As per Relevance of the word configured, we have this rfc below:
Network Working Group R.
Request for Comments: 2185 Cascade Communications Co
Category: Informational D.
Bay Networks Inc
September 1997
Routing Aspects Of IPv6
Status of this
This memo provides information for the Internet community. This
does not specify an Internet standard of any kind. Distribution
this memo is unlimited
This document gives an overview of the routing aspects of the IPv
transition. It is based on the protocols defined in the
"Transition Mechanisms for IPv6 Hosts and Routers" [1].
should be familiar with the transition mechanisms before reading
document
The proposals contained in this document are based on the work of
Ngtrans working group
1.
This paper uses the following terminology
node - a protocol module that implements IPv4 or IPv6.
router - a node that forwards packets not
addressed to itself
host - any node that is not a router
border router - a router that forwards packets
routing domain boundaries
link - a communication facility or medium over
nodes can communicate at the link layer, i.e., the
immediately below internet layer
interface - a node's attachment to a link
address - an network layer identifier for an interface
a group of interfaces
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RFC 2185 Routing Aspects Of IPv6 Transition September 1997
neighbors - nodes attached to the same link
routing domain - a collection of routers which
routing knowledge using a single routing protocol
routing region (or just "region") - a collection of
interconnected by a single internet protocol (e.g. IPv6)
and coordinating their routing knowledge using
protocols from a single internet protocol stack.
routing region may be a superset of a routing domain
tunneling - encapsulation of protocol A within protocol B
such that A treats B as though it were a datalink layer
reachability information - information describing the set
reachable destinations that can be used for
forwarding decisions
routing information - same as reachability information
address prefix - the high-order bits in an address
routing prefix - address prefix that expresses
which have addresses with the matching address prefixes
It is used by routers to advertise what systems they
capable of reaching
route leaking - advertisement of network layer
information across routing region boundaries
2. ISSUES AND
This document gives an overview of the routing aspects of IPv4
IPv6 transition. The approach outlined here is designed to
compatible with the existing mechanisms for IPv6 transition [1].
During an extended IPv4-to-IPv6 transition period, IPv6-based
must coexist with the installed base of IPv4 systems. In such a
internetworking protocol environment, both IPv4 and IPv6
infrastructure will be present. Initially, deployed IPv6-
domains might not be globally interconnected via IPv6-
internet infrastructure and therefore may need to communicate
IPv4-only routing regions. In order to achieve dynamic routing
such a mixed environment, there need to be mechanisms to
distribute IPv6 network layer reachability information
dispersed IPv6 routing regions. The same techniques can be used
later stages of IPv4-to-IPv6 transition to route IPv4 packets
isolated IPv4-only routing region over IPv6 infrastructure
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The IPng transition provides a dual-IP-layer transition, augmented
use of encapsulation where necessary and appropriate. Routing
related to this transition include
(1) Routing for IPv4
(2) Routing for IPv6
(2a) IPv6 packets with IPv6-native
(2b) IPv6 packets with IPv4-compatible
(3) Operation of manually configured static
(4) Operation of automatic
(4a) Locating
(4b) Ensuring that routing is consist
Basic mechanisms required to accomplish these goals include: (i
Dual-IP-layer Route Computation; (ii) Manual configuration of point
to-point tunnels; and (iii) Route leaking to support
encapsulation
The basic mechanism for routing of IPv4 and IPv6 involves dual-IP
layer routing. This implies that routes are separately calculated
IPv4 addresses and for IPv6 addressing. This is discussed in
detail in section 3.1.
Tunnels (either IPv4 over IPv6, or IPv6 over IPv4) may be
configured. For example, in the early stages of transition this
be used to allow two IPv6 domains to interact over an IPv
infrastructure. Manually configured static tunnels are treated as
they were a normal data link. This is discussed in more detail
section 3.2.
Use of automatic encapsulation, where the IPv4 tunnel
address is determined from the IPv4 address embedded in the IPv4-
compatible destination address of IPv6 packet, requires
of routes between IPv4 and IPv6 routing domains for
using IPv4-compatible addresses. For example, consider a packet
starts off as an IPv6 packet, but then is encapsulated in an IPv
packet in the middle of its path from source to destination.
packet must locate an encapsulator at the correct part of its path
Also, this packet has to follow a consistent route for the
path from source to destination. This is discussed in more detail
section 3.3.
The mechanisms for tunneling IPv6 over IPv4 are defined in
transition mechanisms specification [1].
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3. MORE DETAIL OF BASIC
3.1 Basic Dual-IP-layer
In the basic dual-IP-layer transition scheme, routers
independently support IPv4 and IPv6 routing. Other parts of
transition, such as DNS support, and selection by the source host
which packet format to transmit (IPv4 or IPv6) are discussed in [1].
Forwarding of IPv4 packets is based on routes learned through
IPv4-specific routing protocols. Similarly, forwarding of IPv
packets (including IPv6-packets with IPv4-compatible addresses)
based on routes learned through running IPv6-specific
protocols. This implies that separate instances of routing
are used for IPv4 and for IPv6 (although note that this could
of two instances of OSPF and/or two instances of RIP, since both
and RIP are capable of supporting both IPv4 and IPv6 routing).
A minor enhancement would be to use an single instance of
integrated routing protocol to support routing for both IPv4
IPv6. At the time that this is written there is no protocol
has yet been enhanced to support this. This minor enhancement
not change the basic dual-IP-layer nature of the transition
For initial testing of IPv6 with IPv4-compatible addresses, it may
useful to allow forwarding of IPv6 packets without running any IPv6-
compatible routing protocol. In this case, a dual (IPv4 and IPv6)
router could run routing protocols for IPv4 only. It then
IPv4 packets based on routes learned from IPv4 routing protocols
Also, it forwards IPv6 packets with an IPv4-compatible
address based on the route for the associated IPv4 address. There
a couple of drawbacks with this approach: (i) It does
specifically allow for routing of IPv6 packets via IPv6-
routers while avoiding and routing around IPv4-only routers; (ii)
does not produce routes for "non-compatible" IPv6 addresses.
this method the routing protocol does not tell the router
neighboring routers are IPv6-compatible. However, neighbor
may be used to determine this. Then if an IPv6 packet needs to
forwarded to an IPv4-only router it can be encapsulated to
destination host
3.2 Manually Configured Static
Tunneling techniques are already widely deployed for bridging non-
network layer protocols (e.g. AppleTalk, CLNP, IPX) over IPv4
infrastructure. IPv4 tunneling is an encapsulation of
packets inside IPv4 datagrams that are forwarded over IPv
infrastructure between tunnel endpoints. For a tunneled protocol,
tunnel appears as a single-hop link (i.e. routers that establish
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tunnel over a network layer infrastructure can inter-operate over
tunnel as if it were a one-hop, point-to-point link). Once a
is established, routers at the tunnel endpoints can establish
adjacencies and exchange routing information. Describing
protocols for performing encapsulation is outside the scope of
paper (see [1]). Static point-to-point tunnels may also
established between a host and a router, or between two hosts. Again
each manually configured point-to-point tunnel is treated as if
was a simple point-to-point link
3.3 Automatic
Automatic tunneling may be used when both the sending and
nodes are connected by IPv4 routing. In order for
tunneling to work, both nodes must be assigned IPv4-compatible IPv
addresses. Automatic tunneling can be especially useful where
source or destination hosts (or both) do not have any adjacent IPv6-
capable router. Note that by "adjacent router", this
routers which are logically adjacent by virtue of a
configured point-to-point tunnel (which is treated as if it is
simple point-to-point link).
With automatic tunneling, the resulting IPv4 packet is forwarded
IPv4 routers as a normal IPv4 packet, using IPv4 routes learned
routing protocols. There are therefore no special issues related
IPv4 routing in this case. There are however routing issues
to how IPv6 routing works in a manner which is compatible
automatic tunneling, and how tunnel endpoint addresses are
during the encapsulation process. Automatic tunneling is useful
a source host to the destination host, from a source host to
router, and from a router to the destination host. Mechanisms
automatic tunneling from a router to another router are not
defined
3.3.1 Host to Host Automatic
If both source and destination hosts make use of IPv4-compatible IPv
addresses, then it is possible for automatic tunneling to be used
the entire path from the source host to the destination host. In
case, the IPv6 packet is encapsulated in an IPv4 packet by the
host, and is forwarded by routers as an IPv4 packet all the way
the destination host. This allows initial deployment of IPv6-
hosts to be done prior to the update of any routers
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A source host may make use of Host to Host automatic
provided that the following are both true
- the source address is an IPv4-compatible IPv6 address
- the destination address is an IPv4-compatible IPv6 address
- the source host does know of one or more neighboring IPv4-
capable routers, or the source and destination are on
same subnet
If all of these requirements are true, then the source host
encapsulate the IPv6 packet in an IPv4 packet, using a source IPv
address which is extracted from the associated source IPv6 address
and using a destination IPv4 address which is extracted from
associated destination IPv6 address
Where host to host automatic tunneling is used, the packet
forwarded as a normal IPv4 packet for its entire path, and
decapsulated (i.e., the IPv4 header is removed) only by
destination host
3.3.2 Host to Router Configured Default
In some cases "configured default" tunneling may be used
encapsulate the IPv6 packet for transmission from the source host
an IPv6-backbone. However, this requires that the source host
configured with an IPv4 address to use for tunneling to the backbone
Configured default tunneling is particularly useful if the
host does not know of any local IPv6-capable router (implying
the packet cannot be forwarded as a normal IPv6 packet directly
the link layer), and when the destination host does not have
IPv4-compatible IPv6 address (implying that host to host
cannot be used).
Host to router configured default tunneling may optionally also
used even when the host does know of a local IPv6 router. In
case it is a policy decision whether the host prefers to send
native IPv6 packet to the IPv6-capable router or prefers to send
encapsulated packet to the configured tunnel endpoint
Similarly host to router default configured tunneling may be
even when the destination address is an IPv4-compatible IPv6 address
In this case for example a policy decision may be made to
tunneling for part of the path and native IPv6 for part of the path
or alternatively to use tunneling for the entire path from
host to destination host
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RFC 2185 Routing Aspects Of IPv6 Transition September 1997
A source host may make use of host to router configured
tunneling provided that ALL of the following are true
- the source address is an IPv4-compatible IPv6 address
- the source host does know of one or more neighboring IPv4-
capable
- the source host has been configured with an IPv4 address
an dual router which can serve as the tunnel endpoint
If all of these requirements are true, then the source host
encapsulate the IPv6 packet in an IPv4 packet, using a source IPv
address which is extracted from the associated source IPv6 address
and using a destination IPv4 address which corresponds to
configured address of the dual router which is serving as the
endpoint
When host to router configured default tunneling is used, the
is forwarded as a normal IPv4 packet from the source host to the
router serving as tunnel endpoint, is decapsulated by the
router, and is then forwarded as a normal IPv6 packet by the
endpoint
3.3.2.1 Routing to the Endpoint for the Configured Default
The dual router which is serving as the end point of the host
router configured default tunnel must advertise reachability
IPv4 routing sufficient to cause the encapsulated packet to
forwarded to it
The simplest approach is for a single IPv4 address to be assigned
use as a tunnel endpoint. One or more dual routers, which
connectivity to the IPv6 backbone and which are capable of serving
tunnel endpoint, advertise a host route to this address into IPv
routing in the IPv4-only region. Each dual host in the
IPv4-only region is configured with the address of this
endpoint and selects a route to this address for
encapsulated packet to a tunnel end point (for example, the
tunnel end point, based on whatever metric(s) the local
protocol is using).
Finally, in some cases there may be some reason for specific hosts
prefer one of several tunnel endpoints, while allowing all
tunnel endpoints to serve as backups in case the preferred
is not reachable. In this case, each dual router with IPv6
connectivity which is serving as potential tunnel endpoint is given
unique IPv4 address taken from a single IPv4 address block (where
IPv4 address block is assigned either to the
administering the IPv4-only region, or to the
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RFC 2185 Routing Aspects Of IPv6 Transition September 1997
administering the local part of the IPv6 backbone). In the
case that there are much less than 250 such dual routers serving
tunnel endpoints, we suggest using multiple IPv4 addresses
from a single 24-bit IPv4 address prefix for this purpose. Each
router then advertises two routes into the IPv4 region: A host
corresponding to the tunnel endpoint address specifically assigned
it, and also a standard (prefix) route to the associated IPv4
block. Each dual host in the IPv4-only region is configured with
tunnel endpoint address which corresponds to the preferred
endpoint for it to use. If the associated dual router is operating
then the packet will be delivered to it based upon the host
that it is advertising into the IPv4-only region. However, if
associated dual router is down, but some other dual router serving
a potential tunnel endpoint is operating, then the packet will
delivered to the nearest operating tunnel endpoint
3.3.3 Router to Host Automatic
In some cases the source host may have direct connectivity to one
more IPv6-capable routers, but the destination host might not
direct connectivity to any IPv6-capable router. In this case
provided that the destination host has an IPv4-compatible IPv
address, normal IPv6 forwarding may be used for part of the packet'
path, and router to host tunneling may be used to get the packet
an encapsulating dual router to the destination host
In this case, the hard part is the IPv6 routing required to
the IPv6 packet from the source host to the encapsulating router.
this to happen, the encapsulating router has to
reachability for the appropriate IPv4-compatible IPv6 addresses
the IPv6 routing region. With this approach, all IPv6
(including those with IPv4-compatible addresses) are routed
routes calculated from native IPv6 routing. This implies
encapsulating routers need to advertise into IPv6 routing
route entries corresponding to any IPv4-compatible IPv6
that belong to dual hosts which can be reached in an
IPv4-only region. This requires manual configuration of
encapsulating routers to control which routes are to be injected
IPv6 routing protocols. Nodes in the IPv6 routing region would
such a route to forward IPv6 packets along the routed path toward
router that injected (leaked) the route, at which point packets
encapsulated and forwarded to the destination host using normal IPv
routing
Depending upon the extent of the IPv4-only and dual routing regions
the leaking of routes may be relatively simple or may be
complex. For example, consider a dual Internet backbone,
via one or two dual routers to an IPv4-only stub routing domain.
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RFC 2185 Routing Aspects Of IPv6 Transition September 1997
this case, it is likely that there is already one summary
prefix which is being advertised into the Internet backbone in
to summarize IPv4 reachability to the stub domain. In such a case
the border routers would be configured to announce the IPv4
prefix into the IPv4 routing within the backbone, and also
the corresponding IPv4-compatible IPv6 address prefix into IPv
routing within the backbone
A more difficult case involves the border between a major
backbone which is IPv4-only, and a major Internet backbone
supports both IPv4 and IPv6. In this case, it requires that
(i) the entire IPv4 routing table be fed into IPv6 routing in
dual routing domain (implying a doubling of the size of the
tables in the dual domain); or (ii) Manual configuration is
to determine which of the addresses contained in the Internet
table include one or more IPv6-capable systems, and only
addresses be advertised into IPv6 routing in the dual domain
3.3.4 Example of How Automatic Tunnels May be
Clearly tunneling is useful only if communication can be achieved
both directions. However, different forms of tunneling may be used
each direction, depending upon the local environment, the form
address of the two hosts which are exchanging IPv6 packets, and
policies in use
Table 1 summarizes the form of tunneling that will result given
possible combination of host capabilities, and given one possible
of policy decisions. This table is derived directly from
requirements for automatic tunneling discussed above
The example in table 1 uses a specific set of policy decisions: It
assumed in table 1 that the source host will transmit a native IPv
where possible in preference over encapsulation. It is also
that where tunneling is needed, host to host tunneling will
preferred over host to router tunneling. Other combinations
therefore possible if other policies are used
Due to a specific policy choice, the default sending rules in [1]
not be followed
Note that IPv6-capable hosts which do not have any local IPv6
must be given an IPv4-compatible v6 address in order to make use
their IPv6 capabilities. Thus, there are no entries for IPv6-
hosts which have an incompatible IPv6 address and which also do
have any connectivity to any local IPv6 router. In fact, such
could communicate with other IPv6 hosts on the same local
without the use of a router. However, since this document focuses
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RFC 2185 Routing Aspects Of IPv6 Transition September 1997
routing and router implications of IPv6 transition,
communication between two hosts on the same local network without
intervening router is outside the scope of this document
Also, table 1 does not consider manually configured point-to-
tunnels. Such tunnels are treated as if they were normal point-to
point links. Thus any two IPv6-capable devices which have a
configured tunnel between them may be considered to be
connected
-----------------+------------------+--------------------------
Host A | Host B |
-----------------+------------------+--------------------------
v4-compat. addr. | v4-compat. addr. | host to host
no local v6 rtr. | no local v6 rtr. | in both
-----------------+------------------+--------------------------
v4-compat. addr. | v4-compat. addr. | A->B: host to host
no local v6 rtr. | local v6 rtr. | B->A: v6 forwarding
| | rtr->host
-----------------+------------------+--------------------------
v4-compat. addr. | incompat. addr. | A->B: host to rtr
no local v6 rtr. | local v6 rtr. | plus v6
| | B->A: v6 forwarding
| | rtr to host
-----------------+------------------+--------------------------
v4-compat. addr. | v4-compat. addr. | end to end native v
local v6 rtr. | local v6 rtr. | in both
-----------------+------------------+--------------------------
v4-compat. addr. | incompat. addr. | end to end native v
local v6 rtr. | local v6 rtr. | in both
-----------------+------------------+--------------------------
incompat. addr. | incompat. addr. | end to end native v
local v6 rtr. | local v6 rtr. | in both
-----------------+------------------+--------------------------
Table 1: Summary of Automatic Tunneling
3.3.5
Figure 2 illustrates an example network with two regions A and B
Region A is dual, meaning that the routers within region A
capable of forwarding both IPv4 and IPv6. Region B is IPv4-only
implying that the routers within region B are capable of routing
IPv4. The illustrated routers R1 through R4 are dual. The
routers r5 through r9 are IPv4-only. Also assume that hosts H
through H8 are dual. Thus H7 and H8 have been upgraded to be IPv6-
capable, even though they exist in a region in which the routers
not IPv6-capable. However, host h1 and h2 are IPv4-only
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RFC 2185 Routing Aspects Of IPv6 Transition September 1997
......................... .......................
. . . .
. h1 . . |-h2 .
. | . . | .
. H3---R1--------R2---------------r5----r9----+ .
. | | . . | |-H7 .
. | | . . | .
. | | . . | .
. H4---R3--------R4---------------r6----r8-----H8 .
. . . .
......................... .......................
Region A (Dual Routers) Region B (IPv4-only Rtrs
Figure 2: Example of Automatic
Consider a packet from h1 to H8. In this case, since h1 is IPv4-only
it will send an IPv4 packet. This packet will traverse regions A
B as a normal IPv4 packet for the entire path. Routing will
place using normal IPv4 routing methods, with no change from
operation of the current IPv4 Internet (modulo normal advances in
operation of IPv4, of course). Similarly, consider a return
from H8 to h1. Here again H8 will transmit an IPv4 packet, which
be forwarded as a normal IPv4 packet for the entire path
Consider a packet from H3 to H8. In this case, since H8 is in
IPv4-only routing domain, we can assume that H8 uses an IPv4-
compatible IPv6 address. Since both source and destination are IPv6-
capable, H3 may transmit an IPv6 packet destined to H8. The
will be forwarded as far as R2 (or R4) as an IPv6 packet
Router R2 (or R4) will then encapsulate the full IPv6 packet in
IPv4 header for delivery to H8. In this case it is necessary
routing of IPv6 within region A to be capable of delivering
packet correctly to R2 (or R4). As explained in section 3.3,
R2 and R4 may inject routes to IPv4-compatible IPv6 addresses
the IPv6 routing used within region A corresponding to the
which are available via IPv4 routing within region B
Consider a return packet from H8 to H3. Again, since both source
destination are IPv6-capable, a IPv6 packet may be transmitted by H8.
However, since H8 does not have any direct connectivity to an IPv6-
capable router, H8 must make use of an automatic tunnel. Which
of automatic tunnel will be used depends upon the type of
assigned to H3.
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RFC 2185 Routing Aspects Of IPv6 Transition September 1997
If H3 is assigned an IPv4-compatible address, then the
specified in section 3.3.1 will all be satisfied. In this case
H8 may encapsulate the full IPv6 packet in an IPv4 header using
source IPv4 address extracted from the IPv6 address of H8, and
a destination IPv4 address extracted from the IPv6 address of H3.
If H3 has an IPv6-only address, then it is not possible for H8
extract an IPv4 address to use as the destination tunnel address
the IPv6 address of H3. In this case H8 must use host to
tunneling, as specified in section 3.3.2. In this case one or both
R2 and R4 must have been configured with a tunnel endpoint IPv
address (R2 and R4 may use either the same address or
addresses for this purpose). R2 and/or R4 therefore
reachability to the tunnel endpoint address to r5 and r
(respectively), which advertise this reachability information
region B. Also, H8 must have been configured to know which
endpoint address to use for host to router tunneling. This
result in the IPv6 packet, encapsulated in an IPv4 header, to
transmitted as far as the border router R2 or R4. The border
will then strip off the IPv4 header, and forward the remaining IPv
packet as a normal IPv6 packet using the normal IPv6 routing used
region A
4. SECURITY
Use of tunneling may violate firewalls of underlying
infrastructure
No other security issues are discussed in this paper
5.
[1] Gilligan, B. and E. Nordmark. Transition Mechanisms for IPv
Hosts and Routers, Sun Microsystems, RFC 1933, April 1996.
6. AUTHORS'
Ross
Cascade Communications Co
5 Carlisle
Westford, MA 01886
email: rcallon@casc.
Callon & Haskin Informational [Page 12]
RFC 2185 Routing Aspects Of IPv6 Transition September 1997
Dimitry
Bay Networks, Inc
2 Federal
Billerica, MA 01821
email: dhaskin@baynetworks.
Callon & Haskin Informational [Page 13]
if you see any problems within the linking, don't worry be happy,
this is version 0.1 of the Relevance System and you gotta expect some crappy subroutines sometimes,
just be content we did not write this in Java, which would have made this "bigger and better" HAHAHHA.
RFC documents can be found at I.E.T.F.
Relevance System Copyright © 2002 Spectrum WorldResearch
other technical nosh by ServerMasters Corporation
collaboration of BobX
