Application of Postel's Law to XML (as Re: Trust and control )

["Costello, Roger L." <costello@mitre.org>]

Hi Folks,

Postel's Law says:

    Be conservative in what you do, 
    be liberal in what you accept from 
    others.

The intent of Postel's Law was to maximize interoperability between network service implementations, particularly in the face of ambiguous or incomplete specifications [1].

I want to fully understand what it means to apply Postel's Law to XML.

In particular, I want to compile a list of:

- Ways to be conservative in what you do.
- Ways to be liberal in what you accept from others.

Below I have made a start at such a list. Would you add to the list please?

In the following list I provide 3 things:

a. A specification that is ambiguous or incomplete, thus requiring senders and receivers to make a judgment call: how should I interpret the specification -- liberally or conservatively?

b. A way for senders to be conservative in their interpretation of the specification.

c. A way for recipients to be liberal in their interpretation of the specification.

----------
Way #1
----------
a. An ambiguous or incomplete specification:

      The data that is exchanged between
      sender and receiver must be formatted
      using markup.

b. A way for senders to be conservative:

       Send out only well-formed markup documents.

c. A way for recipients to be liberal:

      Accept and process markup documents, even if
      they are not well-formed.

----------
Way #2
----------
a. An ambiguous or incomplete specification:

      The data that is exchanged between
      sender and receiver must be well-formed
      XML.

b. A way for senders to be conservative:

       Send out only well-formed, schema-valid XML documents.

c. A way for recipients to be liberal:

      Accept and process well-formed XML documents, 
      even if they are not schema-valid.

----------
Way #3
----------
a. An ambiguous or incomplete specification:

      The value of the <country> element
      must be the name of a country.

b. A way for senders to be conservative:

       Send out only ISO-standard, 2-letter country 
       codes, e.g., US, CA, GB, DE.

c. A way for recipients to be liberal:

      Accept and process any string, e.g., US, 
      United States, U.S.A.

What other ways can specifications be ambiguous or incomplete, senders conservative, and recipients liberal?

/Roger

[1] http://queue.acm.org/detail.cfm?id=1999945