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rfc:rfc3021

Network Working Group A. Retana Request for Comments: 3021 R. White Category: Standards Track Cisco Systems

                                                             V. Fuller
                                                   GTE Internetworking
                                                          D. McPherson
                                                        Amber Networks
                                                         December 2000
         Using 31-Bit Prefixes on IPv4 Point-to-Point Links

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2000).  All Rights Reserved.

Abstract

 With ever-increasing pressure to conserve IP address space on the
 Internet, it makes sense to consider where relatively minor changes
 can be made to fielded practice to improve numbering efficiency.  One
 such change, proposed by this document, is to halve the amount of
 address space assigned to point-to-point links (common throughout the
 Internet infrastructure) by allowing the use of 31-bit subnet masks
 in a very limited way.

1. Introduction and Motivation

 The perceived problem of a lack of Internet addresses has driven a
 number of changes in address space usage and a number of different
 approaches to solving the problem:
  1. More stringent address space allocation guidelines, enforced by the

IANA and the regional address assignment authorities [RFC2050].

  1. Use of Network Address Translators (NATs), where a small number of

IANA-compliant addresses are shared by a larger pool of private,

   non-globally routed addresses topologically behind a NAT box
   [RFC1631].

Retana, et al. Standards Track [Page 1] RFC 3021 31-Bit Prefixes on IPv4 Links December 2000

  1. Deployment of a new Internet Protocol to increase the size of the

address space. One such protocol, IPv6 [RFC2460], has been through

   the IETF process but has yet to see production deployment.  Should
   it be, deployed, it will still face a many year transition period.
 Prior to the availability of a larger address space, it seems prudent
 to consider opportunities for making more efficient use of the
 existing address space.
 One such (small) opportunity is to change the way that point-to-point
 links are numbered.  One option, which is used today on some parts of
 the Internet, is to simply not number point-to-point links between
 routers.  While this practice may seem, at first, to handily resolve
 the problem, it causes a number of problems of its own, including the
 inability to consistently manage the unnumbered link or reach a
 router through it, difficulty in management and debugging of those
 links, and the lack of standardization [RFC1812].
 In current practice, numbered Internet subnets do not use longer than
 a 30-bit subnet mask (in most cases), which requires four addresses
 per link - two host addresses, one all-zeros network, and one all-
 ones broadcast.  This is unfortunate for point-to-point links, since
 they can only possibly have two identifying endpoints and don't
 support the notion of broadcast - any packet which is transmitted by
 one end of a link is always received by the other.
 A third option is to use host addresses on both ends of a point-to-
 point link.  This option provides the same address space savings as
 using a 31-bit subnet mask, but may only be used in links using PPP
 encapsulation [RFC1332].  The use of host addresses allows for the
 assignment of IP addresses belonging to different networks at each
 side of the link, causing link and network management not to be
 straight forward.
 This document is based on the idea that conserving IP addresses on
 point-to-point links (using longer than a 30-bit subnet mask) while
 maintaining manageability and standard interaction is possible.
 Existing documentation [RFC950] has already hinted at the possible
 use of a 1-bit wide host-number field.
 The savings in address space resulting from this change is easily
 seen--each point-to-point link in a large network would consume two
 addresses instead of four.  In a network with 500 point-to-point
 links, for example, this practice would amount to a savings of 1000
 addresses (the equivalent of four class C address spaces).

Retana, et al. Standards Track [Page 2] RFC 3021 31-Bit Prefixes on IPv4 Links December 2000

2. Considerations of 31-Bit Prefixes

 This section discusses the possible effects, on Internet routing and
 operations, of using 31-bit prefixes on point-to-point links.  The
 considerations made here are also reflected in Section 3.
 For the length of this document, an IP address will be interpreted
 as:
      <Network-number><Host-number>
 where the <Host-number> represents the unmasked portion of the
 address and it SHOULD be at least 1 bit wide.  The "-1" notation is
 used to mean that the field has all 1 bits.  For purposes of this
 discussion, the routing system is considered capable of classless, or
 CIDR [RFC1519], routing.

2.1. Addressing

 If a 31-bit subnet mask is assigned to a point-to-point link, it
 leaves the <Host-number> with only 1 bit.  Consequently, only two
 possible addresses may result:
      {<Network-number>, 0} and {<Network-number>, -1}
 These addresses have historically been associated with network and
 broadcast addresses (see Section 2.2).  In a point-to-point link with
 a 31-bit subnet mask, the two addresses above MUST be interpreted as
 host addresses.

2.2. Broadcast and Network Addresses

 There are several historically recognized broadcast addresses
 [RFC1812] on IP segments:
    (a) the directed broadcast
         {<Network-number>, -1}
         {<Network-number>, 0}
       The network address itself {<Network-number>, 0} is an
       obsolete form of directed broadcast, but it may still be used
       by older hosts.

Retana, et al. Standards Track [Page 3] RFC 3021 31-Bit Prefixes on IPv4 Links December 2000

    (b) the link local (or limited) broadcast
         {-1, -1}
         {0, 0}
       The {0, 0} form of a limited broadcast is obsolete, but may
       still be present in a network.
 Using a 31-bit prefix length leaves only two numbering possibilities
 (see Section 2.1), eliminating the use of a directed broadcast to the
 link (see Section 2.2.1).  The limited broadcast MUST be used for all
 broadcast traffic on a point-to-point link with a 31-bit subnet mask
 assigned to it.
 The <Network-number> is assigned by the network administrator as
 unique to the local routing domain.  The decision as to whether a
 destination IP address should be a directed broadcast or not is made
 by the router directly connected to the destination segment.  Current
 forwarding schemes and algorithms are not affected in remote routers.
 The intent of this document is to discuss the applicability and
 operation of 31-bit prefixes on point-to-point links.  The effects
 (if any) on other types of interfaces are not considered.

2.2.1. Directed Broadcast

 When a device wants to reach all the hosts on a given (remote, rather
 than directly connected) subnet, it may set the packet's destination
 address to the link's subnet broadcast address.  This operation is
 not possible for point-to-point links with a 31-bit prefix.
 As discussed in Section 6, the loss of functionality of a directed
 broadcast may actually be seen as a beneficial side effect, as it
 slightly enhances the network's resistance to a certain class of DoS
 Attacks [RFC2644, SMURF].

2.3. Impact on Current Routing Protocols

 Networks with 31-bit prefixes have no impact on current routing
 protocols.  Most of the currently deployed routing protocols have
 been designed to provide classless routing.  Furthermore, the
 communication between peers is done using multicast, limited
 broadcast or unicast addresses (all on the local network), none of
 which are affected with the use of 31-bit subnet masks.

Retana, et al. Standards Track [Page 4] RFC 3021 31-Bit Prefixes on IPv4 Links December 2000

3. Recommendations

 The considerations presented in Section 2 affect other published
 work.  This section details the updates made to other documents.

3.1. "Requirements for Internet Hosts – Communication Layers" [RFC1122]

 Section 3.2.1.3 (e) is replaced with:
    (e)  { <Network-number>, <Subnet-number>, -1 }
       Directed broadcast to the specified subnet.  It MUST NOT be
       used as a source address, except when the originator is one of
       the endpoints of a point-to-point link with a 31-bit mask.
 A new section (numbered 3.2.1.3 (h)) is added:
    (h)  { <Network-number>, <Subnet-number>, 0 }
       Subnetwork number.  SHOULD NOT be used as a source address,
       except when the originator is one of the endpoints of a point-
       to-point link with a 31-bit mask.  For other types of links, a
       packet with such a destination SHOULD be silently discarded.
       If these packets are not silently discarded, they MUST be
 treated
       as IP broadcasts [RFC1812].

3.2. "Assigned Numbers" [RFC1700]

 Sub-section (e) of the "Special Addresses" section in the
 "Introduction" is replaced with:
    (e)   {<Network-number>, <Subnet-number>, -1}
       Directed broadcast to specified subnet.  Can only be used as a
       destination address.  However, in the case where the originator
       is one of the endpoints of a point-to-point link with a 31-bit
       mask, it can also be used as a source address.

3.3. "Requirements for IP Version 4 Routers" [RFC1812]

 Section 4.2.2.11 (d) is replaced with:
    (d) { <Network-prefix>, -1 }
       Directed Broadcast - a broadcast directed to the specified
       network prefix.  It MUST NOT be used as a source address,
       except when the originator is one of the endpoints of a point-

Retana, et al. Standards Track [Page 5] RFC 3021 31-Bit Prefixes on IPv4 Links December 2000

       to-point link with a 31-bit mask.  A router MAY originate
       Network Directed Broadcast packets.  A router MAY have a
       configuration option to allow it to receive directed broadcast
       packets, however this option MUST be disabled by default, and
       thus the router MUST NOT receive Network Directed Broadcast
       packets unless specifically configured by the end user.
 The text above includes the update made by [RFC2644].
 A new section (numbered 4.2.2.11 (f)) is added:
    (f)  { <Network-number>, <Subnet-number>, 0 }
       Subnetwork number.  SHOULD NOT be used as a source address,
       except when the originator is one of the endpoints of a point-
       to-point link with a 31-bit mask.  For other types of links, a
       packet with such a destination SHOULD be silently discarded.
       If these packets are not silently discarded, they MUST be
       treated as IP broadcasts.
 Sections 4.2.3.1 (1), (2) and (4) are replaced with:
    (1) MUST treat as IP broadcasts packets addressed to
    255.255.255.255 or { <Network-prefix>, -1 }.
    In a point-to-point link with a 31-bit mask, a packet addressed to
    { <Network-prefix>, -1 } corresponds to one of the endpoints of
    such link, it MUST be treated as directed to the router on which
    the address is applied.
    (2) SHOULD silently discard on receipt (i.e., do not even deliver
    to applications in the router) any packet addressed to 0.0.0.0 or
    { <Network-prefix>, 0 }.  If these packets are not silently
    discarded, they MUST be treated as IP broadcasts (see Section
    [5.3.5]).  There MAY be a configuration option to allow receipt of
    these packets.  This option SHOULD default to discarding them.
    In a point-to-point link with a 31-bit mask, a packet addressed to
    { <Network-prefix>, 0 } corresponds to one of the endpoints of
    such link, it MUST be treated as directed to the router on which
    the address is applied.
    (4) SHOULD NOT originate datagrams addressed to 0.0.0.0 or {
    <Network-prefix>, 0 }.  There MAY be a configuration option to
    allow generation of these packets (instead of using the relevant
    1s format broadcast).  This option SHOULD default to not
    generating them.

Retana, et al. Standards Track [Page 6] RFC 3021 31-Bit Prefixes on IPv4 Links December 2000

    In a point-to-point link with a 31-bit mask, the configuration of
    such a mask SHOULD allow for the generation of datagrams addressed
    to { <Network-prefix>, 0 }.
 The following text is added to section 4.3.3.9:
    The 255.255.255.255 IP broadcast address MUST be used for
    broadcast Address Mask Replies in point-to-point links with 31-bit
    subnet masks

4. Operational Experience

 The recommendations presented in this document have been implemented
 by several router vendors in beta code.  The implementation has been
 tested by at least three ISPs with positive results (i.e., no
 problems have been found).  Among the routing protocols tested
 successfully are OSPF, IS-IS, BGP and EIGRP.
 It is expected that the implementation will be officially released
 within the next few months and that other vendors will adopt it.

5. Deployment Considerations

 The intent of this document is to discuss the applicability and
 operation of 31-bit prefixes on point-to-point links.  The effects
 (if any) on other types of interfaces are not considered.  Note that
 a point-to-point link in which only one end supports the use of 31-
 bit prefixes may not operate correctly.

6. Security Considerations

 In the light of various denial of service (DoS) attacks on various
 networks within the Internet, security has become a major concern.
 The use of 31-bit subnet masks within the core of the Internet will
 reduce the number of physical links against which a DoS attack
 relying on packet replication through the use of directed broadcasts
 can be launched [RFC2644, SMURF].
 Overall, implementation of this document recommendation will improve
 the Internet's resilience to these types of DoS attacks.

7. Acknowledgements

 The authors of this document do not make any claims on the
 originality of the ideas described.  Among other people, we would
 like to acknowledge Alex Zinin for his comments, and the many people
 who have tested 31 bit subnet masks in their labs and networks.

Retana, et al. Standards Track [Page 7] RFC 3021 31-Bit Prefixes on IPv4 Links December 2000

8. References

 [RFC950]  Mogul, J. and J. Postel, "Internet Standard Subnetting
           Procedure", STD 5, RFC 950, August 1985.
 [RFC1122] Braden, R., "Requirements for Internet Hosts --
           Communication Layers", STD 3, RFC 1122, October 1989.
 [RFC1332] McGregor, G., "The PPP Internet Protocol Control Protocol
           (IPCP)", RFC 1332, May 1992.
 [RFC1519] Fuller, V., Li, T., Yu, J. and K. Varadhan, "Classless
           Inter-Domain Routing (CIDR): an Address Assignment and
           Aggregation Strategy", RFC 1519, September 1993.
 [RFC1631] Egevang, K. and P. Francis, "The IP Network Address
           Translator (NAT)", RFC 1631, May 1994.
 [RFC1700] Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC
           1700, October 1994.
 [RFC1812] Baker, F., "Requirements for IP Version 4 Routers", RFC
           1812, June 1995.
 [RFC2050] Hubbard, K., Kosters, M., Conrad, D., Karrenberg, D. and J.
           Postel, "Internet Registry IP Allocation Guidelines", BCP
           12, RFC 2050, November 1996.
 [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
           (IPv6) Specification", RFC 2460, December 1998.
 [RFC2644] Senie, D., "Changing the Default for Directed Broadcasts in
           Routers", BCP 34, RFC 2644, August 1999.
 [SMURF]   Huegen, C., "The Latest in Denial of Service Attacks:
           'Smurfing':  Description and Information to Minimize
           Effects", URL:
           http://users.quadrunner.com/chuegen/smurf.cgi

Retana, et al. Standards Track [Page 8] RFC 3021 31-Bit Prefixes on IPv4 Links December 2000

9. Authors' Addresses

 Alvaro Retana
 Cisco Systems, Inc.
 7025 Kit Creek Rd.
 Research Triangle Park, NC 27709
 EMail: aretana@cisco.com
 Russ White
 Cisco Systems, Inc.
 7025 Kit Creek Rd.
 Research Triangle Park, NC 27709
 EMail: riw@cisco.com
 Vince Fuller
 GTE Internetworking
 3801 E. Bayshore Rd.
 Palo Alto, CA, 94303
 EMail: vaf@valinor.barrnet.net
 Danny McPherson
 Amber Networks
 2465 Augustine Drive
 Santa Clara, CA  95054
 EMail: danny@ambernetworks.com

Retana, et al. Standards Track [Page 9] RFC 3021 31-Bit Prefixes on IPv4 Links December 2000

Full Copyright Statement

 Copyright (C) The Internet Society (2000).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Retana, et al. Standards Track [Page 10]

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