v6ops Working Group                                      M-K. Shin (ed.)
INTERNET DRAFT                                                 Y-G. Hong                                                 ETRI/NIST
Expires: September December 2004                                         Y-G. Hong
                                                               J. Hagino
                                                               P. Savola
                                                            E. M. Castro
                                                               June 2004

                 Application Aspects of IPv6 Transition

Status of this Memo

     This document is an Internet-Draft

     By submitting this Internet-Draft, I certify that any applicable
     patent or other IPR claims of which I am aware have been disclosed,
     and is any of which I become aware will be disclosed, in full conformance accordance
     all provisions of Section 10 of RFC2026.

     Internet Drafts RFC 3668.

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     This Internet-Draft will expire on December 2004.


     As IPv6 networks are deployed and the network transition discussed,
     one should also consider how to enable IPv6 support in applications
     running on IPv6 hosts, and the best strategy to develop IP protocol
     support in applications.  This document specifies scenarios and
     aspects of application transition.  It also proposes guidelines on
     how to develop IP version-independent applications during the
     transition period.

Table of Contents:

     1. Introduction .............................................. 3
     2. Overview of IPv6 Application Transition ................... 3
     3. Problems with IPv6 Application Transition ................. 5
      3.1 IPv6 support in the OS and applications are unrelated.... 5
      3.2 DNS does not indicate which IP version will be used ..... 5
      3.3 Supporting many versions of an application is difficult ..6
     4. Description of Transition Scenarios and Guidelines ........ 6
      4.1 IPv4 Applications in a Dual-stack Node .................. 7
      4.2 IPv6 Applications in a Dual-stack Node .................. 7
      4.3 IPv4/IPv6 Applications in a Dual-stack Node .............10
      4.4 IPv4/IPv6 Applications in an IPv4-only Node .............11
     5. Application Porting Considerations ........................11
      5.1 Presentation Format for an IP address ...................12
      5.2 Transport Layer API .....................................13
      5.3 Name and Address Resolution .............................14
      5.4 Specific IP Dependencies ............................... 14
       5.4.1 IP Address Selection .................................14
       5.4.2 Application Framing ..................................15
       5.4.3 Storage of IP addresses ..............................15
      5.5 Multicast Applications ..................................16
     6. Developing IP version-independent Applications ............17
      6.1 IP version-independent Structures .......................17
      6.2 IP version-independent APIs .............................17 .............................18
       6.2.1 Example of Overly Simplistic TCP Server Application ..18 ..19
       6.2.2 Example of Overly Simplistic TCP Client Application ..19 ..20
       6.2.3 Binary/Presentation Format Conversion ................20 ................21
      6.3 Iterated Jobs for Finding the Working Address ...........21 ...........22
       6.3.1 Example of TCP Server Application ....................21 ....................22
       6.3.2 Example of TCP Client Application ....................24 ....................23
     7. Transition Mechanism Considerations .......................24
     8. Security Considerations ...................................24 ...................................25
     9. Acknowledgements  .........................................25
     10. References ...............................................25
     Authors' Addresses ...........................................27
     Appendix A. Other Binary/Presentation Format Conversions .....27 .....28
      A.1 Binary to Presentation using inet_ntop() ................28
      A.2 Presentation to Binary using inet_pton() ................28 ................29

1. Introduction

     As IPv6 is introduced in the IPv4-based Internet, several general
     issues arise such as routing, addressing, DNS, scenarios, etc.

     One important key to a successful IPv6 transition is the
     compatibility with the large installed base of IPv4 hosts and
     routers. This issue had been already been extensively studied, and
     the work is still in progress. In particular, [2893BIS] describes
     the basic transition mechanisms, dual-stack deployment and
     tunneling.  In addition, various kinds of transition mechanisms
     have been developed for the transition to an IPv6 network.
     However, these transition mechanisms take no stance on whether
     applications support IPv6 or not.

     This document specifies application aspects of IPv6 transition.
     That is, two inter-related topics are covered:

         1.  How different network transition techniques affect
             applications, and what are the strategies for applications
             to support IPv6 and IPv4.

         2.  How to develop IPv6-capable or protocol-independent
             applications ("application porting guidelines").

     Applications will need to be modified to support IPv6 (and IPv4),
     using one of a number of techniques described in sections 2-4.
     Some guidelines to develop such application are then presented in
     sections 5 and 6.

2. Overview of IPv6 Application Transition

     The transition of an application can be classifed using four
     different cases (excluding the first case when there is no IPv6
     support either in the application or the operating system), as

      |       appv4       | (appv4 - IPv4-only applications)
      | TCP / UDP / others| (transport protocols - TCP, UDP,
      +-------------------+  SCTP, DCCP, etc.)
      |    IPv4 | IPv6    | (IP protocols supported/enabled in the OS)

      Case 1. IPv4 applications in a dual-stack node
      +-------------------+ (appv4 - IPv4-only applications)
      |  appv4  |  appv6  | (appv6 - IPv6-only applications)
      | TCP / UDP / others| (transport protocols - TCP, UDP,
      +-------------------+             SCTP, DCCP, etc.)
      |    IPv4 | IPv6    | (IP protocols supported/enabled in the OS)

      Case 2. IPv4-only applications and IPv6-only applications
              in a dual-stack node

      |     appv4/v6      | (appv4/v6 - applications supporting
      +-------------------+             both IPv4 and IPv6)
      | TCP / UDP / others| (transport protocols - TCP, UDP,
      +-------------------+             SCTP, DCCP, etc.)
      |    IPv4 | IPv6    | (IP protocols supported/enabled in the OS)

      Case 3. Applications supporting both IPv4 and IPv6
              in a dual-stack node

      |     appv4/v6      | (appv4/v6 - applications supporting
      +-------------------+             both IPv4 and IPv6)
      | TCP / UDP / others| (transport protocols - TCP, UDP,
      +-------------------+             SCTP, DCCP, etc.)
      |       IPv4        | (IP protocols supported/enabled in the OS)

      Case 4. Applications supporting both IPv4 and IPv6
              in an IPv4-only node

         Figure 1. Overview of Application Transition

     Figure 1 shows the cases of application transition.

      Case 1 : IPv4-only applications in a dual-stack node.
               IPv6 protocol is introduced in a node, but
               applications are not yet ported to support IPv6.

      Case 2 : IPv4-only applications and IPv6-only applications
               in a dual-stack node.
               Applications are ported for IPv6-only. Therefore
               there are two similar applications, one for each
               protocol version (e.g., ping and ping6).

      Case 3 : Applications supporting both IPv4 and IPv6 in a dual
               stack node.
               Applications are ported for both IPv4 and IPv6 support.
               Therefore, the existing IPv4 applications can be

      Case 4 : Applications supporting both IPv4 and IPv6 in an
               IPv4-only node.
               Applications are ported for both IPv4 and IPv6 support,
               but the same applications may also have to work when
               IPv6 is not being used (e.g. disabled from the OS).

     Note that this draft does not address DCCP and SCTP considerations
     at this phase.

3. Problems with IPv6 Application Transition

     There are several reasons why the transition period between IPv4
     and IPv6 applications may not be straightforward.  These issues are
     described in this section.

3.1 IPv6 support in the OS and applications are unrelated

     Considering the cases described in the previous section, IPv4 and
     IPv6 protocol stacks in a node is likely to co-exist for a long

     Similarly, most applications are expected to be able to handle both
     IPv4 and IPv6 during another, unrelated long time period.  That is,
     the operating system being dual stack does not mean having both
     IPv4 and IPv6 applications.  Therefore, IPv6-capable application
     transition may be independent of protocol stacks in a node.

     It is even probable that applications capable of both IPv4 and IPv6
     will have to work properly in IPv4-only nodes (whether the IPv6
     protocol is completely disabled or there is no IPv6 connectivity at

3.2 DNS does not indicate which IP version will be used

     The role of the DNS name resolver in a node is to get the list of
     destination addresses. DNS queries and responses are sent using
     either IPv4 or IPv6 to carry the queries, regardless of the
     protocol version of the data records [DNSTRANS].

     The issue of DNS name resolution related to application transition,
     is that a client application can not be certain of the version of
     the peer application by only doing a DNS name lookup. For example,
     if a server application does not support IPv6 yet, but runs on a
     dual-stack machine for other IPv6 services, and this host is listed
     with a AAAA record in the DNS, the client application will fail to
     connect to the server application. This is caused by a mis-match
     between the DNS query result (i.e. IPv6 addresses) and a server
     application version (i.e. IPv4).

     It is bad practise

     Using SRV records would avoid these problems.  Unfortunately, they
     are not sufficiently widely used to add be applicable in most cases.
     Hence an AAAA record for operational technique is to use "service names" in the
     DNS, that is, if a node that does not
     support all the services using IPv6 (rather, an AAAA record is offering multiple services, but only
     some of them over IPv6, add a DNS name for each of these services
     (with the
     specific service associated A/AAAA records), not just a single name and address should be used). for
     the whole node, also including the AAAA records.  However, the
     applications cannot depend on "good practise", and this must be
     handled. such operational practices.

     In consequence, the application should request all IP addresses
     without address family constraints and try all the records returned
     from the DNS, in some order, until a working address is found.  In
     particular, the application has to be able to handle all IP
     versions returned from the DNS. This issue is discussed in more
     detail in [DNSOPV6].

3.3 Supporting many versions of an application is difficult

     During the application transition period, system administrators may
     have various versions of the same application (an IPv4-only
     application, an IPv6-only application, or an application supporting
     both IPv4 and IPv6).

     Typically one cannot know which IP versions must be supported prior
     to doing a DNS lookup *and* trying (see section 3.2) the addresses
     returned.  Therefore, the local users have a difficulty selecting
     the right application version supporting the exact IP version
     required if multiple versions of the same application are

     To avoid problems with one application not supporting the specified
     protocol version, it is desirable to have hybrid applications
     supporting both of the protocol versions.


     One could argue that an alternative approach is for local client
     applications could be to have a "wrapper application" which
     performs certain tasks (like figures out which protocol version
     will be used) and calls the IPv4/IPv6-only applications as
     necessary.  In other words, such applications would perform
     connection establishment (or similar), and pass the opened socket
     to the other application.  However, as these wrapper applications will actually would
     have to do more than just perform a DNS lookup or figure out the
     literal IP address given.  Thus, given, they may will get complex, and only work for
     certain kinds of, usually simple, applications.

     Nonetheless, there should be some reasonable logic to enable the
     users to use the complex -- likely much more
     complex than writing a hybrid application.  Furthermore, "wrapping"
     applications which perform complex operations with any supported protocol version;
     the users should IP addresses
     (like FTP clients) might be even more challenging or even
     impossible.  In summary, wrapper applications does not have to select from various versions look like a
     robust approach for application transition.

4. Description of
     applications, some supporting only IPv4, others only IPv6, and yet
     some both versions by themselves.

4. Description of Transition Scenarios Transition Scenarios and Guidelines

     Once the IPv6 network is deployed, applications supporting IPv6 can
     use IPv6 network services and establish IPv6 connections.  However,
     upgrading every node to IPv6 at the same time is not feasible and
     transition from IPv4 to IPv6 will be a gradual process.

     Dual-stack nodes are one of the ways to maintain IPv4 compatibility
     in unicast communications. In this section we will analyze
     different application transition scenarios (as introduced in
     section 2) and guidelines to maintain interoperability between
     applications running in different types of nodes.

4.1 IPv4 Applications in a Dual-stack Node

     This scenario happens if the IPv6 protocol is added in a node but
     IPv6-capable applications aren't yet available or installed.
     Although the node implements the dual stack, IPv4 applications can
     only manage IPv4 communications.  Then, IPv4 applications can only
     accept/establish connections from/to nodes which implement an IPv4

     In order to allow an application to communicate with other nodes
     using IPv6, the first priority is to port applications to IPv6.

     In some cases (e.g. no source code is available), existing IPv4
     applications can work if the Bump-in-the-Stack [BIS] or Bump-in-
     the-API [BIA] mechanism is installed in the node. However,  We strongly
     recommend that application developers sould not use these
     mechanisms should not be used when application source code is available to prevent their mis-use, for
     example, available.  Also, it
     should not be used as an excuse not to port software. software or delay

     When [BIA] or [BIS] is used, the problem described in section 3.2
     --the IPv4 client in a [BIS]/[BIA] node trying to connect to an
     IPv4 server in a dual stack system-- arises. However, one can rely
     on the [BIA]/[BIS] mechanism, which should cycle through all the
     addresses instead of applications.

     [BIS] or [BIA] does not work with all kinds of applications. In
     particular, the applications which exchange IP addresses as
     application data (e.g., FTP). These mechanisms provide IPv4
     temporary addresses to the applications and locally make a
     translation between IPv4 and IPv6 communication. Hence, these IPv4
     temporary addresses are only valid in the node scope."

4.2 IPv6 Applications in a Dual-stack Node

     As we have seen in the previous section, applications should be
     ported to IPv6.  The easiest way to port an IPv4 application is to
     substitute the old IPv4 API references with the new IPv6 APIs with
     one-to-one mapping. This way the application will be IPv6-only.
     This IPv6-only source code can not work in IPv4-only nodes, so the
     old IPv4 application should be maintained in these nodes.  Then, we
     will get two similar applications working with different protocol
     versions, depending on the node they are running (e.g., telnet and
     telnet6).  This case is undesirable since maintaining two versions
     of the same source code per application could be a difficult task.
     In addition, this approach would cause problems for the users when
     having to select which version of the application to use, as
     described in section 3.3.

     Most implementations of dual stack allow IPv6-only applications to
     interoperate with both IPv4 and IPv6 nodes. IPv4 packets going to
     IPv6 applications on a dual-stack node reach their destination
     because their addresses are mapped to IPv6 ones using IPv4-mapped
     IPv6 addresses: the IPv6 address ::FFFF:x.y.z.w represents the IPv4
     address x.y.z.w.

        | +------------------------------------------+ |
        | |                                          | |
        | |        IPv6-only applications            | |
        | |                                          | |
        | +------------------------------------------+ |
        |                      |                       |
        | +------------------------------------------+ |
        | |                                          | |
        | |   TCP / UDP / others (SCTP, DCCP, etc.)  | |
        | |                                          | |
        | +------------------------------------------+ |
        |    IPv4-mapped    |        |    IPv6         |
        |  IPv6 addresses   |        |   addresses     |
        | +--------------------+ +-------------------+ |
        | |        IPv4        | |      IPv6         | |
        | +--------------------+ +-------------------+ |
        |   IPv4       |                 |             |
        |   adresses   |                 |             |
                       |                 |
                  IPv4 packets      IPv6 packets

     We will analyze the behaviour of IPv6-applications which exchange
     IPv4 packets with IPv4 applications using the client/server model.
     We consider the default case when the IPV6_V6ONLY socket option has
     not been set. This default behavior of IPv6 applications in these
     dual-stack nodes allows a limited amount of IPv4 communication
     using the IPv4-mapped IPv6 addresses.

      IPv6-only server:
          When an IPv4 client application sends data to an
          IPv6-only server application running on a dual-stack
          node using the wildcard address, the IPv4 client address
          is interpreted as the IPv4-mapped IPv6 address in the
          dual-stack node. This allows the IPv6 application to
          manage the communication. The IPv6 server will use this
          mapped address as if it were a regular IPv6 address, and
          a usual IPv6 connection. However, IPv4 packets will be
          exchanged between the nodes.  Kernels with dual stack
          properly interpret IPv4-mapped IPv6 addresses as IPv4
          ones and vice versa.

      IPv6-only client:
          IPv6-only client applications in a dual-stack node will
          not get IPv4-mapped addresses from the hostname
          resolution API functions unless a special hint,
          AI_V4MAPPED, is given. If given, the IPv6 client will
          use the returned mapped address as if it were a regular
          IPv6 address, and a usual IPv6 connection. However, again
          IPv4 packets will be exchanged between applications.

     Respectively, with IPV6_V6ONLY set, an IPv6-only server application
     will only communicate with IPv6 nodes, and an IPv6-only client with
     IPv6 servers, as the mapped addresses have been disabled. This
     option could be useful if applications use new IPv6 features, such
     as Flow Label. If communication with IPv4 is needed, either
     IPV6_V6ONLY must not be used, or dual-stack applications be used,
     as described in section 4.3.

     There are some implementations of dual-stack which do not allow
     IPv4-mapped IPv6 addresses to be used for interoperability between
     IPv4 and IPv6 applications. In that case, there are two ways to
     handle the problem:

      1. deploy two different versions of the application (possibly
         attached with '6' in the name), or

      2. deploy just one application supporting both protocol versions
         as described in the next section.

     The first method is not recommended because of a significant amount
     of problems associated with selecting the right applications. This
     problems are described in sections 3.2 and 3.3.

     Therefore, there are actually two distinct cases to consider when
     writing one application to support both protocols:

      1. whether the application can (or should) support both IPv4
         and IPv6 through IPv4-mapped IPv6 addresses, or should the
         applications support both explicitly (see section 4.3), and

      2. whether the systems where the applications are used support
         IPv6 at all or not (see section 4.4).

     Note that some systems will disable (by default) support for
     internal IPv4-mapped IPv6 addresses.  The security concerns
     regarding IPv4-mapped IPv6 addresses on the wire are legitimate but
     disabling it internally breaks one transition mechanism for server
     applications which were originally written to bind() and listen()
     to a single socket using a wildcard address. This forces the
     software developer to rewrite the daemon to create 2 separate
     sockets, one for IPv4 only and the other for IPv6 only, and then
     use select().  However, enabling mapping of IPv4 addresses on any
     particular system is controlled by the OS owner and not
     necessarilly by a developer.  This complicates the developer's work
     as he now has to rewrite the daemon network code to handle both
     environments, even for the same OS.

4.3 IPv4/IPv6 Applications in a Dual-stack Node

     Applications should be ported to support both IPv4 and IPv6; such
     applications are sometimes called IP version-independent
     applications.  After that, the existing IPv4-only applications
     could be removed.  Since we have only one version of each
     application, the source code will be typically easy to maintain and
     to modify, and there are no problems managing which application to
     select for which communication.

     This transition case is the most advisable.  During the IPv6
     transition period applications supporting both IPv4 and IPv6 should
     be able to communicate with other applications, irrespective of the
     versions of the protocol stack or the application in the node.
     Dual applications allow more interoperability between heterogeneous
     applications and nodes.

     If the source code is written in a protocol-independent way,
     without dependencies on either IPv4 or IPv6, applications will be
     able to communicate with any combination of applications and types
     of nodes.

     Implementations typically by-default prefer IPv6 if the remote node
     and application support it.  However, if IPv6 connections fail,
     version-independent applications will automatically try IPv4 ones.
     The resolver returns a list of valid addresses for the remote node
     and applications can iterate through all of them until connection

     Applications writers should be aware of this typical by-default
     ordering, but the applications themselves typically need not be
     aware of the the local protocol ordering [RFC 3484].

     If the source code is written in a protocol-dependent way, the
     application will support IPv4 and IPv6 explicitly using 2 separate
     sockets.  Note that there are some differences in bind()
     implementation, whether you can first bind to the IPv6, and then
     IPv4, wildcard addresses.  It can be a pain to write applications
     that cope with this. If IPV6_V6ONLY is implemented, this becomes
     simpler. The reason the IPv4 wildcard bind fails on some systems is
     that the IPv4 address space is embedded into IPv6 address space
     when using IPv4-mapped IPv6 addresses.

     A more detailed porting guideline is described in section 6.

4.4. IPv4/IPv6 Applications in an IPv4-only Node

     As the transition is likely to happen over a longer timeframe,
     applications that have already been ported to support both IPv4 and
     IPv6 may be run on IPv4-only nodes. This would typically be done to
     avoid having to support two application versions for older and
     newer operating systems, or to support the case that the user wants
     to disable IPv6 for some reason.

     The most important case is the application support on systems where
     IPv6 support can be dynamically enabled or disabled by the users.
     Applications on such a system should be able to handle the
     situation where IPv6 would not be enabled.  The secondary scenario
     is when an application could be deployed on older systems which do
     not support IPv6 at all (even the basic getaddrinfo etc. APIs).  In
     that case the application designer has to make a case-by-case
     judgement call whether it makes sense to have compile-time toggle
     between an older and newer API (having to support both in the
     code), or whether to provide getaddrinfo etc. function support on
     older platforms as part of the application libraries.

     Depending on how application/operating system support is done, some
     may want to ignore this case, but usually no assumptions can be
     made and applications should also work in this scenario.

     An example is an application that issues a socket() command, first
     trying AF_INET6 and then AF_INET.  However, if the kernel does not
     have IPv6 support, the call will result in an EPROTONOSUPPORT or
     EAFNOSUPPORT error. Typically, encountering errors like these leads
     to exiting the socket loop, and AF_INET will not even be tried.
     The application will need to handle this case or build the loop in
     such a way that errors are ignored until the last address family.

     So, this case is just an extension of the IPv4/IPv6 support in the
     previous case, covering one relatively common but often ignored

5. Application Porting Considerations

     The minimum changes to IPv4 applications to work with IPv6 are
     based on the different size and format of IPv4 and IPv6 addresses.

     Applications have been developed with the assumption they would use
     IPv4 as their network protocol. This assumption results in many IP
     dependencies through source code.

     The following list summarizes the more common IP version
     dependencies in applications:

      a) Presentation format for an IP address: it is an ASCII string
         which represents the IP address, dotted-decimal string
         for IPv4 and hexadecimal string for IPv6.

      b) Transport layer API: functions to establish communications
         and to exchange information.

      c) Name and address resolution: conversion functions between
         hostnames and IP addresses, and vice versa.

      d) Specific IP dependencies: more specific IP version
         dependencies, such as: IP address selection,
         application framing, storage of IP addresses.

      e) Multicast applications: one must find the IPv6 equivalents to
         the IPv4 multicast addresses, and use the right socket
         configuration options.

     In the following subsections, the problems with the aforementioned
     IP version dependencies are analyzed. Although application source
     code can be ported to IPv6 with minimum changes related to IP
     addresses, some recommendations are given to modify the source code
     in a protocol independent way, which will allow applications to
     work using both IPv4 and IPv6.

5.1 Presentation Format for an IP Address

     Many applications use IP addresses to identify network nodes and to
     establish connections to destination addresses. For instance, using
     the client/server model, clients usually need an IP address as an
     application parameter to connect to a server. This IP address is
     usually provided in the presentation format, as a string.  There
     are two problems, when porting the presentation format for an IP
     address: the allocated memory and the management of the
     presentation format.

     Usually, the allocated memory to contain an IPv4 address
     representation as a string is unable to contain an IPv6 address.
     Applications should be modified to prevent buffer overflows made
     possible by the larger IPv6 address.

     IPv4 and IPv6 do not use the same presentation format. IPv4 uses a
     dot (.) to separate the four octets written in decimal notation and
     IPv6 uses a colon (:) to separate each pair of octets written in
     hexadecimal notation. notation [RFC 3513].  In order cases where it one must be
     able to support both IPv4 and IPv6, specify e.g., port numbers with the
     management functions of presentation format, such as IP address
     parsers, should (see below), it
     may be changed desirable to be compliant with both of require placing the address inside the formats square
     brackets [TextRep].

     A particular problem with IP address parsers comes when the input
     is actually a combination of IP address and port number.  With IPv4
     these are often coupled with a semi-colon colon such as "".
     However, such an approach would be ambiguous with IPv6 as colons
     are already used to structure the address.

     Therefore, the IP address parsers which take the port number
     separated with a colon should represent IPv6 addresses somehow.
     One way is to enclose the address in brackets, as is done with
     Uniform Resource Locators (URLs) [RFC 2732], like

     Prefix/len format should be

     Some applications also considered if surrounding brackets
     are used.  In order need to avoid ambiguity, specify IPv6 prefixes and lengths;
     the format, prefix length should be inserted outside of the square
     brackets, if used, like [2001:db8::]/64 or 2001:db8::/64 -- not for
     example [2001:db8::/64].  Note that prefix/length notation is recommended.
     syntactically indistinguishable from a legal URI; therefore the
     prefix/length notation must not be used when it isn't clear from
     the context that it's used to specify the prefix and length and
     not e.g., a URI.

     In some specific cases, it may be necessary to give a zone
     identifier as part of the address, like fe80::1%eth0.  In general,
     applications should not need to parse these identifiers.

     The IP address parsers should support enclosing the IPv6 address in
     brackets even when it's not used in conjunction with a port number,
     but requiring that the user always gives a literal IP address
     enclosed in brackets is not recommended.

     One should note that some applications may also represent IPv6
     address literals differently; for example, SMTP [RFC 2821] uses

     Note that the use of address literals is strongly discouraged for
     general purpose direct input to the applications; host names and
     DNS should be used instead.

5.2 Transport Layer API

     Communication applications often include a transport module that
     establishes communications.  Usually this module manages everything
     related to communications and uses a transport layer API, typically
     as a network library. When porting an application to IPv6, most
     changes should be made in this application transport module in
     order to be adapted to the new IPv6 API.

     In the general case, porting an existing application to IPv6
     requires an examination of the following issues related to the API:

      - Network information storage: IP address data structures.
        The new structures must contain 128-bit IP addresses. The use of
        generic address structures, which can store any address family,
        is recommended.

        Sometimes special addresses are hard-coded in the application
        source code; developers should pay attention to them in order to
        use the new address format. Some of these special IP addresses
        are: wildcard local, loopback and broadcast. IPv6 does not have
        the broadcast addresses, so applications can use multicast

      - Address conversion functions.
        The address conversion functions convert the binary address
        representation to the presentation format and vice versa. The
        new conversion functions are specified to the IPv6 address

      - Communication API functions.
        These functions manage communications. Their signatures are
        defined based on a generic socket address structure. The
        same functions are valid for IPv6, however, the IP address data
        structures used when calling these functions require the

      - Network configuration options.
        They are used when configuring different communication models
        for Input/Output (I/O) operations (blocking/nonblocking, I/O
        multiplexing, etc.) and should be translated to the IPv6 ones.

5.3 Name and Address Resolution

     From the application point of view, the name and address resolution
     is a system-independent process. An application calls functions in
     a system library, the resolver, which is linked into the
     application when this is built. However, these functions use IP
     address structures, which are protocol dependent, and must be
     reviewed to support the new IPv6 resolution calls.

     There are two basic resolution functions. The first function
     returns a list of all configured IP addresses for a hostname. These
     queries can be constrained to one protocol family, for instance
     only IPv4 or only IPv6 addresses.  However, the recommendation is
     that all configured IP addresses should be obtained to allow
     applications to work with every kind of node.  And the second
     function returns the hostname associated to an IP address.


5.4 Specific IP Dependencies

5.4.1 IP Address Selection

     IPv6 promotes the configuration of multiple IP addresses per node,
     which is a difference when compared with the IPv4 model; however
     applications only use a destination/source pair for a
     communication.  Choosing the right IP source and destination
     addresses is a key factor that may determine the route of IP

     Typically nodes, not applications, automatically solve the source
     address selection. A node will choose the source address for a
     communication following some rules of best choice, [RFC 3484], but
     also allowing applications to make changes in the ordering rules.

     When selecting the destination address, applications usually ask a
     resolver for the destination IP address. The resolver returns a set
     of valid IP addresses from a hostname. Unless applications have a
     specific reason to select any particular destination address, they
     should just try each element in the list until the communication

     In some cases, the application may need to specify its source
     address.  Then the destination address selection process picks the
     best destination for the source address (instead of picking the
     best source address for the chosen destination address).  Note that
     there may be an increase in complexity for IP-version independent
     applications which have to specify the source address (especially
     for client applications; fortunately, specifying the source address
     is not typically required), if it is not yet known which protocol
     will be used for communication.

5.4.2 Application Framing

     The Application Level Framing (ALF) architecture controls
     mechanisms that traditionally fall within the transport layer.
     Applications implementing ALF are often responsible for packetizing
     data into Application Data Units (ADUs). The application problem
     when using ALF is the ADU size selection to obtain better

     Application framing is typically needed by applications using
     connectionless protocols (such as UDP). The application will  Such applications have
     three choices: (1) to know, or be able to detect, the use packet sizes which can be sent
     and received, end-to-end, on the network.

     Applications can use 1280 octets as a data length: every no larger than IPv6 link
     must have a IPv6
     minimum Maximum Transmission Unit (MTU) of 1280 octets bytes [RFC2460],
     (2) to use whatever packet sizes but force IPv6
     fragmentation/reassembly when necessary, or
     greater [RFC 2460]. However, in order (3) to get better performance,
     ADU optimize the
     packet size should and avoid unnecessary fragmentation/reassembly, guess
     or find out the optimal packet sizes which can be calculated based sent and
     received, end-to-end, on the length of transmission
     unit of underlying protocols. network.  This memo takes no stance on
     which approach to adopt.

     Note that the most optimal ALF depends on dynamic factors such as
     Path MTU or whether IPv4 or IPv6 is being used (due to different
     header sizes, possible IPv6-in-IPv4 tunneling overhead, etc.).
     These have to be taken into consideration when implementing
     application framing.

5.4.3 Storage of IP Addresses

     Some applications store IP addresses as information of remote
     peers. For instance, one of the most popular ways to register
     remote nodes in collaborative applications is based on using IP
     addresses as registry keys.

     Although the source code that stores IP addresses can be modified
     to IPv6 following the previous basic porting recommendations, there
     are some reasons why applications should not store IP addresses:

      - IP addresses can change throughout time, for instance
        after a renumbering process.

      - The same node can reach a destination host using different
        IP addresses, possibly with a different protocol version.

     When possible, applications should store names such as FQDNs, or
     other protocol-independent identities instead of storing addresses.
     In this case applications are only bound to specific addresses at
     run time, or for the duration of a cache lifetime.  Other types of
     applications, such as massive peer to peer systems with their own
     rendezvous and discovery mechanisms, may need to cache addresses
     for performance reasons, but cached addresses should not be treated
     as permanent, reliable information.  In highly dynamic networks any
     form of name resolution may be impossible, and here again addresses
     must be cached.

5.5 Multicast Applications

     There is an additional problem in porting multicast applications.
     When using multicast facilities some changes must be carried out to
     support IPv6. First, applications must change the IPv4 multicast
     addresses to IPv6 ones, and second, the socket configuration
     options must be changed.

     All the IPv6 multicast addresses encode scope; the scope was only
     implicit in IPv4 (with multicast groups in 239/8).  Also, while a
     large number of application-specific multicast addresses have been
     assigned with IPv4, this has been (luckily enough) avoided in IPv6.
     So, there are no direct equivalents for all the multicast
     addresses.  For link-local multicast, it's possible to pick almost
     anything within the link-local scope.  The global groups could use
     unicast-prefix-based addresses [RFC 3306].  All in all, this may
     force the application developers to write more protocol dependent

     Another problem is/has been that IPv6 multicast does not yet have a
     standardized mechanism for traditional Any Source Multicast for
     Interdomain multicast.  The models for Any Source Multicast (ASM)
     or Source-Specific Multicast (SSM) are generally similar between
     IPv4 and IPv6, but it is possible that PIM-SSM will become more
     widely deployed in IPv6 due to its simpler architecture.

     So, it might be beneficial to port the applications to use SSM
     semantics, requiring off-band source discovery mechanisms and the
     use of a different API [RFC 3678].  Inter-domain ASM service is
     available only through a method embedding the Rendezvous Point
     address in the multicast address [Embed-RP].

     Another generic problem for multiparty conferencing applications,
     which is similar to the issues with peer-to-peer applications, is
     that all the users of the session must use the same protocol
     version (IPv4 or IPv6), or some form of proxies or translators must
     be used (e.g., [MUL-GW]).

6. Developing IP version-independent Applications

     As we have seen before, dual applications working with both IPv4
     and IPv6 are recommended. These applications should avoid IP
     dependencies in the source code.  However, if IP dependencies are
     required, one of the best solutions is based on building a
     communication library which provides an IP version independent API
     to applications and hides all dependencies.

     In order to develop IP version independent applications, the
     following guidelines should be considered.

6.1 IP version-independent Structures

     All of the memory structures and APIs should be IP version-
     independent. In that sense, one should avoid structs in_addr,
     in6_addr, sockaddr_in and sockaddr_in6.

     Suppose you pass a network address to some function, foo().  If you
     use struct in_addr or struct in6_addr, you will end up with an
     extra parameter to indicate address family, as below:

      struct in_addr in4addr;
      struct in6_addr in6addr;
       /* IPv4 case */
      foo(&in4addr, AF_INET);
       /* IPv6 case */
      foo(&in6addr, AF_INET6);

     However, this leads to duplicated code and having to consider each
     scenario from both perspectives independently; this is difficult to
     maintain. So, we should use struct sockaddr_storage like below.

      struct sockaddr_storage ss;
      int sslen;
      /* AF independent! - use sockaddr when passing a pointer */
      /* note: it's typically necessary to also pass the length
         explicitly */
      foo((struct sockaddr *)&ss, sslen);

6.2 IP version-independent APIs

     getaddrinfo() and getnameinfo() are new address independent
     variants that hide the gory details of name-to-address and
     address-to-name address-
     to-name translations.  They implement functionalities of the
     following functions:


     They also obsolete the functionality of gethostbyname2(), defined
     in [RFC2133].

     These can perform hostname/address and service name/port lookups,
     though the features can be turned off if desirable. Getaddrinfo()
     can return multiple addresses, as below:

       localhost.      IN A
                       IN A
                       IN AAAA ::1

     In this example, if IPv6 is preferred, getaddrinfo returns first
     ::1, and then both and is in a random order.

     Getaddrinfo() and getnameinfo() can query hostname as well as
     service name/port at once.

     It is not preferred to hardcode AF-dependent knowledge into the
     program. The construct like below should be avoided:

       /* BAD EXAMPLE */
       switch (sa->sa_family) {
       case AF_INET:
               salen = sizeof(struct sockaddr_in);

     Instead, we should use the ai_addrlen member of the addrinfo
     structure, as returned by getaddrinfo().

     The gethostbyname(), gethostbyaddr(), getservbyname(), and
     getservbyport() are mainly used to get server and client sockets.
     Following, we will see simple examples to create these sockets
     using the new IPv6 resolution functions.

6.2.1 Example of Overly Simplistic TCP Server Application

     A simple TCP server socket at service name (or port number string)

       * BAD EXAMPLE: does not implement the getaddrinfo loop as
       * specified in 6.3. This may result in one of the following:
       *  - an IPv6 server, listening at the wildcard address,
       *    allowing IPv4 addresses through IPv4-mapped IPv6 addresses.
       *  - an IPv4 server, if IPv6 is not enabled,
       *  - an IPv6-only server, if IPv6 is enabled but IPv4-mapped IPv6
       *    addresses are not used by default, or
       *  - no server at all, if getaddrinfo supports IPv6, but the
       *    system doesn't, and socket(AF_INET6, ...) exits with an
       *    error.
      struct addrinfo hints, *res;
      int error, sockfd;

      memset(&hints, 0, sizeof(hints));
      hints.ai_flags = AI_PASSIVE;
      hints.ai_family = AF_UNSPEC;
      hints.ai_socktype = SOCK_STREAM;

      error = getaddrinfo(NULL, SERVICE, &hints, &res);
      if (error != 0) {
         /* handle getaddrinfo error */

      sockfd = socket(res->family, res->ai_socktype, res->ai_protocol);
      if (sockfd < 0) {
         /* handle socket error */

      if (bind(sockfd, res->ai_addr, res->ai_addrlen) < 0) {
         /* handle bind error */

      /* ... */

6.2.2 Example of Overly Simplistic TCP Client Application

     A simple TCP client socket connecting to a server which is running
     at node name (or IP address presentation format) SERVER_NODE and
     service name (or port number string) SERVICE:

       * BAD EXAMPLE: does not implement the getaddrinfo loop as
       * specified in 6.3. This may result in one of the following:
       *  - an IPv4 connection to an IPv4 destination,
       *  - an IPv6 connection to an IPv6 destination,
       *  - an attempt to try to reach an IPv6 destination (if AAAA
       *    record found), but failing -- without fallbacks -- because:
       *     o getaddrinfo supports IPv6 but the system does not
       *     o IPv6 routing doesn't exist, so falling back to e.g. TCP
       *       timeouts
       *     o IPv6 server reached, but service not IPv6-enabled or
       *       firewalled away
       *  - if the first destination is not reached, there is no
       *    fallback to the next records
      struct addrinfo hints, *res;
      int error, sockfd;

      memset(&hints, 0, sizeof(hints));
      hints.ai_family = AF_UNSPEC;
      hints.ai_socktype = SOCK_STREAM;

      error = getaddrinfo(SERVER_NODE, SERVICE, &hints, &res);
      if (error != 0) {
           /* handle getaddrinfo error */

      sockfd = socket(res->family, res->ai_socktype, res->ai_protocol);
      if (sockfd < 0) {
           /* handle socket error */

      if (connect(sockfd, res->ai_addr, res->ai_addrlen) < 0 ) {
           /* handle connect error */

      /* ... */


6.2.3 Binary/Presentation Format Conversion

     In addition, we should consider the binary and presentation address
     format conversion APIs.  The following functions convert network
     address structure in its presentation address format and vice


     Both are from the basic socket extensions for IPv6. However, these
     conversion functions are protocol-dependent; instead it is better
     to use getnameinfo()/getaddrinfo() as follows (inet_pton and
     inet_ntop equivalents are described in Appendix A).

     Conversion from network address structure to presentation format
     can be written:

      struct sockaddr_storage ss;
      char addrStr[INET6_ADDRSTRLEN];
      char servStr[NI_MAXSERV];
      int error;

      /* fill ss structure */

      error = getnameinfo((struct sockaddr *)&ss, sizeof(ss),
                          addrStr, sizeof(addrStr),
                          servStr, sizeof(servStr),

     Conversions from presentation format to network address structure
     can be written as follows:

      struct addrinfo hints, *res;
      char addrStr[INET6_ADDRSTRLEN];
      int error;

      /* fill addrStr buffer */

      memset(&hints, 0, sizeof(hints));
      hints.ai_family = AF_UNSPEC;

      error = getaddrinfo(addrStr, NULL, &hints, &res);
      if (error != 0) {
          /* handle getaddrinfo error */

      /* res->ai_addr contains the network address structure */
      /* ... */

6.3 Iterated Jobs for Finding the Working Address

     In a client code, when multiple addresses are returned from
     getaddrinfo(), we should try all of them until connection succeeds.
     When a failure occurs with socket(), connect(), bind(), or some
     other function, the code should go on to try the next address.

     In addition, if something is wrong with the socket call because the
     address family is not supported (i.e., in case of section 4.4),
     applications should try the next address structure.

     Note: in the following examples, the socket() return value error
     handling could be simplied by substituting special checking of
     specific error numbers by always continuing on with the socket

6.3.1 Example of TCP Server Application

     The previous example TCP server example should be written:

      #define MAXSOCK 2
      struct addrinfo hints, *res;
      int error, sockfd[MAXSOCK], nsock=0;

      memset(&hints, 0, sizeof(hints));
      hints.ai_flags = AI_PASSIVE;
      hints.ai_family = AF_UNSPEC;
      hints.ai_socktype = SOCK_STREAM;

      error = getaddrinfo(NULL, SERVICE, &hints, &res);
      if (error != 0) {
          /* handle getaddrinfo error */

      for (aip=res; aip && nsock < MAXSOCK; aip=aip->ai_next) {
          sockfd[nsock] = socket(aip->ai_family,

          if (sockfd[nsock] < 0) {
              switch errno {
                   case EAFNOSUPPORT:
                   case EPROTONOSUPPORT:
                        *  e.g., skip the errors until
                        *  the last address family,
                        *  see section 4.4.
                        if (aip->ai_next)
                        else {
                               /* handle unknown protocol errors */
                        /* handle other socket errors */

          } else {
              int on = 1;
              /* optional: works better if dual-binding to wildcard
                 address */
              if (aip->ai_family == AF_INET6) {
                  setsockopt(sockfd[nsock], IPPROTO_IPV6, IPV6_V6ONLY,
                             (char *)&on, sizeof(on));
                  /* errors are ignored */
              if (bind(sockfd[nsock], aip->ai_addr,
                                      aip->ai_addrlen) < 0 ) {
                  /* handle bind error */
              if (listen(sockfd[nsock], SOMAXCONN) < 0) {
                  /* handle listen errors */

      /* check that we were able to obtain the sockets */

6.3.2 Example of TCP Client Application

     The previous TCP client example should be written:

      struct addrinfo hints, *res, *aip;
      int sockfd, error;

      memset(&hints, 0, sizeof(hints));
      hints.ai_family   = AF_UNSPEC;
      hints.ai_socktype = SOCK_STREAM;

      error = getaddrinfo(SERVER_NODE, SERVICE, &hints, &res);
      if (error != 0) {
          /* handle getaddrinfo error */
      for (aip=res; aip; aip=aip->ai_next) {

          sockfd = socket(aip->ai_family,

          if (sockfd < 0) {
              switch errno {
                   case EAFNOSUPPORT:
                   case EPROTONOSUPPORT:
                        *  e.g., skip the errors until
                        *  the last address family,
                        *  see section 4.4.
                        if (aip->ai_next)
                        else {
                               /* handle unknown protocol errors */

                        /* handle other socket errors */

          } else {
              if (connect(sockfd, aip->ai_addr, aip->ai_addrlen) == 0)

              /* handle connect errors */

      if (sockfd > 0) {
          /* socket connected to server address */

          /* ... */


7. Transition Mechanism Considerations

     A mechanism, [NAT-PT], introduces a special set of addresses,
     formed of NAT-PT prefix and an IPv4 address; this refers to IPv4
     addresses, translated by NAT-PT DNS-ALG.  In some cases, one might
     be tempted to handle these differently.

     However, IPv6 applications must not be required to distinguish
     "normal" and "NAT-PT translated" addresses (or any other kind of
     special addresses, including the IPv4-mapped IPv6-addresses): that
     would be completely impractical, and if such distinction must be
     made, it must be done elsewhere (e.g. kernel, system libraries).

8. Security Considerations

     There are a number of security considerations with IPv6 transition
     but those are outside the scope of this memo.

     To ensure the availability and robustness of the service even when
     transitioning to IPv6, this memo described a number of ways to make
     applications more resistant to failures by cycling through
     addresses until a working one is found.  Doing this properly is
     critical to avoid unavailability and loss of service.

     One particular point about application transition is how IPv4-
     mapped IPv6-addresses
     IPv4-mapped IPv6 addresses are handled.  The use in the API can be
     seen as both a merit (easier application transition) and as a
     burden (difficulty in ensuring whether the use was legimate) Note
     that some systems will disable (by default) support for internal
     IPv4-mapped IPv6 addresses.  The security concerns regarding
     IPv4-mapped IPv6 addresses on the wire are legitimate but disabling
     it internally breaks one transition mechanism for server
     applications which were originally written to bind() and listen()
     to a single socket using a wildcard address [V6MAPPED].  This
     should be considered in more detail when designing applications.

9. Acknowledgements

     Some of guidelines for development of IP version-independent
     applications (section 6) were first brought up by [AF-APP].  Other
     work to document application porting guidelines has also been in
     progress, for example [IP-GGF] and [PRT].  We would like to thank
     the members of the the v6ops working group and the application area
     for helpful comments.  Special thanks are due to Brian E.
     Carpenter, Antonio Querubin, Stig Venaas, Chirayu Patel, and Jordi
     Palet, and Jason Lin for extensive review of this document.  We
     acknowledge Ron Pike for proofreading the document.

10. References

 Normative References

 [RFC 3493]  R. Gilligan, S. Thomson, J. Bound, W. Stevens, "Basic
             Socket Interface Extensions for IPv6," RFC 3493, February

 [RFC 3542]  W. Stevens, M. Thomas, E. Nordmark, T. Jinmei, "Advanced
             Sockets Application Program Interface (API) for IPv6,"
             RFC 3542, May 2003.

 [BIS]       K. Tsuchiya, H. Higuchi, Y. Atarashi, "Dual Stack Hosts
             using the "Bump-In-the-Stack" Technique (BIS)," RFC 2767,
             February 2000.

 [BIA]       S. Lee, M-K. Shin, Y-J. Kim, E. Nordmark, A. Durand,
             "Dual Stack Hosts using "Bump-in-the-API" (BIA)," RFC
             3338, October 2002.

 [RFC 2460]  S. Deering, R. Hinden, "Internet Protocol, Version 6
             (IPv6) Specification,", Specification," RFC 2460, December 1998.

 [RFC 3484]  R. Draves, "Default Address Selection for IPv6,"
             RFC 3484, February 2003.

 [RFC 3513]  R. Hinden, S. Deering, "Internet Protocol Version 6
             (IPv6) Addressing Architecture," RFC 3513, April 2003.

 Informative References

 [2893BIS]   E. Nordmark, R. E. Gilligan, "Basic Transition Mechanisms
             for IPv6 Hosts and Routers," <draft-ietf-v6ops-mech-v2-
             02.txt>, January
             03.txt>, June 2004, Work-in-progress.

 [RFC 2732]  R. Hinden, B. Carpenter, L. Masinter, "Format for Literal
             IPv6 Addresses in URL's," RFC 2732, December 1999.

 [RFC 2821]  J. Klensin, "Simple Mail Transfer Protocol," RFC 2821,
             April 2001.

 [TextRep]   A. Main, "Textual Representation of IPv4 and IPv6
             Addresses," <draft-main-ipaddr-text-rep-01.txt>, Oct 2003,
             Work in Progress.

 [NAT-PT]    G. Tsirtsis, P. Srisuresh, "Network Address Translation
             - Protocol Translation (NAT-PT)," RFC 2766, February 2000.

 [DNSTRANS]  A. Durand, J. Ihren, "DNS IPv6 transport operational
             guidelines," <draft-ietf-dnsop-ipv6-transport-guidelines-
             00.txt>, June 2003,
             02.txt>, March 2004, Work in Progress.

 [DNSOPV6]   A. Durand, J. Ihren, P. Savola, "Operational Considerations
             and Issues with IPv6 DNS," <draft-ietf-dnsop-ipv6-dns-
             issues-03.txt>, November 2003,
             issues-07.txt>, May 2004, Work in Progress.

 [AF-APP]    J. Hagino, "Implementing AF-independent application",
             http://www.kame.net/newsletter/19980604/, 2001.

 [V6MAPPED]  J. Hagino, "IPv4 mapped address considered harmful",
             <draft-itojun-v6ops-v4mapped-harmful-02.txt>, Apr 2002,
             Work in Progress.

 [IP-GGF]    T. Chown, J. Bound, S. Jiang, P. O'Hanlon, "Guidelines for
             IP version independence in GGF specifications," Global
             Grid Forum(GGF) Documentation, September 2003, Work in

 [Embed-RP]  P. Savola, B. Haberman, "Embedding the Address of RP in
             IPv6 Multicast Address," <draft-ietf-mboned-embeddedrp-
             00.txt>, October 2003, Work in Progress.

 [RFC 3306]  B. Haberman, D. Thaler, "Unicast-Prefix-based IPv6
             Multicast Addresses," RFC 3306, August 2002.

 [RFC 3678]  D. Thaler, B. Fenner, B. Quinn, "Socket Interface
             Extensions for Multicast Source Filters, RFC 3678, January

 [MUL-GW]    S. Venaas, "An IPv4 - IPv6 multicast gateway," <draft-
             venaas-mboned-v4v6mcastgw-00.txt>, February 2003,
             Work in Progress.

 [PRT]       E. M. Castro, "Programming guidelines on transition to
             IPv6, LONG project, January 2003.

Authors' Addresses

       Myung-Ki Shin
  161 Gajeong-Dong, Yuseong-Gu, Daejeon 305-350, Korea
       820 West Diamond Avenue
       Gaithersburg, MD 20899, USA
       Tel : +82 42 860 4847 +1 301 975-3613
       Fax : +82 42 861 5404 +1 301 590-0932
       E-mail : mkshin@pec.etri.re.kr mshin@nist.gov

       Yong-Guen Hong
       ETRI PEC
       161 Gajeong-Dong, Yuseong-Gu, Daejeon 305-350, Korea
       Tel : +82 42 860 6447
       Fax : +82 42 861 5404
       E-mail : yghong@pec.etri.re.kr

       Jun-ichiro itojun HAGINO
       Research Laboratory, Internet Initiative Japan Inc.
       Takebashi Yasuda Bldg.,
       3-13 Kanda Nishiki-cho,
       Chiyoda-ku,Tokyo 101-0054, JAPAN
       Tel: +81-3-5259-6350
       Fax: +81-3-5259-6351
       E-mail: itojun@iijlab.net

       Pekka Savola
       Espoo, Finland
       E-mail: psavola@funet.fi

       Eva M. Castro
       Rey Juan Carlos University (URJC)
       Departamento de Informatica, Estadistica y Telematica
       C/Tulipan s/n
       28933 Madrid - SPAIN
       E-mail: eva@gsyc.escet.urjc.es

Appendix A. Other binary/Presentation Format Conversions

     Section 6.2.3 described the preferred way of performing
     binary/presentation format conversions; these can also be done
     using inet_pton() and inet_ntop() by writing protocol-dependent
     code.  This is not recommended, but provided here for reference and

     Note that inet_ntop()/inet_pton() lose the scope identifier (if
     used e.g. with link-local addresses) in the conversions, contrary
     to the getaddrinfo()/getnameinfo() functions.

A.1 Binary to Presentation using inet_ntop()

     Conversions from network address structure to presentation format
     can be written:

      struct sockaddr_storage ss;
      char addrStr[INET6_ADDRSTRLEN];

      /* fill ss structure */

      switch (ss.ss_family) {

           case AF_INET:
                         &((struct sockaddr_in *)&ss)->sin_addr,

           case AF_INET6:
                          &((struct sockaddr_in6 *)&ss)->sin6_addr,


                /* handle unknown family */

     Note, the destination buffer addrStr should be long enough to
     contain the presentation address format: INET_ADDRSTRLEN for IPv4
     and INET6_ADDRSTRLEN for IPv6. Since INET6_ADDRSTRLEN is longer
     than INET_ADDRSTRLEN, the first one is used as the destination
     buffer length.

A.2 Presentation to Binary using inet_pton()

     Conversions from presentation format to network address structure
     can be written as follows:

      struct sockaddr_storage ss;
      struct sockaddr_in *sin;
      struct sockaddr_in6 *sin6;
      char addrStr[INET6_ADDRSTRLEN];

      /* fill addrStr buffer and ss.ss_family */

      switch (ss.ss_family) {
            case AF_INET:
                  sin = (struct sockaddr_in *)&ss;
                            (sockaddr *)&sin->sin_addr));

            case AF_INET6:
                  sin6 = (struct sockaddr_in6 *)&ss;
                            (sockaddr *)&sin6->sin6_addr);

                /* handle unknown family */

     Note, the address family of the presentation format must be known.

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