QOS Question - TOS/DSCP

Unanswered Question
Sep 25th, 2007


I notice that the TOS bits are in the same spot in the "tos byte" that the DSCP bits are in....

how does the router differentiate between TOS vs DSCP being set?

Is there a bit somewhere in the packet that makes the router decide that the first 3 bits in the "tos byte" are either TOS or DSCP?



I have this problem too.
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IanTarasevitsch1995 Tue, 09/25/2007 - 09:10

Nope. Bits is bits. It's all in how the router chooses to read them.... DSCP combines the old TOS and IPP fields into one larger field. You cannot do both at once, with the exception that the IPP values map directly to DSCP Class Selectors (CS6 through CS1).



lgontarsk Wed, 09/26/2007 - 05:39

very cool, thank you.

So basically one router will set the bits based on whether its coded to mark TOS or DSCP, and another separate router will interpret the bits as whatever it is configured for, TOS or DSCP?

Basically its important than that all routers are configured for either TOS OR DSCP, and that they agree with eachother.


Lisa G

IanTarasevitsch1995 Wed, 09/26/2007 - 16:24

Absolutely. For the most part, TOS has been deprecated. At Layer 2, it has been replaced with 802.1Q/p Class of Service (CoS) marking. At Layer 3 in a DiffServ environment, the equivalent is the IP Precedence (IPP) field. DiffServ uses the entire TOS byte (including the IPP bits) to create both the DSCP code point marking (6 bits, 0-63), and the last two bits for Explicit Congestion Notification (ECN), kinda like you'd see in a Frame Relay cloud. If you're running Differentiated Services for your QoS, you should running DSCP at Layer 3 and (if you need it) CoS at Layer 2.

For the record, there are 14 standard DSCP code points for Per Hop Behavior (PHB), plus IPP level 1 through 6 map directly to Class Selectors, special DSCP code points with the last three bits set to all zeros.

Of course, your upstream ISP will most likely remark your traffic once it hits his administrative domain, but that's to be expected.

Tim Szigeti and Christina Hattingh co-wrote an excellent QoS book that ties it all in together. It has been well worth the money for me as both a teaching and reference source.



lgontarsk Thu, 09/27/2007 - 05:35

Thank you, Ian. That was very helpful.

Isn't it interesting... I just bought their QOS book, and started reading it. That book is precisely what made me ask the question... I started reading and thinking about it. :-)

Thanks again!

Lisa G


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