Question on the DSCP RFC

http://kb.smartvox.co.uk/wp-content/uploads/dscp_ecn_field.jpg

 

 

http://www.ietf.org/rfc/rfc2474.txt

 

As seen above the DSCP field is shown with BIT number ZERO all the way on the left.

Now assuming that i am right that Bit ZERO is the lower order bit ie with value 1 why oh why does it show on all the way on the left instead of all the way on the right when it comes to the TOS Byte???

why choose to to reverse it in the diagram in this ONE instance when every other time bit zero shows up all the way on the right??

 

Tox!

 

Comments

  •  

     The DS field structure is presented below:


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

    DSCP: differentiated services codepoint
    CU: currently unused
    
    
    
    
    
    
    
    




    .
    .
    .
    .

    the above is copy and paste from the RFC
    
    
    again ..makes it look as if bit zero has value 128   
    
    
    
    
    
    





    Tox!

     

  • I probably don't understand your question completely but I have a feeling the answer is related to how the TOS byte was redefind, which is what RFC2474 did.  The original TOS byte had a TOS field made up of the 4 bits after the 3 IPP bits.  These 4 bits were different flags but I don't believe they were used much.  The last bit was Currently Unused

     

    RFC2474 comes along and redefines the TOS byte into the DS Field in which the 8 bits are reclassified into 6 DSCP bits (providing our 64 different available classes with DSCP) and 2 bits for ECN.

     

    These details confused the crap out of me when I first learned QoS so I made a document with pictures and stuff.  If your interested I will send it to you or post it on here if someone explained how to do that.

     

    Ben,

  • I probably don't understand your question completely but I have a feeling the answer is related to how the TOS byte was redefind, which is what RFC2474 did.  The original TOS byte had a TOS field made up of the 4 bits after the 3 IPP bits.  These 4 bits were different flags but I don't believe they were used much.  The last bit was Currently Unused

     

    RFC2474 comes along and redefines the TOS byte into the DS Field in which the 8 bits are reclassified into 6 DSCP bits (providing our 64 different available classes with DSCP) and 2 bits for ECN.

     

    These details confused the crap out of me when I first learned QoS so I made a document with pictures and stuff.  If your interested I will send it to you or post it on here if someone explained how to do that.

     

    Ben,

    no you did not answer my question

    ok when you write out an IP address....is bit number ZERO all the way to the left or all the way to the right??

     

    its all the way to the right...correct?

     

     

    then why does this RFC put bit zero all the way to the left...ie in what we'd thnk of as the bit 7 position (  ie the 128 value position)

    get it?

     

    Tox!

     

  • I'm not familiar with RFC2474 defining TOS that way but I understand why that would be confusing.

  • I'm not familiar with RFC2474 defining TOS that way but I understand why that would be confusing.

     

    well I ws reading the Wendell Odom book and i saw it that way...and it confused me so eventually I went and checked the RFC...

     

    just type out DSCP and hit google images...50% of the images show it the way its shown in the diagram above...

     

    which for a good minute there had my mind blown...and then a CCIE buddy of mine set me straight....

     

    seriously...why do these people do this??   

     

    Tox!

     

  • I'm not familiar with RFC2474 defining TOS that way but I understand why that would be confusing.

     

    well I ws reading the Wendell Odom book and i saw it that way...and it confused me so eventually I went and checked the RFC...

     

    just type out DSCP and hit google images...50% of the images show it the way its shown in the diagram above...

     

    which for a good minute there had my mind blown...and then a CCIE buddy of mine set me straight....

     

    seriously...why do these people do this??   

     

    Tox!

     

  • http://kb.smartvox.co.uk/wp-content/uploads/dscp_ecn_field.jpg

     

     

    http://www.ietf.org/rfc/rfc2474.txt

     

    As seen above the DSCP field is shown with BIT number ZERO all the way on the left.

    Now assuming that i am right that Bit ZERO is the lower order bit ie with value 1 why oh why does it show on all the way on the left instead of all the way on the right when it comes to the TOS Byte???

    why choose to to reverse it in the diagram in this ONE instance when every other time bit zero shows up all the way on the right??

     

    Tox!

     


     

    Actually isn't that how IP/UDP/TCP header format is presented too? Bit 0 the left-most bit is most significant bit. And the right-most bit is the least significant. That's also how you translate DSCP to TOS value, by multiplying it by 4.

    http://en.wikipedia.org/wiki/IPv4_header#Header

  •  

     

    Actually isn't that how IP/UDP/TCP header format is presented too? Bit 0 the left-most bit is most significant bit. And the right-most bit is the least significant. That's also how you translate DSCP to TOS value, by multiplying it by 4.

    http://en.wikipedia.org/wiki/IPv4_header#Header

    yeah seems so...

    so what you are saying is...in the IP address  (so i know for sure that i have this absolutely dead right)

    128.0.0.0 ..its BIT NUMBER ZERO ...thats on...the highest order bit...

     

    correct?

     

     

     

    Tox!

     

     

  •  

     

    Actually isn't that how IP/UDP/TCP header format is presented too? Bit 0 the left-most bit is most significant bit. And the right-most bit is the least significant. That's also how you translate DSCP to TOS value, by multiplying it by 4.

    http://en.wikipedia.org/wiki/IPv4_header#Header

    yeah seems so...

    so what you are saying is...in the IP address  (so i know for sure that i have this absolutely dead right)

    128.0.0.0 ..its BIT NUMBER ZERO ...thats on...the highest order bit...

     

    correct?

     

     

     

    Tox!

     

     


    Think so.

    128.0.0.0 == 1000000.00000000.00000000.00000000

  •  

     

    Actually isn't that how IP/UDP/TCP header format is presented too? Bit 0 the left-most bit is most significant bit. And the right-most bit is the least significant. That's also how you translate DSCP to TOS value, by multiplying it by 4.

    http://en.wikipedia.org/wiki/IPv4_header#Header

    yeah seems so...

    so what you are saying is...in the IP address  (so i know for sure that i have this absolutely dead right)

    128.0.0.0 ..its BIT NUMBER ZERO ...thats on...the highest order bit...

     

    correct?

     

     

     

    Tox!

     

     


    Think so.

    128.0.0.0 == 10000000.00000000.00000000.00000000

  •  

     

    Actually isn't that how IP/UDP/TCP header format is presented too? Bit 0 the left-most bit is most significant bit. And the right-most bit is the least significant. That's also how you translate DSCP to TOS value, by multiplying it by 4.

    http://en.wikipedia.org/wiki/IPv4_header#Header

    yeah seems so...

    so what you are saying is...in the IP address  (so i know for sure that i have this absolutely dead right)

    128.0.0.0 ..its BIT NUMBER ZERO ...thats on...the highest order bit...

     

    correct?

     

     

     

    Tox!

     

     


    Think so.

    128.0.0.0 == 10000000.00000000.00000000.00000000

  •  

     

    Think so.

    128.0.0.0 == 1000000.00000000.00000000.00000000

     

    alright man...i will change where i consider the high order bit to be......for the longest time...high order meant left hand side to me...

     

    christ...

     

    although I sure do hope one of the big names chimes in

     

     

    Tox!

     

  • cisco seems to disagree btw

     

    http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a00800a67f5.shtml

     

    The right most bit, or least significant bit, of an octet holds a value of 20.

     

    according to cisco the 1 bit position is the least siginificant bit or in other words...the low order bit and that would be bit numbered zero...

     

    ????

     

     

     

    Tox!

  • As I understand it... RFC numbers bytes from 0 upwards. Here 0 means first byte transferred on wire. Within a byte, 0 bit is the first bit transferred on wire. Correct me if wrong....Dinesh

  • http://kb.smartvox.co.uk/wp-content/uploads/dscp_ecn_field.jpg

     

     

     

    also if you look at the above diagram...bit number 0 1 and 2 are precedence bits...

     

    if those bits had value 128 64 and 32 respectively then 

    001 would not yield precedence value 1

    not would 111 yield a precedence 7 ...111 would be 224

     

     

    hense the 0 1 and 2 bit positions are of value 1 2 and 4 respectively....

     

    which is when all 3 are turned on you get an ip precedence of 7

     

    it just written very annoyingly....so i'm taking back to me conforming to thinking that the zero bit is valued 128...no..its value is 1

     

     

     

     

     

     

    Tox!

     

  • My understanding is that the left-most bit in your diagram above equals 128.

    Every now and then you need to calculate a full ToS byte value (e.g. for extended ping), and I do that on the basis of bit 0 in your ASCII art is worth 128.

    Don't get too hung up here though Tox - you look to be diving down a rathole again...just get what you _need_ out of this, don't get too tied up here.

  • Oh and yes there are different levels of numbering in here - the IP Precedence numbering is different to calculating the whole ToS byte value.

    Narbik explains it as "there are little green guys inside the Cisco routers, and developers are also little green guys. They're pissed off at the world because they're short and ugly...so they try and get their own back by putting in lots of inconsistencies"

  • My understanding is that the left-most bit in your diagram above equals 128.

    Every now and then you need to calculate a full ToS byte value (e.g. for extended ping), and I do that on the basis of bit 0 in your ASCII art is worth 128.

    Don't get too hung up here though Tox - you look to be diving down a rathole again...just get what you _need_ out of this, don't get too tied up here.

     

    actually THAT(in bold) freakin absolutely (i think) cleared it up...since we are using 3 bits for Prec even though the bit values are 128 64 and 32 we look at them as 4 2 and 1 since its 3 bits....

     

    the problem with me is Northland that I dont get that " ok now i know this" feeling till i really know and understand and comprehend something..i cant just "dabble" in something...which makes me prone to ratholes...but you brought it home for me...and its something i knew when i read the QOS book 1 1/2 years ago...and had forgotton but that sentence in bold rammed it home :)

     

    thankyou

     

     

     

    <edit> I probably wouldn't have slept proper tonight if not for THAT sentence in bold...as it is..i'm typing this at 12:09 am...

     

    thankyou again! :)  its a great feeling when you solve a puzzle!

     

    Tox!

     

  • Oh and yes there are different levels of numbering in here - the IP Precedence numbering is different to calculating the whole ToS byte value.

    Narbik explains it as "there are little green guys inside the Cisco routers, and developers are also little green guys. They're pissed off at the world because they're short and ugly...so they try and get their own back by putting in lots of inconsistencies"

     

    i have much more succinct word for em...

     

    virgins...

     

     

     

     

    Tox!

     

  • cisco seems to disagree btw

     

    http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a00800a67f5.shtml

     

    The right most bit, or least significant bit, of an octet holds a value of 20.

     

    according to cisco the 1 bit position is the least siginificant bit or in other words...the low order bit and that would be bit numbered zero...

     

    ????

     Tox!


    Hmmm..I failed to see where the contradiction lies.

    Left most == most significant; right most == least significant

    I thought that's the part we all agreed upon. Right?

  • cisco seems to disagree btw

     

    http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a00800a67f5.shtml

     

    The right most bit, or least significant bit, of an octet holds a value of 20.

     

    according to cisco the 1 bit position is the least siginificant bit or in other words...the low order bit and that would be bit numbered zero...

     

    ????

     Tox!


     

    Hmmm..I failed to see where the contradiction lies.

    Left most == most significant; right most == least significant

    I thought that's the part we all agreed upon. Right?

    sorry man...it ws late at night and i wasn't thinking clearly...my bad...

     

    i'm just glad Northland cleared it up

     

     

    Tox!

  • We're here to help each other - glad to hear that things are making sense, and you're getting sleep again :D

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