Sunday, January 10, 2010

QoS, Classification and Marking

Wow, 2 days worth of notes to write on here so I will probably just get started.  Again, they probably will not help anyone else out, but I found that if I watch videos/read, and take notes while doing it I get pretty good results.  But if I take those notes, and let them sit for a few days while I ponder them, and them place them into this blog I retain the information much better.  So here we go.

ToS, or type of service markings are used primarily as L3 markings, whereas Cos, or Class of service is L2.
Here are the bits/meanings of them:

Sorry, not the best chart I have ever made up, but this was the interpretation I got from the videos that I watched.  I will see what the exam study guide says bout it in the next couple of days.  Some common L2 markings are as such:
  • Frame Relay Discard Eligible (DE) bit (a 1 or 0, 0 if not eligible for discard, 1 if it is)
  • MPLS experimental bit
  • Ethernet trunk CoS (3 bits)
L3 markings are these two:
  • DSCP (differentiated services code point)
  • IP precedence (older)
So, ToS markings basically can be carried from router to router as they are L3 markings that will survive the hop, unlike L2 markings.  To alleviate some of this, a conversion is done from L2 CoS to L3 ToS, such as a CoS to DSCP conversion.  Traditionally IP precedence could use up to 8 bits, however only used the left-most 3 out of the eight.  DSCP took this a bit further saying "hey, we can use all 8 of those and have some more granular results."  What this became is this:

000       |         000       |        00
PHB (per-hop behavior)       |    drop reliability     |        flow control

Now DSCP is backwards-compatible with ip precedence because of those 3 left-most bits.  Where DSCP differs itself is with the next 3 bits, the drop reliability.  If you look back to that chart above you will see that decimal values 7 and 6 are reserved for network traffic, and that each 0-5 are assigned named values like assured or expedited forwarding.  Now here higher is better; i.e. 101 or 5 is the best rating the packet can get according to the RFC.  000 being the worst.  Drop probability on the other hand is opposite...higher is worse.  Now drop reliability does NOT use all 3 of those bits...only the two left-most ones, so there are three possible combinations:
  • 11x (High drop packet)
  • 10x (Medium drop packet)
  • 01x (Low drop)
So these two fields combined make up your DSCP value.  So if you have a
AF13, you have assured forwarding 1, with a drop rating of high
AF41, you have assured forwarding 4, with a drop rating of low
If a router had to choose one to drop it would drop AF13 every time because it has a lower rating in both the IP precedence and drop rating.

They can also be viewed as decimal values:
10 = 001 010 

So be careful when comparing two decimal values in terms of DSCP because higher or lower is not necessarily always better.  Tomorrow night....dadada NBAR.  Cant wait, see you then.

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