6MAN B. Carpenter, Ed.
Internet-Draft Univ. of Auckland
Intended status: Informational T. Chown
Expires: August 10, 2014 Univ. of Southampton
F. Gont
SI6 Networks / UTN-FRH
S. Jiang
Huawei Technologies Co., Ltd
A. Petrescu
CEA, LIST
A. Yourtchenko
cisco
February 6, 2014
Analysis of the 64-bit Boundary in IPv6 Addressing
draft-carpenter-6man-why64-01
Abstract
The IPv6 unicast addressing format includes a separation between the
prefix used to route packets to a subnet and the interface identifier
used to specify a given interface connected to that subnet.
Historically the interface identifier has been defined as 64 bits
long, leaving 64 bits for the prefix. This document discusses the
reasons for this fixed boundary and the issues involved in treating
it as a variable boundary.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 10, 2014.
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Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
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to this document. Code Components extracted from this document must
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Scenarios for prefixes longer than /64 . . . . . . . . . . . 3
2.1. Insufficient address space delegated . . . . . . . . . . 4
2.2. Concerns over ND cache exhaustion . . . . . . . . . . . . 4
3. Interaction with IPv6 specifications . . . . . . . . . . . . 5
4. Possible areas of breakage . . . . . . . . . . . . . . . . . 7
5. Experimental observations . . . . . . . . . . . . . . . . . . 8
5.1. Survey of the processing of Neighbor Discovery options
with prefixes other than /64 . . . . . . . . . . . . . . 8
5.2. Other Observations . . . . . . . . . . . . . . . . . . . 10
6. Privacy issues . . . . . . . . . . . . . . . . . . . . . . . 11
7. Implementation and deployment issues . . . . . . . . . . . . 11
8. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . 13
9. Security Considerations . . . . . . . . . . . . . . . . . . . 13
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 14
12. Change log [RFC Editor: Please remove] . . . . . . . . . . . 14
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
13.1. Normative References . . . . . . . . . . . . . . . . . . 14
13.2. Informative References . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19
1. Introduction
IPv6 addresses were originally chosen to be 128 bits long to provide
flexibility and new possibilities, rather than simply relieving the
IPv4 address shortage by doubling the address size to 64 bits. The
notion of a 64-bit boundary in the address was introduced after the
initial design was done. There were two motivations for introducing
it. One was the original "8+8" proposal [DRAFT-odell] that
eventually led to ILNP [RFC6741], which required a fixed point for
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the split between local and wide-area parts of the address. The
other was the expectation that EUI-64 MAC addresses would become
widespread in place of 48-bit addresses, coupled with the plan at
that time that auto-configured addresses would normally be based on
interface identifiers derived from MAC addresses.
The IPv6 addressing architecture [RFC4291] specifies that a unicast
address is divided into n bits of subnet prefix followed by (128-n)
bits of interface identifier (IID). Since IPv6 routing is entirely
based on variable length subnet masks, there is no architectural
assumption that n has any particular fixed value. However, RFC 4291
also describes a method of forming interface identifiers from IEEE
EUI-64 hardware addresses [IEEE802] and this does specify that such
interface identifiers are 64 bits long. Various other methods of
forming interface identifiers also specify a length of 64 bits. This
has therefore become the de facto length of almost all IPv6 interface
identifiers. One exception is documented in [RFC6164], which
standardises 127-bit prefixes for inter-router links.
Recently it has been clarified that the bits in an IPv6 interface
identifier have no particular meaning and should be treated as opaque
values [I-D.ietf-6man-ug]. Therefore, there are no bit positions in
the currently used 64 bits that need to be preserved. The addressing
architecture as modified by [I-D.ietf-6man-ug] now states that "For
all unicast addresses, except those that start with the binary value
000, Interface IDs are required to be 64 bits long. If derived from
an IEEE MAC-layer address, they must be constructed in Modified
EUI-64 format."
The question is often asked why the boundary is set rigidly at /64.
This limits the length of a routing prefix to 64 bits, whereas
architecturally, and from the point of view of routing protocols, it
could be anything (in theory) between /1 and /128 inclusive. Here,
we only discuss the question of a shorter IID, allowing a longer
routing prefix.
The purpose of this document is to analyse the issues around this
question. We make no proposal for change, but we do analyse the
possible effects of a change.
2. Scenarios for prefixes longer than /64
In this section we describe existing scenarios where prefixes longer
than /64 have been used or proposed.
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2.1. Insufficient address space delegated
A site may not be delegated a sufficiently large prefix from which to
allocate a /64 prefix to all of its internal subnets. In this case
the site may either determine that it does not have enough address
space to number all its network elements and thus, at the very best,
be only partially operational, or it may choose to use internal
prefixes longer than /64 to allow multiple subnets and the hosts
within them to be configured with addresses.
In this case, the site might choose, for example, to use a /80 per
subnet, in combination with hosts using either manually configured
addressing or DHCPv6.
Scenarios that have been suggested where an insufficient prefix might
be delegated include home or small office networks, vehicles,
building services and transportation services (road signs, etc.). It
should be noted that the homenet architecture text
[I-D.ietf-homenet-arch] states that a CPE should consider the lack of
sufficient address space to be an error condition, rather than using
prefixes longer than /64 internally.
Another scenario occasionally suggested is one where the Internet
address registries actually begin to run out of IPv6 prefix space,
such that operators can no longer assign reasonable prefixes to users
in accordance with [RFC6177]. We mention this scenario here for
completeness, and we briefly analyze it in Section 7.
2.2. Concerns over ND cache exhaustion
A site may be concerned that it is open to neighbour discovery (ND)
cache exhaustion attacks, whereby an attacker sends a large number of
messages in rapid succession to a series of (most likely inactive)
host addresses within a specific subnet, in an attempt to fill a
router's ND cache with ND requests pending completion, in so doing
denying correct operation to active devices on the network.
An example would be to use a /120 prefix, limiting the number of
addresses in the subnet to be similar to an IPv4 /24 prefix, which
should not cause any concerns for ND cache exhaustion. Note that the
prefix does need to be quite long for this scenario to be valid. The
number of theoretically possible ND cache slots on the segment needs
to be of the same order of magnitude as the actual number of hosts.
Thus small increases from the /64 prefix length do not have a
noticeable impact: even 2^32 potential entries, a factor of two
billion decrease compared to 2^64, is still more than enough to
exhaust the memory on current routers.
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As in the previous scenario, hosts would likely be manually
configured with addresses, or use DHCPv6.
It should be noted that several other mitigations of the ND cache
attack are described in [RFC6583], and that limiting the size of the
cache and the number of incomplete entries allowed would also defeat
the attack.
3. Interaction with IPv6 specifications
The precise 64-bit length of the Interface ID is widely mentioned in
numerous RFCs describing various aspects of IPv6. It is not
straightforward to distinguish cases where this has normative impact
or affects interoperability. This section aims to identify
specifications that contain an explicit reference to the 64-bit size.
Regardless of implementation issues, the RFCs themselves would all
need to be updated if the 64-bit rule was changed, even if the
updates were small.
First and foremost, the RFCs describing the architectural aspects of
IPv6 addressing explicitly state, refer and repeat this apparently
immutable value: Addressing Architecture [RFC4291], Reserved
Interface Identifiers [RFC5453], ILNP [RFC6741]. Customer Edge
routers impose /64 for their interfaces [RFC7084]. Only the IPv6
Subnet Model [RFC5942] refers to the assumption of /64 prefix length
as a potential implementation error.
Numerous IPv6-over-foo documents make mandatory statements with
respect to the 64-bit length of the Interface ID to be used during
the Stateless Autoconfiguration. These documents include [RFC2464]
(Ethernet), [RFC2467] (FDDI), [RFC2470] (Token Ring), [RFC2492]
(ATM), [RFC2497] (ARCnet), [RFC2590] (Frame Relay), [RFC3146] (IEEE
1394), [RFC4338] (Fibre Channel), [RFC4944] (IEEE 802.15.4),
[RFC5072] (PPP), [RFC5121] [RFC5692] (IEEE 802.16), [RFC2529]
(6over4), [RFC5214] (ISATAP), [I-D.templin-aerolink] (AERO),
[I-D.ietf-6lowpan-btle], [I-D.ietf-6man-6lobac],
[I-D.brandt-6man-lowpanz].
To a lesser extent, the address configuration RFCs themselves may in
some way assume the 64-bit length of an Interface ID (SLAAC for the
link-local addresses, DHCPv6 for the potentially assigned
EUI-64-based IP addresses, Default Router Preferences [RFC4191] for
its impossibility of Prefix Length 4, Optimistic Duplicate Address
Detection [RFC4429] which computes 64-bit-based collision
probabilities).
The MLDv2 protocol [RFC3810] mandates all queries be sent with the
fe80::/64 link-local source address prefix and subsequently bases the
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querier election algorithm on the link-local subnet prefix length of
length /64.
The IPv6 Flow Label Specification [RFC6437] gives an example of a
20-bit hash function generation which relies on splitting an IPv6
address in two equally-sized 64bit-length parts.
The basic transition mechanisms [RFC4213] refer to IIDs of length 64
for link-local addresses, and other transition mechanisms such as
Teredo [RFC4380] assume the use of IIDs of length 64. Similar
assumptions are found in 6to4 [RFC3056] and 6rd [RFC5969].
Translation-based transition mechanisms such as NAT64 and NPTv6 have
some dependency on prefix length, discussed below.
The proposed method [I-D.ietf-v6ops-64share] of extending an assigned
/64 prefix from a smartphone's cellular interface to its WiFi link
relies on prefix length, and implicitely on the length of the
Interface ID, to be valued at 64.
The CGA and HBA specifications rely on the 64-bit identifier length
(see below), as do the Privacy extensions [RFC4941] and some examples
in IKEv2bis [RFC5996].
464XLAT [RFC6877] explicitly mentions acquiring /64 prefixes.
However, it also discusses the possibility of using the interface
address on the device as the endpoint for the traffic, thus
potentially removing this dependency.
[RFC2526] reserves a number of subnet anycast addresses by reserving
some anycast IIDs. An anycast IID so reserved cannot be less than 7
bits long. This means that a subnet prefix length longer than /121
is not possible, and a subnet of exactly /121 would be useless since
all its identifiers are reserved. It also means that half of a /120
is reserved for anycast. This could of course be fixed in the way
described for /127 in [RFC6164], i.e., avoiding the use of anycast
within a /120 subnet.
While preparing this document, it was noted that many other IPv6
specifications refer to mandatory alignment on 64-bit boundaries,
64-bit data structures, 64-bit counters in MIBs, 64-bit sequence
numbers and cookies in security, etc. Finally, the number "64" may
be considered "magic" in some RFCs, e.g., 64k limits in DNS and
Base64 encodings in MIME. None of this has any influence on the
length of the IID, but might confuse a careless reader.
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4. Possible areas of breakage
This section discusses several specific aspects of IPv6 where we can
expect operational breakage with subnet prefixes other than /64.
o Multicast: [RFC3306] defines a method for generating IPv6
multicast group addresses based on unicast prefixes. This method
assumes a longest network prefix of 64 bits. If a longer prefix
is used, there is no way to generate a specific multicast group
address using this method. In such cases the administrator would
need to use an "artificial" prefix from within their allocation (a
/64 or shorter) from which to generate the group address. This
prefix would not correspond to a real subnet.
Similarly [RFC3956], which specifies Embedded-RP, allowing IPv6
multicast rendezvous point addresses to be embedded in the
multicast group address, would also fail, as the scheme assumes a
maximum prefix length of 64 bits.
o CGA: The Cryptographically Generated Address format (CGA,
[RFC3972]) is heavily based on a /64 interface identifier.
[RFC3972] has defined a detailed algorithm how to generate 64-bit
interface identifier from a public key and a 64-bit subnet prefix.
Breaking the /64 boundary would certainly break the current CGA
definition. However, CGA might benefit in a redefined version if
more bits are used for interface identifier (which means shorter
prefix length). For now, 59 bits are used for cryptographic
purposes. The more bits are available, the stronger CGA could be.
Conversely, longer prefixes would weaken CGA.
o NAT64: Both stateless [RFC6052] NAT64 and stateful NAT64 [RFC6146]
are flexible for the prefix length. [RFC6052] has defined
multiple address formats for NAT64. In Section 2 "IPv4-Embedded
IPv6 Prefix and Format" of [RFC6052], the network-specific prefix
could be one of /32, /40, /48, /56, /64 and /96. The remaining
part of the IPv6 address is constructed by a 32-bit IPv4 address,
a 8-bit u byte and a variable length suffix (there is no u byte
and suffix in the case of 96-bit Well-Known Prefix). NAT64 is
therefore OK with a boundary out to /96, but not longer.
o NPTv6: IPv6-to-IPv6 Network Prefix Translation [RFC6296] is also
bound to /64 boundary. NPTv6 maps a /64 prefix with other /64
prefix. When the NPTv6 Translator is configured with a /48 or
shorter prefix, the 64-bit interface identifier is kept unmodified
during translation. However, the /64 boundary might be broken as
long as the "inside" and "outside" prefix has the same length.
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o ILNP: Identifier-Locator Network Protocol (ILNP) [RFC6741] is
designed around the /64 boundary, since it relies on locally
unique 64-bit interface identifiers. While a re-design to use
longer prefixes is not inconceivable, this would need major
changes to the existing specification for the IPv6 version of
ILNP.
o shim6: The Multihoming Shim Protocol for IPv6 (shim6) [RFC5533] in
its insecure form treats IPv6 address as opaque 128-bit objects.
However, to secure the protocol against spoofing, it is essential
to either use CGAs (see above) or Hash-Based Addresses (HBA)
[RFC5535]. Like CGAs, HBAs are generated using a procedure that
assumes a 64-bit identifier. Therefore, in effect, secure shim6
is affected by the /64 boundary exactly like CGAs.
o others?
It goes without saying that if prefixes longer than /64 are to be
used, all hosts must be capable of generating IIDs shorter than 64
bits, in order to follow the auto-configuration procedure correctly
[RFC4862]. There is however the rather special case of the link-
local prefix. While RFC 4862 is careful not to define any specific
length of link-local prefix within fe80::/10, operationally there
would be a problem unless all hosts on a link use IIDs of the same
length to configure a link-local address during reboot. Typically
today the choice of 64 bits for the link-local IID length is hard-
coded per interface. There might be no way to change this except
conceivably by manual configuration, which will be impossible if the
host concerned has no local user interface.
5. Experimental observations
5.1. Survey of the processing of Neighbor Discovery options with
prefixes other than /64
This section provides a survey of the processing of Neighbor
Discovery options which include prefixes that are different than /64.
The behavior of nodes was assessed with respect to the following
options:
o PIO-A: Prefix Information Option (PIO) [RFC4861] with the A bit
set.
o PIO-L: Prefix Information Option (PIO) [RFC4861] with the L bit
set.
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o PIO-AL: Prefix Information Option (PIO) [RFC4861] with both the A
and L bits set.
o RIO: Route Information Option (RIO) [RFC4191].
In the tables below, the following notation is used:
NOT-SUP:
This option is not supported (i.e., it is ignored no matter the
prefix length used).
LOCAL:
The corresponding prefix is considered "on-link".
ROUTE
The corresponding route is added to the IPv6 routing table.
IGNORE:
The Option is ignored as an error.
+--------------------+--------+-------+--------+---------+
| Operating System | PIO-A | PIO-L | PIO-AL | RIO |
+--------------------+--------+-------+--------+---------+
| FreeBSD 9.0 | IGNORE | LOCAL | LOCAL | NOT-SUP |
+--------------------+--------+-------+--------+---------+
| Linux 3.0.0-15 | IGNORE | LOCAL | LOCAL | NOT-SUP |
+--------------------+--------+-------+--------+---------+
| Linux-current | IGNORE | LOCAL | LOCAL | NOT-SUP |
+--------------------+--------+-------+--------+---------+
| NetBSD 5.1 | IGNORE | LOCAL | LOCAL | NOT-SUP |
+--------------------+--------+-------+--------+---------+
| OpenBSD-current | IGNORE | LOCAL | LOCAL | NOT-SUP |
+--------------------+--------+-------+--------+---------+
| Win XP SP2 | IGNORE | LOCAL | LOCAL | ROUTE |
+--------------------+--------+-------+--------+---------+
| Win 7 Home Premium | IGNORE | LOCAL | LOCAL | ROUTE |
+--------------------+--------+-------+--------+---------+
Table 1: Processing of ND options with prefixes longer than /64
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+--------------------+--------+-------+--------+---------+
| Operating System | PIO-A | PIO-L | PIO-AL | RIO |
+--------------------+--------+-------+--------+---------+
| FreeBSD 9.0 | IGNORE | LOCAL | LOCAL | NOT-SUP |
+--------------------+--------+-------+--------+---------+
| Linux 3.0.0-15 | IGNORE | LOCAL | LOCAL | NOT-SUP |
+--------------------+--------+-------+--------+---------+
| Linux-current | IGNORE | LOCAL | LOCAL | NOT-SUP |
+--------------------+--------+-------+--------+---------+
| NetBSD 5.1 | IGNORE | LOCAL | LOCAL | NOT-SUP |
+--------------------+--------+-------+--------+---------+
| OpenBSD-current | IGNORE | LOCAL | LOCAL | NOT-SUP |
+--------------------+--------+-------+--------+---------+
| Win XP SP2 | IGNORE | LOCAL | LOCAL | ROUTE |
+--------------------+--------+-------+--------+---------+
| Win 7 Home Premium | IGNORE | LOCAL | LOCAL | ROUTE |
+--------------------+--------+-------+--------+---------+
Table 2: Processing of ND options with prefixes shorter than /64
The results obtained can be summarized as follows:
o the "A" bit in the Prefix Information Options is honored only if
the prefix length is 64.
o the "L bit in the Prefix Information Options is honored for any
arbitrary prefix length (whether shorter or longer than /64).
o nodes that support the Route Information Option, allow such routes
to be specified with prefixes of any arbitrary length (whether
shorter or longer than /64)
5.2. Other Observations
Participants in the V6OPS working group have indicated that some
forwarding devices have been shown to work correctly with long prefix
masks such as /80 or /96. Indeed, it is to be expected that longest
prefix match based forwarding will work for any prefix length, and no
reports of this failing have been noted. Also, DHCPv6 is in
widespread use without any dependency on the /64 boundary.
Reportedly, there are deployments of /120 subnets configured using
DHCPv6.
It has been reported that at least one type of switch has a content-
addressable memory limited to 144 bits. This means that filters
cannot be defined based on 128-bit addresses and two 16-bit port
numbers; the longest prefix that could be used in such a filter is /
112.
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There have been unconfirmed assertions that some routers have a
performance drop-off for prefixes longer than /64, due to design
issues.
More input with practical observations is welcomed.
6. Privacy issues
The length of the interface identifier has implications for privacy
[I-D.ietf-6man-ipv6-address-generation-privacy]. In any case in
which the value of the identifier is intended to be hard to guess,
whether or not it is cryptographically generated, it is apparent that
more bits are better. For example, if there are only 20 bits to be
guessed, at most just over a million guesses are needed, today well
within the capacity of a low cost attack mechanism. It is hard to
state in general how many bits are enough to protect privacy, since
this depends on the resources available to the attacker, but it seems
clear that a privacy solution needs to resist an attack requiring
billions rather than millions of guesses. Trillions would be better,
suggesting that at least 40 bits should be available. Thus we can
argue that subnet prefixes longer than say /80 might raise privacy
concerns by making the IID guessable.
A prefix long enough to limit the number of addresses comparably to
an IPv4 subnet, such as /120, would create exactly the same situation
for privacy as IPv4. In particular, a host would be forced to pick a
new IID when roaming to a new network, to avoid collisions. An
argument could be made that since this reduces traceability, it is a
good thing from a privacy point of view.
7. Implementation and deployment issues
From an early stage, implementations and deployments of IPv6 assumed
the /64 subnet size, even though routing was based on variable-length
subnet masks of any length. As shown above, this became anchored in
many specifications (Section 3) and in important aspects of
implementations commonly used in local area networks (Section 5). In
fact, a programmer might be lulled into assuming a comfortable rule
of thumb that subnet prefixes are always /64 and an IID is always of
length 64. Apart from the limited evidence in Section 5.1, we cannot
tell without code inspections or tests whether existing stacks are
able to handle a flexible IID length, or whether they would require
modification to do so.
The main practical consequence of the existing specifications is that
deployments in which longer subnet prefixes are used cannot make use
of SLAAC-configured addresses, and require either statically
configured addresses or DHCPv6. To reverse this argument, if it was
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considered desirable to allow auto-configured addresses with subnet
prefixes longer than /64, all of the specifications identified above
as depending on /64 would have to be modified, with due regard to
interoperability with unmodified stacks. In fact
[I-D.ietf-6man-stable-privacy-addresses] allows for this possibility.
Then modified stacks would have to be developed and deployed. It
might be the case that some stacks contain dependencies on the /64
boundary which are not directly implied by the specifications, and
any such hidden dependencies would also need to be found and removed.
Typical IP Address Management (IPAM) tools treat /64 as the default
subnet size, but allow users to specify longer subnet prefixes if
desired. Clearly, all IPAM tools and network management systems
would need to be checked in detail.
Some implementation issues concerning prefix assignment are worth
mentioning.
1. It is sometimes suggested that assigning a prefix such as /48 or
/56 to every user site (including the smallest) as recommended by
[RFC6177] is wasteful. In fact, the currently released unicast
address space, 2000::/3, contains 35 trillion /48 prefixes
((2**45 = 35,184,372,088,832). With 2000::/3 and 0::/3 currently
committed, we still have 75% of the address space in reserve.
Thus there is no objective risk of prefix depletion by assigning
/48 or /56 prefixes. This should be considered when evaluating
the scenario of Section 2.1.
2. Some have argued that more prefix bits are needed to allow a
hierarchical addressing scheme within a campus or corporate
network. However, flat routing is widely and successfully used
within rather large networks, with hundreds of routers and
thousands of end systems. Therefore there is no objective need
for additional prefix bits to support hierarchy and aggregation.
3. Some network operators wish to know and audit which nodes are
active on a network, especially those that are allowed to
communicate off link or off site. They may also wish to limit
the total number of active addresses and sessions that can be
sourced from a particular host, LAN or site, in order to prevent
potential resource depletion attacks or other problems spreading
beyond a certain scope of control. It has been argued that this
type of control would be easier if only long network prefixes
with relatively small numbers of possible hosts per network were
used, reducing the discovery problem.
We now list some practical effects of the fixed /64 boundary.
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o Everything is the same. Compared to IPv4, there is no more
calculating (leaf) subnet sizes, no more juggling between subnets,
fewer errors.
o There are always enough addresses in any subnet to add one or more
devices. There might be other limits, but addressing will never
get in the way.
o Adding a subnet is easy - just take the next /64. No estimates,
calculations, consideration or judgment is needed.
o Router configurations are easier to understand.
o Documentation is easier to write and easier to read; training is
easier.
8. Conclusion
Summary of pros and cons; risks (write this bit last!)
9. Security Considerations
In addition to the privacy issues mentioned in Section 6, and the
issues mentioned with CGAs and HBAs in Section 4, the length of the
subnet prefix affects the matter of defence against scanning attacks
[I-D.ietf-opsec-ipv6-host-scanning]. Assuming the attacker has
discovered or guessed the prefix length, a longer prefix reduces the
space that the attacker needs to scan, e.g., to only 256 addresses if
the prefix is /120. On the other hand, if the attacker has not
discovered the prefix length and assumes it to be /64, routers can
trivially discard attack packets that do not fall within an actual
subnet.
However, assume that an attacker finds one valid address A and then
starts a scanning attack by scanning "outwards" from A, by trying
A+1, A-1, A+2, A-2, etc. This attacker will easily find all hosts in
any subnet with a long prefix, because they will have addresses close
to A. We therefore conclude that any prefix containing densely packed
valid addresses is vulnerable to a scanning attack, without the
attacker needing to guess the prefix length. Therefore, to preserve
IPv6's advantage over IPv4 in resisting scanning attacks, it is
important that subnet prefixes are short enough to allow sparse
allocation of identifiers within each subnet. The considerations are
similar to those for privacy, and we can again argue that prefixes
longer than say /80 might significantly increase vulnerability.
Ironically, this argument is exactly converse to the argument for
longer prefixes to resist an ND cache attack, as described in
Section 2.2.
Carpenter, et al. Expires August 10, 2014 [Page 13]
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Denial of service attacks related to Neighbor Discovery are discussed
in [RFC6583]. One of the mitigations suggested by that document is
"sizing subnets to reflect the number of addresses actually in use",
but the fact that this greatly simplifies scanning attacks is not
noted. For further discussion of scanning attacks, see
[I-D.ietf-opsec-ipv6-host-scanning].
Note that, although not known at the time of writing, there might be
other resource exhaustion attacks available, similar in nature to the
ND cache attack. We cannot exclude that such attacks might be
exacerbated by sparsely populated subnets such as a /64. It should
also be noted that this analysis assumes a conventional deployment
model with a significant number of end-systems located in a single
LAN broadcast domain. Other deployment models might lead to
different conclusions.
10. IANA Considerations
This document requests no action by IANA.
11. Acknowledgements
This document was inspired by a vigorous discussion on the V6OPS
working group mailing list with at least 20 participants. Later,
valuable comments were received from Lorenzo Colitti, David Farmer,
Ray Hunter, Mark Smith, Fred Templin, Stig Venaas, and other
participants in the IETF.
This document was produced using the xml2rfc tool [RFC2629].
12. Change log [RFC Editor: Please remove]
draft-carpenter-6man-why64-01: WG comments, added experimental
results, implementation/deployment text, 2014-02-06.
draft-carpenter-6man-why64-00: original version, 2014-01-06.
13. References
13.1. Normative References
[I-D.ietf-6man-ug]
Carpenter, B. and S. Jiang, "Significance of IPv6
Interface Identifiers", draft-ietf-6man-ug-06 (work in
progress), December 2013.
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[I-D.ietf-opsec-ipv6-host-scanning]
Gont, F. and T. Chown, "Network Reconnaissance in IPv6
Networks", draft-ietf-opsec-ipv6-host-scanning-03 (work in
progress), January 2014.
[RFC2464] Crawford, M., "Transmission of IPv6 Packets over Ethernet
Networks", RFC 2464, December 1998.
[RFC2467] Crawford, M., "Transmission of IPv6 Packets over FDDI
Networks", RFC 2467, December 1998.
[RFC2470] Crawford, M., Narten, T., and S. Thomas, "Transmission of
IPv6 Packets over Token Ring Networks", RFC 2470, December
1998.
[RFC2492] Armitage, G., Schulter, P., and M. Jork, "IPv6 over ATM
Networks", RFC 2492, January 1999.
[RFC2497] Souvatzis, I., "Transmission of IPv6 Packets over ARCnet
Networks", RFC 2497, January 1999.
[RFC2526] Johnson, D. and S. Deering, "Reserved IPv6 Subnet Anycast
Addresses", RFC 2526, March 1999.
[RFC2529] Carpenter, B. and C. Jung, "Transmission of IPv6 over IPv4
Domains without Explicit Tunnels", RFC 2529, March 1999.
[RFC2590] Conta, A., Malis, A., and M. Mueller, "Transmission of
IPv6 Packets over Frame Relay Networks Specification", RFC
2590, May 1999.
[RFC3056] Carpenter, B. and K. Moore, "Connection of IPv6 Domains
via IPv4 Clouds", RFC 3056, February 2001.
[RFC3146] Fujisawa, K. and A. Onoe, "Transmission of IPv6 Packets
over IEEE 1394 Networks", RFC 3146, October 2001.
[RFC3306] Haberman, B. and D. Thaler, "Unicast-Prefix-based IPv6
Multicast Addresses", RFC 3306, August 2002.
[RFC3810] Vida, R. and L. Costa, "Multicast Listener Discovery
Version 2 (MLDv2) for IPv6", RFC 3810, June 2004.
[RFC3956] Savola, P. and B. Haberman, "Embedding the Rendezvous
Point (RP) Address in an IPv6 Multicast Address", RFC
3956, November 2004.
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[RFC3972] Aura, T., "Cryptographically Generated Addresses (CGA)",
RFC 3972, March 2005.
[RFC4191] Draves, R. and D. Thaler, "Default Router Preferences and
More-Specific Routes", RFC 4191, November 2005.
[RFC4213] Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
for IPv6 Hosts and Routers", RFC 4213, October 2005.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[RFC4338] DeSanti, C., Carlson, C., and R. Nixon, "Transmission of
IPv6, IPv4, and Address Resolution Protocol (ARP) Packets
over Fibre Channel", RFC 4338, January 2006.
[RFC4380] Huitema, C., "Teredo: Tunneling IPv6 over UDP through
Network Address Translations (NATs)", RFC 4380, February
2006.
[RFC4429] Moore, N., "Optimistic Duplicate Address Detection (DAD)
for IPv6", RFC 4429, April 2006.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.
[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862, September 2007.
[RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy
Extensions for Stateless Address Autoconfiguration in
IPv6", RFC 4941, September 2007.
[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4
Networks", RFC 4944, September 2007.
[RFC5072] Varada, S., Haskins, D., and E. Allen, "IP Version 6 over
PPP", RFC 5072, September 2007.
[RFC5121] Patil, B., Xia, F., Sarikaya, B., Choi, JH., and S.
Madanapalli, "Transmission of IPv6 via the IPv6
Convergence Sublayer over IEEE 802.16 Networks", RFC 5121,
February 2008.
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[RFC5214] Templin, F., Gleeson, T., and D. Thaler, "Intra-Site
Automatic Tunnel Addressing Protocol (ISATAP)", RFC 5214,
March 2008.
[RFC5453] Krishnan, S., "Reserved IPv6 Interface Identifiers", RFC
5453, February 2009.
[RFC5533] Nordmark, E. and M. Bagnulo, "Shim6: Level 3 Multihoming
Shim Protocol for IPv6", RFC 5533, June 2009.
[RFC5535] Bagnulo, M., "Hash-Based Addresses (HBA)", RFC 5535, June
2009.
[RFC5692] Jeon, H., Jeong, S., and M. Riegel, "Transmission of IP
over Ethernet over IEEE 802.16 Networks", RFC 5692,
October 2009.
[RFC5942] Singh, H., Beebee, W., and E. Nordmark, "IPv6 Subnet
Model: The Relationship between Links and Subnet
Prefixes", RFC 5942, July 2010.
[RFC5969] Townsley, W. and O. Troan, "IPv6 Rapid Deployment on IPv4
Infrastructures (6rd) -- Protocol Specification", RFC
5969, August 2010.
[RFC5996] Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
"Internet Key Exchange Protocol Version 2 (IKEv2)", RFC
5996, September 2010.
[RFC6052] Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052,
October 2010.
[RFC6146] Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
NAT64: Network Address and Protocol Translation from IPv6
Clients to IPv4 Servers", RFC 6146, April 2011.
[RFC6164] Kohno, M., Nitzan, B., Bush, R., Matsuzaki, Y., Colitti,
L., and T. Narten, "Using 127-Bit IPv6 Prefixes on Inter-
Router Links", RFC 6164, April 2011.
[RFC6177] Narten, T., Huston, G., and L. Roberts, "IPv6 Address
Assignment to End Sites", BCP 157, RFC 6177, March 2011.
[RFC6296] Wasserman, M. and F. Baker, "IPv6-to-IPv6 Network Prefix
Translation", RFC 6296, June 2011.
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[RFC6437] Amante, S., Carpenter, B., Jiang, S., and J. Rajahalme,
"IPv6 Flow Label Specification", RFC 6437, November 2011.
[RFC6741] Atkinson,, RJ., "Identifier-Locator Network Protocol
(ILNP) Engineering Considerations", RFC 6741, November
2012.
[RFC7084] Singh, H., Beebee, W., Donley, C., and B. Stark, "Basic
Requirements for IPv6 Customer Edge Routers", RFC 7084,
November 2013.
13.2. Informative References
[DRAFT-odell]
O'Dell, M., "8+8 - An Alternate Addressing Architecture
for IPv6", draft-odell-8+8.00 (work in progress), October
1996.
[I-D.brandt-6man-lowpanz]
Brandt, A. and J. Buron, "Transmission of IPv6 packets
over ITU-T G.9959 Networks", draft-brandt-6man-lowpanz-02
(work in progress), June 2013.
[I-D.ietf-6lowpan-btle]
Nieminen, J., Savolainen, T., Isomaki, M., Patil, B.,
Shelby, Z., and C. Gomez, "Transmission of IPv6 Packets
over BLUETOOTH Low Energy", draft-ietf-6lowpan-btle-12
(work in progress), February 2013.
[I-D.ietf-6man-6lobac]
Lynn, K., Martocci, J., Neilson, C., and S. Donaldson,
"Transmission of IPv6 over MS/TP Networks", draft-ietf-
6man-6lobac-01 (work in progress), March 2012.
[I-D.ietf-6man-ipv6-address-generation-privacy]
Cooper, A., Gont, F., and D. Thaler, "Privacy
Considerations for IPv6 Address Generation Mechanisms",
draft-ietf-6man-ipv6-address-generation-privacy-00 (work
in progress), October 2013.
[I-D.ietf-6man-stable-privacy-addresses]
Gont, F., "A Method for Generating Semantically Opaque
Interface Identifiers with IPv6 Stateless Address
Autoconfiguration (SLAAC)", draft-ietf-6man-stable-
privacy-addresses-16 (work in progress), December 2013.
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[I-D.ietf-homenet-arch]
Chown, T., Arkko, J., Brandt, A., Troan, O., and J. Weil,
"IPv6 Home Networking Architecture Principles", draft-
ietf-homenet-arch-11 (work in progress), October 2013.
[I-D.ietf-v6ops-64share]
Byrne, C., Drown, D., and V. Ales, "Extending an IPv6 /64
Prefix from a 3GPP Mobile Interface to a LAN link", draft-
ietf-v6ops-64share-09 (work in progress), October 2013.
[I-D.templin-aerolink]
Templin, F., "Transmission of IPv6 Packets over AERO
Links", draft-templin-aerolink-01 (work in progress),
January 2014.
[IEEE802] IEEE, "IEEE Standard for Local and Metropolitan Area
Networks: Overview and Architecture", IEEE Std 802-2001
(R2007), 2007.
[RFC2629] Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
June 1999.
[RFC6583] Gashinsky, I., Jaeggli, J., and W. Kumari, "Operational
Neighbor Discovery Problems", RFC 6583, March 2012.
[RFC6741] Atkinson,, RJ., "Identifier-Locator Network Protocol
(ILNP) Engineering Considerations", RFC 6741, November
2012.
[RFC6877] Mawatari, M., Kawashima, M., and C. Byrne, "464XLAT:
Combination of Stateful and Stateless Translation", RFC
6877, April 2013.
Authors' Addresses
Brian Carpenter (editor)
Department of Computer Science
University of Auckland
PB 92019
Auckland 1142
New Zealand
Email: brian.e.carpenter@gmail.com
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Tim Chown
University of Southampton
Southampton, Hampshire SO17 1BJ
United Kingdom
Email: tjc@ecs.soton.ac.uk
Fernando Gont
SI6 Networks / UTN-FRH
Evaristo Carriego 2644
Haedo, Provincia de Buenos Aires 1706
Argentina
Email: fgont@si6networks.com
Sheng Jiang
Huawei Technologies Co., Ltd
Q14, Huawei Campus
No.156 Beiqing Road
Hai-Dian District, Beijing 100095
P.R. China
Email: jiangsheng@huawei.com
Alexandru Petrescu
CEA, LIST
CEA Saclay
Gif-sur-Yvette, Ile-de-France 91190
France
Email: Alexandru.Petrescu@cea.fr
Andrew Yourtchenko
cisco
7a de Kleetlaan
Diegem 1830
Belgium
Email: ayourtch@cisco.com
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