Introduction to FreeS/WAN

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# Linux FreeS/WAN Compatibility Guide

Much of this document is quoted directly from the Linux FreeS/WAN mailing list. Thanks very much to the community of testers, patchers and commenters there, especially the ones quoted below but also various contributors we haven't quoted.

## Implemented parts of the IPsec Specification

In general, do not expect Linux FreeS/WAN to do everything yet. This is a work-in-progress and some parts of the IPsec specification are not yet implemented.

### In Linux FreeS/WAN

Things we do, as of version 1.96:

- key management methods
  manually keyed
    using keys stored in /etc/ipsec.conf
  automatically keyed
    Automatically negotiating session keys as required. All connections are automatically re-keyed periodically. The Pluto daemon implements this using the IKE protocol.
- Methods of authenticating gateways for IKE
  shared secrets
    stored in ipsec.secrets(5)
  RSA signatures
    For details, see pluto(8) .
  looking up RSA authentication keys from DNS.
    Note that this technique cannot be fully secure until secure DNS is widely deployed.
- groups for Diffie-Hellman key negotiation
  group 2, modp 1024-bit
  group 5, modp 1536-bit
    We implement these two groups.

    In negotiating a keying connection (ISAKMP SA, Phase 1) we propose both groups when we are the initiator, and accept either when a peer proposes them. Once the keying connection is made, we propose only the alternative agreed there for data connections (IPsec SA's, Phase 2) negotiated over that keying connection.

- encryption transforms
  DES
    DES is in the source code since it is needed to implement 3DES, but single DES is not made available to users because DES is insecure.
  Triple DES
    implemented, and used as the default encryption in Linux FreeS/WAN.
- authentication transforms
  HMAC using MD5
    implemented, may be used in IKE or by by AH or ESP transforms.
  HMAC using SHA
    implemented, may be used in IKE or by AH or ESP transforms.

  In negotiations, we propose both of these and accept either.

- compression transforms
  IPComp
    IPComp as described in RFC 2393 was added for FreeS/WAN 1.6. Note that Pluto becomes confused if you ask it to do IPComp when the kernel cannot.

All combinations of implemented transforms are supported. Note that some form of packet-level authentication is required whenever encryption is used. Without it, the encryption will not be secure.

### Deliberately omitted

We do not implement everything in the RFCs because some of those things are insecure. See our discussions of avoiding bogus security.

Things we deliberately omit which are required in the RFCs are:

- null encryption (to use ESP as an authentication-only service)
- single DES
- DH group 1, a 768-bit modp group

Since these are the only encryption algorithms and DH group the RFCs require, it is possible in theory to have a standards-conforming implementation which will not interpoperate with FreeS/WAN. Such an implementation would be inherently insecure, so we do not consider this a problem.

Anyway, most implementations sensibly include more secure options as well, so dropping null encryption, single DES and Group 1 does not greatly hinder interoperation in practice.

We also do not implement some optional features allowed by the RFCs:

- aggressive mode for negotiation of the keying channel or ISAKMP SA. This mode is a little faster than main mode, but exposes more information to an eavesdropper.

In theory, this should cause no interoperation problems since all implementations are required to support the more secure main mode, whether or not they also allow aggressive mode.

In practice, it does sometimes produce problems with implementations such as Windows 2000 where aggressive mode is the default. Typically, these are easily solved with a configuration change that overrides that default.

### Not (yet) in Linux FreeS/WAN

Things we don't yet do, as of version 1.96:

- key management methods
  - authenticate key negotiations via local PKI server, but see links to user patches
  - authenticate key negotiations via secure DNS
  - unauthenticated key management, using Diffie-Hellman key agreement protocol without authentication. Arguably, this would be worth doing since it is secure against all passive attacks. On the other hand, it is vulnerable to an active man-in-the-middle attack.
- encryption transforms

  Currently Triple DES is the only encryption method Pluto will negotiate.

  No additional encryption transforms are implemented, though the RFCs allow them and some other IPsec implementations support various of them. We are not eager to add more. See this FAQ question.

  AES, the successor to the DES standard, is an excellent candidate for inclusion in FreeS/WAN, see links to user patches.

- authentication transforms

  No optional additional authentication transforms are currently implemented. Likely SHA-256, SHA-384 and SHA-512 will be added when AES is.

- Policy checking on decrypted packets

  To fully comply with the RFCs, it is not enough just to accept only packets which survive any firewall rules in place to limit what IPsec packets get in, and then pass KLIPS authentication. That is what FreeS/WAN currently does.

  We should also apply additional tests, for example ensuring that all packets emerging from a particular tunnel have source and destination addresses that fall within the subnets defined for that tunnel, and that packets with those addresses that did not emerge from the appropriate tunnel are disallowed.

  This will be done as part of a KLIPS rewrite. See these links and the design mailing list for discussion.

## Our PF-Key implementation

We use PF-key Version Two for communication between the KLIPS kernel code and the Pluto Daemon. PF-Key v2 is defined by RFC 2367.

The "PF" stands for Protocol Family. PF-Inet defines a kernel/userspace interface for the TCP/IP Internet protocols (TCP/IP), and other members of the PF series handle Netware, Appletalk, etc. PF-Key is just a PF for key-related matters.

### PF-Key portability

PF-Key came out of Berkeley Unix work and is used in the various BSD IPsec implementations, and in Solaris. This means there is some hope of porting our Pluto(8) to one of the BSD distributions, or of running their photurisd(8) on Linux if you prefer Photuris key management over IKE.

It is, however, more complex than that. The PK-Key RFC deliberately deals only with keying, not policy management. The three PF-Key implementations we have looked at -- ours, OpenBSD and KAME -- all have extensions to deal with security policy, and the extensions are different. There have been discussions aimed at sorting out the differences, perhaps for a version three PF-Key spec. All players are in favour of this, but everyone involved is busy and it is not clear whether or when these discussions might bear fruit.

## Kernels other than the latest 2.2.x and 2.4.y

We develop and test on Redhat Linux using the most recent kernel in the 2.2 and 2.4 series. In general, we recommend you use the latest kernel in one of those series. Complications and caveats are discussed below.

### 2.0.x kernels

Consider upgrading to the 2.2 kernel series. If you want to stay with the 2.0 series, then we strongly recommend 2.0.39. Some useful security patches were added in 2.0.38.

Various versions of the code have run at various times on most 2.0.xx kernels, but the current version is only lightly tested on 2.0.39, and not at all on older kernels.

Some of our patches for older kernels are shipped in 2.0.37 and later, so they are no longer provided in FreeS/WAN. This means recent versions of FreeS/WAN will probably not compile on anything earlier than 2.0.37.

### 2.2 and 2.4 kernels

FreeS/WAN 1.0
  ran only on 2.0 kernels
FreeS/WAN 1.1 to 1.8
  ran on 2.0 or 2.2 kernels
  ran on some development kernels, 2.3 or 2.4-test
FreeS/WAN 1.9 to 1.96
  runs on 2.0, 2.2 or 2.4 kernels

In general, we suggest the latest 2.2 kernel or 2.4 for production use.

Of course no release can be guaranteed to run on kernels more recent than it is, so quite often there will be no stable FreeS/WAN for the absolute latest kernel. See the FAQ for discussion.

## Intel Linux distributions other than Redhat

We develop and test on Redhat 6.1 for 2.2 kernels, and on Redhat 7.1 or 7.2 for 2.4, so minor changes may be required for other distributions.

### Redhat 7.0

There are some problems with FreeS/WAN on Redhat 7.0. They are soluble, but we recommend you upgrade to a later Redhat instead..

Redhat 7 ships with two compilers.

- Their gcc is version 2.96. Various people, including the GNU compiler developers and Linus, have said fairly emphatically that using this was a mistake. 2.96 is a development version, not intended for production use. In particular, it will not compile a Linux kernel.
- Redhat therefore also ship a separate compiler, which they call kgcc, for compiling kernels.

Kernel Makefiles have gcc as a default, and must be adjusted to use kgcc before a kernel will compile on 7.0. This mailing list message gives details:

    Subject: Re: AW: Installing IPsec on Redhat 7.0
       Date: Thu, 1 Feb 2001 14:32:52 -0200 (BRST)
      From: Mads Rasmussen <mads@cit.com.br>

    > From www.redhat.com/support/docs/gotchas/7.0/gotchas-7-6.html#ss6.1

    cd to /usr/src/linux and open the Makefile in your favorite editor. You
    will need to look for a line similar to this:

    CC = $(CROSS_COMPILE)gcc -D__KERNEL__ -I$(HPATH)

    This line specifies which C compiler to use to build the kernel. It should
    be changed to:

    CC = $(CROSS_COMPILE)kgcc -D__KERNEL__ -I$(HPATH)

    for Red Hat Linux 7. The kgcc compiler is egcs 2.91.66. From here you can
    proceed with the typical compiling steps.

Check the mailing list archive for more recent news.

### SuSE Linux

SuSE 6.3 and later versions, at least in Europe, ship with FreeS/WAN included.

FreeS/WAN packages distributed for SuSE 7.0-7.2 were somehow miscompiled. You can find fixed packages on Kurt Garloff's page.

Here are some notes for an earlier SuSE version.

#### SuSE Linux 5.3

    Date: Mon, 30 Nov 1998
    From: Peter Onion <ponion@srd.bt.co.uk>

    ... I got Saturdays snapshot working between my two SUSE5.3 machines at home.

    The mods to the install process are quite simple.  From memory and looking at
    the files on the SUSE53 machine here at work....

    And extra link in each of the /etc/init.d/rc?.d directories called K35ipsec
    which SUSE use to shut a service down.

    A few mods in /etc/init.d/ipsec  to cope with the different places that SUSE
    put config info, and remove the inculsion of /etc/rc.d/init.d/functions and .
    /etc/sysconfig/network as they don't exists and 1st one isn't needed anyway.

    insert ". /etc/rc.config" to pick up the SUSE config info and use

      if test -n "$NETCONFIG" -a "$NETCONFIG" != "YAST_ASK" ; then

    to replace

      [ ${NETWORKING} = "no" ] && exit 0

    Create /etc/sysconfig  as SUSE doesn't have one.

    I think that was all (but I prob forgot something)....

You may also need to fiddle initialisation scripts to ensure that /var/run/pluto/pluto.pid is removed when rebooting. If this file is present, Pluto does not come up correctly.

### Slackware

    Subject: Re: linux-IPsec: Slackware distribution
      Date:  Thu, 15 Apr 1999 12:07:01 -0700
      From:  Evan Brewer <dmessiah@silcon.com>

    > Very shortly, I will be needing to install IPsec on at least gateways that
    > are running Slackware. . . .

    The only trick to getting it up is that on the slackware dist there is no
    init.d directory in /etc/rc.d .. so create one.  Then, what I do is take the
    IPsec startup script which normally gets put into the init.d directory, and
    put it in /etc/rc.d and name ir rc.ipsec .. then I symlink it to the file
    in init.d.  The only file in the dist you need to really edit is the
    utils/Makefile, setup4:

    Everything else should be just fine.

A year or so later:

    Subject: Re: HTML Docs- Need some cleanup?
       Date: Mon, 8 Jan 2001
       From: Jody McIntyre <jodym@oeone.com>

    I have successfully installed FreeS/WAN on several Slackware 7.1 machines.
    FreeS/WAN installed its rc.ipsec file in /etc/rc.d.  I had to manually call
    this script from rc.inet2.  This seems to be an easier method than Evan
    Brewer's.

### Debian

A recent (Nov 2001) mailing list points to a web page on setting up several types of tunnel, including IPsec, on Debian.

Some older information:

    Subject: FreeS/WAN 1.0 on Debian 2.1
       Date: Tue, 20 Apr 1999
      From:  Tim Miller <cerebus+counterpane@haybaler.sackheads.org>

            Compiled and installed without error on a Debian 2.1 system
    with kernel-source-2.0.36 after pointing RCDIR in utils/Makefile to
    /etc/init.d.

            /var/lock/subsys/ doesn't exist on Debian boxen, needs to be
    created; not a fatal error.

            Finally, IPsec scripts appear to be dependant on GNU awk
    (gawk); the default Debian awk (mawk-1.3.3-2) had fatal difficulties.
    With gawk installed and /etc/alternatives/awk linked to /usr/bin/gawk
    operation appears flawless.

The scripts in question have been modified since this was posted. Awk versions should no longer be a problem.

### Caldera

    Subject: Re: HTML Docs- Need some cleanup?
       Date: Mon, 08 Jan 2001
       From: Andy Bradford <andyb@calderasystems.com>

    On Sun, 07 Jan 2001 22:59:05 EST, Sandy Harris wrote:

    >     Intel Linux distributions other than Redhat 5.x and 6.x
    >         Redhat 7.0
    >         SuSE Linux
    >             SuSE Linux 5.3
    >         Slackware
    >         Debian

    Can you please include Caldera in this list?  I have tested it since
    FreeS/Wan 1.1 and it works great with our systems---provided one
    follows the FreeS/Wan documentation. :-)

    Thank you,
    Andy

## CPUs other than Intel

FreeS/WAN has been run sucessfully on a number of different CPU architectures. If you have tried it on one not listed here, please post to the mailing list.

### Corel Netwinder (StrongARM CPU)

    Subject: linux-ipsec: Netwinder diffs
    Date: Wed, 06 Jan 1999
    From: rhatfield@plaintree.com

    I had a mistake in my IPsec-auto, so I got things working this morning.

    Following are the diffs for my changes.  Probably not the best and cleanest way
    of doing it, but it works. . . .

These diffs are in the 0.92 and later distributions, so these should work out-of-the-box on Netwinder.

### Yellow Dog Linux on Power PC

    Subject:  Compiling FreeS/WAN 1.1 on YellowDog Linux (PPC)
       Date:  11 Dec 1999
       From:  Darron Froese <darron@fudgehead.com>

    I'm summarizing here for the record - because it's taken me many hours to do
    this (multiple times) and because I want to see IPsec on more linuxes than
    just x86.

    Also, I can't remember if I actually did summarize it before... ;-) I'm
    working too many late hours.

    That said - here goes.

    1. Get your linux kernel and unpack into /usr/src/linux/ - I used 2.2.13.
    <http://www.kernel.org/pub/linux/kernel/v2.2/linux-2.2.13.tar.bz2>

    2. Get FreeS/WAN and unpack into /usr/src/freeswan-1.1
    <ftp://ftp.xs4all.nl/pub/crypto/freeswan/freeswan-1.1.tar.gz>

    3. Get the gmp src rpm from here:
    <ftp://ftp.yellowdoglinux.com//pub/yellowdog/champion-1.1/SRPMS/SRPMS/gmp-2.0.2-9a.src.rpm>

    4. Su to root and do this: rpm --rebuild gmp-2.0.2-9a.src.rpm

    You will see a lot of text fly by and when you start to see the rpm
    recompiling like this:

    Executing: %build
    + umask 022
    + cd /usr/src/redhat/BUILD
    + cd gmp-2.0.2
    + libtoolize --copy --force
    Remember to add `AM_PROG_LIBTOOL' to `configure.in'.
    You should add the contents of `/usr/share/aclocal/libtool.m4' to
    `aclocal.m4'.
    + CFLAGS=-O2 -fsigned-char
    + ./configure --prefix=/usr

    Hit Control-C to stop the rebuild. NOTE: We're doing this because for some
    reason the gmp source provided with FreeS/WAN 1.1 won't build properly on
    ydl.

    cd /usr/src/redhat/BUILD/
    cp -ar gmp-2.0.2 /usr/src/freeswan-1.1/
    cd /usr/src/freeswan-1.1/
    rm -rf gmp
    mv gmp-2.0.2 gmp

    5. Open the freeswan Makefile and change the line that says:
    KERNEL=$(b)zimage (or something like that) to
    KERNEL=vmlinux

    6. cd ../linux/

    7. make menuconfig
    Select an option or two and then exit - saving your changes.

    8. cd ../freeswan-1.1/ ; make menugo

    That will start the whole process going - once that's finished compiling,
    you have to install your new kernel and reboot.

    That should build FreeS/WAN on ydl (I tried it on 1.1).

And a later message on the same topic:

    Subject: Re: FreeS/WAN, PGPnet and E-mail
       Date: Sat, 22 Jan 2000
       From: Darron Froese <darron@fudgehead.com>

    on 1/22/00 6:47 PM, Philip Trauring at philip@trauring.com wrote:

    > I have a PowerMac G3 ...

    The PowerMac G3 can run YDL 1.1 just fine. It should also be able to run
    FreeS/WAN 1.2patch1 with a couple minor modifications:

    1. In the Makefile it specifies a bzimage for the kernel compile - you have
    to change that to vmlinux for the PPC.

    2. The gmp source that comes with FreeS/WAN (for whatever reason) fails to
    compile. I have gotten around this by getting the gmp src rpm from here:

    ftp://ftp.yellowdoglinux.com//pub/yellowdog/champion-1.1/SRPMS/SRPMS/gmp-2.0.2-9a.src.rpm

    If you rip the source out of there - and place it where the gmp source
    resides it will compile just fine.

FreeS/WAN no longer includes GMP source.

### Mklinux

One user reports success on the Mach-based micro kernel Linux.

    Subject: Smiles on sparc and ppc
       Date: Fri, 10 Mar 2000
       From: Jake Hill <jah@alien.bt.co.uk>

    You may or may not be interested to know that I have successfully built
    FreeS/WAN on a number of non intel alpha architectures; namely on ppc
    and sparc and also on osfmach3/ppc (MkLinux). I can report that it just
    works, mostly, with few changes.

### Alpha 64-bit processors

    Subject: IT WORKS (again) between intel & alpha :-)))))
       Date: Fri, 29 Jan 1999
       From: Peter Onion <ponion@srd.bt.co.uk>

    Well I'm happy to report that I've got an IPsec connection between by intel & alpha machines again :-))

    If you look back on this list to 7th of December I wrote...

    -On 07-Dec-98 Peter Onion wrote:
    ->
    -> I've about had enuf of wandering around inside the kernel trying to find out
    -> just what is corrupting outgoing packets...
    -
    -Its 7:30 in the evening .....
    -
    -I FIXED IT  :-))))))))))))))))))))))))))))))))
    -
    -It was my own fault :-((((((((((((((((((
    -
    -If you ask me very nicly I'll tell you where I was a little too over keen to
    -change unsigned long int __u32 :-)  OPSE ...
    -
    -So tomorrow it will full steam ahead to produce a set of diffs/patches against
    -0.91
    -
    -Peter Onion.

In general (there have been some glitches), FreeS/WAN has been running on Alphas since then.

### Sun SPARC processors

Several users have reported success with FreeS/WAN on SPARC Linux. Here is one mailing list message:

    Subject: Smiles on sparc and ppc
       Date: Fri, 10 Mar 2000
       From: Jake Hill <jah@alien.bt.co.uk>

    You may or may not be interested to know that I have successfully built
    FreeS/WAN on a number of non intel alpha architectures; namely on ppc
    and sparc and also on osfmach3/ppc (MkLinux). I can report that it just
    works, mostly, with few changes.

    I have a question, before I make up some patches. I need to hack
    gmp/mpn/powerpc32/*.s to build them. Is this ok? The changes are
    trivial, but could I also use a different version of gmp? Is it vanilla
    here?

    I guess my only real headache is from ipchains, which appears to stop
    running when IPsec has been started for a while. This is with 2.2.14 on
    sparc.

This message, from a different mailing list, may be relevant for anyone working with FreeS/WAN on Suns:

    Subject: UltraSPARC DES assembler
       Date: Thu, 13 Apr 2000
       From: svolaf@inet.uni2.dk (Svend Olaf Mikkelsen)
         To: coderpunks@toad.com

    An UltraSPARC assembler version of the LibDES/SSLeay/OpenSSL des_enc.c
    file is available at http://inet.uni2.dk/~svolaf/des.htm.

    This brings DES on UltraSPARC from slower than Pentium at the same
    clock speed to significantly faster.

### MIPS processors

We know FreeS/WAN runs on at least some MIPS processors because Lasat manufacture an IPsec box based on an embedded MIPS running Linux with FreeS/WAN. We have no details.

### Transmeta Crusoe

The Merilus Firecard, a Linux firewall on a PCI card, is based on a Crusoe processor and supports FreeS/WAN.

### Motorola Coldfire

    Subject: Re: Crypto hardware support
       Date: Mon, 03 Jul 2000
       From: Dan DeVault <devault@tampabay.rr.com>

    .... I have been running
    uClinux with FreeS/WAN 1.4 on a system built by Moreton Bay  (
    http://www.moretonbay.com )  and it was using a Coldfire processor
    and was able to do the Triple DES encryption at just about
    1 mbit / sec rate.......  they put a Hi/Fn 7901 hardware encryption
    chip on their board and now their system does over 25 mbit of 3DES
    encryption........ pretty significant increase if you ask me.

## Multiprocessor machines

FreeS/WAN is designed to work on SMP (symmetric multi-processing) Linux machines and is regularly tested on dual processor x86 machines.

We do not know of any testing on multi-processor machines with other CPU architectures or with more than two CPUs. Anyone who does test this, please report results to the mailing list .

The current design does not make particularly efficient use of multiprocessor machines; some of the kernel work is single-threaded.

## Support for crypto hardware

Supporting hardware cryptography accelerators has not been a high priority for the development team because it raises a number of fairly complex issues:

- Can you trust the hardware? If it is not Open Source, how do you audit its security? Even if it is, how do you check that the design has no concealed traps?
- If an interface is added for such hardware, can that interface be subverted or misused?
- Is hardware acceleration actually a performance win? It clearly is in many cases, but on a fast machine it might be better to use the CPU for the encryption than to pay the overheads of moving data to and from a crypto board.
- the current KLIPS code does not provide a clean interface for hardware accelerators

That said, we have a report of FreeS/WAN working with one crypto accelerator and some work is going on to modify KLIPS to create a clean generic interface to such products. See this web page for some of the design discussion.

More recently, a patch to support some hardware accelerators has been posted:

    Subject: [Design] [PATCH] H/W acceleration patch
       Date: Tue, 18 Sep 2001
       From: "Martin Gadbois" <martin.gadbois@colubris.com>

    Finally!!
    Here's a web site with H/W acceleration patch for FreeS/WAN 1.91, including
    S/W and Hifn 7901 crypto support.

    http://sources.colubris.com/

    Martin Gadbois

Hardware accelerators could take performance well beyond what FreeS/WAN can do in software (discussed here ). Here is some discussion off the IETF IPsec list, October 2001:

     ... Currently shipping chips deliver, 600 mbps throughput on a single
     stream of 3DES IPsec traffic.  There are also chips that use multiple
     cores to do 2.4 gbps.  We (Cavium) and others have announced even faster
     chips. ... Mid 2002 versions will handle at line rate (OC48 and OC192)
     IPsec and SSL/TLS traffic not only 3DES CBC but also AES and arc4.

The patches to date support chips that have been in production for some time, not the state-of-the-art latest-and-greatest devices described in that post. However, they may still outperform software and they almost certainly reduce CPU overhead.

## IP version 6 (IPng)

The Internet currently runs on version four of the IP protocols. IPv4 is what is in the standard Linux IP stack, and what FreeS/WAN was built for. In IPv4, IPsec is an optional feature.

The next version of the IP protocol suite is version six, usually abbreviated either as "IPv6" or as "IPng" for "IP: the next generation". For IPv6, IPsec is a required feature. Any machine doing IPv6 is required to support IPsec, much as any machine doing (any version of) IP is required to support ICMP.

There is a Linux implementation of IPv6 in Linux kernels 2.2 and above. For details, see the FAQ. It does not yet support IPsec. The USAGI project are also working on IPv6 for Linux.

FreeS/WAN was originally built for the current standard, IPv4, but we are interested in seeing it work with IPv6. Some progress has been made, and a patched version with IPv6 support is available. For more recent information, check the mailing list.

### IPv6 background

IPv6 has been specified by an IETF working group. The group's page lists over 30 RFCs to date, and many Internet Drafts as well. The overview is RFC 2460. Major features include:

- expansion of the address space from 32 to 128 bits,
- changes to improve support for
  - mobile IP
  - automatic network configuration
  - quality of service routing
  - ...
- improved security via IPsec

A number of projects are working on IPv6 implementation. A prominent Open Source effort is KAME, a collaboration among several large Japanese companies to implement IPv6 for Berkeley Unix. Other major players are also working on IPv6. For example, see pages at:

- Sun
- Cisco
- Microsoft

The 6bone (IPv6 backbone) testbed network has been up for some time. There is an active IPv6 user group.

One of the design goals for IPv6 was that it must be possible to convert from v4 to v6 via a gradual transition process. Imagine the mess if there were a "flag day" after which the entire Internet used v6, and all software designed for v4 stopped working. Almost every computer on the planet would need major software changes! There would be huge costs to replace older equipment. Implementers would be worked to death before "the day", systems administrators and technical support would be completely swamped after it. The bugs in every implementation would all bite simultaneously. Large chunks of the net would almost certainly be down for substantial time periods. ...

Fortunately, the design avoids any "flag day". It is therefore a little tricky to tell how quickly IPv6 will take over. The transition has certainly begun. For examples, see announcements from NTT and Nokia. However, it is not yet clear how quickly the process will gain momentum, or when it will be completed. Likely large parts of the Internet will remain with IPv4 for years to come.

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