Multi-Link Subnet Issues
RFC 4903
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RFC - Informational
(June 2007; No errata)
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Last updated |
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2015-10-14
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IAB
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plain text
pdf
html
bibtex
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IAB state
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(None)
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Consensus Boilerplate |
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Unknown
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RFC Editor Note |
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Network Working Group D. Thaler
Request for Comments: 4903 Internet Architecture Board
Category: Informational June 2007
Multi-Link Subnet Issues
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
There have been several proposals around the notion that a subnet may
span multiple links connected by routers. This memo documents the
issues and potential problems that have been raised with such an
approach.
Table of Contents
1. Introduction ....................................................2
2. Issues ..........................................................3
2.1. IP Model ...................................................3
2.2. TTL/Hop Limit Issues .......................................4
2.3. Link-scoped Multicast and Broadcast ........................6
2.4. Duplicate Address Detection Issues .........................7
3. Security Considerations .........................................8
4. Recommendations .................................................9
4.1. IP Link Model ..............................................9
4.2. IPv6 Address Assignment ...................................10
4.3. Duplicate Address Detection Optimizations .................12
5. Normative References ...........................................12
6. Informative References .........................................13
Thaler Informational [Page 1]
RFC 4903 Multi-Link Subnet Issues June 2007
1. Introduction
The original IPv4 address definition [RFC791] consisted of a Network
field, identifying a network number, and a Local Address field,
identifying a host within that network. As organizations grew to
want many links within their network, their choices were (from
[RFC950]) to:
1. Acquire a distinct Internet network number for each cable;
subnets are not used at all.
2. Use a single network number for the entire organization, but
assign host numbers without regard to which LAN a host is on
("transparent subnets").
3. Use a single network number, and partition the host address
space by assigning subnet numbers to the LANs ("explicit
subnets").
[RFC925] was a proposal for option 2 that defined a specific type of
Address Resolution Protocol (ARP) proxy behavior, where the
forwarding plane had the properties of decrementing the Time To Live
(TTL) to prevent loops when forwarding, not forwarding packets
destined to 255.255.255.255, and supporting subnet broadcast by
requiring that the ARP-based bridge maintain a list of recent
broadcast packets. This approach was never standardized, although
[RFC1027] later documented an implementation of a subset of [RFC925].
Instead, the IETF standardized option 3 with [RFC950], whereby hosts
were required to learn a subnet mask, and this became the IPv4 model.
Over the recent past, there have been several newer protocols
proposing to extend the notion of a subnet to be able to span
multiple links, similar to [RFC925].
Early versions of the IPv6 scoped address architecture [SCOPID]
proposed a subnet scope above the link scope, to allow for multi-link
subnets. This notion was rejected by the WG due to the issues
discussed in this memo, and as a result the final version [RFC4007]
has no such notion.
There was also a proposal to define multi-link subnets [MLSR] for
IPv6. However, this notion was abandoned by the IPv6 WG due to the
issues discussed in this memo, and that proposal was replaced by a
different mechanism that preserves the notion that a subnet spans
only one link [RFC4389].
Thaler Informational [Page 2]
RFC 4903 Multi-Link Subnet Issues June 2007
However, other WGs continued to allow for this concept even though it
had been rejected in the IPv6 WG. Mobile IPv6 [RFC3775] allows
tunnels to mobile nodes to use the same subnet as a home link, with
the Home Agent doing layer 3 forwarding between them.
The notion also arises in Mobile Ad-hoc NETworks (MANETs) with
proposals that an entire MANET is a subnet, with routers doing layer
3 forwarding within it.
The use of multi-link subnets has also been considered by other
working groups, including NetLMM, 16ng, and Autoconf, and by other
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