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rfc:rfc5952

Internet Engineering Task Force (IETF) S. Kawamura Request for Comments: 5952 NEC BIGLOBE, Ltd. Updates: 4291 M. Kawashima Category: Standards Track NEC AccessTechnica, Ltd. ISSN: 2070-1721 August 2010

       A Recommendation for IPv6 Address Text Representation

Abstract

 As IPv6 deployment increases, there will be a dramatic increase in
 the need to use IPv6 addresses in text.  While the IPv6 address
 architecture in Section 2.2 of RFC 4291 describes a flexible model
 for text representation of an IPv6 address, this flexibility has been
 causing problems for operators, system engineers, and users.  This
 document defines a canonical textual representation format.  It does
 not define a format for internal storage, such as within an
 application or database.  It is expected that the canonical format
 will be followed by humans and systems when representing IPv6
 addresses as text, but all implementations must accept and be able to
 handle any legitimate RFC 4291 format.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc5952.

Kawamura & Kawashima Standards Track [Page 1] RFC 5952 IPv6 Text Representation August 2010

Copyright Notice

 Copyright (c) 2010 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Kawamura & Kawashima Standards Track [Page 2] RFC 5952 IPv6 Text Representation August 2010

Table of Contents

 1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   1.1.  Requirements Language  . . . . . . . . . . . . . . . . . .  4
 2.  Text Representation Flexibility of RFC 4291  . . . . . . . . .  4
   2.1.  Leading Zeros in a 16-Bit Field  . . . . . . . . . . . . .  4
   2.2.  Zero Compression . . . . . . . . . . . . . . . . . . . . .  5
   2.3.  Uppercase or Lowercase . . . . . . . . . . . . . . . . . .  6
 3.  Problems Encountered with the Flexible Model . . . . . . . . .  6
   3.1.  Searching  . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.1.1.  General Summary  . . . . . . . . . . . . . . . . . . .  6
     3.1.2.  Searching Spreadsheets and Text Files  . . . . . . . .  6
     3.1.3.  Searching with Whois . . . . . . . . . . . . . . . . .  6
     3.1.4.  Searching for an Address in a Network Diagram  . . . .  7
   3.2.  Parsing and Modifying  . . . . . . . . . . . . . . . . . .  7
     3.2.1.  General Summary  . . . . . . . . . . . . . . . . . . .  7
     3.2.2.  Logging  . . . . . . . . . . . . . . . . . . . . . . .  7
     3.2.3.  Auditing: Case 1 . . . . . . . . . . . . . . . . . . .  8
     3.2.4.  Auditing: Case 2 . . . . . . . . . . . . . . . . . . .  8
     3.2.5.  Verification . . . . . . . . . . . . . . . . . . . . .  8
     3.2.6.  Unexpected Modifying . . . . . . . . . . . . . . . . .  8
   3.3.  Operating  . . . . . . . . . . . . . . . . . . . . . . . .  8
     3.3.1.  General Summary  . . . . . . . . . . . . . . . . . . .  8
     3.3.2.  Customer Calls . . . . . . . . . . . . . . . . . . . .  9
     3.3.3.  Abuse  . . . . . . . . . . . . . . . . . . . . . . . .  9
   3.4.  Other Minor Problems . . . . . . . . . . . . . . . . . . .  9
     3.4.1.  Changing Platforms . . . . . . . . . . . . . . . . . .  9
     3.4.2.  Preference in Documentation  . . . . . . . . . . . . .  9
     3.4.3.  Legibility . . . . . . . . . . . . . . . . . . . . . .  9
 4.  A Recommendation for IPv6 Text Representation  . . . . . . . . 10
   4.1.  Handling Leading Zeros in a 16-Bit Field . . . . . . . . . 10
   4.2.  "::" Usage . . . . . . . . . . . . . . . . . . . . . . . . 10
     4.2.1.  Shorten as Much as Possible  . . . . . . . . . . . . . 10
     4.2.2.  Handling One 16-Bit 0 Field  . . . . . . . . . . . . . 10
     4.2.3.  Choice in Placement of "::"  . . . . . . . . . . . . . 10
   4.3.  Lowercase  . . . . . . . . . . . . . . . . . . . . . . . . 10
 5.  Text Representation of Special Addresses . . . . . . . . . . . 11
 6.  Notes on Combining IPv6 Addresses with Port Numbers  . . . . . 11
 7.  Prefix Representation  . . . . . . . . . . . . . . . . . . . . 12
 8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
 9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
   10.1. Normative References . . . . . . . . . . . . . . . . . . . 12
   10.2. Informative References . . . . . . . . . . . . . . . . . . 13
 Appendix A.  For Developers  . . . . . . . . . . . . . . . . . . . 14

Kawamura & Kawashima Standards Track [Page 3] RFC 5952 IPv6 Text Representation August 2010

1. Introduction

 A single IPv6 address can be text represented in many ways.  Examples
 are shown below.
    2001:db8:0:0:1:0:0:1
    2001:0db8:0:0:1:0:0:1
    2001:db8::1:0:0:1
    2001:db8::0:1:0:0:1
    2001:0db8::1:0:0:1
    2001:db8:0:0:1::1
    2001:db8:0000:0:1::1
    2001:DB8:0:0:1::1
 All of the above examples represent the same IPv6 address.  This
 flexibility has caused many problems for operators, systems
 engineers, and customers.  The problems are noted in Section 3.  A
 canonical representation format to avoid problems is introduced in
 Section 4.

1.1. Requirements Language

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in [RFC2119].

2. Text Representation Flexibility of RFC 4291

 Examples of flexibility in Section 2.2 of [RFC4291] are described
 below.

2.1. Leading Zeros in a 16-Bit Field

    'It is not necessary to write the leading zeros in an individual
    field.'
 Conversely, it is also not necessary to omit leading zeros.  This
 means that it is possible to select from representations such as
 those in the following example.  The final 16-bit field is different,
 but all of these addresses represent the same address.

Kawamura & Kawashima Standards Track [Page 4] RFC 5952 IPv6 Text Representation August 2010

    2001:db8:aaaa:bbbb:cccc:dddd:eeee:0001
    2001:db8:aaaa:bbbb:cccc:dddd:eeee:001
    2001:db8:aaaa:bbbb:cccc:dddd:eeee:01
    2001:db8:aaaa:bbbb:cccc:dddd:eeee:1

2.2. Zero Compression

    'A special syntax is available to compress the zeros.  The use of
    "::" indicates one or more groups of 16 bits of zeros.'
 It is possible to select whether or not to omit just one 16-bit 0
 field.
    2001:db8:aaaa:bbbb:cccc:dddd::1
    2001:db8:aaaa:bbbb:cccc:dddd:0:1
 In cases where there is more than one field of only zeros, there is a
 choice of how many fields can be shortened.
    2001:db8:0:0:0::1
    2001:db8:0:0::1
    2001:db8:0::1
    2001:db8::1
 In addition, Section 2.2 of [RFC4291] notes,
    'The "::" can only appear once in an address.'
 This gives a choice on where in a single address to compress the
 zero.
    2001:db8::aaaa:0:0:1
    2001:db8:0:0:aaaa::1

Kawamura & Kawashima Standards Track [Page 5] RFC 5952 IPv6 Text Representation August 2010

2.3. Uppercase or Lowercase

 [RFC4291] does not mention any preference of uppercase or lowercase.
    2001:db8:aaaa:bbbb:cccc:dddd:eeee:aaaa
    2001:db8:aaaa:bbbb:cccc:dddd:eeee:AAAA
    2001:db8:aaaa:bbbb:cccc:dddd:eeee:AaAa

3. Problems Encountered with the Flexible Model

3.1. Searching

3.1.1. General Summary

 A search of an IPv6 address if conducted through a UNIX system is
 usually case sensitive and extended options that allow for regular
 expression use will come in handy.  However, there are many
 applications in the Internet today that do not provide this
 capability.  When searching for an IPv6 address in such systems, the
 system engineer will have to try each and every possibility to search
 for an address.  This has critical impacts, especially when trying to
 deploy IPv6 over an enterprise network.

3.1.2. Searching Spreadsheets and Text Files

 Spreadsheet applications and text editors on GUI systems rarely have
 the ability to search for text using regular expression.  Moreover,
 there are many non-engineers (who are not aware of case sensitivity
 and regular expression use) that use these applications to manage IP
 addresses.  This has worked quite well with IPv4 since text
 representation in IPv4 has very little flexibility.  There is no
 incentive to encourage these non-engineers to change their tool or
 learn regular expression when they decide to go dual-stack.  If the
 entry in the spreadsheet reads, 2001:db8::1:0:0:1, but the search was
 conducted as 2001:db8:0:0:1::1, this will show a result of no match.
 One example where this will cause a problem is, when the search is
 being conducted to assign a new address from a pool, and a check is
 being done to see if it is not in use.  This may cause problems for
 the end-hosts or end-users.  This type of address management is very
 often seen in enterprise networks and ISPs.

3.1.3. Searching with Whois

 The "whois" utility is used by a wide range of people today.  When a
 record is set to a database, one will likely check the output to see
 if the entry is correct.  If an entity was recorded as 2001:db8::/48,

Kawamura & Kawashima Standards Track [Page 6] RFC 5952 IPv6 Text Representation August 2010

 but the whois output showed 2001:0db8:0000::/48, most non-engineers
 would think that their input was wrong and will likely retry several
 times or make a frustrated call to the database hostmaster.  If there
 was a need to register the same prefix on different systems, and each
 system showed a different text representation, this would confuse
 people even more.  Although this document focuses on addresses rather
 than prefixes, it is worth mentioning the prefix problems because the
 problems encountered with addresses and prefixes are mostly equal.

3.1.4. Searching for an Address in a Network Diagram

 Network diagrams and blueprints often show what IP addresses are
 assigned to a system devices.  In times of trouble shooting there may
 be a need to search through a diagram to find the point of failure
 (for example, if a traceroute stopped at 2001:db8::1, one would
 search the diagram for that address).  This is a technique quite
 often in use in enterprise networks and managed services.  Again, the
 different flavors of text representation will result in a time-
 consuming search leading to longer mean times to restoration (MTTR)
 in times of trouble.

3.2. Parsing and Modifying

3.2.1. General Summary

 With all the possible methods of text representation, each
 application must include a module, object, link, etc. to a function
 that will parse IPv6 addresses in a manner such that no matter how it
 is represented, they will mean the same address.  Many system
 engineers who integrate complex computer systems for corporate
 customers will have difficulties finding that their favorite tool
 will not have this function, or will encounter difficulties such as
 having to rewrite their macros or scripts for their customers.

3.2.2. Logging

 If an application were to output a log summary that represented the
 address in full (such as 2001:0db8:0000:0000:1111:2222:3333:4444),
 the output would be highly unreadable compared to the IPv4 output.
 The address would have to be parsed and reformed to make it useful
 for human reading.  Sometimes logging for critical systems is done by
 mirroring the same traffic to two different systems.  Care must be
 taken so that no matter what the log output is, the logs should be
 parsed so they are equivalent.

Kawamura & Kawashima Standards Track [Page 7] RFC 5952 IPv6 Text Representation August 2010

3.2.3. Auditing: Case 1

 When a router or any other network appliance machine configuration is
 audited, there are many methods to compare the configuration
 information of a node.  Sometimes auditing will be done by just
 comparing the changes made each day.  In this case, if configuration
 was done such that 2001:db8::1 was changed to 2001:0db8:0000:0000:
 0000:0000:0000:0001 just because the new engineer on the block felt
 it was better, a simple diff will show that a different address was
 configured.  If this was done on a wide scale network, people will be
 focusing on 'why the extra zeros were put in' instead of doing any
 real auditing.  Lots of tools are just plain diffs that do not take
 into account address representation rules.

3.2.4. Auditing: Case 2

 Node configurations will be matched against an information system
 that manages IP addresses.  If output notation is different, there
 will need to be a script that is implemented to cover for this.  The
 result of an SNMP GET operation, converted to text and compared to a
 textual address written by a human is highly unlikely to match on the
 first try.

3.2.5. Verification

 Some protocols require certain data fields to be verified.  One
 example of this is X.509 certificates.  If an IPv6 address field in a
 certificate was incorrectly verified by converting it to text and
 making a simple textual comparison to some other address, the
 certificate may be mistakenly shown as being invalid due to a
 difference in text representation methods.

3.2.6. Unexpected Modifying

 Sometimes, a system will take an address and modify it as a
 convenience.  For example, a system may take an input of
 2001:0db8:0::1 and make the output 2001:db8::1.  If the zeros were
 input for a reason, the outcome may be somewhat unexpected.

3.3. Operating

3.3.1. General Summary

 When an operator sets an IPv6 address of a system as 2001:db8:0:0:1:
 0:0:1, the system may take the address and show the configuration
 result as 2001:DB8::1:0:0:1.  Someone familiar with IPv6 address
 representation will know that the right address is set, but not
 everyone may understand this.

Kawamura & Kawashima Standards Track [Page 8] RFC 5952 IPv6 Text Representation August 2010

3.3.2. Customer Calls

 When a customer calls to inquire about a suspected outage, IPv6
 address representation should be handled with care.  Not all
 customers are engineers, nor do they have a similar skill level in
 IPv6 technology.  The network operations center will have to take
 extra steps to humanly parse the address to avoid having to explain
 to the customers that 2001:db8:0:1::1 is the same as
 2001:db8::1:0:0:0:1.  This is one thing that will never happen in
 IPv4 because IPv4 addresses cannot be abbreviated.

3.3.3. Abuse

 Network abuse reports generally include the abusing IP address.  This
 'reporting' could take any shape or form of the flexible model.  A
 team that handles network abuse must be able to tell the difference
 between a 2001:db8::1:0:1 and 2001:db8:1::0:1.  Mistakes in the
 placement of the "::" will result in a critical situation.  A system
 that handles these incidents should be able to handle any type of
 input and parse it in a correct manner.  Also, incidents are reported
 over the phone.  It is unnecessary to report if the letter is
 uppercase or lowercase.  However, when a letter is spelled uppercase,
 people tend to specify that it is uppercase, which is unnecessary
 information.

3.4. Other Minor Problems

3.4.1. Changing Platforms

 When an engineer decides to change the platform of a running service,
 the same code may not work as expected due to the difference in IPv6
 address text representation.  Usually, a change in a platform (e.g.,
 Unix to Windows, Cisco to Juniper) will result in a major change of
 code anyway, but flexibility in address representation will increase
 the work load.

3.4.2. Preference in Documentation

 A document that is edited by more than one author may become harder
 to read.

3.4.3. Legibility

 Capital case D and 0 can be quite often misread.  Capital B and 8 can
 also be misread.

Kawamura & Kawashima Standards Track [Page 9] RFC 5952 IPv6 Text Representation August 2010

4. A Recommendation for IPv6 Text Representation

 A recommendation for a canonical text representation format of IPv6
 addresses is presented in this section.  The recommendation in this
 document is one that complies fully with [RFC4291], is implemented by
 various operating systems, and is human friendly.  The recommendation
 in this section SHOULD be followed by systems when generating an
 address to be represented as text, but all implementations MUST
 accept and be able to handle any legitimate [RFC4291] format.  It is
 advised that humans also follow these recommendations when spelling
 an address.

4.1. Handling Leading Zeros in a 16-Bit Field

 Leading zeros MUST be suppressed.  For example, 2001:0db8::0001 is
 not acceptable and must be represented as 2001:db8::1.  A single 16-
 bit 0000 field MUST be represented as 0.

4.2. "::" Usage

4.2.1. Shorten as Much as Possible

 The use of the symbol "::" MUST be used to its maximum capability.
 For example, 2001:db8:0:0:0:0:2:1 must be shortened to 2001:db8::2:1.
 Likewise, 2001:db8::0:1 is not acceptable, because the symbol "::"
 could have been used to produce a shorter representation 2001:db8::1.

4.2.2. Handling One 16-Bit 0 Field

 The symbol "::" MUST NOT be used to shorten just one 16-bit 0 field.
 For example, the representation 2001:db8:0:1:1:1:1:1 is correct, but
 2001:db8::1:1:1:1:1 is not correct.

4.2.3. Choice in Placement of "::"

 When there is an alternative choice in the placement of a "::", the
 longest run of consecutive 16-bit 0 fields MUST be shortened (i.e.,
 the sequence with three consecutive zero fields is shortened in 2001:
 0:0:1:0:0:0:1).  When the length of the consecutive 16-bit 0 fields
 are equal (i.e., 2001:db8:0:0:1:0:0:1), the first sequence of zero
 bits MUST be shortened.  For example, 2001:db8::1:0:0:1 is correct
 representation.

4.3. Lowercase

 The characters "a", "b", "c", "d", "e", and "f" in an IPv6 address
 MUST be represented in lowercase.

Kawamura & Kawashima Standards Track [Page 10] RFC 5952 IPv6 Text Representation August 2010

5. Text Representation of Special Addresses

 Addresses such as IPv4-Mapped IPv6 addresses, ISATAP [RFC5214], and
 IPv4-translatable addresses [ADDR-FORMAT] have IPv4 addresses
 embedded in the low-order 32 bits of the address.  These addresses
 have a special representation that may mix hexadecimal and dot
 decimal notations.  The decimal notation may be used only for the
 last 32 bits of the address.  For these addresses, mixed notation is
 RECOMMENDED if the following condition is met: the address can be
 distinguished as having IPv4 addresses embedded in the lower 32 bits
 solely from the address field through the use of a well-known prefix.
 Such prefixes are defined in [RFC4291] and [RFC2765] at the time of
 this writing.  If it is known by some external method that a given
 prefix is used to embed IPv4, it MAY be represented as mixed
 notation.  Tools that provide options to specify prefixes that are
 (or are not) to be represented as mixed notation may be useful.
 There is a trade-off here where a recommendation to achieve an exact
 match in a search (no dot decimals whatsoever) and a recommendation
 to help the readability of an address (dot decimal whenever possible)
 does not result in the same solution.  The above recommendation is
 aimed at fixing the representation as much as possible while leaving
 the opportunity for future well-known prefixes to be represented in a
 human-friendly manner as tools adjust to newly assigned prefixes.
 The text representation method noted in Section 4 should be applied
 for the leading hexadecimal part (i.e., ::ffff:192.0.2.1 instead of
 0:0:0:0:0:ffff:192.0.2.1).

6. Notes on Combining IPv6 Addresses with Port Numbers

 There are many different ways to combine IPv6 addresses and port
 numbers that are represented in text.  Examples are shown below.
 o  [2001:db8::1]:80
 o  2001:db8::1:80
 o  2001:db8::1.80
 o  2001:db8::1 port 80
 o  2001:db8::1p80
 o  2001:db8::1#80
 The situation is not much different in IPv4, but the most ambiguous
 case with IPv6 is the second bullet.  This is due to the "::"usage in

Kawamura & Kawashima Standards Track [Page 11] RFC 5952 IPv6 Text Representation August 2010

 IPv6 addresses.  This style is NOT RECOMMENDED because of its
 ambiguity.  The [] style as expressed in [RFC3986] SHOULD be
 employed, and is the default unless otherwise specified.  Other
 styles are acceptable when there is exactly one style for the given
 context and cross-platform portability does not become an issue.  For
 URIs containing IPv6 address literals, [RFC3986] MUST be followed, as
 well as the rules defined in this document.

7. Prefix Representation

 Problems with prefixes are the same as problems encountered with
 addresses.  The text representation method of IPv6 prefixes should be
 no different from that of IPv6 addresses.

8. Security Considerations

 This document notes some examples where IPv6 addresses are compared
 in text format.  The example on Section 3.2.5 is one that may cause a
 security risk if used for access control.  The common practice of
 comparing X.509 data is done in binary format.

9. Acknowledgements

 The authors would like to thank Jan Zorz, Randy Bush, Yuichi Minami,
 and Toshimitsu Matsuura for their generous and helpful comments in
 kick starting this document.  We also would like to thank Brian
 Carpenter, Akira Kato, Juergen Schoenwaelder, Antonio Querubin, Dave
 Thaler, Brian Haley, Suresh Krishnan, Jerry Huang, Roman Donchenko,
 Heikki Vatiainen, Dan Wing, and Doug Barton for their input.  Also, a
 very special thanks to Ron Bonica, Fred Baker, Brian Haberman, Robert
 Hinden, Jari Arkko, and Kurt Lindqvist for their support in bringing
 this document to light in IETF working groups.

10. References

10.1. Normative References

 [RFC2119]      Bradner, S., "Key words for use in RFCs to Indicate
                Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2765]      Nordmark, E., "Stateless IP/ICMP Translation Algorithm
                (SIIT)", RFC 2765, February 2000.
 [RFC3986]      Berners-Lee, T., Fielding, R., and L. Masinter,
                "Uniform Resource Identifier (URI): Generic Syntax",
                STD 66, RFC 3986, January 2005.

Kawamura & Kawashima Standards Track [Page 12] RFC 5952 IPv6 Text Representation August 2010

 [RFC4291]      Hinden, R. and S. Deering, "IP Version 6 Addressing
                Architecture", RFC 4291, February 2006.

10.2. Informative References

 [ADDR-FORMAT]  Bao, C., "IPv6 Addressing of IPv4/IPv6 Translators",
                Work in Progress, July 2010.
 [RFC4038]      Shin, M-K., Hong, Y-G., Hagino, J., Savola, P., and E.
                Castro, "Application Aspects of IPv6 Transition",
                RFC 4038, March 2005.
 [RFC5214]      Templin, F., Gleeson, T., and D. Thaler, "Intra-Site
                Automatic Tunnel Addressing Protocol (ISATAP)",
                RFC 5214, March 2008.

Kawamura & Kawashima Standards Track [Page 13] RFC 5952 IPv6 Text Representation August 2010

Appendix A. For Developers

 We recommend that developers use display routines that conform to
 these rules.  For example, the usage of getnameinfo() with flags
 argument NI_NUMERICHOST in FreeBSD 7.0 will give a conforming output,
 except for the special addresses notes in Section 5.  The function
 inet_ntop() of FreeBSD7.0 is a good C code reference, but should not
 be called directly.  See [RFC4038] for details.

Authors' Addresses

 Seiichi Kawamura
 NEC BIGLOBE, Ltd.
 14-22, Shibaura 4-chome
 Minatoku, Tokyo  108-8558
 JAPAN
 Phone: +81 3 3798 6085
 EMail: kawamucho@mesh.ad.jp
 Masanobu Kawashima
 NEC AccessTechnica, Ltd.
 800, Shimomata
 Kakegawa-shi, Shizuoka  436-8501
 JAPAN
 Phone: +81 537 23 9655
 EMail: kawashimam@necat.nec.co.jp

Kawamura & Kawashima Standards Track [Page 14]

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