One document matched: draft-ietf-ipngwg-scoping-arch-01.txt
Differences from draft-ietf-ipngwg-scoping-arch-00.txt
IPNGWG Working Group S. Deering
Internet Draft Cisco Systems
draft-ietf-ipngwg-scoping-arch-01.txt B. Haberman
March 2000 Nortel Networks
Expires September 2000 B. Zill
Microsoft
IP Version 6 Scoped Address Architecture
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
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reference material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
Abstract
This document specifies the architectural characteristics, expected
behavior, and usage of IPv6 addresses of different scopes
1. Introduction
The Internet Protocol version 6 (IPv6) introduces the concept of
limited scope addresses to the IP lexicon. While operational
practice with IPv4 has included the concept of a private address
space (net 10, etc.), the design of IPv6 incorporates such addresses
into its base architecture. This document defines terms associated
with such addresses and describes mechanisms for their behavior.
Deering, Haberman, Zill 1
Internet Draft IPv6 Scoped Address Architecture September 2000
2. Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in [RFC 2119].
3. Basic Terminology
The terms link, interface, node, host, and router are defined in
[RFC 2460]. The definitions of unicast address scopes (link-local,
site-local, and global) and multicast address scopes (node-local,
link-local, etc.) are contained in [RFC 2373].
4. Address Scope
Every IPv6 address has a specific scope, that is, a topological
"distance" within which the address may be used as a unique
identifier for an interface. The scope of an address is encoded as
part of the address, as specified in [RFC 2373].
For unicast addresses, there are three defined scopes:
o Link-local scope, for uniquely identifying interfaces
within a single link only.
o Site-local scope, for uniquely identifying interfaces
within a single site only. A "site" is, by intent, not
rigorously defined, but is typically expected to cover a
region of topology that belongs to a single organization
and is located within a single geographic location, such
as an office, an office complex, or a campus. A personal
residence may be treated as a site (for example, when the
residence obtains Internet access via a public Internet
service provider), or as a part of a site (for example,
when the residence obtains Internet access via an
employer's or school's site).
o Global scope, for uniquely identifying interfaces
anywhere in the Internet.
For multicast addresses, there are fourteen possible scopes, ranging
from node-local to global (including both link-local and site-
local). A node-local multicast address serves as a unique
identifier for an interface within a single node only; such an
address is used only for "loopback" delivery of multicasts within a
single node, for example, as a form of inter-process communication
within a computer.
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Internet Draft IPv6 Scoped Address Architecture September 2000
There is an ordering relationship among scopes:
o for unicast scopes, link-local is a smaller scope than
site-local, and site-local is a smaller scope than
global.
o for multicast scopes, scopes with lesser values in the
"scop" subfield of the multicast address [RFC 2373,
section 2.7] are smaller than scopes with greater values,
with node-local being the smallest and global being the
largest.
However, two scopes of different size may cover the exact same
region of topology, for example, a site may consist of a single
link, in which both link-local and site-local scope effectively
cover the same topological "distance".
5. Scope Zones
A scope zone, or a simply a zone, is a connected region of topology
of a given scope. For example, the set of links connected by
routers within a particular site, and the interfaces attached to
those links, comprise a single zone of site-local scope. To
understand the distinction between scopes and zones, observe that
the topological regions within two different sites are considered to
be two DIFFERENT zones, but of the SAME scope.
Addresses of a given (non-global) scope may be re-used in different
zones of that scope. The zone to which a particular non-global
address pertains is not encoded in the address itself, but rather is
determined by context, such as the interface from which it is sent
or received.
Zones of the different scopes are defined as follows:
o A node-local zone (for multicast only) consists of a
single interface on a node. [Note: node-local scope
would have been more accurately named interface-local.]
o A link-local zone (for unicast and multicast) consists of
a single link and all the interfaces attached to that
link.
o There is a single zone of global scope (for both unicast
and multicast), comprising all the links and interfaces
in the Internet.
o The boundaries of zones of scope other than node-local,
link-local, and global must be defined and configured by
network administrators. The only required such
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Internet Draft IPv6 Scoped Address Architecture September 2000
boundaries are site boundaries. A site boundary serves
for both unicast and multicast.
Zone boundaries are relatively static features, not changing in
response to short-term changes in topology. Thus, the requirement
that the topology within a zone be "connected" is intended to
include links and interfaces that may be only occasionally
connected. For example, a residential node or network that obtains
Internet access by dial-up to an employer's site may be treated as
part of the employer's site-local zone even when the dial-up link is
disconnected. Similarly, a failure of a router, interface, or link
that causes a zone to become partitioned does not split that zone
into multiple zones; rather, the different partitions are still
considered to belong to the same zone.
Zones have the following additional properties:
o Zone boundaries cut through nodes, not links. (There are
two exceptions: the global zone has no boundary, and the
boundary of a node-local zone conceptually cuts through
an interface between a node and a link.)
o Zones of the same scope cannot overlap, i.e., they can
have no links or interfaces in common.
o A zone of a given scope (less than global) falls
completely within zones of larger scope, i.e., a smaller
scope zone cannot include more topology than any larger
scope zone with which it shares any links or interfaces.
Each interface belongs to one node-local zone, one link-local zone,
one site-local zone, and the global zone. Each link belongs to one
link-local zone, one site-local zone, and the global zone. An
interface or link only belongs to additional (i.e., multicast) zones
if it falls within the configured boundaries of such additional
zones.
6. Zone Indices
Because the same address of a given (non-global) scope can be re-
used in different zones of that scope, a node must have a means --
other than examining the address itself -- of associating non-global
addresses with particular zones when sending, receiving, or
forwarding packets containing such addresses. This is accomplished
by assigning a local "zone index" to each zone to which a node is
attached. Each attached zone of the same scope must be assigned a
different index value; attached zones of different scopes can re-use
the same index values.
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Internet Draft IPv6 Scoped Address Architecture September 2000
The assignment of zone indices is illustrated in the example in the
figure below:
---------------------------------------------------------------
| a node |
| |
| |
| |
| |
| |
| /--site1--\ /--------------site2--------------\ /--site3--\ |
| |
| /--link1--\ /--------link2--------\ /--link3--\ /--link4--\ |
| |
| intf1 intf2 intf3 intf4 intf5 |
---------------------------------------------------------------
: | | | |
: | | | |
: | | | |
the ================= a point- a
loopback an Ethernet to-point tunnel
link link
Figure 1 : Zone Indices Example
This example node has five interfaces:
o A loopback interface, which can be thought of as an
interface to a phantom link -- the "loopback link" --
that goes nowhere,
o Two interfaces to the same Ethernet,
o An interface to a point-to-point link, and
o A tunnel interface (e.g., the abstract endpoint of an
IPv6-over-IPv6 tunnel [RFC 2473], presumably established
over either the Ethernet or the point-to-point link.)
It is thus attached to five node-local zones, identified by the
interface indices 1 through 5.
Because the two Ethernet interfaces are attached to the same link,
the node is attached to only four link-local zones, identified by
link indices 1 through 4.
It is attached to three site-local zones: one imaginary one to which
the loopback interface belongs, one to which the Ethernet and the
point-to-point link belong, and one to which the tunnel belongs
(perhaps because it is a tunnel to another organization). These
site-local zones are identified by the site indices 1 through 3.
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Internet Draft IPv6 Scoped Address Architecture September 2000
The zone indices are strictly local to the node. For example, the
node on the other end of the point-to-point link may well be using
entirely different interface, link, and site index values for that
link.
The zone index values are arbitrary. An implementation may use any
value it chooses to label a zone as long as it maintains the
requirement that the index value of each attached zone of the same
scope must be unique within the node. Implementations choosing to
follow the recommended basic API [RFC 2553] will also want to
restrict their index values to those that can be represented by the
sin6_scope_id field of a sockaddr_in6.
An implementation may also support the concept of a "default" zone
for each scope. It is convenient to reserve the index value zero,
at each scope, to mean "use the default zone". This default index
can also be used to identify the zone for any scopes for which the
node has not assigned any indices, such as the various multicast-
only scopes.
There is at present no way for a node to automatically determine
which of its interfaces belongs to the same zones, e.g., the same
link or the same site. In the future, protocols may be developed to
determine that information. In the absence of such protocols, an
implementation must provide a means for manual assignment and/or
reassignment of zone indices. Furthermore, to avoid the need to
perform manual configuration in most cases, an implementation
should, by default, initially assign zone indices as follows:
o A unique interface index for each interface
o A unique link index for each interface
o A single site index for all interfaces
Then, manual configuration would be necessary only for the less
common cases of nodes with multiple interfaces to a single link,
interfaces to different sites, or interfaces to zones of different
(multicast-only) scopes.
7. Sending Packets
When an upper-layer protocol sends a packet to a non-global
destination address, the node must also identify the intended zone
to be used for transmission.
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Internet Draft IPv6 Scoped Address Architecture September 2000
Note that there is one exception to the above statement: when
sending to the IPv6 unicast loopback address, ::1, there is no
need to identify the intended zone, even though that address is
non-global. Conceptually, the unicast loopback address is a link-
local address for a node's loopback interface, and is never
assigned to any other interface. Therefore, it unambiguously
identifies a single zone of link-scope, that being the phantom
loopback link.
Although identification of an outgoing interface is sufficient to
identify an intended zone (because each interface is attached to no
more than one zone of each scope), that is more specific than
desired in many cases. For example, when sending to a site-local
unicast address, from a node that has more than one interface to the
intended site, the upper layer protocol may not care which of those
interfaces is used for the transmission, but rather would prefer to
leave that choice to the routing function in the IP layer. Thus,
the upper-layer requires the ability to specify a zone index, rather
than an interface index, when sending to a non-global, non-loopback
destination address.
There may also be cases where the upper-layer wishes to restrict the
choice of outgoing interface to those belonging to a zone of smaller
scope than the destination address. For example, when sending to a
site-local destination, the upper-layer may wish to specify a
specific link on which the packet should be transmitted, but leave
the choice of which specific interface to use on that link to the IP
layer. One possible reason for such behavior is that the source
address chosen by the upper-layer is of smaller scope than the
destination, e.g., when using a link-local source address and a
site-local destination address. Thus, the upper layer requires the
ability, when sending a packet, to specify any zone of scope less
than or equal to the scope of the destination address, including the
case in which the destination address is of global scope. For this
reason, an implementation might find it useful to assign a distinct
value for each zone index, so that they are unique across all zones,
regardless of scope.
(Authors' note to selves: Think about distinct values
for default at each scope level.)
8. Receiving Packets
When an upper-layer protocol receives a packet containing a non-
global source or destination address, the zone to which that address
pertains can be determined from the arrival interface, because the
arrival interface can attached to only one zone of the same scope as
the address under consideration.
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Internet Draft IPv6 Scoped Address Architecture September 2000
9. Forwarding
When a router receives a packet addressed to a node other than
itself, it must take the zone of the destination and source
addresses into account as follows:
o The zone of the destination address is determined by the
scope of the address and arrival interface of the packet.
The next-hop interface is chosen by looking up the
destination address in a (conceptual) routing table
specific to that zone. That routing table is restricted
to refer only to interfaces belonging to that zone.
o After the next-hop interface is chosen, the zone of the
source address is considered. As with the destination
address, the zone of the source address is determined by
the scope of the address and arrival interface of the
packet. If transmitting the packet on the chosen next-
hop interface would cause the packet to leave the zone of
the source address, i.e., cross a zone boundary of the
scope of the source address, then the packet is discarded
and an ICMP Destination Unreachable message [RFC 2463]
with Code 2 ("beyond scope of source address") is sent to
the source of the packet.
Note that the above procedure applies for addresses of all scopes,
including link-local. Thus, if a router receives a packet with a
link-local destination address that is not one of the router's own
link-local addresses on the arrival link, the router is expected to
try to forward the packet to the destination on that link (subject
to successful determination of the destination's link-layer address
via the Neighbor Discovery protocol [RFC 2461]). The forwarded
packet may be transmitted back out the arrival interface, or out any
other interface attached to the same link.
A node that receives a packet addressed to itself and containing a
Routing Header with more than zero Segments Left [RFC 2460, section
4.4] swaps the original destination address with the next address in
the Routing Header. Then the above forwarding rules are applied,
using the new destination address. An implementation MUST NOT
examine additional addresses in the Routing header to determine
whether they are crossing boundaries for their scopes. Thus, it is
possible, though generally inadvisable, to use a Routing Header to
convey a non-global address across its associated zone boundary.
10. Routing
When a routing protocol determines that it is operating on a zone
boundary, it MUST protect inter-zone integrity and maintain intra-
zone connectivity.
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Internet Draft IPv6 Scoped Address Architecture September 2000
In order to maintain connectivity, the routing protocol must be able
to create forwarding information for the global prefixes as well as
for all of the zone prefixes for each of its attached zones. The
most straightforward way of doing this is to create (conceptual)
forwarding tables for each specific zone.
To protect inter-zone integrity routers must be selective in the
prefix information that is shared with neighboring routers. Routers
routinely exchange routing information with neighboring routers.
When a router is transmitting this routing information, it must not
include any information about zones other than the zones assigned to
the interface used to transmit the information.
* *
* *
* ----------- Site ID = X *
* | | *
+-*---|-------|-----+ *
| * i/f 1 i/f 2 | *
| * | *
| * i/f 5 - *
| *******************************
| |
| Router |
******************* *******************
| * * |
Site ID = Y - i/f 3 * * i/f 4 - Site ID = Z
| * * |
******************* *******************
+-------------------+
Figure 2: Multi-Sited Router
As an example, the router in Figure 2 must exchange routing
information on five interfaces. The information exchanged is as
follows:
o Interface 1
o All global prefixes
o All site prefixes learned from Interfaces 1, 2, and 5
o Interface 2
o All global prefixes
o All site prefixes learned from Interfaces 1, 2, and 5
Deering, Haberman, Zill 9
Internet Draft IPv6 Scoped Address Architecture September 2000
o Interface 3
o All global prefixes
o All site prefixes learned from Interface 3
o Interface 4
o All global prefixes
o All site prefixes learned from Interface 4
o Interface 5
o All global prefixes
o All site prefixes learned from Interfaces 1, 2, and 5
By imposing route exchange rules, zone integrity is maintained by
keeping all zone-specific routing information contained within the
zone.
11. Related Documents
The following list is a set of documents that are related to IPv6
address scope:
o Site Prefixes in Neighbor Discovery, draft-ietf-ipngwg-
site-prefixes-03.txt
o An Extension of Format for IPv6 Scoped Addresses, draft-
ietf-ipngwg-scopedaddr-format-00.txt
o Default Address Selection for IPv6, draft-ietf-ipngwg-
default-addr-select-00.txt
o Basic Socket Interface Extensions for IPv6, RFC 2553
o Advanced Sockets API for IPv6, draft-ietf-ipngwg-
rfc2292bis-01.txt
12. Mobility
TBD
13. Textual Representation
TBD
Deering, Haberman, Zill 10
Internet Draft IPv6 Scoped Address Architecture September 2000
14. Security Considerations
The routing section of this document specifies a set of guidelines
that allow routers to prevent zone-specific information from leaking
out of each site. If site boundary routers allow site routing
information to be forwarded outside of the site, the integrity of
the site could be compromise
15. References
[RFC 2119] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, BCP14, March 1999.
[RFC 2373] Hinden, R., and Deering, S., "IP Version 6 Addressing
Architecture", RFC 2373, July 1998.
[RFC 2460] Deering, S., and Hinden, R., "Internet Protocol Version
6 (IPv6) Specification", RFC 2460, December 1998.
[RFC 2473] Conta, A., and Deering, S., "Generic Packet Tunneling in
IPv6 Specification", RFC 2473, December 1998.
[RFC 2463] Conta, A., and Deering, S., "Internet Control Message
Protocol (RFC 2463) for Internet Protocol Version 6
(IPv6)", RFC 2463, December 1998.
[RFC 2461] Narten, T., Nordmark, E., and Simpson, W., "Neighbor
Discovery for IP Version 6 (IPv6)", RFC 2461, December
1998.
[RFC 2553] Gilligan, R., Thomson, S., Bound, J., and Stevens, W.,
"Basic Socket Interface Extensions for IPv6", RFC 2553,
March 1999.
Acknowledgements
Authors' Addresses
Stephen E. Deering
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134-1706
USA
Phone: +1-408-527-8213
Fax: +1-408-527-8213
Deering, Haberman, Zill 11
Internet Draft IPv6 Scoped Address Architecture September 2000
Email: deering@cisco.com
Brian Haberman
Nortel Networks
4309 Emperor Blvd.
Suite 200
Durham, NC 27703
USA
Phone: +1-919-992-4439
Email: haberman@nortelnetworks.com
Brian D. Zill
Microsoft Research
One Microsoft Way
Redmond, WA 98052-6399
USA
Phone: +1-425-703-3568
Fax: +1-425-936-7329
Email: bzill@microsoft.com
Deering, Haberman, Zill 12
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