Implementation Note

	KAME Project
	http://www.kame.net/
	$KAME: IMPLEMENTATION,v 1.216 2001/05/25 07:43:01 jinmei Exp $
	$FreeBSD$

NOTE: The document tries to describe behaviors/implementation choices
of the latest KAME/*BSD stack.  The description here may not be
applicable to KAME-integrated *BSD releases, as we have certain amount
of changes between them.  Still, some of the content can be useful for
KAME-integrated *BSD releases.

Table of Contents

	1. IPv6
	1.1 Conformance
	1.2 Neighbor Discovery
	1.3 Scope Zone Index
	1.3.1 Kernel internal
	1.3.2 Interaction with API
	1.3.3 Interaction with users (command line)
	1.4 Plug and Play
	1.4.1 Assignment of link-local, and special addresses
	1.4.2 Stateless address autoconfiguration on hosts
	1.4.3 DHCPv6
	1.5 Generic tunnel interface
	1.6 Address Selection
	1.6.1 Source Address Selection
	1.6.2 Destination Address Ordering
	1.7 Jumbo Payload
	1.8 Loop prevention in header processing
	1.9 ICMPv6
	1.10 Applications
	1.11 Kernel Internals
	1.12 IPv4 mapped address and IPv6 wildcard socket
	1.12.1 KAME/BSDI3 and KAME/FreeBSD228
	1.12.2 KAME/FreeBSD[34]x
	1.12.2.1 KAME/FreeBSD[34]x, listening side
	1.12.2.2 KAME/FreeBSD[34]x, initiating side
	1.12.3 KAME/NetBSD
	1.12.3.1 KAME/NetBSD, listening side
	1.12.3.2 KAME/NetBSD, initiating side
	1.12.4 KAME/BSDI4
	1.12.4.1 KAME/BSDI4, listening side
	1.12.4.2 KAME/BSDI4, initiating side
	1.12.5 KAME/OpenBSD
	1.12.5.1 KAME/OpenBSD, listening side
	1.12.5.2 KAME/OpenBSD, initiating side
	1.12.6 More issues
	1.12.7 Interaction with SIIT translator
	1.13 sockaddr_storage
	1.14 Invalid addresses on the wire
	1.15 Node's required addresses
	1.15.1 Host case
	1.15.2 Router case
	1.16 Advanced API
	1.17 DNS resolver
	2. Network Drivers
	2.1 FreeBSD 2.2.x-RELEASE
	2.2 BSD/OS 3.x
	2.3 NetBSD
	2.4 FreeBSD 3.x-RELEASE
	2.5 FreeBSD 4.x-RELEASE
	2.6 OpenBSD 2.x
	2.7 BSD/OS 4.x
	3. Translator
	3.1 FAITH TCP relay translator
	3.2 IPv6-to-IPv4 header translator
	4. IPsec
	4.1 Policy Management
	4.2 Key Management
	4.3 AH and ESP handling
	4.4 IPComp handling
	4.5 Conformance to RFCs and IDs
	4.6 ECN consideration on IPsec tunnels
	4.7 Interoperability
	4.8 Operations with IPsec tunnel mode
	4.8.1 RFC2401 IPsec tunnel mode approach
	4.8.2 draft-touch-ipsec-vpn approach
	5. ALTQ
	6. Mobile IPv6
	6.1 KAME node as correspondent node
	6.2 KAME node as home agent/mobile node
	6.3 Old Mobile IPv6 code
	7. Coding style
	8. Policy on technology with intellectual property right restriction

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 past and latest 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: (see RFC2893)
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
    * see RFC3542
RFC2362: Protocol Independent Multicast-Sparse Mode (PIM-SM)
    * RFC2362 defines the packet formats and the protcol of PIM-SM.
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 1.9 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: (see RFC3493)
RFC2671: Extension Mechanisms for DNS (EDNS0)
    * see USAGE for how to use it.
    * not supported on kame/freebsd4 and kame/bsdi4.
RFC2673: Binary Labels in the Domain Name System
    * KAME/bsdi4 supports A6, DNAME and binary label to some extent.
    * KAME apps/bind8 repository has resolver library with partial A6, DNAME
      and binary label support.
RFC2675: IPv6 Jumbograms
    * See 1.7 in this document for details.
RFC2710: Multicast Listener Discovery for IPv6
RFC2711: IPv6 router alert option
RFC2732: Format for Literal IPv6 Addresses in URL's
    * The spec is implemented in programs that handle URLs
      (like freebsd ftpio(3) and fetch(1), or netbsd ftp(1))
RFC2874: DNS Extensions to Support IPv6 Address Aggregation and Renumbering
    * KAME/bsdi4 supports A6, DNAME and binary label to some extent.
    * KAME apps/bind8 repository has resolver library with partial A6, DNAME
      and binary label support.
RFC2893: 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.
RFC2894: Router renumbering for IPv6
RFC3041: Privacy Extensions for Stateless Address Autoconfiguration in IPv6
RFC3056: Connection of IPv6 Domains via IPv4 Clouds
    * So-called "6to4".
    * "stf" interface implements it.  Be sure to read
      draft-itojun-ipv6-transition-abuse-01.txt
      below before configuring it, there can be security issues.
RFC3142: An IPv6-to-IPv4 transport relay translator
    * FAITH tcp relay translator (faithd) implements this.  See 3.1 for more
      details.
RFC3152: Delegation of IP6.ARPA
    * libinet6 resolvers contained in the KAME snaps support to use
      the ip6.arpa domain (with the nibble format) for IPv6 reverse
      lookups.
RFC3484: Default Address Selection for IPv6
    * the selection algorithm for both source and destination addresses
      is implemented based on the RFC, though some rules are still omitted.
RFC3493: Basic Socket Interface Extensions for IPv6
    * IPv4 mapped address (3.7) and special behavior of IPv6 wildcard bind
      socket (3.8) are,
	- supported and turned on by default on KAME/FreeBSD[34]
	  and KAME/BSDI4,
	- supported but turned off by default on KAME/NetBSD and KAME/FreeBSD5,
	- not supported on KAME/FreeBSD228, KAME/OpenBSD and KAME/BSDI3.
      see 1.12 in this document for details.
    * The AI_ALL and AI_V4MAPPED flags are not supported.
RFC3542: Advanced Sockets API for IPv6 (revised)
    * For supported library functions/kernel APIs, see sys/netinet6/ADVAPI.
    * Some of the updates in the draft are not implemented yet.  See
      TODO.2292bis for more details.
RFC4007: IPv6 Scoped Address Architecture
    * some part of the documentation (especially about the routing
      model) is not supported yet.
    * zone indices that contain scope types have not been supported yet.

draft-ietf-ipngwg-icmp-name-lookups-09: IPv6 Name Lookups Through ICMP
draft-ietf-ipv6-router-selection-07.txt:
	Default Router Preferences and More-Specific Routes
    * router-side: both router preference and specific routes are supported.
    * host-side: only router preference is supported.
draft-ietf-pim-sm-v2-new-02.txt
	A revised version of RFC2362, which includes the IPv6 specific
	packet format and protocol descriptions.
draft-ietf-dnsext-mdns-00.txt: Multicast DNS
    * kame/mdnsd has test implementation, which will not be built in
      default compilation.  The draft will experience a major change in the
      near future, so don't rely upon it.
draft-ietf-ipngwg-icmp-v3-02.txt: ICMPv6 for IPv6 specification (revised)
    * See 1.9 in this document for details.
draft-itojun-ipv6-tcp-to-anycast-01.txt:
	Disconnecting TCP connection toward IPv6 anycast address
draft-ietf-ipv6-rfc2462bis-06.txt: IPv6 Stateless Address
	Autoconfiguration (revised)
draft-itojun-ipv6-transition-abuse-01.txt:
	Possible abuse against IPv6 transition technologies (expired)
    * KAME does not implement RFC1933/2893 automatic tunnel.
    * "stf" interface implements some address filters.  Refer to stf(4)
      for details.  Since there's no way to make 6to4 interface 100% secure,
      we do not include "stf" interface into GENERIC.v6 compilation.
    * kame/openbsd completely disables IPv4 mapped address support.
    * kame/netbsd makes IPv4 mapped address support off by default.
    * See section 1.12.6 and 1.14 for more details.
draft-itojun-ipv6-flowlabel-api-01.txt: Socket API for IPv6 flow label field
    * no consideration is made against the use of routing headers and such.

1.2 Neighbor Discovery

Our implementation of 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 an Unsolicited Neighbor Advertisement transmission command as
an administration tool.

Duplicated Address Detection (DAD) will be performed when an IPv6 address
is assigned to a network interface, or the network interface is enabled
(ifconfig up).  It is documented in RFC2462 5.4.
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.  We may try to improve failure recovery
in future KAME code.

A successor version of RFC2462 (called rfc2462bis) clarifies the
behavior when DAD fails (i.e., duplicate is detected): if the
duplicate address is a link-local address formed from an interface
identifier based on the hardware address which is supposed to be
uniquely assigned (e.g., EUI-64 for an Ethernet interface), IPv6
operation on the interface should be disabled.  The KAME
implementation supports this as follows: if this type of duplicate is
detected, the kernel marks "disabled" in the ND specific data
structure for the interface.  Every IPv6 I/O operation in the kernel
checks this mark, and the kernel will drop packets received on or
being sent to the "disabled" interface.  Whether the IPv6 operation is
disabled or not can be confirmed by the ndp(8) command.  See the man
page for more details.

DAD procedure may not be effective on certain network interfaces/drivers.
If a network driver needs long initialization time (with wireless network
interfaces this situation is popular), and the driver mistakingly raises
IFF_RUNNING before the driver becomes ready, DAD code will try to transmit
DAD probes to not-really-ready network driver and the packet will not go out
from the interface.  In such cases, network drivers should be corrected.

Some of network drivers loop multicast packets back to themselves,
even if instructed not to do so (especially in promiscuous mode).  In
such cases DAD may fail, because the DAD engine sees inbound NS packet
(actually from the node itself) and considers it as a sign of
duplicate.  In this case, drivers should be corrected to honor
IFF_SIMPLEX behavior.  For example, you may need to check source MAC
address on an inbound packet, and reject it if it is from the node
itself.

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 IPv4 network code.  To implement the behavior in
RFC2461 section 6.3.6 (3), the kernel needs to know the default
outgoing interface.  To configure the default outgoing interface, use
commands like "ndp -I de0" as root.  Then the kernel will have a
"default" route to the interface with the cloning "C" bit being on.
This default route will cause to make a neighbor cache entry for every
destination that does not match an explicit route entry.

Note that we intentionally disable configuring the default interface
by default.  This is because we found it sometimes caused inconvenient
situation while it was rarely useful in practical usage.  For example,
consider a destination that has both IPv4 and IPv6 addresses but is
only reachable via IPv4.  Since our getaddrinfo(3) prefers IPv6 by
default, an (TCP) application using the library with PF_UNSPEC first
tries to connect to the IPv6 address.  If we turn on RFC 2461 6.3.6
(3), we have to wait for quite a long period before the first attempt
to make a connection fails.  If we turn it off, the first attempt will
immediately fail with EHOSTUNREACH, and then the application can try
the next, reachable address.

The notion of the default interface is also disabled when the node is
acting as a router.  The reason is that routers tend to control all
routes stored in the kernel and the default route automatically
installed would rather confuse the routers.  Note that the spec misuse
the word "host" and "node" in several places in Section 5.2 of RFC
2461.  We basically read the word "node" in this section as "host,"
and thus believe the implementation policy does not break the
specification.

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 the KAME team and/or modify
nd6_maxndopt in sys/netinet6/nd6.c.  If there are high demands we may
provide a sysctl knob for the variable.

Proxy Neighbor Advertisement support is implemented in the kernel.
For instance, you can configure it by using the following command:
	# ndp -s fe80::1234%ne0 0:1:2:3:4:5 proxy
where ne0 is the interface which attaches to the same link as the
proxy target.
There are certain limitations, though:
- It does not send unsolicited multicast NA on configuration.  This is MAY
  behavior in RFC2461.
- It does not add random delay before transmission of solicited NA.  This is
  SHOULD behavior in RFC2461.
- We cannot configure proxy NDP for off-link address.  The target address for
  proxying must be link-local address, or must be in prefixes configured to
  node which does proxy NDP.
- RFC2461 is unclear about if it is legal for a host to perform proxy ND.
  We do not prohibit hosts from doing proxy ND, but there will be very limited
  use in it.

Starting mid March 2000, we support Neighbor Unreachability Detection
(NUD) on p2p interfaces, including tunnel interfaces (gif).  NUD is
turned on by default.  Before March 2000 the KAME stack did not
perform NUD on p2p interfaces.  If the change raises any
interoperability issues, you can turn off/on NUD by per-interface
basis.  Use "ndp -i interface -nud" to turn it off.  Consult ndp(8)
for details.

RFC2461 specifies upper-layer reachability confirmation hint.  Whenever
upper-layer reachability confirmation hint comes, ND process can use it
to optimize neighbor discovery process - ND process can omit real ND exchange
and keep the neighbor cache state in REACHABLE.
We currently have two sources for hints: (1) setsockopt(IPV6_REACHCONF)
defined by the RFC3542 API, and (2) hints from tcp(6)_input.

It is questionable if they are really trustworthy.  For example, a
rogue userland program can use IPV6_REACHCONF to confuse the ND
process.  Neighbor cache is a system-wide information pool, and it is
bad to allow a single process to affect others.  Also, tcp(6)_input
can be hosed by hijack attempts.  It is wrong to allow hijack attempts
to affect the ND process.

Starting June 2000, the ND code has a protection mechanism against
incorrect upper-layer reachability confirmation.  The ND code counts
subsequent upper-layer hints.  If the number of hints reaches the
maximum, the ND code will ignore further upper-layer hints and run
real ND process to confirm reachability to the peer.  sysctl
net.inet6.icmp6.nd6_maxnudhint defines the maximum # of subsequent
upper-layer hints to be accepted.
(from April 2000 to June 2000, we rejected setsockopt(IPV6_REACHCONF) from
non-root process - after a local discussion, it looks that hints are not
that trustworthy even if they are from privileged processes)

If inbound ND packets carry invalid values, the KAME kernel will
drop these packet and increment statistics variable.  See
"netstat -sn", icmp6 section.  For detailed debugging session, you can
turn on syslog output from the kernel on errors, by turning on sysctl MIB
net.inet6.icmp6.nd6_debug.  nd6_debug can be turned on at bootstrap
time, by defining ND6_DEBUG kernel compilation option (so you can
debug behavior during bootstrap).  nd6_debug configuration should
only be used for test/debug purposes - for a production environment,
nd6_debug must be set to 0.  If you leave it to 1, malicious parties
can inject broken packet and fill up /var/log partition.

1.3 Scope Zone Index

IPv6 uses scoped addresses.  It is therefore very important to
specify the scope zone index (link index for a link-local address, or
site index for a site-local address) with an IPv6 address.  Without a
zone index, a scoped IPv6 address is ambiguous to the kernel, and
the kernel would not be able to determine the outbound zone for a
packet to the scoped address.  KAME code tries to address the issue in
several ways.

The entire architecture of scoped addresses is documented in RFC4007.
One non-trivial point of the architecture is that the link scope is
(theoretically) larger than the interface scope.  That is, two
different interfaces can belong to a same single link.  However, in a
normal operation, we can assume that there is 1-to-1 relationship
between links and interfaces.  In other words, we can usually put
links and interfaces in the same scope type.  The current KAME
implementation assumes the 1-to-1 relationship.  In particular, we use
interface names such as "ne1" as unique link identifiers.  This would
be much more human-readable and intuitive than numeric identifiers,
but please keep your mind on the theoretical difference between links
and interfaces.

Site-local addresses are very vaguely defined in the specs, and both
the specification and the KAME code need tons of improvements to
enable its actual use.  For example, it is still very unclear how we
define a site, or how we resolve host names in a site.  There is work
underway to define behavior of routers at site border, but, we have
almost no code for site boundary node support (neither forwarding nor
routing) and we bet almost noone has.  We recommend, at this moment,
you to use global addresses for experiments - there are way too many
pitfalls if you use site-local addresses.

1.3.1 Kernel internal

In the kernel, the link index for a link-local scope address is
embedded into the 2nd 16bit-word (the 3rd and 4th bytes) in the IPv6
address.
For example, you may see something like:
	fe80:1::200:f8ff:fe01:6317
in the routing table and the interface address structure (struct
in6_ifaddr).  The address above is a link-local unicast address which
belongs to a network link whose link identifier is 1 (note that it
eqauls to the interface index by the assumption of our
implementation).  The embedded index enables us to identify IPv6
link-local addresses over multiple links effectively and with only a
little code change.

The use of the internal format must be limited inside the kernel.  In
particular, addresses sent by an application should not contain the
embedded index (except via some very special APIs such as routing
sockets).  Instead, the index should be specified in the sin6_scope_id
field of a sockaddr_in6 structure.  Obviously, packets sent to or
received from must not contain the embedded index either, since the
index is meaningful only within the sending/receiving node.

In order to deal with the differences, several kernel routines are
provided.  These are available by including <netinet6/scope_var.h>.
Typically, the following functions will be most generally used:

- int sa6_embedscope(struct sockaddr_in6 *sa6, int defaultok);
  Embed sa6->sin6_scope_id into sa6->sin6_addr.  If sin6_scope_id is
  0, defaultok is non-0, and the default zone ID (see RFC4007) is
  configured, the default ID will be used instead of the value of the
  sin6_scope_id field.  On success, sa6->sin6_scope_id will be reset
  to 0.

  This function returns 0 on success, or a non-0 error code otherwise.
 
- int sa6_recoverscope(struct sockaddr_in6 *sa6);
  Extract embedded zone ID in sa6->sin6_addr and set
  sa6->sin6_scope_id to that ID.  The embedded ID will be cleared with
  0.

  This function returns 0 on success, or a non-0 error code otherwise.

- int in6_clearscope(struct in6_addr *in6);
  Reset the embedded zone ID in 'in6' to 0.  This function never fails, and
  returns 0 if the original address is intact or non 0 if the address is
  modified.  The return value doesn't matter in most cases; currently, the
  only point where we care about the return value is ip6_input() for checking
  whether the source or destination addresses of the incoming packet is in
  the embedded form.

- int in6_setscope(struct in6_addr *in6, struct ifnet *ifp,
                   u_int32_t *zoneidp);
  Embed zone ID determined by the address scope type for 'in6' and the
  interface 'ifp' into 'in6'.  If zoneidp is non NULL, *zoneidp will
  also have the zone ID.

  This function returns 0 on success, or a non-0 error code otherwise.

The typical usage of these functions is as follows:

sa6_embedscope() will be used at the socket or transport layer to
convert a sockaddr_in6 structure passed by an application into the
kernel-internal form.  In this usage, the second argument is often the
'ip6_use_defzone' global variable.

sa6_recoverscope() will also be used at the socket or transport layer
to convert an in6_addr structure with the embedded zone ID into a
sockaddr_in6 structure with the corresponding ID in the sin6_scope_id
field (and without the embedded ID in sin6_addr).

in6_clearscope() will be used just before sending a packet to the wire
to remove the embedded ID.  In general, this must be done at the last
stage of an output path, since otherwise the address would lose the ID
and could be ambiguous with regard to scope.

in6_setscope() will be used when the kernel receives a packet from the
wire to construct the kernel internal form for each address field in
the packet (typical examples are the source and destination addresses
of the packet).  In the typical usage, the third argument 'zoneidp'
will be NULL.  A non-NULL value will be used when the validity of the
zone ID must be checked, e.g., when forwarding a packet to another
link (see ip6_forward() for this usage).

An application, when sending a packet, is basically assumed to specify
the appropriate scope zone of the destination address by the
sin6_scope_id field (this might be done transparently from the
application with getaddrinfo() and the extended textual format - see
below), or at least the default scope zone(s) must be configured as a
last resort.  In some cases, however, an application could specify an
ambiguous address with regard to scope, expecting it is disambiguated
in the kernel by some other means.  A typical usage is to specify the
outgoing interface through another API, which can disambiguate the
unspecified scope zone.  Such a usage is not recommended, but the
kernel implements some trick to deal with even this case.

A rough sketch of the trick can be summarized as the following
sequence.

   sa6_embedscope(dst, ip6_use_defzone);
   in6_selectsrc(dst, ..., &ifp, ...);
   in6_setscope(&dst->sin6_addr, ifp, NULL);

sa6_embedscope() first tries to convert sin6_scope_id (or the default
zone ID) into the kernel-internal form.  This can fail with an
ambiguous destination, but it still tries to get the outgoing
interface (ifp) in the attempt of determining the source address of
the outgoing packet using in6_selectsrc().  If the interface is
detected, and the scope zone was originally ambiguous, in6_setscope()
can finally determine the appropriate ID with the address itself and
the interface, and construct the kernel-internal form.  See, for
example, comments in udp6_output() for more concrete example.

In any case, kernel routines except ones in netinet6/scope6.c MUST NOT
directly refer to the embedded form.  They MUST use the above
interface functions.  In particular, kernel routines MUST NOT have the
following code fragment:

	/* This is a bad practice.  Don't do this */
	if (IN6_IS_ADDR_LINKLOCAL(&sin6->sin6_addr))
		sin6->sin6_addr.s6_addr16[1] = htons(ifp->if_index);

This is bad for several reasons.  First, address ambiguity is not
specific to link-local addresses (any non-global multicast addresses
are inherently ambiguous, and this is particularly true for
interface-local addresses).  Secondly, this is vulnerable to future
changes of the embedded form (the embedded position may change, or the
zone ID may not actually be the interface index).  Only scope6.c
routines should know the details.

The above code fragment should thus actually be as follows:

	/* This is correct. */
	in6_setscope(&sin6->sin6_addr, ifp, NULL);
	(and catch errors if possible and necessary)

1.3.2 Interaction with API

There are several candidates of API to deal with scoped addresses
without ambiguity.

The IPV6_PKTINFO ancillary data type or socket option defined in the
advanced API (RFC2292 or RFC3542) can specify
the outgoing interface of a packet.  Similarly, the IPV6_PKTINFO or
IPV6_RECVPKTINFO socket options tell kernel to pass the incoming
interface to user applications.

These options are enough to disambiguate scoped addresses of an
incoming packet, because we can uniquely identify the corresponding
zone of the scoped address(es) by the incoming interface.  However,
they are too strong for outgoing packets.  For example, consider a
multi-sited node and suppose that more than one interface of the node
belongs to a same site.  When we want to send a packet to the site,
we can only specify one of the interfaces for the outgoing packet with
these options; we cannot just say "send the packet to (one of the
interfaces of) the site."

Another kind of candidates is to use the sin6_scope_id member in the
sockaddr_in6 structure, defined in RFC2553.  The KAME kernel
interprets the sin6_scope_id field properly in order to disambiguate scoped
addresses.  For example, if an application passes a sockaddr_in6
structure that has a non-zero sin6_scope_id value to the sendto(2)
system call, the kernel should send the packet to the appropriate zone
according to the sin6_scope_id field.  Similarly, when the source or
the destination address of an incoming packet is a scoped one, the
kernel should detect the correct zone identifier based on the address
and the receiving interface, fill the identifier in the sin6_scope_id
field of a sockaddr_in6 structure, and then pass the packet to an
application via the recvfrom(2) system call, etc.

However, the semantics of the sin6_scope_id is still vague and on the
way to standardization.  Additionally, not so many operating systems
support the behavior above at this moment.

In summary,
- If your target system is limited to KAME based ones (i.e. BSD
  variants and KAME snaps), use the sin6_scope_id field assuming the
  kernel behavior described above.
- Otherwise, (i.e. if your program should be portable on other systems
  than BSDs)
  + Use the advanced API to disambiguate scoped addresses of incoming
    packets.
  + To disambiguate scoped addresses of outgoing packets,
    * if it is okay to just specify the outgoing interface, use the
      advanced API.  This would be the case, for example, when you
      should only consider link-local addresses and your system
      assumes 1-to-1 relationship between links and interfaces.
    * otherwise, sorry but you lose.  Please rush the IETF IPv6
      community into standardizing the semantics of the sin6_scope_id
      field.

Routing daemons and configuration programs, like route6d and ifconfig,
will need to manipulate the "embedded" zone index.  These programs use
routing sockets and ioctls (like SIOCGIFADDR_IN6) and the kernel API
will return IPv6 addresses with the 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.

getaddrinfo(3) and getnameinfo(3) support an extended numeric IPv6
syntax, as documented in RFC4007.  You can specify the outgoing link,
by using the name of the outgoing interface as the link, like
"fe80::1%ne0" (again, note that we assume there is 1-to-1 relationship
between links and interfaces.)  This way you will be able to specify a
link-local scoped address without much trouble.

Other APIs like inet_pton(3) and inet_ntop(3) are inherently
unfriendly with scoped addresses, since they are unable to annotate
addresses with zone identifier.

1.3.3 Interaction with users (command line)

Most of user applications now support the extended numeric IPv6
syntax.  In this case, you can specify outgoing link, by using the name
of the outgoing interface like "fe80::1%ne0" (sorry for the duplicated
notice, but please recall again that we assume 1-to-1 relationship
between links and interfaces).  This is even the case for some
management tools such as route(8) or ndp(8).  For example, to install
the IPv6 default route by hand, you can type like
	# route add -inet6 default fe80::9876:5432:1234:abcd%ne0
(Although we suggest you to run dynamic routing instead of static
routes, in order to avoid configuration mistakes.)

Some applications have command line options for specifying an
appropriate zone of a scoped address (like "ping6 -I ne0 ff02::1" to
specify the outgoing interface).  However, you can't always expect such
options.  Additionally, specifying the outgoing "interface" is in
theory an overspecification as a way to specify the outgoing "link"
(see above).  Thus, we recommend you to use the extended format
described above.  This should apply to the case where the outgoing
interface is specified.

In any case, when you specify a scoped address to the command line,
NEVER write the embedded form (such as ff02:1::1 or fe80:2::fedc),
which should only be used inside the kernel (see Section 1.3.1), and 
is not supposed to work.

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 address (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::%ed0/64                 link#1                    UC           ed0
fe80::%ep0/64                 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 a router or a host (router if it is 1, host if it is 0).

It is NOT recommended to change net.inet6.ip6.forwarding while the node
is in operation.  IPv6 specification defines behavior for "host" and "router"
quite differently, and switching from one to another can cause serious
troubles.  It is recommended to configure the variable at bootstrap time only.

The first step in stateless address configuration is Duplicated Address
Detection (DAD).  See 1.2 for more detail on DAD.

When a host hears Router Advertisement from the router, a host may
autoconfigure itself by stateless address autoconfiguration.  This
behavior can be controlled by the net.inet6.ip6.accept_rtadv sysctl
variable and a per-interface flag managed in the kernel.  The latter,
which we call "if_accept_rtadv" here, can be changed by the ndp(8)
command (see the manpage for more details).  When the sysctl variable
is set to 1, and the flag is set, the host autoconfigures itself.  By
autoconfiguration, network address prefixes for the receiving
interface (usually global address prefix) are added.  The 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 an RS packet at any time, use the
"rtsol" command.  The "rtsold" daemon is also available. "rtsold"
generates Router Solicitation whenever necessary, and it works greatly
for nomadic usage (notebooks/laptops).  If one wishes to ignore Router
Advertisements, use sysctl to set net.inet6.ip6.accept_rtadv to 0.
Additionally, ndp(8) command can be used to control the behavior
per-interface basis.

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

Note that the IPv6 specification assumes the following items and that
nonconforming cases are left unspecified:
- Only hosts will listen to router advertisements
- Hosts have a single network interface (except loopback)
This is therefore unwise to enable net.inet6.ip6.accept_rtadv on routers,
or multi-interface hosts.  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 hosts have a single
					interface only, autoconfigred hosts
					with multiple interfaces are
					out-of-scope)
	1		1		invalid, or experimental
					(out-of-scope of spec)

The if_accept_rtadv flag is referred only when accept_rtadv is 1 (the
latter two cases).  The flag does not have any effects when the sysctl
variable is 0.

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 premature (for example, this does NOT implement
address lease/release), and it is not in default compilation tree on
some platforms. 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 may need to remove IPv6 link-local address
automatically assigned to the gif interface.

1.6 Address Selection

1.6.1 Source Address Selection

The KAME kernel chooses the source address for an outgoing packet
sent from a user application as follows:

1. if the source address is explicitly specified via an IPV6_PKTINFO
   ancillary data item or the socket option of that name, just use it.
   Note that this item/option overrides the bound address of the
   corresponding (datagram) socket.

2. if the corresponding socket is bound, use the bound address.

3. otherwise, the kernel first tries to find the outgoing interface of
   the packet.  If it fails, the source address selection also fails.
   If the kernel can find an interface, choose the most appropriate
   address based on the algorithm described in RFC3484.

   The policy table used in this algorithm is stored in the kernel.
   To install or view the policy, use the ip6addrctl(8) command.  The
   kernel does not have pre-installed policy.  It is expected that the
   default policy described in the draft should be installed at the
   bootstrap time using this command.

   This draft allows an implementation to add implementation-specific
   rules with higher precedence than the rule "Use longest matching
   prefix."  KAME's implementation has the following additional rules
   (that apply in the appeared order):

   - prefer addresses on alive interfaces, that is, interfaces with
     the UP flag being on.  This rule is particularly useful for
     routers, since some routing daemons stop advertising prefixes
    (addresses) on interfaces that have become down.

   - prefer addresses on "preferred" interfaces.  "Preferred"
     interfaces can be specified by the ndp(8) command.  By default,
     no interface is preferred, that is, this rule does not apply.
     Again, this rule is particularly useful for routers, since there
     is a convention, among router administrators, of assigning
     "stable" addresses on a particular interface (typically a
     loopback interface).

   In any case, addresses that break the scope zone of the
   destination, or addresses whose zone do not contain the outgoing
   interface are never chosen.

When the procedure above fails, the kernel usually returns
EADDRNOTAVAIL to the application.

In some cases, the specification explicitly requires the
implementation to choose a particular source address.  The source
address for a Neighbor Advertisement (NA) message is an example.
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 rule.

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).

As documented in the source address selection document, temporary
addresses for privacy extension are less preferred to public addresses
by default.  However, for administrators who are particularly aware of
the privacy, there is a system-wide sysctl(3) variable
"net.inet6.ip6.prefer_tempaddr".  When the variable is set to
non-zero, the kernel will rather prefer temporary addresses.  The
default value of this variable is 0.

1.6.2 Destination Address Ordering

KAME's getaddrinfo(3) supports the destination address ordering
algorithm described in RFC3484.  Getaddrinfo(3) needs to know the
source address for each destination address and policy entries
(described in the previous section) for the source and destination
addresses.  To get the source address, the library function opens a
UDP socket and tries to connect(2) for the destination.  To get the
policy entry, the function issues sysctl(3).

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 the task.  To name a few:
- mbuf pkthdr.len field is typed as "int" in 4.4BSD, so it cannot 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 numeric 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 careful updates 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).

In addition to this, we restrict the number of extension headers
(including the IPv6 header) in each incoming packet, in order to
prevent a DoS attack that tries to send packets with a massive number
of extension headers.  The upper limit can be configured by the sysctl
value net.inet6.ip6.hdrnestlimit.  In particular, if the value is 0,
the node will allow an arbitrary number of headers. As of writing this
document, the default value is 50.

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.

RFC2463 requires rate limitation for ICMPv6 error packets generated by a
node, to avoid possible DoS attacks.  KAME kernel implements two rate-
limitation mechanisms, tunable via sysctl:
- Minimum time interval between ICMPv6 error packets
	KAME kernel will generate no more than one ICMPv6 error packet,
	during configured time interval.  net.inet6.icmp6.errratelimit
	controls the interval (default: disabled).
- Maximum ICMPv6 error packet-per-second
	KAME kernel will generate no more than the configured number of
	packets in one second.  net.inet6.icmp6.errppslimit controls the
	maximum packet-per-second value (default: 200pps)
Basically, we need to pick values that are suitable against the bandwidth
of link layer devices directly attached to the node.  In some cases the
default values may not fit well.  We are still unsure if the default value
is sane or not.  Comments are welcome.

1.10 Applications

For userland programming, we support IPv6 socket API as specified in
RFC2553/3493, RFC3542 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.)

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