v6ops WG S. Gundavelli
Internet-Draft M. Townsley
Intended status: Standards Track O. Troan
Expires: January 26, 2011 W. Dec
Cisco
July 25, 2010
Unicast Transmission of IPv6 Multicast Messages on Link-layer
draft-gundavelli-v6ops-l2-unicast-01.txt
Abstract
When transmitting an IPv6 packet to a multicast group, the
destination address in the link-layer header is typically set to the
corresponding mapped address of the destination address from the IP
header. However, it is not mandatory that the destination address in
the link-layer header is always a mapped multicast equivalent of its
IP destination address. There are various deployment scenarios where
there a need to transmit an IPv6 multicast message as an unicast
message on the link-layer. Unfortunately, the IPv6 specifications do
not clearly state this. This document explicitly clarifies this
point and makes such packet construct and transmission legal and
valid.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 26, 2011.
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
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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
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Sending and Receiving IPv6 Multicast Packets . . . . . . . . . 6
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1. Normative References . . . . . . . . . . . . . . . . . . . 10
7.2. Informative References . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
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1. Introduction
This document is about a clarification to the construction and
processing rules of IPv6 multicast messages. This clarification
makes it valid for an IPv6 receiver node to consider a received IPv6
multicast message with a multicast destination address in the IPv6
header, but containing a unicast destination address in the link-
layer header, to be valid withstanding all other validity
considerations specified in the IPv6 standards specifications.
Consequentially, it is also legal for an IPv6 sender node to transmit
an IPv6 multicast message as a unicast message on the link-layer.
This change follows the principles of protocol layer design more
tightly.
There are various deployment scenarios where there is a need to
transmit an IPv6 multicast message as an unicast message on the link-
layer. For example, an 802.11 wireless access point may be hosting
multiple IPv6 subnets/VLAN's and it would need the ability to
selectively advertise hosted IPv6 prefixes on a per-node basis. Such
segregation can only be possible by ensuring the Router
Advertisements received by any IPv6 node includes only those prefixes
that are associated with their respective layer-3 subnet. This
essentially requires the ability to transmit IPv6 multicast messages
as unicast messages on the link-layer. Another such example where
this semantic is needed is in ISATAP [RFC5214] for sending a unicast
Router Advertisement message on ISTAP interfaces. However, it is
ambiguous from the protocol specification perspective, on the
legality of such transmission and any discussions on this subject
always resulted in differing opinions. Therefore, it is the intent
of this document to make the specification clear on this aspect.
The function of the link-layer is purely for transmitting the frame
to a peer or to a set of peers on a given media. A received
multicast message may have been transmitted as a unicast message on
the link-layer and so the destination address in the link-layer will
be a unicast address, while the destination address in the IP header
can be a multicast address. It is inconsequential for the network
layer protocols to go across the layers and check the semantics of
message delivery in the link-layer header. Any such check is a
violation of the principles of protocol layering and does not serve
any purpose. Unfortunately, [RFC4861] or [RFC2464] does not
explicitly state this. However, we have verified this on many open
source and commercial IPv6 implementations on the behavior of the
existing IPv6 stacks, firewalls and we could not find any
implementation that drops IPv6 packets sent to a multicast
destination address in the IP header, but with a unicast destination
address in the link-layer header. Case and Point:
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o Microsoft Windows Vista
o Linux Operating System
o Cisco IOS Operating System
o BSD Variants based on IPv6 KAME implementation
As a result of this analysis, it appears to be quite safe to
explicitly state that such message construct is valid, so future
implementations do not drop packets based on these checks. Section
3.0 of this document defines the additional normative considerations
for IPv6 sender and receiver nodes for allowing this mode of packet
transmission.
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2. Conventions
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 [RFC2119].
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3. Sending and Receiving IPv6 Multicast Packets
The following additional considerations MUST be applied by all IPv6
nodes when sending and receiving IPv6 multicast messages.
o An IPv6 receiver node SHOULD NOT drop a received IPv6 multicast
message containing a multicast destination address in the IPv6
header, but with a unicast destination address in the link-layer
header, withstanding all other validity considerations as
specified in the relevant IPv6 standards specifications.
o An IPv6 sender node MAY choose to transmit an IPv6 multicast
message as a link-layer unicast message. In this case, the
destination address in the IPv6 header will be a multicast group
address, but the destination address in the link-layer header will
be an unicast address. It is up to to the system architecture as
when to transmit an IPv6 multicast message as an link-layer
unicast message.
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4. IANA Considerations
This specification does not require any IANA actions.
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5. Security Considerations
This document is about a clarification to the construction and
processing rules of IPv6 multicast messages. This clarification
makes it valid for an IPv6 receiver node to consider a received IPv6
multicast message with a multicast destination address in the IPv6
header, but containing an unicast destination address in the link-
layer header, to be valid withstanding all other validity
considerations specified in the IPv6 standards specifications. This
change follows the principles of protocol layer design more tightly
and does not introduce any security vulnerabilities.
Network firewalls and Deep Packet inspection tools that perform any
such improper checks matching the destination address types in IP
header and link-layers have to modified to allow such packet
transmission. However, the authors of this document could not find a
single such implementation that drops IP packets based on this check.
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6. Acknowledgements
The authors would like to acknowledge Stig Venaas, Fred Baker, Hemant
Singh, Olaf Bonness, Suresh Krishnan, Behcet Sarikaya, Eric Levy,
Pascal Thubert and Eric Voit for all the discussions on this topic.
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7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.
7.2. Informative References
[RFC2464] Crawford, M., "Transmission of IPv6 Packets over Ethernet
Networks", RFC 2464, December 1998.
[RFC5214] Templin, F., Gleeson, T., and D. Thaler, "Intra-Site
Automatic Tunnel Addressing Protocol (ISATAP)", RFC 5214,
March 2008.
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Authors' Addresses
Sri Gundavelli
Cisco
170 West Tasman Drive
San Jose, CA 95134
USA
Email: sgundave@cisco.com
Mark Townsley
Cisco
L'Atlantis, 11, Rue Camille Desmoulins
ISSY LES MOULINEAUX, ILE DE FRANCE 92782
France
Email: townsley@cisco.com
Ole Troan
Cisco
Skoyen Atrium, Drammensveien 145A
Oslo, N-0277
Norway
Email: otroan@cisco.com
Wojciech Dec
Cisco
Haarlerbergweg 13-19
Amsterdam, Noord-Holland 1101 CH
Netherlands
Email: wdec@cisco.com
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