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IETF RFC 5579
Transmission of IPv4 Packets over Intra-Site Automatic Tunnel Addressing Protocol (ISATAP) Interfaces
Last modified on Thursday, February 18th, 2010
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Independent Submission F. Templin, Ed.
Request for Comments: 5579 Boeing Research & Technology
Category: Informational February 2010
ISSN: 2070-1721
Transmission of IPv4 Packets over
Intra-Site Automatic Tunnel Addressing Protocol (ISATAP) Interfaces
 Abstract
The Intra-Site Automatic Tunnel Addressing Protocol (ISATAP)
specifies a Non-Broadcast, Multiple Access (NBMA) interface type for
the transmission of IPv6 packets over IPv4 networks using automatic
IPv6-in-IPv4 encapsulation. The original specifications make no
provisions for the encapsulation and transmission of IPv4 packets,
however. This document specifies a method for transmitting IPv4
packets over ISATAP interfaces.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This is a contribution to the RFC Series, independently of any other
RFC stream. The RFC Editor has chosen to publish this document at
its discretion and makes no statement about its value for
implementation or deployment. Documents approved for publication by
the RFC Editor are not a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/RFC 5579.
IESG Note
This RFC is not a candidate for any level of Internet Standard. The
IETF disclaims any knowledge of the fitness of this RFC for any
purpose and in particular notes that the decision to publish is not
based on IETF review for such things as security, congestion control,
or inappropriate interaction with deployed protocols. The RFC Editor
has chosen to publish this document at its discretion. Readers of
this document should exercise caution in evaluating its value for
implementation and deployment. See RFC 3932 for more information.
Templin Informational PAGE 1 
RFC 5579 IPv4 Packets over ISATAP February 2010
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document.
Table of Contents
1. Introduction ....................................................3
2. Terminology .....................................................3
3. ISATAP Interface Model ..........................................3
4. ISATAP Interface MTU ............................................4
5. IPv6 Operation ..................................................4
6. IPv4 Operation ..................................................4
6.1. ISATAP IPv4 Address Configuration ..........................4
6.2. IPv4 Route Configuration ...................................5
6.3. ISATAP Interface Determination .............................5
6.4. Next-Hop Resolution ........................................5
6.5. Encapsulation and Transmission .............................6
6.6. IPv4 Multicast Mapping .....................................6
6.7. Recursive Encapsulation Avoidance ..........................7
7. Security Considerations .........................................7
8. Acknowledgements ................................................7
9. References ......................................................7
9.1. Normative References .......................................7
9.2. Informative References .....................................8
Appendix A. Encapsulation Avoidance ................................9
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RFC 5579 IPv4 Packets over ISATAP February 2010
1. Introduction
The Intra-Site Automatic Tunnel Addressing Protocol (ISATAP)
[RFC 5214] specifies a Non-Broadcast, Multiple Access (NBMA) interface
type for the transmission of IPv6 packets over IPv4 networks using
automatic IPv6-in-IPv4 encapsulation. ISATAP interfaces therefore
typically configure IPv6 addresses and prefixes, but they do not
configure IPv4 addresses and prefixes. In typical implementations
and deployments, an ISATAP interface therefore appears as an ordinary
interface configured for IPv6 operation but with a null IPv4
configuration. This places an unnecessary limitation on the ISATAP
domain of applicability.
ISATAP interfaces perform automatic IPv6-in-IPv4 encapsulation over a
virtual IPv6 overlay that spans a region within a connected IPv4
routing topology (i.e., a "site") comprising ordinary IPv4 routers.
ISATAP interfaces configure IPv6 link-local addresses that
encapsulate an IPv4 address assigned to an underlying IPv4 interface
within the IPv6 link-local prefix "fe80::/10", as specified in
Sections 6 and 7 of [RFC 5214]. ISATAP interfaces may additionally
configure IPv6 addresses from a non-link-local IPv6 prefix in exactly
the same fashion. As a result, [RFC 5214] extends the basic
transition mechanisms specified in [RFC 4213].
This document specifies mechanisms and operational practices that
enable automatic IPv4-in-IPv4 encapsulation over ISATAP interfaces in
the same manner as for IPv6-in-IPv4 encapsulation. As a result, this
document also extends the IPv4-in-IPv4 tunneling mechanisms specified
in [RFC 2003]. These mechanisms are useful in a wide variety of
enterprise network scenarios, e.g., as discussed in the RANGER
[RANGER] and VET [VET] proposals.
The following sections specify IPv4 operation over ISATAP interfaces.
A working knowledge of the IPv4 and IPv6 protocols ([RFC 791] and
[RFC 2460]), IPv4-in-IPv4 encapsulation [RFC 2003], and IPv6-in-IPv4
encapsulation ([RFC 4213] and [RFC 5214]) is assumed.
2. Terminology
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 [RFC 2119].
3. ISATAP Interface Model
ISATAP interfaces use automatic IPv6-in-IPv4 encapsulation to span a
region within a connected IPv4 routing topology (i.e., a "site") in a
single IPv6 hop. That is to say that the site comprises border nodes
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RFC 5579 IPv4 Packets over ISATAP February 2010
with ISATAP interfaces that forward IPv6-in-IPv4 packets across the
site in a single IPv6 hop, and ordinary IPv4 routers between the
border nodes that decrement the Time to Live (TTL) in the outer IPv4
header. Border nodes that configure ISATAP interfaces within the
same site therefore see each other as single-hop neighbors.
ISATAP interfaces are configured over one or more of the node's
underlying IPv4 interfaces connected to the site. These underlying
IPv4 interfaces configure site- or greater-scoped IPv4 addresses, and
the underlying IPv4 interfaces of two "neighboring" ISATAP interfaces
may be separated by many IPv4 hops within the site.
This specification simply extends the ISATAP interface model to also
support IPv4-in-IPv4 encapsulation. When IPv4-in-IPv4 encapsulation
is used, the ISATAP interface spans exactly the same underlying site
as for IPv6-in-IPv4 encapsulation.
4. ISATAP Interface MTU
ISATAP interface MTU considerations are exactly as specified in
Section 3.2 of [RFC 4213] and apply equally for both IPv6 and IPv4
operation.
5. IPv6 Operation
IPv6 operations over ISATAP interfaces are exactly as specified in
[RFC 5214].
6. IPv4 Operation
The following sections specify IPv4 operation over ISATAP interfaces:
6.1. ISATAP IPv4 Address Configuration
As for IPv6 operation, IPv4 operation requires that all ISATAP
interfaces connected to the same site configure a unique Layer 3 IPv4
address ("L3ADDR") taken from an IPv4 prefix for the site. L3ADDR is
used for next-hop determination, but it may also be used as the
source address for packets that originate from the ISATAP interface
itself.
When a unique "name" for the ISATAP site is required (e.g., to
distinguish it from other ISATAP sites), L3ADDR is taken from a
public IPv4 prefix. Otherwise, it may be taken from a link-local-
scoped IPv4 prefix (e.g., 169.254/16 [RFC 3927]).
Methods for ensuring L3ADDR uniqueness include dynamic allocation
using DHCP [RFC 2131], manual configuration, etc.
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RFC 5579 IPv4 Packets over ISATAP February 2010
6.2. IPv4 Route Configuration
As for any IPv4 interface, IPv4 Forwarding Information Base (FIB)
entries (i.e., IPv4 routes) are configured over ISATAP interfaces via
either administrative or dynamic mechanisms.
Next-hop addresses in FIB entries configured over an ISATAP interface
correspond to the L3ADDR assigned to the ISATAP interface of a
neighbor.
6.3. ISATAP Interface Determination
When the node's IPv4 layer has a packet to send, it performs an IPv4
FIB lookup to determine the outgoing ISATAP interface and the next-
hop L3ADDR. The node then checks the packet length against the MTU
configured on the ISATAP interface.
If the packet is no larger than the MTU, the node admits it into the
ISATAP interface without fragmentation. If the packet is larger than
the MTU, the node examines the "Don't Fragment (DF)" flag in the IPv4
header. If DF=1, it drops the packet and returns an ICMPv4
"fragmentation needed" message to the original source [RFC 1191];
otherwise, it fragments the packet using IPv4 fragmentation and
admits each fragment into the ISATAP interface.
6.4. Next-Hop Determination and Address Mapping
As for ISATAP for IPv6, ISATAP for IPv4 requires a means for
determining the L3ADDR of neighbors on the ISATAP interface that can
provide a next-hop toward the final destination. Next-hop
determination for default routes is through the Potential Router List
(PRL) discovery procedures specified in Section 8.3.2 of [RFC 5214].
Next-hop determination methods for more-specific routes include
forwarding initial packets via a default router until a redirect is
received, name service lookup (e.g., as described in [VET]), etc.
After a next-hop L3ADDR is discovered, the node admits IPv4
packets/fragments into the ISATAP interface and maps the next-hop
L3ADDR into a next-hop Layer 2 address ("L2ADDR"), which in reality
is the IPv4 address of an underlying interface of the ISATAP neighbor
that may be many IPv4 hops away.
Address mapping for IPv4 differs from the IPv6 version in that no
algorithmic mapping between L3ADDR and L2ADDR is possible. ISATAP
for IPv4 therefore requires an L3ADDR->L2ADDR address mapping method
that is coordinated on a per-site basis such that all nodes in the
site follow the same convention. Examples include name service
lookup (e.g., as described in [VET]), static mapping table lookup,
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RFC 5579 IPv4 Packets over ISATAP February 2010
etc.
The node next performs an IPv4 FIB lookup on the next-hop L2ADDR to
determine the correct underlying IPv4 interface. If the FIB lookup
fails, the node drops the packet and returns an ICMPv4 "Destination
Unreachable" message to the original source [RFC 792]; otherwise, it
encapsulates the packet and submits it to the IPv4 layer as described
below.
6.5. Encapsulation and Transmission
After performing the IPv4 FIB lookup on the next-hop L2ADDR, the node
encapsulates the packet as specified in [RFC 2003] with the IPv4
address of the underlying interface as the outer IPv4 source address
and the next-hop L2ADDR as the outer IPv4 destination address. The
node sets the DF flag in the outer IPv4 header according to Section
3.2 of [RFC 4213]. The node also sets the IP protocol field in the
outer IPv4 header to 4 (i.e., ip-protocol-4).
The node then submits the encapsulated packet to the IPv4 layer. The
IPv4 layer fragments the packet if necessary, then forwards each
fragment to the underlying IPv4 interface. The underlying IPv4
interface then performs address resolution on the outer IPv4
destination address (i.e., the next-hop L2ADDR) and submits the
packet for transmission on the underlying link layer.
6.6. IPv4 Multicast Mapping
In many aspects, ISATAP is simply a unicast-only derivative of
"6over4" [RFC 2529]. For various reasons, however, ISATAP has seen
practical wide-scale deployment while the 6over4 approach has been
silently carried forward through ongoing research efforts. This
specification extends the ISATAP interface model to support IPv4
multicast mapping in a manner that exactly parallels IPv6 multicast
mapping in 6over4 (see [RFC 2529], Section 6). Indeed, the approach
might more aptly be named "4over4" were it not for the fact that the
name "ISATAP" has already become ingrained in the widely published
terminology.
IPv4 multicast mapping is available only on ISATAP interfaces
configured over sites that support IPv4 multicast. For such sites,
an IPv4 packet sent on an ISATAP interface with a multicast
destination address DST MUST be encapsulated for transmission on an
underlying IPv4 interface to the IPv4 multicast address of
Organization-Local Scope using the mapping below. The mapped address
SHOULD be taken from the block 239.193.0.0/16, a different sub-block
of the Organization-Local Scope address block, or -- if none of those
are available -- from the expansion blocks defined in [RFC 2365].
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RFC 5579 IPv4 Packets over ISATAP February 2010
Note that when they are formed using the expansion blocks, they use
only a /16-sized block.
+-------+-------+-------+-------+
| 239 | OLS | DST2 | DST3 |
+-------+-------+-------+-------+
DST2, DST3 Last two bytes of IPv4 multicast address.
OLS From the configured Organization-Local
Scope address block. SHOULD be 193;
see [RFC 2365] for details.
Figure 1: ISATAPv4 Multicast Mapping
No new IANA registration procedures are required for the above.
6.7. Recursive Encapsulation Avoidance
The node must take care in managing its IPv4 FIB table entries in
order to avoid looping through recursive encapsulations.
7. Security Considerations
The security considerations specified in [RFC 2003] apply equally to
this document. The security considerations specified in ISATAP
[RFC 5214] and 6over4 [RFC 2529] also apply, with the exception that
ip-protocol-4 encapsulation is used instead of ip-protocol-41.
Updated tunnel security considerations are found in [SECURITY].
8. Acknowledgements
This work extends the ISATAP interface model, which has evolved
through the insights of many contributers over the course of many
decades. Special thanks to Brian Carpenter and Jari Arkko for their
helpful review input.
9. References
9.1. Normative References
[RFC 791] Postel, J., "Internet Protocol", STD 5, RFC 791, September
1981.
[RFC 792] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, September 1981.
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RFC 5579 IPv4 Packets over ISATAP February 2010
[RFC 1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990.
[RFC 2003] Perkins, C., "IP Encapsulation within IP", RFC 2003,
October 1996.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC 2529] Carpenter, B. and C. Jung, "Transmission of IPv6 over IPv4
Domains without Explicit Tunnels", RFC 2529, March 1999.
[RFC 3927] Cheshire, S., Aboba, B., and E. Guttman, "Dynamic
Configuration of IPv4 Link-Local Addresses", RFC 3927, May
2005.
[RFC 4213] Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
for IPv6 Hosts and Routers", RFC 4213, October 2005.
[RFC 5214] Templin, F., Gleeson, T., and D. Thaler, "Intra-Site
Automatic Tunnel Addressing Protocol (ISATAP)", RFC 5214,
March 2008.
9.2. Informative References
[SECURITY] Hoagland, J., Krishnan, S., and D. Thaler, "Security
Concerns With IP Tunneling", Work in Progress, October
2008.
[VET] Templin, F., "Virtual Enterprise Traversal (VET)", RFC
5558, February 2010.
[RANGER] Templin, F., "Routing and Addressing in Networks with
Global Enterprise Recursion (RANGER)", RFC 5720, February
2010.
[RFC 2131] Droms, R., "Dynamic Host Configuration Protocol", RFC
2131, March 1997.
[RFC 2365] Meyer, D., "Administratively Scoped IP Multicast", BCP 23,
RFC 2365, July 1998.
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RFC 5579 IPv4 Packets over ISATAP February 2010
Appendix A. Encapsulation Avoidance
In some instances, an ISATAP interface may be configured over a site
in which the L3ADDRs and L2ADDRs of all ISATAP neighbors are also
known to be routable within the underlying site. In that case, the
ISATAP interface MAY avoid encapsulation and submit the
unencapsulated packets directly to the IPv4 layer. Note however that
this approach does not provide for the use of indirection afforded
through encapsulation.
Author's Address
Fred L. Templin (editor)
Boeing Research & Technology
P.O. Box 3707 MC 7L-49
Seattle, WA 98124
USA
EMail: fltemplin@acm.org
Templin Informational PAGE 9
Transmission of IPv4 Packets over Intra-Site Automatic Tunnel Addressing Protocol (ISATAP) Interfaces
RFC TOTAL SIZE: 18998 bytes
PUBLICATION DATE: Thursday, February 18th, 2010
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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