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Internet Engineering Task Force (IETF)                         D. ThalerRequest for Comments: 6034                                     MicrosoftCategory: Standards Track                                   October 2010ISSN: 2070-1721Unicast-Prefix-Based IPv4 Multicast AddressesAbstract   This specification defines an extension to the multicast addressing   architecture of the IP Version 4 protocol.  The extension presented   in this document allows for unicast-prefix-based assignment of   multicast addresses.  By delegating multicast addresses at the same   time as unicast prefixes, network operators will be able to identify   their multicast addresses without needing to run an inter-domain   allocation protocol.Status of This Memo   This is an Internet Standards Track document.   This document is a product of the Internet Engineering Task Force   (IETF).  It represents the consensus of the IETF community.  It has   received public review and has been approved for publication by the   Internet Engineering Steering Group (IESG).  Further information on   Internet Standards is available inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc6034.Copyright Notice   Copyright (c) 2010 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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.  Code Components extracted from this document must   include Simplified BSD License text as described in Section 4.e of   the Trust Legal Provisions and are provided without warranty as   described in the Simplified BSD License.Thaler                       Standards Track                    [Page 1]

RFC 6034             Uni-Prefix-Based IPv4 Multicast        October 2010Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . .22.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . . .33.  Address Space . . . . . . . . . . . . . . . . . . . . . . . . .34.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . . .45.  Security Considerations . . . . . . . . . . . . . . . . . . . .46.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . .57.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . .58.  References  . . . . . . . . . . . . . . . . . . . . . . . . . .58.1.  Normative References  . . . . . . . . . . . . . . . . . . .58.2.  Informative References  . . . . . . . . . . . . . . . . . .51.  IntroductionRFC 3180 [RFC3180] defines an allocation mechanism (called "GLOP") in   233/8 whereby an Autonomous System (AS) number is embedded in the   middle 16 bits of an IPv4 multicast address, resulting in 256   multicast addresses per AS.  Advantages of this mechanism include the   ability to get multicast address space without an inter-domain   multicast address allocation protocol, and the ease of determining   the AS that was assigned the address for debugging and auditing   purposes.   Some disadvantages of GLOP include:   oRFC 4893 [RFC4893] expands the size of an AS number to 4 bytes,      and GLOP cannot work with 4-byte AS numbers.   o  When an AS covers multiple sites or organizations, administration      of the multicast address space within an AS must be handled by      other mechanisms, such as manual administrative effort or the      Multicast Address Dynamic Client Allocation Protocol (MADCAP)      [RFC2730].   o  During debugging, identifying the AS does not immediately identify      the correct organization when an AS covers multiple organizations.   o  Only 256 addresses are automatically available per AS, and      obtaining any more requires administrative effort.   More recently, a mechanism [RFC3306] has been developed for IPv6 that   provides a multicast range to every IPv6 subnet, which is at a much   finer granularity than an AS.  As a result, the first three   disadvantages above are avoided (and the last disadvantage does not   apply to IPv6 due to the extended size of the address space).Thaler                       Standards Track                    [Page 2]

RFC 6034             Uni-Prefix-Based IPv4 Multicast        October 2010   Another advantage of providing multicast space to a subnet, rather   than just to an entire AS, is that multicast address assignments   within the range need only be coordinated within the subnet.   This document specifies a mechanism similar to [RFC3306], whereby a   range of global IPv4 multicast address space is provided to each   organization that has unicast address space.  A resulting advantage   over GLOP is that the mechanisms in IPv4 and IPv6 become more   similar.   This document does not obsolete or updateRFC 3180, as the mechanism   described inRFC 3180 is still required for organizations with prefix   allocations more specific than /24.  Organizations usingRFC 3180   allocations may continue to do so.  In fact, it is conceivable that   an organization might use bothRFC 3180 allocations and the   allocation method described in this document.2.  Terminology   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 [RFC2119].3.  Address Space   A multicast address with the prefix 234/8 indicates that the address   is a Unicast-Based Multicast (UBM) address.  The remaining 24 bits   are used as follows:   Bits:  |   0 thru 7   |         8 thru N        |   N+1 thru 31   |          +-------+--------------------+-----------------------------+   Value: |     234      |      Unicast Prefix     |    Group ID     |          +-------+--------------------+-----------------------------+   For organizations with a /24 or shorter prefix, the unicast prefix of   the organization is appended to the common /8.  Any remaining bits   may be assigned by any mechanism the organization wishes.   For example, an organization that has a /16 prefix assigned might   choose to assign multicast addresses manually from the /24 multicast   prefix derived from the above method.  Alternatively, the   organization might choose to delegate the use of multicast addresses   to individual subnets that have a /24 or shorter unicast prefix, or   it might choose some other method.   Organizations with a prefix length longer than 24 do not receive any   multicast address space from this mechanism; in such cases, another   mechanism must be used.Thaler                       Standards Track                    [Page 3]

RFC 6034             Uni-Prefix-Based IPv4 Multicast        October 2010   Compared to GLOP, an AS will receive more address space via this   mechanism if it has more than a /16 for unicast space.  An AS will   receive less address space than it does from GLOP if it has less than   a /16.   The organization that is assigned a UBM address can be determined by   taking the multicast address, shifting it left by 8 bits, and   identifying who has been assigned the address space covering the   resulting unicast address.   The embedded unicast prefix MUST be a global unicast prefix (i.e., no   loopback, multicast, link-local, or private-use IP address space).   In addition, since global unicast addresses are not permanently   assigned, UBM addresses MUST NOT be hard-coded in applications.4.  Examples   The following are a few examples of the structure of unicast-prefix-   based multicast addresses.   o  Consider an organization that has been assigned the global unicast      address space 192.0.2.0/24.  This means that organization can use      the global multicast address 234.192.0.2 without coordinating with      any other entity.  Someone who sees this multicast address and      wants to find who is using it can mentally shift the address left      by 8 bits to get 192.0.2.0, and can then look up who has been      assigned unicast address space that includes that address.   o  Consider an organization that has been assigned a larger address      space, x.y.0.0/16.  This organization can use the global multicast      address space 234.x.y.0/24 without coordinating with any other      entity, and can assign addresses within this space by any      mechanism the organization wishes.  Someone who sees a multicast      address (say) 234.x.y.10 and wants to find who is using it can      mentally shift the address left by 8 bits to get x.y.10.0, and can      then look up who has been assigned unicast address space that      includes that address.5.  Security Considerations   The same well-known intra-domain security techniques can be applied   as with GLOP.  Furthermore, when dynamic allocation is used within a   prefix, the approach described here may have the effect of reduced   exposure to denial-of-service attacks, since the topological area   within which nodes compete for addresses within the same prefix is   reduced from an entire AS to only within an individual organization   or an even smaller area.Thaler                       Standards Track                    [Page 4]

RFC 6034             Uni-Prefix-Based IPv4 Multicast        October 20106.  IANA Considerations   IANA has assigned a /8 in the global IPv4 multicast address space for   this purpose.7.  Acknowledgments   This document was updated based on feedback from the MBoneD working   group.  In particular, Tim Chown, Toerless Eckert, Prashant Jhingran,   Peter Koch, John Linn, Dave Meyer, Pekka Savola, Greg Shepherd, and   Stig Venaas provided valuable suggestions on the text.8.  References8.1.  Normative References   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels",BCP 14,RFC 2119, March 1997.8.2.  Informative References   [RFC2730]  Hanna, S., Patel, B., and M. Shah, "Multicast Address              Dynamic Client Allocation Protocol (MADCAP)",RFC 2730,              December 1999.   [RFC3180]  Meyer, D. and P. Lothberg, "GLOP Addressing in 233/8",BCP 53,RFC 3180, September 2001.   [RFC3306]  Haberman, B. and D. Thaler, "Unicast-Prefix-based IPv6              Multicast Addresses",RFC 3306, August 2002.   [RFC4893]  Vohra, Q. and E. Chen, "BGP Support for Four-octet AS              Number Space",RFC 4893, May 2007.Author's Address   Dave Thaler   Microsoft Corporation   One Microsoft Way   Redmond, WA  98052   USA   Phone: +1 425 703 8835   EMail: dthaler@microsoft.comThaler                       Standards Track                    [Page 5]