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RFC 3021
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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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).
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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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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.
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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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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.
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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.
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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
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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
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RFC 3021 31-Bit Prefixes on IPv4 Links December 2000
Full Copyright Statement
Copyright (C) The Internet Society (2000). All Rights Reserved.
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Retana, et al. Standards Track [Page 10]