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RE: Normative figures

2006-01-09 15:38:50
Stewart,

        You realize that the text "example" you gave is meaningless - 
without making some (potentially non-obvious) assumptions, right? 

--> -----Original Message-----
--> From: ietf-bounces(_at_)ietf(_dot_)org 
[mailto:ietf-bounces(_at_)ietf(_dot_)org] 
--> On Behalf Of Stewart Bryant
--> Sent: Monday, January 09, 2006 2:47 PM
--> To: Sam Hartman
--> Cc: Harald Tveit Alvestrand; ietf(_at_)ietf(_dot_)org
--> Subject: Re: Normative figures
--> 
--> Sam Hartman wrote:
--> 
--> >Hi.  With the exception of packet diagrams, I think all 
--> the examples
--> >you bring up benefit significantly from clear textual description.
--> >
--> Sam
--> 
--> I am not saying that clear text is not needed to accompany 
--> a diagram.
--> However a diagram allows a lot less text to be written producing a
--> shorter clearer draft with less clutter.
--> 
--> For example you could say the following in text : router A connects to
--> router B and D, the cost from A to B is 2, and the cost from A to D is
4.

        A --(2)--> B
      |
      +---(4)--> D

--> Router B connects to router C. The cost to router A is 6, and
--> the cost to router C is 10. 

     + A <--(6)--+
     | |         |
     | +--(2)--> B --(10)--> C
     |
     +--(4)--> D

(Assumes "cost" is from router B in both cases)

--> Router C connects to router D. The cost to router B is 9
--> and the cost to router D is 8. 

     + A <--(6)--+
     | |         |
     | +--(2)--> B <--(9)---+
     |           |          |
     |           +--(10)--> C
     |                      |
     +--(4)--> D <----(8)---+

(Assumes "cost" is from router C in both cases)

--> The cost from router D to router A is 16 and ...

  +------+
  |      |
  |  +-> A <--(6)--+
  |  |   |         |
  |  |   +--(2)--> B <--(9)---+
  |  |             |          |
  |  +--(16)----+  +--(10)--> C
  |             |             |
  +------(4)--> D <----(8)----+

--> 
--> ... the cost to router A is 99.

?

(Assuming you meant the cost from D to C - not A)

  +------+
  |      |
  |  +-> A <--(6)--+
  |  |   |         |
  |  |   +--(2)--> B <--(9)---+
  |  |             |          |
  |  +--(16)----+  +--(10)--> C <-+
  |             |             |   |
  +------(4)--> D <----(8)----+   |
                |                 |
                +-----(99)--------+

                (Figure foo?)


-->
--> In fact you would probably need a table to make sure that 
--> you did not make a mistake. In either case it is hard for 
--> most people to figure out the what the topology is much 
--> less imagine packet actions.

Actually, you need a figure to make sure that you did not
make a mistake, and a second set of eyes to compare what 
you said to what you apparently meant.

--> 
--> Now compare this to:
--> 
--> "The network and costs are as shown in figure foo."
--> 

Is the above actually figure foo?

The reality is that it would be better to break it this way:

"In the network in figure foo, the link costs are as follows:

    A -> B =  2
    B -> A =  6             +-> A <----> B <-+
    A -> D =  4             |                |
    D -> A = 16             +-> D <----> C <-+
    B -> C = 10
    C -> B =  9                 Figure foo
    C -> D =  8
    D -> C = 99                                          "

--> Simple text and the visualisation is already done for you
--> so you can focus on the problem.
-->

The reality is that putting things entirely in pictures means
that less validation of the intent of the picture is possible.
Keeping pictures as simple as possible is, therefore, a goal
since it is far easier to make mistakes in complex figures and 
far more difficult to determine that a mistake has been made.

Using a mixture of text and figure(s), tables or - possibly -
equations makes it both more understandable and easier to be
certain of conveying the idea(s).

For example, in the above, I could verify that I understand 
your intent by asking:

"From this it is apparent that the link cost (D->C) is 99, 
 but the actual minimum cost of getting from D to C given by 
 the formula:

 (D->A) + (A->B) + (B->C) = 16 + 2 + 10 = 28

 Is that correct?"

If we choose to use a simple combination of representations,
rather than strictly expecting a figure to explain it all,
the figure is likely to be far simpler.  For example, using
an approach similar to the above, you could probably describe
a much more complex network than 13, or 20 or maybe even 30
nodes using ASCII art.  The purpose of the picture is to be
a simplified referent.

--> 
--> Now I realise the above could be done in ascii art, but we 
--> have problems that need 13 or more nodes to set up.
--> 
--> >  I believe I'd think that even if I could see the diagrams 
--> >  and I believe I have enough experience with visualization 
--> >  (although not sight) to be confident in that belief.
--> >
--> >  
--> >
--> >As such, I don't believe these diagrams should be normative.
--> >
--> >
--> >I actually thin that many of the tunnel overlay network documents
--> >(PWE3, L2VPN, L3VPN) could benefit significantly from more focus on
--> >the text of the descriptions of situations being described.
--> >  
--> >
--> It is probably a question of how we work and the tools that we
--> find most effective.
--> 
--> >If you believe that normative documents are required, I'd 
--> like you to
--> >explain why the diagrams cannot be described in the text, 
--> why doing so
--> >would decrease the quality of the specification, or why 
--> doing so would
--> >not be a useful investment in our time.
--> >
--> >
--> >  
--> >
--> One reason is that it takes a lot of text to describe what 
--> can be drawn
--> with a few lines and symbols. Many people will get lost in 
--> the text and
--> in any case would have to transcribe the text into a diagram for
--> themselves before they could understand what is happening.
--> 
--> - Stewart
--> 
--> 
--> 
--> 
--> 
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