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Network Working Group                                          A. RetanaRequest for Comments: 3021                                      R. WhiteCategory: Standards Track                                  Cisco Systems                                                               V. Fuller                                                     GTE Internetworking                                                            D. McPherson                                                          Amber Networks                                                           December 2000Using 31-Bit Prefixes on IPv4 Point-to-Point LinksStatus 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:   - More stringent address space allocation guidelines, enforced by the     IANA and the regional address assignment authorities [RFC2050].   - 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   - 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 20002. 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 inSection 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 (seeSection 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   (seeSection 2.1), eliminating the use of a directed broadcast to the   link (seeSection 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 inSection 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 20003. Recommendations   The considerations presented inSection 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.   Sections4.2.3.1 (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 tosection 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 masks4. 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 20008. 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,RFC1700, October 1994.   [RFC1812] Baker, F., "Requirements for IP Version 4 Routers",RFC1812, June 1995.   [RFC2050] Hubbard, K., Kosters, M., Conrad, D., Karrenberg, D. and J.             Postel, "Internet Registry IP Allocation Guidelines",BCP12,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.cgiRetana, et al.              Standards Track                     [Page 8]

RFC 3021             31-Bit Prefixes on IPv4 Links         December 20009. 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.comRetana, et al.              Standards Track                     [Page 9]

RFC 3021             31-Bit Prefixes on IPv4 Links         December 2000Full 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]