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RFC4038 - Application Aspects of IPv6 Transition

王朝asp·作者佚名  2008-05-31
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Network Working Group M-K. Shin, Ed.

Request for Comments: 4038 ETRI/NIST

Category: Informational Y-G. Hong

ETRI

J. Hagino

IIJ

P. Savola

CSC/FUNET

E. M. Castro

GSYC/URJC

March 2005

Application ASPects of IPv6 Transition

Status of This Memo

This memo provides information for the Internet community. It does

not specify an Internet standard of any kind. Distribution of this

memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2005).

Abstract

As IPv6 networks are deployed and the network transition is

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 .... 6

3.3. Supporting Many Versions of an Application Is Difficult. 6

4. Description of Transition Scenarios and Guidelines ........... 7

4.1. IPv4 Applications in a Dual-Stack Node ................. 7

4.2. IPv6 Applications in a Dual-Stack Node ................. 8

4.3. IPv4/IPv6 Applications in a Dual-Stack Node ............ 11

4.4. IPv4/IPv6 Applications in an IPv4-only Node ............ 12

5. Application Porting Considerations ........................... 12

5.1. Presentation Format for an IP Address .................. 13

5.2. Transport Layer API .................................... 14

5.3. Name and Address Resolution ............................ 15

5.4. Specific IP Dependencies ............................... 16

5.4.1. IP Address Selection ........................... 16

5.4.2. Application Framing ............................ 16

5.4.3. Storage of IP addresses ........................ 17

5.5. Multicast Applications ................................. 17

6. Developing IP Version - Independent Applications ............. 18

6.1. IP Version - Independent Structures..................... 18

6.2. IP Version - Independent APIs........................... 19

6.2.1. Example of Overly Simplistic TCP Server

Application .................................... 20

6.2.2. Example of Overly Simplistic TCP Client

Application .................................... 21

6.2.3. Binary/Presentation Format Conversion .......... 22

6.3. Iterated Jobs for Finding the Working Address .......... 23

6.3.1. Example of TCP Server Application .............. 23

6.3.2. Example of TCP Client Application .............. 25

7. Transition Mechanism Considerations .......................... 26

8. Security Considerations ...................................... 26

9. Acknowledgments .............................................. 27

10. References ................................................... 27

Appendix A. Other Binary/Presentation Format Conversions ........ 30

A.1. Binary to Presentation Using inet_ntop() ............... 30

A.2. Presentation to Binary Using inet_pton() ............... 31

Authors' Addresses ............................................... 32

Full Copyright Statement ......................................... 33

1. Introduction

As IPv6 is introduced in the IPv4-based Internet, several general

issues will arise, such as routing, addressing, DNS, and scenarios.

An important key to a successful IPv6 transition is compatibility

with the large installed base of IPv4 hosts and routers. This issue

has already been extensively studied, and work is still in progress.

[2893BIS] describes the basic transition mechanisms: dual-stack

deployment and tunneling. Various other kinds of mechanisms have

been developed for the transition to an IPv6 network. However, these

transition mechanisms take no stance on whether applications support

IPv6.

This document specifies application aspects of IPv6 transition. Two

inter-related topics are covered:

1. How different network transition techniques affect

applications, and strategies for applications to support IPv6

and IPv4.

2. How to develop IPv6-capable or protocol-independent

applications ("application porting guidelines") using standard

APIs [RFC3493][RFC3542].

In the context of this document, the term "application" covers all

kinds of applications, but the focus is on those network applications

which have been developed using relatively low-level APIs (such as

the "C" language, using standard libraries). Many such applications

could be command-line driven, but that is not a requirement.

Applications will have to be modified to support IPv6 (and IPv4) by

using one of a number of techniques described in sections 2 - 4.

Guidelines for developing such applications are presented in sections

5 and 6.

2. Overview of IPv6 Application Transition

The transition of an application can be classified by using four

different cases (excluding the first case when there is no IPv6

support in either the application or the operating system):

+-------------------+

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

removed.

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).

The first two cases are not interesting in the longer term; only few

applications are inherently IPv4- or IPv6-specific, and should work

with both protocols without having to care about which one is being

used.

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 are likely to co-exist in a node for a long

time.

Similarly, most applications are eXPected to be able to handle both

IPv4 and IPv6 during another long period. A dual-stack operating

system is not intended to have both IPv4 and IPv6 applications.

Therefore, IPv6-capable application transition may be independent of

protocol stacks in a node.

Applications capable of both IPv4 and IPv6 will probably 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

In a node, the DNS name resolver gathers the list of destination

addresses. DNS queries and responses are sent by using either IPv4

or IPv6 to carry the queries, regardless of the protocol version of

the data records [DNSTRANS].

The DNS name resolution issue related to application transition is

that by only doing a DNS name lookup a client application can not be

certain of the version of the peer application. 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 an AAAA

record in the DNS, the client application will fail to connect to the

server application. This is caused by a mismatch between the DNS

query result (i.e., IPv6 addresses) and a server application version

(i.e., IPv4).

Using SRV records would avoid these problems. Unfortunately, they

are not used widely enough to be applicable in most cases. Hence an

operational solution is to use "service names" in the DNS. If a node

offers multiple services, but only some of them over IPv6, a DNS name

may be added for each of these services or group of services (with

the associated A/AAAA records), not just a single name for the

physical machine, also including the AAAA records. However, the

applications cannot depend on this operational practice.

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 if multiple versions of the same application are

available, the local users have difficulty selecting the right

version supporting the exact IP version required.

To avoid problems with one application not supporting the specified

protocol version, it is desirable to have hybrid applications

supporting both.

An alternative approach for local client applications could be to

have a "wrapper application" that performs certain tasks (such as

figuring out which protocol version will be used) and calls the

IPv4/IPv6-only applications as necessary. This application would

perform connection establishment (or similar tasks) and pass the

opened socket to another application. However, as applications such

as this would have to do more than just perform a DNS lookup or

determine the literal IP address given, they will become complex --

likely much more so than a hybrid application. Furthermore, writing

"wrapping" applications that perform complex operations with IP

addresses (such as FTP clients) might be even more challenging or

even impossible. In short, wrapper applications do not look like a

robust approach for application transition.

4. Description of Transition Scenarios and Guidelines

Once the IPv6 network is deployed, applications supporting IPv6 can

use IPv6 network services to 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 provide one solution to maintaining IPv4

compatibility in unicast communications. In this section we will

analyze different application transition scenarios (as introduced in

section 2) and guidelines for maintaining interoperability between

applications running in different types of nodes.

Note that the first two cases, IPv4-only and IPv6-only applications,

are not interesting in the longer term; only few applications are

inherently IPv4- or IPv6-specific, and should work with both

protocols without having to care about which one is being used.

4.1. IPv4 Applications in a Dual-Stack Node

In this scenario, 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 and accept/establish connections from/to nodes

that implement an IPv4 stack.

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., when 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 not use these mechanisms when application

source code is available. Also, they should not be used as an excuse

not to port software or to delay porting.

When [BIA] or [BIS] is used, the problem described in section 3.2

arises - (the IPv4 client in a [BIS]/[BIA] node tries to connect to

an IPv4 server in a dual stack system). However, one can rely on the

[BIA]/[BIS] mechanism, which should cycle through all the addresses

instead of applications.

[BIS] and [BIA] do not work with all kinds of applications - in

particular, with applications that 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. Therefore, 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 cannot work in IPv4-only nodes, so the old

IPv4 application should be maintained in these nodes. This

necessitates having two similar applications working with different

protocol versions, depending on the node they are running (e.g.,

telnet and telnet6). This case is undesirable, as maintaining two

versions of the same source code per application could be difficult.

This approach would also cause problems for users 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 by 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

addresses

+--------------------------------------------+

IPv4 packets IPv6 packets

We will analyze the behaviour of IPv6-applications that exchange IPv4

packets with IPv4 applications by using the client/server model. We

consider the default case to be when the IPV6_V6ONLY socket option

has not been set. In these dual-stack nodes, this default behavior

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 by 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

receive IPv4-mapped addresses from the hostname resolution API

functions unless a special hint, AI_V4MAPPED, is given. If it

is, the IPv6 client will use the returned mapped address as if

it were a regular IPv6 address, and a usual IPv6 connection.

However, 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 only

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 must be used, as

described in section 4.3.

Some implementations of dual-stack do not allow IPv4-mapped IPv6

addresses to be used for interoperability between IPv4 and IPv6

applications. In these cases, there are two ways to handle the

problem:

1. Deploy two different versions of the application (possibly

attached with '6' in the name).

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 number

of problems associated with selecting the right applications. These

problems are described in sections 3.2 and 3.3.

Therefore, there are 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 the applications

should support both explicitly (see section 4.3), and

2. Whether the systems in which the applications are used support

IPv6 (see section 4.4).

Note that some systems will disable (by default) support for internal

IPv4-mapped IPv6 addresses. The security concerns regarding these

are legitimate, but disabling them internally breaks one transition

mechanism for server applications originally written to bind() and

listen() to a single socket by using a wildcard address. This forces

the software developer to rewrite the daemon to create two separate

sockets, one for IPv4 only and the other for IPv6 only, and then to

use select(). However, mapping-enabling of IPv4 addresses on any

particular system is controlled by the OS owner and not necessarily

by a developer. This complicates developers' work, as they now have

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. Over

time, the existing IPv4-only applications could be removed. As we

have only one version of each application, the source code will

typically be 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

version 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 prefer IPv6 by default 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.

Application writers should be aware of this protocol ordering, which

is typically the default, but the applications themselves usually

need not be [RFC3484].

If the source code is written in a protocol-dependent way, the

application will support IPv4 and IPv6 explicitly by using two

separate sockets. Note that there are some differences in bind()

implementation - that is, in whether one can first bind to IPv6

wildcard addresses, and then to those for IPv4. Writing applications

that cope with this can be a pain. Implementing IPV6_V6ONLY

simplifies this. The IPv4 wildcard bind fails on some systems

because the IPv4 address space is embedded into IPv6 address space

when IPv4-mapped IPv6 addresses are used.

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 take place over a longer time frame,

applications already ported to support both IPv4 and IPv6 may be run

on IPv4-only nodes. This would typically be done to avoid supporting

two application versions for older and newer operating systems, or to

support a case in which 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 a situation

IPv6 would not be enabled. Another scenario is when an application

is deployed on older systems that do not support IPv6 at all (even

the basic APIs such as getaddrinfo). In this case, the application

designer has to make a case-by-case judgment call as to whether it

makes sense to have compile-time toggle between an older and a 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 application/operating system support, 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, errors like these lead 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 so that errors are

ignored until the last address family.

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 for IPv4 applications to work with IPv6 are based

on the different size and format of IPv4 and IPv6 addresses.

Applications have been developed with IPv4 network protocol in mind.

This assumption has resulted 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: An ASCII string that

represents the IP address, a dotted-decimal string for IPv4,

and a 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.

d) Specific IP dependencies: More specific IP version

dependencies, such as IP address selection, application

framing, and 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.

The following subsections describe the problems with the

aforementioned IP version dependencies. 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

with 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 memory allocated 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 [RFC3513]. In cases where one must be able to

specify, for example, 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; for example, "192.0.2.1:80".

However, this approach would be ambiguous with IPv6, as colons are

already used to structure the address.

Therefore, the IP address parsers that take the port number separated

with a colon should distinguish IPv6 addresses somehow. One way is

to enclose the address in brackets, as is done with Uniform Resource

Locators (URLs) [RFC2732]; for example, 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; for example, [2001:db8::]/64 or 2001:db8::/64 and not

[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, for example, a

URI.

In some specific cases, it may be necessary to give a zone identifier

as part of the address; for example, 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 the address is not used in conjunction with a

port number. Requiring that the user always give a literal IP

address enclosed in brackets is not recommended.

Note that some applications may also represent IPv6 address literals

differently; for example, SMTP [RFC2821] 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 an application is ported 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 these 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

These are used when different communication models are

configured for Input/Output (I/O) operations

(blocking/nonblocking, I/O multiplexing, etc.) and should be

translated for IPv6.

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 it is built. However, these functions use IP address

structures, that are protocol dependent and must be reviewed to

support the new IPv6 resolution calls.

With IPv6, there are two new basic resolution functions,

getaddrinfo() and getnameinfo(). The first 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, it is recommended that all configured IP

addresses be oBTained to allow applications to work with every kind

of node. The second function returns the hostname associated to an

IP address.

5.4. Specific IP Dependencies

5.4.1. IP Address Selection

Unlike the IPv4 model, IPv6 promotes the configuration of multiple IP

addresses per node, 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, per [RFC3484], but

will also allow 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 try each element in the list until the communication succeeds.

In some cases, the application may need to specify its source

address. 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 if it

is not yet known which protocol will be used for communication there

may be an increase in complexity for IP version - independent

applications that have to specify the source address (especially for

client applications. Fortunately, specifying the source address is

not typically required).

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 with ALF

arrives from the ADU size selection to obtain better performance.

Applications using connectionless protocols (such as UDP) typically

need application framing. These applications have three choices: (1)

to use packet sizes no larger than the IPv6 minimum Maximum

Transmission Unit (MTU) of 1280 bytes [RFC2460], (2) to use any

packet sizes, but to force IPv6 fragmentation/reassembly when

necessary, or (3) to optimize the packet size and avoid unnecessary

fragmentation/reassembly, and to guess or find out the optimal packet

sizes that can be sent and received, end-to-end, on the network.

This memo takes no stance on that approach is best.

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

factors have to be taken into consideration when application framing

is implemented.

5.4.3. Storage of IP Addresses

Some applications store IP addresses as remote peer information. For

instance, one of the most popular ways to register remote nodes in

collaborative applications uses IP addresses as registry keys.

Although the source code that stores IP addresses can be modified to

IPv6 by following the previous basic porting recommendations,

applications should not store IP addresses for the following reasons:

- 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 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 multicast facilities are used 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 IPv6 multicast addresses encode scope; the scope was only

implicit in IPv4 (with multicast groups in 239/8). Also, although a

large number of application-specific multicast addresses have been

assigned with IPv4, this has been (luckily enough) avoided with 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 [RFC3306]. All in all, this may force the

application developers to write more protocol-dependent code.

Another problem is 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.

It might be beneficial to port the applications to use SSM semantics,

requiring off-band source discovery mechanisms and a different API

[RFC3678]. Inter-domain ASM service is available only through a

method embedding the Rendezvous Point address in the multicast

address [Embed-RP].

Another generic problem with multiparty conferencing applications,

similar to the issues with peer-to-peer applications, is that all

users of the session must use the same protocol version (IPv4 or

IPv6), or some form of proxy or translator (e.g., [MUL-GW]).

6. Developing IP Version - Independent Applications

As stated, 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

better solutions would be to build a communication library that

provides an IP version - independent API to applications and that

hides all dependencies.

To develop IP version - independent applications, the following

guidelines should be considered.

6.1. IP Version - Independent Structures

All memory structures and APIs should be IP version-independent. One

should avoid structs in_addr, in6_addr, sockaddr_in, and

sockaddr_in6.

Suppose a network address is passed to some function, foo(). If one

uses struct in_addr or struct in6_addr, results 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);

This leads to duplicated code and having to consider each scenario

from both perspectives independently, which is difficult to maintain.

So we should use struct sockaddr_storage, as 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

The new address independent variants getaddrinfo() and getnameinfo()

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].

The new variants can perform hostname/address and service name/port

lookups, though the features can be turned off, if desired.

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 first returns ::1;

then both 127.0.0.1 and 127.0.0.2 are in a random order.

Getaddrinfo() and getnameinfo() can query hostname and service

name/port at once.

Hardcoding AF-dependent knowledge is not preferred in the program.

Constructs such as that below should be avoided:

/* BAD EXAMPLE */

switch (sa->sa_family) {

case AF_INET:

salen = sizeof(struct sockaddr_in);

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. In

the following sections, we will see simple examples creating these

sockets by 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 */

}

/* ... */

freeaddrinfo(res);

6.2.2. Example of Overly Simplistic TCP Client Application

A simple TCP client socket connecting to a server running at node

name (or IP address presentation format) SERVER_NODE and service name

(or port number string) SERVICE follows:

/*

* 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);

6.2.3. Binary/Presentation Format Conversion

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. It is better to use

getnameinfo()/getaddrinfo() (inet_pton and inet_ntop equivalents are

described in Appendix A).

Conversion from network address structure to presentation format can

be written as follows:

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);

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 simplified by always continuing on with the socket

loop instead of performing special checking of specific error

numbers.

6.3.1. Example of TCP Server Application

The previous TCP server example should be written as follows:

#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;

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 as follows:

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,

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

The mechanism [NAT-PT] introduces a special set of addresses, formed

of an NAT-PT prefix and an IPv4 address these refer 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): This

would be completely impractical, and if the distinction must be made,

it must be done elsewhere (e.g., kernel, system libraries).

8. Security Considerations

There are a number of security considerations for 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 describes 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

maintain availability and to avoid loss of service.

A special consideration about application transition is how IPv4-

mapped IPv6 addresses are handled. The use in the API can be seen

both as a merit (easier application transition) and as a burden

(difficulty in ensuring whether the use was legitimate). Note that

some systems will disable (by default) support for internal IPv4-

mapped IPv6 addresses. The security concerns regarding these on the

wire are legitimate, but disabling it internally breaks one

transition mechanism for server applications originally written to

bind() and listen() to a single socket by using a wildcard address

[V6MAPPED]. This should be considered in more detail when

applications are designed.

9. Acknowledgments

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 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

10.1. Normative References

[RFC3493] Gilligan, R., Thomson, S., Bound, J., McCann, J., and W.

Stevens, "Basic Socket Interface Extensions for IPv6",

RFC 3493, February 2003.

[RFC3542] Stevens, W., Thomas, M., Nordmark, E., and T. Jinmei,

"Advanced Sockets Application Program Interface (API) for

IPv6", RFC 3542, May 2003.

[BIS] Tsuchiya, K., Higuchi, H., and Y. Atarashi, "Dual Stack

Hosts using the "Bump-In-the-Stack" Technique (BIS)", RFC

2767, February 2000.

[BIA] Lee, S., Shin, M-K., Kim, Y-J., Nordmark, E., and A.

Durand, "Dual Stack Hosts Using "Bump-in-the-API" (BIA)",

RFC 3338, October 2002.

[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6

(IPv6) Specification", RFC 2460, December 1998.

[RFC3484] Draves, R., "Default Address Selection for Internet

Protocol version 6 (IPv6)", RFC 3484, February 2003.

[RFC3513] Hinden, R. and S. Deering, "Internet Protocol Version 6

(IPv6) Addressing Architecture", RFC 3513, April 2003.

10.2. Informative References

[2893BIS] Nordmark, E. and R. E. Gilligan, "Basic Transition

Mechanisms for IPv6 Hosts and Routers", Work in Progress,

June 2004.

[RFC2133] Gilligan, R., Thomson, S., Bound, J., and W. Stevens,

"Basic Socket Interface Extensions for IPv6", RFC 2133,

April 1997.

[RFC2732] Hinden, R., Carpenter, B., and L. Masinter, "Format for

Literal IPv6 Addresses in URL's", RFC 2732, December

1999.

[RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,

April 2001.

[TextRep] Main, A., "Textual Representation of IPv4 and IPv6

Addresses", Work in Progress, October 2003.

[NAT-PT] Tsirtsis, G. and P. Srisuresh, "Network Address

Translation - Protocol Translation (NAT-PT)", RFC 2766,

February 2000.

[DNSTRANS] Durand, A. and J. Ihren, "DNS IPv6 Transport Operational

Guidelines", BCP 91, RFC 3901, September 2004.

[DNSOPV6] Durand, A., Ihren, J. and P. Savola, "Operational

Considerations and Issues with IPv6 DNS", Work in

Progress, May 2004.

[AF-APP] Hagino, J., "Implementing AF-independent application",

http://www.kame.net/newsletter/19980604/, 2001.

[V6MAPPED] Hagino, J., "IPv4 mapped address considered harmful",

Work in Progress, April 2002.

[IP-GGF] Chown, T., Bound, J., Jiang, S. and P. O'Hanlon,

"Guidelines for IP version independence in GGF

specifications", Global Grid Forum(GGF) Documentation,

work in Progress, September 2003.

[Embed-RP] Savola, P. and B. Haberman, "Embedding the Rendezvous

Point (RP) Address in an IPv6 Multicast Address", RFC

3956, November 2004.

[RFC3306] Haberman, B. and D. Thaler, "Unicast-Prefix-based IPv6

Multicast Addresses", RFC 3306, August 2002.

[RFC3678] Thaler, D., Fenner, B., and B. Quinn, "Socket Interface

Extensions for Multicast Source Filters, RFC 3678,

January 2004.

[MUL-GW] Venaas, S., "An IPv4 - IPv6 multicast gateway", Work in

Progress, February 2003.

[PRT] Castro, E. M., "Programming guidelines on transition to

IPv6 LONG project", Work in Progress, January 2003.

Appendix A. Other Binary/Presentation Format Conversions

Section 6.2.3 describes the preferred way to perform

binary/presentation format conversions; these can also be done by

using inet_pton() and inet_ntop() and by writing protocol-dependent

code. This approach is not recommended, but it is 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 as follows:

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,

addrStr,

sizeof(addrStr));

break;

default:

/* handle unknown family */

}

Note that, the destination buffer addrStr should be long enough to

contain the presentation address format: INET_ADDRSTRLEN for IPv4 and

INET6_ADDRSTRLEN for IPv6. As 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 that, the address family of the presentation format must be

known.

Authors' Addresses

Myung-Ki Shin

ETRI/NIST

820 West Diamond Avenue

Gaithersburg, MD 20899, USA

Phone: +1 301 975-3613

Fax: +1 301 590-0932

EMail: mshin@nist.gov

Yong-Guen Hong

ETRI PEC

161 Gajeong-Dong, Yuseong-Gu, Daejeon 305-350, Korea

Phone: +82 42 860 6447

Fax: +82 42 861 5404

EMail: 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

Phone: +81-3-5259-6350

Fax: +81-3-5259-6351

EMail: itojun@iijlab.net

Pekka Savola

CSC/FUNET

Espoo, Finland

EMail: psavola@funet.fi

Eva M. Castro

Rey Juan Carlos University (URJC)

Departamento de Informatica, Estadistica y Telematica

C/Tulipan s/n

28933 Madrid - SPAIN

EMail: eva@gsyc.escet.urjc.es

Full Copyright Statement

Copyright (C) The Internet Society (2005).

This document is subject to the rights, licenses and restrictions

contained in BCP 78, and except as set forth therein, the authors

retain all their rights.

This document and the information contained herein are provided on an

"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS

OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET

ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,

INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE

INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED

WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

The IETF takes no position regarding the validity or scope of any

Intellectual Property Rights or other rights that might be claimed to

pertain to the implementation or use of the technology described in

this document or the extent to which any license under such rights

might or might not be available; nor does it represent that it has

made any independent effort to identify any such rights. Information

on the procedures with respect to rights in RFC documents can be

found in BCP 78 and BCP 79.

Copies of IPR disclosures made to the IETF Secretariat and any

assurances of licenses to be made available, or the result of an

attempt made to obtain a general license or permission for the use of

such proprietary rights by implementers or users of this

specification can be obtained from the IETF on-line IPR repository at

http://www.ietf.org/ipr.

The IETF invites any interested party to bring to its attention any

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rights that may cover technology that may be required to implement

this standard. Please address the information to the IETF at ietf-

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Acknowledgement

Funding for the RFC Editor function is currently provided by the

Internet Society.

 
 
 
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