DSCP value

[sarahr202]

Hi everybody

I hope you guys are doing fine.  I got stumped by dscp value.

Let say we have different traffics that we have mapped to different dscp values;

A packet received with dscp " EF, and another packet is received with dscp CS7

Which packet  has higer priority ( assume default seeting)?

+++++++++++++++++

A packet is received with dscp "ef" and another packet received with cs5.

which packet hashiher priority i.e the packet received with ef or cs7?

thanks

[Jon Marshall · Accepted solution]

Hi Sarah

Hope you are doing well.

The answer to your question is it depends. If you are setting the QOS policy ie. you have not used auto QOS for example, it us up to you as to which gets preferntial treatment.

It is true that certain markings are expected to get a certain treatment but it is still down to what you actually configure on the device ie. you could assign all EF packets to the class default if that is what you wanted to do but usually it would be assigned to the priority queue to be given the best priority.

Not trying to avoid the question, i'm just trying to point out that although there are conventions it still comes down to how you configure the QOS policy.

Jon

[sarahr202]

Thanks Jon.

[John Blakley · Accepted solution]

Sarah,

In addition to what Jon stated, we have a qos agreement with our provider over our MPLS circuits. They've reversed the process, so anything that I mark with EF, they remark to CS1 on their side through their cloud. I'm assuming that they do this to keep customers from arbitrarily marking packets to see if they can get preference through the network. So, yes, the marking can be treated however you want. For example, if you have a marking of EF, you could choose to match on that and give it preference or match on it and drop the traffic; it's up to you.

HTH,
John

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[sarahr202]

Thanks John

[Joseph W. Doherty · Accepted solution]

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Posting

Default ToS treatment often is no difference in treatment for any ToS markings.  Sometimes, defaults will treat IPPrec 7 or 6 better than other markings.

As noted by other posters, usage of ToS is totally as the discretion of network it's being used on.  That said, there are RFC recommendations on how the different markings might be used.  RFC recommendations have changed over time, as ToS has changed over time.

For example, the earlier IP Precedence RFC did "prioritize" higher numbers over lower numbers, so IPPrec 7 was the highest priority and recommended only for traffic critical to the operation of the network.  IPPrec 6 was recommended for traffic important to the operation of the network (and its often used by routing protocols).  IPPrec 5 through 0 were recommended for non-network (user) traffic.

Later, RFCs revised ToS to support DSCP.  For backward compatibility, DSCP CS7 and CS6 are used for the same purposes as IPPrec 7 and 6.  DSCP 0 was used as the "default" for Best Effort (as most user traffic has that marking).  RFCs, though, for AFs, EF, and scavenger traffic, are a little more complicated than IPPrec 5 though 1, but except for scavenger, the recommendations assume DSCP might be processed by IPPrec standard network devices, so higher CS numbers usually get "better" treatment than lower CS numbers, but that's not really required for AF treatments.

Again, though, defaults are often no special treatments, or only special treatments for "network" marked related traffic.

[sarahr202]

Hi JosephDoherty.

First of all thanks for your response.

Let say we have a router that understand dscp markings, Let further assume the router is following rfc recommendations. Our router receives two packets ,p1 and p2. p1 is marked with dscp ef and the other is cs7. Which packet will get better priority treatment based on rfc recommendations?

What is the rfc that deals with dscp recommendations?

Thanks in advance.

[Joseph W. Doherty · Accepted solution]

Disclaimer

The  Author of this posting offers the information contained within this  posting without consideration and with the reader's understanding that  there's no implied or expressed suitability or fitness for any purpose.  Information provided is for informational purposes only and should not  be construed as rendering professional advice of any kind. Usage of this  posting's information is solely at reader's own risk.

Liability Disclaimer

In  no event shall Author be liable for any damages whatsoever (including,  without limitation, damages for loss of use, data or profit) arising out  of the use or inability to use the posting's information even if Author  has been advised of the possibility of such damage.

Posting

There are many RFCs that deal with QoS and DSCP markings.  Some supercede others, some append to others.  It would take me some time to find them all, but if you search the Internet for RFC and DSCP you should find some, and often they will reference prior RFCs.

I'm unsure there's a specific recommendation for CS7 vs. EF. The former would seem the most important, as it might be used for some error condition, within the network, that the networks needs to "know" for correct operation.  The latter, if delayed or dropped, might break a fragile real-time flow.

If you look at Cisco's baseline model (http://www.cisco.com/en/US/technologies/tk543/tk759/technologies_white_paper0900aecd80295a9b.pdf [also has some RFCs noted]), they don't account for CS7.

Cisco's recommendation does account for CS6, and that they treat not at well as EF.