however, or if you are unsure if the devices
in your network support IP DSCP values, use the IP precedence value to mark
your packets. The IP precedence value is likely supported by all devices in
the network.
A user can set up to 8 different IP precedence markings and 64 different IP
DSCP markings.
rate-limit command extension:
set-dscp-continue-Sets the differentiated services code point (DSCP) (0 to
63) and evaluate the next rate-limit command.
set-dscp-transmit-Sends the DSCP and transmit the packet.
http://www.cisco.com/warp/public/cc/so/neso/ienesv/cxne/nbar_wp.htm
Roy
----- Original Message -----
From: "Ahmed Mamoor Amimi" <mamoor@ieee.org>
To: <leah_lynch@lucent.com>; <ccielab@groupstudy.com>
Sent: Tuesday, February 12, 2002 12:41 PM
Subject: Re: CAR and WRED
> can u explain something about DSCP value .... what is that and wat is the
> value for use
>
> -Mamoor
>
>
> ----- Original Message -----
> From: Leah Lynch <leah_lynch@lucent.com>
> To: 'Ahmed Mamoor Amimi' <mamoor@ieee.org>; <ccielab@groupstudy.com>
> Sent: Wednesday, February 13, 2002 1:25 AM
> Subject: RE: CAR and WRED
>
>
> > That's pretty close:
> >
> > Short answer:
> >
> > CAR is a DiffServ packet marking/classification application based on
value
> > in TOS header. It is very configurable and allows you to change the
value
> of
> > the IP precedence bit or DSCP value based on conform or exceed
statements
> > which are configurable.
> >
> >
> > WRED is based on RED (that goes based on Nagle algorithm) where WRED
uses
> > the value of TOS header to drop packet proactively based on their weight
> > (kind of like WFQ) WRED will drop a percentage of packets once
congestion
> > has been detected on the interface. WRED is used to avoid congestion by
> > forcing TCP flows to backoff using TCP windowing.
> >
> > This doc explains a lot about the differences between the two.
> >
> >
>
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fqos
> > _c/qcfintro.htm
> >
> > This is also covered in Cisco IOS Quality of Service and the
Configuration
> > guide but a lot has changed since that was written (and since IOS 12.2).
I
> > personally like the Documentation CD contents best.
> >
> > Hope that helps.
> >
> > Leah Lynch
> >
> > -----Original Message-----
> > From: nobody@groupstudy.com [mailto:nobody@groupstudy.com]On Behalf Of
> > Ahmed Mamoor Amimi
> > Sent: Tuesday, February 12, 2002 11:42 AM
> > To: ccielab@groupstudy.com
> > Subject: CAR and WRED
> >
> >
> > my thinking about CAR and WRED :
> >
> > CAR
> > By CAR u can set normal and max burst then tell if the rate drop from
> > normal then apply precedence or drop or ATM-CLP or QoS or trasmit.
> > General assumption:
> > set min xxx
> > max xxx
> > if min<flow of packet
> > then
> > drop or clp or prec or qos or transmit
> > end
> >
> >
> > WRED
> > Use the random-detect command to enable WRED, which randomly discards
> > packets during congestion based on IP precedence settings
> > General assumption:
> > if RED<flow of packet
> > then
> > {(set precedence 0-7) and (threshold 1-4096)}
> > or
> > set precedence to rsvp packets
> >
> >
> >
> > my thinking is that they both are some what same but CAR is more
flexible
> as
> > it gives u the upper and lower bond of limit
> > to traffic.
> >
> >
> > -Mamoor
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