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RFC 2474 uses the ToS byte for Differentiated Services Code Point:

    0   1   2   3   4   5   6   7
  +---+---+---+---+---+---+---+---+
  |      DSCP field       |CU/ECN | 
  +---+---+---+---+---+---+---+---+

Further, the DSCP field has more divisions:

    0   1   2   3   4   5   
  +---+---+---+---+---+---+
  |   Class   |  Drop | 0 |
  +---+---+---+---+---+---+
  |___________ ___________|
              V
         DSCP field

My question is now, HOW is the 6. bit, the bit with the number 5, called? The only thing I know is that it is 0. But no name or anything for the 6. Bit. I'm reading the book 'Deploying QoS for cisco IP and Next-Generation-Networks' and there is where I found the table.

The same ToS Byte is also (and originally) used for IP Precedence (RFC 791), and there every bit is described and named:

    0 + 1 + 2     3         4          5             6  +  7
  +-----------+-------+----------+-----------+--------------------+
  |Precedence | Delay |Throughput|Reliability|Reserved and unused |
  +-----------+-------+----------+-----------+--------------------+

I need this for further understanding of how QoS works and how the mapping from IP-Precedence-to-DSCP works.

HopelessN00b
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thz
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  • I really do understand that is one tough question and it might not fit in here but I thought it would be well fitting relative to the other sites (like Superuser, theoretical cs etc.) – thz Feb 07 '13 at 09:19
  • Have no definitve answer. https://datatracker.ietf.org/wg/diffserv/ has some more documents on Differentiated Services. – rhasti Feb 07 '13 at 13:01
  • Unfortunately I couldn't find anything that would describe the name. Thanks anyway. – thz Feb 07 '13 at 13:54

1 Answers1

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Currently it doesn't have a specific name or use, it should always be 0.

Your best bet for understanding the relation between IP Precedence and DSCP (and COS/etc) is to make a table showing all the mappings. I have an excel sheet I can upload somewhere if you want a copy.

cpt_fink
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  • No, no, it's alright. I can get those mapping tables somewhere, that;s not a problem. But if this bit should always be 0, then you've solved the puzzle in my head :D I never would have thought about this oppurtunity, that this bit just isn't used. Whatever, thanks very much! – thz Feb 11 '13 at 06:52