Topic: TOS values???????????????

[hayc59]

so Cablenut can you post some good tos settings for
windows98se/comcast.net 1800down/256up?
thanks...

[woutertal]

Hi again & thanks for your answer.  I have 2 questions:

question 1.  If I understand correctly the options are as follow:

The IP Type of Service has the following fields:

  Bits 0-2:  Precedence.
  Bit  3:  0 = Normal Delay, 1 = Low Delay.
  Bit  4:  0 = Normal Throughput, 1 = High Throughput.
  Bit  5:  0 = Normal Relibility, 1 = High Relibility.
 Bit  6:  0 = normal cost, 1 = minimize cost
  Bit  7:  Reserved for Future Use.

  Bits 0,1,2 = Priority Control:
 
  111 - Network Control
  110 - Internetwork Control
  101 - CRITIC/ECP
  100 - Flash Override
  011 - Flash
  010 - Immediate
  001 - Priority
  000 - Routine

Now, please correct me if I am wrong.  Value 90, as currently adviced in most threats on this forum, would give you this:

Bits 0,1,2: 010 = Immediate Priority
Bit 3: 1 = Low Delay
Bit 4: 1 = High Throughput
Bit 6: 1 = minimize cost

Decimal: 64+16+8+2 = 90

But, as you wrote: 'Might I remind everyone that IANA states that you can only use one TOS option every IP header, so that means you have your choice of only one option such as "High Bandwidth", or "Low Delay". '

Doesn't that mean that 90 is not a correct value?, and that the choices would be between 80, 72, 66 and 64?

question 2.  What is there against using 111 (decimal 128) for priority control?

Sorry for being a pain, but I am fascinated by this stuff, I want to understand.  

[cablenut]

Yes, correct one TOS option per datagram. So 90 isn't invalid it's using more than one TOS option so who knows what option you really are trying to use when you set three of them.

A better option is to choose your priority level, and then one TOS option such as low delay.  Priority 111, or Network Control is 128+64+32 = 224.

Bit 6 is for "cost", and bit 7 is determined as "MBZ field: Must Be Zero".

From Paul Ferguson, Cisco Systems:

INTERNET-DRAFT         Simple Differential Services        March 12, 1998


3.3 Drop Preference

   There is at least one known implementation of preferential
   packet drop which uses the values expressed in the IP precedence
   subfield of the TOS byte, as described in [5], and this draft
   effectively modifies the semantics of the existing IP precedence
   values defined below [5].  Instead, a suggested interpretation
   of these values is provided below which modifies the existing
   semantics when used in conjunction with a differential services
   implementation:


    Bit      RFC791               Relative
   Value    Semantics          Drop Preference

    111 - Network Control         Lowest          
    110 - Internetwork Control       .
    101 - CRITIC/ECP                 .
    100 - Flash Override             .
    011 - Flash                      .
    010 - Immediate                  .
    001 - Priority                   .
    000 - Routine                 Highest

What this is saying that relating to how your datagram is treated in the router "queue". 111, giving you the lowest possible chance of your datagram being discarded by the router.

[Plox]

I guess what everyone is getting at is which is better?

72 which is Immediate, High Throughput, 010 010 00
80  which is Immediate, Low Delay,  010 100 00
88 which is Immediate, Low Delay, High Throughput, 010 110 00
90 which is Immediate, Low Delay, Low Cost 010 10 10
92 which is Immediate, Low Delay, High Throughput, High Reliability 010 111 00

And whoever suggested using Low Cost is plain wrong.
Thats for like NNTP, news servers. Not really important stuff takes a slow low cost route.

Reading RCF 1349, it states you have 5 choices in TOS.

            0000    (all normal)                         Use default metric
            1000    (minimize delay)                  Use delay metric
            0100    (maximize throughput)          Use default metric
            0010    (maximize reliability)            Use reliability metric
            0001    (minimize monetary cost)      Use cost metric
            other                                                  Use default metric

So if when you choose DTR and cost, you can only choose one. Otherwise it looks like defualt.

Example 92, 010 111 00 would look 010 000 00.

1110 would go to defualt of 0000 according to RCF 1349, minimize delay.

If you choose 80, 010 100 00 then it would look the same.

I was recently reading section 4 of RCF 2474.
I maybe wrong, but it looks like cost field does nothing.
It is totally ignored.
It also looks like DTR is now ignored too.

It looks like, according to RCF 2474, if you were trying 010 110 10 the router would just see 101 000 00.

Someone asked about why not increase priority higher?

Anything higher then 010, Immediate, are ment for internal network use. Meaning packets that are not ment to route to the internet.

I'd just use 80 or 72, and stick with that.