Internet Engineering Task Force Ross Callon
Internet Draft Dimitry Haskin
Expires May 1997 Bay Networks Inc.
November 1996
Routing Aspects Of IPv6 Transition
(draft-ietf-ngtrans-routing-aspects-02.txt)
Status of this memo
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Abstract
This internet draft gives an overview of the routing aspects
of the IPv6 transition. It is based on the protocols defined
in the document "Transition Mechanisms for IPv6 Hosts and
Routers" [1]. Readers should be familiar with the transition
mechanisms before reading this document.
The proposals contained in this document are based on the
work of the Ngtrans working group.
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1. TERMINOLOGY
This paper uses the following terminology:
node - a protocol module that implements IPv4 or IPv6.
router - a node that forwards packets not explicitly
addressed to itself.
host - any node that is not a router.
border router - a router that forwards packets across
routing domain boundaries.
link - a communication facility or medium over which
nodes can communicate at the link layer, i.e.,
the layer immediately below internet layer.
interface - a node's attachment to a link.
address - an network layer identifier for an interface or
a group of interfaces.
neighbors - nodes attached to the same link.
routing domain - a collection of routers which coordinate
routing knowledge using a single routing
protocol.
routing region (or just "region") - a collection of routers
interconnected by a single internet protocol
(e.g. IPv6) and coordinating their routing
knowledge using routing protocols from a single
internet protocol stack. A 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 of
reachable destinations that can be used for
packet forwarding decisions.
routing information - same as reachability information.
address prefix - the high-order bits in an address.
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routing prefix - address prefix that expresses destinations
which have addresses with the matching address
prefixes. It is used by routers to advertise what
systems they are capable of reaching.
route leaking - advertisement of network layer reachability
information across routing region boundaries.
2. ISSUES AND OUTLINE
This internet draft gives an overview of the routing aspects of
IPv4 to IPv6 transition. The approach outlined here is designed
to be compatible with the existing mechanisms for IPv6
transition [1].
During an extended IPv4-to-IPv6 transition period, IPv6-based
systems must coexist with the installed base of IPv4 systems. In
such a dual internetworking protocol environment, both IPv4 and
IPv6 routing infrastructure will be present. Initially, deployed
IPv6-capable domains might not be globally interconnected via
IPv6-capable internet infrastructure and therefore may need to
communicate across IPv4-only routing regions. In order to achieve
dynamic routing in such a mixed environment, there need to be
mechanisms to globally distribute IPv6 network layer reachability
information between dispersed IPv6 routing regions. The same
techniques can be used in later stages of IPv4-to-IPv6 transition
to route IPv4 packets between isolated IPv4-only routing region
over IPv6 infrastructure.
The IPng transition provides a dual-IP-layer transition, augmented
by use of encapsulation where necessary and appropriate. Routing
issues related to this transition include:
(1) Routing for IPv4 packets
(2) Routing for IPv6 packets
(2a) IPv6 packets with IPv6-native addresses
(2b) IPv6 packets with IPv4-compatible addresses
(3) Operation of manually configured static tunnels
(4) Operation of automatic encapsulation
(4a) Locating encapsulators
(4b) Ensuring that routing is consist with
encapsulation
Basic mechanisms required to accomplish these goals include:
(i) Dual-IP-layer Route Computation; (ii) Manual configuration of
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point-to-point tunnels; and (iii) Route leaking to support
automatic encapsulation.
The basic mechanism for routing of IPv4 and IPv6 involves
dual-IP-layer routing. This implies that routes are separately
calculated for IPv4 addresses and for IPv6 addressing. This is
discussed in more detail in section 3.1.
Tunnels (either IPv4 over IPv6, or IPv6 over IPv4) may be manually
configured. For example, in the early stages of transition this
may be used to allow two IPv6 domains to interact over an IPv4
infrastructure. Manually configured static tunnels are treated as
if they were a normal data link. This is discussed in more detail
in section 3.2.
Use of automatic encapsulation, where the IPv4 tunnel endpoint
address is determined from the IPv4 address embedded in the IPv4-
compatible destination address of IPv6 packet, requires
of routes between IPv4 routes and IPv6 routes for destinations using
IPv4-compatible addresses. For example, consider a packet which
starts off as an IPv6 packet, but then is encapsulated in an IPv4
packet in the middle of its path from source to destination. This
packet must locate an encapsulator at the correct part of its
path. Also, this packet has to follow a consistent route for the
entire path from source to destination. This is discussed in more
detail in section 3.3.
The mechanisms for tunneling IPv6 over IPv4 are defined in the
transition mechanisms specification [1].
3. MORE DETAIL OF BASIC APPROACHES
3.1 Basic Dual-IP-layer Operation
In the basic dual-IP-layer transition scheme, routers may
independently support IPv4 and IPv6 routing. Other parts of the
transition, such as DNS support, and selection by the source host
of which packet format to transmit (IPv4 or IPv6) are discussed
in [1]. Forwarding of IPv4 packets is based on routes learned
through running IPv4-specific routing protocols. Similarly,
forwarding of IPv6 packets (including IPv6-packets with
IPv4-compatible addresses) is based on routes learned through
running IPv6-specific routing protocols. This implies that
separate instances of routing protocols are used for IPv4 and for
IPv6 (although note that this could consist of two instances of
OSPF and/or two instances of RIP, since both OSPF and RIP are
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capable of supporting both IPv4 and IPv6 routing).
A minor enhancement would be to use an single instance of an
integrated routing protocol to support routing for both IPv4 and
IPv6. At the time that this is written there is no protocol which
has yet been enhanced to support this. This minor enhancement
does not change the basic dual-IP-layer nature of the transition.
For initial testing of IPv6 with IPv4-compatible addresses, it
may be 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 forwards IPv4 packets based on routes learned from IPv4
routing protocols. Also, it forwards IPv6 packets with an
IPv4-compatible destination address based on the route for the
associated IPv4 address. There are a couple of drawbacks with
this approach: (i) It does not specifically allow for routing of
IPv6 packets via IPv6-capable routers while avoiding and routing
around IPv4-only routers; (ii) It does not produce routes for
"non-compatible" IPv6 addresses. With this method the routing
protocol does not tell the router whether neighboring routers
are IPv6-compatible. However, neighbor discovery may be used to
determine this. Then if an IPv6 packet needs to be forwarded to
an IPv4-only router it can be encapsulated to the destination
host.
3.2 Manually Configured Static Tunnels
Tunneling techniques are already widely deployed for bridging
non-IP network layer protocols (e.g. Appletalk, CLNP, IPX) over
IPv4 routed infrastructure. IPv4 tunneling is an encapsulation of
arbitrary packets inside IPv4 datagrams that are forwarded over
IPv4 infrastructure between tunnel endpoints. For a tunneled
protocol, a tunnel appears as a single-hop link (i.e. routers
that establish a tunnel over a network layer infrastructure can
inter-operate over the tunnel as if it were a one-hop, point-to-
point link). Once a tunnel is established, routers at the tunnel
endpoints can establish routing adjacencies and exchange routing
information. Describing the protocols for performing
encapsulation is outside the scope of this paper (see [1]).
Static point-to-point tunnels may also be established between a
host and a router, or between two hosts. Again, each manually
configured point-to-point tunnel is treated as if it was a simple
point-to-point link.
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3.3 Automatic Tunnels
Automatic tunneling may be used when both the sending and
destination nodes are connected by IPv4 routing. In order for
automatic tunneling to work, both nodes must be assigned IPv4-
compatible IPv6 addresses. Automatic tunneling can be especially
useful where either source or destination hosts (or both) do not
have any adjacent IPv6-capable router. Note that by "adjacent
router", this includes routers which are logically adjacent by
virtue of a manually configured point-to-point tunnel (which is
treated as if it is a simple point-to-point link).
With automatic tunneling, the resulting IPv4 packet is forwarded
by IPv4 routers as a normal IPv4 packet, using IPv4 routes
learned from routing protocols. There are therefore no special
issues related to IPv4 routing in this case. There are however
routing issues relating to how IPv6 routing works in a manner
which is compatible with automatic tunneling, and how tunnel
endpoint addresses are selected during the encapsulation process.
Automatic tunneling is useful from a source host to the destination
host, from a source host to a router, and from a router to the
destination host. Mechanisms for automatic tunneling from a router
to another router are not currently defined.
3.3.1 Host to Host Automatic Tunneling
If both source and destination hosts make use of IPv4-compatible
IPv6 addresses, then it is possible for automatic tunneling to be
used for the entire path from the source host to the destination
host. In this case, the IPv6 packet is encapsulated in an IPv4
packet by the source host, and is forwarded by routers as an IPv4
packet all the way to the destination host. This allows initial
deployment of IPv6-capable hosts to be done prior to the update
of any routers.
A source host may make use of Host to Host automatic tunneling
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 the
same subnet.
If all of these requirements are true, then the source host may
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encapsulate the IPv6 packet in an IPv4 packet, using a source
IPv4 address which is extracted from the associated source IPv6
address, and using a destination IPv4 address which is extracted
from the associated destination IPv6 address.
Where host to host automatic tunneling is used, the packet is
forwarded as a normal IPv4 packet for its entire path, and is
decapsulated (i.e., the IPv4 header is removed) only by the
destination host.
If the source host has a neighboring IPv6 router, or if the
source and destination are on the same subnet, then automatic
tunneling need not be used. The packet can be sent in "raw
IPv6" form and forwarded via IPv6 routing.
3.3.2 Host to Router Configured Default Tunneling
In some cases "configured default" tunneling may be used to
encapsulate the IPv6 packet for transmission from the source
host to an IPv6-backbone. However, this requires that the
source host be configured with an IPv4 address to use for
tunneling to the backbone.
Configured default tunneling is particularly useful if the source
host does not know of any local IPv6-capable router (implying that
the packet cannot be forwarded as a normal IPv6 packet directly
over the link layer), and when the destination host does not have
an IPv4-compatible IPv6 address (implying that host to host
tunneling cannot be used).
Host to router configured default tunneling may optionally
also be used even when the host does know of a local IPv6
router. In this case it is a policy decision whether the
host prefers to send a native IPv6 packet to the IPv6-capable
router or prefers to send an encapsulated packet to the
configured tunnel endpoint.
Similary host to router default configured tunneling may be
used even when the destination address is an IPv4-compatible
IPv6 address. In this case for example a policy decision may
be made to prefer tunneling for part of the path and native
IPv6 for part of the path, or alternatively to use tunneling
for the entire path from source host to destination host.
A source host may make use of host to router configured default
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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 routers
- the source host has been configured with an IPv4 address of
an dual router which can serve as the tunnel endpoint.
If all of these requirements are true, then the source host may
encapsulate the IPv6 packet in an IPv4 packet, using a source
IPv4 address which is extracted from the associated source IPv6
address, and using a destination IPv4 address which corresponds to
the configured address of the dual router which is serving as
the tunnel endpoint.
When host to router configured default tunneling is used, the
packet is forwarded as a normal IPv4 packet from the source host
to the dual router serving as tunnel endpoint, is decapsulated by
the dual router, and is then forwarded as a normal IPv6 packet by
the tunnel endpoint.
3.3.2.1 Routing to the Endpoint for the Configured Default Tunnel
The dual router which is serving as the end point of the host to
router configured default tunnel must advertise reachability into
IPv4 routing sufficient to cause the encapsulated packet to be
forwarded to it.
The simplest approach is for a single IPv4 address to be assigned
for use as a tunnel endpoint. One or more dual routers, which
have connectivity to the IPv6 backbone and which are capable of
serving as tunnel endpoint, advertise a host route to this address
into IPv4 routing in the IPv4-only region. Each dual host in the
associated IPv4-only region is configured with the address of this
tunnel endpoint and selects a route to this address for forwarding
encapsulated packet to a tunnel end point (for example, the nearest
tunnel end point, based on whatever metric(s) the local routing
protocol is using).
Finally, in some cases there may be some reason for specific
hosts to prefer one of several tunnel endpoints, while allowing
all potential tunnel endpoints to serve as backups in case the
preferred endpoint is not reachable. In this case, each dual
router with IPv6 backbone connectivity which is serving as
potential tunnel endpoint is given a unique IPv4 address taken
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from a single IPv4 address block (where the IPv4 address block is
assigned either to the organization administering the IPv4-only
region, or to the organization administering the local part of
the IPv6 backbone). In the likely case that there are much less
than 250 such dual routers serving as tunnel endpoints, we
suggest using multiple IPv4 addresses selected from a single
24-bit IPv4 address prefix for this purpose. Each dual router
then advertises two routes into the IPv4 region: A host route
corresponding to the tunnel endpoint address specifically
assigned to it, and also a standard (prefix) route to the
associated IPv4 address block. Each dual host in the IPv4-only
region is configured with a tunnel endpoint address which
corresponds to the preferred tunnel endpoint for it to use. If
the associated dual router is operating, then the packet will be
delivered to it based upon the host route that it is advertising
into the IPv4-only region. However, if the associated dual router
is down, but some other dual router serving as a potential tunnel
endpoint is operating, then the packet will be delivered to the
nearest operating tunnel endpoint.
3.3.3 Router to Host Automatic Tunneling
In some cases the source host may have direct connectivity to one
or more IPv6-capable routers, but the destination host might not
have direct connectivity to any IPv6-capable router. In this
case, provided that the destination host has an IPv4-compatible
IPv6 address, normal IPv6 forwarding may be used for part of the
packet's path, and router to host tunneling may be used to get
the packet from an encapsulating dual router to the destination
host.
In this case, the hard part is the IPv6 routing required to
deliver the IPv6 packet from the source host to the encapsulating
router. For this to happen, the encapsulating router has to
advertise reachability for the appropriate IPv4-compatible IPv6
addresses into the IPv6 routing region. With this approach, all
IPv6 packets (including those with IPv4-compatible addresses) are
routed using routes calculated from native IPv6 routing. This
implies that encapsulating routers need to advertise into IPv6
routing specific route entries corresponding to any IPv4-compatible
IPv6 addresses that belong to dual hosts which can be reached in
an neighboring IPv4-only region. This requires manual configuration
of the encapsulating routers to control which routes are to be
injected into IPv6 routing protocols. Nodes in the IPv6 routing
region would use such a route to forward IPv6 packets along
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the routed path toward the router that injected (leaked) the route,
at which point packets are encapsulated and forwarded to the
destination host using normal IPv4 routing.
Depending upon the extent of the IPv4-only and dual routing
regions, the leaking of routes may be relatively simple or may be
more complex. For example, consider a dual Internet backbone,
connected via one or two dual routers to an IPv4-only stub
routing domain. In this case, it is likely that there is already
one summary address prefix which is being advertised into the
Internet backbone in order to summarize IPv4 reachability to the
stub domain. In such a case, the border routers would be configured
to announce the IPv4 address prefix into the IPv4 routing within the
backbone, and also announce the corresponding IPv4-compatible
IPv6 address prefix into IPv6 routing within the backbone.
A more difficult case involves the border between a major Internet
backbone which is IPv4-only, and a major Internet backbone which
supports both IPv4 and IPv6. In this case, it requires that either
(i) the entire IPv4 routing table be fed into IPv6 routing in the
dual routing domain (implying a doubling of the size of the routing
tables in the dual domain); or (ii) Manual configuration is required
to determine which of the addresses contained in the Internet routing
table include one or more IPv6-capable systems, and only these
addresses be advertised into IPv6 routing in the dual domain.
3.3.4 Example of How Automatic Tunnels May be Combined
Clearly tunneling is useful only if communication can be achieved
in both directions. However, different forms of tunneling may be
used in each direction, depending upon the local environment,
the form of address of the two hosts which are exchanging IPv6
packets, and the policies in use.
Table 1 summarizes the form of tunneling that will result given
each possible combination of host capabilities, and given one
possible set of policy decisions. This table is derived directly
from the requirements for automatic tunneling discussed above.
The example in table 1 uses a specific set of policy decisions:
It is assumed in table 1 that the source host will transmit a
native IPv6 where possible in preference over encapsulation. It
is also assumed that where tunneling is needed, host to host
tunneling will be preferred over host to router tunneling. Other
combinations are therefore possible if other policies are used.
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Note that IPv6-capable hosts which do not have any local IPv6
router must be given an IPv4-compatible v6 address in order to
make use of their IPv6 capabilities. Thus, there are no entries
for IPv6-capable hosts which have an incompatible IPv6 address
and which also do not have any connectivity to any local IPv6
router. In fact, such hosts could communicate with other IPv6
hosts on the same local network without the use of a router.
However, since this internet draft focuses on routing and router
implications of IPv6 transition, direct communication between two
hosts on the same local network without any intervening router is
outside the scope of this internet draft.
Also, table 1 does not consider manually configured point-to-point
tunnels. Such tunnels are treated as if they were normal point-
to-point links. Thus any two IPv6-capable devices which have a
manually configured tunnel between them may be considered to be
directly connected.
-----------------+------------------+--------------------------
Host A | Host B | Result
-----------------+------------------+--------------------------
v4-compat. addr. | v4-compat. addr. | host to host tunneling
no local v6 rtr. | no local v6 rtr. | in both directions
-----------------+------------------+--------------------------
v4-compat. addr. | v4-compat. addr. | A->B: host to host tunnel
no local v6 rtr. | local v6 rtr. | B->A: v6 forwarding plus
| | rtr->host tunnel
-----------------+------------------+--------------------------
v4-compat. addr. | incompat. addr. | A->B: host to rtr tunnel
no local v6 rtr. | local v6 rtr. | plus v6 forwarding
| | B->A: v6 forwarding plus
| | rtr to host tunnel
-----------------+------------------+--------------------------
v4-compat. addr. | v4-compat. addr. | end to end native v6
local v6 rtr. | local v6 rtr. | in both directions
-----------------+------------------+--------------------------
v4-compat. addr. | incompat. addr. | end to end native v6
local v6 rtr. | local v6 rtr. | in both directions
-----------------+------------------+--------------------------
incompat. addr. | incompat. addr. | end to end native v6
local v6 rtr. | local v6 rtr. | in both directions
-----------------+------------------+--------------------------
Table 1: Summary of Automatic Tunneling Combinations
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4. EXAMPLE
Figure 2 illustrates an example network with two regions A and B.
Region A is dual, meaning that the routers within region A are
capable of forwarding both IPv4 and IPv6. Region B is IPv4-only,
implying that the routers within region B are capable of routing
only IPv4. The illustrated routers R1 through R4 are dual. The
illustrated routers r5 through r9 are IPv4-only. Also assume
that hosts H3 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 are not IPv4-capable. However, host h1 and
h2 are IPv4-only.
......................... .......................
. . . .
. 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 Tunneling
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 and B as a normal IPv4 packet for the entire path.
Routing will take place using normal IPv4 routing methods, with
no change from the operation of the current IPv4 Internet (modulo
normal advances in the operation of IPv4, of course). Similarly,
consider a return packet from H8 to h1. Here again H8 will
transmit an IPv4 packet, which will 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 an
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
packet 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
an IPv4 header for delivery to H8. In this case it is necessary
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for routing of IPv6 within region A to be capable of delivering
this packet correctly to R2 (or R4). As explained in section 3.3,
routers R2 and R4 may inject routes to IPv4-compatible IPv6
addresses into the IPv6 routing used within region A corresponding
to the routes which are available via IPv4 routing within region B.
Consider a return packet from H8 to H3. Again, since both source
and 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 form of automatic tunnel will be used depends upon the type
of address assigned to H3.
If H3 is assigned an IPv4-compatible address, then the
requirements specified in section 3.3.1 will all be satisfied. In
this case host H8 may encapsulate the full IPv6 packet in an
IPv4 header using a source IPv4 address extracted from the IPv6
address of H8, and using 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 to
extract an IPv4 address to use as the destination tunnel address
from the IPv6 address of H3. In this case H8 must use host to
router tunneling, as specified in section 3.3.2. In this case one
or both of R2 and R4 must have been configured with a tunnel
endpoint IPv4 address (R2 and R4 may use either the same address
or different addresses for this purpose). R2 and/or R4 therefore
advertise reachability to the tunnel endpoint address to r5 and r6
(respectively), which advertise this reachability information into
region B. Also, H8 must have been configured to know which tunnel
endpoint address to use for host to router tunneling. This will
result in the IPv6 packet, encapsulated in an IPv4 header, to be
transmitted as far as the border router R2 or R4. The border router
will then strip off the IPv4 header, and forward the remaining IPv6
packet as a normal IPv6 packet using the normal IPv6 routing used
in region A.
5. INTERACTION BETWEEN IPv4 AND IPv6 INTER-DOMAIN ROUTING
As discussed above, if route leaking is employed then IPv4 routes
which are acquired by an inter-region dual router may need to be
injected into an IPv6 routing region. Such an inter-region dual
router may use BGP-4 for IPv4 inter-domain routing. Since the
Inter-Domain Routing Protocol (IDRP) has been adopted for IPv6
inter-domain routing, IDRP may need to be used to propagate the
IPv4 route into IPv6 routing.
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When routes learned with BGP are injected into IDRP, it would be
highly desirable to preserve routing attributes associated with
the routes in order to minimize the effect of the inter-region
route leaking onto the routing integrity. Since practically all
routing attributes that are carried in BGP-4 are also present in
IDRP, the mapping of the BGP-4 attributes to the IDRP attributes
is straightforward. However, since addresses and routing domain
identifiers that are carried by IDRP and BGP-4 are assigned from
different number spaces there is a need to ensure that 32-bit
IPv4 addresses and 16-bit routing domain identifiers (Autonomous
System numbers in IPv4 terminology) are uniquely represented in
the larger IPv6 number space.
Since IPv6 domain identifiers are allocated from the 128-bit IPv6
address space and IPv6 addresses with 96 leading zero bits are
reserved to represent addresses assigned from the IPv4 address
space, it will be natural to reserve IPv6 addresses (for use as
routing domain identifiers) with 112 leading zero bits to uniquely
represent IPv4 Autonomous System numbers.
6. SECURITY CONSIDERATIONS
Use of tunneling may violate firewalls of underlying routing
infrastructure.
No other security issues are discussed in this paper.
7. REFERENCES
[1] Transition Mechanisms for IPv6 Hosts and Routers,
Bob Gilligan and Erik Nordmark, Sun Microsystems,
RFC 1933, April 8, 1996.
8. AUTHORS' ADDRESSES
Ross Callon
Bay Networks Inc.
3 Federal Street
Billerica, MA 01821
email: rcallon@baynetworks.com
Dimitry Haskin
Bay Networks, Inc.
2 Federal Street
Billerica, MA 01821
email: dhaskin@baynetworks.com
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