[idn] Modified IPTR

[Hongbo Shi <shi@goto.info.waseda.ac.jp>]

Dear all, 

 According to your suggestions and thinkings including mine, I updated 
 the IPTR document. :)
 
 About the "PTR MUST contain an ACE domain name", it is based on a
 consideration that PTR MUST NOT be NULL. I think "PTR contain an ACE 
 domain name" is a good solution. Moreover, I think the key point at here 
 not only ACE but also the number of domain name, ONE. In short,  ONLY ONE 
 ACE domain name is enough for the PTR RR, though maybe there are a lot iDNs 
 in UTF-8 stored in IPTR RRs.
 
 About the "IPTR Usage", I added some IPTR samples. I hope I explain them 
 clearly. :P 

ADDITION: 

6. IDN Consideration

IPTR described above is based on "a set of IDNs" consideration. Strictly
speaking, a set of canonical IDNs. On the other hand, confusion about IDN,
such as "IDN must exist with ASCII domain name" has lead to a belief that
PTR record should have exactly RRs in its RRSet. In short, the phenomenon
"ONLY IDN" will exist. Thus, the exception handling is REQUIRED in PTR.

7. PTR Extension

In the "ONLY IDN" case, if IPTR RR is not NULL, PTR RR MUST contain an ACE 
domain name.

8. IPTR Usage

The "XXX.com" in following samples will be the same characters or not
the same characters.

4.3.2.1.IN-ADDR.ARPA  IPTR  "zh-tw" "XXX.com in utf8"
                      IPTR  "zh-cn" "XXX.com in utf8"
                      IPTR  "ja"    "XXX.com in utf8"
                      IPTR  "ko"    "XXX.com in utf8"

Furthermore,

4.3.2.1.IN-ADDR.ARPA  IPTR  "zh-tw" "XXX.com in utf8"
                      IPTR  "zh-tw" "xxx.com in utf8"
                      ...
                      IPTR  "zh-cn" "XXX.com in utf8"
                      IPTR  "zh-cn" "xxx.com in utf8"
                      ...
                      IPTR  "ja"    "XXX.com in utf8"
                      IPTR  "ja"    "xxx.com in utf8"
                      ...
                      IPTR  "ko"    "XXX.com in utf8"
                      IPTR  "ko"    "xxx.com in utf8"
                      ...

will exist too, same as the current usage of PTR [RFC2181]. In short, in every
single language, it is possible that more than ONE domain name exist for the 
same host. Thus "xxx.com" MUST be different from "XXX.com". And "xxx.com" in 
the samples above will be the same characters or not the same characters.  

 Any comment is welcome. Thanks! 

Best Regards. 

Hongbo Shi