QoS, DSCP and IPprecedence backward compatibility

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Apr 4th, 2008
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I don't have a clear concept of backward compatibility.


Suppose that node A is DSCP capable and node B is IPprecedence capable only.

What happens if A sends frames to B, marking those using a specific DSCP value (ex. AF11)?

1) Node B maps AF11(001010) marked frames to IP-Precedence value equal to 1 or

2)Node B will consider the priority of the frames, only if A is marking using Class Selector (CS1 in this case [010000])

3) Neither 1 or 2....(your explanation here ;))

I found the behaviour in the inverse direction (B to A) but nothing about a to B

Here's the behaviour of A to B:

Class selector DSCPs are values that are backward compatible with IP precedence. When converting between IP precedence and DSCP, match the three most significant bits. In other words:

IP Prec 5 (101) maps to IP DSCP 101 000



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Joseph W. Doherty Fri, 04/04/2008 - 18:11
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What might help clarify this is to realize that the ToS byte and DiffServ Field is the same byte/field, it's just the interpretation of the bits differ between the older and newer standards. The newer standard, DSCP, attempts to use the first 3 bits, those used by IP Precedence, in much the same way, for the packet's priority class. This so packets might be treated alike whether using the older standard or the newer standard.

Second, also realize, it's left to the equipment whether it can support either standard, and to the configuration whether anything is done with this byte on the equipment.

By default, this byte should pass through all equipment as set by the source. However, each device that understands the byte, might or might not adjust the packet's processing, and/or might or might not change the value. This regardless of whether the old or new standard is being used.

"What happens if A sends frames to B, marking those using a specific DSCP value (ex. AF11)?"

The answer is:

"3) Neither 1 or 2....(your explanation here"

Node B, if using IP Precedence, will see the packet as an IP Precence 1. But it could also see it also marked for "high throughput".


BTW: Cisco supported using one more ToS bit for min-monetary-cost.

Marcofbbr Sat, 04/05/2008 - 02:29
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Hi Joseph,

Thanks for your comment.

About you answer, you clarified to me that CS marking is not stictly necessary for backward-compatibility.

But I partially agree with your explanation

" it could also see it also marked for high throughput"

It sounds quite bad to me. An IP-Precedence only node, without considerering how it reacts to the last 3 bit, should assign the priority reading the first 3 bits only.

Do you have any reference?

Marcofbbr Sat, 04/05/2008 - 02:02
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Ops, I made a mistake..

"Here's the behaviour of A to B: " is "Here's the behaviour of B to A: "

Joseph W. Doherty Sat, 04/05/2008 - 04:37
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The official reference for ToS would be found in RFC791. (There's a link to it in your reference.)

Now somewhat confused, whether you want A to B or B to A. Your question all seemed to be a DSCP marked packet being seen on a non-DSCP device.

If there's still any confusion, feel free to ask another question.


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