rfc3021.txt

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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:

   - 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].




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   - 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).






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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.







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      (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.






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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-



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