path: root/sys/netinet6/IMPLEMENTATION

$OpenBSD: IMPLEMENTATION,v 1.2 1999/12/20 08:26:32 itojun Exp $

# NOTE: this is from original KAME distribution.
# Some portion of this document is not applicable to the code merged into
# OpenBSD-current.  Check sys/netinet6/TODO as well.

			Implementation Note

			KAME Project
			http://www.kame.net/
			KAME Date: 1999/12/20 08:23:13

1. IPv6

1.1 Conformance

The KAME kit conforms, or tries to conform, to the latest set of IPv6
specifications.  For future reference we list some of the relevant documents
below (NOTE: this is not a complete list - this is too hard to maintain...).
For details please refer to specific chapter in the document, RFCs, manpages
come with KAME, or comments in the source code.

Conformance tests have been performed on the KAME STABLE kit
at TAHI project.  Results can be viewed at http://www.tahi.org/report/KAME/.
We also attended Univ. of New Hampshire IOL tests (http://www.iol.unh.edu/)
in the past, with our past snapshots.

RFC1639: FTP Operation Over Big Address Records (FOOBAR)
    * RFC2428 is preferred over RFC1639.  ftp clients will first try RFC2428,
      then RFC1639 if failed.
RFC1886: DNS Extensions to support IPv6
RFC1933: Transition Mechanisms for IPv6 Hosts and Routers
    * IPv4 compatible address is not supported.
    * automatic tunneling (4.3) is not supported.
    * "gif" interface implements IPv[46]-over-IPv[46] tunnel in a generic way,
      and it covers "configured tunnel" described in the spec.
      See 1.5 in this document for details.
RFC1981: Path MTU Discovery for IPv6
RFC2080: RIPng for IPv6
    * KAME-supplied route6d, bgpd and hroute6d support this.
RFC2283: Multiprotocol Extensions for BGP-4
    * so-called "BGP4+".
    * KAME-supplied bgpd supports this.
RFC2292: Advanced Sockets API for IPv6
    * For supported library functions/kernel APIs, see sys/netinet6/ADVAPI.
RFC2362: Protocol Independent Multicast-Sparse Mode (PIM-SM)
    * RFC2362 defines packet formats for PIM-SM.  draft-ietf-pim-ipv6-01.txt
      is written based on this.
RFC2373: IPv6 Addressing Architecture
    * KAME supports node required addresses, and conforms to the scope
      requirement.
RFC2374: An IPv6 Aggregatable Global Unicast Address Format
    * KAME supports 64-bit length of Interface ID.
RFC2375: IPv6 Multicast Address Assignments
    * Userland applications use the well-known addresses assigned in the RFC.
RFC2428: FTP Extensions for IPv6 and NATs
    * RFC2428 is preferred over RFC1639.  ftp clients will first try RFC2428,
      then RFC1639 if failed.
RFC2460: IPv6 specification
RFC2461: Neighbor discovery for IPv6
    * See 1.2 in this document for details.
RFC2462: IPv6 Stateless Address Autoconfiguration
    * See 1.4 in this document for details.
RFC2463: ICMPv6 for IPv6 specification
    * See 1.8 in this document for details.
RFC2464: Transmission of IPv6 Packets over Ethernet Networks
RFC2465: MIB for IPv6: Textual Conventions and General Group
    * Necessary statistics are gathered by the kernel.  Actual IPv6 MIB
      support is provided as patchkit for ucd-snmp.
RFC2466: MIB for IPv6: ICMPv6 group
    * Necessary statistics are gathered by the kernel.  Actual IPv6 MIB
      support is provided as patchkit for ucd-snmp.
RFC2467: Transmission of IPv6 Packets over FDDI Networks
RFC2472: IPv6 over PPP
RFC2492: IPv6 over ATM Networks
    * only PVC is supported.
RFC2497: Transmission of IPv6 packet over ARCnet Networks
RFC2545: Use of BGP-4 Multiprotocol Extensions for IPv6 Inter-Domain Routing
RFC2553: Basic Socket Interface Extensions for IPv6
    * IPv4 mapped address (3.7) and special behavior of IPv6 wildcard bind
      socket (3.8) are,
	- supported on KAME/FreeBSD3x,
	- supported on KAME/NetBSD,
	- supported on KAME/BSDI4,
	- not supported on KAME/FreeBSD228, KAME/OpenBSD and KAME/BSDI3.
      see 1.12 in this document for details.
RFC2675: IPv6 Jumbograms
    * See 1.7 in this document for details.
RFC2710: Multicast Listener Discovery for IPv6
RFC2711: IPv6 router alert option
draft-ietf-ipngwg-router-renum-08: Router renumbering for IPv6
draft-ietf-ipngwg-icmp-namelookups-02: IPv6 Name Lookups Through ICMP
draft-ietf-ipngwg-icmp-name-lookups-03: IPv6 Name Lookups Through ICMP
draft-ietf-pim-ipv6-01.txt: PIM for IPv6
    * pim6dd implements dense mode.  pim6sd implements sparse mode.
draft-ietf-dhc-dhcpv6-14.txt: DHCPv6
draft-ietf-dhc-v6exts-11.txt: Extensions for DHCPv6
    * kame/dhcp6 has test implementation, which will not be compiled in
      default compilation.
draft-itojun-ipv6-tcp-to-anycast-00:
	Disconnecting TCP connection toward IPv6 anycast address
draft-yamamoto-wideipv6-comm-model-00
    * See 1.6 in this document for details.
draft-ietf-ipngwg-scopedaddr-format-00.txt:
	An Extension of Format for IPv6 Scoped Addresses

1.2 Neighbor Discovery

Neighbor Discovery is fairly stable.  Currently Address Resolution,
Duplicated Address Detection, and Neighbor Unreachability Detection
are supported.  In the near future we will be adding Proxy Neighbor
Advertisement support in the kernel and Unsolicited Neighbor Advertisement
transmission command as admin tool.

If DAD fails, the address will be marked "duplicated" and message will be
generated to syslog (and usually to console).  The "duplicated" mark
can be checked with ifconfig.  It is administrators' responsibility to check
for and recover from DAD failures.
The behavior should be improved in the near future.

Some of the network driver loops multicast packets back to itself,
even if instructed not to do so (especially in promiscuous mode).
In such cases DAD may fail, because DAD engine sees inbound NS packet
(actually from the node itself) and considers it as a sign of duplicate.
You may want to look at #if condition marked "heuristics" in
sys/netinet6/nd6_nbr.c:nd6_dad_timer() as workaround (note that the code
fragment in "heuristics" section is not spec conformant).

Neighbor Discovery specification (RFC2461) does not talk about neighbor
cache handling in the following cases:
(1) when there was no neighbor cache entry, node received unsolicited
    RS/NS/NA/redirect packet without link-layer address
(2) neighbor cache handling on medium without link-layer address
    (we need a neighbor cache entry for IsRouter bit)
For (1), we implemented workaround based on discussions on IETF ipngwg mailing
list.  For more details, see the comments in the source code and email
thread started from (IPng 7155), dated Feb 6 1999.

IPv6 on-link determination rule (RFC2461) is quite different from assumptions
in BSD network code.  At this moment, KAME does not implement on-link
determination rule when default router list is empty (RFC2461, section 5.2,
last sentence in 2nd paragraph - note that the spec misuse the word "host"
and "node" in several places in the section).

To avoid possible DoS attacks and infinite loops, KAME stack will accept
only 10 options on ND packet.  Therefore, if you have 20 prefix options
attached to RA, only the first 10 prefixes will be recognized.
If this troubles you, please contact KAME team and/or modify
nd6_maxndopt in sys/netinet6/nd6.c.  If there are high demands we may
provide sysctl knob for the variable.

1.3 Scope Index

IPv6 uses scoped addresses.  Therefore, it is very important to
specify scope index (interface index for link-local address, or
site index for site-local address) with an IPv6 address.  Without
scope index, scoped IPv6 address is ambiguous to the kernel, and
kernel will not be able to determine the outbound interface for a
packet.

Ordinary userland applications should use advanced API (RFC2292) to
specify scope index, or interface index.  For similar purpose,
sin6_scope_id member in sockaddr_in6 structure is defined in RFC2553.
However, the semantics for sin6_scope_id is rather vague.  If you
care about portability of your application, we suggest you to use
advanced API rather than sin6_scope_id.

In the kernel, an interface index for link-local scoped address is
embedded into 2nd 16bit-word (3rd and 4th byte) in IPv6 address.
For example, you may see something like:
	fe80:1::200:f8ff:fe01:6317
in the routing table and interface address structure (struct
in6_ifaddr).  The address above is a link-local unicast address
which belongs to a network interface whose interface identifier is 1.
The embedded index enables us to identify IPv6 link local
addresses over multiple interfaces effectively and with only a
little code change.
Routing daemons and configuration programs, like route6d and
ifconfig, will need to manipulate the "embedded" scope index.
These programs use routing sockets and ioctls (like SIOCGIFADDR_IN6)
and the kernel API will return IPv6 addresses with 2nd 16bit-word
filled in.  The APIs are for manipulating kernel internal structure.
Programs that use these APIs have to be prepared about differences
in kernels anyway.

When you specify scoped address to the command line, NEVER write the
embedded form (such as ff02:1::1 or fe80:2::fedc).  This is not supposed
to work.  Always use standard form, like ff02::1 or fe80::fedc, with
command line option for specifying interface (like "ping6 -I ne0 ff02::1).
In general, if a command does not have command line option to specify
outgoing interface, that command is not ready to accept scoped address.
This may seem to be opposite from IPv6's premise to support "dentist office"
situation.  We believe that specifications need some improvements for this.

Some of the userland tools support extended numeric IPv6 syntax, as
documented in draft-ietf-ipngwg-scopedaddr-format-00.txt.  You can specify
outgoing link, by using name of the outgoing interface like "fe80::1@ne0".
This way you will be able to specify link-local scoped address without much
trouble.
To use this extension in your program, you'll need to use getaddrinfo(3),
and getnameinfo(3) with NI_WITHSCOPEID.
The implementation currently assumes 1-to-1 relationship between a link and an
interface, which is stronger than what specs say.

1.4 Plug and Play

The KAME kit implements most of the IPv6 stateless address
autoconfiguration in the kernel.
Neighbor Discovery functions are implemented in the kernel as a whole.
Router Advertisement (RA) input for hosts is implemented in the
kernel.  Router Solicitation (RS) output for endhosts, RS input
for routers, and RA output for routers are implemented in the
userland.

1.4.1 Assignment of link-local, and special addresses

IPv6 link-local address is generated from IEEE802 adddress (ethernet MAC
address).  Each of interface is assigned an IPv6 link-local address
automatically, when the interface becomes up (IFF_UP).  Also, direct route
for the link-local address is added to routing table.

Here is an output of netstat command:

Internet6:
Destination                   Gateway                   Flags      Netif Expire
fe80:1::/64                   link#1                    UC           ed0
fe80:2::/64                   link#2                    UC           ep0

Interfaces that has no IEEE802 address (pseudo interfaces like tunnel
interfaces, or ppp interfaces) will borrow IEEE802 address from other
interfaces, such as ethernet interfaces, whenever possible.
If there is no IEEE802 hardware attached, last-resort pseudorandom value,
which is from MD5(hostname), will be used as source of link-local address.
If it is not suitable for your usage, you will need to configure the
link-local address manually.

If an interface is not capable of handling IPv6 (such as lack of multicast
support), link-local address will not be assigned to that interface.
See section 2 for details.

Each interface joins the solicited multicast address and the
link-local all-nodes multicast addresses (e.g.  fe80::1:ff01:6317
and ff02::1, respectively, on the link the interface is attached).
In addition to a link-local address, the loopback address (::1) will be
assigned to the loopback interface.  Also, ::1/128 and ff01::/32 are
automatically added to routing table, and loopback interface joins
node-local multicast group ff01::1.

1.4.2 Stateless address autoconfiguration on hosts

In IPv6 specification, nodes are separated into two categories:
routers and hosts.  Routers forward packets addressed to others, hosts does
not forward the packets.  net.inet6.ip6.forwarding defines whether this
node is router or host (router if it is 1, host if it is 0).

When a host hears Router Advertisement from the router, a host may
autoconfigure itself by stateless address autoconfiguration.
This behavior can be controlled by net.inet6.ip6.accept_rtadv
(host autoconfigures itself if it is set to 1).
By autoconfiguration, network address prefix for the receiving interface
(usually global address prefix) is added.  Default route is also configured.
Routers periodically generate Router Advertisement packets.  To request
an adjacent router to generate RA packet, a host can transmit Router
Solicitation.  To generate a RS packet at any time, use the "rtsol" command.
"rtsold" daemon is also available.  "rtsold" generates Router Solicitation
whenever necessary, and it works great for nomadic usage (notebooks/laptops).
If one wishes to ignore Router Advertisements, use sysctl to set
net.inet6.ip6.accept_rtadv to 0.

To generate Router Advertisement from a router, use the "rtadvd" daemon.

Note that, IPv6 specification assumes the following items, and nonconforming
cases are left unspecified:
- Only hosts will listen to router advertisements
- Hosts have single network interface (except loopback)
Therefore, this is unwise to enable net.inet6.ip6.accept_rtadv on routers,
or multi-interface host.  A misconfigured node can behave strange
(KAME code allows nonconforming configuration, for those who would like
to do some experiments).

To summarize the sysctl knob:
	accept_rtadv	forwarding	role of the node
	---		---		---
	0		0		host (to be manually configured)
	0		1		router
	1		0		autoconfigured host
					(spec assumes that host has single
					interface only, autoconfigred host with
					multiple interface is out-of-scope)
	1		1		invalid, or experimental
					(out-of-scope of spec)

RFC2462 has validation rule against incoming RA prefix information option,
in 5.5.3 (e).  This is to protect hosts from malicious (or misconfigured)
routers that advertise very short prefix lifetime.
There was an update from Jim Bound to ipngwg mailing list (look
for "(ipng 6712)" in the archive) and KAME implements Jim's update.

See 1.2 in the document for relationship between DAD and autoconfiguration.

1.4.3 DHCPv6

We supply a tiny DHCPv6 server/client in kame/dhcp6.  However, the
implementation is very premature (for example, this does NOT
implement address lease/release), and it is not in default compilation
tree.  If you want to do some experiment, compile it on your own.

DHCPv6 and autoconfiguration also needs more work.  "Managed" and "Other"
bits in RA have no special effect to stateful autoconfiguration procedure
in DHCPv6 client program ("Managed" bit actually prevents stateless
autoconfiguration, but no special action will be taken for DHCPv6 client).

1.5 Generic tunnel interface

GIF (Generic InterFace) is a pseudo interface for configured tunnel.
Details are described in gif(4) manpage.
Currently
	v6 in v6
	v6 in v4
	v4 in v6
	v4 in v4
are available.  Use "gifconfig" to assign physical (outer) source
and destination address to gif interfaces.
Configuration that uses same address family for inner and outer IP
header (v4 in v4, or v6 in v6) is dangerous.  It is very easy to
configure interfaces and routing tables to perform infinite level
of tunneling.  Please be warned.

gif can be configured to be ECN-friendly.  See 4.5 for ECN-friendliness
of tunnels, and gif(4) manpage for how to configure.

If you would like to configure an IPv4-in-IPv6 tunnel with gif interface,
read gif(4) carefully.  You will need to remove IPv6 link-local address
automatically assigned to the gif interface.

1.6 Source Address Selection

Source selection of KAME is scope oriented (there are some exceptions -
see below).  For a given destination, a source IPv6 address is selected
by the following rule:
    1. If the source address is explicitly specified by the user
       (e.g. via the advanced API), the specified address is used.
    2. If there is an address assigned to the outgoing interface
       (which is usually determined by looking up the routing table)
       that has the same scope as the destination address, the address
       is used.
       This is the most typical case.
    3. If there is no address that satisfies the above condition,
       choose a global address assigned to one of the interfaces
       on the sending node.
    4. If there is no address that satisfies the above condition and
       there is no global address on the sending node, choose the
       address associated with the routing table entry for the destination.
       This is the last resort, which may cause scope violation.

For instance, ::1 is selected for ff01::1, fe80:1::200:f8ff:fe01:6317
for fe80:1::2a0:24ff:feab:839b (note that embedded interface index -
described in 1.3 - helps us choose the right source address.  Those
embedded indices will not be on the wire).
If the outgoing interface has multiple address for the scope,
a source is selected longest match basis (rule 3).  Suppose
3ffe:501:808:1:200:f8ff:fe01:6317 and 3ffe:2001:9:124:200:f8ff:fe01:6317
are given to the outgoing interface.  3ffe:501:808:1:200:f8ff:fe01:6317
is chosen as the source for the destination 3ffe:501:800::1.

Note that the above rule is not documented in the IPv6 spec.  It is
considered "up to implementation" item.
There are some cases where we do not use the above rule.  One
example is connected TCP session, and we use the address kept in tcb
as the source.
Another example is source address for Neighbor Advertisement.
Under the spec (RFC2461 7.2.2) NA's source should be the target
address of the corresponding NS's target.  In this case we follow
the spec rather than the above longest-match rule.

For new connections (when rule 1 does not apply), deprecated addresses
(addresses with preferred lifetime = 0) will not be chosen as source address
if other choises are available.  If no other choices are available,
deprecated address will be used as a last resort.  If there are multiple
choice of deprecated addresses, the above scope rule will be used to choose
from those deprecated addreses.  If you would like to prohibit the use
of deprecated address for some reason, configure net.inet6.ip6.use_deprecated
to 0.  The issue related to deprecated address is described in RFC2462 5.5.4
(NOTE: there is some debate underway in IETF ipngwg on how to use
"deprecated" address).

1.7 Jumbo Payload

KAME supports the Jumbo Payload hop-by-hop option used to send IPv6
packets with payloads longer than 65,535 octets.  But since currently
KAME does not support any physical interface whose MTU is more than
65,535, such payloads can be seen only on the loopback interface(i.e.
lo0).

If you want to try jumbo payloads, you first have to reconfigure the
kernel so that the MTU of the loopback interface is more than 65,535
bytes; add the following to the kernel configuration file:
	options		"LARGE_LOMTU"		#To test jumbo payload
and recompile the new kernel.

Then you can test jumbo payloads by the ping6 command with -b and -s
options.  The -b option must be specified to enlarge the size of the
socket buffer and the -s option specifies the length of the packet,
which should be more than 65,535.  For example, type as follows; 
	% ping6 -b 70000 -s 68000 ::1

The IPv6 specification requires that the Jumbo Payload option must not
be used in a packet that carries a fragment header.  If this condition
is broken, an ICMPv6 Parameter Problem message must be sent to the
sender.  KAME kernel follows the specification, but you cannot usually
see an ICMPv6 error caused by this requirement.

If KAME kernel receives an IPv6 packet, it checks the frame length of
the packet and compares it to the length specified in the payload
length field of the IPv6 header or in the value of the Jumbo Payload
option, if any.  If the former is shorter than the latter, KAME kernel
discards the packet and increments the statistics. You can see the
statistics as output of netstat command with `-s -p ip6' option:
	% netstat -s -p ip6
	ip6:
		(snip)
		1 with data size < data length

So, KAME kernel does not send an ICMPv6 error unless the erroneous
packet is an actual Jumbo Payload, that is, its packet size is more
than 65,535 bytes.  As described above, KAME kernel currently does not
support physical interface with such a huge MTU, so it rarely returns an
ICMPv6 error.

TCP/UDP over jumbogram is not supported at this moment.  This is because
we have no medium (other than loopback) to test this.  Contact us if you
need this.

IPsec does not work on jumbograms.  This is due to some specification twists
in supporting AH with jumbograms (AH header size influences payload length,
and this makes it real hard to authenticate inbound packet with jumbo payload
option as well as AH).

There are fundamental issues in *BSD support for jumbograms.  We would like to
address those, but we need more time to finalize these.  To name a few:
- mbuf pkthdr.len field is typed as "int" in 4.4BSD, so it will not hold
  jumbogram with len > 2G on 32bit architecture CPUs.  If we would like to
  support jumbogram properly, the field must be expanded to hold 4G +
  IPv6 header + link-layer header.  Therefore, it must be expanded to at least
  int64_t (u_int32_t is NOT enough).
- We mistakingly use "int" to hold packet length in many places.  We need
  to convert them into larger integral type.  It needs a great care, as we may
  experience overflow during packet length computation.
- We mistakingly check for ip6_plen field of IPv6 header for packet payload
  length in various places.  We should be checking mbuf pkthdr.len instead.
  ip6_input() will perform sanity check on jumbo payload option on input,
  and we can safely use mbuf pkthdr.len afterwards.
- TCP code needs a careful update in bunch of places, of course.

1.8 Loop prevention in header processing

IPv6 specification allows arbitrary number of extension headers to
be placed onto packets.  If we implement IPv6 packet processing
code in the way BSD IPv4 code is implemented, kernel stack may
overflow due to long function call chain.  KAME sys/netinet6 code
is carefully designed to avoid kernel stack overflow.  Because of
this, KAME sys/netinet6 code defines its own protocol switch
structure, as "struct ip6protosw" (see netinet6/ip6protosw.h).
IPv4 part (sys/netinet) remains untouched for compatibility.
Because of this, if you receive IPsec-over-IPv4 packet with massive
number of IPsec headers, kernel stack may blow up.  IPsec-over-IPv6 is okay.

1.9 ICMPv6

After RFC2463 was published, IETF ipngwg has decided to disallow ICMPv6 error
packet against ICMPv6 redirect, to prevent ICMPv6 storm on a network medium.
KAME already implements this into the kernel.

1.10 Applications

For userland programming, we support IPv6 socket API as specified in
RFC2553, RFC2292 and upcoming internet drafts.

TCP/UDP over IPv6 is available and quite stable.  You can enjoy "telnet",
"ftp", "rlogin", "rsh", "ssh", etc.  These applications are protocol
independent.  That is, they automatically chooses IPv4 or IPv6
according to DNS.

1.11 Kernel Internals

 (*) TCP/UDP part is handled differently between operating system platforms.
     See 1.12 for details.

The current KAME has escaped from the IPv4 netinet logic.  While
ip_forward() calls ip_output(), ip6_forward() directly calls
if_output() since routers must not divide IPv6 packets into fragments.

ICMPv6 should contain the original packet as long as possible up to
1280.  UDP6/IP6 port unreach, for instance, should contain all
extension headers and the *unchanged* UDP6 and IP6 headers.
So, all IP6 functions except TCP6 never convert network byte
order into host byte order, to save the original packet.

tcp6_input(), udp6_input() and icmp6_input() can't assume that IP6
header is preceding the transport headers due to extension
headers.  So, in6_cksum() was implemented to handle packets whose IP6
header and transport header is not continuous.  TCP/IP6 nor UDP/IP6
header structure don't exist for checksum calculation.

To process IP6 header, extension headers and transport headers easily,
KAME requires network drivers to store packets in one internal mbuf or
one or more external mbufs.  A typical old driver prepares two
internal mbufs for 100 - 208 bytes data, however, KAME's reference
implementation stores it in one external mbuf.

"netstat -s -p ip6" tells you whether or not your driver conforms
KAME's requirement.  In the following example, "cce0" violates the
requirement. (For more information, refer to Section 2.)

        Mbuf statistics:
                317 one mbuf
                two or more mbuf::
                        lo0 = 8
			cce0 = 10
                3282 one ext mbuf
                0 two or more ext mbuf

Each input function calls IP6_EXTHDR_CHECK in the beginning to check
if the region between IP6 and its header is
continuous.  IP6_EXTHDR_CHECK calls m_pullup() only if the mbuf has
M_LOOP flag, that is, the packet comes from the loopback
interface.  m_pullup() is never called for packets coming from physical
network interfaces.

TCP6 reassembly makes use of IP6 header to store reassemble
information.  IP6 is not supposed to be just before TCP6, so
ip6tcpreass structure has a pointer to TCP6 header.  Of course, it has
also a pointer back to mbuf to avoid m_pullup().

Like TCP6, both IP and IP6 reassemble functions never call m_pullup().

xxx_ctlinput() calls in_mrejoin() on PRC_IFNEWADDR.  We think this is
one of 4.4BSD implementation flaws.  Since 4.4BSD keeps ia_multiaddrs
in in_ifaddr{}, it can't use multicast feature if the interface has no
unicast address.  So, if an application joins to an interface and then
all unicast addresses are removed from the interface, the application
can't send/receive any multicast packets.  Moreover, if a new unicast
address is assigned to the interface, in_mrejoin() must be called.
KAME's interfaces, however, have ALWAYS one link-local unicast
address.  These extensions have thus not been implemented in KAME.

1.12 IPv4 mapped address and IPv6 wildcard socket

RFC2553 describes IPv4 mapped address (3.7) and special behavior
of IPv6 wildcard bind socket (3.8).  The spec allows you to:
- Transmit IPv4 packet over AF_INET6 socket by using special form of
  the address like ::ffff:10.1.1.1.
- Accept IPv4 connections by AF_INET6 wildcard bind socket.
but the spec itself is very complicated and does not specify how the
socket layer should behave.

We KAME team have 4 OS platforms right now, and behavior is slightly
different between them.  To summarize:
- All KAME implementations treat tcp/udp port number space separately
  between IPv4 and IPv6.
- KAME/BSDI3, KAME/OpenBSD and KAME/FreeBSD228 does not support IPv4 mapped
  address, nor special wildcard bind on AF_INET6.
- KAME/FreeBSD3x supports IPv4 mapped address, and special wildcard bind on
  AF_INET6.  It is enabled by default.  You can disable those two by runtime
  and kernel compile configuration.
  (you can't enable only one of them: they come together)
- KAME/NetBSD supports both.  This is always enabled.
- KAME/BSDI4 supports both.  This is always enabled.

The following sections will give you the details, and how you can
configure the behavior.

Advise to application implementers: to implement a portable IPv6 application
(which works on multiple IPv6 kernels), we believe that the following
is the key to the success:
- NEVER hardcode AF_INET nor AF_INET6.
- Use getaddrinfo() and getnameinfo() throughout the system.
  Never use gethostby*(), getaddrby*(), inet_*() or getipnodeby*().
- If you would like to listen to connections, use getaddrinfo() (maybe
  with AI_PASSIVE), and make sockets for all the "struct addrinfo" returned.
- If you would like to connect to destination, use getaddrinfo() and try
  all the destination returned, like telnet does.
- Some of the IPv6 stack is shipped with buggy getaddrinfo().  Ship a minimal
  working version with your application and use that as last resort.
- Try to avoid use of IPv4 mapped address (waiting for IPv4 connection on
  AF_INET6 socket).  Listen to both AF_INET socket and AF_INET6 socket,
  if you support both address families.
It looks that RFC2553 talks too little on wildcard bind issue,
especially on the port space issue, failure mode and relationship
between AF_INET/INET6 wildcard bind.  There can be several separate
interpretation for this RFC (see 1.12.2 - we have two different
implementation for this, and RFC2553 seems to fit to both of them).
So, to implement portable application you should assume nothing
about the behavior in the kernel.  Using getaddrinfo() is the safest way.
Port number space and wildcard bind issues were discussed in detail
on ipv6imp mailing list, in mid March 1999 and it looks that there's
no concrete consensus (means, up to implementers).  You may want to
check the mailing list archives.

We supply a tool called "bindtest" that explores the behavior of
kernel bind(2).  The tool will not be compiled by default.

1.12.1 KAME/BSDI3 and KAME/FreeBSD228

The platform do not support IPv4 mapped address.
The IPv4 mapped address support needs tweaked implementation in
DNS support libraries, as documented in RFC2553 6.1.  However, since
the platforms do not support this, you do not need to worry about
RFC2553 6.1 and story goes much simpler.
(KAME library actually implements the tweaks, but it is safe to ignore that)

Port number space is totally separate between AF_INET and
AF_INET6 sockets.  You can always perform wildcard bind on both of
the adderss families, on the same port.

If a server application would like to accept IPv4 and IPv6 connections,
it should use AF_INET and AF_INET6 socket (you'll need two sockets).
Applicsations should use proper socket for connections.  IPv4 connections
must be made on AF_INET socket, and IPv6 connections must be made
on AF_INET6 socket.  getaddrinfo() library helps you in writing
AF-independent application, and managing sockets with different AFs.
(some of the implementers think that we should totally get rid of gethostby*
family of the functions and migrate to get{addr,name}info, since
it is very clean and helps you support new AFs in the future)

1.12.2 KAME/FreeBSD3x

The platform can be configured to support IPv4 mapped address/special AF_INET6
wildcard bind (enabled by default).  If you disable it, it behaves as described
in 1.12.1.

The IPv4 mapped address support needs tweaked implementation in
DNS support libraries.  This is documented in RFC2553 6.1.
KAME libraries (namely libinet6.a) actually support that.

RFC2553 does not talk about how port number space should be designed
(i.e. should they be separate between AF_INET and AF_INET6, or
should they be common)
In KAME with the behavior enabled, port number space is separate
between AF_INET and AF_INET6 sockets in most cases.  The only
exception is wildcard bind socket, where the special behavior appears.

If a server application would like to accept IPv4 and IPv6 connections,
it can use IPv6 socket with wildcard bind, or use two sockets (for
AF_INET6 and AF_INET).  You can handle IPv4 and IPv6 connections
by using AF_INET6 socket.  Porting of an application can be simpler in
this case, like:
- change AF_INET into AF_INET6
- use gethostbyname2(hostname, AF_INET6) or getipnodebyname(), instead of
  gethostbyname(hostname)
- use struct sockaddr_in6 instead of sockaddr_in

To provide services to both IPv4 and IPv6 clients, you will run a single
server which binds to single AF_INET6 wildcard socket.  This server will
accept both IPv4 and IPv6 connections to the tcp/udp port.
If you run two daemons, which binds to AF_INET6 wildcard socket and
AF_INET socket (say, sendmail4 and sendmail6), story start to look
a bit complicated.  The next sections have the detail.

Wildcard bind on AF_INET6 behaves like "wildcard bind between two
address families".  It will grab IPv4 connection if and only if
there is no socket that binds to more specific destination.
Here, wildcard bind on AF_INET is regarded as "more specific bind"
than wildcard bind on AF_INET6.
In other words, wildcard bind on AF_INET6 is the only thing that
has special behavior.  It will not affect wildcard bind on AF_INET.
If the following events happen, IPv4 connection will be routed
to application B, and IPv6 connection will be routed to application A.
- application A perform wildcard bind on AF_INET6, port X
- application B perform wildcard bind on AF_INET, port X
- IPv4 connection arrives
- IPv6 connection arrives
If the following events happen, the behavior is the same.  IPv4
connection will be routed to application B, and IPv6 connection
will be routed to application A.
- application B perform wildcard bind on AF_INET, port X
- application A perform wildcard bind on AF_INET6, port X
- IPv4 connection arrives
- IPv6 connection arrives

If the following events happen, KAME/FreeBSD3x will behave like this:
- sendmail4 is running.  It is doing wildcard bind on AF_INET.
- Invoke sendmail6.  It will do a wildcard bind on AF_INET6.
- Stop sendmail4.  Here, on KAME/FreeBSD3x, IPv4 and IPv6 conections will
  be routed to sendmail6.  This is different from KAME/FreeBSD228.

1.12.4 KAME/NetBSD

KAME/NetBSD uses shared tcp4/6 code (from sys/netinet/tcp*) and shared
udp4/6 code (from sys/netinet/udp*).  The implementation is made differently
from KAME/FreeBSD3x.  KAME/NetBSD uses separate inpcb/in6pcb structures,
while KAME/FreeBSD3x uses merged inpcb structure.
Supports for IPv4 mapped address/special AF_INET6 wildcard bind are
enabled by default.  At this moment there is no way to disable it.

1.12.5 KAME/BSDI4

KAME/BSDI4 uses NRL-based TCP/UDP stack and inpcb source code,
which was derived from NRL IPv6/IPsec stack.  I guess it supports IPv4 mapped
address and speical AF_INET6 wildcard bind.  The implementation is, again,
different from other KAME/*BSDs.
Note that NRL inpcb layer has different behavior than KAME implementation,
namely:
- If you bind(2) a socket to IPv6 wildcard address (::) then bind(2)
  another socket to IPv4 wildcard address (0.0.0.0), the latter will fail
  with EADDRINUSE.
- If you bind(2) to IPv4 wildcard address then IPv6 wildcard address,
  both will success.  However, all IPv4 traffic (and IPv6 traffic) will be
  captured by IPv6 wildcard socket.

1.12.6 KAME/OpenBSD

KAME/OpenBSD uses NRL-based TCP/UDP stack and inpcb source code,
which was derived from NRL IPv6/IPsec stack.  However, KAME/OpenBSD
disables special behavior on AF_INET6 wildcard bind for security reasons
(if IPv4 traffic toward AF_INET6 wildcard bind is allowed, access control
will become much harder).  KAME/BSDI4 uses NRL-based TCP/UDP stack as well,
however, the behavior is different.

As a result the behavior of KAME/OpenBSD is similar to KAME/BSDI3 and
KAME/FreeBSD228 (see 1.12.1 for more detail).

1.12.7 configuration and implementation

On KAME/FreeBSD3x, the behavior is configurable by following procedure.
To enable it:
- Add the "MAPPED_ADDR_ENABLED" kernel config option into your
  kernel config file (see "sys/i386/conf/GENERIC.v6" sample file)
  and build your kernel, and
- set sysctl variable appropriately, like:
	# sysctl -w net.inet6.ip6.mapped_addr=1
Note that, to enable the behavior you'll need to do the both of the above.
If you do not do the both, the behavior is disabled.

1.13 sockaddr_storage

When RFC2553 was about to be finalized, there was discusson on how struct
sockaddr_storage members are named.  One proposal is to prepend "__" to the
members (like "__ss_len") as they should not be touched.  The other proposal
was that don't prepend it (like "ss_len") as we need to touch those members
directly.  There was no clear consensus on it.

As a result, RFC2553 defines struct sockaddr_storage as follows:
	struct sockaddr_storage {
		u_char	__ss_len;	/* address length */
		u_char	__ss_family;	/* address family */
		/* and bunch of padding */
	};
On the contrary, XNET draft defines as follows:
	struct sockaddr_storage {
		u_char	ss_len;		/* address length */
		u_char	ss_family;	/* address family */
		/* and bunch of padding */
	};

In December 1999, it was agreed that RFC2553bis should pick the latter (XNET)
definition.

KAME kit prior to December 1999 used RFC2553 definition.  KAME kit after
December 1999 (including December) will conform to XNET definition,
based on RFC2553bis discusson.

If you look at multiple IPv6 implementations, you will be able to see
both definitions.  As an userland programmer, the most portable way of
dealing with it is to:
(1) ensure ss_family and/or ss_len are available on the platform, by using
    GNU autoconf,
(2) have -Dss_family=__ss_family to unify all occurences (including header
    file) into __ss_family, or
(3) never touch __ss_family.  cast to sockaddr * and use sa_family like:
	struct sockaddr_storage ss;
	family = ((struct sockaddr *)&ss)->sa_family

2. Network Drivers

KAME requires three items to be added into the standard drivers:

(1) mbuf clustering requirement. In this stable release, we changed
    MINCLSIZE into MHLEN+1 for all the operating systems in order to make
    all the drivers behave as we expect.  

(2) multicast.  If "ifmcstat" yields no multicast group for a
    interface, that interface has to be patched.

To avoid troubles, we suggest you to comment out the device drivers
for unsupported/unnecessary cards, from the kernel configuration file.
If you accidentally enable unsupported drivers, some of the userland
tools may not work correctly (routing daemons are typical example).

In the following sections, "official support" means that KAME developers
are using that ethernet card/driver frequently.

(NOTE: In the past we required all pcmcia drivers to have a call to
in6_ifattach().  We have no such requirement any more)

2.1 FreeBSD 2.2.x-RELEASE

Here is a list of FreeBSD 2.2.x-RELEASE drivers and its conditions:

	driver	mbuf(1)		multicast(2)	official
						support?
	---	---		---		---
	(Ethernet)
	ar	looks ok	-		-
	cnw	ok		ok		yes (*)
	ed	ok		ok		yes
	ep	ok		ok		yes
	fe	ok		ok		yes
	sn	looks ok	-		-   (*)
	vx	looks ok	-		-
	wlp	ok		ok		-   (*)
	xl	ok		ok		yes
	zp	ok		ok		-
	(FDDI)
	fpa	looks ok	?		-
	(ATM)
	en	ok		ok		yes
	(Serial)
	lp	?		-		not work
	sl	?		-		not work
	sr	looks ok	ok		-   (**)

You may want to add an invocation of "rtsol" in "/etc/pccard_ether",
if you are using notebook computers and PCMCIA ethernet card.

(*) These drivers are distributed with PAO (http://www.jp.freebsd.org/PAO/).

(**) There was some report says that, if you make sr driver up and down and
then up, the kernel may hang up.  We have disabled frame-relay support from
sr driver and after that this looks to be working fine.  If you need
frame-relay support to come back, please contact KAME developers.

2.2 BSD/OS 3.x

The following lists BSD/OS 3.x device drivers and its conditions:

	driver	mbuf(1)		multicast(2)	official
						support?
	---	---		---		---
	(Ethernet)
	cnw	ok		ok		yes
	de	ok		ok		-
	df	ok		ok		-
	eb	ok		ok		-
	ef	ok		ok		yes
	exp	ok		ok		-
	mz	ok		ok		yes
	ne	ok		ok		yes
	we	ok		ok		-
	(FDDI)
	fpa	ok		ok		-
	(ATM)
	en	maybe		ok		-
	(Serial)
	ntwo	ok		ok		yes
	sl	?		-		not work
	appp	?		-		not work

You may want to use "@insert" directive in /etc/pccard.conf to invoke
"rtsol" command right after dynamic insertion of PCMCIA ethernet cards.

2.3 NetBSD

The following table lists the network drivers we have tried so far.

	driver		mbuf(1)	multicast(2)	official
						support?
	---		---	---		---
	(Ethernet)
	ne pci/i386	ok	ok		yes
	ep pcmcia/i386	ok	ok		-
	le sbus/sparc	ok	ok		yes
	bah zbus/amiga	NG(*)
	(ATM)
	en pci/i386	ok	ok		-

(*) This may need some fix, but I'm not sure what arcnet interfaces assume...

2.4 FreeBSD 3.x-RELEASE

Here is a list of FreeBSD 3.x-RELEASE drivers and its conditions:

	driver	mbuf(1)		multicast(2)	official
						support?
	---	---		---		---
	(Ethernet)
	fe	ok		ok		yes
	fxp	ok		ok		yes
	wi	ok		ok		yes
	lnc	?		ok		-
	cnw	ok		ok		-(*)
	ep	ok		ok		-
	sn	?		?		-(*)
	xl	?		ok		-
	ed	?		ok		-

(*) These drivers are distributed with PAO as PAO3
    (http://www.jp.freebsd.org/PAO/).

More drivers will just simply work on KAME FreeBSD 3.x-RELEASE but have not
been checked yet.

2.5 OpenBSD 2.x

Here is a list of OpenBSD 2.x drivers and its conditions:

	driver		mbuf(1)		multicast(2)	official
							support?
	---		---		---		---
	(Ethernet)
	ne pci/i386	ok		ok		yes
	ne pcmcia/i386	ok		ok		yes
	le sbus/sparc	ok		ok		yes

3. Translator

We categorize IPv4/IPv6 translator into 4 types.

Translator A --- It is used in the early stage of transition to make
it possible to establish a connection from an IPv6 host in an IPv6
island to an IPv4 host in the IPv4 ocean.

Translator B --- It is used in the early stage of transition to make
it possible to establish a connection from an IPv4 host in the IPv4
ocean to an IPv6 host in an IPv6 island.

Translator C --- It is used in the late stage of transition to make it
possible to establish a connection from an IPv4 host in an IPv4 island
to an IPv6 host in the IPv6 ocean.

Translator D --- It is used in the late stage of transition to make it
possible to establish a connection from an IPv6 host in the IPv6 ocean
to an IPv4 host in an IPv4 island.

KAME provides an TCP relay translator for category A.  This is called
"FAITH".  We also provide IP header translator for category A.

3.1 FAITH TCP relay translator

FAITH system uses TCP relay daemon called "faithd" helped by the KAME kernel.
FAITH will reserve an IPv6 address prefix, and relay TCP connection
toward that prefix to IPv4 destination.

For example, if the reserved IPv6 prefix is 3ffe:0501:0200:ffff::, and
the IPv6 destination for TCP connection is 3ffe:0501:0200:ffff::163.221.202.12,
the connection will be relayed toward IPv4 destination 163.221.202.12.

	destination IPv4 node (163.221.202.12)
	  ^
	  | IPv4 tcp toward 163.221.202.12
	FAITH-relay dual stack node
	  ^
	  | IPv6 TCP toward 3ffe:0501:0200:ffff::163.221.202.12
	source IPv6 node

faithd must be invoked on FAITH-relay dual stack node.

For more details, consult kame/kame/faithd/README.

3.2 IPv6-to-IPv4 header translator

(to be written)

4. IPsec

# NOTE: This section does not apply to OpenBSD-current.

IPsec is mainly organized by three components.

(1) Policy Management
(2) Key Management
(3) AH and ESP handling

Note that KAME/OpenBSD does NOT include support for KAME IPsec code,
as OpenBSD team has their home-brew IPsec stack and they have no plan
to replace it.  IPv6 support for IPsec is, therefore, lacking on KAME/OpenBSD.
KAME/BSDI4 lacks IPsec at this moment (both NRL and KAME).  In the near
future we will be adding KAME IPSec code support into KAME/BSDI4.

4.1 Policy Management

The kernel implements experimental policy management code.  There are two way
to to manage security policy.  One is to configure per-socket policy using
setsockopt(3).  In this cases, policy configuration is described in
ipsec_set_policy(3).  The other is to configure kernel packet filter-based
policy using PF_KEY interface, via setkey(8).

The policy entry is not re-ordered with its
indexes, so the order of entry when you add is very significant.

4.2 Key Management

The key management code implemented in this kit (sys/netkey) is a
home-brew PFKEY v2 implementation.  This conforms to RFC2367.

The home-brew IKE daemon, "racoon" is included in the kit
(kame/kame/racoon).
Basically you'll need to run racoon as daemon, then setup a policy
to require keys (like ping -P 'out ipsec esp/transport//use').
The kernel will contact racoon daemon as necessary to exchange keys.

4.3 AH and ESP handling

IPsec module is implemented as "hooks" to the standard IPv4/IPv6
processing.  When sending a packet, ip{,6}_output() checks if ESP/AH
processing is required by checking if a matching SPD (Security
Policy Database) is found.  If ESP/AH is needed,
{esp,ah}{4,6}_output() will be called and mbuf will be updated
accordingly.  When a packet is received, {esp,ah}4_input() will be
called based on protocol number, i.e. (*inetsw[proto])().
{esp,ah}4_input() will decrypt/check authenticity of the packet,
and strips off daisy-chained header and padding for ESP/AH.  It is
safe to strip off the ESP/AH header on packet reception, since we
will never use the received packet in "as is" form.

By using ESP/AH, TCP4/6 effective data segment size will be affected by
extra daisy-chained headers inserted by ESP/AH.  Our code takes care of
the case.

Basic crypto functions can be found in directory "sys/crypto".  ESP/AH
transform are listed in {esp,ah}_core.c with wrapper functions.  If you
wish to add some algorithm, add wrapper function in {esp,ah}_core.c, and
add your crypto algorithm code into sys/crypto.

Tunnel mode is partially supported in this release, with the following
restrictions:
- IPsec tunnel is not combined with GIF generic tunneling interface.
  It needs a great care because we may create an infinite loop between
  ip_output() and tunnelifp->if_output().  Opinion varies if it is better
  to unify them, or not.
- MTU and Don't Fragment bit (IPv4) considerations need more checking, but
  basically works fine.
- Authentication model for AH tunnel must be revisited.  We'll need to
  improve the policy management engine, eventually.

4.4 Conformance to RFCs and IDs

The IPsec code in the kernel conforms (or, tries to conform) to the
following standards:
    "old IPsec" specification documented in rfc182[5-9].txt
    "new IPsec" specification documented in rfc240[1-6].txt, rfc241[01].txt,
	rfc2451.txt and draft-mcdonald-simple-ipsec-api-01.txt (draft expired,
	but you can take from ftp://ftp.kame.net/pub/internet-drafts/).
	(NOTE: IKE specifications, rfc241[7-9].txt are implemented in userland,
	as "racoon" IKE daemon)

Currently supported algorithms are:
    old IPsec AH
	null crypto checksum (no document, just for debugging)
	keyed MD5 with 128bit crypto checksum (rfc1828.txt)
	keyed SHA1 with 128bit crypto checksum (no document)
	HMAC MD5 with 128bit crypto checksum (rfc2085.txt)
	HMAC SHA1 with 128bit crypto checksum (no document)
    old IPsec ESP
	null encryption (no document, similar to rfc2410.txt)
	DES-CBC mode (rfc1829.txt)
    new IPsec AH
	null crypto checksum (no document, just for debugging)
	keyed MD5 with 96bit crypto checksum (no document)
	keyed SHA1 with 96bit crypto checksum (no document)
	HMAC MD5 with 96bit crypto checksum (rfc2403.txt
	HMAC SHA1 with 96bit crypto checksum (rfc2404.txt)
    new IPsec ESP
	null encryption (rfc2410.txt)
	DES-CBC with derived IV
		(draft-ietf-ipsec-ciph-des-derived-01.txt, draft expired)
	DES-CBC with explicit IV (rfc2405.txt)
	3DES-CBC with explicit IV (rfc2451.txt)
	BLOWFISH CBC (rfc2451.txt)
	CAST128 CBC (rfc2451.txt)
	RC5 CBC (rfc2451.txt)
	each of the above can be combined with:
	    ESP authentication with HMAC-MD5(96bit)
	    ESP authentication with HMAC-SHA1(96bit)

The following algorithms are NOT supported:
    old IPsec AH
	HMAC MD5 with 128bit crypto checksum + 64bit replay prevention
		(rfc2085.txt)
	keyed SHA1 with 160bit crypto checksum + 32bit padding (rfc1852.txt)

IPsec (in kernel) and IKE (in userland as "racoon") has been tested
at several interoperability test events, and it is known to interoperate
with many other implementations well.  Also, KAME IPsec has quite wide
coverage for IPsec crypto algorithms documented in RFC (we cover
algorithms without intellectual property issues only).

4.5 ECN consideration on IPsec tunnels

KAME IPsec implements ECN-friendly IPsec tunnel, described in
draft-ipsec-ecn-00.txt.
Normal IPsec tunnel is described in RFC2401.  On encapsulation,
IPv4 TOS field (or, IPv6 traffic class field) will be copied from inner
IP header to outer IP header.  On decapsulation outer IP header
will be simply dropped.  The decapsulation rule is not compatible
with ECN, since ECN bit on the outer IP TOS/traffic class field will be
lost.
To make IPsec tunnel ECN-friendly, we should modify encapsulation
and decapsulation procedure.  This is described in
http://www.aciri.org/floyd/papers/draft-ipsec-ecn-00.txt, chapter 3.

KAME IPsec tunnel implementation can give you three behaviors, by setting
net.inet.ipsec.ecn (or net.inet6.ipsec6.ecn) to some value:
- RFC2401: no consideration for ECN (sysctl value -1)
- ECN forbidden (sysctl value 0)
- ECN allowed (sysctl value 1)
Note that the behavior is configurable in per-node manner, not per-SA manner
(draft-ipsec-ecn-00 wants per-SA configuration, but it looks too much for me).

The behavior is summarized as follows (see source code for more detail):

		encapsulate			decapsulate
		---				---
RFC2401		copy all TOS bits		drop TOS bits on outer
		from inner to outer.		(use inner TOS bits as is)

ECN forbidden	copy TOS bits except for ECN	drop TOS bits on outer
		(masked with 0xfc) from inner	(use inner TOS bits as is)
		to outer.  set ECN bits to 0.

ECN allowed	copy TOS bits except for ECN	use inner TOS bits with some
		CE (masked with 0xfe) from	change.  if outer ECN CE bit
		inner to outer.			is 1, enable ECN CE bit on
		set ECN CE bit to 0.		the inner.

General strategy for configuration is as follows:
- if both IPsec tunnel endpoint are capable of ECN-friendly behavior,
  you'd better configure both end to "ECN allowed" (sysctl value 1).
- if the other end is very strict about TOS bit, use "RFC2401"
  (sysctl value -1).
- in other cases, use "ECN forbidden" (sysctl value 0).
The default behavior is "ECN forbidden" (sysctl value 0).

For more information, please refer to:
	http://www.aciri.org/floyd/papers/draft-ipsec-ecn-00.txt
	RFC2481 (Explicit Congestion Notification)
	KAME sys/netinet6/{ah,esp}_input.c

(Thanks goes to Kenjiro Cho <kjc@csl.sony.co.jp> for detailed analysis)

4.6 Interoperability

Here are (some of) platforms we have tested IPsec/IKE interoperability
in the past.  Note that both ends (KAME and others) may have modified their
implementation, so use the following list just for reference purposes.
	Altiga, Ashley-laurent (vpcom.com), Data Fellows (F-Secure), Ericsson
	ACC, FreeS/WAN, HITACHI, IBM AIX, IIJ, Intel, Microsoft WinNT, NIST
	(linux IPsec + plutoplus), Netscreen, OpenBSD, RedCreek, Routerware,
	SSH, Secure Computing, Soliton, Toshiba, VPNet, Yamaha RT100i

5. IPComp

# NOTE: This section does not apply to OpenBSD-current.

IPComp stands for IP payload compression protocol.  This is aimed for
payload compression, not the header compression like PPP VJ compression.
This may be useful when you are using slow serial link (say, cell phone)
with powerful CPU (well, recent notebook PCs are really powerful...).
The protocol design of IPComp is very similar to IPsec.

KAME implements the following specifications:
- RFC2393: IP Payload Compression Protocol (IPComp)
- RFC2394: IP Payload Compression Using DEFLATE

Here are some points to be noted:
- IPComp is treated as part of IPsec protocol suite, and SPI and
  CPI space is unified.  Spec says that there's no relationship
  between two so they are assumed to be separate.
- IPComp association (IPCA) is kept in SAD.
- It is possible to use well-known CPI (CPI=2 for DEFLATE for example),
  for outbound/inbound packet, but for indexing purposes one element from
  SPI/CPI space will be occupied anyway.
- pfkey is modified to support IPComp.  However, there's no official
  SA type number assignment yet.  Portability with other IPComp
  stack is questionable (anyway, who else implement IPComp on UN*X?).
- Spec says that IPComp output processing must be performed before IPsec
  output processing, to achieve better compression ratio and "stir" data
  stream before encryption.  However, with manual SPD setting, you are able to
  violate the ordering requirement (KAME code is too generic, maybe).
- Though MTU can be significantly decreased by using IPComp, no special
  consideration is made about path MTU (spec talks nothing about MTU
  consideration).  IPComp is designed for serial links, not ethernet-like
  medium, it seems.
- You can change compression ratio on outbound packet, by changing
  deflate_policy in sys/netinet6/ipcomp_core.c.  You can also change history
  buffer size by changing deflate_window in the same source code.
  (should it be sysctl accessible?  or per-SAD configurable?)
- Tunnel mode IPComp is not working right.  KAME box can generate tunnelled
  IPComp packet, however, cannot accept tunneled IPComp packet.

6. ALTQ

KAME kit includes ALTQ 2.0 code, which supports FreeBSD2, FreeBSD3 and
NetBSD.  For other BSDs, ALTQ does not work.
ALTQ in KAME supports (or tries to support) IPv6.  ALTQ-related userland
tools must be built manually, using ports/altq or pkgsrc/net/altq.

						 <end of IMPLEMENTATION>