Network Working Group F. Templin
Internet-Draft Nokia
Expires: July 20, 2004 T. Gleeson
Cisco Systems K.K.
M. Talwar
D. Thaler
Microsoft Corporation
January 20, 2004
Intra-Site Automatic Tunnel Addressing Protocol (ISATAP)
draft-ietf-ngtrans-isatap-17.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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This Internet-Draft will expire on July 20, 2004.
Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
The Intra-Site Automatic Tunnel Addressing Protocol (ISATAP) connects
IPv6 neighbors/routers over IPv4 networks. ISATAP views the IPv4
network as a Non-Broadcast, Multiple Access (NBMA) link layer for
IPv6 and views other nodes on the network as potential IPv6
neighbors/routers. ISATAP interfaces support automatic tunneling
whether globally assigned or private IPv4 addresses are used. ISATAP
supports an abstraction for tunnel interface management similar to
the ATM Permanent/Switched Virtual Circuit (PVC/SVC) model.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Model of Operation . . . . . . . . . . . . . . . . . . . . . . 4
5. Node Requirements . . . . . . . . . . . . . . . . . . . . . . 4
6. Addressing Requirements . . . . . . . . . . . . . . . . . . . 4
7. Configuration and Management Requirements . . . . . . . . . . 6
8. Automatic Tunneling . . . . . . . . . . . . . . . . . . . . . 12
9. Neighbor Discovery . . . . . . . . . . . . . . . . . . . . . . 17
10. Other Requirements for Control Plane Signalling . . . . . . . 19
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
12. Security considerations . . . . . . . . . . . . . . . . . . . 20
13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 20
Normative References . . . . . . . . . . . . . . . . . . . . . 21
Informative References . . . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 24
A. Major Changes . . . . . . . . . . . . . . . . . . . . . . . . 25
B. Interface Identifier Construction . . . . . . . . . . . . . . 26
Intellectual Property and Copyright Statements . . . . . . . . 28
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1. Introduction
This document specifies a simple mechanism called the Intra-Site
Automatic Tunnel Addressing Protocol (ISATAP) that connects IPv6
[RFC2460] neighbors/routers over IPv4 [RFC0791] networks. ISATAP
allows dual-stack (IPv6/IPv4) nodes to automatically tunnel packets
to the IPv6 next-hop address through IPv4, i.e., ISATAP sees the IPv4
network as a link layer for IPv6 and views other nodes on the network
as potential IPv6 neighbors/routers. ISATAP supports an abstraction
for tunnel interface management similar to the Non-Broadcast,
Multiple Access (NBMA) [RFC2491] and ATM Permanent/Switched Virtual
Circuit (PVC/SVC) [RFC2492] paradigms.
The main objectives of this document are to: 1) describe the
operational model for ISATAP, 2) specify addressing requirements, 3)
discuss configuration and management requirements, 4) specify
automatic tunneling using ISATAP, 5) specify operational aspects of
IPv6 Neighbor Discovery, and 6) discuss IANA; Security
considerations.
2. Requirements
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
document, are to be interpreted as described in [RFC2119].
3. Terminology
The terminology of [RFC1122][RFC2460][RFC2461][RFC3582] applies to
this document. The following additional terms are defined:
ISATAP node:
a dual-stack (IPv6/IPv4) node that implements this specification.
ISATAP driver:
an ISATAP node's network driver module that provides an engine for
encapsulation, decapsulation and forwarding of packets between
tunnel interfaces and the IPv4 stack; it also implements an API
for tunnel interface management.
ISATAP server daemon:
an ISATAP node's process that sends/receives tunnel configuration
control plane messages, and configures/manages tunnel interfaces
via the ISATAP driver API; often will be the same server daemon
used for IPv6 neighbor/router discovery.
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ISATAP interface:
an ISATAP node's point-to-multipoint IPv6 interface used for
IPv6-in-IPv4 tunneling of control plane traffic; may also be used
to carry data plane traffic in some deployments scenarios, e.g.,
certain enterprise networks.
ISATAP interface identifier:
an IPv6 interface identifier with an embedded IPv4 address
constructed as specified in Section 6.1.
ISATAP address:
an IPv6 unicast address assigned to an ISATAP interface with an
on-link prefix and an ISATAP interface identifier.
4. Model of Operation
ISATAP interfaces provide a point-to-multipoint abstraction for
IPv6-in-IPv4 tunneling. They are commonly used as a nexus for
automatic configuration of point-to-point tunnels via tunnel
configuration control plane messages (e.g., IPv6 Neighbor Discovery
and other ICMPv6 messages). For each tunneled packet received, the
node's ISATAP driver examines a local forwarding table to determine
the correct interface to receive the packet after decapsulation. It
forwards tunnel configuration control plane messages via an ISATAP
interface to the node's ISATAP server daemon, and forwards data
messages to applications via configured tunnel interfaces based on a
specific match for the encapsulating IPv4 source address.
The ISATAP server daemon sends and receives control plane messages,
and configures/manages tunnels via the ISATAP driver API. Each such
configured tunnel provides a nexus for multiplexing one or more
applications between the remote and local tunnel endpoints using IPv6
addresses as application identifiers. Each such application
identifier provides a nexus for multiplexing one or more sessions. In
summary, each configured tunnel provides a single point-to-point
connection between peers that can be used to carry multiple
applications and multiple instances of each application.
5. Node Requirements
Nodes that use this specification implement the common functionality
required by [NODEREQ] as well as the additional features specified in
this document.
6. Addressing Requirements
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6.1 ISATAP Interface Identifiers
ISATAP interface identifiers are constructed in Modified EUI-64
format as specified in ([ADDR-ARCH], section 2.5.1). They are formed
by appending a 32-bit IPv4 address to the 32-bit leading token
'0000:5EFE', then setting the universal/local ("u") bit as follows:
When the IPv4 address is known to be globally unique, the "u" bit is
set to 1 and the leading token becomes '0200:5EFE'. When the IPv4
address is from a private allocation or not otherwise known to be
globally unique, the "u" bit is set to 0 and the leading token
remains as '0000:5EFE'. See: Appendix B for additional non-normative
details.
6.2 ISATAP Addresses
Any IPv6 unicast address ([ADDR-ARCH], section 2.5) that has an
ISATAP interface identifier and an on-link prefix on an ISATAP
interface is considered an ISATAP address. ISATAP addresses are
constructed as follows:
| 64 bits | 32 bits | 32 bits |
+------------------------------+---------------+----------------+
| prefix | 000[0/2]:5EFE | IPv4 Address |
+------------------------------+---------------+----------------+
6.3 Multicast/Anycast
ISATAP interfaces recognize a node's required IPv6 multicast/anycast
addresses ([ADDR-ARCH], section 2.8).
Section 8.2 discusses encapsulation for multicast/anycast packets.
6.4 Source/Target Link Layer Address Options
Source/Target Link Layer Address Options ([RFC2461], section 4.6.1)
for ISATAP have the following format:
+-------+-------+-------+-------+-------+-------+-------+--------+
| Type |Length | 0 | 0 | IPv4 Address |
+-------+-------+-------+-------+-------+-------+-------+--------+
Type:
1 for Source Link-layer address.
2 for Target Link-layer address.
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Length:
1 (in units of 8 octets).
IPv4 Address:
A 32 bit IPv4 address, in network byte order ([RFC2223bis],
section 3.4).
7. Configuration and Management Requirements
7.1 Network Management
ISATAP nodes MAY support network management; ISATAP nodes that
support network management SHOULD support the following MIBs:
[FTMIB][IPMIB][TUNNELMIB]. The configuration objects cited throughout
the remainder of this document were selected to match the names of
MIB objects. ISATAP nodes that do not support network management MAY
choose their own local representation of these objects.
7.2 ISATAP Driver API
The ISATAP driver provides an API for tunnel interface configuration
and management that may be accessed by processes running on the
ISATAP node, e.g., startup scripts, manual command line entry, kernel
processes, ISATAP server daemons, etc. Access MUST be restricted to
privileged users and applications. The API provides the following
primitives; operational details are given in the subsections that
follow:
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'TUNNEL_CREATE':
creates a tunnel interface. Takes as parameters a tunnel
encapsulation method, parameters for setting read-write objects
for the tunnel, and a list of receive addresses to initialize a
forwarding entry in the system's ifRcvAddressTable. Returns an
index for the new tunnel interface, or a failure code.
'TUNNEL_DELETE':
deletes an existing tunnel interface. Takes as parameter an
index of the tunnel interface to be deleted. Returns success
or a failure code.
'TUNNEL_MODIFY':
adds or deletes attributes for an existing tunnel interface, and
its corresponding forwarding entry in the ifRcvAddressTable. Takes
the same list of parameters as for TUNNEL_CREATE, plus a flag that
denotes the operation (i.e., "add" or "delete"). Returns success
or a failure code.
'TUNNEL_DUP':
duplicates an existing tunnel interface. Takes as parameter the
index of the tunnel interface to be duplicated. Returns an index
for the newly-created tunnel interface, or a failure code.
'TUNNEL_GET':
copies configuration attributes from system table entries
associated with the specified tunnel interface into a user's
buffer. Takes as parameter an index of a tunnel interface.
Returns the number of system table entry data bytes written
into the application's buffer or a failure code.
7.2.1 TUNNEL_CREATE
ISATAP drivers implement a 'TUNNEL_CREATE' primitive that provides a
means for configuring the 'tunnelIfEncapsMethod', all read-write
objects associated with the 'tunnelIfEntry', and a list of receive
addresses for the tunnel which consist of an IPv4 address and an
index for the interface to which the address is assigned (i.e,. an
IPv4 address-to-interface mapping).
When a process on the ISATAP node issues 'TUNNEL_CREATE' primitive,
it includes a parameter for configuring the 'tunnelIfEncapsMethod'
object, and MAY include parameters for configuring other read-write
objects in the 'tunnelIfEntry'. It MAY also include one or more
receive address parameters. (Any required configuration parameters
not included in the 'TUNNEL_CREATE' primitive are to be issued in a
subsequent 'TUNNEL_MODIFY' primitive.)
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When the ISATAP driver processes a 'TUNNEL_CREATE' primitive, it
creates an entry in the 'tunnelInetConfigTable', which results in the
simultaneous creation of a 'tunnelIfEntry' in the 'tunnelIfTable' and
an 'ifEntry' in the appropriate 'ifTable'. Next, it sets the
'tunnelIfEncapsMetod' object to the 'IANAtunnelType' specified by the
primitive, and sets any other "read-write" objects in the
'tunnelIfEntry' based on parameters included.
After configuring the 'tunnelIfEntry', the driver uses each receive
address parameter included to locate a preferred 'ipAddressEntry' in
the system's 'ipAddressTable'. It then creates an entry for the new
tunnel interface in the 'ifRcvAddressTable' that includes the list of
selected 'ipAddressEntry's, 'tunnelLocalInetAddress',
'tunnelRemoteInetAddress', 'tunnelIfEncapsMethod', and the 'ifIndex'
for the tunnel interface.
After performing the above actions, the ISATAP driver returns either
an interface index for the newly-created tunnel interface or a
failure code.
7.2.2 TUNNEL_DELETE
ISATAP drivers implement a 'TUNNEL_DELETE' primitive that provides a
means for deleting all table entries associated with a tunnel
interface.
When an ISATAP node's process issues a 'TUNNEL_DELETE' primitive, it
includes an index for the tunnel interface returned via a previous
'TUNNEL_CREATE' or 'TUNNEL_DUP' primitive.
When the ISATAP driver processes a 'TUNNEL_DELETE' primitive, it
locates the 'tunnelInetConfigEntry' for the tunnel interface based on
the interface index parameter and deletes the entry from the
'tunnelInetConfigTable'. This has the result of simultaneously
deleting the 'tunnelIfEntry' and 'ifEntry' from their respective
tables. The driver also removes the corresponding forwarding table
entry for the tunnel interface from the 'ifRcvAddressTable'.
After performing the above actions, the ISATAP driver returns either
success or a failure code.
7.2.3 TUNNEL_MODIFY
ISATAP drivers implement a 'TUNNEL_MODIFY' primitive that provides a
means for modifying all read-write objects associated with the
'tunnelIfEntry' and for adding or deleting entries from the list of
receive addresses for the tunnel. The primitive also provides a flag
for specifying whether the desired operation is "add" or "delete".
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(For vector objects, the "add"/"delete" operations have the meaning
intended by their names; for scalar objects, the ISATAP driver
interprets an "add" operation as: "change to new value" and a
"delete" operation as: "reset to default".)
When an ISATAP node's process issues a 'TUNNEL_MODIFY' primitive, it
includes an index for the tunnel interface returned via a previous
'TUNNEL_CREATE' or 'TUNNEL_DUP' primitive, and also includes a flag
that specifies "add" or delete". It MAY include one or more parameter
for configuring read-write objects in the 'tunnelIfEntry' and MAY
also include one or more receive address (formatted as for
'TUNNEL_CREATE').
When the ISATAP driver processes a 'TUNNEL_MODIFY' primitive, it
locates the correct 'tunnelIfEntry' for the interface index parameter
and modifies objects for the entry based on any included parameters.
If one or more receive address parameters are included, the driver
also adds or deletes receive addresses from the forwarding table
entry in the 'ifRcvAddressTable' corresponding to the
'tunnelIfEntry'. If no parameters are included, the result is a
NO-OP.
After performing the above actions, the ISATAP driver returns either
success or a failure code.
7.2.4 TUNNEL_DUP
ISATAP drivers implement a 'TUNNEL_DUP' primitive that creates a new
tunnel interface by duplicating a set of system table entries from an
existing tunnel interface.
When a user application or a system process issues a 'TUNNEL_MODIFY'
primitive, it includes an index for the tunnel interface to be
duplicated from a previous 'TUNNEL_CREATE' or 'TUNNEL_DUP' primitive.
When the ISATAP driver processes a 'TUNNEL_DUP' primitive, it creates
a new entry in the 'tunnelInetConfigTable' exactly as for
'TUNNEL_CREATE' (see: Section 7.2.1). Next, it locates the
'tunnelIfEntry' and 'ifEntry' for the tunnel interface to be
duplicated and copies all attributes from those objects into the
newly-created 'tunnelIfEntry' and 'ifEntry'. The driver also creates
a duplicate forwarding table entry in the 'ifRcvAddressTable' using
the existing entry identified by the interface index parameter as a
prototype, then sets the newly-created forwarding entry's index to
the 'ifIndex' for the newly-created tunnel interface.
After performing the above actions, the ISATAP driver returns either
an interface index for the newly-created tunnel interface or a
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failure code.
7.2.5 TUNNEL_GET
To aid network administrators, ISATAP drivers SHOULD implement a
'TUNNEL_GET' primitive that returns the current configuration of all
tables in the system associated with the specified tunnel interface.
When a user application issues a 'TUNNEL_GET' primitive, it includes
an index for a tunnel interface from a previous 'TUNNEL_CREATE' or
'TUNNEL_DUP' primitive, a pointer to a character buffer to receive
the configuration information, and an integer indicating the
available space in the buffer.
When the ISATAP driver processes a 'TUNNEL_GET' primitive, it
prepares a character string that includes the concatenation of the
'tunnelIfEntry' and the 'ifRcvAddressTable' entry for the tunnel
interface identified by the index parameter. (The 'ifEntry' is not
concatenated, since its contents may be examined via primitives from
other APIs.) Next, the driver copies the character string to the
server daemon's character buffer up to the specified available buffer
space.
After performing the above actions, the ISATAP driver returns either
the number of bytes copied or a failure code (to include a code that
indicates "operation not supported").
7.3 ISATAP Interface Configuration
ISATAP interfaces are normally configured by an ISATAP node's system
startup scripts or via manual configuration, but may also be created
by a dynamic process. When a node creates (or later modifies) an
ISATAP interface, it assigns to the interface one or more receive
address that consists of an IPv4 address and an index for the
interface to which the address is assigned (i.e,. an IPv4
address-to-interface mapping). Each receive address assigned MUST
represent a mapping for the same site (or, MUST represent a mapping
that is routable on the global Internet), i.e., the receive addresses
assigned to a single tunnel interface MUST NOT span multiple sites.
ISATAP nodes issue 'TUNNEL_CREATE' and 'TUNNEL_MODIFY' primitives for
ISATAP interfaces the same as for any tunnel interface;
'TUNNEL_CREATE' primitives include a parameter to set
'tunnelIfEncapsMethod' to an 'IANATunnelType' code for "isatap". A
'TUNNEL_CREATE' or 'TUNNEL_MODIFY' primitive that includes parameters
to set 'tunnelIfLocalInetAddress' to an IPv4 address that will be
used as one of the interface's receive addresses, and
'tunnelIfRemoteInetAddress' to 0.0.0.0 to denote wildcard match for
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remote tunnel endpoints SHOULD be issued before the IPv6 interface
associated with the tunnel interface is enabled (see below).
When an ISATAP interface is created, the ISATAP driver also creates
an 'ipv6InterfaceEntry' as the companion 'ifEntry' to the
'tunnelIfEntry'. After setting the required objects in the
'tunnelIfEntry' (see above), the ISATAP node configures objects in
the 'ipv6InterfaceEntry' for an ISATAP interface the same as for any
IPv6 interface. For ISATAP interfaces (and other tunnel interfaces
that use IPv4 as a link layer for IPv6 ), the node sets the
'ipv6InterfaceType' object to "tunnel". Next, the node sets the
'ipv6InterfacePhysicalAddress' object to an IPv4 address that will be
used as one of the tunnel interface's receive addresses; this object
MUST be formatted as a 4-octet entity containing an IPv4 address in
network byte order ([RFC2223bis], section 3.4). The node next sets
the 'ipv6ScopeZoneIndexLinkLocal' object to a zone index identifier
that denotes the site for which the tunnel interface's receive
addresses are valid. Finally, the node configures all other required
read-write parameters in the 'ipv6InterfaceEntry' as for any IPv6
interface, and sets 'ipv6InterfaceAdminStatus' to "up".
After configuring the ISATAP interface, the node sets the interface's
'ipv6InterfaceForwarding' object (and, if necessary, the node's
'ip6Forwarding' object) to "forwarding". The node also creates an
'ipv6RouterAdvertEntry' in the 'ipv6RouterAdvertTable' and sets the
'ipv6RouterAdvertIfIndex' object to the same value as
'ipv6InterfaceIfIndex'. Objects in the 'ipv6RouterAdvertEntry' for an
ISATAP interface are configured as for any IPv6 router, however
'ipv6RouterAdvertLinkMTU' SHOULD NOT be set to a value other than 0
unless the ISATAP driver will monitor the IPv4 reassembly cache and
report fragmentation of tunneled packets to the source by sending
IPv6 Router Advertisements with MTU options (see: Section 8.3).
Configuration of objects relating to IPv6 forwarding is normally
managed by the ISATAP server daemon.
7.4 Dynamic Creation of Configured Tunnels
Configured tunnels are normally created through ISATAP driver API
calls issued by an ISATAP server daemon in dynamic response to a
tunnel creation request. Configured tunnel interfaces are created as
for ISATAP interfaces (see: Section 7.3), except that the
'tunnelIfRemoteInetAddress' for the new entry is normally set to a
specific IPv4 address for a remote node at the far end of the tunnel,
i.e., configured tunnels are normally configured as point-to-point.
As for ISATAP interfaces, configured tunnels MUST NOT select a list
of receive address mappings that span multiple sites.
Processes that create configured tunnels may find the 'TUNNEL_DUP'
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primitive useful (and, in some cases essential) for reducing
administrative complexity. An ISATAP interface may be used as the
prototype for the 'TUNNEL_DUP' primitive; the configured tunnel
interface inherits the attributes of the ISATAP interface, including
the forwarding table entry in the system's 'ifRecvAddressTable'.
After creating a configured tunnel via the 'TUNNEL_DUP' primitive,
the process uses the 'TUNNEL_MODIFY' primitive to modify specific
attributes.
7.5 Reconfigurations Due to IPv4 Address Changes
When an 'ipAddressEntry' becomes deprecated (e.g., when an IPv4
address is removed from an IPv4 interface) the 'ipAddressEntry' MUST
be removed from all forwarding entries in the 'ifRcvAddressTable'
that referenced it. Also, all 'tunnelIfEntry's that used
'ipAddressAddr' as 'tunnelIfLocalInetAddress' and
'ipv6InterfaceEntry's that used 'ipAddressAddr' as
'ipv6InterfacePhysicalAddress' MUST select a different IPv4 address
for those objects from their remaining list of receive addresses.
Methods for triggering the mandatory changes are implementor's
choice.
When a new IPv4 address is added to an IPv4 interface, the node MAY
add the new 'ipAddressEntry' to the list of receive addresses for
forwarding entries in 'ifRcvAddressTable', and MAY set
'tunnelIfLocalInetAddress' and/or 'ipv6InterfacePhysicalAddress' for
interfaces referenced by the updated forwarding entries to the new
address.
8. Automatic Tunneling
ISATAP nodes use the basic tunneling mechanisms specified in [MECH].
The following additional specifications are used for ISATAP:
8.1 Encapsulation
The ISATAP driver is responsible for inserting the outermost IPv4
encapsulating header for all tunneled packets. Tunnel interfaces that
use various encapsulation methods (e.g., 6over4 [RFC2529], 6to4
[RFC3056], teredo, IP encapsulation within IP [RFC2003], minimal
encapsulation within IP [RFC2004], basic IPv6-in-IPv4 encapsulation
[MECH], ISATAP encapsulation itself, etc.) can all be configured as
encapsulation clients of the ISATAP driver.
The ISATAP driver performs AH [RFC2402] and ESP [RFC2406] processing
for tunnels that use IPsec, and may also perform header compression
prior to encapsulation.
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8.2 Multicast/Anycast
ISATAP interfaces tunnel only those packets with IPv6 multicast/
anycast destinations to include a node's required multicast/anycast
addresses, the 'All_DHCP_Relay_Agents_and_Servers' and
'All_DHCP_Servers' multicast addresses [RFC3315] and multicast
addresses discovered via MLD [RFC2710]. Packets with unrecognized
IPv6 multicast/anycast destinations are silently dropped.
ISATAP interfaces automatically tunnel IPv6 multicast packets with
the 'All_DHCP_Relay_Agents_and_Servers' and 'All_DHCP_Servers' using
the IPv4 'all hosts' broadcast address (i.e., 0xffffffff broadcast
address) as the destination address in the encapsulating IPv4 header
under the assumption that DHCPv6 servers will be co-located with
DHCPv4 servers.
For other IPv6 multicast destinations, ISATAP interfaces
automatically tunnel packets using a mapped Organization-Local Scope
IPv4 multicast address ([RFC2529], section 6) as the destination
address in the encapsulating IPv4 header. ISATAP nodes join the
Organization-Local Scope IPv4 multicast groups required to support
IPv6 Neighbor Discovery ([RFC2529], Appendix A) on interfaces that
may receive IPv4 packets to be forwarded to an ISATAP interface.
NOTE: When the ISATAP node enables one or more 6over4 interfaces
[RFC2529], the 6over4 interfaces MAY be used (i.e., instead of ISATAP
interfaces) for sending and receiving multicast packets.
8.3 Tunnel MTU and Fragmentation
The specification in ([MECH], section 3.2) is not used; the
specification in this section is used instead:
ISATAP interfaces set a static MTU of 1280 bytes, i.e., the minimum
MTU for IPv6 interfaces ([RFC2460], section 5) and do not set the
Don't Fragment bit in the encapsulating IPv4 headers of tunneled
packets. ISATAP interfaces MAY provide a configuration knob for
setting a larger MTU, but larger MTUs MUST NOT be configured other
than for certain constrained deployments, e.g., in some enterprise
networks). Interfaces that may receive IPv4 packets to be forwarded
to an ISATAP interface SHOULD configure an Effective MTU to Receive
(EMTU_R) [RFC1122], section 3.3.2) of at least 1500 bytes, i.e., they
SHOULD be able to reassemble IPv4 packets of 1500 bytes or larger.
1280 bytes was chosen as the IPv6 interface minimum MTU [DEERING97]
to allow extra room for link layer encapsulations without exceeding
the Ethernet MTU of 1500 bytes, i.e., the practical physical cell
size of the Internet. The 1280 byte MTU provides a fixed upper bound
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for the size of IPv6 packets/fragments with a maximum
store-and-forward delay budget of ~1 second assuming worst-case link
speeds of ~10Kbps [RFC3150], thus allowing a convenient value for use
in reassembly buffer timer settings. Finally, the 1280 byte MTU
allows transport connections (e.g., TCP) to configure a large-enough
maximum segment size for improved performance even if the IPv4
interface that will send the tunneled packets uses a smaller MTU.
When the size of the IPv6 destination's receive buffer is known,
applications MAY send IPv6 packets up to that size using IPv6
fragmentation (or, fragmentation via an alternate form of
encapsulation) with a maximum fragment size that is no larger than
the minimum of the MTU of the IPv4 interface used for tunneling and
1280 bytes. Even so, IPv4 fragmentation MAY still occur along some
paths; in particular, since the minimum IPv4 fragment size is only 8
bytes ([RFC0791], section 2), middleboxes with unusual
implementations of IPv4 fragmentation could shatter the tunneled
packets into as many as 187 IPv4 fragments to accommodate a 1500 byte
IPv4 packet. Such sustained bursts of small packets could result in
poor performance due to increased loss probability on paths with
non-negligible packet loss due to, e.g., link errors, congested
router queues, etc.
Therefore, ISATAP nodes that anticipate or experience poor
performance along some paths MAY choose to adaptively vary the
maximum size for the packets/fragments they send. For example,
implementations may choose to employ a "fragment size slow start"
scheme that begins with as little as 8 bytes (i.e., the minimum IPv4
fragment size) and varies the size of the fragments using, e.g., an
additive-increase, multiplicative-decrease strategy to determine the
size that yields the best performance. The process can be made to
converge more quickly when next-hop IPv6 routers are configured to
send Router Advertisements with MTU options when they experience IPv4
fragmentation, since the sender is made aware that fragmentation is
occurring, and the MTU option can be used to return the size of the
largest IPv4 fragment observed which may help the sender determine
the optimal fragment size.
Since many nodes are expected to implement this specification, an
overall increase in small packets in the Internet may occur as more
nodes with tunnel interfaces implement schemes such as the one
described above to avoid IPv4 fragmentation-related performance
issues. For this reason, network equipment manufacturers and network
administrators are encouraged to observe the Recommendations on Queue
Management and Congestion Avoidance in the Internet [RFC2309]. In
particular, byte mode queue averaging for RED is encouraged.
With reference to the above, it is RECOMMENDED that ISATAP nodes use
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adaptive techniques to minimize IPv4 fragmentation and use IPv6
fragmentation/reassembly (or, fragmentation/reassembly via an
alternate form of encapsulation) to manage the size of the tunneled
packets they send. It is also RECOMMENDED that ISATAP nodes monitor
the IPv4 reassembly cache in order to give early indications of IPv4
network fragmentation by sending Router Advertisements with MTU
options to the source of the IPv4 fragments. The MTU options should
include a value to indicate the size of the largest packet that can
be expected to arrive without incurring IPv4 fragmentation. Finally,
it is RECOMMENDED that ISATAP nodes set small timeout values, e.g. 1
second, for IPv4 reassembly of tunneled packets.
8.4 Handling IPv4 ICMP Errors
ISATAP interfaces SHOULD process ARP failures and persistent ICMPv4
errors as link-specific information indicating that a path to a
neighbor may have failed ([RFC2461], section 7.3.3).
8.5 Link-Local Addresses
The specification in ([MECH], section 3.7) is not used; the
specification in Section 6.1 of this document is used instead.
8.6 Neighbor Discovery over Tunnels
The specification in ([MECH], section 3.8) is not used; the
specification in Section 9 of this document is used instead.
8.7 Decapsulation/Filtering
ISATAP nodes arrange for the ISATAP driver to received all tunneled
packets that use an IPv4 header as the outermost layer of
encapsulation. Examples include ip-protocol-41 (6to4, 6over4, isatap,
etc.), ip-protocol-4 (IP encapsulation within IP, minimal
encapsulation within IP, etc.), UDP port 3544 (teredo, etc.) and
others. The ISATAP driver determines the correct tunnel interface to
receive each packet via a lookup in the 'ifRcvAdddressTable' for the
packet's IPv4 source address, destination address, an index for the
receiving IPv4 interface and the type of encapsulation. Packets for
which the correct tunnel interface cannot be determined are silently
discarded.
After determining the correct tunnel interface, the ISATAP driver
verifies that the packet's link-layer (IPv4) source address is
correct for the network-layer (IPv6) source address. For configured
tunnels, the IPv4 and IPv6 source addresses can be checked directly
against the configured tunnel's addresses. For ISATAP interfaces, the
packet's link-layer source address is correct if one (or more) of the
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following are true:
o the network-layer source address is an ISATAP address that embeds
the link-layer source address in its interface identifier.
o the network-layer source address is an IPv6 neighbor on an
interface that has the same 'ipv6ScopeZoneIndexLinkLocal' as the
receiving ISATAP interface.
o the link-layer source address is a member of the Potential Router
List (see: Section 9.1).
Packets for which the link-layer source address is incorrect are
discarded and, if permitted by the current status of ICMPv6 message
rate limiting parameters [ICMPV6], section 2.4, paragraph f), an
ICMPv6 Destination Unreachable message SHOULD be generated and sent
to the IPv6 source in the inner header of the encapsulated packet.
The error message SHOULD include only enough bytes from the invoking
packet to convey the IPv6 header information, i.e., it SHOULD NOT
include up to the minimum IPv6 MTU.
After determining the correct tunnel interface to receive the packet,
the ISATAP driver examines the IPv6 and IPv4 source addresses to
determine whether a rewrite is required. If the IPv6 source address
is an ISATAP address with the 'u/l' and 'g' bits set to 0 (see:
Section 6.1), and the IPv4 source address does not match the IPv4
address encoded in the ISATAP interface identifier, the ISATAP driver
copies the IPv4 source address over the IPv4 address embedded in the
IPv6 address and sets the 'u/l' bit to 1. Other forms of rewrites
(e.g., rewrites for multicast rendezvous points based on the 'u' and
'g' bit) MAY be specified in other documents.
Next, the ISATAP driver discards the encapsulating IPv4 header and
locates any existing host-pair information, e.g., via the IPv6 Flow
Label [FLOW]. Then:
o If header compression is indicated, the packet's inner header(s)
are reconstituted.
o If a security association is indicated, AH [RFC2402] or ESP
[RFC2406] processing is applied.
o If the packet is a fragment, it is placed in a buffer for
reassembly. The buffer may be, e.g., the IPv6 reassembly cache, an
application's own data buffer [RFC3542], etc.
Finally, when a whole packet has been received, it is delivered to
the correct tunnel interface. If there is clear evidence that
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reassembly of a fragmented packet has stalled, an ICMPv6 "packet too
big" message [RFC1981] is sent to the packet's source address
(subject to ICMPv6 rate-limiting) with an MTU value indicating a size
that is likely to incur successful reassembly.
9. Neighbor Discovery
ISATAP nodes use the neighbor discovery mechanisms specified in
[RFC2461] along with securing mechanisms such as [SEND] to create/
change neighbor cache entries and to provide control plane signalling
for automatic tunnel configuration. ISATAP interfaces also implement
the following specifications:
9.1 Conceptual Model Of A Host
To the list of Conceptual Data Structures ([RFC2461], section 5.1),
ISATAP interfaces add:
Potential Router List
A set of entries about potential routers; used to support the
mechanisms specified in Section 9.2.2.1. Each entry ("PRL(i)")
has an associated timer ("TIMER(i)"), and an IPv4 address
("V4ADDR(i)") that represents a router's advertising ISATAP
interface.
9.2 Router and Prefix Discovery
9.2.1 Router Specification
As permitted by ([RFC2461], section 6.2.6), the ISATAP server daemon
SHOULD send unicast Router Advertisement messages to the soliciting
node's address when the solicitation's source address is not the
unspecified address.
9.2.2 Host Specification
9.2.2.1 Host Variables
To the list of host variables ([RFC2461], section 6.3.2), ISATAP
interfaces add:
PrlRefreshInterval
Time in seconds between successive refreshments of the PRL after
initialization. It SHOULD be no less than 3600 seconds. The
designated value of all 1's (0xffffffff) represents infinity.
Default: 3600 seconds
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MinRouterSolicitInterval
Minimum time in seconds between successive solicitations of the
same advertising ISATAP interface. It SHOULD be no less than 900
seconds. The designated value of alll 1's (0xffffffff) represents
infinity.
Default: 900 seconds
9.2.2.2 Interface Initialization
The ISATAP node joins the all-nodes multicast address on ISATAP
interfaces, as for multicast-capable interfaces ([RFC2461], section
6.3.3) and MAY also join other multicast groups, e.g., see: Section
8.2
Additionally, the node provisions the ISATAP interface's PRL with
IPv4 addresses it discovers via manual configuration, a DNS
fully-qualified domain name (FQDN) [RFC1035], a DHCPv4 option, a
DHCPv4 vendor-specific option, or an unspecified alternate method.
ISATAP nodes establish FQDNs via manual configuration or an
unspecified alternate method. Nodes resolves FQDNs into IPv4
addresses through lookup in a static host file, querying the DNS
service, or an unspecified alternate method. When DNS is used, client
resolvers use the IPv4 transport.
After the node provisions the ISATAP interface's PRL with IPv4
addresses, it sets PrlRefreshIntervalTimer to PrlRefreshInterval
seconds. The node re-initializes the PRL (i.e., as specified above)
when PrlRefreshIntervalTimer expires, or when an asynchronous
re-initialization event occurs. When the node re-initializes the PRL,
it resets PrlRefreshIntervalTimer to PrlRefreshInterval seconds.
9.2.2.3 Processing Received Router Advertisements
The ISATAP server daemon processes Router Advertisements (RAs)
exactly as specified in ([RFC2461], section 6.3.4). Router
Advertisement messages received on a point-to-point tunnel interface
that contain an MTU option with a value less than 1280 bytes cause
the interface to reduce its MTU to the lesser value, but Router
Advertisements received on an ISATAP interface MUST NOT cause the
ISATAP interface to reduce its MTU to a value less than 1280 bytes.
For Router Advertisement messages received on an ISATAP interface
that include prefix options and/or non-zero values in the Router
Lifetime, the server daemon reset TIMER(i) to schedule the next
solicitation event (see: Section 9.2.2.4). Let "MIN_LIFETIME" be the
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minimum value in the Router Lifetime or the lifetime(s) encoded in
options included in the RA message. Then, TIMER(i) is reset as
follows:
TIMER(i) = MAX((0.5 * MIN_LIFETIME), MinRouterSolicitInterval)
9.2.2.4 Sending Router Solicitations
To the list of events after which RSs may be sent ([RFC2461], section
6.3.2), ISATAP interfaces add:
o TIMER(i) for some PRL(i) expires.
Additionally, the ISATAP server daemon MAY send Router Solicitations
to an ISATAP link-local address that embeds V4ADDR(i) for some PRL(i)
instead of the All-Routers multicast address.
9.3 Address Resolution and Neighbor Unreachability Detection
9.3.1 Address Resolution
The specification in ([RFC2461], section 7.2) is used. ISATAP
addresses for which the neighbor/router's link-layer address cannot
otherwise be determined (e.g., from a neighbor cache entry) are
resolved to link-layer addresses by a static computation, i.e., the
last four octets are treated as an IPv4 address.
Hosts SHOULD perform an initial reachability confirmation by sending
Neighbor Solicitation message(s) and receiving a Neighbor
Advertisement message (NS messages are sent to the target's unicast
address). Routers MAY perform this initial reachability confirmation,
but this might not scale in all environments.
As specified in ([RFC2461], section 7.2.4), all nodes MUST send
solicited Neighbor Advertisements on ISATAP interfaces.
9.3.2 Neighbor Unreachability Detection
Hosts SHOULD perform Neighbor Unreachability Detection ([RFC2461],
section 7.3). Routers MAY perform neighbor unreachability detection,
but this might not scale in all environments.
10. Other Requirements for Control Plane Signalling
10.1 Node Information Queries
ISATAP nodes SHOULD implement Node Information Queries as specified
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in [NIQUERY].
Node Information Queries/Responses provide the following advantages:
o the querier receives unambiguous confirmation that the responder
supports the protocol.
o the querier receives assurance that responses are coming from the
correct responder.
o the querier discovers some subset of the responder's addresses.
10.2 Linklocal Multicast Name Resolution (LLMNR)
ISATAP nodes SHOULD implement Link Local Multicast Name Resolution
[LLMNR], since they will commonly be deployed in environments (e.g.,
home networks, ad-hoc networks, etc.) with no access to a Domain Name
System (DNS) server.
11. IANA Considerations
The IANA is advised to specify construction rules for IEEE EUI-64
addresses formed from the Organizationally Unique Identifier (OUI)
"00-00-5E" in the IANA "ethernet-numbers" registry. The non-normative
text in Appendix B is offered as an example specification.
12. Security considerations
The security considerations in the normative references apply.
Additionally, administrators MUST ensure that lists of IPv4 addresses
representing the advertising ISATAP interfaces of PRL members are
well maintained.
13. Acknowledgments
Most of the basic ideas in this document are not original; the
authors acknowledge the original architects of those ideas. Portions
of this work were sponsored through SRI International internal
projects and government contracts. Government sponsors include Monica
Farah-Stapleton and Russell Langan (U.S. Army CECOM ASEO), and Dr.
Allen Moshfegh (U.S. Office of Naval Research). SRI International
sponsors include Dr. Mike Frankel, J. Peter Marcotullio, Lou
Rodriguez, and Dr. Ambatipudi Sastry.
The following are acknowledged for providing peer review input: Jim
Bound, Rich Draves, Cyndi Jung, Ambatipudi Sastry, Aaron Schrader,
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Ole Troan, Vlad Yasevich.
The following are acknowledged for their significant contributions:
Alain Durand, Hannu Flinck, Jason Goldschmidt, Nathan Lutchansky,
Karen Nielsen, Mohan Parthasarathy, Chirayu Patel, Art Shelest, Pekka
Savola, Margaret Wasserman, Brian Zill.
The authors acknowledge the work of Quang Nguyen [VET] under the
guidance of Dr. Lixia Zhang that proposed very similar ideas to those
that appear in this document. This work was first brought to the
authors' attention on September 20, 2002.
The following individuals are acknowledged for their helpful insights
on path MTU discovery: Jari Arkko, Iljitsch van Beijnum, Jim Bound,
Ralph Droms, Alain Durand, Jun-ichiro itojun Hagino, Brian Haberman,
Bob Hinden, Christian Huitema, Kevin Lahey, Hakgoo Lee, Matt Mathis,
Jeff Mogul, Erik Nordmark, Soohong Daniel Park, Chirayu Patel,
Michael Richardson, Pekka Savola, Hesham Soliman, Mark Smith, Dave
Thaler, Michael Welzl, Lixia Zhang and the members of the Nokia NRC/
COM Mountain View team.
"...and I'm one step ahead of the shoe shine,
Two steps away from the county line,
Just trying to keep my customers satisfied,
Satisfi-i-ied!" - Simon and Garfunkel
Normative References
[ADDR-ARCH]
Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", draft-ietf-ipv6-addr-arch-v4-00 (work in
progress), October 2003.
[ICMPV6] Conta, A. and S. Deering, "Internet Control Message
Protocol (ICMPv6) for the Internet Protocol Version 6
(IPv6) Specification", draft-ietf-ipngwg-icmp-v3 (work in
progress), November 2001.
[LLMNR] Esibov, L., Aboba, B. and D. Thaler, "Linklocal Multicast
Name Resolution", draft-ietf-dnsext-mdns (work in
progress), January 2004.
[MECH] Gilligan, R. and E. Nordmark, "Basic Transition Mechanisms
for IPv6 Hosts and Routers", draft-ietf-v6ops-mech-v2-00
(work in progress), February 2003.
[NIQUERY] Crawford, M., "IPv6 Node Information Queries",
draft-ietf-ipngwg-icmp-name-lookups (work in progress),
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Internet-Draft ISATAP January 2004
June 2003.
[NODEREQ] Loughney, J., "IPv6 Node Requirements",
draft-ietf-ipv6-node-requirements (work in progress),
October 2003.
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, September
1981.
[RFC1122] Braden, R., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989.
[RFC1981] McCann, J., Deering, S. and J. Mogul, "Path MTU Discovery
for IP version 6", RFC 1981, August 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC2461] Narten, T., Nordmark, E. and W. Simpson, "Neighbor
Discovery for IP Version 6 (IPv6)", RFC 2461, December
1998.
[RFC2529] Carpenter, B. and C. Jung, "Transmission of IPv6 over IPv4
Domains without Explicit Tunnels", RFC 2529, March 1999.
[RFC3150] Dawkins, S., Montenegro, G., Kojo, M. and V. Magret,
"End-to-end Performance Implications of Slow Links", BCP
48, RFC 3150, July 2001.
[RFC3542] Stevens, W., Thomas, M., Nordmark, E. and T. Jinmei,
"Advanced Sockets Application Program Interface (API) for
IPv6", RFC 3542, May 2003.
Informative References
[DEERING97]
Deering, S., "http://www.cs-ipv6.lancs.ac.uk/ipv6/
mail-archive/IPng/1997-12/0052.html", November 1997.
[FLOW] Rajahalme, J., Conta, A., Carpenter, B. and S. Deering,
"IPv6 Flow Label Specification",
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Internet-Draft ISATAP January 2004
draft-ietf-ipv6-flow-label (work in progress), December
2003.
[FTMIB] Haberman, B. and M. Wasserman, "IP Forwarding Table MIB",
draft-ietf-ipv6-rfc2096-update (work in progress), August
2003.
[IPMIB] Routhier, S., "Management Information Base for the
Internet Protocol (IP)", draft-ietf-ipv6-rfc2011-update
(work in progress), September 2003.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC2003] Perkins, C., "IP Encapsulation within IP", RFC 2003,
October 1996.
[RFC2004] Perkins, C., "Minimal Encapsulation within IP", RFC 2004,
October 1996.
[RFC2223bis]
Reynolds, J. and R. Braden, "Instructions to Request for
Comments (RFC) Authors", draft-rfc-editor-rfc2223bis (work
in progress), August 2003.
[RFC2309] Braden, B., Clark, D., Crowcroft, J., Davie, B., Deering,
S., Estrin, D., Floyd, S., Jacobson, V., Minshall, G.,
Partridge, C., Peterson, L., Ramakrishnan, K., Shenker,
S., Wroclawski, J. and L. Zhang, "Recommendations on Queue
Management and Congestion Avoidance in the Internet", RFC
2309, April 1998.
[RFC2402] Kent, S. and R. Atkinson, "IP Authentication Header", RFC
2402, November 1998.
[RFC2406] Kent, S. and R. Atkinson, "IP Encapsulating Security
Payload (ESP)", RFC 2406, November 1998.
[RFC2486] Aboba, B. and M. Beadles, "The Network Access Identifier",
RFC 2486, January 1999.
[RFC2491] Armitage, G., Schulter, P., Jork, M. and G. Harter, "IPv6
over Non-Broadcast Multiple Access (NBMA) networks", RFC
2491, January 1999.
[RFC2492] Armitage, G., Schulter, P. and M. Jork, "IPv6 over ATM
Networks", RFC 2492, January 1999.
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[RFC2710] Deering, S., Fenner, W. and B. Haberman, "Multicast
Listener Discovery (MLD) for IPv6", RFC 2710, October
1999.
[RFC3056] Carpenter, B. and K. Moore, "Connection of IPv6 Domains
via IPv4 Clouds", RFC 3056, February 2001.
[RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C. and
M. Carney, "Dynamic Host Configuration Protocol for IPv6
(DHCPv6)", RFC 3315, July 2003.
[RFC3582] Abley, J., Black, B. and V. Gill, "Goals for IPv6
Site-Multihoming Architectures", RFC 3582, August 2003.
[SEND] Arkko, J., Kempf, J., Sommerfield, B., Zill, B. and P.
Nikander, "Secure Neighbor Discovery (SEND)",
draft-ietf-send-ndopt (work in progress), October 2003.
[TUNNELMIB]
Thaler, D., "IP Tunnel MIB",
draft-ietf-ipv6-inet-tunnel-mib (work in progress),
January 2004.
[VET] Nguyen, Q., "http://irl.cs.ucla.edu/vet/report.ps", spring
1998.
Authors' Addresses
Fred L. Templin
Nokia
313 Fairchild Drive
Mountain View, CA 94110
US
Phone: +1 650 625 2331
EMail: ftemplin@iprg.nokia.com
Tim Gleeson
Cisco Systems K.K.
Shinjuku Mitsu Building
2-1-1 Nishishinjuku, Shinjuku-ku
Tokyo 163-0409
Japan
EMail: tgleeson@cisco.com
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Mohit Talwar
Microsoft Corporation
One Microsoft Way
Redmond, WA> 98052-6399
US
Phone: +1 425 705 3131
EMail: mohitt@microsoft.com
Dave Thaler
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052-6399
US
Phone: +1 425 703 8835
EMail: dthaler@microsoft.com
Appendix A. Major Changes
Major changes from earlier versions to version 17:
o added tunnel driver API
o expanded section on MTU and fragmentation
o expanded sections on encapsulation/decapsulation
o specified relation to IPv6 Node Requirements
o specified use of additional control plane signalling
o specified multicast mappings.
o specified link layer address option format.
o specified setting of "u" bit in interface id's.
o removed obsoleted appendix sections.
o re-organized major sections to match normative references.
o revised neighbor discovery, address autoconfiguration, security
considerations sections. Added new subsections on interface
management, decapsulation/filtering, address lifetime expiry.
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Appendix B. Interface Identifier Construction
This section provides an example specification for constructing EUI64
addresses from the Organizationally-Unique Identifier (OUI) owned by
the Internet Assigned Numbers Authority (IANA). It can be used to
construct both modified EUI-64 format interface identifiers for IPv6
unicast addresses ([ADDR-ARCH], section 2.5.1) and "native" EUI64
addresses for future use:
|0 2|2 3|3 3|4 6|
|0 3|4 1|2 9|0 3|
+------------------------+--------+--------+------------------------+
| OUI ("00-00-5E"+u+g) | TYPE | TSE | TSD |
+------------------------+--------+--------+------------------------+
Where the fields are:
OUI IANA's OUI: 00-00-5E with "u" and "g" bits (3 octets)
TYPE Type field; specifies use of (TSE, TSD) (1 octet)
TSE Type-Specific Extension (1 octet)
TSD Type-Specific Data (3 octets)
And the following interpretations are specified based on TYPE:
TYPE (TSE, TSD) Interpretation
---- -------------------------
0x00-0xFD RESERVED for future IANA use
0xFE (TSE, TSD) together contain an IPv4 address
0xFF TSD is interpreted based on TSE as follows:
TSE TSD Interpretation
--- ------------------
0x00-0xFD RESERVED for future IANA use
0xFE TSD contains 24-bit EUI-48 intf id
0xFF RESERVED by IEEE/RAC
Using this example specification, if TYPE=0xFE, then TSE is an
extension of TSD. If TYPE=0xFF, then TSE is an extension of TYPE.
(Other values for TYPE, and other interpretations of TSE, TSD are
reserved for future IANA use.) When TYPE='0xFE' the EUI64 address
embeds an IPv4 address, encoded in network byte order.
For Modified EUI64 format interface identifiers in IPv6 unicast
addresses ([ADDR-ARCH], Appendix A) using IANA's OUI, when TYPE=0xFE
and the IPv4 address is a globally unique (i.e., provider-assigned)
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unicast address, the "u" bit is set to 1 to indicate universal scope.
When TYPE=0xFE and the IPv4 address is from a private allocation, the
"u" bit is set to 0 to indicate local scope. Thus, when the first
four octets of the interface identifier in an IPv6 unicast address
are either: '02-00-5E-FE' or: '00-00-5E-FE', the next four octets
embed an IPv4 address and the interface identifier is said to be in
"ISATAP format".
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