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Versions: (draft-shin-v6ops-application-transition)
00 01 02 03 RFC 4038
v6ops Working Group M-K. Shin (ed.)
INTERNET DRAFT ETRI/NIST
Expires: December 2004 Y-G. Hong
ETRI
J. Hagino
IIJ
P. Savola
CSC/FUNET
E. M. Castro
GSYC/URJC
June 2004
Application Aspects of IPv6 Transition
<draft-ietf-v6ops-application-transition-03.txt>
Status of this Memo
By submitting this Internet-Draft, I certify that any applicable
patent or other IPR claims of which I am aware have been disclosed,
and any of which I become aware will be disclosed, in accordance
with RFC 3668.
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The list of Internet-Draft Shadow Directories can be accessed at
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This Internet-Draft will expire on December 2004.
Abstract
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.
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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 .............................18
6.2.1 Example of Overly Simplistic TCP Server Application ..19
6.2.2 Example of Overly Simplistic TCP Client Application ..20
6.2.3 Binary/Presentation Format Conversion ................21
6.3 Iterated Jobs for Finding the Working Address ...........22
6.3.1 Example of TCP Server Application ....................22
6.3.2 Example of TCP Client Application ....................23
7. Transition Mechanism Considerations .......................24
8. Security Considerations ...................................25
9. Acknowledgements .........................................25
10. References ...............................................25
Authors' Addresses ...........................................27
Appendix A. Other Binary/Presentation Format Conversions .....28
A.1 Binary to Presentation using inet_ntop() ................28
A.2 Presentation to Binary using inet_pton() ................29
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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
follows:
+-------------------+
| 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
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+-------------------+ (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
removed.
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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
time.
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
all).
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).
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Using SRV records would avoid these problems. Unfortunately, they
are not sufficiently widely used to be applicable in most cases.
Hence an operational technique is to use "service names" in the
DNS, that is, if a node is offering multiple services, but only
some of them over IPv6, add a DNS name for each of these services
(with the associated A/AAAA records), not just a single name for
the whole node, also including the AAAA records. However, the
applications cannot depend on 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
available.
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 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 applications would
have to do more than just perform a DNS lookup or figure out the
literal IP address given, they will get complex -- likely much more
complex than writing a hybrid application. Furthermore, "wrapping"
applications which perform complex operations with IP addresses
(like FTP clients) might be even more challenging or even
impossible. In summary, wrapper applications does not look like a
robust approach for application transition.
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4. Description of 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
stack.
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. We strongly
recommend that application developers sould not use these
mechanisms when application source code is available. Also, it
should not be used as an excuse not to port software or delay
porting.
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
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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
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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).
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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
succeeds.
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
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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
case.
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.
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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
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hexadecimal notation [RFC 3513]. In cases where it one must be
able to specify e.g., port numbers with the address (see below), it
may be desirable to require placing the address inside the 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 colon such as "192.0.2.1:80".
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
http://[2001:db8::1]:80.
Some applications also need to specify IPv6 prefixes and lengths;
the 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
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
[IPv6:2001:db8::1].
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.
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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
instead.
- 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
format.
- 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
updates.
- 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.
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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
datagrams.
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
succeeds.
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
performance.
Application framing is typically needed by applications using
connectionless protocols (such as UDP). Such applications have
three choices: (1) to use packet sizes no larger than IPv6 IPv6
minimum Maximum Transmission Unit (MTU) of 1280 bytes [RFC2460],
(2) to use whatever packet sizes but force IPv6
fragmentation/reassembly when necessary, or (3) to optimize the
packet size and avoid unnecessary fragmentation/reassembly, guess
or find out the optimal packet sizes which can be sent and
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received, end-to-end, on the 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
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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
code.
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 */
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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 translations. They implement functionalities of the
following functions:
gethostbyname()
gethostbyaddr()
getservbyname()
getservbyport()
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 127.0.0.1
IN A 127.0.0.2
IN AAAA ::1
In this example, if IPv6 is preferred, getaddrinfo returns first
::1, and then both 127.0.0.1 and 127.0.0.2 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);
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break;
}
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)
SERVICE:
/*
* 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 */
}
/* ... */
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freeaddrinfo(res);
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 */
}
/* ... */
freeaddrinfo(res);
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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
versa:
inet_ntop()
inet_pton()
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),
NI_NUMERICHOST);
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 */
/* ... */
freeaddrinfo(res);
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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
loop.
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,
aip->ai_socktype,
aip->ai_protocol);
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)
continue;
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else {
/* handle unknown protocol errors */
break;
}
default:
/* 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 */
close(sockfd[nsock]);
continue;
}
if (listen(sockfd[nsock], SOMAXCONN) < 0) {
/* handle listen errors */
close(sockfd[nsock]);
continue;
}
}
nsock++;
}
freeaddrinfo(res);
/* 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 */
}
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for (aip=res; aip; aip=aip->ai_next) {
sockfd = socket(aip->ai_family,
aip->ai_socktype,
aip->ai_protocol);
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)
continue;
else {
/* handle unknown protocol errors */
break;
}
default:
/* handle other socket errors */
;
}
} else {
if (connect(sockfd, aip->ai_addr, aip->ai_addrlen) == 0)
break;
/* handle connect errors */
close(sockfd);
sockfd=-1;
}
}
if (sockfd > 0) {
/* socket connected to server address */
/* ... */
}
freeaddrinfo(res);
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.
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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 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, 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
2003.
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[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," 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-
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-
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-07.txt>, May 2004, Work in Progress.
[AF-APP] J. Hagino, "Implementing AF-independent application",
http://www.kame.net/newsletter/19980604/, 2001.
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[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
Progress.
[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
2004.
[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
ETRI/NIST
820 West Diamond Avenue
Gaithersburg, MD 20899, USA
Tel : +1 301 975-3613
Fax : +1 301 590-0932
E-mail : 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
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Fax: +81-3-5259-6351
E-mail: itojun@iijlab.net
Pekka Savola
CSC/FUNET
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
comparison.
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:
inet_ntop(ss.ss_family,
&((struct sockaddr_in *)&ss)->sin_addr,
addrStr,
sizeof(addrStr));
break;
case AF_INET6:
inet_ntop(ss.ss_family,
&((struct sockaddr_in6 *)&ss)->sin6_addr,
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addrStr,
sizeof(addrStr));
break;
default:
/* 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;
inet_pton(ss.ss_family,
addrStr,
(sockaddr *)&sin->sin_addr));
break;
case AF_INET6:
sin6 = (struct sockaddr_in6 *)&ss;
inet_pton(ss.ss_family,
addrStr,
(sockaddr *)&sin6->sin6_addr);
break;
default:
/* handle unknown family */
}
Note, the address family of the presentation format must be known.
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Shin et al. Expires December 2004 [Page 30]
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